Basal Igalpha/Igbeta signals trigger the coordinated initiation of pre-B cell antigen receptor-dependent processes

The pro-B to pre-B transition during B cell development is dependent upon surface expression of a signaling competent pre-B cell Ag receptor (pre-BCR). Although the mature form of the BCR requires ligand-induced aggregation to trigger responses, the requirement for ligand-induced pre-BCR aggregation in promoting B cell development remains a matter of significant debate. In this study, we used transmission electron microscopy on murine primary pro-B cells and pre-B cells to analyze the aggregation state of the pre-BCR. Although aggregation can be induced and visualized following cross-linking by Abs to the pre-BCR complex, our analyses indicate that the pre-BCR is expressed on the surface of resting cells primarily in a nonaggregated state. To evaluate the degree to which basal signals mediated through nonaggregated pre-BCR complexes can promote pre-BCR-dependent processes, we used a surrogate pre-BCR consisting of the cytoplasmic regions of Igalpha/Igbeta that is targeted to the inner leaflet of the plasma membrane of primary pro-B cells. We observed enhanced proliferation in the presence of low IL-7, suppression of V(H)(D)J(H) recombination, and induced kappa light (L) chain recombination and cytoplasmic kappa L chain protein expression. Interestingly, Igalpha/Igbeta-mediated allelic exclusion was restricted to the B cell lineage as we observed normal TCRalphabeta expression on CD8-expressing splenocytes. This study directly demonstrates that basal signaling initiated through Igalpha/Igbeta-containing complexes facilitates the coordinated control of differentiation events that are associated with the pre-BCR-dependent transition through the pro-B to pre-B checkpoint. Furthermore, these results argue that pre-BCR aggregation is not a requirement for pre-BCR function.