Thymocyte differentiation in gamma-irradiated severe-combined immunodeficient mice: characterization of intermediates and products of V(D)J recombination at the T cell receptor alpha locus.

Treatment with DNA-damaging agents promotes rescue of V(D)J recombination, limited thymocyte differentiation, and development of thymic lymphomas in severe-combined immunodeficient (SCID) mice. One intriguing aspect of this system is that irradiation rescues rearrangements at the T cell receptor (TCR) beta, gamma and delta loci, but not at the TCR alpha locus. Current models posit that only those loci that are recombinationally active at the time of irradiation can be rescued. Here, we employ sensitive, semiquantitative ligation-mediated polymerase chain reaction assays to detect a specific class of recombination intermediates, hairpin coding ends, at the TCR alpha locus. We found that J alpha-coding ends are undetectable in unirradiated SCID thymocytes, but accumulate after irradiation at times coincident with the emergence of a CD4+ CD8+ thymocyte population. Coding joints produced by joining of these ends, however, are extremely rare. To test whether the presence of hairpin coding ends at TCR alpha is sufficient for irradiation-mediated rescue of coding joint formation, we administered a second dose of gamma-irradiation after abundant CD4+ CD8+ thymocytes and hairpin TCR alpha coding ends had accumulated. This treatment failed to stimulate rescue of TCR alpha coding joints. Thus, the presence of hairpin coding ends at the time of irradiation, while perhaps necessary, is not sufficient for rescue of V(D)J rearrangements. These results support a refined model for irradiation-mediated rescue of TCR rearrangements in SCID mice.