Pre-clinical evaluation of the malaria vaccine candidate P. falciparum MSP142 formulated with novel adjuvants or with alum☆
Introduction
Despite significant efforts over the past 50 years to control malaria, there are approximately 300–500 million infections and between 1 and 3 million deaths each year attributable to Plasmodium falciparum [1]. The development of an effective malaria vaccine has been hindered by the complexity of the malaria parasite. This unicellular protozoan parasite has fourteen chromosomes, approximately 5300 genes, and four developmental stages in the infected host. An effective vaccine against the erythrocytic stage of P. falciparum would be expected to induce both high antibody titers and T-cell responses, which would limit parasite multiplication rates and thereby reduce morbidity and mortality.
Proteins on the surface of the merozoite, such as MSP1, have been considered prime targets for an erythrocyte stage vaccine since they are susceptible to specific humoral immune responses. MSP1 is processed by proteolytic cleavage at or just prior to merozoite release into 83, 30, 38 and 42 kDa fragments [2], [3], [4], [5], [6]. Just before completion of erythrocyte invasion, the C-terminal 42 kDa fragment, attached to the merozoite surface, is further processed into 33 and 19 kDa fragments with only the 19 kDa fragment being carried into the new erythrocyte [7], [8], [9]. Both MSP142 and MSP119 are under consideration as erythrocyte stage vaccines. Indeed, antibodies against MSP142 or MSP119 are able to inhibit the invasion of erythrocytes in vitro [7], [10], [11], [12]. More importantly, immunization with MSP142 or MSP119 antigens provides some degree of protection from erythrocyte stage parasite challenges in New World monkey malaria models of P. falciparum malaria [13], [14], [15], [16], [17], [18].
MSP1 vaccines mixed with complete Freund’s adjuvant are protective in animal models, but not suitable for human use [13], [14], [15], [16], [17], [18]. Adjuvant selection for clinical use is an important issue in vaccine development. Since its first use in the 1940s [19], alum is still the only adjuvant approved by the US Food & Drug Administration. Weak adjuvant activity for both humoral and cellular immunity and the induction of Th2 responses limit alum’s usefulness. The need for more potent and nontoxic adjuvants to replace alum is clearly recognized. New adjuvants are being developed and clinically tested. Among them, MF59 (squalene oil in water emulsion), virosome, immune stimulating complexes (ISCOM), QS21 (saponin derivative), monophosphoryl lipid A (MPL) and CpG oligodeoxynucleotides (ODN) containing CpG motifs, appear promising [20], [21], [22], [23], [24], [25]. MF59-adjuvanted influenza vaccine has recently been licensed for human use in Europe. Unlike complete Freund’s adjuvant, however, MF59 combined with P. falciparum MSP142 failed to induce protection and generated a poor antibody response [17]. QS21, MPL and CpG ODN are immunomodulators capable of inducing Th1 responses when used alone or in combination with other adjuvants. Recent clinical studies have demonstrated that 3′-deacylated MPL combined with alum-adjuvanted HBsAg are significantly more immunogenic than that response antigen formulated only with alum [26], [27]. A multi-component adjuvant, AS02A (an oil-in-water emulsion containing QS21 and MPL) when combined with the malaria circumsporozoite-based vaccine, RTS,S, has shown promising results with a protective efficacy of 41–86% in challenge trials [28], [29], [30], and 71% in a field trial, but efficacy is short lived [31]. RTS,S formulated with alum or MPL plus alum had initially failed to confer significant protection [32]. Here, we evaluated the safety and immunogenicity of a P. falciparum MSP142 vaccine formulated with four proprietary adjuvants and compared them with alum-adjuvanted vaccine.
Section snippets
Vaccine and immunization
Clinical grade recombinant MSP142 protein from the 3D7 strain of P. falciparum was produced in Escherichia coli [33]. Fermentation, purification and vialing were performed under Good Manufacturing Practices (GMP) at the BioProduction Facility, Walter Reed Army Institute of Research (WRAIR), Forest Glen, MD. The purity of the final product was greater than 95% as assessed by SDS–PAGE. The MSP142 protein contained some correct structure based on its binding to conformation-dependent mAbs that
Reactogenicity and systemic toxicity of MSP142-based vaccines
Most monkeys experienced transient muscle induration, erythema and skin swelling from vaccination. These reactions were very mild after the first dose, and more notable after 2nd dose of vaccine. Generally, the reactogenicity after the 2nd, 3rd, 4th and 5th doses of vaccine was similar. No statistical differences between vaccine groups were noted after the first dose, and overall the novel adjuvants were comparable to alum. Reactogenicity assessments after the 2nd dose of vaccine are shown in
Discussion
The mechanisms of protective immunity mediated by experimental malaria erythrocytic stage vaccines probably involve humoral and cell-mediated mechanisms. Previous studies in small animal models suggest that high levels of antibody confer protection against blood stage infection, although the specific antibody isotype involved is unclear [37]. However, in some non-human primate studies, protection occurred in animals with low antibody levels, suggesting that T cell-mediated responses may
Acknowledgements
We thank Kosol Yongvanitchit, Panita Saengkrai, Utaiwan Kum-arb for their technical assistance with the cellular assays, Dr. Bergmann-Leitner, Michelle Gusick, and Marla Evans with the IFAs, Prapatsorn Ngaowichit for administrative support and Prinya Yoophasook for preparing figures. We also thank the animal caretakers in the Department of Veterinary Medicine, AFRIMS, for providing excellent animal care. This work was supported by the United States Army Medical Research and Materiel Command,
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The views in this paper are those of the authors and do not purport to reflect official policy of the US Army or Department of Defense.