GAB functions as a bioenergetic and signalling gatekeeper to control T cell inflammation.

γ-Aminobutyrate (GAB), the biochemical form of (GABA) γ-aminobutyric acid, participates in shaping physiological processes, including the immune response. How GAB metabolism is controlled to mediate such functions remains elusive. Here we show that GAB is one of the most abundant metabolites in CD4 T helper 17 (T17) and induced T regulatory (iT) cells. GAB functions as a bioenergetic and signalling gatekeeper by reciprocally controlling pro-inflammatory T17 cell and anti-inflammatory iT cell differentiation through distinct mechanisms. 4-Aminobutyrate aminotransferase (ABAT) funnels GAB into the tricarboxylic acid (TCA) cycle to maximize carbon allocation in promoting T17 cell differentiation. By contrast, the absence of ABAT activity in iT cells enables GAB to be exported to the extracellular environment where it acts as an autocrine signalling metabolite that promotes iT cell differentiation. Accordingly, ablation of ABAT activity in T cells protects against experimental autoimmune encephalomyelitis (EAE) progression. Conversely, ablation of GABA receptor in T cells worsens EAE. Our results suggest that the cell-autonomous control of GAB on CD4 T cells is bimodal and consists of the sequential action of two processes, ABAT-dependent mitochondrial anaplerosis and the receptor-dependent signalling response, both of which are required for T cell-mediated inflammation.