Activation of B lymphocytes by platelet-associated CD154. (A) Platelet-associated CD154 is competent to induce the proliferation of B lymphocytes. Activated platelets from 4 selected healthy donors or ITP patients were cocultured for 4 days with tonsillar B lymphocytes. In parallel wells, CD40 was blocked (anti-CD40) or not blocked. Other controls included incubating tonsillar B lymphocytes with L-cell CD154 transfectants (CD154 tr). The proliferation ratio was obtained by dividing the thymidine incorporation obtained from tested sample (B lymphocytes plus activated platelets) by the thymidine incorporation obtained from thrombin-treated B lymphocytes alone. (B) Platelets from ITP patients drive the production of anti-GPIIb/IIIa antibody (Ab) in a CD154-dependent manner. Activated platelets from healthy donors (control), ITP patients (group 1), and patients with non-ITP autoimmune diseases (group 2) were cultured in the presence of autologous B lymphocytes. The production of anti-GPIIb/IIIa antibodies was measured using a MAIPA assay. Ratio (R) values were obtained according to the formula, R = mean test OD/mean control OD of supernatants. Threshold of positivity was calculated from 4 controls (control B cells + control platelets) as the mean ± 3 SD. For each patient, all control wells included were below the threshold of positivity. Statistical significance (P < .05; Student t test) was observed on comparing group 1 (no antibody) with group 1 (+ anti-CD40 antibody), group 1 (no antibody) and controls (no antibody), and group 1 (no antibody) and group 2 (no antibody).