Because nitric oxide related species have been found in the inflamed joints of patients with arthritis, we investigated whether protein nitrotyrosine (a marker of tissue exposure to peroxynitrite) is present in their synovial tissues.
Protein nitrotyrosine was detected immunohistochemically and by Western blot analysis. Synovial tissues removed surgically from 12 patients with rheumatoid arthritis (RA) (mean age 63.7 yrs) and 20 with osteoarthritis (OA) (mean age 66.6 yrs) were studied.
Nitrated proteins were detected immunohistochemically in all of 18 tissues examined. Diffuse staining of the stroma was seen in all patients, with more extensive staining in RA than OA (p = 0.008). Intense staining was detected in some lymphocytes, but not in others, even within a single lymphoid aggregate. Neutrophils did not stain for nitrotyrosine. Vascular endothelial cells stained for nitrotyrosine but adjoining smooth muscle cells did not. Both cytoplasmic and nuclear staining was seen in macrophages, endothelial cells, and lymphocytes. Numerous bands of nitrated proteins were detected by Western blot analysis of 15 synovial tissue extracts. Inducible nitric oxide synthase (iNOS) was detected immunohistochemically in endothelial cells, macrophages, vascular smooth muscle cells, and synoviocytes.
Nitrotyrosine-containing proteins were found in essentially all synovia from RA and OA patients. The most prominent site of nitration in all cases was the stroma. iNOS, the likely source of the nitrating species, was found in a variety of cell types.