Autoreactive B cell receptors mimic autonomous pre-B cell receptor signaling and induce proliferation of early B cells.

The majority of early immature B cells express autoreactive B cell receptors (BCRs) that are, according to the current view, negatively selected to avoid the production of self-reactive antibodies. Here, we show that polyreactive BCRs, which recognize multiple self-antigens, induced autonomous signaling and selective expansion of B cell precursors in a manner comparable to the pre-BCR. We found that the pre-BCR was capable of recognizing multiple self-antigens and that a signaling-deficient pre-BCR lacking the non-Ig region of the surrogate-light-chain component lambda5 was rescued by the complementarity-determining region 3 derived from heavy chains of polyreactive receptors. Importantly, bone marrow B cells from mice carrying Ig transgenes for an autoreactive BCR showed increased cell-cycle activity, which could not be detected in cells lacking the transgenic BCR. Together, the pre-BCR has evolved to ensure self-recognition because autoreactivity is required for positive selection of B cell precursors.