Plasmodium vivax malaria is characterized by periodic relapses of symptomatic blood stage parasite infections likely initiated by activation of dormant liver stage parasites -hypnozoites. The lack of tractable animal models for P. vivax constitutes a severe obstacle to investigate this unique aspect of its biology and to test drug efficacy against liver stages. We show that the FRG KO huHep liver-humanized mice support P. vivax sporozoite infection, development of liver stages, and the formation of small non-replicating hypnozoites. Cellular characterization of P. vivax liver stage development in vivo demonstrates complete maturation into infectious exo-erythrocytic merozoites and continuing persistence of hypnozoites. Primaquine prophylaxis or treatment prevents and eliminates liver stage infection. Thus, the P. vivax/FRG KO huHep mouse infection model constitutes an important new tool to investigate the biology of liver stage development and dormancy and might aid in the discovery of new drugs for the prevention of relapsing malaria.

Background:To survive, Plasmodium falciparum parasites export proteins into their host cell. Results: We have characterized the localization, synthesis, and macromolecular-arrangement of the protein export machinery in Plasmodium falciparum. Conclusion: This machinery is carried into the host-cell and is present as a large macromolecular complex. Significance: These data fill current gaps in the field relating to the biochemical nature of Plasmodium falciparum protein export.

Falciparum malaria is initiated when Anopheles mosquitoes transmit the Plasmodium sporozoite stage during a blood meal. Irradiated sporozoites confer sterile protection against subsequent malaria infection in animal models and humans. This level of protection is unmatched by current recombinant malaria vaccines. However, the live-attenuated vaccine approach faces formidable obstacles, including development of accurate, reproducible attenuation techniques. We tested whether Plasmodium falciparum could be attenuated at the early liver stage by genetic engineering. The P. falciparum genetically attenuated parasites (GAPs) harbor individual deletions or simultaneous deletions of the sporozoiteexpressed genes P52 and P36. Gene deletions were done by double-cross-over recombination to avoid genetic reversion of the knockout parasites. The gene deletions did not affect parasite replication throughout the erythrocytic cycle, gametocyte production, mosquito infections, and sporozoite production rates. However, the deletions caused parasite developmental arrest during hepatocyte infection. The double-gene deletion line exhibited a more severe intrahepatocytic growth defect compared with the single-gene deletion lines, and it did not persist. This defect was assessed in an in vitro liver-stage growth assay and in a chimeric mouse model harboring human hepatocytes. The strong phenotype of the double knockout GAP justifies its human testing as a whole-organism vaccine candidate using the established sporozoite challenge model. GAPs might provide a safe and reproducible platform to develop an efficacious whole-cell malaria vaccine that prevents infection at the preerythrocytic stage.genetically attenuated parasites ͉ malaria vaccine ͉ P36 ͉ P52 ͉ sporozoite M alaria is a formidable global health problem, affecting 300 million to 500 million people worldwide annually (1). The resulting Ϸ1 million deaths per year are mainly caused by Plasmodium falciparum infections. Eradication of malaria will in large part depend on an effective vaccine that prevents infection by Plasmodium, but such a vaccine has remained elusive. The parasites' preerythrocytic stages, encompassing the mosquito-inoculated sporozoites and liver stages that develop from sporozoites after their invasion of hepatocytes, are attractive targets for antiinfection vaccines, because at this stage the number of infected host cells is low, and further transmission of the parasite is not yet possible. Occurrence of blood-stage infection after sporozoite challenge is completely preventable by immunization with radiation-attenuated sporozoites in mouse models of malaria (2). This was a landmark finding that set the standards for malaria preerythrocytic vaccine development. Radiation-attenuated sporozoites arrest in development during hepatocyte infection, but their safety and efficacy are dependent on a precise irradiation dose. Humans immunized with P. falciparum radiation-attenuated sporozoites have been effectively protected from subsequent challenge with homologous an...

Plasmodium vivax hypnozoites persist in the liver, cause malaria relapse and represent a major challenge to malaria elimination. Our previous transcriptomic study provided a novel molecular framework to enhance our understanding of the hypnozoite biology (Voorberg-van der Wel A, et al., 2017). In this dataset, we identified and characterized the Liver-Specific Protein 2 (LISP2) protein as an early molecular marker of liver stage development. Immunofluorescence analysis of hepatocytes infected with relapsing malaria parasites, in vitro (P. cynomolgi) and in vivo (P. vivax), reveals that LISP2 expression discriminates between dormant hypnozoites and early developing parasites. We further demonstrate that prophylactic drugs selectively kill all LISP2-positive parasites, while LISP2-negative hypnozoites are only sensitive to anti-relapse drug tafenoquine. Our results provide novel biological insights in the initiation of liver stage schizogony and an early marker suitable for the development of drug discovery assays predictive of anti-relapse activity.

Relapses of Plasmodium dormant liver hypnozoites compromise malaria eradication efforts. New radical cure drugs are urgently needed, yet the vast gap in knowledge of hypnozoite biology impedes drug discovery. We previously unraveled the transcriptome of 6 to 7 day-old P. cynomolgi liver stages, highlighting pathways associated with hypnozoite dormancy (Voorberg-van der Wel et al., 2017). We now extend these findings by transcriptome profiling of 9 to 10 day-old liver stage parasites, thus revealing for the first time the maturation of the dormant stage over time. Although progression of dormancy leads to a 10-fold decrease in transcription and expression of only 840 genes, including genes associated with housekeeping functions, we show that pathways involved in quiescence, energy metabolism and maintenance of genome integrity remain the prevalent pathways active in mature hypnozoites.

Within the liver, Plasmodium sporozoites traverse cells searching for a “suitable” hepatocyte, invading these cells through a process that results in the formation of a parasitophorous vacuole (PV), within which the parasite undergoes intracellular replication as a liver stage. It was previously established that two members of the Plasmodium s48/45 protein family, P36 and P52, are essential for productive invasion of host hepatocytes by sporozoites as their simultaneous deletion results in growth-arrested parasites that lack a PV. Recent studies point toward a pathway of entry possibly involving the interaction of P36 with hepatocyte receptors EphA2, CD81, and SR-B1. However, the relationship between P36 and P52 during sporozoite invasion remains unknown. Here we show that parasites with a single P52 or P36 gene deletion each lack a PV after hepatocyte invasion, thereby pheno-copying the lack of a PV observed for the P52/P36 dual gene deletion parasite line. This indicates that both proteins are equally important in the establishment of a PV and act in the same pathway. We created a Plasmodium yoelii P36mCherry tagged parasite line that allowed us to visualize the subcellular localization of P36 and found that it partially co-localizes with P52 in the sporozoite secretory microneme organelles. Furthermore, through co-immunoprecipitation studies in vivo, we determined that P36 and P52 form a protein complex in sporozoites, indicating a concerted function for both proteins within the PV formation pathway. However, upon sporozoite stimulation, only P36 was released as a secreted protein while P52 was not. Our results support a model in which the putatively glycosylphosphatidylinositol (GPI)-anchored P52 may serve as a scaffold to facilitate the interaction of secreted P36 with the host cell during sporozoite invasion of hepatocytes.

Summary The human malaria parasite Plasmodium vivax remains vastly understudied, mainly due to the lack of suitable laboratory models. Here, we report a humanized mouse model to test interventions that block P. vivax parasite transition from liver stage infection to blood stage infection. Human liver-chimeric FRGN huHep mice infected with P. vivax sporozoites were infused with human reticulocytes, allowing transition of exo-erythrocytic merozoites to reticulocyte infection and development into all erythrocytic forms, including gametocytes, in vivo. In order to test the utility of this model for preclinical assessment of interventions, the invasion blocking potential of a monoclonal antibody targeting the essential interaction of the P. vivax Duffy Binding Protein with the Duffy antigen receptor was tested by passive immunization. This antibody inhibited invasion by over 95%, providing unprecedented in vivo evidence that PvDBP constitutes a promising blood stage vaccine candidate and proving our model highly suitable to test blood stage interventions.

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