IL4I1 overexpression protects against nonalcoholic fatty liver disease in part by inhibiting the AKT/FOXO1 pathway-mediated Th17 cell differentiation.

Nonalcoholic fatty liver disease (NAFLD) has posed a huge threat to public health globally, but there are currently no approved drugs available. Growing evidence has proved the close association between increased Th17 cells and NAFLD progression. Interleukin-4 induced protein 1 (IL4I1), an amino acid oxidase secreted by immune cells, was reported to regulate the Th17 cells, but its exact role in NAFLD progression has not been fully explained yet. We found that IL4I1 was highly expressed in the liver of C57BL/6J mice with NAFLD induced by an 8-weeks western diet. To explore the IL4I1's effect, mice were injected with AAV8 encoding IL4I1 1 week before western diet administration. The results showed that IL4I1 overexpression inhibited the NAFLD progression, demonstrated by relieved liver damage and lipid accumulation. The underlying mechanism in which IL4I1 acts on NAFLD might be attributed to the inactivated AKT/forkhead box protein O1 (FOXO1) signaling pathway-mediated decrease of Th17 cells in liver tissues. Subsequently, by culturing naive CD4(+) T cells isolated from the spleen of mice in Th17 cell-polarizing conditions, we determined that IL4I1 overexpression inhibited Th17 cell differentiation by inactivating the AKT/FOXO1 pathway, whereas its knockdown exhibited opposite effects. Further, the AKT activator SC79 reversed the effect of IL4I1 overexpression on Th17 cell differentiation. Collectively, our study supports that compensatory upregulation of IL4I1 protects against liver damage and lipid accumulation in NAFLD progression, partially by inhibiting the activated AKT/FOXO1 signaling pathway-induced Th17 cell differentiation.