Pre-clinical evaluation of the malaria vaccine candidate P. falciparum MSP1₄₂ formulated with novel adjuvants or with alum

Abstract

We compared the safety and immunogenicity of the recombinant Plasmodium falciparum MSP1₄₂ antigen formulated with four novel adjuvant systems (AS01B, AS02A, AS05 and AS08) to alum in rhesus monkeys. All five formulations of MSP1₄₂ were safe and immunogenic. Whereas, all MSP1₄₂ formulations tested generated high stimulation indices for lymphocyte proliferation (ranging from 27 to 50), the AS02A and AS01B formulations induced the highest levels of specific anti-MSP1₄₂ antibody. ELISPOT assays showed that the AS02A and AS01B vaccine formulations-induced different cytokine response profiles. Using the ratio of IFN-γ/IL-5 secreting cells as the metric, the AS01B formulation induced a strong Th1 response, whereas the AS02A formulation induced a balanced Th1/Th2 response. The IFN-γ response generated by AS02A and AS01B formulations persisted at least 24 weeks after final vaccination. The notable difference in Th1/Th2 polarization induced by the AS02A and AS01B formulations warrants comparative clinical testing.

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.