Abstract
The invention of liver-humanized mouse models has made it possible to directly study the preerythrocytic stages of Plasmodium falciparum. In contrast, the current models to directly study blood stage infection in vivo are extremely limited. Humanization of the mouse blood stream is achievable by frequent injections of human red blood cells (hRBCs) and is currently the only system with which to study human malaria blood stage infections in a small animal model. Infections have been primarily achieved by direct injection of P. falciparum-infected RBCs but as such, this modality of infection does not model the natural route of infection by mosquito bite and lacks the transition of parasites from liver stage infection to blood stage infection. Including these life cycle transition points in a small animal model is of relevance for testing therapeutic interventions. To this end, we used FRGN KO mice that were engrafted with human hepatocytes and performed a blood exchange under immune modulation to engraft the animals with more than 50% hRBCs. These mice were infected by mosquito bite with sporozoite stages of a luciferase-expressing P. falciparum parasite, resulting in noninvasively measurable liver stage burden by in vivo bioluminescent imaging (IVIS) at days 5–7 postinfection. Transition to blood stage infection was observed by IVIS from day 8 onward and then blood stage parasitemia increased with a kinetic similar to that observed in controlled human malaria infection. To assess the utility of this model, we tested whether a monoclonal antibody targeting the erythrocyte invasion ligand reticulocyte-binding protein homolog 5 (with known growth inhibitory activity in vitro) was capable of blocking blood stage infection in vivo when parasites emerge from the liver and found it highly effective. Together, these results show that a combined liver-humanized and blood-humanized FRGN mouse model infected with luciferase-expressing P. falciparum will be a useful tool to study P. falciparum preerythrocytic and erythrocytic stages and enables the testing of interventions that target either one or both stages of parasite infection.
Highlights
More than 200 million clinical cases of malaria are reported each year, with children under the age of 5 being susceptible to illness and death
One injection of human red blood cells (hRBCs) on day 6 postinfection allowed parasites to transition from liver stage to blood stage infection
As shown by inhibition studies using a monoclonal antibody (mAb) targeting reticulocyte-binding protein homolog 5 (RH5), this combined model will be useful to study the effect of novel therapeutics on the different life cycle stages of human Plasmodium parasites as well as stage transitions in vivo
Summary
More than 200 million clinical cases of malaria are reported each year, with children under the age of 5 being susceptible to illness and death. One option to study P. falciparum blood stages in vivo is the injection of in vitro cultured asexual stage parasites into immunodeficient mice that have been preloaded with hRBCs [10, 11] By combining this approach with the injection of macrophage and neutrophil-depleting chemicals, the development of gametocytes that sequester in spleen and bone marrow could be observed, somewhat mimicking human infection [12]. The transition from liver to blood stage is a critical step in the parasite life cycle and provides a target for intervention with drugs and vaccines in order to prevent the establishment of a blood stage infection It should be included in a mouse model for malaria drug and vaccine testing. All animals were bled 200 μl, and received an i.v. injection of 500 μl hRBCs [70% O + human erythrocytes in RPMI 1640 (25 mM HEPES, 2 mM l-glutamine) supplemented with 50 μM hypoxanthine plus 10% human
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