Effect of murine interferon gamma on murine toxoplasmosis

Heterologous immunization targeting the CST1 antigen confers better protection than ROP18 in mice

Aim: To evaluate the protective efficacy induced by heterologous immunization with recombinant baculoviruses or virus-like particles targeting the CST1 and ROP18 antigens of Toxoplasma gondii.Materials & methods: Recombinant baculovirus and virus-like particle vaccines expressing T. gondii CST1 or ROP18 antigens were developed to evaluate protective immunity in mice upon challenge infection with 450 Toxoplasma gondii (ME49).Results: Immunization with CST1 or ROP18 vaccines induced similar levels of T. gondii-specific IgG and IgA responses. Compared with ROP 18, CST1 vaccine showed better antibody-secreting cell response, germinal center B cell activation, and significantly reduced brain cyst burden and body weight loss.Conclusion: Our findings suggest that CST1 heterologous immunization elicited better protection than ROP18, providing important insight into improving the toxoplasmosis vaccine design strategy.

IFNs in host defence and parasite immune evasion during Toxoplasma gondii infections

Interferons (IFNs) are a family of cytokines with diverse functions in host resistance to pathogens and in immune regulation. Type II IFN, i.e. IFN-γ, is widely recognized as a major mediator of resistance to intracellular pathogens, including the protozoan Toxoplasma gondii. More recently, IFN-α/β, i.e. type I IFNs, and IFN-λ (type III IFN) have been identified to also play important roles during T. gondii infections. This parasite is a widespread pathogen of humans and animals, and it is a model organism to study cell-mediated immune responses to intracellular infection. Its success depends, among other factors, on the ability to counteract the IFN system, both at the level of IFN-mediated gene expression and at the level of IFN-regulated effector molecules. Here, I review recent advances in our understanding of the molecular mechanisms underlying IFN-mediated host resistance and immune regulation during T. gondii infections. I also discuss those mechanisms that T. gondii has evolved to efficiently evade IFN-mediated immunity. Knowledge of these fascinating host-parasite interactions and their underlying signalling machineries is crucial for a deeper understanding of the pathogenesis of toxoplasmosis, and it might also identify potential targets of parasite-directed or host-directed supportive therapies to combat the parasite more effectively.

Immune response against toxoplasmosis-some recent updates RH: Toxoplasma gondii immune response

AIMS

Cytokines, soluble mediators of immunity, are key factors of the innate and adaptive immune system. They are secreted from and interact with various types of immune cells to manipulate host body's immune cell physiology for a counter-attack on the foreign body. A study was designed to explore the mechanism of Toxoplasma gondii (T. gondii) resistance from host immune response.

METHODS AND RESULTS

The published data on aspect of host (murine and human) immune response against T. gondii was taken from Google scholar and PubMed. Most relevant literature was included in this study. The basic mechanism of immune response starts from the interactions of antigens with host immune cells to trigger the production of cytokines (pro-inflammatory and anti-inflammatory) which then act by forming a cytokinome (network of cytokine). Their secretory equilibrium is essential for endowing resistance to the host against infectious diseases, particularly toxoplasmosis. A narrow balance lying between Th1, Th2, and Th17 cytokines (as demonstrated until now) is essential for the development of resistance against T. gondii as well as for the survival of host. Excessive production of pro-inflammatory cytokines leads to tissue damage resulting in the production of anti-inflammatory cytokines which enhances the proliferation of Toxoplasma. Stress and other infectious diseases (human immunodeficiency virus (HIV)) that weaken the host immunity particularly the cellular component, make the host susceptible to toxoplasmosis especially in pregnant women.

CONCLUSION

The current review findings state that in vitro harvesting of IL12 from DCs, Np and MΦ upon exposure with T. gondii might be a source for therapeutic use in toxoplasmosis. Current review also suggests that therapeutic interventions leading to up-regulation/supplementation of SOCS-3, IL12, and IFNγ to the infected host could be a solution to sterile immunity against T. gondii infection. This would be of interest particularly in patients passing through immunosuppression owing to any reason like the ones receiving anti-cancer therapy, the ones undergoing immunosuppressive therapy for graft/transplantation, the ones suffering from immunodeficiency virus (HIV) or having AIDS. Another imortant suggestion is to launch the efforts for a vaccine based on GRA6Nt or other similar antigens of T. gondii as a probable tool to destroy tissue cysts.

Overexpression screen of interferon-stimulated genes identifies RARRES3 as a restrictor of Toxoplasma gondii infection

Toxoplasma gondii is an important human pathogen infecting an estimated one in three people worldwide. The cytokine interferon gamma (IFNγ) is induced during infection and is critical for restricting T. gondii growth in human cells. Growth restriction is presumed to be due to the induction of interferon-stimulated genes (ISGs) that are upregulated to protect the host from infection. Although there are hundreds of ISGs induced by IFNγ, their individual roles in restricting parasite growth in human cells remain somewhat elusive. To address this deficiency, we screened a library of 414 IFNγ induced ISGs to identify factors that impact T. gondii infection in human cells. In addition to IRF1, which likely acts through the induction of numerous downstream genes, we identified RARRES3 as a single factor that restricts T. gondii infection by inducing premature egress of the parasite in multiple human cell lines. Overall, while we successfully identified a novel IFNγ induced factor restricting T. gondii infection, the limited number of ISGs capable of restricting T. gondii infection when individually expressed suggests that IFNγ-mediated immunity to T. gondii infection is a complex, multifactorial process.

Nanos gigantium humeris insidentes: old papers informing new research into Toxoplasma gondii

Since Nicolle, Manceaux and Splendore first described Toxoplasma gondii as a parasite of rodents and rabbits in the early 20th century, a diverse and vigorous research community has been built around studying this fascinating intracellular parasite. In addition to its importance as a pathogen of humans, livestock and wildlife, modern researchers are attracted to T. gondii as a facile experimental system to study many aspects of evolutionary biology, cellular biology, host-microbe interactions, and host immunity. For new researchers entering the field, the extensive literature describing the biology of the parasite, and the interactions with its host, can be daunting. In this review, we examine four foundational studies that describe various aspects of T. gondii biology, presenting a 'journal club'-style analysis of each. We have chosen a paper that established the beguiling life cycle of the parasite (Hutchison et al., 1971), a paper that described key features of its cellular biology that the parasite shares with related organisms (Gustafson et al., 1954), a paper that characterised the origin of the unique compartment in which the parasite resides within host cells (Jones and Hirsch, 1972), and a paper that established a key mechanism in the host immune response to parasite infection (Pfefferkorn, 1984). These interesting and far-reaching studies set the stage for subsequent research into numerous facets of parasite biology. As well as providing new researchers with an entry point into the literature surrounding the parasite, revisiting these studies can remind us of the roots of our discipline, how far we have come, and the new directions in which we might head.

Novel Murine Pancreatic Tumor Model Demonstrates Immunotherapeutic Control of Tumor Progression by a Toxoplasma gondii Protein

Pancreatic ductal adenocarcinoma is the fourth leading cause of cancer-related death in the United States, with few effective treatments available and only 10% of those diagnosed surviving 5 years. Although immunotherapeutics is a growing field of study in cancer biology, there has been little progress in its use for the treatment of pancreatic cancer. Pancreatic cancer is considered a nonimmunogenic tumor because the tumor microenvironment does not easily allow for the immune system, even when stimulated, to attack the cancer. Infection with the protozoan parasite Toxoplasma gondii has been shown to enhance the immune response to clear cancer tumors. A subset of T. gondii proteins called soluble Toxoplasma antigen (STAg) contains an immunodominant protein called profilin. Both STAg and profilin have been shown to stimulate an immune response that reduces viral, bacterial, and parasitic burdens. Here, we use STAg and profilin to treat pancreatic cancer in a KPC mouse-derived allograft murine model. These mice exhibit pancreatic cancer with both Kras and P53 mutations as subcutaneous tumors. Pancreatic cancer tumors in C57BL/6J mice with a wild-type background showed a significant response to treatment with either profilin or STAg, exhibiting a decrease in tumor volume accompanied by an influx of CD4⁺ and CD8⁺ T cells into the tumors. Both IFN-γ^-/- mice and Batf3^-/- mice, which lack conventional dendritic cells, failed to show significant decreases in tumor volumes when treated. These results indicate that gamma interferon (IFN-γ) and dendritic cells may play critical roles in the immune response necessary to treat pancreatic cancer.

Efficacy and safety of intratracheal IFN-γ treatment to reverse stroke-induced susceptibility to pulmonary bacterial infections

Stroke-induced immunosuppression contributes to the development of stroke-associated pneumonia (SAP). Experiments in mice demonstrated that apoptosis of IFN-γ producing cells and reduced IFN-γ secretion resulted in impaired immune responses and the development of pneumonia after middle cerebral artery occlusion (MCAo). In the present study, we investigated the efficacy of intratracheal IFN-γ treatment to prevent SAP and demonstrated that modest benefits on pulmonary cytokine response in IFN-γ treated stroke mice did not prevent spontaneously developing infections and even slightly reduced bacterial clearance of aspirated pneumococci. Our results suggest that pulmonary IFN-γ treatment is not an effective preventive measure for SAP.

Immediate Interferon Gamma Induction Determines Murine Host Compatibility Differences between Toxoplasma gondii and Neospora caninum

Rodents are critical for the transmission of Toxoplasma gondii to the definitive feline host via predation, and this relationship has been extensively studied as a model for immune responses to parasites. Neospora caninum is a closely related coccidian parasite of ruminants and canines but is not naturally transmitted by rodents. We compared mouse innate immune responses to N. caninum and T. gondii and found marked differences in cytokine levels and parasite growth kinetics during the first 24 h postinfection (hpi). N. caninum-infected mice produced significantly higher levels of interleukin-12 (IL-12) and interferon gamma (IFN-γ) by as early as 4 hpi, but the level of IFN-γ was significantly lower or undetectable in T. gondii-infected mice during the first 24 hpi. "Immediate" IFN-γ and IL-12p40 production was not detected in MyD88-/- mice. However, unlike IL-12p40-/- and IFN-γ-/- mice, MyD88-/- mice survived N. caninum infections at the dose used in this study. Serial measures of parasite burden showed that MyD88-/- mice were more susceptible to N. caninum infections than wild-type (WT) mice, and control of parasite burdens correlated with a pulse of serum IFN-γ at 3 to 4 days postinfection in the absence of detectable IL-12. Immediate IFN-γ was partially dependent on the T. gondii mouse profilin receptor Toll-like receptor 11 (TLR11), but the ectopic expression of N. caninum profilin in T. gondii had no impact on early IFN-γ production or parasite proliferation. Our data indicate that T. gondii is capable of evading host detection during the first hours after infection, while N. caninum is not, and this is likely due to the early MyD88-dependent recognition of ligands other than profilin.

A study on the status of inflammatory systems in camels naturally infected with Toxoplasma gondii

Toxoplasma gondii is a unique intracellular parasite with a worldwide distribution. This parasite infects a variety of cells in a wide range of animal species such as dromedary camels (Camelus dromedarius). In order to evaluate the pattern of possible changes in the blood level of some inflammatory mediators and antioxidant enzymes in camels infected with T. gondii, blood samples were taken from a total of 493 dromedary camels and serum concentrations of inflammatory mediators, acute phase proteins and antioxidant enzymes were measured. According to serological data, no seropositivity was found for anti-T. gondii IgM in serum samples; however, 49 camels (9.93 %) showed positive titrations for anti-Toxoplasma IgG. The analyses of data in seropositive animals showed significant increases (P < 0.05) in the serum level of IL-1β and adenosine deaminase activity; however, IFN-γ and TNF-α demonstrated no significant changes in serum samples of the infected camels. In addition, while major acute phase proteins (haptoglobin (Hp) and serum amyloid A (SAA)) were markedly elevated in infected camels, the activity of antioxidant enzymes (SOD and GPX) was remarkably decreased in the blood samples of infected animals. Thus, during the chronic infection in camels, T. gondii can promote significant rises in concentrations of some cytokines (such as IL-1β), acute phase proteins and adenosine deaminase.

Toxoplasma gondii triggers phosphorylation and nuclear translocation of dendritic cell STAT1 while simultaneously blocking IFNγ-induced STAT1 transcriptional activity

The protozoan Toxoplasma gondii actively modulates cytokine-induced JAK/STAT signaling pathways to facilitate survival within the host, including blocking IFNγ-mediated STAT1-dependent proinflammatory gene expression. We sought to further characterize inhibition of STAT1 signaling in infected murine dendritic cells (DC) because this cell type has not previously been examined, yet is known to serve as an early target of in vivo infection. Unexpectedly, we discovered that T. gondii infection alone induced sustained STAT1 phosphorylation and nuclear translocation in DC in a parasite strain-independent manner. Maintenance of STAT1 phosphorylation required active invasion but intracellular parasite replication was dispensable. The parasite rhoptry protein ROP16, recently shown to mediate STAT3 and STAT6 phosphorylation, was not required for STAT1 phosphorylation. In combination with IFNγ, T. gondii induced synergistic STAT1 phosphorylation and binding of aberrant STAT1-containing complexes to IFNγ consensus sequence oligonucleotides. Despite these findings, parasite infection blocked STAT1 binding to the native promoters of the IFNγ-inducible genes Irf-1 and Lrg47, along with subsequent gene expression. These results reinforce the importance of parasite-mediated blockade of IFNγ responses in dendritic cells, while simultaneously showing that T. gondii alone induces STAT1 phosphorylation.

Mechanisms of Toxoplasma gondii persistence and latency

Toxoplasma gondii is an obligate intracellular protozoan parasite that causes opportunistic disease, particularly in immunocompromised individuals. Central to its transmission and pathogenesis is the ability of the proliferative stage (tachyzoite) to convert into latent tissue cysts (bradyzoites). Encystment allows Toxoplasma to persist in the host and affords the parasite a unique opportunity to spread to new hosts without proceeding through its sexual stage, which is restricted to felids. Bradyzoite tissue cysts can cause reactivated toxoplasmosis if host immunity becomes impaired. A greater understanding of the molecular mechanisms orchestrating bradyzoite development is needed to better manage the disease. Here, we will review key studies that have contributed to our knowledge about this persistent form of the parasite and how to study it, with a focus on how cellular stress can signal for the reprogramming of gene expression needed during bradyzoite development.

FlowCytomix analysis for Toxoplasma gondii infection in pregnant women in central Taiwan

The study was to determine the seroprevalence of toxoplasmosis in the sera of pregnant women in central Taiwan and to investigate the levels of cytokine in the sera of pregnant women with Toxoplasma gondii infection. The 220 blood samples were collected from pregnant women. The haematological parameters of peripheral blood were analysed by a haematology analyser. Serum samples of the pregnant women were analysed by a commercially available anti-T. gondii IgM/IgG antibody enzyme-linked immunosorbent assay (ELISA) kit and FlowCytomix assays. Six (2.7%) of the sera samples had IgM anti-T. gondii antibodies, and twenty (9.1%) had T. gondii IgG seropositive. All six IgM seropositive samples had low IgG avidity, indicative of acute infection. Total white blood cells and eosinophils were statistically significantly increased (p<0.05) in pregnant women with T. gondii infection, as compared with healthy pregnant women. Th1 cytokines IFN-γ, IL-1β, IL-2 and IL-12 p70, and Th2 cytokines IL-10 in pregnant women with T. gondii IgM/IgG seropositive were significantly increased (p<0.05), as compared with healthy pregnant women. These results showed that both of Th1 and Th2 cytokines play an important role in the toxoplasmosis of pregnant women.

Toxoplasma gondii: 25 years and 25 major advances for the field

This article is an attempt to identify the most significant highlights of Toxoplasma research over the last 25 years. It has been a period of enormous progress and the top 25 most significant advances, in the view of this author, are described. These range from the bench to the bedside and represent a tremendous body of work from countless investigators. And, having laid out so much that has been discovered, it is impossible not to also reflect on the challenges that lie ahead. These, too, are briefly discussed. Finally, while every effort has been made to view the field as a whole, the molecular biology background of the author almost certainly will have skewed the relative importance attached to past and future advances. Despite this, it is hoped that the reader will agree with, or at least not disagree too strongly with, most of the choices presented here.

Further analysis of protection induced by the MIC3 DNA vaccine against T. gondii: CD4 and CD8 T cells are the major effectors of the MIC3 DNA vaccine-induced protection, both Lectin-like and EGF-like domains of MIC3 conferred protection

The present study was conducted mainly to evaluate the contribution of the cellular and the humoral responses in protection conferred by the MIC3 DNA vaccine (pMIC3i) that was proved as a potent vaccine against toxoplasmosis. We performed the adoptive transfer of CD4(+) and CD8(+) T lymphocytes from pMIC3i immunized mice to naive ones and the role of humoral immunity was evaluated by in vitro invasion assays. We also constructed plasmids encoding the EGF-like domains and the Lectin-like domain of MIC3, to define which domains of MIC3 are involved in the protection. Furthermore, the adjuvant effect of the GM-CSF-expressing vector (granulocyte-macrophage colony-stimulating factor) required the precise temporal and spatial codelivery of GM-CSF with antigen, thus, we constructed a bicistronic plasmid expressing MIC3 and GM-CSF. In conclusion, the protection induced by pMIC3i was mainly mediated by CD4(+) and CD8(+) T lymphocytes and both EGF and Lectin domains of MIC3 conferred protection. Furthermore, the codelivery of GM-CSF by a bicistronic plasmid appeared to be a most effective way for enhancing the adjuvant properties of GM-CSF.

Secretion of toxic oxygen products by macrophages: regulatory cytokines and their effects on the oxidase

We are attempting to identify cytokines that regulate macrophage secretion of reactive oxygen intermediates (ROI) and to analyse the biochemical basis of their effects. In both humans and mice, interferon-gamma (IFN-gamma) appears to be the chief factor secreted by clonally unselected lymphocytes that enhances macrophage oxidative metabolism and antiprotozoal activity. In vivo administration of recombinant IFN-gamma enhances the ROI secretory capacity of monocytes in humans, and the secretion of ROI and killing of protozoa by peritoneal macrophages in mice. A protein secreted by murine tumours and certain non-malignant cells exerts opposing effects. This macrophage deactivation factor (MDF) both blocks the induction of activation by IFN-gamma and reverses pre-existent activation. MDF action is non-toxic and selective, suppressing the secretion of ROI, killing of intracellular protozoa, and expression of Ia antigen, without inhibiting secretion of several other products, or synthesis of protein, ingestion of particles or adherence to culture vessels. The suppressive effect of MDF is reversed over several days after its removal. This reversal is hastened by IFN-gamma. Profound suppression of oxidative metabolism accompanies the differentiation of murine monocytes into Kupffer cells. The capacity of Kupffer cells to secrete ROI and kill intracellular protozoa remains deficient even after exposure to IFN-gamma. Thus, four states of macrophage activation can provisionally be discerned: the transition of mouse peritoneal macrophages from the non-activated to the activated state is accompanied by a ninefold increase in affinity of the superoxide-producing enzyme for NADPH, without a marked increase in cellular Vmax or content of cytochrome b559. The MDF-induced transition of mouse peritoneal macrophages from the activated to the deactivated state is accompanied by both an increase in Km and a decrease in apparent V max of the oxidase. There are no changes in the phorbol myristate acetate receptor number or affinity, glucose transport, NADPH levels, cytochrome b559 content, catalase (EC 1.11.1.6) GSH, GSH peroxidase (EC 1.11.1.9), GSH reductase (EC 1.6.4.2) or myeloperoxidase, consistent with the suppressed ROI secretory capacity and antiprotozoal activity of these cells. The Kupffer cell, whose non-responsiveness to IFN-gamma may mark it as inactivated, appears to lack detectable NADPH oxidase activity, despite the probable presence of cytochrome b559, and in this regard differs from both non-activated and deactivated macrophages.(ABSTRACT TRUNCATED AT 400 WORDS)

Macrophage migration inhibitory factor is up-regulated in human first-trimester placenta stimulated by soluble antigen of Toxoplasma gondii, resulting in increased monocyte adhesion on villous explants

Considering the potential role of macrophage migration inhibitory factor (MIF) in the inflammation process in placenta when infected by pathogens, we investigated the production of this cytokine in chorionic villous explants obtained from human first-trimester placentas stimulated with soluble antigen from Toxoplasma gondii (STAg). Parallel cultures were performed with villous explants stimulated with STAg, interferon-gamma (IFN-gamma), or STAg plus IFN-gamma. To assess the role of placental MIF on monocyte adhesiveness to human trophoblast, explants were co-cultured with human myelomonocytic THP-1 cells in the presence or absence of supernatant from cultures treated with STAg (SPN), SPN plus anti-MIF antibodies, or recombinant MIF. A significantly higher concentration of MIF was produced and secreted by villous explants treated with STAg or STAg plus IFN-gamma after 24-hour culture. Addition of SPN or recombinant MIF was able to increase THP-1 adhesion, which was inhibited after treatment with anti-MIF antibodies. This phenomenon was associated with intercellular adhesion molecule expression by villous explants. Considering that the processes leading to vertical dissemination of T. gondii remain widely unknown, our results demonstrate that MIF production by human first-trimester placenta is up-regulated by parasite antigen and may play an essential role as an autocrine/paracrine mediator in placental infection by T. gondii.

IL-12 induction of resistance to pulmonary blastomycosis

BACKGROUND

Why severity varies in blastomycosis outbreaks remains unresolved. In experimental pulmonary blastomycosis, susceptibility varied in mouse strains. In susceptible BALB/c the response is Th-2, in immunized, resistance is associated with Th-1. Can susceptibility be redirected by IL-12? Methods, results: BALB/c bronchoalveolar and peritoneal macrophages (PM) were shown deficient in IL-12 production in response to IFN-gamma+LPS. High dose IL-12 (1 microg, subcutaneously) treatment of BALB/c infected intranasally with Blastomyces resulted in enhanced survival (P<0.008). Since IL-12 was poorly tolerated, a new protocol for infected mice, IL-12 0.1 or 0.3 microg, every other day, resulted in minimal toxicity; almost all treated mice survived (P<0.002 vs. controls). When lungs of surviving mice were cultured, the 0.1 microg regimen resulted in fewer (P<0.02) cfu. For weeks after treatment, in vitro IFN-gamma treatment enabled PM Blastomyces killing. After infection spleen cells from IL-12 treated mice produced 4-fold more IFN-gamma and 3-fold less IL-10 in response to Blastomyces. IL-10 abrogated activation of macrophages by IFN-gamma for enhanced Blastomyces killing.

CONCLUSIONS

A proper IL-12 treatment protocol induces resistance (survival and decreased growth in lungs), low toxicity, macrophage responsiveness to IFN-gamma for killing Blastomyces, up-regulation of IFN-gamma and down-regulation of IL-10 production.

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.

Cytokine gene expression in response to SnSAG1 in horses with equine protozoal myeloencephalitis

Equine protozoal myeloencephalitis (EPM) is a neurologic syndrome seen in horses from the Americas and is mainly caused by Sarcocystis neurona. Recently, a 29-kDa surface antigen from S. neurona merozoites was identified as being highly immunodominant on a Western blot. This antigen has been sequenced and cloned, and the expressed protein has been named SnSAG1. In a previous study, cell-mediated immune responses to SnSAG1 were shown to be statistically significantly reduced in horses with EPM in comparison to EPM-negative control horses. It therefore appears as though the parasite is able to induce immunosuppression towards parasite-derived antigens as parasite-specific responses are decreased. Isolated peripheral blood lymphocytes from 21 EPM (cerebrospinal fluid [CSF] Western blot)-negative horses with no clinical signs and 21 horses with clinical signs of EPM (CSF Western blot positive) were cocultured with SnSAG1 for 48 and 72 h, and the effect on cytokine production was investigated by means of reverse transcriptase PCR. Cytokines assayed include gamma interferon (IFN-gamma), tumor necrosis factor alpha, interleukin (IL)-2, IL-4, and IL-6. beta-Actin was used as the housekeeping gene. A Wilcoxon signed-rank test of the findings indicated that there was a statistically significant decrease in IFN-gamma production after 48 h in culture for samples from horses with clinical disease. There was also a statistically significant increase in IL-4 production after 72 h in culture for samples from horses with EPM. These results further support the notion that this parasite is able to subvert the immune system in horses with clinical disease.

IL-10 is not required to prevent immune hyperactivity during memory responses to Toxoplasma gondii

Primary infection of IL-10 knockout (KO) mice with the protozoan parasite Toxoplasma gondii leads to a CD4(+)-T-cell dependent shock-like reaction with high systemic levels of IL-12 and IFN-gamma, severe liver pathology and death of mice. In the present study, this immune-mediated pathology was prevented by treatment of IL-10 KO mice with the anti-parasitic drug sulfadiazine, allowing these mice to progress to the chronic phase of infection. To address the role of endogenous IL-10 in the regulation of secondary immune responses to T. gondii, IL-10 KO mice were infected with the avirulent Me49 strain of this parasite, treated with sulfadiazine for 2 weeks starting at day 3 p.i., and were rechallenged 6 weeks p.i. with RH, a highly virulent strain of T. gondii. In these studies, chronically infected IL-10 KO mice survived secondary infection with RH and controlled parasite load. Although serum levels of IL-12 and IFN-gamma were higher in IL-10 KO mice than in wild type (WT) mice 8 days after RH rechallenge, these levels were well controlled in the absence of endogenous IL-10, suggesting that IL-10 is not required to down-regulate cytokine production during the memory response. Antigen-specific ex vivo recall responses further revealed that splenocytes from chronically infected WT and IL-10 KO mice responded to parasite antigen with similar production of IL-12 and IFN-gamma, and there was also no significant difference in ex vivo production of these cytokines by splenocytes in response to parasite antigen 7 days after secondary infection with T. gondii. Furthermore, IL-10 KO mice immunized with the Ts-4 vaccine-strain of T. gondii were protected when rechallenged with the virulent RH strain. Together, these studies demonstrate that the inhibitory effect of IL-10, which is required to prevent immune-mediated pathology during primary infection, is not required to prevent immune hyperactivity during a secondary response to T. gondii, and a highly effective memory response is generated in the absence of endogenous IL-10.

Cell-Mediated Immune Responses in Horses with Equine Protozoal Myeloencephalitis

Equine protozoal myeloencephalitis (EPM) is a neurologic syndrome seen in horses from the Americas and is mainly caused by Sarcocystis neurona. Cell-mediated immune responses to mitogens have been shown to be reduced in horses with EPM, although it is not known whether the parasite causes this immunosuppression or if the immunosuppression is required for disease manifestation. Recently, a 29-kDa surface antigen from S. neurona merozoites was identified as being highly immunodominant on Western blot. This antigen has been sequenced and cloned, and the expressed protein has been named SnSAG1. Isolated peripheral blood lymphocytes from 43 EPM-negative horses and 28 horses with clinical EPM were cocultured with a mitogen or SnSAG1, and lymphocyte blastogenic responses to these antigens was measured by tritiated thymidine uptake. The ability of SnSAG1 to induce gamma-interferon (gammaIFN) production was also investigated with reverse transcriptase-polymerase chain reaction. There was no significant differences between EPM-positive and -negative horses in lymphocyte responses to ConcanavalinA. However, lymphocytes from EPM-negative horses responded significantly higher to SnSAG1 than lymphocytes from EPM-positive horses. GammaIFN production was detectable by 24 hr in culture in response to SnSAG1 in all EPM-negative horses. There was still no detectable gammaIFN production in EPM-positive horses after 72 hr in culture. It appears that the parasite is also able to induce an immunosuppression toward parasite-derived antigens as parasite-specific responses are decreased.

Pathogenicity of Toxoplasma gondii through B-2 cell-mediated downregulation of host defense responses

IFN-gamma is the primary mediator of anti-parasite effector mechanisms against Toxoplasma gondii. After intraperitoneal infection with the Fukaya strain of T. gondii, unirradiated IFN-gamma knock-out (GKO) mice transferred with wild type (WT) CD8+ effector T cells from infected mice failed to induce the production of IFN-gamma and died, whereas irradiated (IR) GKO mice transferred with WT CD8+ T cells induced IFN-y production and survived more than 6 months. IR GKO mice transferred with WT CD8+ T cells together with GKO B-2 cells died 8 days after infection, whereas those transferred with WT CD8+ T cells together with B-la or T cells survived. B-2 cells of infected GKO mice activated CD11b+ cells for IL-4 production, and down-regulated NO release, STAT1 phosphorylation, and interferon regulatory factor-1 expression in the peritoneal exudates cells of IR GKO mice transferred with WT CD8+ T cells together with GKO B-2 cells after infection. Thus, B-2 cells in T. gondii-infected mice act as suppressor cells in the host defense of infected mice.

Contribution of interleukin-12 (IL-12) and the CD28/B7 and CD40/CD40 ligand pathways to the development of a pathological T-cell response in IL-10-deficient mice

The ability of interleukin-10 (IL-10) to suppress accessory cell functions required for optimal T-cell activation makes it an important inhibitor of cell-mediated immunity. Thus, after infection with the protozoan parasite Toxoplasma gondii, IL-10 knockout (KO) mice develop a CD4(+)-T-cell-dependent shock-like reaction with high levels of IL-12 and gamma interferon (IFN-gamma) in serum, leading to death of mice during the acute phase of infection. Previous studies from this laboratory have shown that simultaneous blockade of CD28 and CD40 can prevent this lethal reaction by inhibiting the production of IFN-gamma. However, the blockade of costimulation did not affect systemic levels of IL-12. To better understand the relationship between IL-12 and the CD28 and CD40 pathways in mediating immune hyperactivity, antagonists of these factors were used to determine their effects on the development of a pathological T-cell response in IL-10 KO mice. Blockade of IL-12 or the CD28/B7 interaction alone did not affect survival; however, the combined blockade of both pathways resulted in decreased production of IFN-gamma and the survival of IL-10 KO mice. To assess the role of the two ligands for CD28, B7.1 and B7.2, IL-10 KO mice were treated with alphaIL-12 plus alphaB7.1 or alphaB7.2 or the combination of all three antibodies. These studies revealed that blockade of both B7 molecules is required for decreased production of IFN-gamma and survival of infected IL-10 KO mice, suggesting that B7.1 and B7.2 can contribute to the lethal shock-like reaction in IL-10 KO mice. In contrast, neutralization of IL-12 and blockade of the CD40/CD40 ligand (CD40L) interaction in vivo did not alter the production of IFN-gamma and only resulted in a small delay in time to death of mice. Together, these data suggest that the CD28/B7 interaction has a central role in the development of a pathological T-cell response in IL-10 KO mice, which is distinct from the role of the CD40/CD40L and IL-12 pathways.

Dendritic cells as effector cells: gamma interferon activation of murine dendritic cells triggers oxygen-dependent inhibition of Toxoplasma gondii replication

Toxoplasma gondii is an obligate intracellular parasite that infects a wide variety of nucleated cells in its numerous intermediate hosts, including humans. Much interest has focused on the ability of gamma interferon (IFN-gamma)-activated macrophages to prevent intracellular replication, but some other cells (e.g., fibroblasts, endothelial cells, microglial cells, astrocytes, enterocytes and retinal pigment cells) can also be activated to induce this inhibition of proliferation. Dendritic cells are generally known to be involved in the induction of immune responses, but no previous study had investigated the possibility that dendritic cells may act as effector cells of this system. Our results show that IFN-gamma-activation inhibits the replication of T. gondii in dendritic cells, with the inhibition being dose dependent. Neither nitrogen derivatives nor tryptophan starvation appears to be involved in the inhibition of parasite replication by IFN-gamma. Experiments with oxygen scavengers indicate that intracellular T. gondii replication is oxygen dependent. Our findings suggest that, in addition to their essential role in stimulating the immune system, dendritic cells probably act as effector cells in the first line of defense against pathogen invasion.

Effect of administration of a recombinant adenovirus expressing the genes for IFN-gamma and interleukin-12 on acute murine toxoplasmosis

The effect of recombinant murine interferon-gamma (rMuIFN-gamma) produced from an adenovirus construct on Toxoplasma gondii in tissue culture and on the outcome of a T. gondii infection in mice was determined. Supernatants from AdCMVMuIFN-gamma-infected mouse lung epithelial (MuLE) cells were evaluated for the ability to produce biologically active IFN-gamma by measuring the capacity of the supernatants to activate peritoneal macrophages for killing of T. gondii. The bioactivity of IFN-gamma in supernatants increased with increasing multiplicity of infection (moi). Replication was inhibited 43%, 67%, and 70% by supernatants from MuLE cells infected with AdCMVMuIFN-gamma moi 5, 10, and 50, respectively, (p < 0.01 compared with controls). Bioactivity of IFN-gamma also increased as the length of time after infection increased. T. gondii replication was inhibited 28% and 36%, respectively, by AdCMVMuIFN-gamma-infected MuLE cell supernatants recovered at 24 and 48 h (p < 0.01 compared with control). In vivo administration of AdCMVMuIFN-gamma exhibited 33% mortality by day 9 in mice acutely infected with T. gondii compared with 100% mortality in control mice (p = 0.045). Administration of AdCMVIL-12 reduced mortality to 40% compared with control mice. However, this reduction was not significant (p = 0.08). Overall survival was extended 2 days with AdCMVMuINF-gamma administration and 5 days with AdCMVIL-12. AdCMVMuIFN-gamma in vitro inhibits T. gondii, and in vivo AdCMVMuIFN-gamma and AdCMVIL-12 lead to increased survival in mice.

Distribution of parasite stages in tissues of Toxoplasma gondii infected SCID mice and human peripheral blood lymphocyte-transplanted SCID mice

The establishment of Toxoplasma gondii infection in the tissues of SCID mice and SCID mice transplanted with human peripheral blood lymphocytes (PBL) was investigated. The presence of bradyzoites and tachyzoites was analysed in hu-PBL SCID mice using Southern blot of reverse transcriptase-polymerase chain reaction products for the expression of B1, BAG1 and SAG1 of T. gondii. BAG1 was present by week 1 in brain, lung, liver and spleen of some animals; by week 3, BAG1 was present in all animals and in all of these tissues. In contrast, SAG1 was rarely detected until week 2 (mainly in the lung and brain) and by week 3, some animals still did not have detectable SAG1 in brain, lung, liver and spleen. SAG1 expression was increased in the lungs of animals transplanted with human PBL compared to nontransplanted SCID mice. Human PBL engraftment was demonstrated, initially in uninfected mice, by the presence of human CD3+ T cells in the spleen (3.1 x 10(5) positive cells) and peritoneal cavity (3.4 x 10(5) cells) 4 weeks after transplantation. The final outcome of infection was not influenced by the presence of human PBL, with similar mortality in human PBL transplanted and nontransplanted mice. These studies provide a detailed analysis of the kinetics and distribution of both the cyst and tachyzoite stage of T. gondii. This system has been established to allow evaluation of therapies against T. gondii immunodeficient mice in the presence of human immune cells.

Protective immunity against congenital toxoplasmosis with recombinant SAG1 protein in a guinea pig model

Primary infection with Toxoplasma gondii during pregnancy can induce fetal pathology and abortion in both humans and animals. The present study describes the development of an experimental model of congenital toxoplasmosis in the guinea pig. In this animal model, we evaluated the protective effect of vaccination with a recombinant form of SAG1 against maternofetal transmission of tachyzoites. The presence of parasites in fetuses was determined by nested PCRs and by an in vivo readout after fetal brain homogenate injections in mice. The absence of parasites was demonstrated in 66 to 86% of fetuses derived from adult guinea pigs immunized with SAG1 and challenged with the mildly virulent T. gondii strain C56. In contrast, more than 80% of fetuses from mock-immunized guinea pigs were infected. The protection was not correlated with titers of antibody to SAG1. Our results indicated that this experimental model constitutes a relevant model for evaluation of vaccine candidates against congenital toxoplasmosis and that SAG1 elicits significant protection against maternofetal transmission.

IP-10 is critical for effector T cell trafficking and host survival in Toxoplasma gondii infection

The generation of an adaptive immune response against intracellular pathogens requires the recruitment of effector T cells to sites of infection. Here we show that the chemokine IP-10, a specific chemoattractant for activated T cells, controls this process in mice naturally infected with Toxoplasma gondii. Neutralization of IP-10 in infected mice inhibited the massive influx of T cells into tissues and impaired antigen-specific T cell effector functions. This resulted in >1000-fold increase in tissue parasite burden and a marked increase in mortality compared to control antibody-treated mice. These observations suggest that IP-10 may play a broader role in the localization and function of effector T cells at sites of Th1 inflammation.

Augmentation of the CD8+ T Cell Response by IFN-γ in IL-12-Deficient Mice During Toxoplasma gondii Infection

The importance of IFN-γ in regulating the host CD8+ T cell response during microbial infection has not been delineated. Mice deficient for the p40 chain of the IL-12 heterodimer have impaired IFN-γ production and are susceptible to infection with the intracellular parasite Toxoplasma gondii. The administration of exogenous IFN-γ to parasite-infected p40−/− mice increases survival and up-regulates the depressed CD8+ T cell response following infection. CD8+ T cells isolated from cytokine-treated p40−/− mice exhibit an increase in both precursor CTL frequency and IFN-γ production compared with untreated controls. The enhancement of the CD8+ T cell response is independent of CD4+ T cell help. These CD8+ T cells induce protective immunity against a lethal challenge when adoptively transferred into naive p40−/− and IFN-γ−/− mice. These observations indicate that IFN-γ can regulate the CD8+ T cell response during T. gondii infection.

Role of gamma interferon in cellular immune response against murine Encephalitozoon cuniculi infection

Microsporidia are obligate intracellular protozoan parasites that cause a wide variety of opportunistic infection in patients with AIDS. Because it is able to grow in vitro, Encephalitozoon cuniculi is currently the best-studied microsporidian. T cells mediate protective immunity against this parasite. Splenocytes obtained from infected mice proliferate in vitro in response to irradiated parasites. A transient state of hyporesponsiveness to parasite antigen and mitogen was observed at day 17 postinfection. This downregulatory response could be partially reversed by addition of nitric oxide (NO) antagonist to the culture. Mice infected with E. cuniculi secrete significant levels of gamma interferon (IFN-gamma). Treatment with antibody to IFN-gamma or interleukin-2 (IL-12) was able to neutralize the resistance to the parasite. Mutant animals lacking the IFN-gamma or IL-12 gene were highly susceptible to infection. However, mice unable to secrete NO withstood high doses of parasite challenge, similar to normal wild-type animals. These studies describe an IFN-gamma-mediated protection against E. cuniculi infection that is independent of NO production.

Inducible Nitric Oxide Synthase Is Not Required for Long-Term Vaccine-Based Immunity Against Toxoplasma gondii

Induction of reactive nitrogen intermediates by IFN-γ is presumed an important mechanism of host resistance against acute and chronic infection with Toxoplasma gondii. Although nitric oxide (NO) has been shown to be important in the control of parasite replication in vivo, the role of this molecule in vaccine-based immunity against T. gondii is unknown. Mice with a targeted disruption of inducible NO synthase (iNOS) were immunized with an avirulent temperature-sensitive strain of this parasite (ts-4). Both the parental C57BL/6 and the iNOS−/− mice survived infection with the ts-4 mutant. Oral challenge of the vaccinated mice with a lethal dose of cysts containing bradyzoites resulted in reduced parasite burden and increased survival compared with nonvaccinated control mice. Host immunity in the iNOS−/− mice, similar to that observed in the parental strain, appears dependent upon both IFN-γ and CD8+ T cells. These findings suggest that although vaccine-based long-term immunity against T. gondii is dependent upon the induction of IFN-γ, it does not rely upon the anti-microbial effect of NO.

Toxoplasma gondii: kinetics of the dissemination in the host tissues during the acute phase of infection of mice and rats

Mice and rats differ in their susceptibility to Toxoplasma gondii infection. Here we have compared the parasitological parameters of acute infection in both mice and rats infected either orally with cysts of Prugniaud strain or intraperitoneally (ip) with tachyzoites of the RH strain. The animals were killed at regular interval during the acute phase, and the parasitic burden in mesenteric lymph nodes, spleen, liver, diaphragm, heart, lungs, brain, and blood was assessed by a tissue culture method in MRC5 fibroblast cells. Mice infected with the RH strain showed a drastic increase of the parasitic load in all organs (up to 10 (8) parasites / g of organs), and then died. When mice were infected with 50 cysts of Prugniaud strain, parasites were first observed in mesenteric lymph nodes, spleen, and lungs and then in other organs. In rats, infection with 1200 cysts of the same strain led to a similar pattern; however, the chronology of the infected organs changed when they were infected with RH strain tachyzoites. These results suggest that the parasites, present first in the peritoneal cavity in the case of RH ip infection, infect all the adjacent organs and then the blood which disseminates the tachyzoites all over the organism. In contrast, after an oral infection, the parasite crosses the intestinal barrier to reach the mesenteric lymph nodes and then the spleen and are finally distributed by the blood throughout the organism. We also showed that following infection with a lethal or a sublethal doses of the Prugniaud strain, the parasitic burden in the studied organs was similar and therefore does not directly correlate with the death of the mice.

Role of interferon-gamma in murine cytomegalovirus infection

Interferon-gamma has well-documented antiviral and immunomodulatory activity, but its role in the control of cytomegalovirus (CMV) infection is not well studied. In a mouse model of murine CMV (MCMV) disease, interferon-gamma concentrations in serum but not in bronchoalveolar lavage fluid increased in response to viral infection. Serum interferon-gamma levels peaked at day 2 in the relatively resistant C57BL/6 mice, and, in contrast, did not peak until day 6 in susceptible BALB/c mice. Mice genetically lacking interferon-gamma (GKO) were more susceptible to MCMV, although strain differences persisted, with C57BL/6 GKO mice experiencing less severe MCMV disease than BALB/c GKO mice. Treatment of MCMV-infected BALB/c mice with exogenous interferon-gamma starting 2 days after viral infection had a modest protective effect at lower interferon-gamma doses (10(4) units), but interferon-gamma therapy markedly increased morbidity and mortality when higher doses (10(5) units) were used. We conclude that interferon-gamma plays a significant role in host response to MCMV and that the cytokine has dose- and time-dependent beneficial and adverse effects.

A dichotomous role for nitric oxide during acute Toxoplasma gondii infection in mice

Production of nitric oxide by macrophages is believed to be an important microbicidal mechanism for a variety of intracellular pathogens, including Toxoplasma gondii. Mice with a targeted disruption of the inducible nitric oxide synthase gene (iNOS) were infected orally with T. gondii tissue cysts. Time to death was prolonged compared with parental controls. Histologic analysis of tissue from infected mice showed scattered small foci of inflammation with parasites in various tissues of iNOS-/- mice, whereas tissue from the parental C57BL/6 mice had more extensive tissue inflammation with few visible parasites. In particular, extensive ulceration and necrosis of distal small intestine and fatty degeneration of the liver was seen in the parental mice at day 7 postinfection, as compared with the iNOS-/- mice where these tissues appeared normal. Serum interferon gamma and tumor necrosis factor alpha levels postinfection were equally elevated in both mouse strains. Treatment of the parental mice with a NO synthase inhibitor, aminoguanidine, prevented early death in these mice as well as the hepatic degeneration and small bowel necrosis seen in acutely infected control parentals. These findings indicate that NO production during acute infection with T. gondii can kill intracellular parasites but can be detrimental, even lethal, to the host.

Mechanisms of innate resistance to Toxoplasma gondii infection

The interaction of protozoan parasites with innate host defences is critical in determining the character of the subsequent infection. The initial steps in the encounter of Toxoplasma gondii with the vertebrate immune system provide a striking example of this important aspect of the host-parasite relationship. In immuno-competent individuals this intracellular protozoan produces an asymptomatic chronic infection as part of its strategy for transmission. Nevertheless, T. gondii is inherently a highly virulent pathogen. The rapid induction by the parasite of a potent cell-mediated immune response that both limits its growth and drives conversion to a dormant cyst stage explains this apparent paradox. Studies with gene-deficient mice have demonstrated the interleukin-12 (IL-12)-dependent production of interferon gamma (IFN-gamma) to be of paramount importance in controlling early parasite growth. However, this seems to be independent of nitric oxide production as mice deficient in inducible nitric oxide synthase (iNOS) and tumour necrosis factor receptor were able to control early growth of T. gondii, although, they later succumbed to infection. Nitric oxide does, however, seem to be important in controlling persistent infection; treating chronic infection with iNOS metabolic inhibitors results in disease reactivation. Preliminary evidence implicates neutrophils in effector pathways against this parasite distinct from that described for macrophages. Once initiated, IL-12-dependent IFN-gamma production in synergy with other proinflammatory cytokines can positively feed back on itself to induce 'cytokine shock'. Regulatory cytokines, particularly IL-10, are essential to down-regulate inflammation and limit host pathology.

Altered energy balance and cytokine gene expression in a murine model of chronic infection with Toxoplasma gondii

The temporal pattern of changes in energy balance and cytokine mRNA expression in spleen and brain were examined in a mouse model of infection with Toxoplasma gondii. During days 1-7 postinfection, food intake was unaltered, but energy expenditure was significantly increased, and this was associated with elevated tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1, IL-5, and interferon (IFN)-gamma. The hypermetabolic state persisted during subsequent anorexia, whose onset coincided with elevated IL-2, and at the end of the acute phase of cachexia, the dual anorexic and hypermetabolic states were associated with the cytokines examined: TNF-alpha, IL-1 beta, IL-2, IL-4, IL-5, IL-6, IL-10, and IFN-gamma. In the chronic phase of the infection, the mice showed either partial weight recovery (gainers) or no weight regain (nongainers). The infected gainers, though still hypophagic, were no longer hypermetabolic, and their cytokine mRNA was no longer elevated, except for TNF-alpha and IL-10. In contrast, the infected nongainers continued to show both anoroxia and hypermetabolism, which were associated with elevations in all cytokines examined and particularly those of the TH2 profile (IL-4 and IL-5) and IL-6. Taken together, these studies reveal a distinct pattern of cytokine mRNA expression underlying 1) hypermetabolism vs. anorexia, 2) acute vs. chronic cachexia, and 3) stable weight loss vs. partial weight recovery.

Inducible nitric oxide is essential for host control of persistent but not acute infection with the intracellular pathogen Toxoplasma gondii

The induction by IFN-gamma of reactive nitrogen intermediates has been postulated as a major mechanism of host resistance to intracellular pathogens. To formally test this hypothesis in vivo, the course of Toxoplasma gondii infection was assessed in nitric oxide synthase (iNOS)-/- mice. As expected, macrophages from these animals displayed defective microbicidal activity against the parasite in vitro. Nevertheless, in contrast to IFN-gamma-/- or IL-12 p40-/- animals, iNOS-deficient mice survived acute infection and controlled parasite growth at the site of inoculation. This early resistance was ablated by neutralization of IFN-gamma or IL-12 in vivo and markedly diminished by depletion of neutrophils, demonstrating the existence of previously unappreciated NO independent mechanisms operating against the parasite during early infection. By 3-4 wk post infection, however, iNOS knockout mice did succumb to T. gondii. At that stage parasite expansion and pathology were evident in the central nervous system but not the periphery suggesting that the protective role of nitric oxide against this intracellular infection is tissue specific rather than systemic.

Does human toxoplasmosis involve an imbalance in T1/T2 cytokines?

The T1 (interferon-gamma, interleukin-12, interleukin-2) and T2 (interleukin-4, interleukin-10, interleukin-6) cytokine groups constitute two polar responses of the immune system. The T1 group is a predominantly cellular response, while the T2 group response is mainly humoral. The hypothesis forwarded here links these subgroups of induced cytokines to the various clinical forms of human toxoplasmosis. Ocular toxoplasmosis in immunocompetent patients could be attributed to a T1 hyper-response, whereas congenital toxoplasmosis, toxoplasmic encephalitis (in immunodeficient patients) and active chronic toxoplasmosis (with persistent lymphadenophathy) would be characterized by a predominantly T2 response. Confirmation that this kind of immunological imbalance effectively underlies the various clinical forms of toxoplasmosis would open the way for a new range of treatments based on immunomodulation.

Mechanisms of interferon-induced inhibition of Toxoplasma gondii replication in human retinal pigment epithelial cells

Inflammation associated with retinochoroiditis is a major complication of ocular toxoplasmosis in infants and immunocompetent individuals. Moreover, Toxoplasma gondii-induced retinal disease causes serious complications in patients with AIDS and transplant patients. The retinal pigment epithelial (RPE) cell is an important regulatory cell within the retina and is one of the cells infected with T. gondii in in vivo. We have developed a human RPE (HRPE) cell in vitro model system to evaluate T. gondii replication and the regulation of this replication by cytokines. T. gondii replication was quantitated by counting the foci of infection (plaque formation) and the numbers of tachyzoites released into the supernatant fluids. Pretreatment of cultures with recombinant human tumor necrosis factor alpha, alpha interferon (IFN-alpha), IFN-beta, or IFN-gamma for 24 h prior to inoculation inhibited T. gondii replication in a dose-dependent manner. Of these cytokines, IFN-gamma was the most potent, and T. gondii replication was completely inhibited at a concentration of 100 U/ml. The anti-toxoplasmotic activity of IFN-gamma was significantly blocked by monoclonal antibody to IFN-gamma. Treatment of the cultures with IFN-gamma from day 1 or 2 postinoculation with T. gondii also offered protection against the parasite. The anti-toxoplasmotic activity of tumor necrosis factor alpha or IFN-alpha, -beta, or -gamma in these cultures was found to be independent of the nitric oxide (NO) pathway, since NO production was not found in HRPE cells treated with these cytokines. However, addition of tryptophan to IFN-gamma-treated cells significantly reversed the inhibitory effects of IFN-gamma, suggesting that IFN-gamma acts by depleting cellular tryptophan. This effect was further confirmed by reverse transcription-PCR and Northern (RNA) blot analysis, which indicated induction of indoleamine 2,3-dioxygenase (IDO), an enzyme that converts tryptophan to kynurenine. These results indicated that interferons inhibited T. gondii replication in HRPE by NO-independent but IDO-dependent mechanisms. This in vitro model of T. gondii replication in HRPE may be useful in evaluating the effects of cytokines and drugs on T. gondii replication within the retina.

Nonionic block copolymers potentiate activities of drugs for treatment of infections with Toxoplasma gondii

We studied the interaction between drugs and the nonionic block copolymers CRL 8131 and CRL 8142 in the treatment of toxoplasmosis in murine models of the disease. Treatment of acute toxoplasmosis with copolymers alone caused slight prolongation of time to death but not survival. In contrast, significant survival occurred when mice were treated with either copolymer combined with doses of sulfadiazine, pyrimethamine, clindamycin, or atovaquone, which did not prevent mortality when used alone. Treatment with CRL 8131 plus sulfadiazine or pyrimethamine resulted in 50 or 40% survival, respectively. Treatment with the same copolymer plus a dose of clindamycin that protected 40% of the mice when used alone resulted in 100% survival. Treatment of toxoplasmic encephalitis with CRL 8131 plus an ineffective dose of atovaquone reduced the inflammation and numbers of Toxoplasma gondii cysts in the brain. Studies to investigate the drug-enhancing activity of CRL 8131 revealed that mice immunized with toxoplasma lysate plus copolymer had lymphocyte proliferation responses to T. gondii antigens significantly higher than those in mice immunized with lysate alone. Challenge of immunized mice with a lethal inoculum of T. gondii resulted in significant survival. Administration of CRL 8131 alone appeared to cause a down-regulation in the production of gamma interferon and up-regulation in the production of interleukin-2. No differences were noted in the production of tumor necrosis factor alpha between mice treated with CRL 8131 and controls.

Monoclonal rat antibodies directed against Toxoplasma gondii suitable for studying tachyzoite-bradyzoite interconversion in vivo

We previously reported the in vitro analysis of stage differentiation of Toxoplasma gondii in murine bone marrow-derived macrophages. The purpose of this study was to generate monoclonal rat antibodies that might be suitable for investigating tachyzoite-bradyzoite interconversion in vivo with the murine model. Immunization of Fischer rats with cysts of T. gondii NTE resulted in the generation of seven monoclonal antibodies of the immunoglobulin G2a, G2b, or M isotype, which were further characterized by the immunoblot technique, immunofluorescence assay, immunohistology, and immunoelectron microscopy. Immunoblots demonstrated specific reactivity of five monoclonal antibodies with proteins with molecular masses of 40, 52, 55, 60, 64, 65, and 115 kDa. One antibody (CC2) appeared to recognize a differently expressed antigen depending on the parasite stage, reacting with a 40-kDa molecule in tachyzoites and a 115-kDa antigen in bradyzoites and oocysts. Several other monoclonal antibodies were shown to be stage specific and to react in immunofluorescence assays or in immunoblots with either tachyzoites or bradyzoites. Kinetics of stage conversion in vitro could be monitored by immunofluorescence with two of these monoclonal antibodies. Preliminary immunohistological investigations of tissue sections from infected mice demonstrated the possible usefulness of these monoclonal antibodies for future in vivo studies on stage differentiation of T. gondii in the murine system.

Kinetics of interferon gamma production in vivo during infection with the S48 vaccine strain of Toxoplasma gondii

The kinetics of interferon gamma (IFN gamma) production in vivo was examined in sheep during a primary and secondary infection with the sheep vaccine strain (S48) of Toxoplasma gondii. Efferent lymph plasma from a node draining the site of inoculation was tested for anti-viral activity which could be neutralized with monoclonal antibodies against IFN gamma. Within 2 to 5 days of primary infection IFN gamma was detected in each of five sheep and persisted for 6 to 9 days. Accelerated production of IFN gamma occurred after secondary infection, the cytokine being detected in the first 24 h, and persisting in lymph for a further 4-5 days. From day 6 onwards after primary infection, efferent lymph cells produced IFN gamma when stimulated in vitro with a crude T. gondii antigen. These results show that IFN gamma is induced in sheep after infection with the S48 strain of T. gondii.

Sex-determined resistance to Toxoplasma gondii is associated with temporal differences in cytokine production

Examination of a wide range of inbred mice of diverse genetic backgrounds and major histocompatibility complex haplotypes revealed a dramatic difference in the susceptibilities of males and females to Toxoplasma gondii infection. Female mice were found to be more susceptible to acute infection, as determined by higher mortality levels, than male mice, while those female mice surviving to have chronic infections harbored more cysts in their brains than did surviving males. This phenomenon was therefore investigated in greater depth immunologically in the BALB/K mouse, a strain showing moderate susceptibility to infection with T. gondii. Plasma tumor necrosis factor alpha (TNF-alpha) levels were elevated in both male and female BALB/K mice on days 8 and 10 postinfection, but not thereafter, with males producing significantly higher levels than females. However, it was not until day 12 postinfection that the first deaths occurred, and these were among female mice, indicating that TNF-alpha production was not responsible for mortality. In vitro examination of T. gondii-specific T-cell proliferative responses from day 15 postinfection onwards revealed significantly higher stimulation indices in male mice than in their female counterparts. This difference was most apparent in splenocyte cultures initiated at day 15 postinfection, where complete suppression of proliferation was noted in the splenocytes from female mice but not from male mice. Analysis of tissue culture supernatants from these cultures revealed distinct differences in the kinetics of production as well as the quantities of gamma interferon (IFN-gamma) and interleukin 10 (IL-10) produced. Spleen cells from male mice produced higher levels of IFN-gamma in the early stages of infection than those from female mice. IFN-gamma levels were highest in the supernatants from male splenocyte cultures initiated at day 15 postinfection. Similar levels of IFN-gamma were not obtained from the supernatants of female splenocyte cultures until day 22 postinfection. IL-10 production, on the other hand, peaked at maximal levels in the cell cultures from both sexes initiated at day 22 postinfection. These results suggest that, in male mice, a rapid response to infection with high levels of TNF-alpha and IFN-gamma helps to control parasite multiplication, after which IL-10 production may be important in down regulating these potentially harmful inflammatory mediators. The failure of female mice to respond quickly in terms of T-cell proliferation and IFN-gamma production compared with their male counterparts may account for their poor survival rates and higher cyst burdens.

Toxoplasma gondii-specific CD4+ T cell clones from healthy, latently infected humans display a Th0 profile of cytokine secretion

Human Toxoplasma gondii (Tg)-specific T cell clones were raised by infecting peripheral blood mononuclear cells (MNC) from two healthy, latently infected individuals with Tg trophozoites. All of the clones had a CD4+ immunophenotype and produced simultaneously interleukin (IL)-2, interferon (IFN)-gamma, IL-4 and IL-5 upon mitogen or antigen stimulation. Tg-specific T cell clones were classified as T helper of type 0 (Th0) since most of them released roughly comparable amounts of IFN-gamma and IL-4. In some clones, a trend to an increased production of IFN-gamma following antigen-specific as compared to non-specific stimulation was observed. The Th0 phenotype was also expressed by T cell clones that had been raised from bulk cultures performed in the presence of IL-4 or IFN-gamma. All of the Tg-specific T cell clones were cytolytic in a non-specific assay which involves the triggering of the CD3-T cell receptor (TcR) complex. Some clones specifically lysed an autologous lymphoblastoid cell line (LCL) that had been infected with Tg trophozoites. Finally, most of the Tg-specific T cell clones produced IL-10, irrespective of whether they had been raised from bulk cultures incubated in the presence or absence of IL-4 or IFN-gamma. Taken together, these findings suggest that Tg-specific Th0 helper cell clones from healthy, latently infected individuals, beside activating toxoplasmacidal mechanisms through IFN-gamma release, might limit the magnitude of the immune response of the parasite by killing Tg-infected antigen-presenting cells and by releasing IL-10.

A commercial vaccine for ovine toxoplasmosis

This paper describes the development of the first commercial vaccine for toxoplasmosis. The vaccine comprises live tachyzoites of the S48 'incomplete' strain of Toxoplasma gondii and is deployed to control toxoplasma abortion in sheep. A discussion of protective immune mechanisms and recent studies on host responses to the vaccine is also included.