Chemical Resolution of an Epitope Recognized by Blood Group Antibodies Capable of Binding Both A and B Red Blood...

The high specificity of human antibodies to blood group A and B antigens is impressive, especially when considering the structural difference between these antigens (tetrasaccharides) is a NHAc versus a hydroxyl group on the terminal monosaccharide residue. It is well established that in addition to anti-A and anti-B there is a third antibody, anti-A,B capable of recognizing both A and B antigens. To analyze this AB specificity, we synthesized a tetrasaccharide, where the NHAc of the A antigen was replaced with an NH₂. This NH₂ group was then used to attach the glycan to an affinity resin, creating an AB epitope (AB^ep) adsorbent where the critical site for recognition by A and B antibodies was not accessible, while the rest of the (conformationally compact) tetrasaccharide remained accessible. Anti-AB^ep antibodies were then isolated from blood group O donors and found to have expected A,B specificity against immobilized and red cell bound synthetic antigens, including AB^ep, and were able to agglutinate both A and B red cells. The amount of these anti-AB^ep (anti-A,B) antibodies found in the blood of group O donors was comparable to levels of anti-A and anti-B found in group B and A individuals. Using STD-NMR the location for the AB epitope on the tetrasaccharide was found.