Accumulation of MZ B cells in p100^−/− mice is cell intrinsic. (A) Lethally irradiated C57BL/6 recipients were reconstituted with either wild-type (wt → B6) or p100^−/− bone marrow cells (p100^−/− → B6). At 6 weeks after transplantation, splenocytes from chimeras were stained for CD23, CD21, and IgM and analyzed for MZ B cells (CD23⁻CD21^hiIgM^hi). Only CD23⁻ lymphocytes are shown. (B) MZ B-cell accumulation in p100^−/− mice is dependent on relB. Splenocytes from wild-type, p100^−/−relB^−/+, and p100^−/−relB^−/− mice were analyzed for MZ B cells as described in panel A.