Background: Circulating immune complexes can be elevated in serum samples of patients with sarcoidosis and are associated with disease activity, but their diagnostic significance is not understood.
Methods: The different classes of circulating immune complexes containing immunoglobulin A, G, or M, and the content of complement in circulating immune complexes (polyethylene glycol precipitation) as well as levels of complement binding circulating immune complexes (complement binding assay) were determined in 19 patients with active, untreated pulmonary sarcoidosis. The results were compared with other parameters in the serum (soluble interleukin 2 receptor, angiotensin converting enzyme, immunoglobulin A, G, and M) and the bronchoalveolar lavage fluid (lymphocytes, helper cells, suppressor cells, activated T cells), and with radiological stage and functional parameters (FEV1, vital capacity, total lung capacity, transfer coefficient (KCO), and the alveolar-arterial oxygen difference during exercise).
Results: In all patients circulating immune complexes could be detected by polyethylene glycol precipitation and were similar to control subjects. The content of C1q in circulating immune complexes was higher than in controls, yet in all but one of the cases was still within normal limits. In contrast, elevated levels of complement binding circulating immune complexes were found in 67% of the patients. No correlation was seen between circulating immune complexes and any of the other parameters in the serum, bronchoalveolar lavage fluid, or lung function values. No differences were found between radiological type I and II presentations of sarcoidosis.
Conclusions: The complement binding assay showed a much higher sensitivity for the detection of circulating immune complexes in active pulmonary sarcoidosis than the polyethylene glycol precipitation method. As there was no correlation between levels of circulating immune complexes and other parameters of the disease they are probably not useful for the assessment of disease activity.