A post-transcriptional regulatory checkpoint controls the response of tumor-infiltrating cytotoxic CD4 ⁺ T cells to immunotherapy

Acquisition of cytotoxic activity in CD4 ⁺ T cells (T _CTX ) can promote potent anti-tumor activity thus holding promise as a therapeutic approach. However, how this activity is regulated remains poorly understood. Here, we demonstrate that tumor-infiltrating CD4 ⁺ T _CTX activity is restrained by a post-transcriptional regulatory checkpoint. In untreated tumors, CD4 ⁺ T _CTX exist in a poised state, characterized by abundant Gzmb mRNA but limited Granzyme B (GzmB) protein. Differentiation into poised T _CTX is regulated by the Blimp-1-Bcl6 axis and requires type-I interferon signaling. Treatment with anti-CTLA-4 or anti-LAG-3 plus anti-PD-1 removed the block to GzmB protein production by repressing expression of the post-transcriptional regulator Zfp36l1 . Constitutive Zfp36l1 expression abrogated the effects of anti-CTLA-4 while deletion of Zfp36l1 and its paralog Zfp36 triggered GzmB protein production and promoted tumor control. These data identify ZFP36/ZFP36L1 as a key post-transcriptional regulatory checkpoint of CD4 ⁺ T _CTX activity and a potential immunotherapy target in cancer.