Uptake of Bloodborne Bacteria by Pulmonary Intravascular Macrophages and Consequent Inflammatory Responses in Sheep

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Abstract
Bacteria are primarily removed from the bloodstream of most species by mononuclear phagocytic cells in the liver and spleen. We have recently described large numbers of pulmonary intravascular macrophages in several ruminant species that also remove inert particles from the bloodstream. To determine the role of these cells in removal of bacteria from the bloodstream, sheep and rats were injected intravenously with a single dose of Pseudomonas aeruginosa, and kinetics of clearance, relative uptake among organs, and morphologic changes in the lungs and liver 1 h after injection were compared. In rats, bacteria were removed primarily by the liver, and pathologic changes were found in hepatic sinusoids but not in the lungs. However, in sheep, bacteria were removed primarily by the lungs, and pulmonary capillaries became filled with neutrophils, platelets, and fibrin deposits. We propose that uptake of bacteria by pulmonary intravascular macrophages and subsequent release of inflammatory mediators are central to the pathologic changes produced in ruminant models of sepsis-induced adult respiratory distress syndrome.