Malaria is a life-threatening global epidemic disease and has caused more than 400,000 deaths in 2019. To control and prevent malaria, the development of a vaccine is a potential method. An effective malaria vaccine should either combine antigens from all stages of the malaria parasite’s life cycle, or epitopes of multiple key antigens due to the complexity of the Plasmodium parasite. Malaria’s random constructed antigen-1 (M.RCAg-1) is one of the recombinant vaccines, which was selected from a DNA library containing thousands of diverse multi-epitope chimeric antigen genes. Moreover, besides selecting an antigen, using an adjuvant is another important procedure for most vaccine development procedures. Freund’s adjuvant is considered an effective vaccine adjuvant for malaria vaccine, but it cannot be used in clinical settings because of its serious side effects. Traditional adjuvants, such as alum adjuvant, are limited by their unsatisfactory immune effects in malaria vaccines, hence there is an urgent need to develop a novel, safe and efficient adjuvant. In recent years, Pickering emulsions have attracted increasing attention as novel adjuvant. In contrast to classical emulsions, Pickering emulsions are stabilized by solid particles instead of surfactant, having pliability and lateral mobility. In this study, we selected aluminum hydroxide gel (termed as “alum”) as a stabilizer to prepare alum-stabilized Pickering emulsions (ALPE) as a malaria vaccine adjuvant. In addition, monophosphoryl lipid A (MPLA) as an immunostimulant was incorporated into the Pickering emulsion (ALMPE) to further enhance the immune response. In vitro tests showed that, compared with alum, ALPE and ALMPE showed higher antigen load rates and could be effectively endocytosed by J774a.1 cells. In vivo studies indicated that ALMPE could induce as high antibody titers as Freund’s adjuvant. The biocompatibility study also proved ALMPE with excellent biocompatibility. These results suggest that ALMPE is a potential adjuvant for a malaria vaccine.
Chitosan with pH sensitivity and biocompatibility was selected to prepare chitosan nanoparticle-stabilized Pickering emulsion (CSPE). The flexibility of CSPE enables stress deformation when in contact with cell membranes, thereby mimicking the deformability of natural pathogens and facilitating their efficient uptake by cells. In the acidic environment of lysosomes, the amino groups of chitosan molecules are protonated, and the water solubility increases. CSPE transforms from particle-stabilized to polymer chain-stabilized, its subsequent swelling and proton accumulation lead to lysosome rupture. The experimental results evaluating CSPE as an adjuvant shows that CSPE could efficiently load antigens, promote endocytosis and antigen cross-presentation, recruit antigen-presenting cells at the injection site, boost T-cell activation, and enhance both humoral and cellular immune responses. In the prophylactic and therapeutic tumor models of E.G7-OVA lymphoma and B16-MUC1 melanoma, CSPE significantly inhibited tumor growth and prolonged the survival of mice. In summary, antigenic lysosomal escape resulted from the chitosan molecular state transition is the key to the enhancement of cellular immunity by CSPE, and CSPE is a promising vaccine adjuvant.
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