Conclusions

The gut microbiome is emerging as a modulator of immune checkpoint inhibitor outcomes in melanoma, influencing both therapeutic efficacy and immune-related toxicity through interactions involving microbial composition, metabolic function, and host immune response. Current evidence supports the potential of the microbiome as a predictive biomarker and therapeutic target, but findings remain heterogeneous and context-dependent, and most of the underlying evidence carries at least a moderate risk of bias. Large, prospective, multi-center, multi-omic studies that account for key confounding factors are needed before microbiome profiling can be integrated into clinical decision-making for melanoma immunotherapy.