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 Ag did not compromise tolerance. In this study, we determined that murine T cells that failed clonal deletion were rendered functionally impaired in the thymus. Programmed cell death protein 1 (PD-1) was induced in the thymus and was required to establish cell-intrinsic tolerance to tissue-restricted Ag in CD8+ thymocytes independently of clonal deletion. In bone marrow chimeras, tolerance was not observed in PD-L1-deficient recipients, but tolerance was largely maintained following adoptive transfer of tolerant thymocytes or T cells to PD-L1-deficient recipients. However, CRISPR-mediated ablation of PD-1 in tolerant T cells resulted in broken tolerance, suggesting different PD-1 signaling requirements for establishing versus maintaining tolerance. Finally, we showed that chronic exposure to high-affinity Ag supported the long-term maintenance of tolerance. Taken together, our study identifies a critical role for PD-1 in establishing central tolerance in autoreactive T cells that escape clonal deletion. It also sheds light on potential mechanisms of action of anti-PD-1 pathway immune checkpoint blockade and the development of immune-related adverse events.

γδ Thymocyte Maturation and Emigration in Adult Mice.

Several unique waves of γδ T cells are generated solely in the fetal/neonatal thymus, whereas additional γδ T cell subsets are generated in adults. One intriguing feature of γδ T cell development is the coordination of differentiation and acquisition of effector function within the fetal thymus; however, it is less clear whether this paradigm holds true in adult animals. In this study, we investigated the relationship between maturation and thymic export of adult-derived γδ thymocytes in mice. In the Rag2pGFP model, immature (CD24+) γδ thymocytes expressed high levels of GFP whereas only a minority of mature (CD24-) γδ thymocytes were GFP+ Similarly, most peripheral GFP+ γδ T cells were immature. Analysis of γδ recent thymic emigrants (RTEs) indicated that most γδ T cell RTEs were CD24+ and GFP+, and adoptive transfer experiments demonstrated that immature γδ thymocytes can mature outside the thymus. Mature γδ T cells largely did not recirculate to the thymus from the periphery; rather, a population of mature γδ thymocytes that produced IFN-γ or IL-17 remained resident in the thymus for at least 60 d. These data support the existence of two populations of γδ T cell RTEs in adult mice: a majority subset that is immature and matures in the periphery after thymic emigration, and a minority subset that completes maturation within the thymus prior to emigration. Additionally, we identified a heterogeneous population of resident γδ thymocytes of unknown functional importance. Collectively, these data shed light on the generation of the γδ T cell compartment in adult mice.