Extensive analysis of D-J-C arrangements allows the identification of different mechanisms enhancing the diversity in sheep T cell receptor beta-chain repertoire.
BACKGROUND
In most species of mammals, the TRB locus has the common feature of a library of TRBV genes positioned at the 5'- end of two in tandem aligned D-J-C gene clusters, each composed of a single TRBD gene, 6-7 TRBJ genes and one TRBC gene. An enhancer located at the 3'end of the last TRBC and a well-defined promoter situated at the 5'end of the TRBD gene and/or a undefined promoter situated at the 5'end of the TRBD2 are sufficient to generate the full recombinase accessibility at the locus. In ruminant species, the 3'end of the TRB locus is characterized by the presence of three D-J-C clusters, each constituted by a single TRBD, 5-7 TRBJ and one TRBC genes with the center cluster showing a structure combined with the clusters upstream and downstream, suggesting that a unequal crossover occurred in the duplication. An enhancer downstream the last TRBC, and a promoter at the 5'-end of each TRBD gene are also present.


RESULTS
In this paper we focused our attention on the analysis of a large number of sheep TR beta-chain transcripts derived from four different lymphoid tissues of three diverse sheep breed animals to certify the use and frequency of the three gene clusters in the beta-chain repertoire. As the sheep TRB locus genomic organization is known, the exact interpretation of the V-D-J rearrangements was fully determined. Our results clearly demonstrate that sheep beta-chain constitutes a level of variability that is substantially larger than that described in other mammalian species. This is due not only to the increase of the number of D and J genes available to the somatic recombination, but also to the presence of the trans-rearrangement process. Moreover, the functional complexity of beta-chain repertoire is resolved by other mechanisms such as alternative cis- and trans-splicing and recombinational diversification that seems to affect the variety of the constant region.


CONCLUSIONS
All together our data demonstrate that a disparate set of molecular mechanisms operate to perform a diversified repertoire in the sheep beta-chain and this could confer some special biological properties to the corresponding alphabeta T cells in the ruminant lineage.
Di Tommaso
Silvia
S
Dipartimento di Scienze e Tecnologie Biologiche ed Ambientali, Universita' del Salento, Lecce, Italy. silvia.dt@libero.it;
Antonacci
Rachele
R
Ciccarese
Salvatrice
S
Massari
Serafina
S
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D022801:Complementarity Determining Regions / Q000235:genetics; D015723:Gene Library; D015333:Gene Rearrangement, beta-Chain T-Cell Antigen Receptor; D008969:Molecular Sequence Data; D005810:Multigene Family; D020033:Protein Isoforms / Q000235:genetics; D016693:Receptors, Antigen, T-Cell, alpha-beta / Q000235:genetics*; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D034561:Sheep, Domestic / Q000235:genetics*
D022801:Complementarity Determining Regions; D020033:Protein Isoforms; D016693:Receptors, Antigen, T-Cell, alpha-beta