Abstract 4027: From tumor progression to therapeutic control: Combinatorial targeting of immune vulnerabilities in TNBC

Abstract Triple-negative breast cancer (TNBC) is an aggressive form of breast cancer, associated with limited treatment options and high mortality rates. Although immune checkpoint inhibitors (ICIs) show promise, early clinical results indicate benefits in only a specific subset of patients with TNBC. Preclinical evidence indicates that infiltrating macrophages contribute to breast tumor progression by restricting CD8+ T cell infiltration, activation, and cytotoxicity within tumor microenvironments. Preliminary data has revealed that immune-competent mammary tumor-bearing MMTV-PyMT transgenic mice have slowed tumor progression following sequential combination of a microtubule poison (paclitaxel; PTX) to enhance tumor immunogenicity, plus a macrophage-depleting/reprogramming agent such as a colony-stimulating factor 1 receptor blocking antibody (αCSF1R), and undergo primary tumor regression in 60% of transgenic mice when an ICI targeting PD-1 is applied (3x therapy) by a CD8+ T cell-dependent mechanism. Response to 3x therapy is associated with localized CD8+ T effector and resident memory cell expansion, and increased antigen-specific clonal expansion at primary tumor sites. We hypothesized that epigenetic plasticity was limiting T cell memory and thus impeding long-term tumor control. Using a class 1 benzamide histone deacetylase (HDAC) inhibitor (e. g. , entinostat) sequentially added to PTX/αCSF1R/αPD-1 therapy (4x) resulted in primary tumor stasis in 100% of mice, and significantly enhanced overall survival, in two transgenic mouse models of breast cancer (MMTV-PyMT and C3- (1) -TAg). Cohorts were analyzed at distinct time points for primary and metastatic tumor burden, in addition to immunophenotyping (flow cytometry), transcriptional (bulk and scRNA-Seq), epigenetic (sciATAC-Seq), and T cell receptor (TCR) changes correlating with 4x responses. Improved outcome and tumor stasis was associated with increased presence of long-lived central and stem-like memory CD8+ T cell infiltration, intra-tumoral B memory cell, and CD4+ T follicular helper cell infiltration. Tumor stasis was achieved by B and T cell-dependent mechanisms where B cell depletion led to decreased T cell cytotoxicity. Adoptive serum transfer (AST) from 4x-treated MMTV-PyMT mice into either 3x-treated or untreated MMTV-PyMT mice elicited tumor stasis and control. These preclinical studies indicate the complexity of myeloid, B cell and T cell interactions that are co-opted by tumors, and identify novel paths to therapeutic tumor control by targeting vulnerable immune communication programs. Citation Format: Amanda Poissonnier, Eivind Valen Egeland, Wesley Horton, Shamilene Sivagnanam, Orian Stapleton, Rossin Erbe, Nell Kirchberger, Konjit Betre, Alyza Skaist, Ludmila Danilova, Andrew Fields, Peter Ordentlich, Andrew C. Adey, Elana J. Fertig, Lisa M. Coussens. From tumor progression to therapeutic control: Combinatorial targeting of immune vulnerabilities in TNBC 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 (6Suppl): Abstract nr 4027.