Diversity and evolution of T-cell receptor variable region genes in mammals and birds

The receptor of a T lymphocyte (TCR) recognizes nonself antigens in the com pany of major histocompatibility complex (MHC) molecules presented to it by the antigen-presenting cell. The variable region of TCR is encoded by eith er a concatenation of variable region (TCR-V), diversity region (TCR-D), an d joining region (TCR-J) genes, or a concatenation of TCR-V and TCR-J genes . The TCR-V genes exist as a multigene family in vertebrate species. Here w e study the evolutionary relationships of TCR-V genes from humans, sheep, c attle, rabbits, mice, and chicken. These six species can be classified into two groups according to the frequency of gamma delta T-cells in their peri pheral T-cell populations. The "gamma delta low" group of species includes humans and mice, in which gamma delta T-cells constitute very limited porti on of the T-cell population. The "gamma delta high'' group includes sheep, cattle, rabbits, and chicken, in which gamma delta T-cells comprise up to 6 0% of the T-cell population. Here, we compiled TCR-V sequences from the six species and conducted a phylogenetic analysis. We identified various TCR-V gene subgroups based on the analysis. We found that humans and mice have r epresentatives from nearly all of the subgroups identified, while other spe cies have lost subgroups to different extent. Therefore, the ya low species have a high degree of diversity of TCR-V genes, while ya high species all have limited diversity of TCR-V genes. This pattern is similar to that foun d for immunoglobulin variable region (IGV) genes.