α₂AP is required for platelet FXIII-A to stabilize thrombi. (A) (△) FXIII-depleted thrombi were prepared with (▪) 5 × 10⁸ plts/mL, (○) 5 × 10⁸ plts/mL with neutralizing antibody to α₂AP, or (●) neutralizing antibody to α₂AP with no added platelets. **P < .01; ***P < .001 vs FXIII-depleted thrombi + 5 × 10⁸ plts/mL (n = 7). (B) (△) α₂AP-depleted thrombi were prepared with (▪) 5 × 10⁸ plts/mL, (□) with 5 × 10⁸ plts/mL and TG inhibitor, and (▲) with TG inhibitor and no platelets. The dashed line represents PNP thrombus lysis (n = 3). (C) FXIII-depleted thrombi with and without the addition of 5 × 10⁸ plts/mL were dissolved and separated under reducing conditions by sodium dodecyl sulfate- polyacrylamide gel electrophoresis; fibrinogen γ-chain (57 kDa), α-chain (60 kDa), or α₂AP (70 kDa) antigens were detected by western blotting. Representative image of two separate experiments.