Bordetellae, Gram-negative bacilli causing respiratory tract infections of mammals and birds include B. pertussis, B. parapertussis and B. bronchiseptica. The licensed pertussis vaccines confer incomplete efficacy on an individual basis, probably because pertussis toxin antibodies do not kill the organism directly, however herd immunity contributes to the almost complete protection with wide vaccine usage. The presence of bactericidal antibodies would increase vaccine effectiveness on an individual basis. Based on the concept that IgG anti-LPS provides immunity to non-capsulated Gram-negative bacteria we studied chemical, serological and immunological properties of LPS-derived saccharides of B. pertussis and B. bronchiseptica, -reported to share the same LPS core-, obtained by different degradation procedures and their protein conjugates. B. pertussis LPS is composed of a branched dodecasaccharide core bound to Lipid A. B. bronchiseptica LPS core is structurally the same but is further substituted by the O-specific polysaccharide (O-SP): a linear polymer of 1,4-linked 2,3-diacetamido-2,3-dideoxy-alpha-galacturonic acid. Two types of B. bronchiseptica O-SPs were identified based on the identity of their non-reducing end saccharide;no cross-reaction between these two types was found. Competitive inhibition assays of whole cell induced antisera showed that 95% of the antibodies were directed to the non-reducing end of these O-SP. Conjugates of B. bronchiseptica O-SPs were prepared by two methods: using the Kdo residue exposed by mild acid hydrolysis of the LPS or the core glucosamine residue exposed by deamination 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 but not to the core. An isolated B. bronchiseptica core fraction without its O-SP and subjected to ESI-MS and NMR analysis confirmed its structural similarity to that of the B. pertussis core. Small variations were found: the core Fuc4NMe was 50% methylated in B. bronchiseptica, 100% in B. pertussis and the core Hep was about 30% phosphorylated in B. bronchiseptica, non phosphorylated in B. pertussis. Both B. pertussis and B. bronchiseptica cores were conjugated to aminooxylated BSA via their terminal Kdo. Injected into mice, both conjugates induced similar IgG anti B. pertussis LPS levels, significantly higher than a conjugate of B. brochiseptica core + O-SP. Because B. bronchiseptica grows faster than B. pertussis, with high yields and on simple culture media it was further investigated as a potential pertussis vaccine source. Mutants deficient in O-SP production were used: 1. RB50 delta (RB50-derived mutant, with a deletion spanning the wbmB, wbmC, wbmD and wbmE genes - this strain lacks the O-SP but its core structure is identical to that of the parent strain, 2. RBA2b (RB50-derived wbmA mutant producing LPS with no O-SP, but with the three non-reducing end core saccharides repeated several times). We prepared fractions of the B. bronchiseptica core with 1 to 4 repeats of this terminal trisaccharide and bound them to BSA at different densities. All conjugates were immunogenic in mice, the highest antibody levels were obtained by conjugates containing 10-15 saccharide chains per protein and with one repeat of the terminal trisaccharide. Conjugate-induced sera were bactericidal against B. pertussis, their titers correlated roughly with IgG anti LPS levels measured by ELISA.
