Requirement of T cells for production of PF4/heparin-specific antibodies. (A) Impaired production of PF4/heparin-specific antibodies in CD4 T-cell–depleted mice. Wild-type mice were treated with anti-mouse CD4 antibody GK1.5 or PBS, followed by immunization with PF4/heparin complexes. Sera were collected at the indicated time points after immunization and total PF4/heparin-specific IgG levels were measured by enzyme-linked immunosorbent assay (ELISA). Each dot represents a mouse and the horizontal lines indicate the mean values. Data shown are obtained from 10 mice in each group. (B) Normal T-cell–independent responses in CD4 T-cell–depleted mice. GK1.5- or PBS-treated 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 6 mice in each group. (C) Impaired T-cell–dependent responses in CD4 T-cell–depleted mice. GK1.5- or PBS-treated 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 6 mice in each group. (D) Failure of adoptively transferred splenic B cells to produce PF4/heparin-specific antibodies in the absence of T cells. Splenic B cells isolated from wild-type mice were mixed with splenocytes isolated from μMT or Rag1-deficient mice at a 1:1 ratio, and then adoptively transferred into partially irradiated Rag1-deficient mice. The recipients were immunized with PF4/heparin complexes. Sera were collected at the indicated time points, and the levels of total PF4/heparin-specific IgG were measured by ELISA. Data shown are obtained from 10 recipients in each group. Each dot represents a mouse and the horizontal lines indicate the mean values.