Brian M. J. Foxwell

Tenascin-C is an endogenous activator of Toll-like receptor 4 that is essential for maintaining inflammation in arthritic joint disease.

Although there have been major advances in the treatment of rheumatoid arthritis with the advent of biological agents, the mechanisms that drive cytokine production and sustain disease chronicity remain unknown. Tenascin-C (encoded by Tnc) is an extracellular matrix glycoprotein specifically expressed at areas of inflammation and tissue damage in inflamed rheumatoid joints. Here we show that mice that do not express tenascin-C show rapid resolution of acute joint inflammation and are protected from erosive arthritis. Intra-articular injection of tenascin-C promotes joint inflammation in vivo in mice, and addition of exogenous tenascin-C induces cytokine synthesis in explant cultures from inflamed synovia of individuals with rheumatoid arthritis. Moreover, in human macrophages and fibroblasts from synovia of individuals with rheumatoid arthritis, tenascin-C induces synthesis of proinflammatory cytokines via activation of Toll-like receptor 4 (TLR4). Thus, we have identified tenascin-C as a novel endogenous activator of TLR4-mediated immunity that mediates persistent synovial inflammation and tissue destruction in arthritic joint disease.

Tumor necrosis factor-α triggers a cytokine cascade yielding postoperative cognitive decline

Cognitive decline following surgery in older individuals is a major clinical problem of uncertain mechanism; a similar cognitive decline also follows severe infection, chemotherapy, or trauma and is currently without effective therapy. A variety of mechanisms have been proposed, and exploring the role of inflammation, we recently reported the role of IL-1β in the hippocampus after surgery in mice with postoperative cognitive dysfunction. Here, we show that TNF-α is upstream of IL-1 and provokes its production in the brain. Peripheral blockade of TNF-α is able to limit the release of IL-1 and prevent neuroinflammation and cognitive decline in a mouse model of surgery-induced cognitive decline. TNF-α appears to synergize with MyD88, the IL-1/TLR superfamily common signaling pathway, to sustain postoperative cognitive decline. Taken together, our results suggest a unique therapeutic potential for preemptive treatment with anti-TNF antibody to prevent surgery-induced cognitive decline.

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-α 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-α and IL-6 production. However, real-time PCR analysis showed that STAT3 DN expression did not affect immediate suppression of TNF-α mRNA, but did reverse the suppression observed at later time points, suggesting a biphasic regulation of TNF-α 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.

Disordered macrophage cytokine secretion underlies impaired acute inflammation and bacterial clearance in Crohn's disease

The cause of Crohns disease (CD) remains poorly understood. Counterintuitively, these patients possess an impaired acute inflammatory response, which could result in delayed clearance of bacteria penetrating the lining of the bowel and predispose to granuloma formation and chronicity. We tested this hypothesis in human subjects by monitoring responses to killed Escherichia coli injected subcutaneously into the forearm. Accumulation of 111In-labeled neutrophils at these sites and clearance of 32P-labeled bacteria from them were markedly impaired in CD. Locally increased blood flow and bacterial clearance were dependent on the numbers of bacteria injected. Secretion of proinflammatory cytokines by CD macrophages was grossly impaired in response to E. coli or specific Toll-like receptor agonists. Despite normal levels and stability of cytokine messenger RNA, intracellular levels of tumor necrosis factor (TNF) were abnormally low in CD macrophages. Coupled with reduced secretion, these findings indicate accelerated intracellular breakdown. Differential transcription profiles identified disease-specific genes, notably including those encoding proteins involved in vesicle trafficking. Intracellular destruction of TNF was decreased by inhibitors of lysosomal function. Together, our findings suggest that in CD macrophages, an abnormal proportion of cytokines are routed to lysosomes and degraded rather than being released through the normal secretory pathway.

Muramyl dipeptide and toll-like receptor sensitivity in NOD2-associated Crohn's disease

Both NOD2 (CARD15) alleles are mutated in roughly 15% of patients with Crohns disease, but functional effects are unclear. We analysed the cytokine response of peripheral blood mononuclear cells to muramyl dipeptide (MDP), the ligand for NOD2. MDP induced little TNFalpha or interleukin 1beta, but strong interleukin-8 secretion. MDP also substantially upregulated secretion of TNFalpha and interleukin 1beta induced by toll-like receptor ligands. These effects were abolished by the most common Crohns NOD2 double mutant genotypes at low nanomolar MDP concentrations, and provide the basis to develop a test of NOD2 functional deficiency. In Crohns disease, there are defects in neutrophil recruitment driven by NOD2 and interleukin 8 and in cross talk between the NOD2 and toll-like receptor pathways, which suggests that the immune system fails to receive an early priming signal.

The Pyridinyl Imidazole Inhibitor SB203580 Blocks Phosphoinositide-dependent Protein Kinase Activity, Protein Kinase B Phosphorylation, and Retinoblastoma Hyperphosphorylation in Interleukin-2-stimulated T Cells Independently of p38 Mitogen-activated Protein Kinase

Pyridinyl imidazole inhibitors, particularly SB203580, have been widely used to elucidate the roles of p38 mitogen-activated protein (MAP) kinase (p38/HOG/SAPKII) in a wide array of biological systems. Studies by this group and others have shown that SB203580 can have antiproliferative activity on cytokine-activated lymphocytes. However, we recently reported that the antiproliferative effects of SB203580 were unrelated to p38 MAP kinase activity. This present study now shows that SB203580 can inhibit the key cell cycle event of retinoblastoma protein phosphorylation in interleukin-2-stimulated T cells. Studies on the proximal regulator of this event, the phosphatidylinositol 3-kinase/protein kinase B (PKB)(Akt/Rac) kinase pathway, showed that SB203580 blocked the phosphorylation and activation of PKB by inhibiting the PKB kinase, phosphoinositide-dependent protein kinase 1. The concentrations of SB203580 required to block PKB phosphorylation (IC50 3–5 μm) are only approximately 10-fold higher than those required to inhibit p38 MAP kinase (IC50 0.3–0.5 μm). These data define a new activity for this drug and would suggest that extreme caution should be taken when interpreting data where SB203580 has been used at concentrations above 1–2 μm.

Functional consequences of a polymorphism affecting NF-kappaB p50-p50 binding to the TNF promoter region.

ABSTRACT Stimulation of the NF-κB pathway often causes p65-p50 and p50-p50 dimers to be simultaneously present in the cell nucleus. A natural polymorphism at nucleotide −863 in the human TNF promoter (encoding tumor necrosis factor [TNF]) region provides an opportunity to dissect the functional interaction of p65-p50 and p50-p50 at a single NF-κB binding site. We found that this site normally binds both p65-p50 and p50-p50, but a single base change specifically inhibits p50-p50 binding. Reporter gene analysis in COS-7 cells expressing both p65-p50 and p50-p50 shows that the ability to bind p50-p50 reduces the enhancer effect of this NF-κB site. Using an adenoviral reporter assay, we found that the variant which binds p50-p50 results in a reduction of lipopolysaccharide-inducible gene expression in primary human monocytes. This finding adds to a growing body of experimental evidence that p50-p50 can inhibit the transactivating effects of p65-p50 and illustrates the potential for genetic modulation of inflammatory gene regulation in humans by subtle nucleotide changes that alter the relative binding affinities of different forms of the NF-κB complex.

VEGF expression in human macrophages is NF-kappaB-dependent: studies using adenoviruses expressing the endogenous NF-kappaB inhibitor IkappaBalpha and a kinase-defective form of the IkappaB kinase 2.

Vascular endothelial growth factor (VEGF) is the most endothelial cell-specific angiogenic factor characterised to date, and it is produced by a variety of cell types. In macrophages, VEGF has been shown to be upregulated by the inflammatory mediator lipopolysaccharide (LPS) and by engagement of CD40 by CD40 ligand (CD40L). Because LPS and CD40L activate nuclear factor-κB (NF-κB) in monocytes, we investigated in this study whether VEGF production in macrophages, when stimulated with either LPS or CD40L, is NF-κB-dependent. We used adenoviral constructs over-expressing either IκBα (AdvIκBα), the endogenous inhibitor of NF-κB, or a kinase-defective mutant of IKK-2 (AdvIKK-2dn), an upstream activator of IκBα, to infect normal human monocyte-derived macrophages. We observed that LPS-induced production of VEGF in human macrophages was almost completely inhibited (>90%) following adenoviral transfer of IκBα. In addition, we observed significant inhibition of the CD40L-induced VEGF production in macrophages following infection with AdvIκBα. Expression of IKK-2dn in macrophages decreased VEGF production in response to LPS or CD40L by approximately 50%, suggesting that in addition to IKK-2, other kinases might be involved in NF-κB activation. These results show for the first time that VEGF production in human macrophages is NF-κB dependent. NF-κB regulates many of the genes involved in immune and inflammatory responses, and our study adds the angiogenic cytokine VEGF to the list of NF-κB-dependent cytokines.

Interleukin-10 suppression of myeloid cell activation — a continuing puzzle

Efforts to identify the signal transduction pathways used by interleukin‐10 (IL‐10) have resulted in limited success. The anti‐inflammatory effects elicited by IL‐10, and the mechanisms by which these are mediated, are still relatively unknown. Understanding the signalling mechanisms behind the suppression of cytokine expression by IL‐10 could be of potential therapeutic interest. Although the consensus is that the Janus kinase, Jak1, as well as the signal transducer and activator of transcription STAT3 are central, much controversy exists about the participation and roles of many other signalling pathways targeted by IL‐10. The mechanisms of cytokine suppression proposed by various groups have included transcriptional, post‐transcriptional and post‐translational regulation of IL‐10 target genes; nevertheless no unifying model has emerged thus far. Here we would like to highlight novel findings and discuss their implications in the context of current understanding of IL‐10 signalling.

Is targeting Toll-like receptors and their signaling pathway a useful therapeutic approach to modulating cytokine-driven inflammation?

Summary:  Cytokine‐driven inflammation and tissue destruction is a common theme of chronic inflammatory diseases such as rheumatoid arthritis, Crohns disease, ulcerative colitis, psoriasis, chronic obstructive pulmonary disease, and atherosclerosis. Research over the last two decades demonstrated the importance of cytokines that are not only expressed chronically but also are capable of signaling at sites of chronic inflammation. Cytokines thus regulate major pathological processes that include inflammation, angiogenesis, tissue remodeling, and fibrosis. This research led to the identification of key cytokines involved in these processes, two of which, tumor necrosis factor‐α and interleukin‐1, have also been successfully targeted in the clinic. However, what triggers and maintains cytokine gene expression in chronic inflammation remains a mystery. In this article, we review current progress in the understanding of cytokine‐driven inflammation and discuss current evidence implicating Toll‐like receptors (TLRs), recently identified as the receptors recognizing self versus non‐self molecular patterns, in the regulation of cytokine‐driven inflammation. Whether targeting TLRs and their downstream signaling pathway will prove to be a successful approach for the treatment of these devastating diseases remains to be determined.