A checkpoint for autoreactivity in human IgM⁺ memory B cell development

Autoantibodies are removed from the repertoire at two checkpoints during B cell development in the bone marrow and the periphery. Despite these checkpoints, up to 20% of the antibodies expressed by mature naive B cells in healthy humans show low levels of self-reactivity. To determine whether self-reactive antibodies are also part of the antigen-experienced memory B cell compartment, we analyzed recombinant antibodies cloned from single circulating human IgM⁺ memory B cells. Cells expressing antibodies specific for individual bacterial polysaccharides were expanded in the IgM⁺ memory compartment. In contrast, B cells expressing self-reactive and broadly bacterially reactive antibodies were removed from the repertoire in the transition from naive to IgM⁺ memory B cell. Selection against self-reactive antibodies was implemented before the onset of somatic hypermutation. We conclude that a third checkpoint selects against self-reactivity during IgM⁺ memory B cell development in humans.