Requirement of the CD40-CD40 ligand interaction between B and T cells for production of PF4/heparin-specific antibodies. BM cells from wild-type (CD40^+/+) or CD40-deficient (CD40^−/−) mice were mixed with BM cells from μMT mice at a 1:4 ratio, and then transplanted into lethally irradiated μMT mice. Eight weeks after transplantation, the chimeric mice were immunized with the indicated antigens. (A) Failure of chimeric mice with B-cell–specific deficiency of CD40 to produce PF4/heparin-specific antibodies. The chimeric mice were immunized with PF4/heparin complexes. Sera were collected at the indicated time points after immunization, and the levels of total PF4/heparin-specific IgG were measured by ELISA. Data shown are obtained from 10 chimeric mice at each group. (B) Normal T-cell–independent responses in chimeric mice with B-cell–specific deficiency of CD40. The chimeric mice were immunized with the T-cell–independent antigen TNP-Ficoll. Sera were collected at the indicated time points after immunization, and TNP-specific IgM (left) and IgG3 (right) were measured by ELISA. Data shown are obtained from 5 mice in each group. (C) Impaired T-cell–dependent responses in chimeric mice with B-cell–specific deficiency of CD40. The chimeric mice were immunized with the T-cell–dependent antigen NP-CGG. Sera were collected at the indicated time points after immunization, and NP-specific IgM (left) and IgG1 (right) were measured by ELISA. Data shown are obtained from 5 mice in each group. Each dot represents a mouse and the horizontal lines indicate the mean values.