Abstract 7782: Cbl-b-Notch1 axis blockers rewire immunosuppressive tumors to drive cytotoxic and anti-angiogenic immune responses

Abstract Background. Immunotherapies aimed at enhancing antitumor immune responses or delay immune cell exhaustion have shown encouraging results across multiple cancer types. However, immunotherapy still presents limited and variable response rates in immunosuppressive solid tumors, where cancer cells develop multiple strategies to evade and block antitumor immune responses. As a result, options to treat immunosuppressive tumors remain limited and this calls for the development of novel targeted immunotherapies to meet this critical need. Our group recently proved that an immunosuppressive pathway, named as the Cbl-b-Notch1 axis, exists in T cells and acts to constrain T cell function by means of increased Notch1 degradation by Casitas B-lineage lymphoma proto-oncogene B (Cbl-b), following T cell activation. We also found that inhibiting Cbl-b using small-molecule inhibitors effectively restores Notch1 level and enhances CD8 and CD4 T cell functions in in vitro and ex vivo conditions. In this study, we aimed to demonstrate the effectiveness and the underlying mechanisms though which blocking the Cbl-b-Notch1 axis rewires the immunosuppressive tumor microenvironment to mount protective antitumor immune responses. Experimental procedures and results. This work leveraged a combination of genetic editing, targeted immune cell depletion, in silico, in vivo, ex vivo models and functional assays in primary cells. We found that blocking the Cbl-b-Notch1 axis, via Cbl-b inhibition, is effective in hindering tumor growth in immunosuppressive Triple-Negative Breast Cancer (TNBC) models (C0321 and 4T1) and pre-clinical tumor-derived organoids. We found that Cbl-b inhibition exerts its antitumor effects primarily by conferring a cytotoxic Th9 phenotype to CD4 T cells, via Notch1 stimulation which regulates cytotoxic and Th9 features, including Granzyme B (GZMB) and IL9 secretion. Cbl-b-inhibitor-primed CD4 T cells reshape the tumor microenvironment through multifaceted mechanisms: (i) exert direct cytotoxicity against tumor cells via GZMB, (ii) recruit and enable CD8 T cell cytotoxic responses, (iii) reprogram macrophages to produce Thrombospondin-1 (Thbs1), via Notch1-dependend release of IL9 in the tumor microenvironment and (iv) hence leads to strong anti-angiogenesis effects that cut off the nutrients supply within the tumor. These findings are consistent with TNBC patient data in which increased Notch1, IL9 and Thbs1 expression are predictive of a better prognosis and longer survival. Conclusions. Overall, our work identifies Cbl-b-Notch1 axis blockers, like Cbl-b inhibitors, as a promising strategy to reprogram anticancer immune responses, remodel the tumor microenvironment, and suppress tumor growth, providing a promising avenue for next-generation targeted immunotherapies. Citation Format: Zhi Huang, Mairah Khan, Laura Naldi, Brionna King, Luis Del Valle, Silvana Leit, Beth Browning, Christine Loh, Lucio Miele, Giulia Monticone. Cbl-b-Notch1 axis blockers rewire immunosuppressive tumors to drive cytotoxic and anti-angiogenic immune responses [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 7782.