Abstract 6556: Targeting mesenchymal tumor cell-intrinsic factors sensitizes refractory tumors to immune checkpoint blockade therapy

Abstract Immune checkpoint blockade (ICB) therapy has emerged as a promising approach for treating breast cancer, however, only a fraction of patients derives clinical benefit and the underlying reasons for heterogeneous responses remain elusive. The epithelial-to-mesenchymal transition (EMT) enables these carcinomas to metastasize and acquire resistance to chemotherapy. We have recently demonstrated that the EMT program also confers resistance to immunotherapies. Specifically, epithelial tumors recruit CD8+ T-cells to the tumor microenvironment and are sensitive to anti-CTLA4 ICB. In contrast, mesenchymal tumors assemble an immunosuppressive tumor microenvironment and are resistant to the same treatment. Furthermore, mesenchymal tumors express multiple immunosuppressive paracrine factors relative to their epithelial counterparts. Of these, abrogation of CSF1 or SPP1 from mesenchymal cancer cells generates partial responses to ICB therapy. Strikingly, abrogation of CD73 from mesenchymal carcinoma cells completely sensitizes otherwise refractory tumors to immunotherapy. While targeting cancer cell-intrinsic factors can sensitize mesenchymal tumors to ICB, the mechanisms that dictate partial versus complete responses are unclear. Furthermore, whether abrogation of CD73 can also sensitize mesenchymal tumors to other forms of ICB remains unknown. By performing immunofluorescent analysis of tumor sections, we observed that knockout of CD73 promoted the greatest infiltration of both CD4+ and CD8+ T-cells in response to anti-CTLA4 ICB compared to tumors knocked out for CSF1 and SPP1, which showed only intermediate levels of infiltration. Moreover, treatment of mesenchymal tumor-bearing mice with anti-CD73 generated synergistic responses with anti-CTLA4 ICB, but not with anti-PD1, or combinations of anti-CTLA4 and anti-PD1. In conclusion, our results demonstrate that the EMT program in cancer cells is predictive of responses to ICB. Targeting CD73 in mesenchymal breast tumors promotes an influx of both CD4+ and CD8+ T-cells and potentiates the efficacy of anti-CTLA4 ICB. Through mining of canine mammary tumor RNA sequencing data, we have found that canine tumors associated with EMT have increased expression of CD73, which suggests that these mechanisms of immune evasion may translate across species. These findings could promote therapies for both animals and humans that sensitize highly refractory populations of more-mesenchymal cancer cells. Citation Format: Kimaya M. Bakhle, Caitie H. Sams, Anushka Dongre. Targeting mesenchymal tumor cell-intrinsic factors sensitizes refractory tumors to immune checkpoint blockade therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6556.