Cross Reacting Polysaccharides (H. influenzae types a and b, and B. pumilus)

总结

Summary The structures of the 6 capsular polysaccharide (CP) types, a, b, c, d, e, and f of Haemophilus influenzae (Hi) have been elucidated. Systemic infections were due mostly to H. influenzae type b (Hib). A critical level of serum IgG anti-Hib CP confers type-specific immunity by complement-mediated bacteriolysis. Herd immunity followed wide spread usage of Hib conjugate vaccine. This near-elimination of Hib led to the speculation that other Hi types may emerge as causes of meningitis. For example, in Brazil, the incidence of Hib meningitis decreased 69% during 1 year after initiation of Hib conjugate immunization, while the incidence of Hia meningitis increased 8-fold. The Netherlands Reference Laboratory reported that type a was only observed in <4 year-olds. Properties of Hia and Hib CP Hi can be divided into 3 virulence groups of two, related to the structures of their CP: types a and b; the most virulent, are composed of a neutral sugar, an alcohol (ribitol), and a phosphodiester; types c and f are composed of an N-acetylated aminosugar, a monosaccharide, and phosphodiester; types d and e have a repeat unit of an N-acetylglucosamine and N-acetylmannosamineuronic acid. Hib and Hia have greater resistance to the bactericidal effects of complement alone than the other four types. Challenge of infant rats showed that the 50% Infective Dose (ID50) for bacteremia of both Hib and Hia was several logs lower than of the other types. Intranasal challenge of infant rats with type b or a strains resulted in 55 to 90% bacteremia with type b, and 35% with type a strains. The other types were not invasive. Invasive Diseases Caused By Hia Rates of Hia disease have been constant in the US regardless of Hib vaccination. From 1998 to 2002, The Emerging Infections Program, of CDC conducted active laboratory- and population-based surveillance for Hi disease in data from 9 sites with approximately 35 million people. Seventeen of 1,743 invasive isolates were Hia. Hia is an important cause of meningitis in certain populations such as White Mountain Apache Indian children who have an annual incidence of 254 cases/100,000 children of Hia meningitis. Hammittet al. reported an out break of invasive Hia disease among Native Alaska infants. During a 6-month period in 2 nearby villages, 5 Hia cases were documented. In Brazil, an incidence of Hia meningitis of 0.16 cases/100,000 person-years was reported. South Africa and native populations in Manitoba Canada also report Hia meningitis in children. Hia Vaccine The number of cases is too low for a randomized, double-blinded and controlled trial. There is precedence for adding types within a species to a vaccine without evidence for efficacy. Several pneumococcal types, meningococcal groups Y and W135, and poliovirus type 2 were licensed based upon their safety and immunogenicity. The structural, experimental and clinical properties of Hia CP resemble closely that of type b. The increasing number of reports of Hia invasive diseases suggest that development of a Hia conjugate is warranted. Methods for conjugating type b CP to a protein are applicable to Hia. D-1,5-ribitolphosphate is a constituent of the CPs of Hia and Hib. We reported that the cell wall polysaccharide (PS) of B. pumilus Sh18 contains a poly-1,5-ribitolphosphate as a major component and antibodies induced in mice by this PS conjugate cross-reacted with both Hia and Hib. We synthesized polyribitolphosphate chains containing either 8 or 12 repeat units, with the terminal keto groups used for conjugation to aminooxylated BSA or tetanus toxoid. These conjugates were injected into mice, 3 times 2 weeks a part at 2.5g/mouse and sera obtained a week later. Elisa demonstrated antibodies to both Hia and Hib, with the octamer conjugate being a better immunogen than dodecamer conjugate. Their bactericidal activity is being studied.