Insights on CD8+ T cell self-tolerance from studies of a polyclonal melanocyte-specific population using a novel mouse model

Abstract

Self-specific CD8+ T cells can escape clonal deletion, but the properties and capabilities of such cells and the mechanisms restraining them require further clarification. Much of the data on self-tolerance originate from mice with genetically-manipulated TCR repertoires, and there is a need for improved models that better recapitulate normal physiology. We established a novel self-tolerance model focused on polyclonal CD8+ T cells specific for tyrosinase-related protein 2 (Trp2), a melanocyte enzyme encoded by the dopachrome tautomerase gene (Dct). We examined mice expressing (wild-type [WT]) or lacking (Dct-/-) this enzyme; Trp2 represents a self-antigen in WT mice and a foreign antigen in Dct-/- mice. Phenotypic and gene expression profiles of pre-immune Trp2/Kb-specific cells were similar; the population size was only slightly reduced in WT relative to Dct-/- mice. Despite comparable initial responses to Trp2 immunization, WT Trp2/Kb-specific cells showed blunted expansion and less readily differentiated into a CD25+ proliferative population. Functional self-tolerance clearly emerged when assessing immunopathology: adoptively transferred WT Trp2/Kb-specific cells mediated vitiligo much less efficiently. Examination of the T cell receptor (TCR) repertoire as characterized by single-cell sequencing revealed clonotypes shared both within and between the WT and Dct-/- Trp2/Kb-specific populations early after activation. Clonal expansions were more frequent among the Dct-/- repertoire, suggesting that higher-performing cells were more likely to be eliminated in WT animals. Our physiologically-relevant model demonstrates that CD8+ T cells can exhibit functional, cell-intrinsic self-tolerance while sharing many features with non-tolerant cells. This model will be useful in further mechanistic investigation of self-tolerance that may ultimately lead to more effective therapeutic manipulation of the immune system.