BM B-1 cells also lacked expression of CD138, a marker of terminal differentiated

plasma cells (Fig. 5A and data not shown). While BM B-1 cells were roughly comparable in size to conventional plasma cells by FSC (Fig. 5A) and Giemsa staining (Fig. 5B), their cytoplasm content was smaller than that seen for plasma cells, but larger than that of the resting B-2 cells. Together with the expression EMD 1214063 of surface IgM (Figs. 2–4), the data indicate that BM B-1 cells are at a differentiation state distinct from that of antigen-induced plasma cells. Taken together, we have identified a population of natural IgM-secreting B-1 cells that are responsible for spontaneous IgM secretion in the BM in steady-state and that resemble most closely B-2 cell-derived pre-plasmablasts 47. Natural antibody production is controlled by poorly understood mechanisms that maintain serum antibody-titers even during or following antigenic challenge 5, 26. In humans and in mice these antibodies are produced mainly by B-1 cells 25, 28, 30. Whether natural antibody secretion is a property of all B-1 cells, or of only a subset is the current subject of debate

29–34, 36–38, 48. Our study identifies a distinct population of natural IgM-secreting B-1 cells responsible for spontaneous IgM secretion in steady-state BM (Fig. 4). BM B-1 cells are shown here to be phenotypically and functionally similar to IgM-secreting B-1 cells in the spleen, but distinct from the non/little IgM-secreting PerC B-1 cells Epacadostat (Figs. 2 and 3). Their phenotypic profiles make them distinct also from terminally differentiated conventional plasma cells (Fig. 5), and overall indicate that these cells are at an intermediate step of differentiation. The fact that the BM B-1 cells did not express phenotypic markers of terminal

differentiation (Figs. 3 and 5) is consistent with the known ability of peritoneal cavity and spleen B-1 cells to self-replenish, i.e. to slowly proliferate 25. It remains to be determined whether IgM-secreting B-1 cells are turning over like their counterparts in these other tissues, whether they are replenished from non-secreting cells in the peritoneal or pleural cavities and/or other sites, or whether they are long-lived, like BM plasma cells generated in germinal centers from the conventional B cells following antigen C-X-C chemokine receptor type 7 (CXCR-7) encounter 49, 50. It is well established that the BM is a major tissue of residence for long-lived antibody-secreting plasma cells 49, 50. Stromal cells support the survival of plasma cells in the BM and the tissue architecture allows the direct deposition of secreted antibodies into the blood stream 51, 52. Given these features, the BM is also an ideal location for natural IgM-secreting B-1 cells. The red pulp of the spleen, the other tissue in which spontaneous-IgM-secreting B-1 cells are found (Figs. 1 and 2 34), is reported to have many of the same features and is known to support B-1 and B-2 cell-derived plasma cells 38, 53.