TLR triggering on tolerogenic dendritic cells results in TLR2 up-regulation and a reduced proinflammatory immune program.

Tolerogenic dendritic cells (TDC) offer a promising therapeutic potential to ameliorate autoimmune diseases. Reported to inhibit adaptive immune responses, little is known about their innate immunity receptor repertoire. In this study, we compared three types of human TDC (IL-10-DC, dexamethasone (DX)-DC, and 1,25(OH)(2)D(3)-DC) by their TLR expression and response to a set of TLR ligands. TDC are endowed with the same TLR set as standard monocyte-derived dendritic cells but respond differentially to the TLR stimuli Pam3CSK4, polyinosinic-polycytidylic acid, LPS, and flagellin. TDC expressed low or no IL-12-related cytokines and remarkably elevated IL-10 levels. Interestingly, only TDC up-regulated the expression of TLR2 upon stimulation. This boosted the tolerogenic potential of these cells, because IL-10 production was up-regulated in TLR2-stimulated, LPS-primed DX-DC, whereas IL-12 and TNF-alpha secretion remained low. When comparing the TDC subsets, DX-DC and 1,25(OH)(2)D(3)-DC up-regulated TLR2 irrespective of the TLR triggered, whereas in IL-10-DC this effect was only mediated by LPS. Likewise, DX-DC and 1,25(OH)(2)D(3)-DC exhibited impaired ability to mature, reduced allostimulatory properties, and hampered capacity to induce Th1 differentiation. Therefore, both DX-DC and 1,25(OH)(2)D(3)-DC display the strongest tolerogenic and anti-inflammatory features and might be most suitable tools for the treatment of autoimmune diseases.

12/15-lipoxygenase counteracts inflammation and tissue damage in arthritis.

Eicosanoids are essential mediators of the inflammatory response and contribute both to the initiation and the resolution of inflammation. Leukocyte-type 12/15-lipoxygenase (12/15-LO) represents a major enzyme involved in the generation of a subclass of eicosanoids, including the anti-inflammatory lipoxin A(4) (LXA(4)). Nevertheless, the impact of 12/15-LO on chronic inflammatory diseases such as arthritis has remained elusive. By using two experimental models of arthritis, the K/BxN serum-transfer and a TNF transgenic mouse model, we show that deletion of 12/15-LO leads to uncontrolled inflammation and tissue damage. Consistent with these findings, 12/15-LO-deficient mice showed enhanced inflammatory gene expression and decreased levels of LXA(4) within their inflamed synovia. In isolated macrophages, the addition of 12/15-LO-derived eicosanoids blocked both phosphorylation of p38MAPK and expression of a subset of proinflammatory genes. Conversely, 12/15-LO-deficient macrophages displayed significantly reduced levels of LXA(4), which correlated with increased activation of p38MAPK and an enhanced inflammatory gene expression after stimulation with TNF-alpha. Taken together, these results support an anti-inflammatory and tissue-protective role of 12/15-LO and its products during chronic inflammatory disorders such as arthritis.

IL-27 is a key regulator of IL-10 and IL-17 production by human CD4+ T cells.

Although the physiologic pathways that control regulatory T cells (Foxp3-expressing regulatory T cells, IL-10-secreting Tr1 cells) and Th17 cells in rodents have been defined, the factors that control these differentiation pathways in humans are not well understood. In this study, we show that IL-27 promotes the differentiation of IL-10-secreting Tr1 cells while inhibiting Th17 generation and molecules associated with Th17 function. Furthermore, IL-27 inhibits IL-17-polarizing cytokines on dendritic cells, which in turn decrease IL-17 secretion from T cells. Our results demonstrate that IL-27 plays a key role in human T cells by promoting IL-10-secreting Tr1 cells and inhibiting Th17 cells and thus provides a dual regulatory mechanism to control autoimmunity and tissue inflammation.

The cathelicidin LL-37 activates human mast cells and is degraded by mast cell tryptase: counter-regulation by CXCL4.

The cathelicidin LL-37 represents a potent antimicrobial and cell-stimulating agent, most abundantly expressed in peripheral organs such as lung and skin during inflammation. Because mast cells (MC) overtake prominent immunomodulatory roles in these organs, we wondered whether interactions exist between MC and LL-37. In this study, we show for the first time to our knowledge that physiological concentrations of LL-37 induce degranulation in purified human lung MC. Intriguingly, as a consequence LL-37 rapidly undergoes limited cleavage by a released protease. The enzyme was identified as beta-tryptase by inhibitor studies and by comparison to the recombinant protease. Examining the resulting LL-37 fragments for their functional activity, we found that none of the typical capacities of intact LL-37, i.e., MC degranulation, bactericidal activity, and neutralization of LPS, were retained. Conversely, we found that another inflammatory protein, the platelet-derived chemokine CXCL4, protects LL-37 from cleavage by beta-tryptase. Interestingly, CXCL4 did not act as a direct enzyme inhibitor, but destabilized active tetrameric beta-tryptase by antagonizing the heparin component required for the integrity of the tetramer. Altogether our results suggest that interaction of LL-37 and MC initiates an effective feedback loop to limit cathelicidin activity during inflammation, whereas CXCL4 may represent a physiological counter-regulator of beta-tryptase activity.

Murine B cell response to TLR7 ligands depends on an IFN-beta feedback loop.

Type I IFNs play an important, yet poorly characterized, role in systemic lupus erythematosus. To better understand the interplay between type I IFNs and the activation of autoreactive B cells, we evaluated the effect of type I IFN receptor (IFNAR) deficiency in murine B cell responses to common TLR ligands. In comparison to wild-type B cells, TLR7-stimulated IFNAR(-/-) B cells proliferated significantly less well and did not up-regulate costimulatory molecules. By contrast, IFNAR1(-/-) B cells did not produce cytokines, but did proliferate and up-regulate activation markers in response to other TLR ligands. These defects were not due to a difference in the distribution of B cell populations or a failure to produce a soluble factor other than a type I IFN. Instead, the compromised response pattern reflected the disruption of an IFN-beta feedback loop and constitutively low expression of TLR7 in the IFNAR1(-/-) B cells. These results highlight subtle differences in the IFN dependence of TLR7 responses compared with other TLR-mediated B cell responses.

MicroRNA-146a feedback inhibits RIG-I-dependent Type I IFN production in macrophages by targeting TRAF6, IRAK1, and IRAK2.

Upon recognition of viral components by pattern recognition receptors, including TLRs and retinoic acid-inducible gene I (RIG-I)- like helicases, cells are activated to produce type I IFN and proinflammatory cytokines. These pathways are tightly regulated by host to prevent inappropriate cellular response, but viruses can down-regulate these pathways for their survival. Recently, identification of negative regulators for cytoplasmic RNA-mediated antiviral signaling, especially the RIG-I pathway, attract much attention. However, there is no report about negative regulation of RIG-I antiviral pathway by microRNAs (miRNA) to date. We found that vesicular stomatitis virus (VSV) infection up-regulated miR-146a expression in mouse macrophages in TLR-myeloid differentiation factor 88-independent but RIG-I-NF-kappaB-dependent manner. In turn, miR-146a negatively regulated VSV-triggered type I IFN production, thus promoting VSV replication in macrophages. In addition to two known miR-146a targets, TRAF6 and IRAK1, we proved that IRAK2 was another target of miR-146a, which also participated in VSV-induced type I IFN production. Furthermore, IRAK1 and IRAK2 participated in VSV-induced type I IFN production by associating with Fas-associated death domain protein, an important adaptor in RIG-I signaling, in a VSV infection-inducible manner. Therefore, we demonstrate that miR-146a, up-regulated during viral infection, is a negative regulator of the RIG-I-dependent antiviral pathway by targeting TRAF6, IRAK1, and IRAK2.

Self-termination/anergic mechanisms in human basophils and mast cells.

All cellular reactions include feedback loops, both positive and negative. Following all receptor-mediated stimulation there are events that activate the cell response and events that dampen the receptor response back to resting conditions. IgE-mediated activation of mast cells and basophils necessarily includes a variety of signaling events that serve to terminate the activation processes. Some of these negative feedback loops are active even in resting cells and likely serve to establish a set point for the cell's response to IgE-mediated stimulation. But many negative feedback loops become active only after the cell is stimulated. By and large, it will be these processes that will be discussed in this review.

c-Maf increases apoptosis in peripheral CD8 cells by transactivating Caspase 6.

In addition to transactivation of interleukin-4 (IL-4), cellular muscular aponeurotic fibrosarcoma (c-Maf) enhances CD4 cell apoptosis by limiting Bcl-2 expression. The CD8 cells also express c-Maf and peripheral CD8 cell numbers are reduced in c-Maf transgenic mice, suggesting that c-Maf may influence CD8 cell survival in a manner similar to CD4 cells. Here we confirm that, similar to CD4 cells, c-Maf enhances CD8 cell susceptibility to apoptosis induced by multiple stimuli, independent of IL-4. However, unlike CD4 cells, c-Maf enhancement of apoptosis is independent of Bcl-2, suggesting that c-Maf uses other mechanisms to regulate CD8 cell apoptosis. Real-time reverse transcription-polymerase chain reaction reveals that the pro-apoptotic gene Caspase 6 is upregulated in c-Maf transgenic CD8 cells, suggesting that Caspase 6 is a novel c-Maf target gene. Luciferase reporter assays and site-directed mutagenesis reveal a functional c-Maf recognition element (MARE) within the first intron of Caspase 6. Binding of c-Maf to the MARE site is detectable by chromatin immunoprecipitation using non-transgenic T-cell lysates, so c-Maf can interact with the Caspase 6 MARE site in normal T cells. Furthermore, caspase 6 activity is increased among CD8 cells from c-Maf transgenic mice following T-cell receptor engagement. As expected, activity of the downstream caspases 3 and 7 is also increased. Consistent with the ability of caspase 6 to participate in positive feedback loops, cytochrome c release and caspase 8 activation are also increased. Together these results indicated that c-Maf increases CD8 cell sensitivity to apoptotic stimuli, at least in part, by direct transactivation of Caspase 6, providing increased substrate for Caspase 6-dependent apoptosis pathways.

Calcineurin inactivation leads to decreased responsiveness to LPS in macrophages and dendritic cells and protects against LPS-induced toxicity in vivo.

Microbial components such as lipopolysaccharide (LPS) bind to Toll-like receptors (TLRs) and activate innate and inflammatory responses. Responses to LPS and other microbial components are limited by the activation of negative feedback mechanisms that reduce responsiveness to subsequent LPS exposure, often termed LPS tolerance. Our laboratory has previously shown that calcineurin, a phosphatase known for its activation of T cells via NFAT, negatively regulates the TLR pathway in macrophages; consequently, calcineurin inhibitors (FK506 and cyclosporin A) mimic TLR ligands in activating the TLR pathway, NF-KB, and associated innate and inflammatory responses. This study investigated the physiological consequences of calcineurin inactivation for LPS-induced inflammatory responses in vitro and in vivo using two models: calcineurin inhibition by FK506 (tacrolimus) and myeloid cell-specific calcineurin deletion. Activation of dendritic cells and macrophages with FK506 in vitro was shown to induce a state of reduced responsiveness to LPS (i.e. a form of LPS tolerance). Similarly, macrophages from FK506-treated mice or from mice in which the calcineurin B1 (CnB1) subunit was conditionally knocked out in myeloid cells were found to have diminished LPS-induced inflammatory responses. In addition, mice with CnB1-deficient myeloid cells and mice undergoing FK506 treatment showed improved survival and recovery when challenged with high doses of systemic LPS compared to controls. These results demonstrate that inactivation of calcineurin in macrophages and other myeloid cells by inhibition or deletion can induce a form of LPS tolerance and protect the host from LPS toxicity in vivo.

Ncf1 provides a reactive oxygen species-independent negative feedback regulation of TLR9-induced IL-12p70 in murine dendritic cells.

Permanent exposure to pathogens requires decisions toward tolerance or immunity as a prime task of dendritic cells. The molecular mechanisms preventing uncontrolled immune responses are not completely clear. We investigated the regulatory function of Ncf1, an organizing protein of NADPH oxidase, in the signaling cascade of Toll-like receptors. TLR9-stimulated spleen cells from both Ncf1-deficient and B10.Q mice with a point mutation in exon 8 of Ncf1 exhibited increased IL-12p70 secretion compared with controls. This finding was restricted to stimulatory CpG2216 and not induced by CpG2088. Because only CpG/TLR9-induced IL-12p70 was regulated by Ncf1, we used TRIF(-/-) and MyD88(-/-) cells to show that TLR9/MyD88 was primarily affected. Interestingly, additional experiments revealed that spleen cells from NOX2/gp91(phox)-deficient mice and the blocking of electron transfer by diphenylene iodonium had no influence on CpG-induced IL-12p70, confirming an NADPH oxidase-independent function of Ncf1. Finally, proving the in vivo relevance CpG adjuvant-guided OVA immunization resulted in a strong augmentation of IL-12p70-dependent Th1 IFN-gamma response only in Ncf1-deficient mice. These data suggest for the first time an important role for Ncf1 in the fine tuning of the TLR9/MyD88 pathway in vitro and in vivo that is independent of its role as an activator of NOX2.

Role of Fc Receptors as a therapeutic target.

It has been forty years since the discovery of Fc Receptors and their function. Fc Receptors include the IgG receptors (FcgammaR), high-affinity IgE receptor (FcepsilonRI), IgA and IgA/IgM receptors, and neonatal Fc receptor for IgG (FcRn). In particular, the FcgammaRs have been well known to play an important role in many biologic processes including those associated with the response to infection and cancer as well as in the pathogenesis of immune-mediated diseases. Both positive and negative regulatory function has ascribed to Fc receptors and FcgammaRs in particular which serve to establish a threshold for immune cell activation. In other cases, Fc receptors such as FcRn possess a novel structure and function by playing a major role in the transport of IgG across polarized epithelial barriers at mucosal surfaces and in the regulation of IgG half-life. These diverse functions highlight the potential effectiveness of targeting Fc receptors for therapeutic purposes. This review summarizes new information available in the therapeutic applications of this biology.

Feedback loop of immune regulation by CD4+CD25+ Treg.

Naturally occurring regulatory T cells (Tregs), residing in CD4+CD25+ fraction, are important in the maintenance of immune homeostasis. One of the functional characteristics of Tregs is close relationship between suppressive activity and anergy in vitro. Meanwhile, many in vitro assays have observed Treg proliferation and suppressive activities in different settings, i.e., in the absence and presence of CD25(-) responder cells. If the presence of responder cells affect the proliferation of Tregs, comparison between the two settings would be inappropriate. In the present study, we traced proliferation as well as suppressive activities of Tregs in the same setting of coculture in response to varying concentrations of anti-CD3 and anti-CD28. Quantitative analysis using two parameters, precursor frequency and CD25 mean fluorescence intensity, reflecting early and late proliferative responsiveness, respectively, showed that proliferation of Tregs was dependent on the responder cells and proliferating Tregs preserved suppressive activities. Transwell assay and neutralization assay showed that the enhancement of Treg proliferation by the responder cells was mediated through secreted IL-2. Quantitative analysis also showed distinct mode of suppression by Tregs according to the presence or absence of anti-CD28. In the absence of anti-CD28, Tregs suppressed the initial proliferation, whereas in the presence of anti-CD28, Tregs suppressed only the late expansion of the responder cells by lowering CD25 expression. Considering that Tregs cannot produce IL-2 by themselves while they constitutively express CD25 (IL-2Ralpha), dependency of Tregs on their target of suppression (responder cells) for proliferation supports the model for feedback loop of immune regulation by Tregs.

Local production of IFN-gamma by invariant NKT cells modulates acute Lyme carditis.

The Lyme disease spirochete Borrelia burgdorferi is the only known human pathogen that directly activates invariant NKT (iNKT) cells. The number and activation kinetics of iNKT cells vary greatly among different strains of mice. We now report the role of the iNKT cell response in the pathogenesis of Lyme disease using C57BL/6 mice, a strain with optimal iNKT cell activation that is resistant to the development of spirochetal-induced inflammation. During experimental infection of B6 mice with B. burgdorferi, iNKT cells localize to the inflamed heart where they are activated by CD1d-expressing macrophages. Activation of iNKT cells in vivo results in the production of IFN-gamma, which we demonstrate ameliorates the severity of murine Lyme carditis by at least two mechanisms. First, IFN-gamma enhances the recognition of B. burgdorferi by macrophages, leading to increased phagocytosis of the spirochete. Second, IFN-gamma activation of macrophages increases the surface expression of CD1d, thereby facilitating further iNKT activation. Collectively, our data demonstrate that in the resistant background, B6, iNKT cells modulate the severity of murine Lyme carditis through the action of IFN-gamma, which appears to self-renew through a positive feedback loop during infection.

The mDial formin is required for neutrophil polarization, migration, and activation of the LARG/RhoA/ROCK signaling axis during chemotaxis.

Neutrophil chemotaxis depends on actin dynamics, but the roles for specific cytoskeleton regulators in this response remain unclear. By analysis of mammalian diaphanous-related formin 1 (mDia1)-deficient mice, we have identified an essential role for this actin nucleator in neutrophil chemotaxis. Lack of mDia1 was associated with defects in chemoattractant-induced neutrophil actin polymerization, polarization, and directional migration, and also with impaired activation of RhoA, its downstream target p160-Rho-associated coil-containing protein kinase (ROCK), and the leukemia-associated RhoA guanine nucleotide exchange factor (LARG). Our data also revealed mDia1 to be associated with another cytoskeletal regulator, Wiskott-Aldrich syndrome protein (WASp), at the leading edge of chemotaxing neutrophils and revealed polarized morphology and chemotaxis to be more mildly impaired in WAS(-/-) than in mDia1(-/-) neutrophils, but essentially abrogated by combined mDia1/WASp deficiency. Thus, mDia1 roles in neutrophil chemotaxis appear to be subserved in concert with WASp and are realized at least in part by activation of the LARG/RhoA/ROCK signaling pathway.

The transcription repressor, ZEB1, cooperates with CtBP2 and HDAC1 to suppress IL-2 gene activation in T cells.

Activation of T cells leads to the induction of many cytokine genes that are required for appropriate immune responses, including IL-2, a key cytokine for T cell proliferation and homeostasis. The activating transcription factors such as nuclear factor of activated T cells, nuclear factor kappaB/Rel and activated protein-1 family members that regulate inducible IL-2 gene expression have been well documented. However, negative regulation of the IL-2 gene is less studied. Here we examine the role of zinc finger E-box-binding protein (ZEB) 1, a homeodomain/Zn finger transcription factor, as a repressor of IL-2 gene transcription. We show here that ZEB1 is expressed in non-stimulated and stimulated T cells and using chromatin immunoprecipitation assays we show that ZEB1 binds to the IL-2 promoter. Over-expression of ZEB1 can repress IL-2 promoter activity, as well as endogenous IL-2 mRNA production in EL-4 T cells, and this repression is dependent on the ZEB-binding site at -100. ZEB1 cooperates with the co-repressor C-terminal-binding protein (CtBP) 2 and with histone deacetylase 1 to repress the IL-2 promoter and this cooperation depends on the ZEB-binding site in the promoter as well as the Pro-X-Asp-Leu-Ser protein-protein interaction domain in CtBP2. Thus, ZEB1 may function to recruit a repressor complex to the IL-2 promoter.

Tumor-necrosis factor-alpha induces retinoic acid-inducible gene-I in rheumatoid fibroblast-like synoviocytes.

Tumor-necrosis factor-alpha (TNF-alpha) is a potent proinflammtory cytokine and a key molecule in the pathogenesis of rheumatoid arthritis (RA). Retinoic acid-inducible gene-I (RIG-I) is a DExH box protein, which is known to play a role in the inflammatory and immune reactions. We previously reported about potential involvement of RIG-I in synovial inflammation in RA. In the present study, we demonstrated the expression of RIG-I in fibroblast-like synoviocytes stimulated with TNF-alpha. RNA interference against interferon (IFN)-beta abolished the TNF-alpha-induced RIG-I expression. In addition, knockdown of RIG-I partially inhibited the TNF-alpha-induced expression of CC chemokine ligand (CCL) 5, a chemokine with chemotactic activity toward lymphocytes and monocytes. These findings suggest that the TNF-alpha/IFN-beta/RIG-I/CCL5 pathway may be involved in the pathogenesis of synovial inflammation in RA.

Crosstalk between components of the innate immune system: promoting anti-microbial defenses and avoiding immunopathologies.

Because it reaches full functional efficacy rapidly upon encounter with a pathogen, the innate immune system is considered as the first line of defense against infections. The sensing of microbes or of transformed or infected cells, through innate immune recognition receptors (referred to as activating I2R2), initiates pro-inflammatory responses and innate immune effector functions. Other I2R2 with inhibitory properties bind self-ligands constitutively expressed in host. However, this dichotomy in the recognition of foreign or induced self versus constitutive self by I2R2 is not always respected in certain non-infectious conditions reminiscent of immunopathologies. In this review, we discuss that immune mechanisms have evolved to avoid inappropriate inflammatory disorders in individuals. Molecular crossregulation exists between components of I2R2 signaling pathways, and intricate interactions between cells from both innate and adaptive immune systems set the bases of controlled immune responses. We also pinpoint that, like T or B cells, some cells of the innate immune system must go through education processes to prevent autoreactivity. In addition, we illustrate how gene expression profiling of immune cell types is a useful tool to find functional homologies between cell subsets of different species and to speculate about unidentified functions of these cells in the responses to pathogen infections.

DAP10- and DAP12-associated receptors in innate immunity.

The DAP10 and DAP12 signaling subunits are highly conserved in evolution and associate with a large family of receptors in hematopoietic cells, including dendritic cells, plasmacytoid dendritic cells, neutrophils, basophils, eosinophils, mast cells, monocytes, macrophages, natural killer cells, and some B and T cells. Some receptors are able to associate with either DAP10 or DAP12, which contribute unique intracellular signaling functions. Studies of humans and mice deficient in these signaling subunits have provided surprising insights into the physiological functions of DAP10 and DAP12, demonstrating that they can either activate or inhibit immune responses. DAP10- and DAP12-associated receptors have been shown to recognize both host-encoded ligands and ligands encoded by microbial pathogens, indicating that they play an important role in innate immune responses.

Systems biology of innate immunity.

Systems biology is the comprehensive and quantitative analysis of the interactions between all of the components of biological systems over time. Systems biology involves an iterative cycle, in which emerging biological problems drive the development of new technologies and computational tools. These technologies and tools then open new frontiers that revolutionize biology. Innate immunity is well suited for systems analysis, because the relevant cells can be isolated in various functional states and their interactions can be reconstituted in a biologically meaningful manner. Application of the tools of systems biology to the innate immune system will enable comprehensive analysis of the complex interactions that maintain the difficult balance between host defense and inflammatory disease. In this review, we discuss innate immunity in the context of the systems biology concepts, emergence, robustness, and modularity, and we describe emerging technologies we are applying in our systems-level analyses. These technologies include genomics, proteomics, computational analysis, forward genetics screens, and analyses that link human genetic polymorphisms to disease resistance.