Immunohistochemical detection of factor XIIIa and factor XIIIs in synovial membranes of patients with rheumatoid arthritis or osteoarthritis

In spite of differences in etiology, RA and OA lead to astonishingly similar synovitic alterations. Fibroblastic transformation of the synovial membrane and an increase in monocytes constitute a rare but highly characteristic feature of RA. Monocytes synthesize factor (F) XIII, implying that FXIII (a and s) in synovial tissue might help to differentiate between RA and OA. Biopsies were obtained at open surgery from 98 unselected patients with the clinical diagnosis of RA (n=54) or OA (n=44). In a three-stage (ABC) immunoperoxidase technique, polyclonal antisera against factor XIIIa and factor XIIIs were investigated. Compared to OA sections, RA synovium showed more FXIIIa-positve cells - monocytes, fibrocytes, fibroblasts and synovial lining cells. In the subsynovial layer, band-like structure of FXIIIa-stained cells was observed in 27.8% of the RA patients, but in only one OA specimen. Higher proportions of FXIIIa-positive monocytes, macrophages, histiocytes and fibroblasts, as well as positive Langhans' giant cells and vascular wall regions (except endothelial cells), were observed in RA. OA specimens revealed more intense FXIIIa labeling of these cells with a lower percentage of stained cells. Overall, labeling with FXIIIs antibody resulted in less intense staining. In conclusion, distinction between synovitis caused by RA and synovitis due to OA is possible, as the former show higher numbers of FXIIIa-positive cells, including monocytes, fibroblasts, fibrocytes and synovial lining cells. Furthermore, RA tissue is stained less intensely than OA tissue. There is evidence for continuous excretion of FXIII in the synovial membrane by the above-mentioned cell systems.