Plasmodium parasites are the etiological agents of malaria, a disease responsible for over half a million deaths annually. Successful completion of the parasite's life cycle in the vertebrate host and transmission to a mosquito vector is contingent upon the ability of the parasite to evade the host's defenses. The extracellular stages of the parasite, including gametes and sporozoites, must evade complement attack in both the mammalian host and in the blood ingested by the mosquito vector. Here, we show that Plasmodium falciparum gametes and sporozoites acquire mammalian plasminogen and activate it into the serine protease plasmin to evade complement attack by degrading C3b. Complement-mediated permeabilization of gametes and sporozoites was higher in plasminogen-depleted plasma, suggesting that plasminogen is important for complement evasion. Plasmin also facilitates gamete exflagellation through complement evasion. Furthermore, supplementing serum with plasmin significantly increased parasite infectivity to mosquitoes and lowered the transmission-blocking activity of antibodies to Pfs230, a potent vaccine candidate currently in clinical trials. Finally, we show that human factor H, previously shown to facilitate complement evasion by gametes, also facilitates complement evasion by sporozoites. Plasmin and factor H simultaneously cooperate to enhance complement evasion by gametes and sporozoites. Taken together, our data show that Plasmodium falciparum gametes and sporozoites hijack the mammalian serine protease plasmin to evade complement attack by degrading C3b. Understanding of the mechanisms of complement evasion by the parasite is key to the development of novel effective therapeutics. IMPORTANCE Current approaches to control malaria are complicated by the development of antimalarial-resistant parasites and insecticide-resistant vectors. Vaccines that block transmission to mosquitoes and humans are a plausible alternative to overcome these setbacks. To inform the development of efficacious vaccines, it is imperative to understand how the parasite interacts with the host immune response. In this report, we show that the parasite can co-opt host plasmin, a mammalian fibrinolytic protein to evade host complement attack. Our results highlight a potential mechanism that may reduce efficacy of potent vaccine candidates. Taken together, our results will inform future studies in developing novel antimalarial therapeutics.
Keywords: complement; complement evasion; gamete; malaria; plasmin; sporozoite; transmission.