Host immunity modulates the efficacy of microbiota transplantation for treatment of Clostridioides difficile infection.

Fecal microbiota transplantation (FMT) is a successful therapeutic strategy for treating recurrent Clostridioides difficile infection. Despite remarkable efficacy, implementation of FMT therapy is limited and the mechanism of action remains poorly understood. Here, we demonstrate a critical role for the immune system in supporting FMT using a murine C. difficile infection system. Following FMT, Rag1 heterozygote mice resolve C. difficile while littermate Rag1 mice fail to clear the infection. Targeted ablation of adaptive immune cell subsets reveal a necessary role for CD4 Foxp3 T-regulatory cells, but not B cells or CD8 T cells, in FMT-mediated resolution of C. difficile infection. FMT non-responsive mice exhibit exacerbated inflammation, impaired engraftment of the FMT bacterial community and failed restoration of commensal bacteria-derived secondary bile acid metabolites in the large intestine. These data demonstrate that the host's inflammatory immune status can limit the efficacy of microbiota-based therapeutics to treat C. difficile infection.