CHANGES IN LOCUS-SPECIFIC V(D)J RECOMBINASE ACTIVITY-INDUCED BY IMMUNOGLOBULIN GENE-PRODUCTS DURING B-CELL DEVELOPMENT

The process of V(D)J recombination is crucial for regulating the devel opment of B cells and for determining their eventual antigen specifici ty. Here we assess the developmental regulation of the V(D)J recombina se directly, by monitoring the double-stranded DNA breaks produced in the process of V(D)J recombination. This analysis provides a measure o f recombinase activity at immunoglobulin heavy and light chain loci ac ross defined developmental stages spanning the process of B cell devel opment. We find that expression of a complete immunoglobulin heavy cha in protein is accompanied by a drastic change in the targeting of V(D) J recombinase activity, from being predominantly active at the heavy c hain locus in pro-B cells to being exclusively restricted to the light chain loci in pre-B cells. This switch in locus-specific recombinase activity results in allelic exclusion at the immunoglobulin heavy chai n locus. Allelic exclusion is maintained by a different mechanism at t he light chain locus. We find that immature, but not mature, B cells t hat already express a functional light chain protein can undergo conti nued light chain gene rearrangement, by replacement of the original re arrangement on the same allele. Finally, we find that the developmenta lly regulated targeting of V(D)J recombination is unaffected by enforc ed rapid transit through the cell cycle induced by an E mu-myc transge ne.