Expansion of a malarial parasite antigen family in the mammalian host can be explained by synthetic viability of a lethal Plasmodium berghei cysteine protease defect

Abstract text

During life cycle progression of malarial parasites, efficient and timely Plasmodium exit events from replication-permissive compartments are central steps in parasite stage conversion. Papain-like cysteine proteases of a family of Plasmodium-specific serine repeat antigens (SERAs) are central mediators of parasite egress and conserved across the genus Plasmodium. One member, termed SERA3 in the murine malaria model, is likely essential during blood infection. In this study, we employed a systematic reverse genetics approach to characterize the function of P. berghei SERA3. As predicted, we failed to generate P. berghei parasites lacking SERA3 by targeted gene deletion. Surprisingly, in the presence of additional gene deletions, i.e. deletion of both active site serine (SERAser) members of the protease family, SERA1 and 2, parasites were viable and progressed through the parasite life cycle. Systematic phenotyping of sera1-3(-) parasites revealed that infected mice fail to develop signature symptoms of cerebral malaria and are able to clear an infection. Importantly, parasite clearance cannot be attributed to reduced virulence, since it only occurs when initiated by sporozoite infection, but not via transfusion of sera1-3(-) blood stages. Parasite clearance is strictly B- and T-cell dependent and elicits high titres of anti-parasite antibodies for up to one year. Intriguingly, the sera1-3 triple knockout is a genocopy of the avian malarial parasite SERA repertoire. Apparently, SERA3 dysfunction is incompatible with asexual parasite growth in the presence of functional SERAser proteins. This suggests that the up to six SERAser protease family members could only evolve after fixation of SERA3 in the mammalian genomes. This genetic observation, known as synthetic viability, leads us to propose that expansion of the SERA gene family was vital to establish chronic and repeated Plasmodium infections in the mammalian host.