Navigating the gut-metabolite-immune axis: enhancing efficacy and mitigating toxicity of immune checkpoint inhibitors

Immune checkpoint inhibitors (ICIs) have revolutionized the oncological landscape by disrupting inhibitory pathways, notably programmed cell death protein-1/programmed death-ligand 1 (PD-1/PD-L1) and cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) pathways, thereby reinvigorating host antitumor immunity. Although these agents have emerged as frontline standard therapies for malignancies, their clinical utility remains limited. Interpatient therapeutic variability is inextricably linked to the composition and functional capacity of the gut microbiome. The underlying mechanisms appear to involve a complex dialogue between the microbiota and host immune system, where microbial metabolites serve as critical mediators in remodeling the tumor microenvironment. Despite these insights, progression in the field remains constrained due to heterogeneity in study cohorts and sample-processing methodologies, hindering the establishment of reproducible individualized predictive models and clinical intervention strategies. Consequently, there is an urgent need to systematically delineate the microbiome-metabolite-immune axis to optimize the balance between ICI efficacy and systemic toxicity. By synthesizing the latest evidence, this review aimed to highlight the pivotal roles of specific taxa, including Bacteroides, Bifidobacterium, and Akkermansia muciniphila, in ICI efficacy. These microbes and their metabolic byproducts potentiate therapeutic responses by enhancing dendritic cell cross-presentation and promoting CD⁸⁺ T-cell infiltration, often via activation of the cyclic GMP-AMP synthase-stimulator of interferon genes or nucleotide-binding oligomerization domain-containing protein 2 signaling pathways. Furthermore, these microbial components demonstrate the ability to protect the heart and colon against inflammation and barrier disruption, thereby mitigating immune-related adverse events. Although the feasibility and safety of interventions such as fecal microbiota transplantation and supplementation with next-generation encapsulated probiotics, postbiotics, or dietary fiber have been demonstrated in preclinical and Phase I trials, substantial hurdles remain. Future progress requires large-scale, multicenter, standardized, longitudinal studies integrating metagenomics and metabolomics to construct robust cross-cancer and cross-population predictive models. Such rigorous validation would enable the development of precise microbial interventions that maximize therapeutic gains while minimizing the incidence of adverse reactions.