The Role of TNF-stimulated Gene 6 (TSG-6) in Inflammation

Inflammatory processes are associated with the pathogenesis of many diseases, they are part of the response to infection and trauma and play a central role in the development of many autoimmune diseases. Chronic inflammation leads to tissue remodeling and destruction. The proinflammatory cytokines IL-1 and TNF-alpha are the primary mediators of the inflammatory response and act through the activation of a large set of genes in a wide variety of cell types. Among the genes activated by both cytokines is TNF-stimulated gene 6 (TSG-6). The TSG-6 cDNA encodes a secreted 35 kDa glycoprotein which is abundant in synovial fluids of patients with various forms of arthritis and detectable in serum of patients with different inflammatory and autoimmune disorders. The TSG-6 protein consists of two domains, the N-terminal link module and the C-terminal CUB domain. TSG-6 binds to the glycosaminoglycan hyaluronan through its link module, a domain shared with several other hyaluronan-binding proteins. The TSG-6 protein forms a stable complex with components of the plasma protein inter-alpha-inhibitor (IαI), a Kunitz-type serine protease inhibitor. TSG-6 is responsible for the transfer of IαI polypeptide chains (termed heavy chains, but not related to Ig heavy chains) from IαI to hyaluronan, resulting in a permanent modification of this component of the extracellular matrix.

Recombinant human TSG-6 protein exerts a potent antiinflammatory effect in a murine model of acute inflammation and in collagen-induced arthritis, a murine model of rheumatoid arthritis. The effect of recombinant TSG-6 is comparable to, or even superior, to the effect of anti-TNF agents in this experimental model of rheumatoid arthritis. TSG-6-transgenic mice are much less susceptible to develop collagen-induced arthritis than their non-transgenic littermates.

Activation of the TSG-6 gene by proinflammatory cytokines, presence of TSG-6 protein at inflammatory sites and its antiinflammatory effect suggest a role for TSG-6 in a negative feed-back control of the inflammatory response. We are studying the physiological and pathophysiological functions of TSG-6 at the molecular and cellular level and in animal models with particular focus on its role in inflammation. The elucidation of the mechanism of TSG-6 action may eventually contribute to a better understanding of inflammatory processes and result in the development of new concepts for the treatment of inflammatory diseases.

The GAG-IαI-TSG-6 system

Because the complex interactions between glycosaminoglycans (GAGs), IαI, and TSG-6 result in the formation of complexes of various compositions that are often linked by covalent bonds, we propose the use of the term GAG- IαI -TSG-6 system. Besides mediating the transfer of IαI heavy chains to hyaluronan, TSG-6 itself also forms a stable bond with this GAG in a strictly temperature-dependent manner. The structure of this stable bond still awaits elucidation. The interactions within the GAG-IαI-TSG-6 system are likely to be involved in the anti-inflammatory and chondroprotective effects of TSG-6 and the stabilization of the cumulus cell-oocyte complex during ovulation (female TSG-6-deficient mice do not form stable cumulus cell-oocyte complexes and are infertile). Stable complexes with GAGs may serve as a reservoir of TSG-6 and IαI heavy chains in tissues, and may bind additional functional proteins, e.g., pentraxin-3 and thrombospondin, or modulate the activity of furin. TSG-6, in addition to hyaluronan, is able to bind to all common isoforms of chondroitin sulfate (CS). This suggests that TSG-6 is likely to interact with a variety of proteoglycans containing chondroitin sulfate side chains, e.g., aggrecan, a major constituent of cartilage and a target of proteolytic damage in arthritis patients.

TSG-6 as a potential biomarker for disease progression, prognosis and assessment of therapy.

TSG-6 protein is detectable in synovial fluids of patients with various forms of arthritis and in plasma of patients with certain autoimmune diseases. There are currently no sensitive quantitative assays for the determination of TSG-6 protein in body fluids, such as sera or synovial fluids, available. The task of developing such an assay is difficult due to the presence of a TSG-6 binding protein (IαI) and GAGs in these samples. We are currently evaluating a promising assay suitable for the quantitative determination of TSG-6 in biological fluids. This assay combines the principles of an enzyme assay, a GAG binding assay, and an ELISA and overcomes the problems of sample heterogeneity by a novel method of standardization.

We are planning to use this assay to evaluate the potential role of TSG-6 as a biomarker of disease progression and as a prognostic tool in arthritis, SLE and prostate cancer.