Immunotherapy targeting different immune compartments in combination with radiation therapy induces regression of resistant tumors

Abstract Radiation therapy (RT) increases tumor response to CTLA-4 inhibition (CTLA4i) in mice and in some patients, yet deep responses are rare. This raises the possibility that effector T cells are insufficiently activated by the combination. To address this question we use mouse models of triple negative breast cancer highly resistant to immunotherapy. We find that CTLA4i promotes the expansion of CD4+ T helper cells, whereas RT enhances T cell clonality and enriches for CD8+ T cells with an exhausted phenotype. Combination therapy decreases regulatory CD4+ T cells and increases CD8+ T cells with effector memory, early activation and precursor exhausted phenotype. A combined gene signature comprising these three CD8+ T cell clusters is associated with survival in triple negative breast cancer patients. Despite the expression of multiple immune checkpoints on intratumoral T cells, response to RT+CTLA4i is not increased by targeting additional checkpoints, including PD1. In contrast, agonistic CD40 therapy enhances responses in mice, indicating the need to target different immune compartments to increase responses to combinations of RT and CTLA4i.