Whole platelets are not necessary for the delivery of CD154 signal. CD154^−/− mice were injected with 5 × 10⁷ activated platelets (AP) or activated platelet supernatant (AP Sup) from 5 × 10⁸ wild-type (B6) or CD154^−/− platelets. Positive controls were given 500 μg anti-CD40 antibody (1C10) IP. Mice were immunized on day (−1) with 10⁸ pfu of adenovirus. On day 0, all mice received one injection of either AP or AP Sup. A second injection was given to half the mice on day 6 (X1 indicates mice receiving one injection; X2, mice receiving 2 injections). Serum was collected on day 9 for adenovirus-specific IgG analysis by ELISA. This experiment was performed 3 times with 5 mice per group. The graph is from a representative experiment (Wilcoxon rank sum test: *2-tailed P = .002, **2-tailed P = .006).