Macrophage Damage in Relation to the Pathogenesis of Lung Diseases

Abstract

Pulnonary macrophages are important since their migratory patterns and behavior are often pivotal events in the pathogenesis of pulmonary disease. Alveolar macrophages act to decrease the probability of particle penetration through epithelial barriers, and their phagocytic and lytic potentials provide most of the known bactericidal properties of the lungs. Macrophages are also involved in immune responses and in defense against neoplasms. Increased inert or infectious particles stimulate the recruitment of additional macrophages. Most free cells containing particles eventualy reach the airways and are quickly carried to the pharynx and swallowed. In addition, evidence has now accumulated that macrophages play a part in the pathogenesis of pulmonary diseases. For example, the ingestion of some particles by macrophages causes a release of lysosomal enzymes into the macrophage cytoplasm. These enzymes may kill the macrophage, and dead or dying macrophages release a substance which attracts fibroblasts that elicit fibrogenic responses. Other toxic particles, such as cigarette smoke, may lead to a release of proteases and other toxic enzymes. All particles are capable of competitive inhibition of phagocytosis in macrophages and many may be cytotoxic and further depress phagocytosis. In addition, connective tissue macrophages may contribute to lung disease by concentrating and storing potent carcinogens or other toxic particles close to a reactive bronchial epithelium for long periods. Thus, even though macrophages serve as a first line of defense for the alveolar surface, they may also be capable of injuring the host while exercising their defensive role.