Self-specific CD8+T cells can escape clonal deletion, but the properties and capabilities of such cells in a physiological setting are unclear. We characterized polyclonal CD8+ T cells specific for the melanocyte antigen tyrosinase-related protein 2 (Trp2) in mice expressing or lacking this enzyme (due to deficiency in Dct, which encodes Trp2). Phenotypic and gene expression profiles of pre-immune Trp2/Kb-specific cells were similar; the size of this population was only slightly reduced in wild-type (WT) compared to Dct-deficient (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. Hence, CD8+ T cell self-specificity is poorly predicted by precursor frequency, phenotype, or even initial responsiveness, while deficient activation-induced CD25 expression and other gene expression characteristics may help to identify functionally tolerant cells.

“Virtual” memory CD8+ T cells are a subset of immune cells produced by homeostatic mechanisms involving response to self‐antigens, raising the possibility that these cells could mediate autoimmunity. New work by Drobek et al demonstrates that virtual memory T cells are indeed favored by stronger T‐cell receptor signals but exhibit minimal autoreactivity while maintaining self‐tolerance.

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Self-specific CD8+ T cells often escape clonal deletion, but the properties and capabilities of such cells in a physiological setting are unclear. We characterized polyclonal CD8+ T cells specific for the melanocyte antigen tyrosinase-related protein 2 (Trp2) in mice that express or lack this enzyme due to deficiency in Dct, which encodes Trp2. The size, phenotype, and gene expression profile of the pre-immune Trp2/Kb-specific pool were similar in wild-type (WT) and Dct-deficient (Dct-/-) mice. Despite comparable initial responses to Trp2 immunization, WT Trp2/Kb-specific cells showed blunted expansion, and scRNAseq revealed WT cells 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. Hence, CD8+ T cell self-specificity is poorly predicted by precursor frequency, phenotype or even initial responsiveness, while deficient activation-induced CD25 expression and other gene expression characteristics may help to identify functionally tolerant cells.

The mechanisms governing non-deletional CD8+ T cell tolerance are poorly understood. We are using a physiologically-relevant mouse model to better understand the regulation of self-specific CD8+ T cells, which impact both autoimmune disease and cancer. We find a substantial number of polyclonal cells specific for an epitope from the melanocyte/melanoma-associated enzyme tyrosinase-related protein 2 (Trp2) in both wild-type (WT) C57BL/6 and Trp2-deficient (Trp2 KO) mice. In pre-immune mice, these cells have a naïve phenotype in both strains, and they respond similarly for the first several days after in vivo stimulation with Trp2 peptide alone or in combination with adjuvants (TriVax). However, significantly fewer WT Trp2-specific cells are present at an effector timepoint after TriVax or recombinant Trp2-expressing Listeria monocytogenes relative to KO Trp2-specific cells. WT Trp2-specific cells also show a deficiency in mediating vitiligo compared with KO cells when TriVax-generated effectors are transferred into recipient WT mice primed with TriVax and dinitrofluorobenzene (DNFB). Single-cell RNA sequencing at day three after stimulation reveals that KO Trp2-specific cells efficiently differentiate into a proliferative population, whereas few WT Trp2-specific cells demonstrate this phenotype. As a population, WT Trp2-specific CD8+ T cells exhibit covert anergy: despite a naïve appearance at steady state and initial acquisition of activation markers after stimulation, these self-specific cells are unable to expand productively or to efficiently mediate functional anti-melanocyte activity. These findings have implications for recently-described non-deletional CD8+ T cell tolerance in humans.