p21 Prevents the Exhaustion of CD4+ T Cells Within the Antitumor Immune Response Against Colorectal Cancer

Background and aims T cells are crucial for the antitumor response against colorectal cancer (CRC). T cell reactivity to CRC is nevertheless limited by T cell exhaustion. However, molecular mechanisms regulating T cell exhaustion are only poorly understood. Methods We investigated the functional role of cyclin-dependent kinase 1a (Cdkn1a or p21) in CD4+ T cells using murine CRC models. Furthermore, we evaluated the expression of p21 in patients with stage I–IV CRC. In vitro co-culture models were used to understand the effector function of p21-deficient CD4+ T cells. Results We observed that the activation of cell cycle regulator p21 is crucial for CD4+ T cell cytotoxic function and that p21-deficiency in Th1 cells leads to increased tumor growth in murine CRC. Similarly, low p21 expression in CD4+ T cells infiltrated into tumors of CRC patients is associated with reduced cancer-related survival. In mouse models of CRC, p21-deficient Th1 cells show signs of exhaustion, where an accumulation of effector/effector memory T cells and CD27/CD28 loss are predominant. Immune reconstitution of tumor-bearing Rag1-/- mice using ex vivo treated p21-deficient T cells with Palbociclib, an inhibitor of CDK4/6, restored cytotoxic function and prevented exhaustion of p21-deficient CD4+ T cells as a possible concept for future immunotherapy of human disease. Conclusion Our data reveal the importance of p21 in controlling cell cycle and preventing exhaustion of Th1 cells. Furthermore, we unveil the therapeutic potential of CDK inhibitors such as Palbociclib to reduce T cell exhaustion for future treatment of patients with colorectal cancer.