FIG 2.
Analysis of IgG memory B cells (MBCs) reactive to SARS-CoV-2 and non-SARS-CoV-2 proteins in non-SARS-CoV-2-exposed and COVID-19 convalescent subjects. PBMCs for MBC analysis were collected from (i) 21 healthy donors sampled from 2011 to 2014 (HD) and (ii) 26 COVID-19 convalescent subjects sampled 4 to 9 weeks after symptom onset (Conv). PBMCs were stimulated in vitro to induce MBC differentiation into Ab-secreting cells. Antigen-specific quantitation of MBC-derived Ab (IgG)-secreting cells (MASCs) by ELISpot assay or of MBC-derived polyclonal (IgG) Abs (MPAbs) by ELISA provided a measure of the abundance of specific IgG MBCs. (A) IgG MBCs reactive to the SARS-CoV-2 spike (S), receptor binding domain (RBD), and nucleocapsid (N) in convalescent subjects. The assigned cutoff for positivity is shown by the horizontal gray bar. (B) IgG MBCs reactive to the influenza virus H1 hemagglutinin and TTd in convalescent subjects. (C) Proportions of IgG MBCs reactive to the SARS-CoV-2 RBD, S2, and N for individual convalescent subjects. A bar representing the mean value for the HD cohort is included for comparison. In all HD samples, MPAb IgG levels against RBD, S2, and N were below the cutoff for assay positivity. (D) Comparison of serum IgG concentrations (upper panels) and numbers of IgG MBCs (lower panels) reactive to the SARS-CoV-2 S (left-hand side) and N (right-hand side) proteins. Serum IgG was measured by ELISA; IgG MBC numbers were based on ELISA of MPAbs. Dilution curves are shown for individual convalescent subjects; curves for 4 subjects are shown in different colors to identify particular response patterns. (E to H) IgG MBCs reactive to the S proteins of HCoVs OC43 (E) and 229E (F), the H1 hemagglutinin (G), and TTd (H) in convalescent and HD subjects. Significance (*, P < 0.05; ns, not significant) for comparisons of IgG MBC numbers between subject groups was determined by the Wilcoxon rank sum test.