Fig. 2

Plasmablasts contribute to antibody development and blocking activity is associated with disease severity. a–e Luminex-based multiplex assay was used to detect IgM, IgA, and IgG antibodies against S1-, RBD, S2-, or N-antigen of SARS-CoV-2 in patient sera. a–c Percentage of seroconverted ICU (n = 58) and CONV (n = 28) patients for SARS-CoV-2-specific IgM (a), IgA (b), and IgG (c) antibodies. The threshold for positive samples was calculated based on the mean fluorescent intensity (MFI) of antibodies from 36 UEs + 2× standard deviation. d Antibody levels from UE (n = 36), ICU (n = 58), and CONV (n = 28) are displayed as MFI. e Correlation analysis between S- and N-specific antibodies from ICU patients and plasmablasts proportions. f Percentage of ICU (n = 58) and CONV (n = 28) which developed blocking antibodies against SARS-CoV-2 RBD (left) and efficacy of blocking activity (right), which was assessed by competitive ELISA. Efficient blocking was expressed as the percent blocking at a 1:50 serum dilution relative to a UE serum control. g Correlation analysis between COVID-19 severity markers (CRP levels, SOFA-, WHO score, and PF ratio) and blocking efficiency from ICU patients. Black triangles represent last samples from deceased patients. Statistical analysis: multigroup comparisons were performed using ANOVA test with Turkey multiple comparison test or Kruskal–Wallis with test with Dunn’s multiple comparison test. Two-group comparison was performed using Mann–Whitney test; Spearman correlation. *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001