Involvement of suppressor of cytokine signaling-3 as a mediator of the inhibitory effects of IL-10 on lipopolysaccharide-induced macrophage activation

Construction of an adeno-associated viral vector serotype 2/1 containing human interleukin-10 and its expression in donor liver

AIM: To construct an adeno-associated viral vector serotype 2/1 (AAV2/1) containing human interleukin-10 (hIL-10) gene and to observe its expression in donor liver.

METHODS: hIL-10cDNA amplified by reverse transcription polymerase chain reaction (RT-PCR) from human peripheral blood mononuclear cells was cloned into vector pMD18-T. After confirming the sequence, hIL-10cDNA was isolated and inserted into eukaryotic expression vector pSNAV. The recombinant plasmid pSNAV-hIL-10 was transfected into BHK21 cells. BHK21 cells which contained ITR-hIL-10-ITR were obtained by G418 screening. Then the cells were transfected with rHSV/r2c1 containing rep2-cap1 gene. The cells were cultured and purified to obtain rAAV2/1-hIL-10. The expression of hIL-10 gene was detected after this vector was transfer into donor liver in vivo.

RESULTS: The sequence of cloned hIL-10cDNA was identical with that published on GenBank. A new adeno-associated virus vector containing hIL-10cDNA was constructed. And the transcription and expression of hIL-10 were detected in donor liver for 24 weeks. hIL-10 was significantly expressed in test group than that in empty and rAAV2/1-GFP controls 24 wk after transferred (219.15±45.83 ng/L vs 40.02, 38.64 ng/L, P<0.05).

CONCLUSION: The adeno-associated viral vector serotype 2/1 of hIL-10 is successfully established, which provides the basis for applying IL-10 in clinical organ transplantation.

Lipopolysaccharide primes neutrophils for a rapid response to IL-10

Responsiveness of human neutrophils to IL-10 was recently shown to be strictly dependent on the levels of IL-10R1 expression. Activation of signal transducer and activator of transcription 3 (STAT3) phosphorylation and induction of suppressor of cytokine signaling (SOCS)-3 protein by IL-10 are in fact negligible in circulating or freshly isolated ("time 0") neutrophils, but become readily measurable in neutrophils cultured for 4 h in the presence or absence of LPS. In this study, we show that modulation by IL-10 of LPS-induced TNF-alpha, CXCL8/IL-8 and IL-1 receptor antagonist (IL-1ra) mRNA accumulation in neutrophils already expressing a functional IL-10R and antigenic SOCS-3 (i.e. in "4-h-cultured" neutrophils) occurs with kinetics that are similar to those observed in "time 0" neutrophils, depends on de novo protein synthesis, but does not require SOCS-1, SOCS-3, heme oxygenase and Bcl-3 induction. By contrast, we show that IL-10 alone rapidly modulates the expression of TNF-alpha, CXCL8/IL-8 and IL-1ra mRNA, without any new protein synthesis requirement, if neutrophils have been previously exposed to LPS for at least 4 h. These findings suggest that LPS prepares neutrophils to optimally respond to IL-10 in terms of rapid gene modulation via mechanisms that, presumably, depend on specific LPS-induced protein(s).

Development and validation of a novel IL-10 deficient cell transfer model for colitis

A number of rodent models for inflammatory bowel disease (IBD) have been developed, but most cannot be used to develop and validate new therapies for IBD. From the models developed, the IL-10 deficient mouse model is the one that results in a disease similar to human IBD; however, in this model, colitis occurs with variable incidence taking 3-4 months to develop. These are serious problems with the model when evaluating a new therapy because of the large-scale experiments required and the difficulty in performing an accurate pharmacological analysis. In this study, the IL-10 deficient mouse model was modified by transferring whole spleen and mesenteric lymph node cells from IL-10 deficient mice to CB-17 SCID mice. In this IL-10 deficient cell transfer model, chronic intestinal inflammation developed in all recipients within 2-3 weeks, which was far earlier than in donor IL-10 deficient mice. The pathological phenotypes were similar to those of IL-10 deficient mice and CD45RBhi T cell-transfer models. In addition, we assessed several agents for inflammatory bowel disease to validate the general utility of this cell transfer model. It is worth noting that TNFR-Ig or prednisolone, which is effective for treatment of patients with severe-fulminant Crohn's disease, markedly attenuated pathological clinical indices in this colitis model, whereas the immunosuppressive agents, azathioprine, tacrolimus, and cyclosporine A produced no significant effect. These results suggest that the IL-10 deficient cell transfer model is a good experimental model to use for developing new and effective therapies for active IBD.

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.

Signal transduction activator of transcription 5 (STAT5) dysfunction in autoimmune monocytes and macrophages

Autocrine granulocyte macrophage-colony stimulating factor (GM-CSF) sequentially activates intracellular components in monocyte/macrophage production of the pro-inflammatory and immunoregulatory prostanoid, prostaglandin E2 (PGE2). GM-CSF first induces STAT5 signaling protein phosphorylation, then prostaglandin synthase 2 (COX2/PGS2) gene expression, and finally IL-10 production, to downregulate the cascade. Without activation, monocytes of at-risk, type 1 diabetic (T1D), and autoimmune thyroid disease (AITD) humans, and macrophages of nonobese diabetic (NOD) mice have aberrantly high GM-CSF, PGS2, and PGE2 expression, but normal levels of IL-10. After GM-CSF stimulation, repressor STAT5A and B isoforms (80-77kDa) in autoimmune human and NOD monocytes and activator STAT5A (96-94kDa) and B (94-92kDa) isoforms in NOD macrophages stay persistently tyrosine phosphorylated. This STAT5 phosphorylation persisted despite treatment in vitro with IL-10, anti-GM-CSF antibody, or the JAK2/3 inhibitor, AG490. Phosphorylated STAT5 repressor isoforms in autoimmune monocytes had diminished DNA binding capacity on GAS sequences found in the PGS2 gene enhancer. In contrast, STAT5 activator isoforms in NOD macrophages retained their DNA binding capacity on these sites much longer than in healthy control strain macrophages. These findings suggest that STAT5 dysfunction may contribute to dysregulation of GM-CSF signaling and gene activation, including PGS2, in autoimmune monocytes and macrophages.

Immunoregulatory effects of regulated, lung-targeted expression of IL-10 in vivo

Regulation of pulmonary inflammation involves an intricate balance of both pro- and anti-inflammatory mediators. Acute lung injury can result from direct pulmonary insults that activate alveolar macrophages to respond with increased cytokine expression. Such cytokine gene expression is mediated in part via NF-κB. IL-10 has been previously identified as an important endogenous anti-inflammatory cytokine in vivo on the basis of inhibiting NF-κB activation; however, the mechanism of this inhibition remains incompletely defined. We hypothesized that IL-10 regulated NF-κB activation in vivo via IκK inhibition. A bitransgenic mouse that allowed for externally regulated, lung-specific human IL-10 overexpression was generated. In the bitransgenic mice, introduction of doxycycline induced lung-specific, human IL-10 overexpression. Acute induction of IL-10 resulted in significant decreases in bronchoalveolar lavage fluid neutrophils (48%, P = 0.03) and TNF (62%, P < 0.01) following intratracheal LPS compared with bitransgenic negative mice. In vitro kinase assays showed this decrease to correlate to diminished lung IκK activity. Furthermore, we also examined the effect of chronic IL-10 overexpression in these transgenic mice. Results show that IL-10 overexpression in lungs of mature mice increased the number of intrapulmonary cells the phenotype of which was skewed toward increased B220+/CD45+ B cells and CD4+ T cells and was associated with increased CC chemokine expression. Thus regulated, lung-specific IL-10 overexpression may have a variety of complex immunologic effects depending on the timing and duration of expression.

Yersinia enterocolitica induces apoptosis and inhibits surface molecule expression and cytokine production in murine dendritic cells

Yersinia enterocolitica evades innate immunity by expression of a variety of pathogenicity factors. Therefore, adaptive immunity including CD4(+) T cells plays an important role in defense against Y. enterocolitica. We investigated whether Y. enterocolitica might target dendritic cells (DC) involved in adaptive T-cell responses. For this purpose, murine DC were infected with Y. enterocolitica wild-type and mutant strains prior to incubation with ovalbumin (OVA) as antigen and 5-(6)-carboxyfluorescein diacetate N-succinimidyl ester-labeled OVA-specific T cells from DO11.10 mice. While T-cell proliferation was partially affected by infection of DC with plasmid-cured and YopP-deficient Yersinia mutant strains, no T-cell proliferation occurred after infection of DC with wild-type Y. enterocolitica. Infection of DC with Y. enterocolitica wild type resulted in decreased up-regulation of major histocompatibility complex class II, CD54 (intercellular adhesion molecule 1), CD 80, and CD86 expression. Experiments with plasmid-cured Y. enterocolitica or a YopP-deficient mutant strain revealed that YopP accounts for inhibition of surface molecule expression. Wild-type Y. enterocolitica suppressed the release of KC, tumor necrosis factor alpha, interleukin-10 (IL-10), and IL-12 by DC, while infection of DC with plasmid-cured Y. enterocolitica or with the YopP-deficient mutant resulted in the production of these cytokines. Moreover, infection with wild-type Y. enterocolitica induced apoptosis in DC mediated by YopP. Apoptosis occurred despite translocation of NF-kappaB to the nucleus, as demonstrated by electromobility shift assays. Together, these data demonstrate that Y. enterocolitica targets functions of murine DC that are required for T-cell activation. This might contribute to evasion of adaptive immune responses by Y. enterocolitica.

IL-2 Overcomes the Unresponsiveness but Fails to Reverse the Regulatory Function of Antigen-Induced T Regulatory Cells

Intranasal administration of peptide Ac1-9[4Y], based on the N-terminal epitope of myelin basic protein, can induce CD4(+) T cell tolerance, and suppress experimental autoimmune encephalomyelitis induction. The peptide-induced regulatory T (PI-T(Reg)) cells failed to produce IL-2, but expressed IL-10 in response to Ag and could suppress naive T cell responses in vitro. Analysis of Jak-STAT signaling pathways revealed that the activation of Jak1, STAT3, and STAT5 were induced in tolerant T cells after Ag stimulation in vivo. In addition, the expression of suppressor of cytokine signaling 3 was induced in tolerant T cells, suggesting that cytokines regulate the tolerant state of the PI-T(Reg) cells. Stimulation of PI-T(Reg) cells in vitro with IL-10 induced Jak1 and STAT3 activation, but not STAT5, suggesting that IL-10 is important, but not the only cytokine involved in the development of T cell tolerance. Although IL-2 expression was deficient, stimulation with IL-2 in vitro induced Jak1 and STAT5 activation in PI-T(Reg) cells, restored their proliferative response to antigenic stimulation, and abrogated PI-T(Reg)-mediated suppression in vitro. However, the addition of IL-2 could not suppress IL-10 expression, and the IL-2 gene remained inactive. After withdrawal of IL-2, the PI-T(Reg) cells regained their nonproliferative state and suppressive ability. These results underline the ability of the immune system to maintain tolerance to autoantigens, but at the same time having the ability to overcome the suppressive phenotype of tolerant T cells by cytokines, such as IL-2, during the protective immune response to infection.

Regulation of the immune system by SOCS family adaptor proteins

Signal transduction via cytokine receptors is regulated by several mechanisms that control initiation, magnitude and duration of the signaling pathways. Cytokine-induced SOCS family adaptors function as feedback inhibitors of cytokine receptor signaling by inhibiting the JAK-STAT signal transduction pathway. Specific gene-targeted mice have unveiled critical, non-overlapping functions for SOCS1 and SOCS3 in lymphocyte development and homeostasis, and in the regulation of macrophage and dendritic cell functions. In this review, we will discuss the structure of SOCS proteins, mechanisms by which they control the JAK-STAT pathway and their role in immune regulation.

Heat shock protein 10 inhibits lipopolysaccharide-induced inflammatory mediator production

Heat shock protein 10 (Hsp10) and heat shock protein 60 (Hsp60) were originally described as essential mitochondrial proteins involved in protein folding. However, both proteins have also been shown to have a number of extracellular immunomodulatory activities. Here we show that purified recombinant human Hsp10 incubated with cells in vitro reduced lipopolysaccharide (LPS)-induced nuclear factor-kappaB activation and secretion of several inflammatory mediators from RAW264.7 cells, murine macrophages, and human peripheral blood mononuclear cells. Induction of tolerance by contaminating LPS was formally excluded as being responsible for Hsp10 activity. Treatment of mice with Hsp10 before endotoxin challenge resulted in the reduction of serum tumor necrosis factor-alpha and RANTES (regulated upon activation, normal T cell expressed and secreted) levels and an elevation of serum interleukin-10 levels. Hsp10 treatment also delayed mortality in a murine graft-versus-host disease model, where gut-derived LPS contributes to pathology. We were unable to confirm previous reports that Hsp10 has tumor growth factor properties and suggest that Hsp10 exerts anti-inflammatory activity by inhibiting Toll-like receptor signaling possibly by interacting with extracellular Hsp60.

New insights into the molecular mechanism of interleukin-10-mediated immunosuppression

Interleukin-10 (IL-10) is an important immunomodulatory cytokine, which has attracted much attention because of its anti-inflammatory properties. It reduces antigen presentation and inhibits T cell activation. IL-10-treated myeloid cells lose their ability to respond toward the endotoxin lipopolysaccharide (LPS) with the production of several proinflammatory mediators. Thereby, IL-10 limits excessive inflammatory reactions in response to endotoxin as it occurs in colitis or endotoxin shock. Mice can be tolerized toward endotoxin shock when pretreated with a sublethal dose of LPS. This can be mimicked in vitro as LPS desensitization, resulting in a similar LPS hyporesponsiveness as observed with IL-10 pretreatment. However, an early block in LPS signaling characterizes LPS desensitization, whereas IL-10 seems to target late events. Controversial reports have been published where IL-10 would interfere with the induction of proinflammatory mediators, and little is known about the molecular mechanisms behind the anti-inflammatory activities of IL-10. Some recent publications have tried to gain more insight into the molecular mechanism of IL-10 by gene-expression profiling and functional studies in myeloid-derived cells. These results are reviewed here and compared with the progress that has been made to understand the induction of endotoxin tolerance by LPS itself.

A new anti-inflammatory agent KL-1037 represses proinflammatory cytokine and inducible nitric oxide synthase (iNOS) gene expression in activated microglia

Excessive proinflammatory cytokine and NO production by activated microglia play a role in neurodegenerative disorders. In this study, we found that a new compound KL-1037 suppressed LPS-induced NO release/inducible nitric oxide synthase expression in BV2 mouse microglial cells. In addition, KL-1037 prominently diminished LPS-induced production of pro-inflammatory cytokines such as TNF-alpha, IL-1 beta and IL-6, while it increased anti-inflammatory IL-10 and TGF-beta 1 production. By RNase protection assay and RT-PCR, we showed that KL-1037 regulated iNOS and cytokines at transcriptional or post-transcriptional level. Further analysis of molecular mechanisms revealed that KL-1037 prominently increased intracellular cAMP levels and potentiated LPS-induced pCREB expression. However, LPS-induced MAP kinase or NF-kappa B activities were slightly or little changed by KL-1037. Treatment with cAMP antagonist or IL-10 neutralizing antibody completely reversed upregulation of IL-10 and partially repression of TNF-alpha or NO induced by KL-1037. These data suggest that microglial inactivation by KL-1037 is at least in part due to activation of PKA pathway and/or upregulation of IL-10. Thus, repressing proinflammatory cytokines and iNOS gene expression in activated microglia by KL-1037 may provide potential therapeutic strategies for various neurodegenerative diseases including ischemic cerebral disease.

BCL10 Mediates Lipopolysaccharide/Toll-like Receptor-4 Signaling through Interaction with Pellino2

Toll-like receptor (TLR) pathways signal through microbial components stimulation to induce innate immune responses. Herein, we demonstrate that BCL10, a critical molecule that signals between the T cell receptor and IkappaB kinase complexes, is involved in the innate immune system and is required for appropriate TLR4 pathway and nuclear factor-kappaB (NF-kappaB) activation. In response to lipopolysaccharide (LPS) stimulation, BCL10 was recruited to TLR4 signaling complexes and associated with Pellino2, an essential component down-stream of BCL10 in the TLR4 pathway. In a BCL10-deficient macrophage cell line, LPS-induced NF-kappaB activation was consistently defective, whereas activator protein-1 and Elk-1 signaling was intact. In addition, we found that BCL10 was targeted by SOCS3 for negative regulation in LPS signaling. The recruitment of BCL10 to TLR4 signaling complexes was attenuated by induced expression of SOCS3 in a feedback loop. Furthermore, ectopic SOCS3 expression blocked the interaction between BCL10 and Pellino2 together with BCL10-generated NF-kappaB activation and inducible nitric-oxide synthase expression. Together, these data define an important role of BCL10 in the innate immune system.

Distinct transcriptional programs activated by interleukin-10 with or without lipopolysaccharide in dendritic cells: induction of the B cell-activating chemokine, CXC chemokine ligand 13

To understand the modulation of dendritic cell (DC) function by IL-10, gene expression profiling was performed by using Affymetrix technology (Santa Clara, CA) in human monocyte-derived DC treated with IL-10, alone or in combination with LPS. The modulation of selected genes was validated by real-time PCR, Northern blot, and protein production. IL-10 regulated in DC the expression of a limited number of genes, including IL-7, the receptors for transferrin and vitamin D(3), structural matrix proteins, and signal transduction elements. The combined treatment with LPS plus IL-10 modulated a number of genes comparable to LPS alone, but the expression profiles were distinct. As expected, IL-10 suppressed the expression of several LPS-inducible proinflammatory molecules. Among genes uniquely modulated by the concomitant treatment with LPS plus IL-10, phosphatidylinositol 3-kinase gamma was down-regulated while the suppressor of cytokine signaling 3, signaling lymphocytic activation molecule, regulator of G protein signaling 16, and the chemokine, CXC chemokine ligand (CXCL) 13, were up-regulated. Overall, four distinct transcriptional programs were identified, related to: 1) control of immunity and inflammation; 2) tuning of cytokine receptor and G protein-coupled receptor signaling; 3) remodeling of extracellular matrix; and 4) B cell function and lymphoid tissue neogenesis. Among the latter genes, we further demonstrate that IL-10 synergizes with TLR ligands for the production of functionally active B cell-attracting chemokine, CXCL13, in both myeloid and plasmacytoid DC. This novel finding reveals that IL-10 sustains humoral immunity by inducing the production in APCs of the chemokine, CXCL13, which amplifies B cell recruitment and promotes lymphoid tissue neogenesis.

SOCS5 is expressed in primary B and T lymphoid cells but is dispensable for lymphocyte production and function

Suppressors of cytokine signaling (SOCSs) are key regulators of cytokine-induced responses in hematopoietic as well as nonhematopoietic cells. SOCS1 and SOCS3 have been shown to modulate T-cell responses, whereas the roles of other SOCS family members in the regulation of lymphocyte function are less clear. Here, we report the generation of mice with a targeted disruption of the Socs5 gene. Socs5(-/-) mice were born in a normal Mendelian ratio and were healthy and fertile. We found that SOCS5 is expressed in primary B and T cells in wild-type mice. However, no abnormalities in the lymphocyte compartment were seen in SOCS5-deficient mice. We examined antigen- and cytokine-induced proliferative responses in B and T cells in the absence of SOCS5 and found no deviations from the responses seen in wild-type cells. Because SOCS5 has been implicated in Th1 differentiation, we also investigated the importance of SOCS5 in T helper cell responses. Unexpectedly, SOCS5-deficient CD4 T cells showed no abnormalities in Th1/Th2 differentiation and Socs5(-/-) mice showed normal resistance to infection with Leishmania major. Therefore, although SOCS5 is expressed in primary B and T cells, it appears to be dispensable for the regulation of lymphocyte function.

Inhibition of lipopolysaccharide-stimulated TNF-alpha promoter activity by S-adenosylmethionine and 5'-methylthioadenosine

S-adenosylmethionine (SAM) is the principal biological methyl donor and precursor for polyamines. SAM is known to be hepatoprotective in many liver disease models in which TNF-alpha is implicated. The present study investigated whether and how SAM inhibited LPS-stimulated TNF-alpha expression in Kupffer cells (hepatic macrophages). SAM downregulated TNF-alpha expression in LPS-stimulated Kupffer cells at the transcriptional level as suggested by a transfection experiment with a TNF-alpha promoter-reporter gene. This inhibition was not mediated through decreased NF-kappaB binding to four putative kappaB binding elements located within the promoter. The inhibited promoter activity was neither prevented by overexpression of p65 and/or its coactivator p300 nor enhanced by overexpression of coactivator-associated arginine methyltransferase-1, an enzyme that methylates p300 and inhibits a p65-p300 interaction. SAM did not lead to DNA methylation at the most common CpG target sites in the TNF-alpha promoter. Moreover, 5'-methylthioadenosine (MTA), which is derived from SAM but does not serve as a methyl donor, recapitulated SAM's effect with more potency. These data demonstrate that SAM inhibits TNF-alpha expression at the level downstream of NF-kappaB binding and at the level of the promoter activity via mechanisms that do not appear to involve the limited availability of p65 or p300. Furthermore, our study is the first to demonstrate a potent inhibitory effect on NF-kappaB promoter activity and TNF-alpha expression by a SAM's metabolite, MTA.

Insulin Induces SOCS-6 Expression and Its Binding to the p85 Monomer of Phosphoinositide 3-Kinase, Resulting in Improvement in Glucose Metabolism

The suppressors of cytokine signaling (SOCS) family is thought to act largely as a negative regulator of signaling by cytokines and some growth factors. Surprisingly, the SOCS-6 transgenics had no significant defects in the cytokine signaling and hematopoietic system but displayed significant improvements in glucose metabolism. Insulin stimulation of Akt/protein kinase B was also potentiated. Biochemical analysis showed that, after insulin stimulation, SOCS-6 interacted with the monomeric p85 subunit of class-Ia phosphoinositide (PI) 3-kinase but not with p85/p110 dimers. Furthermore, SOCS-6 expression is transiently increased by serum and insulin in normal fibroblasts. However, both the mRNA and protein of SOCS-6 were rapidly degraded after induction by insulin. The degradation of the SOCS-6 protein was partially inhibited by a proteasome inhibitor, suggesting a proteasome-mediated degradation mechanism. In contrast, SOCS-6-associated p85 was not degraded and could be recruited to the newly synthesized SOCS-6 molecules in the presence of insulin, suggesting that SOCS-6 expression and its interaction with p85, but not the degradation, is regulated by insulin. The phenotype of SOCS-6 transgenic mice bears a striking resemblance to p85 knock-out mouse models in which glucose metabolism stimulated by insulin is significantly improved despite reduced activation of PI 3-kinase. This suggests that monomeric p85 might play a physiologically important role in attenuating signaling through PI 3-kinase-dependent pathways in unstimulated cells. Therefore, our results indicate that SOCS-6 may provide a dynamically regulated mechanism by which insulin can transiently overcome the negative effects that p85 monomers have on signaling via PI 3-kinase-dependent signaling pathways.

Stat3 Activation in Acute Lung Injury

Stat3 plays diverse roles in biological processes including cell proliferation, survival, apoptosis, and inflammation. Very little is known regarding its activation and function in the lung during acute inflammation. We now show that Stat3 activation was triggered in lungs and in alveolar macrophages after intrapulmonary deposition of IgG immune complexes in rats. Low levels of constitutive Stat3 were observed in normal rat lungs as determined by the EMSA. Stat3 activity in whole lung extracts increased 2 h after initiation of IgG immune complex deposition, reaching maximal levels by 4 h, whereas Stat3 activation was found in alveolar macrophages as early as 30 min after onset of injury. Expression and activation of Stat3 mRNA, protein, and protein phosphorylation was accompanied by increased gene expression of IL-6, IL-10, and suppressor of cytokine signaling-3 in whole lung tissues. Both Tyr(705) and Ser(727) phosphorylation were involved in Stat3 activation as assessed in whole lung extracts. C5a (complement 5, fragment a) per se can induce phosphorylation of Ser(727) of Stat3. In vivo, Stat3 activation was dramatically suppressed by depletion of neutrophils or lung macrophages, resulting in reduced gene expression of IL-6 and IL-10 in whole lung tissues. Using blocking Abs to IL-6, IL-10, and C5a, Stat3 activation induced by IgG immune complexes was markedly diminished. These data suggest in the lung injury model used that activation of Stat3 in lungs is macrophage dependent and neutrophil dependent. IL-6, IL-10, and C5a contribute to Stat3 activation in inflamed rat lung.

CCL17 and IL-10 as effectors that enable alternatively activated macrophages to inhibit the generation of classically activated macrophages

Classically activated macrophages (CAMphi) have been described as a major effector cell on the host's innate immunities. However, CAMphi are not generated in immunocompromised hosts whose alternatively activated macrophages (AAMphi) predominate. In this study, the mechanism by which AAMphi suppress the ability of resident macrophages (RMphi) to generate CAMphi was investigated. AAMphi were isolated from peritoneal exudates of mice 2 days after third-degree thermal injuries affecting 15% total body surface area. CAMphi were generated from RMphi (peritoneal Mphi from normal mice) through stimulation with CpG DNA, a typical CAMphi inducer. RMphi did not polarize to CAMphi when they were cultured with AAMphi in a dual-chamber Transwell even when supplemented with CpG DNA. In addition, RMphi stimulated with CpG DNA did not convert to CAMphi when they were cultured with the culture fluids of AAMphi (AAMphi Culture-Sup). AAMphi Culture-Sup contained IL-6, IL-10, CCL17, PGE(2), and TGF-beta. Among these, CCL17 and IL-10 inhibited CAMphi generation. The ability of AAMphi Culture-Sup to inhibit CAMphi generation was eliminated when the Culture-Sup was treated with a mixture of mAbs directed against CCL17 and IL-10. These results indicate that CCL17 and IL-10 released from AAMphi inhibit CAMphi generation from RMphi stimulated with CpG DNA.

Macrophage-stimulating protein, the ligand for the stem cell-derived tyrosine kinase/RON receptor tyrosine kinase, inhibits IL-12 production by primary peritoneal macrophages stimulated with IFN-gamma and lipopolysaccharide

IL-12, produced by APCs during the initial stages of an immune response, plays a pivotal role in the induction of IFN-gamma by NK and gammadeltaT cells and in driving the differentiation of Th1 cells, thus providing a critical link between innate and acquired immunity. Due to the unique position occupied by IL-12 in the regulation of immunity, many mechanisms have evolved to modulate IL-12 production. We have shown previously that macrophage-stimulating protein (MSP), the ligand for the stem cell-derived tyrosine kinase/recepteur d'origine nantais (RON) receptor, inhibits NO production by macrophages in response to IFN-gamma and enhances the expression of arginase. Mice lacking RON exhibit increased inflammation in a delayed-type hypersensitivity reaction and increased susceptibility to endotoxic shock. In this study we demonstrate that pretreatment of macrophages with MSP before IFN-gamma and LPS results in the complete inhibition of IL-12 production due to suppression of p40 expression. This response is mediated by the RON receptor, and splenocytes from RON(-/-) animals produce increased levels of IFN-gamma. MSP pretreatment of macrophages resulted in decreased tyrosine phosphorylation of Stat-1 and decreased expression of IFN consensus sequence binding protein in response to inflammatory cytokines. In addition to IL-12, the expression of IL-15 and IL-18, cytokines that are also dependent on IFN consensus sequence binding protein activation, is inhibited by pretreatment with MSP before IFN-gamma and LPS. We also show that the ability of MSP to inhibit IL-12 production is independent of IL-10. Taken together, these results suggest that MSP may actively suppress cell-mediated immune responses through its ability to down-regulate IL-12 production and thus inhibit classical activation of macrophages.

Protein disulfide isomerase suppresses the transcriptional activity of NF-κB

We report here that the transcriptional activity of NF-kappaB is negatively regulated by protein disulfide isomerase (PDI). Over-expression of PDI in RAW 264.7 cells strongly suppressed the LPS-induced production of inflammatory cytokines as well as NF-kappaB-dependent luciferase activity. This negative regulation of NF-kappaB was reversed by bacitracin, a PDI inhibitor. Interestingly, NF-kappaB/DNA complex formation and phosphorylation of NF-kappaB subunits was intact in PDI-expressing cells following stimulation with LPS. In addition, PDI and another redox regulator, thioredoxin (TRX), had opposite effects on NF-kappaB-dependent gene expression: activation of the NF-kappaB pathway by TRX was suppressed by expression of PDI in a dose-dependent manner. Finally, PDI expression was induced by the anti-inflammatory cytokine IL-10, and IL-10-mediated inhibition of LPS-induced IL-6 expression was reduced by bacitracin. These findings clearly demonstrate that PDI is a negative regulator of NF-kappaB, and may act downstream of IL-10 in this signaling pathway.

The role of interleukin-10 in the regulation of the systemic inflammatory response following trauma-hemorrhage

Pro-inflammatory cytokine release after shock is central in the development of subsequent multiple organ dysfunction syndrome. Some studies suggest that interleukin-10 (IL-10) is an immunosuppressive mediator after injury or sepsis, while others suggest that IL-10 is an important regulator of the pro-inflammatory response. We hypothesized that in a model of trauma and hemorrhagic shock (TH), IL-10 regulates pro-inflammatory cytokine activity via an autocrine effect on cytokine mRNA transcription in Kupffer cells early after TH. To study this, male C3H/HeN mice were sham-operated or subjected to TH. Plasma levels of TNF-alpha, IL-6 and PGE(2) were elevated following TH. A sharp peak in IL-10 levels was observed at 2 h after the insult. Kupffer cell (KC) depletion prior to TH reduced plasma IL-6, IL-10 and TNF-alpha levels, whereas treatment with anti-IL-10 after TH increased IL-6 and TNF-alpha levels. Kupffer cell mRNA expression for IL-6, IL-10 and TNF-alpha was elevated in the TH group and further increased by anti-IL-10 treatment. These findings indicate that KC-dependent IL-10 regulates the early systemic inflammatory response after TH. Thus, while IL-10 is an important mediator of immunosuppression following traumatic injury, it also is beneficial with regard to its ability to counter-regulate the early inflammatory response under such conditions.

Immunoregulation of dendritic cells by IL-10 is mediated through suppression of the PI3K/Akt pathway and of IkappaB kinase activity

Interleukin-10 (IL-10) has potent immunoregulatory effects on the maturation and the antigen-presenting cell (APC) function of dendritic cells (DCs). The molecular basis underlying these effects in DCs, however, is ill defined. It is well established that the transcription factor NF-kappaB is a key regulator of DC development, maturation, and APC function. This study was initiated to determine the effects of IL-10 on the NF-kappaB signaling pathway in immature DCs. IL-10 pretreatment of myeloid DCs cultured from bone marrow resulted in reduced DNA binding and nuclear translocation of NF-kappaB after anti-CD40 antibody or lipopolysaccharide (LPS) stimulation. Furthermore, inhibited NF-kappaB activation was characterized by reduced degradation, phosphorylation, or both of IkappaBalpha and IkappaBepsilon but not IkappaBbeta and by reduced phosphorylation of Ser536, located in the trans-activation domain of p65. Notably, IL-10-mediated inhibition of NF-kappaB coincided with suppressed IkappaB kinase (IKK) activity in vitro. Furthermore, IL-10 blocked inducible Akt phosphorylation, and inhibitors of phosphatidylinositol 3-kinase (PI3K) effectively suppressed the activation of Akt, IKK, and NF-kappaB. These findings demonstrate that IL-10 targets IKK activation in immature DCs and that suppressing the PI3K pathway in part mediates blockade of the pathway.

Innate immune discrimination of apoptotic cells: repression of proinflammatory macrophage transcription is coupled directly to specific recognition

Physiological cell death is a process the purpose of which is the elimination of functionally inappropriate cells in a manner that does not elicit an inflammatory response. We have shown previously that the ability of apoptotic corpses to be recognized by macrophages and to modulate the proinflammatory responses of those cells represents paradoxically a gain-of-function acquired during the physiological cell death process. Cells that die pathologically (that is, necrotic vs apoptotic corpses) also are recognized by macrophages but do not down-regulate macrophage inflammatory responses; the recognition of these two classes of native dying cells occurs via distinct and noncompeting mechanisms. We have examined the apoptotic modulation of proinflammatory cytokine gene transcription in macrophages (by real-time RT-PCR analysis) and the corresponding modulation of transcriptional activators (by transcriptional reporter analyses). Our data demonstrate that apoptotic cells target the proinflammatory transcriptional machinery of macrophages with which they interact, without apparent effect on proximal steps of Toll-like receptor signaling. The modulatory activity of the corpse is manifest as an immediate-early inhibition of proinflammatory cytokine gene transcription, and is exerted directly upon binding to the macrophage, independent of subsequent engulfment and soluble factor involvement. Recognition and inflammatory modulation represent key elements of an innate immune response that discriminates live from effete cells, and without regard to self.

Signal transducer and activator of transcription 3 is the dominant mediator of the anti-inflammatory effects of IL-10 in human macrophages

The signaling mechanism by which the anti-inflammatory cytokine IL-10 mediates suppression of proinflammatory cytokine synthesis remains largely unknown. Macrophage-specific STAT3-null mice have demonstrated that STAT3 plays a critical role in the suppression of LPS-induced TNF-alpha release, although the mechanism by which STAT3 mediates this inhibition is still not clear. Using an adenoviral system, we have expressed a dominant negative (DN) STAT3 in human macrophages to broaden the investigation to determine the role of STAT3 in IL-10-mediated anti-inflammatory signaling and gene expression. Overexpression of STAT3 DN completely inhibited IL-10-induced suppressor of cytokine signaling 3, tissue inhibitor of MMP-1, TNF receptor expression, and the recently identified IL-10-inducible genes, T cell protein tyrosine phosphatase and signaling lymphocyte activation molecule. STAT3 DN also blocked IL-10-mediated inhibition of MHC class II and COX2 expression. In agreement with the studies in STAT3-null mice, overexpression of the STAT3 DN completely reversed the ability of IL-10 to inhibit LPS-mediated TNF-alpha and IL-6 production. However, real-time PCR analysis showed that STAT3 DN expression did not affect immediate suppression of TNF-alpha mRNA, but did reverse the suppression observed at later time points, suggesting a biphasic regulation of TNF-alpha mRNA levels by IL-10. In conclusion, although STAT3 does appear to be the dominant mediator of the majority of IL-10 functions, there are elements of its anti-inflammatory activity that are STAT3 independent.

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.

Putative Mechanism of Hemorrhage-Induced Leukocyte Hyporesponsiveness: Induction of Suppressor of Cytokine Signaling-3

BACKGROUND

After hemorrhagic shock, macrophages are less responsive to lipopolysaccharide (LPS) regarding cytokine production and receptor expression. However, mechanisms responsible for this are poorly understood. Suppressors of cytokine signaling (SOCS) proteins have been found to play a prominent role in LPS tolerance and cytokine desensitization in macrophages. Therefore, the purpose of this study was to determine whether hemorrhagic shock induced SOCS expression.

METHODS

Male C3H/HeN mice were subjected to hemorrhage or sham hemorrhage. Twenty-four hours after each procedure, tissues were harvested, the cells were processed for protein, and SOCS expression was examined.

RESULTS

Our data show that SOCS-1 expression does not change after hemorrhage, but SOCS-3 is up-regulated in a tissue and cell population (e.g., macrophage)-specific manner.

CONCLUSION

These data suggest that cytokines or other inflammatory mediators present during the first 24 hours after the induction of shock have the ability to induce tolerance to LPS or cytokines and suppress the function of immune cells by up-regulating SOCS-3.

Negative regulation of cytokine signaling by CIS/SOCS family proteins and their roles in inflammatory diseases

Immune and inflammatory systems are controlled by multiple cytokines, including interleukins (ILs) and interferons. These cytokines exert their biological functions through Janus tyrosine kinases (JAKs) and STAT transcription factors. The CIS (cytokine-inducible SH2 protein) and SOCS (suppressors of cytokine signaling) are a family of intracellular proteins, several of which have emerged as key physiological regulators of cytokine responses, including those that regulate the inflammatory systems. In this review, we focused on the molecular mechanism of the action of CIS/SOCS family proteins and their roles in inflammatory diseases. Furthermore, we illustrate several approaches for treating inflammatory diseases by modulating extracellular and intracellular signaling pathways.

Interleukin-10 inhibits interleukin-12 p40 gene transcription by targeting a late event in the activation pathway

Interleukin-10 (IL-10) is a potent anti-inflammatory cytokine that suppresses the induction of proinflammatory cytokine genes, including the IL-12 p40 gene. Despite considerable effort examining the effect of IL-10 on specific transcription factors and signaling molecules, the mechanism by which IL-10 inhibits gene transcription has remained elusive. To provide a different perspective to this problem, we examined the effect of IL-10 on molecular events occurring at the endogenous IL-12 p40 locus in lipopolysaccharide-stimulated peritoneal macrophages. IL-10 abolished recruitment of RNA polymerase II to the p40 promoter. However, it only modestly reduced binding of C/EBPbeta, as monitored by genomic footprinting and chromatin immunoprecipitation. It also had little effect on NF-kappaB complexes that are critical for p40 induction. A substantial reduction in nucleosome remodeling at the p40 promoter was observed, but the magnitude of this reduction appeared insufficient to account for the strong inhibition of transcription. Finally, a lipopolysaccharide-inducible DNase I hypersensitive site identified 10 kb upstream of the start site was unaffected by IL-10. Thus, despite a dramatic reduction in p40 transcription, several events required for activation of the endogenous p40 gene occurred relatively normally. These findings suggest that IL-10 blocks one or more events that occur after p40 locus decondensation and nucleosome remodeling and after, or in parallel with, the binding of a subset of p40 transcriptional activators.

Molecular mechanisms of interleukin-10-mediated inhibition of NF-kappaB activity: a role for p50

Nuclear factor kappa B (NF-kappaB) is a transcription factor pivotal for the development of inflammation. A dysregulation of NF-kappaB has been shown to play an important role in many chronic inflammatory diseases including rheumatoid arthritis, inflammatory bowel disease and psoriasis. Although classical NF-kappaB, a heterodimer composed of the p50 and p65 subunits, has been well studied, little is known about gene regulation by other hetero- and homodimeric forms of NF-kappaB. While p65 possesses a transactivation domain, p50 does not. Indeed, p50/p50 homodimers have been shown to inhibit transcriptional activity. We have recently shown that Interleukin-10 exerts its anti-inflammatory activity in part through the inhibition of NF-kappaB by blocking IkappaB kinase activity and by inhibiting NF-kappaB already found in the nucleus. Since the inhibition of nuclear NF-kappaB could not be explained by an increase of nuclear IkappaB, we sought to further investigate the mechanisms involved in the inhibition of NF-kappaB by IL-10. We show here that IL-10 selectively induced nuclear translocation and DNA-binding of p50/p50 homodimers in human monocytic cells. TNF-alpha treatment led to a strong translocation of p65 and p50, whereas pretreatment with IL-10 followed by TNF-alpha blocked p65 translocation but did not alter the strong translocation of p50. Furthermore, macrophages of p105/p50-deficient mice exhibited a significantly decreased constitutive production of MIP-2alpha and IL-6 in comparison to wild type controls. Surprisingly, IL-10 inhibited high constitutive levels of these cytokines in wt macrophages but not in p105/p50 deficient cells. Our findings suggest that the selective induction of nuclear translocation and DNA-binding of the repressive p50/p50 homodimer is an important anti-inflammatory mechanism utilized by IL-10 to repress inflammatory gene transcription.

Dendritic Cell Maturation Requires STAT1 and Is under Feedback Regulation by Suppressors of Cytokine Signaling

In this study we show that activation of STAT pathways is developmentally regulated and plays a role in dendritic cell (DC) differentiation and maturation. The STAT6 signaling pathway is constitutively activated in immature DC (iDC) and declines as iDCs differentiate into mature DCs (mDCs). However, down-regulation of this pathway during DC differentiation is accompanied by dramatic induction of suppressors of cytokine signaling 1 (SOCS1), SOCS2, SOCS3, and cytokine-induced Src homology 2-containing protein expression, suggesting that inhibition of STAT6 signaling may be required for DC maturation. In contrast, STAT1 signaling is most robust in mDCs and is not inhibited by the up-regulated SOCS proteins, indicating that STAT1 and STAT6 pathways are distinctly regulated in maturing DC. Furthermore, optimal activation of STAT1 during DC maturation requires both IL-4 and GM-CSF, suggesting that synergistic effects of both cytokines may in part provide the requisite STAT1 signaling intensity for DC maturation. Analyses of STAT1(-/-) DCs reveal a role for STAT1 in repressing CD86 expression in precursor DCs and up-regulating CD40, CD11c, and SOCS1 expression in mDCs. We further show that SOCS proteins are differentially induced by IL-4 and GM-CSF in DCs. SOCS1 is primarily induced by IL-4 through a STAT1-dependent mechanism, whereas SOCS3 is induced mainly by GM-CSF. Taken together, these results suggest that cytokine-induced maturation of DCs is under feedback regulation by SOCS proteins and that the switch from constitutive activation of the STAT6 pathway in iDCs to predominant use of STAT1 signals in mDC is mediated in part by STAT1-induced SOCS expression.

Endothelial cells require STAT3 for protection against endotoxin-induced inflammation

Endothelial cells (ECs) are believed to be an important component in the protection from lipopolysaccharide (LPS)-induced endotoxic shock. However, the cellular and molecular mechanism is not well defined. Here, we report that signal transducer and activator of transcription (STAT) 3 is an essential regulator of the antiinflammatory function of ECs in systemic immunity. Because STAT3 deficiency results in early embryonic lethality, we have generated mice with a conditional STAT3 deletion in endothelium (STAT3^E−/−). STAT3^E−/− mice are healthy and fertile, and isolated ECs initiate normal tube formation in vitro. Conditional endothelial but not organ-specific (i.e., hepatocyte or cardiomyocyte) STAT3 knockout mice show an increased susceptibility to lethality after LPS challenge. The LPS response in STAT3^E−/− mice shows exaggerated inflammation and leukocyte infiltration in multiple organs combined with elevated activity of serum alanine aminotransferase and aspartate aminotransferase, indicating organ damage. Concomitantly, proinflammatory cytokines are produced at an exaggerated level and for a prolonged period. This defect cannot be explained by lack of antiinflammatory cytokines, such as interleukin 10 and transforming growth factor β. Instead, we have shown that a soluble activity derived from endothelia and dependent on STAT3 is critical for suppression of interferon γ. These data define STAT3 signaling within endothelia as a critical antiinflammatory mediator and provide new insight to the protective function of ECs in inflammation.

Regulation of cytokine signaling by SOCS family molecules

Cytokine signaling is dependent on the activation of intracellular molecules, including JAKs (Janus family kinases) and STATs (signal transducers and activators of transcription). Since the discovery of the SOCS (suppressor of cytokine signaling) family molecules in 1997, extensive investigation of these molecules has given important insights into the as yet unclarified regulatory mechanisms of cytokine signaling. Recent gene targeting analyses of SOCS molecules have revealed that they do indeed have essential roles in vivo (more specific than those expected from overexpression studies in vitro) in the negative regulation of various cytokines. Moreover, consistent with the pathological actions of cytokines in human diseases, recent lines of evidence also indicate that SOCS molecules are implicated in autoimmunity, allergy and cancers.

IL-10-inducible Bcl-3 negatively regulates LPS-induced TNF-alpha production in macrophages

Interleukin-10 (IL-10) plays an important role in prevention of chronic inflammation in vivo. However, the molecular mechanism by which IL-10 exerts its anti-inflammatory response is poorly understood. Here, we performed a microarray analysis and identified Bcl-3 as an IL-10-inducible gene in macrophages. Lentiviral vector-mediated expression of Bcl-3 inhibited lipopolysaccharide (LPS)-induced production of tumor necrosis factor alpha (TNF-alpha), but not IL-6, in macrophages. In Bcl-3-transduced and IL-10-pretreated macrophages, LPS-induced nuclear translocation of nuclear factor kappaB (NF-kappaB) p65 was not impaired. However, DNA binding by NF-kappaB p50/p65 was profoundly inhibited. Nuclear localization of Bcl-3 was associated with inhibition of LPS-induced TNF-alpha production. Overexpression of Bcl-3 suppressed activation of the TNF-alpha promoter, but not the IL-6 promoter. Bcl-3 interacted with NF-kappaB p50 and was recruited to the TNF-alpha promoter, but not the IL-6 promoter, indicating that Bcl-3 facilitates p50-mediated inhibition of TNF-alpha expression. Furthermore, Bcl-3-deficient macrophages showed defective IL-10-mediated suppression of LPS induction of TNF-alpha, but not IL-6. These findings suggest that IL-10-induced Bcl-3 is required for suppression of TNF-alpha production in macrophages.

A Novel Synthetic Cannabinoid Derivative Inhibits Inflammatory Liver Damage via Negative Cytokine Regulation

The therapeutic potential of cannabinoids has been described previously for several inflammatory diseases, but the molecular mechanisms underlying the anti-inflammatory properties of cannabinoids are not well understood. In this study, we investigated the mechanism of action of a novel synthetic cannabinoid, [(+)(6aS,10aS)-6,6-Dimethyl-3-(1,1-dimethylheptyl)-1-hydroxy-9-(1H-imidazol-2-ylsulfanylmethyl]-6a,7,10,10a-tetrahydro-6H-dibenzo[b,d]pyran (PRS-211,092) that has no psychotropic effects but exhibits immunomodulatory properties. Treatment with PRS-211,092 significantly decreased Concanavalin A-induced liver injury in mice that was accompanied by: 1) promotion of early gene expression of interleukin (IL)-6 and IL-10 that play a protective role in this model; 2) induction of early gene expression of the suppressors of cytokine signaling (SOCS-1 and 3), followed by 3) inhibition of several pro-inflammatory mediators, including IL-2, monocyte chemoattractant protein-1 (MCP-1), IL-1beta, interferon-gamma, and tumor necrosis factor alpha. Based on these results, we propose a mechanism by which PRS-211,092 stimulates the expression of IL-6, IL-10 and the SOCS proteins that, in turn, negatively regulates the expression of pro-inflammatory cytokines. Negative regulation by PRS-211,092 was further demonstrated in cultured T cells, where it inhibited IL-2 production and nuclear factor of activated T cells activity. These findings suggest that this cannabinoid derivative is an immunomodulator that could be developed as a potential drug for hepatitis as well as for other short- or long-term inflammatory diseases.

Statins induce suppressor of cytokine signaling-3 in macrophages

Our previous study has shown that lipophilic 3-hydroxy-3-methyl-glutaryl coenzyme A reductase inhibitors of statins can inhibit interferon-gamma-induced inducible nitric oxide synthase gene expression in RAW264.7 macrophages. In this study, we showed that lovastatin and fluvastatin are able to upregulate the mRNA expression of the suppressor of cytokine signaling-3 (SOCS-3) gene. This effect is specific for SOCS-3 and could be blocked by mevalonate, farnesyl pyrophosphate and geranylgeranyl pyrophosphate, while it was not affected by inhibitors of protein kinase C and A, mitogen-activated protein/extracellular signal-regulated kinase kinase, p38 mitogen-activated protein kinase, c-Jun N-terminal kinase, Src, Raf and Rho kinase. SOCS-3 expression results in the inhibition of interferon-gamma-, interleukin-6- and macrophage colony-stimulating factor-elicited signal transducer and activator of transcription phosphorylation, suggesting a novel anti-inflammatory mechanism of statins to down-modulate the functions of interferon-gamma-activated macrophages.

Sepsis-induced SOCS-3 expression is immunologically restricted to phagocytes

We have shown that immune cells from septic mice exhibit a suppressed response to exogenous stimuli in vitro. The suppressors of the cytokine signaling (SOCS) family are proteins that block intracellular signaling and can be induced by inflammatory mediators. Therefore, we hypothesized that SOCS-3 is up-regulated in immune cells in response to a septic challenge induced by cecal ligation and puncture (CLP). Mice were subjected to CLP or sham-CLP, and 2-48 h later, the blood, thymus, spleen, lung, and peritoneal leukocytes were harvested and examined. SOCS-3 was undetectable in thymocytes or blood leukocytes. In contrast, SOCS-3 was up-regulated in the spleen, lung, and peritoneal leukocytes in a time-dependent manner. Further examination revealed that only the macrophages and neutrophils expressed SOCS-3. These data suggest that cytokines and bacterial toxins present during sepsis have the ability to suppress the cytokine and/or lipopolysaccharide response and the function of immune cells by up-regulating SOCS-3.

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.

Effects of endotoxin and macrophage-related cytokines on the contractile activity of the gravid murine uterus

Immune cell trafficking and activity are implicated in the parturition process, but little is known about the role of macrophages in control of uterine contractility at term. In the present study, we tested the hypothesis that endotoxin (lipopolysaccharide [LPS]) enhances uterine contractile activity through a mechanism that involves activation of resident macrophages. Various uterotonins and anti-inflammatory mediators were added to a standard muscle bath preparation that contained strips of uterus from Day 15 pregnant C3H/HeN mice. Spontaneous and agonist-induced contractile activity was enhanced following LPS treatment. LPS increased amplitude but not frequency of contractions. Addition of anti-inflammatory cytokines, interleukin 10 or transforming growth factor beta, to suppress macrophage activation did not block LPS-induced increases in contractility. By contrast, indomethacin given to block prostaglandin production suppressed the LPS-induced increase in amplitude of contractions. These findings suggest that an inflammatory response, possibly mediated by activation of macrophages and prostaglandins, participates in the regulation of amplitude but not frequency of contractile activity by the murine uterus before onset of parturition.

Interleukin-10-induced CCR5 expression in macrophage like HL-60 cells: involvement of Erk1/2 and STAT-3

As an immunosuppressive and anti-inflammatory cytokine, IL-10 was recently reported to play roles in CCR5 expression in human monocytes. CCR5 promoter regions contain Oct-2, TCF-1alpha, GATA, and STAT binding sites. Here, we studied the signals involved in the CCR5 expression in IL-10-stimulated cells using the HL-60 cell line. HL-60 cells were stimulated with PMA and differentiated to macrophage-like cells, then stimulated with IL-10. IL-10 induced significant expression of CCR5 protein and CCR5 mRNA in these cells. The induction of CCR5 by IL-10 was inhibited by a MEK-1 inhibitor, PD98059. In addition, IL-10 induced tyrosine (Tyr) phosphorylation of Erk, as well as serine (Ser) and Tyr phosphorylation of STAT-3. Tyr phosphorylation of Erk and Ser phosphorylation of STAT-3 were inhibited by PD98059, while Tyr phosphorylation of STAT-3 was not inhibited by PD98059. DNA binding activity of STAT-3 was observed by the stimulation with IL-10, which was inhibited by PD98059. These results first indicate that Erk1/2 and STAT-3 regulate CCR5 expression, and that Erk-mediated phosphorylation of Ser is required for full stimulation of STAT-3 in CCR5 expression.

Regulation of chemokine mRNA stability by lipopolysaccharide and IL-10

IL-10 has been reported to inhibit the expression of LPS-induced proinflammatory cytokines and chemokines by altering the rate of specific mRNA decay although the molecular target(s) for its action remain unknown. In the present study, using primary peritoneal exudate macrophages and a cell culture model in which a tetracycline-responsive promoter controls transcription of CXC ligand 1 (KC) mRNA, we demonstrate that LPS promotes a time-dependent increase in KC mRNA stability. Although IL-10 had no direct effect on mRNA decay, this treatment antagonized the stabilizing action of LPS. The mechanisms involved were further explored using a cell-free mRNA degradation system. A 5'-capped, polyadenylated in vitro transcript derived from the 3'-untranslated region of KC mRNA exhibited time-dependent decay in the presence of protein extracts prepared from untreated RAW264.7 macrophages. Extracts prepared from LPS-treated RAW264.7 cells had reduced decay activity and this change was antagonized if the cells were costimulated with IL-10. A substrate in which the AU-rich element motifs were mutated exhibited minimal decay that did not vary using extracts prepared from cells treated with LPS or LPS and IL-10. A nonadenylated RNA substrate was also degraded and that activity was diminished by LPS. In concert, these findings demonstrate that KC mRNA stability is regulated by LPS-induced alterations in activities that govern both deadenylation and degradation of the mRNA body. The effects of IL-10 on KC mRNA stability reflect antagonism of the response to LPS.

Interleukin-10 therapy--review of a new approach

Interleukin (IL)-10 is an important immunoregulatory cytokine produced by many cell populations. Its main biological function seems to be the limitation and termination of inflammatory responses and the regulation of differentiation and proliferation of several immune cells such as T cells, B cells, natural killer cells, antigen-presenting cells, mast cells, and granulocytes. However, very recent data suggest IL-10 also mediates immunostimulatory properties that help to eliminate infectious and noninfectious particles with limited inflammation. Numerous investigations, including expression analyses in patients, in vitro and animal experiments suggest a major impact of IL-10 in inflammatory, malignant, and autoimmune diseases. So IL-10 overexpression was found in certain tumors as melanoma and several lymphomas and is considered to promote further tumor development. Systemic IL-10 release is a powerful tool of the central nervous system to prevent hyperinflammatory processes by activation of the neuro-endocrine axis following acute stress reactions. In contrast, a relative IL-10 deficiency has been observed and is regarded to be of pathophysiological relevance in certain inflammatory disorders characterized by a type 1 cytokine pattern such as psoriasis. Recombinant human IL-10 has been produced and is currently being tested in clinical trials. This includes rheumatoid arthritis, inflammatory bowel disease, psoriasis, organ transplantation, and chronic hepatitis C. The results are heterogeneous. They give new insight into the immunobiology of IL-10 and suggest that the IL-10/IL-10 receptor system may become a new therapeutic target.

SOCS3 regulates the plasticity of gp130 signaling

Suppressor of cytokine signaling (SOCS) proteins are feedback inhibitors of the Janus kinase (JAK) and signal transducer and activator of transcription (STAT) signaling pathway. SOCS3 is upregulated by several signals in macrophages and has been implicated as a regulator of various signaling pathways. Here we show that phosphorylation of STAT3 is prolonged in mouse Socs3-deficient macrophages after stimulation with interleukin-6 (IL-6) but not IL-10, indicating that SOCS3 specifically affects signaling mediated by IL-6 and gp130. IL-6 induces a wider transcriptional response in Socs3-deficient macrophages than in wild-type cells; this response is dominated by interferon (IFN)-regulated genes owing to an excess of STAT1 phosphorylation. Thus, SOCS3 functions to control the quality of the response to IL-6 and prevents the activation of an IFN-induced program of gene expression.

IL-6 induces an anti-inflammatory response in the absence of SOCS3 in macrophages

Whereas interleukin-6 (IL-6) is a proinflammatory cytokine, IL-10 is an anti-inflammatory cytokine. Although signal transducer and activator of transcription 3 (STAT3) is essential for the function of both IL-6 and IL-10, it is unclear how these two cytokines have such opposing functions. Here we show that suppressor of cytokine signaling 3 (SOCS3) is a key regulator of the divergent action of these two cytokines. In macrophages lacking the Socs3 gene or carrying a mutation of the SOCS3-binding site in gp130, the lipopolysaccharide-induced production of tumor necrosis factor (TNF) and IL-12 is suppressed by both IL-10 and IL-6. SOCS3 specifically prevents activation of STAT3 by IL-6 but not IL-10. Taken together, these data indicate that SOCS3 selectively blocks signaling by IL-6, thereby preventing its ability to inhibit LPS signaling.

Different modes of IL-10 and TGF-beta to inhibit cytokine-dependent IFN-gamma production: consequences for reversal of lipopolysaccharide desensitization

LPS hyporesponsiveness is characterized by a diminished production of proinflammatory cytokines which can be caused by pretreatment with either LPS (=LPS desensitization) or the combination of the anti-inflammatory cytokines IL-10 and TGF-beta. However, the resulting hyporesponsive states differ regarding their reversibility by the IFN-gamma-inducing cytokine IL-12. Therefore, we aimed at studying the reasons for this differential IL-12 responsiveness of IFN-gamma-producing cells and its consequences for LPS hyporesponsiveness in more detail. In an in vitro IL-12/IL-18 responsiveness model, we demonstrated that IL-10, if permanently present, does not directly inhibit IL-12/IL-18 responsiveness in T/NK cells but indirectly interferes with IFN-gamma production in the presence of monocytes. In contrast, TGF-beta acted directly on IFN-gamma-producing cells by interfering with IL-12/IL-18 responsiveness. After removal of IL-10 but not of TGF-beta, LPS hyporesponsiveness can be reverted by IL-12/IL-18. Consequently, the addition of recombinant TGF-beta during LPS desensitization rendered PBMCs hyporesponsive to a reversal by IL-12/IL-18. Our data suggest that the persistence of IL-10 and the presence of TGF-beta determine the level of IFN-gamma inhibition and may result in different functional phenotypes of LPS desensitization and LPS hyporesponsiveness in vitro and in vivo.

Selective modulation of lipopolysaccharide-stimulated cytokine expression and mitogen-activated protein kinase pathways by dibutyryl-cAMP in BV2 microglial cells

Cyclic AMP is a very important regulator in a wide range of biological processes, including inflammatory reactions. To investigate the role of cAMP in microglia, we examined the effect of dibutyryl-cAMP (dbcAMP) on lipopolysaccharide (LPS)-stimulated cytokine expression and signaling pathways in murine BV2 microglial cells. DbcAMP strongly suppressed LPS-induced TNF-alpha expression, without affecting NO, IL-6 or TGF-beta1 expression. In contrast, LPS-induced IL-1beta or IL-10 expressions were dramatically increased by dbcAMP. We further examined the effect of elevated cAMP on signaling molecules such as MAP kinases (p38 MAPK, ERK and JNK), NF-kappaB and AP1, which are involved in the regulation of inflammatory responses. DbcAMP decreased the LPS-induced phosphorylation of p38 MAPK, while it modestly enhanced the ERK activity. JNK phosphorylation was slightly reduced by dbcAMP only at the later time point. Electrophoretic mobility shift assay revealed that the elevated cAMP potentiated AP-1 binding activity by enhancing c-fos binding. On the other hand, dbcAMP repressed NF-kappaB-mediated transcription without affecting NF-kappaB binding. Treatment with H89, a selective inhibitor of protein kinase A, completely reversed cAMP-induced IL-10 and IL-1beta upregulation but only partially reversed the cAMP-induced repression of TNF-alpha. Thus, the effect of dbcAMP in BV2 cells appears to be mediated through both protein kinase A-dependent and -independent pathways. Taken together, our results demonstrate that cAMP modulates microglia activation in a diverse and complex manner.

Toll-like receptor-dependent production of IL-12p40 causes chronic enterocolitis in myeloid cell-specific Stat3-deficient mice

Stat3 plays an essential role in IL-10 signaling pathways. A myeloid cell-specific deletion of Stat3 resulted in inflammatory cytokine production and development of chronic enterocolitis with enhanced Th1 responses in mice. In this study, we analyzed the mechanism by which a Stat3 deficiency in myeloid cells led to the induction of chronic enterocolitis in vivo. Even in the absence of Stat1, which is essential for IFN-gamma signaling pathways, Stat3 mutant mice developed chronic enterocolitis. TNF-alpha/Stat3 double-mutant mice developed severe chronic enterocolitis with enhanced Th1 cell development. IL-12p40/Stat3 double-mutant mice, however, showed normal Th1 responses and no inflammatory change in the colon. RAG2/Stat3 double-mutant mice did not develop enterocolitis, either. These findings indicate that overproduction of IL-12p40, which induces potent Th1 responses, is essential for the development of chronic enterocolitis in Stat3 mutant mice. Furthermore, enterocolitis was significantly improved and IFN-gamma production by T cells was reduced in TLR4/Stat3 double-mutant mice, indicating that TLR4-mediated recognition of microbial components triggers aberrant IL-12p40 production by myeloid cells, leading to the development of enterocolitis. Thus, this study clearly established a sequential innate and acquired immune mechanism for the development of Th1-dependent enterocolitis.