Regulation of B-1 lymphocyte trafficking through the C5a/C5aR1 axis

在过去的几年里，关于补体系统的知识显著增加。令人惊讶的发现和临床突破突破了补体系统知识的经典“教科书”的界限。传统上，人们认为补体的主要来源是血液循环；目前，局部细胞补体来源越来越受到人们的关注。因此，补体以前未被认识到的功能变得越来越重要。在这项提议中，我们想要描述免疫细胞网络中C5a的细胞调节功能，并明确这种功能的机制，特别关注B-1 B淋巴细胞。B-1 B淋巴细胞介导针对几种类型感染的广泛和组成性抗体免疫反应。我的小组产生的初步和已发表的数据表明，补体成分C5a的受体C5a受体1 （C5aR1）在B-1淋巴细胞生物学中具有重要的新作用。我们首次发现C5aR1在B-1淋巴细胞上表达，并且该受体的缺乏在几个层面上对B-1淋巴细胞生物学有功能上的影响。在稳态条件下，c5ar1缺陷小鼠的腹膜腔（PeC）中B-1淋巴细胞减少，主要是由于CXCL13水平降低。体外实验显示，这很可能是由于C5aR1与TLR2和IL-10R一起驱动腹腔巨噬细胞产生CXCL13的新发现途径受到干扰。与此同时，C5aR1-/-小鼠脾脏中B-1淋巴细胞数量增加，这伴随着个体免疫球蛋白（Ig） M特异性（称为天然(n) IgM抗体）的增加。由于它们是组成性产生的，因此nIgM和B-1淋巴细胞被认为是先天免疫的重要组成部分，它们与补体系统一起作为抵抗感染因子的第一道防线。此外，nIgM通过介导氧化应激或代谢过程中蛋白质修饰产生的凋亡细胞、错误折叠蛋白和自身抗原的清除，发挥重要的内源性功能。在炎症条件下，C5aR1似乎是B-1淋巴细胞迁移到远处组织（如脾脏）所必需的，目前的出版物尚未充分讨论这一主题。在这个项目中，我们的目标是确定C5aR1在单个B-1淋巴细胞亚群的发育、功能和迁移中的作用。此外，我们将确定C5a/C5aR1轴如何调节B-1淋巴细胞nIgM的产生，以及它是否通过调节BCR活性来调节B-1淋巴细胞的功能。最后，我们将在B-1淋巴细胞依赖的人类疾病动物模型中测试我们的发现的生物学相关性。

In the past years the knowledge about the complement system increased significantly. Surprising discoveries and clinical breakthroughs pushed the boundaries of classical “text book”-knowledge of the complement system. Classically, it was thought that the main source of complement is within the circulation; nowadays local cellular complement sources are getting more attention. Therefore, previously unappreciated functions of complement are gaining more importance. In this proposal we want to delineate the cell-regulatory functions of C5a within the immune cell network and define the mechanisms underlying such functions with a specific focus on B-1 B lymphocytes. B-1 B lymphocytes mediate broad and constitutive antibody-based immune responses against several types of infections. Preliminary and published data generated in my group show important novel roles for the receptor for the complement component C5a, the C5a receptor 1 (C5aR1), in B-1 lymphocyte biology. We showed for the first time C5aR1 expression on B-1 lymphocytes and that the lack of this receptor has functional consequences for the B-1 lymphocyte biology on several levels. Under homeostatic conditions, C5aR1-deficient mice show less B-1 lymphocytes in their main compartment, the peritoneal cavity (PeC), mainly due to decreased levels of CXCL13. In vitro experiments revealed that this is most likely due to a disturbance of a newly discovered pathway in which C5aR1 drives, together with TLR2 and IL-10R, CXCL13 production in peritoneal macrophages. At the same time C5aR1-/- mice show increased numbers of B-1 lymphocytes in the spleen, which is accompanied with elevated amounts of individual immunoglobulin (Ig) M specificities known as natural (n) IgM antibodies. Since they are constitutively produced, nIgM and B-1 lymphocytes are considered as an important part of innate immunity that, together with the complement system, act as first line of defense against infectious agents. Moreover, nIgM performs essential housekeeping functions by mediating the clearance of apoptotic cells, misfolded proteins and autoantigens generated under oxidative stress or due to protein modification in the course of metabolic processes. Under inflammatory conditions C5aR1 seems to be necessary for B-1 lymphocyte migration to remote tissues like the spleen, a topic not yet adequately addressed in current publications. In this project, we aim to define the role of C5aR1 in the development, function and migration of individual B-1 lymphocyte subsets. Moreover, we will determine, how the C5a/C5aR1 axis regulates B-1 lymphocyte nIgM production and whether it modulates B-1 lymphocyte functions by tuning BCR activity. Finally, we will test the biological relevance of our findings in B-1 lymphocyte-dependent animal models of human diseases.