Immunity https://doi.org/10.1016/j.immuni.2018.12.015 (2018)

The host microbiota helps shape the composition of the gut immune system, which in turn can affect disease susceptibility in complex ways. In Immunity, Faith and colleagues use fecal microbiota transfer (FMT) from healthy donors and those with inflammatory bowel disease (IBD) into germ-free mice to understand mechanistic changes in the gut immune system. At low resolution, the broad composition of the microbiota from donors with IBD and that from healthy donors looks similar; however, they induce distinct immune cell populations. IBD-derived FMT results in many more cells of the TH17 and TH2 helper T cell subsets but fewer regulatory T cells expressing the transcription factor RORγt. Mice that received IBD-derived FMT also exhibit worse transfer-colitis sequelae, with the relative frequency of induction of RORγt+ regulatory T cells versus that of TH17 cells being predictive of disease severity.