Immunoregulatory role of mast cells in infections of the cerebrospinal fluid compartment: Investigations on a mouse model of pneumococcal meningitis

细菌性脑膜炎是全球感染相关死亡的前十大原因之一。欧洲最常见的病原体是肺炎链球菌。肺炎球菌感染充满脑脊液(CSF)的蛛网膜下腔，会产生医学上已知的最强大的炎症反应之一。尽管这种反应背后的分子机制在很大程度上已经被描述出来，但对于脑膜炎症的细胞启动者仍然存在不确定性。在旨在第一次深入了解肥大细胞在肺炎球菌脑膜炎中的作用的试点实验中，我们观察到肥大细胞缺陷W/W-v小鼠的脑脊液细胞吞噬能力意外显著增加，这是通过肥大细胞重组可逆的。对脑细胞因子表达的分析显示，白介素1的水平也有平行的增加。用IL-1受体拮抗剂治疗W/W-v小鼠后，脑脊液白细胞计数恢复到野生型小鼠的水平。这些数据表明，在肺炎球菌脑膜炎中，肥大细胞可能通过控制IL-1的生物活性来限制中性粒细胞炎症。然而，脑膜肥大细胞限制IL-1？水平的机制(S)尚需阐明(S)。本研究项目的目的是明确肥大细胞在肺炎球菌脑膜炎中的功能。首先，我们计划评估另一种肥大细胞缺陷小鼠品系W-sh小鼠的表型，这种小鼠患有肺炎球菌脑膜炎。我们预计W-sh小鼠将表现出W/W-v小鼠的疾病表型，这进一步证明该表型是由于肥大细胞缺乏而不是其他与小鼠品系相关的因素所致。此外，这将使我们能够在进一步的实验中使用W-sh小鼠。W-sh小鼠是可生育的，并且基于经过充分研究的C57BL/6遗传背景，这有助于使用转基因小鼠品系进行计划中的肥大细胞重建实验。下一步，我们计划确定肥大细胞在肺炎球菌脑膜炎中抗炎活性的机制，从而集中于肥大细胞衍生的抗炎细胞因子(如IL-10)和丝氨酸肽酶(如类胰蛋白酶或糜酶)的作用，后者最近被证明能够将几种细胞因子降解为非活性分子。最后，我们计划研究培养的肥大细胞对肺炎球菌攻击的反应。在此过程中，骨髓来源的肥大细胞应在模拟脑脊液填充的蛛网膜下腔特征的条件下培养。这些细胞计划用于研究[i]它们对肺炎球菌诱导的小鼠巨噬细胞释放IL-1？的影响，以及[ii]它们降解成熟IL-1？的潜力。在我们看来，这一研究项目将显著提高我们对中枢神经系统内免疫调节机制的了解。

Bacterial meningitis is among the top 10 causes of infection-related deaths worldwide. The most frequent etiologic agent in Europe is Streptococcus pneumoniae. Pneumococcal infection of the cerebrospinal fluid (CSF)-filled subarachnoid space generates one of the most powerful inflammatory responses known in medicine. Although the molecular mechanisms underlying this response have been largely characterized, there is still uncertainty about the cellular initiators of meningeal inflammation. In pilot experiments aimed at getting first insight into the role of mast cells in pneumococcal meningitis, we observed an unexpected, marked increase in CSF pleocytosis in mast cell deficient W/W-v mice which was reversible by mast cell reconstitution. Analyses of brain cytokine expression revealed a parallel increase in interleukin (IL)-1ß levels. Treatment of W/W-v mice with an IL-1 receptor antagonist restored CSF leukocyte counts to levels seen in wild type mice. These data suggest that, in pneumococcal meningitis, mast cells may limit neutrophilic inflammation through control of IL-1ß bioactivity. The mechanism(s) underlying the restriction of IL-1ß levels by meningeal mast cells, however, need(s) to be clarified. The aim of this research project is to definitively characterize the function of mast cells in pneumococcal meningitis. In the first place, we plan to evaluate the phenotype of an additional mast cell-deficient mouse strain, the W-sh mice, with pneumococcal meningitis. We expect that W-sh mice will display the disease phenotype of W/W-v mice, providing further evidence that the phenotype is due to mast cell deficiency rather than other mouse strain-related factors. Moreover, it would enable us to use W-sh mice in further experiments. W-sh mice are fertile and on the well-studied C57BL/6 genetic background, facilitating the use of transgenic mouse lines for the planned mast cell reconstitution experiments. In the next step, we plan to identify the mechanisms responsible for the anti-inflammatory activity of mast cells in pneumococcal meningitis, thereby focussing on the role of mast cell-derived anti-inflammatory cytokines (e.g., IL-10) and serine peptidases (like tryptase or chymase), the latter of which have recently been shown to be able to degrade several cytokines into inactive molecules. Finally, we plan to investigate the response of cultured mast cells upon pneumococcal challenge. In doing so, bone marrow-derived mast cells shall be cultured under conditions mimicking features of the CSF-filled subarachnoid space. These cells are planned to be used for investigations of [i] their impact on the pneumococci-induced IL-1ß release by murine macrophages, as well as [ii] their potential to degrade mature IL-1ß. In our opinion, this research project will markedly improve our knowledge about mechanisms of immunoregulation within the central nervous system.