Abstract 6694: Gut microbial drivers of response to bladder-sparing therapy in human muscle-invasive bladder cancer enhance immunotherapy efficacy and limit immunosuppression following fecal microbiome transplantation in vivo

Abstract BACKGROUND: Radiation therapy (RT) is a bladder-sparing option for Muscle-Invasive Bladder Cancer (MIBC), yet encounters a 30% non-response rate, with half of the patients succumbing to metastasis. Preclinical models demonstrate enhanced antitumor responses with RT combined with PD-1/PD-L1 blockade but give limited insight into the determinants of success for such combination therapies (CT). The gut microbiome, pivotal in conditioning local and peripheral immunity, influences PD-1-based immunotherapy efficacy in various cancers. Immune profiling indicates heightened systemic and antitumor immunity in responders with a favorable gut microbiome, enriched in Akkermansia muciniphila, Bifidobacterium, and Faecalibacterium. Oral gavage of immunogenic bacteria further enhances combined RT and anti-PD-L1 therapy. We thus aim to 1) elucidate the role of patients’ microbiome in shaping anti-tumor immune responses to CT, and 2) use its composition as a predictive factor for CT success. METHODS: We performed shotgun metagenomics on microbial DNA from fecal materials of MIBC RT responders (R, n=26) and non-responders (NR, n=11). Fecal homogenates from donors were gavaged into germ-free (GF) mice. Three weeks post-gavage, MB49 tumor cells were subcutaneously delivered to mice, randomized into four groups: control, anti-PD-L1, RT, and RT + anti-PD-L1. Tumors were harvested seven days post-treatment for single-cell RNA-seq and TCR sequencing (10X Genomics), while stool samples were collected weekly for 16S sequencing. RESULTS: Significant gut microbial disparities were observed between R and NR MIBC patients, with particular enrichment in known promoters of anti-PD-L1 response, such as Phocaeicola dorei in R. Transplantation of R microbiome into GF mice resulted in significant increases in predicted survival, tumor growth delay, and counts of effector CD4+ T cell infiltration in the RT+PD-L1 arm. Conversely, NR microbiome transplantation correlated with increased neutrophil and Treg infiltration, abrogating the survival tumor growth delay benefits of the combination arm. Spatial tissue analyses revealed higher neutrophil densities around CD8+ T cells in all NR arms. Single-cell RNA-seq of tumor-infiltrating immune cells unveiled skewed CD4 and CD8 polarization between R and NR in vivo batches. SIGNIFICANCE: To our knowledge, this is the first study to use FMT as a modulator of response in the context of radiation therapy combinations in MIBC. Our results highlight that the gut microbiome alone may condition a favorable immune terrain for combinatorial therapies involving radiation. This offers strong predictive value for personalized therapeutic approaches in selecting patients who will benefit the most from bladder-sparing therapies. Citation Format: Eva Michaud, Sabina Fehric, Cynthia Faubert, Bertrand Routy, Irah King, José Joao Mansure, Wassim Kassouf. Gut microbial drivers of response to bladder-sparing therapy in human muscle-invasive bladder cancer enhance immunotherapy efficacy and limit immunosuppression following fecal microbiome transplantation in vivo 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 (6Suppl): Abstract nr 6694.