Recognition of type 1 chain oligosaccharides and lacto-series glycolipids by an antibody to human secretory component

Binding of the mouse IgM antibody 6C4 is lost after treatment of human free secretory component with peptide N-glycosidase F (Bakos et al. (1991) J. Immunol. 146, 162-168) or periodate, suggesting that asparagine-linked oligosaccharides contain the epitope recognized by this antibody. Inhibition of antibody binding to free secretory component by milk oligosaccharides established that lacto-N-tetraose is the minimum structure recognized by the antibody, but larger oligosaccharides with terminal Galβ1-3GlcNAc sequences bind with much higher affinity. Antibody binding is enhanced by substitution with the Lewis Fucα1-4 and is inhibited by Fucα1-2Ga1 substitution. Free secretory component, however, does not bind other antibodies that recognize Le^a or Le^b oligosaccharides, and binding is lost after digestion with a β- galactosidase that cleaves Galβ1-3 linkages but not after digestion with α- L-fucosidase. Therefore, the major epitope recognized by 6C4 on free secretory component is probably not an asparagine-linked Le^a oligosaccharide. The antibody also binds to human milk lactoferrin, some human mucins, and lacto-series glycolipids including III⁴αFuc- lactotetraosyl ceramide and lactotetraosyl ceramide. Based on affinity chromatography of oligosaccharities released from free secretory component, the epitope recognized by antibody 6C4 is present on approximately 3.5% of the asparagine-linked oligosaccharides.