Biased T-cell receptor delta element recombination in scid thymocytes

Thymocytes in mutant mice with severe combined immunodeficiency (scid thymocytes) show ongoing recombination of some T-cell receptor delta gene elements, generating signal joints quantitatively and qualitatively indistinguishable from those in wild-type fetal thymocytes. Excised D delta 2-J delta 1 and D delta 1-D delta 2 rearrangements are detectable at levels equivalent to or greater than those in thymocytes from wild-type mice on fetal day 15. Signal junctional modification, shown here to occur frequently in wild-type adult but not newborn excised D delta 2-J delta 1 junctions, can occur normally in adult scid thymocytes. Excised D delta 1-D delta 2 scid junctions, similar to wild-type thymocytes, include pseudonormal coding junctions as well as signal junctions. Inversional D delta 1-D delta 2 rearrangements, generating conventional hybrid junctions, are also reproducibly detectable in scid thymus DNA. These hybrids, unlike those reported for artificial recombination constructs, do not show extensive nucleotide loss. In contrast to the normal or high incidences of D delta 1-, D delta 2-, and J delta 1-associated signal junctions in scid thymocytes, V delta 1, V gamma 3, and V gamma 1.2 signal products are undetectable in scid thymocytes or are detectable at levels at least 10-fold lower than the levels in wild-type fetal thymocytes. These findings confirm biased T-cell receptor element recombination by V(D)J recombinase activity of nontransformed scid thymocytes and indicate that analysis of in vivo-mediated gene rearrangements is important for full understanding of how the scid mutation arrests lymphocyte development.