Update on Macrophage Clearance of Inhaled Micro- and Nanoparticles

Background: Lung macrophages, that is, the intravascular, interstitial, pleural, and surface macrophages, are part of the mononuclear phagocyte system. They are derived from the hematopoietic stem cell in the bone marrow with the monocytes as their putative precursors. Macrophages residing on the inner surfaces of the lungs and immersed within the lung lining layer, that is, the alveolar and the airway macrophages, are constantly exposed to the environment; it is those cells that are recognized as first line of cellular host defense. Methods and Results: Phagocytic uptake of inhaled and deposited particles is the main mechanism to remove insoluble micrometer-sized particles from the lung surfaces, where mucociliary transport, cough, or sneezing fail or are absent. Phagocytosis requires an intact cytoskeleton and is most efficient when mediated by Fc-receptors, but complement and scavenger receptors like MARCO and CD206 are just as important. The main pathway for the clearance of macrophage-associated particles is by mucociliary transport; to a lesser degree and species specific, particle-containing macrophages may reenter into the interstitium and go from there to the lymphatics. Inhaled nanometer-sized particles that deposit along the entire respiratory tract, however, are not efficiently phagocytosed by surface macrophages. Conclusions: Uptake by spontaneous or stimulated (macro-) pinocytosis or electrokinetic's phenomena may become more important. In addition, translocation of nanometer-sized particles into the interstitium and to the blood circulation brings them into contact with other fluids; altered particle properties may influence particle uptake. Moreover, translocated particles may interact with lung macrophage populations that were previously not considered of great significance for the clearance of inhaled particles.