Neisseria meningitidis causes explosive epidemics in sub-Saharan Africa. Most are caused by capsular group A strains. However, group W-135 and X strains also cause epidemics in this region, and these strains may emerge once mass immunization with a group A polysaccharide-protein conjugate vaccine is introduced. Our goal is to develop a meningococcal vaccine for Africa that targets strains from all capsular groups. Our approach will be to use novel protein antigens identified for """"""""group B vaccines"""""""", which also elicit protective antibodies against strains with other capsules. These """"""""unconventional"""""""" antigens will be presented in simple outer membrane vesicles (OMV) that have potent natural adjuvants. Our studies will build on previous experience with detergent-extracted OMV vaccines, which are proven to be safe and effective in humans. Their major limitation is that they elicit serum bactericidal antibodies primarily directed at PorA, which is antigenically variable. To extend protection to strains with heterologous PorA, we prepared mutants of group B strains that were engineered to over-express factor H binding protein (fHbp), which is a novel antigen in two promising group B recombinant protein vaccines. By introducing an additional mutation in LPS biosynthesis, we attenuated endotoxin activity. In mice, non-detergent-treated OMV vaccines prepared from the mutants elicited serum bactericidal antibody responses against genetically diverse group B strains, as well as epidemic group A, W-135 and X strains from Africa. Our hypothesis is that a native OMV vaccine prepared from mutant strains from Africa will elicit even broader bactericidal antibodies directed at PorA, fHbp and other antigens expressed by strains from Africa. Further, the LPS mutation will eliminate the need for detergent extraction of the OMV, which is used to decrease LPS in conventional OMV vaccines, but also extracts desirable antigens such as fHbp.
In Aim 1, we will investigate genetic lineages and sequence diversity of genes encoding fHbp, PorA and other vaccine antigens among 200 meningococcal isolates from a geographically diverse collection of strains from Africa.
In Aim 2, we will measure antigen expression by a quantitative capture ELISA, and antigen surface-accessibility on live bacteria by flow cytometry.
In Aim 3, we will create mutants of recent African epidemic strains, which will be engineered to express more than one PorA molecule, over-express fHbp, and have attenuated endotoxin. The vaccine strains also will be selected for naturally high expression of an adhesin/invasin, NadA. We will prepare native OMV vaccines from the mutants, and assess OMV toxicity by measuring inflammatory cytokine responses of human PBMCs incubated in vitro with the vaccine. We will immunize mice and infant primates and measure serum bactericidal antibody responses against strains from Africa. The results will provide proof of principle that the OMV vaccine is likely to be well-tolerated in humans and elicit broad protective immunity. These findings would support an application to test the OMV vaccine in humans for control of meningococcal epidemics in sub-Sahara caused by strains from all capsular groups.
Meningococci cause explosive epidemics of meningitis in sub-Sahara Africa that can involve more than 100,000 cases in a few months. Most epidemics have been caused by encapsulated group A strains but strains from other capsular groups also have begun to cause epidemics in the region. A polysaccharide conjugate vaccine against group A disease is being developed for Africa but there is grave concern that strains with other capsules may emerge and cause epidemics once mass immunization with the group A conjugate vaccine is introduced. We propose to develop an outer membrane vesicle vaccine from mutant meningococcal strains, engineered for over-expression of promising protein vaccine antigens, as a universal meningococcal vaccine for Africa against disease caused by strains from all capsular groups.
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