GPVI-activated platelets release ERp57. (A) Detection of ERp57 in platelets and platelet releasates by Western blot analysis. Cellular lysates and supernatants were obtained from platelets treated with either phosphate-buffered saline buffer, control IgG (RmC7H8), GPVI antibody HGP4C9 (1 μg/mL), or ADP (20μM) for 10 minutes, and ERp57 expression (57-kDa protein) was determined by Western blot analysis. (B) GPVI-activated platelets release functionally active ERp57, which stimulates blood coagulation in the presence of recombinant tissue factor (rTF). Supernatants (P-SNs) recovered from HGP4C9 (1 μg/mL)–activated platelets (2 × 10⁷) were incubated with recombinant human TF (Innovin) plus anti-ERp57 or control antibody (IgG) for 15 minutes. TF-induced factor Xa formation was quantified by hydrolysis of the specific substrate S2222 (n = 3, mean ± SEM, *P < .05 IgG vs anti-PDI antibody).