Breast milk oligosaccharides shape the intestinal environment by affecting mucosal immunity and bacterial colonization. To clarify the role of milk oligosaccharide sialyl(α2,3)lactose (3SL) in intestinal physiology and disease, we investigated colitis development in Il10 −/− mice exposed to normal or 3SL-deficient milk during lactation. Onset and progression of intestinal inflammation were delayed in Il10 −/− mice deficient for the α2,3 sialyltransferase 4 (ST3GAL4) responsible for 3SL biosynthesis. The proinflammatory role of 3SL was confirmed by showing that oral supplementation of newborn Il10 −/− ;St3gal4 −/− mice with 3SL increased colitis severity. Conversely, fostering of newborn Il10 −/− mice to lactating St3gal4 −/− mothers reduced colitis severity. 3SL directly stimulated mesenteric lymph node CD11c + dendritic cells and induced production of cytokines required for expansion of T H 1 and T H 17 T cells. The stimulatory effect of 3SL was attenuated in Tlr4-deficient CD11c + cells, demonstrating that 3SL induces inflammation through Toll-like receptor 4 (TLR4) signaling. Thus, 3SL directly modulates mucosal immunity, which increases susceptibility to colitis.carbohydrate | glycan | prebiotics | mouse innate immunity I nflammatory bowel disease (IBD) affects up to 0.8% of the Western population and this number is constantly growing worldwide (1). The etiology of IBD is not fully understood, although a number of genetic and environmental factors leading to aberrant mucosal immune responses have been identified (2). Nutrition and especially breastfeeding affects the risk for IBD (3). Breastfeeding for at least 3 mo, accordingly, contributes to a lower incidence for IBD (4); however, these data still are controversial. Oligosaccharides are major constituents of breast milk fulfilling various functions, such as promoting growth of beneficial bacteria, acting as soluble receptors preventing attachment of pathogens in the gastrointestinal tract, and reducing adhesion of leukocytes (4, 5). The exact functions of individual oligosaccharides remain however largely unknown.The limited structural diversity of mouse milk oligosaccharides, including only sialyl(α2,3)lactose (3SL) and sialyl(α2,6)lactose (6SL), enables assessing the specific functional contribution of these oligosaccharides to intestinal homeostasis (6). A recent study provided unique evidence that milk-derived 3SL, but not 6SL, increased susceptibility to dextran sulfate sodium (DSS)-induced acute colitis (6). These findings indicate that individual milk oligosaccharides may not only mediate protective effects, but may promote inflammation as well.Mucosal innate immunity has a pivotal role in regulating inflammatory responses (2). Dendritic cells (DCs) and macrophages sense luminal antigens and provide signals for induction of tolerance to ingested antigens and commensal bacteria or for initiation of inflammatory immune responses, facilitating activation of adaptive immunity (7,8). Intestinal DCs consist of two functionally distinct subsets based on e...