Platelet-monocyte interactions increase the secretion of inflammatory mediators in COVID-19. (A) Monocytes from healthy volunteers (control monocyte) were incubated with platelets from patients with severe COVID-19 (COVID-19 platelets) or from a different healthy volunteer (control platelets) for 18 hours, and the indicated inflammatory mediators were quantified in the supernatants. Monocytes from patients with COVID-19 (COVID-19 monocyte) were also incubated with platelets from healthy volunteers (control platelets). The concentration of TNF-α (B), IL-1β (C), IL-10 (D), PGE₂ (E), IL-6 (F), MCP-1/CCL2 (G), PDGF (H), and PF4/CXCL4 (I) are shown. Bars represent mean ± standard error of the mean of 6 to 12 platelet and monocyte combinations from patients with COVID-19 or control participants. All experiments were repeated with cells from 2 independent control participants exposed to platelets or monocytes from the same patients with COVID-19 with similar results, and a representative data from 1 of the donors is shown. *P < .05 between selected groups.