Inter-relationship between tumour necrosis factor-alpha (TNF-alpha) and TNF soluble receptors in pulmonary sarcoidosis.

Background: The importance of tumour necrosis factor-alpha (TNF-alpha) in the pathogenesis of pulmonary sarcoidosis has remained uncertain because of the paucity of clinical features associated with excessive levels of this cytokine. Increased levels of soluble TNF receptors (TNF-R), which are known to inhibit TNF-alpha activity, were recently described in the lungs of subjects with sarcoidosis. We hypothesised that TNF-alpha bioactivity may be inhibited in sarcoidosis by the presence of TNF-R. A study was therefore undertaken to investigate for the first time the relationship between soluble receptors and TNF-alpha bioactivity in the lungs of subjects with sarcoidosis.

Methods: Alveolar macrophages (AMs) from 16 subjects with histologically proven sarcoidosis and 13 healthy controls were cultured in the presence and absence of lipopolysaccharide (LPS). The subjects with sarcoidosis were grouped by radiological assessment into stage I (n = 6) and stage II/III (n = 10). The cell culture supernatants and bronchoalveolar lavage (BAL) fluid were assayed for TNF bioactivity using the WEHI 164 clone 13 assay. Immunoreactive (bound and free) TNF-alpha and free TNF-Rs (p55 and p75) were determined by ELISA.

Results: Bioactive TNF-alpha was undetectable in the BAL fluid of all the subjects with sarcoidosis and most of the healthy controls. However, there was significantly more immunoreactive TNF-alpha in the BAL fluid from subjects with sarcoidosis than from the controls (median values 0.304 ng/ml and 0.004 ng/ml, respectively, 95% CI 0. 076 to 0.455, p<0.001). The levels of both p55 and p75 in the BAL fluid were higher in both sarcoidosis groups than in the controls (p<0.0005 and p<0.001, respectively). In LPS stimulated AM supernatants reduced TNF-alpha bioactivity was seen in subjects with stage I sarcoidosis compared with those with stage II/III disease and healthy controls (median 0.333 ng/ml vs 1.362 ng/ml and 2.385 ng/ml, respectively, p<0.01). This contrasted with increased p55 levels in the AM supernatants derived from subjects with stage I sarcoidosis compared with those with stage II/III disease and healthy controls (median 0.449 ng/ml vs 0.058 ng/ml and 0.078 ng/ml, respectively, p<0.01). The levels of p75 were increased in unstimulated AM cultures in subjects with stage II/III disease compared with those with stage I disease and healthy controls (median 0.326 ng/ml vs 0.064 ng/ml and 0.102 ng/ml, p<0.05).

Conclusions: These results indicate that TNF-alpha bioactivity may be inhibited by increased soluble TNF-R in the lungs of subjects with sarcoidosis, and this inhibition may be greater in patients with stage I sarcoidosis than in those with stage II/III disease. This may represent a homeostatic mechanism which protects the lung from excessive TNF production characteristic of chronic inflammation.