Structural and genetic basis for the binding of a mouse monoclonal antibody to Flavobacterium psychrophilum lipopolysaccharide.

A mouse monoclonal antibody (mAb FL100A) previously prepared against Flavobacterium psychrophilum (Fp) CSF259-93 has now been examined for binding to lipopolysaccharides (LPS) of this strain and Fp 950106-1/1. The corresponding O-polysaccharides (O-PS) of these strains are formed by identical trisaccharide repeats composed of l-Rhamnose (l-Rha), 2-acetamido-2-deoxy-l-fucose (l-FucNAc) and 2-acetamido-4-R₁-2,4-dideoxy-d-quinovose (d-Qui2NAc4NR₁) where R₁ represents a dihydroxyhexanamido moiety. The O-PS loci of these strains are also identical except for the gene (wzy1 or wzy2) that encodes the polysaccharide polymerase. Accordingly, adjacent O-PS repeats are joined through d-Qui2NAc4NR₁ and l-Rha by wzy2-dependent α(1-2) linkages in Fp CSF259-93 versus wzy1-dependent β(1-3) linkages in Fp 950106-1/1. mAb FL100A reacted strongly with Fp CSF259-93 O-PS and LPS but weakly or not at all with Fp 950106-1/1 LPS and O-PS. Importantly, it also labelled cell surface blebs on the former but not the latter strain. Additionally, mAb binding was approximately 5-times stronger to homologous Fp CSF259-93 LPS than to LPS from a strain with a different R-group gene. A conformational epitope for mAb FL100A binding was suggested from molecular dynamic simulations of each O-PS. Thus, Fp CSF259-93 O-PS formed a stable well-defined compact helix in which the R₁ groups were displayed in a regular pattern on the helix exterior while unreactive Fp 950106-1/1 O-PS adopted a flexible extended linear conformation. Taken together, the findings establish the specificity of mAb FL100A for Wzy2-linked F. psychrophilum O-PS and LPS.