Impaired germinal center formation in immunized B7-H2^–/– mice. (A) Cryosections of spleens from wild-type, CD28^–/–, and B7-H2^–/– mice were stained with biotin-conjugated PNA and counterstained with 1% hematoxylin to identify germinal center and follicular B cells 8 days after the secondary immunization. Original magnification, × 100. (B) Flow cytometry analysis of PNA-binding germinal center B cells in the spleens of wild-type and B7-H2^–/– mice 7 days after the secondary antigenic challenge. Cell sample from similarly immunized CD28^–/– mice is included as a control. Numbers indicate the percent of lymphocytes.