The role of short homology repeats and TdT in generation of the invariant γδ antigen receptor repertoire in the fetal thymus

Fetal thymic and adult epithelial Vγ3⁺ and Vγ4⁺ T cells express γδ antigen receptors (TCR) with invariant junctions lacking N nucleotides. Using transgenic recombination substrates, we show that di- or trinucleotide repeats, either in the coding region or in P elements, have strong effects on the site of recombination. In other mice bearing a terminal deoxynucleotidyl transferase (TdT) transgene under the control of the CD2 promoter, we found that the frequency of canonical junctions was markedly reduced with a concomitant increase in in-frame noncanonical junctions with N nucleotides. Together, our results show that short homology repeats direct the site of rearrangement and thus play a critical role in the generation of γδ T cell receptor canonical junctions. Increased TdT activity in Vγ3⁺ T cells has a inhibitory effect on junctional homogeneity in these cells.