Immune checkpoint blockade (ICB) therapy has demonstrated good efficacy in many cancer types. In cancers such as non-resectable advanced or metastatic triple-negative breast cancer (TNBC), it has recently been approved as a promising treatment. However, clinical data shows overall response rates (ORRs) from ~ 3–40% in breast cancer patients, depending on subtype, previous treatments, and mutation status. Composition of the host-microbiome has a significant role in cancer development and therapeutic responsiveness. Some bacterial families are conducive to oncogenesis and progression, while others aid innate and therapeutically induced anti-tumor immunity. Modeling microbiome effects on anti-tumor immunity in ex vivo systems is challenging, forcing the use of in vivo models, making it difficult to dissect direct effects on immune cells from combined effects on tumor and immune cells. We developed a novel immune-enhanced tumor organoid (iTO) system to study factors affecting ICB response. Using the 4T1 TNBC murine cell line and matched splenocytes, we demonstrated ICB-induced response. Further administration of bacterial-derived metabolites from species found in the immunomodulatory host-microbiome significantly increased ICB-induced apoptosis of tumor cells and altered immune cell receptor expression. These outcomes represent a method to isolate individual factors that alter ICB response and streamline the study of microbiome effects on ICB efficacy.
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