Cloning, expression and purification of Plasmodium knowlesi circumsporozoite protein and immunoblot analysis with P. knowlesi strain A1H1 protein extract.

Circumsporozoite protein (CSP) is a sporozoite major surface protein of Plasmodium species. The protein showed promising protection level as a vaccine candidate against Plasmodium falciparum infection. There is a lack of studies on P. knowlesi CSP (PkCSP) as a vaccine candidate due to the high polymorphic characteristic of central repeat region. Recent studies showed the protein has a relatively conserved region at the C-terminal, which consists of T- and B-cell epitopes. This could be the target region for vaccine development against the pre-erythrocytic stage of the parasite. In this study, recombinant PkCSP was expressed using Escherichia coli system. Recombinant PkCSP was immunized in animal models and the antiserum was evaluated using immunoblot analysis. Results showed that PkCSP can be successfully expressed using the bacterial system. Endpoint titre of the antiserum were ranged up to 1:819200. Immunoblot analysis showed the antiserum recognized recombinant PkCSP but not total protein extract from P. knowlesi erythrocytic stage. In conclusion, PkCSP could elicit strong immune response in animal models. However, serum antibodies could not recognize protein from the parasite's erythrocytic stage extract indicating it is not expressed at the erythrocytic stage. Therefore, PkCSP remains as a potential pre-erythrocytic vaccine candidate against P. knowlesi infection.