V-H gene replacement in thymocytes

The quasi-monoclonal (QM) mouse has a functionally rearranged H chain gene inserted into its natural position in the IgH locus. In this position, the H chain gene is subject to many of the same activities as normally arranged H chain genes, including somatic hypermutation, V-H gene replacement, and class switch recombination. Here, we have used this mouse strain to determi ne some of the rules that govern the V(D)J recombination activity of the Ig H locus in thymus. We focused on the requirements for V-H gene replacement. In normal mice, thymic DJ(H) rearrangements are common, but VDJ(H) rearran gements are not, We found intermediate products of V-H replacement in doubl e-positive CD4(+)CD8(+) cells of the QM thymus, demonstrating that the inse rted V-H gene was accessible and ruling out the possibility that a V-H gene per se cannot be rearranged in the thymus. We found transcripts from the k nocked-in H chain gene of QM, but no mu H chain protein was detectable in t hymocytes. Cloning and sequencing of these transcripts revealed that some h ad been generated by V-H gene replacement. Corresponding signal joints coul d also be identified. These results suggest that neither a B cell-specific signal nor an Ig protein are necessary to activate V-H-to-VDJ(H) joining in thymocytes. Possible mechanisms remaining to account for overcoming the ba rrier to V-H joining in thymocytes include the insertion of a transcription ally active gene segment and/or the inactivation of a silencer.