Molecular and functional analysis of human splenic marginal zone B cells

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In mice, the B lymphocyte pool is divided into three main B cell lineages, namely B-1, follicular (B-2) and marginal zone (MZ) B cells. MZ B cells populate specialized histological structures like the splenic marginal zone (sMZ), fulfill an important role in immune protection from blood-borne pathogens and participate in apoptotic cell clearance. sMZ B cells respond fast and efficiently to innate stimulation, however, are also capable of germinal center (GC) maturation and production of high affinity antibodies. Based on this combination of innate and adaptive propensities, sMZ B cells are thought to function as lymphocytes that cross over the boundaries between innate and adaptive immune system. The current view on the molecular mechanisms underlying the functional versatility of sMZ B cells is primarily based on mouse models, showing that sMZ B cells require a distinct genetic program for development and function. Moreover, their histological presentation, priming by innate cells and a restricted B cell receptor (BCR) repertoire are mandatory. In contrast to mouse models, the generation and function of human sMZ B cells are poorly understood. Human sMZ B cells show a high similarity to memory B cells and hence, it is currently debated whether these cells are GC-experienced or represent a distinct lineage as in mice or if human sMZ B cells are composed of more than one subset. In line with their unresolved development, the molecular mechanisms guiding human sMZ B cell functions are largely unexplored.Last year, we collected a cohort of > 80 human spleen biopsies of different ages. Our preliminary results on single cell transcription profiling of human sMZ B cells support the idea that they are composed of distinct B cell subsets and importantly, this picture changes with age. In this proposal, we aim at deepening this analysis to reveal a putative molecular or functional distinctness among these subsets. In addition, we plan to follow up our functional characterization of infant and adult human sMZ B cells in vitro, including the analysis of BCR-repertoires, the specificity for encapsulated bacteria strains and the responsiveness of sMZ B cells towards innate versus adaptive stimulation. Using our established methods to measure cytokine secretion, migration and cell adhesion, we aim at identifying the signals that govern sMZ homing and retention. Moreover, we want to deepen our preliminary findings that suggest a close interaction of activated sMZ B cells with red-pulp-macrophages and TH1 cells. Finally, we plan to quantify clonally related BCR-rearrangements among our collection of paired splenic and peripheral blood (PB) B cell samples to determine, how much the human sMZ and PB IgM memory B cell pools are connected.