Abstract 5101: Inhibition of Notch reverses immunosuppression in basal-like breast cancer

Abstract

Abstract Aberrant Notch activation is a defining feature of basal-like breast cancer (BLBC), which has poor prognosis compared to other breast cancer molecular subtypes. Despite the clinical success of immune checkpoint blockade (ICB) in many malignancies including melanoma and small cell lung cancer, ICB has failed to demonstrate similar response in BLBCs, where most cases are highly infiltrated by tumor-associated macrophages (TAMs). There is increasing evidence that Notch is decisive in regulating intercellular communication in the tumor immune microenvironment (TIME). This includes the recruitment of TAMs which contribute to an immunosuppressive TIME, illuminating the potential of Notch-inhibition as adjuvant immunotherapy in BLBC. To examine the immune phenotype and therapeutic response of BLBC to combined Notch-inhibition and ICB, we employed an in vivo tumorgraft model using murine basal-like mammary tumor 4T1 cells. Briefly, tumor cells were orthotopically injected into the mammary fat pads of BALB/c mice. Using a crossover design, mice were randomly allocated to treatment with either Notch inhibitor (γ-secretase inhibitor, LY411575), anti-PD1 (RMP1-14), or control treatment for 12-days (stage-1), followed by randomization to a second 12-day period (stage 2) with the same, or one of the other treatments. Results: Despite the low response rate of BLBC to ICB alone, Notch inhibition reduced TAMs and induced responsiveness to sequential ICB. This response was characterized by increased cytotoxic T lymphocytes (GrB+, CTL) infiltration of the primary tumor. Similar results were observed when Notch-regulated cytokines (IL-1β and CCL2), crucial to TAM recruitment, were inhibited. Moreover, a more impressive therapeutic effect of sequential treatment was observed in lung metastasis, whereby TAM depletion and increased CTL infiltration were accompanied with near-complete abolition of metastases. Mechanistically, tumor cell Notch signaling upregulates a group of circulating cytokines including IL-1β and CCL2, which prime the lung for metastases. Additionally, compared to primary tumor cells, PD ligand 1 is up-regulated in lung metastases, rendering them profoundly sensitive to sequential ICB treatment. These findings highlight the potential of sequential Notch inhibition and ICB as a novel immunotherapeutic strategy in BLBC. Citation Format: Qiang Shen, Kiichi Murakami, Pamela Ohashi, Michael Reedijk. Inhibition of Notch reverses immunosuppression in basal-like breast cancer. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5101.