Inhibition of thrombus formation by the anti-FcγRIIa mAb, IV.3. Whole blood drawn from healthy human volunteers or mice was incubated with mepacrine to label the platelets and then perfused over microchannels that had been coated with a mixed matrix composed of 50 μg/mL type I collagen and 25 μg/mL fibrinogen. Blood samples contained either 10 ug/mL mAb IV.3 Fabs or an isotype-matched control Fab, as indicated. Blood was initially perfused at a high shear rate 2888 s⁻¹ for 3 minutes, and then adjusted down to 880 s⁻¹ for an additional 3 minutes to emulate partial wound closure. Epifluorescent microscopic images of platelet adhesion and thrombus formation were acquired in rea time at a frame rate of 1 frame per second. (A) Shown is 1 of 3 time-course experiments of thrombi formed from whole blood derived from WT FcγRIIa^neg and FcγRIIa^pos mice. (B) Time-course images of thrombi formed using human blood. The experiment shown is representative of 3 such experiments performed using blood from 3 different donors. (C-D) Quantification of platelet thrombi formed was performed using Metamorph software. Results are expressed as the mean percentage of surface coverage or total integrated fluorescence intensity ± SEM (n ≥ 3 per group). Statistically significant differences between the means were determined using the Student t test. *P < .05, **P < .01. Note that thrombus formation of both human platelets (which are FcγRIIa-positive) and FcγRIIa-positive (but not FcγRIIa^neg) murine platelets is markedly reduced in the presence of mAb IV.3 Fabs.