Conventional B-2 cell-derived plasma cells are surface Ig negative, CD19low/negative and express high levels of the plasma cell marker find more CD138 and slightly higher level of CD43 than B-1 cells (Fig. 5 and 40). BM B-1 cells, >80% of which spontaneously secrete IgM in vitro (Fig. 4C), are surface IgM+IgDlow/negative, and express relatively high levels of CD19 but are CD138− (Figs. 2, 3, 5). Our data are consistent with earlier reports on the phenotype of IgM-secreting B cells 25, 33 and through the use of the allotype chimeras we now identify these BM cells

unequivocally as B-1 cells (Figs. 3 and 4). In addition, as we show here (Figs. 1 and 4), these cells produce antibodies that recognize influenza virus, a specificity we have previously linked to B-1 cell-derived antibodies 5, 26, 27. Staining with antibodies recognizing a B-1a cell-specific Ig-idiotype (T-15) binding to phosphorylcholine, as well as staining with phosphatidylcholine-containing liposomes identified small numbers of BM B-1 cells (data not shown), further confirming their similarity to known B-1 cells with regard to specificity. Notably, these Opaganib mouse cells are distinct from the IgMloIgDhi

sinusoidal BM B cells, which were described recently as rapid IgM secreters following challenge with blood-borne T-independent antigens 42. FACS-sorting experiments did learn more not reveal significant spontaneous IgM secretion among IgMloIgDhi B cells in our

non-challenged mice (Fig. 2). In contrast to BM and spleen, PerC B-1 cells from BALB/c mice or from allotype chimeras were not significant sources of spontaneous IgM secretion (Figs. 1 and 4). Thus, our data are consistent with several in vivo studies that indicated the inability of PerC B-1 cells to produce natural IgM 33, 36, 37. It is remarkable, however, that PerC B-1 cells secrete or shed small amounts of IgM, resulting in large numbers of pinhead-size ELISPOTs (Fig. 1), also noted by others 31, 32, without significant amounts of secreted product amassing in the culture supernatants (Figs. 1 and 3). Such “leakiness” of B cells was not noted for cells harvested from any other tissue, for example the PLNs (Fig. 1). This might explain the apparent discrepancies in the literature regarding IgM secretion by PerC B-1 cells 31–37. Counting of these very small dots by PerC B-1 cells, might lead to an over-estimation of the ability of these cells to secrete significant amounts of natural IgM.