Functional genomics of gam56: characterisation of the role of a 56 kilodalton sexual stage antigen in oocyst wall formation in Eimeria maxima

Proteomic Analysis of Fractionated Eimeria tenella Sporulated Oocysts Reveals Involvement in Oocyst Wall Formation

Eimeria tenella is the most pathogenic intracellular protozoan parasite of the Eimeria species. Eimeria oocyst wall biogenesis appears to play a central role in oocyst transmission. Proteome profiling offers insights into the mechanisms governing the molecular basis of oocyst wall formation and identifies targets for blocking parasite transmission. Tandem mass tags (TMT)-labeled quantitative proteomics was used to analyze the oocyst wall and sporocysts of E. tenella. A combined total of 2865 E. tenella proteins were identified in the oocyst wall and sporocyst fractions; among these, 401 DEPs were identified, of which 211 were upregulated and 190 were downregulated. The 211 up-regulated DEPs were involved in various biological processes, including DNA replication, fatty acid metabolism and biosynthesis, glutathione metabolism, and propanoate metabolism. Among these proteins, several are of interest for their likely role in oocyst wall formation, including two tyrosine-rich gametocyte proteins (EtGAM56, EtSWP1) and two cysteine-rich proteins (EtOWP2, EtOWP6). Concurrently, 96 uncharacterized proteins may also participate in oocyst wall formation. The present study significantly expands our knowledge of the proteome of the oocyst wall of E. tenella, thereby providing a theoretical basis for further understanding of the biosynthesis and resilience of the E. tenella oocyst wall.

Examination of gametocyte protein 22 localization and oocyst wall formation in Eimeria necatrix using laser confocal microscopy and scanning electron microscopy

BACKGROUND

Eimeria parasite infection occurs via ingestion of oocysts. The robust, bilayer oocyst wall is formed from the contents of wall-forming bodies (WFBs), WFB1 and WFB2, located exclusively in macrogametocytes. Eimeria necatrix gametocyte proteins 22 and 59 (EnGAM22 and EnGAM59) have been found to localize to WFBs and the oocyst wall. However, the exact localization of these two proteins is not clear.

METHODS

WFBs of E. necatrix were extracted from purified gametocytes using a cutoff filter and the extracts of purified WFBs and gametocytes were analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting. Then, the localization of EnGAM22 and EnGAM59 proteins was determined using an indirect immunofluorescence assay. Finally, the development of macrogametocytes and the oocyst wall of E. necatrix was analyzed using laser confocal microscopy and scanning electron microscopy.

RESULTS

Purified WFBs had the same shape and size as those observed in macrogametocytes. EnGAM22 protein localized to WFB1, whereas EnGAM59 protein localized to WFB2. Both EnGAM22 and EnGAM59 native proteins were detected in the extracts of WFBs and gametocytes. The outer layer of the oocyst wall was formed by the release of the contents of WFB1 at the surface of the macrogametocyte to form an anti-EnGAM22 positive layer. WFB2 then appeared to give rise to the inner layer, which was anti-EnGAM59 positive.

CONCLUSIONS

EnGAM22 and EnGAM59 proteins localized to WFB1 and WFB2 and were involved in the formation of the outer and inner layers of the oocyst wall of E. necatrix, respectively. The processes of macrogametogenesis and oocyst wall formation of E. necatrix are similar to other Eimeria parasites. The anti-EnGAM22 antibody could be used as a tool to track the transport and secretion of proteins in WFB1 during oocyst development.

Recombinant GMA56 and ROP17 of Eimeria magna conferred protection against infection by homologous species

One of the most common rabbits coccidia species, Eimeria magna is mainly parasitic in the ileal and jejunal epithelial cells. E. magna infection can affect the growth performance of rabbits or cause other secondary diseases. Traditional methods of anticoccidial treatment typically result in drug resistance and drug residue. Therefore, vaccination is a promising alternative. Gametocyte antigen 56 (GAM56) and rhoptry kinase family proteins (ROPs) are involved in oocyst wall formation and parasite invasion, respectively. A virulence factor, ROP17 contains a serine/threonine kinase catalytic domain. In this study, recombinant E. magna GAM56 (rEmGAM56) and ROP17 (rEmROP17) proteins were obtained from a prokaryotic expression system and their reactogenicity was investigated with immunoblotting. To assess the potential of rEmGAM56 and rEmROP17 as coccidiosis vaccines, New Zealand White rabbits were subcutaneously immunized with 100 μg rEmGAM56 (rGC group) or rEmROP17 (rRC group) twice at 2-week intervals followed by homologous oocyst challenge. The rabbit serum was collected weekly to detect the specific antibody levels. The cytokine levels of pre-challenge serum were measured by enzyme-linked immunosorbent assay and the rabbits were observed and recorded post-challenge for the onset of clinical symptoms. The weight gain, oocyst output, and feed conversion ratio were calculated at the end of the experiment. The results showed that both rEmGAM56 and rEmROP17 had good reactogenicity. The rEmGAM56- or rEmROP17-immunized rabbits had milder clinical symptoms and feed conversion ratios of 3.27:1 and 3.37:1, respectively. The rEmGAM56-immunized rabbits had 81.35% body weight gain and 63.85% oocyst output reduction; the rEmROP17-immunized rabbits had 79.03% body weight gain and 80.10% oocyst output reduction. The ACI of rGC and rRC groups were 162.35 and 171.03, respectively. The specific antibody levels increased rapidly after immunization. Significantly increased interleukin (IL)-2, interferon (IFN)-γ, and IL-17 levels were evident in the rGC and rRC groups (p < 0.05). The rEmGAM56 and rEmROP17 elicited humoral and cellular responses, which protected against E. magna infection in rabbits. Thus, rEmGAM56 and rEmROP17 are potential vaccine candidates against E. magna, and rEmROP17 performed better than rEmGAM56.

Identification and characterization of a cDNA encoding a gametocyte-specific protein of the avian coccidial parasite Eimeria necatrix

Gametocyte proteins of Eimeria spp. are essential components of the oocyst wall, and some of these proteins have been analysed to identify targets of transmission-blocking vaccines against avian coccidiosis. In the present study, a cDNA from E. necatrix gametocytes was cloned and sequenced. The cDNA is 1473 bp in length and encodes a 490-amino-acid protein containing a tyrosine-serine (Tyr/Ser)-rich domain and a proline-methionine (Pro/Met)-rich domain. A quantitative real-time PCR (qPCR) analysis showed that the cDNA is expressed only during gametogenesis. A fragment containing the Tyr/Ser-rich domain (rEnGAM59) was expressed in Escherichia coli BL21 (DE3) cells. Immunoblotting showed that rEnGAM59 was recognized by the serum of convalescent chickens after infection with E. necatrix, and that an anti-rEnGAM59 antibody recognized a ∼59 kDa protein and two other proteins (∼35 kDa and ∼33 kDa) in gametocyte extracts. An immunofluorescence assay showed that the anti-rEnGAM59 antibody recognized wall-forming bodies in the macrogametocytes and oocyst walls. An in vivo vaccination and challenge trial was conducted to test the potential utility of rEnGAM59 as a vaccine. Immunized chickens performed better than the unimmunized and challenged (positive control) chickens. The intestinal lesion scores were significantly lower in the immunized groups than in the positive control group (P < 0.05). In contrast, the body weight gains (BWG) were significantly higher in the immunized groups than in the positive control group (P < 0.05). There were no significant differences in the lesion scores and BWG between the groups immunized with rEnGAM59 protein or with live oocysts (P> 0.05). Chickens immunized with rEnGAM59 protein had a significantly higher antigen-specific serum IgY response (P < 0.05). rEnGAM59 protein can be used as candidate antigen to develop a recombinant coccidiosis vaccine.

Correlative light and electron microscopy of wall formation in Eimeria nieschulzi

Coccidian parasites possess complex life cycles involving asexual proliferation followed by sexual development leading to the production of oocysts. Coccidian oocysts are persistent stages which are secreted by the feces and transmitted from host to host guaranteeing life cycle progression and disease transmission. The robust bilayered oocyst wall is formed from the contents of two organelles, the wall-forming bodies type I and II (WFBI, WFBII), located exclusively in the macrogametocyte. Eimeria nieschulzi has been used as a model parasite to study and follow gametocyte and oocyst development. In this study, the gametocyte and oocyst wall formation of E. nieschulzi was analyzed by electron microscopy and immuno-histology. A monoclonal antibody raised against the macrogametocytes of E. nieschulzi identified a tyrosine-rich glycoprotein (EnGAM82) located in WFBII. Correlative light and electron microscopy was used to examine the vesicle-specific localization and spatial distribution of GAM82-proteins during macrogametocyte maturation by this monoclonal antibody. In early and mid-stages, the GAM82-protein is ubiquitously distributed in WFBII. Few hours later, the protein is arranged in subvesicular structures. It was possible to show that the substructure of WFBII and the spatial distribution of GAM82-proteins probably represent pre-synthesized cross-linked materials prior to the inner oocyst wall formation. Dityrosine-cross-linked gametocyte proteins can also be confirmed and visualized by fluorescence microscopy (UV light, autofluorescence of WFBII).

Immunoprophylactic evaluation of recombinant gametocyte 22 antigen of Eimeria tenella in broiler chickens

Gametocyte proteins are being explored as potential vaccine candidates against Eimeria sp. in chicken since they are the components of the resilient oocyst wall. The aim of this study was to investigate the immunoprophylactic efficacy of recombinant Eimeria tenella gametocyte antigen 22 (EtGam22) in chickens against homologous oocyst challenge. Broiler chicks were subcutaneously immunized individually with 100 μg of recombinant EtGam22 adjuvanted with Montanide ISA 71 VG at 7 days of age and boosted 2 weeks later. The immunized chickens were challenged individually with 1 × 10⁴ sporulated oocysts of E. tenella 1 week post-booster immunization. The anti-EtGam22 IgY and serum cytokine response was measured post-immunization. The results showed that the anti-EtGam22 IgY antibody, serum IFN-γ, IL-2, TGF-β, and IL-4 levels in chickens vaccinated with recombinant protein were significantly increased post-immunization as compared to unimmunized challenged controls (P < 0.05). The peripheral blood lymphocyte proliferation activity was also found significantly higher in EtGam22-immunized group on day 28, i.e., pre-challenge (P < 0.05). Upon homologous oocyst challenge, chickens immunized with rEtGam22 exhibited a significant drop in the total oocyst output per bird (246.78 ± 36.9 × 10⁶, 45.23% reduction) and a significantly higher weight gain (497.7 ± 19.2 g) as compared to unimmunized challenged controls. Taken together, these data indicate that EtGam22 is a potent immunogen for use as a subunit vaccine against cecal coccidiosis in chickens as it induces a diverse and robust immune response involving multiple cytokines and strong antibody titers.

Identification and analysis of Eimeria nieschulzi gametocyte genes reveal splicing events of gam genes and conserved motifs in the wall-forming proteins within the genus Eimeria (Coccidia, Apicomplexa)

The genus Eimeria (Apicomplexa, Coccidia) provides a wide range of different species with different hosts to study common and variable features within the genus and its species. A common characteristic of all known Eimeria species is the oocyst, the infectious stage where its life cycle starts and ends. In our study, we utilized Eimeria nieschulzi as a model organism. This rat-specific parasite has complex oocyst morphology and can be transfected and even cultivated in vitro up to the oocyst stage. We wanted to elucidate how the known oocyst wall-forming proteins are preserved in this rodent Eimeria species compared to other Eimeria. In newly obtained genomics data, we were able to identify different gametocyte genes that are orthologous to already known gam genes involved in the oocyst wall formation of avian Eimeria species. These genes appeared putatively as single exon genes, but cDNA analysis showed alternative splicing events in the transcripts. The analysis of the translated sequence revealed different conserved motifs but also dissimilar regions in GAM proteins, as well as polymorphic regions. The occurrence of an underrepresented gam56 gene version suggests the existence of a second distinct E. nieschulzi genotype within the E. nieschulzi Landers isolate that we maintain.

Use of fluorescent nanoparticles to investigate nutrient acquisition by developing Eimeria maxima macrogametocytes

The enteric disease coccidiosis, caused by the unicellular parasite Eimeria, is a major and reoccurring problem for the poultry industry. While the molecular machinery driving host cell invasion and oocyst wall formation has been well documented in Eimeria, relatively little is known about the host cell modifications which lead to acquisition of nutrients and parasite growth. In order to understand the mechanism(s) by which nutrients are acquired by developing intracellular gametocytes and oocysts, we have performed uptake experiments using polystyrene nanoparticles (NPs) of 40 nm and 100 nm in size, as model NPs typical of organic macromolecules. Cytochalasin D and nocodazole were used to inhibit, respectively, the polymerization of the actin and microtubules. The results indicated that NPs entered the parasite at all stages of macrogametocyte development and early oocyst maturation via an active energy dependent process. Interestingly, the smaller NPs were found throughout the parasite cytoplasm, while the larger NPs were mainly localised to the lumen of large type 1 wall forming body organelles. NP uptake was reduced after microfilament disruption and treatment with nocodazole. These observations suggest that E. maxima parasites utilize at least 2 or more uptake pathways to internalize exogenous material during the sexual stages of development.

Cryptosporidiuminfections: molecular advances

Cryptosporidiumhost cell interaction remains fairly obscure compared with other apicomplexans such asPlasmodiumorToxoplasma. The reason for this is probably the inability of this parasite to complete its life cyclein vitroand the lack of a system to genetically modifyCryptosporidium. However, there is a substantial set of data about the molecules involved in attachment and invasion and about the host cell pathways involved in actin arrangement that are altered by the parasite. Here we summarize the recent advances in research on host cell infection regarding the excystation process, attachment and invasion, survival in the cell, egress and the available data on omics.

Cloning and characterization of an Eimeria necatrix gene encoding a gametocyte protein and associated with oocyst wall formation

Background

Gametocyte proteins of Eimeria (E.) spp. are important components of the oocyst wall and some have been used to develop transmission-blocking vaccines against avian coccidiosis.

Methods

Total RNA isolated from E. necatrix gametocytes was utilized as templates for RT-PCR amplification and sequencing of cDNA encoding a gametocyte protein using gene-specific primers. The cDNA was cloned into the bacterial expression vector pET28a(+) and expressed in E. coli BL21 cells. The antigenicity of the recombinant gametocyte protein and its localization in different E. necatrix life-cycle stages were determined by western blot and indirect immunofluorescence analyses, respectively.

Results

A 731-nucleotide sequence of cDNA [GenBank: KF649255] of E. necatrix had 97.7% identity to that of Etgam22 of E. tenella. The cDNA ORF encoded a 186-amino acid protein containing a histidine-proline-rich region. The recombinant gametocyte protein (rEnGAM22) was predominately expressed in the insoluble inclusion body and recognized by antiserum from chickens immunized with oocysts of E. necatrix, E. maxima and E. tenella. A specific antibody to the rEnGAM22 protein recognized the wall-forming bodies in macrogametocytes and the walls of oocysts and sporocysts.

Conclusions

The gene cloned from E. necatrix gametocytes is an ortholog to Etgam22 of E. tenella and presents a potential target for future recombinant subunit vaccines against coccidiosis.

Sex andEimeria: a molecular perspective

Eimeriais a common genus of apicomplexan parasites that infect diverse vertebrates, most notably poultry, causing serious disease and economic loss. Like all apicomplexans, eimerians have a complex life cycle characterized by asexual divisions that amplify the parasite population in preparation for sexual reproduction. This can be divided into three events: gametocytogenesis, producing gametocytes from merozoites; gametogenesis, producing microgametes and macrogametes from gametocytes; and fertilization of macrogametes by microgametes, producing diploid zygotes with ensuing meiosis completing the sexual phase. Sexual development inEimeriadepends on the differential expression of stage-specific genes, rather than presence or absence of sex chromosomes. Thus, it involves the generation of specific structures and, implicitly, storage of proteins and regulation of protein expression in macrogametes, in preparation for fertilization. InEimeria, the formation of a unique, resilient structure, the oocyst wall, is essential for completion of the sexual phase and parasite transmission. In this review, we piece together the molecular events that underpin sexual reproduction inEimeriaand use additional details from analogous events inPlasmodiumto fill current knowledge gaps. The mechanisms governing sexual stage formation and subsequent fertilization may represent targets for counteracting parasite transmission.

Efficacy of a DNA vaccine carrying Eimeria maxima Gam56 antigen gene against coccidiosis in chickens

To control coccidiosis without using prophylactic medications, a DNA vaccine targeting the gametophyte antigen Gam56 from Eimeria maxima in chickens was constructed, and the immunogenicity and protective effects were evaluated. The ORF of Gam56 gene was cloned into an eukaryotic expression vector pcDNA3.1(zeo)+. Expression of Gam56 protein in COS-7 cells transfected with recombinant plasmid pcDNA-Gam56 was confirmed by indirect immunofluorescence assay. The DNA vaccine was injected intramuscularly to yellow feathered broilers of 1-week old at 3 dosages (25, 50, and 100 µg/chick). Injection was repeated once 1 week later. One week after the second injection, birds were challenged orally with 5×10(4) sporulated oocysts of E. maxima, then weighed and killed at day 8 post challenge. Blood samples were collected and examined for specific peripheral blood lymphocyte proliferation activity and serum antibody levels. Compared with control groups, the administration of pcDNA-Gam56 vaccine markedly increased the lymphocyte proliferation activity (P<0.05) at day 7 and 14 after the first immunization. The level of lymphocyte proliferation started to decrease on day 21 after the first immunization. A similar trend was seen in specific antibody levels. Among the 3 pcDNA-Gam56 immunized groups, the median dosage group displayed the highest lymphocyte proliferation and antibody levels (P<0.05). The median dosage group had the greatest relative body weight gain (89.7%), and the greatest oocyst shedding reduction (53.7%). These results indicate that median dosage of DNA vaccine had good immunogenicity and immune protection effects, and may be used in field applications for coccidiosis control.

The spatial organization and extraction of the wall-forming bodies ofEimeria maxima

Eimeria maximahas been used as a model apicomplexan parasite to study sexual stage development and oocyst wall formation. A complete understanding of the wall's biochemical and biophysical properties is of great interest in research on all apicomplexan parasites. Purified gametocytes, zygotes and oocysts were analysed by three-dimensional confocal microscopy, and wide-field fluorescent microscopy was used to investigate the appearance and spatial organization of the 2 types of wall-forming bodies (WFBs). In addition, a variety of staining procedures and immunoassays were used to assess the biosynthesis, metabolic activity, intactness and molecular composition of the WFBsin situ. WFBs were extracted from gametocytes/zygotes and their composition was assessed by microscopy and SDS-PAGE analysis. It was concluded that isolated gametocytes are intact and metabolically active. Additionally, it was observed that the Type 1 WFBs are aligned at the periphery of the parasite and fuse together producing neutral lipid rich patches that appear to be inserted into the space between 2 parasite-specific membranes. Finally, it was shown that the WFBs extracted from purified gametocytes had the same shape, size and staining properties as those observedin situ, and contained the major glycoprotein antigens known to be present in these organelles.

Anti-Recombinant Gametocyte 56 Protein IgY Protected Chickens from Homologous Coccidian Infection

Coccidiosis is caused by intra-cellular infection of Eimeria spp., which goes through a complex life cycle in the intestinal mucosa of infected hosts. Specific immunoglobulins (IgY) could be produced in egg yolk by immunizing hens with specific antigens. In the present study, we cloned the E. maxima gam56 gene, expressed the GST-GAM56 fusion protein and raised IgY to GST-GAM56 in hens. The anti-GST-GAM56 IgY antibody was isolated and used to treat chickens infected with E. maxima oocysts. Intramuscular injection of the antibodies provided minimal protection against parasite infection. However, oral dosing of the IgY 3 or 5 d after oocyst inoculation significantly improved body weight gain, reduced oocyst output and intestinal lesion score were reduced at 3 or 5 d after oocyst challenging, compared to the untreated control group. Our findings suggest that the IgY to gam56 could be an effective prophylactic or therapeutic agent against E. maxima infection in chickens and should have a practical application value.

Proteomic analysis of fractionated Toxoplasma oocysts reveals clues to their environmental resistance

Toxoplasma gondii is an obligate intracellular parasite that is unique in its ability to infect a broad range of birds and mammals, including humans, leading to an extremely high worldwide prevalence and distribution. This work focuses on the environmentally resistant oocyst, which is the product of sexual replication in felids and an important source of human infection. Due to the difficulty in producing and working with oocysts, relatively little is known about how this stage is able to resist extreme environmental stresses and how they initiate a new infection, once ingested. To fill this gap, the proteome of the wall and sporocyst/sporozoite fractions of mature, sporulated oocysts were characterized using one-dimensional gel electrophoresis followed by LC-MS/MS on trypsin-digested peptides. A combined total of 1021 non-redundant T. gondii proteins were identified in the sporocyst/sporozoite fraction and 226 were identified in the oocyst wall fraction. Significantly, 172 of the identified proteins have not previously been identified in Toxoplasma proteomic studies. Among these are several of interest for their likely role in conferring environmental resistance including a family of small, tyrosine-rich proteins present in the oocyst wall fractions and late embryogenesis abundant domain-containing (LEA) proteins in the cytosolic fractions. The latter are known from other systems to be key to enabling survival against desiccation.

The Glycosylation Pathway of Eimeria tenella Is Upregulated during Gametocyte Development and May Play a Role in Oocyst Wall Formation

Sexual-stage glycoproteins of Eimeria are important components of the oocyst wall, a structure that ensures the efficient transmission of these and related parasites. In this study, the primary enzyme in the glycosylation pathway of Eimeria tenella , glucosamine:fructose-6-phosphate aminotransferase (EtGFAT), has been characterized as a macrogamete-specific protein. Although the transcription of Et GFAT was observed early in macrogamete development, protein expression was restricted to mature macrogametes, prior to their conversion into unsporulated oocysts. Genes coding for three other enzymes required for N -acetylgalactosamine (GalNAc) synthesis were also transcribed during E. tenella macrogamete development. Gene transcription of the enzyme responsible for the O-linked transfer of GalNAc to proteins, EtGalNAc-T, was upregulated primarily in unsporulated oocyst stages, and accordingly, a significant increase in GalNAc levels was observed in E. tenella gametocytes and oocysts. Gam56 and Gam82, two well-characterized glycoproteins of Eimeria macrogametes and the oocyst wall, contain high levels of GalNAc and represent probable targets of GalNAc O linkage. It appears that the glycosylation pathway, specifically relating to the formation of GalNAc O links, is dramatically upregulated in E. tenella sexual stages and may play a role in directing a number of macrogamete proteins to the developing oocyst wall.

Conservation of proteins involved in oocyst wall formation in Eimeria maxima, Eimeria tenella and Eimeria acervulina

Vaccination with proteins from gametocytes of Eimeria maxima protects chickens, via transfer of maternal antibodies, against infection with several species of Eimeria. Antibodies to E. maxima gametocyte proteins recognise proteins in the wall forming bodies of macrogametocytes and oocyst walls of E. maxima, Eimeria tenella and Eimeria acervulina. Homologous genes for two major gametocyte proteins - GAM56 and GAM82 - were found in E. maxima, E. tenella and E. acervulina. Alignment of the predicted protein sequences of these genes reveals that, as well as sharing regions of tyrosine richness, strong homology exists in their amino-terminal regions, where protective antibodies bind. This study confirms the conservation of the roles of GAM56 and GAM82 in oocyst wall formation and shows that antibodies to gametocyte antigens of E. maxima cross-react with homologous proteins in other species, helping to explain cross-species maternal immunity.

Excystation of Eimeria tenella sporozoites impaired by antibody recognizing gametocyte/oocyst antigens GAM22 and GAM56

Eimeria tenella is the causative agent of coccidiosis in poultry. Infection of the chicken intestine begins with ingestion of sporulated oocysts releasing sporocysts, which in turn release invasive sporozoites. The monoclonal antibody E2E5 recognizes wall-forming body type II (WFBII) in gametocytes and the WFBII-derived inner wall of oocysts. Here we describe that this antibody also binds to the stieda body of sporocysts and significantly impairs in vitro excystation of sporozoites. Using affinity chromatography and protein sequence analysis, E2E5 is shown to recognize EtGAM56, the E. tenella ortholog of the Eimeria maxima gametocyte-specific GAM56 protein. In addition, this antibody was used to screen a genomic phage display library presenting E. tenella antigens as fusion proteins with the gene VIII product on the surfaces of phagemid particles and identified the novel 22-kDa histidine- and proline-rich protein EtGAM22. The Etgam22 mRNA is expressed predominantly at the gametocyte stage, as detected by Northern blotting. Southern blot analysis in combination with data from the E. tenella genome project revealed that Etgam22 is an intronless multicopy gene, with approximately 12 to 22 copies in head-to-tail arrangement. Conspicuously, Etgam56 is also intronless and is localized adjacent to another gam56-like gene, Etgam59. Our data suggest that amplification is common for genes encoding oocyst wall proteins.

The coccidian oocyst: a tough nut to crack!

Coccidian parasites are transmitted between hosts by the ingestion of food or water contaminated with oocysts, followed by the release of infectious sporozoites and invasion of the gastro-intestinal tract. In the external environment, sporozoites are protected from desiccation and chemical disinfection by the oocyst wall. This unique structure guarantees successful disease transmission and is as vital to the coccidian parasite as the exoskeleton is to insects--without it they would die. Here, we revisit the early work and combine it with newer molecular data to describe our present understanding of the coccidian oocyst wall.

Characterisation of the antigenic and immunogenic properties of bacterially expressed, sexual stage antigens of the coccidian parasite, Eimeria maxima

Coccidiosis in poultry is caused by the intestinal parasite Eimeria; it causes significant financial losses to the commercial poultry industry worldwide. CoxAbic is the first commercially available subunit vaccine against coccidiosis. The vaccine consists of affinity purified sexual stage (gametocyte) antigens (APGA) isolated from Eimeria maxima. Production of this vaccine is time-consuming and laborious and, therefore, a recombinant subunit vaccine substitute for CoxAbic is desirable. The genes encoding the two immunodominant components of CoxAbic, gam56 and gam82, were cloned into the bacterial expression vector, pTRCHisB, and the proteins expressed and purified. Both recombinant proteins were recognised by protective chicken antibodies that were raised to APGA, by immunoblotting. In a competitive ELISA, a combination of the recombinant proteins inhibited the binding of anti-APGA antibodies to APGA by 76%, which was comparable to the inhibition of 98% observed when APGA was used as the competing protein in the assay. In two breeds of chicken (Australorp and Cobb500), the recombinant proteins alone, or in combination, elicited a dose-dependent, antibody response that recognised APGA by ELISA, and gametocytes by immunoblotting. Together, the results suggested that the development of a recombinant subunit vaccine that maintains the antigenic and immunogenic properties of the native protein vaccine, CoxAbic, is feasible.

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.

The development of the macrogamete and oocyst wall in Eimeria maxima: immuno-light and electron microscopy

We have identified, and followed the development of three macrogamete organelles involved in the formation of the oocyst wall of Eimeria maxima. The first were small lucent vacuoles that cross-reacted with antibodies to the apple domains of the Toxoplasma gondii microneme protein 4. They appeared early in development and were secreted during macrogamete maturation to form an outer veil and were termed veil forming bodies. The second were the wall forming bodies type 1, large, electron dense vacuoles that stained positively only with antibodies raised to an enriched preparation of the native forms of 56 (gam56), 82 (gam82) and 230 kDa (gam230) gametocyte antigens (termed anti-APGA). The third were the wall forming bodies type 2, which appeared before the wall forming bodies type 1 but remain enclosed within the rough endoplasmic reticulum and stained positively with antibodies raised to recombinant versions of gam56 (anti-gam56), gam82 (anti-gam82) and gam230 (anti-gam230) plus anti-APGA. At the initiation of oocyst wall formation, the anti-T. gondii microneme protein 4 positive outer veil detached from the surface. The outer layer of the oocyst wall was formed by the release of the contents of wall forming bodies type 1 at the surface to form an electron dense, anti-APGA positive layer. The wall forming bodies type 2 appeared, subsequently, to give rise to the electron lucent inner layer. Thus, oocyst wall formation in E. maxima represents a sequential release of the contents of the veil forming bodies, wall forming bodies types 1 and 2 and this may be controlled at the level of the rough endoplasmic reticulum/Golgi body.

Characterization of a developmentally regulated oocyst protein from Eimeria tenella

Changes in proteins during sporulation of Eimeria tenella oocysts were investigated. Unsporulated E. tenella oocysts collected from cecal tissue at 7 days postinoculation were sporulated in aerated media at 28 C for 0-48 hr. Gel analysis of soluble protein extracts prepared from oocysts from their respective time points indicated the presence of 2 prominent bands with relative molecular weight (Mr) in the range of 30 kDa and making up 20% of the total protein. These 2 bands, designated as major oocyst proteins (MOPs), were absent or barely detectable by 21 hr of sporulation. MOP bands were weakly reactive with glycoprotein stain but showed no mobility shift on deglycosylation. By gel analysis it was shown that the purified MOPs consisted of 2 bands of Mr 28.7 and 30.1 kDa. However, by matrix-assisted laser deabsorption-time of flight analysis it was shown that masses were about 17% lower. Internal sequence analysis of the 28.7-kDa protein generated 2 peptides of 17 and 14 amino acids in length, consistent with a recently described protein coded by the gam56 gene and expressed in E. maxima gametocytes. Rabbit antibodies made against MOPs were localized to outer portions of sporocysts before excystment and to the apical end of in vitro-derived sporozoites. These same antibodies were found to react with bands of Mr 101 and 65 kDa by Western blot but did not recognize MOPs in soluble or insoluble sporozoite extracts. The data suggest that the MOPs are derived from part of a gametocyte protein similar to that coded by gam56 and are processed during sporulation into sporocyst and sporozoite proteins. Alternatively, the binding of anti-MOP to 101- and 65-kDa proteins may result from alternatively spliced genes as the development of parasite proceeds.

Roles of tyrosine-rich precursor glycoproteins and dityrosine- and 3,4-dihydroxyphenylalanine-mediated protein cross-linking in development of the oocyst wall in the coccidian parasite Eimeria maxima

The oocyst wall of apicomplexan parasites protects them from the harsh external environment, preserving their survival prior to transmission to the next host. If oocyst wall formation could be disrupted, then logically, the cycle of disease transmission could be stopped, and strategies to control infection by several organisms of medical and veterinary importance such as Eimeria, Plasmodium, Toxoplasma, Cyclospora, and Neospora could be developed. Here, we show that two tyrosine-rich precursor glycoproteins, gam56 and gam82, found in specialized organelles (wall-forming bodies) in the sexual stage (macrogamete) of Eimeria maxima are proteolytically processed into smaller glycoproteins, which are then incorporated into the developing oocyst wall. The identification of high concentrations of dityrosine and 3,4-dihydroxyphenylalanine (DOPA) in oocyst extracts by high-pressure liquid chromatography, together with the detection of a UV autofluorescence in intact oocysts, implicates dityrosine- and possibly DOPA-protein cross-links in oocyst wall hardening. In addition, the identification of peroxidase activity in the wall-forming bodies of macrogametes supports the hypothesis that dityrosine- and DOPA-mediated cross-linking might be an enzyme-catalyzed event. As such, the mechanism of oocyst wall formation in Eimeria, is analogous to the underlying mechanisms involved in the stabilization of extracellular matrices in a number of organisms, widely distributed in nature, including insect resilin, nematode cuticles, yeast cell walls, mussel byssal threads, and sea urchin fertilization membranes.

Cloning and characterization of the 82 kDa tyrosine-rich sexual stage glycoprotein, GAM82, and its role in oocyst wall formation in the apicomplexan parasite, Eimeria maxima

The sexual (macrogamete/macrogametocyte) stage antigen, GAM82, in the apicomplexan parasite Eimeria maxima, has an apparent molecular mass of 82 kDa, and has been implicated in protective immunity against coccidiosis in poultry. The gene encoding this protein, gam82, was cloned and sequenced. It is a single-copy, intronless gene, which localizes to a 2145 bp transcript, and is first detected at 130 h post-infection. The gene predicts two distinct domains rich in the residues tyrosine and serine, amino acids that have been implicated in oocyst wall formation in other Eimeria spp., and in the extraorganismic sclerotization of structural proteins throughout the animal kingdom. A high number of small amino acids, predominantly alanine and proline, were detected in the intervening sequence between these two domains. The inference that GAM82 is involved in oocyst wall formation in Eimeria was confirmed when it was shown that a specific antibody to a recombinant version of GAM82 recognized the wall forming bodies in macrogametes, and the walls of oocysts in E. maxima. A closer biochemical analysis of the role of GAM82 in oocyst wall formation by sodium dodecyl sulfate polyacrylamide gel electrophoresis and immunoblotting showed that the antibodies to the recombinant version of GAM82 recognized an 82 kDa protein in macrogametocyte extracts, and a 30 kDa protein in unsporulated and sporulated oocyst extracts, as well as in purified oocyst wall fragments. Together, these findings indicate that the 82 kDa macrogametocyte antigen, GAM82, is a tyrosine and serine rich precursor protein that is proteolytically processed during development to give rise to a 30 kDa protein, that is incorporated into the oocyst wall. In addition, these findings provide evidence that the oocyst wall of Eimeria species is composed of a family of tyrosine rich proteins, that arise from precursor proteins found in the wall forming bodies of macrogametes.