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T lymphocytes are the principal actors of vertebrates’ cell-mediated immunity. Like B cells, they can recognize an unlimited number of foreign molecules through their antigen-specific heterodimer receptors (TRs), which consist of αβ or γδ chains. The diversity of the TRs is mainly due to the unique organization of the genes encoding the α, β, γ, and δ chains. For each chain, multi-gene families are arranged in a TR locus, and their expression is guaranteed by the somatic recombination process. A great plasticity of the gene organization within the TR loci exists among species. Marked structural differences affect the TR γ (TRG) locus. The recent sequencing of multiple whole genome provides an opportunity to examine the TR gene repertoire in a systematic and consistent fashion. In this review, we report the most recent findings on the genomic organization of TRG loci in mammalian species in order to show differences and similarities. The comparison revealed remarkable diversification of both the genomic organization and gene repertoire across species, but also unexpected evolutionary conservation, which highlights the important role of the T cells in the immune response.

These data are presented in support of structural and evolutionary analysis of the published article entitled “The occurrence of three D-J-C clusters within the dromedary TRB locus highlights a shared evolution in Tylopoda, Ruminantia and Suina” (Antonacci et al., 2017) [1]. Here we describe the genomic structure and the gene content of the T cell receptor beta chain (TRB) locus in Camelus dromedarius. As in the other species of mammals, the general genomic organization of the dromedary TRB locus consists of a pool of TRBV genes located upstream of in tandem TRBD-J-C clusters, followed by a TRBV gene with an inverted transcriptional orientation. A peculiarity of the dromedary TRB locus structure is the presence of three TRBD-J-C clusters, which is a common feature of sheep, cattle and pig sequences.

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The α/β T cell receptor (TR) is a complex heterodimer that recognizes antigenic peptides and binds to major histocompatibility complex (MH) molecules. Both α and β chains are encoded by different genes localized on two distinct chromosomal loci: TRA and TRB. The present study employed the recent release of the swine genome assembly to define the genomic organization of the TRB locus. According to the sequencing data, the pig TRB locus spans approximately 400 kb of genomic DNA and consists of 38 TRBV genes belonging to 24 subgroups located upstream of three in tandem TRBD-J-C clusters, which are followed by a TRBV gene in an inverted transcriptional orientation. Comparative analysis confirms that the general organization of the TRB locus is similar among mammalian species, but the number of germline TRBV genes varies greatly even between species belonging to the same order, determining the diversity and specificity of the immune response. However, sequence analysis of the TRB locus also suggests the presence of blocks of conserved homology in the genomic region across mammals. Furthermore, by analysing a public cDNA collection, we identified the usage pattern of the TRBV, TRBD, and TRBJ genes in the adult pig TRB repertoire, and we noted that the expressed TRBV repertoire seems to be broader and more diverse than the germline repertoire, in line with the presence of a high level of TRBV gene polymorphisms. Because the nucleotide differences seems to be principally concentrated in the CDR2 region, it is reasonable to presume that most T cell β-chain diversity can be related to polymorphisms in pig MH molecules. Domestic pigs represent a valuable animal model as they are even more anatomically, genetically and physiologically similar to humans than are mice. Therefore, present knowledge on the genomic organization of the pig TRB locus allows the collection of increased information on the basic aspects of the porcine immune system and contributes to filling the gaps left by rodent models.

Background: In humans and mice ("γδ low species") less than 5% of the peripheral blood T lymphocytes are gamma/delta T cells, whereas in chicken and artiodactyls ("γδ high species") gamma/delta T cells represent about half of the T cells in peripheral blood. In cattle and sheep (Bovidae) two paralogous T cell receptor gamma loci (TRG1 and TRG2) have been found. TRG1 is located on 4q3.1, within a region of homology with the human TRG locus on chromosome 7, while TRG2 localizes on 4q2.2 and appears to be unique to ruminants. The purpose of this study was the sequencing of the genomic regions encompassing both loci in a "γδ high" organism and the analysis of their evolutionary history.