Genomic organization of the sheep TRG1@ locus and comparative analyses of Bovidae and human variable genes

Abstract

γδ T cells commonly account for 0.5%–5% of human (γδ low species) circulating T cells, whereas they are very common in chickens, and they may account for > 70% of peripheral cells in ruminants (γδ high species). We have previously reported the ovine TRG2@ locus structure, the first complete physical map of any ruminant animal TCR locus. Here we determined the TRG1@ locus organization in sheep, reported all variable ( V) gamma gene segments in their germline configuration and included human and cattle sequences in a three species comparison. The TRG1@ locus spans about 140 kb and consists of three clusters named TRG5, TRG3, and TRG1 according to the constant ( C) genes. The predicted tertiary structure of cattle and sheep V proteins showed a remarkably high degree of conservation between the experimentally determined human Vγ9 and the proteins belonging to TRG5 Vγ subgroup. However systematic comparison of primary and tertiary structure highligthed that in Bovidae the overall conformation of the γδ TCR, is more similar to the Fab fragment of an antibody than any TCR heterodimer. Phylogenetic analysis showed that the evolution of cattle and sheep V genes is related to the rearrangement process of V segments with the relevant C, and consequentely to the appartenence of the V genes to a given cluster. The TRG cluster evolution in cattle and sheep pointed out the existence of a TRG5 ancient cluster and the occurrence of duplications of its minimal structural scheme of one V, two joining ( J), and one C.