ANTI-ALPHA-GALACTOSYL IMMUNOGLOBULIN-A (IGA), IGG, AND IGM IN HUMAN SECRETIONS

Anti-alpha-galactosyl (anti-Gal) is a natural human serum antibody that binds to the carbohydrate Gal alpha 1,3Gal beta 1,4GlcNAc-R (alpha-galactosyl epitope) and is synthesized by 1% of circulating B lymphocytes in response to immune stimulation by enteric bacteria, We were able to purify secretory anti-Gal from human colostrum and bile by affinity chromatography on silica-linked Gal alpha 1,3Gal beta 1,4GlcNAc. We found similar secretory anti-Gal antibodies in human milk, saliva, and vaginal washings, Secretory anti-Gal from milk and saliva was exclusively immunoglobulin A (IgA); that from colostrum and bile also contained IgG and IgM isotypes. Serum was also found to contain anti-Gal IgM and IgA in addition to the previously reported IgG, Anti-Gal IgA purified from colostrum and bile had both IgA1 and IgA2, Secretory anti-Gal from saliva, milk, colostrum, and bile agglutinated rabbit erythrocytes (RRBC) and bound to bovine thyroglobulin, both of which have abundant alpha-galactosyl epitopes, The RRBC-hemagglutinating capacity of human saliva, milk, bile, and serum was specifically adsorbed by immobilized Gal alpha 1,3Gal beta 1,4GlcNAc but not by Gal alpha 1;4Gal beta 1,4GlcNAc, Gal beta 1,3GalNAc, Gal beta 1,4GlcNAc, Gal beta 1,4GlcNAc alpha 1,2Man, or Fuc alpha 1,2Gal beta 1,4GlcNAc. No RRBC-hemagglutinating activity could be detected in rat milk, rat bile, cow milk, or rabbit bile, suggesting a restricted species distribution for secretory anti-Gal similar to that found for serum anti-Gal, Colostral anti-Gal IgA bound strongly to a sample of gram-negative bacteria isolated from the throats and stools of well children as well as to an Escherichia coli K-l blood isolate. Colostral anti-Gal IgA inhibited the binding of a Neisseria meningitidis strain to human buccal epithelial cells, suggesting that this antibody may play a protective role at the mucosal surface.