p50-/- mice contain reduced numbers of MZ B cells that bind high levels of IgM-IC A. B cell subsets and IgM-IC binding. Splenocytes from B6 or p50-/- mice were analyzed for B cell subsets by FACS using CD21/CD23 or B220/CD1d expression. Histograms on the right show binding of IgM-IC to MZ and FO B cells 1 h after i.v. injection (shaded region); clear profiles represent uninjected control mice. Although spleen cells from p50-/- mice contained reduced numbers of MZ B cells, the cells present in the MZ B cell gate bind IgM-IC at high levels over background, similar to levels of binding of IgM-IC to B6 MZ B cells. B. In vitro binding assay showing binding of IgM-IC to MZ B cells (shaded region) compared with FO B cells (clear region). MZ B cells from both B6 and p50-/- mice bind high levels of IgM-IC compared to FO B cells. C. Immunofluorescence shows the presence of IgMhi cells (red) in the MZ as demarcated by MOMA-1 (marginal metallophilic macrophages, blue) staining. p50-/- mice possess some IgM+ cells in the MZ outside of the MOMA-1 ring, consistent with the presence of appropriately positioned MZ B cells in these mice.