Programmable HMO biosynthesis enabled by human cell-based glycoengineering.

The lactating mammary gland is uniquely adapted to synthesize unconjugated free glycans known as human milk oligosaccharides (HMOs). HMO biosynthesis is initiated by the formation of lactose, the common precursor from which structurally diverse neutral and acidic oligosaccharides are generated through the sequential action of Golgi-resident glycosyltransferases. Although microbial fermentation and chemical synthesis have enabled production of selected HMOs, human cell-based platforms remain largely unexplored despite providing the native glycosylation machinery required for HMO assembly. Consequently, the contributions of individual glycosyltransferases to HMO biosynthesis have remained difficult to define. Here, we describe the reconstruction of HMO biosynthesis in human embryonic kidney (HEK293) cells through reconstitution of the lactose synthase complex and systematic engineering of downstream glycosyltransferases. This modular approach enables programmable production of structurally defined HMOs, including both simple sialylated oligosaccharides and more complex type-I and type-II structures, while providing a tractable platform for assigning glycosyltransferase functions within the HMO biosynthetic pathway. Together, this establishes mammalian glycoengineering as a versatile system for investigating HMO biosynthesis and for the tailored production of biologically relevant HMOs.