Abstract 4164: Relieving immune suppressive pathways in breast cancer to improve outcomes

Abstract Triple negative breast cancer (TNBC)represents the most aggressive subset of breast cancer with limited treatment options leading to high mortality. While treatment of metastatic cancers with immune checkpoint inhibitors (ICIs) offers promise for many, early results from clinical trials evaluating ICIs in TNBC showed benefit for only a subset of patients. Preclinical data indicate that infiltrating macrophages foster tumor progression by limiting CD8+ T cell infiltration, activation and cytotoxicity in tumor microenvironments. We hypothesized that response to ICI therapy could be enhanced by sequentially combining a microtubule poison (paclitaxel; PTX) that enhances tumor immunogenicity, with a macrophage-depleting/reprograming agent, e.g. colony-stimulating factor 1 receptor blocking antibody (aCSF1R), in addition to an ICI targeting the PD-1/PD-L1 axis. Immune-competent mammary tumor-bearing MMTV-PyMT transgenic mice were randomized at day 80, and enrolled into experimental groups evaluating PTX, aCSF1R mAb, αPD-1 mAb, as monotherapy and in combinations. Cohorts were analyzed at distinct time points for primary and metastatic tumor burden, in addition to immunophenotyping (flow cytometry), transcriptional (RNA-Seq), epigenetic (sciATAC-Seq), and T cell receptor (TCR) changes correlating with outcomes. These studies revealed tumor regression and improved outcome for 60% of mice receiving PTX/aCSF1R/aPD-1, associated with localized CD8+ T effector and resident memory cell expansion, increased antigen-specific clonal expansion at the primary tumor site. To address epigenetic plasticity as a limiting factor for long-term T cell memory, a class 1 benzamide histone deacetylase (HDAC) inhibitor (e.g., entinostat) was sequentially added to PTX/aCSF1R/aPD-1 triple therapy resulting in primary tumor stasis in 100% of mice, and significantly enhanced overall survival. Improved outcome and tumor stasis were associated with an increased presence of long-lived central and stem-like memory CD8+ T cell infiltration, associated with increased intra-tumoral B memory cell and CD4+ T follicular helper cell infiltration. Improved outcomes were CD8+ T cell-dependent; depletion of CD19+/CD20+ B cells resulted in complete loss of granzyme B + CD8+ T cell infiltration into primary tumors, and rapid tumor expansion. These preclinical studies indicate that therapies targeting myeloid-mediated T cell suppression can be leveraged to improve anti-tumor T cell memory, and that durable long-term T cell memory can be instated upon addition of an epigenetic modulator. Citation Format: Amanda Poissonnier, Eivind Valen Egeland, Rossin Erbe, Wesley Horton, Ana Howells-Ferreira, Shamilene Sivagnanam, Nell Kirchberger, Dhaarini Murugan, Andrew Fields, Peter Ordentlich, Andrew C. Adey, Elana J. Fertig, Lisa M. Coussens. Relieving immune suppressive pathways in breast cancer to improve outcomes. [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 4164.