Immunogenicity and bactericidal activity in mice of an outer membrane protein vesicle vaccine against Neisseria meningitidis serogroup A disease☆
Introduction
Neisseria meningitidis of serogroup A is responsible for recurring epidemics of meningitis in a region of sub-Saharan Africa traditionally called the meningitis belt [1]. Most recent epidemics in this region have been caused by serogroup A meningococci of subgroup III; which was introduced in the African continent for the first time after the pilgrimage to Mecca in 1987 [2], [3], [4]. By the multilocus sequence typing method [5] subgroup III strains are mainly classified as being either sequence type 5 (ST-5) or 7 (ST-7), of which ST-7 strains now are emerging in Africa [6]. Molecular epidemiological studies of the serogroup A strains have shown that they are genotypically diverse [7] and frequently import foreign DNA [8]. In spite of their capability for rapid genetic change, epidemic serogroup A strains of subgroup III mainly express a single PorA subtype (P1.20,9) [9] and a single PorB serotype (4/21) [10].
Vaccination continues to be one of the most effective means to prevent infectious diseases, and offers the best cost–benefit measure to society. A polysaccharide vaccine against serogroup A meningococcal disease was developed by Gotschlich and co-workers [11]. This vaccine is safe and has been used to control serogroup A epidemics in the meningitis belt, and is nowadays usually combined as a bi-, tri- or tetravalent vaccine as AC, ACW135 or ACYW135, respectively. The serogroup A polysaccharide vaccine is strongly protective in adults and children above 5 years beyond the first year after vaccination, and was shown to be protective in children aged 3 months and over in Finland and New Zealand [12]. Protection correlates with serum bactericidal activity [13].Whether the vaccine confers long-term protection, herd immunity and what the level of efficacy is in children below 2 years, is debated in the scientific community [12], [14]. While the WHO has judged the vaccine as sub-optimal for routine immunisation and recommends mass reactive immunisation campaigns at epidemic threshold levels [15], others oppose this strategy and call for a reassessment of the utilisation of serogroup A polysaccharide vaccine while waiting for the conjugate vaccines to be introduced [14].
Expecting immunological memory and improved protection of young children, the serogroup A polysaccharide was converted into a T-cell dependent antigen by conjugating it to a carrier protein. Several clinical trials have later been performed with such vaccines in both developed and developing countries [16], [17], [18], [19], [20], [21], [22], [23], [24]. These vaccines are well tolerated, and they induce significant levels of anti-serogroup A polysaccharide antibodies and serum bactericidal activity in all age groups. Studies have shown that these levels are either higher than, similar to or close to those induced by serogroup A polysaccharide vaccines [17], [18], [20], [21], [22], [23], [24], [25], but as long as a protective threshold level is reached, the clinical significance of such difference is not known. Whilst the serogroup C-conjugate vaccine induces immunological memory, there is conflicting evidence whether this is true for the serogroup A-conjugate vaccine [21], [22], [24], [25]. The introduction of a serogroup A-conjugate vaccine for Africa has been delayed, as the development and production of this vaccine for the African market was considered as of low commercial interest by major US and EU vaccine manufacturers, and public incentives to overcome this barrier were insufficient [26]. Even if such vaccines prove superior to serogroup A polysaccharide vaccines, a major challenge to their widespread introduction in the meningitis belt will be to produce them at a price affordable for developing countries.
Efficacious serogroup B meningococcal vaccines based on outer membrane vesicles (OMVs) have been developed, and clinical trials have been performed in Cuba, Norway, Brazil, Iceland and Chile [27], [28], [29], [30], [31], [32]. Serogroup B OMV vaccines provided ≥4-fold increases in bactericidal antibody response against the homologous strain in 96–98% of the vaccinees in all age groups studied and induced immunological memory [32], [33]. Serogroup B OMV-based vaccines might therefore be effective to control epidemic situations where the epidemic is caused by a single strain, e.g. as in New Zealand where this approach is now being tried [34]. Since epidemics of serogroup A meningococci have been shown to be basically clonal [3], there should be a rationale for an OMV vaccine in preventing meningitis epidemics in Africa. The manufacturing process of an OMV vaccine is relatively simple and the price per dose is likely to be low.
Our aim with this study was to characterise a collection of serogroup A strains causing recent epidemics in Africa in order to evaluate the antigenic variation within subgroup III strains, and to select a representative isolate to prepare a serogroup A OMV (MenA OMV) vaccine. An investigational vaccine was produced, characterised, and its properties were tested in mice, focusing on immunogenicity and bactericidal activity.
Section snippets
Strains
Fifteen strains of serogroup A N. meningitidis strains belonging to the subgroup III on the basis of their multilocus genotypes determined by multilocus enzyme electrophoresis [4] were selected from the collection of the WHO Collaborating Centre for Reference and Research on Meningococci in Oslo, Norway (Table 1). The strains were isolated from patients in different African countries, collected between 1994 and 1998. The strain 44/76 (B:15:P1.7,16) used for production of the Norwegian serogroup
Strain characterisation
Results of the dot-blot analyses with mAbs of the 15 subgroup III strains (Table 1) showed that 14 strains were serosubtype P1.20,9; all were serotype 21 and reacted with the serotype 4 Mab MNG21.17, but not with mAb 15-1-P4. Eleven were clearly OpcA positive, 11 were L3,7,9 positive, three were clearly L10 positive, and four were clearly positive both with L8 and L11. One strain isolated from a patient in Angola in 1998 (Mk 190/98) was A:4/21:NST:L10 and OpcA÷. This strain did not express
Discussion
The antigen screening of this selection of 15 subgroup III meningococcal isolates from epidemics in different countries in the Meningitis Belt from 1994 to 1998 showed that 14 strains were A:4/21:P1.20,9 (Table 1). This is in agreement with previous studies [2], [4], [6], [43]. Strain Mk 190/98 from Angola did not express a PorA P1.20,9 protein, a phenomenon previously observed in a few subgroup III strains [44]. Sequencing of the variable region of the porA gene [45] of this PorA-deficient
Acknowledgements
Drs. W.D. Zollinger, C. Frasch and J. Kolberg kindly provided the monoclonal antibodies. We thank Nina Bjæring Hansen for the initial experiments on production of the vaccine. Kirsten Konsmo and Berit Nyland are thanked for their technical assistance. This work was supported in part by grant 146185/730 from the Research Council of Norway.
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Data from this manuscript were presented in part at the 13th International Pathogenic Neisseria Conference in Oslo, Norway, September 2002.