CalDAG-GEFI deficiency protects from FcγRIIA-dependent platelet aggregation. (A-B) Representative aggregation traces in response to anti-CD9. (A) WT (black dashed line) and transgenic hFcR platelets were stimulated with 5 μg/mL anti-CD9 in the presence (dotted black line) or absence (black line) of 75 μg/mL of the αIIbβ3 blocking antibody Leo.H4. (B) Black line represents hFcR; light gray line, hFcR treated with the P2Y12 inhibitor 2-MeSAMP; dark gray line, hFcR/CDGI^−/−; and dashed dark gray trace, hFcR/CDGI^−/− treated with 2-MeSAMP. Platelets were stimulated with low-dose (top panel, 1 μg/mL) or high-dose (bottom panel, 10 μg/mL) anti-CD9. Traces are representative of 3 independent experiments. (C) Dose-response of anti–CD9-induced platelet aggregation. Maximum aggregation (%, top panel) and delay of aggregation (minutes, bottom panel) measured in PRP from hFcR (black bar), hFcR preincubated with 100μM 2-MeSAMP (light gray bar), and hFcR/CDGI^−/− platelets (dark gray bar) stimulated with increasing doses of anti-CD9 antibodies. Data are mean ± SEM (n = 5 or 6). *P < .05. **P < .01. (D) Maximum aggregation measured in PRP from WT (black bar), WT preincubated with 100μM 2-MeSAMP (light gray bar), and CDGI^−/− platelets (dark gray bar) stimulated with increasing doses of PAR4 receptor-activating peptide. Data are mean ± SEM (n = 5).