Analysis of the human immune response against the Merozoite Surface Protein (MSP)-1 from Plasmodium falciparum – a malaria vaccine candidate

Abstract

Malaria affects almost half of the world`s population causing more than 200 million clinical cases each year and thus, remains one of the major infectious diseases of mankind. The protozoan parasite Plasmodium falciparum transmitted by infected Anopheles mosquitoes is responsible for 99 % of deaths, of which most occur in African children. An effective vaccine is urgently required but not yet available. The malaria vaccine candidate Merozoite Surface Protein (MSP)-1 from P. falciparum is an essential and highly abundant protein on the surface of parasite blood stages, which has been associated with protection against malaria in various epidemiological and immunization studies and is currently tested in a clinical Phase I trial. The aim of this work is to characterize the human humoral and cellular immune response against the vaccine candidate PfMSP-1D in African adults with naturally acquired immunity against malaria and after experimental infection of malaria-naive volunteers with Pf wildtype sporozoites. Antibodies against Plasmodium merozoites are considered as the main actors of naturally acquired immunity against malaria and can target the parasite via different immune mechanisms, such as direct growth inhibition or opsonization and recruitment of effector cells. By using sera from eleven semi-immune adults from Burkina Faso we show that a few sera were capable of directly inhibiting the growth of P. falciparum blood stages in vitro and MSP-1 specific antibodies partly contributed to this effect. Further antigens presumably responsible for the observed growth inhibitory activity were identified via western blot and mass spectrometry analysis. Furthermore, we demonstrate that serum antibodies from all African individuals could opsonize P. falciparum merozoites, recruit neutrophils and elicit antibody-dependent respiratory burst (ADRB). The antibody level against MSP-1 correlated with ADRB activity and MSP-1 specific antibodies, obtained by affinity-purification, induced neutrophil respiratory burst. Opsonizing antibodies effective via ADRB appear to be mainly cross-reactive and – by affinity purification of specific antibodies and antigen-reversal ADRB – we identified MSP-1 and its largest subunit MSP-183 as important targets. Additionally, we show for the first time that opsonizing antibodies can be elicited in non-human primates by immunization with recombinant PfMSP-1D, suggesting that a MSP-1 based vaccine may function via the induction of ADRB-effective antibodies. Since MSP-1 is initially synthesized in Plasmodium infected hepatocytes at late liver stage, it can be a target of the cellular immune response. Cytotoxic CD8+ T-cells secreting IFNγ are supposed to be the key players eliminating infected hepatocytes. By stimulating PBMCs from semi-immune individuals from Burkina Faso and from malaria-naive European controls with previously identified HLA-A0201 restricted CD8+ T-cell epitopes within MSP-1D, MSP-1 specific IFNγ-secreting CD8+ T-cells were detected in African adults via IFNγ ELISPOT assay. Aiming at a HLA-independent approach, potential CD8+ T-cell epitopes to frequent HLA-types were predicted within MSP-1D and the IFNγ ELISPOT assay was adapted for the use of full-length MSP-1D protein. Furthermore, the immune response of malaria-naive volunteers experimentally infected once with cryopreserved P. falciparum wildtype sporozoites during the TUECHMI I study was investigated. Antibodies against MSP-1, MSP-6 and MSP-7 were induced by Pf sporozoite infection as well as CD8+ T-cells, which target the cross-stage antigen MSP-1 at comparable levels as the known pre-erythrocytic antigens LSA-1 and CSP. Overall, this work provides a detailed characterization of different immune mechanisms targeting PfMSP-1D in humans with naturally acquired immunity against malaria or after single experimental infection with Pf sporozoites. Moreover, the established immunological assays can be employed to analyze the immune response induced by vaccination with PfMSP-1D during clinical trials.