ICU COVID-19 IgGs induce procoagulant PLTs, GPVI cleavage, increased thrombin generation and thrombus formation on collagen. (A) HC, ICU non-COVID-19, and ICU COVID-19 IgG-induced changes in wPLTs externalization of PS and expression of CD62p were analyzed via annexin V-FITC and CD62p-APC antibody staining, respectively. Data are shown as mean percentage ± SEM of annexin V-FITC and or CD62p-APC-positive-labeled wPLTs. (B) ICU-COVID-19 IgG-induced reductions in the expression of GPVI on the PLT surface were analyzed by GPVI-PE antibody staining and compared as percentage of GPVI-negative PLTs ± SEM to the controls. (C) PRP from healthy individuals was preincubated with HC or ICU COVID-19 IgG and analyzed for thrombin generation using CAT. Each curve represents the amounts of generated thrombin over time induced by HC (dotted blue line) or IgG from different ICU COVID-19 patients (red line) (D) PRP from healthy individuals with the blood group O was incubated with HC, ICU non-COVID-19 control, or ICU COVID-19 IgG, labeled with FITC conjugated calcein and perfused through microfluidic channels at a shear rate of 1500⁻¹ (60 dyne) for 5 minutes after reconstitution into autologous whole blood. Images were acquired at x20 magnification in the fluorescent (upper panel) as well as in the BF channel (lower panel). Scale bar 50 µm. (E) Mean percentage of surface are covered (SAC) by thrombus ± SEM in the presence of HC, ICU non-COVID-19 control, and ICU COVID-19 IgG after 5 minutes perfusion time. The number of patients and healthy donors tested is reported in each graph. See Figure 1 for P values and abbreviation definitions.