Abstract LB-B012: Signals from the tumor immune microenvironment induce a potent immune modulatory gene expression program in senescent breast tumor cells

Abstract Breast tumor cells enter cellular senescence to survive chemotherapy treatment and escape cell death induced by apoptosis, mitotic catastrophe, and immune surveillance, which contributes to poor patient survival. Critically important to the senescence response is the in vivo expression of interferon-dependent genes that can suppress immune surveillance through PD-L1 expression, but also promote immunogenicity through expression of antigen presentation genes. These genes, including PD-L1, are not induced in vitro without the addition of exogenous Ifnγ. This finding suggests that an immune cell within the tumor microenvironment (TME) is required to induce tumor cell gene expression changes that promote both immune evasion and visibility. Here we show in both murine mammary tumor models and human breast cancers that CD4+ and CD8+ cells are the source of Ifnγ. To determine the importance of these immune cell populations in senescent tumors that survive chemotherapy, we transplanted mammary tumors into various mouse host strains. Surprisingly, we found that a tiny population of potent CD4+ cells was critically important for tumor cell-specific expression of several hundred immune modulatory and antigen presentation genes induced in senescence, including Cd274 (PD-L1), Cxcl9/10 and Cd74, Tap1/2, respectively. This remarkably potent CD4+ cell population identified in mice, characterized by genes such as Ifng, Pdcd1, and Lag3, was also present in human breast cancers. The CD8+ cells, while able to produce Ifnγ, were not the primary driver of gene expression changes in the tumor cells post-chemotherapy and their absence only yielded a partial reduction of the immune modulatory and antigen presentation genes. The loss of CD4+ cells did not result in a collapse of the tumor immune microenvironment, and, indeed, intratumoral injections of Ifnγ or CD4+ cells rescued gene expression in hosts that lack all T-cells, B-cells, and NK-cells. Lastly, CD4+ cells were required for expression of immune checkpoint genes that predict response to immunotherapy, and when absent, senescent tumors were refractory to anti-PD-L1 treatment. In sum, we have identified a potent, rare, interferon producing population of CD4+ cells in human tumors and mouse models, that exerts a powerful gene expression effect on senescent tumor cells, sensitizing them to immunotherapy. These findings suggest possible strategies for improving immunotherapies to achieve better patient outcomes. Citation Format: Raegan M. Kvadas, Calvin W.T. Adam, Fang-Yen Chiu, Timothy Kayes, Di Tian, Dorota Wyczechowska, Nathan Ungerleider, Ashkan Shahbandi, Wesley Frey, James G. Jackson. Signals from the tumor immune microenvironment induce a potent immune modulatory gene expression program in senescent breast tumor cells abstract. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr LB-B012.