Comprehensive analysis and characterization of the TCR α chain sequences in the common marmoset

A new method for quantitative analysis of the T cell receptor V region repertoires in healthy common marmosets by microplate hybridization assay

The common marmoset, Callithrix jacchus, is one of the smallest primates and is increasingly used for an experimental nonhuman primate model in many research fields. Analysis of T cell receptor (TCR) repertoires is a powerful tool to investigate T cell immunity in terms of antigen specificity and variability of TCR expression. However, monoclonal antibodies specific for many TCR Vα or Vβ chains have not been created. We have recently identified a large number of TCRα chain variable (TRAV) and TCRβ chain variable (TRBV) sequences from a cDNA library of common marmosets. The purpose of this study is to develop a new method for analysis of TCR repertoires in the common marmoset using this sequence information. This method is based on a microplate hybridization technique using 32 TRAV-specific and 32 TRBV-specific oligoprobes following an adaptor-ligation PCR. This enables the easy quantitation of the respective TRAV and TRBV expression levels. No cross-hybridization among specific-oligoprobes and very low variances in repeated measures of the same samples was found, demonstrating high specificity and reproducibility. Furthermore, this method was validated by an antihuman Vβ23 antibody which specifically bound to marmoset Vβ23. Using this method, we analyzed TCR repertoires from various tissue samples (PBMCs, spleen, lymph node and thymus) and isolated T cell subpopulations (CD4+CD8+, CD4+CD8− and CD4−CD8+) from the thymus of 10 common marmosets. Neither tissue-specific nor T cell subpopulation-specific differences was found in TRAV and TRBV repertoires. These results suggest that, unlike mice, TCR repertoires in the common marmoset are not affected by endogenous superantigens and are conserved among individuals, among tissues, and among T cell subpopulations. Thus, TCR repertoire analysis with high specificity and reproducibility is a very useful technique, with the potential to replace flow cytometric analysis using a panel of TRV-specific antibodies, many of which remain unavailable.

Increased positive selection pressure within the complementarity determining regions of the T-cell receptor β gene in New World monkeys

Because of the long-term co-evolution of TCR and MHC molecules, numerous nucleotide substitutions have accumulated within the domains of TCRβ genes. We previously found that nonsynonymous nucleotide substitutions occurred more frequently in complementarity determining region (CDR)β than in CDRα, even though only a limited number of common marmoset (Callithrix jacchus) and human T-cell receptor β variable (TRBV) sequences were compared. This interesting finding raised the question of whether the increased selective pressure within CDRβ was species-specific. In this study, we identified 21 TRBV region sequences from the common marmoset and performed comparative sequence analyses of the T-cell receptor α variable (TRAV) and TRBV regions from human, chimpanzee, rhesus monkey, cotton-top tamarin, Ma's night monkey, and common marmoset. The ratios of the number of nonsynonymous nucleotide substitutions per site (d(N) ) to the d(S) values (d(N) /d(S) ) were less than 1 within the framework regions (FRs) of TRAV and TRBV region sequences, suggesting that purifying selection is largely dominant within the FRs. In contrast, the d(N) values were statistically significantly greater for CDRβ than for CDRα only in New World monkeys. Also, increased d(N) /d(S) ratios (d(N) /d(S) >1) were observed within CDRβ between humans and New World monkeys and, interestingly, between New World monkeys, which share a relatively recent common ancestor. Moreover, phylogenetic analysis by maximum likelihood analysis provided firm evidence to support that positive selection occurred within CDRβ along New World monkey lineages. These results suggest that increased positive selection pressure within CDRβ is common in New World monkeys rather than being species-specific. This study provides an intriguing insight into the co-evolution of TCR and MHC molecules within primates.