Production of interleukin-6 by synovial fibroblasts in rheumatoid arthritis.

Persistent inflammation, as exemplified by rheumatoid arthritis (RA), draws the attention of the afflicted patient, the treating physician, and the investigator to three basic questions: what causes the disease, what maintains the inflammatory response, and what treatment will provide relief or recovery? RA is a systemic illness, the major manifestation of which is a destructive inflammatory process of the peripheral synovial joints,1 which shows the same physiological and biochemical features that characterize inflammatory processes in other body tissues.2 Whereas inflammation may in some instances be caused by infection or local tissue injury. In the case of RA, an autoimmune reaction leads to progressive recruitment of inflammatory cells (monocytes, T and B cells, and neutrophils) from the circulation to the synovium.3 Interactions between the immigrating leukocytes and resident synoviocytes, which include monocyte-like, fibroblast-like (FLS), and dendritic cells, establish a local milieu of cytokines, chemokines, and other inflammatory mediators and metabolites,-12 which provides stimulatory and proliferative signals to synovial cells and promotes the degradation of cartilage and bone.13'14 RA can also elicit a systemic acute phase reaction resulting in increased concentrations of acute-phase plasma proteins that may influence the inflammatory process.2'15 During the last decade, the roles of many cytokines in control of specific aspects of the inflammatory reaction have been defined, particularly in regard to their engagement in intercellular communication cascades and in determining disease progression. Tumor necrosis factor (TNF)-a and interleukin (IL)-13, as initiating inflammatory cytokines derived from activated monocytes, act on synovial T cells4 and FLS.16 These cells, together with immigrating activated leukocytes, produce a second wave of inflammatory cytokines that include, among others, IL-6, leukemia-inhibitory factor, oncostatin M,17 granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, and to a variable extent, the anti-inflammatory cytokines IL-10, transforming growth factor-f, and IL-4.18,19 High concentrations of IL-6 in RA synovial fluid correlate well with the severity of disease. As IL-6 in the circulation of RA patients is also elevated, it has been suggested that this cytokine serves as a marker for RA activity. Fibroblast-like synovial cells have been identified as significant producers of IL-6 by in situ immunocytochemistry and mRNA hybridization and by biochemical analysis of primary cultures of synoviocytes. Expression of IL-6 by these cells in culture is enhanced by treatment with TNF-a, IL-1i, prostaglandin E2, and various activators of the intracellular signaling pathways acting through protein kinase A (PKA) or protein kinase C (PKC), suggesting that the IL-6 gene is a target for multiple intracellular regulatory mechanisms. Insight into the mechanisms regulating these pathways has been obtained from molecular characterization of the IL-6 gene promoter.20-23 Several DNA sequences within the 5' flanking region of the IL-6 gene have been identified that serve as binding sites of transcription factors that are specifically activated by cytokines and that mediate transcriptional induction of IL-6. These regulatory elements, which have been localized to the following positions relative to the transcription initiation site, include from -283 to -277, the interaction site for the activating protein-1 (AP-1), from -173 to -151, the enhancer for regulation by PKA and PKC, from -158 to -145, the binding site for CCAAT/enhancerbinding proteins-,B, and from -73 to -64, the binding site for nuclear factor (NF)-KB. The promoter region containing the last binding site proved to be most critical for producing transcriptional induction by TNF-a and IL-ip as a result of the activating effects of these cytokines on NF-KB in synovial fibroblasts and many other cell types. Among the various NF-KB isoforms, the dimer of p65 is a particularly effective inducer of IL-6.24 Indeed, markedly elevated DNA binding activity of NF-KB is observed in RA synovial tissue.25'26 Recently, a binding site for the