The microbiome–nitric oxide axis in cancer immunotherapy: a targeted qualitative systematic review of mechanistic and translational evidence

Background Immune checkpoint inhibitors (ICIs) have transformed cancer treatment, but responses vary substantially among patients. Clinical studies have associated features of the gut microbiome with ICI outcomes, while experimental evidence suggests that microbial metabolites and nitric oxide (NO)-related pathways may influence antitumor immunity and tumor vascular biology. This qualitative systematic review evaluated the evidence supporting a proposed microbiome–NO–ICI framework. Methods Five databases were searched for peer-reviewed studies published between January 2000 and August 2025. Fifty-four distinct primary studies met the eligibility criteria, comprising 21 clinical studies, 19 animal studies and 14 in vitro mechanistic studies. Evidence was synthesized narratively according to study type and thematic domain. Results Several clinical cohorts associated enrichment of Akkermansia muciniphila , Faecalibacterium prausnitzii , Bifidobacterium species and other short-chain fatty acid–producing taxa with favorable ICI outcomes; however, the reported microbial signatures varied across cohorts and analytical platforms. Microbial-transfer experiments and early non-randomized fecal microbiota transplantation studies provide proof-of-concept evidence that microbiome composition can influence antitumor immune responses, although the clinical intervention evidence remains preliminary. Animal and cellular studies indicate that microbial metabolites, host nitric oxide synthase pathways and reactive nitrogen signaling can affect immune-cell function, vascular regulation and tumor hypoxia. Direct clinical evidence linking microbiota-mediated nitrate reduction or dietary nitrate supplementation to improved ICI outcomes is lacking. Human dietary nitrate studies conducted outside oncology demonstrate increased circulating nitrate and nitrite and measurable vascular effects but do not establish anticancer or ICI efficacy. Conclusion The microbiome–NO–ICI axis should be regarded as a testable integrative model linking microbial ecology, immune regulation and tumor vascular biology. Current evidence supports biological plausibility but is insufficient to establish NO- or microbiome-directed interventions as effective adjuncts to ICI therapy. Prospective mechanistic studies and biomarker-driven clinical trials are required before therapeutic translation.