A toolbox to study liver stage malaria

Differential effects of translation inhibitors on Plasmodium berghei liver stage parasites

Increasing numbers of antimalarial compounds are being identified that converge mechanistically at inhibition of cytoplasmic translation, regardless of the molecular target or mechanism. A deeper understanding of how their effectiveness as liver stage translation inhibitors relates to their chemoprotective potential could prove useful. Here, we probed that relationship using the Plasmodium berghei-HepG2 liver stage infection model. After determining translation inhibition EC50s for five compounds, we tested them at equivalent effective concentrations to compare the parasite response to, and recovery from, a brief period of translation inhibition in early schizogony, followed by parasites to 120 h post-infection to assess antiplasmodial effects of the treatment. We show compound-specific heterogeneity in single parasite and population responses to translation inhibitor treatment, with no single metric strongly correlated to the release of hepatic merozoites for all compounds. We also demonstrate that DDD107498 is capable of exerting antiplasmodial effects on translationally arrested liver stage parasites and uncover unexpected growth dynamics during the liver stage. Our results demonstrate that translation inhibition efficacy does not determine antiplasmodial efficacy for these compounds.

Disclosure of cinnamic acid/4,9-diaminoacridine conjugates as multi-stage antiplasmodial hits

4,9-diaminoacridines with reported antiplasmodial activity were coupled to different trans-cinnamic acids, delivering a new series of conjugates inspired by the covalent bitherapy concept. The new compounds were more potent than primaquine against hepatic stages of Plasmodium berghei, although this was accompanied by cytotoxic effects on Huh-7 hepatocytes. Relevantly, the conjugates displayed nanomolar activities against blood stage P. falciparum parasites, with no evidence of hemolytic effects below 100 µM. Moreover, the new compounds were at least 25-fold more potent than primaquine against P. falciparum gametocytes. Thus, the new antiplasmodial hits disclosed herein emerge as valuable templates for the development of multi-stage antiplasmodial drug candidates.

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.

Translation inhibition efficacy does not determine the Plasmodium berghei liver stage antiplasmodial efficacy of protein synthesis inhibitors

Protein synthesis is a core cellular process, necessary throughout the complex lifecycle of Plasmodium parasites, thus specific translation inhibitors would be a valuable class of antimalarial drugs, capable of both treating symptomatic infections in the blood and providing chemoprotection by targeting the initial parasite population in the liver, preventing both human disease and parasite transmission back to the mosquito host. As increasing numbers of antiplasmodial compounds are identified that converge mechanistically at inhibition of cytoplasmic translation, regardless of molecular target or mechanism, it would be useful to gain deeper understanding of how their effectiveness as liver stage translation inhibitors relates to their chemoprotective potential. Here, we probed that relationship using the P. berghei-HepG2 liver stage infection model. Using o-propargyl puromycin-based labeling of the nascent proteome in P. berghei-infected HepG2 monolayers coupled with automated confocal feedback microscopy to generate unbiased, single parasite image sets of P. berghei liver stage translation, we determined translation inhibition EC50s for five compounds, encompassing parasite-specific aminoacyl tRNA synthetase inhibitors, compounds targeting the ribosome in both host and parasite, as well as DDD107498, which targets Plasmodium eEF2, and is a leading antimalarial candidate compound being clinically developed as cabamiquine. Compounds were then tested at equivalent effective concentrations to compare the parasite response to, and recovery from, a brief period of translation inhibition in early schizogony, with parasites followed up to 120 hours post-infection to assess liver stage antiplasmodial effects of the treatment. Our data conclusively show that translation inhibition efficacy per se does not determine a translation inhibitor's antiplasmodial efficacy. DDD107498 was the least effective translation inhibitor, yet exerted the strongest antimalarial effects at both 5x- and 10x EC50 concentrations. We show compound-specific heterogeneity in single parasite and population responses to translation inhibitor treatment, with no single metric strongly correlated to release of hepatic merozoites for all compound, demonstrate that DDD107498 is capable of exerting antiplasmodial effects on translationally arrested liver stage parasites, and uncover unexpected growth dynamics during the liver stage. Our results demonstrate that translation inhibition efficacy cannot function as a proxy for antiplasmodial effectiveness, and highlight the importance of exploring the ultimate, as well as proximate, mechanisms of action of these compounds on liver stage parasites.

Comparative analyses of functional antibody-mediated inhibition with anti-circumsporozoite monoclonal antibodies against transgenic Plasmodium berghei

BACKGROUND

Acquired functional inhibitory antibodies are one of several humoral immune mechanisms used to neutralize foreign pathogens. In vitro bioassays are useful tools for quantifying antibody-mediated inhibition and evaluating anti-parasite immune antibodies. However, a gap remains in understanding of how antibody-mediated inhibition in vitro translates to inhibition in vivo. In this study, two well-characterized transgenic Plasmodium berghei parasite lines, PbmCh-luc and Pb-PfCSP(r), and murine monoclonal antibodies (mAbs) specific to P. berghei and Plasmodium falciparum circumsporozoite protein (CSP), 3D11 and 2A10, respectively, were used to evaluate antibody-mediated inhibition of parasite development in both in vitro and in vivo functional assays.

METHODS

IC50 values of mAbs were determined using an established inhibition of liver-stage development assay (ILSDA). For the in vivo inhibition assay, mice were passively immunized by transfer of the mAbs and subsequently challenged with 5.0 × 103 sporozoites via tail vein injection. The infection burden in both assays was quantified by luminescence and qRT-PCR of P. berghei 18S rRNA normalized to host GAPDH.

RESULTS

The IC50 values quantified by relative luminescence of mAbs 3D11 and 2A10 were 0.396 µg/ml and 0.093 µg/ml, respectively, against transgenic lines in vitro. Using the highest (> 90%) inhibitory antibody concentrations in a passive transfer, an IC50 of 233.8 µg/ml and 181.5 µg/ml for mAbs 3D11 and 2A10, respectively, was observed in vivo. At 25 µg (250 µg/ml), the 2A10 antibody significantly inhibited liver burden in mice compared to control. Additionally, qRT-PCR of P. berghei 18S rRNA served as a secondary validation of liver burden quantification.

CONCLUSIONS

Results from both experimental models, ILSDA and in vivo challenge, demonstrated that increased concentrations of the homologous anti-CSP repeat mAbs increased parasite inhibition. However, differences in antibody IC50 values between parasite lines did not allow a direct correlation between the inhibition of sporozoite invasion in vitro by ILSDA and the inhibition of mouse liver stage burden. Further studies are needed to establish the conditions for confident predictions for the in vitro ILSDA to be a predictor of in vivo outcomes using this model system.

Identification of disease-related genes in Plasmodium berghei by network module analysis

BACKGROUND

Plasmodium berghei has been used as a preferred model for studying human malaria, but only a limited number of disease-associated genes of P. berghei have been reported to date. Identification of new disease-related genes as many as possible will provide a landscape for better understanding the pathogenesis of P. berghei.

METHODS

Network module analysis method was developed and applied to identify disease-related genes in P. berghei genome. Sequence feature identification, gene ontology annotation, and T-cell epitope analysis were performed on these genes to illustrate their functions in the pathogenesis of P. berghei.

RESULTS

33,314 genes were classified into 4,693 clusters. 4,127 genes shared by six malaria parasites were identified and are involved in many aspects of biological processes. Most of the known essential genes belong to shared genes. A total of 63 clusters consisting of 405 P. berghei genes were enriched in rodent malaria parasites. These genes participate in various stages of parasites such as liver stage development and immune evasion. Combination of these genes might be responsible for P. berghei infecting mice. Comparing with P. chabaudi, none of the clusters were specific to P. berghei. P. berghei lacks some proteins belonging to P. chabaudi and possesses some specific T-cell epitopes binding by class-I MHC, which might together contribute to the occurrence of experimental cerebral malaria (ECM).

CONCLUSIONS

We successfully identified disease-associated P. berghei genes by network module analysis. These results will deepen understanding of the pathogenesis of P. berghei and provide candidate parasite genes for further ECM investigation.

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

Plasmodium parasite resistance to existing antimalarial drugs poses a devastating threat to the lives of many who depend on their efficacy. New antimalarial drugs and novel drug targets are in critical need, along with novel assays to accelerate their identification. Given the essentiality of protein synthesis throughout the complex parasite lifecycle, translation inhibitors are a promising drug class, capable of targeting the disease-causing blood stage of infection, as well as the asymptomatic liver stage, a crucial target for prophylaxis. To identify compounds capable of inhibiting liver stage parasite translation, we developed an assay to visualize and quantify translation in the P. berghei-HepG2 infection model. After labeling infected monolayers with o-propargyl puromycin (OPP), a functionalized analog of puromycin permitting subsequent bioorthogonal addition of a fluorophore to each OPP-terminated nascent polypetide, we use automated confocal feedback microscopy followed by batch image segmentation and feature extraction to visualize and quantify the nascent proteome in individual P. berghei liver stage parasites and host cells simultaneously. After validation, we demonstrate specific, concentration-dependent liver stage translation inhibition by both parasite-selective and pan-eukaryotic active compounds, and further show that acute pre-treatment and competition modes of the OPP assay can distinguish between direct and indirect translation inhibitors. We identify a Malaria Box compound, MMV019266, as a direct translation inhibitor in P. berghei liver stages and confirm this potential mode of action in P. falciparum asexual blood stages.

Conjugated carbon monoxide-releasing molecules have broad-spectrum antimicrobial activity

Aim: To test the antimicrobial effect of carbon monoxide-releasing molecules (CORMs) conjugated with azoles on different microorganisms. Methods & results: We used broth microdilution, checkerboard and cytotoxicity assays, as well as imaging, fluorescence and bioluminescence experiments to study [Re(CO)3(2,2'-bipyridyl)(Ctz)]+ (also known as ReBpyCtz). ReBpyCtz exhibits a low minimum inhibitory concentration value, increases the intracellular formation of reactive oxygen species and causes significant alterations on Staphylococcus aureus's membrane. ReBpyCtz is active against fungi, having a more prolonged fungicidal effect on Candida glabrata than clotrimazole and is selectively active on blood-stage malaria parasites, at a concentration that is not toxic to kidney epithelial cells. Conclusion: Conjugated CORMs have the potential to be active against different types of pathogens, thus constituting a promising class of broad-spectrum antimicrobials.

CRISPR/Cas advancements for genome editing, diagnosis, therapeutics, and vaccine development for Plasmodium parasites, and genetic engineering of Anopheles mosquito vector

Malaria as vector-borne disease remains important health concern with over 200 million cases globally. Novel antimalarial medicines and more effective vaccines must be developed to eliminate and eradicate malaria. Appraisal of preceding genome editing approaches confirmed the CRISPR/Cas nuclease system as a novel proficient genome editing system and a tool for species-specific diagnosis, and drug resistance researches for Plasmodium species, and gene drive to control Anopheles population. CRISPR/Cas technology, as a handy tool for genome editing can be justified for the production of transgenic malaria parasites like Plasmodium transgenic lines expressing Cas9, chimeric Plasmodium transgenic lines, knockdown and knockout transgenic parasites, and transgenic parasites expressing alternative alleles, and also mutant strains of Anopheles such as only male mosquito populations, generation of wingless mosquitoes, and creation of knock-out/ knock-in mutants. Though, the incorporation of traditional methods and novel molecular techniques could noticeably enhance the quality of results. The striking development of a CRISPR/Cas-based diagnostic kit that can specifically diagnose the Plasmodium species or drug resistance markers is highly required in malaria settings with affordable cost and high-speed detection. Furthermore, the advancement of genome modifications by CRISPR/Cas technologies resolves contemporary restrictions to culturing, maintaining, and analyzing these parasites, and the aptitude to investigate parasite genome functions opens up new vistas in the better understanding of pathogenesis.

Block Copolymer Micelles Encapsulating Au(III) Bis(Dithiolene) Complexes as Promising Nanostructures with Antiplasmodial Activity

Block copolymer micelles (BCMs) can be used to improve the solubility of lipophilic drugs and increase their circulation half-life. Hence, BCMs assembled from MePEG-b-PCL were evaluated as drug delivery systems of gold(III) bis(dithiolene) complexes (herein AuS and AuSe) to be employed as antiplasmodial drugs. These complexes exhibited remarkable antiplasmodial activity against liver stages of the Plasmodiumberghei parasite, and low toxicity in a model of zebrafish embryos. To improve the complexes' solubility, BCMs were loaded with AuS, AuSe, and the reference drug primaquine (PQ). PQ-BCMs (D_h = 50.9 ± 2.8 nm), AuSe-BCMs (D_h = 87.1 ± 9.7 nm), and AuS-BCMs (D_h = 72.8 ± 3.1 nm) were obtained with a loading efficiency of 82.5%, 55.5%, and 77.4%, respectively. HPLC analysis and UV-Vis spectrophotometry showed that the compounds did not suffer degradation after encapsulation in BCMs. In vitro release studies suggest that AuS/AuSe-BCMs present a more controlled release compared with PQ-loaded BCMs. The antiplasmodial hepatic activity of the drugs was assessed in vitro and results indicate that both complexes present higher inhibitory activity than PQ, although encapsulated AuS and AuSe presented lower activity than their non-encapsulated counterparts. Nevertheless, these results suggest that the use of BCMs as delivery vehicles for lipophilic metallodrugs, particularly AuS and AuSe, could enable the controlled release of complexes and improve their biocompatibility, constituting a promising alternative to conventional antimalarial treatments.

Derivatives of Amaryllidaceae Alkaloid Ambelline as Selective Inhibitors of Hepatic Stage of Plasmodium berghei Infection In Vitro

The incidence rate of malaria and the ensuing mortality prompts the development of novel antimalarial drugs. In this work, the activity of twenty-eight Amaryllidaceae alkaloids (1-28) belonging to seven different structural types was assessed, as well as twenty semisynthetic derivatives of the β-crinane alkaloid ambelline (28a-28t) and eleven derivatives of the α-crinane alkaloid haemanthamine (29a-29k) against the hepatic stage of Plasmodium infection. Six of these derivatives (28h, 28m, 28n and 28r-28t) were newly synthesized and structurally identified. The most active compounds, 11-O-(3,5-dimethoxybenzoyl)ambelline (28m) and 11-O-(3,4,5-trimethoxybenzoyl)ambelline (28n), displayed IC₅₀ values in the nanomolar range of 48 and 47 nM, respectively. Strikingly, the derivatives of haemanthamine (29) with analogous substituents did not display any significant activity, even though their structures are quite similar. Interestingly, all active derivatives were strictly selective against the hepatic stage of infection, as they did not demonstrate any activity against the blood stage of Plasmodium infection. As the hepatic stage is a bottleneck of the plasmodial infection, liver-selective compounds can be considered crucial for further development of the malaria prophylactics.

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.

The reciprocal influence of the liver and blood stages of the malaria parasite's life cycle

While the liver and blood stages of the Plasmodium life cycle are commonly regarded as two separate fields of malaria research, several studies have pointed towards the existence of a bidirectional cross-talk, where one stage of mammalian infection may impact the establishment and progression of the other. Despite the constraints in experimentally addressing concurrent liver and blood stage Plasmodium infections, animal models and clinical studies have unveiled a plethora of molecular interactions between the two. Here, we review the current knowledge on the reciprocal influence of hepatic and erythrocytic infection by malaria parasites, and discuss its impacts on immunity, pathology and vaccination against this deadly disease.

Five decades of clinical assessment of whole-sporozoite malaria vaccines

In 1967, pioneering work by Ruth Nussenzweig demonstrated for the first time that irradiated sporozoites of the rodent malaria parasite Plasmodium berghei protected mice against a challenge with infectious parasites of the same species. This remarkable finding opened up entirely new prospects of effective vaccination against malaria using attenuated sporozoites as immunization agents. The potential for whole-sporozoite-based immunization in humans was established in a clinical study in 1973, when a volunteer exposed to X-irradiated P. falciparum sporozoites was found to be protected against malaria following challenge with a homologous strain of this parasite. Nearly five decades later, much has been achieved in the field of whole-sporozoite malaria vaccination, and multiple reports on the clinical evaluation of such candidates have emerged. However, this process has known different paces before and after the turn of the century. While only a few clinical studies were published in the 1970’s, 1980’s and 1990’s, remarkable progress was made in the 2000’s and beyond. This article reviews the history of the clinical assessment of whole-sporozoite malaria vaccines over the last forty-nine years, highlighting the impressive achievements made over the last few years, and discussing some of the challenges ahead.

In vitro models for human malaria: targeting the liver stage

The Plasmodium liver stage represents a vulnerable therapeutic target to prevent disease progression as the parasite resides in the liver before clinical representation caused by intraerythrocytic development. However, most antimalarial drugs target the blood stage of the parasite's life cycle, and the few drugs that target the liver stage are lethal to patients with a glucose-6-phosphate dehydrogenase deficiency. Furthermore, implementation of in vitro liver models to study and develop novel therapeutics against the liver stage of human Plasmodium species remains challenging. In this review, we focus on the progression of in vitro liver models developed for human Plasmodium spp. parasites, provide a brief review on important assay requirements, and lastly present recommendations to improve models to enhance the discovery process of novel preclinical therapeutics.

Preparation of Decoquinate Solid Dispersion by Hot-Melt Extrusion as an Oral Dosage Form Targeting Liver-Stage Plasmodium Infection

Liver-stage Plasmodium in humans is an early stage of malarial infection. Decoquinate (DQ) has a potent multistage antimalarial activity. However, it is practically water insoluble. In this study, the hot-melt extrusion (HME) approach was employed to prepare solid dispersions of DQ to improve oral bioavailability. The DQ dispersions were homogeneous in an aqueous suspension that contained most DQ (>90%) in the aqueous phase. Soluplus, a solubilizer, was found compatible with DQ in forming nanoparticle formulations during the HME process. Another excipient HPMC AS-126 was also proven to be suitable for making DQ nanoparticles through HME. Particle size and antimalarial activity of HME DQ suspensions remained almost unchanged after storage at 4°C for over a year. HME DQ was highly effective at inhibiting Plasmodium infection in vitro at both the liver stage and blood stage. HME DQ at 3 mg/kg by oral administration effectively prevented Plasmodium infection in mice inoculated with Plasmodium berghei sporozoites. Orally administered HME DQ at 2,000 mg/kg to mice showed no obvious adverse effects. HME DQ at 20 mg/kg orally administered to rats displayed characteristic distributions of DQ in the blood with most DQ in the blood cells, revealing the permeability of HME DQ into the cells in relation to its antimalarial activity. The DQ dispersions may be further developed as an oral formulation targeting Plasmodium infection at the liver stage.

Impact of Dietary Protein Restriction on the Immunogenicity and Efficacy of Whole-Sporozoite Malaria Vaccination

Malaria remains one of the world’s most prevalent infectious diseases. Several vaccination strategies currently under investigation aim at hampering the development of the Plasmodium parasite during the clinically silent liver stage of its life cycle in the mammalian host, preventing the subsequent disease-associated blood stage of infection. Immunization with radiation-attenuated sporozoites (RAS), the liver-infecting parasite forms, can induce sterile protection against malaria. However, the efficacy of vaccine candidates in malaria-naïve individuals in high-income countries is frequently higher than that found in populations where malaria is endemic. Malnutrition has been associated with immune dysfunction and with a delay or impairment of the immune response to some vaccines. Since vaccine efficacy depends on the generation of competent immune responses, and malaria-endemic regions are often associated with malnutrition, we hypothesized that an inadequate host nutritional status, specifically resulting from a reduction in dietary protein, could impact on the establishment of an efficient anti-malarial immune response. We developed a model of RAS immunization under low protein diet to investigate the impact of a reduced host protein intake on the immunogenicity and protective efficacy of this vaccine. Our analysis of the circulating and tissue-associated immune compartments revealed that a reduction in dietary protein intake during immunization resulted in a decrease in the frequency of circulating CD4+ T cells and of hepatic NK cells. Nevertheless, the profile of CD8+ T cells in the blood, liver and spleen was robust and minimally affected by the dietary protein content during RAS immunization, as assessed by supervised and in-depth unsupervised X-shift clustering analysis. Although mice immunized under low protein diet presented higher parasite liver load upon challenge than those immunized under adequate protein intake, the two groups displayed similar levels of protection from disease. Overall, our data indicate that dietary protein reduction may have minimal impact on the immunogenicity and efficacy of RAS-based malaria vaccination. Importantly, this experimental model can be extended to assess the impact of other nutrient imbalances and immunization strategies, towards the refinement of future translational interventions that improve vaccine efficacy in malnourished individuals.

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.

Elucidating Spatially-Resolved Changes in Host Signaling During Plasmodium Liver-Stage Infection

Upon transmission to the human host, Plasmodium sporozoites exit the skin, are taken up by the blood stream, and then travel to the liver where they infect and significantly modify a single hepatocyte. Low infection rates within the liver have made proteomic studies of infected hepatocytes challenging, particularly in vivo, and existing studies have been largely unable to consider how protein and phosphoprotein differences are altered at different spatial locations within the heterogeneous liver. Using digital spatial profiling, we characterized changes in host signaling during Plasmodium yoelii infection in vivo without disrupting the liver tissue. Moreover, we measured alterations in protein expression around infected hepatocytes and identified a subset of CD163⁺ Kupffer cells that migrate towards infected cells during infection. These data offer the first insight into the heterogeneous microenvironment that surrounds the infected hepatocyte and provide insights into how the parasite may alter its milieu to influence its survival and modulate immunity.

Structural activity relationship of metallo-aminoquines as a next generation antimalarials

Apicomplexian parasite of the genus Plasmodium is the causative agent of malaria, one of the most devastating, furious and common infectious disease throughout the world. According to the latest World malaria report, there were 229 million cases of malaria in 2019 majorly consisting of children under 5 years of age. Some of known analogues viz. quinine, quinoline-containing compounds have been used for last century in the clinical treatment of malaria. Past few decades have witnessed the emergence of multi-drug resistance (MDR) strains of Plasmodium species to existing antimalarials pressing the need for new drug candidates. For the past few decades bioorganometallic approach to malaria therapy has been introduced which led to the discovery of noval metalcontaining aminoquinolines analogues viz. ferroquine (FQ or 1), Ruthenoquine (RQ or 2) and other related potent metal-analogues. It observed that some metal containing analogues (Fe-, Rh-, Ru-, Re-, Au-, Zn-, Cr-, Pd-, Sn-, Cd-, Ir-, Co-, Cu-, and Mn-aminoquines) were more potent; however, some were equally potent as Chloroquine (CQ) and 1. This is probably due to the intertion of metals in the CQ via various approaches, which might be a very attractive strategy to develop a SAR of novel metal containing antimalarials. Thus, this review aims to summarize the SAR of metal containing aminoquines towards the discovery of potent antimalarial hybrids to provide an insight for rational designs of more effective and less toxic metal containing amoniquines.

Synthesis of the new analogs of morpholine and their antiplasmodial evaluation against the human malaria parasite Plasmodium falciparum

A series of morpholine analogs functionalized with hydroxyethylamine (HEA) pharmacophore was synthesized and assayed for the initial screening against Plasmodium falciparum 3D7 in culture, which suggested that analog 6k is a hit molecule with an inhibitory concentration of 5.059 ± 0.2036 μM.

Suppression of Plasmodium MIF-CD74 signaling protects against severe malaria

The deadliest complication of infection by Plasmodium parasites, cerebral malaria, accounts for the majority of malarial fatalities. Although our understanding of the cellular and molecular mechanisms underlying the pathology remains incomplete, recent studies support the contribution of systemic and neuroinflammation as the cause of cerebral edema and blood-brain barrier (BBB) dysfunction. All Plasmodium species encode an orthologue of the innate cytokine, Macrophage Migration Inhibitory Factor (MIF), which functions in mammalian biology to regulate innate responses. Plasmodium MIF (PMIF) similarly signals through the host MIF receptor CD74, leading to an enhanced inflammatory response. We investigated the PMIF-CD74 interaction in the onset of experimental cerebral malaria (ECM) and liver stage Plasmodium development by using a combination of CD74 deficient (Cd74^-/- ) hosts and PMIF deficient parasites. Cd74^-/- mice were found to be protected from ECM and the protection was associated with the inability of brain microvessels to present parasite antigen to sequestered and pathogenic Plasmodium-specific CD8⁺ T cells. Infection of WT hosts with PMIF-deficient sporozoites or infection of Cd74^-/- hosts with WT sporozoites impacted the survival of infected hepatocytes and subsequently reduced blood-stage associated inflammation, contributing to protection from ECM. We recapitulated these finding with a novel pharmacologic PMIF-selective antagonist that reduced PMIF/CD74 signaling and fully protected mice from ECM. These findings reveal a conserved mechanism for Plasmodium usurpation of host CD74 signaling and suggest a tractable approach for new pharmacologic intervention.

A New Thienopyrimidinone Chemotype Shows Multistage Activity against Plasmodium falciparum, Including Artemisinin-Resistant Parasites

Human malaria infection begins with a one-time asymptomatic liver stage followed by a cyclic symptomatic blood stage. For decades, the research for novel antimalarials focused on the high-throughput screening of molecules that only targeted the asexual blood stages. In a search for new effective compounds presenting a triple action against erythrocytic and liver stages in addition to the ability to block the transmission of the disease via the mosquito vector, 2-amino-thienopyrimidinone derivatives were synthesized and tested for their antimalarial activity. One molecule, named gamhepathiopine (denoted as “M1” herein), was active at submicromolar concentrations against both erythrocytic (50% effective concentration [EC₅₀] = 0.045 μM) and liver (EC₅₀ = 0.45 μM) forms of Plasmodium falciparum. Furthermore, gamhepathiopine efficiently blocked the development of the sporogonic cycle in the mosquito vector by inhibiting the exflagellation step. Moreover, M1 was active against artemisinin-resistant forms (EC₅₀ = 0.227 μM), especially at the quiescent stage. Nevertheless, in mice, M1 showed modest activity due to its rapid metabolization by P450 cytochromes into inactive derivatives, calling for the development of new parent compounds with improved metabolic stability and longer half-lives. These results highlight the thienopyrimidinone scaffold as a novel antiplasmodial chemotype of great interest to search for new drug candidates displaying multistage activity and an original mechanism of action with the potential to be used in combination therapies for malaria elimination in the context of artemisinin resistance.

IMPORTANCE This work reports a new chemical structure that (i) displays activity against the human malaria parasite Plasmodium falciparum at 3 stages of the parasitic cycle (blood stage, hepatic stage, and sexual stages), (ii) remains active against parasites that are resistant to the first-line treatment recommended by the World Health Organization (WHO) for the treatment of severe malaria (artemisinins), and (iii) reduces transmission of the parasite to the mosquito vector in a mouse model. This new molecule family could open the way to the conception of novel antimalarial drugs with an original multistage mechanism of action to fight against Plasmodium drug resistance and block interhuman transmission of malaria.

Gold(iii) bis(dithiolene) complexes: from molecular conductors to prospective anticancer, antimicrobial and antiplasmodial agents

The anticancer, antimicrobial and antiplasmodial activities of six gold(iii) bis(dithiolene) complexes were studied. Complexes 1-6 showed relevant anticancer properties against A2780/A2780cisR ovarian cancer cells (IC50 values of 0.08-2 μM), also being able to overcome cisplatin resistance in A2780cisR cells. Complex 1 also exhibited significant antimicrobial activity against Staphylococcus aureus (minimum inhibitory concentration (MIC) values of 12.1 ± 3.9 μg mL-1) and both Candida glabrata and Candida albicans (MICs of 9.7 ± 2.7 and 19.9 ± 2.4 μg mL-1, respectively). In addition, all complexes displayed antiplasmodial activity against the Plasmodium berghei parasite liver stages, even exhibiting better results than the ones obtained using primaquine, an anti-malarial drug. Mechanistic studies support the idea that thioredoxin reductase, but not DNA, is a possible target of these complexes. Complex 1 is stable under biological conditions, which would be important if this compound is ever to be considered as a drug. Overall, the results obtained evidenced the promising biological activity of complex 1, which might have potential as a novel anticancer, antimicrobial and antiplasmodial agent to be used as an alternative to current therapeutics.

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.

Plasmodium vinckei genomes provide insights into the pan-genome and evolution of rodent malaria parasites

Background

Rodent malaria parasites (RMPs) serve as tractable tools to study malaria parasite biology and host-parasite-vector interactions. Among the four RMPs originally collected from wild thicket rats in sub-Saharan Central Africa and adapted to laboratory mice, Plasmodium vinckei is the most geographically widespread with isolates collected from five separate locations. However, there is a lack of extensive phenotype and genotype data associated with this species, thus hindering its use in experimental studies.

Results

We have generated a comprehensive genetic resource for P. vinckei comprising of five reference-quality genomes, one for each of its subspecies, blood-stage RNA sequencing data for five P. vinckei isolates, and genotypes and growth phenotypes for ten isolates. Additionally, we sequenced seven isolates of the RMP species Plasmodium chabaudi and Plasmodium yoelii, thus extending genotypic information for four additional subspecies enabling a re-evaluation of the genotypic diversity and evolutionary history of RMPs.

The five subspecies of P. vinckei have diverged widely from their common ancestor and have undergone large-scale genome rearrangements. Comparing P. vinckei genotypes reveals region-specific selection pressures particularly on genes involved in mosquito transmission. Using phylogenetic analyses, we show that RMP multigene families have evolved differently across the vinckei and berghei groups of RMPs and that family-specific expansions in P. chabaudi and P. vinckei occurred in the common vinckei group ancestor prior to speciation. The erythrocyte membrane antigen 1 and fam-c families in particular show considerable expansions among the lowland forest-dwelling P. vinckei parasites. The subspecies from the highland forests of Katanga, P. v. vinckei, has a uniquely smaller genome, a reduced multigene family repertoire and is also amenable to transfection making it an ideal parasite for reverse genetics. We also show that P. vinckei parasites are amenable to genetic crosses.

Conclusions

Plasmodium vinckei isolates display a large degree of phenotypic and genotypic diversity and could serve as a resource to study parasite virulence and immunogenicity. Inclusion of P. vinckei genomes provide new insights into the evolution of RMPs and their multigene families. Amenability to genetic crossing and transfection make them also suitable for classical and functional genetics to study Plasmodium biology.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-00995-5.

Unraveling Cell Death Pathways during Malaria Infection: What Do We Know So Far?

Malaria is a parasitic disease (caused by different Plasmodium species) that affects millions of people worldwide. The lack of effective malaria drugs and a vaccine contributes to this disease, continuing to cause major public health and socioeconomic problems, especially in low-income countries. Cell death is implicated in malaria immune responses by eliminating infected cells, but it can also provoke an intense inflammatory response and lead to severe malaria outcomes. The study of the pathophysiological role of cell death in malaria in mammalians is key to understanding the parasite-host interactions and design prophylactic and therapeutic strategies for malaria. In this work, we review malaria-triggered cell death pathways (apoptosis, autophagy, necrosis, pyroptosis, NETosis, and ferroptosis) and we discuss their potential role in the development of new approaches for human malaria therapies.

An insight into the recent development of the clinical candidates for the treatment of malaria and their target proteins

Malaria is an endemic disease, prevalent in tropical and subtropical regions which cost half of million deaths annually. The eradication of malaria is one of the global health priority nevertheless, current therapeutic efforts seem to be insufficient due to the emergence of drug resistance towards most of the available drugs, even first-line treatment ACT, unavailability of the vaccine, and lack of drugs with a new mechanism of action. Intensification of antimalarial research in recent years has resulted into the development of single dose multistage therapeutic agents which has advantage of overcoming the antimalarial drug resistance. The present review explored the current progress in the development of new promising antimalarials against prominent target proteins that have the potential to be a clinical candidate. Here, we also reviewed different aspects of drug resistance and highlighted new drug candidates that are currently in a clinical trial or clinical development, along with a few other molecules with excellent antimalarial activity overs ACTs. The summarized scientific value of previous approaches and structural features of antimalarials related to the activity are highlighted that will be helpful for the development of next-generation antimalarials.

Half-Sandwich Cyclopentadienylruthenium(II) Complexes: A New Antimalarial Chemotype

A small library of "half-sandwich" cyclopentadienylruthenium(II) compounds of the general formula [(η⁵-C₅R₅)Ru(PPh₃)(N-N)][PF₆], a scaffold hitherto absent from the toolbox of antiplasmodials, was screened for activity against the blood stage of CQ-sensitive 3D7-GFP, CQ-resistant Dd2, and artemisinin-resistant IPC5202 Plasmodium falciparum strains and the liver stage of Plasmodium berghei. The best-performing compounds displayed dual-stage activity, with single-digit nanomolar IC₅₀ values against blood-stage malaria parasites, nanomolar activity against liver-stage parasites, and residual cytotoxicity against HepG2 and Huh7 mammalian cells. The parasitic absorption/distribution of 7-nitrobenzoxadiazole-appended fluorescent compounds Ru4 and Ru5 was investigated by confocal fluorescence microscopy, revealing parasite-selective absorption in infected erythrocytes and nuclear accumulation of both compounds. The lead compound Ru2 impaired asexual parasite differentiation, exhibiting fast parasiticidal activity against both ring and trophozoite stages of a synchronized culture of the P. falciparum 3D7 strain. These results point to cyclopentadienylruthenium(II) complexes as a highly promising chemotype for the development of dual-stage antiplasmodials.

In vitro Cultivation and Visualization of Malaria Liver Stages in Primary Simian Hepatocytes

Human liver is the primary and obligatory site for malaria infection where sporozoites invade host hepatocytes. Malaria hepatic stages are asymptomatic and represent an attractive target for development of anti-malarial interventions and vaccines. However, owing to lack of robust and reproducible in vitro culture system, it is difficult to target and study this imperative malaria liver stage. Here, we describe a procedure that allow cultivation and visualization of malaria hepatic stages including dormant hypnozoites using primary simian hepatocytes. This method enables sensitive and quantitative assessment of different hepatic stages in vitro.

Repurposing Drugs to Fight Hepatic Malaria Parasites

Malaria remains one of the most prevalent infectious diseases worldwide, primarily affecting some of the most vulnerable populations around the globe. Despite achievements in the treatment of this devastating disease, there is still an urgent need for the discovery of new drugs that tackle infection by Plasmodium parasites. However, de novo drug development is a costly and time-consuming process. An alternative strategy is to evaluate the anti-plasmodial activity of compounds that are already approved for other purposes, an approach known as drug repurposing. Here, we will review efforts to assess the anti-plasmodial activity of existing drugs, with an emphasis on the obligatory and clinically silent liver stage of infection. We will also review the current knowledge on the classes of compounds that might be therapeutically relevant against Plasmodium in the context of other communicable diseases that are prevalent in regions where malaria is endemic. Repositioning existing compounds may constitute a faster solution to the current gap of prophylactic and therapeutic drugs that act on Plasmodium parasites, overall contributing to the global effort of malaria eradication.

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.

Vancomycin improves Plasmodium yoelii malaria parasite in vitro liver stage cultures by controlling Elizabethkingia anophelis, a bacterium in the microbiome of lab-reared Anopheles mosquitoes

Studies of Plasmodium sporozoites and liver stages require dissection of Anopheles mosquitoes to obtain sporozoites for experiments. Sporozoites from the rodent parasite P. yoelii are routinely used to infect hepatocytes for liver stage culture, but sometimes these cultures become contaminated. Using standard microbiological techniques, a single colony type of Gram-negative rod-shaped bacteria was isolated from contaminated cultures. Mass spectrometry and sequencing of the bacterial 16S ribosomal RNA gene identified the contaminant as Elizabethkingia spp. Based on sequence comparison and published studies of the Anopheles microbiome, the best match was E. anophelis. Culture contamination was not ameliorated by density gradient purification of sporozoites. However, the addition of vancomycin to the culture media consistently reduced contamination and improved culture outcomes as measured by liver stage parasite size. Thus, mosquito salivary gland-derived E. anophelis is identified a potential contaminant of Plasmodium liver stage cultures that can be mitigated by the addition of antibiotics.

Current Challenges in the Identification of Pre-Erythrocytic Malaria Vaccine Candidate Antigens

Plasmodium spp.-infected mosquitos inject sporozoites into the skin of a mammalian host during a blood meal. These enter the host's circulatory system and establish an infection in the liver. After a silent metamorphosis, merozoites invade the blood leading to the symptomatic and transmissible stages of malaria. The silent pre-erythrocytic malaria stage represents a bottleneck in the disease which is ideal to block progression to clinical malaria, through chemotherapeutic and immunoprophylactic interventions. RTS,S/AS01, the only malaria vaccine close to licensure, although with poor efficacy, blocks the sporozoite invasion mainly through the action of antibodies against the CSP protein, a major component of the pellicle of the sporozoite. Strikingly, sterile protection against malaria can be obtained through immunization with radiation-attenuated sporozoites, genetically attenuated sporozoites or through chemoprophylaxis with infectious sporozoites in animals and humans, but the deployability of sporozoite-based live vaccines pose tremendous challenges. The protection induced by sporozoites occurs in the pre-erythrocytic stages and is mediated mainly by antibodies against the sporozoite and CD8⁺ T cells against peptides presented by MHC class I molecules in infected hepatocytes. Thus, the identification of malaria antigens expressed in the sporozoite and liver-stage may provide new vaccine candidates to be included, alone or in combination, as recombinant protein-based, virus-like particles or sub-unit virally-vectored vaccines. Here I review the efforts being made to identify Plasmodium falciparum antigens expressed during liver-stage with focus on the development of parasite, hepatocyte, mouse models, and resulting rate of infection in order to identify new vaccine candidates and to improve the efficacy of the current vaccines. Finally, I propose new approaches for the identification of liver-stage antigens based on immunopeptidomics.

The Impact of Antiretroviral Therapy on Malaria Parasite Transmission

Coendemicity between the human immunodeficiency virus (HIV) and Plasmodium parasites, the causative agents of acquired immunodeficiency syndrome (AIDS) and malaria, respectively, occurs in several regions around the world. Although the impact of the interaction between these two organisms is not well understood, it is thought that the outcome of either disease may be negatively influenced by coinfection. Therefore, it is important to understand how current first-line antiretroviral therapies (ART) might impact Plasmodium infection in these regions. Here, we describe the effect of 18 antiretroviral compounds and of first-line ART on the blood and sporogonic stages of Plasmodium berghei in vitro and in vivo. We show that the combination zidovudine + lamivudine + lopinavir/ritonavir (LPV/r), employed as first-line HIV treatment in the field, has a strong inhibitory activity on the sporogonic stages of P. berghei and that several non-nucleoside reverse transcriptase inhibitors (NNRTI) have a moderate effect on this stage of the parasite’s life cycle. Our results expose the effect of current first-line ART on Plasmodium infection and identify potential alternative therapies for HIV/AIDS that might impact malaria transmission.

Organoids for Liver Stage Malaria Research

Plasmodium parasites cause malaria and are maintained between Anopheles mosquitoes and mammalian hosts in a complex life cycle. Malaria parasites occupy tissue niches that can be difficult to access, and models to study them can be challenging to recapitulate experimentally, particularly for Plasmodium species that infect humans. 2D culture models provide extremely beneficial tools to investigate Plasmodium biology but they have limitations. More complex 3D structural networks, such as organoids, have unveiled new avenues for developing more physiological tissue models, and their application to malaria research offers great promise. Here, we review current models for studying Plasmodium infection with a key focus on the obligate pre-erythrocytic stage that culminates in blood infection, causing malaria, and discuss how organoids should fulfil an important and unmet need.

Flexible 3D Cell-Based Platforms for the Discovery and Profiling of Novel Drugs Targeting Plasmodium Hepatic Infection

The restricted pipeline of drugs targeting the liver stage of Plasmodium infection reflects the scarcity of cell models that mimic the human hepatic phenotype and drug metabolism, as well as Plasmodium hepatic infection. Using stirred-tank culture systems, spheroids of human hepatic cell lines were generated, sustaining a stable hepatic phenotype over 4 weeks of culture. Spheroids were employed in the establishment of 3D Plasmodium berghei infection platforms that relied on static or dynamic culture conditions. P. berghei invasion and development were recapitulated in the hepatic spheroids, yielding blood-infective merozoites. The translational potential of the 3D platforms was demonstrated by comparing the in vitro minimum inhibitory concentration of M5717, a compound under clinical development, with in vivo plasma concentrations that clear liver stage P. berghei in mice. Our results show that the 3D platforms are flexible and scalable and can predict the efficacy of antiplasmodial therapies, constituting a powerful tool for integration in drug discovery programs.

Trypanosoma brucei infection protects mice against malaria

Sleeping sickness and malaria are parasitic diseases with overlapping geographical distributions in sub-Saharan Africa. We hypothesized that the immune response elicited by an infection with Trypanosoma brucei, the etiological agent of sleeping sickness, would inhibit a subsequent infection by Plasmodium, the malaria parasite, decreasing the severity of its associated pathology. To investigate this, we established a new co-infection model in which mice were initially infected with T. brucei, followed by administration of P. berghei sporozoites. We observed that a primary infection by T. brucei significantly attenuates a subsequent infection by the malaria parasite, protecting mice from experimental cerebral malaria and prolonging host survival. We further observed that an ongoing T. brucei infection leads to an accumulation of lymphocyte-derived IFN-γ in the liver, limiting the establishment of a subsequent hepatic infection by P. berghei sporozoites. Thus, we identified a novel host-mediated interaction between two parasitic infections, which may be epidemiologically relevant in regions of Trypanosoma/Plasmodium co-endemicity.

Despite the geographical overlap between the parasites that cause sleeping sickness and malaria, the reciprocal impact of a co-infection by T. brucei and Plasmodium had hitherto not been assessed. We hypothesized that the strong immune response elicited by a T. brucei infection could potentially limit the ability of Plasmodium parasites to infect the same host. In this study, we showed that a primary infection by T. brucei significantly attenuates a subsequent infection by the malaria parasite. Importantly, a significant proportion of the co-infected mice do not develop Plasmodium parasitemia, and those few that do, do not display symptoms of severe malaria and survive longer than their singly infected counterparts. We further showed that the prevention or delay in appearance of malaria parasites in the blood results from a dramatic impairment of the preceding liver infection by Plasmodium, which is mediated by the strong immune response mounted against the primary T. brucei infection. Our study provides new insights for a novel inter-pathogen interaction that may bear great epidemiological significance in regions of Trypanosoma/Plasmodium co-endemicity.

Designer Parasites: Genetically Engineered Plasmodium as Vaccines To Prevent Malaria Infection

A highly efficacious malaria vaccine that prevents disease and breaks the cycle of infection remains an aspirational goal of medicine. Whole parasite vaccines based on the sporozoite forms of the parasite that target the clinically silent pre-erythrocytic stages of infection have emerged as one of the leading candidates. In animal models of malaria, these vaccines elicit potent neutralizing Ab responses against the sporozoite stage and cytotoxic T cells that eliminate parasite-infected hepatocytes. Among whole-sporozoite vaccines, immunization with live, replication-competent whole parasites engenders superior immunity and protection when compared with live replication-deficient sporozoites. As such, the genetic design of replication-competent vaccine strains holds the promise for a potent, broadly protective malaria vaccine. In this report, we will review the advances in whole-sporozoite vaccine development with a particular focus on genetically attenuated parasites both as malaria vaccine candidates and also as valuable tools to interrogate protective immunity against Plasmodium infection.

Proteomic Analysis of Plasmodium Merosomes: The Link between Liver and Blood Stages in Malaria

The pre-erythrocytic liver stage of the malaria parasite, comprising sporozoites and the liver stages into which they develop, remains one of the least understood parts of the lifecycle, in part owing to the low numbers of parasites. Nonetheless, it is recognized as an important target for antimalarial drugs and vaccines. Here we provide the first proteomic analysis of merosomes, which define the final phase of the liver stage and are responsible for initiating the blood stage of infection. We identify a total of 1879 parasite proteins, and a core set of 1188 proteins quantitatively detected in every biological replicate, providing an extensive picture of the protein repertoire of this stage. This unique data set will allow us to explore key questions about the biology of merosomes and hepatic merozoites.

Hepatic spheroids used as an in vitro model to study malaria relapse

Hypnozoites are the liver stage non-dividing form of the malaria parasite that are responsible for relapse and acts as a natural reservoir for human malaria Plasmodium vivax and P. ovale as well as a phylogenetically related simian malaria P. cynomolgi. Our understanding of hypnozoite biology remains limited due to the technical challenge of requiring the use of primary hepatocytes and the lack of robust and predictive in vitro models. In this study, we developed a malaria liver stage model using 3D spheroid-cultured primary hepatocytes. The infection of primary hepatocytes in suspension led to increased infectivity of both P. cynomolgi and P. vivax infections. We demonstrated that this hepatic spheroid model was capable of maintaining long term viability, hepatocyte specific functions and cell polarity which enhanced permissiveness and thus, permitting for the complete development of both P. cynomolgi and P. vivax liver stage parasites in the infected spheroids. The model described here was able to capture the full liver stage cycle starting with sporozoites and ending in the release of hepatic merozoites capable of invading simian erythrocytes in vitro. Finally, we showed that this system can be used for compound screening to discriminate between causal prophylactic and cidal antimalarials activity in vitro for relapsing malaria.

The Plasmodium liver-specific protein 2 (LISP2) is an early marker of liver stage development

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

A Plasmodium berghei sporozoite-based vaccination platform against human malaria

There is a pressing need for safe and highly effective Plasmodium falciparum (Pf) malaria vaccines. The circumsporozoite protein (CS), expressed on sporozoites and during early hepatic stages, is a leading target vaccine candidate, but clinical efficacy has been modest so far. Conversely, whole-sporozoite (WSp) vaccines have consistently shown high levels of sterilizing immunity and constitute a promising approach to effective immunization against malaria. Here, we describe a novel WSp malaria vaccine that employs transgenic sporozoites of rodent P. berghei (Pb) parasites as cross-species immunizing agents and as platforms for expression and delivery of PfCS (PbVac). We show that both wild-type Pb and PbVac sporozoites unabatedly infect and develop in human hepatocytes while unable to establish an infection in human red blood cells. In a rabbit model, similarly susceptible to Pb hepatic but not blood infection, we show that PbVac elicits cross-species cellular immune responses, as well as PfCS-specific antibodies that efficiently inhibit Pf sporozoite liver invasion in human hepatocytes and in mice with humanized livers. Thus, PbVac is safe and induces functional immune responses in preclinical studies, warranting clinical testing and development.

A genetically engineered parasite, related to malaria-causing Plasmodium falciparum, excels as a vaccine in preclinical tests. A team led by Miguel Prudêncio, of the University of Lisbon, Portugal, developed a genetically altered vaccine candidate based on Plasmodium berghei, which is pathogenic to rodents but, in humans, fails to progress from a harmless, transient liver infection to causing full, blood-borne malaria. The candidate expresses a human form of ‘circumsporozoite protein,’ a known antigen, and is designed to provoke a more comprehensive immune response as it presents a whole pathogen to the host. In preclinical tests, the candidate generated antibodies able to neutralize infection in human hepatocytes and also provoked a cellular immune response in rabbits. The team’s candidate proved safe and efficacious, warranting further trials and clinical testing.

Metabolomic changes in vertebrate host during malaria disease progression

Metabolomics refers to top-down systems biological analysis of metabolites in biological specimens. Phenotypic proximity of metabolites makes them interesting candidates for studying biomarkers of environmental stressors such as parasitic infections. Moreover, the host-parasite interaction directly impinges upon metabolic pathways since the parasite uses the host metabolite pool as a biosynthetic resource. Malarial infection, although not recognized as a classic metabolic disorder, often leads to severe metabolic changes such as hypoglycemia and lactic acidosis. Thus, metabolomic analysis of the infection has become an invaluable tool for promoting a better understanding of the host-parasite interaction and for the development of novel therapeutics. In this review, we summarize the current knowledge obtained from metabolomic studies of malarial infection in rodent models and human patients. Metabolomic analysis of experimental rodent malaria has provided significant insights into the mechanisms of disease progression including utilization of host resources by the parasite, sexual dimorphism in metabolic phenotypes, and cellular changes in host metabolism. Moreover, these studies also provide proof of concept for prediction of cerebral malaria. On the other hand, metabolite analysis of patient biofluids generates extensive data that could be of use in identifying biomarkers of infection severity and in monitoring disease progression. Through the use of metabolomic datasets one hopes to assess crucial infection-specific issues such as clinical severity, drug resistance, therapeutic targets, and biomarkers. Also discussed are nascent or newly emerging areas of metabolomics such as pre-erythrocytic stages of the infection and the host immune response. This review is organized in four broad sections-methodologies for metabolomic analysis, rodent infection models, studies of human clinical specimens, and potential of immunometabolomics. Data summarized in this review should serve as a springboard for novel hypothesis testing and lead to a better understanding of malarial infection and parasite biology.

Humanized Mouse Models for the Study of Human Malaria Parasite Biology, Pathogenesis, and Immunity

Malaria parasite infection continues to inflict extensive morbidity and mortality in resource-poor countries. The insufficiently understood parasite biology, continuously evolving drug resistance and the lack of an effective vaccine necessitate intensive research on human malaria parasites that can inform the development of new intervention tools. Humanized mouse models have been greatly improved over the last decade and enable the direct study of human malaria parasites in vivo in the laboratory. Nevertheless, no small animal model developed so far is capable of maintaining the complete life cycle of Plasmodium parasites that infect humans. The ultimate goal is to develop humanized mouse systems in which a Plasmodium infection closely reproduces all stages of a parasite infection in humans, including pre-erythrocytic infection, blood stage infection and its associated pathology, transmission as well as the human immune response to infection. Here, we discuss current humanized mouse models and the future directions that should be taken to develop next-generation models for human malaria parasite research.