Frontier research and clinical application prospects of microbiome biomarkers in autoimmune diseases.

The microbiome is increasingly recognized as a master regulator of immune homeostasis and a key environmental factor associated with the pathogenesis of autoimmune diseases (ADs). This review comprehensively synthesizes current knowledge on how microbial communities and their metabolites may contribute to ADs' development through microbial-immune interactions, dysbiosis, and the involvement of viral and fungal components within an integrated inter-kingdom ecosystem. We propose an operational definition of microbiome biomarkers as measurable microbiome-associated features reflecting disease susceptibility, activity, prognosis, or therapeutic response and categorize them into three classes: taxonomic, functional/metabolic, and host-microbiome interaction-derived biomarkers. We critically evaluate the evidence for specific microbial signatures as biomarkers for early diagnosis, disease monitoring, and prediction of therapeutic responses, incorporating evidence grading that distinguishes validated biomarkers from those that remain exploratory and discussing shared versus disease-specific signatures across ADs. The translational potential of microbiome-targeted interventions, including probiotics, prebiotics, and fecal microbiota transplantation, is examined within a personalized medicine framework, with barriers to clinical implementation explicitly addressed. Key confounding factors such as diet, geographic origin, and medication use are highlighted as critical variables shaping microbiome signatures independently of disease. Looking forward, the convergence of multi-omics technologies and artificial intelligence for biomarker discovery, multi-omics integration, and clinical validation promises to unravel the complex microbiome-immune crosstalk, enabling more accurate diagnosis, prognostic stratification, and ultimately, individualized microbiota-informed therapy.