To investigate the expression of the stroma cell product stem cell factor (SCF) in synovial fibroblasts (SFB) in patients with rheumatoid arthritis (RA) and osteoarthritis (OA), and to analyze the capacity of SFB to induce mast cell (MC) chemotaxis.
Synovial tissue was obtained from 29 patients with RA and 25 patients with OA. Tissue was dispersed by enzymatic digestion using collagenase. SFB were grown in serial passage and exposed to tumor necrosis factor alpha (TNFalpha) or control medium. Expression of SCF in cultured SFB was analyzed by reverse transcription-polymerase chain reaction (RT-PCR), enzyme-linked immunosorbent assay (ELISA), and immunostaining. The ability of SFB (supernatants) to induce MC migration was analyzed using a double-chamber chemotaxis assay and the human mast cell line HMC-1. In situ expression of SCF in synovial tissue from patients with RA (n = 6) and OA (n = 6) was examined by double immunohistochemistry using antibodies against SCF and the fibroblast-specific antibody AS02.
In both RA and OA, cultured SFB were found to express SCF messenger RNA, as assessed by RT-PCR. In addition, the SCF protein was detectable in cell lysates and supernatants of SFB by ELISA. Incubation of SFB with TNFalpha resulted in an increased expression and release of SCF. Recombinant human SCF (rHuSCF) and SFB supernatants induced significant migration of HMC-1 cells above control levels. In addition, exposure of SFB to TNFalpha led to an increased migration of HMC-1, and a blocking anti-SCF antibody inhibited the rHuSCF- and SFB-induced migration of HMC-1. In situ double immunostaining revealed expression of SCF in AS02-positive SFB in the synovium of patients with RA.
Our results show that SFB (in RA and OA) express SCF and induce MC chemotaxis. Furthermore, TNFalpha was found to augment SCF expression in SFB. It is hypothesized that these cellular interactions play an important role in MC accumulation and related events in RA.