Expression of decay-accelerating factor on synovial lining cells in inflammatory and degenerative arthritides.

The decay-accelerating factor (DAF) is a complement regulatory cell surface protein that protects cells from complement-mediated lysis. We analysed synovial tissue biopsies from patients with chronic arthritides for the presence of DAF using immunohistochemistry. DAF was expressed in the synovial lining cell layer both in rheumatoid arthritis (RA) and in osteoarthritis (OA). DAF was also on vascular endothelial cells of synovial tissue. A significant correlation was found between the expression of DAF and of HLA-DR in the lining layer, suggesting that DAF may be induced during a local inflammatory response. In addition, C5b-9 terminal complement complexes were found in several DAF-positive cases, suggesting that complement activation might, in itself, induce DAF expression. We propose that the occurrence of DAF may represent a physiological mechanism for local complement regulation in synovial tissue.