PD-1 regulates latent effector differentiation of thymic cytotoxic CD8⁺ T cells

Durable T cell immunity against cancer depends on the continual replenishment of effector CD8⁺ T cells. Thymic output has been associated with favorable prognosis in cancer patients across a range of ages, suggesting that the thymus is an important source for replenishing T cells capable of controlling cancer progression. However, whether CD8⁺ T cells acquire effector potential within the thymus, and how thymic output of effector CD8⁺ T cells contribute to peripheral tumor immunity, remain unclear. In this study, we discover that thymic single-positive (SP) CD8⁺ T cells undergo latent effector differentiation following thymic selection, but this process is subject to PD-1 regulation. We further demonstrate that PD-1 limits the contribution of thymic output of CD8⁺ T cells in shaping the TCR repertoire within the tumor tissues for tumor immunosurveillance. Although PD-1 inhibition facilitates the expansion of effector CD8⁺ T cells in the periphery, these cells gradually lose antitumor activity within tumors due to accelerated exhaustion in the absence of PD-1. Thus, while latent effector differentiation of thymic CD8⁺ T cells enables a rapid response to malignant cells in the periphery, PD-1 restrains this process to prevent overt or terminal effector differentiation, which may compromise balanced and durable peripheral immunity.