Adjuvant Effects Of Plasmodium Excretory And Secretory Products In Enhancing The Immune Protection Against Malaria

Malaria caused by infection with Plasmodium parasite is one of the most serious infectious diseases, which causes 1 million deaths every year. At present, more than 109 countries of the world are suffering from the risk of malaria infection. Malaria is transmitted by Anopheles, which bite human body and inject the parasites into blood circulation. Malaria infection causes severe clinical symptoms including anemia, hypoglycemia and fever. There are four species of Plasmodium that infect human. The rodent malaria parasite, Plasmodium chabaudi, is one of the model to study the Plasmodium falciparum infection.Many vaccines against malaria are currently developed, among which subunit vaccines are most studied. The subunit vaccines can induce strong humoral immune response, but the cell-mediated immunity is weak and the protection is not long lasting. Adjuvants are often needed to help subunit vaccines to strengthen immune responses. Merozoite surface protein1 (MSP1) is a leading candidate vaccine antigen and has been shown to confer certain protection. MSP1 undergoes a series of proteolytic cleavages and finally the C-terminal GPI-anchored MSP119 remains on the parasite surface as it enters the RBC. It was reported that the supernatants from Plasmodium culture medium are recognized by the sera from malaria-infected adults.In this study, we intend to determine the coimmune effects of P. chabaudi culture supernatant and MSP1. We first expressed the 11 kDa fragment of MSP1 C-terminal domain of Plasmodium chabaudi AS in Escherichia coli expression system. A protocol of short-term in vitro culture of Plasmodium chabaudi was used to acquire the excretory and secretary products (ES). After infecting mouse with MSP1 plus ES, we monitored the parasitemia and measured antibody titers using ELISA assay. In addition, spleen cells of the immunized mice were cultured to study the cytokine secretion and immune cell activation. We observed that: (a) immunization with MSP1 using ES as adjuvant effectively inhibited the parasitemia and induced higher levels of IgG1, IgG2c and IgG2b antibody titers; (b) CD11c+DC cells were activated, which showed up-regulated expression of CD40,CD86,but down-regulated expression of MHC-II; (c) the CD4+T cells were activated and CD62L were consequently down-regulated, and (d) splenocytes proliferated more vigorously and produced high levels of IFN-γ,IL-12 and IL-10. We report here first time that the excretory and secretary products can be used as a new immune adjuvant. Our results reveal the complex mechanisms of induction of anti-malarial immunity and may have important implication in development of malaria vaccines.