Monoclonal antipeptide antibodies against amino acid residues 1101-1106 of human C4 distinguish C4A from C4B

Comparison of amino acid sequences of the α-chain fragment of human C4, C4d, has shown C4A- and C4B-specific sequences at residues 1101-1106 in which the aspartic acid-histidine substitution at position 1106 may be related to the amide and ester bond forming properties of these molecules. Peptides containing twelve amino acid residues of the C4A- or C4B-specific sequences were synthesized and injected into female Balb/c mice. Serum from 2 mice, one immunized with the C4A-specific peptide and the other with the C4B-specific peptide, gave strong isotype-specific responses in an enzyme-linked immunosorbent assay against affinity-purified C4A3 and C4B2B1. Spleen cells from these mice were fused with the mouse myeloma SP2/0-Ag 14, and two cloned cell lines, AII-1 and BII-1, were established from hybrids. Enzyme-linked immunosorbent assay and western blotting of monoclonal antibodies AII-1 and BII-1 show that the former reacts with the C4A but not with the C4B α-chain and the latter with C4B but not with the C4A α-chain. Furthermore, immunoblotting of C4 allelic variants showed that AII-1 reacted with all C4A allotypes tested, including A6, A4, A3 and A2, whereas BII-1 reacted with all C4B allotypes tested, including B5, B3, B2, and B1.