Divergent immune responses in behaviorally-inhibited vs. non-inhibited male rats.

Stable behavioral traits (temperament, personality) often predict health outcomes. Temperament-specific differences in immune function could explain temperament-specific health outcomes, however, we have limited information on whether immune function varies by personality. In the present study, we examined the relationship between a basic behavioral trait (behavioral-inhibition vs. non-inhibition) and two immune responses (innate inflammation and delayed-type hypersensitivity, DTH) in a rodent model. In humans, behavioral inhibition (fearful temperament) is associated with altered stress physiology and allergies. In laboratory rats, the trait is associated with elevated glucocorticoid production. We hypothesized that behavioral inhibition is associated with glucocorticoid resistance and dampened T-helper 1 cell responses often associated with chronic stress and allergies. Further, this immune profile would predict poorly-regulated innate inflammation and dampened DTH. In male Sprague-Dawley rats, we quantified consistent behavioral phenotypes by measuring latency to contact two kinds of novelty (object vs. social), then measured lipopolysaccharide(LPS)-induced innate inflammation or keyhole limpet hemocyanin(KLH)-induced DTH. Behaviorally-inhibited rats had heightened glucocorticoid and interleukin-6 responses to a low/moderate dose of LPS and reduced DTH swelling to KLH re-exposure compared to non-inhibited rats. These results suggest that behavioral inhibition is associated with a glucocorticoid resistant state with poorly regulated innate inflammation and dampened cell-mediated immune responses. This immune profile may be associated with exaggerated T-helper 2 responses, which could set the stage for an allergic/asthmatic/atopic predisposition in inhibited individuals. Human and animal models of temperament-specific immune responses represent an area for further exploration of mechanisms involved in individual differences in health.

Asthma Induction During Development and Adult Lung Function, Behavior and Brain Gene Expression.

In developing youth, allergic asthma is the most common chronic condition, with 9%-10% of youth affected. Asthma onset during childhood and adolescence is further associated with other health issues, particularly psychiatric conditions. To understand causal mechanisms by which developmental asthma may lead to altered behavior, brain and health trajectories, we developed a mouse model of developmental allergic asthma. In the current study, we tested for potential long-term effects of developmental asthma on adult lung function and behavior and brain gene expression associated with emotion and stress regulation. We manipulated airway inflammation (AI) and methacholine (MCH)-induced bronchospasm (resulting in labored breathing, LB) in young male and female BALB/cJ mice and measured adult outcomes 3 months after final asthma manipulations. Results indicated that allergen exposure, used to cause AI, and which ended on post-natal day 56 (P56), led to persistent lung AI, mucus buildup and gene expression related to allergic asthma 3 months after final allergen exposure. In addition, at this same age, early allergen exposure led to altered brain gene expression related to stress regulation (prefrontal corticotropin releasing hormone receptor 1, Crhr1 and hippocampal glucocorticoid receptor, GR) and serotonin function (brainstem serotonin transporter, SERT). On the other hand, LB events during development led to altered anxiety-related behavior. Importantly, sex and pre-asthma fear-related behavior (ultrasonic vocalization, USV rates) modulated these adult outcomes. Asthma that develops during childhood/adolescence may have long-term impacts on emotion and stress regulation mechanisms, and these influences may be moderated by sex and pre-asthma temperament.

Peri-adolescent asthma symptoms cause adult anxiety-related behavior and neurobiological processes in mice.

Human and animal studies have shown that physical challenges and stressors during adolescence can have significant influences on behavioral and neurobiological development associated with internalizing disorders such as anxiety and depression. Given the prevalence of asthma during adolescence and increased rates of internalizing disorders in humans with asthma, we used a mouse model to test if and which symptoms of adolescent allergic asthma (airway inflammation or labored breathing) cause adult anxiety- and depression-related behavior and brain function. To mimic symptoms of allergic asthma in young BALB/cJ mice (postnatal days [P] 7-57; N=98), we induced lung inflammation with repeated intranasal administration of house dust mite extract (most common aeroallergen for humans) and bronchoconstriction with aerosolized methacholine (non-selective muscarinic receptor agonist). Three experimental groups, in addition to a control group, included: (1) "Airway inflammation only", allergen exposure 3 times/week, (2) "Labored breathing only", methacholine exposure once/week, and (3) "Airway inflammation+Labored breathing", allergen and methacholine exposure. Compared to controls, mice that experienced methacholine-induced labored breathing during adolescence displayed a ∼20% decrease in time on open arms of the elevated plus maze in early adulthood (P60), a ∼30% decrease in brainstem serotonin transporter (SERT) mRNA expression and a ∼50% increase in hippocampal serotonin receptor 1a (5Htr1a) and corticotropin releasing hormone receptor 1 (Crhr1) expression in adulthood (P75). This is the first evidence that experimentally-induced clinical symptoms of adolescent asthma alter adult anxiety-related behavior and brain function several weeks after completion of asthma manipulations.

T lymphocyte proliferation is suppressed by the opioid growth factor ([Met(5)]-enkephalin)-opioid growth factor receptor axis: implication for the treatment of autoimmune diseases.

Opioid peptides function as immunomodulatory molecules. Reports have linked the opioid growth factor (OGF), [Met(5)]-enkephalin, and its receptor OGFr to autoimmune diseases. OGF repressed the incidence and magnitude of myelin oligodendrocyte-induced experimental autoimmune encephalomyelitis in mice. Given the extensive connection between the immune system and autoimmune diseases, the present study was conducted to examine the relationship of the OGF-OGFr axis and T lymphocyte proliferation. Splenic-derived mouse lymphocytes were stimulated with phytohemagglutin (PHA). All non-stimulated and PHA-stimulated T lymphocytes had immunoreactivity for OGF-like enkephalin and OGFr. OGF markedly suppressed T lymphocyte number in a dose-dependent manner. However, PHA-stimulated T lymphocytes were not altered in cell number by a variety of natural and synthetic opioid-related compounds, some specific for µ, δ, and κ opioid receptors. Persistent blockade of opioid receptors with the general opioid antagonist naltrexone (NTX), as well as antibody neutralization of OGF-like peptides, had no effect on cell number. Non-stimulated T lymphocytes exhibited no change in cell number when subjected to OGF or NTX. Treatment of T lymphocytes with siRNAs for µ, δ, or κ opioid receptors did not affect cell number, and the addition of OGF to these siRNA-exposed cultures depressed the population of cells. T lymphocytes treated with OGFr siRNA also had a comparable number of cells to control cultures, but the addition of OGF did not alter cell number. DNA synthesis in PHA-stimulated T lymphocytes exposed to OGF was markedly decreased from PHA-stimulated cultures receiving vehicle, but the number of cells undergoing apoptosis or necrosis in these cultures was similar to control levels. T lymphocytes subjected to siRNA for p16 and/or p21 had a comparable number of cells compared to controls, and treatment with OGF did not depress cell number in preparations transfected with both p16 and p21 siRNA. These data reveal that the OGF-OGFr axis is present in T lymphocytes and is capable of suppressing cell proliferation. However, T lymphocytes are not dependent on the regulation of cell proliferation by this system. The results showing that the OGF-OGFr axis is an immunosuppressant, offers explanation for reports that autoimmune diseases can be modulated by this system.

A marked reduction in priming of cytotoxic CD8+ T cells mediated by stress-induced glucocorticoids involves multiple deficiencies in cross-presentation by dendritic cells.

Protracted psychological stress elevates circulating glucocorticoids, which can suppress CD8(+) T cell-mediated immunity, but the mechanisms are incompletely understood. Dendritic cells (DCs), required for initiating CTL responses, are vulnerable to stress/corticosterone, which can contribute to diminished CTL responses. Cross-priming of CD8(+) T cells by DCs is required for initiating CTL responses against many intracellular pathogens that do not infect DCs. We examined the effects of stress/corticosterone on MHC class I (MHC I) cross-presentation and priming and show that stress/corticosterone-exposed DCs have a reduced ability to cross-present OVA and activate MHC I-OVA(257-264)-specific T cells. Using a murine model of psychological stress and OVA-loaded ß(2)-microglobulin knockout "donor" cells that cannot present Ag, DCs from stressed mice induced markedly less Ag-specific CTL proliferation in a glucocorticoid receptor-dependent manner, and endogenous in vivo T cell cytolytic activity generated by cross-presented Ag was greatly diminished. These deficits in cross-presentation/priming were not due to altered Ag donation, Ag uptake (phagocytosis, receptor-mediated endocytosis, or fluid-phase uptake), or costimulatory molecule expression by DCs. However, proteasome activity in corticosterone-treated DCs or splenic DCs from stressed mice was partially suppressed, which limits formation of antigenic peptide-MHC I complexes. In addition, the lymphoid tissue-resident CD11b(-)CD24(+)CD8α(+) DC subset, which carries out cross-presentation/priming, was preferentially depleted in stressed mice. At the same time, CD11b(-)CD24(+)CD8α(-) DC precursors were increased, suggesting a block in development of CD8α(+) DCs. Therefore, glucocorticoid-induced changes in both the cellular composition of the immune system and intracellular protein degradation contribute to impaired CTL priming in stressed mice.

B lymphocyte proliferation is suppressed by the opioid growth factor-opioid growth factor receptor axis: Implication for the treatment of autoimmune diseases.

Endogenous opioids are known to repress the incidence and progression of autoimmune diseases. One native opioid peptide, [Met5]-enkephalin, termed the opioid gowth factor (OGF), interacts with the OGF receptor (OGFr) to suppress the expression of experimental autoimmune encephalomyelitis. The present study examined the role of the OGF-OGFr axis in the regulation of B lymphocyte proliferation. Murine B lymphocytes were stimulated with lipopolysaccharide. Both OGF and OGFr were present in all B lymphocytes. OGF had a dose-dependent effect on growth, with cell number inhibited by up to 43% at 72 h; no other synthetic or native opioid altered cell proliferation. Exogenous OGF depressed cell number in cultures treated with siRNAs for the classical opioid receptors, MOR (µ), DOR (δ), and KOR (κ), however this peptide had no effect in preparations exposed to siRNA for OGFr. The decrease in cell number by exogenous OGF was dependent on p16 or p21 cyclin-dependent inhibitory kinase pathways. Exposure to the opioid antagonist, naltrexone, did not change cell number from control levels. These results suggest that the OGF-OGFr axis is present and functional in B lymphocytes, but this system is not an autocrine regulator of cell proliferation. Thus, at least exogenous OGF and perhaps endogenous OGF by paracrine/endocrine sources, can be an immunosuppressant. Modulation of the OGF-OGFr axis may be a novel paradigm for the treatment of autoimmune diseases.

Regulation of natural killer T-cell development by deubiquitinase CYLD.

Natural killer T (NKT) cells modulate immune responses against pathogens and tumours, as well as immunological tolerance. We show here that CYLD, a tumour suppressor with deubiquitinase function, has a pivotal and cell-intrinsic function in NKT cell development. Unlike other known NKT regulators, CYLD is dispensable for intrathymic NKT cell maturation but is obligatory for the survival of immature NKT cells. Interestingly, CYLD deficiency impairs the expression of ICOS, a costimulatory molecule required for the survival and homeostasis of NKT cells, and this molecular defect is associated with attenuated response to an NKT-survival cytokine, IL-7, due to reduced expression of IL-7 receptor. We show, for the first time, that IL-7 induces the expression of ICOS in NKT cells, which is largely dependent on CYLD. Interestingly, loss of CYLD causes constitutive NF-kappaB activation in developing NKT cells, which contributes to their defective IL-7 response and attenuated ICOS expression. These findings establish CYLD as a critical regulator of NKT cell development and provide molecular insights into this novel function of CYLD.

Stress-induced glucocorticoids at the earliest stages of herpes simplex virus-1 infection suppress subsequent antiviral immunity, implicating impaired dendritic cell function.

The systemic elevation of psychological stress-induced glucocorticoids strongly suppresses CD8(+) T cell immune responses resulting in diminished antiviral immunity. However, the specific cellular targets of stress/glucocorticoids, the timing of exposure, the chronology of immunological events, and the underlying mechanisms of this impairment are incompletely understood. In this study, we address each of these questions in the context of a murine cutaneous HSV infection. We show that exposure to stress or corticosterone in only the earliest stages of an HSV-1 infection is sufficient to suppress, in a glucocorticoid receptor-dependent manner, the subsequent antiviral immune response after stress/corticosterone has been terminated. This suppression resulted in early onset and delayed resolution of herpetic lesions, reduced viral clearance at the site of infection and draining popliteal lymph nodes (PLNs), and impaired functions of HSV-specific CD8(+) T cells in PLNs, including granzyme B and IFN-gamma production and the ability to degranulate. In knockout mice lacking glucocorticoid receptors only in T cells, we show that these impaired CD8(+) T cell functions are not due to direct effects of stress/corticosterone on the T cells, but the ability of PLN-derived dendritic cells to prime HSV-1-specific CD8(+) T cells is functionally impaired. These findings highlight the susceptibility of critical early events in the generation of an antiviral immune response to neuroendocrine modulation and implicate dendritic cells as targets of stress/glucocorticoids in vivo. These findings also provide insight into the mechanisms by which the clinical use of glucocorticoids contributes to altered immune responses in patients with viral infections or tumors.

Opioid growth factor suppresses expression of experimental autoimmune encephalomyelitis.

Naltrexone, an opioid antagonist, has been shown to modulate expression of experimental autoimmune encephalomyelitis (EAE), an animal model of MS, suggesting that endogenous opioids are inhibitory trophic factors in EAE. In the present study, we investigated the effects of one native opioid peptide, opioid growth factor ([Met(5)]-enkephalin), on the onset and progression of EAE. C57Bl/6 mice injected with myelin oligodendrocyte glycoprotein (MOG) received daily injections of 10 mg/kg OGF (MOG+OGF) or saline (MOG+Vehicle). Over 60% of the MOG+OGF animals did not exhibit behavioral signs of disease (EAE) in contrast to 100% of the mice in the MOG+Vehicle group. The severity and disease indices of EAE in the OGF-treated mice were markedly reduced from MOG+Vehicle cohorts. By day 30, 60% of MOG+OGF mice had a remission, relative to 4% in the MOG+Vehicle group. MOG-injected mice receiving OGF had significant reductions in activated astrocytes and damaged neurons compared to MOG+Vehicle animals. Unlike MOG+Vehicle and MOG+OGF mice with behavioral signs of disease, MOG+OGF animals without manifestation of disease had no lumbar spinal cord demyelination. Both OGF and OGF receptor were detected in splenic-derived T lymphocytes by immunohistochemistry. OGF treatment decreased both DNA synthesis and cell proliferation in comparison to vehicle-treated T cell lymphocyte cultures. These results indicate that an endogenous opioid, OGF, inhibits the onset and progression of EAE, and suggest that clinical studies on the use of OGF treatment for MS are merited.

The effect of Helicobacter hepaticus infection on immune responses specific to herpes simplex virus type 1 and characteristics of dendritic cells.

Infection of mice with Helicobacter hepaticus is common in research colonies, yet little is known about how this persistent infection affects immunologic research. The goal of this study was to determine whether H. hepaticus infection status can modulate immune responses specific to herpes simplex virus type 1 (HSV1) and the phenotypic and functional characteristics of dendritic cells (DC) of mice. We compared virus-specific antibody and T cell-mediated responses in H. hepaticus-infected and noninfected mice that were inoculated intranasally with HSV1. The effect of H. hepaticus on the HSV1-specific antibody and T cell-mediated immune responses in superficial cervical and tracheobronchal lymph nodes (LN) did not reach statistical significance. Surface expression of the maturation-associated markers CD40, CD80, CD86, and MHC II and percentages of IL12p40- and TNFalpha-producing DC from spleen and colic LN in H. hepaticus-infected mice and noninfected mice were measured in separate experiments. Expression of CD40, CD86, and MHC II and percentages of IL12p40- and TNFalpha-producing DC from colic LN were decreased in H. hepaticus-infected mice. In contrast, H. hepaticus infection did not reduce the expression of these molecules by splenic DC. Expression of CD40, CD80, CD86, and MHC II on splenic DC from H. hepaticus-infected mice was increased after in vitro lipopolysaccharide stimulation. These results indicate that H. hepaticus infection can influence the results of immunologic assays in mice and support the use of H. hepaticus-free mice in immunologic research.

Psychological stress impairs the local CD8+ T cell response to mucosal HSV-1 infection and allows for increased pathogenicity via a glucocorticoid receptor-mediated mechanism.

Psychological stress and its associated increases in corticosterone are generally immunosuppressive and contribute to increased herpes simplex virus (HSV)-associated pathogenicity. However, the impact of stress on local control of the initial mucosal-based HSV infection has not been elucidated, nor have the ramifications of such failures of the immune response in terms of viral spread. To address these gaps in knowledge, the studies described herein sought to determine how psychological stress and associated increases in corticosterone may increase susceptibility to HSV encephalitis by allowing for increased viral titers at the site of initial infection. We have shown that in mice intranasally infected with HSV-1, a cell-mediated immune response occurs in the nasopharyngeal-associated lymphoid tissue (NALT), mediastinal lymph nodes (MLN), and superficial cervical lymph nodes (CLN). However, psychological stress induced by restraint decreased the number of lymphocytes in these tissues in HSV-infected mice. Surprisingly, the effects of this restraint stress on HSV-specific CTL function varied by immune tissue. Increased viral titers were found in the nasal cavity of stressed mice, an observation which correlated with an increased CD8+ cell response in the CLN. These findings led us to extend our studies to also determine the ramifications of decreased numbers of locally derived lymphocytes on viral titers following infection. Using an approach in which the NALT was surgically removed prior to infection, we confirmed that decreased numbers of NALT-derived lymphocytes at the time of infection allows for increased viral replication. We conclude that the increased viral titers observed in mice experiencing psychological stress are the consequence of a glucocorticoid-mediated reduction in the numbers of lymphocytes responsible for resolving the initial infection.

Psychological stress exacerbates primary vaginal herpes simplex virus type 1 (HSV-1) infection by impairing both innate and adaptive immune responses.

Chronic psychological stress is generally immunosuppressive and contributes to an increase in herpes simplex virus (HSV) pathogenicity. We have previously shown that mice experiencing stress at the time of intranasal HSV infection have increased levels of infectious virus in their nasal cavity, as compared to control mice that were not subjected to stress. We have extended our studies to determine the effects of stress at another clinically-relevant mucosal site by examining the immune response to and pathogenesis of vaginal HSV infection. Mice experiencing psychological stress during vaginal HSV infection exhibited an increase in both vaginal viral titers and the pathology associated with this HSV infection. We demonstrate that these observations result from the failure of both the innate and HSV-specific adaptive immune responses. At 2 days post-infection, NK cell numbers were significantly decreased in mice experiencing restraint stress. Studies examining the adaptive immune response revealed a decrease in the number of HSV-specific CD8(+) T cells in not only the vaginal tissue itself but also the draining iliac lymph nodes (ILN). Furthermore, the number of functional cells, in terms of both their degranulation and interferon-gamma production, in the ILN of stressed mice was decreased as compared to non-stressed mice. We conclude that psychological stress, through its suppression of both innate and adaptive immune responses, may be an important factor in the ability to control vaginal HSV infection.

Elevated maternal corticosterone during lactation hinders the neonatal adaptive immune response to herpes simplex virus (HSV) infection.

The neonate's immune system is relatively immature. For short-term protection against pathogens the neonate is reliant primarily on maternally derived antibodies delivered via the mother's milk. However, neonates soon acquire the ability to generate adaptive immune responses for long-term protection. Products of the nervous and endocrine systems that are elicited by psychological stress are known to modulate a variety of immune responses. Additionally, psychological stressors are well recognized for their ability to increase corticosterone levels. The studies described herein examined the effects of increases in maternally derived corticosterone on the neonatal cell-mediated immune response to, and pathogenicity of, herpes simplex virus (HSV) infection. Water containing corticosterone was made available to nursing mothers for a period of 6 consecutive days beginning on either the day of or 6 days post-delivery. At 12 days of age, neonates were infected with HSV-1 in the rear footpads. These neonates exhibited a decrease in the proliferative ability of splenic-derived cells due to the reduction of IL-2 production and IL-2 receptor alpha subunit (IL-2R alpha) expression by these cells. These neonates also exhibited a decrease in the number and function of popliteal lymph node-resident HSV-1 gB(498-505) peptide-specific CD8(+) T cells as measured by tetramer analysis, CTL lytic activity, expression of CD107a, cytokine production, and proliferation. Additionally, these HSV-infected neonates exhibited increased morbidity and mortality. Together, these studies indicate that exposure of neonates to maternally derived corticosterone via the milk hinders their ability to generate an adaptive cell-mediated immune response to a viral infection and illustrate the potential importance of maternal stress in neonatal resistance to infectious pathogens.

Corticosterone impairs dendritic cell maturation and function.

Dendritic cells (DC) play a critical role in initiating and directing adaptive immune responses against pathogens and tumours. Immature DC are thought to act as sentinels in peripheral tissues where their main function is to capture antigen at sites of infection, whereas mature DC are highly efficient at priming T-cell-mediated immune responses against infectious pathogens. The DC maturation process is thought to be an important step in the efficient generation of cytotoxic T lymphocytes (CTL). It is well established that many aspects of immune function, including CTL-mediated antiviral immunity, are modulated by neuroendocrine-derived products. Corticosterone (CORT), an adrenal hormone produced at increased concentrations during a stress response, has been shown to play a role in impaired CTL responses in stressed animals, leading to high mortality in mice normally resistant to viral infection. While direct effects of neuroendocrine mediators on CTL have been studied, little is known about their effects on DC that are critical for CTL priming. Here, we found that physiologically relevant concentrations of CORT, acting via the glucocorticoid receptor, functionally compromise DC maturation. DC exposed to CORT remained phenotypically and functionally immature after stimulation with lipopolysaccharide and were impaired for the production of interleukin (IL)-6, IL-12, and tumour necrosis factor-alpha. These effects were biologically significant, as CORT treatment resulted in a marked reduction in the ability of DC to prime naive CD8(+) T cells in vivo. These findings offer a potential mechanism underlying stress-associated immunosuppression.

Psychological stress compromises CD8+ T cell control of latent herpes simplex virus type 1 infections.

Recurrent HSV-1 ocular disease results from reactivation of latent virus in trigeminal ganglia, often following immunosuppression or exposure to a variety of psychological or physical stressors. HSV-specific CD8+ T cells can block HSV-1 reactivation from latency in ex vivo trigeminal ganglia cultures through production of IFN-gamma. In this study, we establish that either CD8+ T cell depletion or exposure to restraint stress permit HSV-1 to transiently escape from latency in vivo. Restraint stress caused a reduction of TG-resident HSV-specific CD8+ T cells and a functional compromise of those cells that survive. Together, these effects of stress resulted in an approximate 65% reduction of cells capable of producing IFN-gamma in response to reactivating virus. Our findings demonstrate persistent in vivo regulation of latent HSV-1 by CD8+ T cells, and strongly support the concept that stress induces HSV-1 reactivation from latency at least in part by compromising CD8+ T cell surveillance of latently infected neurons.

Modulation of microglia and CD8(+) T cell activation during the development of stress-induced herpes simplex virus type-1 encephalitis.

The central nervous system (CNS) has been shown to be vulnerable to a variety of insults in animals exposed to glucocorticoids. For example, psychological stress, a known inducer of glucocorticoid production, enhances the susceptibility of mice to herpes simplex virus type-1 (HSV-1) infection and results in the development of HSV-1 encephalitis (HSE). To determine the immune mechanisms by which stress promotes the development of HSE, we examined the role of the glucocorticoid receptor (GR) and the N-methyl-d-aspartate (NMDA) receptor in the development of HSE. Our findings demonstrate that blockade of either the GR or the NMDA receptor enhances survival following HSV-1 infection in stressed mice to levels similar to non-stressed mice. Subsequent studies determined the effect of GR and NMDA receptor blockade on immune function by specifically examining both microglia and CD8(+) T cell activation. Stress inhibited the expression of MHC class I by microglia and other brain-derived antigen presenting cells (CD45(hi)) independent of either the glucocorticoid receptor or the NMDA receptor, suggesting that stress-induced suppression of MHC class I expression in the brain does not affect survival during HSE. Blockade of the NMDA receptor, however, diminished HSV-1-induced increases in class I expression by CD45(hi) cells, suggesting that blockade of the NMDA receptor may limit CNS inflammation. Also, while CD8(+) T cell activation and function in the brain were not affected by stress, the number of CD8(+) T cells in the superficial cervical lymph nodes (SCLN) was decreased in stressed mice via GR-mediated mechanisms. These findings indicate that stress-induced hypocellularity is mediated by the GR while NMDA receptor activation is responsible for enhancing CNS inflammation. The combined effects of GR-mediated hypocellularity of the SCLN and NMDA receptor-mediated CNS inflammation during stress promote the development of HSE.

Twenty years of psychoneuroimmunology and viral infections in Brain, Behavior, and Immunity.

For 20 years, Brain, Behavior, and Immunity has provided an important venue for the publication of studies in psychoneuroimmunology. During this time period, psychoneuroimmunology has matured into an important multidisciplinary science that has contributed significantly to our knowledge of mind, brain, and body interactions. This review will not only focus on the primary research papers dealing with psychoneuroimmunology, viral infections, and anti-viral vaccine responses in humans and animal models that have appeared on the pages of Brain, Behavior, and Immunity during the past 20 years, but will also outline a variety of strategies that could be used for expanding our understanding of the neuroimmune-viral pathogen relationship.

Stress presents a problem for dendritic cells: corticosterone and the fate of MHC class I antigen processing and presentation.

Corticosterone (cortisol in humans), a glucocorticoid hormone released into circulation in response to psychological stress via the hypothalamic-pituitary-adrenal axis, can undermine primary and memory CD8(+) cytotoxic T lymphocyte (CTL) responses. These CTL responses are vital for fighting intracellular pathogens, such as viruses, and some tumors. Dendritic cells (DCs) play a pivotal role in the generation of both primary and memory CTL responses. DCs are specialized for antigen acquisition (by direct infection or uptake from neighboring cells), transport, processing, and MHC class I-restricted presentation of antigen to CTL. These are critical events that are an absolute requirement for the generation of CTL responses regardless of any other immune responses that may be occurring. This minireview provides an overview of the components of MHC class I antigen processing and presentation pathway and describes our recent published work on the effects of corticosterone on this process in virally infected DCs. Corticosterone impairs the efficiency with which antigen is presented on DCs. The mechanism of this impairment is shown to be via a reduction in the generation of antigenic peptide from virally expressed protein. This impairment of antigen processing and presentation by corticosterone was also observed in non-immune cells, suggesting that stress may affect essential cellular protein management functions in all cells, and having possible implications for neurological or other diseases that may result from aberrant protein processing.

Stress-induced elevation of glucocorticoids increases microglia proliferation through NMDA receptor activation.

The immunosuppressive nature of glucocorticoids has been well documented both in vitro and in vivo. This glucocorticoid-mediated immunosuppression has also been observed in immune cells within the central nervous system (CNS). For example, microglia have previously been shown to exhibit decreased proliferation, cytokine production, and antigen presentation upon treatment with glucocorticoids in vitro. Despite these in vitro findings, the impact of glucocorticoids on microglia function in vivo has not been fully investigated. To determine the interaction between glucocorticoids and microglia within the CNS, we used a restraint model of psychological stress to elevate corticosterone levels in mice. Quantification of microglia from stressed mice indicated that four sessions of stress induced the proliferation of microglia. This proliferation was a function of corticosterone-induced activation of the N-methyl-D-aspartate (NMDA) receptor within the CNS since blockade of corticosterone synthesis, the glucocorticoid receptor, or the NMDA receptor each prevented stress-induced increases in microglia number. In addition, the NMDA receptor antagonist MK-801 prevented increases in microglia following exogenous corticosterone administration to non-stressed mice. We conclude that activation of the NMDA receptor and subsequent microglia proliferation is a downstream effect of elevated corticosterone levels. These findings demonstrate that elevated levels of glucocorticoids are able to activate microglia in vivo and suggest that stress is able to induce a pro-inflammatory response within the CNS. A pro-inflammatory microglia response may be a contributing factor in the development of various stress-induced inflammatory conditions in the CNS.