Development of a pre-erythrocytic P. vivax vaccine to prevent clinical relapse

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ABSTRACT More than 3 billion people are at risk for contracting malaria caused by Plasmodium vivax (Pv). Pv infection differs from other Plasmodium species in that it develops dormant liver stage forms called hypnozoites. Hypnozoites can reactivate and cause blood stage malaria months to years after primary infection. It is estimated that nearly 90% of active blood stage Pv infections are due to relapse infection and not primary vector-mediated infection. As such, the dormant form is a major driver of Pv transmission and accounts for nearly the entire clinical disease burden. Therefore, a vaccine that reduces or eliminates the formation of hypnozoites, and hence reduces relapse infection, would have a significant impact on both disease burden and transmission rates. Importantly, models suggest that this can be accomplished even in the absence of sterilizing immunity, because hypnozoites only form in a fraction of the infected hepatocytes. Currently there are no clinically advanced vaccines to prevent Pv infection or relapse. Development efforts have been hampered by the inherent difficulty of working with Pv in the lab, the lack of non-CSP antigens, and the lack of a biologically relevant model system that mimics Pv infection. Our long term goal is the development of a pre-erythrocytic vaccine against Pv by creating novel PvCSP vaccines and P. vivax/P. falciparum genetically attenuated parasite (GAP) vaccines that can reduce or prevent relapse infection. To this end, we put together a research program that addresses all the major roadblocks of Pv vaccine development. We propose to evaluate the efficacy of PvCSP vaccines against relapse, and will study novel, non-CSP vaccine candidates that were recently identified to be part of the surface proteome. We will identify those that can augment anti-PvCSP-mediated immunity and reduce or block the formation of hypnozoites. Finally, we will engineer PvCSP (and potentially novel antigens) into the existing PfGAP platform that is under clinical evaluation. Importantly, we have partnered with Mahidol University in Thailand to enable us to work with wild type Pv sporozoites. Additionally, we propose to conduct our vaccine development in a completely humanized model system. We will evaluate our vaccines in humanized immunoglobulin mice and test efficacy in humanized liver mouse models of relapse infection, allowing for a more reliable translation of results in this study toward the eventual deployment of the vaccine into the clinic. Our ultimate goal is the development of a near clinic-ready vaccine that is effective in reducing or eliminating Pv relapse infection.