SARS-CoV-2 activates platelets in a TF-dependent manner. Platelet responses to β-propiolactone–inactivated SARS-CoV-2 (CoV2) obtained from human A549 (ACE2) cell infection or conditioned media (C.Med) solution (A549 [ACE2] cell culture supernatant obtained after 4 days of culture and treatment with β-propiolactone [1:1000 vol/vol]). (A) TF activity was assessed by TF/FVII-mediated conversion of FX to FXa and subsequent conversion of FXa substrate. TF activity was measured in an undiluted sample. The specific TF activity indicated is calculated as total TF activity measured in the presence of control Ig minus the total TF activity measured in the presence of anti-TF antibody HTF-1. Samples were preincubated for 15 minutes with control Ig or HTF-1. The numbers above bars indicate the estimated TF activity in pM. N = 4 (replicates measured in 4 separate assays). n.d. = not detected. (B) Platelet stimulation with CoV2 (1:25 vol/vol) in the presence of control Ig (control Ig, 10 µg/mL) or anti-TF antibody HTF-1 (10 µg/mL) and in the presence of PFP (0.5%). CoV2 was preincubated for 15 minutes with control Ig or HTF-1 before incubation with PFP. N = 6. (C) PF4 measurement by ELISA in platelet supernatant. N = 6. (D) 5-HT (serotonin) measurement by ELISA in platelet supernatant. N = 6. (E) Platelet stimulation with CoV2-Δ in the presence of control Ig (control Ig, 10 µg/mL) or anti-TF antibody HTF-1 (10 µg/mL) and in the presence of PFP (0.5%). CoV2-Δ was preincubated for 15 minutes with control Ig or HTF-1 before incubation with PFP. N = 3. Data are represented as mean ± SD. Statistical analysis: (B-D) Wilcoxon test. (E) Ordinary two-way ANOVA followed by Sidak’s multiple comparison test. *P < .05 and **P < .01.