Role of pericytes in the immunopathogenesis of pneumococcal meningitis

肺炎球菌性脑膜炎（PM）是中枢神经系统（CNS）最严重的传染病之一。肺炎球菌中枢神经系统感染会产生大量炎症反应，可导致脑损伤，从而导致不利的疾病结果。实验工作证明，对于肺炎链球菌和血源性白细胞来说，毛细血管后静脉是进入脑脊液（CSF）的主要部位。毛细血管后小静脉由一层特化的内皮细胞、基膜和周细胞组成。血管被充满液体的空间包围，其中充满免疫能力细胞，如巨噬细胞或肥大细胞。最近的研究显示，肥大细胞缺乏或巨噬细胞耗竭在PM模型中没有或只有部分影响，这表明在肺炎球菌CSF感染时，存在于血管周围生态位的额外细胞对免疫激活有贡献。在初步实验中，我们观察到脑周细胞通过改变细胞因子释放来响应肺炎链球菌的攻击。通过使用抑制策略，我们发现周细胞通过toll样受体识别肺炎链球菌存在的证据。此外，小鼠脑切片的免疫组织化学研究表明，在PM过程中，既定周细胞标记物的染色模式发生了实质性变化。基于这些数据，我们假设周细胞可能是PM中高炎症反应和相关组织损伤的调节因子。首先，我们计划在体外鉴定周细胞在肺炎球菌感染中的功能作用。我们将更详细地比较暴露于不同血清型肺炎链球菌的小鼠初级脑周细胞与人类脑周细胞的反应性。由于周细胞被认为是多能的，我们也将评估周细胞在肺炎球菌攻击时是否采用巨噬细胞样表型。我们将进一步研究周细胞是否能够通过接触和非接触共培养系统调节巨噬细胞对肺炎球菌刺激的反应。下一步，我们计划在小鼠模型中确定周细胞对PM临床病程的功能重要性。首先，我们将使用pdgfrβ - egfp转基因报告小鼠，用周细胞和内皮标记物进行双重免疫染色，以及两种技术的结合来评估脑血管周细胞的覆盖范围。为了更深入地了解它们在PM中的功能作用，我们将通过使用三重策略来评估周细胞消耗对疾病表型的影响：鞘内给药PDGFR抑制剂伊马替尼，使用转基因PDGFR β -tk小鼠（用更昔洛韦内处理），以及使用PDGF-B保留基序敲除（Pdgfbetaret/ret）。我们认为，该研究项目将显著提高我们对中枢神经系统免疫调节机制的认识。

Pneumococcal meningitis (PM) is one of the most serious infectious diseases of the central nervous system (CNS). Pneumococcal CNS infection generates a massive inflammatory reaction which can cause brain damage and thus contributes to unfavorable disease outcome. Experimental work has provided evidence that postcapillary venules are the primary site of entry into the cerebrospinal fluid (CSF) for both, S. pneumoniae and blood-borne leukocytes. Postcapillary venules consist of a layer of specialized endothelial cells, a basement membrane, and pericytes. The vessels are surrounded by a fluid-filled space populated by immunocompetent cells, such as macrophages or mast cells. Recent studies showed no or only partial effects of mast cell deficiency or macrophage depletion in PM models, suggesting contribution of additional cells residing in the perivascular niche to immune activation upon pneumococcal CSF infection. In pilot experiments, we observed that brain pericytes responded to S. pneumoniae challenge by altered cytokine release. By using an inhibition strategy, we found evidence that pericytes recognize the presence of S. pneumoniae by means of Toll-like receptors. Moreover, immunohistochemical investigations on murine brain sections suggested substantial changes in the staining pattern for established pericyte markers during the course of PM. Based on these data, we hypothesize that pericytes could be regulators of the hyper-inflammatory reaction and related tissue damage in PM. In the first place, we plan to characterize the functional role of pericytes in pneumococcal infection in vitro. We will compare the responsiveness of primary murine brain pericytes upon exposure to different serotypes of S. pneumoniae with that of human brain pericytes in more detail. Since pericytes have been suggested to be multipotent, we will also evaluate whether pericytes adopt a macrophage-like phenotype upon pneumococcal challenge. We will further investigate whether pericytes are capable of modulating the response of macrophages to pneumococcal stimulation using contact and non-contact co-culture systems. As a next step, we plan to determine the functional importance of pericytes for the clinical course of PM in a mouse model. First, we will assess pericyte coverage of brain vessels using PDGFRbeta-eGFP transgenic reporter mice, double immunostaining with pericyte and endothelial markers, as well as a combination of both techniques. To get more insight into their functional role in PM, we will then evaluate the impact of pericyte depletion on the disease phenotype by using a triple strategy: by intrathecal administration of the PDGFR inhibitor imatinib, by using transgenic PDGFRbeta-tk mice (treated intracisternally with ganciclovir), and by using PDGF-B retention motif knockouts (Pdgfbetaret/ret). In our opinion, this research project will markedly improve our knowledge about mechanisms of immunoregulation within the CNS.