FXIII requires α₂AP to down-regulate fibrinolysis. (A) Plasma thrombi were prepared from FXIII depleted plasma in the absence (●) and presence (○) of a neutralizing antibody to α₂AP or α₂AP depleted plasma (▴). Various concentrations of FXIII (0, 0.1, 0.3, 1 U/mL) were added before thrombus formation and thrombi were lysed with 1 μg/mL tissue plasminogen activator (tPA). Lysis was monitored as release of fluorescence and expressed as mean lysis rate per minute ± SEM (n = 4). (B) Plasma thrombi prepared as described in panel A in the presence of FXIII (1 U/mL) or neutralizing antibody to α₂AP (Mab) were solubilized in reducing sample buffer. After separating on 7.5% acrylamide gels samples were immunoblotted with antibodies to α₂AP, the α-chain or the γ-chain of fibrinogen.