Matera, University of Turin, Department of Internal Medicine,
A.M. Dogliotti 14, 10126 Turin, Italy
Rapaport, Mount Sinai Hospital, Diabetes Center,
Fifth Avenue 9, New York, NY 10029, USA
by: Elsevier Science
Biology: Volume 2: Growth and Lactogenic Hormones
Arnason, Chicago, IL
Barnes, London, UK
Bartfai, La Jolla, CA
Bertok, Budapest, Hungary
Besedovsky, Marburg , Germany
Bienenstock, Hamilton, Canada
Blatteis, Memphis, TN
Buckingham, London, UK
Chawnshang, Rochester, NY
Dardenne, Paris, France
Gaillard, Lausanne, Switzerland
Good, Tampa, FL
Gorczynski, Toronto, Canada
Heijnen, Utrecht, The Netherlands
Hori, Fukuoka, Japan
Jancso, Szeged, Hungary
Kendall, Cambridge, UK
Korneva, St. Petersburg, Russia
Kovacs, Toronto, Canada
Kunkel, Berlin, Germany
Matera, Turin, Italy
Nance, Winnipeg, Canada
Ovadia, Jerusalem, Israel
Phelps, Tampa, FL
Prockop, Tampa, FL
Rapaport, New York, NY
Reichlin, Tucson, AZ
Skwarlo-Sonta, Warsaw, Poland
Sternberg, Bethesda, MD
Talmage, Denver, CO
Walker, Columbia, MO
Zapata, Madrid, Spain
more than seventy years evidence has accumulated documenting the existence
of interaction between growth hormone and the immune system. In the past
twenty years there has been a tremendous proliferation of information detailing
the function of the growth hormone and insulin-like growth factor axis.
A multitude of growth factors and binding proteins have been identified.
More and more evidence supporting the important role of the growth hormone
- IGF axis in the well functioning of the normal immune system has been
documented. Clearly the challenge today is not to prove, but to understand,
the neuroimmune regulatory role of growth and lactogenic hormones in its
ultimate goal of this volume and of all the other volumes of this series
is to promote the understanding of the science of Neuroimmune Biology and
to ease human suffering.
The Neuroimmune Biology of Growth and Lactogenic Hormones.
of Corresponding Authors
by Prolactin - An Introduction.
GLH Biology, Development & Receptors
Growth Hormone - Insulin-Like Growth Factor - I Axis and Immunity.
Mejia Naranjo, Myriam Sanchez-Gomez, Derek Le Roith
Interactions between the GH/IGF-1 System and Cytokines.
de Benedetti, Mauro Bozzola
Significance of Insulin-Like Growth Factor Binding Proteins.
Alan Weinzimer, Pinchas Cohen
Expression and Function of GH/IGF-I Receptors in the Immune System.
Tenore, Giuliana Valero
Hormone and Insulin-Like Growth factor-1 Production by Cells of the Immune
Applications of Growth Hormone in Promoting Immune Reconstitution.
J. Murphy, Lisbeth Welniak, Rui Sun
Transduction by PRL Receptors.
Transduction and Modulation of Gene Expression by Prolactin in Human Leukocytes.
Hooghe, S. de Vos, Z. Dogusan, E.L. Hooghe-Peters
of PRL Release by Cytokines and Immunomodifiers: Interrelationship between
Leptin and Prolactin Secretion. Functional Implications.
Gualillo, Eduardo Caminos, Ruben Nogueiras, Celia Pombo, Fransica Lago,
Felipe F. Casanueva, Carlos Diéguez
Expression in the Immune Cells.
Kooijman, Sarah Gerlo
Hemopoiesis and Development
as a Promoter of Growth and Differentiation of Hemopoietic Cells.
Hormone/Insulin-Like Growth Factors and Hematopoiesis.
Moghaddas, Robert Rapaport
prolactin Family: Immunological Regulators of Viviparity.
Ain, Heiner Müller, Namita Sahgal, Guoli Dai, Michael J. Soares
GLH and the Immune Response
of Prolactin on Natural Killer and MHC-restricted :Cytotoxic Cells.
Matera, Stefano Buttiglieri, Francesco Moro, Massimo Geuna
Vivo Changes of PRL Levels During the T-cell Dependent Immune Response.
Perez Castro, Marcelo Páez Pereda, Johannes M.H.M. Reul, Günther
K. Stalla, Florian Holsboer, Eduardo Arzt
Regulates Macrophage and NK Cell Mediated Inflammation and Cytotoxic Response
Chattopadhyay, Ratna Biswas
GLH and Disease
and Immune Function.
Colao, Diego Ferone, Paolo Marzullo, Gaetano Lombardi
Hormone and Insulin-Like Growth Factor-1 in Human Immunodeficiency Virus
prolactin as an Immunohematopoietic Factor: Implications for the Clinic.
of Bromocriptine in the Treatment of Autoimmune Diseases.
pathogenic Role of prolactin in Patients with Rheumatoid Arthritis.
reprint used with permission, NIB 2002;2:v-xiv)
Neuroimmune Biology of Growth and Lactogenic Hormones.
Growth hormone (GH) has long been shown in animals to stimulate immune
and inflammatory reactions. However, clinicians did not find immune abnormalities
in pituitary dwarf individuals, which raised serious doubts about the role
of GH in immune function [1,2]. To this date it is difficult to demonstrate
immune alterations in children after GH therapy, although transient responses
can be demonstrated. However, in vitro observations with human
lymphocytes indicate the role of GH in immunoregulation .
For the immunoregulatory role of prolactin (PRL) the first decisive evidence
was obtained in hypophysectomized (Hypox) rats, which are immunodeficient
. Replacement doses of PRL or GH completely restored the immune reactivity
of Hypox animals. Moreover, treatment with the dopaminergic drug, bromocriptine,
which inhibits pituitary PRL secretion, was as immunosuppressive as was
Hypox. Again the immune response could be restored with either GH or PRL
treatment [5-12]. Subsequently numerous observations confirmed the immunoregulatory
potential of PRL and GH, as attested for in this volume.
Although much less studied, the evidence available clearly indicates that
placental lactogenic hormones (PL) also have the potential of regulating
the immune system [7,13-15].
TIMELY QUESTIONS AND ANSWERS.
pituitary dwarf individuals are immunocompetent?
Pituitary dwarfs have normal serum PRL levels . Animal experiments
showed that PRL is able to maintain immune function in the absence of GH
[6-8,11]. On this basis it is reasonable to suggest that PRL is responsible
for the maintenance of immunocompetence in dwarf people as well.
dwarf mice are deficient in both PRL and GH, yet show immunocompetence,
These mice are deficient of the pituitary transcription factor, Pit-1,
which controls the production of GH, PRL and of thyroid stimulating hormone
(TSH) secretion. In the recent literature these animals have often been
presented as lacking completely pituitary GH and PRL secretion. However,
low serum level of GH and PRL was detectable in these animals by radioimmunoassay
[17-19]. Moreover, humans with Pit-1 mutations have subnormal levels of
GH, PRL and TSH, and are not negative . It was also shown that Snell
dwarf mice produced less lymphocyte-derived PRL (LPRL) than did their normal
littermates. LPRL could be restored to normal by thyroxin treatment of
lymphocytes in vitro . The production of placental lactogen
was normal . Therefore, it appears that Pit-1 deficient mice and humans
do in fact, have sufficient pituitary hormone levels, which permit survival
and immune function. Clearly, the joint and complete deficiency of pituitary
GH and PRL has not been demonstrated to date in man or in animals. This
point is further illustrated by the observation that Hypox rats are able
to survive for 6-8 months because of the presence of residual PRL in their
serum. If this residual PRL is neutralized by antibodies, the animals will
perish within a few weeks time .
that lack PRL or IGF-I function survive and are immunocompetent, why?
Knockout mice, lacking either PRL or its receptor (PRLR), or IGF-I are
immunocompetent. It was interpreted, therefore, that these hormones are
not obligate immunoregulators, but rather, affect immune reactions as anabolic
and stress modulating agents [23-25]. In actual fact the data obtained
in knockout mice is a powerful confirmation of the original observations
that growth and lactogenic hormones (GLH) show redundancy in the maintenance
of immunocompetence [7-11]. Today a compelling body of experimental evidence,
which is presented in this volume, indicates that indeed this is the case.
Clearly, immune function, as many other functions in the body, are maintained
by multiple genes that show redundancy .
Growth hormone and PRL belong to the type-I cytokine family . Functional
overlap and redundancy is the rule for type I cytokines (and for other
cytokines as well) in the immune system. The receptor for type I cytokines
consists of a ligand specific chain and of a shared signal transducing
chain. For instance in the first group, where IL-2, -4, -7, -9 and -15
belong, there is a common gamma chain , for the second group (IL3, -5 and
GM-CSF) it is called the common beta chain and for the third group
(Il-6, -11, oncostatin M, leukemia inhibitory factor, ciliary neurotropic
factor and cardiotrophin-1) the common chain is glycoprotein 130. Signal
transduction is possible only if the ligand binding and the signal transducing
chains are crosslinked by the specific cytokine. Knockout experiments in
this system showed that the elimination of specific cytokines or their
specific receptor chains produced minimal if any abnormalities. However,
knocking out the shared signal transducing common gamma chain resulted
in severe combined immunodeficiency [27,28]. These observations collectively
indicate that type I cytokines are indispensable as a group
for normal immune function. Apparently there is enough redundancy in this
group to compensate for the lack of any particular cytokine.
Prolactin and growth hormone do not share receptor chains with any of the
above cytokines. However, human GH and other primate GH are known to act
on PRL receptors and to exert lactogenic activity in many species .
Similarly, IGF-I, IGF-II and insulin show functional overlap . These
facts indicate that functional redundancy exists within GLH
hormones, which explains why the disabling of a single gene is of no consequence
for immune function.
The major signal transduction pathway, which involves the Janus kinase
(JAK) and signal transducers and activators of transcription (STAT) nuclear
regulatory factors, is shared between cytokines and growth and lactogenic
hormones. STAT knockout mice show severe developmental and immune deficiencies
[14,27,28,30]. This emphasizes the significance of this signal transduction
pathway in immune development and function [31,32].
The evidence, that has accumulated to date, indicates that GLH
are indispensable as a group for normal development and bodily functions,
including immune function [14,15, 22, 24, 31, 33, 54] . Because the JAK-STAT
transcription pathway of PRL and GH are shared with interleukins and hemopoietic
growth factors [14,27,29], some regard PRL and GH as members of the hemopoietic
cytokine family. However, GLH have a much wider spectrum of biological
activity than any of the type I cytokines. A functional overlap with these
cytokines could simply indicate the capacity of GLH hormones to maintain
the hemopoietic and immune systems at times when cytokines are in short
supply as well as to boost immune activity in situations of emergency.
Female mice that lack PRL or do not respond to it, do not reproduce [23-25].
In this context, one must not forget that without normal immune function
reproduction is not possible. The immune system is involved in the function
of the gonads, in conception, in the normal development of the fetus and
it plays a role in the normal function of the mammary gland. Milk plays
a very important role in the transfer of maternal antibodies and of other
immune factors and PRL itself to the fetus. There is evidence to indicate
that PRL is important for the immunological function of the mammary gland
[15, 33-37]. Therefore, the immunoregulatory function of PRL may be of
special importance in the female reproductive compartment.
ill patients got worse after treatment with GH, why?
In patients with acute phase response (APR) the GH-IGF-I axis is suppressed.
This observation prompted several clinical trials with GH, which were aimed
at restoring this axis in the hope of preventing the severe catabolic state
and to improve immunocompetence in the interest of increased survival.
However, so far this hope did not materialize. In fact a controlled clinical
trial showed that GH treatment of severely ill patients significantly elevated
the proportion that did not survive . Deaths attributed to " septic
shock or uncontrolled infection" occurred nearly four times more commonly
in GH treated patients compared to placebo receiving patients. Although
no data were given regarding immune parameters, the authors suggested that
alterations in immune functions could have contributed to these fatalities.
Critical illness elicits a highly coordinated and powerful acute phase
reaction, whereby the immune system is switched from the adaptive mode
of response to the amplification of natural immune mechanisms. The acute
phase response is characterized by profound elevations of interleukin-1,
interleukin-6 and tumor necrosis factor-alpha,(TNF-alpha), which induce
complex neuroendocrine and metabolic alterations. The hypothalamic - pituitary-
adrenal axis is activated, whereas the serum levels of growth hormone,
insulin-like growth factor and prolactin are suppressed. Tri-iodothyronine
is also diminished (sick euthyroid syndrome). The increased serum level
of cytokines and the array of neuroendocrine changes lead to fever, catabolism
and to the suppression of the T lymphocyte-dependent adaptive immune system.
At the same time natural immune mechanisms are amplified. There is a rapid
rise in serum natural antibodies and liver-derived acute-phase proteins
such as endotoxin-binding protein and C-reactive protein. These antibodies
and acute phase proteins have the capacity to recognize homologous crossreactive
epitopes (homotopes) on microbes and on altered self components in a polyspecific
fashion and activate immune defense mechanisms after combining with the
respective homotope. Host defenses against toxins and other noxious agents
are also increased during the acute phase response [39-41].
The acute phase response is a massive neuroimmune and metabolic response
that mobilizes all the resources of the body in the interest of host defence
and survival. The findings of Takala and co-workers  suggest strongly
that the suppression of the GH - IGF-I axis in APR is required for intense
catabolism to take place. A rapid release of nutrients and of energy is
necessary under these conditions in order to support maximally the defence
system of the body, that includes the hypothalamus - pituitary - adrenal
axis, the sympathetic nerves system, the bone marrow, CD5+ B lymphocytes,
leukocytes and the liver [39-41]. The adaptive immune system is controlled
by thymus-derived (T) lymphocytes and needs several days to a week for
an effective response. During APR no time is available for an adaptive
immune reaction, and therefore this system is shut down, primarily by the
cytokine and endocrine alterations that take place. The thymus and T cell
function is heavily dependent on the GH/PRL - IGF-I axis and it is suppressed
profoundly by the elevated levels of glucocorticoids and cathecolamines
[39-41]. Recent observations showed that GH inhibits the production of
acute phase proteins in rats with burn injury and in human hepatocytes
[42-43]. These findings strongly support the above hypothesis.
One may argue that the most efficient way to fuel the intensive systemic
effort for survival in APR is by the rapid breakdown of bodily tissues.
GH is a powerful anabolic hormone, which supports the T lymphocyte dependent
immune system, and acts as antagonist of the HPA axis that promotes APR
[6,8,11,39-41,44]. The results of this controlled trial supports the hypothesis
that the inhibition of the HPA axis and of catabolism by GH treatment in
APR hampers the bodies defence mechanisms, which may have fatal consequences.
is the role of GLH in the neuroimmune regulatory network?
Current evidence indicates that GLH is required for the normal growth and
development of embryos as well as for the development of the immune system
and the maintenance of immunocompetence. Clearly, GLH supports any adaptive
immune function and natural immunity under physiological conditions. Lymphocyte
precursors do not have receptors for antigen or cytokines and for this
reason must rely on physiological mediators for survival and differentiation.
Even after full differentiation, naïve lymphocytes remain small and
do not synthesise, neither do they respond to immune-derived cytokines.
In the absence of antigenic stimulation these cells must rely on physiological
systemic mediators to survive. The thymus and other lymphoid organs lose
cellularity and weight in Hypox rats, which also show a profound immunosuppresion.
The weight of lymphoid organs and immune reactivity can be normalized in
Hypox animals by replacement doses of either PRL or GH . The situation
is similar in old animals to some extent, although full immune restoration
by GH treatment was not possible . Both GH and PRL are capable of maintaining
immunocompetence, which is antagonized by the hypothalamus-pituitary adrenal
axis . This enables the pituitary gland to exert a true regulatory
effect on the immune system. Clearly, the pituitary gland does not only
maintain immunocompetence, but also, is capable of fine tuning the level
of reactivity and plays a fundamental role in the induction of immunoconversion
during APR [5,6,9, 41,45].
There is compelling evidence to indicate that after activation by antigen
or mitogen lymphocytes produce their own PRL and/or GH. This makes the
rapid proliferation required for an immune response feasible [48-53]. This
situation is similar to the development of the embryo, where placental
lactogenic hormones make it possible for the embryo to grow at a very rapid
rate. The production of placental GLH is independent from the pituitary
gland and is controlled by " placental" promoters. This allows these hormones
to override the regulatory power of the maternal pituitary gland during
pregnancy in the interest of assuring the proper development of the fetus.
Therefore, while conception is clearly dependent on normal pituitary function,
the fetus becomes independent from such influence .
Interestingly placental promoter was found also in association with the
lymphocyte PRL gene . However, Pit-1 was also detected in lymphocytes
. This suggests that once the lymphocyte PRL gene is activated, pituitary
PRL is no longer required for lymphocyte growth or function. Once the immune
response is over, most of the activated lymphocytes will undergo apoptosis,
which is governed by a complex mechanism that involves the delivery of
death signals, primarily by the Fas-FasL system. However, a specialize
subset called memory cells, will survive [57,58].
We observed years ago that the primary antibody response is fully pituitary
dependent, whereas the secondary response shows only partial dependence.
Actually, when the rats were immunized first, hypohysectomized and immunized
again, they produced antibodies in response to the second stimulus, which
was of similar magnitude to the primary response . These results suggest
that memory cells maintained their reactivity after Hypox, but the rapid
recruitment of naïve lymphocytes, which occurs in normal animals,
could not take place.
The mechanism(s) for the long term survival and self-renewal capacity of
memory B and T lymphocytes is not understood. It was hypothesised that
memory B lymphocytes are stimulated by idiotypes, which are unique determinants
of antigen receptors . Major histocompatibility antigens presenting
self peptides were suggested to fulfill a stimulatory role for memory T
lymphocytes . The role of cytokines in T cell longevity is also recognized
and IL-15 was claimed to be necessary for CD8+ memory cells . T lymphocyte
apoptosis is inhibited by interferon-alpha (IFN-alpha) and IFN-beta and
were proposed to play a role in memory cell survival. These cytokines are
able to maintain T cells without an antigenic stimulus . Recently Cho
and co-workers [63.] demonstrated that in recombinase deficient (RAG-1
-/-) mice, which are lymphopenic, naïve T lymphocytes undergo "homeostasis-stimulated"
proliferation, which is MHC restricted, and develop into memory cells in
the absence of antigenic stimulation. These cells acquire the phenotypic
and functional characteristics of antigen-induced memory CD8+ T cells and
lyse target cells directly and respond to lower doses of antigen than naïve
cells and secrete IFN-gamma faster upon restimulation. Interleukin-2 or
co-stimulation by CD28 are not required and effector cells are not formed
during this homeostatic differentiation. These findings indicate that memory
T cells may be generated and maintained under the influence of physiological
immunoregulatory mechanisms, in the complete absence of immune stimulation
Immature thymocytes of rodents are killed by glucocorticoids, whereas mature
thymocytes are saved. The helper, suppressor and killer functions of T
lymphocytes and the production of interleukins by them are all inhibited
by glucocorticoids. In contrast, the function of memory cells and of cells
mediating the graft-versus-host reaction is not inhibited by glucocorticoids
. Prolactin and GH antagonize the immunosuppressive effects of the
ACTH-adrenal axis [6,8,11]. Taking all the evidence in consideration, one
may suggest that naive T cells are maintained in the absence of antigenic
stimulation by pituitary GLH, whereas memory T cells are autonomous and
survive and resist glucocorticoids, most likely because they produce autocrine
GLH and cytokines that enable these cells to survive and to resist adverse
conditions, such as the APR.
This pattern of obtaining gradual independence assures maximal host defence
while the maturation and selection process of lymphocytes in the thymus
and bone marrow is tightly controlled by the neuroimmune regulatory system.
Pituitary GLH is important for the development of lymphocytes and of the
maintenance of mature naïve cells in a state of immunocompetence
[22,45]. It is likely that after activation lymphocyte-derived GLH gradually
assumes a prominent role in the maintenance of lymphocyte function. Finally,
it appears that memory cells rely on autocrine GLH for long term survival
and function. This is to be substantiated further experimentally.
FROM BENCH TO BEDSIDE
The goal of this volume is to present the current evidence for the role
of growth and lactogenic hormones in the neuroimmune regulatory system.
The evidence presented is compelling and shows that all the requirements
for proven biological significance have been fulfilled. Receptors for GLH
on cells of the immune system have been characterized, signal transduction
pathways have been identified and are being characterized, and the immunoregulatory
activity of GLH has been demonstrated in various species, including man.
It is also clear that both PRL and GH are produced within the immune system
by activated cells. Placental, pituitary and tissue derived GLH hormones
all play a role in neuroimmunoregulation. This redundancy serves well the
adaptability and versatility of the neuroimmune regulatory network as well
as of immune function.
Finally, the therapeutic use and manipulation of GLH is currently underway
for the treatment/correction of various human conditions. Therefore, the
ultimate criterion for the success of scientific research, i.e the application
of knowledge obtained on the laboratory bench at the bedside is being fulfilled.
It is very rewarding to witness one's initial research efforts to develop
and reach this critical stage. No reasonable arguments can be raised any
more in the face of this evidence against the fundamental role of growth
and lactogenic hormones in immunoregulation. Clearly the challenge today
is not to prove, but to understand, the neuroimmune regulatory role of
GLH in its entire complexity.
The realization that a third systemic regulator, the immune system, is
included in homeostatic and in allostatic regulation to form the Neuroimmune
regulatory network, provides new foundation to Biology. This network is
immensely complex and powerful and is involved in both physiological (homeostatic)
and pathophysiological (allostatic) regulation. Indeed the entire biological
cycle from conception till death of the individual is subject to this regulatory
system. It is also clear that the defects and abnormalities of this system
is the underlying cause for many diseases that include neural conditions,
endocrine and metabolic diseases immune abnormalities (immunodeficiency,
hypersensitivity conditions and autoimmune diseases, etc) and others .
A better understanding of neuroimmunoregulation is obligatory for obtaining
new insights into the pathogenesis of these conditions and for the development
of more rational approaches to treatment. The ultimate goal of this volume
and of all the other volumes of this series is to promote the understanding
of the science and to ease human suffering.
Dr. Robert Rapaport, who has contributed significantly to the interpretation
of the findings in critically ill patients after GH treatment. Many other
colleagues contributed over the years to experimentation and to the development
of the viewpoints expressed in this article. Notably, I owe special thanks
to Drs Eva Nagy, Edris Sabbadini, Robert Shiu, Henry Friesen, Robert Matusik,
Richard Warrington, Kalman Kovacs and Sylvia Asa.
experimental work discussed here was supported in part by MRC of Canada
and the Arthritis Society of Canada.