Fig. 1

Significant increase of CR Abs IgG in moderate/severe COVID-19 patients’ sera may contribute to cytokine secretion and NET formation in human leukocytes in the presence of RBD. IgG against nucleocapsid, N, RBD and ACE2 proteins in sera from COVID-19 patients and healthy donors (HD) were measured by ELISA. Comparison of (A) the levels of IgG against N, (B) RBD, and (C) ACE2 in mild vs. moderate/severe COVID-19 patients. (D) Anti-ACE2 IgG levels in sera of mild vs. moderate/severe COVID-19 patients after BSA or RBD preadsorption. For the leukocyte activation test, isolated human leukocytes were preincubated with 1:100 diluted HD serum or sera from CR Ab-positive COVID-19 patients (CR serum) as indicated in the presence or absence of additional rACE2 (10 μg/mL) for 30 min. Afterward, unbound antibodies were removed by centrifugation, and the cells were treated with the RBD protein (10 μg/mL) for 24 h. The supernatants were collected to measure the (E) TNF-α, (F) IL-1β, (G) IL-6, (H) IL-8, and (I) MPO levels using ELISA kits. The averages of triplicate cultures ± SD are shown. Multiple comparison of antibody response and leukocyte activation tests (n = 3–5) was conducted by one-way ANOVA and Tukey’s post hoc test. The comparison between BSA and RBD preadsorption was analyzed by paired Student’s t-test; P values were displayed, and for values less than 0.0001, they were represented as ****