Project 3: Regulation of atheroprotective IgM - producing B cells in murine and human atherosclerosis

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Project 3 Summary We and others have extensively studied B-1 cells in mice, clearly showing that these cells and the IgM to oxidation-specific epitopes (OSE) on LDL (IgMOSE) that they produce are anti- inflammatory and block atherosclerosis (AS) development. Yet, despite a wealth of evidence that plasma levels of IgM to the OSE, malondialdehyde-modified LDL (IgMMDA-LDL) in humans are associated with less CAD and fewer CV events, identification of the IgMMDA-LDL producing human B cell has remained elusive. Our novel data from the previous cycle, utilizing the well characterized CAVA cohort and high dimensional phenotyping of circulating B cells, are the first to identify human B cell subtypes that produce IgMMDA-LDL, and demonstrate that they are indeed associated with high plasma levels of IgMMDA-LDL and inversely correlated with CAD severity in humans. One of these subtypes (B27+IgM+24hi cells) trafficked to the spleen and could be stimulated by MDA antigen to produce higher levels of IgMMDA-LDL in vivo. Our findings revealed the novel discovery that the sialogycoprotein, CD24, regulated expression of the chemokine receptor CCR6 and splenic localization of these cells. CD24, a GPI-anchored sialoglycoprotein that resides in lipid rafts and regulates signaling of the IgM BCR, has not previously been implicated in murine or human AS. Utilizing our antibody and transcriptome sequencing core (Core B), we discovered that B27+IgM+24hi cells express more CCR6 and IgM specifically in subjects with low compared to high CAD and we identified signaling molecules downstream of CCR6 and the IgM BCR associated with CAD severity. Accordingly, we hypothesize that humans with severe CAD and rapid CAD progression have fewer B27+IgM+24hi cells and that CD24 acts as a rheostat promoting key surface receptor function and signaling pathways in B27+IgM+ cells leading to enhanced production of IgMOSE and atheroprotection. We will utilize novel clinical cohorts (aim 1) and loss- and gain- of function approaches in vitro and in vivo (aim 2) to define the mechanisms whereby CD24 regulates the B27+IgM+ phenotype with particular emphasis on CCR6, the BCR, key signaling events and the IgM repertoire in atherosclerosis.