The role of cytokines in human lung fibrosis

Fibrosis is a disorder characterized by a qualitative and quantitative alteration of the deposition of extracellular matrix with accumulation of mesenchymal cells in replacement of normal tissue. The sequence of events leading to fibrosis of an organ involves the subsequent processes of injury with inflammation and disruption of the normal tissue architecture, followed by tissue repair with accumulation of mesenchymal cells in this area. A similar sequence of events occurs in wound healing with formation of normal, limited and transient granulation tissue, while in fibrosis, a maladaptive repair leads to an extensive, exaggerated process with functional impairment. Inflammatory cells (mainly mononuclear phagocytes), platelets, endothelial cells, and type II pneumocytes play a direct and indirect role in tissue injury and repair. The evaluation of several human fibrotic lung diseases, five diffuse (idiopathic pulmonary fibrosis (IPF); adult respiratory distress syndrome (ARDS); coal workers' pneumoconiosis (CWP); Hermansky-Pudlak syndrome (HPS); systemic sclerosis (SS)) and two focal (tumour stroma in lung cancer; and obliterative bronchiolitis (OB) after lung transplantation), has shown that several cytokines participate in the local injury and inflammatory reaction (interleukin-1 (IL-1), interleukin-8 (IL-8), monocyte chemotactic protein-1 (MCP-1), and tumour necrosis factor-alpha (TNF-alpha)), while other cytokines are involved in tissue repair and fibrosis (platelet-derived growth factor (PDGF), insulin-like growth factor-1 (IGF-1), transforming growth factor-beta (TGF-beta), and basic-fibroblast growth factor (b-FGF)). A better understanding of the cytokines and cytokine networks involved in lung fibrosis leads to the possibility of new therapeutic approaches.