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

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.

Host cell Ca2+ and protein kinase C regulate innate recognition of Toxoplasma gondii

In healthy hosts, acute infection with the opportunistic pathogen Toxoplasma gondii is controlled by innate production of IL-12, a key cytokine crucial for the development of protective immunity. Previous work has established that the mitogen-activated protein kinases (MAPK), particularly p38 and ERK1/2, are important regulators of T. gondii-induced IL-12 synthesis. Here we report that host cell Ca(2+) is required for activation of MAPK by T. gondii, as well as LPS and CpG, and for parasite-induced synthesis of IL-12. In addition, pharmacological mobilization of Ca(2+) stores in macrophages treated with parasites or LPS enhanced MAPK phosphorylation initiated by these stimuli. Investigation of the upstream mechanism by which Ca(2+) regulates MAPK activation revealed that T. gondii induced acute activation of conventional, Ca(2+)-dependent PKCalpha and PKCbeta, which are required for infection-induced MAPK activation and production of IL-12. Despite these findings, neither acute parasite infection nor LPS initiated a measurable Ca(2+) response in macrophages, suggesting that low levels of Ca(2+) are permissive for initiation of pro-inflammatory signaling. Together these data identify host cell Ca(2+) and PKC as crucial regulators of the innate immune response to microbial stimuli, including T. gondii.

Protozoan encounters with Toll-like receptor signalling pathways: implications for host parasitism

Toll-like receptors (TLRs) have emerged as a major receptor family involved in non-self recognition. They have a vital role in triggering innate immunity and orchestrate the acquired immune response during bacterial and viral infection. However, the role of TLRs during infection with protozoan pathogens is less clear. Nevertheless, our understanding of how these parasitic microorganisms engage the host TLR signalling system has now entered a phase of rapid expansion. This Review describes recent insights into how parasitic protozoans are sensed by TLR molecules, and how the TLR system itself can be targeted by these microbial pathogens for their own survival.

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.

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.

Toxoplasma gondii genotype determines MyD88-dependent signaling in infected macrophages

Infection of mouse macrophages with Toxoplasma gondii elicits MAPK activation and IL-12 production, but host cell signaling pathways have not been clearly delineated. Here, we compared macrophage signaling in response to high virulence type I (RH) vs low virulence type II (ME49) strain infection. Tachyzoites of both strains induced p38 MAPK-dependent macrophage IL-12 release, although ME49 elicited 2- to 3-fold more cytokine than RH. IL-12 production was largely restricted to infected cells in each case. RH-induced IL-12 release did not require MyD88, whereas ME49-triggered IL-12 production was substantially dependent on this TLR/IL-1R adaptor molecule. MyD88 was also not required for RH-stimulated p38 MAPK activation, which occurred in the absence of detectable upstream p38 MAPK kinase activity. In contrast, ME49-driven p38 MAPK activation displayed an MyD88-dependent component. This parasite strain also induced MyD88-dependent activation of MKK4, an upstream activator of p38 MAPK. The results suggest that RH triggers MAPK activation and IL-12 production using MyD88-independent signaling, whereas ME49 uses these pathways as well as MyD88-dependent signaling cascades. Differences in host signaling pathways triggered by RH vs ME49 may contribute to the high and low virulence characteristics displayed by these parasite strains.

Toxoplasma gondii inhibits toll-like receptor 4 ligand-induced mobilization of intracellular tumor necrosis factor alpha to the surface of mouse peritoneal neutrophils

Neutrophils are well-known to rapidly respond to infection through chemotactic infiltration at sites of inflammation, followed by rapid release of microbicidal molecules, chemokines, and proinflammatory cytokines. For tumor necrosis factor alpha (TNF-alpha), we recently found that neutrophils contain intracellular pools of the cytokine and display the capacity to upregulate transcriptional activity of the gene during lipopolysaccharide (LPS) stimulation. We now show that triggering of mouse peritoneal neutrophils with Toll-like receptor 2 (TLR2), TLR4, and TLR9 ligands, but not ligands of TLR3, induces upregulation of surface membrane TNF-alpha. However, neutrophils infected with the protozoan Toxoplasma gondii displayed an inability to respond fully in terms of TLR ligand-induced increases in membrane TNF-alpha expression. Infected neutrophils failed to display decreased levels of intracellular TNF-alpha upon LPS exposure. In contrast to intermediate inhibitory effects in nontreated neutrophils, T. gondii induced a complete blockade in LPS-induced surface TNF-alpha expression in the presence of the protein synthesis inhibitor cycloheximide. Despite these inhibitory effects, the parasite did not affect LPS-induced upregulation of TNF-alpha gene transcription. Collectively, the results show that Toxoplasma prevents TLR ligand-triggered mobilization of TNF-alpha to the neutrophil surface, revealing a novel immunosuppressive activity of the parasite.

CD40 restrains in vivo growth of Toxoplasma gondii independently of gamma interferon

CD40-CD154 interaction is pivotal for resistance against numerous pathogens. However, it is not known if this pathway can also enhance in vivo resistance in gamma interferon (IFN-gamma)-deficient hosts. This is an important question because patients and mice with defects in type 1 cytokine response can control a variety of pathogens. While blockade of endogenous CD154 resulted in a remarkable increase in parasite load in IFN-gamma-/- mice infected with Toxoplasma gondii, in vivo administration of a stimulatory anti-CD40 monoclonal antibody markedly reduced parasite load. This latter effect took place even in T-cell-depleted mice and was accompanied by induction of macrophage toxoplasmacidal activity. CD40 stimulation restricted T. gondii replication independently of STAT1, p47 GTPases, and nitric oxide. In vivo CD40 ligation enhanced tumor necrosis factor alpha (TNF-alpha) production by T. gondii-infected macrophages. In addition, CD40 stimulation required the presence of TNF receptor 2 to reduce parasite load in vivo. These results suggest that CD40-CD154 interaction regulates IFN-gamma-independent mechanisms of host protection through induction of macrophage antimicrobial activity and modulation of TNF-alpha signaling.

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.

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.

Induction of suppressor of cytokine signaling-1 by Toxoplasma gondii contributes to immune evasion in macrophages by blocking IFN-gamma signaling

Toxoplasma gondii is an intracellular parasite that survives and multiplies in professional phagocytes such as macrophages. Therefore, T. gondii has to cope with the panel of antimicrobial host immune mechanisms, among which IFN-gamma plays a crucial role. We report in this study that in vitro infection of murine macrophages with viable, but not with inactivated, parasites results in inhibition of IFN-gamma signaling within the infected cells. Thus, infection of RAW264.7 macrophages with tachyzoites inhibited IFN-gamma-induced STAT-1 tyrosine phosphorylation, mRNA expression of target genes, and secretion of NO. These effects were dependent on direct contact of the host cells with living parasites and were not due to secreted intermediates. In parallel, we report the induction of suppressor of cytokine signaling-1 (SOCS-1), which is a known feedback inhibitor of IFN-gamma receptor signaling. SOCS-1 was induced directly by viable parasites. SOCS overexpression in macrophages did not affect tachyzoite proliferation per se, yet abolished the inhibitory effects of IFN-gamma on parasite replication. The inhibitory effects of T. gondii on IFN-gamma were diminished in macrophages from SOCS-1-/- mice. The results suggest that induction of SOCS proteins within phagocytes due to infection with T. gondii contributes to the parasite's immune evasion strategies.

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.

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.

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.

Balance between NF-κB and JNK/AP-1 activity controls dendritic cell life and death

The life cycle of dendritic cells (DCs) must be precisely regulated for proper functioning of adaptive immunity. However, signaling pathways actively mediating DC death remain enigmatic. Here we describe a novel mechanism of hierarchical transcriptional control of DC life and death. Ligation of tumor necrosis factor receptor superfamily (TNFR-SF) members on DCs and cognate contact with T cells resulted in quantitatively balanced nuclear factor-κB (NF-κB) and c-Jun N-terminal kinase (JNK)–mediated activator protein-1 (AP-1) induction and strongly enhanced DC longevity. Specific blockade of NF-κB in DCs induced strongly augmented JNK/AP-1 activity because of elevated levels of reactive oxygen species. In this scenario, DC activation by TNFR-SF members or T cells induced DC apoptosis. Specific inhibition of JNK/AP-1 rescued DCs from this activation-induced cell death program and restored TNFR-SF member- and T-cell–mediated survival. We conclude that JNK/AP-1 activity is under negative feedback control of NF-κB and can execute apoptosis in DCs. Thus, feedback-controlled signaling amplitudes of 2 transcriptional pathways decide the fate of a DC.

p38 MAPK autophosphorylation drives macrophage IL-12 production during intracellular infection

The intracellular protozoan Toxoplasma gondii triggers rapid MAPK activation in mouse macrophages (Mphi). We used synthetic inhibitors and dominant-negative Mphi mutants to demonstrate that T. gondii triggers IL-12 production in dependence upon p38 MAPK. Chemical inhibition of stress-activated protein kinase/JNK showed that this MAPK was also required for parasite-triggered IL-12 production. Examination of upstream MAPK kinases (MKK) 3, 4, and 6 that function as p38 MAPK activating kinases revealed that parasite infection activates only MKK3. Nevertheless, in MKK3(-/-) Mphi, p38 MAPK activation was near normal and IL-12 production was unaffected. Recently, MKK-independent p38alpha MAPK activation via autophosphorylation was described. Autophosphorylation depends upon p38alpha MAPK association with adaptor protein, TGF-beta-activated protein kinase 1-binding protein-1. We observed TGF-beta-activated protein kinase 1-binding protein-1-p38alpha MAPK association that closely paralleled p38 MAPK phosphorylation during Toxoplasma infection of Mphi. Furthermore, a synthetic p38 catalytic-site inhibitor blocked tachyzoite-induced p38alpha MAPK phosphorylation. These data are the first to demonstrate p38 MAPK autophosphorylation triggered by intracellular infection.

Escherichia coliK1 inhibits proinflammatory cytokine induction in monocytes by preventing NF-κB activation

Phagocytes are well-known effectors of the innate immune system to produce proinflammatory cytokines and chemokines such as tumor necrosis factor alpha (TNF-alpha), interleukin (IL)-1beta, and IL-8 during infections. Here, we show that infection of monocytes with wild-type Escherichia coli K1, which causes meningitis in neonates, suppresses the production of cytokines and chemokines (TNF-alpha, regulated on activation, normal T expressed and secreted, macrophage-inflammatory protein-1beta, IL-1beta, and IL-8). In contrast, infection of monocytes with a mutant E. coli, which lacks outer membrane protein A (OmpA- E. coli) resulted in robust production of cytokines and chemokines. Wild-type E. coli K1 (OmpA+ E. coli) prevented the phosphorylation and its degradation of inhibitor of kappaB, thereby blocking the translocation of nuclear factor (NF)-kappaB to the nucleus. OmpA+ E. coli-infected cells, subsequently subjected to lipopolysaccharide challenge, were crippled severely in their ability to activate NF-kappaB to induce cytokine/chemokine production. Selective inhibitors of the extracellular signal-regulated kinase (ERK) 1/2 pathway and p38 mitogen-activated protein kinase (MAPK), but not Jun N-terminal kinase, significantly reduced the activation of NF-kappaB and the production of cytokines and chemokines induced by OmpA- E. coli, indicating a role for these kinases in the NF-kappaB/cytokine pathway. It is interesting that the phosphorylation of ERK 1/2 and p38 MAPK was notably reduced in monocytes infected with OmpA+ E. coli when compared with monocytes infected with OmpA- E. coli, suggesting that the modulation of upstream events common for NF-kappaB and MAPKs by the bacterium is possible. The ability of OmpA+ E. coli K1 to inhibit the macrophage response temporarily may enable bacterial survival and growth within the host for the onset of meningitis by E. coli K1.

Francisella tularensis LVS initially activates but subsequently down-regulates intracellular signaling and cytokine secretion in mouse monocytic and human peripheral blood mononuclear cells

Monocytic cells constitute an important defense mechanism against invading pathogens by recognizing conserved pathogens components. The recognition leads to activation of intracellular pathways involving nuclear factor kappa B (NF-kappaB) and mitogen-activated protein kinases (MAPK), such as the c-Jun NH2-terminal kinase (JNK), and p38. We show that in vitro infection with Francisella tularensis results in activation of NF-kappaB, phosphorylation of p38 and c-Jun, and secretion of TNF-alpha in adherent mouse peritoneal cells, in the mouse macrophage-like cell line J774A.1, in the human macrophage cell line THP-1, and in human peripheral blood monocytic cells. This occurred after infection with the human live vaccine strain, F. tularensis LVS or a mutant strain denoted deltaiglC, which lacks expression of a 23-kDa protein, or after addition of killed F. tularensis LVS. Addition of purified F. tularensis LPS resulted in no discernible effects on the cells. When the effects were followed up to 5 h, activation persisted in cultures with killed bacteria or infected with the deltaiglC strain. In contrast, the signal transduction activation and secretion of TNF-alpha were down-regulated within the 5h period in mouse peritoneal cells, J774 cells or human peripheral blood mononuclear cells infected with F. tularensis LVS. Together, the results suggest that infection with live F. tularensis LVS bacteria leads to a rapid induction of a proinflammatory response in mouse and human cells but after internalization of bacteria, this response is completely or partly down-regulated in most cell types. This down-regulation does not occur when cells are infected with the mutant deltaiglC.

IL-10-independent STAT3 activation by Toxoplasma gondii mediates suppression of IL-12 and TNF-alpha in host macrophages

Infection of mouse macrophages by Toxoplasma gondii renders the cells resistant to proinflammatory effects of LPS triggering. In this study, we show that cell invasion is accompanied by rapid and sustained activation of host STAT3. Activation of STAT3 did not occur with soluble T. gondii extracts or heat-killed tachyzoites, demonstrating a requirement for live parasites. Parasite-induced STAT3 phosphorylation and suppression of LPS-triggered TNF-alpha and IL-12 was intact in IL-10-deficient macrophages, ruling out a role for this anti-inflammatory cytokine in the suppressive effects of T. gondii. Most importantly, Toxoplasma could not effectively suppress LPS-triggered TNF-alpha and IL-12 synthesis in STAT3-deficient macrophages. These results demonstrate that T. gondii exploits host STAT3 to prevent LPS-triggered IL-12 and TNF-alpha production, revealing for the first time a molecular mechanism underlying the parasite's suppressive effect on macrophage proinflammatory cytokine production.

New lessons from old pathogens: what parasitic infections have taught us about the role of nuclear factor-kappaB in the regulation of immunity

The nuclear factor-kappaB (NF-kappaB) family of transcription factors is activated by many infectious and inflammatory stimuli. This family regulates the expression of multiple genes, whose products include cytokines, chemokines, adhesion molecules, and antiapoptotic factors that are important components of the innate and adaptive immune response. A prominent role of NF-kappaB transcription factors in resistance to a variety of infectious diseases was revealed by studies with mice that lack individual family members. However, little is known about the basis for these effects or about the role of individual family members during a coordinated immune response to infection. Diverse parasites such as Toxoplasma gondii, Leishmania major, and Trichuris muris provide a unique opportunity to understand the role of the NF-kappaB system in the development of innate and adaptive immunity to these infections. The basis for resistance and susceptibility to these parasites is well understood, and studies using these experimental systems have provided unique insights into the role of NF-kappaB in the regulation of T-helper 1 cell (Th1) and Th2 type responses. It has become clear that NF-kappaB family members have cell lineage-specific functions and that their relative importance varies with type of infection as well as route of pathogen entry. Thus, studies with models of parasitic infection have revealed that individual NF-kappaB family members perform distinct, nonoverlapping, and biologically significant functions in the regulation of immunity and inflammation.

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.

Effect of glutamate on inflammatory responses of intestine and brain after focal cerebral ischemia

AIM: To study the modulation of glutamate on post-ischemic intestinal and cerebral inflammatory responses in a ischemic and excitotoxic rat model.

METHODS: Adult male rats were subjected to bilateral carotid artery occlusion for 15 min and injection of monosodium glutamate intraperitoneally, to decapitate them at selected time points. Tumor necrosis factor alpha (TNF-α) level and nuclear factor kappa B (NF-κB) activity were determined by enzyme-linked immunosorbant assay (ELISA) and electrophoretic mobility shift assay (EMSA), respectively. Hemodynamic parameters were monitored continuously during the whole process of cerebral ischemia and reperfusion.

RESULTS: Monosodium glutamate (MSG) treated rats displayed statistically significant high levels of TNF-α in cerebral and intestinal tissues within the first 6 h of ischemia. The rats with cerebral ischemia showed a minor decrease of TNF-α production in cerebral and intestinal tissues. The rats with cerebral ischemia and treated with MSG displayed statistically significant low levels of TNF-α in cerebral and intestinal tissues. These results correlated significantly with NF-κB production calculated at the same intervals. During experiment, the mean blood pressure and heart rates in all groups were stable.

CONCLUSION: Glutamate is involved in the mechanism of intestinal and cerebral inflammation responses. The effects of glutamate on cerebral and intestinal inflammatory responses after ischemia are up-regulated at the transcriptional level, through the NF-κB signal transduction pathway.

Toxoplasma gondii exposes phosphatidylserine inducing a TGF-β1 autocrine effect orchestrating macrophage evasion

Toxoplasmosis is a worldwide disease caused by Toxoplasma gondii. Activated macrophages control T. gondii growth by nitric oxide (NO) production. However, T. gondii active invasion inhibits NO production, allowing parasite persistence. Here we show that the mechanism used by T. gondii to inhibit NO production persisting in activated macrophages depends on phosphatidylserine (PS) exposure. Masking PS with annexin-V on parasites or activated macrophages abolished NO production inhibition and parasite persistence. NO production inhibition depended on a transforming growth factor-beta1 (TGF-beta1) autocrine effect confirmed by the expression of Smad 2 and 3 in infected macrophages. TGF-beta1 led to inducible nitric oxide synthase (iNOS) degradation, actin filament (F-actin) depolymerization, and lack of nuclear factor-kappaB (NF-kappaB) in the nucleus. All these features were reverted by TGF-beta1 neutralizing antibody treatment. Thus, T. gondii mimics the evasion mechanism used by Leishmania amazonensis and also the anti-inflammatory response evoked by apoptotic cells.

TRAF6-dependent mitogen-activated protein kinase activation differentially regulates the production of interleukin-12 by macrophages in response to Toxoplasma gondii

The production of interleukin-12 (IL-12) is critical to the development of innate and adaptive immune responses required for the control of intracellular pathogens. Many microbial products signal through Toll-like receptors (TLR) and activate NF-kappaB family members that are required for the production of IL-12. Recent studies suggest that components of the TLR pathway are required for the production of IL-12 in response to the parasite Toxoplasma gondii; however, the production of IL-12 in response to this parasite is independent of NF-kappaB activation. The adaptor molecule TRAF6 is involved in TLR signaling pathways and associates with serine/threonine kinases involved in the activation of both NF-kappaB and mitogen-activated protein kinase (MAPK). To elucidate the intracellular signaling pathways involved in the production of IL-12 in response to soluble toxoplasma antigen (STAg), wild-type and TRAF6(-/-) mice were inoculated with STAg, and the production of IL-12(p40) was determined. TRAF6(-/-) mice failed to produce IL-12(p40) in response to STAg, and TRAF6(-/-) macrophages stimulated with STAg also failed to produce IL-12(p40). Studies using Western blot analysis of wild-type and TRAF6(-/-) macrophages revealed that stimulation with STAg resulted in the rapid TRAF6-dependent phosphorylation of p38 and extracellular signal-related kinase, which differentially regulated the production of IL-12(p40). The studies presented here demonstrate for the first time that the production of IL-12(p40) in response to toxoplasma is dependent upon TRAF6 and p38 MAPK.

Functional inactivation of immature dendritic cells by the intracellular parasite Toxoplasma gondii

Despite its noted ability to induce strong cellular immunity, and its known susceptibility to IFN-gamma-dependent immune effector mechanisms, the protozoan Toxoplasma gondii is a highly successful parasite, able to replicate, disseminate, and either kill the host or, more commonly, establish resistant encysted life forms before the emergence of protective immune responses. We sought to understand how the parasite gains the advantage. Using transgenic clonal parasite lines engineered to express fluorescent markers in combination with dendritic cells (DC) grown from the bone marrow of wild-type mice or transgenic mice expressing fluorescent protein-tagged MHC class II molecules, we used flow cytometry and fluorescence microscopy to analyze the responses of infected DC to both invasion by the parasite and subsequent DC maturation signals. We found that T. gondii preferentially invades immature dendritic cells but fails to activate them in the process, and renders them resistant to subsequent activation by TLR ligands or the immune-system-intrinsic maturation signal CD40L. The functional consequences of T. gondii-mediated suppression of DC activation are manifested in a relative inability of infected immature DC to activate naive CD4(+) Th lymphocytes, or to secrete cytokines, such IL-12 and TNF-alpha, that play important roles in innate and/or adaptive immunity. The findings reveal that T. gondii suppresses the ability of immature DC to participate in innate immunity and to induce adaptive immune responses. The ability of T. gondii to temporarily evade recognition could provide a selective advantage that permits dissemination and establishment before adaptive immune response initiation.

Inhibition of lipopolysaccharide-induced macrophage IL-12 production by Leishmania mexicana amastigotes: the role of cysteine peptidases and the NF-kappaB signaling pathway

Infection with lesion-derived Leishmania mexicana amastigotes inhibited LPS-induced IL-12 production by mouse bone marrow-derived macrophages. This effect was associated with expression of cysteine peptidase B (CPB) because amastigotes of CPB deletion mutants had limited ability to inhibit IL-12 production, whereas preincubation of cells with a CPB inhibitor, cathepsin inhibitor IV, was able to suppress the effect of wild-type amastigotes. Infection with wild-type amastigotes resulted in a time-dependent proteolytic degradation of IkappaBalpha and IkappaBbeta and the related protein NF-kappaB. This effect did not occur with amastigotes of CPB deletion mutants or wild-type promastigotes, which do not express detectable CPB. NF-kappaB DNA binding was also inhibited by amastigote infection, although nuclear translocation of cleaved fragments of p65 NF-kappaB was still observed. Cysteine peptidase inhibitors prevented IkappaBalpha, IkappaBbeta, and NF-kappaB degradation induced by amastigotes, and recombinant CPB2.8, an amastigote-specific isoenzyme of CPB, was shown to degrade GST-IkappaBalpha in vitro. LPS-mediated IkappaBalpha and IkappaBbeta degradation was not affected by these inhibitors, confirming that the site of degradation of IkappaBalpha, IkappaBbeta, and NF-kappaB by the amastigotes was not receptor-driven, proteosomal-mediated cleavage. Infection of bone marrow macrophages with amastigotes resulted in cleavage of JNK and ERK, but not p38 MAPK, whereas preincubation with a cysteine peptidase inhibitor prevented degradation of these proteins, but did not result in enhanced protein kinase activation. Collectively, our results suggest that the amastigote-specific cysteine peptidases of L. mexicana are central to the ability of the parasite to modulate signaling via NF-kappaB and consequently inhibit IL-12 production.

Inhibition of Farnesyltransferase Prevents Collagen-Induced Arthritis by Down-Regulation of Inflammatory Gene Expression through Suppression of p21ras-Dependent NF-κB Activation

Farnesylation of p21(ras) is an important step in the intracellular signaling pathway of growth factors, hormones, and immune stimulants. We synthesized a potent and selective farnesyltransferase inhibitor (LB42708) with IC(50) values of 0.8 nM in vitro and 8 nM in cultured cells against p21(ras) farnesylation and examined the effects of this inhibitor in the settings of inflammation and arthritis. LB42708 suppressed NF-kappaB activation and iNOS promoter activity by suppressing the I-kappaB kinase activity and I-kappaBalpha degradation. The inhibitor suppressed the expression of inducible NO synthase, cyclooxygenase-2, TNF-alpha, and IL-1beta and the production of NO and PGE(2) in immune-activated macrophages and osteoblasts as well as LPS-administrated mice. Furthermore, in vivo administration of LB42708 significantly decreased the incidence and severity of arthritis as well as mRNA expression of inducible NO synthase, cyclooxygenase-2, TNF-alpha, and IL-1beta in the paws of collagen-induced arthritic mice compared with controls. These observations indicate that the anti-inflammatory and antiarthritic effects of the farnesyltransferase inhibitor may be ascribed to the inhibition of I-kappaB kinase activity and subsequent suppression of NF-kappaB-dependent inflammatory gene expression through the suppression of p21(ras) farnesylation. Together, these findings reveal that the inhibitory effect of LB42708 on p21(ras)-dependent NF-kappaB activation may have potential therapeutic value for arthritis and other inflammatory diseases.

Toxoplasma gondii interferes with lipopolysaccharide-induced mitogen-activated protein kinase activation by mechanisms distinct from endotoxin tolerance

We show in this study that Toxoplasma gondii infection induces rapid activation of p38 mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase 1/2, and stress-activated protein kinase/c-Jun N-terminal kinase MAPK, followed promptly by their deactivation in mouse macrophages. Nevertheless, when infected cells were subsequently subjected to LPS triggering, MAPK activation was severely defective, in particular in the case of p38 MAPK, which is required for LPS-triggered TNF-alpha and IL-12 production. Similar effects occurred during endotoxin tolerance, but the phenomena were distinct. LPS pretriggering failed to activate the major p38 MAPK kinase, MAPK kinase 3/6. Toxoplasma infection, in contrast, resulted in sustained activation of this kinase. Furthermore, endotoxin pre-exposure blocked IkappaBalpha degradation upon subsequent LPS triggering, but this was not the case for Toxoplasma preinfection. Endotoxin-mediated down-regulation of the LPS receptor, Toll-like receptor 4, has been suggested as one possible mechanism contributing to tolerance, and we found in this study that LPS down-modulated Toll-like receptor 4 expression. In contrast, Toxoplasma infection induced up-regulation of this pattern recognition receptor. Our results show that T. gondii blocks LPS-triggered cytokine production in part through MAPK inactivation, and that this occurs through pathways distinct from endotoxin-induced tolerance.

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.

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.

From cells to signaling cascades: manipulation of innate immunity by Toxoplasma gondii

The intracellular opportunistic protozoan Toxoplasma gondii is a potent stimulus for cell-mediated immunity, and IL-12-dependent IFN-gamma induction is vital in resistance to the parasite. Dendritic cells, neutrophils and macrophages are important sources of IL-12 during infection. T. gondii possesses two mechanisms for triggering IL-12. One is dependent upon the common adaptor protein MyD88, and is likely to involve Toll-like receptors. The other is a more unusual pathway that involves triggering through CCR5 by a parasite cyclophilin molecule. Countering these potent pro-inflammatory activities, T. gondii has several mechanisms to down-regulate immunity. Intracellular infection causes a blockade in the NFkappaB macrophage signaling pathway, correlating with reduced capacity for IL-12 and TNF-alpha production. The parasite also prevents STAT1 activity, resulting in decreased levels of IFN-gamma-stimulated MHC surface antigen expression. Furthermore, infection also induces resistance to apoptosis through inhibition of caspase activity. Extracellular pathways of suppression involve soluble mediators such as IL-10 and lipoxins that have potent IL-12 down-regulatory effects. The balance of pro-inflammatory and anti-inflammatory signaling which T. gondii engages is likely dictated by requirements for a stable host-parasite interaction. First, there is a need for Toxoplasma to induce an immune response robust enough to allow host survival and establish long-term chronic infection. Second, the parasite must avoid immune-elimination and induction of pro-inflammatory pathology that can cause lethality if unchecked. The widespread distribution of T. gondii and the normally innocuous nature of infection indicate the skill with which the parasite achieves the two seemingly contrary goals.

Cross-talk in the innate immune system: neutrophils instruct recruitment and activation of dendritic cells during microbial infection

Type I inflammatory cytokines are essential for immunity to many microbial pathogens, including Toxoplasma gondii. Dendritic cells (DC) are key to initiating type 1 immunity, but neutrophils are also a source of chemokines and cytokines involved in Th1 response ignition. We found that T. gondii triggered neutrophil synthesis of CC chemokine ligand (CCL)3, CCL4, CCL5, and CCL20, chemokines that were strongly chemotactic for immature DC. Moreover, supernatants obtained from parasite-stimulated polymorphonuclear leukocytes induced DC IL-12(p40) and TNF-alpha production. Parasite-triggered neutrophils also released factors that induced DC CD40 and CD86 up-regulation, and this response was dependent upon parasite-triggered neutrophil TNF-alpha production. In vivo evidence that polymorphonuclear leukocytes exert an important influence on DC activation was obtained by examining splenic DC cytokine production following infection of neutrophil-depleted mice. These animals displayed severely curtailed splenic DC IL-12 and TNF-alpha production, as revealed by ex vivo flow cytometric analysis and in vitro culture assay. Our results reveal a previously unrecognized regulatory role for neutrophils in DC function during microbial infection, and suggest that cross-talk between these cell populations is an important component of the innate immune response to infection.

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

Mammalian cells infected with Toxoplasma gondii are resistant to apoptosis induced by a variety of stimuli. We have demonstrated that the host transcription factor NF-kappaB plays a pivotal role in the T.-gondii-mediated blockade of apoptosis because inhibition is lost in cells lacking the p65 (RelA) subunit of NF-kappaB (p65-/-). In the present study, we examined the effects of T. gondii infection on NF-kappaB activation and the expression of genes involved in the apoptotic cascade. Infection of wild-type mouse embryonic fibroblasts (MEFs) with T.-gondii-induced nuclear translocation of the p50 and p65 subunits of NF-kappaB as examined by immunoblotting of nuclear extracts, immunofluorescence and electrophoretic mobility shift assays. A comparison of apoptotic gene expression profiles from wild-type and p65-/- MEFs revealed distinct patterns of induction in response to T. gondii infection. In particular, the differences seen in the Bcl-2 and IAP families are consistent with the antiapoptotic responses observed in the resistant wild-type cells compared with the sensitive p65-/- fibroblasts. Consistent with NF-kappaB activation, T. gondii infection promoted phosphorylation of the inhibitor IkappaB. Interestingly, phosphorylated IkappaB was concentrated on the parasitophorous vacuole membrane (PVM), suggesting a parasite-directed event. Results from this study suggest that activation of NF-kappaB plays an important role in stimulation of antiapoptotic gene expression by T. gondii. Furthermore, recruitment of phosphorylated IkappaB to the PVM implies the presence of intrinsic factor(s) in T. gondii that might be used to manipulate the NF-kappaB signaling pathway in the host to elicit a survival response during infection.

Reduced expression of the inducible nitric oxide synthase after infection with Toxoplasma gondii facilitates parasite replication in activated murine macrophages

Production of nitric oxide by activated murine macrophages is thought to represent an important mechanism to restrict replication of the obligate intracellular parasite Toxoplasma gondii. In this study, we characterised the effect of T. gondii on nitric oxide production and expression of the inducible nitric oxide synthase and determined the functional significance of a parasite-induced evasion of this potential effector mechanism. Infection of primary bone marrow-derived macrophages or monocytic/macrophage RAW264.7 cells with a mouse-avirulent T. gondii strain significantly decreased nitric oxide production that had been induced by activation with either interferon-gamma or lipopolysaccharide or interferon-gamma plus lipopolysaccharide. Importantly, down-regulation of nitric oxide production by T. gondii enabled considerable parasite replication in macrophages activated with interferon-gamma alone or lipopolysaccharide alone. Furthermore, supplementation of endogenous nitric oxide by addition of sodium nitroprusside to levels as observed in uninfected interferon-gamma- or lipopolysaccharide-activated macrophages almost completely abrogated replication of T. gondii. Although T. gondii also partially inhibited the vigorous nitric oxide production induced by interferon-gamma along with lipopolysaccharide, the magnitude of inhibition did not suffice to allow intracellular propagation of the parasite in these synergistically activated macrophages. Inhibition of interferon-gamma-, lipopolysaccharide- and interferon-gamma plus lipopolysaccharide-induced nitric oxide production coincided with reduced inducible nitric oxide synthase protein levels. Such down-regulation required the presence of intracellular parasites as determined by immunofluorescence microscopy. Inducible nitric oxide synthase transcripts induced by interferon-gamma alone or in combination with lipopolysaccharide were also dose-dependently down-regulated after infection of RAW264.7 cells with T. gondii. In conclusion, this evasion strategy enables parasite replication in macrophages moderately activated by interferon-gamma or lipopolysaccharide, but does not suffice to evade the anti-parasitic activity of macrophages fully activated by interferon-gamma plus lipopolysaccharide. Nitric oxide production and its partial inhibition by the parasite may modulate the parasite-host equilibrium during toxoplasmosis.

Unique gene expression profiles of human macrophages and dendritic cells to phylogenetically distinct parasites

Monocyte-derived dendritic cells (DCs) and macrophages (Ms) generated in vitro from the same individual blood donors were exposed to 5 different pathogens, and gene expression profiles were assessed by microarray analysis. Responses to Mycobacterium tuberculosis and to phylogenetically distinct protozoan (Leishmania major, Leishmania donovani, Toxoplasma gondii) and helminth (Brugia malayi) parasites were examined, each of which produces chronic infections in humans yet vary considerably in the nature of the immune responses they trigger. In the absence of microbial stimulation, DCs and Ms constitutively expressed approximately 4000 genes, 96% of which were shared between the 2 cell types. In contrast, the genes altered transcriptionally in DCs and Ms following pathogen exposure were largely cell specific. Profiling of the gene expression data led to the identification of sets of tightly coregulated genes across all experimental conditions tested. A newly devised literature-based clustering algorithm enabled the identification of functionally and transcriptionally homogenous groups of genes. A comparison of the responses induced by the individual pathogens by means of this strategy revealed major differences in the functionally related gene profiles associated with each infectious agent. Although the intracellular pathogens induced responses clearly distinct from the extracellular B malayi, they each displayed a unique pattern of gene expression that would not necessarily be predicted on the basis of their phylogenetic relationship. The association of characteristic functional clusters with each infectious agent is consistent with the concept that antigen-presenting cells have prewired signaling patterns for use in the response to different pathogens.

Recent advances on the role of CD40 and dendritic cells in immunity and tolerance

CD40 is a key signaling pathway for the function of B cells, monocytes, and dendritic cells in the immune system, and plays an important role in inflammatory pathways of nonhemopoietic cells. The NFkappaB family of transcription factors is a critical mediator in inflammation. NFkappaB is involved both in the regulation of CD40 expression and in cell signaling after CD40 ligation. This positive feedback loop linking NFkappaB and CD40 plays an important role in the control of the adaptive immune response, with fundamental implications for immunity and tolerance in vivo.

In the belly of the beast: subversion of macrophage proinflammatory signalling cascades during Toxoplasma gondii infection

Macrophages (MØ) are used as the intracellular niche by several bacterial and protozoan microorganisms. Such microbial pathogens adopt diverse strategies to avoid MØ microbicidal effects. Recent insights into the Toxoplasma gondii-MØ interaction reveal novel ways that intracellular parasites subvert MØ function. In contrast to some microbial pathogens, Toxoplasma infection is not silent but induces rapid activation of transcription factors such as STAT-1 and NFkappaB. However, the parasite blocks nuclear translocation of both factors, and MØ cannot produce IL-12 or TNF-alpha when subsequently triggered with lipopolysaccharide. The nuclear import blockade is lifted 24 h after infection, but cells remain actively suppressed in TNF-alpha production. Nevertheless, IL-12 synthesis is initiated at this later time point. Toxoplasma gondii-induced production of this cytokine occurs through both MyD88- and CCR5-dependent pathways. The balance of cytokine subversion and stimulation during infection probably results from the parasite's need to simultaneously avoid immune elimination and trigger immunity to prevent host death.

Francisella tularensis inhibits Toll-like receptor-mediated activation of intracellular signalling and secretion of TNF-alpha and IL-1 from murine macrophages

Microbial ligands, including lipopolysaccharide (LPS) and bacterial lipoproteins, activate Toll-like receptors (TLR) of mononuclear phagocytes, thereby inducing proinflammatory cytokines and antimicrobial activity. We show that Francisella tularensis, an intracellular pathogen, is capable of inhibiting this macrophage response. Infection with the live vaccine strain F. tularensis LVS rendered cells of the murine macrophage-like cell line J774A.1 incapable of secreting TNF-alpha or IL-1beta and mobilizing an antimicrobial activity in response to bacterial lipopeptide or Escherichia coli-derived LPS. Inhibition of TNF-alpha secretion occurred also when J774 cells were infected with F. tularensis LVS in the presence of chloramphenicol, but not when they were infected with a mutant of F. tularensis LVS defective in expression of a 23 kDa protein that is upregulated during intracellular infection. Purified F. tularensis LPS did not show an agonistic or antagonistic effect on the E. coli LPS-induced activation of the J774 cells. Francisella tularensis LVS suppressed the capability of the cells to respond to LPS or bacterial lipopeptide (BLP) with activation of nuclear factor kappa B (NF-kappaB), and degradation of the in-hibitor of NF-kappaB, IkappaB, was blocked during the infection. Also the LPS- or BLP-induced phosphorylation of the mitogen-activated protein kinase p38 and the transcription factor c-Jun was inhibited by F. tularensis LVS but not by the 23 kDa protein mutant. In conclusion, F. tularensis appears capable of abrogating the TNF-alpha and IL-1 responses of macrophages induced by E. coli LPS or BLP via a mechanism that involves suppression of several intracellular pathways and is dependent on expression of a bacterial 23 kDa protein.

Evasion of innate immunity by parasitic protozoa

Parasitic protozoa are a major cause of global infectious disease. These eukaryotic pathogens have evolved with the vertebrate immune system and typically produce long-lasting chronic infections. A critical step in their host interaction is the evasion of innate immune defenses. The ability to avoid attack by humoral effector mechanisms, such as complement lysis, is of particular importance to extracellular parasites, whereas intracellular protozoa must resist killing by lysosomal enzymes and toxic metabolites. They do so by remodeling the phagosomal compartments in which they reside and by interfering with signaling pathways that lead to cellular activation. In addition, there is growing evidence that protozoan pathogens modify the antigen-presenting and immunoregulatory functions of dendritic cells, a process that facilitates their evasion of both innate and adaptive immunity.

Mechanism of entry determines the ability of Toxoplasma gondii to inhibit macrophage proinflammatory cytokine production

Macrophages (Mphi) infected with tachyzoites of the opportunistic protozoan Toxoplasma gondii are blocked in production of the proinflammatory cytokines tumor necrosis factor alpha (TNF-alpha) and interleukin-12 (IL-12) in response to lipopolysaccharide (LPS) triggering, and this is associated with parasite-induced inhibition of NFkappaB translocation. Here, we demonstrate a requirement for active invasion in the ability of the parasite to mediate suppression. Neither soluble tachyzoite antigen nor secreted products were suppressive, and heat-inactivated, antibody-coated tachyzoites, which efficiently entered the cell through receptor-mediated uptake, failed to inhibit LPS responses. Cytochalasin D, a drug blocking tachyzoite invasion of, but not adherence to, Mphi, severely curtailed Toxoplasma-induced suppression. In addition, parasite-induced nonresponsiveness, as measured by TNF-alpha production, was reversed by treating infected cells with the toxoplasmastatic drugs pyrimethamine and 6-thioxanthine prior to LPS stimulation. A divergence in IL-12 and TNF-alpha responses was found during extended incubation of tachyzoites and Mphi in that 24 h of incubation of infected Mphi resulted in IL-12, but not TNF-alpha, secretion, and production of the latter cytokine remained suppressed when these cells were subjected to LPS triggering. Our results demonstrate that active invasion and survival of the parasite within the parasitophorous vacuole are required to induce and maintain Mphi cytokine-specific nonresponsiveness to LPS. They also show that the effects of Toxoplasma on IL-12 and TNF-alpha production are nonidentical, with the parasite exerting a longer-lasting suppression of the latter.

Dendritic cells and immunity to leishmaniasis and toxoplasmosis

There is increased recognition that dendritic cells (DCs) are an important source of the IL-12 required to initiate protective immunity to protozoa, such as Leishmania and Toxoplasma. This article reviews the advances made in the last two years in understanding the pathways that lead to DC activation after infection with these organisms. Interestingly, there appear to be differences in the DC activation pathways utilized by these two intracellular protozoa which also may differ from the pathways utilized by bacteria.

Excretory/secretory products from plerocercoids of Spirometra erinaceieuropaei suppress the TNF-alpha gene expression by reducing phosphorylation of ERK1/2 and p38 MAPK in macrophages

We previously reported that excretory/secretory products from plerocercoids of Spirometra erinaceieuropaei suppress gene expression and production of tumour necrosis factor-alpha in murine macrophages stimulated with lipopolysaccharide. The present study investigated the suppressive mechanisms of tumour necrosis factor-alpha mRNA by excretory/secretory products in lipopolysaccharide-stimulated murine macrophages. Electrophoretic mobility shift assay and supershift assay revealed that neither nuclear translocation of nuclear factor-kappa B nor conformation of the p50/p65 nuclear factor-kappa B subunits was affected by the treatment of excretory/secretory products in lipopolysaccharide-stimulated macrophages. Inhibition of extracellular signal-regulated protein kinase 1/2 with PD98059 or p38 mitogen-activated protein kinase with SB203580 partially reduced tumour necrosis factor-alpha mRNA expression, and a combination of the two inhibitors additionally suppressed the level of tumour necrosis factor-alpha mRNA, revealing that both pathways are crucial for full induction of the gene. Northern blot analysis showed that excretory/secretory products additionally suppressed tumour necrosis factor-alpha mRNA expression in cells treated with PD98059 or SB208530 and, in turn, we found that excretory/secretory products reduced phosphorylation of extracellular signal-regulated protein kinase 1/2 and p38 mitogen-activated protein kinase in lipopolysaccharide-stimulated macrophages by Western blot analysis. This is the first report demonstrating that excretory/secretory products from parasites suppress tumour necrosis factor-alpha mRNA expression by reducing phosphorylation of extracellular signal-regulated protein kinase 1/2 and p38 mitogen-activated protein kinase without any effect on nuclear factor-kappa B activity in macrophages stimulated with lipopolysaccharide. We hypothesise that excretory/secretory products may enable this parasite to survive within the host.

Heat shock protein 70 is a potential virulence factor in murine toxoplasma infection via immunomodulation of host NF-kappa B and nitric oxide

We propose that the 70-kDa heat shock protein (HSP70) protects virulent Toxoplasma gondii from the effects of the host by immunomodulation. This hypothesis was tested using quercetin and antisense oligonucleotides targeting the start codon of the virulent T. gondii HSP70 gene. Oligonucleotides were transiently transfected into two virulent (RH, ENT) and two avirulent (ME49, C) strains of T. gondii, significantly reducing HSP70 expression in treated parasites. Virulent parasites with reduced HSP70 expression displayed reduced proliferation in vivo, as measured by the number of tachyzoites present in spleens of infected mice. They also exhibited an enhanced rate of conversion from tachyzoites to bradyzoites in vitro. Our results implicate HSP70 as a means by which virulent strains of T. gondii evade host proinflammatory responses: when RAW 264.7 cells were exposed to parasites with reduced HSP70 expression, differential expression of inducible NO synthase (iNOS) and cell NO production were observed between infections with normal and HSP70-deficient T. gondii. iNOS message levels were significantly increased when host cells were infected with HSP70 reduced virulent tachyzoites and HSP70-related inhibition of iNOS transcription resulted in altered host NO production by virulent T. gondii infection. Virulent parasites expressing reduced levels of HSP70 initiated significantly more NF-kappa B activation in host splenocytes than infections with untreated parasites. Neither proliferative ability nor conversion from tachyzoites to bradyzoites was affected by lack of HSP70 in avirulent strains of T. gondii. Furthermore, avirulent T. gondii strains induced high levels of host iNOS expression and NO production, regardless of HSP70 expression in these parasites, and inhibition of HSP70 had no significant effects on translocation of NF-kappa B to the nucleus. Therefore, the 70-kDa parasite stress protein may be part of an important survival strategy by which virulent strains down-regulate host parasiticidal mechanisms.

The role of cytokines and their signaling pathways in the regulation of immunity to Toxoplasma gondii

The development of a strong cellular immune response is critical for the control of the intracellular pathogen Toxoplasma gondii. This occurs by activation of a complex, integrated immune response, which utilizes cells of the innate and adaptive immune systems. In the last two decades there have been major advances in our understanding of the role of cytokines in the initiation and maintenance of protective immunity to T. gondii, and IFN-gamma has been identified as the major mediator of resistance to this pathogen. This article provides an overview of the biology of toxoplasmosis and focuses on the pivotal role of cytokines and their signaling pathways during infection. It also addresses the role of cytokines in modulating other immune functions that are critical in determining the balance between a protective and a pathological immune response.

NF-kappaB family of transcription factors: central regulators of innate and adaptive immune functions

Transcription factors of the Rel/NF-kappaB family are activated in response to signals that lead to cell growth, differentiation, and apoptosis, and these proteins are critical elements involved in the regulation of immune responses. The conservation of this family of transcription factors in many phyla and their association with antimicrobial responses indicate their central role in the regulation of innate immunity. This is illustrated by the association of homologues of NF-kappaB, and their regulatory proteins, with resistance to infection in insects and plants (M. S. Dushay, B. Asling, and D. Hultmark, Proc. Natl. Acad. Sci. USA 93:10343-10347, 1996; D. Hultmark, Trends Genet. 9:178-183, 1993; J. Ryals et al., Plant Cell 9:425-439, 1997). The aim of this review is to provide a background on the biology of NF-kappaB and to highlight areas of the innate and adaptive immune response in which these transcription factors have a key regulatory function and to review what is currently known about their roles in resistance to infection, the host-pathogen interaction, and development of human disease.

Cutting edge: identification of c-Rel-dependent and -independent pathways of IL-12 production during infectious and inflammatory stimuli

The production of IL-12 is required for immunity to many intracellular pathogens. Recent studies have shown that c-Rel, a member of the NF-kappaB family of transcription factors, is essential for LPS-induced IL-12p40 production by macrophages. In this study, we demonstrate that c-Rel is also required for IL-12p40 production by macrophages in response to Corynebacterium parvum, CpG oligodeoxynucleotides, anti-CD40 and low molecular weight hyaluronic acid. However, c-Rel(-/-) mice infected with Toxoplasma gondii produce comparable amounts of IL-12p40 to infected wild-type mice and have an IL-12-dependent mechanism of resistance to this infection. Furthermore, c-Rel was not required for IL-12p40 production by macrophages or dendritic cells in response to soluble Toxoplasma Ag, and neutrophils from c-Rel(-/-) mice contain normal amounts of preformed IL-12p40. Together these studies reveal the presence of c-Rel-dependent pathways critical for IL-12p40 production in response to inflammatory stimuli and demonstrate a novel c-Rel-independent pathway of IL-12p40 production during toxoplasmosis.