Development of an effective malaria vakne poses a majorscientific challenge both in the laboratory and in the field,This thesis addresses several aspects of malaria vaccinedevelopment, by focusing on two differentasexualblood stage antigens ofPlusmedium falicpanrm , Pf332and Pfl155/RESA.
Antibodies play an important role in protective immunity tothis parasite. Antigens Pf332 and Pf155/RESA are recognized byhuman antibodies with protective capacity. The first part ofthis thesis is devoted to the description of different types ofsubunit vaccine strategies for a live attenuatedSalmonella typhimutium-based and for a DNA-based vaccineaimed at induction of anti-Pf332 and anti-Pfl55/RESA humoralresponses. For both theSalmonellaand the DNA vaccine constructs, the effect ofdifferent cellular localization of the antigen on the antibodyresponses was investigated in mice. In each case, significantIevels ofthe expressed recombinant antigens were detectedin vitro,and upon immunization, high titered andlong-lasting specific antibody responses were generated.Furthermore, the antibody responses induced by both types ofvaccine constructs were efficiently boosted, indicating thepresence of immunologital B- ancUor T-cell memory.
Both natural and vaccine-induced immunity are hampered bythe capacity of the parasite to vary critical antigenicstructures, an obstacle in malaria vaccine development which ismagnified by the polymorphism of the human MHC system. Thesecond part of this thesis addresses some of these problems bystudying the antigen EB200 as a vaccine candidate model. Thisantigen is derived from the repeated amino acid sequence ofPf332. The H-2 restriction of the immune responses to EB200 wasinvestigated in congenic mice where several dominant T-cellepitopes were identified. Moreover, evidente for a sharedepitope motif by H-2dand H-2kfrom strutturally related malaria antigen repeatsis provided. This study is complemented witb an analysis of thegenetic polymorphism of the EB200 gene in field isolates fromHonduras, in a region whereP.falciparumtransmission is low and seasonal. TheHonduran parasites displayed limited polymorphism in the EB200gene suggesting that this may be a conserved region in Pf332.Finally, the antibody specificities to EB200 in malaria-exposedindividuals were examined. Although prevalent in mostindividuals, the antibody reactivities to EB200 increased withage and correlated with the increase observed in reactivitiesto crude malaria antigen, suggesting that continous malariaexposure results in high levels of antibodies to EB200. Takentogether, these results suggest that EB200 is conserved in theparask and is immunogenic in humans.
In summary, subunit vaccines based on live recombinantSalmonella typhimutiumor plastnid DNA were designed andevaluated for induction, enhancement and modulation of antibodyresponses to Pf332 and Pfl55/RESA. Moreover, a Pf332-derivedfragment was examined for its potential as a component of amalaria subunit vaccine.
Key words:Malaria, vaccine development, subunitvaccine,Plasmodium falciparum, Salmonella typhimurium,DNAvaccine, Pf332, Pfl55/RESA, EB200, antibody response, geneticpolymorphism, genetic restriction, staphylococcal protein A,streptococcal protein B
Stockholm: Bioteknologi , 1998. , 71 p.