Functional heterogeneity in liver and lung macrophages

Although initially considered merely "scavenger cells" that participate in immunologic responses only after B and T lymphocytes have performed their b iological tasks, more recent evidence suggests that macrophages play a key role in host defense as well as in the maintenance of normal tissue structu re and function. For macrophages to perform their biological functions, the y must be activated. This involves up-regulation of an array of signaling p athways resulting in altered gene expression and increased biochemical and functional activity. Macrophages have been identified in almost all tissues of the body. However, the basal activity of these cells, as well as their ability to respond to inflammatory mediators, varies considerably with thei r location. In addition, even within a particular tissue, there is evidence of macrophage heterogeneity. The largest populations of macrophages in the body are located in the liver and lung. Because of the unique attributes o f these tissues, hepatic and pulmonary macrophages play essential roles not only in nonspecific host defense but also in the homeostatic responses of these tissues. In this review, the functional and biochemical activities of macrophages localized in the liver and lungs are compared. Evidence sugges ts that these represent distinct cell populations with unique functions and responsiveness to inflammatory agents.