Circulating immune complexes in patients with usual interstitial pulmonary fibrosis: partial characterization and relationship with Thermoactinomyces vulgaris.

Sixty-six sera were analysed by solid-phase conglutinin binding assay, to detect the levels of circulating immune complexes (CIC), and by enzyme-linked immunosorbent assay (ELISA) to show a correlation with antibodies to Thermoactinomyces vulgaris. Sixty per cent of patients with usual interstitial pulmonary fibrosis (UIP), were positive for CIC; and T. vulgaris antibodies were detected in 60% of the same patients. In comparison, there was a low frequency of positive results in bronchitis patients (5% for CIC and 35% for T. vulgaris), and in normal blood donors (0% for CIC and 30% for T. vulgaris). Furthermore 31% of patients with lung cancer were found positive for CIC, but not for T. vulgaris. Immune complexes purified on Protein A-Sepharose and by sucrose density gradient from patients with UIP, showed a sedimentation coefficient higher than 19 S. The purified material was found to contain IgG and IgM as antibodies. Binding of immune complexes, purified by sedimentation on sucrose gradient, to conglutinin was inhibited by the presence of T. vulgaris antigen; thus suggesting that this antigen might be present in the complexes.