EtMIC4: a microneme protein from Eimeria tenella that contains tandem arrays of epidermal growth factor-like repeats and thrombospondin type-I repeats

Inhibition of Cell Apoptosis by Apicomplexan Protozoa-Host Interaction in the Early Stage of Infection

Apicomplexan protozoa, which are a group of specialized intracellular parasitic protozoa, infect humans and other animals and cause a variety of diseases. The lack of research on the interaction mechanism between Apicomplexan protozoa and their hosts is a key factor restricting the development of new drugs and vaccines. In the early stages of infection, cell apoptosis is inhibited by Apicomplexan protozoa through their interaction with the host cells; thereby, the survival and reproduction of Apicomplexan protozoa in host cells is promoted. In this review, the key virulence proteins and pathways are introduced regarding the inhibition of cell apoptosis by the interaction between the protozoa and their host during the early stage of Apicomplexan protozoa infection. It provides a theoretical basis for the development of drugs or vaccines for protozoal diseases.

Identification of Eimeria tenella sporozoite immunodominant mimotopes by random phage-display peptide libraries-a proof of concept study

INTRODUCTION

Coccidiosis, caused by parasites of numerous Eimeria species, has long been recognized as an economically significant disease in the chicken industry worldwide. The rise of anti-coccidian resistance has driven a search for other parasite management techniques. Recombinant antigen vaccination presents a highly feasible alternative. Properly identifying antigens that might trigger a potent immune response is one of the major obstacles to creating a viable genetically modified vaccine.

METHODS

This study evaluated a reverse immunology approach for the identification of B-cell epitopes. Antisera from rabbits and hens inoculated with whole-sporozoites of E. tenella were used to identify Western blot antigens. The rabbit IgG fraction from the anti-sporozoite serum exhibited the highest reactogenicity; consequently, it was purified and utilized to screen two random Phage-display peptide libraries (12 mer and c7c mer). After three panning rounds, 20 clones from each library were randomly selected, their nucleotide sequences acquired, and their reactivity to anti-sporozoite E. tenella serum assessed. The selected peptide clones inferred amino acid sequences matched numerous E. tenella proteins.

RESULTS AND CONCLUSIONS

The extracellular domain of the epidermal growth factor-like (EGF-like) repeats, and the thrombospondin type-I (TSP-1) repeats of E. tenella micronemal protein 4 (EtMIC4) matched with the c7c mer selected clones CNTGSPYEC (2/20) and CMSTGLSSC (1/20) respectively. The clone CSISSLTHC that matched with a conserved hypothetical protein of E. tenella was widely selected (3/20). Selected clones from the 12-mer phage display library AGHTTQFNSKTT (7/20), GPNSAFWAGSER (2/20) and HFAYWWNGVRGP (8/20) showed similarities with a cullin homolog, elongation factor-2 and beta-dynein chain a putative E. tenella protein, respectively. Four immunodominant clones were previously selected and used to immunize rabbits. By ELISA and Western blot, all rabbit anti-clone serums detected E. tenella native antigens.

DISCUSSION

Thus, selected phagotopes contained recombinant E. tenella antigen peptides. Using antibodies against E. tenella sporozoites, this study demonstrated the feasibility of screening Phage-display random peptide libraries for true immunotopes. In addition, this study looked at an approach for finding novel candidates that could be used as an E. tenella recombinant epitope-based vaccine.

What Do We Know about Surface Proteins of Chicken Parasites Eimeria?

Poultry is the first source of animal protein for human consumption. In a changing world, this sector is facing new challenges, such as a projected increase in demand, higher standards of food quality and safety, and reduction of environmental impact. Chicken coccidiosis is a highly widespread enteric disease caused by Eimeria spp. which causes significant economic losses to the poultry industry worldwide; however, the impact on family poultry holders or backyard production-which plays a key role in food security in small communities and involves mainly rural women-has been little explored. Coccidiosis disease is controlled by good husbandry measures, chemoprophylaxis, and/or live vaccination. The first live vaccines against chicken coccidiosis were developed in the 1950s; however, after more than seven decades, none has reached the market. Current limitations on their use have led to research in next-generation vaccines based on recombinant or live-vectored vaccines. Next-generation vaccines are required to control this complex parasitic disease, and for this purpose, protective antigens need to be identified. In this review, we have scrutinised surface proteins identified so far in Eimeria spp. affecting chickens. Most of these surface proteins are anchored to the parasite membrane by a glycosylphosphatidylinositol (GPI) molecule. The biosynthesis of GPIs, as well as the role of currently identified surface proteins and interest as vaccine candidates has been summarised. The potential role of surface proteins in drug resistance and immune escape and how these could limit the efficacy of control strategies was also discussed.

Thrombospondin Related Anonymous Protein Superfamily in Vector-Borne Apicomplexans: The Parasite’s Toolkit for Cell Invasion

Apicomplexan parasites transmitted by vectors, including Babesia spp. and Plasmodium spp., cause severe disease in both humans and animals. These parasites have a complex life cycle during which they migrate, invade, and replicate in contrasting hosts such as the mammal and the invertebrate vector. The interaction of parasites with the host cell is mediated by adhesive proteins which play a key role in the different cellular processes regarding successful progression of the life cycle. Thrombospondin related anonymous protein (TRAP) is a superfamily of adhesins that are involved in motility, invasion and egress of the parasite. These proteins are stored and released from apical organelles and have either one or two types of adhesive domains, namely thrombospondin type 1 repeat and von Willebrand factor type A, that upon secretion are located in the extracellular portion of the molecule. Proteins from the TRAP superfamily have been intensively studied in Plasmodium species and to a lesser extent in Babesia spp., where they have proven to be functionally relevant throughout the entire parasite’s journey both in the arthropod vector and in the mammalian host. In recent years new findings provided answers to the role of TRAP proteins and in some cases the function of these adhesins during the parasite’s life cycle was redefined. In this review we will discuss the current knowledge of the diverse roles of the TRAP superfamily in vector-borne parasites from Class Aconoidasida. We will focus on the varied approaches that allowed the understanding of protein function and the relevance of TRAP- superfamily throughout the entire parasite’s cell cycle.

Dual RNA-seq transcriptome analysis of caecal tissue during primary Eimeria tenella infection in chickens

BACKGROUND

Coccidiosis is an infectious disease with large negative impact on the poultry industry worldwide. It is an enteric infection caused by unicellular Apicomplexan parasites of the genus Eimeria. The present study aimed to gain more knowledge about interactions between parasites and the host immune system during the early asexual replication phase of E. tenella in chicken caeca. For this purpose, chickens were experimentally infected with E. tenella oocysts, sacrificed on days 1-4 and 10 after infection and mRNA from caecal tissues was extracted and sequenced.

RESULTS

Dual RNA-seq analysis revealed time-dependent changes in both host and parasite gene expression during the course of the infection. Chicken immune activation was detected from day 3 and onwards with the highest number of differentially expressed immune genes recorded on day 10. Among early (days 3-4) responses up-regulation of genes for matrix metalloproteinases, several chemokines, interferon (IFN)-γ along with IFN-stimulated genes GBP, IRF1 and RSAD2 were noted. Increased expression of genes with immune suppressive/regulatory effects, e.g. IL10, SOCS1, SOCS3, was also observed among early responses. For E. tenella a general up-regulation of genes involved in protein expression and energy metabolism as well as a general down-regulation genes for DNA and RNA processing were observed during the infection. Specific E. tenella genes with altered expression during the experiment include those for proteins in rhoptry and microneme organelles.

CONCLUSIONS

The present study provides novel information on both the transcriptional activity of E. tenella during schizogony in ceacal tissue and of the local host responses to parasite invasion during this phase of infection. Results indicate a role for IFN-γ and IFN-stimulated genes in the innate defence against Eimeria replication.

Curcumin reduces enteric isoprostane 8-iso-PGF2α and prostaglandin GF2α in specific pathogen-free Leghorn chickens challenged with Eimeria maxima

The purpose of this pilot study was to evaluate and determine the concentration of prostaglandin GF2α (PGF2α) and isoprostane 8‐iso‐PGF2α in plasma and intestine of specific pathogen-free (SPF) Leghorn chickens challenged with Eimeria maxima, with or without dietary supplementation of curcumin using solid‐phase microextraction and ultra‐performance liquid chromatography/tandem mass spectrometry. Eighty 1-day-old male SPF chickens were randomly allocated to one of four groups with four replicates (n = 5 chickens/replicate). Groups consisted of: (1) Control (no challenge), (2) Curcumin (no challenge), (3) Eimeria maxima (challenge), and (4) Eimeria maxima (challenge) + curcumin. At day 28 of age, all chickens in the challenge groups were orally gavaged with 40,000 sporulated E. maxima oocysts. No significant differences (P > 0.05) were observed in the groups regardless of the treatment or challenge with E. maxima. Enteric levels of both isoprostane 8‐iso‐PGF2α and PGF2α at 7 days and 9 days post-challenge were significantly increased (P < 0.01) compared to the non-challenge control chickens. Interestingly, the enteric levels of both isoprostane 8‐iso‐PGF2α and PGF2α at 7 days post-challenge were significantly reduced in chickens fed curcumin, compared to control chickens challenge with E. maxima. At 9 days post-challenge, only levels of isoprostane 8‐iso‐PGF2α in the enteric samples were significantly reduced in chickens challenged with E. maxima supplemented with curcumin, compared with E. maxima challenge chickens. No differences of isoprostane 8‐iso‐PGF2α or PGF2α were observed in plasma at both days of evaluation. Similarly, no significant differences were observed between the challenge control or chickens challenge with E. maxima and supplemented with curcumin at both times of evaluation. The results of this pilot study suggests that the antioxidant anti-inflammatory properties of curcumin reduced the oxidative damage and subsequent intestinal mucosal over-production of lipid oxidation products. Further studies to confirm and extend these results in broiler chickens are required.

Specific EtMIC3-binding peptides inhibit Eimeria tenella sporozoites entry into host cells

Avian coccidiosis caused by Eimeria leads to huge economic losses on the global poultry industry. In this study, microneme adhesive repeat regions (MARR) bc1 of E. tenella microneme protein 3 (EtMIC3-bc1) was used as ligand, and peptides binding to EtMIC3 were screened from a phage display peptide library. The positive phage clones were checked by enzyme-linked immunosorbent assay (ELISA). Competitive ELISA was applied to further verify the binding capability between the positive phages and recombinant EtMIC3-bc1 protein or sporozoites protein. The inhibitory effects of target peptides on sporozoites invasion of MDBK cells were measured in vitro. Chickens were orally administrated with target positive phages and the protective effects against homologous challenge were evaluated. The model of three-dimensional (3D) structure for EtMIC3-bc1 was conducted, and molecular docking between target peptides and EtMIC3-bc1 model was analyzed. The results demonstrated that three selected positive phages specifically bind to EtMIC3-bc1 protein. The three peptides A, D and W effectively inhibited invasion of MDBK cells by sporozoites, showing inhibited ratio of 71.8%, 54.6% and 20.8%, respectively. Chickens in the group orally inoculated with phages A displayed more protective efficacies against homologous challenge than other groups. Molecular docking showed that amino acids in three peptides, especially in peptide A, insert into the hydrophobic groove of EtMIC3-bc1 protein, and bind to EtMIC3-bc1 through intermolecular hydrogen bonds. Taken together, the results suggest EtMIC3-binding peptides inhibit sporozoites entry into host cells. This study provides new idea for exploring novel strategies against coccidiosis.

Identification and Characterization of a Novel Apical Membrane Antigen 3 in Eimeria tenella

Eimeria tenella is an obligate intracellular parasite in the phylum Apicomplexa. As described for other members of Apicomplexa, apical membrane antigen 1 (AMA1) has been shown to be critical for sporozoite invasion of host cells by E. tenella. Recently, an E. tenella paralogue of AMA1 (EtAMA1), dubbed sporoAMA1 (EtAMA3), was identified in proteomic and transcriptomic analyses of E. tenella, but not further characterized. Here, we show that EtAMA3 is a type I integral membrane protein that has 24% -38% identity with other EtAMAs. EtAMA3 has the same pattern of Cys residues in domains I and II of AMA1 orthologs from apicomplexan parasites, but high variance in domain III, with all six invariant Cys residues absent. EtAMA3 expression was developmentally regulated at the mRNA and protein levels. EtAMA3 protein was detected in sporulated oocysts and sporozoites, but not in the unsporulated oocysts or second-generation merozoites. EtAMA3 is secreted by micronemes and is primarily localized to the apical end of sporozoites during host-cell invasion. Additionally, pretreatment of sporozoites with rEtAMA3-specific antibodies substantially impeded their invasion into host cells. These results suggest EtAMA3 is a sporozoite-specific protein that is involved in host-cell sporozoite invasion.

Prolonging and enhancing the protective efficacy of the EtMIC3-C-MAR against eimeria tenella through delivered by attenuated salmonella typhimurium

The microneme adhesive repeats (MAR) of Eimeria tenella microneme protein 3 (EtMIC3) are associated with binding to and invasion of host cells. Adhesion and invasion-related proteins or domains are often strongly immunogenic, immune responses mounted against these factors that play a key role in blocking invasion. In the present study, an oral live vaccine consisting of attenuated Salmonella typhimurium X4550 carrying two MAR domains fragment (St-X4550-MAR) was constructed and its protective efficacies were evaluated. The results showed that St-X4550-MAR was more immunogenic and conferred a higher degree of protection than recombinant MAR polypeptide as reflected by increased body weight, decreased oocyst shedding and lesion scores, increased serum IgG and cecal sIgA antibody production, and increasing levels of interferon-γ and interleukin-10. Thus, MAR domains are highly immunogenic and St-X4550-MAR had moderate activity against E. tenella infection by stimulating humoral, mucosal and cellular immunity. Chickens immunized with our constructed live vaccine provided considerable protections as early as at 10 d post-immunization (ACI: 155.17), and maintained higher protection levels at 20 d post-immunization (ACI: 173.66), and at 30 d post-immunization (ACI: 162.4). While the protective efficacy of chickens immunized with the recombinant MAR peptides showed a decreased trend as the post immunization time prolonging. Thus, using live-attenuated S. typhimurium X4550 as a vaccine expression and delivery system can significantly improve the protective efficacy and duration of protective immunity of MAR of EtMIC3.

Identification of a novel thrombospondin-related anonymous protein (BoTRAP2) from Babesia orientalis

BACKGROUND

The thrombospondin-related anonymous protein (TRAP) was first discovered in the sporozoite of Plasmodium falciparum and TRAP family proteins are secreted by micronemes and transported to the parasite surface to participate in the invasion process. Various TRAP proteins have been identified in apicomplexan protozoans, but there have been few reports about TRAP proteins in Babesia orientalis.

METHODS

The functional domain of TRAP2 in B. orientalis was cloned, sequenced, characterized and compared to the TRAP sequences of related apicomplexan parasites. The functional domain of BoTRAP2 was truncated, named BoTRAP2-1, and then cloned into the pET-28a expression vector. Rabbit anti-rBoTRAP2-1 polyclonal antibody was produced by immunizing three rabbits. Western blot analysis was used to identify the native form and immunogenicity of BoTRAP2. The localization of BoTRAP2 was identified by indirect fluorescence assay (IFA).

RESULTS

The amplified genes of BoTRAP2 are 2817 bp in length, encoding a functional domain of about 938 aa with two vWFA domains, one TSP domain and one transmembrane domain. The amino acid sequence of BoTRAP2 has a high similarity with that of B. bovis and B. gibsoni. The predicted tertiary structure of truncated BoTRAP2-1 confirmed that BoTRAP2 contains two vWFA domains and a TSP domain, the main functional areas of the protein. The native BoTRAP2 was identified from B. orientalis lysate by using rabbit polyclonal anti-rBoTRAP2-1. A band corresponding to rBoTRAP2-1 was detected by reaction with serum from a B. orientalis-infected water buffalo, indicating that the protein has a high immunogenicity. IFA showed that BoTRAP2 is mainly localized on the apical end of parasites by rabbit anti-rBoTRAP2-1 polyclonal serum.

CONCLUSIONS

The rBoTRAP2 could differentiate serum from B. orientalis-infected water buffalo and normal water buffalo, implicating that BoTRAP2 has high immunogenicity and could serve as a candidate antigen for diagnosis of B. orientalis infection in buffalo.

Quantitative proteomic studies in resistance mechanisms of Eimeria tenella against polyether ionophores

Polyether ionophores are widely used to treat and control coccidiosis in chickens. Widespread use of anticoccidials resulted in worldwide resistance. Mechanisms of resistance development and expansion are complex and poorly understood. Relative proteomic quantification using LC-MS/MS was used to compare sensitive reference strains (Ref-1, Ref-2) with putatively resistant and moderately sensitive field strains (FS-R, FS-mS) of Eimeria tenella after isotopic labelling with tandem mass tags (TMT). Ninety-seven proteins were identified, and 25 of them were regulated. Actin was significantly upregulated in resistant strains in comparison with their sensitive counterparts. On the other hand, microneme protein (MIC4) was downregulated in resistant strains. Optimization of labelling E. tenella sporozoites by TMT might identify further proteins that play a role in the obvious complex mechanism leading to resistance against Monensin.

Evaluation of the protective effect of pVAX-EtMIC3-recombined plasmid against E. tenella in chicken

Coccidiosis caused by protozoan parasites of the genus Eimeria has a severe economic impact on commercial production worldwide. Micronemes of Eimeria play important roles in invading intestinal cell processes. In this study, the DNA vaccine expressing Eimeria tenella microneme protein 3 (EtMIC3) was constructed to evaluate its immune protective effect against E. tenella infection in chickens. The results demonstrated that chickens immunized with pVAX-EtMIC3 produced strong immune responses in the body, as shown by significant lymphocyte proliferation, cytokine production, and antibody responses. The average body weight gains of chickens in all the vaccinated groups were higher than those of non-vaccinated and challenged groups. In general, oocyst shedding was reduced, and bloody feces and gut lesion scores decreased. In addition, the survival rate of the immunized chickens increased compared to that of the unvaccinated and challenged control chickens. In summary, this study indicated that pVAX-EtMIC3 could induce protective immune effects against coccidiosis and that EtMIC3 is a potential vaccine candidate against coccidiosis.

Breadth of humoral response and antigenic targets of sporozoite-inhibitory antibodies associated with sterile protection induced by controlled human malaria infection

The development of an effective malaria vaccine has remained elusive even until today. This is because of our incomplete understanding of the immune mechanisms that confer and/or correlate with protection. Human volunteers have been protected experimentally from a subsequent challenge by immunization with Plasmodium falciparum sporozoites under drug cover. Here, we demonstrate that sera from the protected individuals contain neutralizing antibodies against the pre-erythrocytic stage. To identify the antigen(s) recognized by these antibodies, a newly developed library of P. falciparum antigens was screened with the neutralizing sera. Antibodies from protected individuals recognized a broad antigenic repertoire of which three antigens, PfMAEBL, PfTRAP and PfSEA1 were recognized by most protected individuals. As a proof of principle, we demonstrated that anti-PfMAEBL antibodies block liver stage development in human hepatocytes. Thus, these antigens identified are promising targets for vaccine development against malaria.

Efficacy of chimeric DNA vaccines encoding Eimeria tenella 5401 and chicken IFN-γ or IL-2 against coccidiosis in chickens

Chimeric DNA vaccines encoding Eimeria tenella (E. tenella) surface antigen 5401 were constructed and their efficacies against E. tenella challenge were studied. The open reading frame (ORF) of 5401 was cloned into the prokaryotic expression vector pGEX-4T2 to express the recombinant protein and the expressed recombinant protein was identified by Western blot. The ORF of 5401 and chicken cytokine gene IFN-γ or IL-2 were cloned into the eukaryotic expression vector pVAX1 consecutively to construct DNA vaccines pVAX-5401-IFN-γ, pVAX-5401-IL-2 and pVAX-5401. The expression of aim genes in vivo was detected by reverse transcription-polymerase chain reaction and Western blot. Fourteen-day-old chickens were inoculated twice at an interval of 7 days with 100 µg of plasmids pVAX-5401, pVAX-5401-IFN-γ and pVAX-5401-IL-2 or 200 µg of recombinant 5401 protein by leg intramuscular injection, respectively. Seven days after the second inoculation, all chickens except the unchallenged control group were challenged orally with 5 × 10(4) sporulated oocysts of E. tenella. Seven days after challenge, all chickens were weighted and slaughtered to determine the effects of immunization. The results showed the recombinant protein was about 90 kDa and reacted with antiserum against soluble sporozoites. The animal experiment showed that all the DNA vaccines pVAX-5401, pVAX-5401-IFN-γ or pVAX-5401-IL-2 and the recombinant 5401 protein could obviously alleviate body weight loss and cecal lesions as compared with non-vaccinated challenged control and empty vector pVAX1control. Furthermore, pVAX-5401-IFN-γ or pVAX-5401-IL-2 induced anti-coccidial index (ACI) of 180.01 or 177.24 which were significantly higher than that of pVAX-5401. The results suggested that 5401 was an effective candidate antigen for vaccine. This finding also suggested that chicken IFN-γ or IL-2 could effectively improve the efficacies of DNA vaccines against avian coccidiosis.

Coccidiosis: recent advancements in the immunobiology of Eimeria species, preventive measures, and the importance of vaccination as a control tool against these Apicomplexan parasites

Coccidiosis, caused by parasites of the genus Eimeria, is probably the most expensive parasitic disease of poultry. Species of Eimeria are ubiquitous where poultry are raised and are known to cause drastic reductions in performance and induce mortality, thereby affecting the overall health status of poultry. Chemotherapy has been the predominant form of disease control for many years, even though vaccination is steadily gaining importance as a feasible control method. The objective of this review is to highlight recent advancements in understanding the role of host immunity against coccidiosis. In addition, pros and cons associated with chemotherapy and the role of vaccination as an increasingly popular disease control method are discussed. Finally, the role played by recombinant vaccines as a potential vaccination tool is highlighted. With interest growing rapidly in understanding host–parasite biology, recent developments in designing recombinant vaccines and potential epitopes that have shown promise are mentioned.

Milestones in avian coccidiosis research: a review

This article describes some of the milestones in research concerned with protozoan parasites of the genus Eimeria that infect birds and cause the disease coccidiosis. The time period covered is from 1891, when oocysts were first found in the ceca of diseased chickens, to the present. Progress in our understanding has lagged behind that of other protozoan parasites such as Toxoplasma and Plasmodium despite the enormous importance of Eimeria to animal livestock production. Nevertheless, applied research by universities, government agencies, and private industry has resulted in the successful development of methods of control, research that continues today. The topics covered and the references provided are selective and include life cycles and biology, pathology, ultrastructure, biochemistry, immunity, genetics, host cell invasion, species identification, taxonomy, chemotherapy, vaccination, and literature concerned with avian coccidiosis. This review is primarily concerned with the avian species of Eimeria that infect poultry, but some important advances, principally in immunology, have been made using species that infect rodents and rabbits. These are included where appropriate.

Strategies for anti-coccidial prophylaxis

Coccidiosis, a serious disease resulting from infection with parasitic protozoa of the genusEimeria, causes significant economic losses to the poultry industry, where intensive rearing facilitates transmission of infectious oocysts via the fecal/oral route. Current control relies primarily on prophylactic drugs in feed but, whilst cost effective, the rise of drug resistance and public demands for residue-free meat has encouraged development of alternative control strategies. Chickens that recover from infection withEimeriadevelop solid immunity that is directed against the early asexual stages of the parasite life cycle. This has allowed development of a number of vaccines that utilize deliberate infection with controlled doses of virulent oocysts or reproductively attenuated lines ofEimeria.The latter are immunogenic but non-pathogenic. The realization that both prophylactic drugs and attenuated vaccines control but do not eradicate infection withEimeriaencouraged development of a vaccine based upon maternal immunity. Laying hens exposed toEimeriaare able to transfer protective antibodies to hatchlings via egg yolks and these antibodies have been used to identify parasite proteins that are conserved across the genus. When delivered maternally, these provide an economical means of preventing coccidiosis, offering immediate protection to newly hatched chicks.

Eimeria tenella rhomboid 3 has a potential role in microneme protein cleavage

Invasion in several apicomplexan parasites, including Eimeria tenella, is accompanied by shedding of surface adhesins by intramembrane proteolysis mediated by rhomboid protease. We have previously identified E. tenella rhomboid 3 (EtROM3), but its precise role has not been elucidated. In this study, the interactions between EtROM3 and microneme (MIC) proteins were analyzed using the yeast two hybrid technique. The results showed that c-Myc-ROM3 fusion protein interacted with EtMIC4 protein in co-transformed AH109 yeasts, which was further confirmed by immunoprecipitation assay. Smaller EtMIC4 band from co-transformed cells suggested that EtROM3 was an active protease and involved in the cleavage of EtMIC4.

Poultry coccidiosis: recent advancements in control measures and vaccine development

Coccidiosis is recognized as the major parasitic disease of poultry and is caused by the apicomplexan protozoan Eimeria. Coccidiosis seriously impairs the growth and feed utilization of infected animals resulting in loss of productivity. Conventional disease control strategies rely heavily on chemoprophylaxis and, to a certain extent, live vaccines. Combined, these factors inflict tremendous economic losses to the world poultry industry in excess of USD 3 billion annually. Increasing regulations and bans on the use of anticoccidial drugs coupled with the associated costs in developing new drugs and live vaccines increases the need for the development of novel approaches and alternative control strategies for coccidiosis. This paper aims to review the current progress in understanding the host immune response to Eimeria and discuss current and potential strategies being developed for coccidiosis control in poultry.

The long view: a selective review of 40 years of coccidiosis research

This selective review of 40 years of coccidiosis research is one of a number on important diseases of poultry to celebrate the 40th anniversary of the birth of Avian Pathology, the journal of the World Veterinary Poultry Association, and is written for the non-specialist. The intention is to provide a flavour of the field problems and intellectual challenges, with emphasis in the areas of immunology and vaccinology that drove research in the 1970s, and to reflect on research progress since.

Characterization of Neospora caninum microneme protein 10 (NcMIC10) and its potential use as a diagnostic marker for neosporosis

Improvements in the serological diagnosis of neosporosis are needed to differentiate acute versus chronic Neospora caninum infections. In the present study, N. caninum microneme protein 10 (NcMIC10), similar to other microneme proteins, was shown to be released in a calcium-dependent manner. NcMIC10 may be discharged during active invasion of host cells by the parasite, and thus represent an excellent marker for the diagnosis of neosporosis. In order to test this hypothesis, recombinant NcMIC10 (rNcMIC10) was expressed in Escherichia coli, and polyclonal antibodies were generated against non-overlapping fragments of the protein. A capture ELISA was developed using these antibodies, and was found to be highly accurate and reproducible with a detection range of 10-10,000 pg/ml. The anti-rNcMIC10 antibodies used in this study did not cross-react with the Toxoplasma gondii antigens. NcMIC10 was detected by the ELISA in sera of 9 out of 10 goats (90%) experimentally infected with N. caninum tachyzoites. In general, goats infected with a lower dose (10(4)) of the parasite displayed a peak in NcMIC10 levels between weeks 4 and 5 post infection. Goats infected with a higher parasite dose (10(6)) displayed a more rapid increase in NcMIC10 levels. In most animals, NcMIC10 decreased to undetectable levels by week 6 post infection. This is the first circulating Neospora antigen-based assay which may complement the existing antibody-based assays for a rapid and cost-effective definitive diagnosis of neosporosis in livestock.

Identification and partial characterization of a serine protease inhibitor (serpin) of Eimeria tenella

Serine protease inhibitors (serpins) mediate many biological processes, including immune responses to pathogenic infection. In this study, a member of the serpin superfamily was identified from the common poultry parasite Eimeria tenella by expressed sequence tag analysis and the rapid amplification of cDNA ends technique. The full-length cDNA was 1,918 bp and had an open reading frame of 1,248 bp encoding a polypeptide of 415 amino acids with the theoretical isoelectric point of 5.26 and predicted molecular weight of 45.5 kDa. Real-time quantitative PCR analysis revealed that the serpin gene was expressed at higher levels in sporozoites than in the other developmental stages (unsporulated oocysts, sporulated oocysts, and second-generation merozoites). The sequence encoding the mature protein was amplified by PCR, cloned into the pET28(a) vector, and expressed in Escherichia coli. Specific antiserum generated against the recombinant protein was prepared and used to determine invasion inhibition capacity and localization; the results suggested that the serpin may play an important role in invasion and survival of the sporoziotes in the host.

A new thrombospondin-related anonymous protein homologue in Neospora caninum (NcMIC2-like1)

Neospora caninum is an Apicomplexan protozoan that has the dog as a definitive host and cattle (among other animals) as intermediate hosts. It causes encephalopathy in dogs and abortion in cows, with significant loss in worldwide livestock. As any Apicomplexan, the parasite invades the cells using proteins contained in the phylum-specific organelles, like the micronemes, rhoptries and dense granules. The aim of this study was the characterization of a homologue (denominated NcMIC2-like1) of N. caninum thrombospondin-related anonymous protein (NcMIC2), a micronemal protein previously shown to be involved in the attachment and connection with the intracellular motor responsible for the active process of invasion. A polyclonal antiserum raised against the recombinant NcMIC2-like1 functional core (thrombospondin and integrin domains) recognized the native form of NcMIC2-like1, inhibited the in vitro invasion process and localized NcMIC2-like1 at the apical complex of the parasite by confocal immunofluorescence, indicating its micronemal localization. The new molecule, NcMIC2-like1, has features that differentiates it from NcMIC2 in a substantial way to be considered a homologue†.

Eimeria tenella: effects of diclazuril treatment on microneme genes expression in second-generation merozoites and pathological changes of caeca in parasitized chickens

The effects of diclazuril on mRNA expression levels of invasion-related microneme genes were examined in second-generation merozoites of Eimeria tenella (E. tenella) by quantitative real-time (QRT) PCR. Diclazruil treatment of infected chickens significantly decreased the number of second-generation merozoites by 65.13%, and resulted in downregulation of EtMIC genes: EtMIC1 by 65.63%, EtMIC2 by 64.12%, EtMIC3 by 56.82%, EtMIC4 by 73.48%, and EtMIC5 by 78.17%. SEM images of caecum tissue from uninfected chickens showed regular intestinal villus structure. In infected chickens, a distinct loss of the superficial epithelium, with a flattened mucosa and large-area necrosis and anabrosis, was evident. In diclazruil-treated chickens, a decrease in merozoite number and a visibly improved appearance of the caeca were noted. These improvements appeared to be mediated in part by downregulation of the expression of invasion-related EtMIC genes in response to diclazuril.

Proteomic comparison of four Eimeria tenella life-cycle stages: unsporulated oocyst, sporulated oocyst, sporozoite and second-generation merozoite

We report the proteomes of four life-cycle stages of the Apicomplexan parasite Eimeria tenella. A total of 1868 proteins were identified, with 630, 699, 845 and 1532 found in early oocysts (unsporulated), late oocysts (sporulated), sporozoites and second-generation merozoites, respectively. A multidimensional protein identification technology shotgun approach identified 812 sporozoites, 1528 merozoites and all of the oocyst proteins, whereas 2-D gel proteomics identified 230 sporozoites and 98 merozoite proteins. Comparing the invasive stages, we find moving junction components RON2 in both, whereas AMA-1 and RON4 are found only in merozoites and AMA-2 and RON5 are only found in sporozoites, suggesting stage-specific moving junction proteins. During early oocyst to sporozoite development, refractile body and most "glideosome" proteins are found throughout, whereas microneme and most rhoptry proteins are only found after sporulation. Quantitative analysis indicates glycolysis and gluconeogenesis are the most abundant metabolic groups detected in all stages. The mannitol cycle "off shoot" of glycolysis was not detected in merozoites but was well represented in the other stages. However, in merozoites we find more protein associated with oxidative phosphorylation, suggesting a metabolic shift mobilising greater energy production. We find a greater abundance of protein linked to transcription, protein synthesis and cell cycle in merozoites than in sporozoites, which may be residual protein from the preceding massive replication during schizogony.

Eimeria tenella microneme protein EtMIC4: capture of the full-length transcribed sequence and comparison with other microneme proteins

EtMIC4 is a microneme protein of Eimeria tenella, an intracellular apicomplexan protozoan that can cause severe enteritis in chickens. The EtMIC4 gene has been partially characterised, and in this study, we used a combined strategy of rapid amplification of cDNA ends (5'RACE) and reverse transcription-polymerase chain reaction to identify the authentic 5' end of the transcribed sequence (accession number AJ306453.2). Comparison of the predicted EtMIC4 transcription start site with predicted start sites for EtMIC1, 2 and 3 genes identified comparable initiator regions that each conform to the consensus sequence for a transcriptional initiator element. The EtMIC4 gene is organised over 11 exons and analysis of the full-length predicted protein identified a new N-terminal region that comprises a hydrophobic signal peptide followed by four thrombospondin-type 1 modules that are similar to those previously described further downstream in the protein. Best-fit analysis shows that EtMIC4 shares high homology with the Eimeria maxima protein EmTFP250 and with TgMIC12, a predicted Toxoplasma gondii microneme protein. EtMIC4 and EmTFP250 share 70% amino acid identity and all predicted structural domains are conserved between the two. EtMIC4 and TgMIC12 share 48% identity and they have very similar domain organisation and conservation of intron/exon boundaries.

Roles of proteases during invasion and egress by Plasmodium and Toxoplasma

Apicomplexan pathogens replicate exclusively within the confines of a host cell. Entry into (invasion) and exit from (egress) these cells requires an array of specialized parasite molecules, many of which have long been considered to have potential as targets of drug or vaccine-based therapies. In this chapter the authors discuss the current state of knowledge regarding the role of parasite proteolytic enzymes in these critical steps in the life cycle of two clinically important apicomplexan genera, Plasmodium and Toxoplasma. At least three distinct proteases of the cysteine mechanistic class have been implicated in egress of the malaria parasite from cells of its vertebrate and insect host. In contrast, the bulk of the evidence indicates a prime role for serine proteases of the subtilisin and rhomboid families in invasion by both parasites. Whereas proteases involved in egress may function predominantly to degrade host cell structures, proteases involved in invasion probably act primarily as maturases and 'sheddases', required to activate and ultimately remove ligands involved in interactions with the host cell.

Microneme proteins in apicomplexans

Microneme secretion supports several key cellular processes including gliding motility, active cell invasion and migration through cells, biological barriers, and tissues. The modular design of microneme proteins enables these molecules to assist each other in folding and passage through the quality control system, accurately target to the micronemes, oligimerizing with other parasite proteins, and engaging a variety of host receptors for migration and cell invasion. Structural and biochemical analyses of MIC domains is providing new perspectives on how adhesion is regulated and the potentially distinct roles MICs might play in long or short range interactions during parasite attachment and entry. New access to complete genome sequences and ongoing advances in genetic manipulation should provide fertile ground for refining current models and defining exciting new roles for MICs in apicomplexan biology.

Surface antigens and potential virulence factors from parasites detected by comparative genomics of perfect amino acid repeats

Background

Many parasitic organisms, eukaryotes as well as bacteria, possess surface antigens with amino acid repeats. Making up the interface between host and pathogen such repetitive proteins may be virulence factors involved in immune evasion or cytoadherence. They find immunological applications in serodiagnostics and vaccine development. Here we use proteins which contain perfect repeats as a basis for comparative genomics between parasitic and free-living organisms.

Results

We have developed Reptile , a program for proteome-wide probabilistic description of perfect repeats in proteins. Parasite proteomes exhibited a large variance regarding the proportion of repeat-containing proteins. Interestingly, there was a good correlation between the percentage of highly repetitive proteins and mean protein length in parasite proteomes, but not at all in the proteomes of free-living eukaryotes. Reptile combined with programs for the prediction of transmembrane domains and GPI-anchoring resulted in an effective tool for in silico identification of potential surface antigens and virulence factors from parasites.

Conclusion

Systemic surveys for perfect amino acid repeats allowed basic comparisons between free-living and parasitic organisms that were directly applicable to predict proteins of serological and parasitological importance. An on-line tool is available at .

The Microneme Proteins EtMIC4 and EtMIC5 of Eimeria tenella Form a Novel, Ultra-high Molecular Mass Protein Complex That Binds Target Host Cells

Eimeria tenella, in common with other parasitic protozoa of the phylum Apicomplexa, invades host cells using an actinomyosin-powered "glideosome" complex and requires the secretion of adhesive proteins from the microneme organelles onto the parasite surface. Microneme proteins of E. tenella include EtMIC4, a transmembrane protein that has multiple thrombospondin type I domains and calcium-binding epidermal growth factor-like domains in its extracellular domain, and EtMIC5, a soluble protein composed of 11 tandemly repeated domains that belong to the plasminogen-apple-nematode superfamily. We show here that EtMIC4 and EtMIC5 interact to form an oligomeric, ultrahigh molecular mass protein complex. The complex was purified from lysed parasites by non-denaturing techniques, and the stoichiometry was shown to be [EtMIC4](2):[EtMIC5](1), with an octamer of EtMIC4 bound non-covalently to a tetramer of EtMIC5. The complex is formed within the parasite secretory pathway and is maintained after secretion onto the surface of the parasite. The purified complex binds to a number of epithelial cell lines in culture. Identification and characterization of this complex contributes to an overall understanding of the role of multimolecular protein complexes in specific interactions between pathogens and their hosts during infection.

Toxoplasma MIC2 is a major determinant of invasion and virulence

Like its apicomplexan kin, the obligate intracellular protozoan Toxoplasma gondii actively invades mammalian cells and uses a unique form of gliding motility. The recent identification of several transmembrane adhesive complexes, potentially capable of gripping external receptors and the sub-membrane actinomyosin motor, suggests that the parasite has multiple options for host-cell recognition and invasion. To test whether the transmembrane adhesin MIC2, together with its partner protein M2AP, participates in a major invasion pathway, we utilized a conditional expression system to introduce an anhydrotetracycline-responsive mic2 construct, allowing us to then knockout the endogenous mic2 gene. Conditional suppression of MIC2 provided the first opportunity to directly determine the role of this protein in infection. Reduced MIC2 expression resulted in mistrafficking of M2AP, markedly defective host-cell attachment and invasion, the loss of helical gliding motility, and the inability to support lethal infection in a murine model of acute toxoplasmosis. Survival of mice infected with MIC2-deficient parasites correlated with lower parasite burden in infected tissues, an attenuated inflammatory immune response, and induction of long-term protective immunity. Our findings demonstrate that the MIC2 protein complex is a major virulence determinant for Toxoplasma infection and that MIC2-deficient parasites constitute an effective live-attenuated vaccine for experimental toxoplasmosis.

Metagenomic analysis of mesopelagic Antarctic plankton reveals a novel deltaproteobacterial group

Phylogenetic screening of 3200 clones from a metagenomic library of Antarctic mesopelagic picoplankton allowed the identification of two bacterial 16S-rDNA-containing clones belonging to the Deltaproteobacteria, DeepAnt-1F12 and DeepAnt-32C6. These clones were very divergent, forming a monophyletic cluster with the environmental sequence GR-WP33-58 that branched at the base of the myxobacteria. Except for the possession of complete rrn operons without associated tRNA genes, DeepAnt-1F12 and DeepAnt-32C6 were very different in gene content and organization. Gene density was much higher in DeepAnt-32C6, whereas nearly one-third of DeepAnt-1F12 corresponded to intergenic regions. Many of the predicted genes encoded by these metagenomic clones were informational (i.e. involved in replication, transcription, translation and related processes). Despite this, a few putative cases of horizontal gene transfer were detected, including a transposase. DeepAnt-1F12 contained one putative gene encoding a long cysteine-rich protein, probably membrane-bound and Ca2+-binding, with only eukaryotic homologues. DeepAnt-32C6 carried some predicted genes involved in metabolic pathways that suggested this organism may be anaerobic and able to ferment and to degrade complex compounds extracellularly.

Inhibitory activities of epidermal growth factor receptor tyrosine kinase-targeted dihydroxyisoflavone and trihydroxydeoxybenzoin derivatives on Sarcocystis neurona, Neospora caninum, and Cryptosporidium parvum development

Several gene sequences of parasitic protozoa belonging to protein kinase gene families and epidermal growth factor (EGF)-like peptides, which act via binding to receptor tyrosine kinases of the EGF receptor (EGFR) family, appear to mediate host-protozoan interactions. As a clue to EGFR protein tyrosine kinase (PTK) mediation and a novel approach for identifying anticoccidial agents, activities against Sarcocystis neurona, Neospora caninum, and Cryptosporidium parvum grown in BM and HCT-8 cell cultures of 52 EGFR PTK inhibitor isoflavone analogs (dihydroxyisoflavone and trihydroxydeoxybenzoine derivatives) were investigated. Their cytotoxicities against host cells were either absent, mild, or moderate by a nitroblue tetrazolium test. At concentrations ranging from 5 to 10 microg/ml, 20 and 5 analogs, including RM-6427 and RM-6428, exhibited an in vitro inhibitory effect of > or = 95% against at least one parasite or against all three, respectively. In immunosuppressed Cryptosporidium parvum-infected Mongolian gerbils orally treated with either 200 or 400 mg of agent RM-6427/kg of body weight/day for 8 days, fecal microscopic oocyst shedding was abolished in 6/10 animals (P of <0.001 versus untreated controls) and mean shedding was reduced by 90.5% (P of <0.0001) and 92.0% (P of <0.0001), respectively, higher levels of inhibition than after nitazoxanide (200 mg/kg/day for 8 days) or paromomycin (100 mg/kg/day for 8 days) treatment (55.0%, P of <0.001, and 17.5%, P of >0.05, respectively). After RM-6427 therapy (200 mg/kg/day for 8 days), the reduction in the ratio of animals with intracellular parasites was nearly significant in ileum (P = 0.067) and more marked in the biliary tract (P < 0.0013) than after nitazoxanide or paromomycin treatment (0.05 < P < 0.004). RM-6428 treatment at a regimen of 400 mg/kg/day for 12 days inhibited oocyst shedding, measured using flow cytometry from day 4 (P < 0.05) to day 12 (P < 0.02) of therapy, when 2/15 animals had no shedding (P < 0.0001) and 11/15 were free of gut and/or biliary tract parasites (P < 0.01). No mucosal alteration was microscopically observed for treated or untreated infected gerbils. To our knowledge, this report is the first to suggest that the isoflavone class of agents has the potential for anticoccidial therapy.

Calcium binding activity of the epidermal growth factor-like domains of the apicomplexan microneme protein EtMIC4

Microneme proteins are secreted from apicomplexan parasites during invasion of host cells and they play crucial roles in parasite-host cell adhesion. EtMIC4 is a 240 kDa transmembrane protein from Eimeria tenella that contains 31 tandemly arranged epidermal growth factor (EGF), like repeats within its extracellular domain. The majority of these repeats have calcium binding (cb) consensus sequences. Little is known about cbEGFs in apicomplexan parasites but their presence in microneme proteins suggests that they may contribute to parasite-host interactions. To investigate the potential role of cbEGFs we have expressed and correctly refolded a cbEGF triplet from EtMIC4 (cbEGF7-9) and demonstrated that this triplet binds calcium. Circular dichroism spectroscopic analysis of cbEGF7-9 demonstrates that the molecule undergoes a gradual change in conformation with increasing levels of calcium. In the presence of calcium, the triplet becomes resistant to proteolytic degradation by a variety of proteases, a characteristic feature of cbEGF repeats from higher eukaryotic proteins, such as fibrillin, suggesting that calcium binding induces the formation of a rigid conformation. Moreover, mass spectrometric mapping of the cleavage sites that are protected by calcium shows that these sites are located both close to and distant from the calcium binding sites, indicating that protection is not due to steric hindrance by calcium ions, but rather due to the overall conformation adopted by the triplet in the presence of calcium. Thus, the tandemly-arranged cbEGF repeats within EtMIC4 provide a mechanism whereby, in the calcium-rich extracellular environment, the molecule could adopt a protease-resistant, rigid structure that could favour its interaction with host cell ligands.

Eimeria tenella: identification of secretory and surface proteins from expressed sequence tags

To identify new vaccine candidates, Eimeria tenella expressed sequence tags (ESTs) from public databases were analysed for secretory molecules with an especially developed automated in silico strategy termed DNAsignalP. A total of 12,187 ESTs were clustered into 2881 contigs followed by a blastx search, which resulted in a significant number of E. tenella contigs with homologies to entries in public databases. Amino acid sequences of appropriate homologous proteins were analysed for the occurrence of an N-terminal signal sequence using the algorithm signalP. The resulting list of 84 entries comprised 51 contigs whose deduced proteins showed homologies to proteins of apicomplexan parasites. Based on function or localisation, we selected candidate proteins classified as (i) secreted proteins of Apicomplexa parasites, (ii) secreted enzymes, and (iii) transport and signalling proteins. To verify our strategy experimentally, we used a functional complementation system in yeast. For five selected candidate proteins we found that these were indeed secreted. Our approach thus represents an efficient method to identify secretory and surface proteins out of EST databases.

Eimeria tenella microneme protein EtMIC3: identification, localisation and role in host cell infection

The gene coding for Eimeria tenella protein EtMIC3 was cloned by screening a sporozoite cDNA library with two independent monoclonal antibodies raised against the oocyst stage. The deduced sequence of EtMIC3 is 988 amino acids long. The protein presents seven repeats in tandem, with four highly conserved internal repeats and three more divergent external repeats. Each repeat is characterised by a tyrosine kinase phosphorylation site, WRCY, and a reminiscent motif of the thrombospondin1 (TSP1)-type I domain, CXXXCG. The protein EtMIC3 is localised at the apex of free parasite stages. It is not detected in the early intracellular parasite stage but is synthesised in mature schizonts. Secretion of the protein is induced when sporozoites are incubated in complete medium at 41 degrees C. Strangely enough, the two independent mAb that allow cloning of EtMIC3 interfere with parasitic growth in different ways. One is able to inhibit parasite invasion whereas the other inhibits development. Expression and localisation of the protein EtMIC3 are consistent with a protein involved in the invasion process as is expected for a microneme protein.

Plasmodium sporozoite molecular cell biology

Plasmodium sporozoites display complex phenotypes including gliding motility and invasion of and transmigration through cells in the mosquito vector and the vertebrate host. Sporozoite studies have been difficult to perform because of technical concerns. Nevertheless, they have already provided insights into several aspects of sporozoite biology, shared in part with other apicomplexan invasive stages. Structure/function analysis of the thrombospondin-related anonymous protein paved the way to the understanding of the molecular mechanisms of apicomplexan gliding motility and host cell invasion. Functional studies of circumsporozoite protein revealed its role in Plasmodium sporozoite morphogenesis in addition to its well-known function in host cell invasion. Transcriptional surveys, which facilitate the investigation of gene expression programs that control sporozoite phenotypes, have revealed a high degree of previously unappreciated complexity and novel proteins that mediate sporozoite host cell infection.

The Biology of Avian Eimeria with an Emphasis on their Control by Vaccination

Studies on the biology of the avian species of Eimeria are currently benefiting from the availability of a comprehensive sequence for the nuclear genome of Eimeria tenella. Allied to some recent advances in transgenic technologies and genetic approaches to identify protective antigens, some elements are now being assembled that should be helpful for the development of a new generation of vaccines. In the meantime, control of avian coccidiosis by vaccination represents a major success in the fight against infections caused by parasitic protozoa. Live vaccines that comprise defined populations of oocysts are used routinely and this form of vaccination is based upon the long-established fact that chickens infected with coccidial parasites rapidly develop protective immunity against challenge infections with the same species. Populations of wild-type Eimeria parasites were the basis of the first live vaccines introduced around 50 years ago and the more recent introduction of safer, live-attenuated, vaccines has had a significant impact on coccidiosis control in many areas of the world. In Europe the introduction of vaccination has coincided with declining drug efficacy (on account of drug resistance) and increasing concerns by consumers about the inclusion of in-feed medication and prospects for drug residues in meat. The use of attenuated vaccines throughout the world has also stimulated a greater interest in the vaccines that comprise wild-type parasites and, during the past 3 years worldwide, around 3x10(9) doses of each type of vaccine have been used. The need for only small numbers of live parasites to induce effective protective immunity and the recognition that Eimeria spp. are generally very potent immunogens has stimulated efforts to develop other types of vaccines. None has succeeded except for the licensing, within several countries in 2002, of a vaccine (CoxAbic vaccine; Abic, Israel) that protects via the maternal transfer of immunoglobulin to the young chick. Building on the success of viral vaccines that are delivered via the embryonating egg, an in ovo coccidiosis vaccine (Inovocox, Embrex Inc.) is currently in development. Following successful field trials in 2001, the product will be ready for Food and Drug Administration approval in 2005 and a manufacturing plant will begin production for sale in late 2005. Limited progress has been achieved towards the development of subunit or recombinant vaccines. No products are available and studies to identify potential antigens remain compromised by an absence of effective in vitro assays that correlate with the induction of protective immunity in the host. To date, only a relatively small portfolio of molecules has been evaluated for an ability to induce protection in vivo. Although Eimeria are effective immunogens, it is probable that to date none of the antigens that induce potent protective immune responses during the course of natural infection has been isolated.

Host cell invasion by the apicomplexans: the significance of microneme protein proteolysis

Intracellular life-style has been adopted by many pathogens as a successful immune evasion mechanism. To gain entry to a large variety of host cells and to establish an intracellular niche, Toxoplasma gondii and other apicomplexans rely on an active process distinct from phagocytosis. Calcium-regulated secretion of microneme proteins and parasite actin polymerization together with the action of at least one myosin motor act in concert to generate the gliding motility necessary to propel the parasite into host cells. During this active penetration, host cell transmembrane proteins are excluded from the forming parasitophorous vacuole hence conferring the resistance to acidification and degradative fusion. Apicomplexans possess a large repertoire of microneme proteins that contribute to invasion, but their precise role and the level of functional redundancy remain to be evaluated. Remarkably, most microneme proteins are proteolytically cleaved during biogenesis and post-exocytosis. The significance of the processing events and the identification of the proteases implicated are the object of intensive investigations. These proteases may constitute potential drug targets for intervention against malaria and other diseases caused by these parasites.

The unicellular ancestry of animal development

The transition to multicellularity that launched the evolution of animals from protozoa marks one of the most pivotal, and poorly understood, events in life's history. Advances in phylogenetics and comparative genomics, and particularly the study of choanoflagellates, are yielding new insights into the biology of the unicellular progenitors of animals. Signaling and adhesion gene families critical for animal development (including receptor tyrosine kinases and cadherins) evolved in protozoa before the origin of animals. Innovations in transcriptional regulation and expansions of certain gene families may have allowed the integration of cell behavior during the earliest experiments with multicellularity. The protozoan perspective on animal origins promises to provide a valuable window into the distant past and into the cellular bases of animal development.

Gene expression during early ascidian metamorphosis requires signalling by Hemps, an EGF-like protein

Hemps, a novel epidermal growth factor (EGF)-like protein, is expressed during larval development and early metamorphosis in the ascidian Herdmania curvata and plays a direct role in triggering metamorphosis. In order to identify downstream genes in the Hemps pathway we used a gene expression profiling approach, in which we compared post-larvae undergoing normal metamorphosis with larval metamorphosis blocked with an anti-Hemps antibody. Molecular profiling revealed that there are dynamic changes in gene expression within the first 30 minutes of normal metamorphosis with a significant portion of the genome (approximately 49%) being activated or repressed. A more detailed analysis of the expression of 15 of these differentially expressed genes through embryogenesis, larval development and metamorphosis revealed that while there is a diversity of temporal expression patterns, a number of genes are transiently expressed during larval development and metamorphosis. These and other differentially expressed genes were localised to a range of specific cell and tissue types in Herdmania larvae and post-larvae. The expression of approximately 24%of the genes that were differentially expressed during early metamorphosis was affected in larvae treated with the anti-Hemps antibody. Knockdown of Hemps activity affected the expression of a range of genes within 30 minutes of induction, suggesting that the Hemps pathway directly regulates early response genes at metamorphosis. In most cases, it appears that the Hemps pathway contributes to the modulation of gene expression, rather than initial gene activation or repression. A total of 151 genes that displayed the greatest alterations in expression in response to anti-Hemps antibody were sequenced. These genes were implicated in a range of developmental and physiological roles, including innate immunity, signal transduction and in the regulation of gene transcription. These results suggest that there is significant gene activity during the very early stages of H. curvata metamorphosis and that the Hemps pathway plays a key role in regulating the expression of many of these genes.

Eimeria maxima TRAP family protein EmTFP250: subcellular localisation and induction of immune responses by immunisation with a recombinant C-terminal derivative

EmTFP250 is a high molecular mass, asexual stage antigen from Eimeria maxima strongly associated with maternally derived immunity to this protozoan parasite in hatchling chickens. Cloning and sequence analysis has predicted the antigen to be a novel member of the thrombospondin-related anonymous protein (TRAP) family of apicomplexan parasites. Members of the TRAP family are microneme proteins and are associated with host cell invasion and apicomplexan gliding motility. In order to assess the immunogenicity of EmTFP250, a C-terminal derivative encoding a low complex, hydrophilic region and putative transmembrane domain/cytosolic tail was expressed in a bacterial host system. The recombinant protein was used to immunise mice and chickens and found to induce strong IgG responses in both animal models as determined by specific ELISAs. Using Western blotting, protective maternal IgG antibodies previously shown to recognise native EmTFP250 recognised the recombinant protein and, in addition, antibodies raised against the recombinant protein were shown to recognise native EmTFP250. Localisation studies employing immuno-light microscopy and immuno-electron microscopy showed that antibodies to the recombinant protein specifically labeled micronemes within merozoites of E. maxima. Furthermore, antibodies to the recombinant EmTFP250 derivative showed similar labeling of micronemes within merozoites of Eimeria tenella. This study is further suggestive of a functional importance for EmTFP250 and underscores its potential as a candidate for a recombinant vaccine targeting coccidiosis in chickens.

Eimeria tenella sporozoites and merozoites differentially express glycosylphosphatidylinositol-anchored variant surface proteins

Little is known about glycosylphosphatidylinositol (GPI)-linked surface proteins in the coccidian parasite Eimeria tenella. Examination of 28,550 EST sequences from the sporozoite and second merozoite developmental stages of the parasite led to the identification of 37 potential GPI-linked variant surface proteins, termed EtSAGs. Analysis of the complete nucleotide sequences of 23 EtSAG genes separated them into two multi-gene families. All the predicted EtSAG proteins (which vary in length from 228 to 271 residues) have an N-terminal hydrophobic signal peptide, a C-terminal hydrophobic GPI signal-anchor peptide and an extracellular domain organised around six cysteine residues, the positions of which are conserved within each family. Using specific antibodies against a small number of recombinant-expressed EtSAGs, the surface localisation and GPI-anchorage of members of both families was confirmed experimentally. Expression of EtSAGs is differentially regulated between the oocyst/sporozoite and second generation merozoite stages, with only one expressed specifically in the sporozoite, a small number expressed in both stages and the majority expressed specifically in the second generation merozoite. Preliminary data support a model in which multiple variant surface antigens are co-expressed on individual parasites, rather than a model of antigenic switching. The biological role(s) of EtSAGs and the effect(s) that expression of a complex repertoire of variant surface antigens by the second generation merozoite has on host adapted immunity are unknown.

Molecular characterisation of EmTFP250: a novel member of the TRAP protein family in Eimeria maxima

We have previously described a high molecular mass, asexual stage antigen from Eimeria maxima (EmTFP250), implicated as a target of maternal antibodies produced by breeding hens infected with this protozoan parasite. Following partial purification of the protein by ion exchange chromatography, N-terminal and internal peptide sequences were generated and used in the design of degenerate PCR primers. Using a rapid amplification of cDNA ends PCR-based strategy, the cDNA encoding EmTFP250 has been cloned and sequenced. Translation predicts a mature polypeptide with a molecular mass of 246kDa and an isoelectric point of 4.2. Analysis of the amino acid sequence has revealed a novel member of the TRAP (thrombospondin-related anonymous protein) family, containing 16 thrombospondin type-1 repeats and 31 epidermal growth factor-like calcium binding domains. EmTFP250 also contains two low complex, hydrophilic regions rich in glutamic acid and glycine residues, and a transmembrane domain/cytosolic tail associated with parasite gliding motility that is highly conserved within apicomplexan microneme proteins. The protein has 61% identity (71% similarity) with EtMIC4, a 218kDa microneme protein of Eimeria tenella also rich in epidermal growth factor-like and thrombospondin type-1 domains. Using Southern blotting, the gene encoding EmTFP250 has been determined to be present as a single copy within the genome, and reverse transcriptase-PCR has shown that expression is confined to the asexual stages of development. By employing a PCR-based method, a region of the E. maxima Houghton strain EmTFP250 gene was found conserved in Australian isolates of several (at least four) Eimeria species that parasitise chickens. The characterisation of EmTFP250 adds to the expanding apicomplexan TRAP family and suggests a functional significance for the protein.