Boston Children’s Hospital researchers reported Phase I results showing that oral encapsulated fecal microbiome transplantation (FMT) may raise peanut tolerance in adults with peanut allergy. In an open-label trial (NCT02960074) published in Science Translational Medicine, 15 adults received a one-time 36-capsule FMT dosing strategy, with and without antibiotic pretreatment. Six of the 15 participants met a secondary efficacy endpoint, with response durability tracked through the four-month clinical endpoint and no grade 3 or higher adverse events reported. Mechanistically, responders showed immune shifts consistent with restored oral tolerance, including increased tolerogenic RORγt-positive regulatory T cells and decreased type 2 helper T cells. The study also explored donor-microbe metabolite biology in parallel mouse work, highlighting bile acid metabolites as a possible mediator of tolerance—an actionable direction for future biomarker-driven trials.