Professor of Immunology Mayo Clin. Col. of Med. and Sci. Rochester, Minnesota, United States
Introduction/Rationale: Developing T cells receive fate-determining signals through thymic selection to ensure self-tolerance and functional competence. While the proliferative and migratory capacities of mature thymic CD8⁺ T cells have been well characterized, the mechanisms underlying their acquirement of effector competence remain poorly understood. PD-1 signaling has been implicated in modulating positive selection of thymocytes; but its role in shaping the effector potential of thymic CD8⁺ T cells is not yet fully elucidated.
Methods: We utilized a CD8 (E8I)-specific conditional Pdcd1 KO mouse model and performed scRNA-seq, flow cytometry, and T cell cytotoxicity assays to assess functional and phenotypic changes. Additionally, scTCR-seq was employed to analyze the TCR repertoire overlap between thymic and peripheral CD8⁺ T cells.
Results: We identified a distinct effector-competent gene signature imprinted in single-positive (SP) CD8⁺ thymocytes following positive selection. In the absence of PD-1, these SP CD8⁺ T cells exhibited elevated expression of cytotoxic effector molecules and enhanced cytotoxic activity. Notably, T cell repertoire of intratumoral CD8⁺ T cells was directly traced to thymic SP CD8⁺ T cells in PD-1-deficient mice, indicating a link of clonal relationship and developmental trajectory. However, PD-1 deficiency increased expression of exhaustion-related genes, which compromised a durable anitumor activity of CD8⁺ T cellsin peripheral.
Conclusion: Our findings reveal a previously unrecognized facet of thymic T cell development—namely, the emergence of effector-competent cytotoxic CD8⁺ thymocytes under the regulation of PD-1. While the imprinting of effector competence during thymic development equips CD8⁺ T cells to rapidly acquire effector functions upon encountering cognate antigens in the periphery, PD-1 functions as a critical checkpoint during the selection of SP CD8⁺ thymocytes, serving to restrain excessive effector activity and maintain immune homeostasis.
