Regulation of host cell survival by intracellular Plasmodium and Theileria parasites

Mixed Wolbachia infections resolve rapidly during in vitro evolution

The intracellular symbiont Wolbachia pipientis evolved after the divergence of arthropods and nematodes, but it reached high prevalence in many of these taxa through its abilities to infect new hosts and their germlines. Some strains exhibit long-term patterns of co-evolution with their hosts, while other strains are capable of switching hosts. This makes strain selection an important factor in symbiont-based biological control. However, little is known about the ecological and evolutionary interactions that occur when a promiscuous strain colonizes an infected host. Here, we study what occurs when two strains come into contact in host cells following horizontal transmission and infection. We focus on the faithful wMel strain from Drosophila melanogaster and the promiscuous wRi strain from Drosophila simulans using an in vitro cell culture system with multiple host cell types and combinatorial infection states. Mixing D. melanogaster cell lines stably infected with wMel and wRi revealed that wMel outcompetes wRi quickly and reproducibly. Furthermore, wMel was able to competitively exclude wRi even from minuscule starting quantities, indicating that this is a nearly deterministic outcome, independent of the starting infection frequency. This competitive advantage was not exclusive to wMel's native D. melanogaster cell background, as wMel also outgrew wRi in D. simulans cells. Overall, wRi is less adept at in vitro growth and survival than wMel and its in vivo state, revealing differences between cellular and humoral regulation. These attributes may underlie the observed low rate of mixed infections in nature and the relatively rare rate of host-switching in most strains. Our in vitro experimental framework for estimating cellular growth dynamics of Wolbachia strains in different host species, tissues, and cell types provides the first strategy for parameterizing endosymbiont and host cell biology at high resolution. This toolset will be crucial to our application of these bacteria as biological control agents in novel hosts and ecosystems.

Recent Advances in Host-Directed Therapies for Tuberculosis and Malaria

Tuberculosis (TB), caused by the bacterium Mycobacterium tuberculosis, and malaria, caused by parasites from the Plasmodium genus, are two of the major causes of death due to infectious diseases in the world. Both diseases are treatable with drugs that have microbicidal properties against each of the etiologic agents. However, problems related to treatment compliance by patients and emergence of drug resistant microorganisms have been a major problem for combating TB and malaria. This factor is further complicated by the absence of highly effective vaccines that can prevent the infection with either M. tuberculosis or Plasmodium. However, certain host biological processes have been found to play a role in the promotion of infection or in the pathogenesis of each disease. These processes can be targeted by host-directed therapies (HDTs), which can be administered in conjunction with the standard drug treatments for each pathogen, aiming to accelerate their elimination or to minimize detrimental side effects resulting from exacerbated inflammation. In this review we discuss potential new targets for the development of HDTs revealed by recent advances in the knowledge of host-pathogen interaction biology, and present an overview of strategies that have been tested in vivo, either in experimental models or in patients.

Host-targeted Interventions as an Exciting Opportunity to Combat Malaria

Terminal and benign diseases alike in adults, children, pregnant women, and others are successfully treated by pharmacological inhibitors that target human enzymes. Despite extensive global efforts to fight malaria, the disease continues to be a massive worldwide health burden, and new interventional strategies are needed. Current drugs and vector control strategies have contributed to the reduction in malaria deaths over the past 10 years, but progress toward eradication has waned in recent years. Resistance to antimalarial drugs is a substantial and growing problem. Moreover, targeting dormant forms of the malaria parasite Plasmodium vivax is only possible with two approved drugs, which are both contraindicated for individuals with glucose-6-phosphate dehydrogenase deficiency and in pregnant women. Plasmodium parasites are obligate intracellular parasites and thus have specific and absolute requirements of their hosts. Growing evidence has described these host necessities, paving the way for opportunities to pharmacologically target host factors to eliminate Plasmodium infection. Here, we describe progress in malaria research and adjacent fields and discuss key challenges that remain in implementing host-directed therapy against malaria.

Theileria annulata Cyclophilin1 (TaCyp1) Interacts With Host Cell MED21

Host cells infected by Theileria annulata schizonts show the character of permanent proliferation in vitro, also named transformation. To explore the molecular mechanism a T. annulata Cyp1 (TaCyp1) protein potentially involved in regulating cell transformation was used as bait to screen for its interacting proteins by yeast-two-hybrid assay. Additional GST-pull down experiments confirmed that only MED21 specifically interacted with TaCyp1. Moreover, the distribution of TaCyp1 around T. annulata schizonts facilitated interaction with host cell MED21. As a component of mediator complex, MED21 is normally involved in regulating the transcription of nearly all RNA polymerase II-dependent genes. Therefore, to explore its influence on NF-κB signaling MED21 RNA interference and parasite killing with BW720c treatment were performed. Knock down of MED21 resulted in a significant decrease in NF-κB1/2 mRNA expressions, but no significant change in P105, P52 levels, nor detectable alteration in levels of phosphorylated IκBα/β. By contrast, BW720c treatment induced an obvious decrease in the phosphorylation status of P52 and IκBα/β, but no obvious change in that of P105. This suggests that BW720c-induced parasite death had a significant negative influence on NF-κB signaling, whereas knock down of MED21 had no obvious effect on NF-κB signaling. Characterization of TaCyp1 provides information on the function of parasite cyclophilins and leads to a better understanding of the interactions between T. annulata and its host leukocytes.

Manipulating Eryptosis of Human Red Blood Cells: A Novel Antimalarial Strategy?

Malaria is a major global health burden, affecting over 200 million people worldwide. Resistance against all currently available antimalarial drugs is a growing threat, and represents a major and long-standing obstacle to malaria eradication. Like many intracellular pathogens, Plasmodium parasites manipulate host cell signaling pathways, in particular programmed cell death pathways. Interference with apoptotic pathways by malaria parasites is documented in the mosquito and human liver stages of infection, but little is known about this phenomenon in the erythrocytic stages. Although mature erythrocytes have lost all organelles, they display a form of programmed cell death termed eryptosis. Numerous features of eryptosis resemble those of nucleated cell apoptosis, including surface exposure of phosphatidylserine, cell shrinkage and membrane ruffling. Upon invasion, Plasmodium parasites induce significant stress to the host erythrocyte, while delaying the onset of eryptosis. Many eryptotic inducers appear to have a beneficial effect on the course of malaria infection in murine models, but major gaps remain in our understanding of the underlying molecular mechanisms. All currently available antimalarial drugs have parasite-encoded targets, which facilitates the emergence of resistance through selection of mutations that prevent drug-target binding. Identifying host cell factors that play a key role in parasite survival will provide new perspectives for host-directed anti-malarial chemotherapy. This review focuses on the interrelationship between Plasmodium falciparum and the eryptosis of its host erythrocyte. We summarize the current knowledge in this area, highlight the different schools of thoughts and existing gaps in knowledge, and discuss future perspectives for host-directed therapies in the context of antimalarial drug discovery.

The secreted Theileria annulata Ta9 protein contributes to activation of the AP-1 transcription factor

Theileria annulata is an obligate intracellular protozoan parasite of the phylum Apicomplexa. Theileria sporozoites invade bovine leukocytes and develop into a multinucleate syncytial macroschizont that causes uncontrolled proliferation and dissemination of infected and transformed leukocytes. Activator protein 1 (AP-1) is a transcription factor driving expression of genes involved in proliferation and dissemination and is therefore a key player in Theileria-induced leukocytes transformation. Ta9 possesses a signal peptide allowing it to be secreted into the infected leukocyte cytosol and be presented to CD8 T cells in the context of MHC class I. First, we confirmed that Ta9 is secreted into the infected leukocyte cytosol, and then we generated truncated versions of GFP-tagged Ta9 and tested their ability to activate AP-1 in non-infected HEK293T human kidney embryo cells. The ability to activate AP-1-driven transcription was found to reside in the C-terminal 100 amino acids of Ta9 distant to the N-terminally located epitopes recognised by CD8+ T cells. Secreted Ta9 has therefore, not only the ability to stimulate CD8+ T cells, but also the potential to activate AP-1-driven transcription and contribute to T. annulata-induced leukocyte transformation.

Screening and identification of host proteins interacting with Theileria annulata cysteine proteinase (TaCP) by yeast-two-hybrid system

BACKGROUND

Theileria annulata can infect monocytes/macrophages and B lymphocytes and causes severe lymphoproliferative disease in ruminants. Meanwhile, infection by T. annulata leads to the permanent proliferation of cell population through regulating signaling pathways of host cells. Cysteine proteinases (CPs) are one kind of protein hydrolase and usually play critical roles in parasite virulence, host invasion, nutrition and host immune response. However, the biological function of T. annulata CP (TaCP) is still unclear. In this study, a yeast-two-hybrid assay was performed to screen host proteins interacting with TaCP, to provide information to help our understanding of the molecular mechanisms between T. annulata and host cells.

METHODS

The cDNA from purified bovine B cells was inserted into pGADT7-SfiI vector (pGADT7-SfiI-BcDNA, Prey plasmid) for constructing the yeast two-hybrid cDNA library. TaCP was cloned into the pGBKT7 vector (pGBKT7-TaCP) and was considered as bait plasmid after evaluating the expression, auto-activation and toxicity tests in the yeast strain Y2HGold. The yeast two-hybrid screening was carried out via co-transforming bait and prey plasmids into yeast strain Y2HGold. Sequences of positive preys were analyzed using BLAST, Gene Ontology, UniProt and STRING.

RESULTS

Two host proteins, CRBN (Bos taurus cereblon transcript variant X2) and Ppp4C (Bos indicus protein phosphatase 4 catalytic subunit) were identified to interact with TaCP. The results of functional analysis showed that the two proteins were involved in many cellular processes, such as ubiquitylation regulation, microtubule organization, DNA repair, cell apoptosis and maturation of spliceosomal snRNPs.

CONCLUSIONS

This study is the first to screen the host proteins of bovine B cells interacting with TaCP, and 2 proteins, CRBN and Ppp4C, were identified using yeast two-hybrid technique. The results of functional analysis suggest that the two proteins are involved in many cellular processes, such as ubiquitylation regulating, microtubule organization, DNA repair, cell apoptosis and maturation of spliceosomal snRNPs. The interaction with CRBN and Ppp4C indicate that TaCP possibly is involved in regulating signaling pathways and cell proliferation, which is helpful for understanding the interaction between T. annulata and host cells.

Potential Sabotage of Host Cell Physiology by Apicomplexan Parasites for Their Survival Benefits

Plasmodium, Toxoplasma, Cryptosporidium, Babesia, and Theileria are the major apicomplexan parasites affecting humans or animals worldwide. These pathogens represent an excellent example of host manipulators who can overturn host signaling pathways for their survival. They infect different types of host cells and take charge of the host machinery to gain nutrients and prevent itself from host attack. The mechanisms by which these pathogens modulate the host signaling pathways are well studied for Plasmodium, Toxoplasma, Cryptosporidium, and Theileria, except for limited studies on Babesia. Theileria is a unique pathogen taking into account the way it modulates host cell transformation, resulting in its clonal expansion. These parasites majorly modulate similar host signaling pathways, however, the disease outcome and effect is different among them. In this review, we discuss the approaches of these apicomplexan to manipulate the host–parasite clearance pathways during infection, invasion, survival, and egress.

Subverting Host Cell P21-Activated Kinase: A Case of Convergent Evolution across Pathogens

Intracellular pathogens have evolved a wide range of strategies to not only escape from the immune systems of their hosts, but also to directly exploit a variety of host factors to facilitate the infection process. One such strategy is to subvert host cell signalling pathways to the advantage of the pathogen. Recent research has highlighted that the human serine/threonine kinase PAK, or p21-activated kinase, is a central component of host-pathogen interactions in many infection systems involving viruses, bacteria, and eukaryotic pathogens. PAK paralogues are found in most mammalian tissues, where they play vital roles in a wide range of functions. The role of PAKs in cell proliferation and survival, and their involvement in a number of cancers, is of great interest in the context of drug discovery. In this review we discuss the latest insights into the surprisingly central role human PAK1 plays for the infection by such different infectious disease agents as viruses, bacteria, and parasitic protists. It is our intention to open serious discussion on the applicability of PAK inhibitors for the treatment, not only of neoplastic diseases, which is currently the primary objective of drug discovery research targeting these enzymes, but also of a wide range of infectious diseases.

Evidence for polymicrobial communities in explanted vascular filters and atheroma debris

RATIONALE

Microbial communities have been implicated in a variety of disease processes and have been intermittently observed in arterial disease; however, no comprehensive unbiased community analysis has been performed. We hypothesize that complex microbial communities may be involved in chronic vascular diseases as well and may be effectively characterized by molecular assays.

OBJECTIVE

The main objective is to survey vascular debris, atheroma, and vascular filters for polymicrobial communities consisting of prokaryotic and eukaryotic microbes, specifically eukaryotic microbes.

METHODS AND RESULTS

We examined vascular aspirates of atheromatous debris or embolic protection filters in addition to matched peripheral blood samples, from fifteen patients, as well as three cadaveric coronary arteries from two separate patients, for microbial communities. General fluorescence microscopy by Höechst and ethidium bromide DNA stains, prokaryotic and eukaryotic community analysis by Next Generation DNA Sequencing (NGS), and a eukaryotic microbial 9 probe multiplexed quantitative PCR were used to detect and characterize the presence of putative polymicrobial communities. No prokaryotes were detected in peripheral blood; however, in 4 of 9 sequenced filters and in 2 of 7 sequenced atheroma debris samples, prokaryotic populations were identified. By DNA sequencing, eukaryotic microbes were detected in 4 of 15 blood samples, 5 of the 9 sequenced filters, and 3 of the 7 atheroma debris samples. The quantitative multiplex PCR detected sequences consistent with eukaryotic microbes in all 9 analyzed filter samples as well as 5 of the 7 atheroma debris samples. Microscopy reveals putative polymicrobial communities within filters and atheroma debris. The main contributing prokaryotic species in atheroma debris suggest a diverse and novel composition. Additionally, Funneliformis mosseae, an arbuscular mycorrhizal fungus in the Glomeraceae family, was detected in the coronary hard plaque from two patients. Well studied biofilm forming bacteria were not detectable in circulating peripheral blood and were not universally present in atheroma or filters. Analyses of the sequenced eukaryotes are consistent with a diverse of array poorly studied environmental eukaryotes. In summary, out of 15 patients, 6 exhibited molecular evidence of prokaryotes and 14 had molecular evidence of eukaryotic and/or polymicrobial communities in vivo, while 2 post-mortem coronary plaque samples displayed evidence of fungi.

CONCLUSION

Prokaryotes are not consistently observed in atheroma debris or filter samples; however, detection of protozoa and fungi in these samples suggests that they may play a role in arterial vascular disease or atheroma formation.

Malaria and the liver: immunological hide-and-seek or subversion of immunity from within?

During the pre-erythrocytic asymptomatic phase of malarial infection, sporozoites develop transiently inside less than 100 hepatocytes that subsequently release thousands of merozoites. Killing of these hepatocytes by cytotoxic T cells (CTLs) confers protection to subsequent malarial infection, suggesting that this bottleneck phase in the parasite life cycle can be targeted by vaccination. During natural transmission, although some CTLs are generated in the skin draining lymph nodes, they are unable to eliminate the parasite, suggesting that the liver is important for the sporozoite to escape immune surveillance. The contribution of the organ to this process is unclear. Based on the known ability of several hepatic antigen-presenting cells (APCs) to induce primary activation of CD8 T cells and tolerance, malarial antigens presented by both infected hepatocytes and/or hepatic cross-presenting APCs should result in tolerance. However, our latest model predicts that due to the low frequency of infected hepatocytes, some T cells recognizing sporozoite epitopes with high affinity should differentiate into CTLs. In this review, we discuss two possible models to explain why CTLs generated in the liver and skin draining lymph nodes are unable to eliminate the parasite: (1) sporozoites harness the tolerogenic property of the liver; (2) CTLs are not tolerized but fail to detect infected cells due to sparse infection of hepatocytes and the very short liver stage. We propose that while malaria sporozoites might use the ability of the liver to tolerize both naive and effector cells, they have also developed strategies to decrease the probability of encounter between CTLs and infected liver cells. Thus, we predict that to achieve protection, vaccination strategies should aim to boost intrahepatic activation and/or increase the chance of encounter between sporozoite-specific CTLs and infected hepatocytes.

Wolbachia infection density in populations of the Asian citrus psyllid (Hemiptera: Liviidae)

The symbiotic relationships between bacteria of the genus Wolbachia (order Rickettsiales) and their arthropod hosts are diverse and can range from mutualism to parasitism. Whereas effects of Wolbachia on host biology are well investigated, little is known about diversity and abundance of Wolbachia in their natural hosts. The phloem-feeding Asian citrus psyllid, Diaphorina citri (Kuwayama) (Hemiptera: Liviidae), is naturally infected with Wolbachia (wDi). In the current study, we calculated the within-host density of Wolbachia in Florida D. citri populations using quantitative polymerase chain reaction for detection of the Wolbachia outer surface protein gene, wsp. Gene quantities were normalized to the D. citri wingless gene (Wg) to estimate Wolbachia abundance in individual D. citri. Using this method, significant geographic differences in Wolbachia densities were detected among Florida D. citri populations, with higher infection levels occurring in male versus female hosts.

Curcumin treatment provides protection against Trypanosoma cruzi infection

Trypanosoma cruzi, the etiologic agent of Chagas disease, causes an acute myocarditis and chronic cardiomyopathy. The current therapeutic agents for this disease are not always effective and often have severe side effects. Curcumin, a plant polyphenol, has demonstrated a wide range of potential therapeutic effects. In this study, we examined the effect of curcumin on T. cruzi infection in vitro and in vivo. Curcumin pretreatment of fibroblasts inhibited parasite invasion. Treatment reduced the expression of the low density lipoprotein receptor, which is involved in T. cruzi host cell invasion. Curcumin treatment of T. cruzi-infected CD1 mice reduced parasitemia and decreased the parasitism of infected heart tissue. This was associated with a significant reduction in macrophage infiltration and inflammation in both the heart and liver; moreover, curcumin-treated infected mice displayed a 100% survival rate in contrast to the 60% survival rate commonly observed in untreated infected mice. These data are consistent with curcumin modulating infection-induced changes in signaling pathways involved in inflammation, oxidative stress, and apoptosis. These data suggest that curcumin and its derivatives could be a suitable drug for the amelioration of chagasic heart disease.

Enforcing host cell polarity: an apicomplexan parasite strategy towards dissemination

The propagation of apicomplexan parasites through transmitting vectors is dependent on effective dissemination of parasites inside the mammalian host. Intracellular Toxoplasma and Theileria parasites face the challenge that their spread inside the host depends in part on the motile capacities of their host cells. In response, these parasites influence the efficiency of dissemination by altering adhesive and/or motile properties of their host cells. Theileria parasites do so by targeting signalling pathways that control host cell actin dynamics. The resulting enforced polar host cell morphology facilitates motility and invasiveness, by establishing focal adhesion and invasion structures at the leading edge of the infected cell. This parasite strategy highlights mechanisms of motility regulation that are also likely relevant for immune or cancer cell motility.

Activation of a PAK-MEK signalling pathway in malaria parasite-infected erythrocytes

Merozoites of malaria parasites invade red blood cells (RBCs), where they multiply by schizogony, undergoing development through ring, trophozoite and schizont stages that are responsible for malaria pathogenesis. Here, we report that a protein kinase-mediated signalling pathway involving host RBC PAK1 and MEK1, which do not have orthologues in the Plasmodium kinome, is selectively stimulated in Plasmodium falciparum-infected (versus uninfected) RBCs, as determined by the use of phospho-specific antibodies directed against the activated forms of these enzymes. Pharmacological interference with host MEK and PAK function using highly specific allosteric inhibitors in their known cellular IC₅₀ ranges results in parasite death. Furthermore, MEK inhibitors have parasiticidal effects in vitro on hepatocyte and erythrocyte stages of the rodent malaria parasite Plasmodium berghei, indicating conservation of this subversive strategy in malaria parasites. These findings have profound implications for the development of novel strategies for antimalarial chemotherapy.

The transforming parasite Theileria co-opts host cell mitotic and central spindles to persist in continuously dividing cells

The transforming protozoan Theileria recruits Plk1, a host kinase that regulates mitosis, to its surface and engages spindle microtubules to secure its division and inheritance into both daughter cells.

The protozoan parasite Theileria inhabits the host cell cytoplasm and possesses the unique capacity to transform the cells it infects, inducing continuous proliferation and protection against apoptosis. The transforming schizont is a multinucleated syncytium that resides free in the host cell cytoplasm and is strictly intracellular. To maintain transformation, it is crucial that this syncytium is divided over the two daughter cells at each host cell cytokinesis. This process was dissected using different cell cycle synchronization methods in combination with the targeted application of specific inhibitors. We found that Theileria schizonts associate with newly formed host cell microtubules that emanate from the spindle poles, positioning the parasite at the equatorial region of the mitotic cell where host cell chromosomes assemble during metaphase. During anaphase, the schizont interacts closely with host cell central spindle. As part of this process, the schizont recruits a host cell mitotic kinase, Polo-like kinase 1, and we established that parasite association with host cell central spindles requires Polo-like kinase 1 catalytic activity. Blocking the interaction between the schizont and astral as well as central spindle microtubules prevented parasite segregation between the daughter cells during cytokinesis. Our findings provide a striking example of how an intracellular eukaryotic pathogen that evolved ways to induce the uncontrolled proliferation of the cells it infects usurps the host cell mitotic machinery, including Polo-like kinase 1, one of the pivotal mitotic kinases, to ensure its own persistence and survival.

As part of their survival tactics, intracellular parasites often resort to cunning mechanisms to manipulate the cells they inhabit. Theileria, an important and particularly artful parasite of cattle in the tropics, transforms parasitized cells (that is, it induces continuous proliferation and protection from apoptosis—a state reminiscent of tumor cells). As a large, strictly intracellular syncytium, the transforming Theileria schizont cannot exit from the infected cell to invade other target cells. How then does the parasite ensure that each daughter cell, generated upon host cell division, remains infected and transformed? Our data show that the parasite co-opts the mitotic apparatus of the host cell and Plk1, a host protein kinase with a central regulatory role in mitosis and cytokinesis. As the host cell enters mitosis, the schizont binds to the microtubules that emanate symmetrically from the two spindle poles. This microtubule binding positions the schizont so that it spans the equatorial region of the mitotic cell where host cell chromosomes assemble. Then, as sister chromatids start to separate, the schizont associates with Plk1 and the central spindle that assembles between the separating chromosomes, with the activity of Plk1 presumably coordinating progression through mitosis with proper schizont positioning. This alignment with the central spindle positions the schizont to be included in the plane of cell division at the onset of cytokinesis, thus ensuring faithful passage of a Theileria schizont on to each daughter cell.

Transinfection of Wolbachia in planthoppers: nymphal injection of cultured Wolbachia and infection dynamics

Wolbachia species are intracellular symbionts that cause reproductive alterations in arthropods. Transinfection experiments have been performed in many arthropod species to elucidate the interaction between Wolbachia and a new host. To ease transinfection of this bacterium to new arthropod hosts, we introduced two techniques: nymphal injection instead of embryonic injection and the use of a cultured source of Wolbachia instead of direct transfer from donors to recipients. Wolbachia in the small brown planthopper Laodelphax striatellus was cultivated in a cell line and injected into the nymphal body cavity of the brown planthopper Nilaparvata lugens together with the cells. By using these techniques, two transinfected planthopper lines were obtained. In one line, Wolbachia disappeared after several generations; in the other line, Wolbachia was retained for >7 yr. Infection rates in this latter transinfected line were approximately 80% in early generations after transinjection but decreased to <10% through 40-60 generations. Subsequent selection for Wolbachia-infected females in this line did not increase the infection rate as a temporary effect. Thus, this transinfected line of N. lugens showed cytoplasmic incompatibility, although the incompatibility level was lower than in L. striatellus, the original host. The method of transinfection presented herein is useful for transmitting intracellular symbionts between small arthropod hosts.

Progress towards understanding the immunobiology of Theileria parasites

The pathogenic Theileria species Theileria parva and T. annulata infect bovine leukocytes and erythrocytes causing acute, often fatal lymphoproliferative diseases in cattle. The parasites are of interest not only because of their economic importance as pathogens, but also because of their unique ability to transform the leukocytes they infect. The latter property allows parasitized leukocytes to be cultured as continuously growing cell lines in vitro, thus providing an amenable in vitro system to study the parasite/host cell relationship and parasite-specific cellular immune responses. This paper summarizes important advances in knowledge of the immunobiology of these parasites over the last 40 years, focusing particularly on areas of relevance to vaccination.

Expression analysis of the Theileria parva subtelomere-encoded variable secreted protein gene family

Background

The intracellular protozoan parasite Theileria parva transforms bovine lymphocytes inducing uncontrolled proliferation. Proteins released from the parasite are assumed to contribute to phenotypic changes of the host cell and parasite persistence. With 85 members, genes encoding subtelomeric variable secreted proteins (SVSPs) form the largest gene family in T. parva. The majority of SVSPs contain predicted signal peptides, suggesting secretion into the host cell cytoplasm.

Methodology/Principal Findings

We analysed SVSP expression in T. parva-transformed cell lines established in vitro by infection of T or B lymphocytes with cloned T. parva parasites. Microarray and quantitative real-time PCR analysis revealed mRNA expression for a wide range of SVSP genes. The pattern of mRNA expression was largely defined by the parasite genotype and not by host background or cell type, and found to be relatively stable in vitro over a period of two months. Interestingly, immunofluorescence analysis carried out on cell lines established from a cloned parasite showed that expression of a single SVSP encoded by TP03_0882 is limited to only a small percentage of parasites. Epitope-tagged TP03_0882 expressed in mammalian cells was found to translocate into the nucleus, a process that could be attributed to two different nuclear localisation signals.

Conclusions

Our analysis reveals a complex pattern of Theileria SVSP mRNA expression, which depends on the parasite genotype. Whereas in cell lines established from a cloned parasite transcripts can be found corresponding to a wide range of SVSP genes, only a minority of parasites appear to express a particular SVSP protein. The fact that a number of SVSPs contain functional nuclear localisation signals suggests that proteins released from the parasite could contribute to phenotypic changes of the host cell. This initial characterisation will facilitate future studies on the regulation of SVSP gene expression and the potential biological role of these enigmatic proteins.

The protozoan parasite Theileria annulata alters the differentiation state of the infected macrophage and suppresses musculoaponeurotic fibrosarcoma oncogene (MAF) transcription factors

The tick-borne protozoan parasite Theileria annulata causes a debilitating disease of cattle called Tropical Theileriosis. The parasite predominantly invades bovine macrophages (m phi) and induces host cell transformation by a mechanism that has not been fully elucidated. Infection is associated with loss of characteristic m phi functions and phenotypic markers, indicative of host cell de-differentiation. We have investigated the effect of T. annulata infection on the expression of the m phi differentiation marker c-maf. The up-regulation of c-maf mRNA levels observed during bovine monocyte differentiation to m phi was suppressed by T. annulata infection. Furthermore, mRNA levels for c-maf and the closely related transcription factor mafB were significantly lower in established T. annulata-infected cell-lines than in bovine monocyte-derived m phi. Treatment of T. annulata-infected cells with the theileriacidal drug buparvaquone induced up-regulation of c-maf and mafB, which correlated with altered expression of down-stream target genes, e.g. up-regulation of integrin B7 and down-regulation of IL12A. Furthermore, T. annulata infection is associated with the suppression of the transcription factors, Pu.1 and RUNX1, and colony stimulating factor 1 receptor (CSF1R) which are also involved in the regulation of monocyte/m phi differentiation. We believe these results provide the first direct evidence that T. annulata modulates the host m phi differentiation state, which may diminish the defence capabilities of the infected cell and/or promote cell proliferation. Musculoaponeurotic fibrosarcoma oncogene (MAF) transcription factors play an important role in cell proliferation, differentiation and survival; therefore, regulation of these genes may be a major mechanism employed by T. annulata to survive within the infected m phi.

High and low temperatures differently affect infection density and vertical transmission of male-killing Spiroplasma symbionts in Drosophila hosts

We investigated the vertical transmission, reproductive phenotype, and infection density of a male-killing Spiroplasma symbiont in two Drosophila species under physiological high and low temperatures through successive host generations. In both the native host Drosophila nebulosa and the nonnative host Drosophila melanogaster, the symbiont infection and the male-killing phenotype were stably maintained at 25 degrees C, rapidly lost at 18 degrees C, and gradually lost at 28 degrees C. In the nonnative host, both the high and low temperatures significantly suppressed the infection density of the spiroplasma. In the native host, by contrast, the low temperature suppressed the infection density of the spiroplasma whereas the high temperature had little effect on the infection density. These results suggested that the low temperature suppresses both the infection density and the vertical transmission of the spiroplasma whereas the high temperature suppresses the vertical transmission preferentially. The spiroplasma density was consistently higher in the native host than in the nonnative host, suggesting that the host genotype may affect the infection density of the symbiont. The temperature- and genotype-dependent instability of the symbiont infection highlights a complex genotype-by-genotype-by-environment interaction and may be relevant to the low infection frequencies of the male-killing spiroplasmas in natural Drosophila populations.

Protozoan parasites: programmed cell death as a mechanism of parasitism

Programmed cell death (PCD) is a potent mechanism to remove parasitized cells, but it has also been shown that protozoan parasites can induce or inhibit apoptosis in host cells. In recent years, it has become clear that unicellular parasites can also undergo PCD, meaning that they commit suicide in response to various stimuli. This review focuses on the role of protozoan PCD and on the interaction between protozoan parasites and the host cell death machinery from the perspective of parasite survival strategies.

Suicide prevention: disruption of apoptotic pathways by protozoan parasites

The modulation of apoptosis has emerged as an important weapon in the pathogenic arsenal of multiple intracellular protozoan parasites. Cryptosporidium parvum, Leishmania spp., Trypanosoma cruzi, Theileria spp., Toxoplasma gondii and Plasmodium spp. have all been shown to inhibit the apoptotic response of their host cell. While the pathogen mediators responsible for this modulation are unknown, the parasites are interacting with multiple apoptotic regulatory systems to render their host cell refractory to apoptosis during critical phases of intracellular infection, including parasite invasion, establishment and replication. Additionally, emerging evidence suggests that the parasite life cycle stage impacts the modulation of apoptosis and possibly parasite differentiation. Dissection of the host-pathogen interactions involved in modulating apoptosis reveals a dynamic and complex interaction that recent studies are beginning to unravel.