DISCUSSION
During a typical CD8+ T cell immune response, a range of clonotypes are recruited, each with their own distinct affinity for the target epitope. In this study we identified an HCV-specific clonotype from an individual with acutely resolved infection that was monoclonal expanded and exhibited a strong affinity for the HLA-B*07:02GPR epitope, with a dissociation rate significantly smaller than the ones of previously reported TCRs13,14. Intriguingly, a polyclonal repertoire from a second individual that also generated a response against the same epitope also exhibited similar dissociation rates, suggesting that affinity alone may not be a sufficient factor regulating whether monoclonal or polyclonal populations arise.
Our findings provide evidence that high-affinity TCRs may be more common than previously observed, or particular epitopes may bias the recruitment of higher affinity receptors. Previous studies of TCR dissociations using the same assay have reported half-lives typically of up to 200 seconds 13,14,23, which was exceeded by all GPR-specific clonotypes in this study.
The affinity between a TCR and its target pMHC is particularly meaningful for dictating the activation potency of CD8+ T cell clones24 which may have contributed to the accelerated differentiation and expansion of the monoclonal GPR-specific clonotype at the expense of recruitment of additional clonotypes. It is plausible that the range of affinities measured from our colony expansions may have been biased to favour subsets such as central memory T cells which have greater capacity for clonal expansion in vitro by cytokine stimulation25 or high affinity clonotypes which are more responsive to memory recall responses 26,27.
The role of affinity in determining T cell fate remains unclear, with contrasting reports on the role of high affinity in driving memory T cell fate28-30. High-affinity clones were found dominating during the acute phase of infection, while under chronic conditions, low-affinity repertoires may be favoured through mechanisms such as mutational escape epitopes upon viral evolution or re-infection18. Our new study is consistent with a model that high-affinity memory cells can be detected in the acute phase of viral infection, and these are phenotypically and transcriptionally distinct from effector cells that contribute to IFN-𝛾 production19. In this study we studied high-risk behaviour individuals who inject drugs, thus it is likely that continuous exposure to HCV may contribute to the formation of memory populations, and therefore to future protective immunity31,32. Nevertheless, more studies are needed to dissect the role of affinity in determining the fate of a cytotoxic T cell response33 and whether the affinity profile of memory populations continue to be shaped by future antigen encounter.
Our findings will benefit the growing field of personalized immunotherapies involving the rational TCR design. Notably, rational engineering of the monoclonal receptor offers the possibility to further enhance its affinity, for example to synthesize specific high-affinity TCRs against neoantigen targets34.