Gut Microbiota Conditioning Promotes Anti-Tumor Immunity and Responsiveness to Immunological Checkpoint Blockade in Colorectal Cancer

Abstract Background Colorectal cancer (CRC) infiltration by T lymphocytes is associated with prolonged survival. However, immune infiltration spontaneously occurs in <20% of tumors and no therapies able to promote it are currently available. CRC tissue is populated by gut bacteria translocated across the neoplastic epithelium. We previously showed that direct contact between tumor cells and bacteria induces overexpression of genes encoding immune cell-recruiting chemokines in tumor cells, and we identified a panel of bacterial species associated with enhanced chemokine expression, high T cell infiltration and favorable prognosis. Aims In this project we tested the ability of identified bacterial species to boost T-cell infiltration and promote tumor regression in orthotopic CRC experimental models. Methods Luciferase-expressing mismatch repair (MMR)-proficient or -deficient murine CRC cells from established cell lines were injected intra-cecum in syngeneic mice. Tumor growth was monitored by bioluminescence. Bacterial species of interest, defined as Operational Taxonomic Units (OTUs), were cultured under anaerobic conditions, and, following tumor development, were administered to tumor-bearing mice by oral gavage. In specific experiments, bacteria administration was combined with immunological checkpoint blockade (ICB), based on injection of anti-PD-1 antibodies. Immune cell infiltration was evaluated by flow cytometry, following tumor harvesting and enzymatic tissue digestion. Results In MMR-deficient tumors, known to be immunogenic and responsive to immunotherapy, administration of the bacterial species OTU8, was sufficient to reduce tumor growth. In MMR-proficient tumors, scarcely infiltrated by immune cells and poorly responsive to ICB, OTU8, did not per se affect tumor growth, but enhanced infiltration by cytotoxic CD8+ T cells, ultimately leading, in combination with anti-PD-1 antibodies, to tumor growth inhibition. Conclusions We demonstrated that administration of defined bacteria promotes endogenous immune responses against CRC. Gut microbiota conditioning might represent an innovative therapeutic strategy to enhance responsiveness to ICB and to broaden its indication beyond MMR-deficient tumors.