Protection of Rhesus Monkeys by a DNA Prime/Poxvirus Boost Malaria Vaccine Depends on Optimal DNA Priming and Inclusion of Blood Stage Antigens

Abstract

BackgroundWe have previously described a four antigen malaria vaccine consisting of DNA plasmids boosted by recombinant poxviruses which protects a high percentage of rhesus monkeys against Plasmodium knowlesi (Pk) malaria. This is a multi-stage vaccine that includes two pre-erythrocytic antigens, PkCSP and PkSSP2(TRAP), and two erythrocytic antigens, PkAMA-1 and PkMSP-1(42kD). The present study reports three further experiments where we investigate the effects of DNA dose, timing, and formulation. We also compare vaccines utilizing only the pre-erythrocytic antigens with the four antigen vaccine.MethodologyIn three experiments, rhesus monkeys were immunized with malaria vaccines using DNA plasmid injections followed by boosting with poxvirus vaccine. A variety of parameters were tested, including formulation of DNA on poly-lactic co-glycolide (PLG) particles, varying the number of DNA injections and the amount of DNA, varying the interval between the last DNA injection to the poxvirus boost from 7 to 21 weeks, and using vaccines with from one to four malaria antigens. Monkeys were challenged with Pk sporozoites given iv 2 to 4 weeks after the poxvirus injection, and parasitemia was measured by daily Giemsa stained blood films. Immune responses in venous blood samples taken after each vaccine injection were measured by ELIspot production of interferon-γ, and by ELISA.Conclusions1) the number of DNA injections, the formulation of the DNA plasmids, and the interval between the last DNA injection and the poxvirus injection are critical to vaccine efficacy. However, the total dose used for DNA priming is not as important; 2) the blood stage antigens PkAMA-1 and PkMSP-1 were able to protect against high parasitemias as part of a genetic vaccine where antigen folding is not well defined; 3) immunization with PkSSP2 DNA inhibited immune responses to PkCSP DNA even when vaccinations were given into separate legs; and 4) in a counter-intuitive result, higher interferon-γ ELIspot responses to the PkCSP antigen correlated with earlier appearance of parasites in the blood, despite the fact that PkCSP vaccines had a protective effect.

High-dose chemotherapy augments the efficacy of recombinant adenovirus vaccines and improves the therapeutic outcome

“…As such, prime/boost strategies, such as those described recently for immunization against HIV and malaria should be developed. 41,42 Although these strategies are difficult to evaluate in murine models owing to the short time frame of the tumor growth, they can be evaluated in human trials where tumor exhibits a longer term of growth.…”

Prime-boost vectored malaria vaccines: Progress and prospects

“…Importantly, although viral vectors have been shown to induce significant protection by targeting the TRAP antigen expressed during the liver-stages there is now considerable interest in the possibility that vectors could induce significant protection by targeting blood-stage antigens as well. 3,4 The first evidence that vectors could provide protection in humans came from a phase IIa challenge study using the orthopox vector NYVAC that encoded seven malaria antigens, including TRAP and CSP. 5 Although antibody immunogenicity was poor, detectable lytic T-cell responses were found to TRAP, CSP and LSA-1 peptides, and 1 out of 35 challenged volunteers was sterilely protected.…”

Poxvirus-based vaccine platforms: getting at those hard-to-reach places

“…This finding translated into similar reduced parasite loads in subsequent human trials using FPV and MVA expressing TRAP and several other antigens [35]. Recently, Phase II trials have begun and interest in the potential inherent in co-administration of poxvirus vectors and DNA vaccines as a new protocol of immunization continues [36].…”