L-Phenylalanine is a metabolic checkpoint of human Th2 cells.

After the primary response, circulating memory CD4(+)T effector and T regulatory (Treg) cells regulate recall responses, typically impaired in allergy. We discovered distinct metabolomes of these cells in humans, differentially enriched in phenylalanine-related metabolites. Energy metabolism assessment in in vitro and ex vivo single-cell analyses revealed that increased intracellular L-phenylalanine boosts glycolysis while limiting oxidative phosphorylation (OXPHOS) in CD4(+)T, memory CD4(+)T, and Th2 cells, but not in Th1, Th17, or Treg cells. L-phenylalanine also restrains proliferation of memory CD4(+)T, Th2, and Th17 cells in an IL4I1-dependent manner and limits Th2 differentiation via inhibition of STAT6 and mechanistic target of rapamycin (mTOR) signaling. RNA sequencing, metabolomics, flow cytometry, and proteomics, validated both in vitro and across patient cohorts, revealed impaired LAT1-dependent transport of L-phenylalanine into Th2 cells in allergy, with increased intracellular processing accompanied by expansion of pathogenic Th2 cells. Thus, our study identifies L-phenylalanine as a checkpoint in Th2 cell development, energy metabolism, and function.