Direct and Indirect Regulators of Epithelial-Mesenchymal Transition (EMT)-mediated Immunosuppression in Breast Carcinomas

Abstract

The epithelial-to-mesenchymal transition (EMT), which conveys epithelial (E) carcinoma cells to quasi-mesenchymal (qM) states, enables them to metastasize and acquire resistance to certain treatments. Murine tumors composed of qM mammary carcinoma cells assemble an immunosuppressive tumor microenvironment (TME) and develop resistance to anti-CTLA4 immune checkpoint blockade therapy (ICB), unlike their E counterparts. Importantly, minority populations of qM cells within a tumor can cross-protect their more E neighbors from immune attack. The underlying mechanisms of immunosuppression and cross-protection have been unclear. We demonstrate that abrogation of qM carcinoma cell-derived factors (CD73, CSF1 or SPP1) prevents the assembly of an immunosuppressive TME and sensitizes otherwise refractory qM tumors partially or completely to anti-CTLA4 ICB. Most strikingly, mixed tumors in which minority populations of carcinoma cells no longer express CD73, are now sensitized to anti-CTLA4 ICB. Finally, loss of CD73 also enhances the efficacy of anti-CTLA4 ICB during the process of metastatic colonization.