Volume 14 Supplement 1
Synovial fibroblasts display an uncontrolled inflammatory and tissue destructive response to TNF-α
© Kalliolias et al.; licensee BioMed Central Ltd. 2012
Published: 29 February 2012
Synovial fibroblasts are key players in the pathogenesis of Rheumatoid Arthritis (RA) and potentially attractive treatment targets. Upon activation within the joint's inflammatory milieu, they gain a transformed phenotype and produce pro-inflammatory cytokines (mainly IL-6) and tissue destructive enzymes .
Materials and methods
Synovial fibroblasts were isolated via enzymatic processing from synovial tissues obtained from patients with RA or Osteoarthritis (OA). Synovial fibroblasts (passages 2-4) were stimulated with TNF-α (10 ng/ml) only on day 1. The expression of TNF-α-target genes was measured by qPCR in time course experiments (1, 3, 6, 24, 48, 72, 96 and 120 hours after TNF-α stimulation).
Human macrophages (Mϕ) generated in vitro (blood derived CD14+ cells stimulated for 48 h with M-CSF) were used in similar time course experiments as controls.
In Mϕ it was observed a rapid (within 1-3 hours) induction of TNF-α-target genes (including TNF-a, IL-1β, IL-6 and IL-8) that was restrained back to the baseline within a few hours (3-24 hours depending on the gene). In stark contrast, synovial fibroblasts displayed a remarkably more sustained response to TNF-α. IL-6 mRNA expression was induced within a few hours by TNF-α, and induction increased continuously for 72-96 h despite the absence of any further exogenous TNF-α stimulation. The levels of IL-6 mRNA induced by TNF-α in synovial fibroblasts were substantially higher compared to human Mϕ, suggesting that within the joint microenvironment, synovial fibroblasts and not Mϕ are the main source of IL-6. By adding the supernatants from 96 h TNF-α-stimulated fibroblast cultures on unstimulated synovial fibroblasts, a similar robust induction of IL-6 mRNA was observed, suggesting that there is a TNF-α-induced soluble factor that mediates the sustained response. A similar pattern of sustained expression was observed for other TNF-α-target genes including IL-1β, IL-8 and MMPs. Interestingly, there was no difference between OA- and RA-derived synovial fibroblasts in their response to TNF-α.
In contrast to human Mϕ, synovial fibroblasts display a sustained inflammatory and tissue destructive response to TNF-α. Our observations suggest that synovial fibroblasts may lack the homeostatic mechanisms that control and terminate the effects of TNF-α on human Mϕ . To support this hypothesis, further investigation is needed at the level of proximal and distal TNF-α signaling events and at the level of epigenetic regulation of TNF-α-target genes in synovial fibroblasts.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.