Prevalence of circulating immune complexes in patients with cystic fibrosis and arthritis.

We report the clinical and laboratory characteristics of eight patients with cystic fibrosis (CF) and arthropathy. Five patients had frequently relapsing, episodic arthritis; one had chronic arthritis with high titers of rheumatoid factors that could not be distinguished from juvenile chronic arthritis. In two patients the arthropathy did not relapse. Forty-six patients with CF but without arthropathy, seen during the same 1-year period, served as control subjects. Results of lung function tests and sputum cultures were similar in both groups. The sera of six of the eight patients with CF-arthropathy had high levels of circulating immune complexes, measured both by C1q-binding and the indirect granulocyte phagocytosis test, whereas only 1 of 12 control patients tested had positive C1q binding (p < 0.023). Serum complement levels were normal in both groups. Synovial biopsies, performed in three patients, showed only scanty lymphocytic infiltrates; immunofluorescent staining showed deposits of IgM, IgG, and components of complement C1q, C3, and C4. These findings suggest that the arthropathy found in CF is an immune-mediated phenomenon.