Isotype conversion of Staphylococcal-specific IgG into IgM broadens the reactivity to other bacterial pathogens.

Therapeutic antibodies are actively explored as alternative to treat or prevent bacterial infections. However, the narrow antigen specificity of IgG in combination with broad diversity in bacterial surface structures currently hampers the development of therapeutic antibodies against bacteria. Here we reveal that isotype conversion of three highly specific anti-staphylococcal antibodies from IgG into IgM does not only affect Fc effector functions but also modifies the interaction of Fab domains with bacterial surface antigens. These converted IgMs gain cross-reactivity for a broad range of bacterial species, including Gram-negatives such as Escherichia coli and Neisseria meningitidis and even protect against invasive infection with Streptococcus pyogenes in vivo. Mechanistic studies show that enhanced cross-specificity by IgM is conferred by changed ligand specificity and multivalent binding to high-density antigens. Altogether, these findings provide important insights for the development of antibody therapy for bacterial infections.