Plasmepsin 4 Deficient Plasmodium berghei Are Virulence-Attenuated and Induce Protective Immunity against Experimental Malaria

Plasmodium parasites lacking plasmepsin 4 (PM4), the aspartic protease that functions in the lysosomal compartment and contributes to hemoglobin digestion, have only a modest decrease in asexual blood-stage growth rate but in the rodent malaria parasite P. berghei are significantly less virulent than the parental parasite. P. berghei Δpm4 parasites failed to induce experimental cerebral malaria (ECM) in ECM-susceptible mice and ECM-resistant mice were able to clear infections. Furthermore, after a single infection all convalescent mice were protected against subsequent parasite challenge for at least one year. Real-time in vivo parasite imaging demonstrated that protective immunity acted through antibody-mediated parasite clearance in the spleen. This work demonstrates, for the first time, that a single Plasmodium gene disruption can generate virulence-attenuated parasites that do not induce cerebral complications and, moreover, are able to stimulate strong protective immunity against subsequent challenge with wild type parasites.