Inhibition of penetration of cultured cells by Eimeria bovis sporozoites by monoclonal immunoglobulin G antibodies against the parasite surface protein P20

The Growth of Eimeria tenella: Characterization and Application of Quantitative Methods to Assess Sporozoite Invasion and Endogenous Development in Cell Culture

In vitro development of the complete life cycle of Eimeria species has been achieved in primary cultures of avian epithelial cells with low efficiency. The use of immortalized cell lines simplifies procedures but only allows partial development through one round of parasite invasion and intracellular replication. We have assessed the suitability of Madin-Darby Bovine Kidney (MDBK) cells to support qualitative and quantitative studies on sporozoite invasion and intracellular development of Eimeria tenella. Analysis of parasite ultrastructure by transmission electron microscopy and serial block face-scanning electron microscopy proved the suitability of the system to generate good quality schizonts and first-generation merozoites. Parasite protein expression profiles elucidated by mass spectrometry corroborated previous findings occurring during the development of the parasite such as the presence of alternative types of surface antigen at different stages and increased abundance of proteins from secretory organelles during invasion and endogenous development. Quantitative PCR (qPCR) allowed the tracking of development by detecting DNA division, whereas reverse transcription qPCR of sporozoite- and merozoite-specific genes could detect early changes before cell division and after merozoite formation, respectively. These results correlated with the analysis of development using ImageJ semi-automated image analysis of fluorescent parasites, demonstrating the suitability and reproducibility of the MDBK culture system. This systems also allowed the evaluation of the effects on invasion and development when sporozoites were pre-incubated with anticoccidial drugs, showing similar effects to those reported before. We have described through this study a series of methods and assays for the further application of this in vitro culture model to more complex studies of Eimeria including basic research on parasite cell biology and host-parasite interactions and for screening anticoccidial drugs.

Transfer of Cystoisospora suis-specific colostral antibodies and their correlation with the course of neonatal porcine cystoisosporosis

Cystoisospora suis is the most pathogenic species of coccidia in suckling piglets, affecting them predominantly within their first three weeks of life. The clinical signs of neonatal cystoisosporosis include watery diarrhea and wasting, leading to significant economic losses for the farmer. Since neonatal piglets have an immature immune system, colostral transfer of maternal factors such as immune cells or antibodies is essential for controlling infections at that age. However, the role of C. suis-specific antibodies transferred from the sow to the piglets and possible correlations between antibody levels in the piglets acquired from colostrum with the clinical outcome of disease are currently not understood. To address this issue, 12 non-infected piglets and 14 piglets experimentally infected with C. suis on the third day of life were examined during their first four weeks of life. IgG, IgA, and IgM titers in the blood serum specific for sporozoites and merozoites of C. suis were evaluated, along with oocyst excretion and fecal consistency. Additionally, the antibody content in the colostrum and milk of three mother sows was determined. A transfer of naturally acquired C. suis-specific antibodies from sows to piglets with the colostrum could be demonstrated. Maternal antibodies in piglets' blood sera did not persist for longer than 14-21 days except for IgG which was present in high titers until the end of the study. Within 2-3 weeks after birth the onset of endogenous antibody production was noticed. Titers in blood serum showed a correlation with the severity of diarrhea which was positive for IgG and IgM (possibly due to increased consumption or loss of these antibodies) and negative for IgA. C. suis-specific mucus antibodies isolated from infected and non-infected piglets (n=6/group) on the 28th day of life were present in both groups, showing significantly higher titers of IgA and IgM in infected piglets. Maternally transferred antibodies acquired by natural infections of sows as observed in this study did not provide protection against the clinical manifestation of disease. The level and effect of transferrable maternal factors necessary for protection still need to be elucidated. However, correlations between antibody titers and fecal consistency in the piglets indicate that C. suis-specific antibodies might be useful markers for the expectable clinical severity of cystoisosporosis.

Identification of lead compounds targeting the cathepsin B-like enzyme of Eimeria tenella

Cysteine peptidases have been implicated in the development and pathogenesis of Eimeria. We have identified a single-copy cathepsin B-like cysteine peptidase gene in the genome database of Eimeria tenella (EtCatB). Molecular modeling of the predicted protein suggested that it differs significantly from host enzymes and could be a good drug target. EtCatB was expressed and secreted as a soluble, active, glycosylated mature enzyme from Pichia pastoris. Biochemical characterization of the recombinant enzyme confirmed that it is cathepsin B-like. Screening of a focused library against the enzyme identified three inhibitors (a nitrile, a thiosemicarbazone, and an oxazolone) that can be used as leads for novel drug discovery against Eimeria. The oxazolone scaffold is a novel cysteine peptidase inhibitor; it may thus find widespread use.

Inhibition of Eimeria tenella CDK-related kinase 2: From target identification to lead compounds

Apicomplexan parasites encompass several human- and animal-pathogenic protozoans such as Plasmodium falciparum, Toxoplasma gondii, and Eimeria tenella. E. tenella causes coccidiosis, a disease that afflicts chickens, leading to tremendous economic losses to the global poultry industry. The considerable increase in drug resistance makes it necessary to develop new therapeutic strategies against this parasite. Cyclin-dependent kinases (CDKs) are key molecules in cell-cycle regulation and are therefore prominent target proteins in parasitic diseases. Bioinformatics analysis revealed four potential CDK-like proteins, of which one-E. tenella CDK-related kinase 2 (EtCRK2)-has already been characterized by gene cloning and expression.1 By using the CDK-specific inhibitor flavopiridol in EtCRK2 enzyme assays and schizont maturation assays (SMA), we could chemically validate CDK-like proteins as potential drug targets. An X-ray crystal structure of human CDK2 (HsCDK2) served as a template to build protein models of EtCRK2 by comparative homology modeling. Structural differences in the ATP binding site between EtCRK2 and HsCDK2, as well as chicken CDK3, were addressed for the optimization of selective ATP-competitive inhibitors. Virtual screening and "wet-bench" high-throughput screening campaigns on large compound libraries resulted in an initial set of hit compounds. These compounds were further analyzed and characterized, leading to a set of four promising lead compounds for development as EtCRK2 inhibitors.

A novel multi-domain mucin-like glycoprotein of Cryptosporidium parvum mediates invasion

Cryptosporidium parvum is a protozoan parasite which produces self-limited disease in immunocompetent hosts and devastating, persistent diarrhea in immunocompromised individuals. There is no effective treatment for cryptosporidiosis and little is known about the basic biology of the organism. Cloning and sequence analysis of the gene encoding GP900, a previously identified > 900 kDa glycoprotein, predicts a mucin-like glycoprotein composed of distal cysteine-rich domains separated by polythreonine domains and a large membrane proximal N-glycosylated core region. A trinucleotide repeat composed predominantly of the triplet ACA encodes the threonine domains. GP900 is stored in micronemes prior to appearance on the surface of invasive forms. The concentration of native GP900 which inhibits 50% (IC50) of invasion in vitro is low picomolar; the IC50 for a recombinant cysteine rich-domain is low nanomolar. These observations indicate that GP900 is a parasite ligand for a host receptor involved in attachment/invasion and suggest that immunotherapy or chemotherapy directed against GP900 may be feasible.

Monoclonal antibodies inhibiting invasion of cultured cells by Eimeria tenella sporozoites

Monoclonal antibodies (MoAbs) secreted by 12 hybridomas and reactive with the surface antigens of E. tenella sporozoites were produced. These MoAbs designated as KC-1 to KC-12 were characterized as IgG3 kappa. In Western blot analysis, these MoAbs reacted with only one polypeptide band of sporozoite antigens having a molecular mass of 25 kDa. All the MoAbs were reactive with sporozoites, trophozoites, immature and mature first generation schizonts, first generation merozoites, and the interior structure of sporulating oocysts of E. tenella, however, not with any of the methanol-fixed sporozoites of E. acervulina, E. mitis, E. maxima, E. brunetti, E. necatrix and E. praecox. Invasion of primary culture of chicken kidney cells by E. tenella sporozoites was significantly inhibited by several of the MoAbs. These results suggest that these MoAbs recognizing the E. tenella-specific surface molecule are involved in the invasion of sporozoites into host cells.

Developmental regulation of an Eimeria bovis mRNA encoding refractile body-associated proteins

Eimeria bovis antigens defined by the monoclonal antibody (mAb) 2.4 are associated with the refractile bodies of sporozoites and are found in the parasitophorous vacuole and host cell cytoplasm during schizogony. Screening of an E. bovis oocyst cDNA library with mAb 2.4 resulted in the identification of a single unique cDNA sequence (Eb-25/50). Comparison of the predicted protein sequence of Eb-25/50 revealed a high degree of identity to an Eimeria tenella refractile body protein and mAb 2.4 was found to cross-react with refractile bodies from Eimeria acervulina, demonstrating that these proteins are highly conserved among eimerian species. Measurements of Eb-25/50 mRNA showed that the multiple proteins recognized by mAb 2.4 are encoded by a single mRNA species whose kinetics of expression during sporulation and schizogony closely correlated with protein expression. Consistent with multiple Eb-25/50 proteins arising from a single polypeptide, results from a Southern analysis of E. bovis genomic DNA indicated that Eb-25/50 mRNA is derived from a single copy gene. The presence of Eb-25/50 proteins in the host cytoplasm during schizogony, the high degree of conservation of these proteins, and the apparent complex post-translational modification raises interesting questions about the biochemistry of these proteins during eimerian development.

Production and characterization of monoclonal antibodies directed against Eimeria falciformis and cross-reactive with sporozoites from two species of avian coccidia

Monoclonal antibodies (mAbs) were obtained against the surface antigens of the Eimeria falciformis sporozoite by immunizing mice with whole homogenized sporozoite. The hybridomas were selected by their reactivities against oocyst extracts, then against glutaraldehyde-treated sporozoites. Three mAbs recognized both the surface of E. falciformis, E. tenella, and E. acervulina and their refractile bodies, whereas a fourth mAb recognized only one epitope on the refractile bodies. All mAbs bound to the same immunoaffinity-purified antigens in Western-blot analysis (P27 for E. falciformis and P25 for E. tenella and E. acervulina). Thus, the mAbs define at least two shared epitopes between sporozoite antigens from different eimerian species. Two of these mAbs are involved in the in vitro phagocytosis of E. falciformis sporozoites by macrophages and also in their lysis by neutrophils. Altogether, these properties showed that the four mAbs came from different activated B-cells. The P27 antigen recognized by our mAbs represents a major target of the in vitro destructive immune response.

Passive transfer of immunity against Cryptosporidium infection in neonatal mice using monoclonal antibodies

Monoclonal antibodies (mAb) against merozoites and sporozoites of the protozoan parasite Cryptosporidium parvum were examined for potential modulation of cryptosporidial infections in vivo by daily oral mAb administration to oocyst-inoculated neonatal mice. Monoclonal-treated neonatal mice were sacrificed four and eight days post infection (pi). Differences in infection rates were observed among the treatment groups at the p < 0.05 level. Suckling mice treated daily with orally administered mixtures of mAbs (ascitic fluids) showed significantly reduced parasite loads compared to control mice at four and eight days pi, while suckling mice receiving mAb Cmg-3 alone showed significant differences only at 4 days pi., suggesting that passive transfer of mAb may be of value in controlling cryptosporidial infections.

Identification and initial characterization of five Cryptosporidium parvum sporozoite antigen genes

Cryptosporidium parvum, an Apicomplexan parasite of gastrointestinal epithelial cells, causes severe disease in persons with AIDS and is a common cause of self-limited diarrhea in children, animal handlers, and residents of developing countries. No approved therapy exists; in research studies, however, hyperimmune bovine colostrum raised to Cryptosporidium oocysts and sporozoites has eradicated disease or decreased parasite burden in some AIDS patients. Although the protective antigens recognized by bovine hyperimmune colostrum have not been defined, protective antigens of other Apicomplexan parasites frequently have been associated with two unique structures of invasive forms, the trilaminar pellicle and the apical complex. In order to identify immunogenic Cryptosporidium proteins that may be protective antigens for use as recombinant immunogens in passive and/or active immunotherapy, we screened two genomic DNA expression libraries with polyspecific anti-Cryptosporidium antibodies. We used an approach to cloning apical complex and pellicle protein antigens that succeeded despite the lack of large numbers of organisms that would be necessitated for conventional biochemical approaches requiring organelle or membrane purification. We report here the molecular cloning of five C. parvum genes and the characterization of the cognate sporozoite proteins having molecular masses of greater than 500, 68/95, 45, 23, and 15/35 kDa. The light microscopic immunofluorescence pattern of antibodies recognizing these protein antigens suggest that they are located in the pellicle or apical complex of Cryptosporidium sporozoites.

Shedding of the immunodominant P20 surface antigen of Eimeria bovis sporozoites

P20 is an immunodominant surface antigen of Eimeria bovis sporozoites. As parasites underwent merogony within cultured bovine monocytes and Madin-Darby bovine kidney (MDBK) cells, P20 appeared to be shed gradually by meronts and was absent in type 1 and 2 first-generation merozoites. Meronts of E. bovis appeared to shed P20 into the parasitophorous vacuole of bovine monocytes, whereas MDBK cells evidently released P20 into the culture medium or destroyed its antigenic determinant.

Protein 20, an immunodominant surface antigen of Eimeria bovis

Autoradiography identified six 125I-labeled proteins, ranging in molecular weight (Mr) from 20,000 to approximately 110,000, on the plasmalemma of Eimeria bovis sporozoites. Immunoblotting with bovine antiserum generated by intravenous inoculations of sporozoites and with immune serum generated by per os inoculations of oocysts revealed that protein 20 (i.e., 20,000 Mr) was an immunodominant antigen on the surface of E. bovis sporozoites.