TLR expression profiles are a function of disease status in rheumatoid arthritis and experimental arthritis.

The role of the innate immune system has been established in the initiation and perpetuation of inflammatory disease, but less attention has been paid to its role in the resolution of inflammation and return to homeostasis. Toll-like receptor (TLR) expression profiles were analysed in tissues with differing disease status in rheumatoid arthritis (RA), ankylosing spondylitis (AS), and in experimental arthritis. TLR gene expression was measured in whole blood and monocytes, before and after TNF blockade. In RA and osteoarthritis synovia, the expression of TLRs was quantified by standard curve qPCR. In addition, four distinct stages of disease were defined and validated in collagen-induced arthritis (CIA), the gold standard animal model for RA - pre-onset, early disease, late disease and immunised mice that were resistant to the development of disease. TLR expression was measured in spleens, lymph nodes, blood cells, liver and the paws (inflamed and unaffected). In RA whole blood, the expression of TLR1, 4 and 6 was significantly reduced by TNF blockade but the differences in TLR expression profiles between responders and non-responders were less pronounced than the differences between RA and AS patients. In RA non-responders, monocytes had greater TLR2 expression prior to therapy compared to responders. The expression of TLR1, 2, 4 and 8 was higher in RA synovium compared to control OA synovium. Circulating cytokine levels in CIA resistant mice were similar to naïve mice, but anti-collagen antibodies were similar to arthritic mice. Distinct profiles of inflammatory gene expression were mapped in paws and organs with differing disease status. TLR expression in arthritic paws tended to be similar in early and late disease, with TLR1 and 2 moderately higher in late disease. TLR expression in unaffected paws varied according to gene and disease status but was generally lower in resistant paws. Disease status-specific profiles of TLR expression were observed in spleens, lymph nodes, blood cells and the liver. Notably, TLR2 expression rose then fell in the transition from naïve to pre-onset to early arthritis. TLR gene expression profiles are strongly associated with disease status. In particular, increased expression in the blood precedes clinical manifestation.

Precipitation of Soluble Uric Acid Is Necessary for In Vitro Activation of the NLRP3 Inflammasome in Primary Human Monocytes.

OBJECTIVE: To investigate the effects of soluble uric acid (UA) on expression and activation of the NOD-like receptor (NLR) pyrin domain containing protein 3 (NLRP3) inflammasome in human monocytes to elucidate the role of hyperuricemia in the pathogenesis of gout. METHODS: Primary human monocytes and the THP-1 human monocyte cell line were used to determine the effects of short- and longterm exposure to UA on activation of the NLRP3 inflammasome and subsequent interleukin 1ß (IL-1ß) secretion by ELISA and cell-based assays. Expression of key NLRP3 components in monocytes from patients with a history of gout were analyzed by quantitative PCR. RESULTS: Precipitation of UA was required for activation of the NLRP3 inflammasome and subsequent release of IL-1ß in human monocytes. Neither monosodium urate (MSU) crystals nor soluble UA had any effect on activation of the transcription factor, nuclear factor-κB. Prolonged exposure of monocytes to soluble UA did not alter these responses. However, both MSU crystals and soluble UA did result in a 2-fold increase in reactive oxygen species. Patients with gout (n = 15) had significantly elevated serum UA concentrations compared to healthy individuals (n = 16), yet secretion of IL-1ß and expression of NLRP3 inflammasome components in monocytes isolated from these patients were not different from those of healthy controls. CONCLUSION: Despite reports indicating that soluble UA can prime and activate the NLRP3 inflammasome in human peripheral blood mononuclear cells, precipitation of soluble UA into MSU crystals is essential for in vitro NLRP3 signaling in primary human monocytes.

Engineering of TIMP-3 as a LAP-fusion protein for targeting to sites of inflammation.

Tissue inhibitor of metalloproteinase (TIMP)-3 is a natural inhibitor of a range of enzymes that degrade connective tissue and are involved in the pathogenesis of conditions such as arthritis and cancer. We describe here the engineering of TIMP-3 using a novel drug-delivery system known as the 'LAP technology'. This involves creating therapeutic proteins in fusion with the latency-associated peptide (LAP) from the cytokine TGF-? to generate proteins that are biologically inactive until cleavage of the LAP to release the therapy. LAP-TIMP-3 was successfully expressed in mammalian cells and the presence of the LAP resulted in a 14-fold increase in the quantity of recombinant TIMP-3 produced. LAP-TIMP-3 was latent until release from the LAP by treatment with matrix metalloproteinase when it could inhibit proteases of the adamalysins and adamalysins with thrombospondin motifs families, but not matrix metalloproteinases, indicating that this version of TIMP-3 is a more specific inhibitor than the native protein. There was sufficient protease activity in synovial fluid from human joints with osteoarthritis to release TIMP-3 from the LAP fusion. These results demonstrate the potential for development of TIMP-3 as a novel therapy for conditions where upregulation of catabolic enzymes are part of the pathology.

Modulation of toll-like receptor function has therapeutic potential in autoimmune disease.

IMPORTANCE OF THE FIELD: The role of toll-like receptors (TLRs) in the immune response to exogenous pathogens is well characterized. These receptors have been suggested to be involved in the initiation and/or perpetuation of many inflammatory autoimmune diseases and have become attractive candidates for the modulation of inflammation. AREAS COVERED IN THIS REVIEW: This review discusses the evidence to support a potential role for TLRs in inflammatory diseases, focusing on rheumatoid arthritis, multiple sclerosis and systemic lupus erythematosus. The approaches to targeting TLR activation are outlined. WHAT THE READER WILL GAIN: An appreciation for the role of TLRs in inflammatory diseases and in particular the contribution of specific TLRs in rheumatoid arthritis, multiple sclerosis and systemic lupus erythematosus. This review focuses on recent developments in targeting TLR activity from ligand binding through to the resultant signaling. TAKE HOME MESSAGE: As initiators of immune responses, TLRs have previously been targeted to increase the immune response with some success. However, targeting TLRs to attenuate immune responses for the treatment of chronic inflammatory diseases will require further evidence of the mechanisms of TLR involvement in the pathophysiology and a better understanding of the potential effects of modulating TLR physiology over a sustained period.

Inhibitors of TLR8 reduce TNF production from human rheumatoid synovial membrane cultures.

The advent of anti-TNF biologicals has been a seminal advance in the treatment of rheumatoid arthritis (RA) and has confirmed the important role of TNF in disease pathogenesis. However, it is unknown what sustains the chronic production of TNF. In this study, we have investigated the anti-inflammatory properties of mianserin, a serotonin receptor antagonist. We discovered mianserin was able to inhibit the endosomal TLRs 3, 7, 8, and 9 in primary human cells and inhibited the spontaneous release of TNF and IL-6 from RA synovial membrane cultures. This suggested a role for these TLRs in production of TNF and IL-6 from RA which was supported by data from chloroquine, an inhibitor of endosomal acidification (a prerequisite for TLRs 3, 7, 8, and 9 activation) which also inhibited production of these cytokines from RA synovial cultures. Only stimulation of TLR 3 or 8 induced TNF from these cultures, indicating that TLR7 and TLR9 were of less consequence in this model. The key observation that indicated the importance of TLR8 was the inhibition of spontaneous TNF production by imiquimod, which we discovered to be an inhibitor of TLR8. Together, these data suggest that TLR8 may play a role in driving TNF production in RA. Because this receptor can be inhibited by small m.w. molecules, it may prove to be an important therapeutic target.

Could toll-like receptors provide a missing link in chronic inflammation in rheumatoid arthritis? Lessons from a study on human rheumatoid tissue.

In the last decade the development of a number of biological therapies has revolutionised the treatment of rheumatic diseases. The first and most widely used of these approaches, tumour necrosis factor (TNF) blockade (infliximab, entanercept, adalimumab), has now been administered to over a million patients. However, the success of these biological therapies has also highlighted their limitations. None of these treatments has shown a 100% patient response; normally responses are in the 50-70% range. As proteins, these drugs cannot be given orally and they are expensive to produce, a cost ultimately borne by the patient/health provider that can seriously limit the availability of these drugs. Lastly, these treatments, whether involving the systemic neutralisation of a cytokine (eg, TNF or IL6 receptor blockade (tocilizumab)), the ablation of a B cell population (anti-CD20, rituximab), or the potential disruption of important cellular interactions as with CTLA4-Ig (abatacept), can cause major perturbations of the immune system, the long-term effects of which are still unclear. At present, treatments such as TNF blockade can result in an increased infectious risk and the reactivation of tuberculosis can be a major issue in certain populations. As with all therapies, there is an increasing large refractory population over time. Therefore, despite the undoubted success of these therapies, there is room for improvement. Although it might be too much to expect any new treatment to affect a "cure" (all the current biological therapies require repeated administrations), there are definite gains to be made in terms of cost, oral bioavailability and a more selective interference with the immune-inflammatory response.

Key differences in TLR3/poly I:C signaling and cytokine induction by human primary cells: a phenomenon absent from murine cell systems.

TLR3 recognizes double-stranded RNA, a product associated with viral infections. Many details of TLR3-induced mechanisms have emerged from gene-targeted mice or inhibition studies in transformed cell lines. However, the pathways activated in human immune cells or cells from disease tissue are less well understood. We have investigated TLR3-induced mechanisms of human primary cells of the innate immune system, including dendritic cells (DCs), macrophages (MØs), endothelial cells (ECs), and synovial fibroblasts isolated from rheumatoid arthritis joint tissue (RA-SFs). Here, we report that while these cells all express TLR3, they differ substantially in their response to TLR3 stimulation. The key antiviral response chemokine IP-10 was produced by all cell types, while DCs and MØs failed to produce the proinflammatory cytokines TNFalpha and IL-6. Unexpectedly, TNFalpha was found secreted by TLR3-stimulated RA-SF. Furthermore, TLR3 stimulation did not activate NFkappaB, MAPKs, or IRF-3 in DCs and MØs, but was able to do so in ECs and RA-SF. These findings were specific for human cells, thereby revealing a complexity not previously expected. This is the first report of such cell type- and species-specific response for any TLR stimulation and helps to explain important difficulties in correlating murine models of inflammatory diseases and human inflammation.

Selective use of TRAM in lipopolysaccharide (LPS) and lipoteichoic acid (LTA) induced NF-kappaB activation and cytokine production in primary human cells: TRAM is an adaptor for LPS and LTA signaling.

TLR signal via Toll-IL-1R (TIR) homology domain-containing adaptor proteins. One of these adaptors, Toll-IL-1R domain-containing adaptor inducing IFN-beta-related adaptor molecule (TRAM), has been shown to be essential for TLR4 signaling in TRAM(-/-) mice and cell lines. Previously, we showed that MyD88 or Mal dominant-negative constructs did not inhibit LPS induction of cytokines in primary human M-CSF-derived macrophages. A possible explanation was redundancy of the adaptors during LPS signaling. TRAM is a suitable candidate to compensate for these adaptors. To investigate a potential role for TRAM in LPS signaling in human M-CSF-derived macrophages, we engineered an adenoviral construct expressing dominant-negative TRAM-C117H (AdTRAMdn). Synovial fibroblasts (SF) and human umbilical endothelial cells (HUVECs) were used as a nonmyeloid comparison. AdTRAMdn inhibited LPS-induced signaling in SFs and HUVECs, reducing NF-kappaB activation and cytokine production, but did not inhibit LPS signaling in M-CSF-derived human macrophages. Further investigation of other TLR ligands showed that AdTRAMdn was also able to inhibit signaling initiated by lipoteichoic acid, a TLR2 ligand, in SFs and HUVECs and lipoteichoic acid and macrophage-activating lipopeptide 2 signaling was also inhibited in TRAM(-/-) murine embryonic fibroblasts. We conclude that TRAM is an adaptor protein for both TLR4 and TLR2/6 signaling in SFs, HUVECs, and murine embryonic fibroblasts, but cannot demonstrate a role in human macrophages.

The Toll-like receptor adaptor proteins MyD88 and Mal/TIRAP contribute to the inflammatory and destructive processes in a human model of rheumatoid arthritis.

The widespread distribution of Toll-like receptors (TLRs) and their ligands raises the question whether they contribute to the production of inflammatory and tissue destructive molecules in rheumatoid arthritis (RA). We examined the expression and function of TLR2 and TLR4 and their downstream signaling adaptors MyD88 and Mal/TIRAP in synovial membrane cultures from RA tissue. Both TLR2 and TLR4 were detected by flow cytometry, and stimulation with TLR2 and TLR4 ligands augmented the spontaneous production of tumor necrosis factor-alpha, interleukin (IL)-6, and IL-8, indicating that TLR2 and TLR4 are functional in these cultures. In addition, overexpression of dominant-negative forms of MyD88 and Mal/TIRAP significantly down-regulated the spontaneous production of cytokines tumor necrosis factor-alpha, IL-6, and vascular endothelial growth factor, and enzymes MMP-1, MMP-2, MMP-3, and MMP-13 in RA synovial membrane cell cultures. Because TLR2 and TLR4 require both MyD88 and Mal/TIRAP for signaling, this study suggests that TLR function may regulate the expression of these factors in the RA synovium. Conditioned media from synovial membrane cell cultures stimulated human macrophages in a MyD88- and Mal-dependent manner, suggesting the release of a TLR ligand(s) from these cells. Thus, TLRs not only protect against infection but may also promote the inflammatory and destructive process in RA.

Toll-like receptors: a new target in rheumatoid arthritis?

Rheumatoid arthritis (RA) is one of the most prevalent autoimmune diseases. It is characterized by chronic inflammation of the joint leading to its destruction. Although the initiating cause remains elusive, environmental factors and genetic background are known to contribute to the etiology of RA. The role of Toll-like receptors (TLRs) in innate immunity and their ability to recognize microbial products has been well characterized. TLRs are able to recognize endogenous molecules released upon cell damage and necrosis, and are present in RA synovial fluid. Although it appears unlikely that a pathogen underlies the pathogenesis or progression of RA, the release of endogenous TLR ligands during inflammation may activate TLRs and perpetuate the disease. An increasing body of circumstantial evidence implicates TLR signaling in RA, although, at present, their involvement is not defined comprehensively. Targeting individual TLRs or their signaling transducers may provide a more specific therapy without global suppression of the immune system.

Molecular therapeutic targets in rheumatoid arthritis.

In an attempt to combat the pain and damage generated by rheumatoid arthritis (ra), new drugs are being developed to target molecular aspects of the disease process. Recently, a major development has been the use of biologicals (antibodies and soluble receptors) that neutralise the activity of tumour necrosis factor alpha (TNF-alpha) and interleukin 1 (IL-1), both of which are involved in disease progression. An increase in our understanding of cell and molecular biology has resulted in the identification and investigation of potential new targets, and also the refinement and improvement of current therapeutic modalities. This review describes therapies that are approved, in clinical trials or under pre-clinical investigation at the laboratory level, particularly focusing on cytokines, although other therapeutic targets of interest are mentioned.

Endotoxin signaling in human macrophages: signaling via an alternate mechanism.

Lipopolysaccharide (LPS) signals through Toll-like receptors (TLRs) in the course of sepsis, resulting in the release of inflammatory factors. In cell lines and murine models, parts of the signaling pathways involved have been elucidated with MyD88, Mal/TIRAP and IKK2 playing an important role in the induction of NF-kappaB. By focusing on primary human cells, we have shown that there are fundamental signaling differences between human and murine macrophages and between cells of myeloid and non-myeloid origins. In primary human cells, there are no available knockouts so we employed the use of dominant negatives to investigate the signaling cascades. We show that in primary human macrophages MyD88, Mal/TIRAP and IKK2-independent alternative pathways activate NF-kappaB and induce the expression of inflammatory cytokines, whereas in non-myeloid synovial fibroblasts MyD88 and/or Mal/TIRAP are essential adaptors for LPS signaling.

Distinct pathways of LPS-induced NF-kappa B activation and cytokine production in human myeloid and nonmyeloid cells defined by selective utilization of MyD88 and Mal/TIRAP.

How lipopolysaccharide (LPS) signals through toll-like receptors (TLRs) to induce nuclear factor (NF)-kappa B inflammatory cytokines in sepsis remains unclear. Major candidates for that process are myeloid differentiation protein 88 (MyD88) and MyD88 adaptor-like/TIR domain-containing adaptor protein (Mal/TIRAP) but their role needs to be further defined. Here, we have examined the role of MyD88 and Mal/TIRAP in primary human cells of nonmyeloid and myeloid origin as physiologically relevant systems. We found that MyD88 and Mal/TIRAP are essential for LPS-induced I kappa B alpha phosphorylation, NF-kappa B activation, and interleukin 6 (IL-6) or IL-8 production in fibroblasts and endothelial cells in a pathway that also requires IKK2. In contrast, in macrophages neither MyD88, Mal/TIRAP, nor I kappa B kinase 2 (IKK2) are required for NF-kappa B activation or tumor necrosis factor alpha (TNF alpha), IL-6, or IL-8 production, although Mal/TIRAP is still involved in the production of interferon beta (IFN beta). Differential usage of TLRs may account for that, as in macrophages but not fibroblasts or endothelial cells, TLR4 is expressed in high levels at the cell surface, and neutralization of TLR4 but not TLR2 blocks LPS signaling. These observations demonstrate for the first time the existence of 2 distinct pathways of LPS-induced NF-kappa B activation and cytokine production in human myeloid and nonmyeloid cells defined by selective utilization of TLR4, MyD88, Mal/TIRAP, and IKK2, and reveal a layer of complexity not previously expected.

Apolipoprotein E (apoE) isoforms differentially induce nitric oxide production in endothelial cells.

Although apolipoprotein E3 (apoE3) is atheroprotective, two common isoforms, apoE2 and apoE4, produce recessive and dominant hyperlipidaemias, respectively. Using a fluorescent assay, we report herein that apoE3 particles secreted from recombinant cells stimulate more nitric oxide release in cultured human EA.hy926 endothelial cells than apoE2 or apoE4 (141% more than controls vs. 61 or 11%). Phosphatidylinositol (PI) 3-kinase inhibitors suppressed the apoE effect, while apoE receptor 2 (apoER2) was tyrosine phosphorylated. We conclude that apoE stimulates endothelial nitric oxide release in an isoform-dependent manner, and propose that tyrosine phosphorylation of apoER2 initiates PI3-kinase signalling and activation of nitric oxide synthase.

Intracellular localization of endothelial cell annexins is differentially regulated by oxidative stress.

Annexins are calcium-dependent phospholipid binding proteins that are implicated in the regulation of both intracellular and extracellular thrombostatic mechanisms in the vascular endothelium. Tight control of annexin gene expression and targeting of annexin proteins is therefore of importance in maintaining the health of the endothelium. Because annexins are abundant in vascular endothelial cells and could be either dysregulated by or contribute to anomalies in Ca2+ signaling, we investigated annexin gene expression and subcellular localization in human umbilical vein endothelial cells (HUVEC) in a model of chronic oxidative stress. HUVEC were cultured under mild hyperoxic conditions in a custom-built chamber to induce oxidative stress over a period of 12 days. Although annexin expression levels did not change significantly in response to hyperoxic stress, immunofluorescence analysis revealed striking effects on the subcellular localization of certain annexins, including the redistribution of annexins 5 and 6 from the cytosol to the nucleus. In addition, oxidative stress modulated the responses of certain annexins to stimulation with a range of pharmacological and physiological Ca2+-mobilizing agonists, in a manner that suggested that annexin localization is regulated via the complex integration of both Ca2+ and intracellular signaling pathways. These results show that differential regulation of annexin localization by oxidative stress may have a causative role in the cellular pathophysiology of vascular endothelial cell disease.