Collagen neoepitopes in sarcoidosis: what do they tell us?

Fibrosis is scarring due to the replacement of tissue architecture by extracellular matrix (ECM), which consists largely of collagen. Scarring progressively destroys organ structure and thereby impairs function. Accumulation of collagen is thought to be due to an increase in production, a reduction in degradation of collagen or a combination of both.1 If so, we might expect the ratio of production to degradation to increase with increasing fibrosis. Biomarkers of type III and VI collagen production (PRO-C3 and PRO-C6) and degradation (C3M and C6M), called neoepitopes, have been developed to measure these processes.1 In diseases such as idiopathic pulmonary fibrosis, where fibrosis is the dominant pathophysiological feature, neoepitope levels align closely with disease progression.2 Whether these collagen markers are relevant in other diseases has been less well explored. In sarcoidosis, fibrosis in affected organs, especially the lungs and heart, accounts for most of the long-term morbidity and mortality that is directly attributable to sarcoidosis itself (rather than due to attempts to treat the disease). Fibrosis in sarcoidosis is thought to occur in the setting of persistent granulomatous inflammation but is likely modulated by other factors besides duration of disease alone since not all patients with chronic sarcoidosis develop substantial fibrosis. A major clinical challenge is to identify whether the dysfunction of an affected organ is predominantly due to granulomatous inflammation or fibrosis. Thus, a blood marker of sarcoidosis fibrosis would be an extremely helpful clinical tool. Recently, Sand and co-workers3 hypothesised that patients with sarcoidosis may have increased plasma levels of collagen neoepitopes and that …