Natural IgM-secreting PCs are of B-1 cell origin. (A, left) Neonatal chimeras were created to differentiate B-1 and B-2 cells and their secreted product by their Ig allotype. Neonatal Igh^a/CD45.2 C57BL/6 mice were treated from birth for 6 wk with anti-IgM^a (DS.1) and received magnetic bead (autoMACS)–enriched donor Igh^b/CD45.1 peritoneal cavity cells (PerC, right) within a few days of birth. Values shown in all FACS plots represent percentage within parent population. FMO, fluorescence minus one control stains for IgM. After the end of treatment, mice are rested for at least another 6 wk to allow for B-2 cell reconstitution. Resulting chimeras have IgM^b/CD45.1 donor B-1 cells and IgM^a/CD45.2 host B-2 cells. (B and C) Shown are 5% contour FACS plots of Ig allotype chimeric mice with CD45.2 IgM^a conventional B cells and (CD45.1, IgM^b) peritoneal cavity donor B-1 cells. (B) BM and (C) spleen PC and B-1 cells among CD45.1+ cells. Plots are representative of three mice. (D) B-1–derived serum IgM^b concentrations ± SD (mg/ml) of chimeras and control C57BL/6 (IgM^b) nontreated mice as measured by ELISA (n = 3). No significant difference between groups, as assessed by an unpaired Student’s t test. Results in B and D are representative of three independent experiments, two of which used FACS-sorted B-1 cells in place of autoMACS-purified B-1 cells.