PfAMA1-expressing chimeric rodent malaria parasites provide an in vivo platform for evaluating multistage interventions against malaria

Abstract

Apical membrane antigen 1 (AMA1) is expressed in the merozoite and sporozoite infectious stages of the malaria parasite, and upon secretion plays essential roles during host cell invasion. AMA1 is a leading candidate for vaccine development, although specific antibodies frequently fail to inhibit the growth of field-isolated Plasmodium falciparum malaria parasites, likely due to the high diversity of polymorphisms in AMA1 surface antigens. A key step for efficient invasion of target cells is tight junction formation through interaction of merozoite-surface AMA1 and rhoptry neck protein 2 (RON2), which is secreted and embedded within the erythrocyte membrane. Antibodies or reagents that disrupt the AMA1-RON2 interaction represent interventions to reduce parasite transmission to humans, as well as to repress clinical symptoms. To create a mouse model system for the evaluation of reagents against P. falciparum AMA1 (PfAMA1), we generated CRISPR/Cas9-engineered rodent malaria parasites in which the endogenous Plasmodium berghei AMA1 (PbAMA1) was replaced with PfAMA1, resulting in a chimeric line Pb_PfAMA1. Pb_PfAMA1 parasites infect mouse liver and erythrocytes as efficiently as the parental line, demonstrating that PfAMA1 functionally complements the essential roles of PbAMA1. AlphaFold-based structure modeling suggested structural compatibility of the heterologous PfAMA1-PbRON2 interaction, and co-immunoprecipitation analyses supported the functional association of the PfAMA1 and PbRON complex required for merozoite invasion of erythrocytes. Utilizing the interaction-inhibitor R1 peptide with Pb_PfAMA1 sporozoites, we demonstrated that the AMA1-RON2 interaction is crucial for sporozoite invasion of hepatocytes. Repeated infection with Pb_PfAMA1 elicited PfAMA1-reactive antibodies, and immune sera inhibited the growth of the P. falciparum lines Pf3D7 and PfHB3B; suggesting that naturally processed parasite-derived PfAMA1 induces antibodies which recognize conserved conformational epitopes. To expand this platform, we replaced circumsporozoite protein PbCSP with PfCSP, to generate dual-chimeric Pb_PfCSP+PfAMA1 parasites. Together, these chimeric parasites establish an in vivo platform for evaluating multistage and multi-antigen interventions against malaria.