Cell numbers and GC formation. (A) Expression of M17 cDNA in developing B cells. Bone marrow–derived lymphocytes were sorted into B220^– cells, B220⁺ IgM^– c-kit⁺ pro-B cells, B220⁺ IgM^– CD25⁺ pre-B cells, and B220⁺ IgM⁺ B cells by fluorescent-activated cell sorting (FACS), and the expression of M17 mRNA transcripts in these cell populations was determined by RT-PCR. (B) The total number of cells in peripheral lymphoid organs. SP indicates spleen; MLN, mesenteric lymph nodes; and PP, Peyer patches. (C) Generation of GC B cells. Mice were immunized with 50 μg NP-CG and analyzed for the presence of CD19⁺PNA⁺Fas⁺ GC B cells 14 days after immunization. Only CD19⁺ cells are shown. Numbers represent the mean in percent plus standard deviation. SP indicates spleen; MLN, mesenteric lymph nodes; and PP, Peyer patches. (D-E) GC architecture in M17^–/– mice. Mice were immunized with 50 μg NP-CG. Frozen splenic sections were prepared on day 14 after immunization and analyzed by immunofluorescence. (D) Sections were incubated with αCD19 mAb (green) and PNA (red) to visualize B-cell follicles and GCs, respectively. (E) Sections were incubated with αBCL-6 mAb (brown) and counterstained with hematoxylin (blue). Representative pictures are shown. SP indicates spleen; PP, Peyer patches. Numbers below pictures indicate the magnification.