Diverse IgG ⁺ MBC-derived RBD-specific antibodies gain potency when expressed as IgM. (A) Panel of eight IgG⁺ MBC-derived IgG mAbs indicating affinity for RBD and NT₅₀ of WA-1 SARS-CoV-2 in PRNT (Rodda et al., 2021). Samples were analyzed in duplicate in at least two separate experiments. (B) Binding kinetics for individual mAbs as determined by BLI using sensor-bound IgG mAbs and serial dilution of soluble RBD protein. Each mAb was tested with six dilutions of RBD. (C and D) Summary of epitope-mapping experiments showing relative competition for RBD between the indicated MBC-derived IgG and well-characterized Fabs (C) or each other (D); individual data are provided in Figs. S1 A and S2 A. (E) Comparison of neutralizing potency of clonally identical IgG and IgM mAbs against pseudovirus. Bars indicate mean and SD for three independent experiments; individual symbols indicate the average of internal duplicates for each experiment. Average fold difference in NT₅₀ potency for IgG vs. IgM for each BCR clone is shown below the graph. (F) Representative results for 297 IgM vs. IgG in neutralization assays using the pseudovirus (left panel; average and SD are shown for three independent experiments, each performed in duplicate) or a WA-1 SARS-CoV-2 PRNT (middle panel; representative of three independent experiments) and summary of NT₅₀ data (right table).