PF4 enhances endothelial thromboinflammation induced by VITT serum in the Endo-chip. (A) Serum concentration of PF4 (ng/mL) in VITT ELISA positive, VITT ELISA negative, VTE without VITT, and vax control samples. One-way ANOVA with Tukey post hoc test was used for comparison. (B) Platelet (green), neutrophil (red), and fibrin (purple) accumulation following a 15-minute blood perfusion in the Endo-chip after treatment with VITT ELISA positive, VITT ELISA negative, or vax control serum without (upper panel) or with (lower panel) the addition of PF4. Representative images. (C) Platelet fluorescence area per field, neutrophil count per field, and fibrin fluorescence area per field following a 15-minute blood perfusion in the Endo-chip after treatment with VITT ELISA positive serum without or with addition of PF4, VITT ELISA negative serum without or with addition of PF4, and vax control serum without or with addition of PF4. A paired t test was used for comparison. (D) Kinetics of the platelet fluorescence area, neutrophil count, and fibrin fluorescence area over 15 minutes of blood perfusion in the Endo-chip after treatment with VITT ELISA positive serum without or with addition of PF4, VITT ELISA negative serum without or with addition of PF4, or vax control serum without or with addition of PF4. VITT ELISA positive n = 7 (patients 1, 2, 4, 5, 6, 8, 11), VITT ELISA negative n = 11 (patients 1, 2, 4, 5, 6, 8, 9, 10, 11, 12, 14), vax controls n = 11 (patients 1, 2, 5, 6, 7, 8, 10, 12, 13, 14, 15). PF4 was added at 25 μg/mL in all experiments.