Human rotavirus-specific igm memory B cells have differential cloning efficiencies and switch capacities and play a role in antiviral immunity in vivo

Protective immunity to rotavirus (RV) is primarily mediated by antibodies produced by RV-specific memory B cells (RV-mBc).Of note, most of these cells express IgM, but the function of this subset is poorly understood. Here, using limiting dilution assaysof highly sort-purified human IgM⁺ mBc, we found that 62% and 21% of total (non-antigen-specific) IgM⁺ and RV-IgM⁺ mBc,respectively, switched in vitro to IgG production after polyclonal stimulation. Moreover, in these assays, the median cloningefficiencies of total IgM⁺(17%) and RV-IgM⁺ (7%) mBc were lower than those of the corresponding switched (IgG⁺ IgA⁺) total(34%) and RV-mBc (17%), leading to an underestimate of their actual frequency. In order to evaluate the in vivo role of IgM⁺RV-mBc in antiviral immunity, NOD/Shi-scid interleukin-2 receptor-deficient (IL-2Rγ^null) immunodeficient mice were adoptivelytransferred highly purified human IgM⁺ mBc and infected with virulent murine rotavirus. These mice developed hightiters of serum human RV-IgM and IgG and had significantly lower levels than control mice of both antigenemia and viremia.Finally, we determined that human RV-IgM⁺ mBc are phenotypically diverse and significantly enriched in the IgM^hi IgD^low subset.Thus, RV-IgM⁺ mBc are heterogeneous, occur more frequently than estimated by traditional limiting dilution analysis, havethe capacity to switch Ig class in vitro as well as in vivo, and can mediate systemic antiviral immunity.