Shigella flexneri O-antigen epitopes: chemical and immunochemical analyses reveal that epitopes of type III and group 6 antigens are identical.

In the present investigation we studied the nature of a Shigella flexneri O antigen, the type III antigen. No structural entity has yet been attributed to this antigen. Phenol-water-extracted lipopolysaccharides (LPSs) from all S. flexneri serotypes possessing either group 6 antigen or both type III antigen and group 6 antigen were subjected to de-O-acetylation by weak alkali treatment. Two-dimensional nuclear magnetic resonance spectra of serotype 3a native LPS and de-O-acetylated LPS were completely resolved, confirming the established structure (L. Kenne, B. Lindberg, K. Petersson, E. Katzenellenbogen, and E. Romanowska, Eur. J. Biochem. 91:279-284, 1978) and proving that no other structural alteration was introduced by the treatment. Both native and de-O-acetylated LPSs of serotypes 1b, 3a, 3b, and 4b were tested in an enzyme-linked immunosorbent assay against four absorbed rabbit anti-S. flexneri antisera and three monoclonal antibodies with specificity for different parts of the S. flexneri type III antigen and group 6 antigen. Upon de-O-acetylation, all binding capacity for the tested antibodies was lost. The chemical and immunological results indicate that the type III and group 6 antigens of S. flexneri are one and the same, a result of the presence of the O-acetyl group in rhamnose III of the repeating unit of the S. flexneri O-antigen chain.