Mechanisms of innate resistance to Toxoplasma gondii infection

Nitric oxide in parasitic infections: a friend or foe?

The complex interaction between the host and the parasite remains a puzzling question. Control of parasitic infections requires an efficient immune response that must be balanced against destructive pathological consequences. Nitric oxide is a nitrogenous free radical which has many molecular targets and serves diverse functions. Apart from being a signaling messenger, nitric oxide is critical for controlling numerous infections. There is still controversy surrounding the exact role of nitric oxide in the immune response against different parasitic species. It proved protective against intracellular protozoa, as well as extracellular helminths. At the same time, it plays a pivotal role in stimulating detrimental pathological changes in the infected hosts. Several reports have discussed the anti-parasitic and immunoregulatory functions of nitric oxide, which could directly influence the control of the infection. Nevertheless, there is scarce literature addressing the harmful cytotoxic impacts of this mediator. Thus, this review provides insights into the most updated concepts and controversies regarding the dual nature and opposing sides of nitric oxide during the course of different parasitic infections.

High resistance to Toxoplasma gondii infection in inducible nitric oxide synthase knockout rats

Nitric oxide (NO) is an important immune molecule that acts against extracellular and intracellular pathogens in most hosts. However, after the knockout of inducible nitric oxide synthase (iNOS^−/−) in Sprague Dawley (SD) rats, these iNOS^−/− rats were found to be completely resistant to Toxoplasma gondii infection. Once the iNOS^−/− rat peritoneal macrophages (PMs) were infected with T. gondii, they produced high levels of reactive oxygen species (ROS) triggered by GRA43 secreted by T. gondii, which damaged the parasitophorous vacuole membrane and PM mitochondrial membranes within a few hours post-infection. Further evidence indicated that the high levels of ROS caused mitochondrial superoxide dismutase 2 depletion and induced PM pyroptosis and cell death. This discovery of complete resistance to T. gondii infection, in the iNOS^−/−-SD rat, demonstrates a strong link between NO and ROS in immunity to T. gondii infection and showcases a potentially novel and effective backup innate immunity system.

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iNOS^−/−-SD rats show strong resistance to Toxoplasma gondii infection

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iNOS^−/−-SD rat PMs resist T. gondii infection through ROS upregulation

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The T. gondii infection results in PM pyroptosis in iNOS^−/−-SD rats

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GRAs play a key role in the activation of resistance in iNOS^−/−-SD rat PMs

Hormonal modulation of Toxoplasma gondii infection: Regulation of hormonal receptors and cytokine production in THP-1 cells

BACKGROUND

Toxoplasma gondii (T. gondii) is an obligate intracellular protozoan able to infect humans and it is common in pregnant women. During pregnancy and lactation, there are changes in the concentration of 17β-estradiol (E2), progesterone (Prg), and prolactin (PRL). It is known that a proinflamatory response reduces the susceptibility to be infected, and this response may change according to hormonal impairment. Monocytes and macrophages are the main barrier against many intracellular microorganisms, due to their ability to produce cytokines. The aim of this work was to determine the effect of E2, progesterone, and PRL on the infective capacity of T. gondii, proinflamatory immune response modulation and the expression of hormonal receptors on THP-1 cell stimulated with T. gondii.

METHODS

The THP-1 cells were infected with 1500 T. gondii tachyzoites, of RH strain. Stimuli were conducted with recombinant PRL (200 ng/mL), E2 (40 nM) y Prg (40 nM). MTT assays were performed to evaluate cellular viability. Western blot assays were carried out to evaluate the expression of the hormonal receptors (PRLR, ERα, and ERβ). Cytokines produced were measured with a magnetic bead kit directed to 17 cytokines.

RESULTS

Stimuli with E2 and Prg increased T. gondii infection in monocytes after 48 h; however, no differences in infection were observed in PRL stimulus. The E2 decreased the secretion of IL-12 and IL-1β and PRL did not modify the production of these cytokines in THP-1 cells stimulated with T. gondii; however, both hormones increased the production of IL-10. Besides, PRL augmented the production of IL-4 and IL-13. In contrast, Prg reduced these cytokines. Our results show that T. gondii induces the expression of ERα and ERβ and lowers PRLR. The hormones modify the expression of the receptors of other hormones: Prg decreases PRLR, ERβ and increases ERα; E2 diminishes PRLR; and PRL decreases ERα and ERβ expression.

CONCLUSION

The hormones can increase T. gondii infection and could be mediating an anti-inflammatory response in THP-1 cells. T. gondii induces changes in the expression of hormonal receptors.

Evaluation of nitazoxanide as a novel drug for the treatment of acute and chronic toxoplasmosis

Toxoplasmosis is a widespread, neglected disease with significant morbidity and mortality. In search of an effective treatment, nitazoxanide (NTZ) was evaluated in the treatment of acute and chronic toxoplasmosis in experimental mice. For this purpose, mice were infected with 20 cysts (acute infection model) or 10 cysts (chronic infection model) of Toxoplasma gondii (ME 49 strain). Treated mice received NTZ (at doses of 100 and 150 mg/kg), starting from the third day (acute model) or the fifth week (chronic model) post-infection, which continued for 14 consecutive days. The effects of NTZ were evaluated in comparison to the pyrimethamine/sulfadiazine combination. Evaluation included mortality rates, brain cyst count, inflammatory scoring and immunological studies. The latter included estimation of interferon-gamma (IFN-γ) and induced nitric oxide synthase (iNOS). In the acute infection model, NTZ at 100 and 150 mg/kg significantly reduced the number of brain cysts by 78 and 87% compared to the infected untreated controls and reduced the mortality rate to 24 and 20%, respectively, compared with 44% in the infected untreated control. In the chronic infection model, cyst reduction reached 32 and 38% for 100 and 150 mg/kg NTZ treatments, respectively. NTZ was significantly able to reduce inflammation caused by acute and chronic T. gondii infection with slight necrosis and few infiltrating mononuclear cells. Additionally, the immunological analysis revealed that NTZ significantly increased the production of serum IFN-γ and enhanced iNOS production in brain tissue, suggesting an immunomodulatory role for the drug. Based on the findings of the present study, it can be concluded that NTZ is a potential drug for the treatment of acute and chronic toxoplasmosis.

Research on the effect and mechanism of antimicrobial peptides HPRP-A1/A2 work against Toxoplasma gondii infection

With increasing antibiotic resistance and drug safety concerns, novel therapeutics are urgently needed. Antimicrobial peptides are promising candidates that could address the spread of multidrug-resistant pathogens. HPRP-A1/A2 are known to display antimicrobial activity against gram-negative bacteria, gram-positive bacteria and some pathogenic fungi, but whether HPRP-A1/A2 work on Toxoplasma gondii (T gondii) is unknown. In this study, we found that the viability of tachyzoites that received HPRP-A1/A2 treatment was significantly decreased, and there was a reduction in the adhesion to and invasion of macrophages by tachyzoites after HPRP-A1/A2 treatment. HPRP-A1/A2 damaged the integrity of tachyzoite membranes, as characterized by membrane disorganization in and cytoplasm outflow from tachyzoites. In addition, in vivo injection with HPRP-A1/A2 resulted in a significantly decreased number of tachyzoites and an accelerated Th1/Tc1 response, and elicited pro-inflammatory cytokines in T gondii-infected mice. Furthermore, HPRP-A1/A2-treated splenocytes exhibited a significantly increased Tc1/Th1 response, and HPRP-A1/A2-stimulated macrophages inhibited the growth of carboxyfluorescein succinimidyl amino ester (CFSE)-labelled tachyzoites, which had higher TNF-α/IL-12 mRNA levels. Altogether, these results imply that HPRP-A1/A2 are effective against T gondii through damaging the structure of tachyzoites and inducing a protective immune response, which could offer an alternative approach against T gondii infection.

Inducible Nitric Oxide Synthase Is a Key Host Factor for Toxoplasma GRA15-Dependent Disruption of the Gamma Interferon-Induced Antiparasitic Human Response

Although Toxoplasma virulence mechanisms targeting gamma interferon (IFN-γ)-induced cell-autonomous antiparasitic immunity have been extensively characterized in mice, the virulence mechanisms in humans remain uncertain, partly because cell-autonomous immune responses against Toxoplasma differ markedly between mice and humans. Despite the identification of inducible nitric oxide synthase (iNOS) as an anti-Toxoplasma host factor in mice, here we show that iNOS in humans is a pro-Toxoplasma host factor that promotes the growth of the parasite. The GRA15 Toxoplasma effector-dependent disarmament of IFN-γ-induced parasite growth inhibition was evident when parasite-infected monocytes were cocultured with hepatocytes. Interleukin-1β (IL-1β), produced from monocytes in a manner dependent on GRA15 and the host's NLRP3 inflammasome, combined with IFN-γ to strongly stimulate iNOS expression in hepatocytes; this dramatically reduced the levels of indole 2,3-dioxygenase 1 (IDO1), a critically important IFN-γ-inducible anti-Toxoplasma protein in humans, thus allowing parasite growth. Taking the data together, Toxoplasma utilizes human iNOS to antagonize IFN-γ-induced IDO1-mediated cell-autonomous immunity via its GRA15 virulence factor.IMPORTANCE Toxoplasma, an important intracellular parasite of humans and animals, causes life-threatening toxoplasmosis in immunocompromised individuals. Gamma interferon (IFN-γ) is produced in the host to inhibit the proliferation of this parasite and eventually cause its death. Unlike mouse disease models, which involve well-characterized virulence strategies that are used by Toxoplasma to suppress IFN-γ-dependent immunity, the strategies used by Toxoplasma in humans remain unclear. Here, we show that GRA15, a Toxoplasma effector protein, suppresses the IFN-γ-induced indole-2,3-dioxygenase 1-dependent antiparasite immune response in human cells. Because NLRP3-dependent production of IL-1β and nitric oxide (NO) in Toxoplasma-infected human cells is involved in the GRA15-dependent virulence mechanism, blocking NO or IL-1β production in the host could represent a novel therapeutic approach for treating human toxoplasmosis.

Implications of infectious agents on results of animal experiments

Report of the Working Group on Hygiene of the Gesellschaft für Versuchstierkunde–Society for Laboratory Animal Science (GV-SOLAS)

GV-SOLAS Working Group on Hygiene: Werner Nicklas (Chairman), Felix R. Homberger, Brunhilde Illgen-Wilcke, Karin Jacobi, Volker Kraft, Ivo Kunstyr, Michael Mähler, Herbert Meyer & Gabi Pohlmeyer-Esch

AMA1-deficient Toxoplasma gondii parasites transiently colonize mice and trigger an innate immune response that leads to long-lasting protective immunity

The apical membrane antigen 1 (AMA1) protein was believed to be essential for the perpetuation of two Apicomplexa parasite genera, Plasmodium and Toxoplasma, until we genetically engineered viable parasites lacking AMA1. The reduction in invasiveness of the Toxoplasma gondii RH-AMA1 knockout (RH-AMA1(KO)) tachyzoite population, in vitro, raised key questions about the outcome associated with these tachyzoites once inoculated in the peritoneal cavity of mice. In this study, we used AMNIS technology to simultaneously quantify and image the parasitic process driven by AMA1(KO) tachyzoites. We report their ability to colonize and multiply in mesothelial cells and in both resident and recruited leukocytes. While the RH-AMA1(KO) population amplification is rapidly lethal in immunocompromised mice, it is controlled in immunocompetent hosts, where immune cells in combination sense parasites and secrete proinflammatory cytokines. This innate response further leads to a long-lasting status immunoprotective against a secondary challenge by high inocula of the homologous type I or a distinct type II T. gondii genotypes. While AMA1 is definitively not an essential protein for tachyzoite entry and multiplication in host cells, it clearly assists the expansion of parasite population in vivo.

MAP kinase phosphatase-2 plays a key role in the control of infection with Toxoplasma gondii by modulating iNOS and arginase-1 activities in mice

The dual specific phosphatase, MAP kinase phosphatase-2 (MKP-2) has recently been demonstrated to negatively regulate macrophage arginase-1 expression, while at the same time to positively regulate iNOS expression. Consequently, MKP-2 is likely to play a significant role in the host interplay with intracellular pathogens. Here we demonstrate that MKP-2(-/-) mice on the C57BL/6 background have enhanced susceptibility compared with wild-type counterparts following infection with type-2 strains of Toxoplasma gondii as measured by increased parasite multiplication during acute infection, increased mortality from day 12 post-infection onwards and increased parasite burdens in the brain, day 30 post-infection. MKP-2(-/-) mice did not, however, demonstrate defective type-1 responses compared with MKP-2(+/+) mice following infection although they did display significantly reduced serum nitrite levels and enhanced tissue arginase-1 expression. Early resistance to T. gondii in MKP-2(+/+), but not MKP-2(-/-), mice was nitric oxide (NO) dependent as infected MKP-2(+/+), but not MKP-2(-/-) mice succumbed within 10 days post-infection with increased parasite burdens following treatment with the iNOS inhibitor L-NAME. Conversely, treatment of infected MKP-2(-/-) but not MKP-2(+/+) mice with nor-NOHA increased parasite burdens indicating a protective role for arginase-1 in MKP-2(-/-) mice. In vitro studies using tachyzoite-infected bone marrow derived macrophages and selective inhibition of arginase-1 and iNOS activities confirmed that both iNOS and arginase-1 contributed to inhibiting parasite replication. However, the effects of arginase-1 were transient and ultimately the role of iNOS was paramount in facilitating long-term inhibition of parasite multiplication within macrophages.

Lower expression of inducible nitric oxide synthase and higher expression of arginase in rat alveolar macrophages are linked to their susceptibility to Toxoplasma gondii infection

Rats are naturally resistant to Toxoplasma gondii infection, particularly the RH strain, while mice are not. Previous studies have demonstrated that inducible nitric oxide synthase (iNOS) and arginase-1 of rodent peritoneal macrophages are linked to the mechanism of resistance. As an increasing number of studies on human and animal infections are showing that pulmonary toxoplasmosis is one of the most severe clinical signs from T. gondii infection, we are interested to know whether T. gondii infection in alveolar macrophages of rats is also linked to the levels of iNOS and arginase-1 activity. Our results demonstrate that T. gondii could grow and proliferate in rat alveolar macrophages, both in vitro and in vivo, at levels higher than resistant rat peritoneal macrophages and at comparable levels to sensitive mouse peritoneal macrophages. Lower activity and expression levels of iNOS and higher activity and expression levels of arginase-1 in rat alveolar macrophages were found to be linked to the susceptibility of T. gondii infection in these cells. These novel findings could aid a better understanding of the pathogenesis of clinical pulmonary toxoplasmosis in humans and domestic animals.

Immune response and immunopathology during toxoplasmosis

Toxoplasma gondii is a protozoan parasite of medical and veterinary significance that is able to infect any warm-blooded vertebrate host. In addition to its importance to public health, several inherent features of the biology of T. gondii have made it an important model organism to study host-pathogen interactions. One factor is the genetic tractability of the parasite, which allows studies on the microbial factors that affect virulence and allows the development of tools that facilitate immune studies. Additionally, mice are natural hosts for T. gondii, and the availability of numerous reagents to study the murine immune system makes this an ideal experimental system to understand the functions of cytokines and effector mechanisms involved in immunity to intracellular microorganisms. In this article, we will review current knowledge of the innate and adaptive immune responses required for resistance to toxoplasmosis, the events that lead to the development of immunopathology, and the natural regulatory mechanisms that limit excessive inflammation during this infection.

Toxoplasma gondii infection in the brain inhibits neuronal degeneration and learning and memory impairments in a murine model of Alzheimer's disease

Immunosuppression is a characteristic feature of Toxoplasma gondii-infected murine hosts. The present study aimed to determine the effect of the immunosuppression induced by T. gondii infection on the pathogenesis and progression of Alzheimer's disease (AD) in Tg2576 AD mice. Mice were infected with a cyst-forming strain (ME49) of T. gondii, and levels of inflammatory mediators (IFN-γ and nitric oxide), anti-inflammatory cytokines (IL-10 and TGF-β), neuronal damage, and β-amyloid plaque deposition were examined in brain tissues and/or in BV-2 microglial cells. In addition, behavioral tests, including the water maze and Y-maze tests, were performed on T. gondii-infected and uninfected Tg2576 mice. Results revealed that whereas the level of IFN-γ was unchanged, the levels of anti-inflammatory cytokines were significantly higher in T. gondii-infected mice than in uninfected mice, and in BV-2 cells treated with T. gondii lysate antigen. Furthermore, nitrite production from primary cultured brain microglial cells and BV-2 cells was reduced by the addition of T. gondii lysate antigen (TLA), and β-amyloid plaque deposition in the cortex and hippocampus of Tg2576 mouse brains was remarkably lower in T. gondii-infected AD mice than in uninfected controls. In addition, water maze and Y-maze test results revealed retarded cognitive capacities in uninfected mice as compared with infected mice. These findings demonstrate the favorable effects of the immunosuppression induced by T. gondii infection on the pathogenesis and progression of AD in Tg2576 mice.

NADPH phagocyte oxidase knockout mice control Trypanosoma cruzi proliferation, but develop circulatory collapse and succumb to infection

(•)NO is considered to be a key macrophage-derived cytotoxic effector during Trypanosoma cruzi infection. On the other hand, the microbicidal properties of reactive oxygen species (ROS) are well recognized, but little importance has been attributed to them during in vivo infection with T. cruzi. In order to investigate the role of ROS in T. cruzi infection, mice deficient in NADPH phagocyte oxidase (gp91(phox) (-/-) or phox KO) were infected with Y strain of T. cruzi and the course of infection was followed. phox KO mice had similar parasitemia, similar tissue parasitism and similar levels of IFN-γ and TNF in serum and spleen cell culture supernatants, when compared to wild-type controls. However, all phox KO mice succumbed to infection between day 15 and 21 after inoculation with the parasite, while 60% of wild-type mice were alive 50 days after infection. Further investigation demonstrated increased serum levels of nitrite and nitrate (NOx) at day 15 of infection in phox KO animals, associated with a drop in blood pressure. Treatment with a NOS2 inhibitor corrected the blood pressure, implicating NOS2 in this phenomenon. We postulate that superoxide reacts with (•)NO in vivo, preventing blood pressure drops in wild type mice. Hence, whilst superoxide from phagocytes did not play a critical role in parasite control in the phox KO animals, its production would have an important protective effect against blood pressure decline during infection with T. cruzi.

IgE mediates killing of intracellular Toxoplasma gondii by human macrophages through CD23-dependent, interleukin-10 sensitive pathway

Background

In addition to helminthic infections, elevated serum IgE levels were observed in many protozoal infections, while their contribution during immune response to these pathogens remained unclear. As IgE/antigen immune complexes (IgE-IC) bind to human cells through FcεRI or FcεRII/CD23 surface molecules, the present study aimed to identify which functional receptor may be involved in IgE-IC interaction with human macrophages, the major effector cell during parasite infection.

Methodology/Principal Findings

Human monocyte-derived macrophages were infected with Toxoplasma gondii before being incubated with IgE-IC. IgE receptors were then identified using appropriate blocking antibodies. The activation of cells and parasiticidal activity were evaluated by mediator quantification and direct counting of infected macrophages. RNAs were extracted and cell supernatants were also collected for their content in tumor necrosis factor (TNF)-α, interleukin-10 (IL-10) and nitrites. Sera from symptomatic infected patients were also tested for their content of IgE, IL-10 and nitrites, and compared to values found in healthy donors. Results showed that IgE-IC induced intracellular elimination of parasites by human macrophages. IgE-mediated effect was FcεRI-independent, but required cross-linking of surface FcεRII/CD23, cell activation and the generation of nitric oxide (NO). Although TNF-α was shown to be produced during cell activation, this cytokine had minor contribution in this phenomenon while endogenous and exogenous IL-10 down-regulated parasite killing. Inverse relationship was found between IL-10 and NO expression by infected human macrophages at both mRNA and mediator levels. The relationship between these in vitro data and in vivo levels of various factors in T. gondii infected patients supports the involvement of CD23 antigen and IL-10 expression in disease control.

Conclusion

Thus, IgE may be considered as immune mediator during antiprotozoal activity of human macrophages through its ability to trigger CD23 signaling. Increased cell activation by IgE-IC may also account for chronic inflammatory diseases observed in some patients.

Increased seroprevalence of Toxoplasma gondii in a population of patients with Bell's palsy: a sceptical interpretation of the results regarding the pathogenesis of facial nerve palsy

Facial nerve oedema and anatomical predisposition to compression within the fallopian tube seem to be the only generally accepted facts in the pathophysiology of Bell's palsy. Several infectious causes have been suggested as possible triggers of this oedema. Most of the suggested pathogens have been associated with facial nerve lesions during latent infections, reinfections or endogenous reactivations. The aim of this study was to investigate the seroprevalence of three such pathogens Toxoplasma gondii, Epstein-Barr virus (EBV) and cytomegalovirus (CMV) in a population of patients with facial nerve palsy. Fifty-six patients with Bell's palsy were included in the study. A group of 25 individuals with similar age and gender distribution was used as control. Seropositivity for T. gondii, EBV viral capsid antigen (VCA) and CMV-specific IgM and IgG antibodies was investigated 2-5 days after the onset of the palsy. Comparisons for both IgM and IgG antibodies against T. gondii attributed significantly higher seroprevalence in the patients' group than in the control group (p = 0.024 and 0.013, respectively). The respective examinations for EBV and CMV attributed no significant results. The roles of EBV and CMV in the pathogenesis of Bell's palsy were not confirmed by this study. However, a significantly higher seroprevalence of IgM- and IgG-specific T. gondii antibodies was detected in patients with Bell's palsy when compared to healthy controls. The possibility that facial nerve palsy might be a late complication of acquired toxoplasmosis may need to be addressed in further studies.

Pyk2 is required for neutrophil degranulation and host defense responses to bacterial infection

The appropriate regulation of neutrophil activation is critical for maintaining host defense and limiting inflammation. Polymorphonuclear neutrophils (PMNs) express a number of cytoplasmic tyrosine kinases that regulate signaling pathways leading to activation. One of the most highly expressed, but least studied, kinases in PMNs is proline rich kinase 2 (Pyk2). By analogy to the related focal adhesion kinase, Pyk2 has been implicated in regulating PMN adhesion and migration; however, its physiologic function has yet to be described. Using pyk2(-/-) mice, we found that this kinase was required for integrin-mediated degranulation responses, but was not involved in adhesion-induced cell spreading or activation of superoxide production. Pyk2-deficient PMNs also manifested reduced migration on fibrinogen-coated surfaces. The absence of Pyk2 resulted in a severe reduction in paxillin and Vav phosphorylation following integrin ligation, which likely accounts for the poor degranulation and cell migration. Pyk2(-/-) mice were unable to efficiently clear infection with Staphylococcus aureus in a skin abscess model, owing in part to the poor release of granule contents at the site of infection. However, Pyk2-deficient PMNs responded normally to soluble agonists, demonstrating that this kinase functions mainly in the integrin pathway. These data demonstrate the unrealized physiologic role of this kinase in regulating the adhesion-mediated release of PMN granule contents.

Toxoplasma gondii: impaired maturation and pro-inflammatory response of dendritic cells in MIF-deficient mice favors susceptibility to infection

Macrophage migration inhibitory factor (MIF) has been found to be involved in host resistance to several parasitic infections. To determine the mechanisms of MIF-dependent responses to Toxoplasma gondii, we investigated host resistance in MIF-/- mice (BALB/c background) during natural oral infection. We focused on the potential involvement of MIF in Dendritic Cell (DC) maturation and IL-12 production. Following oral T. gondii infection, wild type mice developed a strong IL-12 response with an adequate maturation of their draining mesenteric lymph node DC (MLNDC) population and were resistant to challenge with either 40 or 100 cysts (ME49 strain). In contrast, similarly infected MIF-/- mice mounted a weak IL-12 response, displayed immature MLNDCs in the early phases of infection and rapidly succumbed to both type of challenges. Lack of maturation and IL-12 production of DCs in response to T. gondii antigens was confirmed by in vitro studies, and these effects were reversed following treatment with recombinant MIF. These findings demonstrate that MIF-induced early DC maturation and IL-12 production mediate resistance to T. gondii infection.

Identification and functional characterization of cis-regulatory elements in the apicomplexan parasite Toxoplasma gondii

Mining of genomic sequence data of the apicomplexan parasite Toxoplasma gondii identifies putative cis-regulatory elements using a de novo approach.

Background

Toxoplasma gondii is a member of the phylum Apicomplexa, which consists entirely of parasitic organisms that cause several diseases of veterinary and human importance. Fundamental mechanisms of gene regulation in this group of protistan parasites remain largely uncharacterized. Owing to their medical and veterinary importance, genome sequences are available for several apicomplexan parasites. Their genome sequences reveal an apparent paucity of known transcription factors and the absence of canonical cis-regulatory elements. We have approached the question of gene regulation from a sequence perspective by mining the genomic sequence data to identify putative cis-regulatory elements using a de novo approach.

Results

We have identified putative cis-regulatory elements present upstream of functionally related groups of genes and subsequently characterized the function of some of these conserved elements using reporter assays in the parasite. We show a sequence-specific role in gene-expression for seven out of eight identified elements.

Conclusions

This work demonstrates the power of pure sequence analysis in the absence of expression data or a priori knowledge of regulatory elements in eukaryotic organisms with compact genomes.

Toxoplasmosis in Bennett's wallabies (Macropus rufogriseus) in Spain

Toxoplasmosis is one of the more common parasitic zoonoses world-wide. In this study, an epizootic of toxoplasmosis among captive Bennett's wallabies (Macropus rufogriseus) from different locations is reported. By means of light microscopy, Toxoplasma gondii-like tachyzoites were observed associated to interstitial pneumonia, non-suppurative myocarditis, cholangiohepatitis and severe gastroenteritis. The protozoa stained positively with a T. gondii antibody and ultrastructurally were similar to T. gondii. Strikingly, tachyzoites appeared sometimes in an intranuclear location within granulocyte-like cells. Feral cats or reactivation of a latent infection are discussed as the possible sources of infection. As far as we know, this is the first confirmed report of toxoplasmosis in Bennett's wallabies in Spain and Europe, and may constitute a risk of infection for humans since new alimentary habits are being imposed in our countries.

Macrophage migration inhibitory factor (MIF) is critical for the host resistance against Toxoplasma gondii

Macrophage migration inhibitory factor (MIF) exerts either a protective or a deleterious role in the immune response to different pathogens. We analyzed herein the role of MIF in the host control of toxoplasmosis using MIF(-/-) mice backcrossed to either the BALB/c or the C57BL/6 genetic backgrounds. Both, wild-type (WT) BALB/c and MIF(-/-) BALB/c mice were susceptible to infection with highly virulent RH as well as moderately virulent ME49 strains of T. gondii. MIF(-/-) mice, however, showed greater liver damage and more brain cysts, produced less proinflammatory cytokines, and succumbed significantly faster than WT mice. Bone marrow-derived dendritic cells (BMDCs) from MIF(-/-) mice produced less interleukin-1beta, interleukin-12, and tumor necrosis factor-alpha than WT BMDCs after stimulation with soluble Toxoplasma antigen (STAg). Similar observations were made in CD11c(+) low-density cells isolated from the spleens of MIF(-/-) mice challenged with STAg. MIF(-/-) C57BL/6 mice succumbed to ME49 infection faster than their WT counterparts. C57BL/6 mice that succumbed to infection with the ME49 strain produced less MIF than resistant BALB/c mice similarly infected. Interestingly, an analysis of brains from patients with cerebral toxoplasmosis showed low levels of MIF expression. Together, these findings demonstrate that MIF plays a critical role in mediating host resistance against T. gondii.

Toll-like receptor-4-mediated macrophage activation is differentially regulated by progesterone via the glucocorticoid and progesterone receptors

Macrophage function has been demonstrated to be subject to modulation by progesterone. However, as this steroid hormone can act through the glucocorticoid receptor as well as the progesterone receptor, the mechanism of action has not been precisely characterized. To determine the mode of action, we compared the ability of progesterone, norgestrel (a synthetic progesterone-receptor-specific agonist) and dexamethasone (a synthetic glucocorticoid receptor agonist) to modulate macrophage function following stimulation of the Toll-like receptor-4 (TLR-4) ligand lipopolysaccharide (LPS). The results demonstrate that following stimulation of TLR-4 with LPS and cotreatment with either progesterone or dexamethasone, but not norgestrel, there is a significant reduction in nitric oxide (NO) production, indicating that this progesterone-mediated effect is through ligation of the glucocorticoid receptor. In contrast, LPS-induced interleukin-12 (IL-12) production could be downregulated by all three steroids, indicating that ligation by progesterone of either the glucocorticoid or the progesterone receptors or both could mediate this effect. While progesterone downmodulated NO-mediated killing of Leishmania donovani by activated macrophages in vitro, most probably via the glucocorticoid receptor, it had little effect on Toxoplasma gondii growth in these cells. This would suggest that progesterone-mediated increased susceptibility to T. gondii during pregnancy is more likely to be related to the ability of the hormone to downregulate IL-12 production and a type-1 response utilizing the progesterone as well as the glucocorticoid receptors.

Cellular and immunological basis of the host-parasite relationship during infection with Neospora caninum

Neospora caninum is an apicomplexan parasite that is closely related to Toxoplasma gondii, the causative agent of toxoplasmosis in humans and domestic animals. However, in contrast to T. gondii, N. caninum represents a major cause of abortion in cattle, pointing towards distinct differences in the biology of these two species. There are 3 distinct key features that represent potential targets for prevention of infection or intervention against disease caused by N. caninum. Firstly, tachyzoites are capable of infecting a large variety of host cells in vitro and in vivo. Secondly, the parasite exploits its ability to respond to alterations in living conditions by converting into another stage (tachyzoite-to-bradyzoite or vice versa). Thirdly, by analogy with T. gondii, this parasite has evolved mechanisms that modulate its host cells according to its own requirements, and these must, especially in the case of the bradyzoite stage, involve mechanisms that ensure long-term survival of not only the parasite but also of the host cell. In order to elucidate the molecular and cellular bases of these important features of N. caninum, cell culture-based approaches and laboratory animal models are being exploited. In this review, we will summarize the current achievements related to host cell and parasite cell biology, and will discuss potential applications for prevention of infection and/or disease by reviewing corresponding work performed in murine laboratory infection models and in cattle.

BeWo Trophoblasts are Unable to Control Replication of Toxoplasma gondii, Even in the Presence of Exogenous IFN-γ

The ability of RH strain of Toxoplasma gondii to invade and grow into BeWo cells was investigated in the present study using IFN-gamma, l-tryptophan, or alpha-methyl-tryptophan treatments. HeLa cells were used in the same conditions for comparison purposes. It was demonstrated that BeWo cells are more permissive to T. gondii infection, making them more susceptible to this pathogen when compared to HeLa cells. Infection rates of BeWo cells do not show any significant alteration in different protocols using IFN-gamma. In addition, BeWo treated with l-tryptophan was unable to significantly increase parasite growth. In contrast, HeLa cells treated with IFN-gamma or IFN-gamma plus l-tryptophan are able to impair or increase, respectively, parasite replication, providing evidence that this indoleamine-2,3-dioxygenase-dependent phenomenon is operant in these cells, whereas it is inactive in BeWo. Therefore, our data support the hypothesis that the immunological mechanisms controlling infection at the maternal-fetal interface are different from those occurring in the periphery. At the same time that operating regulatory mechanisms work inside and outside the cells located at that microenvironment to prevent maternal rejection of the concept, these events might facilitate the progression of infection caused by intracellular pathogens, as T. gondii.

Toxoplasma gondii and Neospora caninum infections of bovine endothelial cells induce endothelial adhesion molecule gene transcription and subsequent PMN adhesion

Toxoplasma gondii and Neospora caninum are important, closely related coccidian parasites infecting a broad spectrum of hosts and host cells. Infections underly a complex immunological regulation; however, little is known on innate immune reactions to these parasites. To investigate interactions between infected cells and polymorphonuclear neutrophil cells (PMN), PMN adhesion to tachyzoite-infected bovine umbilical vein endothelial cells (BUVECs) under physiological flow conditions and adhesion molecule (E-selectin, P-selectin, VCAM-1, ICAM-1) gene transcription in infected BUVECs were examined in vitro for 72h post-infection (p.i.). BUVECs were rapidly invaded by T. gondii and N. caninum; in general 10-15% of the cells became infected. Tachyzoites were released from 24 and 48h p.i. onwards, for T. gondii and N. caninum, respectively. PMN adhesion to infected cell layers increased early (4h) after infection with both parasites, reached maximum levels 16-24h p.i., but remained enhanced throughout the observation period. PMN adhered to both, infected and non-infected cells within one cell layer, suggesting parasites induced paracrine activation of the BUVECs. Semiquantitative Realtime RT-PCR showed upregulated transcription of the E- and P-selectin genes in BUVECs within 1h p.i. and of ICAM-1 and VCAM-1 genes within 2h p.i. Maximum transcript levels were observed at 4-6h p.i.; the 24h p.i. gene transcription had declined to control levels. In general, T. gondii more strongly induced PMN adhesion and adhesion molecule gene transcription than N. caninum. The data suggest an effective role of PMN in innate immune reactions to these parasites.

Proliferation of Toxoplasma gondii in inflammatory macrophages in vivo is associated with diminished oxygen radical production in the host cell

While reactive oxygen species (ROS) can kill Toxoplasma gondii in vitro the role these molecules play in vivo is not known. We used a flow cytometry-based assay to investigate the relationship between intracellular infection and ROS production during acute peritoneal toxoplasmosis in mice. A distinct population of ROS(+) inflammatory macrophages, detected by the oxidation of hydroethidine, was observed to increase progressively in frequency during the course of infection, and to be inversely correlated with the degree of cell parasitization. These data imply that either intracellular parasites inhibit ROS synthesis or, alternatively, ROS-producing cells contain anti-Toxoplasma activity. The latter interpretation was supported by the finding that uninfected ROS-producing inflammatory macrophages were resistant to infection in vivo. However, in the same animals, ROS-producing macrophages that had previously been parasitized could readily be infected with additional parasites, suggesting that the difference in ROS production between highly infected and less infected cells was not due to ROS-associated killing of parasites within these cells. In addition, macrophages infected with T. gondii in vitro and then briefly transferred to acutely infected mice upregulated ROS production in a manner that was again inversely correlated with the degree of intracellular parasitization. Taken together, these findings suggest that both ROS-associated anti-Toxoplasma activity and parasite-driven inhibition of ROS production underlie the observed pattern of ROS production. ROS function and parasite evasion of this function may contribute significantly to the balance between host defense and disease progression during acute infection.

Toxoplasma gondiiregulates ICAM‐1 mediated monocyte adhesion to trophoblasts

Materno-foetal transmission causes one of the most serious forms of infection with the intracellular protozoan parasite Toxoplasma gondii. In the placenta, trophoblast cells constitute the barrier between maternal circulation and foetal tissue. We looked at the factors that determine the extent of cell adhesion to human BeWo trophoblast cells during T. gondii infection. BeWo monolayers stimulated with the supernatant of T. gondii-infected PBMC showed a large increase in THP-1 cell adhesion and upregulation of the intercellular adhesion molecule (ICAM)-1. Neutralization of cytokines by corresponding antibodies demonstrated that anti-IFN-gamma, but not anti-TNF-alpha or anti-IL-1beta, led to a significant reduction of THP-1 adhesion to a BeWo monolayer. Treatment of BeWo cells with single cytokines failed to induce upregulation of adhesion. In contrast, simultaneous treatment with IFN-gamma and either TNF-alpha or IL-1beta mimicked strongly the effect of infected cell supernatant. The results suggest that IFN-gamma plays a pivotal role in the cell adhesion process through upregulation of ICAM-1 and in the process of congenital transmission of T. gondii.

Interleukin-17/interleukin-17 receptor-mediated signaling is important for generation of an optimal polymorphonuclear response against Toxoplasma gondii infection

We investigated the role of interleukin-17 (IL-17)/IL-17 receptor (IL-17R)-mediated signaling in the protective immunity against Toxoplasma gondii. IL-17R(-/-) mice developed a normal adaptive immunity against the parasite. However, increased mortality in the knockout animals can be attributed to a defect in the migration of polymorphonuclear leukocytes to infected sites during early infection.

Intestinal pathology during acute toxoplasmosis is IL-4 dependent and unrelated to parasite burden

The role of interleukin-4 (IL-4) during the course of Toxoplasma gondii infection was studied using IL-4-/- mice and their wild-type (WT) counterparts on a C57BL/6 background. Following oral infection with T. gondii tissue cysts an exacerbative role for IL-4 was demonstrated and IL-4-/- mice were found to be more resistant to infection than WT mice as measured by significantly reduced mortality. Furthermore pathology in the small intestine was less severe in IL-4-/- mice although conversely liver pathology was greater than in wild-type mice. Significantly, plasma IL-12 and IFN-gamma levels, which peaked at days 6 and 8, respectively, were higher in IL-4-/- mice. The exacerbatory role of IL-4 in the intestine was found by competitive RT-PCR not to be associated with increased parasite burdens but was related to comparative expression of IL-10.

Role of cytokines and major histocompatibility complex restriction in mouse resistance to infection with a natural recombinant strain (type I-III) of Toxoplasma gondii

Herein we characterized various genetic markers and the biological behavior of a natural recombinant strain of Toxoplasma gondii (P-Br). From nine genetic markers analyzed, three (B1, ROP1, and SAG1) and three (cS10-A6, GRA6, and SAG3) markers belong to parasites from the type I and type III lineages, respectively. The SAG2 and L363 loci were shown to be type I-III chimera alleles. The cB2l-4 microsatellite marker showed a unique haplotype. The P-Br strain presented low virulence in the acute phase of infection and was cystogenic during the chronic infection. The interleukin 12/gamma interferon axis and inducible nitric oxide synthase were main determinants of resistance during the acute infection with the P-Br strain. As opposed to infection with the type II strain of T. gondii (ME-49), peroral infection with the P-Br strain led only to a light inflammatory infiltrate and no major lesions in the intestine of the C57BL/6 mice. In addition, the BALB/c (resistant to ME-49) and C57BL/6 (susceptible to ME-49) mice were shown, respectively, to be more susceptible and more resistant to cyst formation and toxoplasmic encephalitis when infected with the P-Br strain. Further, the C57BL/KsJ and DBA2/J congenic strains containing major histocompatibility complex (MHC) haplotype "d" were more resistant than the parental strains (C57BL/6 and DBA1/J), when infected with the ME-49 but not with the P-Br strain. Together, our results indicate that resistance to cyst formation and toxoplasmic encephalitis induced during infection with P-Br is not primarily controlled by the MHC haplotype d, as previously reported for type II strains of T. gondii.

Inhibition of caspase activation and a requirement for NF-kappaB function in the Toxoplasma gondii-mediated blockade of host apoptosis

Mammalian cells infected with the protozoan parasite Toxoplasma gondii are resistant to many apoptotic stimuli transmitted along both the mitochondrial and death receptor pathways. Apoptosis, and its inhibition in infected cells, was examined using multiple morphological, molecular and biochemical approaches. The data strongly indicate manipulation of the host apoptotic machinery at multiple levels, focusing on the inhibition of host caspases. Activation of the pro-apoptotic caspase family of proteases is a biochemical hallmark of apoptosis. Caspase activation occurs in a highly ordered cascade triggered by the initiator caspases 8 and 9, which activate the executioner caspase, caspase 3. Our findings indicate a profound blockade of caspase activation and activity as the molecular basis for the inhibition of apoptosis in T.-gondii-infected cells. Caspase inhibition was demonstrated using multiple intrinsic and synthetic substrates. Although the specific inhibitory molecule remains to be identified, data indicate an absolute requirement for the host transcription factor NF-kappaB and, by extension, genes regulated by it. We propose that T. gondii activates the host survival response, thereby increasing the overall resistance of infected cells to apoptotic stimuli.

Toxoplasma gondii induction of interleukin-12 is associated with acute virulence in mice and depends on the host genotype

The intracellular parasite Toxoplasma gondii can influence host resistance by modulating immune functions in various cell types. The stimulation of interleukin (IL)-12 production in macrophages, dendritic cells and neutrophils by T. gondii has been implicated to be important for skewing anti-parasite immunity early after infection as well as in mediating the pathologic effects induced by the parasite. The present study demonstrates secretion of IL-12 p40 and the bioactive p70 heterodimer by inflammatory macrophages following exposure to live Toxoplasma or tachyzoite lysate. Parasite induction of IL-12 occurred in a dose-dependent manner. Predigestion of T. gondii lysate with proteinase K abrogated its IL-12 inducing activity, thus indicating that a parasite protein(s) triggers this response. Macrophages from various mouse inbred strains showed a differential responsiveness: cells from T. gondii-susceptible mice released more IL-12 upon toxoplasmic challenge than those from resistant mice, although the infection rate and intracellular parasite growth were similar. In triggering macrophage production of IL-12, tachyzoites proved superior to bradyzoites prepared from the same T. gondii isolate. Furthermore, parasites of a mouse-virulent isolate became less efficient inducers of IL-12 following attenuation. The parallel loss in macrophage stimulation in vitro and acute virulence in vivo suggests a linkage of both parasite capacities. Together with the correlation on host side between the genotype-dependent mouse susceptibility to infection and cellular responsiveness to the parasite trigger, these findings indicate that an overproduction of parasite-induced IL-12 might represent a basic mechanism of T. gondii pathogenicity.

Toxoplasma gondii tachyzoites inhibit proinflammatory cytokine induction in infected macrophages by preventing nuclear translocation of the transcription factor NF-kappa B

Control of microbial infection requires regulated induction of NF-kappaB-dependent proinflammatory cytokines such as IL-12 and TNF-alpha. Activation of this important transcription factor is driven by phosphorylation-dependent degradation of the inhibitory IkappaB molecule, an event which enables NF-kappaB translocation from the cytoplasm to the nucleus. In this study, we show that intracellular infection of macrophages with the protozoan parasite Toxoplasma gondii induces rapid IkappaB phosphorylation and degradation. Nevertheless, NF-kappaB failed to translocate to the nucleus, enabling the parasite to invade cells without triggering proinflammatory cytokine induction. Infected cells subsequently subjected to LPS triggering were severely crippled in IL-12 and TNF-alpha production, a result of tachyzoite-induced blockade of NF-kappaB nuclear translocation. Our results are the first to demonstrate the ability of an intracellular protozoan to actively interfere with the NF-kappaB activation pathway in macrophages, an activity that may enable parasite survival within the host.

Attenuation of mouse-virulent Toxoplasma gondii parasites is associated with a decrease in interleukin-12-inducing tachyzoite activity and reduced expression of actin, catalase and excretory proteins

Determinants of Toxoplasma gondii virulence are still unknown, although genetic markers associated with T. gondii pathogenicity or host susceptibility to infection have been identified. To define indicator proteins of mouse virulence, type I strain parasites were attenuated by continuous passage in fibroblast culture and compared with the parental strain passaged in mice. The loss of acute virulence, evident by a 1000-fold higher pathogen dose causing 100% lethality in mice correlated with a less efficient infection of inflammatory cells at the site of inoculation, while parasite proliferation and invasiveness in vitro proved unimpaired. Infection with the attenuated parasites elicited earlier local interleukin-12 and strong interferon-gamma responses in vivo, although the activity that triggers interleukin-12 secretion in macrophages is reduced in the attenuated compared to the virulent strain variant. The interleukin-12-inducing T. gondii stimulus was identified as a protein(s) present in tachyzoite excretory products. Comparative proteome analysis combined with immunodetection and quantitation of a variety of T. gondii antigens indicated that the steady-state levels of actin, catalase, microneme protein 5, as well as dense granule proteins 1, 2, 3, 4, 5, 7, 8 and nucleoside triphosphate hydrolase 1 are decreased in the attenuated phenotype, whereas the surface antigen 1 and rhoptry protein 1 are produced at a similar level by virulent and attenuated parasites. In conclusion, these findings reveal a correlation between the efficient establishment of T. gondii infection in vivo and parasite synthesis of actin, catalase and several excretory proteins, and thus postulate a role for these molecules in acute virulence.

Microarray analysis reveals previously unknown changes in Toxoplasma gondii-infected human cells

Cells infected with the intracellular protozoan parasite Toxoplasma gondii undergo up-regulation of pro-inflammatory cytokines, organelle redistribution, and protection from apoptosis. To examine the molecular basis of these and other changes, gene expression profiles of human foreskin fibroblasts infected with Toxoplasma were studied using human cDNA microarrays consisting of approximately 22,000 known genes and uncharacterized expressed sequence tags. Early during infection (1-2 h), <1% of all genes show a significant change in the abundance of their transcripts. Of the 63 known genes in this group, 27 encode proteins associated with the immune response. These genes are also up-regulated by secreted, soluble factors from extracellular parasites indicating that the early response does not require parasite invasion. Later during infection, genes involved in numerous host cell processes, including glucose and mevalonate metabolism, are modulated. Many of these late genes are dependent on the direct presence of the parasite; i.e. secreted products from either the parasite or infected cells are insufficient to induce these changes. These results reveal several previously unknown effects on the host cell and lay the foundation for detailed analysis of their role in the host-pathogen interaction.

Immunoconversion against Sarcocystis neurona in normal and dexamethasone-treated horses challenged with S. neurona sporocysts

Equine protozoal myeloencephalitis is a common neurologic disease of horses in the Americas usually caused by Sarcocystis neurona. To date, the disease has not been induced in horses using characterized sporocysts from Didelphis virginiana, the definitive host. S. neurona sporocysts from 15 naturally infected opossums were fed to horses seronegative for antibodies against S. neurona. Eight horses were given 5x10(5) sporocysts daily for 7 days. Horses were examined for abnormal clinical signs, and blood and cerebrospinal fluid were harvested at intervals for 90 days after the first day of challenge and analyzed both qualitatively (western blot) and quantitatively (anti-17kDa) for anti-S. neurona IgG. Four of the challenged horses were given dexamethasone (0.1mg/kg orally once daily) for the duration of the experiment. All challenged horses immunoconverted against S. neurona in blood within 32 days of challenge and in CSF within 61 days. There was a trend (P = 0.057) for horses given dexamethasone to immunoconvert earlier than horses that were not immunosuppressed. Anti-17kDa was detected in the CSF of all challenged horses by day 61. This response was statistically greater at day 32 in horses given dexamethasone. Control horses remained seronegative throughout the period in which all challenged horses converted. One control horse immunoconverted in blood at day 75 and in CSF at day 89. Signs of neurologic disease were mild to equivocal in challenged horses. Horses given dexamethasone had more severe signs of limb weakness than did horses not given dexamethasone; however, we could not determine whether these signs were due to spinal cord disease or to effects of systemic illness. At necropsy, mild-moderate multifocal gliosis and neurophagia were found histologically in the spinal cords of 7/8 challenged horses. No organisms were seen either in routinely processed sections or by immunohistochemistry. Although neurologic disease comparable to naturally occurring equine protozoal myeloencephalitis (EPM) was not produced, we had clear evidence of an immune response to challenge both systemically and in the CNS. Broad immunosuppression with dexamethasone did not increase the severity of histologic changes in the CNS of challenged horses. Future work must focus on defining the factors that govern progression of inapparent S. neurona infection to EPM.

Counter-protective role for interleukin-5 during acute Toxoplasma gondii infection

The role of interleukin-5 (IL-5) during Toxoplasma gondii infection was investigated by comparing disease progression in IL-5 gene deficient (IL-5-/-) mice and their wild-type (WT) counterparts on a C57BL/6 background. IL-5-/- mice infected orally with T. gondii were less susceptible to infection than WT mice as demonstrated by reduced mortality rates. Consistent with this data, orally infected IL-5-/- mice had less severe pathological changes in their small intestines than WT mice at 8 days postinfection. At this time, splenocytes and mesenteric lymph node cells derived from IL-5-/- mice produced levels of IL-12, interferon-gamma (IFN-gamma), IL-4, IL-10, and nitric oxide (measured as nitrite) similar to those derived from WT mice when stimulated with Toxoplasma lysate antigen. However, peak serum IL-12 and IFN-gamma levels (at days 6 and 8, respectively) were significantly higher in IL-5-/- mice than in WT mice. In addition, WT mice but not IL-5-/- mice had raised levels of eosinophils in their peripheral blood between days 5 and 8 following infection. Oral administration of N omega-nitro-L-arginine methyl (from day 4 postinfection) increased mortality rates in both IL-5-/- and WT mice, indicating a protective role for nitric oxide during the early stages of oral T. gondii infection. In comparison with oral infection, no difference in mortality was observed between IL-5-/- and WT mice following intraperitoneal infection with T. gondii, with all mice surviving until 35 days postinfection. Similarly, no significant differences were observed in the severity of the meningitis, perivascular cuffing, or number of microglial nodules or parasites in the brains of intraperitoneally infected mice. Together, these results demonstrate a detrimental role for IL-5 during the early stage of oral infection with T. gondii which is associated with increased small-intestine pathology, eosinophilia, and reduced plasma IL-12 and IFN-gamma levels.

Equine protozoal myeloencephalitis

Recent advances in the understanding of the parasite life cycle, epidemiology, clinical signs, diagnosis, treatment, and prevention of EPM are reviewed. The NAHMS Equine '98 study and a controlled retrospective study from The Ohio State University College of Veterinary Medicine identified a number of risk factors associated with development of the disease. The national annual incidence of EPM was 1% or less depending on the primary use of the animals. Increased disease risk was associated with age (1-5 and > 13 years of age), season (lowest in winter months and increasing with ambient temperature), previous stressful events, the presence of opossums, the use of nonsurface water drinking systems, and failure to restrict wildlife access to feed. Horses that received treatment were 10 times more likely to improve, and those that improved were 50 times more likely to survive. A number of recent studies confirmed that horses can be experimentally infected with S. neurona; however, large numbers of sporocysts are apparently necessary to achieve infection, and clinical signs and abnormal CNS histology are only seen inconsistently. Results suggest that CNS infection and positive CSF immunoblot findings may be transient phenomena among naturally infected horses. Although immunosuppression may be involved in the development of EPM, some element of the immune response seems to be necessary for the development of clinical signs. Use of the standard immunoblot test for the detection of anti-S. neurona antibodies in CSF continues to provide the most useful adjunct to a detailed neurologic examination for the diagnosis of EPM. Test sensitivity and specificity were 89% in 295 horses euthanatized because of neurologic disease, of which 123 were confirmed cases of EPM. The PPV was 85%, and the NVP was 92%. A number of promising new EPM treatments are under investigation. In addition to standard SDZ/PYR therapy, toltrazuril, ponazuril, diclazuril, and NTZ have shown promise as possible alternatives.

Investigation into the mechanism of gamma interferon-mediated inhibition of Toxoplasma gondii in murine astrocytes

Toxoplasma gondii is an obligate intracellular parasite that is a common opportunistic pathogen of the central nervous system in AIDS patients. Gamma interferon (IFN-gamma) alone or in combination with interleukin-1 (IL-1), IL-6, or tumor necrosis factor alpha significantly inhibits the growth of T. gondii in murine astrocytes, suggesting these are important nonimmune effector cells in the brain. Inhibition was found to be independent of a nitric oxide-mediated or tryptophan starvation mechanism. Both reactive oxygen intermediates and iron deprivation are IFN-gamma-mediated mechanisms known to operate against intracellular parasites in other cell types. Astrocytes generated from mice genetically deficient in the production of reactive oxygen intermediates (phox(-/-) mice) were found to inhibit growth of T. gondii when stimulated with IFN-gamma alone or in combination with other cytokines. The reactive oxygen inhibitor catalase and the reactive oxygen scavengers mannitol and thiourea failed to reverse the IFN-gamma-induced inhibition of T. gondii in astrocytes. These data indicate that IFN-gamma-induced inhibition in astrocytes is independent of reactive oxygen intermediates. IFN-gamma-induced inhibition could not be reversed by the addition of iron salts, ferric citrate, ferric nitrate, or ferric transferrin. Pretreatment of astrocytes with desferrioxamine also did not induce the inhibition of T. gondii. These data indicate that the mechanism of IFN-gamma inhibition was not due to iron deprivation. IFN-gamma had no effect on T. gondii invasion of astrocytes, but inhibition of growth and loss of tachyzoite vacuoles were evident in IFN-gamma-treated astrocytes by 24 h after invasion. Overall, these data suggest that IFN-gamma-activated astrocytes inhibit T. gondii by an as-yet-unknown mechanism.

Transformation of leukocytes by Theileria parva and T. annulata

Theileria parva and T. annulata provide intriguing models for the study of parasite-host interactions. Both parasites possess the unique property of being able to transform the cells they infect; T. parva transforms T and B cells, whereas T. annulata affects B cells and monocytes/macrophages. Parasitized cells do not require antigenic stimulation or exogenous growth factors and acquire the ability to proliferate continuously. In vivo, parasitized cells undergo clonal expansion and infiltrate both lymphoid and non-lymphoid tissues of the infected host. Theileria-induced transformation is entirely reversible and is accompanied by the expression of a wide range of different lymphokines and cytokines, some of which may contribute to proliferation or may enhance spread and survival of the parasitized cell in the host. The presence of the parasite in the host-cell cytoplasm modulates the state of activation of a number of signal transduction pathways. This, in turn, leads to the activation of transcription factors, including nuclear factor-kappa B, which appear to be essential for the survival of Theileria-transformed T cells.

The host-parasite relationship in neosporosis

Neospora caninum is an apicomplexan parasite which invades many different cell types and tissues. It causes neosporosis, namely stillbirth and abortion in cattle and neuromuscular disease in dogs, and has been found in several other animal species. N. caninum is closely related to Toxoplasma gondii, and controversial opinions exist with respect to its phylogenetical status. Initially, two stages of N. caninum had been identified, namely asexually proliferating tachyzoites and bradyzoites. The sexually produced stage of this parasite, oocysts containing sporozoites, has been found only recently. In order to answer the many open questions regarding its basic biology and its relationship with the host, a number of diagnostic tools have been developed. These techniques are based on the detection of antibodies against parasites in body fluids, the direct visualization of the parasite within tissue samples by immunohistochemistry, or the specific amplification of parasite DNA by PCR. Other studies have been aiming at the identification of specific antigenic components of N. caninum, and the molecular and functional characterization of these antigens with respect to the cell biology of the parasite. Clearly, molecular approaches will also be used increasingly to elucidate the immunological and pathogenetic events during infection, but also to prepare potential new immunotherapeutic tools for future vaccination against N. caninum infection.

A protective role for IL-6 during early infection with Toxoplasma gondii

IL-6 deficient mice were found to be significantly more susceptible to peroral infection with Toxoplasma gondii than their wild-type counterparts as measured by survival, brain cyst burdens and brain pathology at 28 days post infection. The physical manifestations of disease, such as weight loss, were not observed in IL-6 deficient animals until at least seven days later than such changes occurred in wildtype mice. During this early stage of infection IL-6+/+ but not IL-6-/- mice mounted a peripheral blood neutrophilia. Furthermore, between 6-8 days post-infection there was a significant increase in plasma IFN-gamma levels in wild-type but not IL-6 deficient mice. Not until days 18-23 post-infection, concurrent with the majority of deaths in IL-6-/- mice, were plasma IFN-gamma levels substantially and significantly raised in IL-6-/- mice. At this time not only were these plasma IFN-gamma levels 20-fold higher than background but eight-fold greater than peak (6-8 days post-infection) IFN-gamma levels in IL-6+/+ mice. IFN-gamma dependent parasite specific IgG2a levels were also significantly higher in IL-6-/- mice over this period and thereafter. Overall the evidence suggests that in the absence of IL-6 mice are unable to initiate a rapid proinflammatory response against T. gondii, which allows increased parasite growth. Increased mortality in IL-6-/- mice may be directly due to this increased parasite burden and the excessive inflammatory response this induces three weeks post-infection.

The role of IL-4 in adult acquired and congenital toxoplasmosis

The course of Toxoplasma gondii infection was studied in IL-4-deficient mice from three genetic backgrounds and their wild-type counterparts following peroral inoculation of tissue cysts. Survival rates were significantly reduced in disease-susceptible C57 BL/6 mice and F1 (C57BL/6 x 129Sv) mice deficient in IL-4 compared with wild-type controls. In contrast, this difference was not observed in T. gondii-resistant BALB/c mice. However, brain tissue cyst burdens in IL-4-deficient mice were either equivalent to (C57BL/6 and BALB/c mice) or significantly less (B6/129 mice) than similarly infected wild-type mice. Thus strain-specific differences in the course of T. gondii were demonstrated in the absence of IL-4. The course of T. gondii infection was also compared between B6/129 IL-4-deficient mice and their wild-type counterparts following peroral challenge with 20 tissue cysts on day 12 of pregnancy. Age-matched non-pregnant IL-4-/- and IL-4+/+ mice were also infected to assess the role of IL-4 on T. gondii infection during pregnancy. Disease phenotypes, as measured by mortality, were reversed if infections were initiated during pregnancy compared with non-pregnant infection. Thus significant mortality occurred immediately post partum in IL-4+/+ mothers, while all IL-4-/- mothers survived. Cyst burdens 28 days p.i. were significantly lower in IL-4-/- mothers than IL-4+/+ mothers and both IL-4-/- and IL-4+/+ non-pregnant mice. Congenital disease transmission as measured by foetal death or vertical disease transmission was independent of the presence or absence of IL-4. These studies demonstrate a role for IL-4 in pregnancy-induced immunosuppression and the associated increased susceptibility to T. gondii infection.