Splenectomised and spleen intact Aotus monkeys' immune response to Plasmodium vivax MSP-1 protein fragments and their high activity binding peptides.

Two E. coli expressed recombinant polypeptides (rPvMSP-1(14) and rPvMSP-1(20)) contained in the 33kDa fragment, located within Plasmodium vivax merozoite surface protein (PvMSP-1) 42kDa C-terminal region, and a cocktail of high reticulocyte binding synthetic peptides located within these fragments, were evaluated for immunogenicity and protective immune responses in splenectomised and spleen intact Aotus nancymaae monkeys. Thirty splenectomised monkeys who had been previously immunised with either rPvMSP-1(14), rPvMSP-1(20), or a mixture of both recombinant fragments were intravenously challenged with the heterologous P. vivax VCG-1 strain (as determined by DNA sequencing); full protection was observed in five monkeys and low parasitaemia levels were obtained in eight more monkeys. Splenectomised control monkey group rapidly developed high parasitaemia levels, while no significant parasitaemia was obtained in the non-splenectomised control group. Although PvMSP-1 42 and 33kDa fragments were recognised by Western Blot and whole parasites by IFAT when tested with immune monkey sera, no correlation between protection and antibody titres by IFAT and ELISA was observed, suggesting that protection is not being solely mediated by a humoral immune response. This data showed that partial protection against a heterologous strain challenge was best achieved when immunising with a rPvMSP-1(14)-rPvMSP-1(20) mixture (2 were fully protected and 4 with low parasitaemia out of 12) suggesting for the first time, that these fragments could be good candidates for inclusion in a P. vivax multi-stage, multi-antigen vaccine.

Expression, polymorphism analysis, reticulocyte binding and serological reactivity of two Plasmodium vivax MSP-1 protein recombinant fragments.

Among the four parasite species causing malaria in humans, Plasmodium vivax prevails on both the Asian and the American continents. Several antigens from this parasite's erythrocytic stages have been characterised and some of them are considered to be good vaccine candidates. The P. vivax merozoite surface protein-1 (PvMSP-1) is a 200 kDa antigen, thought to mediate the initial contact between the merozoite and the erythrocyte. An effective blockage of this interaction could be important in anti-malarial vaccine design. This study analyses the genetic polymorphism, binding to both reticulocytes and erythrocytes, antigenicity and immunogenicity of two recombinant proteins belonging to the 33 kDa PvMSP-1 proteolytic fragment. Both regions showed very low genetic variation, bound reticulocytes with higher affinity than erythrocytes, were recognised by naturally P. vivax-infected patient sera and were immunogenic when used to immunise rabbits, making them good vaccine candidates against P. vivax, to be further preclinically tested in the Aotus monkey model.