Long Term Immune Responses Induced against P. vivax CSP and MSP-1 chimeric vaccine candidates delivered by novel Adenoviral Vectors

Background: Adenoviral vectors are a promising malaria vaccine platform as they can induce robust cellular immune responses and are capable of long-lasting transgene expression. Materials and Methods: We developed two P. vivax chimeric proteins based on circumsporozoite protein (CSP) and merozoite surface protein 1 (MSP1). The PvCSP chimera includes two C-terminal promiscuous T cell epitopes, and multiple copies of different VK210 type 1 and the VK247 type 2 repeat sequence variants. The PvMSP1 chimera includes five promiscuous T cell epitopes linked to the MSP-119 fragment a target of functional antibodies. To assess the immunogenicity of these chimeric proteins delivered by adenoviral vectors, BALB/c mice were primed with either a recombinant human adenovirus Ad5/3 or a novel simian adenovirus SAd36 (which are both resistant to pre-existing immunity to Ad5), expressing the CSP-MSP1 chimeras as transgenes. After one year, mice were boosted with the heterologous adenoviral vector, followed by two protein boosts 20 and 40 days later. Mice receiving protein immunizations at days 0, 365, 385, and 405, and naïve mice, were used as controls. Finally, 40 days after the final immunization, mice were challenged with P. vivax CSP repeat VK210 transgenic P. berghei sporozoites for assessment of liver parasite burden by PCR. Results: We observed no reduction in antibody titers 1 year after adenoviral priming, while titers elicited by protein immunization reduced by 30%. At 20 days after the final immunization, we observed no difference in titers between the immunization groups. Assessment of T cell functionality via flow cytometry 5 days after the final immunization revealed that both heterologous adenoviral regimens induced CD4 and CD8 T cells capable of producing higher levels of IFN-γ, IL-2, and TNF-α upon ex vivo stimulation with the MSP-1 or CSP chimeras compared to mice receiving protein. Upon assessment of liver parasite burden, both the protein and SAd36 priming regimens displayed significantly reduced parasite loads compared to naïve mice and mice primed with Ad5/3. Conclusions: Taken together, the data demonstrate that our novel multistage malaria vaccine can induce long-lasting humoral and cellular immune responses able to recognize two P. vivaxantigens when delivered by adenoviral vectors.