[Rheumatoid arthritis: new developments in the pathogenesis with special reference to synovial fibroblasts]

Rheumatoid arthritis (RA) is a chronic inflammatory disease, which is mainly characterized by synovial hyperplasia, pathological immune phenomena and progressive destruction of the affected joints. Various cell types are involved in the pathogenesis of RA including T cells, antigen presenting cells, and endothelial cells. Recent experimental evidence suggests that the CD40/CD154 system might play an important role in the development of RA. Our experimental approach focuses on RA synovial fibroblasts (RA-SF) that are able to destroy articular cartilage independent of inflammation. To elucidate the specific role of those cells in RA pathophysiology the following questions are currently addressed: 1. Which mechanisms do activate the RA-SF? 2. How do the activated RA-SF attach to the cartilage? 3. How do RA-SF destroy cartilage and bone? Which mechanisms do activate the RA-SF? The process of activation is poorly understood. It is unclear, how far the synovial hyperplasia of RA resembles tumor diseases. Along this line some contradictory results exist concerning the role of the tumor suppressor protein p53. Some investigations could show the expression of p53 in the synovial lining including p53 mutations in RA synovium and in RASF, while other research groups could not confirm these data. Our group has demonstrated that the tumor suppressor PTEN was less expressed in the synovial lining of RA than in normal synovium, but no PTEN mutations could be found in the RA-SF. In addition, the in vivo and in vitro expression of the anti-apoptotic molecule sentrin suggests a functional resistance of RA-SF to undergo apoptosis. Although it is still unclear, whether certain viruses or viral elements are involved in the pathogenesis of RA (cause, consequence or coincidence?), certain viruses could play a role in the pathogenesis of RA. The endogenous retroviral element L1 was found to be expressed in the synovial lining, at sites of invasion as well as in RA-SF grown in vitro. Moreover, the data indicate that after the initial activation of L1 downstream molecules such as the SAP kinase 4, the met-protoonocogene and the galectin-3 binding protein are upregulated. How do the activated RA-SF attach to the cartilage? It has been suggested that integrins mediate the attachment of RA-SF to fibronectin rich sites of cartilage. Intriguingly, other adhesion molecules such as the vascular cellular adhesion molecule-1 (VCAM) and CS-1, a splice variant of fibronectin, are synthesized by RA-SF. By binding to these adhesion molecules, lymphocytes that express the integrin VLA-4 could be stimulated and thereby maintain the inflammatory process. Osteopontin is an extracellular matrix protein, which is associated with matrix adhesion and metastasis in tumors. In RA synovium, osteopontin was detectable in the synovial lining and at sites of invasion. How do RA-SF destroy cartilage and bone? The destruction of cartilage and bone in RA is mediated by matrix metalloproteinases (MMPs) and cathepsins. MMPs exist as secreted and as membrane bound forms. In vitro models are being developed to simulate the invasive process of RA-SF. In an in vitro model developed in our laboratory, the treatment of RA-SF with anti-CD44 or anti-interleukin-1 (IL-1) minimized matrix degradation of RA-SF. On the other hand, co-culture of RA-SF and U937 cells as well as application of interleukin-1 beta (IL-1 beta) or tumor necrosis factor alpha (TNF alpha) increased the invasiveness of RA-SF. Gene transfer of bovine pancreas trypsin inhibitor (BPMI) or interleukin-10 (IL-10) reduced the invasion of RA-SF, while transduction of interleukin-1 receptor antagonist (IL-1Ra) was chondroprotective. Double gene transfer of IL-10 and IL-1Ra resulted in both inhibition of invasion and chondroprotection.