Published by: Elsevier Science
ISBN:0-444-50851-1
NeuroImmune Biology: Vol.3:The Immune-Neuroendocrine Circuitry:
History and Progress

Description:
The book summarizes the current understanding of the Nervous - Endocrine and Immune systems with emphasis on shared mediators and receptors and functional interaction. In addition to the fundamental physiological and pathophysiological mechanisms, which are presented in detail, some clinically relevant subjects are also presented, such as inflammation, asthma and allergy, autoimmune disease, immunodeficiency and the acute phase response.
 

	A comprehensive presentation of neuroimmune biology.
	Introduces the subject matter to the uninformed reader.
	Contains basic information, theoretical considerations and up-to-date  clinical chapters.
	The clinical chapters will be helpful to practising physicians.

Audience:

Neurologists, psychologists, psychiatrists, immunologists, endocrinologists, physiologists, practising clinicians, veterinarians, animal scientists.
 

Contents:

Foreword:
Istvan Berczi, Andor Szentivanyi

Preface:
Istvan Berczi, Andor Szentivanyi

Part I: The nervous system, receptors, ligands and signal transduction
 

Acknowledgements.

I. Introduction

Introduction 
Andor Szentivanyi, Istvan Berczi, Harry Nyanteh, Allan Goldman 

History
Andor Szentivanyi, Istvan Berczi, Harry Nyanteh, Allan Goldman

The discovery of immune neuroendocrine circuitry - A generation of progress.
Andor  Szentivanyi, Istvan Berczi, Harry Nyanteh, Allan Goldman

II. The Nervous System - A historical perspective.

Altered effector responses.
Andor Szentivanyi, Istvan Berczi, Harry Nyanteh, Allan Goldman

Some evolutionary morphoregulatory and functional aspects of the immune-neuroendocrine circuitry.
Andor Szentivanyi, Istvan Berczi, Harry Nyanteh, Allan Goldman

Virus associated immune and pharmacologic mechanisms in disorders of respiratory and cutaneous atopy.
Andor Szentivanyi, Istvan Berczi, Harry Nyanteh, Allan Goldman

III. Receptors, ligands and signaling

Adhesion molecules
Istvan Berczi, Andor Szentivanyi

Immunoglobulins
Istvan Berczi, Andor Szentivanyi

Growth and lactogenic hormones, insulin-like growth factor and insulin.
Istvan Berczi, Andor Szentivanyi

The hypothalamus-pituitary-adrenal axis and opioid peptides.
Istvan Berczi, Andor Szentivanyi

The hypothalamus-pituitary-thyroid axis.
Istvan Berczi, Andor Szentivanyi

Nerve growth factor, leptin and neuropeptides.
Istvan Berczi, Andor Szentivanyi

Cytokines and chemokines.
Istvan Berczi, Andor Szentivanyi

Steroid hormones.
Istvan Berczi, Eva Nagy, Edward Baral, Andor Szentivanyi 

Regulatory enzymes.
Istvan Berczi, Edris Sabbadini

Part II: Neuroimmune Function and the Neuroimmune Regulatory Network

IV.  Neuroimmune function 

A. Physiology

Immunocompetence
Istvan Berczi, Andor Szentivanyi

Antigen presentation.
Istvan Berczi, Andor Szentivanyi

Immune reactions.
Istvan Berczi, Andor Szentivanyi

The hypothalamus-pituitary-adrenal (HPA) axis: A major mediator of the adaptive responses to stress.
K. Eddie Gabry, George Chrousos, Philip W. Gold 

Immunoregulation by innervation.
Dwight Nance, Brian MacNeil

B. Pathophysiology

Inflammation in the airways.
Peter Barnes

Defensins: antimicrobial peptides with a broad spectrum of biological activity.
Elena A. Korneva, Vladimir N. Kokryakov

The acute phase response.
Istvan Berczi, Andor Szentivanyi

Autoimmune disease. 
Istvan Berczi, Andor Szentivanyi

Immunodeficiency
Istvan Berczi, Andor Szentivanyi

V. Immune-Neuroendocrine Circuitry

The immune-neuroendocrine circuitry.
Istvan Berczi, Andor Szentivanyi
 

(Article used with permission, NIB 2003;3 pp561-592)
The Immune-Neuroendocrine Circuitry.

ABSTRACT.

  That a healthy mind is fundamental to general well being has been recognized since prehistoric times and proverbs analogous to "Healthy body - healthy mind" exist in many languages. Scientific inquiries with regards to mind-body interactions commenced over a century ago. Hans Selye discovered that stress activates the hypothalamus-pituitary-adrenal-thymus axis, which results in the development of the 'general adaptation syndrome'. This syndrome is characterised by elevated resistance of stressed animals to diverse insults [1].   Andor Szentivanyi and colleagues discovered [2-4] that hypothalamic lesions prevent anaphylactic death in guinea pigs.  This was the first experimental evidence for the sweeping regulatory power of the nervous system over violent, life threatening immune reactions. Miklos Jancso and coworkers [5]. described that the nervous system also controls the inflammatory response.

     In the seventies and eighties a handful of laboratories started to re-examine various aspects of neuroimmune-interaction.  It was established that pituitary hormones have the capacity to stimulate, inhibit and modulate immune responses.  Placental and pituitary hormones are also involved in the development of the immune system and maintenance of immunocompetence.  It was also described that lymphoid organs are innervated and that neurotransmitters and neuropeptides are important regulators of immune and inflammatory reactions.  It became gradually apparent that immune derived cytokines and nerve impulses serve as feedback signals towards the neuroendocrine system.  Compelling evidence was produced, indicating that immune reactions may be conditioned in the classical pavlovian sense and that emotions affect immune function.  Evidence is increasing rapidly for the physiological role of cytokines and of immunocytes in the function of various organs and tissues, and in reproduction.  It is also becoming obvious that Selye's general adaptation syndrome really corresponds to the acute phase response.  This is a multi-faceted and highly co-ordinated systemic defence reaction, which involves the conversion of the immune system from a specific, adaptive mode of reactivity to a rapidly amplifiable, poly-specific reaction mediated by natural immune mechanisms.  Immunological (poly)specificity is assured by profoundly elevated levels of natural antibodies and liver-derived acute phase proteins.

     Much has been learned about the regulation of cell activation, growth and function from immunological studies.  Burnet's clonal selectional theory designated the antigen as the sole activator [6]. Bretcher and Cohn recognised first that at least 2 signals are required [7].  This was followed by numerous studies on cell-to-cell interaction within the immune system and led to our current understanding of the importance of cell adhesion molecules and cytokines in cell activation and proliferation.  This, coupled with the available information about the mechanisms of action of hormones and neurotransmitters, of signal transduction and nuclear regulatory pathways paves the way to understanding how higher organisms function in their entire complexity.  It is now apparent that the Nervous- Endocrine- and Immune-Systems form a systemic regulatory network, which is capable of regulating all aspects of bodily functions in health and disease.  Thus, Neuroimmune Biology provides new foundations to Biology.

1. HISTORY

  The first paper on the role of the nervous system in immunological reactions was published in 1898 by Salomonsen and Madsen [8]. This was followed by a book on vagotomy in 1910 [9] that explained anaphylaxis and allergy with a pathological increase in the firing of the vagus nerve. Hyperergic inflammatory reactions were observed in denervated tissue during the 1930s. However, such tissue shows hyperreactivity to neurotransmitters that are present in serum or may reach the tissue through local axonal reflexes. Today it is clear that the sympathetic and the parasympathetic nerveus systems are in a dynamic equilibrium, which changes continuously according to the local regulatory demands of the organs and tissues innervated. Pavlov’s students applied successfully the methodology of conditioning to the phenomenon of anaphylaxis. Unfortunately contemporary scientists rejected these findings.

     Szentivanyi and co-workers during 1949-1952 [2-4] obtained definitive scientific evidence for the dominant regulatory role of the hypothalamus (tuber cinereum) over anaphylactic reactions in guinea pigs. Other investigators confirmed these original observations by producing evidence that both the hypothalamus and the pituitary gland are key regulators of the immune system. Today much evidence is available in the literature to support the role of the neuroendocrine system in immunoregulation as is obvious from the material presented in this book.

2. THEORETICAL PROGRESS.

  The recognition of the immunoregulatory role of the hypothalamus in 1951 coincided with the advent of knowledge about some basic aspects of immunology. The nature of antibody diversity occupied the focus of interest in the light of new genetic studies completed. The “instructionist” theories of immune activation have been abandoned on the basis of new knowledge in favour of “selection theories” as advanced by Talmage in 1957 [10] and Burnet [6]. These theories served as the basis for the elucidation of the genetic and molecular basis of lymphocyte function during immune responses, as we know it today. The recognition that the immune system is highly adaptable through receptor mutation, possesses memory and is capable of responding under in vitro conditions led to the conclusion that it is a virtually autonomous system that defends the body from foreign pathogens.  This view is still maintained by most immunologists who see no need or use for additional systemic regulatory intervention.

     Under these conditions the scientific community at large has ignored the evidence that accumulated slowly with regards to the interaction of the Neuroendocrine and Immune systems. Neverteless, the significance of this relationship has been re-emphasised in 1966 [11] and it was proposed that immune homeostasis and the homeostasis of the organism are closely linked.  Much evidence has accumulated since then to support this hypothesis.

3. THE CELLULAR FOUNDATION OF NERVOUS TISSUE AND ITS ORGANIZATION

     The nervous system and the central organ of the adaptive immune sytem, the thymus originate from the neural crest, and these two systems share numerous adhesion molecules and soluble mediators. All neural tissue consists of nerve cells called neurons, and supporting cells. Neurons have long fibers called axons and short ones named dendrites. Supporting elements are the Schwann cells and   oligodendrocytes that provide the nerve axons with myelin sheaths. Microglia are bone marrow derived and macrophage-related. Astrocytes are plentiful but ill defined in their function. Ependymal cells line the internal cavities of the brain. The blood vessels in the brain are lined with endothelial cells forming tight junctions that form the blood-brain-barrier.

4. THE AUTONOMIC NERVOUS SYSTEM

  The autonomic nervous system may be divided into sympathetic (adrenergic) and parasympathetic (cholinergic) compartments, with norepinephrine and acethylcholine as  neurotransmitters.  Anatomically this system is often referred to as the thoracolumbar or sympathetic and craniosacral or parasympathetic division. Sympathetic chromaffin tissues are the adrenal medullary cells, paraganglia and the carotid body and homologous tissues of the great vessels in the thorax.

     The nervous system uses ionic currents and neurotransmitters to communicate, which are transmitted from one cell to the other through synapses. Synapses may be either electrical (gap junctions) or chemical (neuroeffector junctions). Neuroeffector junctions are formed between nerve cells and non-nervous effector cells. A junctional contact consists of a pre-synaptic membrane of the axon, a post-synaptic membrane of the dendrite or perycaryon and an inter-membrane synaptic cleft (~200- 300 Å). Because of lack of anatomical continuity, the nerve impulse is transmitted from the first to the second cell by specific mediators called neurotransmitters. The transmitters are secreted into the cleft where they reach their receptors on the target cell membrane.

     Adrenergic receptors may be subdivided into alpha and beta types which contain several subtypes, and cholinergic receptors may be muscarinic or nicotinic. Biogenic amines (histamine, serotonin, acethylcholine), amino acids ((gamma-aminobutyric acid - GABA, glycine), peptides (such as substance-P, neurotensin, calcitonin-gene related peptide, somatostatin, vasoactive intestinal peptide, the endogenous opioid peptides, vasopressin and oxytocin and thymic peptides, etc), non-conventional neurotransmitters (nitric oxide, carbon monoxide) and growth factors (nerve growth factor, neurotrophins), kinins (kallikrein, bradykinin, leukokinin) serve as neurotransmitters.

5.THE ENDOCRINE HYP0THALAMUS AND CYTOKINES

     The hypothalamus contains neurons that coordinate and integrate distant organ system activities through the effector functions of the endocrine and autonomic nervous systems ("endocrine"' hypothalamus). Peptide neurotransmitters produced by hypophysiotropic neurons in the hypothalamus control anterior pituitary hormone secretion. Nearly 40 peptides are present in the median eminence (ME)* of the hypothalamus, which are produced in different hypothalamic nuclei: corticotropin­ releasing factor (CRF) in the paraventricular nucleus (PVN); growth hormone ­releasing hormone (GHRH) in the arcuate nucleus; somatostatin (SRIF) in the anterior periventricular area; thyrotropin-releasing hormone (TRH) in the PVN, and dopamine (DA) in the A12 region of the hypothalamus and the A2 and A4 regions of the brain stem. The neurons containing luteinizing hormone-releasing hormone (LHRH) are scattered in the diagonal band of Broca, the medial septum, the medial preoptic and suprachiasmatic areas, and the lateral basal hypothalamus.

     Several cytokines, including IL-lalpha, IL-1beta, IL-2, IL-6, TNF-alpha, and IFN-gamma are now known to affect the release of pituitary hormones by an action on the hypothalamus and/or the pituitary gland. Their predominant effects are to stimulate the hypothalamic -pituitary-adrenal axis and to suppress the hypothalamic-pituitary­-thyroid and gonadal axes. IFN-gamma inhibits the HPA axis. The main effect of cytokines is to stimulate the release of hypothalamic CRF. This effect is then augmented by direct pituitary actions of IL-1 and IL-6 from hypothalamic and/or pituitary sources (possibly from both) on basal and CRF-stimulated ACTH secretion. Conversely,glucocorticoids have been known for some time to have a negative feedback on interleukin synthesis in cells of the immune system. The predominant effect of IL-1 on the hypothalamic-pituitary-gonadal axis is inhibitory via a central action on gonadotropin-releasing hormone (GnRH) secretion. IL-1, TNF-alpha, and IFN-gamma have an inhibitory effect on thyroid hormone secretion acting directly at the hypothalamic-pituitary level. IL-1, IL-6, TNF-alpha, and the interferons (alpha, beta, gamma) all stimulate prolactin release from anterior pituitary cells. IL-6 release is stimulated from the anterior pituitary by IL-1, vasoactive intestinal peptide, pituitary adenylate cyclase-activating polypeptide (PACAP), and calcitonin gene-related peptide. IL-1 predominantly stimulates somatostatin release and this may result in an overall suppression of GH secretion.

6.  ALTERED EFFECTOR RESPONSES

     During neurotransmission, whether synaptic or neuroeffector, the second unit should always be regarded as an effector cell, regardless of its nervous or non-nervous nature. In the autonomic system there are three types of effector cells: the neurons, the smooth muscle, and exocrine gland cells. These are the principal target cells of the pharmacologic mediators of allergic responses. The mediators of an­tigen-antibody responses, when viewed from the stand­point of their physiologic function, are among the natural chemi­cal organizers of autonomic action. Denervation or mediator antagonist drugs cause hypersensitivity in the effector cells towards the transmitter in short supply.

      The hypothalamus has two reciprocally antagonistic divisions: the anterior hypothalamus, which mediates primarily cholinergic responses, and the posterior hypo­thalamus, the stimulation of which results largely in adrenergic responses. A balance between these divisions is thought to be important in maintaining normal autonomic functions (e.g., blood pressure). The posterior hypothalamus is normally suppressed by in­hibitory impulses transmitted from the sinoaortic barore­captors, which, among others, keep the posterior hypothalamus in check. When histamine or acethylcholine is given, blood pressure falls, the sinoaortic tension decreases, causing a shift to sympathetic ac­tivity. This leads to the release of cate­chols, which tends to correct and limit the blood pressure drop. Catecholamines increase blood pressure, which shifts the hypothalamic balance to the cholinergic side. It is possible to produce imbalance of the hypothalamus by applying hista­mine, acethylcholine, or catecholamines directly on hypothalamic structures: by the electrolytic removal or electrical stimula­tion of one of the divisions of the hypothalamus; and by many other stimuli.

7.SOME EVOLUTIONARY MORPHOREGULATORY AND FUNCTIONAL ASPECTS OF THE 
   IMMUNE- NEUROENDOCINE CIRCUITRY

  The immune system is distributed throughout the body and its basic function is local disease prevention and regulation. The nervous system and the thymus epithelium are both develop from the neural crest. Neuronal immunoglobulin gene superfamilies (Ig SFs) and cytokines and their receptors play important roles during the embryonic development of both systems. The neural cell adhesion molecule (N-CAM) belongs to IgSF and plays an important role in the development of the central nervous system by cell-to-cell and cell-to-matrix adhesion and promotion of synapse formation. Extracellular matrix (ECM) proteins have neurite-promoting activity. These ECM proteins are: type IV collagen, fibronectin, laminin, and a newly discovered protein, neurite outgrowth factor (NOF), which promotes neurite outgrowth from sympathetic, parasympathetic, sensory, motor, and central neurons. Cytokines, that are shared by the immune and nervous systems also play a role in neural development and function. They include the cilliary neurotropic factor, the leukemia inhibitory factor, oncostatin M, nerve growth factor, brain-derived neurotropic factor, neurotropin-3 and neurotropins 4/5. Factors known to regulate NGF secretion in astrocytes include IL-1, IL-4 and IL-5. The origin of cytokine receptors also represents a classical example of molecular co-evolution in the immune and nervous systems.

     Immunomodulation through the T-cell lineage in rodents can be achived by the manipulation of hemispheric lateralization in the brain. It appears that he right hemisphere controls the inductive influence on T cells of signal emitted by the left hemisphere.

     Two phenomena are associated with learning and memory in the nervous sytem: habituation and potentiation.  Habituation defines decreased responsiveness to a stimulus  presented repetitively over time and potentiation refers to increased responsiveness to repetitive stimuli. The hippocampus plays a special role in learning. Experiments on hippocampal slices indicate that the biochemical changes in the synapse represent the molecular bases for long-term memory. They are: (1) a monoamine (serotonin) and a glutamate receptor (NMDA receptor) are involved; (2) binding of the neurotransmitter (serotonin, glutamate) by these receptors initiates a cascade of enzymatic reactions; (3) the first step in this cascade is the activation of a G-protein mediated signal transduction that ultimately affects the efficacy of synaptic transmission. In B memory cells the affinity maturation of the antigen receptor takes place that potentiates signal transduction after antigenic stimulation. Other possible connections between neuronal and immunological memory are: the survival gene, bcl-2, plays a significant role in both immune and neuronal memory; so far IL-1alpha has been found to be present only in a subset of T-memory cells and hippocampal neurons; transgenic mice overproducing bcl-2 have a long-term persistence of immunoglobulin-secreting cells and an extended lifetime for memory B cells. This suggests that the biochemical basis of all memory-forming processes are highly similar.

     Lymphocytic IL-1alpha is a cell- and species-specific factor, which is capable of increasing beta-adrenoceptor concentration and induction of its gene. These observations are highly important both for normal airway physiology as well as for the possible nature of the beta-adrenergic dysregulation in asthma by adding an entirely new dimension to the beta -adrenergic theory of the atopic abnormality in bronchial asthma.

     Stress alters immune reactivity through specific interactions at every level of the neuroimmune regulatory system. This is based on four critical features shared by both systems: (1) they are composed of extraordinarily large numbers of phenotypically distinct cells organized into intricate networks. (2) Cells of both systems synthesize, secrete, and/or release shared effector molecules. (3) Recognition of these effector molecules is realized by the same cellular receptors and second messenger mechanisms of both cell systems. (4) These cellular and molecular determinants make a continuous, multi-lateral flow of information, which is the sine qua non of the unique interactions within the immune-neuroendocrine circuitry.

     Neuropeptides take on added significance as immuno­modulators, since it is now known that lymphoid organs are directly innervated with nerves secreting these agents. From the standpoint of the integration of information in the immune - neuroendocrine circuitry, future studies will have to examine these parallel signaling pathways in isolation.

8. VIRUS ASSOCIATED IMMUNE AND PHARMACOLOGIC MECHANISMS IN DISORDER OF THE
    RESPIRATORY AND CUTANEOUS ATOPY

8.1 Infection  associated immune and pharmacologic mechanisms in atopy.

The prototype of the non-atopic immediate hypersensitivity is localized or generalized anaphylaxis, whereas manifestations of atopic immediate hypersensitivity include bronchial asthma, hay fever, allergic rhinitis, chronic urticaria, and atopic dermatitis. Only a minority of the population shows some form of atopic disease in spite of the fact that, by and large, identical conditions of antigens must be presumed to exist for all members of the population.  The nature of the constitutional basis of atopy, that is, of the underlying determinant for the development of atopic disease, is as yet unexplained. Two possible explanations for the pathogenesis of these conditions have survived: 1) atopy is a primary disorder of the immune system with sequelae in the various effector tissues; and 2) a concept of atopy as a primary autonomic imbalance, essentially beta adrenergic in character, with sequelae in effector cells, including those engaged in the production of antibodies.  The autonomic imbalance is perceived as caused not by some disorder of the autonomic nervous system itself but by a defector functioning of its effector cells. These two concepts are not mutually exclusive. The IgE antibody, which mediates allergic reactions, is essentially identical with atopic reagin in various animal species.

     In anaphylaxis we are dealing with a normal (physiologic) antibody response to an unnatural exposure to antigen, whereas in atopic allergic an “abnormal” antibody response to natural antigenic exposure seems to be involved.  Anaphylactic reactivity of the sensitized individual depends on the release of an amount of pharmacologically active effector molecules sufficient to be toxic for most members of the same species.  In contrast, individuals with atopic disease possess a quantitatively and qualitatively abnormal reactivity to otherwise nontoxic concentrations of endogenously released or exogenously administered pharmacologic mediators.  The quantitative change consistently is in the direction of a decreased response when beta-adrenergic agents are the agonists and in the direction of an increased response when any one of the other pharmacologically active effector molecules are involved.

  Infection and a number of unrelated stimuli may play major contributory roles in atopy. Atopic disease with its spontaneous pattern of familial occurrence cannot be induced at will. The essential difference between immediate hypersensitivities of the non atopic and atopic varieties is that the former conditions are mediated by normal immune and pharmacologic mechanisms, whereas atopy is based on abnormal immune and pharmacologic mechanisms.
     Filip, Szentivanyi and Mess [2-4] used hypothalamic “imbalanced” anaphylactic guinea pigs as models for atopic disease.  By electrolytic removal of one hypothalamic division or electric stimulation of the antagonistic division, it was possible to alter profoundly the anaphylactic reactivity of guinea pigs both immunologically and pharmacologically.  From both the immunologic and pharmacologic standpoints, the conditions so produced more closely approximated those of the human atopic state than does anaphylaxis.  Later Szentivanyi and Fishel [12] found that the Bordetella Pertussis- induced hypersensitive state served as a more appropriate model. The hypersensitivity of the pertussis sensitized mouse to pharmacologic mediators was found to be due to an acquired or genetically determined autonomic imbalance caused primarily by a reduced functioning of the adenylate cyclase coupled beta adrenergic receptors and the associated cyclic AMP system.

8.2 The beta-adrenergic theory of atopic disease

     This theory regards atopic disorders (i.e., perennial and seasonal allergic rhinitis, bronchial asthma, and atopic dermatitis) not as immunologic diseases but as unique patterns of altered reactivities to a broad spectrum of immunologic, psychic, infectious, chemical and physical stimuli. The antigen-antibody interaction is given the same role as that of a broad category of nonspecific stimuli that function only to trigger the same defective homeostatic mechanism in the various effector cells involved in immediate hypersensitivities [13]. Regardless of the immunologic or non-immunologic nature of the triggering event, the chemical realization of hypersensitivity would be expected to be brought about by the same mediators. The theory postulated that the constitutional basis of atopy lies in a relatively unresponsive beta-adrenergic effector system and the resultant autonomic imbalance deprives the effector tissues of their normal counter regulatory adjustment.  This leads to a unique pattern of quantitatively and qualitatively altered reactivity to the chemical organizers of autonomic action, mostly in response to trivial trauma. This alters immune reactions, such as chemotaxis, phagocytosis and cytotoxicity. T cell mediated immunity is suppressed, whereas IgE production and Fc receptor expression is stimulated, mast cell mediator release is exaggerated, bronchial smooth muscle constriction is augmented, mucus secretion is increased and eosiniphilia will develop. All these alterations point to the malfunctioning of the adenylate cyclase-coupled beta-adrenergic receptor and the associated cyclic nucleotide complex. Current evidence favors the possibility that there are inherited and/or acquired multiple abnormalities in the receptor - adenylate cyclase - cyclic  AMP system of all effector cells that are critical in the organization of immune reactivities.

     Atopic abnormality may be 1) acquired by functional receptor regulatory shifts caused by hormonal changes, infection (viral, bacterial, etc) allergic tissue injury or other event; 2) genetically determined; or 3) caused by autoimmune disease.  One, two or all three of these effector mechanisms may be operative in a particular disease.

     There is an important relationship between asthma and viral respiratory infection. A history of childhood viral respiratory illness is a risk factor for the development of chronic obstructive airway syndromes in later life. If such infection leads to obstructive airway disease, the resultant manifestation is likely to be a “wheezy” or asthmatic type of obstructive airway disease. Respiratory viruses commonly induce exacerbations of bronchospasm in the older child, and in adults with known asthma. However, not all viruses can be implicated in this phenomenon.  This is a finding that is difficult to reconcile with the damaged epithelium hypothesis (assuming equivalent of infection) and raises the question of other possible mechanisms related to the biochemical properties of the virus. An endotoxin-like action of influenza vaccine has been described and vaccination with purified LPS from E. Coli resulted in decreased number of beta-adrenergic receptors in guinea pig lung. It was also observed that a simple colonization of the respiratory tract by virus was not sufficient to provoke asthma: such attacks occurred only when the infection produced symptoms of fever, malaise, cough or coryza.  The dominant role of fever in these episodes immediately suggests the profound involvement of adrenergic effector mechanisms.

      In patients with atopic dermatitis the increased susceptibility to viral infections is not restricted to dermatotropic viruses but rather reflects an abnormal host response to viral infections in general. Defective cytotoxic T cells and also the association of abnormally functioning macrophages and natural killer cells, appear to have an important role in the impaired host defense against viral infections in atopic dermatitis. A reduced production of IFN-gamma in children with atopic dermatitis  as well as of IFN-gamma in atopic patients with food allergy has recently been demonstrated. Lymphocytic cyclic AMP-phosphodiesterase, that destroys cyclic AMP, is increased in atopic dermatitis and in allergic respiratory disease of adults, and this increased activity correlated closely with histamine release from basophils. A significant elevation of phosphodiesterase activity was reconfirmed in newborns with a positive atopic history in first-degree relatives, compared to newborns with a negative history. However, there was no cor­relation between phosphodiesterase activity and histamine release. This strongly suggests that increased cyclic AMP phosphodiesterase activity plays a primary role in the pathogenesis of atopic disease. Studies of peripheral blood leukocytes and lymphocytes in atopic dermatitis have frequently demonstrated impaired beta adrenergic reactivity.

8.3. The allergic tissue injury as a developmental mechanism of beta adrenergic subsensitivity

     Allergic tissue injury may be initiated by antigen-specific IgE antibodies that combine with FcE receptors on various cell types and trigger mediator release upon encounter with the antigen. Various noxious agents that are capable of triggering asthma are capable of releasing inflammatory mediators from the same target cells. Accounting only for those pharmacologic mediators where the cell-type has been identified, the spectrum of mediator-storing, synthesizing, or transporting cells, includes the neutrophil leucocytes that produce and release slow-reacting substance of anaphylaxis (SRS-A), eosinophil chemotactic factor of anaphylaxis (ECF-A), enzymes, vascular permeability factors, kinin-generating substances, a complement- activating factor, histamine-releasers, and a neutrophil inhibitory factor (NIF); basophilic leucocytes  that release histamine, SRS-A, ECF-A, neutrophil chemo- tactic factor (NCF) and platelet-activating factor (PAF); the murine basophilic leucocyte  releasing histamine, SRS-A, ECF-A, PAF, and serotonin;  eosinophilic leucocytes that secrete histamine, PAF, and possibly SRS-A; the mast cell with histamine, SRS-A, ECF-A, NCF and PAF content, the murine mast cell that contains histamine, SRS-A, ECF-A, PAF, NCF and serotonin, the “chromaffin-positive” mast cells with dopamine content in ruminants - in other mammals possibly norepinephrine; the enterochromaffin cell containing serotonin; the chromaffin cell  containing catecholamines; the platelet, - depending on species, may contain histamine, serotonin, catecholamines, and prostaglandins; the neurosecretory cell with histamine, serotonin, catecholamines, acetylcholine, and prostaglandin cotent, and nerve cells that potentially produce all amine-mediators as well as prostaglandins and kinins. Collectively, these pharmacologically active agents produce an increase in blood flow, in capillary permeability, constriction of smooth muscles, and secretion of mucus by exocrine glands. These are the manifestations that dominate the clinical picture of immediate hypersensitivities and the associated inflammatory responses. Pharmacologically active adrenergic agents can both release as well as inhibit the release of allergic mediators. Such non-immunologic histamine release renders sensitized mast cells incapable of responding to antigen challenge with histamine release.

  Methylxanthines inhibit histamine release, possibly because they are competitive inhibitors of the specific phosphodiesterase that inactivates cyclic 3’, 5’AMP and thereby induce “adrenergic action” by increasing the intracellular concentration of the compound. The adenylate cyclase system therefore must be considered as a critical regulatory system in allergic histamine release. The allergic release of histamine or of other pharmacologic mediators of immediate hypersensitivity is also inhibited by prostaglandins of the E series, prostacycline, adenosine, and histamine. These are substances that interact with cell membrane receptors that activate adenylate cyclase. Current evidence suggests that cyclic AMP acts early in the release process, that it is linked to the obligatory inward flux of calcium, and that it is related to microtubule function.

8.4 Allergic tissue injury, beta-adrenergic subsensitivity and asthma

Recent studies in patients with extrinsic asthma and in animal models of experimental asthma indicated that the allergic tissue injury itself may result in the development of some forms of beta-adrenergic subsensitivity. Beta-adrenergic subsensitivity of airway smooth muscle that was associated with airways hyper-reactivity in a canine asthma model which was due to a deficiency of beta-adrenoceptors. All post receptor beta-adrenergic responses that were measured (cAMP, protein kinase, relaxation) tended to be depressed in animals with airways hyper-reactivity. These observations suggest that the allergic process can cause beta-adrenergic subsensitivity. However, beta-adrenergic subsensitivity in asthma can be shown to occur in the absence of allergic symptoms or beta-adrenergic medication, and under circumstances in which prior or concurrent beta adrenergic medication can be only one contributing factor to defective beta-adrenergic function. This is also reflected by the presence of beta-adrenergic subsensitivity in atopic dermatitis in which beta-adrenergic medication is not used as a therapeutic modality. Nevertheless, endogenous release of catecholamines in response to the allergic tissue injury may contribute to the development of beta-adrenergic subsensitivity through homologous desensitization of the beta-adrenergic receptors. At the same time the endogenous release of other pharmacologic mediators (i.e., histamine) in response to the allergic tissue injury may contribute to the beta-adrenergic subsensitivity through heterologous desensitization.

     Enhanced “releasability” of histamine from basophils and mast cells has been shown to occur in atopic dermatitis and in bronchial asthma. There is also evidence of increased spontaneous in vitro IgE-secretion by lymphocytes from patients with atopic dermatitis. The regulation of IgE secretion in man has not been elucidated in full detail. Furthermore, the exact pathogenetic role of IgE-mediated reactions in atopic dermatitis is still controversial.

8.5 Autoimmunity as a developmental mechanism of beta-adrenergic subsensitivity

  Myasthenia gravis is caused by autoantibodies directed at nicotinic acetylcholine receptors at the neuromuscular end-plates, Graves’ disease involves autoantibodies to the thyrotropin receptor, and the severe insulin resistance in Type II insulin-resistant diabetes that has been ascribed to autoantibodies to the insulin receptor. It has been recently described that autoantibodies to beta adrenoceptors can be identified in the plasma of some subjects with atopic allergy. Such autoantibodies bound to calf lung membranes, and inhibited stereospecific beta-adrenergic radioligand binding to calf lung beta adrenoceptors, and also precipitated solubilized calf lung beta-adrenergic receptors. The presence of such autoantibodies to beta-adrenoceptors in patients correlated well with a reduced beta - and an increased alpha-adrenergic responsiveness. Virus infections can elicit autoantibody formation.

9. RECEPTORS, LIGANDS AND SIGNAL TRANSDUCTION

9.1 Adhesion molecules

  Adhesion molecules are cell membrane receptors that mediate cell-to-cell and cell-to-matrix communication and are involved in organ, tissue and cell specific regulation under physiological and pathophysiological conditions. These molecules are present in all cells of the body, some may be unique to the tissue/cell type, while others are shared amongst numerous tissues/organs and cells. This regulatory system has the power to alter/modify/tune systemic regulatory signals and to generate signals locally in order to match tissue/cell function with local demands in health and disease. Moreover adhesion molecules are involved in immune-activation and function as well as in immune neuroendocrine communication. Four major families of adhesion molecules are distinguished. (i) The immunoglobulin superfamily (IgSF): Fc receptors for immunoglobulins, the CD4 and CD8 co-receptors of T cells, major histocompatibility antigens, and the T and B lymphocyte antigen receptor are members of this family. (ii) Integrins are transmembrane glycoprotein receptors that play critical roles in matrix-to-cell and cell-to-cell signaling and play key roles in the early development of invertebrates and vertebrates, in inflammation and in tumor metastasis. Beta-1 integrins play crucial roles in lymphocyte adhesion, migration, proliferation and differentiation. Beta-2 integrins mediate leukocyte cell-to-cell and cell-to-matrix adhesions during inflammation and immune responses. (iii) Selectins recognize carbohydrate molecules on pathogenic microorganisms as well as on self-components. They play a role in cellular interactions, function as cell surface receptors that mediate leukocyte recirculation and natural killer cell activity as well as play a role in inflammation. Patients with neutrophils deficient in selectin ligand suffer from recurrent infections, persistent leukocytosis and severe growth and mental retardation. (iv) Cadherins are abundant in the central nervous system (CNS), but also present in many other tissues. Classical cadherins, desmosomal cadherins, seven-pass transmembrane cadherins and protocadherins are major families belonging to this superfamily. Cell-to-cell signaling by cadherins affects cytoskeletal proteins and the organization of adherens junctions. Seven-pass transmembrane cadherins signal through G-protein coupled receptors, regulate cell polarity and thus determine neuronal morphology. Protocadherins are numerous and several of them display an immunoglobulin-like genetic structure. The level of cadherin expression by cells determines the strength of adhesions and the type determines the specificity of cell interactions. Cadherins regulate epithelial cell shape and differentiation, contribute to central nervous system (CNS) regionalization, morphogenesis, fiber tract formation and to the maintenance of functional structures. In reproductive structures, cadherins are responsive to steroid hormones.

9.2. Immunoglobulins (Ig)

  Immunoglobulins are serum proteins produced by bone marrow-derived (B) lymphocytes and function as specific antibodies. Five classes are known in man and higher animals: IgA, IgD, IgE, IgG and IgM. Human IgA is further divided into 2, and IgG into 4 subclasses. The dimeric heavy (H) chains, that are unique to each class are designated as alpha, delta, epsilon, gamma, and mu chains, respectively. Each Ig molecule contains two identical light (L) chains, which may belong to 2 major classes: kappa or lamda.  The variable (V) regions of one H and L chain form one antigen combining site,  giving rise to 2 sites per Ig molecule. Both H and L chains have constant (C) regions. The C region of the H chains mediates biological functions, such as complement fixation, phagocytosis, cytotoxicity, secretion, regulation of cellular functions and transportation across membranes. IgA is secreted to mucosal surfaces as a dimer and IgM exists both as pentamer and hexamer in serum and secretions. The mammalian genome contains multiple genes coding for variable, diversity (D), joining (J) and constant regions of immunoglobulins. The germline genes are spliced and joined together to achieve VDJC configuration for H chains and VJC for L chains. During the joining process mutations also occur, which give rise to affinity maturation of antibodies during the immune response. Natural antibodies are produced by CD5+ B lymphocytes and preserve their germ-line cofiguration, without mutations. These antibodies recognize highly conserved homologous epitopes (homotopes) in microbes and self components and provide wide sprectrum, polyspecific protection against pathogens. It is now emerging that autoreactive natural antibodies fulfill important regulatory functions. Protection against cancer and against autoimmune disease are amongst the functions fulfilled by natural antibodies.

9.3. Growth and lactogenic hormones (GLH)

  Prolactin (PRL), growth hormone (GH) and placental lactogenic hormones (PL) belong to this family. These hormones show molecular heterogeneity and the variant hormones differ in their biological activity. GLH receptors are also heterogeneous and share common features with type I cytokine receptors. GLH commonly use signal transducers and activators of transcription (STAT) family transcription factors in the early phase of induction. However, they also act through other signaling systems.

     Growth hormone and prolactin induce insulin-like growth factor-I (IGF-I) in cells of the immune system and in other cells in the body. IGF receptors belong to the transmembrane tyrosine kinase family which signal through phospholipase C-protein kinase C pathway.

     The embryonic development of the immune system is supported by placental lactogenic hormones. After parturition pituitary GH and PRL in conjunction with IGF-I maintain the bone marrow, thymus and the secondary lymphoid tissue (spleen, lymph nodes, mucosal and cutaneous lymphoid system) in a functional state. In the joint and complete absence of pituitary GH and PRL, the immune system loses rapidly cellularity, which is associated with a profound decrease in immunocompetence. Bone marrow function is also decreased. Full restoration is possible with replacement doses of either PRL or GH. Pituitary GLH in conjunction with IGF-I support all immune functions that involve humoral and cell mediated immune responses and also the internal immunoregulatory pathways. On this basis, pituitary GLH have been designated as the hormones of immunocompetence.

  Insulin (INS) is a pleiotropic growth factor, which interacts with both GH and IGF-I. Insulin is required for normal immune development and function. In general, INS stimulates immune reactions but inhibitory effects have also been observed. Insulin appears to be a key regulator of immune function in acute illnesses, which are also characterized by insulin resistance.

9.4. The hypothalamus-pituitary-adrenal (HPA) axis

  Corticotropin releasing factor (CRF) regulates adrenal corticotropic hormone (ACTH) release, integrates the stress response in the CNS and acts centrally as an immunosuppressive agent by causing sympathetic outflow.  Lymphocytes also produce CRF, and it functions within the immune system as an internal regulator.  Eurocortin (UCN) is a novel endogenous ligand for Type II CRF receptors which is produced within the immune system.

      ACTH exerts an immunosuppressive and anti-inflammatory effect mainly by the stimulation of glucocorticoid production by the adrenal glands. Lymphocytes express receptors for ACTH.  ACTH is also produced within the immune system and has a direct regulatory effect on immune function.  In addition to ACTH, beta-endorphin (END) and alpha-melanocyte stimulating hormone (Alpha-MSH) are derived from the pro-opiomelanocortin molecule in the pituitary gland and in other tissues including the immune system.  Kappa, delta and mu-type opioid receptors are present in immune sites.  Endogenous opioids have a variable effect on immune function, which may be the result of direct action on the immune system or the modulation of immune function through the CNS. Alpha-MSH is a major inhibitor of fever and inflammation and an antagonist of proinflammatory cytokines. Alpha-MSH is produced within the immune system, in the skin and in many other tissues and functions locally as an immunoregulator.

9.5 The pituitary-thyroid axis

  Thyrotropin releasing hormone (TRH) is produced in the hypothalamus and also within the immune system. In the pituitary gland, TRH regulates the release of thyroid stimulating hormone (TSH), prolactin and growth hormone. Within the immune system, TRH has an effect on the development of intestinal T lymphocytes, on the production of interleukin-2 and interferon-gamma. TSH also effects immunocytes directly and receptors are detectable at low concentration on B and T lymphocytes, monocytes and NK cells; whereas, high levels were detected on dendritic cells. Human monocytes and lymphocytes synthesize TSH. TSH stimulated IL-2 production by lymphocytes, NK cell cytotoxicity and the phagocytic activity and the production of IL-1beta and IL-12 by dendritic cells. Bone marrow cells also express TSH receptors and respond to TSH by increased cytokine production. Thyroxin (T4) and tri-iodothyronine (T3) directly effect lymphocytes and monocytes through nuclear receptors. Lymphocytes are able to convert T4 to T3 which regulates their metabolism, maturation and function. The immunological effects of thyroid hormones are diverse and somewhat controversial. While thyroid deficiency is usually, but not always, associated with immune deficiency, supplemental treatment with T3 yielded mostly negative results. Primary B lymphocyte development is defective in mice deficient in the TSH receptor and thyroid receptor alpha-negative mice. Other hematopoietic lineages and the development of secondary lymphocytes were normal in these mice.

9.6. Nerve growth factor and leptin and neuropeptides

  Nerve growth factor (NGF) shares tyrosine kinase receptors with other neurotropic factors. These receptors belong to the TNF receptor superfamily and are present in lymphoid cells and cells of the nervous system. In general, NGF stimulates immune responses with the exception of IgE production which is inhibited. Nerve growth factor is stimulatory for mast cells and neutrophilic leukocytes and locally exerts a proinflammatory effect. However, systematically applied NGF suppresses inflammation. During inflammatory reactions, lymphocyte-derived NGF protects the nervous system and other tissues from damage.

  Leptin (LEP) is produced primarily by adipocytes and structurally is related to the GLH cytokine family. LEP signals by a class I cytokine receptor of which two isoforms are known. Leptin regulates energy metabolism, reproductive function and lymphocyte development and function. It stimulates TH-1 mediated immune responses and upregulates phagocytosis and the production of proinflammatory cytokines. During the acute phase response, LEP rises rapidly in response to elevated TNF levels. LEP appears to contribute significantly to survival in sepsis by moderating glucocorticoid and IL-6 production, stimulating IL-1 receptor antagonist and potentiating the immune response.

  Arginine vasopressin (AVP) exerts an immunostimulatory effect through V1 type receptors. AVP is produced by lymphocytes.

  Substance P (SP) is produced in both the nervous system and in the immune system. SP is a major mediator of neurogenic inflammation and is capable of causing mast cell discharge, increased capillary permeability and smooth muscle construction. SP has a stimulatory effect on immune phenomena in general. Its effect on immunoglobulin secretion is variable. SP stimulates bone marrow cytokines and the formation of granuloma tissue.

  Calcitonin gene related peptide (CGRP) stimulates mast cell discharge and inflammatory response whereas it has an inhibitory effect on immune reactions. T lymphocytes synthesize CGRP. Somatostatin (SOM) is an antagonist of SP and inhibits inflammatory and immune reactions. Vasoactive intestinal peptide (VIP) is produced within the immune system and the receptors are present on monocytes and lymphocytes. In general, VIP functions as an immunosuppressive neuropeptide and is involved in the immunosuppressive properties of aqueous humour in the eye. Its effect on immunoglobulin secretion and NK cell activity is variable. VIP downregulated the inflammatory response. Pituitary adenylate cyclase (PACAP) activating peptide has a similar immunosuppressive and antiinflammatory effect.
 

9.7.Cytokines

  These are locally acting tissue hormones that function as autocrine and paracrine regulators. They have a regulatory influence on growth, differentiation, function, inhibition and apoptosis of cells in various tissues.

  Type I Cytokines:  Interleukin (IL)-2, -4, -7, -9 and -15 are immunoregulatory cytokines that are recognized by type I membrane bound glycoprotein receptors.  These receptors are heterodiamers of a cytokine specific chain and a common (c chain, which is involved in signal transduction.  Interleukin -3, -5 and granulocyte-macrophage colony stimulating factor (GM-CSF) are hemopoietic cytokines with similar receptors that share a signaling gamma-c chain.  Interleukin -6, IL-11, oncostatin M (OSM), leukemia inhibitory factor (LIF), ciliary neurotropic factor (CNTF) and cardiotrophin-1 have diverse targets that range from the hematopoietic and immune systems to the cardiovascular and central nervous systems.  Receptors for these cytokines share glycoprotein (gp) 130. Interleukin-12 is a covalent dimer of a 35 and 50 kDa peptide, the latter is analogous to the soluble IL-6R-alpha chain.   Interleukin-12 promotes cellular immunity and hemopoiesis.  IL-21 and 23 are new members of this cytokine family.

  Type II Cytokines: Type I interferons (IFN-alpha, beta, omega) and the Type II IFN-gamma belong to this group.  Type I interferons share receptors and induce the same signals.  They exert anti-proliferative and anti-viral effects and stimulate cytolysis by lymphocytes, NK cells and macrophages.  Interferon-gamma forms a homodimer (34kDa)  and its receptor consists of 2 chains (IFNGR-1 and IFNGR-2).  The IFN-gamma receptor is ubiquitously expressed even by platelets.

      Interleukin-10 is an inhibitory cytokine regulating the growth and differentiation of B cells, T cells, mast cells, granulocytes, dendritic cells, keratinocytes and endothelial cells.  The IL-10 receptor is related to the IFN receptor. IL-19, 20 and 21 are recently discovered cytokines related to IL-10.

  Proinflammatory Cytokines:  1) Tumor necrosis factor family - Tumor necrosis factor (TNF), lymphotoxins (LT-alpha, beta) belong here.  TNF and LT-alpha share receptors and LT-beta has a specific receptor as well.  These receptors, the p75 nerve growth factor receptor and a number of other related receptors share a “death domain”, which is involved in the induction of apoptosis.  LT-alpha and other TNF ligands bind to their receptors as trimers.  TNF exists both as a soluble and membrane bound protein and LT-beta is membrane bound.  Many cells can produce TNF in response to noxious agents, infections or immune stimuli.  These cytokines regulate the growth of some  cells while inhibit others, promote inflammation and exert a neuroendocrine and a metabolic effect during acute phase reactions.

  The interleukin-1 family - IL-1-alpha, IL-1-beta and the IL-1 receptor antagonist (IL-1ra) belong in this family. These cytokines coordinate the host response to infection and injury. IL-1ra is the only known natural cytokine receptor antagonist. Pro-IL-1 is stored in the cytoplasm of various cells and needs to be enzymatically activated before it can be secreted. Various enzymes, including the IL-1-beta converting enzyme (ICE) activate pro-IL-1. Interleukin-1 is secreted in response to diverse stimuli, including infection, immunological and inflammatory processes and endogenous mediators of cell and tissue injury.  Two receptors, IL-1RI and IL-1RII and a receptor associated protein, IL-1RacP, with putative signal transducing function, are known. Type II receptor serves as a “decoy” receptor with no signaling capacity. Immune and inflammatory cells express receptors. IL-1 is a pleiotropic cytokine that coordinates host response to infection and injury and is a major messenger between the neuroendocrine and the immune systems. Interleukin-18 is a new member of the IL-1 family of cytokines.

  Chemokines (CEN) - Chemokines are the largest cytokine group. They show chemotactic and proinflammatory properties. They play a major role in acute and chronic inflammation. Chemokines also promote the immune responses, contribute to lympho- and hematopoiesis, vascularization, cell-to-cell adhesion, leukocyte recirculation and homing. Injury, polyclonal lymphocyte stimulation, antigens and endogenous cytokines stimulate CEM production. Four groups of CEM may be distinguished on the basis of their receptor specificity: CXC(alpha), CC(beta), C(gamma)) and CX3C. These receptors are seven pass transmembrane proteins and are coupled with G-proteins. Chemokines are stored intracellularly as inactive precursors and have to be enzymatically cleaved before secretion in their active form. Interleukin-16 and -17 are novel members of this cytokine family.

  Transforming growth factor-beta (TGF-beta) - This superfamily includes over 30 members, all of which are multifunctional regulating growth, differentiation, inflammation and immune function. Three isotypes are present in mammals: TGF-beta-2 and  TGF-beta-3  are important in embryogenesis and cellular differentiation, whereas TGF-beta-1 is an immunoregulator. TGF-beta is produced as an inactive protein and it is bound to cell surfaces or to matrix components before it is activated enzymatically. Three receptors exist. Type I and type II receptors are transmembrane serine/threonine kinases that are  involved in signal transduction; type III receptors  are beta glycans and present TGF-beta to receptors I and II.  TGF-beta is a powerful immunosuppressor and has a systemic anti-inflammatory effect. It promotes the induction of oral immunological tolerance, promotes tissue repair and wound healing and regulates cell proliferation and differentiation.  Macrophage inhibitory protein-1 (MIC-1) is a divergent member of the TGF-beta superfamily.

  Migration inhibitory factor (MIF) - MIF is a pleiotropic cytokine that is produced within the immune and neuroendocrine systems, including the pituitary gland. MIF is secreted by the pituitary gland during infection and injury  and regulates glucocorticoid action systemically and locally and it is a major regulator of immune function. MIF promotes cytokine production, angiogenesis, and the immune/inflammatory process.

9.8. Steroid hormones

  Glucocorticoids, aldosterone, estrogens, androgens and vitamin D play major roles in the neuroimmune regulatory network. The receptors for these hormones are nuclear transcription factors that directly regulate gene expression.  Glucocorticoids are produced in the adrenal gland and in the thymus and are fundamental to immune function. Physiological levels are required for the normal development and functioning of the immune system. Under pathophysiological conditions the serum level of glucocorticoids is elevated, which plays an important role in the suppression of the adoptive immune response, which is T cell-dependent. Glucocorticoids exert a powerful anti-inflammatory effect. Elevated glucocorticoids during acute phase reactions augment the production of natural antibodies and of acute phase proteins by the liver.

  Luteinizing hormone releasing hormone and gonadotropins exert a direct regulatory influence on the immune system, in addition to the regulation of sex steroid hormones. In turn immune-derived cytokines regulate the production of gonadotropins. These mechanisms insure the coordination of reproduction with health status and prevent inopportune conception.

  Estrogens regulate thymus function and suppress cell-mediated immune reactions. The antibody response and natural immunity (NK cytotoxicity, phagocytosis) are augmented by estradiol. Testosterone is immunosuppressive during trauma and shock. Many of the immunological effects of testosterone are due to its conversion to estradiol by aromatase in the thymus and in other lymphoid organs. The adrenal androgen, dehydroepyandrosterone (DHEA), stimulates immune reactions in experimental animals and in man and antagonizes the immunosuppressive effect of glucocorticoids. The age-related decline of DHEA may contribute to the immunodeficiency that develops in elderly individuals.  Progesterone is a powerful immunosupressive hormone and plays a major role in the protection of the fetus during mammalian reproduction. Progesterone also contributes to the generation of self tolerance and protects against the excessive activation of the immune system.

  1, 25-dihydroxy vitamin D3 (VD3) is a powerful immunosuppressive steroid hormone. Deficiency of VD3 plays an important role in the development of autoimmune disease. It is also required for normal immune function and for proper defense against infectious disease and cancer.

9.9. Enzymes

      Members of the serine protease family induce a wide range of biological effects by activating several hormones and growth factors. They also participate in enzyme cascades for inflammation, blood coagulation, complement activation, and other reactions. The action of these proteases is regulated by the endocrine system and by several inhibitors, which are also under endocrine control. Many inhibitors of these enzymes belong to a single family of proteins called serine protease inhibitors or serpins.

10. NEUROIMMUNE FUNCTION

10.1 Physiology

10.1.1. Immunocompetence

  Immune reactions, which are based on lymphocyte proliferation, are regulated by mechanisms that are involved in the growth control of all cells in higher animals.  Growth and lactogenic hormones (GLH) are required for the development and function of the immune system. GLH deliver the first signal to cells, including lymphocytes, that prepares them for proliferation, differentiation and function. This signal is designated as the competence signal, which  is required for lymphocyte growth and is obligatory for the maintenance of immunocompetence.  The second group of signals, that control cell growth, is delivered by cell-to-cell and cell-to-matrix signaling and are designated as stromal or adherence signals. Adhesion molecules, tissue bound hormones, cytokines and matrix components deliver these signals. Within the immune system antigen presentation represents a dominant signal for which cell-to-cell interaction is obligatory.  Adhesion molecules are fundamental to the organization of multi-cellular organisms and the signals delivered by them serve the basis of species, organ and tissue specific growth and development. The adhesion molecule system has been perfected during evolution from self-recognition to individually specific antigen recognition. Adhesion molecules also play a role in the elimination of degenerated and neoplastic cells.  Cell-to-cell signaling has a dominant power over other signals to commit the cell to proliferation.  The cell cycle is then completed after the delivery of cytokine signals. The nature and combination of these three groups of signals will determine the fate of each cell, which may be survival, proliferation, differentiation and function or alternately apoptosis. Hormones and neurotransmitters, that alter signal delivery, modulate further this basic pattern of animal cell growth.  Current evidence that GLH maintain immunocompetence, which enables the immune system to respond to specific antigenic and mitogenic stimuli.

10.1.2. Antigen presentation

  Antigen presentation takes place by cell-to-cell interaction whereby a complex signaling process via cell surface adhesion molecules initiates the adaptive (antigen specific) immune responses. Without exception the macromolecular antigen undergoes proteolytic breakdown (processing) and peptide fractions ( ~ 9- 24 residues) are presented by antigen presenting cells (APC) in association with surface MHC antigens. External antigens are engulfed by phagocytic mononuclear cells and are digested in endosomes. The peptide fragments are then  joined with MHC-II molecules in endocytic vesicles (endocytic pathway of processing) prior to expression on the cell surface. The APC of this pathway are resposible for the induction of the antibody response and of delayed type hypersensitivity reactions.  Endogenous antigens are processed in the cytoplasm, in enzyme-containing proteosomes (cytosolic pathway) and the peptides generated are associated with MHC-I in the endoplsmic reticulum, which in turn is expressed on the surface of all nucleated cells in the body. Cytotoxic T lymphocytes recognize MHC-I-peptide complexes on the cell surface and destroy infected and cancer cells.

     Because the antigen presenting system requires the digestion of the antigen after phagocytosis, it protects against extracellular and intracellular pathogens, it also exposes hidden antigenic determinents, decreases the impact of mutations by employing short peptides and allows for self-non-self discrimination. Non-classical MHC antigens are also involved in antigen presentation. Specialized surface molecules (CD1) mediate carbohydrate and lipid presentation. Heat shock proteins normally serve to eliminate dead cells from tissues but also form complexes with antigen, which are taken up by APC through a specific receptor (CD91). Such complexes are taken up by macrophages and dendritic cells, digested and are presented by MHC-I. The dominant role of antigen presentation in the adaptive immune system illustrates the  fundamental regulatory function of adherence signals in multi-cellular organisms.

10.1.3. Immune reactions

  Under physiological conditions the immune system provides continuous defence against infectious agents and cancer, and is part of the homeostatic neuroimmune regulatory circuit that co-ordinates the normal function of the entire organism. The immune system provides local and mobile defence and regulation and it has enormous capacity to deliver defense and regulatory molecules to sites that are in need. Every organ and tissue possesses stromal lymphoid elements that intervene locally to control autoimmune reactions, inflammation, and in general, participate in the physiological processes.

  Adaptive cell mediated and humoral immunity and immunological memory are reactions exerted by T and B lymphocytes in concert with members of the leukocyte series. Innate host defense relies on non-immune mechanisms, on specialized immune cells, such as natural killer cells, (gamma-delta-T lymphcytes and CD5+ B cells. The complement system and of T and B lymphocytes activated by alternate pathways are also part of the natural immune system. The natural immune system relies on germ-line coded receptors that recognizes evolutionarily highly preserved homologous epitopes (homotopes) on microbes and also on self components.

     Cell-to-cell interactions by the antigen receptor and of MHC molecules and by other adhesion molecules are fundamental to immune activation as well as to stromal regulation. Cytokines are an essential part of this regulatory system. In addition, the immune system interacts with the neural elements and mediators, parenchymal and stromal cells as part of the local neuroimme circuits that govern the organ/tissue under physiological circumstances.

  Immunoregulation by the HPA axis. Glucocorticoid hormones exert a multitude of functions that affect virtually every cell in the body. The physiological significance of glucocorticoids is most remarkable at times of stress, when the hypothalamic-pituitary-adrenal axis (HPA) is fully mobilized. The same hypothalamic hormone that stimulates the HPA axis, CRF also mediates behavioral, autonomic and neuroendocrine responses to stress in rodents and primates. Hyperfunction of CRF neurons, therefore, appears to underlie a variety of psychiatric, gastrointestinal, cardiovascular, metabolic and reproductive illnesses attributable to stress. Nonpeptide CRF type-1 receptor antagonists are novel tools to moderate CRH activity in the brain after systemic administration [14]

10.2 Pathophysiology

10.2.1. Neurogenic inflammation

  The release of neuropeptides, including tachykinins and calcitonin gene-related peptide, from sensory nerves via an axon or local reflex may have inflammatory effects in the airways. This neurogenic inflammation may be initiated by the activation of sensory nerves by inflammatory mediators and irritants. The phenomenon of neurogenic inflammation is well developed in rodents and may contribute to the inflammatory response to allergens, infections and irritants in animal models. However, the role of neurogenic inflammation in airway inflammatory diseases, such as asthma and chronic obstructive pulmonary disease is still uncertain. There is still little direct evidence for the involvement of sensory neuropeptides in human airways. Initial clinical studies using strategies to block neurogenic inflammation have not been encouraging. In order to clarify the situation, it is necessary to perform prolonged studies of more severe forms of airway disease in the future to explore the role of neurogenic inflammation [15].

10.2.2. Defensins (DEF)

  Defensins are antimicrobial cationic peptides with a cysteine-stabilized amphipathic structure. Defensins are normally localized in phagocytes (neutrophil, monocyte/macrophage) and in the epithelial cells of mucous membranes and skin. Some DEF are released into the blood during the course of infection, inflammation or stress. DEF function not only as endogenous animal antibiotic molecules, killing microbial cells and enveloped viruses, but also as physiological regulators. Defensins are implicated in the regulation of endocytosis, chemotaxis, mast cell degranulation and inflammation. Moreover, these molecules are modulators of hemostasis and neuroendocrine-immune interaction. Defensins lower the stress-induced elevation of corticosteroid levels in the blood, and abolish the stress-induced inhibition of the humoral immune response. These facts support the hypothesis that DEF are antibacterial peptides with a broad spectrum of biological activity [16].

10.2.3. The acute phase response

Mild infection or sublethal dose of endotoxin elicits a brief elevation of GH and PRL in the serum, which are proinflammatory and immunostimulatory.  In severe trauma, sepsis and shock, GH and PRL are suppressed, whereas glucocorticoids and catecholamines are elevated.  Under these conditions an acute phase response (APR) is initiated by immune-derived cytokines, primarily IL-1, IL-6, TNF-alpha. These cytokines elicit a profound neuroendocrine and metabolic response.  Fever and catabolism prevails, whereas the synthesis of acute phase proteins (APP) in the liver, cell proliferation in the bone marrow, and protein synthesis by leukocytes is elevated.  This is an emergency reaction to save the organism after the adaptive immune sytem has failed to protect the host.  During sepsis and endotoxin shock the systemic activation of the complement system and of leukocyte-derived enzymes, tissue-derived bare-down products and highly toxic cytokines seriously threaten survival.  The hypothalamus-pituitary-adrenal axis is activated. Glucocorticoids play a major role in the regulation of  proinflammatory cytokine production and potentiate the secretion of acute phase proteins.  Some APP, such as C reactive protein, LPS binding protein and mannose binding protein in the serum are designed to combine with microorganisms and trigger their destruction by the activation of complement system and of phagocytes.  The increased production of some complement components also helps host resistance.  The rise in serum fibrinogen promotes blood clotting which can serve to isolate the invading agent by triggering thrombosis in infected tissues.  A number of enzyme inhibitors are produced as APP, which are likely to serve to curb the nonspecific damage inflicted by enzymes. Catecholamines are also elevated, which serve to inhibit inflammatory responses and to promote, even initiate, the acute phase response. Serum leptin is also increased, which governs energy metabolism as well as it has an immunostimulatory effect. If the acute phase reaction fails to protect the host, shock will develop.  Patients with subclinical adrenal insufficiency succumb to septic shock almost invariably if glucocorticoid therapy is not given.  However, glucocorticoid treatment of septic patients with normal adrenal function has not been helpful.

     During the acute phase response the T-cell regulated adaptive immune response is switched off and natural immune mechanisms are amplified several hundred to a thousand times within 24-48 hours. This phenomenon has been designated as immunoconversion, which is  initiated by immune derived cytokines, and involves profound neuroendocrine and metabolic changes, all in the interest of host defense. Therefore, natural immunity is essential for a first and last line of defense and the neuroendocrine system is an important promoter  and regulator of this fundamental form of immune defence.

10.2.4. Autoimmunity

  Auto-reactive lymphocytes and natural antibodies do exist physiologically and participate in the homeostatic regulation of bodily functions. Under normal conditions mature T lymphocytes with autoimmune effector potential are in equlibrium with suppressor cells and with other inhibitory elements, the end result of which is self-tolerance. Immune homeostasis does not rely exclusively on internal suppressive mechanisms of the immune system. Immunocompetence is dependent on growth and lactogenic hormones. Adaptive and natural immune responses and inflammation are controlled by neuroendocrine regulatory mechanisms. By definition autoimmune disease develops upon the loss of self-tolerance. An autoimmune reactions are due to de-regulated lymphocyte proliferation and maturation into effector cells directed towards self antigens. There is evidence to indicate that the de-regulation of multiple genes are required for the development of autoimmune disease. It appears that that de-regulation at the cellular level as well as an altered neuroendocrine milieu are necessary for autoimmmune disease to develop.

       Autoimmune reactions may be specific for self-molecules, cells, organs or tissues. These specific reactions are due to de-regulated cells of the adaptive immune system. Rheumatoid diseases are characterized by polyclonal lymphocyte activation, inflammation and acute phase reactants, which are indicative of a disorder of the innate immune system.  Abnormalities of the hypothalamus - growth and lactogenic hormone - insulin like growth factor axis, of the hypothalamus - pituitary - adrenal axis and of the hypothalamus - pituitary - gonadal axis are freequently observed in autoimmune disease. An imbalance of these major immunoregulatory axes of the pituitary gland is very likely to play an etiological role in the pathogenesis of autoimmune disease. It is also becoming apparent that defective regulation by neurotransmitters and neuropeptides contribute significantly to the pathogenesis of autoimmune and inflammatory conditions.

10.2.5. Immunodeficiency

  According to current consensus, immunodeficiency is attributed to internal structural and/or regulatory defects, although it is recognized that some of the deficiencies are complex. Animal experiments and some observations in man clearly indicate that abnormalities of neuroendocrine sytem are also able to cause immunodeficiency.

     In hypohysectomized (Hypox) rats immune function is inhibited. If the residual serum PRL present in such animals is neutralized, the thymus, spleen, adrenals and gonads become atrophic, severe combined immunodeficiency, anemia, wasting and death will occur in 6-8 weeks. The spontaneous occurrence of joint and complete deficiency of pituitary GH and PRL has never been demonstrateed in man or in animals. This suggests that this condition must be lethal.  Memory cells survive hypophysectomy and resist     glucocorticoids. Pituitary PRL and GH support the adaptive T cell dependent immune system and are suppressed during febrile illness (acute phase response), where natural immune mechanisms are amplified.

     The hormones of the HPA axis, especially glucocorticoids and DHEA-S, are required for normal immune function. Animals lacking adrenal function may be killed by excess cytokine production after simple immunization or after minute amounts of endotoxin, which are harmless to normal animals. The HPA axis is also the prime neuroendocrine mediator of APR, which is fundamental for survival in emergency situations (e.g. sepsis), where adaptive immune mechanisms are ineffective. Cytokines (IL-1, -6, TNF-alpha) initiate these neuroendocrine, metabolic and immune alterations in APR. The HPA axis coordinates the nervous, endocrine and immune systems during APR by regulating fever and CNS activity in general. This axis contributes to the conversion of the immune system from the adaptive mode of reactivity to boosting of natural immune mechanisms and to metabolic alterations (catabolism in most tissues).

     Growth hormone treatment of patients with severe acute illness worsened their survival. One possible reason for this is that GH interfered with boosting natural immune defence by inhibiting the HPA axis and catabolism, which are fundamental to APR.

  Estradiol is required for the cytotoxic activity of CD4+ T lymphocytes, which fulfill major immunoregulatory function. Androgens and progesterone are immunosuppressive steroids. Sex hormones are fundamental for reproductive function, where the immune system is an active participant. Disturbances of sex hormone function due to ageing or to other factors contribute to deficient immune function and to the development of disease.

     Whenever the adaptive immune system becomes deficient (e.g. in aging individuals, AIDS patients, or as a consequence of malnutrition or of various insults) the body must resort to natural immune defence, including APR. APR develops at major metabolic costs to the host, and may lead to malnutrition, wasting and eventually to death.  Proper nutrition and hormone supplements (e. g. GH, IGF-I, anabolic steroids) proved useful in at least some of these conditions.

11. THE NEUROIMMUNE REGULATORY NETWORK

11.1 Basic  concepts and principles in neuroimmune regulation

  The rapid accumulation of experimental data in diverse systems that has relevance to neuroimmune regulation has led to contradictions, misconceptions and confusion, which need to be addressed and clarified. Some of the key issues are addressed below both from the theoretical and practical points o view.

11.1.1. Hierarchy

  It is clear that the hypothalamus is the highest regulatory organ of immune and inflammatory reactions. The hypothalamus communicates with all tissues and organs in the body, rather than having a “bi-directional” affair with the immune system, which is commonly held today. Indeed, there is ample evidence to support the proposal that the Nervous-, Endocrine- and Immune systems form a regulatory network that involves the entire organism and governs all functions from conception till death. Network communication is achieved by innervation, by hormones and cytokines distributed through the blood, and by re-circulating cells of the immune system (Figure 1). Innervation is bi-directional, as the CNS sends signals to each organ and tissue in the body and receives feedback signals through sensory nerves and by the vagus nerve. Communication by hormones and cytokines is of multi-directional network type. The immune system is mobile and is capable of “patrolling” the body by T cell recirculation. Other immune cells (B cells, macrophages), together with T cells, are taking up residence in various organs and tissues as stromal regulatory elements and also home to sites of immune/inflammatory reactions. Immune- and tissue-derived cytokines provide feedback towards the hypothalamus. Shared mediators assure efficient communication between members of the regulatory triad as well as with the tissues and organs of the entire organism [18].

11.1.2.Competence

  It is known for over a century that GH is capable of promoting the proportional growth of all tissues and organs in the body. Naturally, this includes the immune organs, such as the spleen, thymus and lymph nodes. Receptors for GLH are expressed on every tissue and organ in the body. Current indications are that all cells require some members of this hormone family for normal growth and function. GLH in conjunction with IGF-I exert a growth stimulatory and an anti-apoptotic effect on all cells and thus assure maintenance in a functional state, i.e. the cells are competent to respond to additional stimuli. On this basis GLH may be defined as competence hormones. The competence signal is the first in the signaling hierarchy of cell activation (No. 1 in figure 1), as it is the pre-requisite for the survival, growth, differentiation and function of each cell in the body. There is evidence to indicate that the immune system is dependent on GLH for the maintenance of immunocompetence. This is related to the general function of these hormones to promote growth and development, as lymphocyte growth is a prerequisite of adaptive immune reactions [19].

     Tissue specific growth factors, cytokines and adhesion molecules play important roles in cell to cell signaling and the regulation of growth and differentiation of various cells in the body. It is apparent that proportional growth, although ultimately controlled by the systemic level of GH, is achieved through the coordinated interaction of systemic and local growth regulatory signals, much of which is tissue/organ specific and also function-related. GH in conjunction with IGF-I promote the survival and growth of all cells in the body and maintain them in a state of competence. Prolactin, which may be regarded as a modified growth hormone, is also capable of providing competence to most tissues and organs, with the possible exception of the skeleton where its growth promoting effect is limited.  Like GH, PRL also induces IGF-I in its target cells. The IGF-I signal may be regarded as the ubiquitous cytokine signal that is needed for the survival of competent cells. On this basis one may suggest that competence hormones maintain their target cells by a direct stimulatory effect on the genome and by the stimulation of IGF-I or equivalent cytokine.

  Prolactin synthesis is detectable in numerous tissues, including lymphoid tissue. There is compelling evidence to suggest that tissue-derived PRL fulfills autocrine/paracrine regulatory functions. Within the immune system, small lymphocytes are in a quiescent state and do not synthesize significant amounts of mediators. They need to be activated in order to do so. Based on current experimental evidence, one may propose that inactive small lymphocytes are dependent for survival on pituitary GH/PRL and IGF-I.  The dependence of cell survival in the thymus, spleen and bone marrow on GH/PRL supports this hypothesis. Moreover, pituitary PRL and GH maintain vital bodily functions and thus must act as survival hormones for the entire organism [19-21]. Animals with joint and total deficiency of GH/PRL do not exist, and such deficiency has not been convincingly demonstrated in man to date [22].

11.1.3. Redundancy

  Failure of the neuroimmune regulatory system invariably leads to the death of the organism.  In order to avoid frequent failures, the system must have multiple and overlapping regulatory pathways with a high degree of flexibility and plasticity. This is achieved by  isologous forms of regulatory molecules, multiple forms of receptors, and by the existence of functionally overlapping or interchangeable regulatory pathways.  This is true for growth and lactogenic hormones for the IGF/insulin system for steroid hormones, neuropeptides, cytokines, chemokines and for the various immunoglobin classes.  It is quite common in immunology that unsuspected redundancy is revealed in the system by knocking out a particular gene. Similarly, the disabling of prolactin or growth hormone, or even IGF-I, would not paralyze immune function. These experiments and clinical observations indicate clearly that the functional integrity of the neuroimmune regulatory network is maintained even after very severe insults/deficiencies. Elaborate physiological and pathophysiological mechanisms assure that this systemic regulatory network remains functional under the most adverse conditions.

11.1.4 Homeostasis

  Healthy individuals and animals maintain their body temperature, blood pressure heart rate metabolism and the concentration of various ingredients in the serum and in tissue fluids within standard physiological ranges, which is characteristic of the species.  This has been termed by Claude Bernard over a century ago as “milieu interieaux” [23], now designated as homeostasis.  Under physiological, or homeostatic conditions two basic forms of immune reactivity can be observed, e.g., innate or natural immunity and adaptive immunity. These function hand-in-hand, providing efficient host defense quietly in total harmony with the organism.

     Emotional and physical stress activates the HPA axis, which has a suppressive effect on the adaptive immune system by the inhibition of the thymus and of T lymphocytes. Severe trauma or infection will induce febrile illness, which is now termed the acute phase response (APR). During APR the innate immune system is amplified and the adaptive, T-cell-dependent system is suppressed. This immunoconversion  is a very intensive and highly co-ordinated process, that requires the catabolic breakdown of body constituents in order to fuel the neuroendocrine and immune systems (bone marrow, leukocytes) and the intensive production of liver-derived acute phase defense molecules [24]. Because the immune-, neuroendocrine- and metabolic alterations are strictly regulated, the use of the term allostasis [25,26], in comparison with homeostasis, is justified. The allostatic milieu assures the promotion of host defense at the expense of other tissues and organs. Immunodeficient individuals rely more and more frequently on APR for host defence, which may create a negative energy balance leading to cachexia and eventually to death.

11.2 Cell-to-cell regulation

      Traditionally the cells of all tissues and organs have been divided to stromal cells, which were thought to provide for the structure of organs and serve as a frame for holding together the parenchymal or functioning cell. It is now apparent that stromal cells interact actively with parenchymal cells and this interaction is fundamental to the functional regulation of the tissue/organ. Invariably the stroma of all tissues and organs contain immune derived elements such as lymphocytes, macrophages or more specialized cells that include the glia cells in the nervous system, Kuppfer cells in the liver, the Langerhans cells in the skin, etc. These lympoid cells play important functional roles both in health and disease.

     Blood vessels and endothelial cells lining the blood vessels are also active participants in lymphocyte recirculation and in local immune/inflammatory reactions. These cells interact both with the circulatory elements of the immune system and locally with elements of the tissue/organ. Cell to cell regulation in tissues is mediated by adhesion molecules that have complimentary binding sites. These molecules are capable of delivering activation or inhibitory signals [26-28].

     Upon lymphocyte activation adhesion molecules and other cell membrane receptors have the capacity to co-aggregate within the semi-fluid cell membrane (capping). This brings the molecules/receptors in close proximity and allows the interaction of immuunoreceptor thyroid based activation motives (ITAM) and -inhibitory motives (ITIM). These motifs promote activation by phosphorylation, or inhibit activation by dephosphorylation of signal transuding molecules, respectively.  According to the phosphorylation/dephosphorylation balance amongst the capped receptors, the cell may be activated or inhibited as the final outcome.  The relevance of this phenomenon to cell function is especially well established for NK cells and lymphocytes. However evidence is increasing that this “higher order” interaction may apply to many other cell types and receptors.   Also, the phenomena “receptor crosstalk” has been observed in several  systems (28-31). Adhesion signals have a dominant power to regulate cell activation, proliferation, differentiation and function, or alternately inhibition or  commit cells to the suicide pathway. Adhesion signals are in second place in the hierarchy of cell signaling (Figure 1).

     The immune system consists of mobile cells that are able to home readily to specific target tissues and also to sites of infection, injury, regeneration and healing. T lymphocytes extravasate through postcapillary venules, enter the tissue compartment and then, after having “patrolled” the tissues, gather through the lymphoid system into the thoracic duct and ultimately into the blood. This T cell recirculation is a continuous phenomenon and is thought to be the basis of “immunological surveillance” to control cancer, and to remove infected and degenerated cells from tissues.  Stromal lymphoid cells play important physiological roles and are fundamental to host defense, regeneration and repair.  Adhesion molecules mediate immunocyte homing and lymphocyte re-circulation.  Blood vessels also provide important barrier function in some tissues and organs that are known as immunologically privileged sites.  The blood brain barrier is very important from the point of view of neuroimmune interaction.
Ultimately, adhesion molecules, that are capable of delivering non-diffusible cell-to-cell or cell-to-matrix signals, determine the proliferation and functional activation of all cells in the body. Adherence signals are capable of regulating cell function specifically on an individual basis. For example the antigen presenting cell delivers regulatory signals to the antigen specific T lymphocyte [18].

11.2. Innervation

  All the immune organs are innervated and functional neurotransmitter receptors are present on immune cells. These facts create the foundation of the research field of neuroimmunology, which examines the homeostatic interactions between the nervous and immune systems in the process of host resistance. Much evidence is available which shows the effects of neurotransmitters on various in vitro measures of immune cells. However, the complex interactions created by neural-immune interactions at sites of antigen challenge and within immune organs cannot be accurately modeled in tissue culture systems. This fact created a need for focusing attention on in vivo model systems, which maintain the local microenvironment created when antigen is encountered in skin or mucosal membranes. Such experiments showed that nerves are important regulators of lymphid organs and also of immune/inflammatory reactions [32-34].

     The central nervous system has the capacity to deliver neurotransmitters and neuropeptides to all tissues and cells in the body. Interestingly the spleen contains only sympathetic efferent nerve fibers. Tissue mast cells are form synapses with nerve fibers.  Neurogenic inflammation is the direct result of the discharge of inflammatory mediators from mast cells after stimulation by neurotransmitters (primarily by substance P) released from sensory  nerve terminals. Neurotransmitters and neuropeptides,  (e.g. cathecolamines, substance P, somatostatin) play major roles in the regulation of immune/inflammatory responses.

     During the acute phase response there is a massive release of catecholamines into the circulation, which is known as “sympathetic outflow”. Catecholamines are important regulators in the acute phase response, which is an emergency defense reaction.  Sensory nerves provide feedback signals towards the CNS from sites of injury, inflammation, and infections.  The vagus nerve carries feedback signals to the CNS from visceral  organs.

11.4 Humoral Communication

  Historically the humoral mediators of cell communication have been classified as hormones that act at distant targets, neurotransmitters and neuropeptides, and locally produced hormone-like mediators, now called cytokines.  One may also include here immunoglobulins, which originate from B lymphocytes. Immunoglobulins have evolved from adhesion molecules. In addition, virtually every cell membrane bound molecule is present in the serum, which includes MHC molecules and receptor-like-binding proteins.

     By now it is clear that “classical” hormones, neurotransmitters and neuropeptides are widely synthesized at various other sites, including the immune system.  Moreover, cytokines, which have been originally discovered within the immune system, are now known to be synthesized in other tissues and organs, including the neuroendocrine system.  Therefore, the historical definition of hormones neurotransmitters and neuropeptides no longer applies. Rather, systemic and locally produced mediators complement each other, so that optimal function is assured both under physiological and pathophysiological conditions.

  Cytokine signals are third in the hierarchy of signals required for cell activation (Figure 1). In the mitotic cycle, third signals are delivered during the late G1 phase. Insulin-like growth factor 1 and insulin are the classical hormones that act at this stage and initiate the synthetic (S) phase, which will then progress to mitosis [35].  It is clear, however, that at least some of the cytokines produced locally in the bone marrow, e.g. IL-3 and GM-CSF are actually capable of stimulating IGF-I in their target cells [36]. Therefore, these cytokines should be considered as competence hormones, delivering signal 1 and not signal 3. It has been also observed on B-cell hybridomas in culture that PRL potentiated the effect of interleukins for the stimulation of proliferation and antibody formation [37]. Moreover, PRL and GH, which are the prototypes of competence hormones, are also produced locally. In cell biology epidermal growth factor, platelet-derived growth factor and fibroblast growth factor have been designated as competence hormones [35]. In addition estradiol is capable of inducing IGF-I in mammary tissue [38].  These observations indicate that competence may be inducible by hormones and cytokines other than GLH. However, in vivo observation in GLH deficient animals strongly indicate that the total lack of GLH leads to wasting disease and death. Therefore, GLH is fundamental to the maintenance of vital bodily functions, including immune function. A simple explanation for this could be that the production of all the other factors that are capable of delivering signal 1 in the mitotic cycle requires competent cells to begin with and the production of such cells rests with the availability of GLH. Indeed, we have observed that PRL disappeared from lymphoid tissue of rats after hypophysectomy [17]. Clearly, GH, PRL, IGF-I and insulin are readily available in significant quatities in the serum at all times, and thus provide competence to all cells in the body whether they are active or in a quiescent state. Further studies are required to elucidate these questions in more detail.

     Numerous receptors in immunology and several hormone receptors need to be cross-linked by the ligand in order to deliver an activation signal to the target cell.  This mechanism provides an important regulatory function in that cross-linking may take place only at an optimal receptor/ligand ratio, whereas low or high ligand concentrations would not be able to activate the receptor. When more than one receptor isotype is available, the homo- and hetero-diamers formed by the specific ligand may have different biological functions. This is the case with steroid hormones. In addition, cross-linking may be one of the important mechanisms that promote capping of the receptors prior to activation.

12. THE SIGNIFICANCE OF NEUROIMMUNE BIOLOGY

      The fundamental significance of the central nervous system for normal development and for the maintenance of health throughout the life cycle of higher organisms is generally accepted by the scientific community and by the general public. These organisms rely continuously on their central nervous system for the regulation of all bodily functions. Some time ago it was realized that the Endocrine system is an essential part of this systemic regulatory circuitry. Now a case can be made for the Immune system, which also belongs to this systemic regulatory network of higher organisms that play a fundamental coordinating role form conception till death. Some important aspects of this regulation are discussed below [17].

     Placental hormones play a major role in the development of the fetus. Indeed, now there is decisive evidence to indicate that placental GLH gradually overrule the maternal pituitary gland and regulate metabolism and the level of developmental hormones even in the maternal organism in the interest of normal fetal development. Virtually all other hormones required for fetal development are produced in the placenta in a self sufficient and autonomous manner [39,40].   These hormones govern fetal development, which include the development of the immune system. In this context it is interesting to note, that fetuses with the congenital lack of the pituitary gland develop normally in utero, but are not viable after parturition [41]. This indicates that placental and perhaps maternal hormones fully support fetal development and that the pituitary gland takes over the regulatory role of the placenta after parturition. Sinha and Vanderlaan treated newborn mice with anti-PRL antibody, which induced wasting disease and death in about 30% of the animals [42]. Mice are born with an immature immune system and probably also, with an immature pituitary gland. It is likely that during the transition period of immune and pituitary maturation the newborn pups rely for survival on PRL received via placental transfer and in the colostrum. If this PRL is neutralized, there is no other competence hormone to take over, which could result in wasting and death. This situation is analogous to the case of  hypophysectomized rats, where there is residual PRL in the serum of long-term survivors. If this PRL is neutralized by antibodies, immune and bone marrow function ceases, the thymus, spleen, adrenals and gonads become very athrophic and a rapid wasting disease and death occurs within 6 weeks [19]. The vital importance of GLH as a group is further emphasized by the fact that joint and complete deficiency of PRL and GH has never been described.  Pit-1 deficient mice and human beings, which have been claimed to be free of pituitary GLH, do in fact have detectable serum hormone levels [43-46]. This explains their survival.

     It is known that old people may be deficient in GH. However, PRL deficiency is virtually unknown. Again, it is possible that those individuals that become deficient in both hormones pass away before their condition would be recognized. Preliminary clinical evidence indicates that the treatment of immunodeficiency-related wasting disease with GH and with anabolic steroids is beneficial. These crucial observations, along with much additional evidence presented in this volume, indicate that GLH maintain vital bodily functions, including immune function, in the entire organism from conception till death. These hormones provide all cells in the body with competence to develop and function. The fundamental importance of these hormones in the biology of higher organisms cannot be over-emphasized.

     In contrast with GLH, which have an overall stimulatory effect on the organism, the hormones of hypothalamus - pituitary - adrenal (HPA) axis act both synergistically with GLH under physiological conditions and antagonistically during pathophysiological events by inhibiting growth, metabolism, immune responses and inflammation. Glucocorticoid and other steroid hormone receptors are nuclear transcription factors and are necessary for normal signal transcription by membrane bound receptors. Thus they are integrated into the signal transduction sequence under physiological conditions. The immune system and other tissues have the capacity to generate bioactive steroid hormones from the adrenal steroid precursor-hormone, dehydroepyandrosterone-sulphate (DHEA-S).

     There is evidence to indicate that the HPA axis begins to function already within the fetus, prior to the activation of pituitary GLH secretion [41]. This suggests that it is of vital importance to have the nuclear control of signaling in place for normal development and function of the organism, including immune function. Animals that lack HPA function respond with exaggerated cytokine production after immunization or if given minute doses of LPS. Such animals die easily, due to cytokine toxicity. The HPA axis plays a fundamental regulatory role also during the acute phase response by promoting the production of acute phase proteins and keeping cytokine production and immune activation at a level, which is compatible with survival. In rheumatoid arthritis and in other chronic inflammatory conditions the HPA axis responds at a subnormal level to inflammatory cytokines, which is likely to play an etiological role in the prolongation of the disease process. DHEA declines with ageing and also during chronic illness, which contributes to the deficiency of adaptive immunity under these circumstances.

     The immunomodulatory role of estradiol and progesterone is fundamental to the success of mammalian reproduction. Thymus function is regulated by estradiol, some of which is testosterone derived in males. Estrogens suppress cellular immunity and enhance the humoral response and the activity of phagocytes. Testosterone has an overall suppressive effect, which appears to be significant for protection against autoimmune reactions, but it is a definite  disadvantage in trauma-relatd immunosuppression. Because castration does not create life threatening immune disturbances, these hormones my be designated as facultative immunomodulators that help to fine tune immune reactions to accommodate some physiological events such as the reproductive cycle in females.

     By now it is clear that the immune system is integrated with the neuroendocrine systems to form a systemic regulatory network in higher organisms. Immune function is required for reproductive success. All tissues and organs contain immune derived stromal cells, which  participate in normal development and function as well as provide protection under pathophysiological conditions. Immune derived cells (such as the glia cells in the CNS, the Langerhans cells in the skin, Kupffer cells in the liver and similar monocyte/macrophage related cells in other tissues) interact with parenchymal cells. They play major roles as cytokine producing regulatory cells that are also capable of phagocytosis and antigen presentation. Parenchymal cells, such as keratinocytes in the skin, neurons in the brain, enterocytes in the gut and liver cells form important local regulatory circuits with immune-derived stromal cells [47-51]. According to recent evidence MHC-specific suppressor and effector T lymphcytes are present in all tissues, balancing against each other during homeostasis. The effector T cells rapidly mount immunity against foreign invaders and against cancer, and also participate in the elimination of degenerated cells. The immune system is fundamental also to regeneration and healing [52].

      Immunological studies in ageing animals revealed a correlation between longevity and the preservation of immune function. In healthy 90 and 100 year old individuals the level of NK cell-mediated cytotoxicity was similar to that of young subjects. Thyroid hormone and vitamin D levels were maintained and lean body mass was preserved [53]. These observations point again to the fundamental importance of  the neuroimmune regulatory triad in the maintenance of long and healthy lifespan. Clearly, this systemic regulatory network is fundamental to the development and normal function of higher organisms throughout the entire life cycle.

13. FUTURE PERSPECTIVES

    The human genome has been sequenced and functional studies of the entire genome are underway (Genomics) with modern methodology. The power of these approaches is unprecedented and is expected to produce major advances to our understanding at the cellular level. The need for the integration of the accumulated knowledge has also been realized and is promoted by several groups world-wide. These efforts run under the term of Neuroimmunology, NeuroImmunoModulation, PsychoNeuroImmunology, Integrative Physiology and the all-inclusive term, Neuroimmune Biology. There is no doubt that these efforts will lead to major advances in our understanding of higher organisms in their entire complexity, as well as yield abundant practical information for more rational approaches to the prevention and management of diseases.

     Major advances will be made in our understanding of the physiology and pathophysiology of higher organisms, including man, at the molecular, cellular and organizational level. The conditions and requirements of health will be recognized, which will make Preventive Medicine the first choice for the management of public health problems. The etiology of most diseases that are not curable today will be clearly delineated. If prevention is not feasible, it will be possible to correct the deficiencies either by replacing the key regulatory substances that are missing or even better, by providing the patient with good genes, healthy cells, tissues or organs that will perform the missing/defective function. Recent developments in the field of molecular science, genetics, immunology, stem cell science and medicine give us legitimate hope that most, if not all the human diseases that cause much suffering today, will be prevented and or eradicated in the brave new world of Neuroimmune Biology. The implications are similar for all other areas of Biology, including Animal Science and Animal Husbandry.
 
 

Click on image for Larger version of diagram Figure 1. The Neuroimmune Regulatory Network

This figure shows the interaction of major systemic neuroimmune regulatory pathways with local paracrine-autocrine regulatory circuits. Some key organs are also shown with their major regulatory input. Each organ/tissue has its local circuits, which interact with the systemic network via innervation, hormones and cytokines. Mobile elements of the immune system home to all organs and tissues and participate in physiological and pathophysiological processes.  Neuroimmune regulation is fundamental to the development and function of all cells in the body from conception till death. Immunoregulation is used as an example for detailed explanation. It is proposed that the development and maintenance of lymphocytes and other cells in the body in a functional state is dependent on competence hormones. Additional signals are required for fine tuned functional regulation that includes hormones, adhesion molecules, neurotransmitters, neuropeptides and cytokines. There is a hierarchy for the 3 major groups of signals as  given below: 1.  Competence signal.  During post-natal life pituitary GH and/or PRL deliver to lymphocytes and to all other cells in the body this signal. Many tissues also produce competence hormones ectopically, including the immune system. Ectopic PRL/GH fulfil a local autocrine/paracrine regulatory function during immune reactions. This local regulatory circuit makes rapid lymphocyte proliferation possible, which is a prerequisite of immune reactions. After lymphocyte activation some cytokines may be able to initiate/potentiate the competence signals within the immune system. This remains to be clarified. 2.  Stromal/adherence signals.  Antigen presentation by specialized cells is an adherence signal and a dominant lymphocyte activator.  This is accompanied by additional co-stimulatory adherence signals, eventually leading to lymphocyte activation.  Adherence signals also play a role in the induction of immunological tolerance, in lymphocyte survival, and in the induction of programmed cell death (PCD). These signals fulfil tissue-, site- and cell-specific regulation in the body, i.e. the function/fate of individual cells are determined at this level. 3.  Cytokine signals.  Lymphocyte activation is completed by cytokine signals, which lead to cell proliferation, differentiation, and functional activation. Cytokines may also perform inhibitory function (e.g. TGF-beta, interferon-gamma) or cause PCD (e.g. tumor necrosis factors).  a.  Signal modulation.  Some steroid hormones, catecholamines, endorphins/ enkephalins and chemokines are capable of modulating the process of signal delivery from the cell membrane to the nucleus by regulating Ca2+ influx, cAMP and cGMP.  b.  Signal regulation.  Glucocorticoids, sex and other steroid hormones (SH), tri-iodothyronin (T3) and vitamin D3 control lymphocyte signalling by the regulation of nuclear transcription factors. These steroid hormones and T3 play a regulatory role also in cell differentiation and in the elimination of unwanted cells via the induction of PCD.  c.  Local hormone activation.  Bioactive thyroid and steroid hormones are locally generated from inactive precursors by immunocytes (e.g. T3, E2, androstenediol, androstenetriol, and vitamin D3) while the primary function of others (corticosteroids, estradiol, progesterone, aldosterone) is systemic immunoregulation. The thymus itself is also a steroidogenic organ.  Quiescent lymphocytes do not synthesize DNA and exert minimum metabolic activity.  Pituitary GH/PRL, some adhesion signals and serum IGF-I play a key role in the maintenance of these lymphocytes until functional activation occurs. Neurotransmitters and neuropeptides are locally acting functional regulators, basically acting as signal modulators and cytokines. Some neuropeptides may be able to deliver competence signals.  The primary and secondary lymphoid organs, the mucosal and cutaneous lymphoid systems contribute leukocytes to the circulation that mediate systemic immunity. Circulating leukocytes also penetrate tissues and become “stromal cells”. These sessile lymphoid elements fulfil important local regulatory function. The adrenal, thyroid and gonads produce important immunoregulatory hormones. The liver contributes to immune function by producing serum IGF-I, by promoting the induction of oral immunological tolerance and it plays a major role in the acute phase response. The pancreas produces insulin and the submndibular gland nerve growth factor, which are major immunoregulators. These glands also produce glandular kallikrein, which have a major immunoregulatory function. All other tissues and organs have an input into the neuroimmune regulatory network via nerve impulses and by cytokine production. Abbreviations: ACTH = adrenocorticotropic hormone; ALD = aldosterone; CAT = cathecolamines; CNS = central nervous system; CTK = cytokines; FSH = follicle stimulating hormone; GC = glucocorticoids; GH= growth hormone; IGF-I insulin-like growth factor-I, INS = insulin; LH = luteinizing hormone; NGF = nerve growth factor; PRL = prolactin; SH = sex hormones; TSH= thyroid stimulating hormone; T4 = thyroxin; VD3 = vitamon D

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