Impaired B1 cell–mediated responses in the absence of Zfx. (A) B-1 cell progenitors in the BM of control and CD19-Cre⁺Zfx^flox/y CKO mice at 8 to 12 weeks. The fraction of lineage (Lin; CD11b/Gr-1/Ly-6C/IgM/Ter119/NK1.1/CD8/TCR)⁻ AA4.1⁺ CD19⁺ B220⁻ B-1 progenitors in the total BM is indicated (mean ± SD of 4 mice); (left) mature recirculating B cells are shown for comparison. (B) B-1 B cells in the peritoneal cavity of control and CD19-Cre⁺Zfx^flox/y CKO mice at 8 to 12 weeks. Representative staining profiles show CD11b⁻ IgD^hi conventional B cells and CD11b^int IgD^int IgM^hi CD5⁺ (B-1a) and CD5⁻ (B-1b) B1 cells, with the percentage among total peritoneal cavity cells indicated (mean ± SD of 3-4 mice). (C) Absolute numbers of B-1 cells, conventional B cells (B220⁺ IgD^hi) and macrophages (CD11b^hi) in the peritoneal cavity of control and CKO mice (mean ± SD of 6 mice). (D) B-1 cell–derived T15 idiotype IgM titers in the sera of naive control and CD19-Cre⁺Zfx^flox/y CKO mice. Symbols represent individual mice and horizontal lines represent the mean values. (E) T15 idiotype and anti-PC IgM titers in control and CD19-Cre⁺Zfx^flox/y CKO mice immunized with PC-KLH. Symbols represent individual mice and horizontal lines represent the mean values.