B-1 cells in the bone marrow are a significant source of natural IgM

Natural IgM antibodies secreted in the absence of antigenic challenge are important contributors to antimicrobial immunity and tissue homeostasis. Early studies identified BM and, to a lesser extent the spleen, as main tissue sources of this spontaneously secreted IgM. However, the responsible B-cell subset has never been identified. Using multicolor flow cytometry, cell sorting and chimeric mice in which B-1 and B-2 cells and their secreted antibodies are distinguished by their Ig-allotype, we unequivocally identify the natural IgM-secreting cells in spleen and, for the first time, in the BM as IgM ⁺ IgD ^lo/-CD19 ^hi CD43 ⁺ CD5 ^+/- B-1 cells. The newly identified population of BM B-1 cells shows many of the phenotypic characteristics of splenic B-1 cells but is distinct from B-1 cells in the peritoneal cavity, which generate at best very small amounts of IgM. Antibody-secreting spleen and BM B-1 cells are distinct also from terminally differentiated plasma cells generated from antigen-induced conventional B cells, as they express high levels of surface IgM and CD19 and lack expression of CD138. Overall, these data identify populations of non-terminally differentiated B-1 cells in spleen and BM as the most significant producers of natural IgM.