A Second TCRδ Locus in Galliformes Uses Antibody-like V Domains: Insight into the Evolution of TCRδ and TCRμ Genes in Tetrapods

Analyses of the available avian genomes revealed the presence of a second TCR delta locus in the Galliformes. This second TCR delta locus is nonsyntenic to the conventional TCR alpha/delta and is unusual in that the V genes are more related to IgH V genes (VH) than to TCR V genes. The second TCR delta is not found in another avian lineage, the passerine zebra finch. Rather the finch's conventional TCR alpha/delta locus contains VH genes that are expressed with the conventional C delta gene, similar to what has been found in amphibians. A comparison between Galliformes and Passeriformes genomic organization suggests an origin of the second TCR delta in the former lineage involving gene duplication. Expression of these atypical TCR delta transcripts with a VH domain paired with C delta was found in lymphoid tissues of both avian lineages. The configuration of the second TCR delta in chicken and turkey is reminiscent of the TCR delta duplication that is present in nonplacental mammals and provides insight into the origin of the uniquely mammalian TCR mu locus. The Journal of Immunology, 2012, 188: 3912-3919.