The STEROL ELEMENT BINDING PROTEIN 1C (SREBP1c) controls oxidative metabolism and suppressive function of regulatory T cells

Aim: Immunosuppressive T regulatory cells limits inflammation in different pathological settings, including atherosclerosis. Several studies have demonstrated that Treg lipid metabolism is critical for their function. Previous studies of our group showed that deficiency of SREBP1c, a key protein regulating intracellular fatty acid (FA) metabolism, worsened Tregs immunosuppressive function by derailing energetic metabolism toward glycolysis and affecting phospholipid metabolism. Therefore, aim of this study was to investigate how SREBP1c controls Treg function by modulating lipid metabolism. Methods: T cells isolated from WT and SREBP-1c KO mice and in vitro induced Treg (iTreg) were analyzed through RT-qPCR, flow-cytometry and lipidomics. iTreg metabolism was studied using the Seahorse XF technology. iTreg suppressor and migratory phenotype were assessed by in vitro assay. Results: Deficiency of Srebp1c resulted in a reduced in vitro generation of iTreg (-6,76%, p<0,0001), inhibition of Tell proliferation (-5%, p<0,01), increased migration (+80% vs CCL19/21, p<0,05) and glycolytic metabolism (+18,8%, p<0,05) compared to WT iTreg. Inhibition of fatty acid oxidation by Etomoxir (5μM) impaired suppressive phenotype of WT but not Srebp1c KO iTreg (-3,93%, p<0,05 for WT), suggesting a defective FA oxidation of the latter. Addition of palmitate improved suppressive phenotype of WT iTreg (+5,6%, p<0,01), but did not revert the functional and metabolic phenotype of Srebp1c KO iTreg. Protein and mRNA expression of CD36 were reduced in Srebp1c KO vs WT iTreg, confirming missed compensation of impaired lipid metabolism by extracellular fatty acids uptake. By lipidomic analysis Srebp1c deficiency resulted in an altered phospholipid composition, showing an accumulation of LPC over PC, which is associated to an impaired Lands cycle. Moreover, LPCAT3 inhibition showed a reduced expression of specific Treg proteins (CD73, AKT) required for their immunosuppressive activity. Conclusion: SREBP1c plays a key role in the immune-metabolic response of Tregs: by modulating lipid cellular metabolism SREBP1c controls energetic and functional phenotype of Treg.