The roles of preB and B cell receptors in the stepwise allelic exclusion of mouse IgH and L chain gene loci

Membrane-bound preBCR of wild-type mice, and probably also preBCR-like V-preB mu H chain complexes in lambda(5)-deficient mice, signal allelic exclusion so that < 0.1% of all preB-II cells and all subsequent B lineage cells express two mu H chains on their surface On the of her hand a large number of mu H chains which are originally generated at the transition of preB-I to preB-II cells cannot pair with surrogate L chains, cannot form a preBCR on the surface and, hence, allow two H chain alleles to be productively rearranged in one B-lineage cell. By contrast membrane-bound BCR on immature B cells does not signal allelic or isotypic exclusion of Ig kappa L and lambda L chain gene loci. This allows the rearrangement machinery to remain active, and secondary L chain rearrangements on one kappa L chain allele are frequently observed. Rapid selection of fitting H/L chain pairs, forming BCR on the surface, allows B-lineage cells to enter the mature B cell pool where the rearrangement machinery is shut off, securing allelic exclusion oft chain loci in most B cells.