Complete Plasmodium falciparum liver-stage development in liver-chimeric mice

Abstract

Plasmodium falciparum, which causes the most lethal form of human malaria, replicates in the host liver during the initial stage of infection. However, in vivo malaria liver-stage (LS) studies in humans are virtually impossible, and in vitro models of LS development do not reconstitute relevant parasite growth conditions. To overcome these obstacles, we have adopted a robust mouse model for the study of P. falciparum LS in vivo: the immunocompromised and fumarylacetoacetate hydrolase-deficient mouse (Fah -/-, Rag2 -/-, Il2rg -/-, termed the FRG mouse) engrafted with human hepatocytes (FRG huHep). FRG huHep mice supported vigorous, quantifiable P. falciparum LS development that culminated in complete maturation of LS at approximately 7 days after infection, providing a relevant model for LS development in humans. The infections allowed observations of previously unknown expression of proteins in LS, including P. falciparum translocon of exported proteins 150 (PTEX150) and exported protein-2 (EXP-2), components of a known parasite protein export machinery. LS schizonts exhibited exoerythrocytic merozoite formation and merosome release. Furthermore, FRG mice backcrossed to the NOD background and repopulated with huHeps and human red blood cells supported reproducible transition from LS infection to blood-stage infection. Thus, these mice constitute reliable models to study human LS directly in vivo and demonstrate utility for studies of LS-to-blood-stage transition of a human malaria parasite. IntroductionPlasmodium falciparum is the most deadly of the human malaria parasites. The disease continues to be a global health crisis and causes more than 250 million new clinical cases annually, resulting in over 800,000 deaths, mostly of children in sub-Saharan Africa (1). Female anopheline mosquitoes introduce infectious sporozoites into the host dermis when taking a blood meal. Sporozoites exit the bite site by migration, enter a blood vessel, and are carried to the liver (2). Here, each sporozoite traverses numerous hepatocytes before it invades a final hepatocyte with the formation of a parasitophorous vacuole (PV) (3). Ensconced in the PV, the parasite undergoes liver-stage (LS, also called exoerythrocytic form [EEF]) development, culminating in the formation and release of tens of thousands of first generation merozoites (4). This preerythrocytic phase of the parasite life cycle is asymptomatic, and all clinical pathologies are caused by the ensuing asexual erythrocytic stage of infection. The erythrocytic stages are routinely studied in vitro, made possible by the development of a continuous culture system that allows asexual parasite replication in human rbc (hurbc) (5). However, studying the biology and pathophysiology of P. falciparum in vivo is difficult and is hampered by the lack of adequate animal models. Sporogonic stages are generated by feeding female Anopheles mosquitoes on in vitro gametocyte cultures, allowing progression of the parasite life cycle in the mosquito and subsequent sporozoite accumulation...

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“…2). This could be caused by an immune response to Ags expressed by late LS parasites (20,43), or alternatively by transient exposure to BS Ags expressed by merozoites as they exit the liver and briefly exist in the circulation before invading RBCs and being cleared by AS. Similarly to AS-ITV, late-arresting GAPs also result in cross-stage protection that is likely mediated by exposure to BS Ags during late LS (20).…”

Vaccine-induced monoclonal antibodies targeting circumsporozoite protein prevent Plasmodium falciparum infection

“…The study of Plasmodium falciparum's liver stage is hampered by the low in vitro infection rate of human or primate host cells and by the need for a specialized insectary to rear and infect Anopheles mosquitoes for the production of sporozoites. The development of a mouse model with fully functional human hepatocytes has made it possible to study the liver stage in a preclinical in vivo setting (5)(6)(7)(8)(9)(10)(11).…”

Human natural killer cells control Plasmodium falciparum infection by eliminating infected red blood cells

“…Furthermore, a recent report shows the development of liver-stage P. falciparum infection in immunocompromised and fumarylacetoacetate hydrolase-deficient (Fah −/− , Rag2 −/− , Il2rg −/− ) (FRG) mice. Backcrossing of FRG mice to the NOD background and supplementing the resulting mice with human RBCs led to reproducible transition from liver-stage infection to blood-stage infection (6). Despite such progress, none of the existing mouse models of human parasite infection has a human immune system.…”