Synovial fibroblasts occur as two phenotypes - intimal and subintimal. The specialised intimal phenotype includes expression of uridine diphosphoglucose dehydrogenase (UDPGD), vascular cell adhesion molecule-1 (VCAM-1) and complement decay-accelerating factor (DAF). These gene products contribute to specialised functions relating to tissue movement and leucocyte traffic.

The aim of this study was to explore the molecular profile of proliferating rheumatoid arthritis synovial fibroblasts (RA-SF). Total RNA was extracted from two cultures of RA-SF (low-density [LD] proliferating cells and high-density [HD] nonproliferating cells) and suppression subtractive hybridization was performed to compare differential gene expression of these two cultures. Subtracted cDNA was subcloned, and nucleotide sequences were analyzed to identify each clone. Differential expression of distinct clones was confirmed by semiquantitative RT-PCR. The expression of certain genes in synovial tissues was examined by in situ hybridization. In both LD and HD cells, 44 clones were upregulated. Of the 88 total clones, 46 were identical to sequences that have previously been characterized. Twenty-nine clones were identical to cDNAs that have been identified, but with unknown functions so far, and 13 clones did not show any significant homology to sequences in GenBank (NCBI). Differential expression of distinct clones was confirmed by RT-PCR. In situ hybridization showed that certain genes, such as S100A4, NFAT5, unr and Fbx3, were also expressed predominantly in synovial tissues from patients with RA but not from normal individuals. The expression of distinct genes in proliferating RA-SF could also be found in RA synovium, suggesting that these molecules are involved in synovial activation in RA. Most importantly, the data indicate that the expression of certain genes in RA-SF depends on the stage of proliferation; therefore, the stage needs to be considered in any analysis of differential gene expression in SF.

Fibroblast biology: Synovial fibroblasts in rheumatoid arthritis - leading role or chorus line?

Macrophages that accumulate in the synovium of rheumatoid arthritis patients play an important role in the pathogenesis of this inflammatory disease. However, the mechanism by which macrophages are attracted into the inflamed synovium and accumulate there has not been completely delineated. The results of this study show that rheumatoid arthritis synovial stromal cells produce the chemokines monocyte chemotactic protein-1 and IL-8, and these have the capacity to attract peripheral monocytes. These results suggest that one of the mechanisms by which macrophages accumulate in the inflamed synovium is by responding to the chemokines produced locally.

Background The first type III repeat of fibronectin is known to be involved in fibronectin matrix assembly, and recombinant proteins from this type III repeat can inhibit cell proliferation, tumor metastasis and angiogenesis. We have analyzed the way rat aortic smooth muscle cells (RASMCs) interact with a recombinant protein encompassing a C-terminal portion of the first type III repeat of fibronectin (protein III1-C). Results Cells are able to adhere to and spread on III1-C coated on a dish. Both β1 integrins and cell surface heparan sulfate proteoglycans serve as receptors for III1-C. For example, cell attachment to III1-C is partially inhibited by agents that block β1 integrins or by heparin. Complete inhibition of cell attachment is seen only when integrin blocking agents are combined with heparin. Affinity chromatography revealed the binding of proteins that likely represent the integrin β1 and α5 submits to a III1-C column. Cell adhesion to III1-C results in robust ERK1/2 activation that is blocked by integrin-blocking agents. In addition, cell adhesion to III1-C and ERK1/2 activation by III1-C are both inhibited by heparan sulfate but not by chondroitin sulfate. Moreover, heparitinase treatment, but not chondroitinase treatment of RASMCs results in reduced cell adhesion and ERK1/2 activation. Affinity chromatography experiments demonstrated that 35SO4-labeled cell surface heparan sulfate proteoglycans bound specifically to III1-C. Conclusions The results suggest that the 1st type III repeat of fibronectin contains a previously unrecognized cell adhesion domain that stimulates robust ERK1/2 activation in RASMCs. Cells interact with this domain through cell surface heparan sulfate proteoglycans and integrins, and both classes of receptors are required for optimal cell adhesion and ERK1/2 activation.

Mechanisms whereby T lymphocytes contribute to synovial inflammation in rheumatoid arthritis are poorly understood. Here we review data that indicate an important role for cell contact between synovial T cells, adjacent macrophages and fibroblast-like synoviocytes (FLS). Thus, T cells activated by cytokines, endothelial transmigration, extracellular matrix or by auto-antigens can promote cytokine, particularly TNFα, metalloproteinase production by macrophages and FLS through cell-membrane interactions, mediated at least through β-integrins and membrane cytokines. Since soluble factors thus induced may in turn contribute directly to T cell activation, positive feedback loops are likely to be created. These novel pathways represent exciting potential therapeutic targets.

We have analyzed the pattern of procoagulant and fibrinolytic gene expression in affected joints during the course of arthritis in two murine models. In both models, we found an increased expression of tissue factor, tissue factor pathway inhibitor, urokinase plasminogen activator, and plasminogen activator inhibitor 1, as well as thrombin receptor. The observed pattern of gene expression tended to favor procoagulant activity, and this pattern was confirmed by functional assays. These alterations would account for persistence of fibrin within the inflamed joint, as is seen in rheumatoid arthritis.

The knowledge of transcription factors and proto-oncogenes has influenced the understanding of cell regulation, cell cycle, and apoptotic cell death in rheumatoid arthritis (RA) synovium. In addition, the development of normal synovial fibroblasts into transformed-appearing aggressive synovial fibroblasts may be triggered by the lack of antiproliferative factors, such as p53, p53-associated molecules, other tumor suppressors, as well as by upregulation of anti-apoptotic genes. Therefore, data derived from experiments such as those performed by Tak and colleagues in this issue of Arthritis Research not only enrich the intensive discussion addressing the impact of p53 on RA pathophysiology, they also may facilitate development of novel therapeutic approaches including p53-targeted gene therapy.

Leukocyte ingress into the synovium is a key process in the pathogenesis of rheumatoid arthritis and other inflammatory conditions. In this review, the role of endothelial cells in leukocyte extravasation will be discussed, including the role of the most relevant cellular adhesion molecules. These molecules play an important role in mediating leukocyte-endothelial interactions. It is likely that different adhesive pathways are involved in different steps of leukocyte adhesion to and migration through endothelia. Targeting of pathological endothelial function, including leukocyte-endothelial adhesion, may be useful for the future management of inflammatory arthritis.

The synovium lines the noncartilaginous surfaces of the diarthrodial joints, and synovial tissue is also found in tendon sheaths and bursae [1]. Several rheumatic diseases are characterized by synovial inflammation. In these conditions, descriptive studies of synovial biopsy specimens may contribute to an understanding of the events that take placein vivo, and they complement experimental animal studies as well as in-vitro studies. Examination of synovial tissue is generally more relevant than synovial fluid analysis, except, for example, the analysis of neutrophils and platelets, and studies of soluble mediators. Recently, there has been an enormous upsurge in investigations of the pathological changes in the synovium [2] because of the availability of new methods to obtain synovial biopsy samples [3,4] and because of the development of immunohistological methods, in-situ hybridization, and the polymerase chain reaction. Moreover, the complementary DNA microarray technology may hold great promise for synovial tissue analysis in the future [5].