Anti-infectious properties of the human milk fat globule membrane.

Secretory immunoglobulin A (sIgA), the predominant antibody fraction of human milk, represents a major protective factor against neonatal infection. Until now, sIgA had been identified only in the humoral fraction of human milk. For bovine milk an association between sIgA and the milk fat globule (MFG) membranes has been demonstrated. The aim of our study was to assess whether sIgA is associated with the MFG membranes in human milk. Using anti-sIgA-agglutinated human MFG and immune fluorescence microscopy, we demonstrated that sIgA is, in fact, associated with human MFG. Subsequently, by electrophoretic separation of human MFG membranes and Western blotting, we demonstrated specific sIgA bands, suggesting that sIgA is truly an integral part of the human MFG membrane. This may be of physiological relevance, as undigested and functional human MFG are found in the stools of the newborn.

Secretory immunoglobulin A is a component of the human milk fat globule membrane.

Secretory immunoglobulin A (sIgA), the predominant antibody fraction of human milk, represents a major protective factor against neonatal infection. Until now, sIgA had been identified only in the humoral fraction of human milk. The aim of this study was to assess whether in human milk sIgA is also associated with the milk fat globule (MFG). Using anti-sIgA-agglutinated human MFG and immune fluorescence microscopy, we could demonstrate that sIgA is, in fact, associated with human MFG. In human MFG membranes separated by Western blotting, sIgA bands were specifically stained, suggesting that sIgA is strongly associated with the human MFG membrane. This may be of physiologic relevance, inasmuch as earlier we could show that a quantity of undigested and functional human MFG are in fact found in the stools of the newborn. This would allow an additional extension of the protective mechanisms of sIgA throughout the whole intestine.

Inhibition of adhesion of S-fimbriated E. coli to buccal epithelial cells by human skim milk is predominantly mediated by mucins and depends on the period of lactation.

Expression of S-fimbriae is frequent in Escherichia coli strains causing sepsis and meningitis in the newborn period. We analysed the ability of human skim milk to inhibit adhesion of S-fimbriated E. coli to human buccal epithelia. Adhesion was inhibited by up to 90% using colostrum (5%) and up to 50% with mature milk (5%), indicating that this anti-infective mechanism depends on the period of lactation. Elimination of up to 99% of immunoglobulins and 91% of lactoferrin by affinity chromatography had no effect on the inhibition of adhesion. After separation of high- (> 10 kD) and low-molecular-weight fractions of skim milk, only the fraction > 10 kD was found to be able to inhibit bacterial adhesion. In order to further characterize receptor molecules for bacteria, we investigated binding of isolated S-fimbriae to glycoprotein bands on Western blot strips. Fimbriae mainly bound to a high-molecular-weight band (> 200 kD). According to molecular weight and staining behaviour, this band most likely represents mucins. We conclude that carbohydrate residues on secreted mucins of human skim milk are able to inhibit bacterial adhesion to mucosal surfaces. This could provide protection against neonatal sepsis and meningitis caused by E. coli.

Inhibition of adhesion of S-fimbriated Escherichia coli to epithelial cells by meconium and feces of breast-fed and formula-fed newborns: mucins are the major inhibitory component.

We investigated the ability of meconium, feces from human milk-fed (HMF) newborns, and feces from formula-fed (FF) newborns to inhibit adhesion of S-fimbriated E. coli to human buccal epithelial cells. S-fimbriae are a common property of E. coli strains causing sepsis and meningitis in neonates. Meconium had the highest content of neuraminic acid and the strongest inhibitory effect on bacterial adhesion. HMF also exerted high inhibitory activity while FF was markedly less active: To achieve inhibitory effects comparable to HMF a sixfold amount of FF was required. Glycoproteins from excretions were separated by gel chromatography. Fractions obtained were analyzed for adhesion-inhibiting activity. In all excretions analyzed, the mucin-containing fraction could be identified as the major inhibitory component. Inhibition was probably mediated by specific interaction of this fraction with S-fimbriae, as shown by binding of isolated fimbriae on Western blots after electrophoretic separation of glycoproteins. In conclusion, our data support the view that the mucin-containing fraction from meconium and human milk exerts antibacterial functions by preventing adhesin-mediated binding of pathogenic bacteria to mucosal epithelia.

Inhibition of adhesion of S-fimbriated Escherichia coli to buccal epithelial cells by human milk fat globule membrane components: a novel aspect of the protective function of mucins in the nonimmunoglobulin fraction.

We investigated the presence of factors in human milk that inhibit invasion of pathogenic bacteria. The effect of human milk fat globule membrane (HMFGM) components on adhesion of cloned S-fimbriated Escherichia coli to human buccal epithelial cells was analyzed. S fimbriae are a common feature of E. coli strains causing sepsis and meningitis in newborns and are bound to epithelia via sialyl-(alpha-2-3)galactoside structures. Human milk fat globules (HMFG) could be agglutinated by the above-mentioned bacteria. Agglutination could be inhibited by fetuin, human glycophorin, and alpha 1-acid glycoprotein. In addition, pretreatment of HMFG with Vibrio cholerae neuraminidase markedly reduced bacterium-induced agglutinations, indicating the involvement of neuraminic acid-containing glycoproteins. In contrast, lipid droplets of infant formula or artificial lipid emulsions (Intralipid) could not be agglutinated. HMFG were present in stools of breast-fed neonates as shown by indirect immunofluorescence staining with a monoclonal antibody directed against carbohydrate residues present on HMFGM. These HMFG could be agglutinated by bacteria. HMFG inhibited E. coli adhesion to buccal epithelial cells. To further characterize relevant E. coli binding structures, HMFGM components were separated by gel chromatography. The mucin fraction showed the most pronounced inhibitory effect on adhesion of S-fimbriated E. coli to human buccal epithelial cells. Our data suggest that HMFG inhibit bacterial adhesion in the entire intestine and thereby may provide protection against bacterial infection.