Pulmonary intravascular macrophages in sheep. Morphology and function of a novel constituent of the mononuclear phagocyte system.

Macrophages resident in the pulmonary capillaries of sheep avidly remove injected particles from the circulating blood. Both sheep and rats were injected intravenously with radiolabeled gold colloid and magnetic iron oxide particles. One hour later, particle uptake in various organs was quantified by gamma counting and magnetometry. Organ localization of both gold and iron oxide particles was predominantly hepatic in rats. In marked contrast, sheep had predominantly pulmonary uptake. Ultrastructural morphology showed that pulmonary iron oxide uptake was by intravascular macrophages. Pulmonary intravascular macrophages were present in ruminant lungs in large numbers. Lungs of sheep given no particles were fixed by intratracheal instillation of glutaraldehyde; randomly chosen tissue samples were routinely processed for electron microscopy and studied with stereological methods. We found that these macrophages occupied 15.3% of the intravascular volume, and had 15.9 m2 of free surface available for contact with blood. Intravascular macrophages were closely applied to 7.1% of the endothelial surface, including numerous short segments with 12 to 15 nm of membrane interspace, increased subplasmalemmal cytoplasmic density, and intercellular electron-dense material. We conclude that pulmonary intravascular macrophages in sheep comprise an important component of their mononuclear phagocyte system. Furthermore, we suggest that these macrophages, through phagocytic uptake of bacteria or endotoxin, may contribute to pulmonary inflammation and injury.