Competitive advantage of MHC II⁻B cells. (A) Competitive bone marrow chimeras were setup by reconstituting myeloablated CD45.1⁺CD45.2⁺ WT (WT) hosts with mixtures of CD45.1⁺ control (donor 1; always WT) and CD45.2⁺ test genotype (donor 2; WT, Itgax^creH2-Ab1^c, or Fcer2a^creH2-Ab1^c) bone marrow. The composition of myeloid and lymphoid cells in reconstituted recipients was then examined by flow cytometry (the key refers to panels B-D). (B-D) B220 and MHC II expression and mean percentage of splenic T cells, B cells, and myeloid cells originating from the host or each of the 2 types of donor, in competitive chimeras between WT and WT donors (n = 5) (B); WT and Itgax^creH2-Ab1^c donors (n = 8) (C); or WT and Fcer2a^creH2-Ab1^c donors (n = 7) (D). (E) Gating of splenic transitional (T) T1, T2, T3, marginal zone (MZ), follicular (FO), and GC B cells. (F) Mean change (plus or minus SEM) from input frequency of CD45.2⁺ donor cells in splenic T cells or T1, T2, T3, MZ, FO, and GC B cells from chimeras between CD45.1⁺ WT control and CD45.2⁺ test genotype donors (WT: WT, n = 12; WT: Itgax^creH2-Ab1^c, n = 6; WT: Fcer2a^creH2-Ab1^c, n = 5). *P < .01; **P < .001 between WT and other genotypes by Student t tests. (G) MHC II expression in splenic FO and overlaid GL7⁺ GC B cells of host and donor origin, distinguished by CD45 allotype expression. FACS, fluorescence-activated cell sorting.