Anti-CD44 IgG subclass determines the specific macrophage FcγRs inhibited. Macrophages were treated with anti-CD44 or controls before incubation with platelets opsonized with anti-GPIIb/IIIa antibodies of different IgG subclasses. (A) Magnitude of untreated macrophage phagocytosis of platelets opsonized with different anti-GPIIb/IIIa IgG subclasses. Nonopsonized platelets (-) were incubated with PBS only during the opsonization step before addition to macrophages. Data are from 6 to 11 experiments (mean ± standard error of the mean [SEM]). Effect of FcγRIII blockade (clone 2.4G2) (B), rat IgG1 anti-CD44 (C), or rat IgG2b anti-CD44 (D) compared with no antibody treatment (-) on macrophage phagocytosis of platelets opsonized with the different IgG subclasses shown. Data are from 3 or 4 experiments (mean ± SEM). The phagocytic index is the average number of phagocytosed platelets per 100 macrophages. ****P < .0001, ***P < .001, **P < .01, vs the nonopsonized group, nonparametric Mann-Whitney t test (A), unpaired t test (B-D). IgG1, rat IgG1 clone MWReg30; IgG2a, mouse IgG2a (clone 5C4 or 2G1); IgG2b, mouse IgG2b clone 10B5; ns, not significant; α-CD44, anti-CD44.