Increased MZ, B1, and GC B-cell numbers are a cell-extrinsic property of AID^−/−µS^−/− B lymphocytes. BM chimeras constructed with a 1:1 ratio of BM cells from CD45.1⁺ AID^−/−µS^−/− donors and CD45.2⁺ BM cells from AID^−/−µS^−/−, WT, AID^−/− (AID), and μS^−/− (μS) donors. A control group was reconstituted with a 1:1 ratio of BM cells from CD45.1⁺ and CD45.2⁺ WT donors. BM chimeras were analyzed 10 to 14 weeks after reconstitution. (A) Serum levels of IgM and total IgGs measured by ELISA at the time of the analysis. (B) Total numbers of CD19⁺ B cells per spleen. (C) Representative dot plots showing the relative proportion of splenic B-cell subsets defined as CD21^hiCD23^lo MZ, CD21^loCD23^lo, and CD21^loCD23^hi follicular (FO) in the different BM chimeras were studied in relation to their genotype, traceable through the expression of CD45.1 and CD45.2 allotypes. (D) Bar charts show proportions of MZ (left), CD21^loCD23^lo (middle), and GC B cells (right) among CD45.1⁺ B cells, quantified by FACS analysis of spleens from the different groups of chimeras. (E) Representative dot plots showing the proportion of GC B cells among the CD45.1⁺ mLN cells taken from chimeras of the different groups. Results are representative of 2 independent experiments; *P ≤ .05; **P ≤ .01; ***P ≤ .005.