ANTI-M MONOCLONAL-ANTIBODIES CROSS-REACTING WITH VARIANT M(G) ANTIGEN- AN EXAMPLE OF MODULATION OF ANTIGENIC PROPERTIES OF PEPTIDE BY ITS GLYCOSYLATION

Some monoclonal antibodies (MoAbs) directed against blood group M-rela ted epitope of glycophorin A (GPA) were found to agglutinate rare vari ant erythrocytes carrying GPA of M(9) type. In contradistinction to no rmal GPA-M or -N, the N-terminal portion of GPA-MS is not glycosylated . Therefore, the multipin peptide synthesis was used for testing the s pecificity of the cross-reacting MoAbs. Among several anti-M and anti- M MoAbs tested, only three anti-M (E3, E6, 425/2B) agglutinated M(9) e rythrocytes and showed binding to the synthetic octapeptides correspon ding to N-terminal sequences of GPA-M (SSTTGVAM), GPA-N (LSTTEVAM), an d GPA-M(9) (LSTNEVAM). Testing multiple peptide analogs (window and re placement analysis) showed that these MoAbs were specific for peptidic epitope in which Met8 and Val6 were the most essential amino acid res idues. The amino acid replacements Ser1 <-> Leu1 or Gly5 <-> Glu5 (M v N) and Thr4 <-> Asn4 (M and N v M(9)) had no or negligible effect on the reaction of synthetic peptides with the MoAbs. However, when Ser2, Thr3, and Thr4 carry O-linked sialooligosaccharides (normal GPA-M or -N), the MoAbs recognize Gly5- and sialic acid-dependent blood group M -related epitope. An interesting finding concerning anti-M/M(9) MoAbs described here is the fact that glycosylation of amino acid residues a djacent to the most important part of peptidic epitope not only differ entially modulates the proper exposure of peptidic epitope, but also a lters the requirement for some amino acid residues present within the epitope. Pathologic conditions, including hematologic disorders, are o ften accompanied by alterations in protein glycosylation, resulting no t only from differences in the structure of antigen polypeptide chain, but also from changes in specificity or expression of enzymes involve d in glycosylation. Our present findings draw attention to possibility of the bidirectional modulation of protein antigenicity by glycosylat ion and may be helpful in interpretation of some results obtained with MoAb used for diagnostic or other purposes. (C) 1994 by The American Society of Hematology.