Cultures of human dermal fibroblasts and synoviocytes have proved relevant to in vitro models for studying surrogate markers of the mechanisms of chronic inflammatory diseases resulting in inflammation and tissue destruction of the joint structures. Synoviocytes are exquisitely sensitive to mediators of inflammation, such as TNF and IL-1β, and react strongly to direct contact with inflammatory T cells [1–6,35]. They are able to secrete numerous proinflammatory molecules, such as PGE2, chemokines (i.e. IL-8 and MCP-1) and proteolytic enzymes involved in extracellular matrix destruction such as MMP-1. The home-ostasis of inflammatory cytokines and metalloproteinases, as well as their respective inhibitors (i.e. IL-1Ra, TNFsR, tissue inhibitor of metalloproteinases-1) controls the process of tissue destruction [3,14,15]. In addition, the process of repair of collagen and proteoglycan is also inhibited by IL-1 and TNF [32,35].
We recently reported that a member of the TNF family, TWEAK, has inflammatory and cytotoxic activities like TNF and lymphotoxin (LT)α . Marsters et al.  have reported that the receptor for TWEAK is the death-domain-containing receptor Apo3/DR-3/TRAMP, whereas Schneider et al.  demonstrated that TWEAK-induced cell death occurs via TNF and TNF receptor I on the Kym-1 cell line. Nakayama et al.  and Kaptein et al.  showed that TWEAK binding and TWEAK-induced cell death occurs in a number of tumour cells deficient in Apo3/DR-3/TRAMP. There must exist a TWEAK receptor that does not contain a death domain because TWEAK also promotes the growth of endothelial cells . In addition, in astrocytes, which proved resistant to TWEAK-induced cell death, the binding of TWEAK results in the increased secretion of IL-8, IL-6 and intercellular adhesion molecule .
The present study demonstrates another noncytotoxic effect of TWEAK in that hTWEAK stimulated dermal fibroblasts and synoviocytes to produce significant amounts of RANTES, IP-10, IL-8 and MMP-1 as well as some PGE2 and IL-6. Although hTWEAK has proinflammatory properties, these are much less pronounced than those of IL-1β and TNF (with the exception for RANTES and IP-10), even though it potentiates the proinflammatory activities of TNF or IL-1β. In contrast with TNF, induction by TWEAK of most of the inflammatory mediators requires a longer time of incubation. Consequently, it cannot be ruled out that the prolonged effect of TWEAK in stimulating these mediators may be due to the rapid induction of one or more intermediate mediators, which are unidentified to date.
The production of PGE2, MMP-1, IL-8, IL-6, RANTES and IP-10 induced by hTWEAK (but not the production induced by TNF and IL-1β) was completely blocked by the mAb to hTWEAK, BCB10, confirming the specificity of hTWEAK. Moreover, blocking anti-TNF antibodies were inactive in the secretion of IL-8, MMP-1, PGE2, RANTES and IP-10 induced by TWEAK, ruling out any involvement of TNF in TWEAK activity. In contrast, BEB3, another mAb to hTWEAK, increased the activity of hTWEAK significantly, possibly by promoting the trimerization and/or stability of TWEAK. Such a mechanism has been observed with a number of ligands of the TNF family. On synoviocytes, but not on dermal fibroblasts, BEB3 alone stimulated the secretion of IL-8, MMP-1 and PGE2, whereas the blocking mAb to TWEAK, BCB10, reduced the basal level of IL-8 and MMP-1, suggesting that synoviocytes secrete endogenous TWEAK. The latency of this activity – observed on synoviocytes whether stimulated with BEB3 alone or not – could indicate that a soluble TWEAK inhibitor may also be secreted by these cells. In dermal fibroblasts, BEB3-crosslinked hTWEAK was shown to increase the mRNA level of three chemokines of the CXC or CC families, IL-8, MCP-1 and RANTES. In synoviocytes, BEB3-crosslinked hTWEAK increased the mRNA level of IL-8, MCP-1, RANTES, IP-10 and MIP-1α. The delayed synthesis of these molecules, contrasting with TNF-induced stimulation, suggests that TWEAK could be relevant to the persistence of the inflammatory response by recruiting neutrophils and macrophages to the site of the inflammation.