The Genomic Organisation of the TRA/TRD Locus Validates the Peculiar Characteristics of Dromedary δ-Chain Expression

The role of gamma/delta T cells in vertebrate immunity is still an unsolved puzzle. Species such as humans and mice display a low percentage of these T lymphocytes (i.e., “ gamma/delta low species”) with a restricted diversity of gamma/delta T cell receptors (TR). Conversely, artiodactyl species (i.e., “ gamma/delta high species”) account for a high proportion of gamma/delta T cells with large gamma and delta chain repertoires. The genomic organisation of the gamma TR (TRG) and delta (TRD) loci has been determined in sheep and cattle, noting that a wide number of germline genes that encode for gamma and delta chains characterise their genomes. Taking advantage of the current improved version of the genome assembly, we have investigated the genomic structure and gene content of the dromedary TRD locus, which, as in the other mammalian species, is nested within the TR alpha (TRA) genes. The most remarkable finding was the identification of a very limited number of variable germline genes (TRDV) compared to sheep and cattle, which supports our previous expression analyses for which the somatic hypermutation mechanism is able to enlarge and diversify the primary repertoire of dromedary delta chains. Furthermore, the comparison between genomic and expressed sequences reveals that D genes, up to four incorporated in a transcript, greatly contribute to the increased diversity of the dromedary delta chain antigen binding-site