Platelets must be responsive to collagen for collagen-mediated CTL augmentation. (A) FcRγ^−/− mice (platelets cannot be activated by collagen) were immunized subcutaneously on the flank with 10⁶ pfu Ad5-mOVA in PBS or 30 mg/mL collagen. After 14 days, spleens were harvested for lytic assay. (B) B6 mice were injected intravenously with 50 μg of the GPVI (platelet collagen receptor)–blocking monoclonal antibody JAQ1. After 24 hours, mice were immunized subcutaneously on the flank with 10⁶ pfu Ad5-mOVA in PBS or 30 mg/mL collagen. Fourteen days later, spleens were harvested for lytic assays. Each panel is representative of 2 experiments of 3 mice per group.