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.

Temporal gene expression during asexual development of the apicomplexan Sarcocystis neurona.

Asexual replication in the apicomplexan Sarcocystis neurona involves two main developmental stages: the motile extracellular merozoite and the sessile intracellular schizont. Merozoites invade host cells and transform into schizonts that undergo replication via endopolygeny to form multiple (64) daughter merozoites that are invasive to new host cells. Given that the capabilities of the merozoite vary significantly from the schizont, the patterns of transcript levels throughout the asexual lifecycle were determined and compared in this study. RNA-Seq data were generated from extracellular merozoites and four intracellular schizont development time points. Of the 6,938 genes annotated in the S. neurona genome, 6,784 were identified in the transcriptome. Of these, 4,111 genes exhibited significant differential expression between the merozoite and at least one schizont development time point. Transcript levels were significantly higher for 2,338 genes in the merozoite and 1,773 genes in the schizont stages. Included in this list were genes encoding the secretory pathogenesis determinants (SPDs), which encompass the surface antigen and SAG-related sequence (SAG/SRS) and the secretory organelle proteins of the invasive zoite stage (micronemes, rhoptries, and dense granules). As anticipated, many of the S. neurona SPD gene transcripts were abundant in merozoites. However, several SPD transcripts were elevated in intracellular schizonts, suggesting roles unrelated to host cell invasion and the initial establishment of the intracellular niche. The hypothetical genes that are potentially unique to the genus Sarcocystis are of particular interest. Their conserved expression patterns are instructive for future investigations into the possible functions of these putative Sarcocystis-unique genes. IMPORTANCE: The genus Sarcocystis is an expansive clade within the Apicomplexa, with the species S. neurona being an important cause of neurological disease in horses. Research to decipher the biology of S. neurona and its host-pathogen interactions can be enhanced by gene expression data. This study has identified conserved apicomplexan orthologs in S. neurona, putative Sarcocystis-unique genes, and gene transcripts abundant in the merozoite and schizont stages. Importantly, we have identified distinct clusters of genes with transcript levels peaking during different intracellular schizont development time points, reflecting active gene expression changes across endopolygeny. Each cluster also has subsets of transcripts with unknown functions, and investigation of these seemingly Sarcocystis-unique transcripts will provide insights into the interesting biology of this parasite genus.

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.

Global transcriptome landscape of the rabbit protozoan parasite Eimeria stiedae.

BACKGROUND: Coccidiosis caused by Eimeria stiedae is a widespread and economically significant disease of rabbits. The lack of studies on the life-cycle development and host interactions of E. stiedae at the molecular level has hampered our understanding of its pathogenesis. METHODS: In this study, we present a comprehensive transcriptome landscape of E. stiedae to illustrate its dynamic development from unsporulated oocysts to sporulated oocysts, merozoites, and gametocytes, and to identify genes related to parasite-host interactions during parasitism using combined PacBio single-molecule real-time and Illumina RNA sequencing followed by bioinformatics analysis and qRT-PCR validation. RESULTS: In total, 12,582 non-redundant full-length transcripts were generated with an average length of 1808 bp from the life-cycle stages of E. stiedae. Pairwise comparisons between stages revealed 8775 differentially expressed genes (DEGs) showing highly significant description changes, which compiled a snapshot of the mechanisms underlining asexual and sexual biology of E. stiedae including oocyst sporulation between unsporulated and sporulated oocysts; merozoite replication between sporulated oocysts and merozoites; and gametophyte development and gamete generation between merozoites and gametocytes. Further, 248 DEGs were grouped into nine series clusters and five groups by expression patterns, and showed that parasite-host interaction-related genes predominated in merozoites and gametocytes and were mostly involved in steroid biosynthesis and lipid metabolism and carboxylic acid. Additionally, co-expression analyses identified genes associated with development and host invasion in unsporulated and sporulated oocysts and immune interactions during gametocyte parasitism. CONCLUSIONS: This is the first study, to our knowledge, to use the global transcriptome profiles to decipher molecular changes across the E. stiedae life cycle, and these results not only provide important information for the molecular characterization of E. stiedae, but also offer valuable resources to study other apicomplexan parasites with veterinary and public significance.

Depletion of the mini-chromosome maintenance complex binding protein allows the progression of cytokinesis despite abnormal karyokinesis during the asexual development of Plasmodium falciparum.

The eukaryotic cell cycle is typically divided into distinct phases with cytokinesis immediately following mitosis. To ensure proper cell division, each phase is tightly coordinated via feedback controls named checkpoints. During its asexual replication cycle, the malaria parasite Plasmodium falciparum undergoes multiple asynchronous rounds of mitosis with segregation of uncondensed chromosomes followed by nuclear division with intact nuclear envelope. The multi-nucleated schizont is then subjected to a single round of cytokinesis that produces dozens of daughter cells called merozoites. To date, no cell cycle checkpoints have been identified that regulate the Plasmodium spp. mode of division. Here, we identify the Plasmodium homologue of the Mini-Chromosome Maintenance Complex Binding Protein (PfMCMBP), which co-purified with the Mini-Chromosome Maintenance (MCM) complex, a replicative helicase required for genomic DNA replication. By conditionally depleting PfMCMBP, we disrupt nuclear morphology and parasite proliferation without causing a block in DNA replication. By immunofluorescence microscopy, we show that PfMCMBP depletion promotes the formation of mitotic spindle microtubules with extensions to more than one DNA focus and abnormal centrin distribution. Strikingly, PfMCMBP-deficient parasites complete cytokinesis and form aneuploid merozoites with variable cellular and nuclear sizes. Our study demonstrates that the parasite lacks a robust checkpoint response to prevent cytokinesis following aberrant karyokinesis.

Cystoisospora suis merozoite development assay for screening of drug efficacy in vitro.

Cystoisospora suis is a common diarrheal pathogen of piglets and typically controlled by metaphylactic toltrazuril application. Recently, toltrazuril resistance has been reported in the field; however, both evaluation of toltrazuril efficacy against field isolates and the anticoccidial drug development for pigs is hampered by costs and labor of animal experimentation. Therefore an in vitro merozoite development assay was developed to evaluate the efficacy of compounds against C. suis in vitro. Monolayers of IPEC-1 cells were infected with sporozoites derived from oocysts of defined C. suis laboratory strains and the optimal infection dose as well as concentration, time point and duration of treatment were evaluated by quantitative real-time PCR. Cell cultures were treated with bumped kinase inhibitor (BKI) 1369 at different time points to evaluate the possibility to delineate effects on different developmental stages in vitro during invasion and early infection, and to determine different inhibitory concentrations (IC50, IC95). BKI 1369 had an IC50 of 35 nM and an IC95 of 350 nM. Dose- and duration-dependent efficacy was seen when developing stages were treated with BKI 1369 after infection (days 0-1, 2-3 and 2-5) but not when sporozoites were pre-incubated with BKI 1369 before infection. Efficacies of further BKIs were also evaluated at 200 nM. BKI 1318, 1708, 1748 and 1862 had an efficacy comparable to that of BKI 1369 (which is also effective in vivo). BKI 1862 showed a more pronounced loss of efficacy in lower concentrations than BKI 1369, signifying pharmacokinetic differences of similar compounds detectable in vitro. In addition, the effects of toltrazuril and its metabolites, toltrazuril sulfoxide and toltrazuril sulfone, on a toltrazuril sensitive and a resistant strain of C. suis were evaluated. Inhibition of merozoite growth in vitro by toltrazuril and its metabolites was dose-dependent only for toltrazuril. Clear differences were noted for the effect on a toltrazuril-sensitive vs. a resistant strain, indicating that this in vitro assay has the capacity to delineate susceptible from resistant strains in vitro. It could also be used to evaluate and compare the efficacy of novel compounds against C. suis and support the determination of the optimal time point of treatment in vivo.

Identification and characterization of a Babesia bigemina thrombospondin-related superfamily member, TRAP-1: a novel antigen containing neutralizing epitopes involved in merozoite invasion.

BACKGROUND: Thrombospondin-related anonymous protein (TRAP) has been described as a potential vaccine candidate for several diseases caused by apicomplexan parasites. However, this protein and members of this family have not yet been characterized in Babesia bigemina, one of the most prevalent species causing bovine babesiosis. METHODS: The 3186-bp Babesia bigemina TRAP-1 (BbiTRAP-1) gene was identified by a bioinformatics search using the B. bovis TRAP-1 sequence. Members of the TRAP and TRAP-related protein families (TRP) were identified in Babesia and Theileria through a search of the TSP-1 adhesive domain, which is the hallmark motif in both proteins. Structural modeling and phylogenetic analysis were performed with the identified TRAP proteins. A truncated recombinant BbiTRAP-1 that migrates at approximately 107 kDa and specific antisera were produced and used in Western blot analysis and indirect fluorescent antibody tests (IFAT). B-cell epitopes with neutralizing activity in BbiTRAP-1 were defined by enzyme-linked immunosorbent assays (ELISA) and invasion assays. RESULTS: Three members of the TRAP family of proteins were identified in B. bigemina (BbiTRAP-1 to -3). All are type 1 transmembrane proteins containing the von Willebrand factor A (vWFA), thrombospondin type 1 (TSP-1), and cytoplasmic C-terminus domains, as well as transmembrane regions. The BbiTRAP-1 predicted structure also contains a metal ion-dependent adhesion site for interaction with the host cell. The TRP family in Babesia and Theileria species contains the canonical TSP-1 domain but lacks the vWFA domain and together with TRAP define a novel gene superfamily. A variable number of tandem repeat units are present in BbiTRAP-1 and could be used for strain genotyping. Western blot and IFAT analysis confirmed the expression of BbiTRAP-1 by blood-stage parasites. Partial recognition by a panel of sera from B. bigemina-infected cattle in ELISAs using truncated BbiTRAP-1 suggests that this protein is not an immunodominant antigen. Additionally, bovine anti-recombinant BbiTRAP-1 antibodies were found to be capable of neutralizing merozoite invasion in vitro. CONCLUSIONS: We have identified the TRAP and TRP gene families in several Babesia and Theileria species and characterized BbiTRAP-1 as a novel antigen of B. bigemina. The functional relevance and presence of neutralization-sensitive B-cell epitopes suggest that BbiTRAP-1 could be included in tests for future vaccine candidates against B. bigemina.

The Effect of Increased Temperatures on Viability, Morphology, Infectivity, and Development of Eimeria tenella.

Commonly found in backyard and commercial poultry production, coccidiosis, caused by Eimeria species, presents a self-limiting intestinal infection based on the number of ingested oocysts. Heat stress (HS) is one of the major environmental stressors in poultry, predisposing broiler chickens to immunosuppression and rendering them susceptible to diseases. There are suggestions that HS reduces Eimeria oocyst shedding in chickens; however, the relationship between HS and coccidiosis is not well elucidated. The objective of this study was to investigate the effect of temperature on viability, morphology, infectivity, and development of Eimeria tenella in vitro, and merozoite production and oocyst shedding in vivo. In vitro exposure of sporozoites to 55 C for at least 60 min reduced sporozoites viability as shown by morphological changes and rendering them unable to invade Mardin-Darbi bovine kidney (MDBK) cells. Intracellular development of merozoites was significantly reduced by an increase in 2 C in the optimal temperature of incubation in vitro. Most importantly, the induction of HS in the live chickens caused significantly lower lesion scores, reduced merozoite production, and oocyst shedding, resulting in a much less severe disease outcome.

PfCERLI1 is a conserved rhoptry associated protein essential for Plasmodium falciparum merozoite invasion of erythrocytes.

The disease-causing blood-stage of the Plasmodium falciparum lifecycle begins with invasion of human erythrocytes by merozoites. Many vaccine candidates with key roles in binding to the erythrocyte surface and entry are secreted from the large bulb-like rhoptry organelles at the apical tip of the merozoite. Here we identify an essential role for the conserved protein P. falciparum Cytosolically Exposed Rhoptry Leaflet Interacting protein 1 (PfCERLI1) in rhoptry function. We show that PfCERLI1 localises to the cytosolic face of the rhoptry bulb membrane and knockdown of PfCERLI1 inhibits merozoite invasion. While schizogony and merozoite organelle biogenesis appear normal, biochemical techniques and semi-quantitative super-resolution microscopy show that PfCERLI1 knockdown prevents secretion of key rhoptry antigens that coordinate merozoite invasion. PfCERLI1 is a rhoptry associated protein identified to have a direct role in function of this essential merozoite invasion organelle, which has broader implications for understanding apicomplexan invasion biology.

Characterization of Cystoisospora suis sexual stages in vitro.

BACKGROUND: The porcine coccidium Cystoisospora suis is characterized by a complex life-cycle during which asexual multiplication is followed by sexual development with two morphologically distinct cell types, the micro- and macrogametes. Genes related to the sexual stages and cell cycle progression were previously identified in related Apicomplexa. Dynein light chain type 1 and male gamete fusion factor HAP2 are restricted to microgametes. Tyrosine-rich proteins and oocyst wall proteins are a part of the oocyst wall. The Rad51/Dmc1-like protein and Nima-related protein kinases are associated with the cell cycle and fertilization process. Here, the sexual stages of C. suis were characterized in vitro morphologically and for temporal expression changes of the mentioned genes to gain insight into this poorly known phase of coccidian development. METHODS: Sexual stages of C. suis developing in vitro in porcine intestinal epithelial cells were examined by light and electron microscopy. The transcriptional levels of genes related to merozoite multiplication and sexual development were evaluated by quantitative real-time PCR at different time points of cultivation. Transcription levels were compared for parasites in culture supernatants at 6-9 days of cultivation (doc) and intracellular parasites at 6-15 doc. RESULTS: Sexual stage of C. suis was detected during 8-11 doc in vitro. Microgamonts (16.8 ± 0.9 µm) and macrogamonts (16.6 ± 1.1 µm) are very similar in shape and size. Microgametes had a round body (3.5 ± 0.5 µm) and two flagella (11.2 ± 0.5 µm). Macrogametes were spherical with a diameter of 12.1 ± 0.5 µm. Merozoite gene transcription peaked on 10 doc and then declined. Genes related to the sexual stages and cell cycle showed an upregulation with a peak on 13 doc, after which they declined. CONCLUSIONS: The present study linked gene expression changes to the detailed morphological description of C. suis sexual development in vitro, including fertilization, meiosis and oocyst formation in this unique model for coccidian parasites. Following this process at the cellular and molecular level will elucidate details on potential bottlenecks of C. suis development (applicable for coccidian parasites in general) which could be exploited as a novel target for control.

The parasitophorous vacuole of the blood-stage malaria parasite.

The pathology of malaria is caused by infection of red blood cells with unicellular Plasmodium parasites. During blood-stage development, the parasite replicates within a membrane-bound parasitophorous vacuole. A central nexus for host-parasite interactions, this unique parasite shelter functions in nutrient acquisition, subcompartmentalization and the export of virulence factors, making its functional molecules attractive targets for the development of novel intervention strategies to combat the devastating impact of malaria. In this Review, we explore the origin, development, molecular composition and functions of the parasitophorous vacuole of Plasmodium blood stages. We also discuss the relevance of the malaria parasite's intravacuolar lifestyle for successful erythrocyte infection and provide perspectives for future research directions in parasitophorous vacuole biology.

Proteomic analysis of the second-generation merozoites of Eimeria tenella under nitromezuril and ethanamizuril stress.

BACKGROUND: Eimeria tenella is a highly pathogenic coccidian that causes avian coccidiosis. Both nitromezuril (NZL) and ethanamizuril (EZL) are novel triazine compounds with high anticoccidial activity, but the mechanisms of their action are still unclear. This study explored the response of E. tenella to NZL and EZL by the study of changes in protein composition of the second-generation merozoites. METHODS: Label-free quantification (LFQ) proteomics of the second-generation merozoites of E. tenella following NZL and EZL treatment were studied by LC-MS/MS to explore the mechanisms of action. The identified proteins were annotated and analyzed by Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis and protein-protein interaction (PPI) networks analysis. RESULTS: A total of 1430 proteins were identified by LC-MS/MS, of which 375 were considered as differential proteins in response to drug treatment (DPs). There were 26 only found in the NZL treatment group (N-group), 63 exclusive to the EZL treatment group (E-group), and 80 proteins were present in both drug groups. In addition, among the DPs, the abundant proteins with significantly altered expression in response to drug treatment (SDPs) were found compared with the C-group, of which 49 were upregulated and 51 were downregulated in the N-group, and 66 upregulated and 79 downregulated in the E-group. Many upregulated proteins after drug treatment were involved in transcription and protein metabolism, and surface antigen proteins (SAGs) were among the largest proportion of the downregulated SDPs. Results showed the top two enriched GO terms and the top one enriched pathway treated with EZL and NZL were related, which indicated that these two compounds had similar modes of action. CONCLUSIONS: LFQ proteomic analysis is a feasible method for screening drug-related proteins. Drug treatment affected transcription and protein metabolism, and SAGs were also affected significantly. This study provided new insights into the effects of triazine anticoccidials against E. tenella.

Transcriptome analysis of Plasmodium berghei during exo-erythrocytic development.

BACKGROUND: The complex life cycle of malaria parasites requires well-orchestrated stage specific gene expression. In the vertebrate host the parasites grow and multiply by schizogony in two different environments: within erythrocytes and within hepatocytes. Whereas erythrocytic parasites are well-studied in this respect, relatively little is known about the exo-erythrocytic stages. METHODS: In an attempt to fill this gap, genome wide RNA-seq analyses of various exo-erythrocytic stages of Plasmodium berghei including sporozoites, samples from a time-course of liver stage development and detached cells were performed. These latter contain infectious merozoites and represent the final step in exo-erythrocytic development. RESULTS: The analysis represents the complete transcriptome of the entire life cycle of P. berghei parasites with temporal detailed analysis of the liver stage allowing comparison of gene expression across the progression of the life cycle. These RNA-seq data from different developmental stages were used to cluster genes with similar expression profiles, in order to infer their functions. A comparison with published data from other parasite stages confirmed stage-specific gene expression and revealed numerous genes that are expressed differentially in blood and exo-erythrocytic stages. One of the most exo-erythrocytic stage-specific genes was PBANKA_1003900, which has previously been annotated as a "gametocyte specific protein". The promoter of this gene drove high GFP expression in exo-erythrocytic stages, confirming its expression profile seen by RNA-seq. CONCLUSIONS: The comparative analysis of the genome wide mRNA expression profiles of erythrocytic and different exo-erythrocytic stages could be used to improve the understanding of gene regulation in Plasmodium parasites and can be used to model exo-erythrocytic stage metabolic networks toward the identification of differences in metabolic processes during schizogony in erythrocytes and hepatocytes.

Endogenous development of Cystoisospora belli in intestinal and biliary epithelium of humans.

Cystoisospora (Isospora) belli is a coccidian parasite of humans. It can cause serious digestive disorders involving infection of intestines, biliary tract and gallbladder, especially in those with depressed immunity. It has a direct fecal-oral transmission cycle. After ingestion of sporulated oocysts, the parasite multiplies asexually and sexually within host epithelial cells, resulting in unsporulated oocysts that are excreted in feces. The details of asexual and sexual stages are not known and certain inclusions in epithelial cells in biopsy samples have been erroneously identified recently as C. belli. Here, we provide details of developmental stages of C. belli in two patients, in duodenal biopsy of one and biliary epithelium of the other. Immature and mature asexual stages (schizonts/meronts) were seen in epithelial cells. The merozoites were seen singly, in pairs and in groups in single parasitophorous vacuole (pv) in host cytoplasm. Immature and mature meronts were seen together in the same pv; up to eight nuclei were seen in meronts that retained elongated crescent shape; round multinucleated schizonts, seen in other coccidians, were not found. Meronts were up to 25 µm long and contained up to ten merozoites that were 8-11 µm long. The merozoites and meronts contained PAS-positive granules. Microgamonts (male) contained up to 30 nuclei that were arranged at the periphery and had condensed chromatin; 1-3 PAS-positive, eosinophilic, residual bodies were left when microgametes were formed. The microgametes were 4 µm long and PAS-negative. All stages of macrogamonts, including oocysts were PAS-positive. The detailed description of the life cycle stages of C. belli reported here should facilitate in histopathologic diagnosis of this parasite.

Nanoparticles show potential to retard bradyzoites in vitro formation of Toxoplasma gondii.

Toxoplasmosis is a common parasitic disease caused by Toxoplasma gondii (Nicolle et Manceaux, 1908), an obligate parasite capable of infecting a range of cell types in almost all warm-blooded animals. Upon infecting an intermediate host, the parasites differentiate into tachyzoites which rapidly infect host tissues. Usually, the invading parasites are cleared by the immune system and administered drugs, but some tachyzoites differentiate into bradyzoites forming tissue cysts. These tissue cysts could serve as a source for re-infection and exacerbations. Currently, treatment for toxoplasmosis is limited and, moreover, there are no drugs for treating the cystic stage thus rendering toxoplasmosis a global burden. Recently, we demonstrated that inorganic nanoparticles showed promising activity against the tachyzoite stage T. gondii. In the present study, we evaluated nanoparticles for effect on bradyzoite formation in vitro. Data revealed that the nanoparticles limited bradyzoite burden in vitro. Further, the nanoparticles decreased the bradyzoite-specific BAG-1 promoter activity relative to the untreated control under a bradyzoite-inducing culture condition, even though this reduction in BAG-1 promoter activity waned with increasing concentrations of nanoparticles. In contrast, a parallel experiment under normal cell culture conditions showed that the nanoparticle treatment mildly increased the BAG-1 promoter activity relative to the untreated control. Taken together, the findings are evidence that nanoparticles not only possess anti-tachyzoite potential but they also have anti-bradyzoite potential in vitro.

Immunomic Identification of Malaria Antigens Associated With Protection in Mice.

Efforts to develop vaccines against malaria represent a major research target. The observations that 1) sterile protection can be obtained when the host is exposed to live parasites and 2) the immunity against blood stage parasite is principally mediated by protective antibodies suggest that a protective vaccine is feasible. However, only a small number of proteins have been investigated so far and most of the Plasmodium proteome has yet to be explored. To date, only few immunodominant antigens have emerged for testing in clinical trials but no formulation has led to substantial protection in humans. The nature of parasite molecules associated with protection remains elusive. Here, immunomic screening of mice immune sera with different protection efficiencies against the whole parasite proteome allowed us to identify a large repertoire of antigens validated by screening a library expressing antigens. The calculation of weighted scores reflecting the likelihood of protection of each antigen using five predictive criteria derived from immunomic and proteomic data sets, highlighted a priority list of protective antigens. Altogether, the approach sheds light on conserved antigens across Plasmodium that are amenable to targeting by the host immune system upon merozoite invasion and blood stage development. Most of these antigens have preliminary protection data but have not been widely considered as candidate for vaccine trials, opening new perspectives that overcome the limited choice of immunodominant, poorly protective vaccines currently being the focus of malaria vaccine researches.

Phosphodiesterase beta is the master regulator of cAMP signalling during malaria parasite invasion.

Cyclic nucleotide signalling is a major regulator of malaria parasite differentiation. Phosphodiesterase (PDE) enzymes are known to control cyclic GMP (cGMP) levels in the parasite, but the mechanisms by which cyclic AMP (cAMP) is regulated remain enigmatic. Here, we demonstrate that Plasmodium falciparum phosphodiesterase ß (PDEß) hydrolyses both cAMP and cGMP and is essential for blood stage viability. Conditional gene disruption causes a profound reduction in invasion of erythrocytes and rapid death of those merozoites that invade. We show that this dual phenotype results from elevated cAMP levels and hyperactivation of the cAMP-dependent protein kinase (PKA). Phosphoproteomic analysis of PDEß-null parasites reveals a >2-fold increase in phosphorylation at over 200 phosphosites, more than half of which conform to a PKA substrate consensus sequence. We conclude that PDEß plays a critical role in governing correct temporal activation of PKA required for erythrocyte invasion, whilst suppressing untimely PKA activation during early intra-erythrocytic development.

Stable transfection of Eimeria necatrix through nucleofection of second generation merozoites.

Eimeria spp., the causative agents of coccidiosis, are the most common protozoan pathogens of chickens. Infection with these parasites can result in poor development or death of animals leading to a devastating economic impact on poultry production. The establishment of transfection protocols for genetic manipulation of Eimeria species and stable expression of genes would help advance the biology of these parasites as well as establish these organisms as novel vaccine delivery vehicles. Here, we report the selection of the first stable transgenic E. necatrix population, EnHA1, consitutively expressing the EYFP reporter following transfection of the 2nd generation merozoites with a linear DNA fragment harboring the EYFP reporter gene, the HA1 gene from the avian influenza virus H9N2 and the TgDHFR-TS selectable marker, which confers resistance to pyrimethamine. Transfected merozoites were inoculated into chickens via the cloacal route, and feces from 18 h to 72 h post inoculation were collected and subjected to subsequent serial passages, FACS sorting and pyrimethamine selection. A gradual increase in the number of EYFP-expressing sporulated oocysts was noticed with more than 90% EYFP + oocysts obtained after five passages. Immunofluorescence assay confirmed successful expression of the HA1 antigen in the EnHA1 population. The ability to genetically manipulate E. necatrix merozoites and express heterologous genes in this parasite will pave the way for possible use of this organism as a vaccine-delivery vehicle.

Shed EBA-175 mediates red blood cell clustering that enhances malaria parasite growth and enables immune evasion.

Erythrocyte Binding Antigen of 175 kDa (EBA-175) has a well-defined role in binding to glycophorin A (GpA) during Plasmodium falciparum invasion of erythrocytes. However, EBA-175 is shed post invasion and a role for this shed protein has not been defined. We show that EBA-175 shed from parasites promotes clustering of RBCs, and EBA-175-dependent clusters occur in parasite culture. Region II of EBA-175 is sufficient for clustering RBCs in a GpA-dependent manner. These clusters are capable of forming under physiological flow conditions and across a range of concentrations. EBA-175-dependent RBC clustering provides daughter merozoites ready access to uninfected RBCs enhancing parasite growth. Clustering provides a general method to protect the invasion machinery from immune recognition and disruption as exemplified by protection from neutralizing antibodies that target AMA-1 and RH5. These findings provide a mechanistic framework for the role of shed proteins in RBC clustering, immune evasion, and malaria.

Concomitant in vitro development of Eimeria zuernii- and Eimeria bovis-macromeronts in primary host endothelial cells.

Eimeria zuernii and E. bovis are host-specific apicomplexan parasites of cattle causing haemorrhagic typhlocolitis in young animals worldwide. During first merogony, both Eimeria species form giant macromeronts (>300 µm) in host endothelial cells containing >120,000 merozoites I in vivo. During the massive intracellular replication of macromeronts, large amounts of cholesterol and fatty acids are indispensable for enormous merozoite I-derived membrane production. From a metabolic perspective, host endothelial cells might be of advantage to the parasite, as transcription of several genes involved in both, cholesterol de novo biosynthesis and low density lipoprotein (LDL)-mediated uptake, are up-regulated in Eimeria macromeront-carrying host endothelial cells. In order to analyse further influence of E. zuernii/E. bovis infections on the metabolism of cholesterol, fatty acids, and glycolysis of the host endothelial cells, suitable in vitro cell culture systems are necessary. So far, in vitro cell culture systems based on primary bovine umbilical vein endothelial cells (BUVEC) are available for E. bovis-macromeront I formation, but have not been evaluated for E. zuernii. A novel E. zuernii (strain A), initially isolated from naturally infected calves in Antioquia, Colombia, was used for sporozoite isolation. Primary BUVEC monolayers were concomitantly infected with E. zuernii- and E. bovis-sporozoites, resulting in large sized macromeronts whose morphological/morphometric characteristics were compared. BUVEC carrying E. zuernii-macromeronts resulted in the release of viable and highly motile merozoites I. Overall, E. zuernii-merozoites I differed morphologically from those of E. bovis. The new E. zuernii (strain A) will allow detailed in vitro investigations not only on the modulation of cellular cholesterol processing (i. e. cholesterol-25-hydroxylase and sterol O-acyltransferase) but also on the surface expression of LDL receptors during macromeront formation.