Immunogenicity and antigenicity of defined linear determinants of the MN sialoglycoprotein glycophorin A

Many epitopes of the blood group MN sialoglycoprotein glycophorin A (GPA) are clinically important and have been implicated in alloimmunization and autoimmune hemolytic anemia. However, the factors that contribute to the immunogenicity and antigenicity of GPA epitopes are poorly understood. To study this question, linear determinants were defined to include all GPA exofacial regions that were free of linked carbohydrate (GPA 27-34, GPA 52-62, and GPA 65-70), and peptides corresponding to these amino acids were synthesized. All defined GPA determinants were shown to contain residues accessible to the immune system. Immunogenicity was measured by binding rabbit antisera, produced by using red cells (RBCs), RBC membrane, and purified GPA as immunogens, to peptide antigens in an enzyme-linked immunosorbent assay. GPA 27-34 and GPA 52-62 were shown to be immunogenic, but GPA 65-70 was not. Comparison of the binding of antisera to peptide and purified GPA antigens showed that the defined determinants were less immunogenic than other, nondefined GPA epitopes. Antigenicity algorithms (hydrophilicity, surface probability, flexibility, and antigenic index) predicted that GPA 27-34 and, to a lesser extent, GPA 52-62 would be antigenic, while GPA 65-70 would be nonantigenic. Antigenicity, measured by the binding of rabbit antipeptide sera to RBCs, RBC membrane, and purified GPA, was demonstrated for the GPA 52-62 determinant alone. Results differed among molecular forms of GPA, which indicated that the specific conformation assumed by GPA is important in determining the immunogenicity and antigenicity of its epitopes.(ABSTRACT TRUNCATED AT 250 WORDS)