CXCR4 maintains a unique bone marrow B-1a population and regulates atheroprotective IgM production

Atherosclerosis is a chronic inflammatory disease of the artery wall that leads to the development of cardiovascular disease (CVD) and its sequelae, which remain the leading cause of death worldwide. The development of CVD varies significantly among individuals with similar traditional risk factors, suggesting that CVD could be further prevented by additional strategies. The completion of the Canakinumab anti-inflammatory thrombosis outcome study (CANTOS) has given validation to the inflammatory hypothesis that reducing inflammation reduces the risk of CVD independent of lipid-lowering therapies, and immunomodulatory therapy holds promise for improving CVD prevention.
B cells have emerged as important immune cells that regulate atherosclerosis in a subset-specific manner. B-1 cells have a known atheroprotective function that is mediated by production of anti-inflammatory IgM antibodies that recognize oxidation specific epitopes (OSE), which accumulate in atherosclerosis. Bone marrow B-1 cells contribute significantly to circulating IgM titers at homeostasis, yet the mechanisms regulating B-1 cell localization to the bone marrow are undetermined. Moreover, factors that regulate anti-OSE IgM production by the putative human B-1 cell subset, and its atheroprotective role in human CVD remain unexplored.
The C-X-C motif chemokine receptor CXCR4 regulates cell trafficking and localization and is broadly expressed on both immune and non-immune cell populations. Genome-wide association studies and murine experimental models have implicated CXCR4 in CVD, yet results have been conflicting due to its broad expression and the relative lack of cell type-specific studies.
In this dissertation, I present evidence demonstrating a role for CXCR4 in regulating B-1 cell production of atheroprotective IgM antibody, and characterize a deeper heterogeneity present in human and murine B-1 cell populations. I present my work in murine models showing that CXCR4 regulated B-1 cell localization to the bone marrow and circulating amount of anti-OSE IgM, and in a human CVD cohort that B-1 CXCR4 expression associated with reduced coronary artery plaque burden and a protected plaque phenotype. Murine studies utilized a B cell-specific CXCR4 knockout mouse model, which revealed fewer B-1a cells and IgM antibody-secreting cells in the bone marrow in the absence of CXCR4. Conversely, retroviral-mediated overexpression of CXCR4 in vivo increased B-1a localization to the bone marrow and associated with increased circulating anti-OSE IgM. Mechanistically, we found that CXCR4 mediated trafficking of circulating peripheral B-1a cells to the bone marrow and regulated bone marrow B-1a cell survival. I additionally examine niche-specific heterogeneity in murine B-1 cell populations, and provide novel characterization of the immunoglobulin repertoire of aged hyperlipidemic ApoE-/- mice, which reveals greater IgM repertoire diversity expressed by bone marrow B-1a cells compared to peritoneal B-1a cells. Finally, analysis of a human CVD cohort demonstrates that CXCR4 expression on the putative human B-1 subset associates with increased circulating anti-OSE IgM antibodies and decreased coronary artery disease. Thus, the evidence presented here supports the hypothesis that the bone marrow B-1a pool uniquely contributes to the pool of available atheroprotective IgM antibodies, and its maintenance is governed by CXCR4, a novel marker associating with protection in human CVD.