S-antigen: identification of the MAbA9-C6 monoclonal antibody binding site and the uveitopathogenic sites

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Abstract
The location of the monoclonal antibody, MAbA9-C6, binding site and two uveitopathogenic sites in S-antigen have been determined. Using cyanogen bromide, S-antigen was cleaved into nine peptides, designated C1 to C9. MAbA9-C6 bound selectively to one large peptide designated C5, consisting of 122 amino acids. Six peptides (20 to 22 amino acids in length) designated 2, 3, K, L, N and M, corresponding to the entire length of peptide C5, were synthesized chemically. In a radioimmunoassay and a dot-binding immunoassay, MAbA9-C6 bound selectively to one of the six peptides, peptide 3, indicating that this region of peptide C5 contains the MAbA9-C6 binding site. Twelve smaller peptides (ten amino acids in length), corresponding to the amino acid sequence of peptide 3, were synthesized and used in a competitive inhibition binding assay. These studies localized the MAbA9-C6 binding site to a small region within peptide 3. In addition, peptide K and peptide M were highly pathogenic for the induction of experimental autoimmune uveitis (EAU). Clinical and histological evidence of a severe uveo-retinitis, indistinguishable from that seen with native S-antigen, was documented in Lewis rats immunized with the synthetic peptides (50 μg), 11-12 days following immunization. Our results show that the MAbA9-C6 binding site and the two uveitopathogenic sites lie in close proximity to each other within the region of S-antigen corresponding to peptide C5. Furthermore, microcomputer analysis of the average hydrophili-city/hydrophobicity values of the amino acid sequence corresponding to peptide C5 shows that the MAbA9-C6 binding site and one uveitopathogenic site (peptide K) lie on the adjacent peaks. The significance of these findings and their relationship to the role of S-antigen in the pathogenesis of EAU and the phototransduction of vision is discussed.