Human Milk Oligosaccharides in the Prevention of Necrotizing Enterocolitis: A Journey From in vitro and in vivo Models to Mother-Infant Cohort Studies

Abstract

Preterm infants who receive human milk instead of formula are 6- to 10-times less likely to develop necrotizing enterocolitis (NEC), one of the most common and devastating intestinal disorders that affects 5-10% of all very-low-birth-weight infants. Combined data from in vitro tissue culture models, in vivo preclinical studies in animal models, as well human mother-infant cohort studies support the hypothesis that human milk oligosaccharides (HMOs), complex sugars that are highly abundant in human milk but not in infant formula, contribute to the beneficial effects of human milk feeding in reducing NEC. The almost 20-year long journey of testing this hypothesis took an interesting turn during HMO in vivo efficacy testing and structure elucidation, suggesting that the original hypothesis may indeed be correct and specific HMO reduce NEC risk, however, the underlying mechanisms are likely different than originally postulated.

Keywords: breast milk; human milk oligosaccharide; infant nutrition; necrotizing enterocolitis; preterm infant.

Figure 1

Flow diagram of proposed NEC pathogenesis and potential benefits of HMOs. HMO, human milk oligosaccharides; NEC, necrotizing enterocolitis; PAF, platelet-activating factor; PNC, platelet-neutrophil complexes; ROS, reactive oxygen species [modified after Hsueh et al. (19)].

Figure 2

Selectin-mediated cell-cell interactions and potential interference with HMOs. (A) Leukocytes decelerate from the blood stream before they adhere and finally transmigrate to the site of inflammation. The initial rolling, the first interaction between leukocytes and activated endothelial cells, is mediated by selectins (box). HMOs (red dots) serve as selectin ligand analogs, reduce selectin ligand binding, and are thought to reduce leukocyte rolling and infiltration (modified after http://ley-leukocyte.liai.org). (B) Activated platelets upregulate expression of P-selectin (CD62P), which binds to P-Selectin Glycoprotein Ligand-1 (PSGL-1) on neutrophils, which establishes platelet-neutrophil-complex (PNC) formation and triggers a signaling cascade with an increase in neutrophil adhesion molecules and production of reactive oxygen species (ROS). Once again, HMOs serve as selectin-ligand analogs, reduce P-selectin-PSGL-1 binding, and neutrophil activation [modified after Cerletti et al. (36)]. CD62E/P, E- or P-Selectin; CD62L, L-Selectin; HMO, human milk oligosaccharide; PSGL-1, P-Selectin Glycoprotein Ligand-1.

Figure 3

Disialyl-lacto-N-tetraose (DSLNT) (A) reduces NEC-like symptoms in neonatal rats, but the HMO does not contain the Sialyl Lewis X (SLe^x) determinant (B). The Sialyl Lewis X is highlighted as an orange background (blue circle, glucose; yellow circle, galactose; blue square, N-acetylglucosamine; red triangle, fucose; purple diamond, sialic acid).

Figure 4

The combination of in vivo HMO efficacy and structure-function testing in neonatal rats with association studies in human mother-infant cohorts led to the identification of DSLNT as the protective HMO in NEC and informs future invention studies.