Angiogenic Cytokines and Fibrinolytic Activity in Parapneumonic Effusions

Formation of parapneumonic effusions (PPE) involves increased pleural vascular permeability induced by the contiguous pneumonia. It has been demonstrated that exposure of pleural mesothelial cells to bacteria or lipopolysaccharide (LPS) leads to increased release of angiogenic factors, including vascular endothelial growth factor (VEGF) and interleukin (IL)-8, which induce vascular hyperpermeability, fluid exudation, and neutrophil influx into the pleural space, and may play a pivotal role in development of PPE. With persistent inflammation and angiogenesis, amplified vascular and mesothelial permeability leads to increased plasma extravasation, activation of the coagulation cascade, and repression of fibrinolytic activity within the pleural cavity, which contribute to the development of a ''complicated'' PPE, manifested with fibrin deposition and pleural fluid loculation. Fibrin turnover in the pleural cavity is greatly affected by fibrinolytic activity mediated by plasmin, which is regulated mainly by the equilibrium between plasminogen activators (PAs) and plasminogen activator inhibitors (PAIs).VEGF induces vascular hyperpermeability and may facilitate the genesis of fibrin gel in PPE. Previous studies reported that VEGF plays a role in the modulation of tPA and PAI-1, and that anti-VEGF antibody attenuates pleurodesis induced by transforming growth factor-β2. These findings suggest that VEGF may be involved in the regulation of fibrin turnover, fluid loculation and tissue fibrosis in the pleural space. Enhanced procoagulant and depressed fibrinolytic activities have been observed in PPE. However, the relationship between angiogenic cytokines and fibrinolytic activity in PPE remains unclear.