Abstract 1423: Harnessing the gut microbiome to modulate immune checkpoint blockade response in triple-negative breast cancer

Abstract Triple-negative breast cancer (TNBC) is one of the most aggressive subtypes of breast cancer. The discovery that this subtype has the highest levels of tumor-infiltrating immune cells and programmed death ligand 1 (PD-L1) expression in the tumor microenvironment (TME) has propelled the use of anti-PD-1 immune checkpoint blockade (ICB) therapy, in combination with chemotherapy, as a treatment for TNBC patients. This has advantageously impacted TNBC patient outcomes such as survival, yet there remains a need to improve patient response to ICB therapies. The gut microbiome, in particular, increased abundance of Akkermansia muciniphila was associated with ICB therapy response in other cancer types. A. muciniphila can degrade mucins and produce short-chain fatty acid (SCFA) metabolites. Therefore, we investigated the interplay between A. muciniphila, SCFAs, and ICB therapy efficacy in TNBC models. As diet is the main modifier of the gut microbiome, we first investigated diet-gut microbiome interactions on ICB efficacy. Using the EO771 syngeneic murine model of TNBC, female C57BL/6 tumor-bearing mice consuming a low-fat control, or high-fat Western, or Mediterranean diet, were treated with 3 doses of 200µg of IgG or anti-PD-L1 antibodies and tumor progression was monitored. To assess changes in tumoral immune cell populations, end-of-study tumor tissue was used for immunohistochemistry (IHC). To evaluate modulation on the gut microbiome, metagenomic sequencing was performed on DNA isolated from fecal samples collected at the end-of-study, and targeted SCFA metabolomic analysis was performed on plasma collected at the study endpoint. Results show mice consuming a Mediterranean diet treated with anti-PD-L1 had significant increases in the abundance of gut A. muciniphila and plasma SCFAs, as well as significantly reduced primary tumor volume and increased immune cell activity within the TME. To then investigate if direct supplementation of A. muciniphila or SCFAs could enhance ICB efficacy, EO771 tumor-bearing mice were treated with 3 doses of 200µg of IgG, anti-PD-L1 or anti-PD-1 antibodies with a subset of mice stratified to also receive A. muciniphila or exogenoussupplementation of SCFAs. Tumor progression was measured and changes in tumoral immune cell populations were assessed with end-of-study tumor tissue prepared for flow cytometry and IHC. Results show mice supplemented with A. muciniphila and treated with anti-PD-L1 therapy demonstrated response to treatment, with a significant reduction in tumor volume as well as increased levels of immune cell populations in the TME. Mice supplemented with SCFAs and treated with a combination of anti-PD-1 and chemotherapy demonstrated enhanced response to treatment, with a significant reduction in tumor volume. In conclusion, data from our lab suggest that increased levels of gut A. muciniphila and SCFAs drive ICB efficacy in TNBC. Citation Format: Kenysha YJ. Clear, Adam S. Wilson, Elizabeth R. Stirling, Yu-Ting Tsai, Valerie Payne, David R. Soto-Pantoja, Katherine L. Cook. Harnessing the gut microbiome to modulate immune checkpoint blockade response in triple-negative breast cancer [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(6_Suppl):Abstract nr 1423.