This study investigates the impact of the gut microbiome on the response to anti-PD-1 immunotherapy in melanoma patients. The researchers collected oral and gut microbiome samples from 112 patients with metastatic melanoma, both responders (R) and non-responders (NR), and analyzed their diversity and composition. They found that the gut microbiome of R patients had significantly higher alpha diversity and a higher abundance of Ruminococcaceae bacteria compared to NR patients. Metagenomic analysis revealed that R patients had enriched anabolic pathways in their gut bacteria, suggesting enhanced systemic and anti-tumor immunity. Immune profiling showed that R patients had higher levels of effector CD4+ and CD8+ T cells and preserved cytokine responses to anti-PD-1 therapy. In contrast, NR patients had higher levels of regulatory T cells and myeloid-derived suppressor cells, leading to impaired immune responses. Fecal microbiome transplantation (FMT) experiments in germ-free mice further confirmed that a favorable gut microbiome, characterized by high diversity and abundance of Ruminococcaceae/Faecalibacterium, improved tumor growth inhibition and anti-PD-1 therapy response. These findings suggest that modulating the gut microbiome may enhance the efficacy of anti-PD-1 immunotherapy in melanoma patients.This study investigates the impact of the gut microbiome on the response to anti-PD-1 immunotherapy in melanoma patients. The researchers collected oral and gut microbiome samples from 112 patients with metastatic melanoma, both responders (R) and non-responders (NR), and analyzed their diversity and composition. They found that the gut microbiome of R patients had significantly higher alpha diversity and a higher abundance of Ruminococcaceae bacteria compared to NR patients. Metagenomic analysis revealed that R patients had enriched anabolic pathways in their gut bacteria, suggesting enhanced systemic and anti-tumor immunity. Immune profiling showed that R patients had higher levels of effector CD4+ and CD8+ T cells and preserved cytokine responses to anti-PD-1 therapy. In contrast, NR patients had higher levels of regulatory T cells and myeloid-derived suppressor cells, leading to impaired immune responses. Fecal microbiome transplantation (FMT) experiments in germ-free mice further confirmed that a favorable gut microbiome, characterized by high diversity and abundance of Ruminococcaceae/Faecalibacterium, improved tumor growth inhibition and anti-PD-1 therapy response. These findings suggest that modulating the gut microbiome may enhance the efficacy of anti-PD-1 immunotherapy in melanoma patients.