Immunogenicity analysis of conserved fragments in Plasmodium ovale species merozoite surface protein 4

Abstract

Background: There is an urgent need for an effective vaccine to control and eradicate malaria, one of the most serious global infectious diseases. Plasmodium merozoite surface protein 4 (MSP4) has been listed as a blood-stage subunit vaccine candidate for malaria. Plasmodium ovale species (spp.) infection is also a source of malaria burden in tropical regions where it is sometimes mixed with other Plasmodium species. However, little is known about P. ovale spp. MSP4. Methods: The msp4 gene was amplified through polymerase chain reaction using genomic DNA extracted from blood samples of 46 patients infected with P. ovale spp. and amplified products were sequenced. Open reading frames predicted as immunogenic peptides consisting of 119 and 97 amino acids of P. ovale curtisi MSP4 (PocMSP4) and P. ovale wallikeri MSP4 (PowMSP4), respectively, were selected for protein expression. Recombinant proteins (rPoMSP4) were expressed in Escherichia coli, purified, analyzed, and immunized in BALB/c mice. The specificity of anti-MSP4-immunoglubulin (Ig) G antibodies was evaluated by Western blot and enzyme-linked immunosorbent assays, and cellular immune responses were analyzed via lymphocyte proliferation assays. Results: Full peptide sequences of PocMSP4 and PowMSP4 were completely conserved in all clinical isolates, except in the epidermal growth factor-like domain at the carboxyl terminus where only one mutation was observed in one P. ovale wallikeri isolate. Furthermore, we successfully expressed the truncated pocmsp4 and powmsp4 and purified as ~32 kDa proteins. Our results showed that PocMSP4 and PowMSP4 induced high antibody responses with end-point titers ranging from 1:10,000 to 1:2,560,000 in all immunized mouse groups and with high IgG avidity to PocMSP4 (80.5%) and PowMSP4 (92.3%). Furthermore, rPocMSP4 and rPowMSP4 cross-reacted with anti-PowMSP4-specific or anti-PocMSP4-specific antibodies. Additionally, anti-PoMSP4 IgG antibodies showed broad immuno-specificity in reacting against rPoMSP1 and rPoAMA1. Lastly, PocMSP4- and PowMSP4-immunized mice induced cellular immune responses with PocMSP4 (36%) and PowMSP4 cells (15.8%) as observed through splenocyte proliferation assays. Conclusion: Our study suggested conservation in PoMSP4 protein sequences and high immunogenicity was observed in rPoMSP4. Furthermore, PocMSP4- and PowMSP4-immunized mice induced cellular immune responses, suggesting that both humoral and cellular immune responses play crucial roles in the protection against any antigen.