Parasitism as a lifestyle: Ultimate intimacy between Apicomplexan protozoans and metazoan hosts.

Editorial: The Apicomplexa parasite Toxoplasma gondii glides on substrate with a helical path and releases material that forms a trail behind. The helical microtubules (green) periodically compress and relax, acting as spring force by coupling with the myosin motor (red).

Theileria secretes proteins to subvert its host leukocyte.

Theileria parasites are classified in the phylum Apicomplexa that includes several genera of medical and veterinary importance such as Plasmodium, Babesia, Toxoplasma and Cryptosporidium. These protozoans have evolved subtle ways to reshape their intracellular niche for their own benefit and Theileria is no exception. This tick transmitted microorganism is unique among all eukaryotes in that its intracellular schizont stage is able to transform its mammalian host leukocytes into an immortalised highly disseminating cell that phenocopies tumour cells. Here, we describe what is known about secreted Theileria-encoded host cell manipulators.

Patatin-like phospholipases in microbial infections with emerging roles in fatty acid metabolism and immune regulation by Apicomplexa.

Emerging lipidomic technologies have enabled researchers to dissect the complex roles of phospholipases in lipid metabolism, cellular signaling and immune regulation. Host phospholipase products are involved in stimulating and resolving the inflammatory response to pathogens. While many pathogen-derived phospholipases also manipulate the immune response, they have recently been shown to be involved in lipid remodeling and scavenging during replication. Animal and plant hosts as well as many pathogens contain a family of patatin-like phospholipases, which have been shown to have phospholipase A2 activity. Proteins containing patatin-like phospholipase domains have been identified in protozoan parasites within the Apicomplexa phylum. These parasites are the causative agents of some of the most widespread human diseases. Malaria, caused by Plasmodium spp., kills nearly half a million people worldwide each year. Toxoplasma and Cryptosporidium infect millions of people each year with lethal consequences in immunocompromised populations. Parasite-derived patatin-like phospholipases are likely effective drug targets and progress in the tools available to the Apicomplexan field will allow for a closer look at the interplay of lipid metabolism and immune regulation during host infection.

Apicomplexan profilins in vaccine development applied to bovine neosporosis.

Neospora caninum, an intracellular protozoan parasite from the phylum Apicomplexa, is the etiologic agent of neosporosis, a disease considered as a major cause of reproductive loss in cattle and neuromuscular disease in dogs. Bovine neosporosis has a great economic impact in both meat and dairy industries, related to abortion, premature culling and reduced milk yields. Although many efforts have been made to restrain bovine neosporosis, there are still no efficacious control methods. Many vaccine-development studies focused in the apicomplexan proteins involved in the adhesion and invasion of the host cell. Among these proteins, profilins have recently emerged as potential vaccine antigens or even adjuvant candidates for several diseases caused by apicomplexan parasites. Profilins bind Toll-like receptors 11 and 12 initiating MyD88 signaling, that triggers IL-12 and IFN-γ production, which may promote protection against infection. Here we summarized the state-of-the-art of novel vaccine development based on apicomplexan profilins applied as antigens or adjuvants, and delved into recent advances on N. caninum vaccines using profilin in the mouse model and in cattle.

Large, rapidly evolving gene families are at the forefront of host-parasite interactions in Apicomplexa.

The Apicomplexa is a phylum of parasitic protozoa, which includes the malaria parasite Plasmodium, amongst other species that can devastate human and animal health. The past decade has seen the release of genome sequences for many of the most important apicomplexan species, providing an excellent basis for improving our understanding of their biology. One of the key features of each genome is a unique set of large, variant gene families. Although closely related species share the same families, even different types of malaria parasite have distinct families. In some species they tend to be found at the ends of chromosomes, which may facilitate aspects of gene expression regulation and generation of sequence diversity. In others they are scattered apparently randomly across chromosomes. For some families there is evidence they are involved in antigenic variation, immune regulation and immune evasion. For others there are no known functions. Even where function is unknown these families are most often predicted to be exposed to the host, contain much sequence diversity and evolve rapidly. Based on these properties it is clear that they are at the forefront of host-parasite interactions. In this review I compare and contrast the genomic context, gene structure, gene expression, protein localization and function of these families across different species.

Identification of potential general markers of disease resistance in American oysters, Crassostrea virginica through gene expression studies.

Several diseases have a significant impact on American oyster populations in the Atlantic coasts of North America. Knowledge about the responses of oysters to pathogenic challenge could help in identifying potential markers of disease resistance and biomarkers of the health status of an oyster population. A previous analysis of the transcriptome of resistant and susceptible American oysters in response to challenge with the bacterial pathogen Roseovarius crassostreae, as well as sequencing of suppression subtractive hybridization libraries from oysters challenged with the protozoan parasite Perkinsus marinus, provided a list of genes potentially involved in disease resistance or susceptibility. We investigated the patterns of inducible gene expression of several of these genes in response to experimental challenge with the oyster pathogens R. crassostreae, Vibrio tubiashii, and P. marinus. Oysters showing differential susceptibility to R. crassostreae demonstrated differential patterns of expression of genes coding for immune (serine protease inhibitor-1, SPI1) and stress-related (heat shock protein 70, HSP70; arginine kinase) proteins 30 days after challenge with this bacterial pathogen. Differential patterns of expression of immune (spi1, galectin and a matrix metalloproteinase) and stress-related (hsp70, histone H4, and arginine kinase) genes was observed in hemocytes from adult oysters challenged with P. marinus, but not with V. tubiashii. While levels of spi1 expression in hemocytes collected 8 and 21 days after P. marinus challenge were negatively correlated with parasite load in oysters tissues at the end of the challenge (62 days), levels of expression of hsp70 in hemocytes collected 1-day after challenge were positively correlated with oyster parasite load at 62 days. Our results confirm previous research on the role of serine protease inhibitor-1 in immunity and disease resistance in oysters. They also suggest that HSP70 and histone H4 could be used as a markers of health status or disease susceptibility in oysters.

Proteases as regulators of pathogenesis: examples from the Apicomplexa.

The diverse functional roles that proteases play in basic biological processes make them essential for virtually all organisms. Not surprisingly, proteolysis is also a critical process required for many aspects of pathogenesis. In particular, obligate intracellular parasites must precisely coordinate proteolytic events during their highly regulated life cycle inside multiple host cell environments. Advances in chemical, proteomic and genetic tools that can be applied to parasite biology have led to an increased understanding of the complex events centrally regulated by proteases. In this review, we outline recent advances in our knowledge of specific proteolytic enzymes in two medically relevant apicomplexan parasites: Plasmodium falciparum and Toxoplasma gondii. Efforts over the last decade have begun to provide a map of key proteotolyic events that are essential for both parasite survival and propagation inside host cells. These advances in our molecular understanding of proteolytic events involved in parasite pathogenesis provide a foundation for the validation of new networks and enzyme targets that could be exploited for therapeutic purposes. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Genetic mapping identifies novel highly protective antigens for an apicomplexan parasite.

Apicomplexan parasites are responsible for a myriad of diseases in humans and livestock; yet despite intensive effort, development of effective sub-unit vaccines remains a long-term goal. Antigenic complexity and our inability to identify protective antigens from the pool that induce response are serious challenges in the development of new vaccines. Using a combination of parasite genetics and selective barriers with population-based genetic fingerprinting, we have identified that immunity against the most important apicomplexan parasite of livestock (Eimeria spp.) was targeted against a few discrete regions of the genome. Herein we report the identification of six genomic regions and, within two of those loci, the identification of true protective antigens that confer immunity as sub-unit vaccines. The first of these is an Eimeria maxima homologue of apical membrane antigen-1 (AMA-1) and the second is a previously uncharacterised gene that we have termed 'immune mapped protein-1' (IMP-1). Significantly, homologues of the AMA-1 antigen are protective with a range of apicomplexan parasites including Plasmodium spp., which suggest that there may be some characteristic(s) of protective antigens shared across this diverse group of parasites. Interestingly, homologues of the IMP-1 antigen, which is protective against E. maxima infection, can be identified in Toxoplasma gondii and Neospora caninum. Overall, this study documents the discovery of novel protective antigens using a population-based genetic mapping approach allied with a protection-based screen of candidate genes. The identification of AMA-1 and IMP-1 represents a substantial step towards development of an effective anti-eimerian sub-unit vaccine and raises the possibility of identification of novel antigens for other apicomplexan parasites. Moreover, validation of the parasite genetics approach to identify effective antigens supports its adoption in other parasite systems where legitimate protective antigen identification is difficult.

Parasite burden and the insect immune response: interpopulation comparison.

The immune response affects host's survival and reproductive success. Insurmountable immune function has not evolved because it is costly and there is a trade-off between other life-history traits. In previous studies several factors such as diet and temperature have been proposed to cause interpopulation differences in immune response. Moreover, the insect immune system may be functionally more protective upon secondary exposure, thus infection history may associate with the immune response. Here we measured how geographical location and parasite burden is related to variation in immune response between populations. We included 13 populations of the Northern Damselfly Coenagrion hastulatum (Odonata: Coenagrionidae) in Finland over a latitudinal range of 880 km to this study. We found that water mites associated strongly with the immune response at interpopulation level: the more the mites, the higher the immune response. Also, in an alternative model based on AIC, latitude and individual size associated with the immune response. In turn, endoparasitic gregarines did not affect the immune response. To conclude, a positive interpopulation association between the immune response and the rate of water mite infection may indicate (i) local adaptation to chronic parasite stress, (ii) effective 'induced' immune response against parasites, or (iii) a combined effect of both of these.

[Advances in the development of vaccines for bovine neosporosis]. / Avances en el desarrollo de vacunas contra la neosporosis bovina.

Neosporosis, a disease caused by the obligate intracellular protozoan Neospora caninum, produces abortions in cattle. The severe economic losses in cattle industry justify the need to develop control measures for preventing bovine abortion. Apicomplexan parasitic resistance is associated with T helper 1 immune response mediated by CD4 cytotoxic T lymphocytes, the production of interferon-gamma, interleukin-12, tumor necrosis factor and immunoglobulin G2. The reduction of vertical transmission in subsequent pregnancies and the low levels of abortion repetition suggests the existence of protective immune mechanisms. Inoculation with live tachyzoites before mating protects against infection and abortion. Antecedents of the development of live vaccines against other protozoa stimulate research to develop a live vaccine against N. caninum. On the other hand, an inactivated vaccine with low efficacy against neosporosis is useful in the prevention of abortion in farms with epizootic disease. A neosporosis vaccine should avoid abortion, transplacental transmission and infection persistence. In the present work, advances in vaccine development including lysate of tachyzoites, live parasites, recombinant antigens and vaccine vectors are reviewed.

CD4+ cytolytic T cell clone confers protection against murine malaria.

A CD4+ T cell clone (A1.6) was derived from spleen cells of mice immunized with irradiated sporozoites. This T cell clone recognizes an antigen that is shared by sporozoites and blood forms of Plasmodium berghei and differs from the circumsporozoite protein. Clone A1.6 displays cytotoxic activity, produces IFN-gamma and IL-2 in vitro, and recognizes the plasmodial antigen in the context of the class II I-Ed molecule. Passive transfer of this CD4+ clone into naive mice resulted in a high degree of protection against sporozoite challenge.

Cell type- and species-specific host responses to Toxoplasma gondii and its near relatives.

Toxoplasma gondii is remarkably unique in its ability to successfully infect vertebrate hosts from multiple phyla and can successfully infect most cells within these organisms. The infection outcome in each of these species is determined by the complex interaction between parasite and host genotype. As techniques to quantify global changes in cell function become more readily available and precise, new data are coming to light about how (i) different host cell types respond to parasitic infection and (ii) different parasite species impact the host. Here we focus on recent studies comparing the response to intracellular parasitism by different cell types and insights into understanding host-parasite interactions from comparative studies on T. gondii and its close extant relatives.

Fluorescent-antibody studies of haplosporidian parasites of oysters in Chesapeake and Delaware bays.

A fluorescent antibody was produced against a plasmodial oyster parasite thought to be a haplosporidian, Minchinia nelsoni, which was termed MSX. The antibody reacted with this type of plasmodia from oysters, Crassostrea virginica ( Gmelin), from all sources, and it also reacted with its sporulating and spore stages. This reaction indicates cospecificity of the stages. No reaction occurred with any stage of another oyster haplosporidian, Minchinia costalis, indi- cating that the M. nelsoni and the M. costalis are antigenically distinct species.

Proteosome-lipopeptide vaccines: enhancement of immunogenicity for malaria CS peptides.

Proteosomes are hydrophobic, membranous, multimolecular preparations of meningococcal outer membrane proteins that are also B cell mitogens. These characteristics suggested that proteosomes may serve as carrier proteins and adjuvants to enhance peptide immunogenicity. Although high titers of malaria circumsporozoite (CS) antibodies protect against malaria, vaccines thus far tested in humans have been insufficiently immunogenic to be clinically useful. Here it is shown that synthetic CS peptides hydrophobically complexed to proteosomes by way of lauroyl-cysteine become highly immunogenic in mice without other adjuvants. The high titers of antibodies produced and the safety of proteosomes in humans suggest that this novel system is widely applicable for the development of peptide vaccines to protect against many diseases.

Epigenetics in Apicomplexa: control of gene expression during cell cycle progression, differentiation and antigenic variation.

Apicomplexan parasites are important disease causing organisms that infect both animals and humans, causing extensive health and economic damage to human populations, particularly those in the developing world. The ability to perform genetic crosses, to engineer transgenic parasites lines, and the wealth of information made available through recent genome sequencing projects have made the laboratory study of these parasites important not only for understanding the diseases that they cause, but also for gaining insights into basic biological processes. The control of gene expression and cellular differentiation are particularly interesting in these organisms, as the apparent lack of large families of recognizable transcription factors typically found in other eukaryotic organisms suggests that they may be unusually reliant on epigenetic mechanisms. Here we review recent advances in the study of epigenetic gene regulation in the apicomplexan parasites Plasmodium falciparum and Toxoplasma gondii.

An Overview of Peripheral Blood Mononuclear Cells as a Model for Immunological Research of Toxoplasma gondii and Other Apicomplexan Parasites.

In biology, models are experimental systems meant to recreate aspects of diseases or human tissue with the goal of generating inferences and approximations that can contribute to the resolution of specific biological problems. Although there are many models for studying intracellular parasites, their data have produced critical contradictions, especially in immunological assays. Peripheral blood mononuclear cells (PBMCs) represent an attractive tissue source in pharmacogenomics and in molecular and immunologic studies, as these cells are easily collected from patients and can serve as sentinel tissue for monitoring physiological perturbations due to disease. However, these cells are a very sensitive model due to variables such as temperature, type of stimulus and time of collection as part of posterior processes. PBMCs have been used to study Toxoplasma gondii and other apicomplexan parasites. For instance, this model is frequently used in new therapies or vaccines that use peptides or recombinant proteins derived from the parasite. The immune response to T. gondii is highly variable, so it may be necessary to refine this cellular model. This mini review highlights the major approaches in which PBMCs are used as a model of study for T. gondii and other apicomplexan parasites. The variables related to this model have significant implications for data interpretation and conclusions related to host-parasite interaction.

Innate Lymphoid Cells in Protection, Pathology, and Adaptive Immunity During Apicomplexan Infection.

Apicomplexans are a diverse and complex group of protozoan pathogens including Toxoplasma gondii, Plasmodium spp., Cryptosporidium spp., Eimeria spp., and Babesia spp. They infect a wide variety of hosts and are a major health threat to humans and other animals. Innate immunity provides early control and also regulates the development of adaptive immune responses important for controlling these pathogens. Innate immune responses also contribute to immunopathology associated with these infections. Natural killer (NK) cells have been for a long time known to be potent first line effector cells in helping control protozoan infection. They provide control by producing IL-12 dependent IFNγ and killing infected cells and parasites via their cytotoxic response. Results from more recent studies indicate that NK cells could provide additional effector functions such as IL-10 and IL-17 and might have diverse roles in immunity to these pathogens. These early studies based their conclusions on the identification of NK cells to be CD3-, CD49b+, NK1.1+, and/or NKp46+ and the common accepted paradigm at that time that NK cells were one of the only lymphoid derived innate immune cells present. New discoveries have lead to major advances in understanding that NK cells are only one of several populations of innate immune cells of lymphoid origin. Common lymphoid progenitor derived innate immune cells are now known as innate lymphoid cells (ILC) and comprise three different groups, group 1, group 2, and group 3 ILC. They are a functionally heterogeneous and plastic cell population and are important effector cells in disease and tissue homeostasis. Very little is known about each of these different types of ILCs in parasitic infection. Therefore, we will review what is known about NK cells in innate immune responses during different protozoan infections. We will discuss what immune responses attributed to NK cells might be reconsidered as ILC1, 2, or 3 population responses. We will then discuss how different ILCs may impact immunopathology and adaptive immune responses to these parasites.

Cytosolic Recognition of Microbes and Pathogens: Inflammasomes in Action.

Infection is a dynamic biological process underpinned by a complex interplay between the pathogen and the host. Microbes from all domains of life, including bacteria, viruses, fungi, and protozoan parasites, have the capacity to cause infection. Infection is sensed by the host, which often leads to activation of the inflammasome, a cytosolic macromolecular signaling platform that mediates the release of the proinflammatory cytokines interleukin-1ß (IL-1ß) and IL-18 and cleavage of the pore-forming protein gasdermin D, leading to pyroptosis. Host-mediated sensing of the infection occurs when pathogens inject or carry pathogen-associated molecular patterns (PAMPs) into the cytoplasm or induce damage that causes cytosolic liberation of danger-associated molecular patterns (DAMPs) in the host cell. Recognition of PAMPs and DAMPs by inflammasome sensors, including NLRP1, NLRP3, NLRC4, NAIP, AIM2, and Pyrin, initiates a cascade of events that culminate in inflammation and cell death. However, pathogens can deploy virulence factors capable of minimizing or evading host detection. This review presents a comprehensive overview of the mechanisms of microbe-induced activation of the inflammasome and the functional consequences of inflammasome activation in infectious diseases. We also explore the microbial strategies used in the evasion of inflammasome sensing at the host-microbe interaction interface.