Malaria affects 198 million people and kills 584,000 each year, predominantly in Sub-Saharan Africa (WHO). The most severe form of malaria is caused by the protozoan parasite Plasmodium falciparum. Development of a vaccine against P. falciparum has been hindered by its complex life cycle with multiple antigenically distinct human and mosquito stages. To effectively prevent disease and reduce the parasite burden in populations, a vaccine will need to target multiple stages, including blocking transmission at the mosquito stage.
Antibodies generated against P. falciparum mosquito stage antigen Pfs25 can prevent parasite transmission from humans to mosquitoes. However, Pfs25 is poorly immunogenic and immunization with the protein with alum as an adjuvant does not provide adequate transmission blocking activity. In this study I used adenovirus vectors (Ad) modified to express Pfs25 as a transgene or with capsid displayed Pfs25 epitopes to elicit a stronger anti-Pfs25 antibody response specifically directed at epitopes highly associated with transmission blocking.
Although antibody titers are closely associated with transmission blocking activity, it is unlikely that all antibodies generated by immunization with whole protein are transmission blocking. Based on molecular models, EGF-like domains 2 and 3 appear to be most surface accessible on the parasite surface. However, antibodies raised against soluble Pfs25 equally recognize all 4 EGF-like domains. Thus, a vaccine involving in situ expression of membrane anchored Pfs25 may better target antibody responses to domains 2 and 3. Additionally, using secondary immunization with capsid displayed Pfs25, I expected to further direct the antibody response to predicted B cell epitopes. Serum from mice immunized with these vectors can provide transmission blocking activity with lower antibody titers.
Innate immune responses to Ad are an important component of generating adaptive immunity and may provide a means to further augment antibody responses to poorly immunogenic protein antigens. Molecules such as Pseudomonas aeruginosa flagellin (FliC) potently activate the innate immune system and could be used to modify vectors to induce greater adaptive immunity. We have generated Ad vectors with FliC attached to protein IX. Using these vectors with capsid displayed Pfs25 epitopes I assessed whether capsid displayed FliC increases antibody titers to Pfs25
