Intravascular pulmonary macrophages: a novel cell removes particles from blood.

Clearance of radiolabeled gold colloid from the blood and relative organ distribution of retained colloid and magnetic iron oxide particles were compared between rats and calves. When unanesthetized calves 1 wk of age were injected intravenously with these two particle types, uptake was predominantly pulmonary. In contrast, in the rat there was overwhelming hepatic uptake. Similar pulmonary localization of injected particulate material was found in adult goats. When lung tissue was examined by electron microscopy, injected iron oxide was found within intravascular pulmonary macrophages. The mononuclear phagocyte system removes particulate material of both exogenous and endogenous origin from the circulation; hepatic and splenic uptake of such material usually predominates. We found that, unlike the species previously studied, ruminants have actively phagocytic cells within the pulmonary vasculature. Thus pulmonary intravascular macrophages constitute an important part of the mononuclear phagocyte system in some species.