David A. Padgett

How stress influences the immune response

Abstract In response to a stressor, physiological changes are set into motion to help an individual cope with the stressor. However, chronic activation of these stress responses, which include the hypothalamic–pituitary–adrenal axis and the sympathetic–adrenal–medullary axis, results in chronic production of glucocorticoid hormones and catecholamines. Glucocorticoid receptors expressed on a variety of immune cells bind cortisol and interfere with the function of NF-κB, which regulates the activity of cytokine-producing immune cells. Adrenergic receptors bind epinephrine and norepinephrine and activate the cAMP response element binding protein, inducing the transcription of genes encoding for a variety of cytokines. The changes in gene expression mediated by glucocorticoid hormones and catecholamines can dysregulate immune function. There is now good evidence (in animal and human studies) that the magnitude of stress-associated immune dysregulation is large enough to have health implications.

Restraint Stress Slows Cutaneous Wound Healing in Mice

The impact of stress on cutaneous wound healing was assessed in a murine model. Female, hairless SKH-1 mice, 6-8 weeks of age were subjected to restraint stress (RST) 3 days before and for 5 days following dorsal application of a 3.5-mm sterile punch wound. Control mice were wounded, but not restrained. Using photography and image analysis, the rate of wound healing was compared between the two groups. Wounds on control mice healed on average 3.10 days sooner than RST-treated mice. In addition, cross-sectional, morphometric analysis of the dermal and epidermal layers revealed reduced inflammation surrounding wounds from RST mice at 1, 3, and 5 days after wounding. In the RST group, serum corticosterone levels averaged 162.5 ng/ml compared to 35.7 ng/ml in the controls. Treatment of RST-stressed animals with the glucocorticoid receptor antagonist RU40555 resulted in healing rates comparable to those of control animals. Thus, the reduction in inflammation and delayed healing correlated with serum corticosterone levels and suggest that disruption of neuroendocrine homeostasis modulates wound healing.

Stress and wound healing.

Over the past decade it has become clear that stress can significantly slow wound healing: stressors ranging in magnitude and duration impair healing in humans and animals. For example, in humans, the chronic stress of caregiving as well as the relatively brief stress of academic examinations impedes healing. Similarly, restraint stress slows healing in mice. The interactive effects of glucocorticoids (e.g. cortisol and corticosterone) and proinflammatory cytokines [e.g. interleukin-1β (IL-1β), IL-1α, IL-6, IL-8, and tumor necrosis factor-α] are primary physiological mechanisms underlying the stress and healing connection. The effects of stress on healing have important implications in the context of surgery and naturally occurring wounds, particularly among at-risk and chronically ill populations. In research with clinical populations, greater attention to measurement of health behaviors is needed to better separate behavioral versus direct physiological effects of stress on healing. Recent evidence suggests that interventions designed to reduce stress and its concomitants (e.g., exercise, social support) can prevent stress-induced impairments in healing. Moreover, specific physiological mechanisms are associated with certain types of interventions. In future research, an increased focus on mechanisms will help to more clearly elucidate pathways linking stress and healing processes.

Social stress increases the susceptibility to endotoxic shock

The influence of social disruption stress (SDR) on the susceptibility to endotoxic shock was investigated. SDR was found to increase the mortality of mice when they were challenged with the bacterial endotoxin lipopolysaccharide (LPS). Histological examination of SDR animals after LPS injection revealed widespread disseminated intravascular coagulation in the brain and lung, extensive meningitis in the brain, severe hemorrhage in the lung, necrosis in the liver, and lymphoid hyperplasia in the spleen, indicating inflammatory organ damage. In situ hybridization histochemical analysis showed that the expression of the glucocorticoid receptor mRNA was down-regulated in the brain and spleen of SDR animals while the ratio of expression of AVP/CRH-the two adrenocorticotropic hormone secretagogue, increased. After LPS injection, the expression of pro-inflammatory cytokines, IL-1beta and TNF-alpha, was found significantly higher in the lung, liver, spleen, and brain of the SDR mice as compared with the LPS-injected home cage control animals. Taken together, these results show that SDR stress increases the susceptibility to endotoxic shock and suggest that the development of glucocorticoid resistance and increased production of pro-inflammatory cytokines are the mechanisms for this behavior-induced susceptibility to endotoxic shock.

Androstenediol and dehydroepiandrosterone protect mice against lethal bacterial infections and lipopolysaccharide toxicity

The protective effects of the hormones androstenediol (androstene-3beta, 17beta,-diol; AED) and dehydroepiandrosterone (5-androsten-3beta-ol-17-one; DHEA) on the pathophysiology of two lethal bacterial infections and endotoxin shock were examined. The infections included a gram-positive organism (Enterococcus faecalis) and a gram-negative organism (Pseudomonas aeruginosa). Both hormones protected mice from the lethal bacterial infections and from lipopolysaccharide (LPS) challenge. Treatment of animals lethally infected with P. aeruginosa with DHEA resulted in a 43% protection whereas treatment with AED gave a 67% protection. Both hormones also protected completely animals infected with an LD50 dose of E. faecalis. Similarly, the 88% mortality rate seen in LPS challenge was reduced to 17% and 8.5%, by treatment with DHEA and AED, respectively. The protective influences of both steroids were shown not to be directly antibacterial, but primarily an indirect antitoxin reaction. DHEA appears to mediate its protective effect by a mechanism that blocks the toxin-induced production of pathophysiological levels of tumour necrosis factor-alpha (TNF-alpha) and interleukin-1. AED usually had greater protective effects than DHEA; however, the AED effect was independent of TNF-alpha suppression, both in vivo and in vitro. The data suggest that both DHEA and AED may have a role in the neuro-endocrine regulation of antibacterial immune resistance.

Repeated Social Defeat Causes Increased Anxiety-Like Behavior and Alters Splenocyte Function in C57BL/6 and CD-1 Mice

The experimental model, social disruption (SDR), is a model of social stress in which mice are repeatedly attacked and defeated in their home cage by an aggressive conspecific. In terms of the impact of this stressor on the immune response, SDR has been reported to cause hyperinflammation and glucocorticoid insensitivity. To this point however, the behavioral consequences of SDR have not been thoroughly characterized. Because social defeat has been reported to cause anxiety- and depressive-like behaviors, the current study was designed to assess whether SDR also causes anxiety- and depressive-like behaviors. Using the light/dark preference test and the open field test as tools to measure behaviors characteristic of anxiety, the data showed that C57BL/6 and CD-1 male mice subjected to SDR displayed increased anxiety-like behavior. The increase in anxiety-like behaviors persisted for at least 1 week after the cessation of the stressor. In contrast, depressive-like behaviors were not elicited by SDR as assessed by the forced swim test or the tail suspension test. These data indicate that social disruption stress causes an increase in anxiety-like behaviors, but not depressive-like behaviors.

Social Disruption, Immunity, and Susceptibility to Viral Infection: Role of Glucocorticoid Insensitivity and NGF

Abstract: Glucocorticoid (cort) responses have been shown to suppress inflammatory reactions by inhibiting the trafficking of immune cells. Recently, it was demonstrated that restraint stress (RST) and psychosocial stress (social reorganization; SRO) differentially affected the pathophysiology and survival in the mouse influenza viral infection model. While both stressors activated the HPA axis, only SRO affected survival. in RST, elevated cort diminished recruitment of inflammatory cells following intranasal challenge of C57BL/6 mice with A/PR8 virus. However, infected SRO mice developed hypercellularity in the lungs and were more likely to die from lung consolidation than controls. Since elevated cort failed to be anti‐inflammatory in SRO mice, the hypothesis that psychosocial stress induced steroid insensitivity was tested. An in vitro cort suppression test was performed by stimulating splenocytes from SRO and control mice with mitogen in the presence or absence of cort. Proliferation of ConA‐stimulated cells was inhibited by cort in a dose‐dependent fashion in controls, but splenocytes from SRO mice stimulated with ConA were resistant to cort‐induced suppression. Thus, psychosocial stress induced a state of steroid insensitivity. SRO also induced the release of nerve growth factor (NGF) from the salivary glands into circulation; plasma NGF correlated with development of steroid insensitivity. NGF has been reported to negatively regulate the expression of type II glucocorticoid receptors, and thus may be a key factor in the induction of steroid insensitivity.

Endocrine regulation of murine macrophage function: effects of dehydroepiandrosterone, androstenediol, and androstenetriol

In these studies, the in vitro influences of dehydroepiandrosterone (DHEA), androstenediol (AED), and androstenetriol (AET) on proinflammatory cytokine production from macrophages was examined. From physiologic to pharmacologic doses, DHEA suppressed secretion of each pro-inflammatory cytokine while AED had little influence on the responses. In sharp contrast, AET augmented TNF-alpha and IL-1 secretion while not influencing IL-6 production. Furthermore, the antiglucocorticoid activity of DHEA, AED, and AET was also investigated. Co-culture with AET counteracted the down-regulatory effect of hydrocortisone on LPS-induced TNF-alpha and IL-1 secretion. These data imply that AET is capable of regulating cytokine secretion from macrophages and may function to counterbalance glucocorticoid function.

Interleukin-6 and the development of social disruption-induced glucocorticoid resistance

Following social disruption (SDR) stress in male mice, corticosterone resistance of splenocytes was accompanied by enhanced LPS-stimulated interleukin (IL)-6 secretion. The present study examined the role of IL-6 in the development of corticosterone resistance. Addition of IL-6 to control splenocyte cultures did not induce corticosterone resistance. SDR also elevated IL-6 in plasma and liver, but not in spleen. IL-6 deficient mice that were exposed to SDR developed glucocorticoid resistance despite the absence of systemic IL-6. These findings suggest that although SDR enhanced IL-6 responses, IL-6 was not essential for the development of stress-induced splenocyte corticosterone resistance.

Social disruption-induced glucocorticoid resistance: kinetics and site specificity

Social disruption (SDR) of male mice has been shown to induce a state of functional glucocorticoid (GC) resistance in splenocytes. The present study demonstrated that GC resistance developed following repeated, but not acute exposure to SDR. GC resistance was long-lasting and persisted for at least 10 days after stress. In contrast, SDR did not alter cytokine secretion from peritoneal mononuclear cells treated with corticosterone. These findings suggest that SDR-induced GC resistance may be restricted to specific sites such as the spleen.