LPA stimulates platelet-monocyte aggregate formation. (A) Dose-response curve of LPA-induced platelet-monocyte aggregate formation in anticoagulated (hirudin) whole blood. Platelet-monocyte aggregate in stirred blood samples were measured 5 minutes after incubation with 1-acyl-LPA (16:0) by FACS analysis. Values represent the mean ± SD, n = 7-19, ^*P < .05, ^**P < .01. (B) Plateletmonocyte aggregate formation is mediated via P-selectin. Effects of ADP receptor antagonists and aspirin. Blood samples were stirred for 5 minutes at 37°C after addition of buffer (basal), or 1-acyl-LPA (16:0) (20 μM). A blocking anti–P-selectin antibody (10 μg/mL) or the ADP receptor antagonists AR-C69931MX (1μM) and MRS2179 (100μM) alone or in combination was added 30 seconds before the addition of buffer or LPA. Blood was incubated with aspirin (1 mM) for 30 to 45 minutes prior to treatment with LPA. Values represent mean ± SD, n = 8-10. ^*P < .05 as compared to control.