The crystal structure of the murine Fab S-20-4 from a protective anti-cholera Ab specific for the lipopolysaccharide Ag of the Ogawa serotype has been determined in its unliganded form and in complex with synthetic fragments of the Ogawa O-specific polysaccharide (O-SP). The upstream terminal O-SP monosaccharide is shown to be the primary antigenic determinant. Additional perosamine residues protrude outwards from the Ab surface and contribute only marginally to the binding affinity and specificity. A complementary water-excluding hydrophobic interface and five Ab-Ag hydrogen bonds are crucial for carbohydrate recognition. The structure reported here explains the serotype specificity of anti-Ogawa Abs and provides a rational basis toward the development of a synthetic carbohydrate-based anti-cholera vaccine.

We used synthetic mono-to hexasaccharides that mimic the fragments of the O-antigen of Ogawa and Inaba O-polysaccharides (2-4), together with certain analogs of their monosaccharides to evaluate specificity. The binding of three immunoglobulins G (two specific for Ogawa and one specific for Ogawa/ Inaba) and of two immunoglobulins A (one specific for Ogawa and one specific for Inaba/Ogawa) were characterized by ligand-induced fluorescence titration or ELISA inhibition. The cDNA sequences of these antibodies are also presented in this report. MATERIALS AND METHODSMonoclonal Antibodies-Murine ascites fluids of A-20-6 and S-20-4 both contain vibriocidal IgG 1 specific for Ogawa-LPS. I-24-2, in contrast, contains IgG 3 specific for both serotypes Ogawa and Inaba-LPS, and it has low vibriocidal activity (5) (clone S-20-4 comes from the same hybridoma cells as clone S-20-3 described in this reference). Murine ascites fluid 2D6 and ZAC-3 contain IgA specific for Ogawa-LPS and IgA specific for Inaba/Ogawa-LPS, respectively. The latter two hybridomas were gifts from Drs. Marian Neutra, Harvard Medical School, and Dr. Richard Weltzin, Oravax, Cambridge, MA (1, 6) and were grown in BALB/c mice. IgGs were purified using ImmunoPure® (G) IgG purification kits (Pierce). Briefly, ascites fluid (2 ml, clarified by centrifugation) was mixed with ImmunoPure® (G) binding buffer (2 ml) and applied to a protein G column. After washing the column with 5 ϫ 2-ml aliquots of the ImmunoPure® (G) binding buffer, the bound IgG was eluted with 6 ml of ImmunoPure® (G) elution buffer, dialyzed against PBS, pH 7.4 (2000 ml) for three changes at 0°C, frozen, and labeled. The purified A-20-6, S-20-4, and I-24-2 showed a single arc of precipitation versus goat anti-mouse IgG 1 and IgG 3 , respectively (heavy chainspecific), and goat anti-whole mouse serum (Sigma) by immunoelectrophoresis. IgAs were purified from ascites fluid by 40% ammonium sulfate precipitation and anion-exchange DEAE-Sephadex A-25 chromatography (7). Monomeric IgA was obtained by mild reduction with 5 mM 1,4-dithiothreitol (Sigma) and alkylation with 11 mM 2-iodoacetamide (Sigma), followed by re-adsorption of the sample on DEAESephadex A-25 and elution with PBS, pH 7.4. The purity of IgAs was also verified by immuno-electrophoresis against anti-mouse IgA and serum and SDS-polyacrylamide gel electrophoresis.LPS and Synthetic Oligosaccharides-V. cholerae O:1 LPSs were obtained from acetone-treated cells of strain 569B, classical biotype, serotype Inaba, lot VC1219; strain 3083, classical biotype, serotype Ogawa; and V. cholerae O:139 Bengal, strain 4450. Salmonella paratyphi A LPS was a field isolate in Nepal, strain NTP-6. All LPSs were purified as described (8) and at 2 mg/ml showed negative tests (Coomassie Blue) for protein. Severely base-degraded V. cholerae O:139 LPS (9) was a gift from Dr. Andrew D. Cox, National Research Council, Ottawa, Canada. De-O-acylated Ogawa-LPS (10) was oxidized in aqueous 0.8% periodate solution for 3 days in the dark, dialyzed, and freezedried a...

The epidemic and pandemic potential of Vibrio cholerae O139 is such that a vaccine against this newly emerged serogroup of V. cholerae is required. A conjugate made of the polysaccharide moiety (O-specific polysaccharide plus core) of the lipopolysaccharide (LPS) of V. cholerae O139 (pmLPS) was prepared by derivatization of the pmLPS with adipic acid dihydrazide and coupling to tetanus toxoid (TT) by carbodiimidemediated condensation. The immunologic properties of the conjugate were tested using BALB/c mice injected subcutaneously three times at 2 weeks interval and then a fourth time 4 weeks later. Mice were bled 7 days after each injection and then once each month for the following 6 months. LPS and TT antibody levels were determined by enzyme-linked immunosorbent assay using immunoplates coated with either O139 LPS or TT. Both pmLPS and pmLPS-TT conjugate elicited low levels of immunoglobulin M (IgM), peaking 5 weeks after the first immunization. The conjugate elicited high levels of IgG antibodies, peaking 3 months after the first immunization and declining slowly during the following 5 months. TT alone, or as a component of conjugate, induced mostly IgG antibodies. Antibodies elicited by the conjugate recognized both capsular polysaccharide and LPS from V. cholerae O139 and were vibriocidal. They were also protective in the neonatal mouse model of cholera infection. The conjugation of the O139 pmLPS, therefore, enhanced its immunogenicity and conferred T-dependent properties to this polysaccharide.Since the appearance of Vibrio cholerae O139 in the suburb of Madras, India, in October 1992, epidemic cholera caused by this strain has spread rapidly throughout the Indian subcontinent (1). Clinical illness associated with V. cholerae O139 infection appears to be virtually identical to that due to V. cholerae O1 E1 Tor infections. However, in contrast to infection with V. cholerae O1, V. cholerae O139 infection has largely affected the adult population in areas of V. cholerae O1 endemicity, indicating a lack of protective immunity against this newly evolved strain (1). Presumably, there are differences between the immune responses against O1 and O139 strains, which may be of considerable importance in terms of protection (33). A quiescent period followed the appearance of V. cholerae O139, and it was thought that it was a one-time event. However, there was an upsurge of cases in Calcutta, India, in 1996, and the O139 serogroup again became the dominant serogroup causing cholera in India by September 1996 (32). The O139 serogroup has remained present in India and Bangladesh since this last outbreak (15) and requires careful monitoring.It has been suggested that the emergence of V. cholerae O139 is the result of a complex chromosomal rearrangement involving the horizontal transfer of genes encoding enzymes involved in O-specific polysaccharide (O-SP) biosynthesis (3,8,14,43). Indeed, the major differences between V. cholerae O1 and V. cholerae O139 reside in their cell surface components. V. cholerae O139, unlike ...

Cholera remains an important public health problem in many parts of the world and the availability of an effective cholera vaccine is important for the prevention of cholera in the countries affected by this disease. Despite the apearance in 1992 of a new serogroup, O139, of Vibrio cholerae, most of the cholera outbreaks are still caused by V. cholerae O1 biotype El Tor. Vaccine trials in Asia from 1968 to 1971, and studies of the production of serotypespecific antiserum in rabbits and of the protective activity of monoclonal antibodies against diarrhoeal disease in neonatal mice, have led to the conclusion that the Ogawa serotype contains a specific antigenic determinant whereas the Inaba serotype contains a different antigenic determinant that cross-reacts with the Ogawa serotype. By studying the binding of anti-Ogawa monoclonal antibodies to synthetic oligosaccharide fragments mimicking the Ogawa O-specific polysaccharide, it has been shown that the terminal monosaccharide, bearing the 2-O-methyl group in the O-specific polysaccharide, is most probably the serotype-specific determinant for the Ogawa strain. However, study of the binding of a monoclonal antibody recognizing both Ogawa and Inaba serotypes suggested partial recognition of the core as well as of the O-specific polysaccharide of the LPS of V. cholerae O1. To further characterize this antigenic determinant that is common to the Ogawa and Inaba serotypes, the core and the O-specific polysaccharide linked to the core of V. cholerae O1 LPS were purified by preparative electrophoresis. The O-specific polysaccharide linked to the core was subjected to periodate oxidation to destroy sugars from the core. Binding studies of these purified saccharide fragments to a monoclonal antibody which is protective in mice and specific to the antigenic determinant common to Ogawa and Inaba serotypes showed that both the core and the O-specific polysaccharide are involved in this common antigenic determinant. This explains how the presence or the absence of the Ogawa-specific antigenic determinant would lead to the expression of two independent antigenic determinants of V. cholerae O1, one specific to the Ogawa serotype and the other common to both Ogawa and Inaba serotypes.