Harald Engler

Stress in puberty unmasks latent neuropathological consequences of prenatal immune activation in mice.

Double Whammy Psychopathologies that cannot be explained by simple genetic or environmental circumstances may sometimes result from complex interplay between multiple inputs. Giovanoli et al. (p. 1095) analyzed the interactions between prenatal and postnatal stressors in mice to see what synergies give rise to psychopathologies in the adult mice. The results suggest that susceptibilities arise when mice are exposed to prenatal infection and also exposed to stressors around puberty. Stressors delivered later in adolescence did not seem to produce the same susceptibility. Although the mechanisms that impose the delay between stressors and psychopathology remain obscure, the timing and sequence of the triggers hint at possible cellular causes. Unfortunate synergies between stressors at vulnerable stages may underlie psychopathologies in mice. Prenatal infection and exposure to traumatizing experiences during peripuberty have each been associated with increased risk for neuropsychiatric disorders. Evidence is lacking for the cumulative impact of such prenatal and postnatal environmental challenges on brain functions and vulnerability to psychiatric disease. Here, we show in a translational mouse model that combined exposure to prenatal immune challenge and peripubertal stress induces synergistic pathological effects on adult behavioral functions and neurochemistry. We further demonstrate that the prenatal insult markedly increases the vulnerability of the pubescent offspring to brain immune changes in response to stress. Our findings reveal interactions between two adverse environmental factors that have individually been associated with neuropsychiatric disease and support theories that mental illnesses with delayed onsets involve multiple environmental hits.

Effects of repeated social stress on leukocyte distribution in bone marrow, peripheral blood and spleen

Leukocyte trafficking between the various body compartments has an important surveillance function that ensures the detection of antigen and enables the immune system to initiate a rapid and effective response. Repeated social defeat of group-housed male mice induced by daily, acute encounters with an aggressive conspecific substantially altered leukocyte trafficking and led to a gradual redistribution of immune cells in bone marrow, peripheral blood and spleen. Recurrent exposure to the stressor over a period of 2, 4 or 6 consecutive days was associated with cell mobilization and increased myelopoiesis in the bone marrow that was paralleled by an accumulation of neutrophils and monocytes in circulation and spleen. Substantial depletion of B cells in bone marrow and blood was associated with an increase in splenic B cells indicating a redirection of this cell type to the spleen. In contrast, T cells were markedly reduced in these immune compartments. The recruitment of CD11b+ leukocytes (i.e., monocytes/macrophages and neutrophils) from the bone marrow to the spleen might play a critical role in the development of functional glucocorticoid resistance in the murine spleen that was reported in context with repeated social defeat.

Dose-Dependent Effects of Endotoxin on Neurobehavioral Functions in Humans

Clinical and experimental evidence document that inflammation and increased peripheral cytokine levels are associated with depression-like symptoms and neuropsychological disturbances in humans. However, it remains unclear whether and to what extent cognitive functions like memory and attention are affected by and related to the dose of the inflammatory stimulus. Thus, in a cross-over, double-blind, experimental approach, healthy male volunteers were administered with either placebo or bacterial lipopolysaccharide (LPS) at doses of 0.4 (n = 18) or 0.8 ng/kg of body weight (n = 16). Pro- and anti-inflammatory cytokines, norephinephrine and cortisol concentrations were analyzed before and 1, 1.75, 3, 4, 6, and 24 h after injection. In addition, changes in mood and anxiety levels were determined together with working memory (n-back task) and long term memory performance (recall of emotional and neutral pictures of the International Affective Picture System). Endotoxin administration caused a profound transient physiological response with dose-related elevations in body temperature and heart rate, increases in plasma interleukin (IL)-6, IL-10, tumor necrosis factor (TNF)-α and IL-1 receptor antagonist (IL-1ra), salivary and plasma cortisol, and plasma norepinephrine. These changes were accompanied by dose-related decreased mood and increased anxiety levels. LPS administration did not affect accuracy in working memory performance but improved reaction time in the high-dose LPS condition compared to the control conditon. In contrast, long-term memory performance was impaired selectively for emotional stimuli after administration of the lower but not of the higher dose of LPS. These data suggest the existence of at least two counter-acting mechanisms, one promoting and one inhibiting cognitive performance during acute systemic inflammation.

Posttraumatic stress disorder is associated with an enhanced spontaneous production of pro-inflammatory cytokines by peripheral blood mononuclear cells

BackgroundPosttraumatic stress disorder (PTSD) is associated with an enhanced risk for cardiovascular and other inflammatory diseases. Chronic low-level inflammation has been suggested as a potential mechanism linking these conditions.MethodsWe investigated plasma cytokine levels as well as spontaneous and lipopolysaccharide (LPS)-stimulated cytokine production by peripheral blood mononuclear cells (PBMCs) in a group of 35 severely traumatized PTSD patients compared to 25 healthy controls.ResultsSpontaneous production of interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α by isolated PBMCs was significantly higher in the PTSD compared to the control group and even correlated with PTSD symptom severity within the PTSD group. In contrast, circulating plasma levels of pro- and anti-inflammatory cytokines such as IL-6, IL-8, IL-10, TNF-α, or monocyte chemotactic protein (MCP)-1 were not significantly altered in PTSD patients compared to healthy controls.ConclusionsOur findings indicate that PBMCs of PTSD patients are already pre-activated in vivo, providing further evidence for low-grade inflammation in PTSD. This might possibly represent one psychobiological pathway from PTSD to poor physical health.

Substantial reduction of naïve and regulatory T cells following traumatic stress.

Posttraumatic stress disorder (PTSD) is associated with an enhanced susceptibility to various somatic diseases. However, the exact mechanisms linking traumatic stress to subsequent physical health problems have remained unclear. This study investigated peripheral T lymphocyte differentiation subsets in 19 individuals with war and torture related PTSD compared to 27 non-PTSD controls (n=14 trauma-exposed controls; n=13 non-exposed controls). Peripheral T cell subpopulations were classified by their characteristic expression of the lineage markers CD45RA and CCR7 into: naïve (CD45RA(+) CCR7(+)), central memory (T(CM): CD45RA(-) CCR7(+)) and effector memory (T(EM): CD45RA(-) CCR7(-) and T(EMRA): CD45RA(-) CCR7(-)) cells. Furthermore, we analyzed regulatory T cells (CD4(+)CD25(+)FoxP3(+)) and ex vivo proliferation responses of peripheral blood mononuclear cells after stimulation with anti-CD3 monoclonal antibody. Results show that the proportion of naïve CD8(+) T lymphocytes was reduced by 32% (p=0.01), whereas the proportions of CD3(+) central (p=0.02) and effector (p=0.01) memory T lymphocytes were significantly enhanced (+22% and +34%, respectively) in PTSD patients compared to non-PTSD individuals. To a smaller extent, this effect was also observed in trauma-exposed non-PTSD individuals, indicating a cumulative effect of traumatic stress on T cell distribution. Moreover, PTSD patients displayed a 48% reduction in the proportion of regulatory T cells (p<0.001). Functionally, these alterations were accompanied by a significantly enhanced (+34%) ex vivo proliferation of anti-CD3 stimulated T cells (p=0.05). The profoundly altered composition of the peripheral T cell compartment might cause a state of compromised immune responsiveness, which may explain why PTSD patients show an increased susceptibility to infections, and inflammatory and autoimmune diseases.

Stress induces the translocation of cutaneous and gastrointestinal microflora to secondary lymphoid organs of C57BL/6 mice.

Mammals are colonized by a vast array of bacteria that reside as part of the hosts microflora. Despite their enormous levels, these microorganisms tend to be restricted to cutaneous and mucosal surfaces. In the current experiment, only a small percentage of non-stressed mice exhibited detectable levels of bacteria in their inguinal lymph nodes (ILN), spleen, liver, or mesenteric lymph nodes (MLN). However, after experiencing repeated social disruption (SDR), a significant increase in the number of animals having bacteria in their ILN and MLN was found. Since SDR involves fighting in which bite wounds on the skin could provide a portal of entry into the host, it was determined whether experimental wounding (full-thickness skin biopsy), chronic restraint (which is a potent stressor that does not disrupt the skin barrier), or wounding combined with restraint would increase the occurrence of bacteria in secondary lymphoid tissues and liver. Wounding did not significantly increase the prevalence of bacteria in the ILN, MLN, or liver. Interestingly, a larger percentage of restrained and restrained plus wounded mice, in comparison to controls, had bacteria in the ILN, MLN, and liver. Although the stressors increased the number of animals that became colonized, the levels of bacteria in the stressed mice were similar to the levels found in the few non-stressed mice that did become colonized. Our results indicate that psychological components of social stress facilitate the translocation of indigenous bacteria into the host, thus identifying an additional facet through which stressors may impact health.

Tissue-specific alterations in the glucocorticoid sensitivity of immune cells following repeated social defeat in mice

Endogenous glucocorticoids (GC) play an important role in the termination of the inflammatory response following infection and tissue injury. However, recent findings indicate that stress can impair the anti-inflammatory capacities of these hormones. Lipopolysaccharide (LPS)-stimulated splenocytes of mice that were repeatedly subjected to social disruption (SDR) stress were less sensitive to the immunosuppressive effects of corticosterone (CORT) as demonstrated by an increased production of pro-inflammatory cytokines and enhanced cell survival. Myeloid cells expressing the marker CD11b were shown to play a key role in this process. Here we investigated the role of the bone marrow as a potential source of the GC-insensitive cells. The study revealed that LPS-stimulated bone marrow cells, in the absence of experimental stress, were virtually GC-resistant and retained high levels of cell viability after treatment with CORT. Recurrent exposure to the acute stressor over a period of 2, 4 or 6 days led to an increase in the GC sensitivity of the bone marrow cells. This increase in GC sensitivity was associated with enhanced mRNA expression of granulocyte-macrophage colony-stimulating factor (GM-CSF), an increase in the number of myeloid progenitors, and a decrease in the proportion of mature CD11b+ cells. The changes in the cellular composition of the bone marrow were accompanied by an increase in splenic CD11b+ cell numbers. Simultaneous assessment of the GC sensitivity in bone marrow and spleen revealed a significant negative correlation between both tissues suggesting that social stress causes the redistribution of GC-insensitive myeloid cells from the bone marrow to the spleen.

Expression of glucocorticoid resistance following social stress requires a second signal

Stimulation of splenocytes from socially stressed mice [social disruption (SDR)] with Gram‐negative bacterial lipopolysaccharide (LPS) revealed a state of functional glucocorticoid (GC) resistance. LPS‐stimulated splenocytes were less sensitive to the inhibitory effects of corticosterone. This study demonstrated that activation signals were required for the expression of splenic GC resistance. The results demonstrated that six cycles of SDR induced splenomegaly and increased the number of CD11b‐positive monocytes. SDR also increased the viability of cultured, nonstimulated splenocytes, and addition of corticosterone reduced the viability of these cells in a dose‐dependent manner. However, following stimulation with LPS, the sensitivity of SDR splenocytes to GC was reduced. Similar results were obtained using lipid A, a fraction of the LPS molecule that binds to Toll‐like receptor (TLR)4. Furthermore, C3H/HeJ mice that do not possess a functional TLR4 molecule responded to SDR with an increased number of CD11b‐positive monocytes in the spleen and increased viability of nonstimulated splenocytes. However, neither LPS nor lipid A stimulation resulted in the expression of GC resistance. Together, these findings suggest that the expression of GC resistance in response to SDR requires a second signal that can be provided by ligation of TLR4.

Interleukin-1 receptor type 1-deficient mice fail to develop social stress-associated glucocorticoid resistance in the spleen

Frequent or chronic stress as a result of repeated or persistent exposure to social challenges has been shown to affect the glucocorticoid (GC) responsiveness of immune cells in mice. Lipopolysaccharide-stimulated splenocytes of mice that were repeatedly subjected to social disruption were less sensitive to the anti-inflammatory actions of GC as evident from an increased production of pro-inflammatory cytokines and enhanced cell survival. The development of functional GC resistance was accompanied by the accumulation of GC-insensitive CD11b(+) cells in the spleen. These cells were shown to exhibit impaired nuclear translocation of the GC receptor and lack of GC-induced suppression of NF-kappaB. Similar impairments in GC receptor function have been reported after in vitro treatment of various cell lines with interleukin (IL)-1. The aim of this study was to elucidate whether IL-1 is a critical factor for the development of GC resistance in socially stressed mice. In the first experiment, we investigated if repeated social stress alters plasma levels and tissue gene expression of IL-1alpha and IL-1beta. It revealed that recurrent stressor exposure significantly increased splenic and hepatic mRNA expression and the plasma protein level of IL-1beta, and hepatic mRNA expression of IL-1alpha. In the second experiment, IL-1 receptor type 1 (IL1R1)-deficient mice were subjected to the stressor and both the tissue distribution of CD11b(+) cells and the GC sensitivity of the splenocytes were compared to wildtype mice. Mice lacking the IL1R1 exhibited adrenal hypertrophy, thymic involution, and elevated serum corticosterone levels in response to the stressor but did not show splenic accumulation of CD11b(+) cells and failed to develop GC resistance. These findings suggest that IL-1 plays a critical role in the development of the social stress-associated GC resistance in the murine spleen.

Endotoxin-induced experimental systemic inflammation in humans: a model to disentangle immune-to-brain communication.

Systemic inflammation is among the most prominent and most frequently observed responses of the immune system. Over the past decades, it has become clear that inflammatory cytokines not only affect immune and metabolic functions but also cause a wide range of behavioral and mood changes. Based on experimental findings in animals and observations in clinical populations it has been hypothesized that inflammation-induced neurocognitive changes contribute to the pathophysiology of neuropsychiatric diseases. However, since certain aspects of human behavior cannot be modeled in laboratory animals, there is a need for human models of systemic inflammation. In this review, we summarize recent studies employing administration of endotoxin as a model to induce transient systemic inflammation in healthy human subjects.