Data analysis and statistical evaluation
Flow cytometry data were analysed using FlowJo version 10.6.2 (BD Biosciences). Gating was performed according to single stains and fluorescent-minus-one controls. The corresponding gating strategy is presented in Figure 2. Lymphocyte identification and doublet exclusion were performed by forward-scatter (FSC) versus sideward parameters (SSC) and FSC/SSC-height (H) versus FSC/SSC-area (A) plotting, respectively. Cells positive for DAPI (dead cells), CD14 (mainly monocytes), or CD3 (T cells) were excluded from further analyses. CD19 expressing cells were further characterized for IgD, CD27, and CD38 expression. Frequencies of SARS-CoV-2-protein specific B cells in recovered COVID-19 patients and healthy donors were compared using two-way repeated measurements ANOVA and Sidak’s multiple comparisons test. Patient and healthy donor age was compared using unpaired two-tailed t-test, and gender was compared using two-tailed Fisher’s exact test. Correlations were calculated using Spearman’s rank coefficient. Statistical analysis and graphical representation were realized with GraphPad Prism v7 and R v3.6.2.