L-Phenylalanine is a metabolic checkpoint of human Th2 cells

After the primary response, circulating memory CD4+T effector and T regulatory cells (Treg) regulate recall responses, which are impaired in allergy. Using mass spectrometry, we discovered distinct metabolomes of these cells in humans and their unique enrichment in amino acids. By assessing energy metabolism in in vitro and ex vivo single-cell analyses, we determined that increased intracellular L-phenylalanine boosts glycolysis while limiting OXPHOS in CD4+T, memory CD4+T and Th2, but not in Treg cells. L-phenylalanine also restrains memory CD4+T proliferation in an IL4I1-dependent manner and inhibits Th2 cell proliferation and differentiation. RNA-sequencing, metabolomics, flow cytometry and proteomics, validated in vitro and across patients' cohorts, revealed an impairment in LAT1-dependent transport of L-phenylalanine into Th2 cells in allergy with an increase in its intracellular processing, accompanied by an expansion of pathogenic Th2 cells. Thus, our study identifies L-phenylalanine as a checkpoint in the development, energy metabolism and function of Th2 cells. ### Competing Interest Statement MS has received research grants from the Swiss National Science Foundation (SNSF nr 310030_189334/1), GSK, Novartis, Stiftung vorm. Bundner Heilstatte Arosa, CK-Care and OM Pharma as well as speaker's fee from AstraZeneca.