Presence of LC3B-positive vesicles in platelets from patients with severe COVID-19. Washed platelets from healthy donors (n = 5) and patients with severe COVID-19 (n = 7) were fixed and labeled with a specific anti-LC3B antibody and analyzed by super-resolution confocal microscopy with the Airyscan module (A). F-actin staining using phalloidin-AF488 allowed platelet visualization. Representative images are shown (bar represents 1 µm). Images reconstructed in 3 dimensions (B) show LC3B spatial distribution in platelets of healthy control and patients with severe COVID-19. A representative western blotting analysis shows the level of LC3B-I and LC3B-II in platelets from 4 heathy donors (H1-H4) representative of 5 and from 4 patients with severe COVID-19 (P1-P4) representative of 7 (C, left panel). The exposure time was adjusted to efficiently distinguish the LC3BI and LC3BII forms. The quantification of the LC3B/actin ratio for LC3B-I and LC3B-II from healthy donors (n = 5) and patients with severe COVID-19 (n = 7) is shown in the right panel. Results are mean ± SEM. **P < .01 ***P < .001 according to the nonparametric Mann-Whitney test. Immunogold labeling of LC3B protein analyzed by TEM shows the number of gold particles by cross-section (30 from n = 3 healthy donors [10 for each] and 30 from n = 3 patients with severe COVID-19 [10 for each]) (D). Results are mean ± SEM. ***P < .001 according to the nonparametric Mann-Whitney test. The presence of LC3B associated with a large vacuole reminiscent of autophagosome (arrows and magnification insert) is shown in a platelet from patient with severe COVID-19 (E). Representative TEM sections of platelets from patients with severe COVID-19 (n = 10) showing structures typical of elongation membrane (EM), autophagosome (AP), and autophagolysosome-like (AL) (F).