The dynamic TCRδ: TCRδ chains in the amphibian Xenopus tropicalis utilize antibody-like V genes

The content and organization of the Xenopus tropicalis TCR alpha/delta locus was determined. This locus is highly conserved among tetrapods, with the genes encoding the TCR delta chains embedded with those encoding TCR alpha. However, the frog TCR alpha/delta is unusual in that it contains V genes that appear indistinguishable from those in the IgH locus (VH). These V genes, termed VH delta, make up 70% of the V genes at the TCR delta locus and are expressed exclusively in TCR delta chains. Finding TCR delta chains that use antibody-like V domains in frogs is similar to the situation in shark TCR delta variants and TCR mu in marsupials. These results suggest that such unconventional TCR may be more widespread across vertebrate lineages than originally thought and raise the possibility of previously unrealized subsets of T cells. We also revealed close linkage of TCR alpha/delta, IgH, and Ig lambda in Xenopus which, in combination with linkage analyses in other species, is consistent with the previous models for the emergence of these antigen receptor loci.