Fine specificities of murine anti-Mg monoclonal antibodies.

The specificities of two murine anti-Mg monoclonal IgG1 antibodies, 3B10 and 2D5, were determined by pepscan analysis. The peptides which correspond to various fragments of amino-terminal portions of glycophorin A of group M (GPA-M), N (GPA-N) and Mg (GPA-Mg), and replacement analogues of some of these peptides, were synthesized on plastic pins and tested for binding of the antibodies. Both antibodies bound strongly to the N-terminal Mg octapeptide 1LSTNEVAM8, but they showed different subspecificities. The essential fragment of the epitope 2D5 are amino acid residues 2STNEV6. Replacement of any of these amino acid residues by Ala, and replacement of Glu5 residue by Gly, abolished or strongly reduced the antibody binding, but replacement of Asn4 by Thr gave only a moderate decrease of peptide activity. In contrast, the Leu1 and Asn4 residues were most essential components of the epitope 3B10, while Ser2, Thr3 and Glu5 seemed to be less important. Our present results and earlier ones on the specificity of human anti-Mg alloantibodies and monoclonal anti-M/Mg antibodies showed that antibodies reacting with Mg antigen recognize different fragments and/or different amino acid residues of the amino- terminal nonglycosylated domain of GPA-Mg. The knowledge of fine specificities of antibodies reacting with Mg antigen is interesting in view of the presence of anti-Mg alloantibodies in 1–2% of human sera.