[Pathogenesis of idiopathic interstitial pneumonia/idiopathic pulmonary fibrosis: cellular and molecular biology of the disease].

Idiopathic interstitial pneumonia (IIP), which is referred to as "idiopathic pulmonary fibrosis (IPF)" in US and European countries or "cryptogenic fibrosing alveolitis (CFA)" in UK, is considered to result from an uncontrolled, chronic inflammatory process in the lower respiratory tract and alveoli of the lung. It is conceivable that some hereditary background may contribute to the disease in susceptible individuals. In IPF, alveolitis involving activation of alveolar macrophages (AM), neutrophils, T- and B-lymphocytes and/or other inflammatory cell types precedes the injury to the lung parenchyma and interstitial fibrosis. Activation and accumulation of cellular components in the disease seem to be modulated by a variety of biological substances such as cytokines, chemotactic factors and adhesion molecules. Exposure to inorganic dusts or viral infection also may trigger activation of AM. The injury to the alveolar epithelium, capillary endothelial cells and extracellular matrix mediated by oxidants and/or proteases produced by AM and neutrophils results in the derangement of the lung parenchyma. Subsequent fibrotic change of the lung interstitium is characterized by the accumulation of mesenchymal cells such as fibroblasts and deposition of their connective tissue produces within the alveolar walls. In this process, AM is considered to play a major role by secreting growth factors for mesenchymal cells, although alveolar epithelial and endothelial cells also seem to participate in activating mesenchymal cell proliferation in IPF. Possibly, connective tissue synthesis and/or degradation may be altered in the process of IPF, although there is no direct evidence for this so far. Thus, IPF is a chronic inflammatory disease in which inflammatory processes of the alveoli injure the lung parenchyma and modulate the proliferation of mesenchymal cells which result in the fibrotic change. The pathogenesis of the disease involves multiple cellular components of the lower respiratory tract through modulation of gene expression and protein synthesis of a variety of biological factors.