Activation of NF-kappaB by Toxoplasma gondii correlates with increased expression of antiapoptotic genes and localization of phosphorylated IkappaB to the parasitophorous vacuole membrane

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.

The Toxoplasma gondii Parasitophorous Vacuole Membrane: Transactions Across the Border

The obligate intracellular protozoan Toxoplasma gondii establishes its replication permissive niche within the infected host cell. This niche, the parasitophorous vacuole (PV), is delimited from the host cell cytoplasm by the PV membrane (PVM). In this chapter we highlight the roles of the PVM in the remodeling of host cell architecture, nutrient acquisition, the manipulation of signaling, and touch upon the potential roles in the parasite developmental cycle. We further present the PVM as a unique and dynamic "organelle" found only within the infected cell where it is established outside the parent organism. Despite its importance little is known about the biology of the PVM. There has, however, been a recent renewal of interest in the PVM, the study of which has become more tractable with the application of both classical approaches as well as genomic and proteomic analyses. In this review we discuss the diverse activities associated with the PVM and present pressing questions that remain to be elucidated regarding this enigmatic organelle.

Toxoplasma gondii inhibits apoptosis in infected cells by caspase inactivation and NF-kappaB activation

Our experiments aimed to clarify the mechanism by which host cell apoptosis is inhibited by infection with the intracellular protozoan parasite, Toxoplasma gondii (T. gondii). Mouse spleen cells were cultured in 6-well plates with RPMI 1640/ 10% FBS at 37?, in a 5% CO2 atmosphere. Apoptosis of spleen cells was induced by actinomycin-D (AD) treatment for 1 h prior to infection with T. gondii. A variety of assays were used to assess the progression of apoptosis: DNA size analysis on agarose gel electrophoresis, flow cytometry with annexin V/PI staining, and analysis of expression levels of Bcl-2 family and NF-kappaB mRNA and proteins by RT-PCR, Western blotting, and EMSA. Additionally, transmission electron microscopy (TEM) was performed to observe changes in cell morphology. Fragmentation of DNA was inhibited in spleen cells treated with AD and T. gondii 5 h and 18 h post infection, respectively, and flow cytometry studies showed a decreased apoptotic rates in AD and T. gondii treated spleen cells. We observed decreased expression of Bax mRNA and protein, while levels of Bcl-2 mRNA remained constant in spleen cells treated with AD and T. gondii. Caspase 3 and PARP were inactivated in cells treated with AD and T. gondii, and increased levels of cleaved caspase 8 were also observed. Analysis of EMSA and Western blot data suggests that activation of transcription factor NF-kappaB may be involved in the blockade of apoptosis by T. gondii. TEM analysis showed nuclear fragmentation and chromatin condensation occurring in spleen cells treated with AD; however, such apoptosis- associated morphological changes were not observed in cells treated with both AD and T. gondii tachyzoites. Together, these data show that T. gondii infection inhibits AD induced apoptosis via caspase inactivation and NF-kappaB activation in mouse spleen cells.

Subversion of innate and adaptive immune responses by Toxoplasma Gondii

The intracellular apicomplexan parasite Toxoplasma gondii is able to survive and persist in immunocompetent intermediate hosts for the host's life span. This is despite the induction of a vigorous humoral and -- more importantly -- cell-mediated immune response during infection. In order to establish and maintain such chronic infections, however, T. gondii has evolved multiple strategies to avoid or to interfere with potentially efficient anti-parasitic immune responses of the host. Such immune evasion includes (1) indirect mechanisms by altering the expression and secretion of immunomodulatory cytokines or by altering the viability of immune cells and (2) direct mechanisms by establishing a lifestyle within a suitable intracellular niche and by interference with intracellular signaling cascades, thereby abolishing a number of antimicrobial effector mechanisms of the host. Despite the parasite's ability to interfere successfully with the host's efforts to eradicate the infection, the immune response is, however, not completely abrogated but is rather partially diminished after infection. T. gondii thus keeps a delicate balance between induction and suppression of the host's immune response in order to guarantee the survival of the host as a safe harbor for parasite development and to allow its transmission to the definitive host.

Pro- and anti-apoptotic activities of protozoan parasites

During infection, programmed cell death, i.e. apoptosis, is an important effector mechanism of innate and adaptive host responses to parasites. In addition, it fulfils essential functions in regulating host immunity and tissue homeostasis. Not surprisingly, however, adaptation of parasitic protozoa to their hosts also involves modulation or even exploitation of cell death in order to facilitate parasite survival in a hostile environment. During recent years, considerable progress has been made in our understanding of apoptosis during parasitic infections and there is now convincing evidence that apoptosis and its modulation by protozoan parasites has a major impact on the parasite-host interaction and on the pathogenesis of disease. This review updates our current knowledge on the diverse functions apoptosis may fulfil during infections with diverse protozoan parasites including apicomplexans, kinetoplastids and amoebae. Furthermore, we also summarize common mechanistic themes of the pro- and anti-apoptotic activities of protozoan parasites. The diverse and complex effects which parasitic protozoa exert on apoptotic cell death within the host highlight fascinating interactions of parasites and their hosts. Importantly, they also stress the importance of further investigations before the modulation of host cell apoptosis can be exploited to combat parasitic infections.

Involvement of NF-κB in the response of embryonic cells to Methotrexate

The involvement of NF-kappaB in the regulation of the apoptotic process was demonstrated previously, however, its exact role has not been established yet. In order to unravel mechanisms underlying teratogen-induced cell death, we tried in our present study to assess the involvement of the p65 subunit of NF-kappaB in the response of mouse embryonic fibroblasts (MEFs) to the anti-cancer drug methotrexate (MTX), using p65 knockout MEFs (p65(-/-)). Indeed, this cell line was found to be more susceptible to the exposure to MTX, demonstrated by more profound changes in cell survival, cell cycle, proliferation and the percentage of apoptotic or necrotic cells, as compared to wild type (WT) MEFs. Also, a different pattern of intracellular localization of p65 in WT cells as well as IkappaBalpha and Bax in both cell lines was detected in response to MTX. Altogether, our results implicate the p65 subunit of NF-kappaB to play an important role in the response of embryonic cells to MTX.

NF-kappaB translocation prevents host cell death after low-dose challenge by Legionella pneumophila

Legionella pneumophila, the causative agent of Legionnaires' disease, grows within macrophages and manipulates target cell signaling. Formation of a Legionella-containing replication vacuole requires the function of the bacterial type IV secretion system (Dot/Icm), which transfers protein substrates into the host cell cytoplasm. A global microarray analysis was used to examine the response of human macrophage-like U937 cells to low-dose infections with L. pneumophila. The most striking change in expression was the Dot/Icm-dependent up-regulation of antiapoptotic genes positively controlled by the transcriptional regulator nuclear factor κB (NF-κB). Consistent with this finding, L. pneumophila triggered nuclear localization of NF-κB in human and mouse macrophages in a Dot/Icm-dependent manner. The mechanism of activation at low-dose infections involved a signaling pathway that occurred independently of the Toll-like receptor adaptor MyD88 and the cytoplasmic sensor Nod1. In contrast, high multiplicity of infection conditions caused a host cell response that masked the unique Dot/Icm-dependent activation of NF-κB. Inhibition of NF-κB translocation into the nucleus resulted in premature host cell death and termination of bacterial replication. In the absence of one antiapoptotic protein, plasminogen activator inhibitor–2, host cell death increased in response to L. pneumophila infection, indicating that induction of antiapoptotic genes is critical for host cell survival.

cDNA microarray analysis of host-pathogen interactions in a porcine in vitro model for Toxoplasma gondii infection

Toxoplasma gondii induces the expression of proinflammatory cytokines, reorganizes organelles, scavenges nutrients, and inhibits apoptosis in infected host cells. We used a cDNA microarray of 420 annotated porcine expressed sequence tags to analyze the molecular basis of these changes at eight time points over a 72-hour period in porcine kidney epithelial (PK13) cells infected with T. gondii. A total of 401 genes with Cy3 and Cy5 spot intensities of >/=500 were selected for analysis, of which 263 (65.6%) were induced >/=2-fold (expression ratio, >/=2.0; P </= 0.05 [t test]) over at least one time point and 48 (12%) were significantly down-regulated. At least 12 functional categories of genes were modulated (up- or down-regulated) by T. gondii. The majority of induced genes were clustered as transcription, signal transduction, host immune response, nutrient metabolism, and apoptosis related. The expression of selected genes altered by T. gondii was validated by quantitative real-time reverse transcription-PCR. These results suggest that significant changes in gene expression occur in response to T. gondii infection in PK13 cells, facilitating further analysis of host-pathogen interactions in toxoplasmosis in a secondary host.

Anti-apoptotic signalling by the Dot/Icm secretion system of L. pneumophila

The Dot/Icm type IV secretion system of Legionella pneumophila triggers robust activation of caspase-3 during early and exponential stages of proliferation within human macrophages, but apoptosis is delayed till late stages of infection, which is novel. As caspase-3 is the executioner of the cell, we tested the hypothesis that L. pneumophila triggers anti-apoptotic signalling within the infected human macrophages to halt caspase-3 from dismantling the cells. Here we show that during early and exponential replication, L. pneumophila-infected human monocyte-derived macrophages (hMDMs) exhibit a remarkable resistance to induction of apoptosis, in a Dot/Icm-dependent manner. Microarray analyses and real-time PCR reveal that during exponential intracellular replication, L. pneumophila triggers upregulation of 12 anti-apoptotic genes that are linked to activation of the nuclear transcription factor kappa-B (NF-kappaB). Our data show that L. pneumophila induces a Dot/Icm-dependent sustained nuclear translocation of the p50 and p65 subunits of NF-kappaB during exponential intracellular replication. Bacterial entry is essential both for the anti-apoptotic phenotype of infected hMDMs and for nuclear translocation of the p65. Using p65-/- and IKKalpha-/- beta-/- double knockout mouse embryonic fibroblast cell lines, we show that nuclear translocation of NF-kappaB is required for the resistance of L. pneumophila-infected cells to apoptosis-inducing agents. In addition, the L. pneumophila-induced nuclear translocation of NF-kappaB requires the activity of IKKalpha and/or IKKbeta. We conclude that although the Dot/Icm secretion system of L. pneumophila elicits an early robust activation of caspase-3 in human macrophages, it triggers a strong anti-apoptotic signalling cascade mediated, at least in part by NF-kappaB, which renders the cells refractory to external potent apoptotic stimuli.

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

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

Toxoplasma gondiitriggers Gi-dependent PI 3-kinase signaling required for inhibition of host cell apoptosis

Infection with the intracellular parasite Toxoplasma gondii renders cells resistant to multiple pro-apoptotic signals, but underlying mechanisms have not been delineated. The phosphoinositide 3-kinase (PI 3-kinase) pathway and the immediate downstream effector protein kinase B (PKB/Akt) play important roles in cell survival and apoptosis inhibition. Here, we show that Toxoplasma infection of mouse macrophages activates PKB/Akt in vivo and in vitro. In a mixed population of infected and non-infected macrophages, activation is only observed in parasite-infected cells. The PI 3-kinase inhibitors wortmannin and LY294002 block parasite-induced PKB phosphorylation. PKB activation occurs independently of Toll-like receptor adaptor protein MyD88 but uncoupling of Gi-protein-mediated signaling with pertussis toxin prevents PKB phosphorylation. Moreover, in the presence of PI 3-kinase inhibitors or pertussis toxin, not only PKB activation but also ERK1/2 activation during T. gondii infection is defective. Most importantly, the parasite's ability to induce macrophage resistance to pro-apoptotic signaling is prevented by incubation with PI 3-kinase inhibitors. This study demonstrates that T. gondii exploits host Gi-protein-dependent PI 3-kinase signaling to prevent induction of apoptosis in infected macrophages.

The host cell transcription factor hypoxia-inducible factor 1 is required for Toxoplasma gondii growth and survival at physiological oxygen levels

Toxoplasma gondii is an obligate intracellular protozoan pathogen. We previously found that genes mediating cellular responses to hypoxia were upregulated in Toxoplasma -infected cells but not in cells infected with another intracellular pathogen, Trypanosoma cruzi. The inducible expression of these genes is controlled by the hypoxia-inducible factor 1 (HIF1) transcription factor, which is the master regulator of cells exposed to low oxygen. Because this response may be important for parasites to grow at physiological oxygen levels, we tested the hypothesis that HIF1 is important for Toxoplasma growth. Here, we demonstrate that Toxoplasma infection rapidly increased the abundance of the HIF1alpha subunit and activated HIF1 reporter gene expression. In addition, we found that Toxoplasma growth and survival was severely reduced in HIF1alpha knockout cells at 3% oxygen. While HIF1alpha was not required for parasite invasion, we determined that HIF1 was required for parasite cell division and organelle maintenance at 3% oxygen. These data indicate that Toxoplasma activates HIF1 and requires HIF1 for growth and survival at physiologically relevant oxygen levels.

Toxoplasma gondii inhibits ultraviolet light-induced apoptosis through multiple interactions with the mitochondrion-dependent programmed cell death pathway

Cells infected with the protozoan parasite Toxoplasma gondii are resistant to diverse apoptotic stimuli. In this study, we perform a detailed analysis of the manipulation of the mitochondrial arm of the apoptotic cascade by the parasite. Apoptosis was induced using irradiation with ultraviolet light (UV), and the kinetics of caspase activation, cytochrome c release and activation of the upstream signalling pathways were examined. The evidence clearly points to T. gondii targeting multiple steps in the transmission [inhibition of c-Jun N-terminal kinase (JNK) activation in response to UV], triggering (inhibition of cytochrome c release by affecting the balance of pro- and anti-apoptotic BCL-2 family members) and execution (inhibition of caspase 9 and caspase 3) phases of the apoptotic cascade. Interestingly, the multilevel pattern of inhibition that emerges suggests that the global inhibition of the mitochondrial arm of apoptosis is not likely to be contributed to by the small subset of mitochondria recruited to the T. gondii parasitophorous vacuole membrane.

Modulation of tumor necrosis factor by microbial pathogens

In response to invasion by microbial pathogens, host defense mechanisms get activated by both the innate and adaptive arms of the immune responses. TNF (tumor necrosis factor) is a potent proinflammatory cytokine expressed by activated macrophages and lymphocytes that induces diverse cellular responses that can vary from apoptosis to the expression of genes involved in both early inflammatory and acquired immune responses. A wide spectrum of microbes has acquired elegant mechanisms to overcome or deflect the host responses mediated by TNF. For example, modulatory proteins encoded by multiple families of viruses can block TNF and TNF-mediated responses at multiple levels, such as the inhibition of the TNF ligand or its receptors, or by modulating key transduction molecules of the TNF signaling pathway. Bacteria, on the other hand, tend to modify TNF-mediated responses specifically by regulating components of the TNF signaling pathway. Investigation of these diverse strategies employed by viral and bacterial pathogens has significantly advanced our understanding of both host TNF responses and microbial pathogenesis. This review summarizes the diverse microbial strategies to regulate TNF and how such insights into TNF modulation could benefit the treatment of inflammatory or autoimmune diseases.

Early transcriptional response of human neutrophils to Anaplasma phagocytophilum infection

Anaplasma phagocytophilum, an unusual obligate intracellular pathogen that persists within neutrophils, causes human anaplasmosis (previously known as human granulocytic ehrlichiosis). To study the effects of this pathogen on the transcriptional profile of its host cell, we performed a comprehensive DNA microarray analysis of the early (4-h) transcriptional response of human neutrophils to A. phagocytophilum infection. A. phagocytophilum infection resulted in the up- and down-regulation of 177 and 67 neutrophil genes, respectively. These data were verified by quantitative reverse transcription-PCR of selected genes. Notably, the up-regulation of many antiapoptotic genes, including the BCL2A1, BIRC3, and CFLAR genes, and the down-regulation of the proapoptotic TNFSF10 gene were observed. Genes involved in inflammation, innate immunity, cytoskeletal remodeling, and vesicular transport also exhibited differential expression. Vascular endothelial growth factor was also induced. These data suggest that A. phagocytophilum may alter selected host pathways in order to facilitate its survival within human neutrophils. To gain further insight into the bacterium's influence on host cell gene expression, this report presents a detailed comparative analysis of our data and other gene expression profiling studies of A. phagocytophilum-infected neutrophils and promyelocytic cell lines.

Neospora caninum and neosporosis — recent achievements in host and parasite cell biology and treatment

Neospora caninum is an apicomplexan parasite, which owes its importance to the fact that it represents the major infectious cause of bovine abortion worldwide. Its life cycle is comprised of three distinct stages: Tachyzoites, representing the proliferative and disease-causing stage, bradyzoites, representing a slowly replicating, tissue cyst-forming stage, and sporozoites, which represent the end product of a sexual process taking place within the intestinal tissue of the final canine host. Tachyzoites are capable of infecting a large variety of host cells in vitro and in vivo, while bradyzoites have been found mainly within the central nervous system. In order to survive, proliferate, and proceed in its life cycle, N. caninum has evolved some amazing features. First, the parasite profits immensely from its ability to interact with, and invade, a large number of host cell types. Secondly, N. caninum exploits its capability to respond to alterations in living conditions by converting into another stage (tachyzoite-to-bradyzoite or vice versa). Thirdly, this parasite has evolved mechanisms that modulate its host cells according to its own requirements, and these must, especially in the case of the bradyzoite stage, involve mechanisms that ensure long term survival of not only the parasite but also of the host cell. These three key events (host cell invasion — stage conversion — host cell modulation) represent potential targets for intervention. In order to elucidate the molecular and cellular bases of these important features of N. caninum, cell culture-based approaches and laboratory animal models are extensively exploited. In this review, we will summarize the present knowledge and achievements related to host cell and parasite cell biology.

Protein kinases as targets for antimalarial intervention: Kinomics, structure-based design, transmission-blockade, and targeting host cell enzymes

The surge of interest in protein kinases as targets for chemotherapeutic intervention in a number of diseases such as cancer and neurodegenerative disorders has stimulated research aimed at determining whether enzymes of this class might also be considered as targets in the context of diseases caused by parasitic protists. Here, we present an overview of recent developments in this field, concentrating (i) on the benefits gained from the availability of genomic databases for a number of parasitic protozoa, (ii) on the emerging field of structure-aided design of inhibitors targeting protein kinases of parasitic protists, (iii) on the concept known as transmission-blockade, whereby kinases implicated in the development of the parasite in their arthropod vector might be targeted to interfere with disease transmission, and (iv) on the possibility of controlling parasitic diseases through the inhibition of host cell protein kinases that are required for the establishment of infection by the parasites.

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.

In vitro efficacies of nitazoxanide and other thiazolides against Neospora caninum tachyzoites reveal antiparasitic activity independent of the nitro group

The thiazolide nitazoxanide [2-acetolyloxy-N-(5-nitro-2-thiazolyl)benzamide] (NTZ) exhibits a broad spectrum of activities against a wide variety of intestinal and tissue-dwelling helminths, protozoa, and enteric bacteria infecting animals and humans. The drug has been postulated to act via reduction of its nitro group by nitroreductases, including pyruvate ferredoxin oxidoreductase. In this study, we investigated the efficacies of nitazoxanide and a number of other thiazolides against Neospora caninum tachyzoites in vitro. We employed real-time-PCR-based monitoring of tachyzoite adhesion, invasion, and intracellular proliferation, as well as electron microscopic visualization of the effects imposed by nitazoxanide. In addition, we investigated several modified versions of this drug. These modifications included on one hand the replacement of the nitro group on the thiazole ring with a bromide, thus removing the most reactive group, and on the other hand the differential positioning of methyl groups on the salicylate ring. We show that the thiazole-associated nitro group is not necessarily required for the action of the drug and that methylation of the salicylate ring can result in complete abrogation of the antiparasitic activity, depending on the positioning of the methyl group. These findings indicate that other mechanisms besides the proposed mode of action involving the pyruvate ferredoxin oxidoreductase enzyme could be responsible for the wide spectrum of antiparasitic activity of NTZ and that modifications in the benzene ring could be important in these alternative mechanisms.

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.

Apoptosis and its modulation during infection with Toxoplasma gondii: molecular mechanisms and role in pathogenesis

Infection with the obligate intracellular protozoan Toxoplasma gondii leads to lifelong persistence of the parasite in its mammalian hosts including humans. Apoptosis plays crucial roles in the interaction between the host and the parasite. This includes innate and adaptive defense mechanisms to restrict intracellular parasite replication as well as regulatory functions to modulate the host's immune response. Not surprisingly, however, T. gondii also extensively modifies apoptosis of its own host cell or of uninfected bystander cells. After infection, apoptosis is triggered in T lymphocytes and other leukocytes, thereby leading to suppressed immune responses to the parasite. T cell apoptosis may be largely mediated by Fas engagement but also occurs independently of Fas under certain conditions. Depending on the magnitude of T cell apoptosis, it is either associated with unrestricted parasite replication and severe pathology or facilitates a stable parasite-host-interaction. However, T. gondii has also evolved strategies to inhibit host cell apoptosis. Apoptosis is blocked by indirect mechanisms in uninfected bystander cells, thereby modulating the inflammatory response to the parasite. In contrast, inhibition of apoptosis in infected host cells by direct interference with apoptosis-signaling cascades is thought to facilitate the intracellular development of T. gondii. Blockade of apoptosis by intracellular parasites may be achieved by different means including interference with the caspase cascade, increased expression of antiapoptotic molecules by infected host cells, and a decreased activity of the poly(ADP-ribose) polymerase. The intriguing dual activity of T. gondii to both promote and inhibit apoptosis requires a tight regulation to promote a stable parasite host-interaction and establishment of persistent toxoplasmosis.

1,25-Dihydroxyvitamin D3 induces splenocyte apoptosis and enhances BALB/c mice sensitivity to toxoplasmosis

The hormonal form of Vitamin D, 1,25-dihydroxyvitamin D3, is well known for its immunosuppressive, anti-proliferative and pro-apoptotic activities. In the present work, we studied the effect of 1,25-dihydroxyvitamin D3 on Toxoplasma gondii-infected mice. We observed that 1,25-dihydroxyvitamin D3 reduces the survival rate of infected mice by up to 37% at day 10 post-infection compared to untreated infected mice (P < 0.0001). IFN-gamma and IL-12p40 levels were significantly reduced by 1,25-dihydroxyvitamin D3 in infected mice sera indicating an inhibition of Th-1-type cytokines. CD4+ T lymphocyte and splenocyte counts were also reduced following 1,25-dihydroxyvitamin D3 treatment and a marked induction of apoptosis, accompanied with down-regulation of the anti-apoptotic proteins Bcl-2 and Bcl-X(L), was observed. The above results indicate that 1,25-dihydroxyvitamin D3 induces splenocyte apoptosis and enhances host susceptibility to toxoplasmosis.

Tpl2/cot signals activate ERK, JNK, and NF-kappaB in a cell-type and stimulus-specific manner

Macrophages and B-cells from Tpl2 knock-out mice exhibit a restricted defect in lipopolysaccharide and death receptor signaling that is limited to the activation of ERK. Here we show that Tpl2-/- MEFs exhibit defects in ERK, JNK, and NF-kappaB activation, or ERK activation only when stimulated with tumor necrosis factor-alpha (TNF-alpha) or interleukin-1beta, respectively. In addition, we show that the activation of Tpl2 by TNF-alpha depends on signals transduced by both TRAF2 and RIP1. Activated Tpl2 phosphorylates MKK4/SEK1 upstream of JNK and stimulates NF-kappaB DNA binding and transcriptional activity by mechanisms that are independent of the nuclear translocation of p50 and p65. Tpl2-transduced TNF-alpha signals instead promote the phosphorylation of p65 at Ser276 and modulate the spectrum of proteins associated with p65. Phosphorylation stimulates the transcriptional activity of NF-kappaB but does not affect its ability to bind DNA, which may be affected by the composition of the nuclear NF-kappaB complexes. These data confirm that defects caused by a single mutation may be cell-type and signal-specific and delineate the role of Tpl2 in the transduction of TNF-alpha signals that activate JNK and NF-kappaB in MEFs.

Survival of protozoan intracellular parasites in host cells

The most common human diseases are caused by pathogens. Several of these microorganisms have developed efficient ways in which to exploit host molecules, along with molecular pathways to ensure their survival, differentiation and replication in host cells. Although the contribution of the host cell to the development of many intracellular pathogens (particularly viruses and bacteria) has been unequivocally established, the study of host-cell requirements during the life cycle of protozoan parasites is still in its infancy. In this review, we aim to provide some insight into the manipulation of the host cell by parasites through discussing the hurdles that are faced by the latter during infection.

A simple method allowing DIC imaging in conjunction with confocal microscopy

Current optical methods to collect Nomarski differential interference contrast (DIC) or phase images with a transmitted light detector (TLD) in conjunction with confocal laser scanning microscopy (CLSM) can be technically challenging and inefficient. We describe for the first time a simple method that combines the use of the commercial product QPm (Iatia, Melbourne Australia) with brightfield images collected with the TLD of a CLSM, generating DIC, phase, Zernike phase, dark-field or Hoffman modulation contrast images. The brightfield images may be collected at the same time as the confocal images. This method also allows the calculation of contrast-enhanced images from archival data. The technique described here allows for the creation of contrast-enhanced images such as DIC or phase, without compromising the intensity or quality of confocal images collected simultaneously. Provided the confocal microscope is equipped with a motorized z-drive and a TLD, no hardware or optical modifications are required. The contrast-enhanced images are calculated with software using the quantitative phase-amplitude microscopy technique (Barone-Nugent et al., 2002). This technique, being far simpler during image collection, allows the microscopist to concentrate on their confocal imaging and experimental procedures. Unlike conventional DIC, this technique may be used to calculate DIC images when cells are imaged through plastic, and without the use of expensive strain-free objective lenses.

Manipulation of mitogen-activated protein kinase/nuclear factor-kappaB-signaling cascades during intracellular Toxoplasma gondii infection

The intracellular protozoan Toxoplasma gondii exerts profound effects on nuclear factor-kappaB (NF-kappaB)- and mitogen-activated protein kinase (MAPK)-signaling cascades in macrophages. During early infection, nuclear translocation of NF-kappaB is blocked, and later, the cells display defects in lipopolysaccharide (LPS)-induced MAPK phosphorylation after undergoing initial activation in response to Toxoplasma itself. Infected macrophages that are subjected to triggering through Toll-like receptor 4 (TLR4) with LPS display defective production of tumor necrosis factor-alpha and IL-12 (IL-12) that likely reflects interference with NF-kappaB- and MAPK-signaling cascades. Nevertheless, T. gondii possesses molecules that themselves induce eventual proinflammatory cytokine synthesis. For interleukin-12, this occurs through both myeloid differentiation factor 88-dependent and chemokine receptor CCR5-dependent pathways. The balance between activation and interference with proinflammatory signaling is likely to reflect the need to achieve an appropriate level of immunity that allows the host and parasite to maintain a stable interaction.

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

The NF-kappaB family of transcription factors is part of an evolutionarily conserved system that plays an important role in the regulation of genes associated with the development of innate and adaptive responses required for the recognition and immunologic control of pathogens [Clin. Microbiol. Rev. 15 (2002) 414; Annu. Rev. Immunol. 16 (1998) 225; Infect. Immun. 70 (2002) 3311]. In addition, NF-kappaB regulates other cellular processes required for a coordinated immune response, such as cellular growth and differentiation, cell adhesion, survival and apoptosis. Recent studies have highlighted the prominent role played by the NF-kappaB system in resistance to Toxoplasma gondii but it is becoming apparent that this intracellular parasite can directly modulate this signalling pathway. This article briefly reviews the biology of NF-kappaB, examines the complex interaction that takes place between T. gondii and these transcription factors in infected cells, and highlights the role of different NF-kappaB family members during the development of a protective immune response to this pathogen.

Mechanisms underlying the manipulation of host apoptotic pathways by Toxoplasma gondii

The establishment of a productive infection by an obligate intracellular pathogen is dependent on subversion of cellular defences. Apoptosis, or programmed cell death, is a property of metazoan cells that plays a critical role in inhibiting the proliferation of invasive organisms and viruses thereby protecting uninfected cells and limiting damage to the host organism. Not surprisingly, manipulation of the machinery of apoptosis plays a critical role in the pathogenesis of several intracellular pathogens. Toxoplasma gondii, arguably one of the most successful protozoan pathogens, has evolved several strategies to inhibit both the initiation and propagation of the apoptotic cascade. Recent work from several groups indicates an exquisite level of sophistication in the mechanisms to inhibit apoptosis along its diverse pathways. Much of this ability appears to centre around the manipulation of host transcription, specifically of genes involved in the pro-survival/anti-apoptotic response effectively manipulating the infected cell into a highly anti-apoptotic state. The implications of these observations extend beyond Toxoplasma biology to the broader area of microbial pathogenesis and cell signalling in mammalian cells.

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

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