Updated publication reference for PubMed record(s): 28580955. Project abstract: Foxp3+ T regulatory (Treg) cells have important functions in suppressing immune cell activation and establishing normal immune homeostasis. How Treg cells maintain their identity is not completely understood. Here we show that Ndfip1, a co-activator of Nedd4-family E3 ubiquitin ligases, is required for Treg cell stability and function. Ndfip1 deletion in Treg cells disrupts immune homeostasis and results in autoinflammatory disease. Ndfip1-deficient Treg cells are highly proliferative and are more likely to lose Foxp3 expression to become IL-4-producing TH2 effector cells. Proteomic analyses indicate that Ndfip1 deficiency alters the metabolic signature of Treg cells. Metabolic profiling reveals elevated glycolysis and increased mTORC1 signalling. Additional data suggest that Ndfip1 restricts Treg cell metabolic capacity and IL-4 production via distinct mechanisms. Thus, Ndfip1 preserves Treg lineage stability by preventing the expansion of highly proliferative and metabolically active cells that can cause immunopathology via secretion of IL-4.