TCR-delta gene rearrangement and selection during fetal thymocyte development

TCR delta-chain gene rearrangements were analyzed at different stages of thymic ontogeny. The VDJ delta junctional sequences of the dominantly expressed TCR delta-chain V7 subfamily are highly conserved in fetal and neonatal thymocytes. They are equally conserved in C delta-targeted mice lacking cell surface expression of gamma delta receptors, indicating evolutionary selection acting at the level of DNA rearrangement. Yet, in C delta-mutant mice, the frequency of in-frame transcripts is reduced to 61%, from 88% in wild-type mice, suggesting cellular selection of this DV7-overexpressing gamma delta thymocyte subset. In contrast, in genomic DNA rearrangements, no difference was found in the frequency of productive rearrangements between mutant (26%) and wild-type mice (31%). This in-frame rate, characteristic of random rearrangements, indicates that positive selection is not required for thymic maturation of gamma delta T cells. The results are discussed with regard to models of alpha beta/gamma delta lineage commitment.