CpG DNA induces self and cross-hyporesponsiveness of RAW264.7 cells in response to CpG DNA and lipopolysaccharide: alterations in IL-1 receptor-associated kinase expression

Modulation of corneal epithelial innate immune response to pseudomonas infection by flagellin pretreatment

PURPOSE

A prior study showed that Toll-like receptor (TLR)-5 recognizes Pseudomonas aeruginosa flagellin and triggers the production of proinflammatory cytokines in human corneal epithelial cells (HCECs). The present study was conducted to determine how the inflammatory response is modulated after TLR activation in HCECs.

METHODS

HUCL cells, a telomerase-immortalized HCEC line, and primary cultures of HCECs were pretreated with low-dose flagellin and then challenged, with either a high dose of flagellin or with Pseudomonas. NF-kappaB activation was determined by the extent of IkappaB-alpha phosphorylation and degradation and of nuclear p65 DNA binding. The amount of cytokines in the culture media was assessed by ELISA. The activation of p38 and JNK and the cellular expression of TLR5 were determined by Western blot analysis. Cell surface distribution of TLR5 was assessed by flow cytometry. The expression and secretion of antimicrobial peptides were assessed by semiquantitative RT-PCR and slot-blot analysis, respectively.

RESULTS

Pre-exposure (12-24 hours) of HCECs to low-dose flagellin induced a state of tolerance, characterized by impaired activation of the NF-kappaB, p38, and JNK pathways and reduced production of IL-8 and TNF-alpha on subsequent challenge with a high dose of flagellin. Flagellin-induced tolerance did not alter the cellular level and surface distribution of TLR5. Furthermore, flagellin priming of HCECs dampened the inflammatory response of HCECs to live Pseudomonas. Pseudomonas-induced upregulation of antimicrobial genes such as hBD2 and LL-37 was augmented, even in tolerized HCECs.

CONCLUSIONS

Pre-exposure of the ocular surface to TLR agonists may induce protective mechanisms that not only modulate the host inflammatory response but also provide an innate defense against bacterial infection in the cornea.

Impaired Plasmacytoid Dendritic Cell Innate Immune Responses in Patients with Herpes Virus-Associated Acute Retinal Necrosis

Plasmacytoid dendritic cells (PDC), the main producers of type I IFNs in the blood, are important for the recognition and control of viral and bacterial infections. Because several viruses induce IFN-alpha production, severe courses of herpes virus infections in nonimmunocompromised patients may be related to numerical or functional PDC deficits. To evaluate this hypothesis, PBMC and PDC were repeatedly isolated from nine patients with acute retinal necrosis (ARN), caused by herpes simplex or varicella zoster virus. The patients experienced meningitis/encephalitis and frequent infections in childhood (n = 2), recurrent herpes virus infections at unusual localizations (n = 2), ocular surgery (n = 1), infections (n = 4), and stress around ARN (n = 6). The median percentage of isolated PDC was significantly lower in patients compared with 18 age-matched healthy controls (p < 0.001), confirmed by FACS analysis using peripheral blood, and was extremely low during acute disease. PDC counts dropped in five controls suffering from respiratory infections or diarrhea. IFN-alpha production in PDC and PBMC exposed to different stimuli was significantly lower in patients than in controls (p < 0.05). Anergy to these stimuli was observed on four occasions, in particular during acute disease. PDC of patients showed up-regulated IFN regulatory factor-7 mRNA levels and evidence of in vivo activation (CD80) and maturation (CD83) (p < 0.05). CD8+ cell responses were significantly lower in patients vs controls (p = 0.04). These data support a risk factor model in which numerical and functional deficits in PDC-mediated innate immune responses contribute to an impaired control of latent herpes virus infections and subsequent development of ARN.

TLR9 cooperates with TLR4 to increase IL-12 release by murine dendritic cells

Toll-like receptors (TLR) are expressed on the surface or intracellularly by dendritic cells (DC) and recognize specifically different pathogen-associated molecular patterns (PAMPs). Increasing evidence suggests that TLR expressed by DC can cooperate to synergize their functions. Here, we describe the cooperation of TLR9 and TLR4 triggering of murine bone marrow derived DC by CpG oligonucleotides and LPS, respectively. The simultaneous DC stimulation of LPS and CpG showed additive effects on the production of IL-12 but not on other cytokines, such as TNF, IL-6 or IL-10. CpG pretreatment before LPS induced five times more IL-12p40 and IL-12p70 production by DC, whereas LPS pretreatment before CpG showed no effect. The optimal time interval between CpG and LPS treatment was 4h and the synergistic effects were dependent on myeloid differentiation factor 88 (MyD88) but independent from the DNA backbone and did not mediate by nucleosome remodeling. The stimulatory effect could be further enhanced by addition of IFN-gamma but not anti-CD40 antibodies. These data show, that TLR4 and TLR9 can cooperate to increase selectively IL-12 production by DC.

Macrophage tolerance induced by stimulation with Toll-like receptor 7/8 ligands

The engagement of Toll-like receptors (TLRs) results in resistance to subsequent challenge with respective ligands in macrophages. Studies have shown that stimulation by ligands for TLR2, TLR4, TLR5 and TLR9 induces this state of hypo-responsiveness (homo-tolerance) towards subsequent stimulation with the same ligands. However, whether homo-tolerance is induced by the ligands of TLR7/8 has not been previously determined. We found that ligands for TLR7/8, namely ss-RNA from HIV and an imidazoquinoline compound, R848, induced macrophage tolerance, as judged by the production of the chemokine MIP-1beta. IRAK-1 phosphorylation was also inhibited in the tolerant cells after subsequent stimulation with R848, although no significant differences were observed in the protein levels of TLR7 between tolerant and non-tolerant cells. These results indicate that macrophage tolerance induced by TLR7/8 ligands is regulated at least at the level of IRAK-1 activation.

Cell surface 4-1BBL mediates sequential signaling pathways 'downstream' of TLR and is required for sustained TNF production in macrophages

The stimulation of Toll-like receptors (TLRs) on macrophages triggers production of the cytokine tumor necrosis factor (TNF). TNF production occurs within 1 h of TLR stimulation and is sustained for 1 d. Here we document a function for the TNF family member 4-1BB ligand (4-1BBL) in sustaining TLR-induced TNF production. TLR signaling induced 4-1BBL, and 4-1BBL interacted with TLRs on the macrophage surface. The influence of 4-1BBL on TNF production was independent of its receptor (4-1BB) and did not require the adaptors MyD88 or TRIF. It did not influence TLR4-induced activation of transcription factor NF-kappaB (an early response) but was required for TLR4-induced activation of transcription factors CREB and C/EBP (a late event). Transient TLR4-MyD88 complexes appeared during the first hour after lipopolysaccharide stimulation, and TLR4-4-1BBL interactions were detected between 2 h and 8 h after lipopolysaccharide stimulation. Our results indicate that two different TLR4 complexes sequentially form and selectively control early and late TNF production.

Co-infection of helminths and malaria: modulation of the immune responses to malaria

Chronic helminth infections induce strong type 2 and regulatory immune responses and are known to influence immune activity to other antigens such as allergens and vaccines. Since malaria and helminth infections often coincide geographically in the same tropical regions, the question arises whether helminth infections modulate the immune responses towards the malaria parasite and affect its course of disease. Here, we will review studies on co-infections in both animal models and in human populations, and discuss the changes in the immune system seen. Furthermore, the implications of helminth infection for the efficacy of malaria vaccines will be discussed.

Toll-like receptor interactions: tolerance of MyD88-dependent cytokines but enhancement of MyD88-independent interferon-beta production

Toll-like receptors (TLRs) signal through two main pathways: a myeloid differentiation factor (MyD)88-dependent pathway that acts via nuclear factor kappaB (NF-kappaB) to induce proinflammatory cytokines such as tumour necrosis factor-alpha (TNF-alpha) and a MyD88-independent pathway that acts via type I interferons to increase the expression of interferon-inducible genes. Repeated signalling through TLR4 and a number of other TLRs has been reported to result in a reduction in the subsequent proinflammatory cytokine response, a phenomenon known as TLR tolerance. In this study we have shown that, whilst NF-kappaB activation and production of TNF-alpha and interleukin-12 by murine RAW264.7 and J774.2 cells in response to stimulation by TLR4, -5, -7 or -9, was reduced by prior stimulation with TLR4, -5, -7 or -9 ligands, the primary stimulation of TLR3, which does not use the MyD88 pathway, did not reduce the TNF-alpha or interleukin-12 responses to subsequent TLR stimulation. The response to TLR3 stimulation was not diminished by prior TLR ligand exposure. Furthermore, the production of interferon-beta (IFN-beta) following stimulation of TLR3 or -4, which is MyD88-independent, was increased by prior activation of TLR4, -5, -7 or -9. In contrast, TLR9 ligand-induced IFN-beta production, which is MyD88-dependent, was tolerized by prior TLR stimulation. These results are consistent with differential regulation of MyD88-dependent and MyD88-independent cytokine production following serial activation of TLRs.

Maturation of murine bone marrow-derived dendritic cells with poly(I:C) produces altered TLR-9 expression and response to CpG DNA

Although poly(I:C) and LPS induced differential dendritic cell (DC) cytokine profiles and toll-like receptor (TLR) expression, all were capable of causing phenotypic and functional DC maturation. Both LPS and poly(I:C) downregulated TLR-4/MD-2 expression on DCs. Although poly(I:C) highly upregulated their cell surface TLR-9 expression, LPS upregulated the intracellular TLR-9 expression. LPS-treated DCs could not produce IL-12p70 in response to subsequent both LPS- and CpG DNA-stimulation. On the other hand, poly(I:C)-treated DCs retained to produce IL-12p70 by subsequent CpG DNA-stimulation, while subsequent LPS-stimulation did not induce IL-12p70 production. Chloroquine, inhibitor of endosomal maturation, completely inhibited cytokine production of LPS-treated DCs as well as unstimulated control in response to subsequent CpG DNA-stimulation, while it failed to delete the IL-12p40 and IL-10 production in poly(I:C)-treated DCs. These data suggest that poly(I:C) may induce a novel DC phenotype that preserves the capacity of cytokine production to subsequent CpG DNA-stimulation.

Endotoxin modulates the capacity of CpG-activated liver myeloid DC to direct Th1-type responses

DC are believed to play important roles in the induction and regulation of immune responses in the liver, an organ implicated in peripheral tolerance. Since the liver is located downstream of the gut, it is constantly exposed to bacterial LPS. Our recent observations indicate that prior exposure to endotoxin modulates subsequent liver DC responses to this TLR4 ligand. In this study, we demonstrate that endotoxin modifies the capacity of mouse liver myeloid DC (MDC) activated by CpG (TLR9 ligand) to direct Th1-type responses. IL-12 production by liver MDC was significantly lower than that of spleen MDC following CpG or Imiquimod (R837; TLR7 ligand) activation in vitro. In addition, allogeneic T cells stimulated by CpG-activated liver MDC secreted significantly lower levels of IFN-gamma than T cells stimulated with CpG-activated spleen MDC. A similar effect on liver DC was observed in response to in vivo CpG administration. This effect may be explained by exposure of the DC to endotoxin, because LPS attenuated IL-12 production by CpG-stimulated liver MDC, both in vitro and in vivo. Moreover, attenuation of the response to CpG was not observed in liver MDC from TLR4-mutant (C3H/HeJ) mice, in which TLR4 signaling is impaired. These data suggest that endotoxin-induced 'cross-tolerance' to TLR ligands in liver DC may contribute to down-regulation of hepatic immune responses.

Disparate regulation and function of the class A scavenger receptors SR-AI/II and MARCO

The macrophage class A scavenger receptors, macrophage receptor with a collagenous structure (MARCO) and type I/II class A scavenger receptor (SR-AI/II), share structural features and roles in host defense, but little is known about their regulation and signaling properties. Ligation of MARCO on mouse thioglycollate-elicited peritoneal macrophages (PEMs) with immobilized mAb costimulated IL-12 production, in contrast to previously reported inhibition by SR-AI/II. PEMs from MARCO-deficient mice exhibited 2.7 times lower IL-12 production in responses to stimulation with LPS and IFN-gamma and lack of significant IL-12 production on stimulation with LPS alone. Conversely, SR-AI/II-deficient PEMs produced 2.4 and 1.8 times more IL-12 than wild-type PEMs in response to LPS or LPS and IFN-gamma, respectively. Corresponding differences in regulation of SR-A and MARCO expression were also observed. Th1 adjuvants (LPS, a CpG motif-containing oligodeoxynucleotide (CpG-ODN), IL-12, and GM-CSF) increased, whereas Th2-polarizing factors (IL-4, M-CSF, and non-CpG ODN) decreased expression of MARCO on J774 macrophage-like cells. Expression of SR-A was regulated in the opposite manner to MARCO or not affected. Whereas MARCO was involved in opsonin-independent phagocytosis in CpG-ODN-pretreated but not in IL-4-pretreated J774 cells, anti-SR-A Abs inhibited particle uptake in untreated and IL-4-pretreated but not in CpG-ODN-pretreated cells. SR-A and MARCO are regulated differently and mediate distinct negative and positive effects on IL-12 production in macrophages. These differences may contribute to sustained Th1 or Th2 polarization of ongoing immune responses.

Immunostimulatory CpG-oligodeoxynucleotides (CpG-ODN) induce early hepatic injury, but provide a late window for protection against endotoxin-mediated liver damage

BACKGROUND/AIMS

An impaired immunologic response to infection has been recognized as a major defect in the pathogenesis of sepsis and multi-organ failure. Sepsis-associated liver dysfunction and damage are main determinants for the course of the disease. CpG-motif-containing DNA-sequences (CpG-ODN) were previously shown to confer protection in models of infection by stimulating both innate and specific immune responses. Herein, we studied the effect of CpG-ODN in lipopolysaccharide (LPS)-associated hepatotoxicity.

METHODS

Sprague Dawley rats pre-treated at day 6 with either CpG-ODN or inert DNA were challenged with E. coli LPS and subsequently studied for liver injury at 6 and 16 h using in vivo fluorescence microscopy and immunohistochemistry. Western blot protein analysis served for assessment of expression of TLR4, TNF receptor-associated factor 6 (TRAF6), NFkappaB and caspase-3. To evaluate CpG-ODN effects during non-septic conditions, additional animals were solely exposed to CpG-ODN and studied after 1 and 6 days.

RESULTS

CpG-ODN application induced marked hepatic microcirculatory deterioration and liver dysfunction at day 1, however, with almost complete recovery to normal at day 6. Interestingly, CpG-ODN pre-treatment decreased LPS-induced leukocyte-endothelial cell interaction, sinusoidal perfusion failure and caspase-3-dependent apoptotic cell death. Although Kupffer cell phagocytic activity was not affected, CpG-ODN pre-treatment in LPS-challenged animals attenuated hepatic protein expression of TRAF6 and NFkappaB and increased TLR4 by almost 100%.

CONCLUSIONS

CpG-containing DNA-sequences induce early hepatic injury, but mediate long-term protection against LPS hepatotoxicity. The mechanism of protection is based on the induction of cross-tolerance, probably via inhibition of the downstream TRAF6-NFkappaB signaling pathway and upregulation of the TLR4 surface receptor.

Decreased intracellular TLR9 confers hyporesponsiveness of RAW264.7 cells to subsequent CpG ODN challenge

Low-dose CpG ODN pretreatment is known to induce effective protective immunity against acute infectious diseases. In the present study, using primary murine peritoneal macrophages and the macrophage-like cell line, RAW264.7, we investigated whether low-dose CpG ODN pretreatment would induce hyporesponsiveness in response to a subsequent high-dose CpG ODN challenge and further investigated the molecular mechanisms underlying this event. Our results showed that pretreatment with a low dose of CpG ODN inhibits TNF-alpha production stimulated by later high-dose CpG ODN stimulation in a dose- and time-dependent manner. Interestingly, anti-mouse TLR9 blocking antibody added prior to CpG ODN pretreatment did not affect TNF-alpha release, but antibody added after CpG ODN pretreatment augmented the pretreatment effect of CpG ODN. This difference suggests that cell-surface TLR9 is indeed functional on activated cells. Flow cytometry revealed that low-dose CpG ODN pretreatment decreased cell-surface binding and internalization of a subsequent high-dose stimulation, suggesting that decreased internalization of succeeding CpG ODN is associated with reduced TNF-alpha release. Although both intracellular and cell-surface TLR9 expression are observed, low dose of CpG ODN pretreatment increased only cell-surface TLR9 levels. Importantly, low-dose CpG ODN pretreatment also significantly inhibited the activation of NF-kappaB, an important downstream regulator of various proinflammatory cytokines. In summary, our results demonstrate that suppression of TNF-alpha production by low dose of CpG ODN pretreatment correlates with decreased binding and internalization of subsequent CpG ODN, decreased intracellular content of TLR9, and inhibition of NF-kappaB activation.

Toll-like receptor 3 ligand attenuates LPS-induced liver injury by down-regulation of toll-like receptor 4 expression on macrophages

This study demonstrates that pretreatment with polyinosinic-polycytidylic acid (poly I:C) significantly decreased the mortality and liver injury caused by injection of lipopolysaccharide (LPS) in the presence of d-galactosamine (d-GalN) in C57BL/6 mice. Depletion of natural killer, natural killer T, and T cells did not change the protective effect of poly I:C on LPS/d-GalN-induced liver injury in vivo. However, depletion of macrophages abolished LPS/d-GalN-induced fulminant hepatitis, which could be restored by adoptive transfer of macrophages but not by transfer of poly I:C-treated macrophages. Treatment with poly I:C down-regulated the expression of the toll-like receptor 4 (TLR4) on macrophages and reduced the sensitivity of macrophages (Kupffer cells and peritoneal macrophages from C57BL/6 mice, or RAW264.7 cells) to LPS stimulation. Poly I:C pretreatment also impaired the signaling of mitogen-activated protein kinases and NF-kappaB induced by LPS in RAW264.7 cells. Blockade of TLR3 with a TLR3 antibody abolished poly I:C down-regulation of TLR4 expression and LPS stimulation of TNF-alpha production in RAW264.7 cells. Taken together, our findings suggest that activation of TLR3 by its ligand, poly I:C, induced LPS tolerance by down-regulation of TLR4 expression on macrophages.

Differential effects of CpG-DNA in Toll-like receptor-2/-4/-9 tolerance and cross-tolerance

Lipopolysaccharide (LPS) tolerance is a state of refractoriness towards a second stimulation by LPS after a preceding stimulation. LPS is recognized by Toll-like receptor-4 (TLR-4), which belongs to a group of pattern recognition receptors mediating activation of innate immunity by microbial components. To date, it is not known in detail to what extent other TLR-dependent stimuli also induce tolerance and whether preceding and challenging stimuli are interchangeable. We have examined tolerance induction in detail for lipoteichoic acid (LTA), LPS and CpG-DNA, which are recognized by TLR-2, -4 and -9, respectively. In RAW264.7 macrophages, all three stimuli induced tolerance towards a subsequent challenge with the same stimulus used for priming, as well as cross-tolerance towards subsequent challenge with other stimuli signalling via different TLRs. However, whereas LPS/LTA cross-tolerance was also functional in an in vivo model of galactosamine (GalN)-primed liver damage, pretreatment with CpG only protected against GalN/CpG challenge and failed to induce cross-tolerance for LPS and LTA. CpG-DNA pretreatment even enhanced tumour necrosis factor (TNF)-alpha production and liver damage upon subsequent challenge with LPS or LTA. Stimulation with CpG-DNA resulted in a peculiar sensitization for interferon (IFN)-gamma secretion. The data indicate that, in contrast to in vitro macrophage desensitization, the in vivo consequences of repeated TLR stimulation greatly differ amongst different TLR ligands.

Inducible binding of PU.1 and interacting proteins to the Toll-like receptor 4 promoter during endotoxemia

We hypothesized that PU.1 and PU.1 interacting proteins (PIP) binding to the Toll-like receptor 4 (TLR4) promoter is involved in endotoxin-induced upregulation of TLR4 gene expression. Our results employing chromatin immunoprecipitation assays indicate that PU.1 binds to the murine TLR4 promoter both in macrophage cells and, most importantly, in whole lung tissue. Treatment of RAW 264.7 cells with endotoxin induced the association of PU.1 and the TLR4 promoter in a time-dependent manner, and this was closely tied to interactions between the TLR4 promoter and the PIP interferon regulatory factors (IRF)4 and IRF8. PU.1 binding was related to increases in steady-state TLR4 mRNA and total TLR4 protein in RAW cells. Endotoxemia in animals caused the similar inducible interaction between PU.1 and IRF4 and the TLR4 promoter in lung tissue of mice that was treated with a single intraperitoneal injection of endotoxin. PU.1 binding to the TLR4 promoter was not enhanced in the lung tissue of endotoxin-resistant C3H/HeJ mice in response to endotoxemia. Transient transfection studies in RAW cells indicate that inducible binding of PU.1 to the TLR4 promoter is abrogated by a Ser148 to Ala mutation in PU.1. These data suggest that induction of PU.1/PIP binding to the TLR4 promoter is involved in endotoxin response in vivo and may mediate transcriptional changes in TLR4 gene expression.

A novel splice variant of interleukin-1 receptor (IL-1R)-associated kinase 1 plays a negative regulatory role in Toll/IL-1R-induced inflammatory signaling

The interleukin-1 (IL-1) receptor-associated kinase 1 (IRAK1) is a member of the IRAK kinase family that plays a pivotal role in the Toll/IL-1 receptor (TIR) family signaling cascade. We have identified a novel splice variant, IRAK1c, which lacks a region encoded by exon 11 of the IRAK1 gene. IRAK1c expression was confirmed by both RNA and protein detection. Although both IRAK1 and IRAK1c are expressed in most tissues tested, IRAK1c is the predominant form of IRAK1 expressed in the brain. Unlike IRAK1, IRAK1c lacks kinase activity and cannot be phosphorylated by IRAK4. However, IRAK1c retains the ability to strongly interact with IRAK2, MyD88, Tollip, and TRAF6. Overexpression of IRAK1c suppressed NF-kappaB activation and blocked IL-1beta-induced IL-6 as well as lipopolysaccharide- and CpG-induced tumor necrosis factor alpha production in multiple cellular systems. Mechanistically, we provide evidence that IRAK1c functions as a dominant negative by failing to be phosphorylated by IRAK4, thus remaining associated with Tollip and blocking NF-kappaB activation. The presence of a regulated, alternative splice variant of IRAK1 that functions as a kinase-dead, dominant-negative protein adds further complexity to the variety of mechanisms that regulate TIR signaling and the subsequent inflammatory response.

Inhibition of inducible nitric-oxide synthase expression by (5R)-5-hydroxytriptolide in interferon-gamma- and bacterial lipopolysaccharide-stimulated macrophages

(5R)-5-Hydroxytriptolide (LLDT-8) is a novel analog of triptolide that has antiarthritic, hepatoprotective, and antiallogenic transplantation-rejective effects. In the present study, we report that LLDT-8 inhibited nitric oxide (NO) production and inducible nitric-oxide synthase (iNOS) expression in macrophages. LLDT-8 significantly attenuated NO production, in a dose-dependent manner, in primary peritoneal macrophages and a macrophage cell line of Raw 264.7 cells following stimulation with interferon (IFN)-gamma, lipopolysaccharide (LPS), and IFN-gamma plus LPS. It also reduced the production of tumor necrosis factor-alpha from LPS-stimulated Raw 264.7 cells. To further elucidate the mechanism responsible for the inhibition of NO, we examined the effect of LLDT-8 on IFN-gamma and LPS-induced iNOS expression. Indeed, LLDT-8 prevented NO generation by inhibiting iNOS expression at mRNA level and protein level, rather than by interfering its enzymatic activity. In IFN-gamma-stimulated Raw 264.7 cells, LLDT-8 suppressed the gene transcription of signal transducer and activator of transcription 1alpha and interferon regulatory factor (IRF)-1, but it displayed no apparent effect on IFN-gamma receptor level on cell surface. After LPS challenge, LLDT-8 further abrogated the expression of LPS receptor complex, including CD14, Toll-like receptor 4, and myeloid differentiation protein-2; decreased the LPS-induced phosphorylation of stress-activated protein kinase/c-Jun NH(2)-terminal kinase, extracellular signal-regulated kinase 1/2, and p38 mitogen-activated protein kinase (MAPK); retarded the degradation of IkappaBalpha; and ameliorated the DNA binding activity of nuclear factor-kappaB (NF-kappaB) to nuclear proteins that accounts for transcriptional regulation of iNOS. Taken together, these results suggest that LLDT-8 reduces NO production and iNOS expression by inhibiting IFN-gamma-triggered IRF-1 expression and LPS-triggered MAPK phosphorylation and NF-kappaB activation.

Expression of tumour necrosis factor receptor and Toll-like receptor 2 and 4 on peripheral blood leucocytes of human volunteers after endotoxin challenge: a comparison of flow cytometric light scatter and immunofluorescence gating

Toll-like receptors (TLRs) are involved in the recognition of bacterial products and thus participate in the induction of the inflammatory cascade. However, much less is known about the evolution of leucocyte TLR expression during human inflammatory stress. We hypothesized that a decrease in leucocyte TLRs could account for the so-called tolerance or hyporesponsiveness state to subsequent stimulation with bacteria-derived products. Because of the profound monocytopenia that ensues after in vivo lipopolysaccharide (LPS) challenge, we also compared monocyte TLR expression using two different techniques of flow cytometric gating. In a first set of experiments, 17 healthy volunteers underwent LPS challenge. Blood was drawn at different time-points and analysed by flow cytometry using light scatter gating and one-colour analysis to assess the expression of the tumour necrosis factor receptor (TNFR) and TLR2 and TLR4 on both monocytes and granulocytes. In a second set of experiments, the assessment of those receptors was made using a more specific gating method that utilized light scatter and CD14 immunofluorescence in a two-colour analysis. This was performed using whole blood drawn from five healthy volunteers and incubated ex vivo for different time periods with or without LPS and in 12 volunteers who underwent LPS challenge in vivo. The pattern of expression for monocyte TNFR was similar for both types of gating. Using only the light scatter gating, an initial drop of TLR 2 and 4 was observed on monocytes. By contrast, when using light scatter x immunofluorescence gating, an up-regulation of these two receptors following both in vivo and in vitro LPS exposure was observed. LPS up-regulates the expression of TLRs on monocytes and granulocytes. Depending upon the methodology utilized, contrasting results were obtained with respect to TLR2 and TLR4 expression. The flow cytometric gating technique used is of importance in determining cellular TLR2 and TLR4 expression, especially in blood samples exhibiting significant monocytopenia.

Cutting Edge: The Acquisition of TLR Tolerance during Malaria Infection Impacts T Cell Activation

An effective immune response to infection requires control of pathogen growth while minimizing inflammation-associated pathology. During malaria infection, this balance is particularly important. Murine malaria is characterized by early production of proinflammatory cytokines, which declines in the face of continuing parasitemia. The mechanism by which this occurs remains poorly understood. In this study, we investigated the role of dendritic cells (DCs) in regulating pro- and anti-inflammatory cytokine responses. As malaria infection progresses, DCs become refractory to TLR-mediated IL-12 and TNF-alpha production, while increasing their ability to produce IL-10 and retaining the capacity for activation of naive T cells. IL-12-secreting DCs from early infection stimulate an IFN-gamma-dominated T cell response, whereas IL-10-secreting DCs from later stages induce an IL-10-dominated T cell response. We suggest that phenotypic changes in DCs during Plasmodium yoelii infection represent a mechanism of controlling host inflammation while maintaining effective adaptive immunity.

Intestinal epithelial Tollerance versus inTollerance of commensals

This brief review summarizes the current understanding of Toll-like receptor (TLRs) mediated intestinal epithelial mechanisms of commensal tolerance versus intolerance and provides an update on the downstream negative control of signaling responses through decreased surface expression, interregulation with NOD2, overexpression of Tollip, various inhibitors of NF-kappaB as well as soluble tolerizing mediators present in lumen and serum which all may maintain or--when dysregulated--impair mucosal homeostasis in health or disease, respectively.

Airway epithelial cell tolerance to Pseudomonas aeruginosa

Background

The respiratory tract epithelium is a critical environmental interface that regulates inflammation. In chronic infectious airway diseases, pathogens may permanently colonize normally sterile luminal environments. Host-pathogen interactions determine the intensity of inflammation and thus, rates of tissue injury. Although many cells become refractory to stimulation by pathogen products, it is unknown whether the airway epithelium becomes either tolerant or hypersensitive in the setting of chronic infection. Our goals were to characterize the response of well-differentiated primary human tracheobronchial epithelial cells to Pseudomonas aeruginosa, to understand whether repeated exposure induced tolerance and, if so, to explore the mechanism(s).

Methods

The apical surface of well-differentiated primary human tracheobronchial epithelial cell cultures was repetitively challenged with Pseudomonas aeruginosa culture filtrates or the bacterial media control. Toxicity, cytokine production, signal transduction events and specific effects of dominant negative forms of signaling molecules were examined. Additional experiments included using IL-1β and TNFα as challenge agents, and performing comparative studies with a novel airway epithelial cell line.

Results

An initial challenge of the apical surface of polarized human airway epithelial cells with Pseudomonas aeruginosa culture filtrates induced phosphorylation of IRAK1, JNK, p38, and ERK, caused degradation of IκBα, generation of NF-κB and AP-1 transcription factor activity, and resulted in IL-8 secretion, consistent with activation of the Toll-like receptor signal transduction pathway. These responses were strongly attenuated following a second Pseudomonas aeruginosa, or IL-1β, but not TNFα, challenge. Tolerance was associated with decreased IRAK1 protein content and kinase activity and dominant negative IRAK1 inhibited Pseudomonas aeruginosa -stimulated NF-κB transcriptional activity.

Conclusion

The airway epithelial cell response to Pseudomonas aeruginosa entails adaptation and tolerance likely mediated, in part, by down-regulation of IRAK1.

HSP60 and CpG-DNA-oligonucleotides differentially regulate LPS-tolerance of hepatic Kupffer cells

BACKGROUND/AIMS

Hepatic Kupffer cells (KC) are major regulators of the immune response to gut-derived bacterial products; uncontrolled activation of KC by bacterial components is of pathogenic relevance in alcoholic hepatitis and septic shock.

METHODS

We examined the role of bacterial lipopolysaccharide (LPS), bacterial and autologous HSP60 and bacterial DNA, which are recognized by innate Toll-like receptors, during activation of murine KC.

RESULTS

In cultivated KC, autologous HSP60 induced a state of LPS-hyporesponsiveness; bacterial DNA did not mitigate the response to subsequent LPS-challenge in vitro; in contrast, pre-treatment of mice with bacterial DNA even significantly increased serum TNF levels, liver function tests and mortality in a model of LPS-induced hemorrhagic liver failure.

CONCLUSION

HSP60 and CpG-DNA differentially modulated the threshold of KC activation by LPS and might therefore contribute to the regulation of inflammatory immunity to gut-derived bacterial compounds.

Augmentation of T(H)-1 type response by immunoactive AT oligonucleotide from lactic acid bacteria via Toll-like receptor 9 signaling

Toll-like receptor 9, which is expressed on the surface of antigen presenting cells and which was recently identified in the cytoplasmic follicle, recognizes bacterial CpG oligodeoxynucleotides (ODNs), resulting in the induction of a potent immune response. However, in our previous study, we found that TLR9 potentially recognizes not only CpG ODN but also non-CpG ODN such as AT ODN. Therefore, in the present study, to investigate this possibility, we elucidated the effects of AT ODN on T(H)-1, T(H)-2 type cytokine induction via TLR9 by real-time quantitative PCR analysis and ELISA of the swine TLR9 transfectant. The results demonstrated that the T(H)-1 type cytokines such as interleukin (IL)-12p70 and interferon (IFN)-gamma were strongly induced by AT ODN compared to the unexposed controls, while T(H)-2 type cytokines were not induced. These results indicate that the AT ODN can augment the T(H)-1 immune response, which plays an important role in prevention of allergic responses. Moreover, the swine TLR9 transfectant demonstrated its usefulness for evaluation of immunostimulation by bacterial DNA through the detection of T(H)-1, T(H)-2 type cytokine induction via TLR9 signaling.

Immunomodulation via novel use of TLR4 by the filarial nematode phosphorylcholine-containing secreted product, ES-62

Filarial nematodes, parasites of vertebrates, including humans, secrete immunomodulatory molecules into the host environment. We have previously demonstrated that one such molecule, the phosphorylcholine-containing glycoprotein ES-62, acts to bias the immune response toward an anti-inflammatory/Th2 phenotype that is conducive to both worm survival and host health. For example, although ES-62 initially induces macrophages to produce low levels of IL-12 and TNF-alpha, exposure to the parasite product ultimately renders the cells unable to produce these cytokines in response to classic stimulators such as LPS/IFN-gamma. We have investigated the possibility that a TLR is involved in the recognition of ES-62 by target cells, because phosphorylcholine, a common pathogen-associated molecular pattern, appears to be responsible for many of the immunomodulatory properties of ES-62. We now demonstrate that ES-62-mediated, low level IL-12 and TNF-alpha production by macrophages and dendritic cells is abrogated in MyD88 and TLR4, but not TLR2, knockout, mice implicating TLR4 in the recognition of ES-62 by these cells and MyD88 in the transduction of the resulting intracellular signals. We also show that ES-62 inhibits IL-12 induction by TLR ligands other than LPS, bacterial lipopeptide (TLR2) and CpG (TLR9), via this TLR4-dependent pathway. Surprisingly, macrophages and dendritic cells from LPS-unresponsive, TLR4-mutant C3H/HeJ mice respond normally to ES-62. This is the first report to demonstrate that modulation of cytokine responses by a pathogen product can be abrogated in cells derived from TLR4 knockout, but not C3H/HeJ mice, suggesting the existence of a novel mechanism of TLR4-mediated immunomodulation.

Depressed tumor necrosis factor alpha and interleukin-12p40 production by peripheral blood mononuclear cells of gastric cancer patients: Association with IL-1R-associated kinase-1 protein expression and disease stage

Our study investigated the ability of peripheral blood mononuclear cells (PBMCs) isolated from patients with different clinical stages of gastric cancer to produce proinflammatory (tumor necrosis factor alpha [TNFalpha], interleukin 12p40 [IL-12p40] and interleukin 6 [IL-6]) and antiinflammatory (interleukin-10 [IL-10]) cytokines after stimulation with lipopolysaccharide (LPS) or tumor cells, and its correlation with IL-1R-associated kinase-1 (IRAK-1) protein expression. The data showed that TNF production by tumor cell-stimulated PBMCs obtained from patients with advanced gastric cancer was significantly depressed in comparison to the control group. The response to LPS was less affected. IL-12p40 production was depressed in all stages of disease, while the release of IL-10 and IL-6 remained unchanged. Depressed tumor cell-induced TNF and IL-12p40 production was associated with diminished IRAK-1 protein expression in PBMC. These findings may suggest that in advanced gastric cancer (at least in some cancer patients) diminished IRAK-1 protein expression may be a novel mechanism responsible for or facilitating downregulation of innate immune response to tumor cells.

Papillomavirus-like particles stimulate murine bone marrow-derived dendritic cells to produce alpha interferon and Th1 immune responses via MyD88

Dendritic cells (DCs) link innate and adaptive immunity by sensing pathogens or vaccinogens and signaling a variety of defense responses. Since human papillomavirus type 16 L1 virus-like particles (HPV16 VLPs) induce a potent, protective immune response after vaccination, we examined their recognition by DCs. HPV16 VLPs cause phenotypic maturation of murine bone marrow-derived DCs (BMDCs), and immunization of mice with HPV16 VLP-loaded BMDCs or HPV16 VLPs alone induced T helper 1 (Th1)-biased immune responses. Analysis of transcriptional responses of murine BMDCs by microarray suggested that alpha/beta interferon (IFN-alpha/beta) transcripts and numerous proinflammatory cytokines and chemokines are up regulated in response to HPV16 VLPs. Indeed, the induction of IFN-alpha, IFN-gamma, and interleukin-12 (IL-12) production by BMDCs after stimulation with HPV16 VLPs was demonstrated by quantitative enzyme-linked immunosorbent assay. Many microbial products that induce proinflammatory responses are recognized via Toll-like receptor (TLR) signaling through the key adaptor protein MyD88 and activation of NF-kappaB, nuclear factor of activated T cells (NF-AT), and activating protein 1 (AP-1). Reporter assays indicated that HPV16 VLPs activated NF-kappaB-, NF-AT-, and AP-1-dependent transcription in the RAW264.7 macrophage cell line. Knockdown of MyD88 transcripts by small interfering RNA in the RAW264.7 macrophage cell line inhibited the activation of NF-kappaB-, NF-AT- and AP-1-dependent transcription by HPV16 VLP. Furthermore, MyD88(-/-) BMDCs failed to up regulate IL-12 and IFN-alpha and -gamma in response to HPV16 VLPs. Finally, Th1-biased immune responses to HPV16 VLPs are dramatically impaired in MyD88 and IFN-alpha/beta receptor-deficient mice. This implicates TLR recognition as central to immune recognition of HPV16 L1 VLPs.

LPS-induced upregulation of SHIP is essential for endotoxin tolerance

An initial exposure to lipopolysaccharide (LPS) induces a transient state of hyporesponsiveness to a subsequent challenge with LPS. The mechanism underlying this phenomenon, termed endotoxin tolerance, remains poorly understood despite a recent resurgence of interest in this area. We demonstrate herein that SHIP(-/-) bone marrow-derived macrophages (BMmphis) and mast cells (BMMCs) do not display endotoxin tolerance. Moreover, an initial LPS treatment of wild-type BMmphis or BMMCs increases the level of SHIP, but not SHIP2 or PTEN, and this increase is critical for the hyporesponsiveness to subsequent LPS stimulation. Interestingly, this increase in SHIP protein is mediated by the LPS-induced production of autocrine-acting TGFbeta and neutralizing antibodies to TGFbeta block LPS-induced endotoxin tolerance. In vivo studies with SHIP(+/+) and SHIP(-/-) mice confirm these in vitro findings and show a correlation between the duration of endotoxin tolerance and elevated SHIP levels.

Yersinia V antigen induces both TLR homo- and heterotolerance in an IL-10-involving manner

The virulence antigen (LcrV) of pathogenic yersiniae "silences" macrophages against stimulation with the TLR2-agonist zymosan A in a CD14/TLR2-dependent fashion via IL-10 induction. This pathogenically important "silencing" resembles TLR tolerance phenomena; in these, pre-exposure to a primary tolerizing TLR-agonist renders macrophages unresponsive to stimulation with a secondary challenging TLR-agonist which may involve either the same (TLR homotolerance) or a different TLR (TLR heterotolerance) as the primary TLR-agonist. Here, we show that rLcrV induces TLR homo- and heterotolerance against TLR2- or TLR4-agonists both in human and murine macrophages, respectively. The underlying mechanism of LcrV-induced tolerance is most likely not due to changes in TLR2- or TLR4 expression, but involves LcrV-mediated IL-10 production, since LcrV-induced TLR homo- and heterotolerance is highly impaired in IL-10(-/-) macrophages. Moreover, the involvement of IL-10 in TLR tolerance induction seems to be a more general phenomenon as shown by experiments using different TLR-agonists in IL-10(-/-) macrophages. Since LcrV acts as a secreted protein upon macrophages without requiring direct cell contact, as shown in transwell assays, we propose that yersiniae exploit IL-10-involving TLR tolerance mechanisms by the virulence factor LcrV.

Impact of immunomodulatory oligodeoxynucleotides on cytokine production in the lipopolysaccharide-stimulated human whole blood model

BACKGROUND

In studying potential immunotherapeutics for sepsis, we used a lipopolysaccharide (LPS)-stimulated whole blood model to test the immunomodulating capacity of cytosine-phospho-guanine oligodeoxynucleotides (CpG ODNs). We hypothesized that CpG ODNs would have considerable counterinflammatory effects on LPS-induced cytokine production.

METHODS

We administered 4 micromol/L of CpG ODNs (2216, D19, 2006, K3, or 1668) or guanine-phospho-cytosine (GpC) ODNs (control D or control K) immediately after LPS (10 ng/mL) stimulation of heparinized human whole blood. Samples were incubated for 1, 3, 6, 12, and 24 hours. Media and LPS were used as negative and positive controls. Cell-free supernatants were obtained and evaluated for interferon gamma (IFN-gamma), interleukin (IL)-12(p40), tumor necrosis factor alpha, IL-6, IL-10, IFN-alpha, IL-8, and IL-1beta by ELISA.

RESULTS

Compared to LPS alone, significantly reduced levels of IFN-gamma, IL-12(p40), tumor necrosis factor alpha, and IL-6 were associated with both CpG and GpC ODNs administration to LPS-stimulated whole blood. IL-10 levels were concomitantly increased. However, IFN-alpha generation was CpG specific as was increased IL-8 levels. Lastly, only 2216 was associated with decreased IL-1beta levels.

CONCLUSIONS

CpG ODNs and GpC ODNs in the LPS-stimulated whole blood model demonstrate differential counterinflammatory effects, but only CpG ODNs were associated with proinflammatory cytokine production. With further examination, we may find that these observed immunomodulatory differences could potentially be exploited for therapeutic benefit.

Immunostimulatory CpG oligodeoxynucleotide confers protection in a murine model of infection with Burkholderia pseudomallei

Although CpG oligodeoxynucleotides (CpG ODNs) are known to enhance resistance against infection in a number of animal models, little is known about the CpG-induced protection against acute fatal sepsis such as that associated with the highly virulent bacterium Burkholderia pseudomallei. We previously demonstrated in an in vitro study that immunostimulatory CpG ODN 1826 enhances phagocytosis of B. pseudomallei and induces nitric oxide synthase and nitric oxide production by mouse macrophages. In the present study, CpG ODN 1826 given intramuscularly to BALB/c mice 2 to 10 days prior to B. pseudomallei challenge conferred better than 90% protection. CpG ODN 1826 given 2 days before the bacterial challenge rapidly enhanced the innate immunity of these animals, judging from the elevated serum levels of interleukin-12 (IL-12)p70 and gamma interferon (IFN-gamma) over the baseline values. No bacteremia was detected on day 2 in 85 to 90% of the CpG-treated animals, whereas more than 80% of the untreated animals exhibited heavy bacterial loads. Although marked elevation of IFN-gamma was found consistently in the infected animals 2 days after the bacterial challenge, it was ameliorated by the CpG ODN 1826 pretreatment (P = 0.0002). Taken together, the kinetics of bacteremia and cytokine profiles presented are compatible with the possibility that protection by CpG ODN 1826 against acute fatal septicemic melioidosis in this animal model is associated with a reduction of bacterial load and interference with the potential detrimental effect of the robust production of proinflammatory cytokines associated with B. pseudomallei multiplication.

Tolerance Induced by the Lipopeptide Pam3Cys Is Due to Ablation of IL-1R-Associated Kinase-1

Stimulation of the human monocytic cell line Mono Mac 6 with the synthetic lipopeptide (S)-(2,3-bis(palmitoyloxy)-(2RS)-propyl)-N-palmitoyl-(R)-Cys-(S)-Ser(S)-Lys(4)-OH, trihydrochloride (Pam(3)Cys) at 10 microg/ml induces a rapid expression of the TNF gene in a TLR2-dependent fashion. Preculture of the cells with Pam(3)Cys at 1 microg/ml leads to a reduced response after subsequent stimulation with Pam(3)Cys at 10 microg/ml, indicating that the cells have become tolerant to Pam(3)Cys. The CD14 and TLR2 expression is not decreased on the surface of the tolerant cells, but rather up-regulated. Analysis of the NF-kappaB binding in Pam(3)Cys-tolerant cells shows a failure to mobilize NF-kappaB-p50p65 heterodimers, while NF-kappaB-p50p50 homodimers remain unchanged. Pam(3)Cys-tolerant cells showed neither IkappaBalpha-Ser(32) phosphorylation nor IkappaBalpha degradation but MyD88 protein was unaltered. However, IRAK-1 protein was absent in Pam(3)Cys-induced tolerance, while IRAK-1 mRNA was still detectable at 30% compared with untreated cells. In contrast, in LPS-tolerized cells, p50p50 homodimers were induced, IRAK-1 protein level was only partially decreased, and p50p65 mobilization remained intact. It is concluded that in Mono Mac 6 monocytic cells, inhibition of IRAK-1 expression at the mRNA and protein levels is the main TLR-2-dependent mechanism responsible for Pam(3)Cys-induced tolerance, but not for TLR-4-dependent LPS-induced tolerance.

Prolonged Toll-like receptor stimulation leads to down-regulation of IRAK-4 protein

Interleukin-1 receptor-associated kinase (IRAK)-4 is a key mediator in the Toll-like receptor (TLR) signaling. We found that stimulation of TLR2, TLR4, or TLR9, but not TLR3, caused a decrease in IRAK-4 protein without affecting its mRNA level in a mouse macrophage cell line, RAW 264. The decrease in IRAK-4 was accompanied by the appearance of a smaller molecular weight protein (32 kD), which was recognized by an anti-IRAK-4 antibody raised against the C-terminal region. The decrease in IRAK-4 and the appearance of the 32-kD protein occurred with slower kinetics than the activation of IRAK-1 and were suppressed by inhibitors of the proteasome, inducible inhibitor of kappaBalpha phosphorylation or protein synthesis, but not by caspase inhibitors. These results indicate that prolonged stimulation of TLR2, TLR4, or TLR9 causes a down-regulation of IRAK-4 protein, which may be mediated through cleavage of IRAK-4 by a protease induced by the activation of nuclear factor-kappaB.

Influence of Coinfecting Pathogens on HIV Expression: Evidence for a Role of Toll-Like Receptors

Immune activation of HIV gene expression as a consequence of the host response to coinfecting pathogens has been implicated as an important factor in AIDS progression. Immune responsiveness to many of the infectious agents associated with HIV has been demonstrated to depend on a family of innate recognition molecules, known as Toll-like receptors (TLR). Therefore, TLR-pathogen interactions could play an indirect role in regulating HIV-associated disease. In this review, we summarize emerging evidence for the influence of TLR recognition on HIV gene activation and AIDS progression.

Molecular mechanisms of macrophage activation and deactivation by lipopolysaccharide: roles of the receptor complex

Bacterial lipopolysaccharide (LPS), the major structural component of the outer wall of Gram-negative bacteria, is a potent activator of macrophages. Activated macrophages produce a variety of inflammatory cytokines. Excessive production of cytokines in response to LPS is regarded as the cause of septic shock. On the other hand, macrophages exposed to suboptimal doses of LPS are rendered tolerant to subsequent exposure to LPS and manifest a profoundly altered response to LPS. Increasing evidence suggests that monocytic cells from patients with sepsis and septic shock survivors have characteristics of LPS tolerance. Thus, an understanding of the molecular mechanisms underlying activation and deactivation of macrophages in response to LPS is important for the development of therapeutics for septic shock and the treatment of septic shock survivors. Over the past several years, significant progress has been made in identifying and characterizing several key molecules and signal pathways involved in the regulation of macrophage functions by LPS. In this paper, we summarize the current findings of the functions of the LPS receptor complex, which is composed of CD14, Toll-like receptor 4 (TLR4), and myeloid differentiation protein-2 (MD-2), and the signal pathways of this LPS receptor complex with regard to both activation and deactivation of macrophages by LPS. In addition, recent therapeutic approaches for septic shock targeting the LPS receptor complex are described.

Activation of RAW264.7 macrophages by bacterial DNA and lipopolysaccharide increases cell surface DNA binding and internalization

Bacterial DNA containing unmethylated CpG motifs is a pathogen-associated molecular pattern (PAMP) that interacts with host immune cells via a toll-like receptor (TLR) to induce immune responses. DNA binding and internalization into cells is independent of TLR expression, receptor-mediated, and required for cell activation. The objective of this study was to determine whether exposure of immune cells to bacterial DNA affects DNA binding and internalization. Treatment of RAW264.7 cells with CpG oligodeoxynucleotide (ODN) for both 18 and 42 h resulted in a significant increase in DNA binding, whereas non-CpG ODN had no effect on DNA binding. Enhanced DNA binding was non-sequence-specific, inhibited by unlabeled DNA, showed saturation, was consistent with increased cell surface DNA receptors, and resulted in enhanced internalization of DNA. Treatment with Escherichia coli DNA or lipopolysaccharide (LPS) also resulted in a significant increase in DNA binding, but treatment with interleukin-1alpha, tumor necrosis factor-alpha, or phorbol 12-myristate 13-acetate had no effect on DNA binding. Soluble factors produced in response to treatment with CpG ODN or LPS did not affect DNA binding. These studies demonstrate that one consequence of activating the host innate immune response by bacterial infection is enhanced binding and internalization of DNA. During this period of increased DNA internalization, RAW264.7 cells were hypo-responsive to continued stimulation by CpG ODN, as assessed by tumor necrosis factor-alpha activity. We speculate the biological significance of increasing DNA binding and internalization following interaction with bacterial PAMPs may provide a mechanism to limit an ongoing immune inflammatory response by enhancing clearance of bacterial DNA from the extracellular environment.

Role of vav1- and src-related tyrosine kinases in macrophage activation by CpG DNA

Macrophage activation by CpG DNA requires toll-like receptor 9 and the adaptor protein MyD88. Gram-negative bacterial lipopolysaccharide also activates macrophages via a toll-like receptor pathway (TLR-4), but we and others have reported that lipopolysaccharide also stimulates tyrosine phosphorylation in macrophages. Herein we report that exposure of RAW 264.7 murine macrophages to CpG DNA (but not non-CpG DNA) provoked the rapid tyrosine phosphorylation of vav1. PP1, a selective inhibitor of src-related tyrosine kinases, blocked both the CpG DNA-mediated tyrosine phosphorylation of vav1 and the CpG DNA-mediated up-regulation of macrophage tumor necrosis factor secretion and inducible nitric-oxide synthase protein accumulation. Furthermore, we found that the inducible expression of any of three dominant interfering mutants of vav1 (a truncated protein, vavC; a form containing a point mutation in the regulatory tyrosine residue, vavYF174; and a form with an in-frame deletion of six amino acids required for the guanidine nucleotide exchange factor (GEF) activity of vav1 for rac family GTPases, vavGEFmt) consistently inhibited CpG DNA-mediated up-regulation of tumor necrosis factor secretion and inducible nitric-oxide synthase protein accumulation in RAW-TT10 macrophages. Finally, we determined that CpG DNA-mediated up-regulation of NF-kappaB activity (but not mitogen-activated protein kinase activation) was inhibited by preincubation with PP1 or by expression of the truncated vavC mutant. Taken together, our results indicate that the tyrosine phosphorylation of vav1 by a src-related tyrosine kinase or kinases plays an important role in the macrophage response to CpG DNA.

A dominant role of Toll-like receptor 4 in the signaling of apoptosis in bacteria-faced macrophages

Conserved bacterial components potently activate host immune cells through transmembrane Toll-like receptors (TLRs), which trigger a protective immune response but also may signal apoptosis. In this study, we investigated the roles of TLR2 and TLR4 as inducers of apoptosis in Yersinia enterocolitica-infected macrophages. Yersiniae suppress activation of the antiapoptotic NF-kappaB signaling pathway in host cells by inhibiting inhibitory kappaB kinase-beta. This leads to macrophage apoptosis under infection conditions. Experiments with mouse macrophages deficient for TLR2, TLR4, or both receptors showed that, although yersiniae could activate signaling through both TLR2 and TLR4, loss of TLR4 solely diminished Yersinia-induced apoptosis. This suggests implication of TLR4, but not of TLR2, as a proapoptotic signal transducer in Yersinia-conferred cell death. In the same manner, agonist-specific activation of TLR4 efficiently mediated macrophage apoptosis in the presence of the proteasome inhibitor MG-132, an effect that was less pronounced for activation through TLR2. Furthermore, the extended stimulation of overexpressed TLR4 elicited cellular death in epithelial cells. A dominant-negative mutant of Fas-associated death domain protein could suppress TLR4-mediated cell death, which indicates that TLR4 may signal apoptosis through a Fas-associated death domain protein-dependent pathway. Together, these data show that TLR4 could act as a potent inducer of apoptosis in macrophages that encounter a bacterial pathogen.

Contrasting activity of cytosin-guanosin dinucleotide oligonucleotides in mice with experimental colitis

Intestinal inflammation in inflammatory bowel disease (IBD) and experimental models of colitis is characterized by a dysregulated intestinal immune response with elevated levels of Th1 cytokines. The luminal flora has been implicated as a major factor contributing to the initiation and perpetuation of inflammation in experimental colitis by mechanisms not known. Bacterial DNA contains unmethylated cytosin-guanosin dinucleotides (CpG) which strongly activate Th1-mediated immune responses. To test whether these CpG-motifs modulate intestinal inflammation we treated mice with dextran sulphate sodium (DSS)-induced colitis with CpG-containing oligodeoxynucleotides (CpG-ODN). CpG-ODN given after the onset of DSS colitis aggravated the disease, as indicated by a significantly increased loss of body weight and a 30% increase of the histological score. Further, we found a severe increase of proinflammatory cytokines (interleukin (IL)-6: 40-fold; interferon (IFN)-gamma: 11-fold). In a pretreatment setting CpG-ODN reduced weight loss significantly and reduced intestinal inflammation by 45%. Colonic IFN-gamma and IL-6 mRNA levels were reduced by 75%, and IL-10 was elevated by 400% compared to controls. The prophylactic CpG-effect was not imitated by IL-12 because IL-12 pretreatment was not protective. In time-course experiments, CpG-ODN pretreatment over 5 days resulted in a tolerance effect concerning its IFN-gamma-inducing quality, and during the following days of colitis induction IL-10 secretion from mesenterial lymph node cells was elevated compared to controls. Therefore, the prophylactic effect of CpG-ODN might be explained by its tolerizing effect and/or the increased ability for IL-10 production during the consecutive intestinal inflammation.

The induction of Toll-like receptor tolerance enhances rather than suppresses HIV-1 gene expression in transgenic mice

Microbial-induced proinflammatory pathways are thought to play a key role in the activation of human immunodeficiency virus type 1 (HIV-1) gene expression. The induction of Toll-like receptor (TLR) tolerance leads to a complex reprogramming in the pattern of inflammatory gene expression and down-modulates tumor necrosis factor alpha (TNF-alpha), interleukin (IL)-1, and IL-6 production. Using transgenic (Tg) mice that incorporate the entire HIV-1 genome, including the long-terminal repeat, we have previously demonstrated that a number of different TLR ligands induce HIV-1 gene expression in cultured splenocytes as well as purified antigen-presenting cell populations. Here, we have used this model to determine the effect of TLR-mediated tolerance as an approach to inhibiting microbial-induced viral gene expression in vivo. Unexpectedly, Tg splenocytes and macrophages, rendered tolerant in vitro to TLR2, TLR4, and TLR9 ligands as assessed by proinflammatory cytokine secretion and nuclear factor-kappaB activation, showed enhanced HIV-1 p24 production. A similar enhancement was observed in splenocytes tolerized and then challenged with heterologous TLR ligands. Moreover, TLR2- and TLR4-homotolerized mice demonstrated significantly increased plasma p24 production in vivo despite lower levels of TNF-alpha. Together, these results demonstrate that HIV-1 expression is enhanced in TLR-reprogrammed host cells, possibly reflecting a mechanism used by the virus to escape the effects of microbial-induced tolerance during natural infection in vivo.

Shielding the double-edged sword: negative regulation of the innate immune system

The innate immune system is evolutionarily conserved among all multicellular organisms and is the first line of defense against microorganisms. It enables the host not only to combat pathogenic organisms but also to cohabit with nonpathogenic microorganisms by balancing the host-microorganism interaction. The innate immune response is activated rapidly (within hours) compared with adaptive immunity. Activation of the innate immune system allows the activation of the adaptive immune response by production of proinflammatory cytokines and by providing stimulatory signals via major histocompatibility complex molecules and costimulatory molecules such as CD40, CD80, or CD86; together, these lead to the full activation of both immune systems to fight against pathogenic microorganisms. Activation of the innate immune system, however, can be a double-edged sword for the host. Proinflammatory cytokines mediate a positive feedback loop on the innate immune system, and overproduction of cytokines, if unchecked, is hazardous to the host and may cause severe outcomes such as hyperthermia, organ failure, and even death in extreme cases. Moreover, if the overproduction of proinflammatory cytokines persists, it may cause chronic inflammatory diseases. During evolution, the innate immune system has acquired complicated regulatory systems to control itself so that this "sword" will not kill the host. Various mechanisms including inhibition of Toll-like receptor signaling by interleukin-1 receptor-associated kinase-M have evolved for this purpose and are important not only to fight against pathogenic microorganisms efficiently but also are critical for the peaceful coexistence with commensal bacterial flora.

How not to get bugged by bugs: mechanisms of cellular tolerance to microorganisms

PURPOSE OF REVIEW

Advances in the characterization of the receptors and signaling pathways involved in the response to infection have led to a more detailed understanding of cellular tolerance to endotoxin and other microbial components. This review summarizes recent progress in the field, particularly in relation to the molecular mechanisms that underlie the development of tolerance to microorganisms.

RECENT FINDINGS

The identification of Toll-like receptors as major sensors of microbial molecules has led to numerous studies of their function in tolerant cells. Decreased Toll-like receptor expression, altered interactions between the Toll-like receptors and intracellular signal transducers, and decreased expression or activity of downstream signaling molecules have all been implicated. Upregulation of specific proteins that inhibit Toll-like receptor signaling has also been described. Apart from these general mechanisms, special features of the intestinal microenvironment and its resident cells also contribute to making the gut hyporesponsive to microorganisms.

SUMMARY

The application of gene knockout technology has highlighted the importance of macrophage tolerance in regulating innate immunity to microbial infection. Such studies have indicated that the failure of tolerance can lead to exaggerated inflammatory responses to intestinal bacteria, raising the possibility that defects in tolerance may be linked to conditions such as inflammatory bowel disease. Further characterization of this link will help in elucidating the pathogenesis of such conditions and in devising new approaches to treatment.

Ubiquitin activated tumor necrosis factor receptor associated factor-6 (TRAF6) is recycled via deubiquitination

Ubiquitination of intermediates in the interleukin-1 (IL-1) signaling cascade plays an important role in activation and regulation of the pathway. Both IL-1 receptor associated kinase-1 (IRAK1) and inhibitor of nuclear factor kappaB-alpha (IkappaBalpha) are rapidly ubiquitinated and degraded. Tumor necrosis factor associated factor-6 (TRAF6) is an ubiquitin ligase that is activated by ubiquitination and a signaling intermediate between IRAK1 and IkappaBalpha. It is unknown whether activated TRAF6 is subsequently degraded. We show that in liver cells IL-1 stimulates TRAF6 poly-ubiquitination. In less than 1 h levels of non-modified TRAF6 return to levels near those observed prior to activation. TRAF6 cannot be reactivated in cells which have been pretreated with IL-1. This observation correlates with decreased levels of IRAK1 in IL-1 pretreated cells. The re-establishment of non-modified TRAF6 levels following activation does not require de novo protein synthesis, strongly suggesting that TRAF6 is recycled via deubiquitination. This indicates a unique mechanism of regulation of TRAF6 activity.