Genetic analysis of the essential components of the immunoprotective response to infection with Eimeria vermiformis

Coccidiosis: Recent Progress in Host Immunity and Alternatives to Antibiotic Strategies

Coccidiosis is an avian intestinal disease caused by several distinct species of Eimeria parasites that damage the host’s intestinal system, resulting in poor nutrition absorption, reduced growth, and often death. Increasing evidence from recent studies indicates that immune-based strategies such as the use of recombinant vaccines and various dietary immunomodulating feed additives can improve host defense against intracellular parasitism and reduce intestinal damage due to inflammatory responses induced by parasites. Therefore, a comprehensive understanding of the complex interactions between the host immune system, gut microbiota, enteroendocrine system, and parasites that contribute to the outcome of coccidiosis is necessary to develop logical strategies to control coccidiosis in the post-antibiotic era. Most important for vaccine development is the need to understand the protective role of the local intestinal immune response and the identification of various effector molecules which mediate anti-coccidial activity against intracellular parasites. This review summarizes the current understanding of the host immune response to coccidiosis in poultry and discusses various non-antibiotic strategies which are being developed for coccidiosis control. A better understanding of the basic immunobiology of pertinent host–parasite interactions in avian coccidiosis will facilitate the development of effective anti-Eimeria strategies to mitigate the negative effects of coccidiosis.

Still naïve or primed: Anticoccidial vaccines call for memory

Despite decades of investigation to clarify protective mechanisms of anticoccidial responses, one crucial field is neglected, that is, protective memory responses in primed birds. Protective memory immunity is critical for host resistance to reinfection and is the basis of modern vaccinology, especially in developing successful subunit vaccines. There are important differences between the immune responses induced by infections and antigens delivered either as killed, recombinant proteins or as live, replicating vector vaccines or as DNA vaccines. Animals immunized with these vaccines may fail to develop protective memory immunity, and is still naïve to Eimeria infection. This may explain why limited success is achieved in developing next-generation anticoccidial vaccines. In this review, we try to decipher the protective memory responses against Eimeria infection, assess immune responses elicited by various anticoccidial vaccine candidates, and propose possible approaches to develop rational vaccines that can induce a protective memory response to chicken coccidiosis.

Poultry Coccidiosis: Design and Interpretation of Vaccine Studies

Eimeria infection impacts upon chicken welfare and economic productivity of the poultry sector. Live coccidiosis vaccines for chickens have been available for almost 70 years, but the requirement to formulate blends of oocysts from multiple Eimeria species makes vaccine production costly and logistically demanding. A multivalent vaccine that does not require chickens for its production and can induce protection against multiple Eimeria species is highly desirable. However, despite the identification and testing of many vaccine candidate antigens, no recombinant coccidiosis vaccine has been developed commercially. Currently, assessment of vaccine efficacy against Eimeria, and the disease coccidiosis, can be done only through in vivo vaccination and challenge experiments but the design of such studies has been highly variable. Lack of a "standard" protocol for assessing vaccine efficacy makes comparative evaluations very difficult, complicating vaccine development, and validation. The formulation and schedule of vaccination, the breed of chicken and choice of husbandry system, the species, strain, magnitude, and timing of delivery of the parasite challenge, and the parameters used to assess vaccine efficacy all influence the outcomes of experimental trials. In natural Eimeria infections, the induction of strong cell mediated immune responses are central to the development of protective immunity against coccidiosis. Antibodies are generally regarded to be of lesser importance. Unfortunately, there are no specific immunological assays that can accurately predict how well a vaccine will protect against coccidiosis (i.e., no "correlates of protection"). Thus, experimental vaccine studies rely on assessing a variety of post-challenge parameters, including assessment of pathognomonic lesions, measurements of parasite replication such as oocyst output or quantification of Eimeria genomes, and/or measurements of productivity such as body weight gain and feed conversion rates. Understanding immune responses to primary and secondary infection can inform on the most appropriate immunological assays. The discovery of new antigens for different Eimeria species and the development of new methods of vaccine antigen delivery necessitates a more considered approach to assessment of novel vaccines with robust, repeatable study design. Careful consideration of performance and welfare factors that are genuinely relevant to chicken producers and vaccine manufacturers is essential.

Involvement of T Cell Immunity in Avian Coccidiosis

Avian coccidiosis is caused by Eimeria, which is an intracellular apicomplexan parasite that invades through the intestinal tract to cause devastating disease. Upon invasion through the intestinal epithelial cells, a strong inflammatory response is induced that results in complete villous destruction, diarrhea, hemorrhage, and in severe cases, death. Since the life cycle of Eimeria parasites is complex and comprises several intra- and extracellular developmental stages, the host immune responses are diverse and complex. Interferon-γ-mediated T helper (Th)1 response was originally considered to be the predominant immune response in avian coccidiosis. However, recent studies on other avian T cell lineages such as Th17 and T regulatory cells have implicated their significant involvement in maintaining gut homeostasis in normal and disease states including coccidiosis. Therefore, there is a need to understand better their role in coccidiosis. This review focuses on research findings concerning the host immune response induced by avian coccidiosis in the context of T cell immunity, including expression of T-cell-related cytokines and surface molecules that determine the phenotype of T lymphocytes.

A Novel Vaccine Delivery Model of the Apicomplexan Eimeria tenella Expressing Eimeria maxima Antigen Protects Chickens against Infection of the Two Parasites

Vaccine delivery is critical in antigen discovery and vaccine efficacy and safety. The diversity of infectious diseases in humans and livestock has required the development of varied delivery vehicles to target different pathogens. In livestock animals, previous strategies for the development of coccidiosis vaccines have encountered several hurdles, limiting the development of multiple species vaccine formulations. Here, we describe a novel vaccine delivery system using transgenic Eimeria tenella expressing immunodominant antigens of Eimeria maxima. In this delivery system, the immune mapped protein 1 of E. maxima (EmIMP1) was delivered by the closely related species of E. tenella to the host immune system during the whole endogenous life cycle. The overexpression of the exogenous antigen did not interfere with the reproduction and immunogenicity of transgenic Eimeria. After immunization with the transgenic parasite, we detected EmIMP1’s and E. maxima oocyst antigens’ specific humoral and cellular immune responses. In particular, we observed partial protection of chickens immunized with transgenic E. tenella against subsequent E. maxima infections. Our results demonstrate that the transgenic Eimeria parasite is an ideal coccidia antigen delivery vehicle and represents a new type of coccidiosis vaccines. In addition, this model could potentially be used in the development of malaria live sporozoite vaccines, in which antigens from different strains can be expressed in the vaccine strain.

Dual RNA-seq reveals no plastic transcriptional response of the coccidian parasite Eimeria falciformis to host immune defenses

Background

Parasites can either respond to differences in immune defenses that exist between individual hosts plastically or, alternatively, follow a genetically canalized (“hard wired”) program of infection. Assuming that large-scale functional plasticity would be discernible in the parasite transcriptome we have performed a dual RNA-seq study of the lifecycle of Eimeria falciformis using infected mice with different immune status as models for coccidian infections.

Results

We compared parasite and host transcriptomes (dual transcriptome) between naïve and challenge infected mice, as well as between immune competent and immune deficient ones. Mice with different immune competence show transcriptional differences as well as differences in parasite reproduction (oocyst shedding). Broad gene categories represented by differently abundant host genes indicate enrichments for immune reaction and tissue repair functions. More specifically, TGF-beta, EGF, TNF and IL-1 and IL-6 are examples of functional annotations represented differently depending on host immune status. Much in contrast, parasite transcriptomes were neither different between Coccidia isolated from immune competent and immune deficient mice, nor between those harvested from naïve and challenge infected mice. Instead, parasite transcriptomes have distinct profiles early and late in infection, characterized largely by biosynthesis or motility associated functional gene groups, respectively. Extracellular sporozoite and oocyst stages showed distinct transcriptional profiles and sporozoite transcriptomes were found enriched for species specific genes and likely pathogenicity factors.

Conclusion

We propose that the niche and host-specific parasite E. falciformis uses a genetically canalized program of infection. This program is likely fixed in an evolutionary process rather than employing phenotypic plasticity to interact with its host. This in turn might limit the potential of the parasite to adapt to new host species or niches, forcing it to coevolve with its host.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4095-6) contains supplementary material, which is available to authorized users.

Immunomodulatory and antiparasitic effects of garlic extract on Eimeria vermiformis-infected mice

We investigated the immunomodulatory and parasiticidal effects of garlic extract on coccidiosis caused by Eimeria vermiformis infection in male ICR mice. One group received garlic extract daily until the end of the experiment by the oral route from 10 days prior to oral infection with 300 sporulated E. vermiformis oocysts (infected-garlic(+)). The other group served as a control positive with E. vermiformis infection alone (infected-garlic(-)). In the infected-garlic(+) group, garlic extract treatment induced a significant reduction in fecal oocyst output when compared with the infected-garlic(-) group. Histopathological, immunohistochemical, and gene expression analysis for inflammatory cytokines in ileal tissues showed that the garlic extract treatment impaired intracellular development of E. vermiformis during the early stages by increasing the number of intraepithelial CD8(+) T cells and decreasing IL-10 expression. This induced cell cytotoxicity which was reflected by a decrease in oocyst numbers in the intestinal villi and the feces, indicating anticoccidial effects of the garlic extract. However, further studies to explore the precise mechanism of the observed effects of garlic treatment during Eimeria infection are needed to verify our results.

Analysis of differentially expressed genes in two immunologically distinct strains of Eimeria maxima using suppression subtractive hybridization and dot-blot hybridization

Background

It is well known that different Eimeria maxima strains exhibit significant antigenic variation. However, the genetic basis of these phenotypes remains unclear.

Methods

Total RNA and mRNA were isolated from unsporulated oocysts of E. maxima strains SH and NT, which were found to have significant differences in immunogenicity in our previous research. Two subtractive cDNA libraries were constructed using suppression subtractive hybridization (SSH) and specific genes were further analyzed by dot-blot hybridization and qRT-PCR analysis.

Results

A total of 561 clones were selected from both cDNA libraries and the length of the inserted fragments was 0.25–1.0 kb. Dot-blot hybridization revealed a total of 86 differentially expressed clones (63 from strain SH and 23 from strain NT). Nucleotide sequencing analysis of these clones revealed ten specific contigs (six from strain SH and four from strain NT). Further analysis found that six contigs from strain SH and three from strain NT shared significant identities with previously reported proteins, and one contig was presumed to be novel. The specific differentially expressed genes were finally verified by RT-PCR and qRT-PCR analyses.

Conclusions

The data presented here suggest that specific genes identified between the two strains may be important molecules in the immunogenicity of E. maxima that may present potential new drug targets or vaccine candidates for coccidiosis.

A selective review of advances in coccidiosis research

Coccidiosis is a widespread and economically significant disease of livestock caused by protozoan parasites of the genus Eimeria. This disease is worldwide in occurrence and costs the animal agricultural industry many millions of dollars to control. In recent years, the modern tools of molecular biology, biochemistry, cell biology and immunology have been used to expand greatly our knowledge of these parasites and the disease they cause. Such studies are essential if we are to develop new means for the control of coccidiosis. In this chapter, selective aspects of the biology of these organisms, with emphasis on recent research in poultry, are reviewed. Topics considered include taxonomy, systematics, genetics, genomics, transcriptomics, proteomics, transfection, oocyst biogenesis, host cell invasion, immunobiology, diagnostics and control.

T cell reactions of Eimeria bovis primary and challenge-infected calves

Eimeria bovis infections commonly have clinical impact only on young animals, as homologous reinfections generally are under immunological control. So far, the nature of the immune responses delivering protection to calves has not been investigated. In this study we therefore analysed local and peripheral proliferative T cell activities of primary and challenge-infected calves and investigated the occurrence of T cell phenotypes in the peripheral blood and in mucosal gut segments isolated either by bioptic means or by necropsies.We show that lymphocytes of E. bovis-infected calves exhibit effective, transient antigen-specific proliferative responses in the course of prepatency of primary infection but fail to react after homologous reinfection suggesting early abrogation of parasite development. Whilst in primary infection an expansion of peripheral CD4+ T cells was observed, reinfection had no effect on the proportions of CD4+, CD8+ subsets or gammadeltaTCR+ T cells. In contrast, both E. bovis primary and challenge infections had an impact on local tissue T cell distribution. Primary infection was characterised by a CD4+ T cell infiltration early in prepatency in ileum and later in colon mucosa, whereas CD8+ T cells were only found accumulating in the latter gut segment. Challenge infection led to infiltration of both CD4+ and CD8+ T cells in small intestine and large intestine segments indicating protective functions of both cell types. In contrast, infiltration of ileum and colon mucosa with gammadeltaTCR+ T cells was restricted to primary infection.

Past and future: vaccination againstEimeria

Eimeriaspp. are the causative agents of coccidiosis, a major disease affecting many intensively-reared livestock, especially poultry. The chicken is host to 7 species ofEimeriathat develop within intestinal epithelial cells and produce varying degrees of morbidity and mortality. Control of coccidiosis by the poultry industry is dominated by prophylactic chemotherapy but drug resistance is a serious problem. Strongly protective but species-specific immunity can be induced in chickens by infection with any of theEimeriaspp. At the Institute of Animal Health in Houghton, UK in the 1980s we showed that all 7Eimeriaspp. could be stably attenuated by serial passage in chickens of the earliest oocysts produced (i.e. the first parasites to complete their endogenous development) and this process resulted in the depletion of asexual development. Despite being highly attenuated, the precocious lines retained their immunizing capacity. Subsequent work led to the commercial introduction of the first live attenuated vaccine, Paracox®, that has now been in use for 20 years. As much work still remains to be done before the development of recombinant vaccines becomes a reality, it is likely that reliance upon live, attenuated vaccines will increase in years to come.

Suppression of airway inflammation by a natural acute infection of the intestinal epithelium

Although chronic intestinal helminth infections may suppress allergen-induced airway pathology by inducing a combination of modified T-helper (Th) 2 and immunosuppressive cytokines, a similar capacity of natural acute intestinal infections has remained untested, despite their global prevalence. Here, we show that allergic airway phenotypes including eosinophilia, eotaxin mRNA, and Th2 cytokines are significantly suppressed in animals that were infected by and that have cleared the intestinal parasite Eimeria vermiformis. Unlike in helminth-infected animals, regulation requires temporal coincidence of infection with sensitization; depends on interferon-gamma; and is not associated with an enhanced antigen-specific immunoglobulin G1 response. Moreover, regulation was effective following allergen sensitization in different anatomical sites, and in young and adult mice. These data highlight a transient anatomical dissemination of "functional immunologic dominance" following infection of the gut mucosa. They strongly support the hypothesis that airway allergies are naturally suppressed by both acute and chronic mucosal pathogens, but by different mechanisms.

The biological and practical significance of antigenic variability in protective T cell responses against Theileria parva

The evolution of antigenically distinct pathogen strains that fail to cross-protect is well documented for pathogens controlled primarily by humoral immune responses. Unlike antibodies, which recognise native proteins, protective T cells can potentially recognise epitopes in a variety of proteins that are not necessarily displayed on the pathogen surface. Moreover, individual hosts of different MHC genotypes generally respond to different sets of epitopes. It is therefore less easy to envisage how strain restricted immunity can arise for pathogens controlled by T cell responses, particularly in antigenically complex parasites. Nevertheless, strain restricted immunity is clearly a feature of a number of parasitic infections, where immunity is known to be mediated by T cell responses. One such parasite is Theileria parva which induces potent CD8 T cell responses that play an important role in immunity. CD8 T cells specific for parasitized lymphoblasts exhibit strain specificity, which appears to correlate with the ability of parasite strains to cross-protect. Studies using recently identified T. parva antigens recognised by CD8 T cells have shown that the strain restricted nature of immunity is a consequence of the CD8 T cell response in individual animals being focused on a limited number of dominant polymorphic antigenic determinants. Responses in animals of different MHC genotypes are often directed to different parasite antigens, indicating that, at the host population level, a larger number of parasite proteins can serve as targets for the protective T cell response. Nevertheless, the finding that parasite strains show overlapping antigenic profiles, probably as a consequence of sexual recombination, suggests that induction of responses to an extended but limited set of antigens in individual animals may overcome the strain restricted nature of immunity.

Challenges in the successful control of the avian coccidia

Eimeria species infect livestock in a host-specific manner and are the cause of the disease, coccidiosis. Control of Eimeria species is essential and is currently dominated by chemotherapy; with vaccination using formulations of live wild-type or attenuated parasites an increasing option. A new generation of subunit, live-vector or DNA vaccination strategies is being sought and determining the identity of suitable antigens remains difficult. Some past and present methods of controlling avian coccidia are discussed briefly and we describe progress with a novel approach to identify immunoprotective antigens as vaccine candidates.

Genetic identification of antigens protective against coccidia

The Eimeria species, causative agents of the disease coccidiosis, are genetically complex protozoan parasites endemic in livestock. Drug resistance remains commonplace among the Eimeria, and alternatives to chemotherapeutic control are being sought. Vaccines based upon live formulations of parasites are effective, but production costs are high, stimulating demand for a recombinant subunit vaccine. The identity of antigens suitable for inclusion in such vaccines remains elusive. Selection of immunoprotective antigens of the Eimeria species as vaccine candidates based upon recognition by the host immune system has been unsuccessful, obscured by the considerable number of molecules that are immunogenic but not immunoprotective. This is a common problem which characterizes work with most eukaryotic parasites. The identification of a selective criterion to directly access genetic loci that encode immunoprotective antigens of Eimeria maxima using a mapping strategy based upon parasite genetics, immune selection and DNA fingerprinting promises to revolutionize the process of antigen discovery. Linkage analyses of DNA markers amplified from populations of recombinant parasites defined by an ability to escape parent-specific deleterious selection by strain-specific immunity and chemotherapy has revealed four discrete regions within the E. maxima genome linked to escape from a protective immune response. These regions now form the basis of detailed study to identify antigens as candidates for inclusion in future vaccination strategies.

The influence of immunizing dose size and schedule on immunity to subsequent challenge with antigenically distinct strains ofEimeria maxima

Eimeria maxima, the most immunogenic of the Eimeriidae that infect the chicken, is characterized by the presence of antigenic diversity within field isolates. In priming/challenge experiments immunity to homologous infection is essentially complete while immunity against challenge by a heterologous strain is often only partial. The phenotype "escape from immune protection" is known to be influenced by both host and parasite genotypes but the impact of varied immunization dose and schedule remains poorly documented. In this manuscript we report that an immunizing dose between <or=5 and <or=20 sporulated E. maxima oocysts is consistently capable of stimulating complete (>99.99%) protective immunity against challenge by 100 oocysts of a homologous strain. In contrast, complete immunity against a heterologous strain was never observed, although increasing the immunizing dose size did frequently reduce oocyst production arising from subsequent heterologous challenge. Differences in cross-protective immunizing capacity between two strains of E. maxima were evident as the H strain consistently stimulated a more potent protective immune response than the W strain. Similarly, increasing the number of immunizing doses of the E. maxima W strain (but not the H strain) increased immune protection against subsequent heterologous challenge. When combined with previously published data the results described here suggest that the E. maxima genome encodes a pool of antigens that are capable of stimulating an immune response cross-protective against more than one strain. These antigens supplement a separate restricted pool of antigens that are capable of stimulating stronger, but strain-specific, protective immune responses.

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.

Age-dependent requirement for gammadelta T cells in the primary but not secondary protective immune response against an intestinal parasite

Between weaning (3 wk of age) and adulthood (7 wk of age), mice develop increased resistance to infection with Eimeria vermiformis, an abundant intestinal parasite that causes coccidiosis. This development of resistance was perturbed in T cell receptor (TCR)δ^−/− mice, which at 4 wk of age remained largely susceptible to infection and prone to infection-associated dehydration. These phenotypes were rescued by the repopulation of γδ cells after adoptive transfer of lymphoid progenitors into newborn recipients. Because αβ T cells are necessary and sufficient for the protection of adult mice against E. vermiformis, the requirement for γδ cells in young mice shows a qualitative difference between the cellular immune responses operating at different ages. An important contribution toward primary immune protection in young hosts may have provided a strong selective pressure for the evolutionary conservation of γδ cells. This notwithstanding, the development of effective, pathogen-specific immunity in young mice requires αβ T cells, just as it does in adult mice.

A single dose of recombinant Salmonella typhimurium induces specific humoral immune responses against heterologous Eimeria tenella antigens in chicken

Salmonella typhimurium vaccine strains were used as antigen delivery system for oral immunisation of chickens against two antigens of the coccidian parasite Eimeria tenella. The cDNAs of the known E. tenella proteins, SO7 and TA4, were isolated from total RNA and subcloned into the expression vectors pQE30 and pTECH2. Subcutaneous immunisation of chickens with Escherichia coli-expressed SO7 and TA4 revealed that both proteins were immunogenic. Both cDNAs were subcloned into plasmids of the pTECH2 vector system, which allows them to be expressed as fusion proteins with the highly immunogenic fragment C of the tetanus toxin under control of the anaerobically inducible nirB promoter. Plasmids were introduced into the S. typhimurium vaccine strains SL3261, C5aroD and C5htrA. SDS-PAGE and Western blot analysis revealed expression of both fusion proteins in all strains under anaerobic culture conditions. Three-week-old white leghorn chickens were orally immunised with 10(9) CFU per animal. The stability of the recombinant bacteria was revealed by recovery of viable Salmonella containing the respective plasmids from the liver of the immunised chickens at day 3 after inoculation. Specific serum IgG antibodies against the SO7-or TA4-antigens were detectable by ELISA 2 weeks after oral immunisation and remained for at least 6 weeks, while specific IgA antibodies were restricted to the bile of the birds. All chickens produced serum IgG and IgA to S. typhimurium lipopolysaccharides. Our data show that a single oral inoculation with recombinant S. typhimurium SL3261, C5aroD and C5htrA can induce specific antibody responses to heterologous Eimeria antigens in chickens, suggesting that recombinant Salmonella are a suitable delivery system for vaccines against Eimeria infections.

Antigenic diversity in Eimeria maxima and the influence of host genetics and immunization schedule on cross-protective immunity

Eimeria spp. are a group of highly successful intracellular protozoan parasites that develop within enterocytes. Eimeria maxima from the chicken is characterized by high immunogenicity (a small priming infection gives complete immunity to subsequent homologous challenge) and naturally occurring antigenically variant populations that do not completely cross-protect. In this study we examined the expression of antigenic diversity in E. maxima, as manifested by cross-strain protection in a series of inbred chicken lines. The IAH line of Light Sussex chickens and all lines of inbred White Leghorns were susceptible to primary infections with either of two strains (H and W) of E. maxima and were protected completely against challenge with the homologous strain of parasite. The extent of cross-protection against the heterologous parasite strain varied from 0 to almost 100% depending on host genetics. Interestingly, in one inbred line of chickens (line 15I) the cross-protective phenotype was directional and intensely influenced by the infection history of the host. The basis for the observed variation in cross-protection is not known, but our results suggest that the major histocompatibility complex is not a major genetic component of the phenotype. These results are discussed in relation to the number of protective antigens presented by complex pathogens and the development of immunoprotective responses in hosts of different genetic backgrounds.

Recent advances in biology and immunobiology of Eimeria species and in diagnosis and control of infection with these coccidian parasites of poultry

Avian coccidiosis, an intestinal disease caused by protozoan parasites of the genus Eimeria, occurs worldwide. It is considered to be one of the most economically important diseases of domestic poultry. For many years, prophylactic use of anticoccidial feed additives has been the primary means of controlling coccidiosis in the broiler industry and has played a major role in the growth of this industry, which now can produce about 7.6 billion chickens annually. However, development of anticoccidial resistance has threatened the economic stability of the broiler industry. Although there has been little effort by the pharmaceutical industry to develop new anticoccidials, the mounting problem of drug resistance of Eimeria species has prompted major research efforts to seek alternative means of control through increased knowledge of parasite biology, host response, and nutritional modulation. As a consequence, important advancements have been made, particularly in defining parasite antigens that have potential use in vaccines, defining the Eimeria genome, understanding the immunology of coccidial infections, and the practical applications of live vaccines. This review describes the progress in these areas, most of which has occurred within the past 10 to 15 years.

In vivo expression profiles of cytokine and iNOS mRNAs in rats infected with Eimeria separata

Studies on cytokine (IFN-gamma, IL-2, IL-4, IL-5, IL-10) and inducible NO-synthase (iNOS) gene transcription in mesenteric lymph nodes (MLN) and the caecum wall were performed 0, 48, and 72h after primary and challenge infections of rats with Eimeria separata using RT-PCR. The amount of IFN-gamma mRNA was elevated in MLN and caeca 72h after primary and 48-72h after challenge infection when compared with uninfected controls. Increased amounts of IL-2 mRNA were only found in MLN of infected rats 72h post-infection (p.i.). In case of IL-10, infections did not affect the amount of mRNA in MLN, but led to markedly increased levels in the caecum wall of both infected groups 48 and 72h p.i. Levels of IL-4 mRNA remained unchanged after infections and IL-5 gene transcripts were undetectable. Amounts of iNOS mRNA (not investigated in MLN) were found strongly enhanced 48 and 72h p.i. in the caecum walls of all infected animals when compared with naive controls. The data are discussed in regard of the cellular source of the cytokines and their immunological role.

Effects of dietary protein types on immune responses and levels of infection with Eimeria vermiformis in mice

The present study reports the dietary effects of bovine alpha whey fraction, bovine casein and soy protein isolate on the immune responsiveness of C57BL/6J mice infected with Eimeria vermiformis. During the patent period, mice fed alpha whey fraction had significantly higher blood total white cell, CD4+ and CD8+ lymphocyte counts and higher Con A-stimulated IFN-gamma production by spleen cells than those fed other protein sources, but there was no significant difference in output of faecal oocysts. Casein-fed mice had significantly higher levels of Con A- stimulated IFN-gamma production and a lower output of faecal oocysts than soy-fed mice. The study demonstrated that dietary proteins have different impacts on immune responsiveness and level of parasitic infection in the gut; however, the mechanisms affecting level of infection are not clear. These effects appeared not to be solely related to nutritional properties of the diets. Further research into the underlying immune mechanisms and possible direct interactions between bioactive proteins and the parasite E. vermiformis should be fruitful.

Lymphocyte subpopulations in the caecum mucosa of rats after infections with Eimeria separata: early responses in naive and immune animals to primary and challenge infections

This study aimed to characterise the local (intestinal) immune response of rats after primary and challenge infections with Eimeria separata. Naive rats and rats which had been immunised by two moderate infections were exposed to a heavy infection with 100000 oocysts per animal. Necropsies were performed 0, 24 and 48 h after infection and lymphocyte subpopulations were microscopically quantified in the caecum mucosa after marking by immunohistological techniques. There was no difference between naive and immune rats concerning the number of CD45R(+) (B) cells, whereas significantly more CD3(+) (T) cells were found in the caecum wall of the immune rats. CD4(+) T cells predominated in animals after primary infection, whereas CD8(+) T cells represented the major T-cell subset in challenged rats. The proportion of TCRgammadelta(+) T cells did not differ in the mucosa between the groups examined, whereas challenged rats showed significantly increased numbers of TCRalphabeta(+) T cells in the caecum wall when compared with animals after a primary infection. Thus, CD4(+) T cells may be particularly involved in the immune response to a primary infection of rats with E. separata whereas immunity to a challenge infection seems to be mediated predominantly by CD8(+) and TCRalphabeta(+) T cells.

Host cell-mediated responses to infection with Cryptosporidium

The coccidian Cryptosporidium infects epithelial cells of a variety of vertebrate hosts and is the causative agent of cryptosporidiosis. In mammals, including humans and domestic animals, C. parvum infects the gastrointestinal tract producing an acute watery diarrhoea and weight loss. CD4+ T-cell-deficient hosts have increased susceptibility to infection with the parasite and may develop severe life-threatening complications. The host responses which induce protective immunity and contribute to pathogenesis are poorly understood. In the immunological control of infection, recent studies with murine infection models suggest that IFN-gamma plays a key role in a partially protective innate immunity against infection identified in immunocompromised mice and also in the elimination of infection mediated by CD4+ T-cells. At the mucosal level, CD4+ intraepithelial lymphocytes are involved in the control of cryptosporidial infection, acting at least in part through production of IFN-gamma which has a direct inhibitory effect on parasite development in enterocytes. Primary infection of ruminants induces an intestinal inflammatory response in which increased numbers of various T-cell subpopulations appear in the villi. In addition, infection results in increased intestinal expression of pro-inflammatory cytokines such as IL-12, IFN-gamma and TNF-alpha. Because these cytokines appear to be important in the aetiology of inflammatory bowel disease, it is possible that they are involved in the mucosal pathogenesis of cryptosporidiosis.

Effects of treatments with cyclooxygenase inhibitors on chickens infected with Eimeria acervulina

Reduced weight gains and feed conversions are major sources of economic losses from avian coccidiosis. Experiments were conducted to determine possible involvement of up-regulation of cyclooxygenase (COX) activity during coccidia infections that might contribute to reduced weight gain. In a series of trials, young (3 to 5 wk of age) cockerels infected with Eimeria acervulina, a duodenal parasite, received oral treatments with indomethacin (IM), an inhibitor of both COX-1 and COX-2, or nimesulide (NIM), a specific inhibitor of COX-2. Infection doses varied with experiment (from 10(5) to 10(6) oocysts per chick). Effects of infection on weight gain, duodenal lesions, plasma carotenoids, and levels of NO2+ NO3- were determined at 6 d postinoculation. Total oocysts were enumerated in feces collected from 5 through 8 d postinoculation from separate replicate groups. In no trials did treatment with IM reverse effects of infection on weight gain or significantly reduce lesion scores. However, in all trials, IM treatment reduced oocyst output per chick (Trial 1, 15%; Trial 2, 19%; Trial 3, 53%; Trial 4, 29%; Trial 5, 28.5%). Supplementation of feed with 400 and 100 ppm NIM significantly reduced weight gain of, and increased oocyst output from, infected chicks, whereas supplementation with 50 ppm NIM had no beneficial effects on weight gain or lesion scores, but reduced oocyst shedding. The inability of the IM and NIM treatments to reverse infection-associated weight gain suppression suggests that this pathological effect is not linked to increased prostanoid synthesis as a result of COX-2 up-regulation during infection. The inhibitory effects of IM treatment on oocyst shedding suggest that COX-2 products may have immunosuppressive effects in coccidia infection at local sites of infection. The stimulating effects of high-dose NIM treatments on oocyst shedding suggest that this compound may inhibit synthesis of other prostanoids as well.

Interleukin-6 is produced during both murine and avian Eimeria infections

The production of interleukin-6 (IL-6) during Eimeria infection was investigated in an attempt to gain a better understanding of the role of this multi-functional cytokine in resistance to this parasite. IL-6 production was measured in both chickens, in which the disease is of economic importance, and the better-characterised murine model system. Systemic and local IL-6 production in mice during E. vermiformis infection was investigated, in the relatively resistant BALB/c strain, and the relatively susceptible C57 BL/6 strain, using a murine IL-6 ELISA and the 7TD1 assay. Enhanced systemic production of IL-6 in serum was seen in infected BALB/c mice when compared to C57 BL/6 mice. This difference was also reflected in the draining lymph node of the site of infection, assessed by testing supernatants from stimulated mesenteric lymph node cells taken from infected mice at different times post-infection. Production of chicken IL-6-like factor activity was investigated using a murine IL-6 7TD1 bioassay. The presence of substantial quantities of IL-6-like factor activity was detected in serum taken from some chickens infected with E. tenella during the course of primary infection and, in a separate experiment, during the first few hours post-infection, a time when the pro-inflammatory capacity of IL-6 would influence the developing immune response. These results suggest that IL-6 is also important in the induction of immune effector responses to Eimeria infections in the chicken.

Vaccination against coccidiosis: host strain-dependent evocation of protective and suppressive subsets of murine lymphocytes

BALB/c mice are normally more resistant than C57BL/6 (B6) mice to infection with Eimeria vermiformis, but these phenotypes can be reversed by oral or parenteral vaccination with a crude antigen prepared from the parasite. Treatment of mice with antibodies specific for CD4+ or CD8+ T cells showed that the increased susceptibility of vaccinated BALB/c mice was associated with the presence of CD4+ T cells. This finding was confirmed when the recipients of CD4+ T cells selected from the mesenteric lymph nodes (MLN) of vaccinated BALB/c mice produced more oocysts after challenge than the recipients of a similar population of cells from sham-vaccinated mice. The residual population of cells (presumably enriched for CD8+ T cells, 'CD8+'), on the other hand, conferred some protection and, in B6 mice, the findings were reversed. Thus, vaccination induced suppressive or protective CD4+ cells and protective or suppressive 'CD8+' cells, depending upon the normal resistance/susceptibility phenotype of the host. Examinations of the isotypes (IgG1, IgG2a) of specific serum antibodies, and of the levels of IFN-gamma and IL-5 cytokines released by MLN cells stimulated ex vivo, did not allow any further characterization of the mechanisms involved.