V-D-J REARRANGEMENTS AT THE T-CELL RECEPTOR DELTA-LOCUS IN MOUSE THYMOCYTES OF THE ALPHA-BETA LINEAGE

The T cell receptor (TCR) delta locus lies within the TCR alpha locus and is excised from the chromosome by V alpha-J alpha rearrangement. We show here that delta sequences persist in a large fraction of the DNA from mature CD4(+)CD8(-)alpha beta(+) mouse thymocytes. Virtually all delta loci in these cells are rearranged and present in extrachromosomal DNA. In immature alpha beta lineage thymocytes (CD3(-/lo)CD4(+)CD8(+)) and in CD4(+)CD8(-)alpha beta(+) thymocytes expressing a transgene-encoded alpha beta receptor, rearranged delta genes are present both in chromosomal and extrachromosomal DNA. Thus, contrary to earlier proposals, commitment to the alpha beta lineage does not require recombinational silencing of the delta locus or its deletion by a site-specific mechanism prior to V alpha-J alpha rearrangement.