BackgroundDepression is a heterogeneous disorder, with the exact neuronal mechanisms causing the disease yet to be discovered. Recent work suggests it is accompanied by neuro-inflammation, characterized, in particular, by microglial activation. However, microglial activation and its involvement in neuro-inflammation and stress-related depressive disorders are far from understood.MethodsWe utilized multiple detection methods to detect the neuro-inflammation in the hippocampus of rats after exposure to chronic mild stress (CMS). Male Sprague Dawley (SD) rats were subjected to chronic mild stressors for 12 weeks. Microglial activation and hippocampal neuro-inflammation were detected by using a combinatory approach of in vivo [18F] DPA-714 positron emission computed tomography (PET) imaging, ionized calcium-binding adapter molecule 1 and translocator protein (TSPO) immunohistochemistry, and detection of NOD-like receptor protein 3 (NLRP3) inflammasome and some inflammatory mediators. Then, the rats were treated with minocycline during the last 4 weeks to observe its effect on hippocampal neuro-inflammation and depressive-like behavior induced by chronic mild stress.ResultsThe results show that 12 weeks of chronic mild stress induced remarkable depressive- and anxiety-like behavior, simultaneously causing hippocampal microglial activation detected by PET, immunofluorescence staining, and western blotting. Likewise, activation of NLRP3 inflammasome and upregulation of inflammatory mediators, such as interleukin-1β (IL-1β), IL-6, and IL-18, were also observed in the hippocampus after exposure to chronic stress. Interestingly, the anti-inflammatory mediators, such as IL-4 and IL-10, were also increased in the hippocampus following chronic mild stress, which may hint that chronic stress activates different types of microglia, which produce pro-inflammatory cytokines or anti-inflammatory cytokines. Furthermore, chronic minocycline treatment alleviated the depressive-like behavior induced by chronic stress and significantly inhibited microglial activation. Similarly, the activation of NLRP3 inflammasome and the increase of inflammatory mediators were not exhibited or significantly less marked in the minocycline treatment group.ConclusionThese results together indicate that microglial activation mediates the chronic mild stress-induced depressive- and anxiety-like behavior and hippocampal neuro-inflammation.

BackgroundIn recent years, proinflammatory cytokine interleukin-1β (IL-1β) was considered to play a critical role in the pathogenesis of depression. In addition, P2X7 receptor (P2X7R), a member of the purinergic receptor family, which is predominantly present on microglia, as well as on astrocytes and neurons in lesser amounts in the central nervous system, was suggested to be involved in the processing and releasing of IL-1β. Here, we investigated the role of P2X7R in the pathogenesis of depression.MethodsMale Sprague-Dawley rats were subjected to chronic unpredictable stressors (CUS) for 3 weeks. At the end of week 1, 2, and 3, extracellular ATP, caspase 1, IL-1β, and components and activation of NLRP3 inflammasome (nucleotide-binding, leucine-rich repeat, pyrin domain containing 3) were evaluated as biomarker of neuroinflammation. In separate experiments, the rats were microinjected with P2X7R agonists ATP, BzATP, and saline into the hippocampus, respectively, or exposed to CUS combined with hippocampal microinjection with P2X7R antagonist, BBG and A438079, and saline, respectively, for 3 weeks, followed by exposed to forced swimming test and open-field test. Moreover, we also evaluated the depressive and anxiety-like behavior of P2X7-null mice in forced swimming test, open-field test, and elevated plus maze.ResultsAlong with stress accumulation, extracellular ATP, cleaved-caspase 1, IL-1β, and ASC were significantly enhanced in the hippocampus, but P2X7R and NLRP3 were not. Immunoprecipitation assay indicated that along with the accumulation of stress, assembly of NLRP3 inflammasome and cleaved caspase 1 in NLRP3 inflammasome were significantly increased. Moreover, antagonists of P2X7R, either BBG or A438079, prevented the development of depressive-like behaviors induced by chronic unpredictable stress in rats. Meanwhile, we could not observe any depressive-like or anxiety-like behaviors of P2X7-null mice after they had been exposed to CUS. The results implied that P2X7 knockout could impede the development of depressive-like and anxiety-like behaviors induced by CUS. In contrast, chronic administration of agonists of P2X7R, either ATP or BzATP, could induce depressive-like behaviors.ConclusionsThe activation of P2X7R and subsequent NLRP3 inflammasome in hippocampal microglial cells could mediate depressive-like behaviors, which suggests a new therapeutic target for the prevention and treatment of depression.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-017-0865-y) contains supplementary material, which is available to authorized users.

IntroductionImmune thrombocytopenic purpura (ITP) is a bleeding disorder characterized by production of autoreactive antibodies to platelet antigens, resulting in both accelerated destruction of platelets and reduced platelet production. 1 While healthy individuals harbor platelet-specific autoreactive T cells that are tolerized in the periphery, 2 patients with ITP possess activated platelet autoreactive T cells and cytokine imbalance, 3-7 suggesting loss of peripheral tolerance in ITP patients. CD4 ϩ regulatory T cells (Tregs) play an important role in maintenance of peripheral tolerance and are characterized by the expression of the CD25 surface marker and the transcription factor forkhead box protein 3 (Foxp3), making up 5% to 10% of the normal CD4 ϩ T-cell population. 8 Different populations of Tregs have been described, including naturally occurring and inducible Tregs. 9 The former are thymically derived and suppress general autoreactive responses under noninflammatory conditions, although they can also become activated and expand in an antigen-specific manner. 10 Inducible Tregs are generated in the periphery through exposure to antigen, but once activated are thought to mediate suppressive activity against other antigens by the local release of specific cytokines. 11 Several reports have demonstrated Treg alterations in a number of autoimmune diseases. [12][13][14][15][16] These reports suggest that circulating Treg frequency and/or function may be used as a marker for evaluating autoimmune status in patients. Recent studies in patients with ITP have shown reduced levels of Foxp3 mRNA 17 and protein 18 in circulating mononuclear cells and abnormal Treg function in spleen biopsies. 19 These studies indicate that deficiency in generation and/or defective functions of Tregs may contribute to loss of immunologic self-tolerance in patients with ITP. To test the hypothesis that the pathogenesis of chronic ITP may be related to the levels or function of circulating peripheral Tregs, we examined the frequency of Tregs in peripheral blood mononuclear cells (PBMCs) from patients with chronic ITP by flow cytometry and performed in vitro assays to assess the immunosuppressive effect of Tregs on CD4 ϩ T-cell proliferation. Methods SubjectsWe enrolled 17 patients with chronic refractory ITP (Table 1) and 16 age-matched and closely age-matched healthy donors in this study, and informed consent was obtained in accordance with the Declaration of Helsinki. The study was approved by the Institutional Review Boards of the Weill Medical College of Cornell University and of the New York Blood Center (NYBC). Cell staining and purificationWithin 2 hours of collection, whole blood was stained with anti-CD4 and anti-CD25 (both from BD Pharmingen, San Diego, CA) followed by Foxp3 staining (clone PCH101; eBioscience, San Diego, CA) according to the manufacturer's instructions and analyzed by flow cytometry (FACSCanto cytometer with FACSDiva software; BD Biosciences, San Jose, CA). Due to the lack of a Treg cell-specific surface ma...

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