To investigate the presence of soluble HLA class I (s-HLA) antigens in serum and synovial fluid (SF) from a large cohort of rheumatic patients.
We studied clinical and analytical data and serum samples from 300 patients [122 patients with rheumatoid arthritis (RA), 38 with osteoarthritis or osteoporosis, 29 with seronegative spondyloarthropathies, 45 patients with other rheumatic diseases] and 66 healthy controls. In addition, we studied 25 paired samples of serum and SF from these groups of subjects. In RA patients, we examined whether the levels of s-HLA in serum and SF were related to the activity of the disease.
The mean concentrations of s-HLA molecules in serum were slightly higher in RA patients (1.2 microg/ml) than in the other four groups (1.08, 1.01, 1.09 and 0.94 microg/ml respectively). We found no correlation between serum s-HLA levels and any variable of inflammatory disease activity in RA patients. s-HLA molecules were found in SF and at levels that correlated with those found in serum (P=0.04; r=0.4). Furthermore, s-HLA levels were higher in SF from patients with RA (1.3 microg/ml) or crystal-induced arthritis (0.98 microg/ml) than in SF from those with osteoarthritis (0.38 microg/ml) (P<0.05 and P<0.005 respectively), and these levels were correlated inversely and significantly with the score on the visual analogue scale of pain (P=0.02), the number of painful joints (P=0.05) and the level of C-reactive protein (P=0.03) in RA patients.
This is the first report to demonstrate the presence of s-HLA molecules in SF at levels that correlate with serum levels. The mean levels of s-HLA molecules were significantly higher in SF from patients with RA and crystal-induced arthritis than in SF from cases of osteoarthritis, and correlated inversely with certain variables of disease activity in RA patients.