PTPN22 expression in platelets (Plt) and its role in hemostasis and thrombus formation. Human or mouse mononuclear cells (MNC) were isolated and used as positive control. Plt or megakaryocytes (Mk) from WT mouse or platelets from healthy individuals were isolated to measure PTPN22 expression by western blot with 2 different antibodies (A) or immunofluorescent staining (B). (C) PTPN22 expression in PTPN22^−/− Plt was measured by western blot. Human megakaryocytic cell line (CMK) was used as human Mk. Data are representative of 3 independent experiments. Scar bar = 10 μm. (D) Tail bleeding time and FeCl₃-induced arterial thrombosis in WT and PTPN22^−/− mice (mean). Representative image of arterial thrombosis at different time point is shown. (E) Platelet adhesion on collagen under flow conditions. Whole blood was labeled with mepacrine and perfused through fibrillar collagen–coated BioFlux plates (Fluxion Biosciences) at 40 dynes/cm² for 5 minutes. Platelet adhesion (covered area) was quantified at different time points (mean ± standard error, n = 6; one-way analysis of variance). **P < .01, ***P < .001, ****P < .0001.