Analysis of peptidic epitopes recognized by the three monoclonal antibodies specific for the same region of glycophorin A but showing different properties.

Analysis of epitopes for the three monoclonal antibodies (GPA105, GPA33, OSK4-1) against glycophorin A (GPA) was performed with the use of proteolytic fragments of GPA, the synthetic nonapeptide with the sequence of amino acid residues 35-43 of GPA, and a series of peptides synthesized on plastic pins. The antibodies were specific for a short peptide sequence RAHE (a.a. 39-42 of GPA, MAbs GPA105 and OSK4-1) or RAHEV (a.a. 39-43 of GPA, MAb GPA33). Despite recognizing the same fragment of GPA, the three antibodies showed differences in fine specificity and in response to antigen desialylation. Reactions with single replacement analogs of the RAHEV sequence showed that immunodominant (unreplaceable) residues for the MAbs GPA33 and OSK4-1 were His and Glu, respectively, whereas no such residue was found for the MAb GPA105. Desialylation of the antigen gave strong enhancement of reactivity with the MAb GPA33, moderate--with the MAb GPA105, and weak or no enhancement of reaction with the MAb OSK4-1. The results showed that monoclonal antibodies directed against the same fragment of the polypeptide chain of densely glycosylated antigen may recognize different subsites which are masked at different degree by sialic acid residues.