Immune-gut-metabolite Interactions in Multiple Sclerosis: A MendelianRandomization and Mediation Study

Introduction:: Multiple sclerosis (MS) is a complex autoimmune disease characterized by chronic inflammation and neurodegeneration. Although immune dysregulation, gut microbiota, and metabolic alterations have been implicated in MS, their causal relationships remain unclear. Methods:: Utilizing publicly available genetic data, we performed two-sample MR analyses to investigate the causal effects of 731 immune cell phenotypes, 91 inflammatory proteins, 473 gut microbiota taxa, and 1,400 plasma metabolites on MS. The inverse-variance weighted (IVW) method was used as the primary analysis. The robustness of the results was verified through various sensitivity analyses. Furthermore, bidirectional MR and two-step mediation analyses were applied to assess potential reverse causality and mediating pathways. Results:: We identified 13 immune cell phenotypes, 3 inflammatory proteins, 16 gut microbiota and 14 plasma metabolites with significant causal associations with MS. Bidirectional MR analysis suggested evidence of reverse causality for specific gut microbiota. Mediation analysis uncovered multiple pathways. For instance, while CD28-DN (CD4-CD8-) %T cells mediated 33.50% of the protective effect against the species Bacteroides darus. Genus UBA644 mediating 17.9% of the risk effect of CD28-DN (CD4-CD8-) % T cells, and 2-hydroxysebacate mediated by CD28- CD8br %CD8br Treg (43%). Conclusion:: Our study provides robust genetic evidence supporting the causal roles of immune cells, inflammatory proteins, gut microbiota, and plasma metabolites in MS pathogenesis. The mediation findings highlight complex interaction networks, offering novel insights into the immune–microbiota–metabolite axis and potential therapeutic targets for MS.