Blockade of αvβ8 integrin synergizes with checkpoint inhibition to enhance anti-tumor immunity through Cxcl9-dependent augmentation of CD8+ T cell cytotoxicity

Abstract Inhibition of the TGF-β activating integrin, αvβ8, can dramatically enhance the effectiveness of checkpoint inhibition in multiple checkpoint resistant tumor models, but the mechanisms underlying this dramatic synergy have not been previously explored. In this study, we used single cell RNA sequencing to investigate the mechanisms of synergy between inhibition of αvβ8 and PD1 in the EMT-6 model, a checkpoint resistant model of triple negative breast cancer. Combination therapy dramatically enhanced interferon gamma (Ifnγ) expression by CD8+ and CD4+ T cells, activated interferon signaling throughout the tumor microenvironment, specifically increased Cxcl9 secretion by tumor macrophages and enhanced Gzmb expression and tumor cell killing by CD8+ T cells. The protective effects of combination therapy were inhibited by Cxcl9 blockade. These findings suggest that Ifnγ-mediated induction of Cxcl9 is an important driver of synergy between αvβ8 and checkpoint inhibition and raise the possibility that Cxcl9 might be useful as an early PD biomarker of effective combination therapy.