Entering the Age of Specificity in T Cell Immunity.

The discovery of MHC restriction, the resolution of peptide-MHC structure, and the identification of the T cell receptor gene quickly centered epitope-level specificity as a core concern of T cell biology. Epitope mapping culminated in functional peptide-based stimulation assays and tetramer staining, which together provide exquisite resolution of individual responses. Despite these tools, quantitative assessment of epitope-resolved responses in complex human infections and tumors has rarely been applied at scale. As a result, no epitope-aware correlates of protection or risk have been established for major infections, and most human T cell profiling still occurs at the level of representative epitopes or bulk assays. In this review, I propose that variation in specificity, defined as distinct epitope-level responses across individuals, represents a largely untested critical element in our assessment of anti-pathogen and antitumor immunity. Advances in TCR repertoire profiling and reverse epitope discovery now make formal tests of this hypothesis tractable, and I discuss approaches to determine the extent to which epitope variation underlies the noise and variation of human immune phenotypes.