Microbial sensing and innate immune activation in B cells

点击翻译按钮获取中文摘要

B-cells are mostly known for their capacity to produce antibodies. However, an increasing body of literature has now shown that they may also participate in the immune response by various antibody-independent mechanisms, such as cytokine production, co-stimulation, and antigen presentation. B-cells can be divided in three different subpopulations: follicular B cells , innate-like marginal zone B-cells (MZB), and B1 B-cells. Follicular B cells are the most prominent B-cell subpopulation and reside in lymphoid follicles of secondary and tertiary lymphoid organs. MZB are located in the marginal zone of the spleen and are endowed with the capacity to bind immune complexes, migrate towards the splenic white pulp, and transfer antigen to follicular dendritic cells. B1 B-cells are known for their secretion of natural antibodies. We have recently reported that B cells can capture the protozoan parasite Leishmania donovani. Upon exposure to the parasite, B cells form clusters and hold L. donovani in extracellular IgM-rich pockets. This close interaction results in polyclonal B cell activation and eventually in cell death after 24-48h. Interestingly, the parasite triggers endosomal Toll-Like Receptors (TLRs), although it is attached extracellularly on the cell surface of the B cell. Activation of endosomal TLRs was required to induce IL-10 and IFN-I expression and to promote hypergammaglobulinemia. The pathways up- and downstream of endosomal TLR activation in B-cells and the contribution of these pathways to the induction of hypergammaglobulinemia are as yet unknown. Definition of these steps will require a deeper understanding of how B cells interact with the parasite. The overall goal of our research is to understand how various B cell subpopulations recognize and react to microorganisms, and how this recognition affects their function. Our objective in this application is to characterize the interaction between L. donovani and B cells in order to understand pathways leading to polyclonal B cell activation. Particularly, we will investigate i) parasite capture and cell surface interaction between L. donovani and various B cell subpopulations; ii) parasite recognition by endosomal toll-like receptors and other microbial sensors (mainly cytosolic) and the downstream pathways they activate; and iii) cell-to-cell communication (e.g. tunnelling nanotubules; receptor/ligand interactions) and role of this communication in the induction of polyclonal B cell activation. Transgenic parasites, knock-out mice, confocal microscopy, flow cytometry, and several biochemical techniques will be used to dissect the various activation and cellular communication pathways in in vitro experiments. The pathways identified here could uncover novel strategies that may also be used by other microorganisms to activate B cells and modulate their functions.