Kim E. Longworth, Ph.D.

Department of Surgical and Radiological Sciences, School of Veterinary Medicine University of California, Davis, CA

Received 4/11/97; Accepted 5/21/97

6. PERSPECTIVE

Experimental studies suggest that pulmonary intravascular macrophages have a dominant role in the mononuclear phagocyte system of mammals in the orders Artiodactyla and Perissodactyla. These cells are involved in clearance of particles and debris from the circulation and the immune response against blood-borne and airborne pathogens. It is less clear what their importance is to animals in the natural environment. Although various hypotheses have been proposed to account for the presence of these macrophages, it is uncertain why such a substantial and distinctive population has evolved in some species and not others (4).

Much of the current research focuses on whether pulmonary intravascular macrophages are induced in humans in certain pathological conditions. Humans normally have few intravascular macrophages; foreign particles in the circulation are localized in the liver, phagocytized by Kupffer cells. However, gram-negative septicemia or endotoxemia in humans often leads to acute lung injury (Adult Respiratory Distress Syndrome, or ARDS) and it not understood how pulmonary inflammation develops from systemically introduced pathogens. Sheep and pigs are both used as experimental models for ARDS in humans; possibly, humans develop intravascular macrophage-type cells in the lung after endotoxemia or liver injury, making their pulmonary circulations behave more like those in species with resident intravascular macrophages. To determine if this is possible, workers are attempting to induce a pulmonary intravascular macrophage population in species without the cells by chronic endotoxin infusion (4, 5, 7, 68).