The gut microbiome as an actionable drug-sensitivity modulator for immune checkpoint blockade: clinical evidence for FMT, live biotherapeutics, and defined consortia

Immune checkpoint inhibitors (ICIs) deliver durable benefit to only a subset of patients and can be limited by immune-related adverse events (irAEs). The gut microbiome has emerged as an actionable, host-level modulator of ICI drug sensitivity and toxicity. This mini-review links microbial ecology to antigen presentation, T-cell priming and fitness, metabolite signaling, and barrier inflammation, and summarizes interventional evidence across three modalities. Responder-derived fecal microbiota transplantation (FMT) provides the strongest proof-of-concept for re-sensitization in anti–PD-1–refractory melanoma. Microbiome repair can also improve refractory ICI-associated colitis. Early trials of live biotherapeutics and defined consortia support scalability but highlight context dependence and design pitfalls, including antibiotic preconditioning. We discuss practical determinants of reproducibility, including co-medications, diet, engraftment and functional readouts, and conclude with safety, regulatory, and reporting priorities for clinically deployable microbiome engineering.