IgM antibodies retain activity against viral spike variants that escape clonally identical IgG. (A) Binding to Wuhan-Hu-1 (WH-1) or Beta variant RBD proteins by purified IgG mAbs in ELISAs, quantified as the area under the curve (AUC) for a 10-dilution series. Results for the panel of eight IgG⁺ MBC-derived mAbs are shown (left). For clarity, simultaneously performed ELISAs testing well-characterized IgG mAbs, or an ACE2-Fc fusion protein (ACE2), are shown separately (right). Data are representative of two independent experiments performed in triplicate. (B) Affinity for Beta RBD of representative antibodies with significantly reduced binding, 203 (top), and moderate reduced binding, 297 (bottom). (C) Neutralization potencies for each mAb as IgG vs. IgM against WA-1 (blue) or Beta (pink) spike pseudovirus. Dashed line illustrates the maximum antibody concentration tested (2 μg/ml). For antibodies that did not approach an NT₅₀, the NT₅₀ is graphed arbitrarily in the shaded area as 4 μg/ml. Each IgG/IgM pair was tested in duplicate against both viruses in three independent experiments. (D) Varying neutralization potency against WA-1, Beta, Delta, and Omicron BA.1 spike pseudoviruses for IgG (left) vs. IgM (right) mAbs. Error bars illustrate mean ± SD for three or more experiments with internal duplicates. (E) Summary neutralization potencies of IgG vs. IgM mAbs for the indicated variants in experiments described in D. Control, malaria-specific IgG or IgM (MaliA01 mAbs; Thouvenel et al., 2021). (F) Neutralization in a Vero cell plaque reduction assay of Delta SARS-CoV-2 for 297 IgG vs. IgM. Representative plot of five independent experiments.