Platelets from COVID-19 patients induce TF expression in monocytes through mechanisms depending on P-selectin and integrin α_IIb/β₃. (A) Monocytes from healthy volunteers were incubated with platelets from severe COVID-19 patients (COVID-19 platelets) or from heterologous healthy volunteers (control platelets) for the indicated time points. The percentage of TF-expressing monocytes is shown. (B-D) Control monocytes were exposed to platelets from severe COVID-19 patients for 2 hours in the presence of anti–P-selectin (anti-CD62P) neutralizing antibody, the anti-α_IIb/β₃ antibody abciximab, or isotype-matched IgG. The percentage of TF-expressing monocytes (B), platelet-monocyte complexes (C), and the percentage and the mean fluorescence intensity (MFI) of P-selectin expression on CD14⁺CD41⁺ monocytes (D) are shown in each condition. (E-F) Control monocytes were exposed to platelets from COVID-19 patients for 2 hours in the presence of aspirin (100 µM) or clopidogrel (300 µM). Bars represent mean plus or minus standard error of the mean of monocytes exposed to platelets from 6 independent COVID-19 patients. All experiments were repeated with monocytes from 2 (B-D), 3 (E-F), or 4 (A) independent healthy volunteers with similar results, and representative data from 1 of the donors are shown. *P < .05 between selected groups.