IDDF2024-ABS-0178 Acetate-producing bifidobacterium catenulatum boosts anti-PD1 efficacy in colorectal cancer via activation of cytotoxic CD8+ cells

<h3>Background</h3> <i>Bifidobacterium catenulatum</i> was reported to attenuate inflammation through modulating gut microbiota. Here, we investigated the roles of <i>B. catenulatum</i> as a potential probiotic in suppressing tumorigenesis and improving the efficacy of immune checkpoint blockade (ICB) in colorectal cancer (CRC). <h3>Methods</h3> <i>B. catenulatum</i> abundance was evaluated in CRC patients (n=308) and healthy subjects (n=288) based on shotgun metagenomic sequencing from multiple cohorts. The effect of <i>B. catenulatum</i> in CRC tumorigenesis was determined in mice induced by azoxymethane (AOM)/dextran sulfate sodium (DSS) injection. The effect of <i>B. catenulatum</i> on anti-PD1 therapeutic efficacy was evaluated in CT26 (MSI-low) and MC38 (MSI-high) syngeneic mouse models. The changes in immune landscapes were identified by multicolor flow cytometry and immunohistochemistry staining. Functional metabolites were identified by non-targeted and targeted metabolomics assays (GC-MS) and verified in human CRC cell lines. The effect of <i>B. catenulatum</i> and its metabolite on anti-PD1 efficacy was assessed in Apc^min/+ mice and AOM/DSS-induced tumorigenesis mice. <h3>Results</h3> <i>B. catenulatum</i> was significantly depleted in stool samples of CRC patients compared to healthy controls (<i>p</i>&lt;0.05). Oral administration of<i> B. catenulatum</i> significantly inhibited AOM/DSS-induced CRC tumorigenesis in mice. <i>B. catenulatum</i> boosted anti-PD-1 therapy in CT26, but not MC38 allografts. Acetate was identified as an enriched metabolite of <i>B. catenulatum</i> by metabolomic profiling of bacteria culture supernatants and stool samples from <i>B. catenulatum</i>-treated mice. We confirmed that <i>B. catenulatum</i> and its derived acetate significantly augmented anti-PD1 efficacy and increased the intratumoral infiltration of cytotoxic CD8⁺ T cells in Apc^min/+ mice. Mechanistically, acetate directly binds to monocarboxy-late transporter (MCT)-4 in CD8⁺ T cells and activates its downstream mitogen-activated protein kinases (MAPK) signaling. Pharmacological blockade of MCT-4 by VB124 significantly abolished the activation of CD8⁺ T cells by acetate <i>in vitro</i>. <h3>Conclusions</h3> <i>B. catenulatum</i> protects against CRC tumorigenesis. <i>B. catenulatum</i> generated acetate boosts anti-PD1 efficacy in MSI-low CRC. Live <i>B. catenulatum</i> is a potential probiotic for CRC prevention and adjuvant immunotherapy.