Azithromycin disrupts apicoplast biogenesis in replicating and dormant liver stages of the relapsing malaria parasites Plasmodium vivax and Plasmodium cynomolgi

The control and elimination of malaria caused by Plasmodium vivax is hampered by the threat of relapsed infection resulting from the activation of dormant hepatic hypnozoites. Currently, only the 8-aminoquinolines, primaquine and tafenoquine, have been approved for the elimination of hypnozoites, although their use is hampered by potential toxicity. Therefore, an alternative radical curative drug that safely eliminates hypnozoites is a pressing need. This study assessed the potential hypnozoiticidal activity of the antibiotic azithromycin, which is thought to exert antimalarial activity by inhibiting prokaryote-like ribosomal translation within the apicoplast, an indispensable organelle. The results show that azithromycin inhibited apicoplast development during liver-stage schizogony in P. vivax and Plasmodium cynomolgi, leading to impaired parasite maturation. More importantly, this study found that azithromycin is likely to impair the hypnozoite's apicoplast, resulting in the loss of this organelle. Subsequently, using a recently developed long-term hepatocyte culture system, this study found that this loss likely induces a delay in the hypnozoite activation rate, and that those parasites that do proceed to schizogony display liver-stage arrest prior to differentiating into hepatic merozoites, thus potentially preventing relapse. Overall, this work provides evidence for the potential use of azithromycin for the radical cure of relapsing malaria, and identifies apicoplast functions as potential drug targets in quiescent hypnozoites.

Protein O-Fucosyltransferase Is Required for the Efficient Invasion of Hepatocytes by Plasmodium berghei Sporozoites

The O-fucosylation of the thrombospondin type I repeat (TSR) domain is important for TSR-containing proteins' optimal folding and stability. However, the importance of Plasmodium O-fucosyltransferase 2 (POFut2) remains unclear due to two different reports. Here, we disrupted the POFut2 gene in Plasmodium berghei and demonstrated that POFut2 KO parasites develop normally in blood and mosquito stages but show reduced infectivity in mice. We found that the reduced infectivity of POFut2 KO sporozoites was due to a diminished level of TRAP that affected the parasite gliding motility and hepatocyte infectivity. Using all-atom MD simulation, we also hypothesize that O-fucosylation impacts the TSR domain's stability more than its heparin binding capacity.

Moving on: How malaria parasites exit the liver

An essential step in the life cycle of malaria parasites is their egress from hepatocytes, which enables the transition from the asymptomatic liver stage to the pathogenic blood stage of infection. To exit the liver, Plasmodium parasites first disrupt the parasitophorous vacuole membrane that surrounds them during their intracellular replication. Subsequently, parasite-filled structures called merosomes emerge from the infected cell. Shrouded by host plasma membrane, like in a Trojan horse, parasites enter the vasculature undetected by the host immune system and travel to the lung where merosomes rupture, parasites are released, and the blood infection stage begins. This complex, multi-step process must be carefully orchestrated by the parasite and requires extensive manipulation of the infected host cell. This review aims to outline the known signaling pathways that trigger exit, highlight Plasmodium proteins that contribute to the release of liver-stage merozoites, and summarize the accompanying changes to the hepatic host cell.

Assessing the Potential Apoptotic Effects of Different Hydatid Cyst Fluids on Human Healthy Hepatocytes and Hepatocellular Carcinoma Cells

Cystic Echinococcosis (CE) is a zoonotic infection caused by the larval form of Echinococcus granulosus in humans. Emerging evidence suggests an intriguing inverse association between E. granulosus infection and the occurrence of cancer. This study aimed to investigate the influence of diverse host-derived hydatid cyst fluids (HCF) with distinct genotypes on human liver hepatocytes (HC) and hepatocellular carcinoma cells (HepG2). Specifically, we examined their effects on cell proliferation, apoptosis sensitivity (BAX/BCL-2), apoptosis-related p53 expression, and the expression of cancer-related microRNA (hsa-miR-181b-3p). Cell proliferation assays, real-time PCR, and ELISA studies were conducted to evaluate potential anti-cancer properties. The findings revealed that animal-origin HCF (G1(A)) induced direct cell death by augmenting the susceptibility of HepG2 cells to apoptosis. Treatment with both G1(A) and G1(H) HCF sensitized HepG2 and HC cell lines to apoptosis by modulating the BAX/BCL-2 ratio, accompanied by upregulation of the p53 gene. Additionally, G1(A) HCF and human-derived HCFs (G1(H), G7(H)) reduced the expression of miR-181b-3p in HepG2 cells. Consequently, this study demonstrates the potential anti-cancer effect of HCF in HepG2 cells and provides the first comparative assessment of HCFs from human and animal sources with diverse genotypes, offering novel insights into this field.

Single-cell quantitative bioimaging of P. berghei liver stage translation

IMPORTANCE

Plasmodium parasites cause malaria in humans. New multistage active antimalarial drugs are needed, and a promising class of drugs targets the core cellular process of translation, which has many potential molecular targets. During the obligate liver stage, Plasmodium parasites grow in metabolically active hepatocytes, making it challenging to study core cellular processes common to both host cells and parasites, as the signal from the host typically overwhelms that of the parasite. Here, we present and validate a flexible assay to quantify Plasmodium liver stage translation using a technique to fluorescently label the newly synthesized proteins of both host and parasite followed by computational separation of their respective nascent proteomes in confocal image sets. We use the assay to determine whether a test set of known compounds are direct or indirect liver stage translation inhibitors and show that the assay can also predict the mode of action for novel antimalarial compounds.

Does hydrogen peroxide contribute to the immunity against Malaria induced by whole attenuated plasmodial sporozoites?

Plasmodium sporozoites can block apoptotic pathways within host hepatocytes, ensuring the survival of the parasite. However, attenuated plasmodial sporozoites are unable to prevent apoptosis, which provides many parasite antigens to immune cells. This exposure leads to protection against Malaria in both human and animal models. If these hosts are later inoculated with infectious sporozoites, apoptosis of infected hepatocytes will occur, preventing parasite development. Considering that hydrogen peroxide can induce apoptosis, it is plausible that it plays a role in the mechanisms associated with the protection mediated by attenuated plasmodial sporozoites. Based on published results that describe the relationship between Plasmodium, hydrogen peroxide, and apoptosis, a rational explanation can be provided for this hypothesis.

Heterogeneity in killing efficacy of individual effector CD8+ T cells against Plasmodium liver stages

Vaccination strategies in mice inducing high numbers of memory CD8+ T cells specific to a single epitope are able to provide sterilizing protection against infection with Plasmodium sporozoites. We have recently found that Plasmodium-specific CD8+ T cells cluster around sporozoite-infected hepatocytes but whether such clusters are important in elimination of the parasite remains incompletely understood. Here, we used our previously generated data in which we employed intravital microscopy to longitudinally image 32 green fluorescent protein (GFP)-expressing Plasmodium yoelii parasites in livers of mice that had received activated Plasmodium-specific CD8+ T cells after sporozoite infection. We found significant heterogeneity in the dynamics of the normalized GFP signal from the parasites (termed 'vitality index' or VI) that was weakly correlated with the number of T cells near the parasite. We also found that a simple model assuming mass-action, additive killing by T cells well describes the VI dynamics for most parasites and predicts a highly variable killing efficacy by individual T cells. Given our estimated median per capita kill rate of k = 0.031/h we predict that a single T cell is typically incapable of killing a parasite within the 48 h lifespan of the liver stage in mice. Stochastic simulations of T cell clustering and killing of the liver stage also suggested that: (i) three or more T cells per infected hepatocyte are required to ensure sterilizing protection; (ii) both variability in killing efficacy of individual T cells and resistance to killing by individual parasites may contribute to the observed variability in VI decline, and (iii) the stable VI of some clustered parasites cannot be explained by measurement noise. Taken together, our analysis for the first time provides estimates of efficiency at which individual CD8+ T cells eliminate intracellular parasitic infection in vivo.

Does attenuated plasmodial sporozoite-mediated protection require peroxynitrite?

Attenuated plasmodial sporozoite-induced immune response includes intrahepatic nitric oxide (NO) production, which promotes apoptosis of infected hepatocytes and consequent parasite clearance. NO in excess reacts with superoxide, forming peroxynitrite, a powerful cytotoxic agent. Here, I suggest that peroxynitrite proapoptotic action may contribute to the attenuated malarial sporozoite-mediated protection.

Rhoptry neck protein 4 plays important roles during Plasmodium sporozoite infection of the mammalian liver

During invasion, Plasmodium parasites secrete proteins from rhoptry and microneme apical end organelles, which have crucial roles in attaching to and invading target cells. A sporozoite stage-specific gene silencing system revealed that rhoptry neck protein 2 (RON2), RON4, and RON5 are important for sporozoite invasion of mosquito salivary glands. Here, we further investigated the roles of RON4 during sporozoite infection of the liver in vivo. Following intravenous inoculation of RON4-knockdown sporozoites into mice, we demonstrated that sporozoite RON4 has multiple functions during sporozoite traversal of sinusoidal cells and infection of hepatocytes. In vitro infection experiments using a hepatoma cell line revealed that secreted RON4 is involved in sporozoite adhesion to hepatocytes and has an important role in the early steps of hepatocyte infection. In addition, in vitro motility assays indicated that RON4 is required for sporozoite attachment to the substrate and the onset of migration. These findings indicate that RON4 is crucial for sporozoite migration toward and invasion of hepatocytes via attachment ability and motility.IMPORTANCEMalarial parasite transmission to mammals is established when sporozoites are inoculated by mosquitoes and migrate through the bloodstream to infect hepatocytes. Many aspects of the molecular mechanisms underpinning migration and cellular invasion remain largely unelucidated. By applying a sporozoite stage-specific gene silencing system in the rodent malarial parasite, Plasmodium berghei, we demonstrated that rhoptry neck protein 4 (RON4) is crucial for sporozoite infection of the liver in vivo. Combined with in vitro investigations, it was revealed that RON4 functions during a crossing of the sinusoidal cell layer and invading hepatocytes, at an early stage of liver infection, by mediating the sporozoite capacity for adhesion and the onset of motility. Since RON4 is also expressed in Plasmodium merozoites and Toxoplasma tachyzoites, our findings contribute to understanding the conserved invasion mechanisms of Apicomplexa parasites.

Development of an ectopic huLiver model for Plasmodium liver stage infection

Early Plasmodium falciparum and P. vivax infection requires parasite replication within host hepatocytes, referred to as liver stage (LS). However, limited understanding of infection dynamics in human LS exists due to species-specificity challenges. Reported here is a reproducible, easy-to-manipulate, and moderate-cost in vivo model to study human Plasmodium LS in mice; the ectopic huLiver model. Ectopic huLiver tumors were generated through subcutaneous injection of the HC-04 cell line and shown to be infectible by both freshly dissected sporozoites and through the bite of infected mosquitoes. Evidence for complete LS development was supported by the transition to blood-stage infection in mice engrafted with human erythrocytes. Additionally, this model was successfully evaluated for its utility in testing antimalarial therapeutics, as supported by primaquine acting as a causal prophylactic against P. falciparum. Presented here is a new platform for the study of human Plasmodium infection with the potential to aid in drug discovery.

The PTEX Pore Component EXP2 Is Important for Intrahepatic Development during the Plasmodium Liver Stage

During vertebrate infection, obligate intracellular malaria parasites develop within a parasitophorous vacuole, which constitutes the interface between the parasite and its hepatocyte or erythrocyte host cells. To traverse this barrier, Plasmodium spp. utilize a dual-function pore formed by EXP2 for nutrient transport and, in the context of the PTEX translocon, effector protein export across the vacuole membrane. While critical to blood-stage survival, less is known about EXP2/PTEX function in the liver stage, although major differences in the export mechanism are suggested by absence of the PTEX unfoldase HSP101 in the intrahepatic vacuole. Here, we employed the glucosamine-activated glmS ribozyme to study the role of EXP2 during Plasmodium berghei liver-stage development in hepatoma cells. Insertion of the glmS sequence into the exp2 3' untranslated region (UTR) enabled glucosamine-dependent depletion of EXP2 after hepatocyte invasion, allowing separation of EXP2 function during intrahepatic development from a recently reported role in hepatocyte invasion. Postinvasion EXP2 knockdown reduced parasite size and largely abolished expression of the mid- to late-liver-stage marker LISP2. As an orthogonal approach to monitor development, EXP2-glmS parasites and controls were engineered to express nanoluciferase. Activation of glmS after invasion substantially decreased luminescence in hepatoma monolayers and in culture supernatants at later time points corresponding to merosome detachment, which marks the culmination of liver-stage development. Collectively, our findings extend the utility of the glmS ribozyme to study protein function in the liver stage and reveal that EXP2 is important for intrahepatic parasite development, indicating that PTEX components also function at the hepatocyte-parasite interface. IMPORTANCE After the mosquito bite that initiates a Plasmodium infection, parasites first travel to the liver and develop in hepatocytes. This liver stage is asymptomatic but necessary for the parasite to transition to the merozoite form, which infects red blood cells and causes malaria. To take over their host cells, avoid immune defenses, and fuel their growth, these obligately intracellular parasites must import nutrients and export effector proteins across a vacuole membrane in which they reside. In the blood stage, these processes depend on a translocon called PTEX, but it is unclear if PTEX also functions during the liver stage. Here, we adapted the glmS ribozyme to control expression of EXP2, the membrane pore component of PTEX, during the liver stage of the rodent malaria parasite Plasmodium berghei. Our results show that EXP2 is important for intracellular development in the hepatocyte, revealing that PTEX components are also functionally important during liver-stage infection.

Single-cell RNA profiling of Plasmodium vivax-infected hepatocytes reveals parasite- and host- specific transcriptomic signatures and therapeutic targets

The resilience of Plasmodium vivax, the most widely-distributed malaria-causing parasite in humans, is attributed to its ability to produce dormant liver forms known as hypnozoites, which can activate weeks, months, or even years after an initial mosquito bite. The factors underlying hypnozoite formation and activation are poorly understood, as is the parasite's influence on the host hepatocyte. Here, we shed light on transcriptome-wide signatures of both the parasite and the infected host cell by sequencing over 1,000 P. vivax-infected hepatocytes at single-cell resolution. We distinguish between replicating schizonts and hypnozoites at the transcriptional level, identifying key differences in transcripts encoding for RNA-binding proteins associated with cell fate. In infected hepatocytes, we show that genes associated with energy metabolism and antioxidant stress response are upregulated, and those involved in the host immune response downregulated, suggesting both schizonts and hypnozoites alter the host intracellular environment. The transcriptional markers in schizonts, hypnozoites, and infected hepatocytes revealed here pinpoint potential factors underlying dormancy and can inform therapeutic targets against P. vivax liver-stage infection.

Mid-Liver Stage Arrest of Plasmodium falciparum Schizonts in Primary Porcine Hepatocytes

During co-evolution Plasmodium parasites and vertebrates went through a process of selection resulting in defined and preferred parasite-host combinations. As such, Plasmodium falciparum (Pf) sporozoites can infect human hepatocytes while seemingly incompatible with host cellular machinery of other species. The compatibility between parasite invasion ligands and their respective human hepatocyte receptors plays a key role in Pf host selectivity. However, it is unclear whether the ability of Pf sporozoites to mature in cross-species infection also plays a role in host tropism. Here we used fresh hepatocytes isolated from porcine livers to study permissiveness to Pf sporozoite invasion and development. We monitored intra-hepatic development via immunofluorescence using anti-HSP70, MSP1, EXP1, and EXP2 antibodies. Our data shows that Pf sporozoites can invade non-human hepatocytes and undergo partial maturation with a significant decrease in schizont numbers between day three and day five. A possible explanation is that Pf sporozoites fail to form a parasitophorous vacuolar membrane (PVM) during invasion. Indeed, the observed aberrant EXP1 and EXP2 staining supports the presence of an atypical PVM. Functions of the PVM include the transport of nutrients, export of waste, and offering a protective barrier against intracellular host effectors. Therefore, an atypical PVM likely results in deficiencies that may detrimentally impact parasite development at multiple levels. In summary, despite successful invasion of porcine hepatocytes, Pf development arrests at mid-stage, possibly due to an inability to mobilize critical nutrients across the PVM. These findings underscore the potential of a porcine liver model for understanding the importance of host factors required for Pf mid-liver stage development.

An improved experimental method for simultaneously isolating hepatocytes and hepatic stellate cells in mouse liver infected with Echinococcus multilocularis

BACKGROUND

Alveolar echinococcosis (AE) is a zoonotic disease caused by the larval stage of Echinococcus multilocularis parasitizing in the human liver, causing local pathological changes in the liver and manifesting as hyperplasia, liver fibrosis, atrophy, degeneration, and necrosis. Here, we report a method that can simultaneously isolate hepatocytes and hepatic stellate cells (HSCs) from mice infected with Echinococcus multilocularis.

METHODS

A mouse model of AE was established. Hepatocytes and HSCs were isolated from mouse liver using a two-step method combining in situ collagenase perfusion and gradient centrifugation. Expressions of Alb, Desmin, and α-SMA were detected with immunofluorescence to identify the isolated hepatocytes and HSCs.

RESULTS

The viability and purity of hepatocytes and HSCs both reached 90% or above. For hepatocytes, clear cell boundaries were observed, and the nuclei were round or oval, with clear nucleoli. There was a homogeneous distribution of the hepatocyte marker Alb in the cytoplasm of hepatocytes. Lipid droplets and Desmin expression were observed in the cytoplasm of freshly isolated HSCs. During the activation of HSCs, the lipid droplets gradually decreased and disappeared with a high expression of α-SMA.

CONCLUSION

Hepatocytes and HSCs are simultaneously isolated. This may provide a research tool to investigate the interaction between hepatocytes and HSCs and to investigate the mechanism of Echinococcus multilocularis infection-induced liver pathological changes.

Echinococcus multilocularis infection induces UBE2N suppression via exosomal emu-miR-4989

Echinococcus multilocularis metacestodes mainly reside in liver in humans and animals, and cause serious damages. UBE2N was herein shown to be downregulated in response to the infection. UBE2N was further shown to be predominantly expressed in the hepatocytes, which was also significantly downregulated during the infection. UBE2N was a target of emu-miR-4989, which was loaded into the exosomes secreted by parasites. These emu-miR-4989-encapsulating exosomes were internalized by hepatocytes, and induced a significant decrease of relative luciferase activity in the cells transfected with the construct containing a wild type of UBE2N 3'-UTR compared to the control (p < 0.05). These results demonstrate that emu-miR-4989 is involved in the UBE2N inhibition in the hepatocytes during E. multilocularis through exosomes.

The power and promise of genetic mapping from Plasmodium falciparum crosses utilizing human liver-chimeric mice

Genetic crosses are most powerful for linkage analysis when progeny numbers are high, parental alleles segregate evenly and numbers of inbred progeny are minimized. We previously developed a novel genetic crossing platform for the human malaria parasite Plasmodium falciparum, an obligately sexual, hermaphroditic protozoan, using mice carrying human hepatocytes (the human liver-chimeric FRG NOD huHep mouse) as the vertebrate host. We report on two genetic crosses-(1) an allopatric cross between a laboratory-adapted parasite (NF54) of African origin and a recently patient-derived Asian parasite, and (2) a sympatric cross between two recently patient-derived Asian parasites. We generated 144 unique recombinant clones from the two crosses, doubling the number of unique recombinant progeny generated in the previous 30 years. The allopatric African/Asian cross has minimal levels of inbreeding and extreme segregation distortion, while in the sympatric Asian cross, inbred progeny predominate and parental alleles segregate evenly. Using simulations, we demonstrate that these progeny provide the power to map small-effect mutations and epistatic interactions. The segregation distortion in the allopatric cross slightly erodes power to detect linkage in several genome regions. We greatly increase the power and the precision to map biomedically important traits with these new large progeny panels.

Plasmodium sporozoites on the move: Switching from cell traversal to productive invasion of hepatocytes

Parasites of the genus Plasmodium, the etiological agent of malaria, are transmitted through the bite of anopheline mosquitoes, which deposit sporozoites into the host skin. Sporozoites migrate through the dermis, enter the bloodstream, and rapidly traffic to the liver. They cross the liver sinusoidal barrier and traverse several hepatocytes before switching to productive invasion of a final one for replication inside a parasitophorous vacuole. Cell traversal and productive invasion are functionally independent processes that require proteins secreted from specialized secretory organelles known as micronemes. In this review, we summarize the current understanding of how sporozoites traverse through cells and productively invade hepatocytes, and discuss the role of environmental sensing in switching from a migratory to an invasive state. We propose that timely controlled secretion of distinct microneme subsets could play a key role in successful migration and infection of hepatocytes. A better understanding of these essential biological features of the Plasmodium sporozoite may contribute to the development of new strategies to fight against the very first and asymptomatic stage of malaria.

Bioengineered Liver Cell Models of Hepatotropic Infections

Hepatitis viruses and liver-stage malaria are within the liver infections causing higher morbidity and mortality rates worldwide. The highly restricted tropism of the major human hepatotropic pathogens-namely, the human hepatitis B and C viruses and the Plasmodium falciparum and Plasmodium vivax parasites-has hampered the development of disease models. These models are crucial for uncovering the molecular mechanisms underlying the biology of infection and governing host-pathogen interaction, as well as for fostering drug development. Bioengineered cell models better recapitulate the human liver microenvironment and extend hepatocyte viability and phenotype in vitro, when compared with conventional two-dimensional cell models. In this article, we review the bioengineering tools employed in the development of hepatic cell models for studying infection, with an emphasis on 3D cell culture strategies, and discuss how those tools contributed to the level of recapitulation attained in the different model layouts. Examples of host-pathogen interactions uncovered by engineered liver models and their usefulness in drug development are also presented. Finally, we address the current bottlenecks, trends, and prospect toward cell models' reliability, robustness, and reproducibility.

Comparative analysis of different extracellular vesicles secreted by Echinococcus granulosus protoscoleces

Extracellular vesicles (EVs) are heterogeneous populations of different membrane-wrapped vesicles in size and encapsulated cargo and have recently emerged as a crucial carrier with the functions in intercellular communication, being involved in host-parasite interactions. However, Echinococcus granulosus EVs are not fully described. To separate EVs with a different size, the culture supernatant of E. granulosus protoscoleces (PSCs) was sequentially centrifuged at 2,000g, 10,000g and 110,000g, and the resulting precipitates were accordingly named as 2K, 10K and 110K EVs, respectively. The size and morphology of three different EVs were identified using ZETASIZER NANO and transmission electron microscopy (TEM), respectively. Then mass spectrometry was applied to define protein cargo of EVs and EV internalization was assessed using fluorescent microscopy and flow cytometry. The results showed that 2K EVs mainly ranged from 450 to 950 nm in diameter, 10K EVs ranged from 220 to 390 nm and 110K EVs from 60 to 150 nm. A total of 901 EV proteins were identified, 328 of which were commonly found in the three types of EVs. GO analysis revealed that these proteins were mainly involved in binding (44%) and catalytic activity (44%). Three types of EVs were different in biomarkers (Enolase and 14-3-3) and in reactivity with anti-echinococcosis positive serum. Moreover, 110K EVs were more easily internalized by hepatic cells than 10K EVs as well as 2K EVs (p < 0.0001). These results reveal the physical and biological discrepancy among 2K, 10K and 110K EVs, suggesting a distinct role in host-parasite interactions.

Hepatic Inflammation Confers Protective Immunity Against Liver Stages of Malaria Parasite

Deciphering the mechanisms by which Plasmodium parasites develop inside hepatocytes is an important step toward the understanding of malaria pathogenesis. We propose that the nature and the magnitude of the inflammatory response in the liver are key for the establishment of the infection. Here, we used mice deficient in the multidrug resistance-2 gene (Mdr2-/-)-encoded phospholipid flippase leading to the development of liver inflammation. Infection of Mdr2-/- mice with Plasmodium berghei ANKA (PbANKA) sporozoites (SPZ) resulted in the blockade of hepatic exo-erythrocytic forms (EEFs) with no further development into blood stage parasites. Interestingly, cultured primary hepatocytes from mutant and wild-type mice are equally effective in supporting EEF development. The abortive infection resulted in a long-lasting immunity in Mdr2-/- mice against infectious SPZ where neutrophils and IL-6 appear as key effector components along with CD8+ and CD4+ effector and central memory T cells. Inflammation-induced breakdown of liver tolerance promotes anti-parasite immunity and provides new approaches for the design of effective vaccines against malaria disease.

Inhibition of Resistance-Refractory P. falciparum Kinase PKG Delivers Prophylactic, Blood Stage, and Transmission-Blocking Antiplasmodial Activity

The search for antimalarial chemotypes with modes of action unrelated to existing drugs has intensified with the recent failure of first-line therapies across Southeast Asia. Here, we show that the trisubstituted imidazole MMV030084 potently inhibits hepatocyte invasion by Plasmodium sporozoites, merozoite egress from asexual blood stage schizonts, and male gamete exflagellation. Metabolomic, phosphoproteomic, and chemoproteomic studies, validated with conditional knockdown parasites, molecular docking, and recombinant kinase assays, identified cGMP-dependent protein kinase (PKG) as the primary target of MMV030084. PKG is known to play essential roles in Plasmodium invasion of and egress from host cells, matching MMV030084's activity profile. Resistance selections and gene editing identified tyrosine kinase-like protein 3 as a low-level resistance mediator for PKG inhibitors, while PKG itself never mutated under pressure. These studies highlight PKG as a resistance-refractory antimalarial target throughout the Plasmodium life cycle and promote MMV030084 as a promising Plasmodium PKG-targeting chemotype.

A Novel Chemically Differentiated Mouse Embryonic Stem Cell-Based Model to Study Liver Stages of Plasmodium berghei

Asymptomatic and obligatory liver stage (LS) infection of Plasmodium parasites presents an attractive target for antimalarial vaccine and drug development. Lack of robust cellular models to study LS infection has hindered the discovery and validation of host genes essential for intrahepatic parasite development. Here, we present a chemically differentiated mouse embryonic stem cell (ESC)-based LS model, which supports complete development of Plasmodium berghei exoerythrocytic forms (EEFs) and can be used to define new host-parasite interactions. Using our model, we established that host Pnpla2, coding for adipose triglyceride lipase, is dispensable for P. berghei EEF development. In addition, we also evaluated in-vitro-differentiated human hepatocyte-like cells (iHLCs) to study LS of P. berghei and found it to be a sub-optimal infection model. Overall, our results present a new mouse ESC-based P. berghei LS infection model that can be utilized to study the impact of host genetic variation on parasite development.

Dual-Luciferase-Based Fast and Sensitive Detection of Malaria Hypnozoites for the Discovery of Antirelapse Compounds

Efforts to eradicate Plasmodium vivax malaria are hampered by the presence of hypnozoites, persisting stages in the liver that can reactivate after prolonged periods of time enabling further transmission and causing renewed disease. Large-scale drug screening is needed to identify compounds with antihypnozoite activity, but current platforms rely on time-consuming high-content fluorescence imaging as read-out, limiting assay throughput. We here report an ultrafast and sensitive dual-luciferase-based method to differentiate hypnozoites from liver stage schizonts using a transgenic P. cynomolgi parasite line that contains Nanoluc driven by the constitutive hsp70 promoter, as well as firefly luciferase driven by the schizont-specific lisp2 promoter. The transgenic parasite line showed similar fitness and drug sensitivity profiles of selected compounds to wild type. We demonstrate robust bioluminescence-based detection of hypnozoites in 96-well and 384-well plate formats, setting the stage for implementation in large scale drug screens.

Hepatocyte-derived IL-10 plays a crucial role in attenuating pathogenicity during the chronic phase of T. congolense infection

Bovine African Trypanosomosis is an infectious parasitic disease affecting livestock productivity and thereby impairing the economic development of Sub-Saharan Africa. The most important trypanosome species implicated is T. congolense, causing anemia as most important pathological feature. Using murine models, it was shown that due to the parasite’s efficient immune evasion mechanisms, including (i) antigenic variation of the variable surface glycoprotein (VSG) coat, (ii) induction of polyclonal B cell activation, (iii) loss of B cell memory and (iv) T cell mediated immunosuppression, disease prevention through vaccination has so far been impossible. In trypanotolerant models a strong, early pro-inflammatory immune response involving IFN-γ, TNF and NO, combined with a strong humoral anti-VSG response, ensures early parasitemia control. This potent protective inflammatory response is counterbalanced by the production of the anti-inflammatory cytokine IL-10, which in turn prevents early death of the host from uncontrolled hyper-inflammation-mediated immunopathologies. Though at this stage different hematopoietic cells, such as NK cells, T cells and B cells as well as myeloid cells (i.e. alternatively activated myeloid cells (M2) or Ly6c^- monocytes), were found to produce IL-10, the contribution of non-hematopoietic cells as potential IL-10 source during experimental T. congolense infection has not been addressed. Here, we report for the first time that during the chronic stage of T. congolense infection non-hematopoietic cells constitute an important source of IL-10. Our data shows that hepatocyte-derived IL-10 is mandatory for host survival and is crucial for the control of trypanosomosis-induced inflammation and associated immunopathologies such as anemia, hepatosplenomegaly and excessive tissue injury.

Bovine African Trypanosomosis is a parasitic disease of veterinary importance that adversely affects the public health and economic development of sub-Saharan Africa. The most important trypanosome species implicated is T. congolense, causing anemia as most important pathological feature and major cause of death. Using murine models, it was shown that the disease is characterized by a well-timed and balanced production of pro-inflammatory cytokine promoting factors followed by an anti-inflammatory response, involving IL-10. The latter is required to attenuate infection-associated pathogenicity and to prevent early host death from uncontrolled hyper-inflammation mediated immunopathologies. However, the cellular source of IL-10 in vivo and the window within which these cells exert their function during the course of African trypanosomiasis remain poorly understood, which hampers the design of effective therapeutic strategies. Using a T. congolense infection mouse model, relevant for bovine trypanosomosis, we demonstrate that during the chronic stage of infection hepatocyte-derived IL-10, but not myeloid cell-derived IL-10, regulates the main infection-associated immunopathologies and ultimately mediates host survival. Hence, strategies that tilt the balance of hepatocyte cytokine production in favor of IL-10 could majorly impact the wellbeing and survival of T. congolense-infected animals. Given the unmet medical need for this parasite infection, our findings offer promise for improved treatment protocols in the field.

Clustering of Activated CD8 T Cells Around Malaria-Infected Hepatocytes Is Rapid and Is Driven by Antigen-Specific Cells

Malaria, a disease caused by parasites of the Plasmodium genus, begins when Plasmodium-infected mosquitoes inject malaria sporozoites while searching for blood. Sporozoites migrate from the skin via blood to the liver, infect hepatocytes, and form liver stages which in mice 48 h later escape into blood and cause clinical malaria. Vaccine-induced activated or memory CD8 T cells are capable of locating and eliminating all liver stages in 48 h, thus preventing the blood-stage disease. However, the rules of how CD8 T cells are able to locate all liver stages within a relatively short time period remains poorly understood. We recently reported formation of clusters consisting of variable numbers of activated CD8 T cells around Plasmodium yoelii (Py)-infected hepatocytes. Using a combination of experimental data and mathematical models we now provide additional insights into mechanisms of formation of these clusters. First, we show that a model in which cluster formation is driven exclusively by T-cell-extrinsic factors, such as variability in "attractiveness" of different liver stages, cannot explain distribution of cluster sizes in different experimental conditions. In contrast, the model in which cluster formation is driven by the positive feedback loop (i.e., larger clusters attract more CD8 T cells) can accurately explain the available data. Second, while both Py-specific CD8 T cells and T cells of irrelevant specificity (non-specific CD8 T cells) are attracted to the clusters, we found no evidence that non-specific CD8 T cells play a role in cluster formation. Third and finally, mathematical modeling suggested that formation of clusters occurs rapidly, within few hours after adoptive transfer of CD8 T cells, thus illustrating high efficiency of CD8 T cells in locating their targets in complex peripheral organs, such as the liver. Taken together, our analysis provides novel insights into and attempts to discriminate between alternative mechanisms driving the formation of clusters of antigen-specific CD8 T cells in the liver.

The Plasmodium berghei serine protease PbSUB1 plays an important role in male gamete egress

The Plasmodium subtilisin-like serine protease SUB1 is expressed in hepatic and both asexual and sexual blood parasite stages. SUB1 is required for egress of invasive forms of the parasite from both erythrocytes and hepatocytes, but its subcellular localisation, function, and potential substrates in the sexual stages are unknown. Here, we have characterised the expression profile and subcellular localisation of SUB1 in Plasmodium berghei sexual stages. We show that the protease is selectively expressed in mature male gametocytes and localises to secretory organelles known to be involved in gamete egress, called male osmiophilic bodies. We have investigated PbSUB1 function in the sexual stages by generating P. berghei transgenic lines deficient in PbSUB1 expression or enzyme activity in gametocytes. Our results demonstrate that PbSUB1 plays a role in male gamete egress. We also show for the first time that the PbSUB1 substrate PbSERA3 is expressed in gametocytes and processed by PbSUB1 upon gametocyte activation. Taken together, our results strongly suggest that PbSUB1 is not only a promising drug target for asexual stages but could also be an attractive malaria transmission-blocking target.

Deciphering host lysosome-mediated elimination of Plasmodium berghei liver stage parasites

Liver stage Plasmodium parasites reside in a parasitophorous vacuole (PV) that associates with lysosomes. It has previously been shown that these organelles can have beneficial as well as harmful effects on the parasite. Yet it is not clear how the association of lysosomes with the parasite is controlled and how interactions with these organelles lead to the antagonistic outcomes. In this study we used advanced imaging techniques to characterize lysosomal interactions with the PV. In host cells harboring successfully developing parasites we observed that these interaction events reach an equilibrium at the PV membrane (PVM). In a population of arrested parasites, this equilibrium appeared to shift towards a strongly increased lysosomal fusion with the PVM witnessed by strong PVM labeling with the lysosomal marker protein LAMP1. This was followed by acidification of the PV and elimination of the parasite. To systematically investigate elimination of arrested parasites, we generated transgenic parasites that express the photosensitizer KillerRed, which leads to parasite killing after activation. Our work provides insights in cellular details of intracellular killing and lysosomal elimination of Plasmodium parasites independent of cells of the immune system.

A tracer-based method enables tracking of Plasmodium falciparum malaria parasites during human skin infection

Introduction: The skin stage of malaria is a vital and vulnerable migratory life stage of the parasite. It has been characterised in rodent models, but remains wholly uninvestigated for human malaria parasites. To enable in depth analysis of not genetically modified (non-GMO) Plasmodium falciparum (Pf) sporozoite behaviour in human skin, we devised a labelling technology (Cy5M2, targeting the sporozoite mitochondrion) that supports tracking of individual non-GMO sporozoites in human skin. Methods: Sporozoite labelling with Cy5M2 was performed in vitro as well as via the feed of infected Anopheles mosquitos. Labelling was validated using confocal microscopy and flow cytometry and the fitness of labelled sporozoites was determined by analysis of infectivity to human hepatocytes in vitro, and in vivo in a rodent infection model. Using confocal video microscopy and custom software, single-sporozoite tracking studies in human skin-explants were performed. Results: Both in vitro and in mosquito labelling strategies yielded brightly fluorescent sporozoites of three different Plasmodium species. Cy5M2 uptake colocalized with MitoTracker® green and could be blocked using the known Translocator protein (TSPO)-inhibitor PK11195. This method supported the visualization and subsequent quantitative analysis of the migration patterns of individual non-GMO Pf sporozoites in human skin and did not affect the fitness of sporozoites. Conclusions: The ability to label and image non-GMO Plasmodium sporozoites provides the basis for detailed studies on the human skin stage of malaria with potential for in vivo translation. As such, it is an important tool for development of vaccines based on attenuated sporozoites and their route of administration.

In Vitro and in Vivo Effect of MAPK Signal Transduction Pathway Inhibitors on Echinococcus multilocularis

To evaluate the effect of mitogen-activated protein kinase (MAPK) signal transduction pathway inhibitors against alveolar echinococcosis in vitro and in vivo, Echinococcus multilocularis metacestode cysts and protoscolices were obtained from infected mice. Protein chip technology was utilized to screen for key highly expressed target proteins in the MAPK pathway in this parasite and their corresponding inhibitors. Four-week-old Balb/c female mice used for the in vivo experiment underwent inoculation of E. multilocularis by intraperitoneal injection, as well as intragastric administration of MAPK inhibitors for 6 wk. We included 6 groups of mice: a phosphate-buffered saline (PBS) group (negative control); an albendazole-treated group (positive group); and 4 experimental groups treated with TRx0237 mesylate, GDC-0994, pifithrin-β hydrobromide, or Selonsertib. Echinococcus multilocularis protoscolices were collected and cultured in 1066 medium with penicillin/streptomycin and 10% fetal bovine serum. The in vitro experiment included a PBS group (negative control), a dimethyl sulfoxide-treated group (solvent group), and 4 inhibitor-treated groups as in the in vivo experiment (experimental groups). Each inhibitor group received 4 drug concentrations (5, 30, 55, and 80 μM), and the experiment was performed in triplicate per sample. Fluorescence microscopy was used to evaluate the survival rate of the protoscolices every 48 hr beginning from the first 24 hr. The same grouping was used to evaluate cytotoxicity on E. multilocularis germinal cells and L02 cells. The average weights of E. multilocularis metacestode cyst tissue from each group of the in vivo experiment were 873 mg (PBS), 335 mg (albendazole), 323 mg (TRx0237 mesylate), 420 mg (GDC-0994), 340 mg (pifithrin-β hydrobromide), and 642 mg (Selonsertib). Results showed albendazole, TRx0237 mesylate, and pifithrin-β hydrobromide had significant inhibitory effects on inhibition of E. multilocularis. We found a positive correlation between drug concentrations and the inhibitory effects seen in the in vitro experiment, with the differences in contrast with the control group becoming statistically significant after 72 hr of treatment ( P < 0.05). The inhibition rates of TRx0237 mesylate to germinal cells by drug concentration were 23.73, 46.59, 74.71, and 77.44%. Other drugs had no effect on germinal cells. All the inhibitors had low toxicity on L02 cells. Inhibitors of the MAPK signal transduction pathway showed significant inhibitory effects on E. multilocularis, suggesting these may be potential candidates for the treatment of alveolar echinococcosis.

Dual RNA-seq identifies human mucosal immunity protein Mucin-13 as a hallmark of Plasmodium exoerythrocytic infection

The exoerythrocytic stage of Plasmodium infection is a critical window for prophylactic intervention. Using genome-wide dual RNA sequencing of flow-sorted infected and uninfected hepatoma cells we show that the human mucosal immunity gene, mucin-13 (MUC13), is strongly upregulated during Plasmodium exoerythrocytic hepatic-stage infection. We confirm MUC13 transcript increases in hepatoma cell lines and primary hepatocytes. In immunofluorescence assays, host MUC13 protein expression distinguishes infected cells from adjacent uninfected cells and shows similar colocalization with parasite biomarkers such as UIS4 and HSP70. We further show that localization patterns are species independent, marking both P. berghei and P. vivax infected cells, and that MUC13 can be used to identify compounds that inhibit parasite replication in hepatocytes. This data provides insights into host-parasite interactions in Plasmodium infection, and demonstrates that a component of host mucosal immunity is reprogrammed during the progression of infection.

The Micronemal Plasmodium Proteins P36 and P52 Act in Concert to Establish the Replication-Permissive Compartment Within Infected Hepatocytes

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.

Plasmodium sporozoites can invade hepatocytic cells independently of the Ephrin receptor A2

Sporozoite forms of the malaria parasite Plasmodium are transmitted by mosquitoes and first infect the liver for an initial round of replication before parasite proliferation in the blood. The molecular mechanisms involved during sporozoite invasion of hepatocytes remain poorly understood. In previous studies, two receptors of the Hepatitis C virus (HCV), the tetraspanin CD81 and the Scavenger Receptor BI (SR-BI), were shown to play an important role during entry of Plasmodium sporozoites into hepatocytic cells. In contrast to HCV entry, which requires both CD81 and SR-BI together with additional host factors, CD81 and SR-BI operate independently during malaria liver infection, as sporozoites can use CD81 and/or SR-BI, depending on the Plasmodium species, to invade hepatocytes. However, the molecular function of CD81 and SR-BI during parasite entry remains unknown. Another HCV entry factor, the Ephrin receptor A2 (EphA2), was recently reported to play a key role as a host cell entry factor during malaria liver infection. Here, we investigated the contribution of EphA2 during CD81-dependent and SR-BI-dependent sporozoite infection. Using small interfering RNA (siRNA) and antibodies against EphA2, combined with direct detection of parasites by flow cytometry or microscopy, we show that blocking EphA2 has no significant impact on P. yoelii or P. berghei host cell infection, irrespective of the entry route. Thus, our findings argue against an important role of EphA2 during malaria liver infection.

Plasmodium parasite exploits host aquaporin-3 during liver stage malaria infection

Within the liver a single Plasmodium parasite transforms into thousands of blood-infective forms to cause malaria. Here, we use RNA-sequencing to identify host genes that are upregulated upon Plasmodium berghei infection of hepatocytes with the hypothesis that host pathways are hijacked to benefit parasite development. We found that expression of aquaporin-3 (AQP3), a water and glycerol channel, is significantly induced in Plasmodium-infected hepatocytes compared to uninfected cells. This aquaglyceroporin localizes to the parasitophorous vacuole membrane, the compartmental interface between the host and pathogen, with a temporal pattern that correlates with the parasite’s expansion in the liver. Depletion or elimination of host AQP3 expression significantly reduces P. berghei parasite burden during the liver stage and chemical disruption by a known AQP3 inhibitor, auphen, reduces P. falciparum asexual blood stage and P. berghei liver stage parasite load. Further use of this inhibitor as a chemical probe suggests that AQP3-mediated nutrient transport is an important function for parasite development. This study reveals a previously unknown potential route for host-dependent nutrient acquisition by Plasmodium which was discovered by mapping the transcriptional changes that occur in hepatocytes throughout P. berghei infection. The dataset reported may be leveraged to identify additional host factors that are essential for Plasmodium liver stage infection and highlights Plasmodium’s dependence on host factors within hepatocytes.

Plasmodium parasites undergo an obligatory morphogenesis and replication within the liver before they invade red blood cells and cause malaria. The liver stage is clinically silent but essential for the Plasmodium parasite to complete its life cycle. During this time, the parasite relies on the host cell to support a massive replication event, yet host factors that are critical to this expansion are largely unknown. We identify human aquaporin-3 (AQP3), a water and glycerol channel, as essential for the proper development of the parasite within the liver cell. AQP3 localizes to the parasitophorous vacuole membrane, the interface between the host cytoplasm and the parasite, possibly aiding in the nutritional uptake for the parasite. Genetic disruption or treatment with the AQP3 inhibitor auphen, reduces parasite load in liver and blood cells.

A comprehensive model for assessment of liver stage therapies targeting Plasmodium vivax and Plasmodium falciparum

Malaria liver stages represent an ideal therapeutic target with a bottleneck in parasite load and reduced clinical symptoms; however, current in vitro pre-erythrocytic (PE) models for Plasmodium vivax and P. falciparum lack the efficiency necessary for rapid identification and effective evaluation of new vaccines and drugs, especially targeting late liver-stage development and hypnozoites. Herein we report the development of a 384-well plate culture system using commercially available materials, including cryopreserved primary human hepatocytes. Hepatocyte physiology is maintained for at least 30 days and supports development of P. vivax hypnozoites and complete maturation of P. vivax and P. falciparum schizonts. Our multimodal analysis in antimalarial therapeutic research identifies important PE inhibition mechanisms: immune antibodies against sporozoite surface proteins functionally inhibit liver stage development and ion homeostasis is essential for schizont and hypnozoite viability. This model can be implemented in laboratories in disease-endemic areas to accelerate vaccine and drug discovery research.

Host cell cytosolic immune response during Plasmodium liver stage development

Recent years have witnessed a great gain in knowledge regarding parasite-host cell interactions during Plasmodium liver stage development. It is now an accepted fact that a large percentage of sporozoites invading hepatocytes fail to form infectious merozoites. There appears to be a delicate balance between parasite survival and elimination and we now start to understand why this is so. Plasmodium liver stage parasites replicate within the parasitophorous vacuole (PV), formed during invasion by invagination of the host cell plasma membrane. The main interface between the parasite and hepatocyte is the parasitophorous vacuole membrane (PVM) that surrounds the PV. Recently, it was shown that autophagy marker proteins decorate the PVM of Plasmodium liver stage parasites and eliminate a proportion of them by an autophagy-like mechanism. Successfully developing Plasmodium berghei parasites are initially also labeled but in the course of development, they are able to control this host defense mechanism by shedding PVM material into the tubovesicular network (TVN), an extension of the PVM that releases vesicles into the host cell cytoplasm. Better understanding of the molecular events at the PVM/TVN during parasite elimination could be the basis of new antimalarial measures.

Three-dimensional hepatocyte culture system for the study of Echinococcus multilocularis larval development

Background

Hepatocyte-based metacestode culture is an attractive method to study alveolar echinococcosis (AE), but it is limited by the relatively short lifespan of cultured hepatocytes in maintaining their normal function.

Methodology/principal findings

We describe a three-dimensional (3D) hepatic culture system developed from co-cultured hepatocytes and mesenchymal stem cells using a collagen scaffold to study the development of Echinococcus multilocularis larvae. This 3D culture system preserved the function of hepatocytes for a longer period of time than their monolayer counterparts, with albumin secretion, 7-ethoxyresorufin O-deethylation activity, urea synthesis, CYP3A4 and CYP2D6 activity being highly preserved for 21–28 days. The expression levels of hepatocyte-specific genes including CLDN-3, Bsep, AFP, G6P, A1AT, CYP3A4 and NR1I3 were significantly higher in the 3D cultured system compared with their monolayer counterparts after 14-days in culture. Additionally, in the presence of 3D cultured hepatocytes, 81.2% of E. multilocularis protoscoleces rapidly de-differentiated into infective vesicles within eight weeks. Transcriptomic analyses revealed 807 differentially expressed genes between cultured vesicles and protoscoleces, including 119 genes uniquely expressed in protoscoleces, and 242 genes uniquely expressed in vesicles. These differentially expressed genes were mainly involved in parasite growth relating to the G-protein coupled receptor activity pathway, substrate-specific transmembrane transporter activity, cell-cell adhesion process, and potentially with neuroactive ligand-receptor interaction.

Conclusions/significance

This culture system provides a valuable advance in prolonging hepatocyte functionality, a foundation for future in-depth analysis of the host-parasite interaction in AE, and a useful model to evaluate potential therapeutic strategies to treat AE.

Alveolar echinococcosis (AE) is one of the world’s most dangerous zoonoses. Although there have been recent advances in some aspects of the molecular biology of E. multilocularis, larval development is far from understood. An in vitro hepatocyte based cultivation system for the metacestode stage of E. multilocularis has been developed to improve our understanding of AE. However, in two-dimensional conventional cultures, hepatocytes rapidly lose key phenotypic and functional characteristics after only approximately seven days. This hinders long-term in vitro studies of E. multilocularis larvae, which require several months for development. Thus, in this paper, a three-dimensional (3D) hepatic model was developed for simulating the organotropism of E. multilocularis toward the liver of its intermediate host. This 3D model can preserve the functions of hepatocytes and results in rapidly developed E. multilocularis larva. Genes uniquely expressed in protoscoleces and vesicles provided key information for the further study of AE. The 3D hepatic model provides a new foundation for E. multilocularis developmental studies and in-depth analysis of the host-parasite interaction in AE.

Cell Traversal Activity Is Important for Plasmodium falciparum Liver Infection in Humanized Mice

Malaria sporozoites are deposited into the skin by mosquitoes and infect hepatocytes. The molecular basis of how Plasmodium falciparum sporozoites migrate through host cells is poorly understood, and direct evidence of its importance in vivo is lacking. Here, we generated traversal-deficient sporozoites by genetic disruption of sporozoite microneme protein essential for cell traversal (PfSPECT) or perforin-like protein 1 (PfPLP1). Loss of either gene did not affect P. falciparum growth in erythrocytes, in contrast with a previous report that PfPLP1 is essential for merozoite egress. However, although traversal-deficient sporozoites could invade hepatocytes in vitro, they could not establish normal liver infection in humanized mice. This is in contrast with NF54 sporozoites, which infected the humanized mice and developed into exoerythrocytic forms. This study demonstrates that SPECT and perforin-like protein 1 (PLP1) are critical for transcellular migration by P. falciparum sporozoites and demonstrates the importance of cell traversal for liver infection by this human pathogen.

Deletion of the rodent malaria ortholog for falcipain-1 highlights differences between hepatic and blood stage merozoites

Proteases have been implicated in a variety of developmental processes during the malaria parasite lifecycle. In particular, invasion and egress of the parasite from the infected hepatocyte and erythrocyte, critically depend on protease activity. Although falcipain-1 was the first cysteine protease to be characterized in P. falciparum, its role in the lifecycle of the parasite has been the subject of some controversy. While an inhibitor of falcipain-1 blocked erythrocyte invasion by merozoites, two independent studies showed that falcipain-1 disruption did not affect growth of blood stage parasites. To shed light on the role of this protease over the entire Plasmodium lifecycle, we disrupted berghepain-1, its ortholog in the rodent parasite P. berghei. We found that this mutant parasite displays a pronounced delay in blood stage infection after inoculation of sporozoites. Experiments designed to pinpoint the defect of berghepain-1 knockout parasites found that it was not due to alterations in gliding motility, hepatocyte invasion or liver stage development and that injection of berghepain-1 knockout merosomes replicated the phenotype of delayed blood stage growth after sporozoite inoculation. We identified an additional role for berghepain-1 in preparing blood stage merozoites for infection of erythrocytes and observed that berghepain-1 knockout parasites exhibit a reticulocyte restriction, suggesting that berghepain-1 activity broadens the erythrocyte repertoire of the parasite. The lack of berghepain-1 expression resulted in a greater reduction in erythrocyte infectivity in hepatocyte-derived merozoites than it did in erythrocyte-derived merozoites. These observations indicate a role for berghepain-1 in processing ligands important for merozoite infectivity and provide evidence supporting the notion that hepatic and erythrocytic merozoites, though structurally similar, are not identical.

Malaria affects hundreds of millions of people and is the cause of hundreds of thousands of deaths each year. Infection begins with the inoculation of sporozoites into the skin during the bite of an infected mosquito. Sporozoites subsequently travel to the liver, where they invade and replicate in hepatocytes, eventually releasing the stage of the parasite that is infectious for red blood cells, termed merozoites. Hepatic merozoites initiate blood stage infection, the stage that is responsible for the clinical symptoms of malaria. The blood stage of the parasite grows through repeated rounds of invasion, development and egress of blood stage merozoites, which then continue the cycle. Proteases are among the enzymes that are essential for parasite survival and their functions range from invasion of red blood cells, to the breakdown of red cell hemoglobin, to the release of parasites from red cells. As the function of the cysteine protease falcipain-1 in the lifecycle of the human malaria parasite Plasmodium falciparum remains poorly understood, we decided to study berghepain-1, the orthologue of the rodent malaria parasite P. berghei by generating a berghepain-1 deletion parasite. Using this mutant, we demonstrate that berghepain-1 has a critical role in both hepatic and erythrocytic merozoite infectivity. Little is known about differences between these two types of merozoites and our data leads us to conclude that these merozoites are not identical.

Laser capture microdissection enables transcriptomic analysis of dividing and quiescent liver stages of Plasmodium relapsing species

Dormant liver stage forms (hypnozoites) of the malaria parasite Plasmodium vivax present major hurdles to control and eradicate infection. Despite major research efforts, the molecular composition of hypnozoites remains ill defined. Here, we applied a combination of state-of-the-art technologies to generate the first transcriptome of hypnozoites. We developed a robust laser dissection microscopy protocol to isolate individual Plasmodium cynomolgi hypnozoites and schizonts from infected monkey hepatocytes and optimized RNA-seq analysis to obtain the first transcriptomes of these stages. Comparative transcriptomic analysis identified 120 transcripts as being differentially expressed in the hypnozoite stage relative to the dividing liver schizont, with 69 and 51 mRNAs being up- or down-regulated, respectively, in the hypnozoites. This lead to the identification of potential markers of commitment to and maintenance of the dormant state of the hypnozoite including three transcriptional regulators of the ApiAP2 family, one of which is unique to P. cynomolgi and P. vivax, and the global translational repressor, eIF2a kinase eIK2, all of which are upregulated in the hypnozoite. Together, this work not only provides a primary experimentally-derived list of molecular markers of hypnozoites but also identifies transcriptional and posttranscriptional regulation of gene expression as potentially being key to establishing and maintaining quiescence.

Plasmodium P36 determines host cell receptor usage during sporozoite invasion

Plasmodium sporozoites, the mosquito-transmitted forms of the malaria parasite, first infect the liver for an initial round of replication before the emergence of pathogenic blood stages. Sporozoites represent attractive targets for antimalarial preventive strategies, yet the mechanisms of parasite entry into hepatocytes remain poorly understood. Here we show that the two main species causing malaria in humans, Plasmodium falciparum and Plasmodium vivax, rely on two distinct host cell surface proteins, CD81 and the Scavenger Receptor BI (SR-BI), respectively, to infect hepatocytes. By contrast, CD81 and SR-BI fulfil redundant functions during infection by the rodent parasite P. berghei. Genetic analysis of sporozoite factors reveals the 6-cysteine domain protein P36 as a major parasite determinant of host cell receptor usage. Our data provide molecular insights into the invasion pathways used by different malaria parasites to infect hepatocytes, and establish a functional link between a sporozoite putative ligand and host cell receptors.

Natural Schistosoma mansoni Infection in the Wild Reservoir Nectomys squamipes Leads to Excessive Lipid Droplet Accumulation in Hepatocytes in the Absence of Liver Functional Impairment

Schistosomiasis is a neglected tropical disease of a significant public health impact. The water rat Nectomys squamipes is one of the most important non-human hosts in the schistosomiasis mansoni transmission in Brazil, being considered a wild reservoir. Cellular mechanisms that contribute to the physiological adaptation of this rodent to the Schistosoma mansoni parasite are poorly understood. Here we identified, for the first time, that a hepatic steatosis, a condition characterized by excessive lipid accumulation with formation of lipid droplets (LDs) within hepatocytes, occurs in response to the natural S. mansoni infection of N. squamipes, captured in an endemic region. Significant increases of LD area in the hepatic tissue and LD numbers/hepatocyte, detected by quantitative histopathological and ultrastructural analyses, were paralleled by increased serum profile (total cholesterol and triglycerides) in infected compared to uninfected animals. Raman spectroscopy showed high content of polyunsaturated fatty acids (PUFAs) in the liver of both groups. MALDI-TOFF mass spectroscopy revealed an amplified pool of omega-6 PUFA arachidonic acid in the liver of infected animals. Assessment of liver functional activity by the levels of hepatic transaminases (ALT and AST) did not detect any alteration during the natural infection. In summary, this work demonstrates that the natural infection of the wild reservoir N. squamipes with S. mansoni elicits hepatic steatosis in the absence of liver functional harm and that accumulation of lipids, markedly PUFAs, coexists with low occurrence of inflammatory granulomatous processes, suggesting that lipid stores may be acting as a protective mechanism for dealing with the infection.

Molecular mechanisms of host cell traversal by malaria sporozoites

Malaria is a pernicious infectious disease caused by apicomplexan parasites of the genus Plasmodium. Each year, malaria afflicts over 200million people, causing considerable morbidity, loss to gross domestic product of endemic countries, and more than 420,000 deaths. A central feature of the virulence of malaria parasites is the ability of sporozoite forms injected by a mosquito to navigate from the inoculation site in the skin through host tissues to infect the liver. The ability for sporozoites to traverse through different host cell types is very important for the successful development of parasites within the mammalian host. Over the past decade, our understanding of the role of host cell traversal has become clearer through important studies with rodent models of malaria. However, we still do not understand the stepwise process of host cell entry and exit or know the molecular mechanisms governing each step. We know even less about cell traversal by malaria parasite species that infect humans. Here, we review current knowledge regarding the role and molecular mechanisms of sporozoite cell traversal and highlight recent advances that prompt new ways of thinking about this important process.

A Comprehensive Analysis of Plasmodium Circumsporozoite Protein Binding to Hepatocytes

Circumsporozoite protein (CSP) is the dominant protein on the surface of Plasmodium sporozoites and plays a critical role in the invasion by sporozoites of hepatocytes. Contacts between CSP and heparin sulfate proteoglycans (HSPGs) lead to the attachment of sporozoites to hepatocytes and trigger signaling events in the parasite that promote invasion of hepatocytes. The precise sequence elements in CSP that bind HSPGs have not been identified. We performed a systematic in vitro analysis to dissect the association between Plasmodium falciparum CSP (PfCSP) and hepatocytes. We demonstrate that interactions between PfCSP and heparin or a cultured hepatoma cell line, HepG2, are mediated primarily by a lysine-rich site in the amino terminus of PfCSP. Importantly, the carboxyl terminus of PfCSP facilitates heparin-binding by the amino-terminus but does not interact directly with heparin. These findings provide insights into how CSP recognizes hepatocytes and useful information for further functional studies of CSP.

Human Liver Infection in a Dish: Easy-To-Build 3D Liver Models for Studying Microbial Infection

Human liver infection is a major cause of death worldwide, but fundamental studies on infectious diseases affecting humans have been hampered by the lack of robust experimental models that accurately reproduce pathogen-host interactions in an environment relevant for the human disease. In the case of liver infection, one consequence of this absence of relevant models is a lack of understanding of how pathogens cross the sinusoidal endothelial barrier and parenchyma. To fill that gap we elaborated human 3D liver in vitro models, composed of human liver sinusoidal endothelial cells (LSEC) and Huh-7 hepatoma cells as hepatocyte model, layered in a structure mimicking the hepatic sinusoid, which enable studies of key features of early steps of hepatic infection. Built with established cell lines and scaffold, these models provide a reproducible and easy-to-build cell culture approach of reduced complexity compared to animal models, while preserving higher physiological relevance compared to standard 2D systems. For proof-of-principle we challenged the models with two hepatotropic pathogens: the parasitic amoeba Entamoeba histolytica and hepatitis B virus (HBV). We constructed four distinct setups dedicated to investigating specific aspects of hepatic invasion: 1) pathogen 3D migration towards hepatocytes, 2) hepatocyte barrier crossing, 3) LSEC and subsequent hepatocyte crossing, and 4) quantification of human hepatic virus replication (HBV). Our methods comprise automated quantification of E. histolytica migration and hepatic cells layer crossing in the 3D liver models. Moreover, replication of HBV virus occurs in our virus infection 3D liver model, indicating that routine in vitro assays using HBV or others viruses can be performed in this easy-to-build but more physiological hepatic environment. These results illustrate that our new 3D liver infection models are simple but effective, enabling new investigations on infectious disease mechanisms. The better understanding of these mechanisms in a human-relevant environment could aid the discovery of drugs against pathogenic liver infection.

Micropatterned coculture of primary human hepatocytes and supportive cells for the study of hepatotropic pathogens

The development of therapies and vaccines for human hepatropic pathogens requires robust model systems that enable the study of host-pathogen interactions. However, in vitro liver models of infection typically use either hepatoma cell lines that exhibit aberrant physiology or primary human hepatocytes in culture conditions in which they rapidly lose their hepatic phenotype. To achieve stable and robust in vitro primary human hepatocyte models, we developed micropatterned cocultures (MPCCs), which consist of primary human hepatocytes organized into 2D islands that are surrounded by supportive fibroblast cells. By using this system, which can be established over a period of days, and maintained over multiple weeks, we demonstrate how to recapitulate in vitro hepatic life cycles for the hepatitis B and C viruses and the Plasmodium pathogens P. falciparum and P. vivax. The MPCC platform can be used to uncover aspects of host-pathogen interactions, and it has the potential to be used for drug and vaccine development.

Flow Cytometry Based Detection and Isolation of Plasmodium falciparum Liver Stages In Vitro

Malaria, the disease caused by Plasmodium parasites, remains a major global health burden. The liver stage of Plasmodium falciparum infection is a leading target for immunological and pharmacological interventions. Therefore, novel approaches providing specific detection and isolation of live P. falciparum exoerythrocytic forms (EEFs) are warranted. Utilizing a recently generated parasite strain expressing green fluorescent protein (GFP) we established a method which, allows for detection and isolation of developing live P. falciparum liver stages by flow cytometry. Using this technique we compared the susceptibility of five immortalized human hepatocyte cell lines and primary hepatocyte cultures from three donors to infection by P. falciparum sporozoites. Here, we show that EEFs can be detected and isolated from in vitro infected cultures of the HC-04 cell line and primary human hepatocytes. We confirmed the presence of developing parasites in sorted live human hepatocytes and characterized their morphology by fluorescence microscopy. Finally, we validated the practical applications of our approach by re-examining the importance of host ligand CD81 for hepatocyte infection by P. falciparum sporozoites in vitro and assessment of the inhibitory activity of anti-sporozoite antibodies. This methodology provides us with the tools to study both, the basic biology of the P. falciparum liver stage and the effects of host-derived factors on the development of P. falciparum EEFs.

A novel mouse model for stable engraftment of a human immune system and human hepatocytes

Hepatic infections by hepatitis B virus (HBV), hepatitis C virus (HCV) and Plasmodium parasites leading to acute or chronic diseases constitute a global health challenge. The species tropism of these hepatotropic pathogens is restricted to chimpanzees and humans, thus model systems to study their pathological mechanisms are severely limited. Although these pathogens infect hepatocytes, disease pathology is intimately related to the degree and quality of the immune response. As a first step to decipher the immune response to infected hepatocytes, we developed an animal model harboring both a human immune system (HIS) and human hepatocytes (HUHEP) in BALB/c Rag2-/- IL-2Rγc-/- NOD.sirpa uPAtg/tg mice. The extent and kinetics of human hepatocyte engraftment were similar between HUHEP and HIS-HUHEP mice. Transplanted human hepatocytes were polarized and mature in vivo, resulting in 20-50% liver chimerism in these models. Human myeloid and lymphoid cell lineages developed at similar frequencies in HIS and HIS-HUHEP mice, and splenic and hepatic compartments were humanized with mature B cells, NK cells and naïve T cells, as well as monocytes and dendritic cells. Taken together, these results demonstrate that HIS-HUHEP mice can be stably (> 5 months) and robustly engrafted with a humanized immune system and chimeric human liver. This novel HIS-HUHEP model provides a platform to investigate human immune responses against hepatotropic pathogens and to test novel drug strategies or vaccine candidates.

CD81 is required for rhoptry discharge during host cell invasion by Plasmodium yoelii sporozoites

Plasmodium sporozoites are transmitted by Anopheles mosquitoes and first infect the liver of their mammalian host, where they develop as liver stages before the onset of erythrocytic infection and malaria symptoms. Sporozoite entry into hepatocytes is an attractive target for anti-malarial prophylactic strategies but remains poorly understood at the molecular level. Apicomplexan parasites invade host cells by forming a parasitophorous vacuole that is essential for parasite development, a process that involves secretion of apical organelles called rhoptries. We previously reported that the host membrane protein CD81 is required for infection by Plasmodium falciparum and Plasmodium yoelii sporozoites. CD81 acts at an early stage of infection, possibly at the entry step, but the mechanisms involved are still unknown. To investigate the role of CD81 during sporozoite entry, we generated transgenic P. yoelii parasites expressing fluorescent versions of three known rhoptry proteins, RON2, RON4 and RAP2/3. We observed that RON2 and RON4 are lost following rhoptry discharge during merozoite and sporozoite entry. In contrast, our data indicate that RAP2/3 is secreted into the parasitophorous vacuole during infection. We further show that sporozoite rhoptry discharge occurs only in the presence of CD81, providing the first direct evidence for a role of CD81 during sporozoite productive invasion.

A cysteine protease inhibitor of plasmodium berghei is essential for exo-erythrocytic development

Plasmodium parasites express a potent inhibitor of cysteine proteases (ICP) throughout their life cycle. To analyze the role of ICP in different life cycle stages, we generated a stage-specific knockout of the Plasmodium berghei ICP (PbICP). Excision of the pbicb gene occurred in infective sporozoites and resulted in impaired sporozoite invasion of hepatocytes, despite residual PbICP protein being detectable in sporozoites. The vast majority of these parasites invading a cultured hepatocyte cell line did not develop to mature liver stages, but the few that successfully developed hepatic merozoites were able to initiate a blood stage infection in mice. These blood stage parasites, now completely lacking PbICP, exhibited an attenuated phenotype but were able to infect mosquitoes and develop to the oocyst stage. However, PbICP-negative sporozoites liberated from oocysts exhibited defective motility and invaded mosquito salivary glands in low numbers. They were also unable to invade hepatocytes, confirming that control of cysteine protease activity is of critical importance for sporozoites. Importantly, transfection of PbICP-knockout parasites with a pbicp-gfp construct fully reversed these defects. Taken together, in P. berghei this inhibitor of the ICP family is essential for sporozoite motility but also appears to play a role during parasite development in hepatocytes and erythrocytes.

D-Glucose concentration is the key factor facilitating liver stage maturation of Plasmodium

The course of malaria infection in mammals begins with transmission of Plasmodium sporozoites into the skin by Anopheles mosquitoes, followed by migration of the sporozoites to the liver. As no symptoms present until hepatic merozoites are released and until they infect erythrocytes in the blood vessels, sporozoites and liver-stage (LS) parasites are promising targets for anti-malaria drugs aiming to prevent mosquito-to-mammal transmission. In vitro LS parasite development system is useful in the screening of candidate drugs on LS parasite development and the elucidation of its underlying molecular mechanisms, which remain unclear. Using rodent malaria parasites (Plasmodium berghei) as a model, this study aimed to develop an optimal in vitro LS culture system for the full maturation of the LS parasite into the hepatic merozoite, the next infective stage in parasite development. As the development of this system required measurement of maturation, a novel quantitative index of LS parasite maturation based on the expression pattern of liver-specific protein 2 (LISP2) was first developed. The use of this index for comparing the effect of incubation in different culture media on LS maturation revealed that the d-glucose concentration of the culture medium is the key factor promoting parasite development in hepatocytes and that a d-glucose concentration of 2000mg/L/day is the threshold concentration at which the maturation of P. berghei into infective hepatic merozoites is achieved. These findings can be utilized to optimize a human malaria LS culture system for drug discovery.