KKO binding to murine platelets in the presence of hPF4. (A) Details are the same as for Figure 1A but mPF4^null platelets were studied with increasing amounts of added hPF4. Diamonds indicate total platelet PF4; squares, total surface immunogenic PF4; and circles, KKO-detectable surface PF4. (B) Details are the same as for panel A for KKO but mPF4^null platelets pretreated with CS ABC were used. (C) Details are the same as for panel A for KKO but genotypes of the mice are as shown and the platelets were incubated with either the FcγRIIA blocking mAb IV.3 or an isotype control (50 μg/mL) for 30 minutes at RT prior to addition of PF4 and KKO. (D) Details are the same as for panel A for KKO with the genotypes of the mice indicated. Relative annexin binding was measured. (E) Time course of KKO and RTO binding (50 μg/mL) in the whole blood samples to transgenic murine platelets. +/+ indicates hPF4^high/FcγRIIA⁺ double-transgenic mouse platelets; +/–, hPF4^high/FcγRIIA^– transgenic mouse platelets. MFI is in absolute values. For panels A-D, the mean ± 1 SD is shown. Each experiment was performed 3 times. In panel E, a study representative of 3 is shown.