Chronic social defeat alters brain vascular-associated cell gene expression patterns leading to vascular dysfunction and immune system activation.

Brain vascular integrity is critical for brain health, and its disruption is implicated in many brain pathologies, including psychiatric disorders. Brain-vascular barriers are a complex cellular landscape composed of endothelial, glial, mural, and immune cells. Yet currently, little is known about these brain vascular-associated cells (BVACs) in health and disease. Previously, we demonstrated that 14 days of chronic social defeat (CSD), a mouse paradigm that produces anxiety and depressive-like behaviors, causes cerebrovascular damage in the form of scattered microbleeds. Here, we developed a technique to isolate barrier-related cells from the mouse brain and subjected the isolated cells to single-cell RNA sequencing. Using this isolation technique, we found an enrichment in BVAC populations, including distinct subsets of endothelial and microglial cells. In CSD compared to non-stress, home-cage control, differential gene expression patterns disclosed biological pathways involving vascular dysfunction, vascular healing, and immune system activation. Overall, our work demonstrates a unique technique to study BVAC populations from fresh brain tissue and suggests that neurovascular dysfunction is a key driver of psychosocial stress-induced brain pathology.

CCR2 monocytes repair cerebrovascular damage caused by chronic social defeat stress.

Immune surveillance of the brain plays an important role in health and disease. Peripheral leukocytes patrol blood-brain barrier interfaces, and after injury, monocytes cross the cerebrovasculature and follow a pattern of pro- and anti-inflammatory activity leading to tissue repair. We have shown that chronic social defeat (CSD) causes scattered vasculature disruptions. Here, we assessed CCR2+ monocyte trafficking to the vascular injury sites in Ccr2wt/rfp reporter mice both during CSD and one week following CSD cessation. We found that CSD for 14 days induced microhemorrhages where plasma fibrinogen leaked into perivascular spaces, but it did not affect the distribution or density of CCR2rfp+ monocytes in the brain. However, after recovery from CSD, many vascularly adhered CCR2+ cells were detected, and gene expression of the CCR2 chemokine receptor ligands CCL7 and CCL12, but not CCL2, was elevated in endothelial cells. Adhered CCR2+ cells were mostly the non-classical, anti-inflammatory Ly6Clo type, and they phagocytosed fibrinogen in perivascular spaces. In CCR2-deficient Ccr2rfp/rfp mice, fibrinogen levels remained elevated in recovery. Fibrinogen infused intracerebroventricularly induced CCR2+ cells to adhere to the vasculature and phagocytose perivascular fibrinogen in Ccr2wt/rfp but not Ccr2rfp/rfp mice. Depletion of monocytes with clodronate liposomes during CSD recovery prevented fibrinogen clearance and blocked behavioral recovery. We hypothesize that peripheral CCR2+ monocytes are not elevated in the brain on day 14 at the end of CSD and do not contribute to its behavioral effects at that time, but in recovery following cessation of stress, they enter the brain and exert restorative functions mediating vascular repair and normalization of behavior.

B-cells are abnormal in psychosocial stress and regulate meningeal myeloid cell activation.

There is increasing interest in how immune cells, including those within the meninges at the blood-brain interface, influence brain function and mood disorders, but little data on humoral immunity in this context. Here, we show that in mice exposed to psychosocial stress, there is increased splenic B cell activation and secretion of the immunoregulatory cytokine interleukin (IL)-10. Meningeal B cells were prevalent in homeostasis but substantially decreased following stress, whereas Ly6Chi monocytes increased, and meningeal myeloid cells showed augmented expression of activation markers. Single-cell RNA sequencing of meningeal B cells demonstrated the induction of innate immune transcriptional programmes following stress, including genes encoding antimicrobial peptides that are known to alter myeloid cell activation. Cd19-/- mice, that have reduced B cells, showed baseline meningeal myeloid cell activation and decreased exploratory behaviour. Together, these data suggest that B cells may influence behaviour by regulating meningeal myeloid cell activation.

The Behavioral Sequelae of Social Defeat Require Microglia and Are Driven by Oxidative Stress in Mice.

Chronic social defeat (CSD) in male mice can produce anxiety and aberrant socialization. Animals susceptible to CSD show activation of microglia, which have elevated levels of oxidative stress markers. We hypothesized that microglia and reactive oxygen species (ROS) production contribute to the CSD stress-induced changes in affective behavior. First, we selectively depleted microglia (99%) by administering the CSF1R (colony-stimulating factor 1 receptor) antagonist PLX5622 before and during the 14 d CSD procedure. Microglia-depleted mice in contrast to nondepleted mice were protected from the stress effects measured by light/dark and social interaction tests. ROS production, measured histochemically following dihydroethidium administration, was elevated by CSD, and the production was reduced to basal levels in mice lacking microglia. The deleterious stress effects were also blocked in nondepleted mice by continuous intracerebral administration of N-acetylcysteine (NAC), a ROS inhibitor. In a second experiment, at the end of the CSD period, PLX5622 was discontinued to allow microglial repopulation. After 14 d, the brain had a full complement of newly generated microglia. At this time, the mice that had previously been protected now showed behavioral deficits, and their brain ROS production was elevated, both in all brain cells and in repopulated microglia. NAC administration during repopulation prevented the behavioral decline in the repopulated mice, and it supported behavioral recovery in nondepleted stressed mice. The data suggest that microglia drive elevated ROS production during and after stress exposure. This elevated ROS activity generates a central state supporting dysregulated affect, and it hinders the restoration of behavioral and neurochemical homeostasis after stress cessation.SIGNIFICANCE STATEMENT Chronic psychosocial stress is associated with psychiatric disorders such as depression and anxiety. Understanding the details of CNS cellular contributions to stress effects could lead to the development of intervention strategies. Inflammation and oxidative stress are positively linked to depression severity, but the cellular nature of these processes is not clear. The chronic social defeat (CSD) paradigm in mice produces mood alterations and microglial activation characterized by elevated reactive oxygen species (ROS) production. The depletion of microglia or ROS inhibition prevented adverse stress effects. Microglial repopulation of the brain post-CSD reintroduced adverse stress effects, and ROS inhibition in this phase protected against the effects. The results suggest that stress-induced microglial ROS production drives a central state that supports dysregulated affective behavior.

Analysis of cerebrovascular dysfunction caused by chronic social defeat in mice.

Psychological stress and affective disorders are clinically associated with hypertension and vascular disease, but the biological links between the conditions have not been fully explored. To examine this relationship, we used chronic social defeat (CSD) stress, which produces anxiety-like and depressive-like behavioral declines in susceptible mice. In such mice, CSD also produces cerebrovascular microbleeds in scattered locations. Here, we showed further evidence of vascular pathology and blood-brain barrier breakdown by visualizing plasma immunoglobulins and erythrocytes within the parenchyma and perivascular spaces of CSD brains. To further characterize the impact of stress on the cerebrovasculature, brain endothelial cells (bECs) were isolated, and global gene expression profiles were generated. Bioinformatic analysis of CSD-induced transcriptional changes in bECs showed enrichment in pathways that delineate the vascular response to injury. These pathways followed a temporal sequence of inflammation, oxidative stress, growth factor signaling, and wound healing (i.e., platelet aggregation, hemostasis, fibrinogen deposition, and angiogenesis). Immunohistochemical staining for markers of fibrinogen deposition and angiogenesis confirmed the existence of the markers at the sites of vascular disruptions. Recovery after CSD cessation was marked by recruitment of leukocytes perhaps participating in vascular repair. The data suggest that co-morbidity of affective disorders and vascular diseases may be attributed in part to a common link in altered endothelial cell function.

Lymphocytes from chronically stressed mice confer antidepressant-like effects to naive mice.

We examined whether cells of the adaptive immune system retain the memory of psychosocial stress and thereby alter mood states and CNS function in the host. Lymphocytes from mice undergoing chronic social defeat stress or from unstressed control mice were isolated and adoptively transferred into naive lymphopenic Rag2(-/-) mice. Changes in affective behavior, hippocampal cell proliferation, microglial activation states, and blood cytokine levels were examined in reconstituted stress-naive mice. The mice receiving lymphocytes from defeated donors showed less anxiety, more social behavior, and increased hippocampal cell proliferation compared with those receiving no cells or cells from unstressed donors. Mice receiving stressed immune cells had reduced pro-inflammatory cytokine levels in the blood relative to the other groups, an effect opposite to the elevated donor pro-inflammatory cytokine profile. Furthermore, mice receiving stressed immune cells had microglia skewed toward an anti-inflammatory, neuroprotective M2-like phenotype, an effect opposite the stressed donors' M1-like pro-inflammatory profile. However, stress had no effect on lymphocyte surface marker profiles in both donor and recipient mice. The data suggest that chronic stress-induced changes in the adaptive immune system, contrary to conferring anxiety and depressive behavior, protect against the deleterious effects of stress. Improvement in affective behavior is potentially mediated by reduced peripheral pro-inflammatory cytokine load, protective microglial activity, and increased hippocampal cell proliferation. The data identify the peripheral adaptive immune system as putatively involved in the mechanisms underlying stress resilience and a potential basis for developing novel rapid-acting antidepressant therapies.

Social defeat induces depressive-like states and microglial activation without involvement of peripheral macrophages.

BACKGROUND: We are interested in the causal interactions between psychological stress and activity within different compartments of the immune system. Psychosocial stress has been reported to not only alter microglia morphology but also produce anxiety-like and depressive-like effects by triggering CNS infiltration of macrophages from the periphery. We sought to test these phenomena in a somewhat different but standardized model of chronic social defeat (SD) stress. METHODS: We used a paradigm of dyadic home pairing of dominant and subordinate mice that has been validated to induce powerful anxiety-like and depressive-like effects manifested by behavior assessed in social tasks. We administered the SD stress for 3 days (acute SD) or 14 days (chronic SD) and looked for monocyte entry into the brain by three independent means, including CD45 activation states assessed by flow cytometry and tracking fluorescently tagged peripheral cells from Ccr2 (wt/rfp) and Ubc (gfp/gfp) reporter mice. We further characterized the effects of SD stress on microglia using quantitative morphometric analysis, ex vivo phagocytosis assays, flow cytometry, and immunochemistry. RESULTS: We saw no evidence of stress-induced macrophage entry after acute or chronic defeat stress. In comparison, brain infiltration of peripheral cells did occur after endotoxin administration. Furthermore, mutant mice lacking infiltrating macrophages due to CCR2 knockout developed the same degree of chronic SD-induced depressive behavior as wildtype mice. We therefore focused more closely on the intrinsic immune cells, the microglia. Using Cx3cr1 (wt/gpf) microglial reporter mice, we saw by quantitative methods that microglial morphology was not altered by stress at either time point. However, chronic SD mice had elevated numbers of CD68(hi) microglia examined by flow cytometry. CD68 is a marker for phagocytic activity. Indeed, these cells ex vivo showed elevated phagocytosis, confirming the increased activation status of chronic SD microglia. Finally, acute SD but not chronic SD increased microglial proliferation, which occurred selectively in telencephalic stress-related brain areas. CONCLUSIONS: In the SD paradigm, changes in CNS-resident microglia numbers and activation states might represent the main immunological component of the psychosocial stress-induced depressive state.

Glucocorticoids orchestrate divergent effects on mood through adult neurogenesis.

Both social defeat stress and environmental enrichment stimulate adrenal glucocorticoid secretion, but they have opposing effects on hippocampal neurogenesis and mood. Hypothalamic-pituitary-adrenal axis dysregulation and decreased neurogenesis are consequences of social defeat. These outcomes are correlated with depressive states, but a causal role in the etiology of depression remains elusive. The antidepressant actions of environmental enrichment are neurogenesis-dependent, but the contribution of enrichment-elevated glucocorticoids is unexplored. Importantly, for both social defeat and environmental enrichment, how glucocorticoids interact with neurogenesis to alter mood is unknown. Here, we investigate causal roles of glucocorticoids and neurogenesis in induction of depressive-like behavior and its amelioration by environmental enrichment in mice. By blocking neurogenesis and surgically clamping adrenal hormone secretions, we showed that neurogenesis, via hypothalamic-pituitary-adrenal axis interactions, is directly involved in precipitating the depressive phenotype after social defeat. Mice adrenalectomized before social defeat showed enhanced behavioral resiliency and increased survival of adult-born hippocampal neurons compared with sham-operated defeated mice. However, mice lacking hippocampal neurogenesis did not show protective effects of adrenalectomy. Moreover, glucocorticoids secreted during environmental enrichment promoted neurogenesis and were required for restoration of normal behavior after social defeat. The data demonstrate that glucocorticoid-dependent declines in neurogenesis drive changes in mood after social defeat and that glucocorticoids secreted during enrichment promote neurogenesis and restore normal behavior after defeat. These data provide new evidence for direct involvement of neurogenesis in the etiology of depression, suggesting that treatments promoting neurogenesis can enhance stress resilience.

Environmental enrichment confers stress resiliency to social defeat through an infralimbic cortex-dependent neuroanatomical pathway.

Enriched environmental (EE) housing dampens stress-induced alterations in neurobiological systems, promotes adaptability, and extinguishes submissive behavioral traits developed during social defeat stress (SD). In the present study, we hypothesized that enrichment before SD can confer stress resiliency and, furthermore, that neuronal activity in the prefrontal cortex (PFC) is requisite for this resiliency. To test these hypotheses, mice were housed in EE, standard (SE), or impoverished (IE) housing and then exposed to SD. EE conferred resilience to SD as measured in several behavioral tasks. EE-housed mice expressed elevated FosB/ΔFosB immunostaining in areas associated with emotional regulation and reward processing, i.e., infralimbic, prelimbic, and anterior cingulate cortices, amygdala, and nucleus accumbens, and this expression was mostly preserved in mice receiving EE followed by SD. In contrast, in SE- or IE-housed animals, SD increased maladaptive behaviors and greatly reduced FosB/ΔFosB staining in the forebrain. We tested the putative involvement of the PFC in mediating resilience by lesioning individual regions of the PFC either before or after EE housing and then exposing the mice to SD. We found that discrete lesions of the infralimbic but not prelimbic or cingulate cortex made before but not after EE abolished the behavioral resiliency to stress afforded by EE and attenuated FosB/ΔFosB expression in the accumbens and amygdala while increasing it in the paraventricular hypothalamic nucleus. These data suggest that pathological ventromedial PFC outputs to downstream limbic targets could predispose an individual to anxiety disorders in stressful situations, whereas enhanced ventromedial PFC outputs could convey stress resilience.

Maternal immune activation by LPS selectively alters specific gene expression profiles of interneuron migration and oxidative stress in the fetus without triggering a fetal immune response.

Maternal immune activation (MIA) is a risk factor for the development of schizophrenia and autism. Infections during pregnancy activate the mother's immune system and alter the fetal environment, with consequential effects on CNS function and behavior in the offspring, but the cellular and molecular links between infection-induced altered fetal development and risk for neuropsychiatric disorders are unknown. We investigated the immunological, molecular, and behavioral effects of MIA in the offspring of pregnant Sprague-Dawley rats given an intraperitoneal (0.25 mg/kg) injection of lipopolysaccharide (LPS) on gestational day 15. LPS significantly elevated pro-inflammatory cytokine levels in maternal serum, amniotic fluid, and fetal brain at 4 h, and levels decreased but remained elevated at 24 h. Offspring born to LPS-treated dams exhibited reduced social preference and exploration behaviors as juveniles and young adults. Whole genome microarray analysis of the fetal brain at 4 h post maternal LPS was performed to elucidate the possible molecular mechanisms by which MIA affects the fetal brain. We observed dysregulation of 3285 genes in restricted functional categories, with increased mRNA expression of cellular stress and cell death genes and reduced expression of developmentally-regulated and brain-specific genes, specifically those that regulate neuronal migration of GABAergic interneurons, including the Distal-less (Dlx) family of transcription factors required for tangential migration from progenitor pools within the ganglionic eminences into the cerebral cortex. Our results provide a novel mechanism by which MIA induces the widespread down-regulation of critical neurodevelopmental genes, including those previously associated with autism.

Cautionary notes on the use of NF-κB p65 and p50 antibodies for CNS studies.

BACKGROUND: The characterization and cellular localization of transcription factors like NF-κB requires the use of antibodies for western blots and immunohistochemistry. However, if target protein levels are low and the antibodies not well characterized, false positive data can result. In studies of NF-κB activity in the CNS, antibodies detecting NF-κB proteins have been used to support the finding that NF-κB is constitutively active in neurons, and activity levels are further increased by neurotoxic treatments, glutamate stimulation, or elevated synaptic activity. The specificity of the antibodies used was analyzed in this study. METHODS: Selectivity and nonselectivity of commonly used commercial and non-commercial p50 and p65 antibodies were demonstrated in western blot assays conducted in tissues from mutant gene knockout mice lacking the target proteins. RESULTS: A few antibodies for p50 and p65 each mark a single band at the appropriate molecular weight in gels containing proteins from wildtype tissue, and this band is absent in proteins from knockout tissues. Several antibodies mark proteins that are present in knockout tissues, indicating that they are nonspecific. These include antibodies raised against the peptide sequence containing the nuclear localization signals of p65 (MAB3026; Chemicon) and p50 (sc-114; Santa Cruz). Some antibodies that recognize target proteins at the correct molecular weight still fail in western blot analysis because they also mark additional proteins and inconsistently so. We show that the criterion for validation by use of blocking peptides can still fail the test of specificity, as demonstrated for several antibodies raised against p65 phosphorylated at serine 276. Finally, even antibodies that show specificity in western blots produce nonspecific neuronal staining by immunohistochemistry. CONCLUSIONS: We note that many of the findings in the literature about neuronal NF-κB are based on data garnered with antibodies that are not selective for the NF-κB subunit proteins p65 and p50. The data urge caution in interpreting studies of neuronal NF-κB activity in the brain.

Induction of IDO by bacille Calmette-Guérin is responsible for development of murine depressive-like behavior.

Chronic inflammation activates the tryptophan-degrading enzyme IDO, which is well known to impair T cell proliferation. We have previously established that bacille Calmette-Guérin (BCG), an attenuated form of Mycobacterium bovis, is associated with persistent activation of IDO in the brain and chronic depressive-like behavior, but a causative role has not been established. In these experiments we used both pharmacologic and genetic approaches to test the hypothesis that IDO activation is responsible for the development of chronic depression that follows BCG infection. BCG induced TNF-alpha, IFN-gamma, and IDO mRNA steady-state transcripts in the brain as well as the enzyme 3-hydroxyanthranilic acid oxygenase (3-HAO) that lies downstream of IDO and generates the neuroactive metabolite, quinolinic acid. Behaviors characteristic of depression were apparent 1 wk after BCG infection. Pretreatment with the competitive IDO inhibitor 1-methyltryptophan fully blocked BCG-induced depressive-like behaviors. Importantly, IDO-deficient mice were completely resistant to BCG-induced depressive-like behavior but responded normally to BCG induction of proinflammatory cytokines. These results are the first to prove that the BCG-induced persistent activation of IDO is accompanied by the induction of 3-hydroxyanthranilic acid oxygenase and that IDO is required as an initial step for the subsequent development of chronic depressive-like behavior.

NF-kappaB activity affects learning in aversive tasks: possible actions via modulation of the stress axis.

The role of altered activity of nuclear factor kappaB (NF-kappaB) in specific aspects of motivated behavior and learning and memory was examined in mice lacking the p50 subunit of the NF-kappaB/rel transcription factor family. Nfkb1-deficient mice are unable to produce p50 and show specific susceptibilities to infections and inflammatory challenges, but the behavioral phenotype of such mice has been largely unexamined, owing in large part to the lack of understanding of the role of NF-kappaB in nervous system function. Here we show that Nfkb1 (p50) knockout mice more rapidly learned to find the hidden platform in the Morris water maze than did wildtype mice. The rise in plasma corticosterone levels after the maze test was greater in p50 knockout than in wildtype mice. In the less stressful Barnes maze, which tests similar kinds of spatial learning, the p50 knockout mice performed similarly to control mice. Adrenalectomy with corticosterone replacement eliminated the differences between p50 knockout and wildtype mice in the water maze. Knockout mice showed increased levels of basal anxiety in the open-field and light/dark box tests, suggesting that their enhanced escape latency in the water maze was due to activation of the stress (hypothalamic-pituitary-adrenal) axis leading to elevated corticosterone production by strongly but not mildly anxiogenic stimuli. The results suggest that, as in the immune system, p50 in the nervous system normally serves to dampen NF-kappaB-mediated intracellular activities, which are manifested physiologically through elevated stress responses to aversive stimuli and behaviorally in the facilitated escape performance in learning tasks.

Insidious adrenocortical insufficiency underlies neuroendocrine dysregulation in TIF-2 deficient mice.

The transcription-intermediary-factor-2 (TIF-2) is a coactivator of the glucocorticoid receptor (GR), and its disruption would be expected to influence glucocorticoid-mediated control of the hypothalamo-pituitary-adrenal (HPA) axis. Here, we show that its targeted deletion in mice is associated with altered expression of several glucocorticoid-dependent components of HPA regulation (e.g., corticotropin-releasing hormone, vasopressin, ACTH, glucocorticoid receptors), suggestive of hyperactivity under basal conditions. At the same time, TIF-2(-/-) mice display significantly lower basal corticosterone levels and a sluggish and blunted initial secretory response to brief emotional and prolonged physical stress. Subsequent analysis revealed this discrepancy to result from pronounced aberrations in the structure and function of the adrenal gland, including the cytoarchitectural organization of the zona fasciculata and basal and stress-induced expression of key elements of steroid hormone synthesis, such as the steroidogenic acute regulatory (StAR) protein and 3beta-hydroxysteroid dehydrogenase (3beta-HSD). In addition, altered expression levels of two nuclear receptors, DAX-1 and steroidogenic factor 1 (SF-1), in the adrenal cortex strengthen the view that TIF-2 deletion disrupts adrenocortical development and steroid biosynthesis. Thus, hyperactivity of the hypothalamo-pituitary unit is ascribed to insidious adrenal insufficiency and impaired glucocorticoid feedback.

Decoding microglia responses to psychosocial stress reveals blood-brain barrier breakdown that may drive stress susceptibility.

An animal's ability to cope with or succumb to deleterious effects of chronic psychological stress may be rooted in the brain's immune responses manifested in microglial activity. Mice subjected to chronic social defeat (CSD) were categorized as susceptible (CSD-S) or resilient (CSD-R) based on behavioral phenotyping, and their microglia were isolated and analyzed by microarray. Microglia transcriptomes from CSD-S mice were enriched for pathways associated with inflammation, phagocytosis, oxidative stress, and extracellular matrix remodeling. Histochemical experiments confirmed the array predictions: CSD-S microglia showed elevated phagocytosis and oxidative stress, and the brains of CSD-S but not CSD-R or non-stressed control mice showed vascular leakage of intravenously injected fluorescent tracers. The results suggest that the inflammatory profile of CSD-S microglia may be precipitated by extracellular matrix degradation, oxidative stress, microbleeds, and entry and phagocytosis of blood-borne substances into brain parenchyma. We hypothesize that these CNS-centric responses contribute to the stress-susceptible behavioral phenotype.

Contributions of the adaptive immune system to mood regulation: Mechanisms and pathways of neuroimmune interactions.

Clinical and basic studies of functional interactions between adaptive immunity, affective states, and brain function are reviewed, and the neural, humoral, and cellular routes of bidirectional communication between the brain and the adaptive immune system are evaluated. In clinical studies of depressed populations, lymphocytes-the principal cells of the adaptive immune system-exhibit altered T cell subtype ratios and CD4+ helper T cell polarization profiles. In basic studies using psychological stress to model depression, T cell profiles are altered as well, consistent with stress effects conveyed by the hypothalamic-pituitary-adrenal axis and sympathetic nervous system. Lymphocytes in turn have effects on behavior and CNS structure and function. CD4+ T cells in particular appear to modify affective behavior and rates of hippocampal dentate gyrus neurogenesis. These observations force the question of how such actions are carried out. CNS effects may occur via cellular and molecular mechanisms whereby effector memory T cells and the cytokine profiles they produce in the blood interact with the blood-brain barrier in ways that remain to be clarified. Understanding the mechanisms by which T cells polarize and interact with the brain to alter mood states is key to advances in the field, and may permit development of therapies that target cells in the periphery, thus bypassing problems associated with bioavailability of drugs within the brain.

Chronic social defeat reduces myelination in the mouse medial prefrontal cortex.

The medial prefrontal cortex (mPFC) plays a key role in top-down control of the brain's stress axis, and its structure and function are particularly vulnerable to stress effects, which can lead to depression in humans and depressive-like states in animals. We tested whether chronic social defeat produces structural alterations in the mPFC in mice. We first performed a microarray analysis of mPFC gene expression changes induced by defeat, and biological pathway analysis revealed a dominant pattern of down-regulation of myelin-associated genes. Indeed, 69% of the most significantly down-regulated genes were myelin-related. The down regulation was confirmed by in situ hybridization histochemistry for two strongly down-regulated genes, myelin oligodendrocyte glycoprotein (Mog) and ermin (Ermn), and by immunohistochemistry for myelin basic protein. To test for stress-induced changes in myelin integrity, aurophosphate (Black Gold) myelin staining was performed on mPFC sections. Quantitative stereologic analysis showed reduced myelinated fiber length and density. Behavioral analysis confirmed that the 14-day social defeat sessions resulted in induction of depressive-like states measured in social interaction and light/dark tests. The combined data suggest that chronic social defeat induces molecular changes that reduce myelination of the prefrontal cortex, which may be an underlying basis for stress-induced depressive states.

Toll-like receptor 4 on nonhematopoietic cells sustains CNS inflammation during endotoxemia, independent of systemic cytokines.

Inflammatory agonists such as lipopolysaccharide (LPS) induce robust systemic as well as CNS responses after peripheral administration. Responses in the innate immune system require triggering of toll-like receptor 4 (TLR4), but the origin of CNS sequelas has been controversial. We demonstrate expression of TLR4 transcripts in mouse brain in the meninges, ventricular ependyma, circumventricular organs, along the vasculature, and in parenchymal microglia. The contribution of TLR4 expressed in CNS resident versus hematopoietic cells to the development of CNS inflammation was examined using chimeric mice. Reciprocal bone marrow chimeras between wild-type and TLR4 mutant mice show that TLR4 on CNS resident cells is critically required for sustained inflammation in the brain after systemic LPS administration. Hematopoietic TLR4 alone supported the systemic release of acute phase cytokines, but transcription of proinflammatory genes in the CNS was reduced in duration. In contrast, TLR4 function in radiation-resistant cells was sufficient for inflammatory progression in the brains of chimeric mice, despite the striking absence of cytokine elevations in serum. Surprisingly, a temporal rise in serum corticosterone was also dependent on TLR4 signaling in nonhematopoietic cells. Our findings demonstrate a requirement for TLR4 function in CNS-resident cells, independent of systemic cytokine effects, for sustained CNS-specific inflammation and corticosterone rise during endotoxemia.

Therapeutic effects of stress-programmed lymphocytes transferred to chronically stressed mice.

Our group has recently provided novel insights into a poorly understood component of intercommunication between the brain and the immune system by showing that psychological stress can modify lymphocytes in a manner that may boost resilience to psychological stress. To demonstrate the influence of the adaptive immune system on mood states, we previously showed that cells from lymph nodes of socially defeated mice, but not from unstressed mice, conferred anxiolytic and antidepressant-like effects and elevated hippocampal cell proliferation when transferred into naïve lymphopenic Rag2(-/-) mice. In the present study, we asked whether similar transfer could be anxiolytic and antidepressant when done in animals that had been rendered anxious and depressed by chronic psychological stress. First, we demonstrated that lymphopenic Rag2(-/-) mice and their wild-type C57BL/6 mouse counterparts had similar levels of affect normally. Second, we found that following chronic (14days) restraint stress, both groups displayed an anxious and depressive-like phenotype and decreased hippocampal cell proliferation. Third, we showed that behavior in the open field test and light/dark box was normalized in the restraint-stressed Rag2(-/-) mice following adoptive transfer of lymph node cells from green fluorescent protein (GFP) expressing donor mice previously exposed to chronic (14days) of social defeat stress. Cells transferred from unstressed donor mice had no effect on behavior. Immunolabeling of GFP+ cells confirmed that tissue engraftment had occurred at 14days after transfer. We found GFP+ lymphocytes in the spleen, lymph nodes, blood, choroid plexus, and meninges of the recipient Rag2(-/-) mice. The findings suggest that the adaptive immune system may play a key role in promoting recovery from chronic stress. The data support using lymphocytes as a novel therapeutic target for anxiety states.