Delayed antibody response to T-dependent antigen after B cell–specific Zfx deletion. (A) Serum Ig levels in naive control and CD19-Cre⁺Zfx^flox/y CKO mice at 8 to 12 weeks. Significant differences between the groups are indicated. (B) Antibody response to T-dependent antigen in control and CD19-Cre⁺Zfx^flox/y CKO mice. Mice were immunized with NP-KLH, and relative anti-NP IgM and IgG1 titers were determined 7 days later. (C) The analysis of GC B cells in control and CKO mice immunized as in panel C. Shown are staining profiles of splenic B220⁺ IgD^lo B cells, with the fraction of PNA⁺ Fas⁺ GC B cells indicated (mean ± SD of 4 mice). The graph shows absolute GC B-cell numbers per spleen. (D) Immunochemical analysis of germinal center (GC) response to T-dependent antigen. Control and CD19-Cre⁺Zfx^flox/y CKO spleens 7 days after immunization with NP-KLH were stained for GC B cells with PNA in combination with anti-IgM. Bars represent 0.5 mm (left) and 0.2 mm (right).