Plasmodium falciparum: studies on mature exoerythrocytic forms in the liver of the chimpanzee, Pan troglodytes

The Plasmodium liver-stage parasitophorous vacuole: A front-line of communication between parasite and host

The intracellular development and differentiation of the Plasmodium parasite in the host liver is a prerequisite for the actual onset of malaria disease pathology. Since liver-stage infection is clinically silent and can be completely eliminated by sterilizing immune responses, it is a promising target for urgently needed innovative antimalarial drugs and/or vaccines. Discovered more than 65 years ago, these stages remain poorly understood regarding their molecular repertoire and interaction with their host cells in comparison to the pathogenic erythrocytic stages. The differentiating and replicative intrahepatic parasite resides in a membranous compartment called the parasitophorous vacuole, separating it from the host-cell cytoplasm. Here we outline seminal work that contributed to our present understanding of the fundamental dynamic cellular processes of the intrahepatic malarial parasite with both specific host-cell factors and compartments.

A Plasmodium α/β-hydrolase modulates the development of invasive stages

The bud emergence (BEM)46 proteins are evolutionarily conserved members of the α/β-hydrolase superfamily, which includes enzymes with diverse functions and a wide range of substrates. Here, we identified a Plasmodium BEM46-like protein (PBLP) and characterized it throughout the life cycle of the rodent malaria parasite Plasmodium yoelii. The Plasmodium BEM46-like protein is shown to be closely associated with the parasite plasma membrane of asexual erythrocytic stage schizonts and exo-erythrocytic schizonts; however, PBLP localizes to unique intracellular structures in sporozoites. Generation and analysis of P. yoelii knockout (Δpblp) parasite lines showed that PBLP has an important role in erythrocytic stage merozoite development with Δpblp parasites forming fewer merozoites during schizogony, which results in decreased parasitemia when compared with wild-type (WT) parasites. Δpblp parasites showed no defects in gametogenesis or transmission to mosquitoes; however, because they formed fewer oocysts there was a reduction in the number of developed sporozoites in infected mosquitoes when compared with WT. Although Δpblp sporozoites showed no apparent defect in mosquito salivary gland infection, they showed decreased infectivity in hepatocytes in vitro. Similarly, mice infected with Δpblp sporozoites exhibited a delay in the onset of blood-stage patency, which is likely caused by reduced sporozoite infectivity and a discernible delay in exo-erythrocytic merozoite formation. These data are consistent with the model that PBLP has an important role in parasite invasive-stage morphogenesis throughout the parasite life cycle.

Plasmodium falciparum full life cycle and Plasmodium ovale liver stages in humanized mice

Experimental studies of Plasmodium parasites that infect humans are restricted by their host specificity. Humanized mice offer a means to overcome this and further provide the opportunity to observe the parasites in vivo. Here we improve on previous protocols to achieve efficient double engraftment of TK-NOG mice by human primary hepatocytes and red blood cells. Thus, we obtain the complete hepatic development of P. falciparum, the transition to the erythrocytic stages, their subsequent multiplication, and the appearance of mature gametocytes over an extended period of observation. Furthermore, using sporozoites derived from two P. ovale-infected patients, we show that human hepatocytes engrafted in TK-NOG mice sustain maturation of the liver stages, and the presence of late-developing schizonts indicate the eventual activation of quiescent parasites. Thus, TK-NOG mice are highly suited for in vivo observations on the Plasmodium species of humans.

Mice engrafted with human cells are useful models for research on human malaria parasites. Here the authors show that the complete life cycle of Plasmodium falciparum and the liver stages of Plasmodium ovale can be studied in mice doubly engrafted with human primary hepatocytes and red blood cells.

Halofuginone - the multifaceted molecule

Halofuginone is an analog of febrifugine-an alkaloid originally isolated from the plant Dichroa febrifuga. During recent years, halofuginone has attracted much attention because of its wide range of beneficial biological activities, which encompass malaria, cancer, and fibrosis-related and autoimmune diseases. At present two modes of halofuginone actions have been described: (1) Inhibition of Smad3 phosphorylation downstream of the TGFβ signaling pathway results in inhibition of fibroblasts-to-myofibroblasts transition and fibrosis. (2) Inhibition of prolyl-tRNA synthetase (ProRS) activity in the blood stage of malaria and inhibition of Th17 cell differentiation thereby inhibiting inflammation and the autoimmune reaction by activation of the amino acid starvation and integrated stress responses. This review deals with the history and origin of this natural product, its synthesis, its known modes of action, and it's various biological activities in pre-clinical animal models and in humans.

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

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.

Mitosis in the human malaria parasite Plasmodium falciparum

Malaria is caused by intraerythrocytic protozoan parasites belonging to Plasmodium spp. (phylum Apicomplexa) that produce significant morbidity and mortality, mostly in developing countries. Plasmodium parasites have a complex life cycle that includes multiple stages in anopheline mosquito vectors and vertebrate hosts. During the life cycle, the parasites undergo several cycles of extreme population growth within a brief span, and this is critical for their continued transmission and a contributing factor for their pathogenesis in the host. As with other eukaryotes, successful mitosis is an essential requirement for Plasmodium reproduction; however, some aspects of Plasmodium mitosis are quite distinct and not fully understood. In this review, we will discuss the current understanding of the architecture and key events of mitosis in Plasmodium falciparum and related parasites and compare them with the traditional mitotic events described for other eukaryotes.

The Effects of radiation on the safety and protective efficacy of an attenuated Plasmodium yoelii sporozoite malaria vaccine

We are developing a radiation attenuated Plasmodium falciparum sporozoite (PfSPZ) malaria vaccine. An important step was to determine the minimum dose of irradiation required to adequately attenuate each sporozoite. This was studied in the Plasmodium yoelii rodent model system. Exposure to 100 Gy completely attenuated P. yoelii sporozoites (PySPZ). Next we demonstrated that immunization of mice intravenously with 3 doses of 750 PySPZ that had received 200 Gy, double the radiation dose required for attenuation, resulted in 100% protection. These results support the contention that a radiation attenuated sporozoite vaccine for malaria will be safe and effective at a range of radiation doses.

Genetic immunisation by liver stage antigen 3 protects chimpanzees against malaria despite low immune responses

Background

The true interest of genetic immunisation might have been hastily underestimated based on overall immunogenicity data in humans and lack of parallelism with other, more classical immunisation methods.

Principal Findings

Using malaria Liver Stage Antigen-3 (LSA-3), we report that genetic immunization induces in chimpanzees, the closest relative of humans, immune responses which are as scarce as those reported using other DNA vaccines in humans, but which nonetheless confer strong, sterile and reproducible protection. The pattern was consistent in 3/4 immunized apes against two high dose sporozoite challenges performed as late as 98 and 238 days post-immunization and by a heterologous strain.

Conclusions

These results should, in our opinion, lead to a revisiting of the value of this unusual means of immunisation, using as a model a disease, malaria, in which virulent challenges of volunteers are ethically acceptable.

Release of hepatic Plasmodium yoelii merozoites into the pulmonary microvasculature

Plasmodium undergoes one round of multiplication in the liver prior to invading erythrocytes and initiating the symptomatic blood phase of the malaria infection. Productive hepatocyte infection by sporozoites leads to the generation of thousands of merozoites capable of erythrocyte invasion. Merozoites are released from infected hepatocytes as merosomes, packets of hundreds of parasites surrounded by host cell membrane. Intravital microscopy of green fluorescent protein-expressing P. yoelii parasites showed that the majority of merosomes exit the liver intact, adapt a relatively uniform size of 12-18 microm, and contain 100-200 merozoites. Merosomes survived the subsequent passage through the right heart undamaged and accumulated in the lungs. Merosomes were absent from blood harvested from the left ventricle and from tail vein blood, indicating that the lungs effectively cleared the blood from all large parasite aggregates. Accordingly, merosomes were not detectable in major organs such as brain, kidney, and spleen. The failure of annexin V to label merosomes collected from hepatic effluent indicates that phosphatidylserine is not exposed on the surface of the merosome membrane suggesting the infected hepatocyte did not undergo apoptosis prior to merosome release. Merosomal merozoites continued to express green fluorescent protein and did not incorporate propidium iodide or YO-PRO-1 indicating parasite viability and an intact merosome membrane. Evidence of merosomal merozoite infectivity was provided by hepatic effluent containing merosomes being significantly more infective than blood with an identical low-level parasitemia. Ex vivo analysis showed that merosomes eventually disintegrate inside pulmonary capillaries, thus liberating merozoites into the bloodstream. We conclude that merosome packaging protects hepatic merozoites from phagocytic attack by sinusoidal Kupffer cells, and that release into the lung microvasculature enhances the chance of successful erythrocyte invasion. We believe this previously unknown part of the plasmodial life cycle ensures an effective transition from the liver to the blood phase of the malaria infection.

Plasmodium falciparum-infected mice: more than a tour de force

Up until recently, the relevance of Plasmodium falciparum-infected humanized mice for malaria studies has been questioned because of the low percentage of mice in which the parasite develops. Advances in the generation of new immunodeficient mouse strains combined with the use of protocols that modulate the innate immune defenses of mice have facilitated the harvesting of exoerythrocytic and intraerythrocytic stages of the parasite. These results renew the hope of working with P. falciparum in a laboratory animal and indicate that the next challenge (i.e. a complete parasite cycle in the same mouse, including transmission to mosquito) could be reached in the future.

Expression of human CD81 differently affects host cell susceptibility to malaria sporozoites depending on the Plasmodium species

Plasmodium sporozoites can enter host cells by two distinct pathways, either through disruption of the plasma membrane followed by parasite transmigration through cells, or by formation of a parasitophorous vacuole (PV) where the parasite further differentiates into a replicative exo-erythrocytic form (EEF). We now provide evidence that following invasion without PV formation, transmigrating Plasmodium falciparum and Plasmodium yoelii sporozoites can partially develop into EEFs inside hepatocarcinoma cell nuclei. We also found that rodent P. yoelii sporozoites can infect both mouse and human hepatocytes, while human P. falciparum sporozoites infect human but not mouse hepatocytes. We have previously reported that the host tetraspanin CD81 is required for PV formation by P. falciparum and P. yoelii sporozoites. Here we show that expression of human CD81 in CD81-knockout mouse hepatocytes is sufficient to confer susceptibility to P. yoelii but not P. falciparum sporozoite infection, showing that the narrow P. falciparum host tropism does not rely on CD81 only. Also, expression of CD81 in a human hepatocarcinoma cell line is sufficient to promote the formation of a PV by P. yoelii but not P. falciparum sporozoites. These results highlight critical differences between P. yoelii and P. falciparum sporozoite infection, and suggest that in addition to CD81, other molecules are specifically required for PV formation during infection by the human malaria parasite.

Plasmodium falciparum infection and exoerythrocytic development in mice with chimeric human livers

The exoerythrocytic stage of Plasmodium falciparum has remained a difficult phase of the parasite life-cycle to study. The host and tissue specificity of the parasite requires the experimental infection of humans or non-human primates and subsequent surgical recovery of parasite-infected liver tissue to analyze this stage of the parasites development. This type of study is impossible in humans due to obvious ethical considerations and the cost and complexity in working with primate models has precluded their use for extensive studies of the exoerythrocytic stage. In this study we assessed, for the first time, the use of transgenic, chimeric mice containing functioning human hepatocytes as an alternative for modeling the in vivo interaction of P. falciparum parasites and human hepatocytes. Infection of these mice with P. falciparum sporozoites produced morphologically and antigenically mature liver stage schizonts containing merozoites capable of invading human red blood cells. Additionally, using microdissection, highly enriched P. falciparum liver stage parasites essentially free of hepatocyte contamination, were recovered for molecular studies. Our results establish a stable murine model for P. falciparum that will have a wide utility for assessing the biology of the parasite, potential anti-malarial chemotherapeutic agents and vaccine design.

Insights into the P. y. yoelii hepatic stage transcriptome reveal complex transcriptional patterns

During their complex life cycle, malaria parasites adopt morphologically, biochemically and immunologically distinct forms. The intra-hepatic form is the least known, yet of established value in the induction of sterile immunity and as a target for chemoprophylaxis. Using Plasmodium yoelii as a model we present here a novel approach to the elucidation of the transcriptome of this poorly studied stage. Sequences from Plasmodium were obtained in 388 of the 3533 inserts (11%) isolated from liver stages cDNA obtained from optimized cultures with high yields. These corresponded to a total of 88 putative P. yoelii genes. The majority of the transcribed genes identified, code for predicted proteins of as yet unknown function. The RT-PCR analysis carried out for 29 of these genes, confirmed expression at the hepatic stage and provided evidence for complex patterns of genes transcription in the distinct stages found in the mosquito and vertebrate host. The results demonstrate the efficacy of the approach that can now be applied to further detailed analysis of the hepatic stage transcriptome of Plasmodium.

Human Plasmodium liver stages in SCID mice: a feasible model?

In a recent issue of Parasitology Today, Stanley and Virgin have stressed the potential of B- and T-cell deficient mice, among which severe combined immunodeficiency (SCID) mice are most frequently used, as models for the study of parasites. One of the most tantalizing prospects has been in the development of liver stages (LS) of human Plasmodium.

The Plasmodium falciparum knob-associated PfEMP3 antigen is also expressed at pre-erythrocytic stages and induces antibodies which inhibit sporozoite invasion

The expression of the pfemp3 gene and the corresponding PfEMP3 knob-associated protein in the pre-erythrocytic stages of Plasmodium falciparum was demonstrated by RT-PCR, Western blots, IFAT and IEM. The antigen was found on the surface of the sporozoite and in the cytoplasm of mature hepatic stage parasites. Immunological cross-reactivity was observed with sporozoites from the rodent malaria parasites Plasmodium yoelii yoelii and Plasmodium berghei and was exploited to assess a potential role of this protein at the pre-erythrocytic stages. Specific antibodies from immune individuals were found to inhibit P. yoelii yoelii and P. berghei sporozoite invasion of primary hepatocyte cultures. PfEMP3 should now be added to the small list of proteins expressed at the pre-erythrocytic stages of P. falciparum, and its vaccine potential now deserves to be investigated.

Protection against Plasmodium falciparum malaria in chimpanzees by immunization with the conserved pre-erythrocytic liver-stage antigen 3

In humans, sterile immunity against malaria can be consistently induced through exposure to the bites of thousands of irradiated infected mosquitoes. The same level of protection has yet to be achieved using subunit vaccines. Recent studies have indicated an essential function for intrahepatic parasites, the stage after the mosquito bite, and thus for antigens expressed during this stage. We report here the identification of liver-stage antigen 3, which is expressed both in the mosquito and liver-stage parasites. This Plasmodium falciparum 200-kilodalton protein is highly conserved, and showed promising antigenic and immunogenic properties. In chimpanzees (Pan troglodytes), the primates most closely related to humans and that share a similar susceptibility to P. falciparum liver-stage infection, immunization with LSA-3 induced protection against successive heterologous challenges with large numbers of P. falciparum sporozoites.

High immunogenicity in chimpanzees of peptides and lipopeptides derived from four new Plasmodium falciparum pre-erythrocytic molecules

We have investigated the immunogenicity in chimpanzees of twelve synthetic peptides derived from four new Plasmodium falciparum molecules expressed at pre-erythrocytic stages of the human malaria parasite. These parasite molecules were initially selected through their ability to be recognized by stage restricted human antibodies. Twelve 20- to 41-mer peptides representing potential human B- or T-cell epitopes were selected from these proteins, and synthesized. Six of these were modified by a C-terminal lipidic chain in order to re-inforce their immunogenicity. Strong B- and T-helper cell responses were induced in chimpanzees by lipopeptides injected without adjuvant and by peptides in Montanide. All twelve peptides induced CD4(+) T-cell proliferative responses, as well as the secretion of IFN-gamma (some of them at very high levels) and eleven peptides induced antibody responses. The immune responses elicited in this way were reactive with native parasite proteins, as shown by recall studies with sporozoite stage proteins, and proved to be long-lasting (up to 10 months after immunization). Our results support the strategy employed to select these four new malarial antigens and the corresponding peptides, and suggest that the immunizing formulations are both efficient and clinically acceptable.

Lipopeptide immunization without adjuvant induces potent and long-lasting B, T helper, and cytotoxic T lymphocyte responses against a malaria liver stage antigen in mice and chimpanzees

We have employed a 26-amino-acid synthetic peptide based on Plasmodium falciparum liver stage antigen-3 to evaluate improvements in immunogenicity mediated by the inclusion of a simple lipid-conjugated amino acid during peptide synthesis. Comparative immunization by the peptide in Freund's adjuvant or by the lipopeptide in saline shows that the addition of a palmitoyl chain can dramatically increase T helper (Th) cell responses in a wide range of major histocompatibility complex (MHC) class II haplotypes, to the extent that responses were induced in mice otherwise unable to respond to the non-modified peptide injected with Freund's adjuvant, and that the increased immunogenicity of the lipopeptide led to high and longer lasting antibody production (studied up to 8 months). B and T cell responses induced by the lipopeptide were reactive with native parasite protein epitopes, and a lipopeptide longer than ten amino acids was endogenously processed to associate with MHC class I and elicit cytotoxic T lymphocyte (CTL) responses. Finally, the lipopeptide was safe and highly immunogenic in chimpanzees, whose immune system is very similar to that of humans. Our results suggest that relatively large synthetic peptides, carefully chosen from pertinent areas of proteins and incorporating a simple palmitoyl-lysine, can induce not only CTL, but also strong Th and antibody responses in genetically diverse populations. Lipopeptides engineered in this way are simple to produce and purify under GMP conditions, they are well tolerated by apes, and with the enhanced immunogenicity without the need for adjuvant that we report here, they offer a quick and relatively low-cost route to provide material for human malaria vaccination trials.

Cloning and characterization of a novel Plasmodium falciparum sporozoite surface antigen, STARP

A novel Plasmodium falciparum sporozoite antigen, STARP (Sporozoite Threonine and Asparagine-Rich Protein), detected consistently on the surface of sporozoites obtained from laboratory strains and field isolates, has been identified and cloned, following a systematic approach aimed at isolating novel non-CS sporozoite surface antigens. The 2.0-kb STARP gene has a 5' miniexon/large central exon structure and contains a complex repetitive region encoding multiple dispersed motifs and tandem 45- and 10-amino acid repeats. In sporozoites, transcription of the STARP gene has been conclusively demonstrated by reverse PCR and Northern blot hybridisation and the 78-kDa protein has been localized by immunofluorescence and immunoelectron microscopy to the sporozoite surface. STARP is also expressed in liver stages, as revealed by immunofluorescence assays using antisera raised either to the central repetitive region or the C-terminal non-repetitive region. Expression is also detected in early ring stages, though not in mature erythrocytic or sexual stages. Identification and elucidation of this novel antigen is a step forward in current efforts aimed at developing an effective preerythrocytic-stage malaria vaccine.

Schizogonic development of Leucocytozoon smithi

The schizogonic development of Leucocytozoon smithi in the liver of experimentally infected turkey poults was examined by electron microscopy. Following intraperitoneal injection, sporozoites migrated to the liver and entered hepatic cells to become intracellular trophozoites. Three to four days post inoculation (PI), trophozoites underwent asexual multiple fission known as merogony or schizogony. Two generations of schizonts were observed. The primary or first generation schizonts, abundant on day 4 PI, appeared as interconnected cytoplasmic masses (pseudocytomeres). Each pseudocytomere was enclosed by a membranous vacuole and contained varying numbers of nuclei. As nuclear division and growth of the schizonts continued, larger discrete cytoplasmic masses or cytomeres were formed with rhoptries and multiple nuclei in various stages of division. Synchronous multiple cytoplasmic cleavage of the schizont resulted in the formation of numerous uninucleate merozoites. Second generation schizonts, which developed from hepatic merozoites released from primary schizonts, were abundant in hepatocytes on day 6 PI. Although tissue samples from liver, lung, spleen, kidney, intestine, brain, blood vessels and lymph nodes were examined, schizogonous forms were observed in liver only. No megaloschizonts were detected in any host tissue examined. Schizogonic development was completed by day 7 PI as merozoites developed into gametocytes within mononuclear phagocytes.

Mouse model for exoerythrocytic stages of Plasmodium falciparum malaria parasite

Research on the exoerythrocytic (EE) stages of human malaria parasites has been hindered because of the lack of an easily available suitable animal model. We report here an approach to produce mature EE-stage Plasmodium falciparum parasites by using severe combined immunodeficient (scid) mice with transplanted human hepatocytes. Transplantation of human hepatocytes into scid mice (scid hu-hep), their subsequent intravenous infection with P. falciparum sporozoites, and the development of mature liver-stage merozoites was achieved. Immunofluorescent staining of scid hu-hep kidney tissue sections demonstrated the presence of circumsporozoite protein (early during infection), merozoite surface antigen 1, and liver schizont antigen 1. The scid hu-hep model can serve as a source of human malaria liver-stage parasites, decreasing the need for nonhuman primates. Use of this model will facilitate characterization of EE-stage antigens and the assessment of stage-specific chemotherapeutic agents and candidate vaccines.

Primary structure and localization of a conserved immunogenic Plasmodium falciparum glutamate rich protein (GLURP) expressed in both the preerythrocytic and erythrocytic stages of the vertebrate life cycle

A gene coding for a 220-kDa glutamate rich protein (GLURP), an exoantigen of Plasmodium falciparum, was isolated and its nucleotide sequence was determined. The deduced amino acid sequence contains 2 repeat regions. The sequence of one of these was shown to be conserved among geographically dispersed isolates, and a fusion protein containing that sequence was able to stimulate B- and T-cells. Antibodies against GLURP stained erythrocytic stages of the parasite as well as the hepatic stage as detected by electron microscopy.

Detection of different developmental stages of malaria parasites by non-radioactive DNA in situ hybridization

A highly sensitive non-radioactive DNA in situ hybridization procedure is described that enables detection and unequivocal identification of various developmental stages of human and rodent malaria parasites. Using biotinylated species-specific DNA probes, erythrocytic parasites can be specifically stained in blood smears. Similarly exoerythrocytic stages can be visualized in cell culture and in sections of paraffin-embedded liver. In blood smears, the hybridization procedure provides a rapid detection of (low) parasitemia and species-determination for experienced microscopists at 100 to 400x magnification. Moreover, the procedure can be applied even after previous Giemsa staining of the preparation, enabling revision of patient smears which were difficult to read after routine Giemsa staining.