Diminished competitive fitness of developing xid B cells in the BM and spleen. BM chimeras were established via transplantation of a total of 2 × 10⁶ B220-depleted BM cells into irradiated B6.Ly5^SJL hosts. Eight weeks later BM and spleen cells were stained for expression of the indicated markers as illustrated in Figure 1A and allele-specific antibodies to Ly5. Donor BM cells were C57BL/6 (B6), B6.xid (both Ly5^B6+), or wild-type B6.Ly5^SJL (Ly5^SJL+) and double chimeras were established by transferring BM cells from B6.Ly5^SJL mice mixed an equal number of BM cells from either C57Bl/6 or B6.xid mice. (A) Representative flow cytometric plots from the indicated double BM chimeras. Gates for each population are as in Figure 1A. Fr E indicates sIgM⁺ AA4⁺ BM B cells. (B) Double BM chimeras were assessed for ratios of wild-type C57BL/6 (solid bars with black diamonds) versus B6.xid (hatched bars with white diamonds) BM or splenic B-cell frequencies within the indicated gate divided by the frequency of B6.Ly5^SJL-derived (Ly5^SJL+) cells in the identical gates. (C) Comparison of the ratios of CD23⁻ to CD23⁺ immature B cells defined as B220⁺ AA4⁺ sIgM⁺ in normal B6 and B6.xid adults and the indicated single and double chimeras. Data are representative of 2 separate experiments.