A CRISPR activation screen identifies CH25H as a restriction factor against influenza viruses by targeting accessible cholesterol.

Influenza A viruses (IAVs) cause severe outbreaks with high mortality in birds and humans. A deeper understanding of cell-intrinsic defense mechanisms against influenza viruses is therefore crucial for developing novel antiviral strategies. Herein, we perform a genome-wide CRISPR activation screen to systematically elucidate host restriction factors against influenza A (H7N9) virus. Among multiple candidates, cholesterol 25-hydroxylase (CH25H) is shown to be induced by influenza virus infection and inhibit viral membrane fusion. Notably, our previous work demonstrated that CH25H blocks the entry of plasma membrane-fusing viruses such as coronaviruses. This inhibition occurs by relocating accessible cholesterol from the plasma membrane (PM) to the endoplasmic reticulum (ER). Here, we extend this finding and show that the same mechanism works against endocytosis-dependent viruses such as influenza viruses. The exogenous supplementation of cholesterol can restore depleted accessible cholesterol and reverse the CH25H-mediated restriction. Additionally, we prove that acyl-CoA:cholesterol acyltransferase (ACAT) is required to recruit the accessible cholesterol in this process. However, how hydrophobic accessible cholesterol is transported remains unclear. Here, we demonstrate that GRAMD1/Aster-mediated non-vesicular cholesterol transport is utilized to mobilize accessible cholesterol upon stimulation of CH25H. 25-hydroxycholesterol (25HC), the catalytic product of CH25H, is a natural metabolite that potently inhibits influenza virus infection both