The known ligands of γδ T cells are diverse and are recognized directly, without the requirement for antigen processing and presentation. However, only a few γδ T-cell ligands have been defined, and it is unclear how they are recognized by γδ-T-cell receptors (γδ-TCRs). Now, reporting in Science, Sunny Shin and colleagues show that a naturally occurring population of γδ T cells have a germline-encoded motif in the complementarity-determining region 3 (CDR3) of the TCR δ-chain, the presence of which correlates strongly with recognition of their ligand. This finding was supported by a crystallography study by Erin Adams and colleagues that was published in the same issue of Science.

Previous studies have shown that the ligands for a high proportion (0.1–2%) of γδ T cells in unimmunized mice are the highly homologous non-classical MHC class I molecules T10 and T22, so Shin et al. set out to determine how this γδ-TCR repertoire is generated. First, they isolated T22-specific γδ T cells from both the spleen and the small intestine, using a T22-tetramer staining reagent, and determined the sequence of individual γδ-TCRs. Because tissue-specific distribution of γδ T cells correlates strongly with Vγ (variable gene segment of TCR-γ) usage, it has been postulated that Vγ-encoded residues mediate the recognition of tissue-specific antigens. However, Shin et al. found that a variety of Vγ and Vδ gene segments were associated with T22-specific TCRs, and after comparing the TCR sequences of T22-tetramer-positive and -negative γδ T cells, they concluded that Vγ usage is reflective of tissue origin rather than antigen specificity.

Although there were no sequences conserved among TCR γ-chains, most T22-specific γδ T cells from the spleen (∼40–60%) and intestine (∼90%) had a conserved sequence — W-(S)EGYEL — in the CDR3 of TCR-δ, and this sequence was absent in T22-tetramer-negative γδ T cells. The motif was found to be encoded by the Vδ or Dδ1 (diversity) gene segment (W), the Dδ2 gene segment (SEGYE; as γδ-TCRs can incorporate two D segments into a single TCR δ-chain) and a P-nucleotide addition (L), indicating that recognition results from a sequence that is mainly germline encoded (SEGYE).

These findings were supported by the crystal structure of T22 complexed with the TCR of the γδ T-cell clone G8, which was determined by Adams and colleagues. Most of the contact residues between G8 (which also contains the TCR-δ CDR3 motif) and T22 are contributed by the CDR3. Moreover, the CDR3 loop seems to be anchored by the W, G, Y, E and L residues of the identified motif.

This finding is in contrast to recognition by the αβ-TCR, which uses all three CDRs to contact antigen and in which the most crucial residues (in the CDR3 of TCR-α and TCR-β) are not germline encoded but encoded by N nucleotides incorporated during junctional recombination.

The germline-encoded nature of the motif is unexpected given that the possible use of two D regions in TCR-δ provides γδ TCRs with the highest possible diversity of all antigen receptors, as a result of the increased potential for junctional diversity. Shin et al. showed that variation in the residues adjacent to the CDR3 motif resulted in the population of T22-specific γδ T cells having a range of affinities for T22. They speculate that this could be useful for flexible responses to the self-antigen T22 and that, if specificity for other antigens is encoded in a similar manner, then γδ T cells are likely to recognize relatively few antigens (compared with B-cell receptors and αβ-TCRs) but at a high frequency.