Figure 6. B-1 cells are responsible for increased natural IgM secretion in Fcmr ^flx/flxCd19-Cre mice.

(a) 5% contour plots with outliers gated for live CD19⁺ spleen B cells from control and Fcmr ^flx/flxCd19-Cre mice. B-1 cells were identified as: CD43⁺ IgM^hi CD19^hi and CD5^+/− (B-1a/B-1b). Graph summarizes numbers of B-1 cells, including B-1a and B-1b cells in spleen, (b) peritoneal cavity and (c) BM (n=4 mice/group). (d–g) Generation of chimeras with Fcmr^−/− B-1 (IgH^b) and wild-type B-2 (IgH^a) cells and controls (n=4 mice/group). (d) (left) 5% contour plots with outliers gated for lived CD19⁺ B cells from peritoneal cavities and spleens. For BM, cells were pre-gated for B-1 cells (CD19⁺ IgM⁺ CD43⁺). The transferred B-1 cells were identified as IgM^b+ IgM^a−. Graph provides summary of IgM^b B-1 cell frequencies in indicated tissues. (e) 5% contour plots with outliers gated on IgM^b+ B-1 cells in spleen. Gates and numbers identify frequencies of CD138⁺ of IgM^b+ B-1 cells. FMO, fluorescence minus one control. (f) Graphs summarize the frequencies of IgM^b secreting B-1 (left) and IgM^a secreting B-2 cells (right) in spleen, and BMs. (g) Serum IgM^b levels in the chimera. Data are representative of at least two independent experiments (a–c) (mean ± SD in a–d, f–g). *p<0.05, **p<0.005, ***p<0.0005 by unpaired two-tailed Student’s t-test.