The Gut–Liver Axis in HCC Immunotherapy

Background/Objectives: Hepatocellular carcinoma (HCC) is the sixth most diagnosed cancer worldwide and a leading cause of cancer-related mortality. The majority of cases arise in the setting of chronic liver disease, where immune checkpoint inhibitors (ICIs) have emerged as a cornerstone of systemic therapy for advanced disease. However, durable clinical benefit remains limited to a minority of patients, and reproducible biomarkers of ICI response are lacking. The gut–liver axis—encompassing bidirectional exchange of microbial products, metabolites, bile acids, and immune signals—has emerged as a biologically plausible determinant of both hepatocarcinogenesis and immunotherapy response. This narrative review synthesises current evidence on the role of the gut–liver axis in HCC and ICI response and proposes a unifying conceptual framework to resolve discrepancies in the existing literature. Methods: A narrative review was conducted through systematic searches of PubMed/MEDLINE, Embase, and Web of Science. Studies were selected based on relevance to the biological mechanisms, clinical associations, and experimental models underpinning gut–liver–immune interactions in HCC, with particular emphasis on studies providing mechanistic insight, addressing immunotherapy outcomes, or highlighting temporal and context-dependent effects. Results: Observational studies consistently associate higher microbial diversity and enrichment of homeostasis-promoting taxa—including Akkermansia, Bifidobacterium, and short-chain fatty acid-producing Ruminococcaceae—with ICI responsiveness in HCC. Functional microbial outputs, particularly short-chain fatty acids and secondary bile acids, exert mechanistically grounded effects on hepatic immune tone and T cell activity that are biologically proximate to ICI effector pathways. Therapeutic modulation of the gut–liver axis through probiotics, dietary interventions, faecal microbiota transplantation, and antibiotic exposure demonstrates context-dependent effects on immune activation and ICI outcomes, with timing and disease severity emerging as critical determinants. The limited reproducibility of microbiome-immunotherapy associations across cohorts is attributable primarily to the dynamic and treatment-sensitive nature of the gut–liver axis rather than a fundamental lack of mechanistic coupling. Conclusions: The gut–liver axis in HCC is best understood as a dynamic, treatment-sensitive system rather than a static baseline trait. This reframing shifts emphasis from single-timepoint taxonomic signatures toward functional and longitudinal readouts and provides a coherent rationale for the heterogeneity observed across existing studies. Longitudinal clinical studies incorporating mechanistic endpoints and functional biomarker assessment are needed to translate this framework into clinically actionable strategies for patient stratification and microbiota-targeted intervention in HCC immunotherapy.