Functionally responsive self‐reactive B cells of low affinity express reduced levels of surface IgM

Somatic gene rearrangement generates a diverse repertoire of B cells, many which have receptors possessing a range of affinities for self-Ag. Newly generated B cells express high and relatively uniform amounts of surface IgM (sIgM), while follicular (FO) B cells express sIgM at widely varying levels. It is plausible, therefore, that downmodulation of sIgM serves as a mechanism to maintain weakly self-reactive B cells in a responsive state by decreasing their avidity for self-Ag. We tested this hypothesis by performing comparative functional tests with FO IgMhi and IgMlo B cells from the unrestricted repertoire of WT C57BL/6 mice. We found that FO IgMlo B cells mobilized Ca2+ equivalently to IgMhi B cells when the same number of sIgM molecules was engaged. In agreement, FO IgMlo B cells were functionally competent to produce an antibody response following adoptive transfer. The FO IgMlo cell population had elevated levels of Nur77 transcript, and was enriched with nuclear-reactive specificities. Hybridoma sampling revealed that these B-cell receptors were of low affinity. Collectively, these results suggest that sIgM downmodulation by low-affinity, self-reactive B cells preserves their immunocompetence and circumvents classical peripheral tolerance mechanisms that would otherwise reduce diversity within the B cell compartment.