Functional genomics and evolution of the gamma/delta T cell receptor loci in Old World Camels

The Camelidae species occupy a peculiar niche within the adaptive immune response and the camel lineage has been proposed as a fascinating model in the evolution of immune systems. In fact, the significant amount of special heavy chain-only antibodies in the serum, in addition to the tetrameric IgGs along with a repertoire, largely diversified by extensive somatic hypermutation, result in novel paratopes different from those of conventional antibodies. Moreover, for the first time in a mammalian organism, it was shown that T cell receptor evolution has been favoured in the dromedary by mutation in the productively rearranged gamma (TRG) and delta (TRD) genes, thus contributing to the repertoire diversity of γδ heterodimer. Under this scenario, in order to gain further insights into the function and evolution of the T cell receptor gamma/delta heterodimer, we investigated the genomic structure and the gene content of the TRG and TRD loci in Old World camels. The availability of recent draft genomes and new re-sequenced data of the three species, Camelus dromedarius (n = 7), Camelus bactrianus (n = 9) and Camelus ferus (n = 9) (at an average 15-fold coverage using the Illumina HiSeq2000) allowed us to determine the map of the entire TRG locus. It spans approximately 198 kb, it is flanked at its 5' end by the AMPH gene and it contains three V-J-C cassettes. Furthermore, the retrieval of relevant contigs, about 900 kb long, confirmed that TRD locus with its V, D, J and C genes, as in eutherians and birds, is clustered within the TRA locus.