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.

During the past four years several livestock farms (sheep, cattle and river buffalo) in the provinces of Naples and Caserta (southern Italy) have been unable to sell their milk and other dairy products due to the levels of dioxins (17 different types) present in the milk mass exceeding the value permitted [3 pg/g of fat, as human WHO 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin) equivalent]. While some farms, especially those showing relatively low levels of dioxins, have managed to reduce the dioxins in the milk below the permitted threshold by changing the diet, many, especially sheep farms, have failed to do so. Indeed, about 12 000 head of cattle, river buffaloes and sheep have so far been culled. In the present study, 34 and 42 sheep from two herds raised in the province of Naples (Acerra municipality) and where high levels of dioxins (50.65 and 39.51 pg/g of fat, respectively) were found in the milk mass, were cytogenetically investigated and compared with 20 sheep (control) raised 80 km away from the exposed area. Increases of both chromosome abnormalities (gap, chromosome and chromatid breaks) (17 and 8 times higher in the two exposed herds, respectively) and sister chromatid exchanges (SCEs) were found in both herds when compared with the control, and the differences were highly significant (P < 0.001). No statistical differences were found when comparing the frequencies of aneuploid cells of exposed animals (16.4 and 17.8%) and control (17.9%). Furthermore, high levels of mortality and abnormal foetuses were recorded in one of the two farms when compared with the control.

In mammals, T cells develop along two discrete pathways characterized by expression of either the αβ or the γδ T cell receptors. Human and mouse display a low peripheral blood γδ T cell percentage ("γδ low species") while sheep, bovine and pig accounts for a high proportion of γδ T lymphocytes ("γδ high species"). While the T cell receptor alpha (TRA) and delta (TRD) genes and the genomic organization of the TRA/TRD locus has been determined in human and mouse, this information is still poorly known in artiodactyl species, such as sheep. The analysis of the current Ovis aries whole genome assembly, Oar_v3.1, revealed that, as in the other mammalian species, the sheep TRD locus is nested within the TRA locus. In the most 5’ part the TRA/TRD locus contains TRAV genes which are intermingled with TRDV genes, then TRD genes which include seven TRDD, four TRDJ genes, one TRDC and a single TRDV gene with an inverted transcriptional orientation, and finally in the most 3’ part, the TRA locus is completed by 61 TRAJ genes and one TRAC gene. Comparative sequence and analysis and annotation led to the identification of 66 TRAV genes assigned to 34 TRAV subgroups and 25 TRDV genes belonging to the TRDV1 subgroup, while one gene was found for each TRDV2, TRDV3 and TRDV4 subgroups. Multiple duplication events within several TRAV subgroups have generated the sheep TRAV germline repertoire, which is substantially larger than the human one. A significant proportion of these TRAV gene duplications seems to have occurred simultaneously with the amplification of the TRDV1 subgroup genes. This dynamic of expansion has also generated novel multigene subgroups, which are species-specific. Ovis aries TRA and TRD genes identified in this study were assigned IMGT definitive or temporary names and were approved by the IMGT/WHO-IUIS nomenclature committee. The completeness of the genome assembly in the 3' part of the locus has allowed us to interpret rearranged CDR3 of cDNA from both TRA and TRD chain repertoires. The involvement of one up to four TRDD genes into a single transcript makes the potential sheep TRD chain much larger than any known TR chain repertoire. The sheep genome, as the bovine genome, contains a large and diverse repertoire of TRA and TRD genes when compared to the “γδ T cell low” species genomes. The composition and length of the rearranged CDR3 in TRD V-delta domains influence the three-dimensional configuration of the antigen-combining site thus suggesting that in ruminants, γδ T cells play a more important and specific role in immune recognition.

In the last 3 years several farms raising cattle, river buffalo and sheep have been unable to sell dairy milk due to the presence of high levels of dioxins. Furthermore, several cases of abortion (around 25% of total births) and abnormal foetuses (2.5% of total births) were recorded in two flocks of sheep raised in the province of Naples where a higher level of dioxins (5.27 pg/g fat, as human WHO TCDD equivalent) have been found in the milk mass than that permitted (3.0 pg/g fat, as human WHO TCDD equivalent). Cytogenetic investigations were carried out on 24 sheep (all females), randomly sampled from the two different flocks, one abnormal foetus and 11 female sheep (control) raised ∼80 km from the area where the two exposed flocks were raised. Frequencies of aneuploid cells, gaps, chromatid breaks, chromosome breaks, fragments and sister chromatid exchange (SCE) were determined. While no differences were observed between the number of aneuploid cells (15% of total cell population) of both exposed animals and controls, significant (P < 0.001) increases in the frequencies of other chromosome abnormalities (mean chromosome abnormality/cell = 0.76 ± 1.1) and SCEs (mean SCE/cell = 9.4 ± 3.7) were found in the exposed animals, compared with the control (mean chromosome abnormality/cell = 0.18 ± 0.4; mean SCE/cell = 7.1 ± 3.0). Significantly higher values of SCEs (mean SCE/cell = 10.9 ± 4.4) were also found in the abnormal foetus compared with the control. Chemical analyses on soil, grass and water at two sites where the two flocks were pastured established that doses of dioxins (17 different types) were below the legally permitted limits.