Gamma interferon-mediated inhibition of Eimeria vermiformis growth in cultured fibroblasts and epithelial cells

Eimeria falciformis infection of the mouse caecum identifies opposing roles of IFNγ-regulated host pathways for the parasite development

Intracellular parasites reprogram host functions for their survival and reproduction. The extent and relevance of parasite-mediated host responses in vivo remains poorly studied, however. We utilized Eimeria falciformis, a parasite infecting the mouse intestinal epithelium, to identify and validate host determinants of parasite infection. Most prominent mouse genes induced during the onset of asexual and sexual growth of parasite comprise interferon γ (IFNγ)-regulated factors, e.g., immunity-related GTPases (IRGA6/B6/D/M2/M3), guanylate-binding proteins (GBP2/3/5/6/8), chemokines (CxCL9-11), and several enzymes of the kynurenine pathway including indoleamine 2,3-dioxygenase 1 (IDO1). These results indicated a multifarious innate defense (tryptophan catabolism, IRG, GBP, and chemokine signaling), and a consequential adaptive immune response (chemokine-cytokine signaling and lymphocyte recruitment). The inflammation- and immunity-associated transcripts were increased during the course of infection, following influx of B cells, T cells, and macrophages to the parasitized caecum tissue. Consistently, parasite growth was enhanced in animals inhibited for CxCr3, a major receptor for CxCL9-11 present on immune cells. Interestingly, despite a prominent induction, mouse IRGB6 failed to bind and disrupt the parasitophorous vacuole, implying an immune evasion by E. falciformis. Furthermore, oocyst output was impaired in IFNγ-R(-/-) and IDO1(-/-) mice, both of which suggest a subversion of IFNγ signaling by the parasite to promote its growth.

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.

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.

Antigen-induced cytokine production in lymphocytes of Eimeria bovis primary and challenge infected calves

Cellular immune responses against Eimeria bovis are highly specific and a key factor for the development of protection against challenge infections. In this study we investigate the cellular immune responses of E. bovis primary and challenge infected calves stimulated in vitro by E. bovis merozoite I-antigen. Primary infection was accompanied by an increase of IFN-gamma and IL-2 gene transcription in whole blood samples, peaking during prepatency (8-12 days p.i.) and declining thereafter, whereas IL-4 gene transcription was induced predominantly in patency. IL-10 mRNA was not influenced by E. bovis infection. Both CD4+ and CD8+ T cells were identified as source of IFN-gamma gene transcripts, whilst IL-2 and IL-4 gene transcription was enhanced mainly in CD4+ T cells. Increased levels of IFN-gamma transcripts and protein were also found in lymphocytes isolated from ileocaecal lymph node biopsy 8 days p.i., and in cell culture supernatants obtained from antigen-stimulated peripheral blood mononuclear cells (PBMC) at days 8 and 12 p.i., respectively. Challenge infections of calves influenced neither IFN-gamma nor IL-2 gene transcription in peripheral blood or in lymph node-derived lymphocytes. In contrast, IL-4 gene transcription was increased in lymphocytes isolated from draining lymph nodes. Besides antigen-specific reactions we also found an infection-triggered induction of the non-specific activation state of PBMC in the course of primary infection as measured by the intracellular IFN-gamma and IL-4 content of phorbol-12-myristate-13-acetate/ionomycin-stimulated PBMC. This may represent a new mechanism of immune cells of E. bovis-infected calves contributing to ongoing immune reactions.

Intestinal intraepithelial lymphocytes sustain the epithelial barrier function against Eimeria vermiformis infection

Eimeria spp. are intracellular protozoa that infect intestinal epithelia of most vertebrates, causing coccidiosis. Intestinal intraepithelial lymphocytes (IEL) that reside at the basolateral site of epithelial cells (EC) have immunoregulatory and immunoprotective roles against Eimeria spp. infection. However, it remains unknown how IEL are involved in the regulation of epithelial barrier during Eimeria sp. infection. Here, we demonstrated two distinct roles of IEL against infection with Eimeria vermiformis, a murine pathogen: production of cytokines to induce protective immunity and expression of junctional molecules to preserve epithelial barrier. The number of IEL markedly increased when oocyst production reached a peak. During infection, IEL increased production of gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha) and decreased transforming growth factor beta (TGF-beta) production. Addition of IFN-gamma and TNF-alpha or supernatants obtained from cultured IEL from E. vermiformis-infected mice reduced transepithelial electrical resistance (TER) in a confluent CMT93 cell monolayer, a murine intestine-derived epithelial line, but antibodies against these cytokines suppressed the decline of TER. Moreover, TGF-beta attenuated the damage of epithelial monolayer and changes in TER caused by IFN-gamma and TNF-alpha. The expression of junctional molecules by EC was decreased when IEL produced a high level of IFN-gamma and TNF-alpha and a low level of TGF-beta in E. vermiformis-infected mice. Interestingly, IEL constantly expressed junctional molecules and a coculture of EC with IEL increased TER. These results suggest that IEL play important multifunctional roles not only in protection of the epithelium against E. vermiformis-induced change by cytokine production but also in direct interaction with the epithelial barrier when intra-EC junctions are down-regulated.

CD8+ T cell-coccidia interactions

Host responses to coccidian parasites involve many facets of the immune system, including antigen-specific as well as antigen-nonspecific components. Hyun Lillehoj and James Trout here review the evidence that cell-mediated responses are probably the main line of defense against coccidial infection.

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.

Advances and prospects for subunit vaccines against protozoa of veterinary importance

Protozoa are responsible for considerable morbidity and mortality in domestic and companion animals. Preventing infection may involve deliberate exposure to virulent or attenuated parasites so that immunity to natural infection is established early in life. This is the basis for vaccines against theilerosis and avian coccidiosis. Vaccination may not be effective or practical with diseases, such as cryptosporidiosis, that primarily afflict the immune-compromised or individuals with an incompletely developed immune system. Strategies for combating these diseases often rely on passive immunotherapy using serum or colostrums containing antibodies to parasite surface proteins. Subunit vaccines offer an attractive alternative to virulent or attenuated parasites for several reasons. These include the use of bacteria or lower eukaryotes to produce recombinant proteins in batch culture, the relative stability of recombinant proteins compared to live parasites, and the flexibility to incorporate only those antigens that elicit "protective" immune responses. Although subunit vaccines offer many theoretical advantages, our lack of understanding of immune mechanisms to primary and secondary infection and the capacity of many protozoa to evade host immunity remain obstacles to developing effective vaccines. This review examines the progress made on developing recombinant proteins of Eimeria, Giardia, Cryptosporidium, Toxoplasma, Neospora, Trypanosoma, Babesia, and Theileria and attempts to use these antigens for vaccinating animals against the associated diseases.

Interferon gamma induces enterocyte resistance against infection by the intracellular pathogen Cryptosporidium parvum

BACKGROUND & AIMS

Interferon (IFN)-gamma plays an important role in the immunologic control of infection by the protozoan enteropathogen Cryptosporidium parvum. We tested the hypothesis that IFN-gamma may directly inhibit infection of enterocytes by this pathogen.

METHODS

HT-29, Caco-2, and H4 human enterocyte cell lines were grown in monolayers and incubated with IFN-gamma before exposure with C. parvum. IFN-gamma receptor expression in the cell lines was determined by Western blot analysis.

RESULTS

IFN-gamma inhibited C. parvum infection of both HT-29 and Caco-2 cells but not H4 cells. Response to IFN-gamma was related to the expression of the IFN-gamma receptor in the respective cell lines. The effect of IFN-gamma was partially reversed by inhibition of the JAK/STAT signaling pathway. IFN-gamma mediated its action by at least 2 mechanisms: (1) inhibition of parasite invasion and (2) by modification of intracellular Fe(2+) concentration. No role for tryptophan starvation or nitric oxide synthase activity was found. TNF-alpha and IL-1beta also had anti-C. parvum activity but had no synergistic effect with IFN-gamma.

CONCLUSIONS

IFN-gamma directly induces enterocyte resistance against C. parvum infection; this observation may have important consequences for our understanding of the mucosal immune response to invasive pathogens.

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.

Eimeria tenella infection induces local gamma interferon production and intestinal lymphocyte subpopulation changes

The role of intestinal lymphocytes and gamma interferon (IFN-gamma) production in protective immunity to Eimeria tenella infection was evaluated in two inbred strains of chickens (SC and TK) that display different patterns of susceptibility to coccidiosis. Oral inoculation of either strain with E. tenella led to parasite invasion of the intestinal cecum and cecal tonsils. Greater fecal oocyst shedding was seen in TK chickens. Flow cytometric analyses of cecal tonsil lymphocytes demonstrated greater numbers of CD4(+) and T-cell receptor gammadelta-positive (TCR1(+)) cells in SC chickens and elevated numbers of CD8(+) and TCR2(+) cells in TK chickens following primary infection. IFN-gamma mRNA expression was significantly increased in cecal tonsil and intraepithelial lymphocytes at days 6 and 8, respectively, after primary infection in SC compared to TK chickens. While no differences were noted between cecal tonsil lymphocytes of the two strains following secondary infection, TK chickens showed elevated IFN-gamma transcript levels in intestinal intraepithelial lymphocytes at this time. Selective depletion of CD4(+), but not CD8(+), cecal tonsil lymphocytes in SC chickens resulted in a reduced IFN-gamma mRNA expression, indicating that CD4(+) cells are the primary source of this cytokine. Collectively, these results indicate that local lymphocyte responses and production of IFN-gamma are influenced by host genetic factors.

Role of T lymphocytes and cytokines in coccidiosis

Development of a vaccine for avian coccidiosis has been hampered by lack of understanding of the various components of the host immune system leading to protective immunity. Clear understanding of the cellular dichotomy in cytokine production in mice and the availability of immunological reagents, as well as gene knock-out mice, now makes in-depth immunological study in this species feasible. From studies of various parasitic infection models in mice, it is becoming clear that complex regulation by cytokines is involved in host immunity. Furthermore, the studies in mice clearly indicated an important role of various effector mechanisms involving T lymphocytes, macrophages, natural killer (NK) cells and cytokines in resistance to coccidiosis. In comparative studies of coccidiosis in chickens, in-vivo and in-vitro studies revealed that interferon-gamma, tumor necrosis factor and transforming growth factor-beta are induced following Eimeria infection. Depletion studies revealed the importance of CD8+TCR-alpha-beta+ T lymphocytes in host protective immunity to avian coccidiosis. Taken together, studies in mice and chickens are providing a better understanding of the role of effector cells and soluble factors which control immune responses to Eimeria parasites.

Progress on developing a recombinant coccidiosis vaccine

The past 10 years of research aimed at developing subunit vaccines against a number of apicomplexans, including Eimeria, Plasmodium and Toxoplasma, have, if anything, revealed the complex nature of parasite-host interactions. The Knowledge gained from this research has shown why developing a subunit vaccine based on a single recombinant antigen from one developmental stage of the parasite was an overly optimistic approach. Many apicomplexan parasites have acquired unique strategies to evade host immunity. The variable expression of genes encoding erythrocyte membrane protein 1 of Plasmodium falciparum [1] (Berendt et al. Parasitology 1994;108:S19-S28) exemplifies one such strategy. The particular mechanism for evading immune destruction depends on a number of interrelated factors, not least of which is the parasite life-cycle and the availability of susceptible hosts. The goal of any vaccine, be it an attenuated organism or a recombinant antigen, is to break the cycle of infection. The development of a recombinant vaccine against apicomplexan parasites will depend on identifying those antigens and intracellular processes that are vital to the parasite survival and those which exist merely as a way of evading immunity. The information that follows is a review of both molecular biology/biochemistry of eimerian parasites and factors that influence host immune responses to coccidia.

Nonspecific immune responses and mechanisms of resistance to Eimeria papillata infections in mice

Severe combined immunodeficient (SCID)-beige mice inoculated with the intracellular parasite Eimeria papillata produced significantly more oocysts during primary infections than inoculated immunodeficient SCID mice. Therefore, the addition of the beige mutation, which detrimentally affects neutrophil and natural killer (NK) cell functions, enhanced the parasites' ability to reproduce within the small intestine. To identify which of these two cell types is responsible for a protective immune response during primary infection, the following groups of mice were inoculated: (i) SCID mice depleted of neutrophils with antigranulocyte monoclonal antibody (RB6-8C5), (ii) C57BL/6 mice depleted of NK cells with the anti-NK-1.1 monoclonal antibody (PK136), and (iii) transgenic Tg epsilon26++ mice (T and NK cell deficient). To identify the mechanisms of immunity during primary and secondary infections, gamma interferon (IFN-gamma) knockout and perforin knockout mice were inoculated. Oocyst output was found to be significantly higher during primary infection for mice depleted of NK cells by administration of anti-NK-1.1 antibodies, for Tg epsilon26++ mice, and for IFN-gamma knockout mice. During secondary infections, only perforin knockout mice produced significantly more oocysts compared to control mice. Our observations suggest that NK cells inhibit E. papillata oocyst output during primary infection by the production of IFN-gamma and that this inhibition is independent of perforin. Immunity to reinfection does not require IFN-gamma but appears to be mediated, at least in part, by a perforin-dependent mechanism.

Avian gut-associated lymphoid tissues and intestinal immune responses to Eimeria parasites

Coccidiosis, an intestinal infection caused by intracellular protozoan parasites belonging to several different species of Eimeria, seriously impairs the growth and feed utilization of livestock and poultry. Host immune responses to coccidial infection are complex. Animals infected with Eimeria spp. produce parasite-specific antibodies in both the circulation and mucosal secretions. However, it appears that antibody-mediated responses play a minor role in protection against coccidiosis. Furthermore, there is increasing evidence that cell-mediated immunity plays a major role in resistance to infection. T lymphocytes appear to respond to coccidial infection through both cytokine production and a direct cytotoxic attack on infected cells. The exact mechanisms by which T cells eliminate the parasites, however, remain unclear. Although limited information is available on the intestinal immune system of chickens, gut lymphoid tissues have evolved specialized features that reflect their role as the first line of defense at mucosal surfaces, including both immunoregulatory cells and effector cells. This review summarizes our current understanding of the avian intestinal immune system and mucosal immune responses to Eimeria spp., providing an overview of the complex cellular and molecular events involved in intestinal immune responses to enteric pathogens.

Cytokines and immunological control of Eimeria spp

Protozoan parasites belonging to the genus Eimeria cause considerable losses in livestock production in which stocking densities are high or environments restricted. The ability of hosts to mount immunological responses which limit parasite reproduction vary according to the particular species of Eimeria. Typically though, immune responses restrict parasite reproduction during primary infection and limit, if not prevent, subsequent infections. Although mechanisms of immunity are unknown, host immune responses have been exploited in the development of a method to control coccidiosis-immunisation with attenuated strains of Eimeria. Limitations of this control method, predominantly the cost of producing the attenuated parasites, necessitates identification of protective immune responses to facilitate selection of antigens for use in non-living vaccines. As in immune responses to many other parasitic infections of the gastrointestinal tract, the role of antibodies is at best minor, whereas T-cells are crucial. Numerous studies have shown that the intestinal mucosal T-cell population is dynamic; the number and phenotype of T-cells changes in response to Eimeria-infection. Specific changes in the intestinal T-cell population have not, however, been correlated with limitation of parasite reproduction. Experiments involving adoptive transfer of T-cell sub-populations and in vivo depletion of specific T-cells have shown that CD4+ T-cells and to a lesser extent CD8+ T-cells are important in immune responses which limit primary infection. In contrast, CD8+ T-cells are more important in subsequent infections with CD4+ T-cells having a lesser role. The effects of T-cells on Eimeria are partially mediated by the cytokines they release. Most attention has concentrated on interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) because these cytokines have been shown to limit other protozoan infections. IFN-gamma is produced in Eimeria-infected hosts but evidence that it is present at the site of infection is limited. Intestinal levels of IFN-gamma increase earlier in response to primary Eimeria-infection in mice which are relatively resistant, than in mice which are relatively susceptible. Neutralisation of endogenously produced IFN-gamma has shown that this cytokine limits oocyst production in either primary or secondary infections depending on the species of Eimeria. Production of TNF-alpha is also increased in infected hosts. In comparison with relatively susceptible mice, TNF-alpha is produced earlier and to a greater extent in the intestines of relatively resistant mice. Unexpectedly, injections of TNF-alpha into infected mice increased oocyst production. It remains to be determined whether the effects of endogenous TNF-alpha are the same as those of exogenous TNF-alpha. Mechanisms by which IFN-gamma and TNF-alpha modulate parasite reproduction have not been identified. A number of lines of experimentation have suggested that it is unlikely that IFN-gamma limits parasite reproduction through induction of the synthesis of reactive oxygen or reactive nitrogen intermediates, since both of these reactive intermediates have the capacity to exacerbate Eimeria-infection.

Immunization against experimental coccidiosis produces contrasting results in inbred mice of differing susceptibility to infection

Pretreatment of inbred mice with intravenous and/or intraperitoneal injection of an antigen prepared from sporozoites of Eimeria vermiformis modulated the course of infection with the parasite in a manner that depended on the resistance-susceptibility phenotype of the host. Mice with a resistant background (BALB) produced more oocysts and those with a susceptible background (C57BL) produced fewer oocysts than their respective controls. The optimum conditions for producing these effects were established, and evidence is presented which suggests that the phenomenon might also apply in the target host, the chicken.

Dynamics of cytokine production during coccidial infections in chickens: colony-stimulating factors and interferon

We assayed two classes of immunoregulatory cytokines, colony-stimulating factors (CSF) and interferon (IFN), during and immediately after a primary coccidial infection in chickens. Coccidial infection induces significant alterations in serum colony-stimulating activity (CSA) and these alterations immediately precede the characteristic biphasic leukocytosis. CSA rose sharply during the first 24 h post-inoculation (PI), but returned to control levels by 48 h PI. At this time, we detected an increase in peripheral blood leukocytes which peaked at 96 h PI. A second phase of CSA increase began 96 h PI and peaked at 120-144 h PI which again preceded the second phase of leukocytosis. We also examined the production of IFN during the first 20 days PI. Splenic T cells from Eimeria maxima-infected chickens produced significantly less IFN on day 5 PI compared to T cells from the coccidia-free controls. By days 10 and 15 PI, there was no significant difference in IFN production between the T cells of infected and non-infected chickens. However, by day 20 PI, IFN production by the T cells of the infected birds produced significantly more IFN than the control T cells. The results of our studies indicated the differential production of two different cytokines by chickens during and following a primary coccidial infection. Based on these experiments, CSF may be some of the first cytokines produced during an E. maxima-infection, while IFN may be one of the later cytokines produced.

Immunity to coccidiosis: genetic influences on lymphocyte and cytokine responses to infection with Eimeria vermiformis in inbred mice

Cellular and cytokine responses to infection with Eimeria vermiformis were compared in BALB/c (resistant) and C57BL/6 (B6-susceptible) inbred mice. Cellular responses in the mesenteric lymph node (MLN) occurred sooner after primary infection in the resistant BALB/c strain. In contrast, proliferative responses occurred earlier after challenge in B6 mice. Resting levels of CD4 + ve and CD8 + ve T-lymphocytes in the MLN differed between the two strains but the relative numbers of each subset remained relatively constant throughout primary infection. MLN cells taken at intervals after infection were assayed for release of the cytokines IFN-gamma, IL-5 and IL-10 after culture in vitro with the mitogen Concanavalin A (Con-A) or with parasite antigen. With either stimulus cells from resistant BALB/c mice released IFN-gamma and IL-5 earlier after infection than did B6 cells. The strains had a comparable absolute ability to produce IFN-gamma but BALB/c cells released more IL-5 than did B6, levels declining, rather than increasing, during primary infection in the latter. Only cells from BALB/c mice released IL-10 during infection. Cells taken after a secondary infection released relatively little cytokine after pulsing in vitro. These data suggest that the difference in response phenotype between the two strains when infected with E. vermiformis reflect a kinetic, rather than a qualitative, difference in ability to mount protective T-helper (Th) cell subset responses. No evidence was found for a Th2-mediated interference with ability to release IFN-gamma, the cytokine most closely associated with protective immunity.

Intestinal changes associated with expression of immunity to challenge with Eimeria vermiformis

To provide more information on the mechanisms involved in the immune inhibition of eimeria infections, NIH mice were adoptively immunized against infection with Eimeria vermiformis by the transfer of mesenteric lymph node cells from primed animals and homologously challenged. Subsequent changes in the architecture and cellular composition of the intestine were compared with those observed in similarly challenged susceptible control mice and correlated with the development of the parasite in the two groups. Actively immunized mice were also examined. In adoptively immunized mice, the development of E. vermiformis was inhibited within 3 days of administering the challenge inoculum. Concurrent changes in the intestine included lymphocytic infiltration, crypt hyperplasia, flattening of the crypt epithelium, and a reduction in the number of Paneth cells. Hyperplasia of goblet and pyroninophilic cells in response to challenge, although accelerated and enhanced in adoptively immunized hosts, occurred after the inhibition of the parasites, and mastocytosis was not observed in these animals, findings which suggest that the activities of goblet, pyroninophilic, and mast cells were not instrumental in reducing the numbers of parasites. The intestines of immunized mice contained fewer intraepithelial lymphocytes at the time of inhibition of the parasites than did those of the controls. The protective effects and intestinal changes described above did not differ appreciably from those seen after challenge of mice that had been immunized by infection.

Cytokines, free radicals and resistance to Eimeria

The cytokine, gamma-interferon (IFN-gamma), which is produced by CD4(+) T cells, plays a crucial role in host resistance to Eimeria infections. Karen Ovington and Nick Smith propose that free oxygen radical generation by leukocytes in response to infection with Eimeria is the result of activation by IFN-gamma. The functional role of free oxygen radicals is unclear but these highly reactive radicals are produced by the leukocytes that infiltrate the intestine in large numbers during infection, and the parasites,enterocytes and cells of the immune system may all be vulnerable to oxidative damage. Gamma-interferon also appears to induce the enterocytes inhabited by Eimeria to turn against the parasite. The authors draw from literature documenting similar effects on other protozoa, especially Leishmania and Plasmodium, and speculate that reactive nitrogen intermediates produced by enterocytes have a functional role in resistance to Eimeria.

Research on avian coccidia: an update

Despite the availability of many anticoccidial drugs, infections caused by species of Eimeria continue to be a source of significant economic loss to the poultry industry. After two decades in which the use world wide of ionophorous antibiotics gave unparalleled control of coccidiosis, drug resistance is once again tipping the balance in favour of the parasites. The realization that even the most spectacularly successful drugs might, after all, have a finite life if not used conservatively, has focused attention on ways in which the life span of drugs can be prolonged. Many drugs with different (if unknown) modes of action are available, and a variety of shuttle and rotation programmes can be considered. In view of the limitations of chemotherapy, particularly for the rearing of replacement flocks, there is considerable interest in the development of vaccines. Prospects for the introduction of live vaccines based on attenuated parasites are now very good, but the availability in the future of genetically engineered vaccines is more uncertain as little is known about the parasite molecules that stimulate protective immunity and, even if isolated, how they can be administered to the host so that it responds in the immunologically correct manner. Current research on Eimeria spp. in the chicken is broadly representative of that being done on other coccidia. Many lines of investigation are not connected with the development of new drugs or vaccination per se (and therefore have no obvious practical applications), but they are providing new insights into the biological complexity of the organisms and the ways in which they interact with their hosts. It remains possible, however, that a more detailed understanding and analysis of the molecules that are essential in the maintenance of the parasitic life style can be exploited in the future to provide alternative targets for chemical or immunological attack. The research topics considered in this review are arbitrarily grouped as studies on: (1) the basic biology of parasites, including aspects of the life cycle, and structure and function of the apical organelles; (2) the molecular biology of the parasites, including analyses of the number and structure of chromosomes, characterization of DNA sequences, and an account of the viral RNA that has been found in some species of Eimeria; and (3) control of coccidiosis, encompassing first immunity and the development of vaccines, and secondly, chemotherapy.

Further investigation of anticoccidial activity of 7-bromo-N-(2-imidazolidinylidene)-1H-indazol-6-amine

The clonidine analog 7-bromo-N-(2-imidazolidinylidene)-1H-indazol-6-amine exhibits potent activity against Eimeria tenella infections in chickens. Disease control was abrogated by a selective alpha 2 antagonist, which is consistent with the dependence of such activity upon binding to receptors with characteristics of the vertebrate alpha 2 adrenoceptor. Lack of significant activity against the parasite in tissue culture and our inability to detect significant binding of alpha 2 adrenergic ligands to E. tenella imply that the anticoccidial action may be an indirect effect mediated by the host. Efficacy varied, depending upon the Eimeria species, being greatest for the cecal species E. tenella and less for the intestinal species. The effects described differ substantially from previous accounts of adrenergic actions on parasitic protozoa. The evidence suggests that we have observed a new mechanism of action for antiparasitic drugs.