Conjugate formation between platelets and PMNs or monocytes is enhanced by LPS or thrombin and dependent on CD18 and PSGL-1 but not on TREM-1. (A,B) Citrated blood samples were incubated in the absence or presence of LPS (1 μg/mL) (A) or thrombin (5 U/mL) (B) for 10 minutes. Binding of platelets on neutrophils, monocytes, or lymphocytes was assessed by staining against platelet marker CD41a by FACS. Cells were characterized by sideward scatter and CD45 staining. Shown is the percentage of platelet binding on the indicated cell population. (C,D) Platelets were purified and stained with calcein, then mixed with PMNs (ratio PLT/PMN, 30:1) measured by FACS. Binding of platelets to PMNs was assessed by percentage of calcein-positive neutrophils. (C) LPS (□) and thrombin (▒) enhance platelet/PMN adhesion; neither rsTREM-1 nor α–TREM-1 mAb interfered in this adhesion. (D) Antibodies against CD18 or PSGL-1 decrease binding between the 2 cell types. Data represent the mean of triplicates plus or minus SD of at least 3 independent experiments. Asterisks indicate statistically significant differences calculated by Student t test (P < .01). (A) Single asterisks indicate statistically significant differences between stimulated and unstimulated cells. Double asterisks show statistically significant different binding of platelets on cell populations in comparison with binding of platelets on lymphocytes.