The development of vaccine candidates against Plasmodium vivax-the most geographically widespread human malaria species-is challenged by technical difficulties, such as the lack of in vitro culture systems and availability of animal models. Chimeric rodent Plasmodium parasites are safe and useful tools for the preclinical evaluation of new vaccine formulations. We report the successful development and characterization of chimeric Plasmodium berghei parasites bearing the type I repeat region of P. vivax circumsporozoite protein (CSP). The P. berghei-P. vivax chimeric strain develops normally in mosquitoes and produces highly infectious sporozoites that produce patent infection in mice that are exposed to the bites of as few as 3 P. berghei-P. vivax-infected mosquitoes. Using this transgenic parasite, we demonstrate that monoclonal and polyclonal antibodies against P. vivax CSP strongly inhibit parasite infection and thus support the notion that these antibodies play an important role in protective immunity. The chimeric parasites we developed represent a robust model for evaluating protective immune responses against P. vivax vaccines based on CSP.T he global burden of malaria due to Plasmodium vivax is approximately 70 to 390 million cases annually (1), with around 2.5 billion people living at risk of infection (2). Even though Plasmodium falciparum remains the leading cause of malaria in Africa, P. vivax malaria is the most geographically widespread of the human malarias (2, 3). It is extensively distributed throughout the tropics, in the Middle East, Asia, the western Pacific, and Latin America (1-4).P. vivax malaria was long considered a benign and non-lifethreatening disease; however, recent publications report an increasing number of severe, complicated cases due to P. vivax infections (5, 6). Efforts toward the development of antimalaria vaccines are still mainly focused on P. falciparum, although P. vivax malaria is "harder to prevent, diagnose, and treat, and both species are coendemic" (7). While the importance of a vaccine targeting P. vivax is recognized, the development of P. vivax malaria vaccines is hampered by insufficient funding and technical difficulties (2,8,9). The lack of in vitro culture systems and suitable animal models, for example, imposes a significant limitation on testing novel vaccine candidates (7, 10).The circumsporozoite protein (CSP) is the most abundant protein on the surfaces of sporozoites and is responsible for eliciting both T-cell and antibody responses. The plasmodial CSP is one of the best studied antigens and is considered to be a major malaria vaccine candidate. RTS,S, the most effective malaria vaccine candidate to date, is based on P. falciparum CSP, thus underscoring CSP's immunogenic properties. CSP is comprised of an immunodominant central repeat region, diverse among Plasmodium species, flanked by two conserved domains: region I at the N terminus of the repeats and a type I thrombospondin repeat (TSR) motif C terminal to the repeat region. Early studies in animal...