Bifidobacterium catenulatum boosts anti-PD-1 efficacy in microsatellite stable colorectal cancer via activating CD8+ T cells

Background: Certain gut bacteria are associated with improved responses to immunotherapy. Objective: We aim to identify bacteria that inhibit colorectal cancer (CRC) progression and enhance immunotherapy efficacy. Design: The abundance of bacteria in CRC patients was evaluated in our in-house cohorts and validated in published datasets. The effect of candidate bacterium with anti-PD-1 therapy was determined in two syngeneic mouse models of MC38 (microsatellite instability-high) and CT26 (microsatellite stable, MSS), transgenic Apc^min/+ mice and azoxymethane/dextran sulfate sodium (AOM/DSS)-induced CRC tumourigenesis model. Immune landscape changes were identified by multicolour flow cytometry and immunohistochemistry staining. Metabolomic profiling was performed on stool, serum and tumour tissues. Results: Bifidobacterium catenulatum was significantly depleted in stool samples of 110 CRC patients compared with 112 healthy controls, which was further validated in 3 published metagenomic datasets comprising 198 CRC patients and 176 normal subjects. Oral administration of B. catenulatum inhibited tumour growths in multiple CRC models including MC38 and CT26 syngeneic models, Apc^min/+ mice and AOM/DSS-induced CRC. Notably, B. catenulatum synergised with anti-PD-1 therapy through enhancing intratumoural CD8⁺ T cell infiltration in MSS CRC models of Apc^min/+ mice and CT26 allografts. B. catenulatum-derived acetate was identified as the functional metabolite. Mechanistically, acetate directly bound to MCT-4 in CD8⁺ T cells and activated mitogen-activated protein kinase signalling. Pharmacological and genetic MCT4 ablation abolished acetate-mediated CD8⁺ T cell activation in vitro. Conclusion: B. catenulatum suppresses colorectal tumourigenesis through generating acetate, which also improves anti-PD-1 efficacy through activating CD8⁺ T cells in MSS CRC. B. catenulatum is a potential adjuvant to improve immunotherapy against CRC.