Host-Microbiome Integration as a Biomarker Framework in Esophageal Cancer: Current Evidence and Translational Challenges

Immune checkpoint inhibitors have improved outcomes in esophageal cancer across settings, yet clinical benefit remains heterogeneous, with current host-derived biomarkers incompletely predicting response. This mini review evaluates recent studies that integrate gut or intratumoral microbial features with host immune, molecular, or metabolic assessment in esophageal cancer. We classify the evidence using a four-level hierarchy of host-microbiome integration: ecological association, functional association, mechanistic integration, and clinical predictive integration. Tissue studies reveal compartment-specific relationships between microbial diversity or individual taxa and immune architecture, whereas treatment cohorts identify bacterial and fungal signatures associated with pathological or immunotherapy response. Mechanistic studies offer the strongest biological evidence, most notably the Lactobacillus salivarius-indole-3-lactic acid-AhR/NF-κB axis, which drives CD8-positive T-cell exhaustion and resistance to anti-PD-1 therapy. However, biological integration is substantially more advanced than clinical response prediction. Small cohorts, heterogeneous regimens, contamination of low-biomass samples, coarse taxonomic (rather than functional) resolution, confounding by histology, multi-omic layers measured in different patients, and lack of external validation currently jeopardize integration of microbiome to guide treatment. Future studies should use longitudinal, multicenter, compartment-matched sampling and test whether microbial genes or metabolites improve patient selection and predict clinical response beyond established clinical and host biomarkers.