The NF-kappaB signaling pathway: immune evasion and immunoregulation during toxoplasmosis

Gut microbiota-related metabolite alpha-linolenic acid mitigates intestinal inflammation induced by oral infection with Toxoplasma gondii

BACKGROUND

Oral infection with cysts is the main transmission route of Toxoplasma gondii (T. gondii), which leads to lethal intestinal inflammation. It has been widely recognized that T. gondii infection alters the composition and metabolism of the gut microbiota, thereby affecting the progression of toxoplasmosis. However, the potential mechanisms remain unclear. In our previous study, there was a decrease in the severity of toxoplasmosis after T. gondii α-amylase (α-AMY) was knocked out. Here, we established mouse models of ME49 and Δα-amy cyst infection and then took advantage of 16S rRNA gene sequencing and metabolomics analysis to identify specific gut microbiota-related metabolites that mitigate T. gondii-induced intestinal inflammation and analyzed the underlying mechanism.

RESULTS

There were significant differences in the intestinal inflammation between ME49 cyst- and Δα-amy cyst-infected mice, and transferring feces from mice infected with Δα-amy cysts into antibiotic-treated mice mitigated colitis caused by T. gondii infection. 16S rRNA gene sequencing showed that the relative abundances of gut bacteria, such as Lactobacillus and Bacteroides, Bifidobacterium, [Prevotella], Paraprevotella and Macellibacteroides, were enriched in mice challenged with Δα-amy cysts. Spearman correlation analysis between gut microbiota and metabolites indicated that some fatty acids, including azelaic acid, suberic acid, alpha-linolenic acid (ALA), and citramalic acid, were highly positively correlated with the identified bacterial genera. Both oral administration of ALA and fecal microbiota transplantation (FMT) decreased the expression of pro-inflammatory cytokines and restrained the MyD88/NF-κB pathway, which mitigated colitis and ultimately improved host survival. Furthermore, transferring feces from mice treated with ALA reshaped the colonization of beneficial bacteria, such as Enterobacteriaceae, Proteobacteria, Shigella, Lactobacillus, and Enterococcus.

CONCLUSIONS

The present findings demonstrate that the host gut microbiota is closely associated with the severity of T. gondii infection. We provide the first evidence that ALA can alleviate T. gondii-induced colitis by improving the dysregulation of the host gut microbiota and suppressing the production of pro-inflammatory cytokines via the MyD88/NF-κB pathway. Our study provides new insight into the medical application of ALA for the treatment of lethal intestinal inflammation caused by Toxoplasma infection. Video Abstract.

Tryptophan metabolism and immune alterations in pregnant Hispanic women with chronic Toxoplasma gondii infection

PROBLEM

Pregnancy markedly modifies women's metabolism and immune functions. We hypothesized that pregnancy might alter the immune and metabolic responses to chronic Toxoplasma gondii infection in pregnancy.

METHOD OF STUDY

A population of 690 pregnant Hispanic women were screened for antibodies to T. gondii and 158 women were positive (23% positivity) with 83% showing high avidity indices. These seropositive women were followed through their pregnancies with four data collection time points and a postpartum collection at two clinics in Tampa, Florida. A T. gondii seronegative group (N = 128) was randomly selected to serve as a control group and measured along pregnancy in the same way. Serum levels of tryptophan, kynurenine, and their ratio, phenylalanine, tyrosine and their ratio, neopterin, and nitrite were measured through pregnancy and the postpartum. A plasma cytokine panel (IFN-γ, TNFα, IL-2, IL-10, IL-12, IL-6, IL-17) was analyzed in parallel.

RESULTS

The major findings suggest that indoleamine 2,3-dioxygenase (IDO-1) was less activated in T. gondii seropositive pregnant Hispanic women with chronic infection. Evidence for IDO-1 suppression was that tryptophan catabolism was less pronounced and there were lower levels of multiple inflammatory cytokines including IFN-γ, which is the major inducer of IDO-1, and higher nitrite concentration, a surrogate marker for nitric oxide, an inhibitor of IDO.

CONCLUSIONS

Latent T. gondii infection was associated with higher plasma tryptophan levels, and lower inflammatory cytokines across pregnancy, suggesting suppression of the IDO-1 enzyme, and possible T cell exhaustion during pregnancy.

Host-pathogen interactions mediated by extracellular vesicles in Toxoplasma gondii infection during pregnancy

Molecular communication between a pathogen and its host is crucial for a successful interplay. Extracellular vesicles (EVs) act as mediators for the delivery of molecular signals among pathogens or between pathogens and the host. Toxoplasma gondii (T. gondii), an intracellular parasite with a worldwide presence, produces EVs itself, or induces the secretion of EVs from infected host cells potentially having capacities to modulate the host immune response. T. gondii infection is particularly important during pregnancy. Depending on the gestational age at the time of infection, the parasite can be transmitted through the placenta to the fetus, causing clinical complications such as jaundice, hepatosplenomegaly, chorioretinitis, cranioencephalic abnormalities, or even death. T. gondii infection is related to a pro-inflammatory immune response in both mother and fetus, which may enhance parasite transmission, but the implication of EV signaling in this process remains unclear. In this review, we summarize the current knowledge on EV release from T. gondii and its human host cells in regard to the immunological consequences and the passage through the placenta.

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.

NF-κB and Its Regulators During Pregnancy

The transcriptional factor NF-κB is a nuclear factor involved in both physiological and pathological processes. This factor can control the transcription of more than 400 genes, including cytokines, chemokines, and their modulators, immune and non-immune receptors, proteins involved in antigen presentation and cell adhesion, acute phase and stress response proteins, regulators of apoptosis, growth factors, other transcription factors and their regulators, as well as different enzymes; all these molecules control several biological processes. NF-κB is a tightly regulated molecule that has also been related to apoptosis, cell proliferation, inflammation, and the control of innate and adaptive immune responses during onset of labor, in which it has a crucial role; thus, early activation of this factor may have an adverse effect, by inducing premature termination of pregnancy, with bad outcomes for the mother and the fetus, including product loss. Reviews compiling the different activities of NF-κB have been reported. However, an update regarding NF-κB regulation during pregnancy is lacking. In this work, we aimed to describe the state of the art around NF-κB activity, its regulatory role in pregnancy, and the effect of its dysregulation due to invasion by pathogens like Trichomonas vaginalis and Toxoplasma gondii as examples.

6-Gingerol attenuates ventilator-induced lung injury via anti-inflammation and antioxidative stress by modulating the PPARγ/NF-κBsignalling pathway in rats

Although mechanical ventilation (MV) is indispensable to life-support therapy in critically ill patients, it may promote or aggravatelunginjury known asventilator-inducedlunginjury(VILI). 6-Gingerol is the principal ingredient of ginger with potential anti-inflammatory and antioxidant properties in various diseases. Nevertheless, the role and mechanism of 6-gingerol in the process of VILI has not been explicitly investigated. In the study, we found that pre-treatment with 6-gingerol significantly improved the histological changes and pulmonary oedema, inhibited neutrophil accumulation and the release of early pro-inflammatory cytokines and MPO, and reduced oxidative stress reactions after high MV. Moreover, 6-gingerol treatment also increased PPARγ expression and decreased NF-κB activation in rats subjected to high MV. Furthermore, GW9662, a specific PPARγ inhibitor, was demonstrated to activatethe NF-κB pathway and cancele the protective role of 6-gingerol in VILI. This indicates that 6-gingerol exerted anti-inflammatory and antioxidative stress effects in VILI by activating PPARγ and inhibiting the NF-κBsignalling pathway.

PA-X protein of H5N1 avian influenza virus inhibits NF-kappaB activity, a potential mechanism for PA-X counteracting the host innate immune responses

PA-X is a fusion protein of influenza virus which plays a crucial role in modulating influenza virus-induced host innate immune response and subsequent pathogenicity. However, the potential mechanism of PA-X regulation of the host innate immune response remains largely unknown. It is well known that NF-κB signal pathway is crucial for the immediate early step of immune responses activation, while the specific role of PA-X in NF-κB transcriptional activity is totally unknown. In this study, we initially showed that PA-X inhibits NF-κB transcription that stimulated by poly(I:C). We then further determined that the inhibitory effect on NF-κB activation mediated by PA-X was characterized by restricting NF-κB p65 nuclear translocation and nuclear NF-κB p65 activity but not by impeding the phosphorylation of NF-κB p65. Correspondingly, PA-X decreases the amount of NF-κB signaling pathway-associated genes, including TNF-α, Nos2, IL-6 and IL-2. Moreover, PA-X also suppresses both the mRNA and protein expression level of IFN-β, suggesting the direct contribution of PA-X to the inhibition of NF-κB-regulated IFN-β expression. Together, our study sheds light on the potential molecular mechanisms underlying the regulation of host NF-κB activity by PA-X and also identifies a novel functional role for PA-X in counteracting the host innate immune response. However, further exploration of the more elaborate mechanism of PA-X-mediated inhibition of NF-κB activity and the associated signaling pathway may help to elucidate its precise mechanism of evading and subverting the host immune response.

The role of NF-κB activation during protection against Leishmania infection

Members of the nuclear factor-κB (NF-κB) family of transcription factors regulate a variety of molecules involved in host defense against pathogens. A prominent role of NF-κB in innate and adoptive immunity is based on the regulation of inducible transcription of various genes whose products are essential components of the immune response such as cytokines, chemokines, and adhesion molecules. Since the discovery of the five members of the NF-κB transcription factor family, RelA, c-Rel, RelB, p50 and p52, considerable progress has been made toward better understanding how the different NF-κB homo- and heterodimers regulate such distinct subsets of target genes. All of the NF-κB molecules are activated by various infectious stimuli; however, there are still open questions related to the selective functions of individual NF-κB family members during a coordinated immune response to infection. Diverse parasites such as Toxoplasma gondii, Leishmania donovani, Leishmania major, and Trichuris muris have been reported to activate NF-κB signaling cascades, and a number of distinct parasite-derived molecules may actively interfere with the pathways that lead to NF-κB activation. In this review, we provide an overview on the role of NF-κB activation in leishmaniasis and discuss how individual NF-κB family members might perform their distinct and non-overlapping functions in the regulation of protective immunity to Leishmania infection.

Chitohexaose activates macrophages by alternate pathway through TLR4 and blocks endotoxemia

Sepsis is a consequence of systemic bacterial infections leading to hyper activation of immune cells by bacterial products resulting in enhanced release of mediators of inflammation. Endotoxin (LPS) is a major component of the outer membrane of Gram negative bacteria and a critical factor in pathogenesis of sepsis. Development of antagonists that inhibit the storm of inflammatory molecules by blocking Toll like receptors (TLR) has been the main stay of research efforts. We report here that a filarial glycoprotein binds to murine macrophages and human monocytes through TLR4 and activates them through alternate pathway and in the process inhibits LPS mediated classical activation which leads to inflammation associated with endotoxemia. The active component of the nematode glycoprotein mediating alternate activation of macrophages was found to be a carbohydrate residue, Chitohexaose. Murine macrophages and human monocytes up regulated Arginase-1 and released high levels of IL-10 when incubated with chitohexaose. Macrophages of C3H/HeJ mice (non-responsive to LPS) failed to get activated by chitohexaose suggesting that a functional TLR4 is critical for alternate activation of macrophages also. Chitohexaose inhibited LPS induced production of inflammatory molecules TNF-α, IL-1β and IL-6 by macropahges in vitro and in vivo in mice. Intraperitoneal injection of chitohexaose completely protected mice against endotoxemia when challenged with a lethal dose of LPS. Furthermore, Chitohexaose was found to reverse LPS induced endotoxemia in mice even 6/24/48 hrs after its onset. Monocytes of subjects with active filarial infection displayed characteristic alternate activation markers and were refractory to LPS mediated inflammatory activation suggesting an interesting possibility of subjects with filarial infections being less prone to develop of endotoxemia. These observations that innate activation of alternate pathway of macrophages by chtx through TLR4 has offered novel opportunities to cell biologists to study two mutually exclusive activation pathways of macrophages being mediated through a single receptor.

Sepsis is one of the leading causes of death contributing to mortality as high as 54 percent in intensive care units across the world. Hyper inflammation induced by bacteria or bacterial products through Toll like receptors leads to sepsis and hence current approaches are directed towards blockade such receptors. While many such candidate antagonists have shown promise they also result in induction of inappropriate innate immune responses thus increasing risk of development of shock leading to death. In this study we describe a novel approach to treat endotoxemia associated with sepsis, fundamentally different from other reports. Chitohexaose a small molecular weight polysaccharide by virtue of its ability to bind to active sites of TLR4 inhibited LPS induced production of inflammatory mediators by murine macrophages and human monocytes. Administration of chitohexaose with LPS blocked endotoxemia leading to mortality of mice. More significantly, Chitohexaose reversed inflammation and protected mice even 24/48 hrs after onset of endotoxemia. Apart from competitively inhibiting LPS induced inflammation chitohexaose also activated alternate pathway of macrophages. Such macrophages are known to display increased phagocytic activity, are resistant to LPS induced activation and associated with resolution of inflammation and tissue repair.

Pathogenesis of chagas' disease: parasite persistence and autoimmunity

Acute Trypanosoma cruzi infections can be asymptomatic, but chronically infected individuals can die of Chagas' disease. The transfer of the parasite mitochondrial kinetoplast DNA (kDNA) minicircle to the genome of chagasic patients can explain the pathogenesis of the disease; in cases of Chagas' disease with evident cardiomyopathy, the kDNA minicircles integrate mainly into retrotransposons at several chromosomes, but the minicircles are also detected in coding regions of genes that regulate cell growth, differentiation, and immune responses. An accurate evaluation of the role played by the genotype alterations in the autoimmune rejection of self-tissues in Chagas' disease is achieved with the cross-kingdom chicken model system, which is refractory to T. cruzi infections. The inoculation of T. cruzi into embryonated eggs prior to incubation generates parasite-free chicks, which retain the kDNA minicircle sequence mainly in the macrochromosome coding genes. Crossbreeding transfers the kDNA mutations to the chicken progeny. The kDNA-mutated chickens develop severe cardiomyopathy in adult life and die of heart failure. The phenotyping of the lesions revealed that cytotoxic CD45, CD8(+) γδ, and CD8α(+) T lymphocytes carry out the rejection of the chicken heart. These results suggest that the inflammatory cardiomyopathy of Chagas' disease is a genetically driven autoimmune disease.

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.

Activation and regulation of toll-like receptors (TLRs) by helminth parasites

Helminth (worm) infections are major public health problems that have important socioeconomic consequences for the more than 2 billion infected individuals. Chronicity (their hallmark) can lead to anemia (in hookworm infection), river blindness (onchcerciasis), cirrhosis (schistosomiasis), and elephantiasis (lymphatic filariasis). Although there have been many studies examining innate immune responses (including TLR expression and function) in response to intracellular pathogens, fewer have examined the interaction of the multicellular helminth parasites and the innate immune system. This review will focus on two "systemic" helminth parasitic infections (lymphatic filariasis and schistosomiasis) and the regulation of TLRs that may contribute to infection outcome.

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.

Horses experimentally infected with Sarcocystis neurona develop altered immune responses in vitro

Equine protozoal myeloencephalitis (EPM) due to Sarcocystis neurona infection is 1 of the most common neurologic diseases in horses in the United States. The mechanisms by which most horses resist disease, as well as the possible mechanisms by which the immune system may be suppressed in horses that develop EPM, are not known. Therefore, the objectives of this study were to determine whether horses experimentally infected with S. neurona developed suppressed immune responses. Thirteen horses that were negative for S. neurona antibodies in serum and cerebrospinal fluid (CSF) were randomly assigned to control (n = 5) or infected (n = 8) treatment groups. Neurologic exams and cerebrospinal fluid analyses were performed prior to, and following, S. neurona infection. Prior to, and at multiple time points following infection, immune parameters were determined. All 8 S. neurona-infected horses developed clinical signs consistent with EPM, and had S. neurona antibodies in the serum and CSF. Both infected and control horses had increased percentages (P < 0.05) of B cells at 28 days postinfection. Infected horses had significantly decreased (P < 0.05) proliferation responses as measured by thymidine incorporation to nonspecific mitogens phorbol myristate acetate (PMA) and ionomycin (I) as soon as 2 days postinfection.

Toxoplasma gondii rhoptry discharge correlates with activation of the early growth response 2 host cell transcription factor

Toxoplasma gondii is a ubiquitous apicomplexan parasite that can cause severe disease in fetuses and immune-compromised patients. Rhoptries, micronemes, and dense granules, which are secretory organelles unique to Toxoplasma and other apicomplexan parasites, play critical roles in parasite growth and virulence. To understand how these organelles modulate infected host cells, we sought to identify host cell transcription factors triggered by their release. Early growth response 2 (EGR2) is a host cell transcription factor that is rapidly upregulated and activated in Toxoplasma-infected cells but not in cells infected with the closely related apicomplexan parasite Neospora caninum. EGR2 upregulation occurred only when live parasites were in direct contact with the host cell and not from exposure to cell extracts that contain dense granule or micronemal proteins. When microneme-mediated attachment was blocked by pretreating parasites with a calcium chelator, EGR2 expression was significantly reduced. In contrast, when host cells were infected with parasites in the presence of cytochalasin D, which allows rhoptry secretion but prevents parasite invasion, EGR2 was activated. Finally, we demonstrate that Toxoplasma activation of host p38 mitogen-activated protein kinase is necessary but not sufficient for EGR2 activation. Collectively, these data indicate that EGR2 is specifically upregulated by a parasite-derived secreted factor that is most likely a resident rhoptry protein.

Activation of NF-kappaB1 by OX40 contributes to antigen-driven T cell expansion and survival

The costimulatory molecule OX40 (CD134) is required in many instances for effective T cell-mediated immunity, controlling proliferation, and survival of T cells after encountering specific Ag. We previously found that the functional targets of OX40 are survivin and aurora B that regulate proliferation and Bcl-2 antiapoptotic family members that regulate survival. However, the intracellular pathways from OX40 that mediate these effects are unclear. In this study, we show that OX40 signaling can target the canonical NF-kappaB (NF-kappaB1) pathway in peripheral Ag-responding CD4 T cells. Phosphorylation of IkappaBalpha, nuclear translocation of NF-kappaB1/p50 and RelA, and NF-kappaB1 activity, are impaired in OX40-deficient T cells. Retroviral transduction of active IkappaB kinase that constitutively activates NF-kappaB1 rescues the poor expansion and survival of OX40-deficient T cells, directly correlating with increased expression and activity of survivin, aurora B, and Bcl-2 family members. Moreover, active IkappaB kinase expression alone is sufficient to restore the defective expansion and survival of OX40-deficient T cells in vivo when responding to Ag. Thus, OX40 signals regulate T cell number and viability through the NF-kappaB1 pathway that controls expression and activity of intracellular targets for proliferation and survival.

Advances in the use of genetically engineered parasites to study immunity to Toxoplasma gondii

Studying in vivo biology and the host immune response to Toxoplasma gondii has yielded many insights into the pathogenesis of this parasitic organism. It is recognized that this infection in immune competent hosts elicits a strong Th1-type response, which is characterized by the generation of parasite-specific CD4(+) and CD8(+) T cells that produce IFN-gamma and provide protective immunity. One of the problems associated with studying resistance to Toxoplasma has been the lack of reagents to track parasite-specific T cell responses with a high degree of specificity. To overcome this difficulty, it is possible to use a combination of transgenic parasites that are engineered to express well-characterized heterologous reporters or antigens, and T cell hybridomas or naïve T cells that express a T cell receptor specific for the processed peptide. These approaches have provided new insights into parasite dissemination, antigen presentation, as well as immune regulation.

Plasmodium circumsporozoite protein promotes the development of the liver stages of the parasite

The liver stages of malaria are clinically silent but have a central role in the Plasmodium life cycle. Liver stages of the parasite containing thousands of merozoites grow inside hepatocytes for several days without triggering an inflammatory response. We show here that Plasmodium uses a PEXEL/VTS motif to introduce the circumsporozoite (CS) protein into the hepatocyte cytoplasm and a nuclear localization signal (NLS) to enter its nucleus. CS outcompetes NFkappaB nuclear import, thus downregulating the expression of many genes controlled by NFkappaB, including those involved in inflammation. CS also influences the expression of over one thousand host genes involved in diverse metabolic processes to create a favorable niche for the parasite growth. The presence of CS in the hepatocyte enhances parasite growth of the liver stages in vitro and in vivo. These findings have far reaching implications for drug and vaccine development against the liver stages of the malaria parasite.

Presentation of Toxoplasma gondii antigens via the endogenous major histocompatibility complex class I pathway in nonprofessional and professional antigen-presenting cells

Challenge with the intracellular protozoan parasite Toxoplasma gondii induces a potent CD8+ T-cell response that is required for resistance to infection, but many questions remain about the factors that regulate the presentation of major histocompatibility complex class I (MHC-I)-restricted parasite antigens and about the role of professional and nonprofessional accessory cells. In order to address these issues, transgenic parasites expressing ovalbumin (OVA), reagents that track OVA/MHC-I presentation, and OVA-specific CD8+ T cells were exploited to compare the abilities of different infected cell types to stimulate CD8+ T cells and to define the factors that contribute to antigen processing. These studies reveal that a variety of infected cell types, including hematopoietic and nonhematopoietic cells, are capable of activating an OVA-specific CD8+ T-cell hybridoma, and that this phenomenon is dependent on the transporter associated with antigen processing and requires live T. gondii. Several experimental approaches indicate that T-cell activation is a consequence of direct presentation by infected host cells rather than cross-presentation. Surprisingly, nonprofessional antigen-presenting cells (APCs) were at least as efficient as dendritic cells at activating this MHC-I-restricted response. Studies to assess whether these cells are involved in initiation of the CD8+ T-cell response to T. gondii in vivo show that chimeric mice expressing MHC-I only in nonhematopoietic compartments are able to activate OVA-specific CD8+ T cells upon challenge. These findings associate nonprofessional APCs with the initial activation of CD8+ T cells during toxoplasmosis.

Host and parasite-derived IKK activities direct distinct temporal phases of NF-kappaB activation and target gene expression following Toxoplasma gondii infection

Activation of NF-kappaB by the intracellular pathogen Toxoplasma gondii is associated with the localization of phosphorylated IkappaB alpha to the parasitophorous vacuole membrane (PVM). This is mediated by a parasite-derived IkappaB kinase (TgIKK) activity and is independent of host IKK function. In the present study, we examined the roles of host IKK and parasite-derived TgIKK on the temporal modulation of NF-kappaB activation. Despite the presence of TgIKK activity at the PVM, nuclear translocation of NF-kappaB and subsequent gene expression exhibited a requirement for the host IKK complex. A detailed kinetic analysis of NF-kappaB activation revealed a biphasic, hierarchical and temporally regulated response. We propose a novel paradigm for the modulation of NF-kappaB-dependent gene expression by T. gondii that involves both the host IKK complex and TgIKK activity at different phases of infection. Thus, T. gondii effectively alters gene expression in a temporal dimension by exploiting the NF-kappaB signaling machinery and subsequently rewiring the activation circuits of the infected host cell.

Toxoplasma gondii and subversion of the immune system

Toxoplasma gondii is an intracellular obligate parasite that enters the host via the gastrointestinal tract. The parasite is able to evade or subvert the immune response of its host via various mechanisms. Here, we discuss a recent in vitro study by Eric Denkers and colleagues that focused on the modulation of gene transcription of host macrophages stimulated by lipopolysaccharide (LPS) following infection with T. gondii. The parasite was able to block the response of macrophages to LPS, a major immunostimulatory component of Gram negative bacteria, thus possibly avoiding the hyperinflammatory response of the host to gut microflora, among which Gram negative bacteria are abundant.

Screening for Toxoplasma gondii-regulated transcriptional responses in lipopolysaccharide-activated macrophages

Toxoplasma gondii-infected macrophages are blocked in production of the proinflammatory cytokines interleukin-12 (IL-12) and tumor necrosis factor alpha (TNF-alpha) upon activation with lipopolysaccharide (LPS). Here, we used pathway-focused cDNA arrays to identify additional T. gondii-regulated transcriptional responses. Parasite infection decreased 57 (inclusive of IL-12 and TNF-alpha) and increased expression of 7 of 77 LPS-activated cytokine and cytokine-related genes. Interestingly, we found that the LPS-induced transcriptional response of the anti-inflammatory cytokine IL-10 was synergistically increased by T. gondii, results that we validated by conventional reverse transcription-PCR and enzyme-linked immunosorbent assay. Importantly, although the parasite exerted disparate effects in LPS-signaling leading to TNF-alpha versus IL-10 production, both responses required functional Toll-like receptor 4. We suggest that these effects represent parasite defense mechanisms to avoid or delay induction of antimicrobial activity and/or T-cell-mediated immunity during Toxoplasma infection.

Regulation of host cell survival by intracellular Plasmodium and Theileria parasites

Plasmodium and Theileria parasites are obligate intracellular protozoa of the phylum Apicomplexa. Theileria infection of bovine leukocytes induces transformation of host cells and infected leukocytes can be kept indefinitely in culture. Theileria-dependent host cell transformation has been the subject of interest for many years and the molecular basis of this unique phenomenon is quite well understood. The equivalent life cycle stage of Plasmodium is the infection of mammalian hepatocytes, where parasites reside for 2-7 days depending on the species. Some of the molecular details of parasite-host interactions in P. berghei-infected hepatocytes have emerged only very recently. Similar to what has been shown for Theileria-infected leukocytes these data suggest that malaria parasites within hepatocytes also protect their host cell from programmed cell death. However, the strategies employed to inhibit host cell apoptotic pathways appear to be different to those used by Theileria. This review discusses similarities and differences at the molecular level of Plasmodium- and Theileria-induced regulation of the host cell survival machinery.

Initiation and termination of NF-kappaB signaling by the intracellular protozoan parasite Toxoplasma gondii

Signaling via the NF-kappaB cascade is critical for innate recognition of microbial products and immunity to infection. As a consequence, this pathway represents a strong selective pressure on infectious agents and many parasitic, bacterial and viral pathogens have evolved ways to subvert NF-kappaB signaling to promote their survival. Although the mechanisms utilized by microorganisms to modulate NF-kappaB signaling are diverse, a common theme is targeting of the steps that lead to IkappaB degradation, a major regulatory checkpoint of this pathway. The data presented here demonstrate that infection of mammalian cells with Toxoplasma gondii results in the activation of IKK and degradation of IkappaB. However, despite initiation of these hallmarks of NF-kappaB signaling, neither nuclear accumulation of NF-kappaB nor NF-kappaB-driven gene expression is observed in infected cells. However, this defect was not due to a parasite-mediated block in nuclear import, as general nuclear import and constitutive nuclear-cytoplasmic shuttling of NF-kappaB remain intact in infected cells. Rather, in T. gondii-infected cells, the termination of NF-kappaB signaling is associated with reduced phosphorylation of p65/RelA, an event involved in the ability of NF-kappaB to translocate to the nucleus and bind DNA. Thus, these studies demonstrate for the first time that the phosphorylation of p65/RelA represents an event downstream of IkappaB degradation that may be targeted by pathogens to subvert NF-kappaB signaling.

Toxoplasma gondii prevents neuron degeneration by interferon-gamma-activated microglia in a mechanism involving inhibition of inducible nitric oxide synthase and transforming growth factor-beta1 production by infected microglia

Interferon (IFN)-gamma, the main cytokine responsible for immunological defense against Toxoplasma gondii, is essential in all infected tissues, including the central nervous system. However, IFN-gamma-activated microglia may cause tissue injury through production of toxic metabolites such as nitric oxide (NO), a potent inducer of central nervous system pathologies related to inflammatory neuronal disturbances. Despite potential NO toxicity, neurodegeneration is not commonly found during chronic T. gondii infection. In this study, we describe decreased NO production by IFN-gamma-activated microglial cells infected by T. gondii. This effect involved strong inhibition of iNOS expression in IFN-gamma-activated, infected microglia but not in uninfected neighboring cells. The inhibition of NO production and iNOS expression were parallel with recovery of neurite outgrowth when neurons were co-cultured with T. gondii-infected, IFN-gamma-activated microglia. In the presence of transforming growth factor (TGF)-beta1-neutralizing antibodies, the beneficial effect of the parasite on neurons was abrogated, and NO production reverted to levels similar to IFN-gamma-activated uninfected co-cultures. In addition, we observed Smad-2 nuclear translocation, a hallmark of TGF-beta1 downstream signaling, in infected microglial cultures, emphasizing an autocrine effect restricted to infected cells. Together, these data may explain a neuropreservation pattern observed during immunocompetent host infection that is dependent on T. gondii-triggered TGF-beta1 secretion by infected microglia.

Differences among the three major strains of Toxoplasma gondii and their specific interactions with the infected host

Toxoplasma gondii is one of the most successful protozoan parasites owing to its ability to manipulate the immune system and establish a chronic infection. There are many T. gondii strains but the majority identified in Europe and North America falls into three distinct clonal lineages. Many studies have investigated the ability of T. gondii to manipulate its host but few have examined directly whether the three lineages differ in this ability.

Detection of a novel parasite kinase activity at the Toxoplasma gondii parasitophorous vacuole membrane capable of phosphorylating host IkappaBalpha

Toxoplasma gondii activates the NF-kappaB pathway in the infected host cell resulting in upregulation of pro-survival genes and prevention of apoptosis. Manipulation of the NF-kappaB cascade by T. gondii correlates with the localization of phosphorylated IkappaB at the parasitophorous vacuole membrane (PVM). This suggests a parasite-mediated event, involving the recruitment and activation of the host IkappaB kinase (IKK) complex, as has been observed with the related protozoan Theileria parva. In contrast to Theileria, confocal microscopy studies showed no apparent hijacking of IKKalpha, IKKbeta, or their activated phosphorylated forms at the T. gondii PVM. Remarkably, phosphorylation of IkappaBalpha at Ser 32/36 was observed at the PVM of T. gondii-infected IKKalpha-/-, IKKbeta-/- and IKKalpha/beta double-knockout (IKKalpha/beta-/-) fibroblasts, suggesting the involvement of a parasite kinase activity independent of host IKK. The presence of a putative T. gondii IkappaB kinase was examined by in vitro kinase assays using GST-IkappaBalpha constructs and protein extracts from both extracellular parasites and PVM fractions. Interestingly, an activity capable of phosphorylating IkappaBalpha at the critical Ser 32/36 sites was identified in parasite extracts, a property restricted to the IKK signalosome. Taken together, our data support the role for a T. gondii kinase involved in phosphorylation of host cell IkappaBalpha and suggest an unusual mechanism utilized by an intracellular pathogen capable of manipulating the NF-kappaB pathway.

Fitness of Toxoplasma gondii is not related to DHFR single-nucleotide polymorphism during congenital toxoplasmosis

Factors that regulate the pathogenesis of Toxoplasma gondii in humans are poorly understood. When acquired during pregnancy, toxoplasmosis can be disastrous, leading to fetal loss or conversely to subclinical disease. In congenitally infected infants, evolution is highly unpredictable. Genotype based virulence patterns have been described in mice, but in humans this classification does not correlate with the gravity of the disease. Mutations on DHFR-TS loci have recently been reported to confer T. gondii fitness cost. In this study, we investigated the relationship between the virulence of the parasite, as measured by clinical outcome in the fetus or newborn, fitness, as measured by parasitic load in amniotic fluid, and allelic polymorphism in DHFR. Six cases of severe congenital toxoplasmosis and 23 cases of mild congenital infections were included in the study. Quantitative PCR was performed to evaluate total T. gondii DNA load in amniotic fluid and detection of mutations was carried out with a LightCycler using hybridisation probes. Parasitic load was significantly higher in severe infections than in mild diseases. Among isolates from severe or non-severe cases of congenital toxoplasmosis, no polymorphism could be detected at loci 36, 83 or 245 of the DHFR gene. The virulent RH strain presented the same melting temperature as the non-virulent PRU strain for codons 36, 83 and 245. Only mutated clones, M2M3 and M2M4 with allelic replacement at these positions, displayed different profiles allowing a clear distinction between wild and mutant types. We concluded that the DHFR gene mutations we investigated do not regulate T. gondii fitness in humans.

Neutrophils, dendritic cells and Toxoplasma

Toxoplasma gondii rapidly elicits strong Type 1 cytokine-based immunity. The necessity for this response is well illustrated by the example of IFN-gamma and IL-12 gene knockout mice that rapidly succumb to the effects of acute infection. The parasite itself is skilled at sparking complex interactions in the innate immune system that lead to protective immunity. Neutrophils are one of the first cell types to arrive at the site of infection, and the cells release several proinflammatory cytokines and chemokines in response to Toxoplasma. Dendritic cells are an important source of IL-12 during infection with T. gondii and other microbial pathogens, and they are also specialized for high-level antigen presentation to T lymphocytes. Tachyzoites express at least two types of molecules that trigger innate immune cell cytokine production. One of these involves Toll-like receptor/MyD88 pathways common to many microbial pathogens. The second pathway is less conventional and involves molecular mimicry between a parasite cyclophilin and host CC chemokine receptor 5-binding ligands. Neutrophils, dendritic cells and Toxoplasma work together to elicit the immune response required for host survival. Cytokine and chemokine cross-talk between parasite-triggered neutrophils and dendritic cells results in recruitment, maturation and activation of the latter. Neutrophil-empowered dendritic cells possess properties expected of highly potent antigen presenting cells that drive T helper 1 generation.