Abstract:
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Background: A DNA prime, poxvirus (COPAK) boost vaccination regime with four antigens, i.e. a combination of
two Plasmodium knowlesi sporozoite (csp/ssp2) and two blood stage (ama1/msp142) genes, leads to self-limited
parasitaemia in 60% of rhesus monkeys and survival from an otherwise lethal infection with P. knowlesi. In the
present study, the role of the blood stage antigens in protection was studied in depth, focusing on antibody
formation against the blood stage antigens and the functionality thereof.
Methods: Rhesus macaques were immunized with the four-component vaccine and subsequently challenged i.v.
with 100 P. knowlesi sporozoites. During immunization and challenge, antibody titres against the two blood stage
antigens were determined, as well as the in vitro growth inhibition capacity of those antibodies. Antigen reversal
experiments were performed to determine the relative contribution of antibodies against each of the two blood
stage antigens to the inhibition.
Results: After vaccination, PkAMA1 and PkMSP119 antibody titres in vaccinated animals were low, which was
reflected in low levels of inhibition by these antibodies as determined by in vitro inhibition assays. Interestingly,
after sporozoite challenge antibody titres against blood stage antigens were boosted over 30-fold in both
protected and not protected animals. The in vitro inhibition levels increased to high levels (median inhibitions of
59% and 56% at 6 mg/mL total IgG, respectively). As growth inhibition levels were not significantly different
between protected and not protected animals, the ability to control infection appeared cannot be explained by
GIA levels. Judged by in vitro antigen reversal growth inhibition assays, over 85% of the inhibitory activity of these
antibodies was directed against PkAMA1.
Conclusions: This is the first report that demonstrates that a DNA prime/poxvirus boost vaccination regimen
induces low levels of malaria parasite growth inhibitory antibodies, which are boosted to high levels upon
challenge. No association could, however, be established between the levels of inhibitory capacity in vitro and
protection, either after vaccination or after challenge. |