Surface Engineering of the RNA Coliphage Qβ to Display Plasmodium Falciparum Derived Asexual Blood Stage Antigens UB05 and Merozoite Surface Protein 3

Abstract

Background: Naturally acquired immune responses to Plasmodium falciparum merozoite surface protein 3 (MSP3) and UB05 are implicated in semi immunity in populations living in malaria endemic areas. Thus designing chimeric malaria vaccine candidates involving MSP-3 and UB05 displayed upon the surface of a phage in its native form could potentiate their immunogenicity and antigenicity. In this study, we have engineered both MSP3 and UB05 upon the Qβ and assessed their antigenicity with plasma from children living in a high malaria transmission region of Cameroon. Methods: The surface of the RNA coliphage Qβ was genetically modified to display three Plasmodium falciparum derived immunogens including MSP3, UB05 and a chimera of the two UB05-MSP3. The resultant recombinant phages including QβMSP3, QβUB05 and QβUB05-MSP3 with surface displayed malaria immunogens were produced after transformation of the E. coli strain HB101. Plasma levels of antigen specific IgG antibody were then determined in samples from malaria positive and negative children living in a high malaria transmission region of Cameroon. Results: To improve yield each recombinant phage was scaled up to 1014 pfu/ml using production strategies previously optimized in our group. This was significantly higher (P<0.001) relative to the 108 pfu/ml of the wild type phage when produced routinely. Conformational integrity of the surface displayed antigens was confirmed in ELISA assays by testing for the specific recognition of the N and C-terminal parts of both UB05 and MSP3 using the recombinant QβMSP3, QβUB05 and chimeric QβUB05-MSP3 phages as antigens and the monoclonal antibodies XQ38G73- N, X-Q0KGH2-N, X-Q38G73-C, X-Q0KGH2-C targeting the N- and C-terminals of both UB05 and MSP3 respectively. Antigen specific naturally acquired IgG antibodies in plasma from both malaria negative and positive children living in a high transmission area of Cameroon recognized all three recombinant phages. However, plasma from children less than five years old contained significantly less plasma levels of antigen specific IgG antibodies. Conclusion: Thus, the novel recombinant phages QβMSP3, QβUB05 and QβUB05-MSP3 can be used as a tool for assessing natural or vaccine-induced antibody responses against malaria. The recombinant chimeric QβUB05- MSP3 phage is validated as a multivalent antigen for tracking semi immunity to malaria.