To test the hypothesis that idiopathic pulmonary fibrosis (IPF) is mediated through collagenase present in the lower respiratory tract, we used the fiberoptic bronchoscope to obtain fluid from the lower respiratory tract of 24 patients with IPF, 18 controls and nine patients with sarcoidosis. The fluid was analyzed for a variety of enzymes, including collagenase. Fifteen of 21 patients with IPF showed collagenase activity, whereas normal controls and patients with sarcoidosis showed none (P<0.001, for all comparisons). In two patients with IPF who were re-evaluated after eight to 24 months, the collagenase activity was persistent. Fluid from patients with IPF also contained elevated levels of a nonspecific neutral protease (P<0.01 compared with controls), but there was no elastase activity in fluid from patients with IPF or from controls. The collagenase found in lavage fluid in IPF cleaved lung collagen into collagenase-specific TCA and TCB fragments. We conclude that in IPF the collagen of the lung is subjected to sustained lysis, followed by disordered resynthesis, and that the presence of active collagenase in the lower respiratory tract is a specific feature of the alveolitis associated with this disease. (N Engl J Med 301:737–742, 1979) IDIOPATHIC pulmonary fibrosis (IPF) is a usually fatal disorder of lung characterized by chronic inflammation of the alveolar structures (“alveolitis”) and progressive interstitial fibrosis.1 2 3 4 5 6 7 8 9 Although the alveolitis of IPF may have some direct influence on the ability of gas-exchange units to function normally, its major role in the pathogenesis of the disease is in inciting the fibrotic process.4,5,8,9 The major importance of the fibrotic process in this disease is the effect it has on deranging the alveolar structures and the resulting impairment of gas exchange.3,4,8,9 Early studies of the fibrotic process of IPF assumed that it deranged gas-exchange units simply.
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