Movement of receptors and ligands through the endocytic apparatus in alveolar macrophages.

The ability of alveolar macrophages to remove potentially injurious agents is essential for normal lung function. One mechanism by which macrophages affect this function is by the internalization and clearance of noxious agents from lung fluid. Extracellular macromolecules may be removed as a result of nonspecific uptake via fluid-phase pinocytosis or as a result of binding to surface receptors followed by a highly efficient internalization of the receptor-ligand complex. The ability of macrophages to bind, internalize, and thus sequester extracellular macromolecules is facilitated by the fact that membrane components involved in this process may be reutilized. That is, once internalized, membrane components are not invariably degraded but can be recycled for further use. In this commentary we examine some of the features known about the recycling process in both macrophages and other cell types. We demonstrate that although the rate of internalization of a membrane component is unique to that component, the rate of movement of internalized components within cells is a function of the endocytic apparatus and is independent of the ligands or receptors contained in the internalized vesicle. We also demonstrate how alterations in the rates of recycling may result in changes in the distribution of membrane proteins expressed on the cell surface and affect the ability of a cell to respond to the environment.