Malaria infection by sporozoite challenge induces high functional antibody titres against blood stage antigens after a DNA prime, poxvirus boost vaccination strategy in Rhesus macaques

Muzamil Mahdi Abdel Hamid,Alan W Thomas,Bart W Faber,Ibrahim M El Hassan,Ayman A Hussain,Clemens Hm Kocken,Edmond J Remarque,Walter R Weiss,David L Narum

doi:10.1186/1475-2875-10-29

Abstract

BackgroundA 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.MethodsRhesus 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.ResultsAfter 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.ConclusionsThis 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.

Highlights

A DNA prime, poxvirus (COPAK) boost vaccination regime with four antigens, i.e. a combination of two Plasmodium knowlesi sporozoite and two blood stage genes, leads to self-limited parasitaemia in 60% of rhesus monkeys and survival from an otherwise lethal infection with P. knowlesi
Approximately 60% (8 out of 13) of monkeys were protected from challenge with P. knowlesi, after receiving three injections with DNA encoding 4 Plasmodium knowlesi antigens and a booster with a mixture of 4 COPAK viruses, encoding the same antigens
Antibody responses induced by DNA prime/viral boost vaccination ELISA titres for the two blood stage antigens (PkMSP119/PkAMA1) were determined in serum samples obtained from csp/ssp2 and csp/ssp2/ama1/msp142 animals, both before and after challenge, and run in a single experiment (Figure 1, Table 2)

Summary

Introduction

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 face of increasing resistance of Plasmodium parasites to anti-malarial (prophylactic) drugs, development of an effective malaria vaccine is generally considered a public health priority [2]. The traditional approach for malaria vaccine development is based on recombinant proteins administered in combination with novel adjuvants, directed either to erythrocytic or pre-erythrocytic stages of the parasite. Protein subunit vaccines do have a number of disadvantages. One is that they require the use of adjuvants that may induce to adverse effects and may be difficult to get access to, due to intellectual property rights. Antigen conformation and stability (with or without adjuvant) at ambient temperatures are major issues that may complicate the use of subunit vaccines

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