Protein hormones and immunity

A number of observations and discoveries over the past 20 years support the concept of important physiological interactions between the endocrine and immune systems. The best known pathway for transmission of information from the immune system to the neuroendocrine system is humoral in the form of cytokines, although neural transmission via the afferent vagus is well documented also. In the other direction, efferent signals from the nervous system to the immune system are conveyed by both the neuroendocrine and autonomic nervous systems. Communication is possible because the nervous and immune systems share a common biochemical language involving shared ligands and receptors, including neurotransmitters, neuropeptides, growth factors, neuroendocrine hormones and cytokines. This means that the brain functions as an immune-regulating organ participating in immune responses. A great deal of evidence has accumulated and confirmed that hormones secreted by the neuroendocrine system play an important role in communication and regulation of the cells of the immune system. Among protein hormones, this has been most clearly documented for prolactin (PRL), growth hormone (GH), and insulin-like growth factor-1 (IGF-I), but significant influences on immunity by thyroid-stimulating hormone (TSH) have also been demonstrated. Here we review evidence obtained during the past 20 years to clearly demonstrate that neuroendocrine protein hormones influence immunity and that immune processes affect the neuroendocrine system. New findings highlight a previously undiscovered route of communication between the immune and endocrine systems that is now known to occur at the cellular level. This communication system is activated when inflammatory processes induced by proinflammatory cytokines antagonize the function of a variety of hormones, which then causes endocrine resistance in both the periphery and brain. Homeostasis during inflammation is achieved by a balance between cytokines and endocrine hormones.     

Neuroendocrine peptide hormones and their receptors in the immune system: production, processing and action

Recent findings indicate that the immune and neuroendocrine systems interact and modulate one another functionally. The mechanism for this seems to be that the 2 systems share a set of receptors and ligands (hormones). Cells of the immune system are able to synthesize neuroendocrine peptide hormones which are biologically active and produced in physiologically significant quantities. Furthermore, leukocytes possess functional receptors for these same neuroendocrine hormones which will specifically modulate immune responses. The structural and functional evidence for these interactions is reviewed and discussed in the context of a bidirectional regulatory circuit between the immune and neuroendocrine systems.     

IL-10 as a mediator in the HPA axis and brain

Certain functional interactions between the nervous, endocrine, and immune systems are mediated by cytokines. The pro-inflammatory cytokines, interleukin-1 (IL-1) and tumor necrosis factor (TNF) were among the first to be recognized in this regard. A modulator of these cytokines, IL-10, has been shown to have a wide range of activities in the immune system; in this review, we describe its production and actions in the hypothalamic–pituitary–adrenal (HPA) axis. IL-10 is produced in pituitary, hypothalamic, and neural tissues in addition to lymphocytes. IL-10 enhances corticotropin releasing factor (CRF) and corticotropin (ACTH) production in hypothalamic and pituitary tissues, respectively. Further downstream in the HPA axis endogenous IL-10 has the potential to contribute to regulation of glucocorticosteroid production both tonically and following stressors. Our studies and those of others reviewed here indicate that IL-10 may be an important endogenous regulator in HPA axis activity and in CNS pathologies such as multiple sclerosis. Thus, in addition to its more widely recognized role in immunity, IL-10's neuroendocrine activities described here point to its role as an important regulator in communication between the immune and neuroendocrine systems.     

Molecular mechanisms and neural pathways mediating the influence of interleukin-1 on the activity of neuroendocrine CRF motoneurons in the rat

The action of immune-system-derived cytokines to stimulate the release of corticotropin-releasing factor (CRF) from the hypothalamus and the consequent elaboration of ACTH and release of corticosteroids has provided an especially useful model to investigate the nature of the intercommunication of neuroendocrine and immunological pathways. Substantial evidence exists to support the production of cytokines, such as interleukin-1 (IL-1) α and β, within the mammalian central nervous system. The mechanisms and neuronal circuitries involved in the effects of these cytokines of peripheral and central origin on the activity of neuroendocrine CRF motoneurons and the hypothalamic-pituitary-adrenal axis are described. Also included is a discussion of the influence of IL-1 on transduction signals controlling the release and the biosynthesis of CRF in the parvocellular division of the paraventricular nucleus of the hypothalamus and the relationship between these two distinct intracellular processes. The relebance of using immediate early genes as indices of neuronal activity in immune-challenged rats and the possible roles of c-fos and NGFI-B within neuroendocrine CRF motoneurons are outlined. Finally, the effects of acute immune response on neuroendocrine functions and brain neuronal activation are presented.     

Neuroendocrine–immune interactions in homeostasis and autoimmunity

Recent experimental evidence confirms the interrelationships between the central nervous, neuroendocrine and immune systems. Indeed, extensive duality exists in the use of neurotransmitters, hormones and receptors each system displays. In the present annotation, the effect of cytokines, soluble mediators of immune function, on the CNS and neuroendocrine systems is addressed and conversely, we discuss the modification of the immune compartment by the sympathetic nervous and neuroendocrine systems, with particular reference to the role of noradrenaline and corticosterone. Dysfunction between the systems is considered in the context of autoimmune conditions, with emphasis on experimental allergic encephalomyelitis and the contribution of corticosterone–driven T–cell apoptosis to recovery from the disease. Finally, we speculate on the relevance of neuroimmune interactions in the pathogenesis of multiple sclerosis.     

Conditioned immunomodulation: research needs and directions

Considering the brief time that psychoneuroimmunology has existed as a bona fide field of research, a great deal of data has been collected in support of the proposition that homeostatic mechanisms are the product of an integrated system of defenses of which the immune system is a critical component. It is now clear that immune function is influenced by autonomic nervous systems activity and by the release of neuroendocrine substances from the pituitary. Conversely, cytokines and hormones released by an activated immune system influence neural and endocrine processes. Regulatory peptides and receptors, once confined to the brain, are expressed by both the nervous and immune systems enabling each system to monitor and modulate the activities of the other. It is hardly surprising, then, that immunologic reactivity can be influenced by stressful life experiences or by Pavlovian conditioning.     

Neural-immune interactions: an integrative view of the bidirectional relationship between the brain and immune systems

This review briefly summarizes a part of the relevant knowledge base of neuroimmunology, with particular emphasis on bidirectional neural-immune interactions. These complex systems interact at multiple levels. Both neuroendocrine (the primary hormonal pathway is hypothalamic-pituitary-adrenal axis) and neuronal (direct sympathetic innervation of the lymphoid organs) pathways are involved in the control of the humoral and cellular immune responses. Although, the recent evidence has been made on immunosuppressive effect of acetylcholine-secreting neurons of the parasympathetic nervous system. The immune system, in turn, influences the central nervous system primarily through cytokines. At the molecular level, neuro- and immune signal molecules (hormones, neurotransmitters, neuropeptides, cytokines) or their receptors are member of the same superfamily which enable the mutual neuroimmune communication. Most extensively studied are cytokine-neuropeptide/neurotransmitter interactions and the subcellular and molecular mechanisms of these interactions. At the system (neuroanatomical) level, advances in neural-immune communication have been made in the role of discrete brain areas related to emotionality. The immunoenhancement, including the antiviral and antitumor cytotoxic activity, related to the "brain reward system", limbic structures and neocortex, offers a new directions for therapy in immune disorders.     

Impact of the neuroendocrine system on thymus and bone marrow function

The nervous, endocrine, and immune systems interact to adapt to infection, inflammation, and tissue injury. Neural control is mediated in several ways, one of them being through the neuroendocrine regulation of the secretion of hypothalamic and pituitary hormones. The hormonal effects on the immune system range from the impact of steroidal hormones, which exhibit inhibitory effects over immune functions, to growth hormone, prolactin and neurohypophyseal hormones, known to stimulate and modulate humoral and cellular aspects of the immune system. This review will discuss the mechanisms behind the immunomodulatory role of the neuroendocrine system, including the critically important feedback loops required to maintain balance for these bidirectional interactions and alterations that occur with age.     

Neuroendocrine dysfunction in Sjogren's syndrome

Interactions among the immune, nervous and endocrine systems, which are mediated by hormones, neuropeptides, neurotransmitters, cytokines and their receptors, appear to play an important role in modulating host susceptibility and resistance to inflammatory disease. The neuroendocrine system has two main components: the central and the peripheral. The central compartment is located in the locus ceruleus, the brainstem centers of the autonomic system and the paraventricular nucleus; the peripheral mainly consists of the sympathetic/adrenomedullary system, the hypothalamic-pituitary-adrenal axis (HPA), the hypothalamic-pituitary-gonadal (HPG) axis, and the neuroendocrine tissue located in several organs throughout the body. Hormones and neuropeptides may influence the activities of lymphoid organs and cells via endocrine and local autocrine/paracrine pathways or alter the function of different cell types in target organs. Recent studies highlighted alterations of the neuroendocrine system in systemic autoimmune diseases, including rheumatoid arthritis, systemic lupus erythematosus and Sjogren's syndrome (SS). SS, a prototype autoimmune disorder, has a wide clinical spectrum, extending from organ involvement (autoimmune exocrinopathy) to systemic disease and B cell lymphoma. In SS, several functions of the neuroendocrine system are impaired. First, the HPA axis appears to be disturbed, since significantly lower basal ACTH and cortisol levels were found in patients with SS and were associated with a blunted pituitary and adrenal response to ovine corticotropin-releasing factor compared to normal controls. Second, HPG axis is also involved, since lack of estrogens is associated with human disease and the development of autoimmune exocrinopathy in several experimental models. Finally, exocrine glands are enriched with neuroendocrine-related molecules, adjacent to local autoimmune lesions. Certain clinical manifestations of the disease, including the sicca manifestations, easy fatigue, fibromyalgia and psychological disturbances can be very well explained by mechanisms directly related to disturbances of the neuroendocrine axis. On the other hand, the molecular and biochemical effects of the inflammatory molecules or cell-to-cell interaction, observed during the local or systemic autoimmune injury with cells and mediators of the neuroendocrine system, are largely unexplored.     

The importance of being receptive

Neuroendocrine system and immune system can communicate via the use of soluble mediators like hormones, neurotransmitters and cytokines. The level of mediators secreted by either of these systems creates the milieu in which immune and neuroendocrine responses take place. For adequate communication between the systems, receptors for hormones, neurotransmitters and cytokines are required. This review describes the role of regulated expression and function of receptors for hormones and neurotransmitters within the immune system in neuroendocrine-immune communication.     

Distribution of type I corticotropin‐releasing factor (CRF1) receptors on GABAergic neurons within the basolateral amygdala

The neuropeptide corticotropin‐releasing factor (CRF) plays a critical role in mediating anxiety‐like responses to stressors, and dysfunction of the CRF system has been linked to the etiology of several psychiatric disorders. Extra‐hypothalamic CRF can also modulate learning and memory formation, including amygdala‐dependent learning. The basolateral nucleus of the amygdala (BLA) contains dense concentrations of CRF receptors, yet the distribution of these receptors on specific neuronal subtypes within the BLA has not been characterized. Here, we quantified the expression of CRF receptors on three nonoverlapping classes of GABAergic interneurons: those containing the calcium‐binding protein parvalbumin (PV), and those expressing the neuropeptides somatostatin (SOM) or cholecystokinin (CCK). While the majority of PV+ neurons and roughly half of CCK+ neurons expressed CRF receptors, they were expressed to a much lesser extent on SOM+ interneurons. Knowledge of the distribution of CRF receptors within the BLA can provide insight into how manipulations of the CRF system modulate fear and anxiety‐like behaviors.     

How cancer hijacks the body’s homeostasis through the neuroendocrine system

During oncogenesis, cancer not only escapes the body’s regulatory mechanisms, but also gains the ability to affect local and systemic homeostasis. Specifically, tumors produce cytokines, immune mediators, classical neurotransmitters, hypothalamic and pituitary hormones, biogenic amines, melatonin, and glucocorticoids, as demonstrated in human and animal models of cancer. The tumor, through the release of these neurohormonal and immune mediators, can control the main neuroendocrine centers such as the hypothalamus, pituitary, adrenals, and thyroid to modulate body homeostasis through central regulatory axes. We hypothesize that the tumor-derived catecholamines, serotonin, melatonin, neuropeptides, and other neurotransmitters can affect body and brain functions. Bidirectional communication between local autonomic and sensory nerves and the tumor, with putative effects on the brain, is also envisioned. Overall, we propose that cancers can take control of the central neuroendocrine and immune systems to reset the body homeostasis in a mode favoring its expansion at the expense of the host.     

Interactions between the nervous and immune systems

Substantial morphologic and functional evidence exists that supports the reciprocal interactions that occur between the nervous and immune systems. The nervous and immune systems have been increasingly found to use a common chemical language in the form of neuropeptides, cytokines, and hormones. Sophisticated immunologic techniques such as the identification and detection of immune cell surface markers enable researchers to determine the origin and activity of diverse cells in the blood and central nervous system. These techniques have elucidated the activity of immune cells in the central nervous system (CNS) that was previously thought to be privileged from immune surveillance in the presence of an intact blood brain barrier. Immune cells in the CNS play a central role in several degenerative diseases such as Alzheimer's disease, Huntington's disease, Multiple sclerosis, AIDS dementia complex, and nerve destruction associated with trauma. Immune cells also play a role in demyelinating peripheral nerve disorders. Cytokines and neuropeptides secreted by peripheral immune cells have profound effects on behavior that is mediated by the CNS. The close integration between immune and nervous system responses is being increasingly recognized in physiologic and pathologic conditions.     

Evidence of conserved neuroendocrine interactions in the thymus: intrathymic expression of neuropeptides in mammalian and non-mammalian vertebrates

The function of lymphoid organs and immune cells is often modulated by hormones, steroids and neuropeptides produced by the neuroendocrine and immune systems. The thymus intrinsically produces these factors and a comparative analysis of the expression of neuropeptides in the thymus of different species would highlight the evolutionary importance of neuroendocrine interaction in T cell development. In this review, we highlight the evidence which describes the intrathymic expression and function of various neuropeptides and their receptors, in particular somatostatin, substance P, vasointestinal polypeptide, calcitonin gene-related peptide and neuropeptide Y, in mammals (human, rodent) and non-mammals (avian, amphibian and teleost), and conclude that neuropeptides play a conserved role in vertebrate thymocyte development.     

The relationship between interleukin-6 and herpes simplex virus type 1: implications for behavior and immunopathology.

Cytokines are hormones once thought to be restricted to the immune system produced solely by hematopoietic-derived cells and acting on receptors expressed by cells of the immune system. However, it is now clear that many cytokines are produced not only by lymphocytes, monocytes, granulocytes, and dendritic cells but are also synthesized by cells outside the realm of the immune system in response to stimuli that may not be associated with immune homeostasis. In fact, there is evidence supporting a role of selected cytokines modifying behavior and neuroendocrine function. Recently, a potential relationship between the cytokine interleukin (IL)-6 and herpes simplex virus type 1 (HSV-1) reactivation has been found. This article discusses the relevance of these findings and considers the potential impact that HSV-1 infection has on behavior and chronic inflammatory processes that can occur in the nervous system during "latent" virus infection as a result of chronic IL-6 expression.     

Presence of endo-oligopeptidase (EC 3.4.22.19), a putative neuropeptide-metabolizing endopeptidase in cells of the immune system.

Neuropeptides have been shown to modulate the bidirectional communication between the central nervous and immune systems. The endooligopeptidase (EC 3.4.22.19), originally isolated and characterized in the nervous tissue, was shown to hydrolyse several neuropeptides and to generate enkephalin from enkephalin-containing peptides. This report shows the presence of endopeptidase 22.19 in the rat immune system using both biochemical and immunochemical methods. The specific activity of endopeptidase 22.19 in soluble fraction of lymphocytes was 3-4-fold higher than the one found in the nervous tissue. Among rat blood cells the highest specific activity of endopeptidase 22.19 was found in T lymphocytes, being 2.5-fold higher than the activity found in other leukocytes. Immunocytochemical studies performed in tissues and cells of the immune system indicate the presence of endopeptidase 22.19-like enzyme in all types of leukocytes. The occurrence of this enzyme in cells of the immune system can be considered an important step in understanding the metabolism of neuropeptides in the immune system as well as its possible participation as a regulatory enzyme in neuroimmunomodulation.     

The immune-hypothalamo-pituitary-adrenal axis and autoimmunity.

Immunoendocrinology is a rapidly expanding field, uncovering numerous bilateral interactions between the immune system and neuroendocrine circuits. Various hormones and neurotransmitters appear to modulate cells of the immune system and likewise cytokines control the function of neuroendocrine system. In the present paper, we discuss some lines of evidence indicating that an immunoendocrine feedback loop, which we term 'immune-hypothalamo-pituitary-adrenal system' is an integral part of the regulation of self tolerance. The finding that pathology of this immunoendocrine feedback loop is related to development of autoimmunity may lead to new prophylactic and therapeutic strategies.     

Production of tumor necrosis factor-alpha, interleukin 1-beta, interleukin 2, and interleukin 6 by rat leukocyte subpopulations after exposure to substance P

The interaction between components of the nervous system and multiple target cells in the cutaneous immune system has been receiving increasing attention. Recently, the involvement of neuropeptides has been demonstrated to play an important role in the inflammatory cascade. Neuropeptides such as Substance P are released by cutaneous neurons and modulate the function of immunocompetent and inflammatory cells as well as epithelial and endothelial cells. Substance P has been shown to function as a mediator for cell proliferation, cytokine production, and as an upregulator of various cell surface receptors. In this study, we show the effect of Substance P on the production of Tumor Necrosis Factor-alpha, Interleukin 1-beta, Interleukin 2, and Interleukin 6 by T-lymphocytes, macrophages and neutrophils. These data demonstrate that pathophysiological levels of Substance P induce production of cytokines in all three cell populations tested. Interestingly, T-cells demonstrated the highest percentage of cells expressing all four cytokines. In contrast, macrophages and neutrophils produced the highest absolute levels of cytokines. The elucidation of mediating mechanisms of Substance P activation of leukocytes is crucial to the understanding of the cutaneous inflammatory cascade and involvement of the peripheral nervous system on the immune system. These findings suggest that Substance P participates in the complex network of mediators that regulate cutaneous inflammation and potentially the rate of wound healing.     

Listening to mutant mice: a spotlight on the role of CRF/CRF receptor systems in affective disorders

Genetically engineered mice were originally generated to delineate the role of a specific gene product in behavioral or neuroendocrine phenotypes, rather than to produce classic animal models of depression. To learn more about the neurobiological mechanisms underlying a clinical condition such as depression, it has proven worthwhile to investigate changes in behaviors characteristic of depressed humans, such as anxiety, regardless of whether or not these alterations may also occur in other disorders besides depression. The majority of patients with mood and anxiety disorders have measurable shifts in their stress hormone regulation as reflected by elevated secretion of central and peripheral stress hormones or by altered hormonal responses to neuroendocrine challenge tests. In recent years, these alterations have been increasingly translated into testable hypotheses addressing the pathogenesis of illness. Refined molecular technologies and the creation of genetically engineered mice have allowed to specifically target individual genes involved in regulation of corticotropin releasing factor (CRF) system elements (e.g. CRF and CRF-related peptides, their receptors, binding protein). Studies performed in such mice have complemented and extended our knowledge. The cumulative evidence makes a strong case implicating dysfunction of these systems in the pathogenesis of depression and leads us beyond the monoaminergic synapse in search of eagerly anticipated strategies to discover and develop better therapies for depression.     

Immune-neuroendocrine circuits: integrative role of cytokines.

Several efficient autoregulatory mechanisms confer a certain degree of autonomy to the immune system. However, increasing evidence shows that immune processes operate in a coordinated fashion with other body systems. In this article, we discuss concepts and facts concerning interactions between immune and neuroendocrine mechanisms. There are clear examples that immune cells can be influenced by hormones, neurotransmitters, and neuropeptides and also by alterations in brain functions. Conversely, immune-derived products such as lymphokines and monokines can affect endocrine, autonomic, and central mechanisms. Neuroendocrine responses occur during the activation of the immune system. These responses can be elicited by innocuous antigens; they can also be detected during pathological conditions involving immune activation, and in many cases are dissociable from the effects of the disease itself and from the stress of being sick. On this basis, we emphasize the multidirectional nature of the communication processes between the immune, endocrine, and nervous systems. The role of lymphokines and monokines as messengers able to convey information to neuro and endocrine structures about the present state of activity of the immune system is stressed. The relevance of immune-neuroendocrine interactions for immunoregulation and host defenses is discussed as well as the active role of the immune system in mediating metabolic and homeostatic adjustments or derangements during the course of certain infectious, inflammatory, and neoplastic processes. The evidence available suggests that complex immune-neuroendocrine networks operate under both physiological and pathological conditions.