Genetically engineered antigen confers partial protection against avian coccidial parasites

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.

Immunostimulatory complexes containing Eimeria tenella antigens and low toxicity plant saponins induce antibody response and provide protection from challenge in broiler chickens

Immunostimulating complexes (ISCOMs) are unique multimolecular structures formed by encapsulating antigens, lipids and triterpene saponins and are one of the most successful antigen delivery systems for microbial antigens. In the current study, both the route of administration and the antigen concentration of ISCOMs, containing Eimeria tenella antigens and saponins from native plants, were evaluated in their ability to stimulate humoral immunity and to protect chickens against a challenge infection with E. tenella. Broiler chickens were immunized with ISCOM preparations containing E. tenella antigens and the purified saponins Gg6, Ah6 and Gp7 isolated from Glycyrrhiza glabra, Aesculus hippocastanum and Gipsophila paniculata, respectively. The effects of the route of administration, dose of antigen and type of saponin used for construction of ISCOMs were evaluated for ability to stimulate serum IgG and IgM and to protect chickens against a homologous challenge. A single intranasal immunization was the most effective route for administering ISCOMs although the in ovo route was also quite effective. Dose titration experiments demonstrated efficacy after single immunization with various ISCOM doses but maximum effects were observed when ISCOMs contain 5-10mug antigen. Immunization of birds by any of the three routes with E. tenella antigens alone or antigens mixed with alum hydroxide adjuvant resulted in lower serum antibody and reduced protection to challenge relative to immunization with ISCOMs. Overall the results of this study confirm that significant immunostimulation and protection to challenge are achieved by immunization of chickens with ISCOMs containing purified saponins and native E. tenella antigens and suggest that ISCOMs may be successfully used to develop a safe and effective vaccine for prevention of avian coccidiosis.

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.

Immunostimulating complexes incorporating Eimeria tenella antigens and plant saponins as effective delivery system for coccidia vaccine immunization

Immunostimulating complexes (ISCOMs) are unique, multimolecular structures formed by encapsulating antigens, lipids, and triterpene saponins of plant origin, and are an effective delivery system for various kinds of antigens. The uses of ISCOMs formulated with saponins from plants collected in Kazakhstan, with antigens from the poultry coccidian parasite Eimeria tenella, were evaluated for their potential use in developing a vaccine for control of avian coccidiosis. Saponins isolated from the plants Aesculus hippocastanum and Glycyrrhiza glabra were partially purified by HPLC. The saponin fractions obtained from HPLC were evaluated for toxicity in chickens and chicken embryos. The HPLC saponin fractions with the least toxicity, compared to a commercial saponin Quil A, were used to assemble ISCOMs. When chicks were immunized with ISCOMs prepared with saponins from Kazakhstan plants and E. tenella antigens, and then challenged with E. tenella oocysts, significant protection was conveyed compared to immunization with antigen alone. The results of this study indicate that ISCOMs formulated with saponins isolated from plants indigenous to Kazakhstan are an effective antigen delivery system which may be successfully used, with low toxicity, for preparation of highly immunogenic coccidia vaccine.

Eimeria tenella: Ginsenosides-enhanced immune response to the immunization with recombinant 5401 antigen in chickens

Three-day-old specific-pathogen-free chickens were subcutaneously immunized with Eimeria tenella recombinant 5401 antigen (100 microg per chicken) with (0.25, 0.5 or 1.0mg per dose) or without ginsenosides, and boosted with the same dosage 14 days later. The chickens were challenged with 6 x 10(4) homologous sporulated oocysts 14 day after the booster. The specific antibody response and lymphocyte proliferation in response to Con A were measured before and 7, 14, 21, 28, 35, 42 days after the immunization. Oocyst output, mortality, and lesion scores were measured to evaluate the protective effects of the immunization. The vaccine containing 0.5 or 1.0mg ginsenosides per dose induces higher antibody response and lymphocyte proliferation in response to Con A than the vaccine without ginsenosides or containing 0.25mg per dose. The oocyst output indicated that recombinant 5401 antigen with ginsenosides (0.5 and 1.0mg per dose) gave a protection rate of 59.38 and 62.5%, respectively. The lesion score in the group vaccinated with recombinant 5401 antigen with 0.5 or 1.0mg ginsenosides per dose were significantly lower than in group without ginsenosides or containing 0.25mg per dose. Therefore, we conclude that ginsenosides have strong adjuvant effects at a dose of 0.5 or 1.0mg when mixed with E. tenella recombinant 5401 antigen, and has a potential as an adjuvant in chicken vaccine.

Partial protection against Eimeria acervulina and Eimeria tenella induced by synthetic peptide vaccine

Coccidiosis is a major parasitic disease of poultry industry and an ideal vaccine should induce long-lasting cross-species protective immunity. Broiler chickens (Cobb 500) were inoculated with single, double or triple injections of a synthetic peptide (derived from sequences of Eimeria acervulina and Eimeria tenella antigens) homogenized in Freund's complete and incomplete adjuvants. The immune responses to the vaccine were assessed by evaluation of antibody and lymphocyte proliferation responses, and the degree of resistance of vaccinated chickens to challenge with sporulated oocysts of E. acervulina or E. tenella determined by comparison of their oocyst output with those of control chickens. The results indicated that the synthetic peptide vaccine induced a high level of antibody and cellular responses associated with partial cross-species protection against challenge with sporulated oocysts of E. acervulina or E. tenella.

Efficacy of a DNA vaccine delivered in attenuated Salmonella typhimurium against Eimeria tenella infection in chickens

The efficacy of an oral DNA vaccine carrying the Eimeria tenella 5401 antigen gene delivered by attenuated Salmonella typhimurium was examined in an experimental challenge study. The DNA vaccine preparation was made by transforming the recombinant plasmid pcDNA3-5401 into the attenuated S. typhimurium strain (Dam(-) and PhoP(-)) (designated hereafter as ZJ111/pcDNA3-5401). The chickens were randomly divided into six groups, 50 per group. Group A were given PBS as control. Chickens in group B were fed with 10(8) colony forming units (CFU) of attenuated S. typhimurium carrying pcDNA3. Group C were immunised with 100 microg of the recombinant 5401 protein via intramuscular injection. Groups D to F orally received ZJ111/pcDNA3-5401 at doses of 10(7), 10(8) and 10(9)CFU per chicken, respectively. All immunisations were boosted 2 weeks later. The immunised chickens were challenged with 6x10(4) homologous sporulated oocysts 14 days after the second immunisation. No significant differences in body weight were detected between the groups before immunisation and at week 4 after the booster immunisation. The ZJ111/pcDNA3-5401 was eventually eliminated from the spleen and liver on week 6 post-immunisation. The plasmid pcDNA3-5401 was stably maintained in over 80% of the attenuated S. typhimurium population after 100 generations of growth in antibiotic-free media. Oral immunisation of chickens with ZJ111/pcDNA3-5401 elicited specific humoral responses and stimulated proliferation of peripheral blood lymphocytes. The lymphocyte proliferation response was significantly higher in all vaccinated groups than in the control chickens. Antibody response was significantly lower in group C than in groups immunised with strain ZJ111/pcDNA3-5401. Vaccination with the strain ZJ111/pcDNA3-5401 at 10(8) (group E) and 10(9) (group F) CFU per chicken provided 55.0 and 57.5% protection against E. tenella challenge, respectively. These results have important implications for the development of DNA vaccines against avian coccidiosis by bacteria-vectored oral delivery system.

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.

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.

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

Micronemes are specialised secretory organelles that release their proteins by a stimulus-coupled exocytosis that occurs when apicomplexan parasites make contact with target host cells. These proteins play crucial roles in motility and invasion, most likely by mediating adhesion between parasite and host cell surfaces and facilitating the transmission of dynamic forces generated by the parasite actinomyosin cytoskeleton. Members of the TRAP family of microneme proteins are characterised by having extracellular domains containing one or more types of cysteine-rich, adhesive modules, highly-conserved transmembrane regions and cytosolic tails that contain one or more tyrosines, stretches of acidic residues and a single tryptophan. In this paper, we describe a novel member of the TRAP family, EtMIC4, a 218 kDa microneme protein from Eimeria tenella. EtMIC4 contains 31 epidermal growth factor (EGF) modules, 12 thrombospondin type-1 (TSP-1) modules and a highly acidic, proline and glycine-rich region in its extracellular region, plus the conserved transmembrane and cytosolic tail. Like EtMIC1, another TRAP family member from E. tenella, EtMIC4 is expressed in sporozoites and all the merozoite stages of the parasite, suggesting that this parasite has a strong requirement for TSP-1 modules. Unlike the other microneme proteins so far studied in E. tenella, EtMIC4 appears to be found constitutively on the sporozoite surface as well as within the micronemes.

Vaccines against the avian enteropathogens Eimeria, Cryptosporidium and Salmonella

The worldwide poultry industry provides a substantial proportion of the nutritional requirement of the human population. To keep pace with the increasing demand for the high-quality, low-cost protein source that poultry provides, intensive rearing practices have been developed within the past few decades. For example, chickens are housed routinely in crowded environments under adverse conditions, and genetic strains have been selected for rapid growth, high protein-to-fat content and superior egg-laying characteristics. A major negative consequence of these practices has been an increase in the incidence of diseases. Enteric diseases in particular have emerged as a major problem threatening the future viability of the poultry industry. A variety of methods have been used to combat avian diseases in the commercial setting, including improved farm management practices, the use of antibiotic drugs, the selection of disease-resistant strains of chickens, and the manipulation of the chicken's immune system. In the latter category, the development of vaccines against the major avian diseases has become a priority in the poultry industry. This review will highlight recent progress in vaccine development against three major avian enteric pathogens: Eimeria, Cryptosporidium and Salmonella.

Intestinal immune responses to coccidiosis

Intestinal parasitism is a major stress factor leading to malnutrition and lowered performance and production efficiency of livestock and poultry. Coccidiosis is an intestinal infection caused by intracellular protozoan parasites belonging to several different species of Eimeria. Infection with coccidia parasites seriously impairs the growth and feed utilization of chickens and costs the US poultry industry more than $1.5 billion in annual losses. Although acquired immunity to Eimeria develops following natural infection, due to the complex life cycle and intricate host immune response to Eimeria, vaccine development has been difficult and a better understanding of the basic immunobiology of pertinent host-parasite interactions is necessary for developing effective immunological control strategies against coccidiosis. Chickens infected with Eimeria produce parasite specific antibodies in both the circulation and mucosal secretions but humoral immunity plays only a minor role in protection against this disease. Rather, recent evidence implicates cell-mediated immunity as the major factor conferring resistance to coccidiosis. This review will summarize current understanding of the avian intestinal immune system and its response to Eimeria as well as provide a conceptual overview of the complex molecular and cellular events involved in intestinal immunity to coccidiosis. It is anticipated that increased knowledge of the interaction between parasites and host immunity will stimulate the birth of novel immunological and molecular biological concepts in the control of intestinal parasitism.

Vaccination against Eimeria tenella infection using a fraction of E. tenella sporozoites selected by the capacity to activate T cells

At 8 days after a primary Eimeria tenella infection, a subset of T cells, of which the protective role is as yet unclear, circulates in the peripheral blood. In order to investigate this, the in vitro cellular responsiveness of these peripheral blood lymphocytes has been used as selection criterion to identify potentially protective E. tenella sporozoite antigens. The hydrophilic protein phase of purified E. tenella sporozoite homogenates obtained by Triton X-114 extraction was fractionated using preparative gel electrophoresis. Nine fractions, separated according to different molecular weight, were tested for their ability to stimulate T-cell responses. Both the proliferation of peripheral blood lymphocytes and the macrophage activating activity released in the culture supernatants were measured. On the basis of this responsiveness, four fractions were selected and used to vaccinate chickens. All vaccine preparations induced strong T-cell responses. One fraction immunised chickens against subsequent challenge infection, in that the caecal lesion scores were significantly lower as compared with that of the unvaccinated controls. This fraction contained hydrophilic polypeptides with a molecular mass that ranged from 26 to 30 kDa.

Immune responses in chickens against Eimeria tenella sporozoite antigen

Two-day old broiler chicks were subcutaneously immunized with Eimeria tenella sporozoite antigen (25 microg per chick) with or without adjuvants on 2 and 18 days of age and the effect of induced immunity was determined by challenging the chickens with 10(4) homologous sporulated oocysts at 32 days of age. Chicks immunized with sporozoite antigen emulsified in Freund's Complete Adjuvant (FCA) showed protection in terms of oocyst production, mortality and mean lesion scores. Antigen emulsified in FCA produced significant cell mediated immune responses (as assessed by lymphocyte migration inhibition test) from 12 to 30 days post-immunization. Antibody responses as assessed by enzyme linked immunosorbent assay were significant from 12 days postimmunization when the antigen was administered with or without adjuvants by subcutaneous route.

B-cell epitope mapping within the MA16 antigenic sequence found in Eimeria acervulina merozoites and sporozoites

Overlapping heptapeptides derived from the MA16 Eimeria acervulina antigenic sequence (Castle et al., 1991) were synthesised on polypropylene pins ('pepskan' technique, Cambridge Research Biochemicals, UK). Binding of antibodies from chickens and rabbits infected and immunised respectively with various species of Eimeria oocysts (E. acervulina, E. tenella, E praecox, E. necatrix and E. maxima), was examined using the coated pins as the solid phase of an enzyme immunoassay (EIA). Antigenicity of the overlapping synthetic heptapeptides was then analysed using a number of algorithms based on the amino acid sequence to predict secondary protein structure, hydrophilicity, acrophilicity and chain flexibility profiles. The antigenicity of this sequence appears to be quite different from that found for the E. tenella GX3264 antigenic sequence (Bhogal et al., 1992) whose profile was similarly examined (Talebi and Mulcahy, 1994) using the same rabbit and chicken anti-Eimeria oocyst sera.

Mapping for B-cell epitopes in the GX3262 antigenic sequence derived from Eimeria tenella sporulated oocysts

Polypropylene pins were impregnated with synthetic overlapping heptapeptides based on the GX3262 Eimeria tenella antigenic sequence (Miller et al., 1989). Using these coated pins as the solid phase of an enzyme immunoassay (EIA), binding of sera from chickens and rabbits infected and immunised respectively with five different species of Eimeria were examined. Antibody reactions to the individual heptapeptides were then analysed by a number of criteria based on the amino acid sequence including hydropathy, chain flexibility and secondary structure.

A review on vaccination against protozoa and arthropods of veterinary importance

The recent advances in immunology and biotechnology have stimulated much research on the control of parasitic diseases through vaccination. This is a review of the state of the art regarding important protozoan and arthropod veterinary parasites. A live oocyst vaccine for avian coccidiosis is still in use but much work has been done on the identification, cloning, and assay of protective antigens. The sporozoites of Eimeria tenella have been the preferred subject and at least four recombinant antigens have already been tested with partial success. Premunization against babesiosis is still widely used in Latin America as is a live vaccine with attenuated parasites in Australia. At least three Babesia bovis and three Babesia bigemina antigens that generate partial protection have been produced as recombinant proteins. A vaccine against canine babesiosis is being commercialized in France. Infection-treatment is still used to vaccinate against Theileria parva and a schizont vaccine against Theileria annulata. Recombinant sporozoite antigens have been assayed with partial success against both species but the identification and administration of protective schizont antigens, regarded as the most important, still requires considerable work. The immunological control of African trypanosomoses is still impaired by the antigenic variation that the parasites experience during the infection. Although some possibilities exist, most specialists are pessimistic about the promise of developing a vaccine in the near future. Control of Boophilus ticks with an occult tick intestine recombinant antigen seems to have potential in inhibiting reproduction of the tick but salivary antigens appear to be more effective at inhibiting feeding and pathogen transmission. Vaccination with a Hypoderma protein, recently cloned, has induced 90% protection against subsequent infestations. It is very likely that effective vaccines against veterinary parasites will become available in the near future.

High-resolution mapping of B-cell epitopes within an antigenic sequence from Eimeria tenella

Overlapping hexapeptides representing part of an Eimeria tenella antigenic sequence, shown to induce partial immunity to homologous challenge in chickens, were synthesized on polypropylene pins (Pepskan technique; Cambridge Research Biochemicals, Cambridge, United Kingdom). The binding to these hexapeptides of antibodies from chickens infected and rabbits immunized with five species of Eimeria was studied, using the coated pins as the solid phase of an enzyme-linked immunoassay. Antibody binding to most regions of the sequence was demonstrated, with peak areas of antigenicity correlating with the most hydrophilic regions. A particularly hydrophilic and antigenic area towards the N terminus of the sequence consists of a peptide motif repeated five times in the native antigen. Homologous antisera (chicken and rabbit anti-E. tenella antisera) differed in their pattern of reactivity from heterologous sera raised against other Eimeria species. While the former bound to fewer of the hexapeptides than the latter, they did so very strongly, indicating affinity maturation of the antibody response to E. tenella-specific sequences. No antibody reactivity to two regions of the sequence was detected. These regions occur in relatively hydrophilic areas and so are unlikely to be situated in transmembrane domains or in the interior of globular proteins. Synthetic peptides, as used in these experiments, make possible analysis of the fine specificity of immune responses and thus have a role to play in the development of novel vaccines for the control of coccidiosis.

Codon usage and bias among individual genes of the coccidia and piroplasms

Codon usage has been analysed in individual gene sequences, derived from a variety of parasitic protozoa in the class Sporozoa of the phylum Apicomplexa using metric multidimensional scaling. The two groups of codon usage patterns detected reflect the two main subgroups of organisms studied (the coccidia and the piroplasms), and it is the pattern of usage of synonymous codons that has the largest influence on overall codon usage in the individual genes, rather than being the pattern of amino acid composition of the gene product. The magnitude of the codon usage bias in the sequences was determined using three commonly used indices-NC, GC3S and B. In general, although relatively low levels of codon usage bias were detected in these gene sequences, codon usage bias does explain at least some of the codon usage patterns observed. Codon usage bias was observed to be dependent on the overall base composition of the genes analysed, which in turn was reflected in the types of codons that were either over- or under-represented in the nucleotide sequences. In keeping with observations on prokaryotic organisms, it is speculated that the codon usage patterns detected in these parasitic protozoa are the result of directional mutation pressure on the base composition of the genomic DNA.

Vaccination against animal parasites

A decade of molecular parasitology is beginning to bear fruit, with the appearance of several new, highly effective, practical vaccines against parasitic diseases. Recombinant antigen vaccines have been developed against cestode, nematode, trematode, protozoan and arthropod parasites. Greatest progress has been made with veterinary vaccines, where the ability to test numerous vaccine formulations in challenge trials has allowed more rapid identification of host-protective antigens than is possible with many medically important parasites. Several quite different approaches to vaccine development have been successful. The traditional approach using live, attenuated parasites continues to provide effective vaccines against several protozoan and nematode parasites. Recombinant DNA technology, monoclonal antibody technology, protein chemistry and immunochemistry have played critical roles in the outstanding success which has been achieved over the last 5 years in the development of defined-antigen vaccines. Two approaches have been successful in research towards defined antigen vaccines against parasites: (1) the 'natural antigen' approach where immune responses are stimulated to parasite molecules which are normally antigenic, and possibly host-protective, in infected hosts; (2) the 'naive antigen' approach where parasite molecules which are not antigenic, or of very low antigenicity, in infected hosts are used to raise immune responses capable of killing the parasite. This review examines the successful approaches taken towards the development of effective anti-parasite vaccines and the vaccines which have been produced to date.

Analysis of dinucleotide frequency and codon usage in the phylum Apicomplexa

Dinucleotide frequency (DiF) and codon usage (cu) were analysed in gene sequences from four parasitic protozoa, Babesia bovis, Theileria parva, Toxoplasma gondii and Eimeria tenella, of the phylum Apicomplexa. In keeping with the 'genome hypothesis', cu was found to be non-random and species specific in these organisms, although cu among members of the same subclass was found to be very similar. Several low-usage (lu) codons were identified, and the usage of lu codons appears to be related to the taxonomic position of the organisms under study. A comparison of the observed/expected DiF ratios obtained from gene coding regions revealed a low frequency of the TA and CG dinucleotides in all organisms studied. A comparison of these DiF ratios with those found in rRNA-encoding genes and in introns, showed that in the parasites, B. bovis and Th. parva (representing the piroplasms), the low frequency of dinucleotides appeared to be the result of coding pressure alone. In T. gondii and E. tenella (representing the coccidia), however, coding pressure could not completely explain differences in DiF.

Colostrum from cows immunized with Eimeria acervulina antigens reduces parasite development in vivo and in vitro

Experiments were undertaken to determine whether passive immunization utilizing hyperimmune bovine colostrum (HBC) specific for Eimeria acervulina (EA) antigens conferred protection against coccidiosis in chickens. The HBC was produced by immunizing three pregnant, nonmilking Jersey cows with EA antigens administered via one intramuscular injection followed by three intramammary infusions at approximately 10, 8, 6, and 4 wk before parturition. One cow was immunized with sporozoites (SZ), the second with merozoites (MZ), and the third with recombinant merozoite antigen (rMZ). A fourth cow, unimmunized, provided normal colostrum (NC) for control purposes. Colostral whey from each cow was tested by ELISA for antibody against SZ, MZ, and rMZ antigens. In all immunized cows, antiparasite titers were elevated above those of the control. Antibodies from MZ- and rMZ-immunized cows recognized both MZ and rMZ antigen. Separate groups of 2-wk-old chickens received two oral doses of anti-SZ, -MZ, or -rMZ HBC or NC or PBS daily from 1 day before through 6 days after oral inoculation (DAI) with EA oocysts. Feces from each group were examined for oocysts. Intestines were examined for lesions 6 DAI. Histologic sections of duodenum were examined for asexual stages and gametocytes utilizing monoclonal antibody and fluorescence microscopy. In Experiments 1 and 2, oocyst production was reduced in all HBC-treated groups, except one treated with rMZ HBC, compared with PBS- or NC-treated groups. In Experiment 2, the severity of lesions was significantly reduced in all HBC-treated groups compared with those that received NC or PBS. Significantly fewer developmental stages were found in histological sections from all chickens treated with anti-SZ and anti-rMZ HBC than from controls. Anti-SZ HBC significantly reduced the number of intracellular SZ found 24 h after their inoculation into cultures of primary chicken kidney cells. These results suggest that HBC specific for certain EA antigens can inhibit parasite development and reduce severity of parasite-related gut lesions.

Cross-protection against four species of chicken coccidia with a single recombinant antigen

A cDNA clone, SO7', from an Eimeria tenella cDNA library was inserted into the high-expression vector pJC264 and was expressed in Escherichia coli as a fusion protein, CheY-SO7', with a molecular mass of approximately 36 kDa. By using the purified recombinant antigen to immunize young chicks, it was demonstrated that a single dose, without adjuvant, not only protected against severe coccidiosis induced by infection with E. tenella but also protected chicks challenged with the heterologous species Eimeria acervulina, E. maxima, and E. necatrix. By using rabbit antiserum raised against recombinant CheY-SO7', Western blot (immunoblot) analysis of sporulated oocysts of all seven major species of chicken coccidia showed that all species tested contained proteins characteristic of the B class of antigens, of which CheY-SO7' is representative. It seems likely that a single B antigen could protect chickens against severe coccidiosis caused by infection with any of these Eimeria species. Although chicks exposed to prolonged, natural infection develop antibodies to B antigen, active immunization of young chicks with a protective dose of CheY-SO7' does not elicit a humoral antibody response, suggesting that the partial protection results from cell-mediated effector mechanisms. In addition, the cross-protective nature of the immunity indicates that the response to B antigen is different from that induced by natural infection, which elicits a species-specific immunity. To date, the protection induced by B antigen immunization, although remarkable for a single recombinant protein, is not sufficient to compete with prophylactic chemotherapy.

Protective immunization against the intestinal parasite Eimeria acervulina with recombinant coccidial antigen

The gene encoding an immunodominant Eimeria acervulina merozoite surface antigen (EAMZ250) was expressed in bacteria as a fusion peptide with the galactose-binding protein (GBP) of Escherichia coli. Recombinant and control antigens were administered to 1-wk-old chickens by peroral inoculation with live nonpathogenic bacteria that were expressing GBP-EAMZ250 or GBP protein. The immunization elicited antigen-specific humoral and cellular immune responses as measured by ELISA and T-cell blastogenesis assay. In addition, chickens immunized with recombinant GBP-EAMZ250 exhibited significant protection against weight loss and intestinal lesions after E. acervulina challenge. Bacterial transformants were recoverable from the upper and middle intestine of inoculated chickens for various times after immunization. These data indicate that oral administration of live E. coli expressing a recombinant E. acervulina antigen is an effective means of inducing resistance to coccidiosis.

Development of a genetically engineered vaccine against poultry coccidiosis

Coccidiosis is caused by infection with Eimeria spp. The disease is responsible for major economic loss to the poultry industry unless infections are controlled by anticoccidial drugs. John Ellis and Fiona Tomley discuss recent research on the characterization and cloning of antigens from Eimeria spp and advances towards the development of genetically engineered vaccines against poultry coccidiosis.