Characterization Of Sheep Erythrocyte Glycosphingolipids Recognized By Human Anti-Forssman Antibodies

The FORS histo-blood group system is the most recently discovered carbohydrate-based human blood group system. FORS is a rare blood group system, and most individuals have naturally occurring anti-FORS1 antibodies in plasma. Screening for anti-FORS1 antibodies is often done by hemagglutination assays using FORS1-expressing sheep erythrocytes, since FORS1-positive human erythrocytes are most often not available. Here, we have characterized the non-acid glycosphingolipids from sheep erythrocytes and isolated subfractions, with mass spectrometry, binding of antibodies and lectins, and by enzymatic hydrolysis. This demonstrated the presence of Forssman and Galili pentaosylceramides, and a Galili heptaosylceramide. Two complex glycosphingolipids recognized by human anti-FORS1 antibodies were characterized as a Forssman neolacto hybrid hexaosylceramide (GalNAc alpha 3GalNAc beta 3Gal beta 4GlcNAc beta 3Gal beta 4Glc beta 1Cer) and a Forssman Galili hybrid heptaosylceramide (GalNAc alpha 3GalNAc beta 3Gal alpha 3Gal beta 4GlcNAc beta 3Gal beta 4Glc beta 1Cer). These are novel glycosphingolipid structures, and to our knowledge, the first case of an elongated Galili antigen. Thus, the anti-Forssman antibodies in human serum bind not only to the classical Forssman pentaosylceramide (GalNAc alpha 3GalNAc beta 3Gal alpha 4Gal beta 4Glc beta 1Cer), but also when the GalNAc alpha 3GalNAc beta 3 sequence is presented on a neolacto core chain and even on a Galili carbohydrate sequence.