Increased platelet-monocyte aggregates in severe COVID-19 patients. (A) Gating strategy for analysis of platelet-monocyte aggregates in patients with COVID-19 or control subjects. (B) Percentage of platelet-monocyte complexes (CD14⁺CD41⁺) among monocytes from SARS-CoV-2⁻ control volunteers, SARS-CoV-2⁺ asymptomatic subjects, or symptomatic patients presenting mild to severe COVID-19 syndrome. The horizontal lines in the box plots represent the median, the box edges represent the interquartile ranges, and the whiskers indicate the minimal and maximal value in each group. *P < .05 between selected groups. (C) The percentage of platelet P-selectin (CD62P) surface expression was plotted against the percentage of platelet-monocyte complexes in the same infected subjects. Linear regression and Pearson correlation were calculated according to the distribution of the dots. (D) Phalloidin-labeled cytospin blood samples from control subjects or COVID-19 patients. Platelets and leukocytes obtained from whole blood after lysis of erythrocytes were labeled with Alexa 488–phalloidin (F actin, green) and DAPI (nuclei, blue). Scale bar, 10 μm; inset, 5 μm. Representative images of platelet-monocyte aggregates are shown. The fourth panel shows orthogonal views from different planes of the confocal image. The green line generated a 2-dimensional (2D) image that is shown in the top green rectangle whereas the red line–generated 2D image is shown in the red rectangle on the right. SSC-H, side scatter height.