Infection as a stressor: a cytokine-mediated activation of the hypothalamo-pituitary-adrenal axis?

Visceral Inflammation and Immune Activation Stress the Brain

Stress refers to a dynamic process in which the homeostasis of an organism is challenged, the outcome depending on the type, severity, and duration of stressors involved, the stress responses triggered, and the stress resilience of the organism. Importantly, the relationship between stress and the immune system is bidirectional, as not only stressors have an impact on immune function, but alterations in immune function themselves can elicit stress responses. Such bidirectional interactions have been prominently identified to occur in the gastrointestinal tract in which there is a close cross-talk between the gut microbiota and the local immune system, governed by the permeability of the intestinal mucosa. External stressors disturb the homeostasis between microbiota and gut, these disturbances being signaled to the brain via multiple communication pathways constituting the gut-brain axis, ultimately eliciting stress responses and perturbations of brain function. In view of these relationships, the present article sets out to highlight some of the interactions between peripheral immune activation, especially in the visceral system, and brain function, behavior, and stress coping. These issues are exemplified by the way through which the intestinal microbiota as well as microbe-associated molecular patterns including lipopolysaccharide communicate with the immune system and brain, and the mechanisms whereby overt inflammation in the GI tract impacts on emotional-affective behavior, pain sensitivity, and stress coping. The interactions between the peripheral immune system and the brain take place along the gut-brain axis, the major communication pathways of which comprise microbial metabolites, gut hormones, immune mediators, and sensory neurons. Through these signaling systems, several transmitter and neuropeptide systems within the brain are altered under conditions of peripheral immune stress, enabling adaptive processes related to stress coping and resilience to take place. These aspects of the impact of immune stress on molecular and behavioral processes in the brain have a bearing on several disturbances of mental health and highlight novel opportunities of therapeutic intervention.

Stress-induced redistribution of immune cells--from barracks to boulevards to battlefields: a tale of three hormones--Curt Richter Award winner

BACKGROUND

The surveillance and effector functions of the immune system are critically dependent on the appropriate distribution of immune cells in the body. An acute or short-term stress response induces a rapid and significant redistribution of immune cells among different body compartments. Stress-induced leukocyte redistribution may be a fundamental survival response that directs leukocyte subpopulations to specific target organs during stress, and significantly enhances the speed, efficacy and regulation of an immune response. Immune responses are generally enhanced in compartments (e.g., skin) that are enriched with leukocytes, and suppressed in compartments that are depleted of leukocytes during/following stress. The experiments described here were designed to elucidate the: (1) Time-course, trajectory, and subpopulation-specificity of stress-induced mobilization and trafficking of blood leukocytes. (2) Individual and combined actions of the principal stress hormones, norepinephrine (NE), epinephrine (EPI), and corticosterone (CORT), in mediating mobilization or trafficking of specific leukocyte subpopulations. (3) Effects of stress/stress hormones on adhesion molecule, L-selectin (CD62L), expression by each subpopulation to assess its adhesion/functional/maturation status.

METHODS

Male Sprague Dawley rats were stressed (short-term restraint, 2-120 min), or adrenalectomized and injected with vehicle (VEH), NE, EPI, CORT, or their combinations, and blood was collected for measurement of hormones and flow cytometric quantification of leukocyte subpopulations.

RESULTS

Acute stress induced an early increase/mobilization of neutrophils, lymphocytes, helper T cells (Th), cytolytic T cells (CTL), and B cells into the blood, followed by a decrease/trafficking of all cell types out of the blood, except neutrophil numbers that continued to increase. CD62L expression was increased on neutrophils, decreased on Th, CTL, and natural killer (NK) cells, and showed a biphasic decrease on monocytes & B cells, suggesting that CD62L is involved in mediating the redistribution effects of stress. Additionally, we observed significant differences in the direction, magnitude, and subpopulation specificity of the effects of each hormone: NE increased leukocyte numbers, most notably CD62L⁻/⁺ neutrophils and CD62L⁻ B cells. EPI increased monocyte and neutrophil numbers, most notably CD62L⁻/⁺ neutrophils and CD62L⁻ monocytes, but decreased lymphocyte numbers with CD62L⁻/⁺ CTL and CD62L⁺ B cells being especially sensitive. CORT decreased monocyte, lymphocyte, Th, CTL, and B cell numbers with CD62L⁻ and CD62L⁺ cells being equally affected. Thus, naïve (CD62L⁺) vs. memory (CD62L⁻) T cells, classical (CD62L⁺) vs. non-classical (CD62L⁻) monocytes, and similarly distinct functional subsets of other leukocyte populations are differentially mobilized into the blood and trafficked to tissues by stress hormones.

CONCLUSION

Stress hormones orchestrate a large-scale redistribution of immune cells in the body. NE and EPI mobilize immune cells into the bloodstream, and EPI and CORT induce traffic out of the blood possibly to tissue surveillance pathways, lymphoid tissues, and sites of ongoing or de novo immune activation. Immune cell subpopulations appear to show differential sensitivities and redistribution responses to each hormone depending on the type of leukocyte (neutrophil, monocyte or lymphocyte) and its maturation/functional characteristics (e.g., non-classical/resident or classical/inflammatory monocyte, naïve or central/effector memory T cell). Thus, stress hormones could be administered simultaneously or sequentially to induce specific leukocyte subpopulations to be mobilized into the blood, or to traffic from blood to tissues. Stress- or stress hormone-mediated changes in immune cell distribution could be clinically harnessed to: (1) Direct leukocytes to sites of vaccination, wound healing, infection, or cancer and thereby enhance protective immunity. (2) Reduce leukocyte traffic to sites of inflammatory/autoimmune reactions. (3) Sequester immune cells in relatively protected compartments to minimize exposure to cytotoxic treatments like radiation or localized chemotherapy. (4) Measure biological resistance/sensitivity to stress hormones in vivo. In keeping with the guidelines for Richter Award manuscripts, in addition to original data we also present a model and synthesis of findings in the context of the literature on the effects of short-term stress on immune cell distribution and function.

The occurrence of preterm delivery is linked to pregnancy-specific distress and elevated inflammatory markers across gestation

There is mounting evidence that stress during pregnancy can have detrimental effects on gestation and birth. Existing studies indicate that prenatal stress may increase levels of circulating inflammatory markers that are associated with prematurity and pregnancy complications, suggesting that stress-related changes in the cytokine milieu may increase the risk of poor pregnancy outcome. Previous studies, however, have not clearly connected stress during pregnancy to changes in inflammatory mediators and, in turn, to clinically-relevant outcomes such as premature delivery. The present study sought to directly connect prenatal stress and changes in inflammatory markers to preterm delivery and gestational age at birth (GAB). A sample of 173 women was recruited during the first trimester of pregnancy and followed through delivery. Overall stress, pregnancy-specific distress, and inflammatory markers were assessed early and later in pregnancy, and the predictive value of these measures for preterm birth and GAB was determined. There were significant differences in pregnancy-specific distress, IL-6, and TNF-α between women who delivered prematurely versus those who delivered at term, and elevated levels of pregnancy-specific distress, IL-6, and TNF-α were predictive of shortened GAB overall. Importantly, in many cases, the effects of overall stress and pregnancy-specific distress on GAB were mediated by levels of circulating inflammatory markers. Collectively, these data provide strong evidence that prenatal stress experiences can affect the timing of parturition via alterations in circulating inflammatory mediators, and underscore the need for ongoing research aimed at further understanding the mechanisms and effects of prenatal stress on maternal and infant health.

Facilitated acquisition of the classically conditioned eyeblink response in male rats after systemic IL-1beta

Exposure to inescapable stressors enhances cue-dependent learning in male rats; enhanced learning is apparent as facilitated acquisition of the classically conditioned eyeblink respouse (CCER). The proinflammatory cytokines, in particular interleukin (IL)-1beta, are presumed to orchestrate a number of acute-phase stress responses in rats, most notably fever, reduced feeding, and inactivity. Little is known of the impact proinflammatory cytokines have on learning and memory processes. Here, we address the effects of IL-1beta treatment on acquisition of the classically conditioned eyeblink response 2 hours [?] after injection in male rats. Training was accomplished with a delay-type paradigm (500-ms conditional stimulus coterminating with a 10-ms periorbital unconditional stimulation). Facilitated acquisition was clearly apparent in rats treated with IL-1beta (3.0 microg/kg). In a second experiment, we compared rats treated with 3.0 microg/kg to those treated with 1.0 microg/kg. Facilitated acquisition was reproduced, but the lower dose did not appreciably affect acquisition. These data further support contentions that IL-1beta has anxiogenic properties, affecting basic new motor learning in a manner similar to that observed after exposure to stress.

Impact of maternal stress on the transmammary transfer and protective capacity of herpes simplex virus-specific immunity

In adults, psychological stress regulates immune responsiveness in part via the increased levels of corticosterone that are produced as a result of hypothalamic-pituitary-adrenal (HPA) axis activation. However, there is a lack of knowledge as to the role such regulation may play in the neonate. Neonates are severely compromised in their ability to generate an immune response to pathogens encountered after birth and therefore rely heavily on maternally derived antibody acquired postnatally through the milk. This passive transfer of antibody is critical for protection of the neonate from severe herpes simplex virus (HSV) infection and mortality. Using a well-established postnatal restraint/light stress model, we determined whether maternal stress and the associated increases in corticosterone would affect the transmammary transfer of antibody and subsequent neonate susceptibility to HSV-associated mortality. Serum corticosterone levels were markedly increased in lactating mice subjected to the restraint/light stress, and increased levels of corticosterone were transferred through the milk of these stressed mothers to their neonates. Despite these increases in corticosterone, the transmammary transfer and accumulation of total and HSV-specific IgG in neonate serum remained intact. This milk-derived, HSV-specific antibody alone protected the neonate from systemic viral spread. Interestingly, postnatal maternal stress significantly increased neonate survival after HSV-2 infection despite no apparent alteration in viral spread. These studies demonstrate that although the transmammary transfer of antibody is unaffected by maternal stress, stress may be enhancing components of antiviral immunity that are effective in protecting neonates from HSV-associated mortality.

T-lymphocyte activation increases hypothalamic and amygdaloid expression of CRH mRNA and emotional reactivity to novelty

Stimulation of T-cells with staphylococcal enterotoxin B (SEB) significantly elevates interleukin-2 (IL-2) and contemporaneous activation of the hypothalamic-pituitary-adrenal (HPA) axis and c-fos in the paraventricular nucleus (PVN) of BALB/cByJ mice. Such neural signaling may promote cognitive and emotional adaptation before or during infectious illness. Because corticotropin-releasing hormone (CRH) is an anxiogenic neuropeptide that may mediate the stressor-like effects of immunological stimuli, we measured neuronal CRH mRNA alterations in mice challenged with SEB. Increased CRH mRNA levels were observed in the PVN and central nucleus of the amygdala (ceA) 4-6 hr after SEB administration. This was associated with plasma ACTH increases, which could be abrogated by the systemic administration of anti-CRH antiserum. Additional experiments did not support a role for IL-2 or prostaglandin synthesis in activating the HPA axis. Behavioral experiments testing for conditioned taste aversion did not confirm that SEB challenge promotes malaise. However, consistent with the notion that central CRH alterations induced by SEB may affect emotionality (e.g., fear), SEB challenge augmented appetitive neophobia in a context-dependent manner, being marked in a novel and stressful environment. It is hypothesized that immunological stimuli generate a cascade of events that solicit integrative neural processes involved in emotional behavior. As such, these data support the contention that affective illness may be influenced by immunological processes and the production of cytokines and are consistent with other evidence demonstrating that autoimmune reactivity is associated with enhanced emotionality.

Blunted sensitivity to sucrose in autoimmune MRL-lpr mice: a curve-shift study

Lupus-prone MRL-lpr mice show an autoimmunity-associated behavioral syndrome that has many features similar to the effects of chronic stress. The present study evaluated whether autoimmune MRL-lpr mice show reduced responsiveness to sucrose, as observed in normal animals exposed to chronic mild stress. Sixteen-week old MRL-lpr mice and their age-matched congenic MRL +/+ controls were given 0%, 0.5%, 1%, 2%, 4%, 8%, or 16% sucrose solution to drink every 48 h in a one-bottle test. The MRL-lpr mice drank less than controls at all concentrations, except at 16%. The amount of sucrose consumed vs. solution concentration followed a saturation curve. Estimates were obtained for the concentration yielding the half-maximum response (X50) and the response at saturating concentration of sucrose (Rmax). The X50 was significantly higher in MRL-lpr than in MRL +/+ mice, indicating a shift to the right of the concentration-intake curve. The Rmax did not differ significantly between substrains, suggesting that the autoimmune process did not affect performance capacity. Pretreatment with the immunosuppressant cyclophosphamide diminished the substrain difference in X50, suggesting that reduced sensitivity to sucrose is related to autoimmune/inflammatory factors. These results support the similarity between autoimmunity-associated behavioral syndrome and behavioral changes produced by chronic stress, and suggest common neuroendocrine mechanisms. Because reduced sensitivity to palatable stimulus may reflect blunted hedonic responsiveness ("anhedonia"), it is hypothesized that an autoimmune/inflammatory factor(s) produces the depression found in human lupus, and some cases of affective disorder.