Abstract
Using newer vaccine platforms which have been effective against malaria in rodent models, we tested five immunization regimens against Plasmodium knowlesi in rhesus monkeys. All vaccines included the same four P. knowlesi antigens: the pre-erythrocytic antigens CSP, SSP2, and erythrocytic antigens AMA1, MSP1. We used four vaccine platforms for prime or boost vaccinations: plasmids (DNA), alphavirus replicons (VRP), attenuated adenovirus serotype 5 (Ad), or attenuated poxvirus (Pox). These four platforms combined to produce five different prime/boost vaccine regimens: Pox alone, VRP/Pox, VRP/Ad, Ad/Pox, and DNA/Pox. Five rhesus monkeys were immunized with each regimen, and five Control monkeys received a mock vaccination. The time to complete vaccinations was 420 days. All monkeys were challenged twice with 100 P. knowlesi sporozoites given IV. The first challenge was given 12 days after the last vaccination, and the monkeys receiving the DNA/Pox vaccine were the best protected, with 3/5 monkeys sterilely protected and 1/5 monkeys that self-cured its parasitemia. There was no protection in monkeys that received Pox malaria vaccine alone without previous priming. The second sporozoite challenge was given 4 months after the first. All 4 monkeys that were protected in the first challenge developed malaria in the second challenge. DNA, VRP and Ad5 vaccines all primed monkeys for strong immune responses after the Pox boost. We discuss the high level but short duration of protection in this experiment and the possible benefits of the long interval between prime and boost.
Highlights
Malaria infects over 200 million people annually and causes almost 1 million deaths [1]
We developed reagents to test immune responses to all 4 P. knowlesi vaccine antigens in order to study their association with protection
In the Control group (Figure 1A), the first blood stage parasites were detected between days 8 to 10 and animals required drug treatment for parasitemia exceeding 2% between days 10 to 12. This is consistent with our previous studies using the 100 P. knowlesi sporozoite challenge in rhesus monkeys [17,18,19]
Summary
Malaria infects over 200 million people annually and causes almost 1 million deaths [1]. The vaccine farthest along in field testing [2]is based on a single malaria antigen, and is not as effective as experimental radiation attenuated whole parasite vaccines [3,4,5,6,7,8]. When immune responses to the protective irradiated parasite vaccines are analyzed, no single target antigen has been identified that explains the full extent of host immunity[9]. This suggests that the protective vaccines work by the summation of many immune responses against multiple antigens on the parasites[9]
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