The cell biology and pathogenic role of pulmonary intravascular macrophages.

Pulmonary intravascular macrophages (PIMs) are an extensive population of mature phagocytic cells adherent to the pulmonary capillary endothelium in selected species. They are not prevalent in lungs of commonly studied laboratory animals, such as rodents, and thus have only been recently appreciated. However, their potential role in host defense and acute lung injury has attracted interest, since a number of studies have demonstrated pulmonary localization of circulating particles, microbes, and endotoxin by PIMs. Those animal species, such as ruminants, that provide useful models of pathogen (or endotoxin)-induced acute lung injury demonstrate rapid pulmonary uptake of bacteria by PIMs. Inflammatory mediators released by activated PIMs may initiate the process and provoke accumulation of neutrophils and platelets. This review summarizes the morphological characteristics of PIMs and their species distribution. The role of these members of the mononuclear phagocyte system, both beneficial and potentially pathogenic, is reviewed. The question of whether PIMs have a role in acute lung injury in humans is also discussed.