Establishment of CD8+ T cell thymic central tolerance to tissue-restricted antigen requires PD-1

Highly self-reactive T cells are censored from the repertoire by both central and peripheral tolerance mechanisms upon receipt of high-affinity TCR signals. Clonal deletion is considered a major driver of central tolerance; however, other mechanisms such as induction of regulatory T cells and functional impairment have been described. An understanding of the interplay between these different central tolerance mechanisms is still lacking. We previously showed that impaired clonal deletion to a model tissue-restricted antigen (TRA) did not compromise tolerance. In this study, we determined that T cells that failed clonal deletion in this model were rendered functionally impaired in the thymus. PD-1 was induced in the thymus and established cell-intrinsic tolerance to TRA in CD8+ thymocytes independently of clonal deletion. PD-1 signaling in developing thymocytes was sufficient to induce tolerance but was dispensable for the initial maintenance of tolerance in the periphery. We showed that chronic exposure to high affinity antigen supported the long-term maintenance of tolerance in this model. Taken together, our study identifies a role for PD-1 in establishing central tolerance in autoreactive T cells that escape clonal deletion and sheds light on potential mechanisms of action of anti-PD-1 pathway immune checkpoint blockade and the development of immune-related adverse events. Significance Statement The establishment of T cell tolerance is critical to prevent autoimmune diseases. Apoptosis of highly self-reactive thymocytes is an important mechanism that enforces central tolerance. However, not all self-reactive thymocytes undergo apoptosis during development, and the fate of cells that evade this process is under-examined. Using bone marrow chimera, adoptive transfer, and thymic transplant experiments, we found acute PD-1 signaling is required to establish tolerance to tissue-restricted antigen (TRA) independently of clonal deletion. This tolerance is maintained by chronic exposure to tolerizing antigen but persists in the absence of PD-1 until late time points. Overall, this study identifies a role for PD-1 in establishing central tolerance, and it provides insight into the mechanism of PD-1 pathwaytargeting cancer immunotherapies.