Regulation and function of T-cell-mediated immunity during Toxoplasma gondii infection

Early response of mucosal epithelial cells during Toxoplasma gondii infection

The innate immune response of mucosal epithelial cells during pathogen invasion plays a central role in immune regulation in the gut. Toxoplasma gondii is a protozoan intracellular parasite that is usually transmitted through oral infection. Although much of the information on immunity to T. gondii has come from i.p. infection models, more recent studies have revealed the importance of studying immunity following infection through the natural peroral route. Oral infection studies have identified many of the key players in the intestinal response; however, they have relied on responses detected days to weeks following infection. Much less is known about how the gut epithelial layer senses and reacts during initial contact with the pathogen. Given the importance of epithelial cells during pathogen invasion, this study uses an in vitro approach to isolate the key players and examine the early response of intestinal epithelial cells during infection by T. gondii. We show that human intestinal epithelial cells infected with T. gondii elicit rapid MAPK phosphorylation, NF-kappaB nuclear translocation, and secretion of IL-8. Both ERK1/2 activation and IL-8 secretion responses were shown to be MyD88 dependent and TLR2 was identified to be involved in the recognition of the parasite regardless of the parasite genotype. Furthermore, we were able to identify additional T. gondii-regulated genes in the infected cells using a pathway-focused array. Together, our findings suggest that intestinal epithelial cells were able to recognize T. gondii during infection, and the outcome is important for modulating intestinal immune responses.

Toll-like receptor initiated host defense against Toxoplasma gondii

Toxoplasma gondii is an intracellular pathogen notable for its ability to establish a stable host-parasite relationship amongst a wide range of host species and in a large percentage of the human population. Toll-like receptor signaling through MyD88 is a critical pathway in initiating defense against this opportunistic protozoan and may also be a mediator of pathology during immune dysfunction. Other MyD88 independent signaling pathways are also involved in the host-parasite interaction. These responses can be triggered by the parasite itself, but interactions with the intestinal microbiota add additional complexity during enteric infection.

T cell-intrinsic role of Nod2 in promoting type 1 immunity to Toxoplasma gondii

Nod2 belongs to the (NOD)-like receptor (NLR) family of proteins, which function as intracellular pathogen sensors in innate immune cells. Nod2-deficiency results in an impaired immune response against bacterial pathogens. However, our understanding of how this protein promotes host defense against intracellular parasites is unknown. Here we found that Nod2^−/− mice showed reduced clearance of Toxoplasma gondii and decreased interferon-γ production. Reconstitution of T-cell deficient mice with Nod2^−/− T cells followed by T. gondii infection revealed a T cell-intrinsic defect. Nod2^−/− CD4⁺ T cells displayed poor helper T cell differentiation, which was associated with impaired IL-2 production and nuclear accumulation of c-Rel. These data revealed a T cell-intrinsic role of Nod2 signaling that is critical for host defense against T. gondii.

Impairment of gamma interferon signaling in human neutrophils infected with Anaplasma phagocytophilum

Anaplasma phagocytophilum, the causative agent of tick-borne human granulocytic anaplasmosis (HGA), is an intracellular bacterium which survives and multiplies inside polymorphonuclear neutrophil granulocytes (PMN). Increased bacterial burden in gamma interferon (IFN-gamma)-deficient mice suggested a major role of IFN-gamma in the control of A. phagocytophilum. Here we investigated whether infection of human PMN with A. phagocytophilum impairs IFN-gamma signaling thus facilitating intracellular survival of the bacterium. The secretion of the IFN-gamma-inducible chemokines IP-10/CXCL10 and MIG/CXCL9 was markedly inhibited in infected neutrophils. Molecular analyses revealed that, compared to uninfected PMN, A. phagocytophilum decreased the expression of the IFN-gamma receptor alpha-chain CD119, diminished the IFN-gamma-induced phosphorylation of STAT1, and enhanced the expression of SOCS1 and SOCS3 in PMN. Since IFN-gamma activates various antibacterial effector mechanisms of PMN, the impaired IFN-gamma signaling in infected cells likely contributes to the survival of A. phagocytophilum inside PMN and to HGA disease development.

Dynamics of T cell, antigen-presenting cell, and pathogen interactions during recall responses in the lymph node

Memory T cells circulate through lymph nodes where they are poised to respond rapidly upon re-exposure to a pathogen; however, the dynamics of memory T cell, antigen-presenting cell, and pathogen interactions during recall responses are largely unknown. We used a mouse model of infection with the intracellular protozoan parasite, Toxoplasma gondii, in conjunction with two-photon microscopy, to address this question. After challenge, memory T cells migrated more rapidly than naive T cells, relocalized toward the subcapsular sinus (SCS) near invaded macrophages, and engaged in prolonged interactions with infected cells. Parasite invasion of T cells occurred by direct transfer of the parasite from the target cell into the T cell and corresponded to an antigen-specific increase in the rate of T cell invasion. Our results provide insight into cellular interactions during recall responses and suggest a mechanism of pathogen subversion of the immune response.

Protective effect of DNA-mediated immunization with liposome-encapsulated GRA4 against infection of Toxoplasma gondii

The dense granule protein 4 (GRA4) is a granular protein from Toxoplasma gondii, and is a candidate for vaccination against this parasite. In this study, the plasmid pcDNA3.1-GRA4 (pGRA4), encoding for the GRA4 antigen, was incorporated by the dehydration-rehydration method into liposomes composed of 16 mmol/L egg phosphatidylcholine (PC), 8 mmol/L dioleoyl phosphatidylethanolamine (DOPE), and 4 mmol/L 1,2-diodeoyl-3-(trimethylammonium) propane (DOTAP). C57BL/6 mice and BALB/c mice were immunized intramuscularly three times with liposome-encapsulated pGRA4 to determine whether DNA immunization could elicit a protective immune response to T. gondii. Enzyme-linked immunosorbent assay (ELISA) of sera from immunized mice showed that liposome-encapsulated pGRA4 generated high levels of IgG antibodies to GRA4. Production of primary interferon (IFN)-gamma and interleukin (IL)-2 in GRA4-stimulated splenocytes from vaccinated mice suggested a modulated Th1-type response. 72.7% of C57BL/6 mice immunized with liposome-encapsulated pGRA4 survived the challenge with 80 tissue cysts of ME49 strain, whereas C57BL/6 mice immunized with pGRA4 had only a survival rate of 54.5%. When immunized BALB/c mice were intraperitoneally challenged with 10(3) tachyzoites of the highly virulent RH strain, the survival time of mice immunized with liposome-encapsulated pGRA4 was markedly longer than that of other groups. Our observations show that liposome-encapsulated pGRA4 enhanced the protective effect against infection of T. gondii.

Apoptosis in Toxoplasma gondii activated T cells: the role of IFNgamma in enhanced alteration of Bcl-2 expression and mitochondrial membrane potential

In the present study we addressed the question whether Toxoplasma gondii could promote apoptosis in T lymphocytes in the acute stage of infection. Using in vivo activated T cells and then culturing them for a short time, we observed activation-induced cell death in T. gondii infected mice. A higher level of activation-induced cell death (AICD) was seen in susceptible C57BL/6 mice than in resistant CBA/J mice following infection with the same P strain of parasite. Apoptosis in T cells of susceptible mice was associated with altered induction of Bcl-2/Bax, loss of Mitochondrial Transmembrane Potential. Both CD4+ and CD8+ T cells were found to be susceptible to apoptosis; CD4+ T cells were sensitive to Fas-mediated death whereas CD8+ T cells were insensitive to this signal. Caspase inhibitors had less effect on DNA fragmentation in CD4+ compared to CD8+ T cells. Exposure of CD4+ T cells to anti-IFNgamma mAb resulted in an increase in the number of T cells that were positive for anti-apoptotic molecule Bcl-2 and DiOC6, a cationic dye that accumulates in intact mitochondria. These changes were less noticeable in CD8+ T cells following treatment with anti-IFNgamma mAb. These findings provide further insight into the mechanisms of T cell apoptosis in T. gondii infection.

Is Toxoplasma gondii a causal agent in migraine?

BACKGROUND

Many different tissues may be parasitized by Toxoplasma gondii, particularly, lung, heart, lymphoid organs, and the central nervous tissues. Tissue cysts of this parasite in the brain may spontaneously rupture, releasing parasites that cause antibody titers to rise. In immunocompetent subjects with acquired toxoplasmosis, the most frequent symptoms were lymphadenopathy and headache. In the neurogenic inflammation theory of the pathogenesis of migraine, the cause of initial release of ions and inflammatory agents has not been established. In this study, we aimed to investigate if T. gondii infection is a possible cause of neurogenic inflammation of migraines.

METHODS

The anti-T. gondii antibody status of 104 patients with migraine were studied and compared with those of control groups, 50 healthy subjects and 50 subjects with headache due to rhinosinusitis, by using a micro-enzyme-linked immunosorbent assay technique.

RESULTS

Forty-six (44.2%) patients with migraine, 13 (26.0%) healthy control subjects, and 12 (24%) control subjects with rhinosinusitis were positive for anti-T. gondii IgG antibody. The rate of positivity in the migraine patient group was statistically different from those of the control groups (P < 0.05).

CONCLUSIONS

The results show the presence of chronic Toxoplasma infection in patients with migraine. Toxoplasma infection may contribute to neurogenic inflammation as the pathogenesis of migraine, as many studies in the literature have reported that Toxoplasma infection causes biochemical and immunologic changes.

Pathogen recognition and inflammatory signaling in innate immune defenses

The innate immune system constitutes the first line of defense against invading microbial pathogens and relies on a large family of pattern recognition receptors (PRRs), which detect distinct evolutionarily conserved structures on pathogens, termed pathogen-associated molecular patterns (PAMPs). Among the PRRs, the Toll-like receptors have been studied most extensively. Upon PAMP engagement, PRRs trigger intracellular signaling cascades ultimately culminating in the expression of a variety of proinflammatory molecules, which together orchestrate the early host response to infection, and also is a prerequisite for the subsequent activation and shaping of adaptive immunity. In order to avoid immunopathology, this system is tightly regulated by a number of endogenous molecules that limit the magnitude and duration of the inflammatory response. Moreover, pathogenic microbes have developed sophisticated molecular strategies to subvert host defenses by interfering with molecules involved in inflammatory signaling. This review presents current knowledge on pathogen recognition through different families of PRRs and the increasingly complex signaling pathways responsible for activation of an inflammatory and antimicrobial response. Moreover, medical implications are discussed, including the role of PRRs in primary immunodeficiencies and in the pathogenesis of infectious and autoimmune diseases, as well as the possibilities for translation into clinical and therapeutic applications.

Improvement of a dendritic cell-based therapeutic cancer vaccine with components of Toxoplasma gondii

The use of dendritic cells (DCs) as a cellular adjuvant is a promising approach to the immunotherapy of cancer. It has previously been demonstrated that DCs pulsed ex vivo with Toxoplasma gondii antigens trigger a systemic Th1-biased specific immune response and induce protective and specific antitoxoplasma immunity. In the present study, we demonstrate that tumor antigen-pulsed DCs matured in the presence of Toxoplasma gondii components induce a potent antitumor response in a mouse model of fibrosarcoma. Bone-marrow derived DCs (BMDCs) were cultured in the presence of granulocyte-macrophage colony-stimulating factor and interleukin-4. After 5 days, tumor lysates with or without the T. gondii lysate were added to the culture for another 2 days. The cytokine production in the BMDC culture and the coculture supernatants of DCs and splenic cells was evaluated. For immunization, 7 days after tumor challenge, different groups of BALB/c mice received different kinds of DCs subcutaneously around the tumor site. Tumor growth was monitored, and 2 weeks after DC immunotherapy, the cytotoxic activity and the infiltration of CD8(+) T cells were monitored in different groups. According to the findings, immunotherapy with T. gondii-matured DCs led to a significant increase in the activity of cytotoxic T cells and decreased the rate of growth of the tumor in immunized animals. Immature DCs did not cause any change in cytotoxic activity or the tumor growth rate compared to that in the healthy controls. The current study suggests that a specific antitumor immune response can be induced by DCs matured with T. gondii components and provide the basis for the use of T. gondii in DC-targeted clinical therapies.

A transmembrane domain-containing surface protein from Toxoplasma gondii augments replication in activated immune cells and establishment of a chronic infection

Toxoplasma gondii mutants identified as defective in the establishment of chronic infection were screened to isolate those specifically impaired in their ability to replicate within activated macrophages. One of the identified mutants contains an insertion in the hypothetical gene TGME49_111670. Genetic complementation restores the ability of the mutant to replicate in immune cells and produce cysts in the brains of mice. While the mutant is more sensitive to nitric oxide than is its parental strain, it is not defective in its ability to suppress nitric oxide. The disrupted protein has no significant homology to proteins with known functions, but is predicted to have one transmembrane domain. Immunofluorescence shows the protein on the parasite surface, even in activated macrophages, colocalizing with a tachyzoite surface antigen, SAG1, and oriented with its C-terminal end external. Western analysis reveals that the protein is downregulated in bradyzoites. Despite the tachyzoite specificity of this protein, mice infected with the mutant succumb to acute infection similarly to those infected with the parent strain. Serum samples from mice with chronic T. gondii infection react to a polypeptide from TGME49_11670, indicating that the protein is seen by the immune system during infection. This study is the first to characterize a T. gondii surface protein that contains a transmembrane domain and show that the protein contributes to parasite replication in activated immune cells and the establishment of chronic infection.

Dynamic Imaging of CD8(+) T cells and dendritic cells during infection with Toxoplasma gondii

To better understand the initiation of CD8⁺ T cell responses during infection, the primary response to the intracellular parasite Toxoplasma gondii was characterized using 2-photon microscopy combined with an experimental system that allowed visualization of dendritic cells (DCs) and parasite specific CD8⁺ T cells. Infection with T. gondii induced localization of both these populations to the sub-capsular/interfollicular region of the draining lymph node and DCs were required for the expansion of the T cells. Consistent with current models, in the presence of cognate antigen, the average velocity of CD8⁺ T cells decreased. Unexpectedly, infection also resulted in modulation of the behavior of non-parasite specific T cells. This TCR-independent process correlated with the re-modeling of the lymph node micro-architecture and changes in expression of CCL21 and CCL3. Infection also resulted in sustained interactions between the DCs and CD8⁺ T cells that were visualized only in the presence of cognate antigen and were limited to an early phase in the response. Infected DCs were rare within the lymph node during this time frame; however, DCs presenting the cognate antigen were detected. Together, these data provide novel insights into the earliest interaction between DCs and CD8⁺ T cells and suggest that cross presentation by bystander DCs rather than infected DCs is an important route of antigen presentation during toxoplasmosis.

Toxoplasma gondii is a protozoan parasite that can infect a wide range of hosts, including humans. Infection with T. gondii is potentially life threatening in immuno-compromised individuals and it can be detrimental during pregnancy, often leading to abortion of the fetus. Dendritic cells are thought to play a vital role in the development of protective immunity to Toxoplasma gondii through their ability to produce immunological signals such as cytokines and also process and present parasite derived peptides to T cells. However, little is known about the actual interactions between these cell types in an intact organ, such as the lymph node, during infection. Using the technology of live imaging by 2-photon microscopy we have identified a very early window of time during infection when dendritic cells and T cells make sustained contacts with one another, which appears crucial for the generation of protective responses. We also show that substantial changes are induced in the lymph node micro-architecture as a result of infection, which in turn could have effects on immune responses to secondary pathogens. Understanding the interaction between these immune cells in vivo that leads to resistance to active infection would help in the design of better strategies to develop protective immune responses against this pathogen in immuno-compromised individuals.

Toxoplasma gondii cyclophilin 18-mediated production of nitric oxide induces Bradyzoite conversion in a CCR5-dependent manner

Toxoplasma gondii modulates pro- and anti-inflammatory responses to regulate parasite multiplication and host survival. Pressure from the immune response causes the conversion of tachyzoites into slowly dividing bradyzoites. The regulatory mechanisms involved in this switch are poorly understood. The aim of this study was to investigate the immunomodulatory role of T. gondii cyclophilin 18 (TgCyp18) in macrophages and the consequences of the cellular responses on the conversion machinery. Recombinant TgCyp18 induced the production of nitric oxide (NO), interleukin-12 (IL-12), and tumor necrosis factor alpha through its binding with cysteine-cysteine chemokine receptor 5 (CCR5) and the production of gamma interferon and IL-6 in a CCR5-independent manner. Interestingly, the treatment of macrophages with TgCyp18 resulted in the inhibition of parasite growth and an enhancement of the conversion into bradyzoites via NO in a CCR5-dependent manner. In conclusion, T. gondii possesses sophisticated mechanisms to manipulate host cell responses in a TgCyp18-mediated process.

Toxoplasma gondii: the role of IFN-gamma, TNFRp55 and iNOS in inflammatory changes during infection

In order to examine the role of IFN-gamma, TNFRp55 and iNOS in inflammatory reaction during toxoplasmosis, IFN-gamma(-/-), TNFRp55(-/-) and iNOS(-/-) mice were experimentally infected with Toxoplasma gondii ME-49 strain. The organs of the mice were evaluated for histology and immunohistochemistry in detection of tissue parasitism and iNOS positive cells. IFN-gamma(-/-) mice presented mild inflammation in peripheral organs associated with a high parasitism and mortality in the acute phase of infection. In contrast, the peripheral organs of WT, TNFRp55(-/-) and iNOS(-/-) mice, presented a significant inflammatory reaction and low tissue parasitism in the same period of infection. The inflammatory lesions and tissue parasitism were increased and more severe in the Central Nervous System (CNS) of TNFRp55(-/-) and iNOS(-/-) with a progression of infection, when compared to WT mice. In these knockout animals, the inflammatory changes were associated with low levels or no expression of iNOS in TNFRp55(-/-) and iNOS(-/-) mice, respectively.

Clinical features of ocular toxoplasmosis during pregnancy

PURPOSE

To evaluate the ocular characteristics of active ocular toxoplasmosis (OT) during pregnancy in nine female patients and to compare these attacks with those in the nonpregnant periods.

METHODS

Retrospective analysis of active OT during pregnancy and the nonpregnant periods. Outcome measurements consisted of the duration of attacks, the size of the retinal lesions, treatment strategies, and diverse ocular features and outcomes.

RESULTS

The 9 patients had 10 attacks during pregnancy and 24 attacks while not pregnant. One woman had recurrences during several pregnancies, and in total, three female patients had attacks only when pregnant. In general, the severity of the attacks during the pregnant and nonpregnant periods did not differ.

CONCLUSION

The attacks during pregnancy were not distinctively different in severity, duration, or outcome from the OT attacks outside pregnancy.

Dynamic imaging of T cell-parasite interactions in the brains of mice chronically infected with Toxoplasma gondii

The intracellular parasite Toxoplasma gondii can establish persistent infection in the brain of a mammalian host, a standoff that involves the active participation of host CD8 T cells to control infection. CD8 T cells generally protect against intracellular pathogens by local delivery of effector molecules upon recognition of specific pathogen Ags on invaded host cells. However, the interactions between CD8 T cells, T. gondii, and APCs in the brain have not yet been examined. In this study we have used a mouse infection model in conjunction with two-photon microscopy of living brain tissue and confocal microscopy of fixed brain sections to examine the interactions between CD8 T cells, parasites, and APCs from chronically infected mice. We found that Ag-specific CD8 T cells were recruited to the brains of infected mice and persisted there in the presence of ongoing Ag recognition. Cerebral CD8 T cells made transient contacts with granuloma-like structures containing parasites and with individual CD11b(+) APCs, including some that did not contain parasites. In contrast, T cells ignored intact Ag-bearing cysts and did not contact astrocytes or neurons, including neurons containing parasites or cysts. Our data represent the first direct observation of the dynamics of T cell-parasite interactions within living tissue and provide a new perspective for understanding immune responses to persistent pathogens in the brain.

Toxoplasma gondii: effects of Artemisia annua L. on susceptibility to infection in experimental models in vitro and in vivo

Considering that the treatment for toxoplasmosis is based on drugs that show limited efficacy due to their substantial side effects, the purpose of the present study was to evaluate the effects of Artemisia annua on in vitro and in vivo Toxoplasma gondii infection. A. annua infusion was prepared from dried herb and tested in human foreskin fibroblasts (HFF) or mice that were infected with the parasite and compared with sulfadiazine treatment. For in vitro experiments, treatment was done on parasite before HFF infection or on cells previously infected with T. gondii and the inhibitory concentration (IC(50)) values for each treatment condition were determined. Viability of HFF cells in the presence of different concentrations of A. annua infusion and sulfadiazine was above 72%, even when the highest concentrations from both treatments were tested. Also, the treatment of T. gondii tachyzoites with A. annua infusion before infection in HFF cells showed a dose-response inhibitory curve that reached up to 75% of inhibition, similarly to the results observed when parasites were treated with sulfadiazine. In vivo experiments with a cystogenic T. gondii strain demonstrated an effective control of infection using A. annua infusion. In conclusion, our results indicate that A. annua infusion is useful to control T. gondii infection, due to its low toxicity and its inhibitory action directly against the parasite, resulting in a well tolerated therapeutic tool.

Anti-CD25 antibody-mediated depletion of effector T cell populations enhances susceptibility of mice to acute but not chronic Toxoplasma gondii infection

Natural regulatory T cells (Tregs) constitutively express the IL-2R alpha-chain (CD25) on their surface. Consequently, administration of anti-CD25 Abs is a commonly used technique to deplete Treg populations in vivo. However, activated effector T cells may also transiently express CD25, and are thus also potential targets for anti-CD25 Abs. In this study using Toxoplasma gondii as a model proinflammatory infection, we have examined the capacity of anti-CD25 Abs to target effector T cell populations during an inflammatory episode, to determine to what extent that this action may modulate the outcome of disease. Anti-CD25 Ab-treated C57BL/6 mice displayed significantly reduced CD4(+) T cell IFN-gamma production during acute T. gondii infection and exhibited reduced weight loss and liver pathology during early acute infection; aspects of infection previously associated with effector CD4(+) T cell responses. In agreement, anti-CD25 Ab administration impaired parasite control and caused mice to succumb to infection during late acute/early chronic stages of infection with elevated tissue parasite burdens. In contrast, anti-CD25 Ab treatment of mice with established chronic infections did not markedly affect brain parasite burdens, suggesting that protective T cell populations do not express CD25 during chronic stages of T. gondii infection. In summary, we have demonstrated that anti-CD25 Abs may directly abrogate effector T cell responses during an inflammatory episode, highlighting important limitations of the use of anti-CD25 Ab administration to examine Treg function during inflammatory settings.

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.

Induction of partial protection against infection with Toxoplasma gondii genotype II by DNA vaccination with recombinant chimeric tachyzoite antigens

Infection with the obligate intracellular parasite Toxoplasma gondii is a significant source of parasitic infections worldwide. In adults, infections may often lead to severe retinochoroiditis. Infection of the foetus causes abortion or congenital pathology that may lead to neurological complications. Although several strategies have been suggested for making a vaccine, none is currently available. Here, we investigate the protection conferred by DNA vaccination with two constructs, pcEC2 (MIC2-MIC3-SAG1) and pcEC3 (GRA3-GRA7-M2AP), encoding chimeric proteins containing multiple antigenic sequences from T. gondii. After challenge with a T. gondii genotype II, but not a genotype III strain, a significant decrease in cerebral cyst load was found compared to the controls. The immune protection involved a cell-mediated immune response with the synthesis of the cytokines IFN-? and IL-10. In silico structure analysis and the expression profile of EC2, suggest an association between antigen stability, the degree of protein secondary structure and induction of cellular immune responses. Intracellular protein degradation is an important step in the pathway leading to presentation of antigenic peptides on Major Histocompatibility Complex molecules. We suggest that degradation of this chimeric protein may have contributed to the induction of a cellular immune response via enhanced presentation of antigenic peptides on Major Histocompatibility Complex class I molecules.

Toxoplasma gondii prevents chromatin remodeling initiated by TLR-triggered macrophage activation

Macrophages infected with the opportunistic protozoan Toxoplasma gondii are unable to up-regulate many proinflammatory cytokine genes, including TNF (TNF-alpha), upon stimulation with LPS and other TLR ligands. In this study, we examined the influence of T. gondii on transcription factors associated with TNF-alpha transcription, as well as phosphorylation and acetylation of histone H3 at distal and proximal regions of the TNF-alpha promoter. During LPS stimulation, we found that Toxoplasma blocks nuclear accumulation of transcription factor c-Jun, but not that of cAMP response element-binding protein or NF-kappaB. However, chromatin immunoprecipitation studies revealed that binding of all of these transcription factors to the TNF promoter was decreased by T. gondii infection. Furthermore, the parasite blocked LPS-induced Ser(10) phosphorylation and Lys(9)/Lys(14) acetylation of histone H3 molecules associated with distal and proximal regions of the TNF-alpha promoter. Our results show that Toxoplasma inhibits TNF-alpha transcription by interfering with chromatin remodeling events required for transcriptional activation at the TNF promoter, revealing a new mechanism by which a eukaryotic pathogen incapacitates proinflammatory cytokine production during infection.

Dendritic cells expressing plasmacytoid marker PDCA-1 are Trojan horses during Toxoplasma gondii infection

Plasmacytoid dendritic cells (pDCs) play a key role in the innate immune response to viral infection, due largely to their ability to produce large quantities of type I IFNs. These cells are also notable for their ability to differentiate into conventional dendritic cells after appropriate stimulation. Here, we show that a splenic population of murine CD11c(+) cells expressing pDC markers Gr-1, B220, and PDCA-1 is preferentially parasitized after infection with the virulent RH strain of Toxoplasma gondii. Although these markers are closely associated with pDCs, the population we identified was unusual because the cells express CD11b and higher than expected levels of CD11c. By adoptive transfer of CD45.1-positive cells into CD45.2 congenic mice, we show that CD11c(+)Gr-1(+) cells migrate from the peritoneal cavity to the spleen. During infection, these cells accumulate in the marginal zone region. Recruitment of infected CD11c(+)Gr-1(+) cells to the spleen is partially dependent upon signaling through chemokine receptor CCR2. Intracellular cytokine staining demonstrates that infected, but not noninfected, splenic CD11c(+)Gr-1(+) dendritic cells are suppressed in their ability to respond to ex vivo TLR stimulation. We hypothesize that Toxoplasma exploits pDCs as Trojan horses, targeting them for early infection, suppressing their cytokine effector function, and using them for dissemination within the host.

Functional aspects of Toll-like receptor/MyD88 signalling during protozoan infection: focus on Toxoplasma gondii

Toll-like receptor (TLR)/MyD88 signalling has emerged as a major pathway of pathogen recognition in the innate immune system. Here, we review recent data that begin to show how this pathway controls the immune response to protozoan infection, with particular emphasis on the opportunistic pathogen Toxoplasma gondii. The various ways that the parasite activates and suppresses TLR/MyD88 signalling defines several key principals that illuminate the complexities of the host-pathogen interaction. We also speculate how TLR/MyD88 signalling might be exploited to provide protection against Toxoplasma, as well as other protozoa and infection in general.

Parasite stage-specific recognition of endogenous Toxoplasma gondii-derived CD8+ T cell epitopes

BACKGROUND

BALB/c mice control infection with the obligate intracellular parasite Toxoplasma gondii and develop a latent chronic infection in the brain, as do immunocompetent humans. Interferon-gamma-producing CD8+ T cells provide essential protection against T. gondii infection, but the epitopes recognized have so far remained elusive.

METHODS

We employed caged major histocompatibility complex molecules to generate approximately 250 H-2L(d) tetramers and to distinguish T. gondii-specific CD8+ T cells in BALB/c mice.

RESULTS

We identified 2 T. gondii-specific H-2L(d)-restricted T cell epitopes, one from dense granule protein GRA4 and the other from rhoptry protein ROP7. H-2L(d)/GRA4 reactive T cells from multiple organ sources predominated 2 weeks after infection, while the reactivity of the H-2L(d)/ROP7 T cells peaked 6-8 weeks after infection. BALB/c animals infected with T. gondii mutants defective in establishing a chronic infection showed altered levels of antigen-specific T cells, depending on the T. gondii mutant used.

CONCLUSIONS

Our results shed light on the identity and the parasite stage-specificity of 2 CD8+ T cell epitopes recognized in the acute and chronic phase of infection with T. gondii.

TLR adaptor MyD88 is essential for pathogen control during oral toxoplasma gondii infection but not adaptive immunity induced by a vaccine strain of the parasite

TLR adaptor MyD88 activation is important in host resistance to Toxoplasma gondii during i.p. infection, but the function of this signaling pathway during oral infection, in which mucosal immunity assumes a predominant role, has not been examined. In this study, we show that MyD88(-/-) mice fail to control the parasite and succumb within 2 wk of oral infection. Early during infection, T cell IFN-gamma production, recruitment of neutrophils and induction of p47 GTPase IGTP (Irgm3) in the intestinal mucosa were dependent upon functional MyD88. Unexpectedly, these responses were MyD88-independent later during acute infection. In particular, CD4(+) T cell IFN-gamma reached normal levels independently of MyD88, despite continued absence of IL-12 in these animals. The i.p. vaccination of MyD88(-/-) mice with an avirulent T. gondii uracil auxotroph elicited robust IFN-gamma responses and protective immunity to challenge with a high virulence T. gondii strain. Our results demonstrate that MyD88 is required to control Toxoplasma infection, but that the parasite can trigger adaptive immunity without the need for this TLR adaptor molecule.

The role of MHC haplotypes H2d/H2b in mouse resistance/susceptibility to cyst formation is influenced by the lineage of infective Toxoplasma gondii strain

Toxoplasma gondii strains displaying the Type I/III genotype are associated with acquired ocular toxoplasmosis in humans. Here, we used a mice model to characterize some immunological mechanisms involved in host resistance to infection with such strains. We have chosen the Type I/III strains D8, G2 and P-Br, which cause a chronic infection in mice that resembles human toxoplamosis. Mice deficient of molecules MyD88, IFN-gamma, and IL-12 were susceptible to all three parasite strains. This finding indicates the importance of innate mechanisms in controlling infection. On the other hand, MHC haplotype did not influenced resistance/susceptibility; since mice lineages displaying a same genetic background but different MHC haplotypes (H2b or H2d) developed similar mortality and cyst numbers after infection with those strains. In contrast, the C57BL/6 genetic background, and not MHC haplotype, was critical for development of intestinal inflammation caused by any of the studied strains. Finally, regarding effector mechanisms, we observed that B and CD8+ T lymphocytes controlled survival,whereas the inducible nitric oxide synthase influenced cyst numbers in brains of mice infected with Type I/III strains. These findings are relevant to further understanding of the immunologic mechanisms involved in host protection and pathogenesis during infection with T. gondii.

Human visceral leishmaniasis expresses Th1 pattern in situ liver lesions

The architectural and infiltrate pattern of liver human visceral leishmaniasis (HVL) have been systematically classified as typical, fibrogenic or nodular. Despite this histopathological classification, the immune response based on cytokines and cellular phenotypes have never been performed. The aim of this study was to determine the immunophenotypic pattern and cytokine profile of the nodular involvement of the liver in HVL. We evaluated nine cases of the nodular form of HVL. In situ immune response was studied through cytokine analysis and immunohistochemical study for phenotype markers: IL-1, IL-4, IL-10, TNF-alpha, IFN-gamma, CD4+ T cells, CD8+ T cells, CD20, CD68, CD57 and macrophage activation was determined by evaluation of iNOS activity. HVL seems to be related to a better immune response. Amastigotes were rarely found on liver sections. Leishmania antigen expression was also rare and located in the inflammatory nodules. The lower expression of IL-4 and IL-10, moderate expression of TNF-alpha and IFN-gamma demonstrate a panorama of Th1 phenotype. The increased expression of NK cells could help in sustaining this model of response. This pattern of immune response is probably responsible for improvement in the parasite's clearance from liver tissue and it is a prognostic marker of human visceral leishmaniasis.

CCR2 receptor is essential to activate microbicidal mechanisms to control Toxoplasma gondii infection in the central nervous system

Chemokines comprise a structurally related family of cytokines that regulate leukocyte trafficking. Because infection with Toxoplasma gondii can induce an important inflammatory reaction that, if left uncontrolled, can lead to death, we investigated the role of the chemokine receptor CCR2 in T. gondii infection. We orally infected CCR2(-/-) mice with five ME-49 T. gondii cysts and monitored morbidity, survival, and immune response thereafter. The CCR2(-/-) mice displayed higher susceptibility to infection as all mice died on day 28 after infection. Despite similar Th1 responses, a more evident anti-inflammatory response was induced in the peripheral organs of CCR2(-/-) mice compared with wild-type C57BL/6 mice. Additionally, CCR2(-/-) mice presented greater parasitism and a milder inflammatory reaction in their peripheral organs with lesser CD4(+) and MAC-1(+) and greater CD8(+) cell migration. The parasite load decreased in these organs in CCR2(-/-) mice but remained uncontrolled in the central nervous system. Additionally, we observed down-regulated inducible nitric oxide synthase expression in peripheral organs from CCR2(-/-) mice that was associated with a small nitric oxide production by spleen macrophages. In conclusion, in the absence of CCR2, another mechanism is activated to control tissue parasitism in peripheral organs. Nevertheless, CCR2 is essential for the activation of microbicidal mediators that control T. gondii replication in the central nervous system.

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.

Cellular and molecular physiopathology of congenital toxoplasmosis: the dual role of IFN-gamma

Toxoplasma gondii is one of the few pathogens that can cross the placenta. Frequency and severity of transmission vary with gestational age. While the control of acquired toxoplasmosis is already well explored, the control of materno-foetal transmission of the parasite remains almost unknown. This is partly due to the lack of an animal model to study this process. This review summarises the studies which have been undertaken and shows that the mouse is a valuable model despite obvious differences to the human case. The paramount role of the cellular immune response has been shown by several experiments. However, IFN-gamma has a dual role in this process. While its beneficial effects in the control of toxoplasmosis are well known, it also seems to have transmission-enhancing effects and can also directly harm the developing foetus. The ultimate goal of these studies is to develop a vaccine which protects both mother and foetus. Therefore, it is useful to study the mechanisms of natural resistance against transmission during a secondary infection. In this setting, the process is more complicated, involving both cellular and also humoral components of the immune system. In summary, even if the whole process is far from being elucidated, important insights have been gained so far which will help us to undertake rational vaccine research.

Gene expression profile of Th1 and Th2 cytokines and their receptors in human and nonhuman primates

BACKGROUND

To date comparative knowledge concerning gene expression profiles of T-helper 1(Th1)/Th2 cytokines and their receptors between human and non-human primates is scarce.

METHODS

We assessed the gene expression level of both Th1 [interleukin-4(IL-4)] and Th2 [IL-12, interferon-gamma(IFN-gamma)] cytokines and the receptors (IL-4Ralpha, IFN-gammaR1, IFN-gammaR2) in peripheral blood mononuclear cells (PBMC) from humans, chimpanzee, baboon, and macaque by a quantitative real-time reverse transcriptase-polymerase chain reaction(RT-PCR).

RESULTS

The expression level of the IFN-gamma gene was markedly lower in humans than that in non-human primates. The IL-4 gene expression was significantly higher, whereas that of IL-12 was distinctly lower, in human/chimpanzee than in baboon/macaque. The IFN-gammaR2 gene expression was especially higher in the macaque than in the other three primates.

CONCLUSIONS

These results indicate distinct gene expression of Th1/Th2 cytokines and their receptors in primates. These also suggest characteristic differences in Th1/Th2 immune responses affecting host defense and/or disease susceptibility among these primates.

Fas/CD95-mediated apoptosis of type II cells is blocked by Toxoplasma gondii primarily via interference with the mitochondrial amplification loop

The intracellular protozoan Toxoplasma gondii induces persistent infections in various hosts and is an important opportunistic pathogen of humans with immature or deficient immune responses. The ability to survive intracellularly largely depends on the blocking of different proapoptotic signaling cascades of its host cell. Fas/CD95 triggers an apoptotic cascade that is crucial for immunity and the outcome of infectious diseases. We have determined the mechanism by which T. gondii counteracts death receptor-mediated cell death in type II cells that transduce Fas/CD95 ligation via caspase 8-mediated activation of the mitochondrial amplification loop. The results showed that infection with T. gondii significantly reduced Fas/CD95-triggered apoptosis in HeLa cells by inhibiting the activities of initiator caspases 8 and 9 and effector caspase 3/7. Parasitic infection dose dependently diminished cleavage of caspase 8, the BH3-only protein Bid, and the downstream caspases 9 and 3. Importantly, interference with Fas/CD95-triggered caspase 8 and caspase 3/7 activities after parasitic infection was largely dependent on the presence of caspase 9. Within the mitochondrial amplification loop, T. gondii significantly inhibited the Fas/CD95-triggered release of cytochrome c into the host cell cytosol. These results indicate that T. gondii inhibits Fas/CD95-mediated apoptosis in type II cells primarily by decreasing the apoptogenic function of mitochondria.

Understanding the multiple functions of Gr-1(+) cell subpopulations during microbial infection

The murine cell surface determinant Gr-1 is expressed at high level on neutrophils. Depletion of polymorphonuclear leukocytes with anti-Gr-1(+) monoclonal antibody results in increased susceptibility and dysregulated immunity to many microbial pathogens, a finding widely interpreted to indicate the importance of neutrophils during infection. Yet, in recent years it has become clear that additional cell types express the Gr-1 determinant, including dendritic cell and monocyte subpopulations. In this review, we evaluate current knowledge on the functional aspects of Gr-1(+) cell populations. We focus on infection with the opportunistic protozoan Toxoplasma gondii, a case where host survival depends on an intact Gr-1(+) cell population.

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.

BeWo trophoblast cell susceptibility to Toxoplasma gondii is increased by interferon-gamma, interleukin-10 and transforming growth factor-beta1

The present study aimed to investigate BeWo trophoblast cell susceptibility to Toxoplasma gondii infection under stimulation with anti-inflammatory cytokines in comparison with HeLa cells. Both cell types were submitted to different treatments with recombinant cytokines [interleukin (IL)-10 and transforming growth factor (TGF)-beta1] or the respective antibodies (anti-IL-10 and anti-TGF-beta) before and after T. gondii infection. The effect of interferon (IFN)-gamma was also assessed alone or in combination with anti-inflammatory cytokines or the respective antibodies after the parasite infection. Cells were fixed, stained and parasites quantified under light microscopy to evaluate intracellular replication (mean number of parasites per cell in 100 infected cells) and infection index (percentage of infected cells per 100 examined cells). In contrast with HeLa cells, treatments with IL-10 or TGF-beta1 induced a considerable augmentation in both T. gondii intracellular replication and invasion into BeWo cells. In addition, treatment with IFN-gamma alone or associated with IL-10 or TGF-beta1 increased the same parameters in BeWo cells, whereas the opposite effect was observed in HeLa cells. When endogenous IL-10 or TGF-beta was blocked, both BeWo and HeLa cells were able to control the parasite infection only in the presence of IFN-gamma. Together, these results indicate that the higher susceptibility of BeWo cells to T. gondii may be due to immunomodulation mechanisms, suggesting that the role of trophoblast cells in maintaining a placental microenvironment favourable to pregnancy may facilitate the infection into the placental tissues.

Protective effect of a multiantigenic DNA vaccine against Toxoplasma gondii with co-delivery of IL-12 in mice

In this study, we constructed a multiantigenic DNA vaccine, pSAG1-ROP2-SAG2 and examined its effect with co-delivery of a plasmid encoding IL-12 (pIL-12) as an adjuvant in BALB/c mice against Toxoplasma gondii. After a lethal challenge of T. gondii RH strain, survival of the mice immunized with this pSAG1-ROP2-SAG2 vaccine was significantly prolonged in comparison to the control groups. Furthermore, the protection was significantly augmented by pIL-12 co-delivery. As demonstrated by lymphocyte proliferation assay, cytokine and antibody level determinations, the humoral and Th1-type cellular responses elicited by this multiantigenic DNA vaccine were significantly stronger than those elicited by double-antigenic, or single-antigenic DNA vaccines. Our data suggest that multiantigenic DNA vaccine with pIL-12 co-delivery is a very effective approach in the protection against T. gondii.