Liver Stage Development Research Articles

Genome-wide gene expression profiles throughout human malaria parasite liver stage development in humanized mice

Gene expression of Plasmodium falciparum (Pf) liver-stage (LS) parasites has remained poorly characterized, although they are major vaccine and drug targets. Using a human liver-chimaeric mouse model and a fluorescent parasite line (PfNF54CSPGFP), we isolated PfLS and performed transcriptomics on key LS developmental phases. We linked clustered gene expression to ApiAP2, a major family of transcription factors that regulate the parasite life cycle. This provided insights into transcriptional regulation of LS infection and expression of essential LS metabolic and biosynthetic pathways. We observed expression of antigenically variant PfEMP1 proteins and the major Pf protein export machine PTEX and identified protein candidates that might be exported by LS parasites. Comparing Pf and P. vivax LS transcriptomes, we uncovered differences in their expression of sexual commitment factors. This data will aid LS research and vaccine and drug target identification for prevention of malaria infection.

Genetically attenuated parasites show promise as a next-generation malaria vaccine.

Metabolically active, genetically attenuated Plasmodium falciparum parasite lines are promising second-generation malaria vaccine candidates. Lamers et al. and Roozen et al. demonstrated in recent Phase 1/2a trials that GA2 parasites, designed to arrest late during liver-stage development and transmitted via mosquito bites, can induce substantial protection against sporozoite challenge infection.

Autophagy protein Atg7 is essential for maintaining malaria parasite cellular homeostasis and organelle biogenesis.

Plasmodium parasites have a complex life cycle that transitions between mosquito and mammalian hosts, and undergo continuous cellular remodeling to adapt to various drastic environments. Following hepatocyte invasion, the parasite discards superfluous organelles for intracellular replication, and the remnant organelles undergo extensive branching and mature into hepatic merozoites. Autophagy is a ubiquitous eukaryotic process that permits the recycling of intracellular components. Here, we show that the Plasmodium berghei autophagy-related E1-like enzyme Atg7 is expressed in the blood, sporozoites, and liver stages, localized to the parasite cytosol, and is essential for the localization of Atg8 on the membrane and the development of parasite blood and liver forms. We found that depleting Atg7 abolishes Atg8 lipidation, exocytosis of micronemes, organelle biogenesis, and the formation of merozoites during liver-stage development. Overall, this study establishes the essential functions of Atg7 in Plasmodium blood and liver stages, and highlights its role in maintaining the parasite's cellular homeostasis and organelle biogenesis.IMPORTANCEThe malaria life cycle involves two hosts, mosquitoes and vertebrates. Plasmodium parasites undergo complex intracellular and extracellular stages during this transition. Here, we report that an autophagy-related E1-like enzyme Atg7 is required to conjugate Atg8 on the apicoplast membrane. Atg7 depletion in Plasmodium berghei resulted in the loss of Atg8 lipidation and multiple defects like clearance of micronemes, organelle biogenesis, and maturation of hepatic schizonts during liver-stage development. The essentiality of Plasmodium Atg7 in blood and liver stages suggests it is a prospective target for developing autophagy-specific inhibitors. These results highlight the importance of autophagy in malaria parasite development.

Plasmodium berghei liver stage parasites exploit host GABARAP proteins for TFEB activation

Plasmodium, the causative agent of malaria, infects hepatocytes prior to establishing a symptomatic blood stage infection. During this liver stage development, parasites reside in a parasitophorous vacuole (PV), whose membrane acts as the critical interface between the parasite and the host cell. It is well-established that host cell autophagy-related processes significantly impact the development of Plasmodium liver stages. Expression of genes related to autophagy and lysosomal biogenesis is orchestrated by transcription factor EB (TFEB). In this study, we explored the activation of host cell TFEB in Plasmodium berghei-infected cells during the liver stage of the parasite. Our results unveiled a critical role of proteins belonging to the Gamma-aminobutyric acid receptor-associated protein subfamily (GABARAP) of ATG8 proteins (GABARAP/L1/L2 and LC3A/B/C) in recruiting the TFEB-blocking FLCN-FNIP (Folliculin-Folliculin-interacting protein) complex to the PVM. Remarkably, the sequestration of FLCN-FNIP resulted in a robust activation of TFEB, reliant on conjugation of ATG8 proteins to single membranes (CASM) and GABARAP proteins. Our findings provide novel mechanistic insights into host cell signaling occurring at the PVM, shedding light on the complex interplay between Plasmodium parasites and the host cell during the liver stage of infection.

A conserved Plasmodium nuclear protein is critical for late liver stage development

Malaria, caused by Plasmodium parasites, imposes a significant health burden and live-attenuated parasites are being pursued as vaccines. Here, we report on the creation of a genetically attenuated parasite by the deletion of Plasmodium LINUP, encoding a liver stage nuclear protein. In the rodent parasite Plasmodium yoelii, LINUP expression was restricted to liver stage nuclei after the onset of liver stage schizogony. Compared to wildtype P. yoelii, P. yoelii LINUP gene deletion parasites (linup—) exhibited no phenotype in blood stages and mosquito stages but suffered developmental arrest late in liver stage schizogony with a pronounced defect in exo-erythrocytic merozoite formation. This defect caused severe attenuation of the liver stage-to-blood stage transition and immunization of mice with linup— parasites conferred robust protection against infectious sporozoite challenge. LINUP gene deletion in the human parasite Plasmodium falciparum also caused a severe defect in late liver stage differentiation. Importantly, P. falciparum linup— liver stages completely failed to transition from the liver stage to a viable blood stage infection in a humanized mouse model. These results suggest that P. falciparum LINUP is an ideal target for late liver stage attenuation that can be incorporated into a late liver stage-arresting replication competent whole parasite vaccine.

A Plasmodium late liver stage arresting GAP provides superior protection in mice.

Liver-stage genetically attenuated malaria parasites (GAPs) are powerful immunogens that provide protection against sporozoite challenge. We previously generated two late liver-stage-arresting GAPs by deleting the stearoyl-CoA desaturase (Scd) or sporozoite conserved orthologous transcript 1 (Scot1) genes in Plasmodium berghei. Immunization with Scd or Scot1 GAP conferred complete protection against a sporozoite challenge. In a safety study, we observed rare breakthrough blood-stage infections in mice inoculated with high doses of sporozoites, indicating that both GAPs were incompletely attenuated. In this study, we generated a Scd/Scot1 GAP by dual gene deletion. This resulted in complete attenuation of the parasites in the liver and did not transition to blood-stage infection despite a high-dose sporozoite challenge. The Scd/Scot1 KO and WT GFP parasites were indistinguishable during blood, mosquito and early liver stage development. Moreover, Scd/Scot1 KO liver-stage schizonts exhibited an abnormal apicoplast biogenesis and nuclear division phenotype, failed to form hepatic merozoites, and exhibited late liver-stage arrest. Compared with early-arresting Speld KO immunization, late-stage liver-arresting Scd/Scot1 KO induces greater and broader CD8+ T-cell responses and elicits stage-transcending immunity that provides superior protection in C57BL/6 mice. These data prove that multiple gene deletions lead to complete attenuation of the parasite and support the development of late liver stage-arresting P. falciparum GAP.

Efficacy of ivermectin and its metabolites against Plasmodium falciparum liver stages in primary human hepatocytes.

Ivermectin, a broad-spectrum anti-parasitic drug, has been proposed as a novel vector control tool to reduce malaria transmission by mass drug administration. Ivermectin and some metabolites have mosquito-lethal effect, reducing Anopheles mosquito survival. Ivermectin inhibits liver stage development in a rodent malaria model, but no inhibition was observed in a primate malaria model or in a human malaria challenge trial. In the liver, cytochrome P450 3A4 and 3A5 enzymes metabolize ivermectin, which may impact drug efficacy. Thus, understanding ivermectin metabolism and assessing this impact on Plasmodium liver stage development is critical. Using primary human hepatocytes (PHHs), we characterized ivermectin metabolism and evaluated the efficacy of ivermectin and its primary metabolites M1 (3″-O-demethyl ivermectin) and M3 (4-hydroxymethyl ivermectin) against Plasmodium falciparum liver stages. Two different modes of ivermectin exposure were evaluated: prophylactic mode (days 0-3 post-infection) and curative mode (days 3-5 post-infection). We used two different PHH donors and modes to determine the inhibitory concentration (IC50) of ivermectin, M1, M3, and the known anti-malarial drug pyrimethamine, with IC50 values ranging from 1.391 to 14.44, 9.95-23.71, 4.767-8.384, and 0.9073-5.416 µM, respectively. In our PHH model, ivermectin and metabolites M1 and M3 demonstrated inhibitory activity against P. falciparum liver stages in curative treatment mode (days 3-5) and marginal activity in prophylactic treatment mode (days 0-3). Ivermectin had improved efficacy when co-administered with ketoconazole, a specific inhibitor of cytochrome P450 3A4 enzyme. Further studies should be performed to examine ivermectin liver stage efficacy when co-administered with CYP3A4 inhibitors and anti-malarial drugs to understand the pharmacokinetic and pharmacodynamic drug-drug interactions that enhance efficacy against human malaria parasites in vitro.

A Micronemal Protein, Scot1, Is Essential for Apicoplast Biogenesis and Liver Stage Development in Plasmodium berghei.

Plasmodium sporozoites invade hepatocytes, transform into liver stages, and replicate into thousands of merozoites that infect erythrocytes and cause malaria. Proteins secreted from micronemes play an essential role in hepatocyte invasion, and unneeded micronemes are subsequently discarded for replication. The liver-stage parasites are potent immunogens that prevent malarial infection. Late liver stage-arresting genetically attenuated parasites (GAPs) exhibit greater protective efficacy than early GAP. However, the number of late liver-stage GAPs for generating GAPs with multiple gene deletions is limited. Here, we identified Scot1 (Sporozoite Conserved Orthologous Transcript 1), which was previously shown to be upregulated in sporozoites, and by endogenous tagging with mCherry, we demonstrated that it is expressed in the sporozoite and liver stages in micronemes. Using targeted gene deletion in Plasmodium berghei, we showed that Scot1 is essential for late liver-stage development. Scot1 KO sporozoites grew normally into liver stages but failed to initiate blood-stage infection in mice due to impaired apicoplast biogenesis and merozoite formation. Bioinformatic studies suggested that Scot1 is a metal-small-molecule carrier protein. Remarkably, supplementation with metals in the culture of infected Scot1 KO cells did not rescue their phenotype. Immunization with Scot1 KO sporozoites in C57BL/6 mice confers protection against malaria via infection. These proof-of-concept studies will enable the generation of P. falciparum Scot1 mutants that could be exploited to generate GAP malaria vaccines.

Flp/FRT-mediated disruption of ptex150 and exp2 in Plasmodium falciparum sporozoites inhibits liver-stage development

Plasmodium falciparum causes severe malaria and assembles a protein translocon (PTEX) complex at the parasitophorous vacuole membrane (PVM) of infected erythrocytes, through which several hundred proteins are exported to facilitate growth. The preceding liver stage of infection involves growth in a hepatocyte-derived PVM; however, the importance of protein export during P. falciparum liver infection remains unexplored. Here, we use the FlpL/FRT system to conditionally excise genes in P. falciparum sporozoites for functional liver-stage studies. Disruption of PTEX members ptex150 and exp2 did not affect sporozoite development in mosquitoes or infectivity for hepatocytes but attenuated liver-stage growth in humanized mice. While PTEX150 deficiency reduced fitness on day 6 postinfection by 40%, EXP2 deficiency caused 100% loss of liver parasites, demonstrating that PTEX components are required for growth in hepatocytes to differing degrees. To characterize PTEX loss-of-function mutations, we localized four liver-stage Plasmodium export element (PEXEL) proteins. P. falciparum liver specific protein 2 (LISP2), liver-stage antigen 3 (LSA3), circumsporozoite protein (CSP), and a Plasmodium berghei LISP2 reporter all localized to the periphery of P. falciparum liver stages but were not exported beyond the PVM. Expression of LISP2 and CSP but not LSA3 was reduced in ptex150-FRT and exp2-FRT liver stages, suggesting that expression of some PEXEL proteins is affected directly or indirectly by PTEX disruption. These results show that PTEX150 and EXP2 are important for P. falciparum development in hepatocytes and emphasize the emerging complexity of PEXEL protein trafficking.

DNA N-glycosylases Ogg1 and EndoIII as components of base excision repair in Plasmodium falciparum organelles

The integrity of genomes of the two crucial organelles of the malaria parasite — an apicoplast and mitochondrion in each cell − must be maintained by DNA repair mediated by proteins targeted to these compartments. We explored the localisation and function of Plasmodium falciparum base excision repair (BER) DNA N-glycosylase homologs PfEndoIII and PfOgg1. These N-glycosylases would putatively recognise DNA lesions prior to the action of apurinic/apyrimidinic (AP)-endonucleases. Both Ape1 and Apn1 endonucleases have earlier been shown to function solely in the parasite mitochondrion. Immunofluorescence localisation showed that PfEndoIII was exclusively mitochondrial. PfOgg1 was not seen clearly in mitochondria when expressed as a PfOgg1leader-GFP fusion, although chromatin immunoprecipitation assays showed that it could interact with both mitochondrial and apicoplast DNA. Recombinant PfEndoIII functioned as a DNA N-glycosylase as well as an AP-lyase on thymine glycol (Tg) lesions. We further studied the importance of Ogg1 in the malaria life cycle using reverse genetic approaches in Plasmodium berghei. Targeted disruption of PbOgg1 resulted in loss of 8-oxo-G specific DNA glycosylase/lyase activity. PbOgg1 knockout did not affect blood, mosquito or liver stage development but caused reduced blood stage infection after inoculation of sporozoites in mice. A significant reduction in erythrocyte infectivity by PbOgg1 knockout hepatic merozoites was also observed, thus showing that PbOgg1 ensures smooth transition from liver to blood stage infection. Our results strengthen the view that the Plasmodium mitochondrial genome is an important site for DNA repair by the BER pathway.

Hemozoin-induced IFN-γ production mediates innate immune protection against sporozoite infection

Hemozoin is a crystal synthesized by Plasmodium parasites during hemoglobin digestion in the erythrocytic stage. The hemozoin released when the parasites egress from the red blood cell, which is complexed with parasite DNA, is cleared from the circulation by circulating and tissue-resident monocytes and macrophages, respectively. Recently, we reported that intravenous administration of purified hemozoin complexed with Plasmodium berghei DNA (HzPbDNA) resulted in an innate immune response that blocked liver stage development of sporozoites that was dose-dependent and time-limited. Here, we further characterize the organismal, cellular, and molecular events associated with this protective innate response in the liver and report that a large proportion of the IV administered HzPbDNA localized to F4/80+ cells in the liver and that the rapid and strong protection against liver-stage development waned quickly such that by 1 week post-HzPbDNA treatment animals were fully susceptible to infection. RNAseq of the liver after IV administration of HzPbDNA demonstrated that the rapid and robust induction of genes associated with the acute phase response, innate immune activation, cellular recruitment, and IFN-γ signaling observed at day 1 was largely absent at day 7. RNAseq analysis implicated NK cells as the major cellular source of IFN-γ. In vivo cell depletion and IFN-γ neutralization experiments supported the hypothesis that tissue-resident macrophages and NK cells are major contributors to the protective response and the NK cell-derived IFN-γ is key to induction of the mechanisms that block sporozoite development in the liver. These findings advance our understanding of the innate immune responses that prevent liver stage malaria infection.

Machine learning for predicting Plasmodium liver stage development in vitro using microscopy imaging

Malaria, a significant global health challenge, is caused by Plasmodium parasites. The Plasmodium liver stage plays a pivotal role in the establishment of the infection. This study focuses on the liver stage development of the model organism Plasmodium berghei, employing fluorescent microscopy imaging and convolutional neural networks (CNNs) for analysis. Convolutional neural networks have been recently proposed as a viable option for tasks such as malaria detection, prediction of host-pathogen interactions, or drug discovery. Our research aimed to predict the transition of Plasmodium-infected liver cells to the merozoite stage, a key development phase, 15 hours in advance. We collected and analyzed hourly imaging data over a span of at least 38 hours from 400 sequences, encompassing 502 parasites. Our method was compared to human annotations to validate its efficacy. Performance metrics, including the area under the receiver operating characteristic curve (AUC), sensitivity, and specificity, were evaluated on an independent test dataset. The outcomes revealed an AUC of 0.873, a sensitivity of 84.6%, and a specificity of 83.3%, underscoring the potential of our CNN-based framework to predict liver stage development of P. berghei. These findings not only demonstrate the feasibility of our methodology but also could potentially contribute to the broader understanding of parasite biology.

Stearoyl-CoA desaturase regulates organelle biogenesis and hepatic merozoite formation in Plasmodium berghei.

Plasmodium is an obligate intracellular parasite that requires intense lipid synthesis for membrane biogenesis and survival. One of the principal membrane components is oleic acid, which is needed to maintain the membrane's biophysical properties and fluidity. The malaria parasite can modify fatty acids, and stearoyl-CoA Δ9-desaturase (Scd) is an enzyme that catalyzes the synthesis of oleic acid by desaturation of stearic acid. Scd is dispensable in P. falciparum blood stages; however, its role in mosquito and liver stages remains unknown. We show that P. berghei Scd localizes to the ER in the blood and liver stages. Disruption of Scd in the rodent malaria parasite P. berghei did not affect parasite blood stage propagation, mosquito stage development, or early liver-stage development. However, when Scd KO sporozoites were inoculated intravenously or by mosquito bite into mice, they failed to initiate blood-stage infection. Immunofluorescence analysis revealed that organelle biogenesis was impaired and merozoite formation was abolished, which initiates blood-stage infections. Genetic complementation of the KO parasites restored merozoite formation to a level similar to that of WT parasites. Mice immunized with Scd KO sporozoites confer long-lasting sterile protection against infectious sporozoite challenge. Thus, the Scd KO parasite is an appealing candidate for inducing protective pre-erythrocytic immunity and hence its utility as a GAP.

Toxoplasma and Plasmodium associate with host Arfs during infection.

The parasites Toxoplasma gondii and Plasmodium live complex intracellular lifestyles where they must acquire essential host nutrients while avoiding recognition. Although previous work has sought to identify the specific nutrients scavenged by apicomplexans, the mechanisms by which host materials are transported to and across the parasite vacuole membrane are largely unknown. Here, we examined members of the host vesicular trafficking network to identify specific pathways subverted by T. gondii and Plasmodium berghei. Our results indicate that T. gondii selectively internalizes host Arfs, a class of proteins involved in intracellular trafficking. For P. berghei, host Arfs were restricted by the parasite's vacuole membrane, but proteins involved in vesicular trafficking were identified as essential for liver stage development. A greater exploration into how and why apicomplexans subvert host vesicular trafficking could help identify targets for host-directed therapeutics.

Naturally acquired antibodies against Plasmodium vivax pre-erythrocytic stage vaccine antigens inhibit sporozoite invasion of human hepatocytes in vitro

In Plasmodium vivax, the most studied vaccine antigens are aimed at blocking merozoite invasion of erythrocytes and disease development. Very few studies have evaluated pre-erythrocytic (PE) stage antigens. The P. vivax circumsporozoite protein (CSP), is considered the leading PE vaccine candidate, but immunity to CSP is short-lived and variant specific. Thus, there is a need to identify other potential candidates to partner with CSP in a multivalent vaccine to protect against infection and disease. We hypothesize that sporozoite antigens important for host cell infection are considered potential targets. In this study, we evaluated the magnitude and quality of naturally acquired antibody responses to four P. vivax PE antigens: sporozoite surface protein 3 (SSP3), sporozoite protein essential for traversal 1 (SPECT1), cell traversal protein of ookinetes and sporozoites (CelTOS) and CSP in plasma of P. vivax infected patients from Thailand. Naturally acquired antibodies to these antigens were prevalent in the study subjects, but with significant differences in magnitude of IgG antibody responses. About 80% of study participants had antibodies to all four antigens and only 2% did not have antibodies to any of the antigens. Most importantly, these antibodies inhibited sporozoite infection of hepatocytes in vitro. Significant variations in magnitude of antigen-specific inhibitory antibody responses were observed with individual samples. The highest inhibitory responses were observed with anti-CelTOS antibodies, followed by anti-SPECT1, SSP3 and CSP antibodies respectively. These data highlight the vaccine potential of these antigens in protecting against hepatocyte infection and the need for a multi-valent pre-erythrocytic vaccine to prevent liver stage development of P. vivax sporozoites.

The effect of dosage on the protective efficacy of whole-sporozoite formulations for immunization against malaria

Immunization with Plasmodium sporozoites, either attenuated or administered under the cover of an antimalarial drug, can induce strong protection against malaria in pre-clinical murine models, as well as in human trials. Previous studies have suggested that whole-sporozoite (WSpz) formulations based on parasites with longer liver stage development induce higher protection, but a comparative analysis of four different WSpz formulations has not been reported. We employed a rodent model of malaria to analyze the effect of immunization dosage on the protective efficacy of WSpz formulations consisting of (i) early liver arresting genetically attenuated parasites (EA-GAP) or (ii) radiation-attenuated sporozoites (RAS), (iii) late arresting GAP (LA-GAP), and (iv) sporozoites administered under chemoprophylaxis, that are eliminated upon release into the bloodstream (CPS). Our results show that, unlike all other WSpz formulations, EA-GAP fails to confer complete protection against an infectious challenge at any immunization dosage employed, suggesting that a minimum threshold of liver development is required to elicit fully effective immune responses. Moreover, while immunization with RAS, LA-GAP and CPS WSpz yields comparable, dosage-dependent protection, protection by EA-GAP WSpz peaks at an intermediate dosage and markedly decreases thereafter. In-depth immunological analyses suggest that effector CD8+ T cells elicited by EA-GAP WSpz immunization have limited developmental plasticity, with a potential negative impact on the functional versatility of memory cells and, thus, on protective immunity. Our findings point towards dismissing EA-GAP from prioritization for WSpz malaria vaccination and enhance our understanding of the complexity of the protection elicited by these WSpz vaccine candidates, guiding their future optimization.

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

BackgroundAcquired 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.MethodsIC50 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.ResultsThe 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.ConclusionsResults 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.

Malaria sporozoites evade host complement attack.

Complement is the first line of the host innate immune response against bacterial and viral infections; however, its role in the development of the malaria liver stage remains undefined. We found that sporozoite infection by either a mosquito bite or intravenous injection activated systemic complement, but neither depletion of C3 nor knockout of C3 had a significant effect on malaria liver stage development. Incubation of mouse serum with trypsin-treated sporozoites, but not naive sporozoites, led to the deposition of a membrane attack complex (MAC) on the surface of sporozoites and greatly reduced the number of exo-erythrocytic forms (EEF). Further studies have shown that the recruitment of complement H factor (CFH) may be associated with the prevention of MAC deposition on the surface of naïve sporozoites. Our data strongly suggest that sporozoites can escape complement attacks and provide us with a novel strategy to prevent malaria infection.

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

BackgroundPlasmodium 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.MethodsNetwork 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.Results33,314 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).ConclusionsWe 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.

Preliminary characterization of Plasmodium vivax sporozoite antigens as pre-erythrocytic vaccine candidates.

Plasmodium vivax pre-erythrocytic (PE) vaccine research has lagged far behind efforts to develop Plasmodium falciparum vaccines. There is a critical gap in our knowledge of PE antigen targets that can induce functionally inhibitory neutralizing antibody responses. To overcome this gap and guide the selection of potential PE vaccine candidates, we considered key characteristics such as surface exposure, essentiality to infectivity and liver stage development, expression as recombinant proteins, and functional immunogenicity. Selected P. vivax sporozoite antigens were surface sporozoite protein 3 (SSP3), sporozoite microneme protein essential for cell traversal (SPECT1), sporozoite surface protein essential for liver-stage development (SPELD), and M2 domain of MAEBL. Sequence analysis revealed little variation occurred in putative B-cell and T-cell epitopes of the PE candidates. Each antigen was tested for expression as refolded recombinant proteins using an established bacterial expression platform and only SPELD failed. The successfully expressed antigens were immunogenic in vaccinated laboratory mice and were positively reactive with serum antibodies of P. vivax-exposed residents living in an endemic region in Thailand. Vaccine immune antisera were tested for reactivity to native sporozoite proteins and for their potential vaccine efficacy using an in vitro inhibition of liver stage development assay in primary human hepatocytes quantified on day 6 post-infection by high content imaging analysis. The anti-PE sera produced significant inhibition of P. vivax sporozoite invasion and liver stage development. This report provides an initial characterization of potential new PE candidates for a future P. vivax vaccine.