A novel strategy for the synthesis of neoglycoconjugates from deacylated deep rough lipopolysaccharides

Subtle Changes in the Combining Site of the Chlamydiaceae-Specific mAb S25-23 Increase the Antibody-Carbohydrate Binding Affinity by an Order of Magnitude

Murine antibodies S25-23, S25-26, and S25-5 derive from a common germ-line origin, and all bind the Chlamydiaceae family-specific epitope αKdo(2→8)αKdo(2→4)αKdo (where Kdo is 3-deoxy-α-d- manno-oct-2-ulosonic acid) with high affinity and specificity. These antibodies recognize the entire trisaccharide antigen in a linkage-dependent manner via a groove composed largely of germ-line residues. Despite sharing identical heavy and light chain genes, S25-23 binds the family-specific epitope with nanomolar affinity, which is an order of magnitude higher than that of S25-26, while S25-5 displays an affinity between those of S25-23 and S25-26. We determined the high-resolution crystal structures of S25-23 and S25-5 antigen binding fragments in complex with a pentasaccharide derived from the LPS of Chlamydia and measured the affinity of S25-5 for chlamydial LPS antigens using isothermal titration microcalorimetry. The 1.75 Å resolution structure of S25-23 shows how subtle conservative mutations Arg(L)-27E to lysine and Ser(H)-56 to threonine lead to an order of magnitude increase in affinity. Importantly, comparison between previous S25-26 structures and the 1.99 and 2.05 Å resolution liganded and unliganded structures of S25-5, respectively, shows how a Ser(L)-27E mutation results in an intermediate affinity due to the reduced enthalpic penalty associated with complex formation that would otherwise be required for arginine in this position. This strategy allows for subtle adjustments in the combining site via affinity maturation that have dramatic consequences for the affinity of an antibody for its antigen.

Groove-type recognition of chlamydiaceae-specific lipopolysaccharide antigen by a family of antibodies possessing an unusual variable heavy chain N-linked glycan

The structure of the antigen binding fragment of mAb S25-26, determined to 1.95 Å resolution in complex with the Chlamydiaceae family-specific trisaccharide antigen Kdo(2→8)Kdo(2→4)Kdo (Kdo = 3-deoxy-α-d-manno-oct-2-ulopyranosonic acid), displays a germ-line-coded paratope that differs significantly from previously characterized Chlamydiaceae-specific mAbs despite being raised against the identical immunogen. Unlike the terminal Kdo recognition pocket that promotes cross-reactivity in S25-2-type antibodies, S25-26 and the closely related S25-23 utilize a groove composed of germ-line residues to recognize the entire trisaccharide antigen and so confer strict specificity. Interest in S25-23 was sparked by its rare high μm affinity and strict specificity for the family-specific trisaccharide antigen; however, only the related antibody S25-26 proved amenable to crystallization. The structures of three unliganded forms of S25-26 have a labile complementary-determining region H3 adjacent to significant glycosylation of the variable heavy chain on asparagine 85 in Framework Region 3. Analysis of the glycan reveals a heterogeneous mixture with a common root structure that contains an unusually high number of terminal αGal-Gal moieties. One of the few reported structures of glycosylated mAbs containing these epitopes is the therapeutic antibody Cetuximab; however, unlike Cetuximab, one of the unliganded structures in S25-26 shows significant order in the glycan with appropriate electron density for nine residues. The elucidation of the three-dimensional structure of an αGal-containing N-linked glycan on a mAb variable heavy chain has potential clinical interest, as it has been implicated in allergic response in patients receiving therapeutic antibodies.

Analysis of cross-reactive and specific anti-carbohydrate antibodies against lipopolysaccharide from Chlamydophila psittaci

Chlamydiae contain a rough-type lipopolysaccharide (LPS) of 3-deoxy-alpha-d-manno-oct-2-ulopyranosonic acid residues (Kdo). Two Kdo trisaccharides, 2.8/2.4- and 2.4/2.4-linked, and a branched 2.4[2.8]2.4-linked Kdo tetrasaccharide occur in Chlamydiaceae. While the 2.8/2.4-linked trisaccharide contains a family-specific epitope, the branched Kdo oligosaccharide occurs only in Chlamydophila psittaci and antibodies against it will be useful in human and veterinarian diagnostics. To overcome the generation of cross-reactive antibodies that bind with high affinity to a dominant epitope formed by 2.4/2.4-linked Kdo, we immunized mice with a synthetic 2.4[2.8]-linked branched Kdo trisaccharide and used phage display of scFv to isolate recombinant antibody fragments (NH2240-31 and SAG506-01) that recognize the branched Kdo oligosaccharide with a K(D) of less than 10 nM. Importantly, although these antibodies used germline genes coding for an inherited Kdo recognition site, they were able clearly to distinguish between 2.4[2.8]2.4- and 2.4/2.4-linked Kdo. Sequence determination, binding data, and X-ray structural analysis revealed the basis for the improved discrimination between similar Kdo ligands and indicated that the alteration of a stacking interaction from a phenylalanine residue in the center of the combining site to a tyrosine residue facing away from the center favors recognition of branched 2.4[2.8]2.4-linked Kdo residues. Immunofluorescence tests of infected cell monolayers using this antibody show specific staining of C. psittaci elementary bodies that allow it to be distinguished from other pathogenic chlamydiae.

Saccharide/protein conjugate vaccines for Bordetella species: preparation of saccharide, development of new conjugation procedures, and physico-chemical and immunological characterization of the conjugates

Bordetellae are Gram-negative bacilli causing respiratory tract infections of mammals and birds. Clinically important are B. pertussis, B. parapertussis and B. bronchiseptica. B. pertussis vaccines have been successful in preventing pertussis in infants and children. Veterinary vaccines against B. bronchiseptica are available, but their efficacy and mode of action are not established. There is no vaccine against B. parapertussis. Based on the concept that immunity to non-capsulated Gram-negative bacteria may be conferred by serum IgG anti-LPS we studied chemical, serological and immunological properties of the O-specific polysaccharides (O-SP) of B. bronchiseptica and B. parapertussis obtained by different degradation procedures. One type of the B. parapertussis and two types of B. bronchiseptica O-SP were recognized based on the structure of their non-reducing end saccharide; no cross-reaction between the two B. bronchiseptica types was observed. Competitive inhibition assays showed the immunodominance of the non-reducing end of these O-SP. Conjugates of B. bronchiseptica and B. parapertussis O-SP were prepared by two methods: using the anhydro-Kdo residue exposed by mild acid hydrolysis of the LPS or the 2,5-anhydromannose residue exposed by deamination of the core glucosamine of the LPS, for binding to an aminooxylated protein. Both coupling methods were carried out at a neutral pH, room temperature, and in a short time. All conjugates, injected as saline solutions at a fraction of an estimated human dose, induced antibodies in mice to the homologous O-SP. These methodologies can be applied to prepare O-SP-based vaccines against other Gram-negative bacteria.