Explore the potential mediating role of plasma metabolites and cytokines in the causal relationship between gut microbiota and the severity of immune-related adverse events: A Mendelian randomization study.

The gut microbiota, particularly genus Ruminiclostridium6, has been implicated in modulating immune-related adverse events (irAEs) associated with immune checkpoint inhibitor therapy. However, the mediating roles of circulating metabolites and cytokines in this relationship remain poorly understood. We performed a two-sample Mendelian randomization study to investigate causal links between genus Ruminiclostridium6, 19 plasma metabolites, 41 cytokines, and high-grade irAEs. Genetic instruments were selected from publicly available genome-wide association studies summary statistics. Inverse variance weighted method served as the primary analysis, supplemented by sensitivity and mediation analyses. No robust causal relationships were found between genus Ruminiclostridium6 and the 19 metabolites. However, high-grade irAEs were causally linked to decreased oleoylcholine levels. Among cytokines, only IL-2RA showed a causal association with high-grade irAEs, but not with genus Ruminiclostridium6. Consequently, two-step Mendelian randomization did not support a mediating role for the studied metabolites or cytokines in the genus Ruminiclostridium6-irAEs pathway. Enrichment analysis highlighted glycine, serine, threonine, arginine, and proline metabolism as potential pathways of interest. This study did not support 19 plasma metabolites or 41 cytokines as major mediators of the protective effect of genus Ruminiclostridium6 against high-grade irAEs. The mechanism may involve local gut-level immunomodulation or microbial metabolites not captured in this study. These findings underscore the complexity of microbiota-irAEs interactions and highlight specific metabolic pathways for further investigation.