Pulmonary removal of circulating endotoxin results in acute lung injury in sheep

Endotoxemia has often been associated with the development of the adult respiratory distress syndrome. Our previous studies have shown that sheep, a popular animal model for adult respiratory distress syndrome, have abundant resident pulmonary intravascular macrophages that rapidly remove inorganic particles and live bacteria from the circulating blood. In this study, we examine the fate of circulating endotoxin in sheep and correlated the site of uptake with early morphologic evidence of tissue injury. Mature sheep and rats received intravenous 125I-labeled lipopolysaccharide (LPS). The dose was 0.8 microgram/kg in sheep and 17.0 micrograms/kg in rats. 125I-LPS clearance from the blood was assayed by gamma-counting of blood samples drawn over 1 hour and was correlated with circulating leukocyte numbers. The distribution of 125I-LPS was determined by gamma-counting of samples of major organs and tissues at time of necropsy. Lungs and liver were examined morphologically in both species. The half-life of circulating 125I-LPS was 2.38 minutes in sheep, and 12.39 minutes in rats. The endotoxin content of the lungs after injection was 77.58% of the total recovered dose in sheep, but only 2.02% in rats. Neutrophil margination occurred in the lungs of both species. In sheep, almost 25 minutes elapsed before peripheral neutrophil numbers decreased by 50%, much longer than the time required for LPS sequestration in the lungs. Rapid LPS uptake by the sheep lungs was associated with early (10-minute) ultrastructural changes including signs of pulmonary intravascular macrophage activation and microvascular congestion. By 60 minutes, many capillaries were occluded with neutrophils, platelets, and fibrin. There was interstitial edema, and endothelial cells showed evidence of severe injury. We conclude that in contrast to the rat, the sheep clears circulating endotoxin more rapidly and preferentially sequesters it in the lungs. Subsequent release of mediators by activated pulmonary intravascular macrophages may then lead to influx of other inflammatory cells and cascading injury.