NEUTROPHIL SEQUESTRATION IN RAT LUNGS

Background - The transit of neutrophils through the pulmonary microvas culature is prolonged compared with red blood cells and is increased f urther during cigarette smoking and in exacerbations of chronic obstru ctive pulmonary disease. The increased residence time (sequestration) of neutrophils in the pulmonary capillaries in these conditions may be the first step leading to the accumulation of cells within the lung i nterstitium and in the bronchoalveolar space, so potentiating lung dam age. A rat model has been developed to investigate the factors which m ay influence neutrophil transit through the lung microvasculature. Met hods - Intratracheal instillation of the heat killed organism Coryneba cterium parvum was used to induce an acute neutrophil alveolitis. Neut rophils and red blood cells were isolated from donor rats, labelled wi th two distinct radioisotopes, and then reinjected into recipient rats to assess their transit through the pulmonary circulation. To ascerta in whether peripheral blood neutrophils were minimally altered by the isolation procedure their functional status in vitro was compared with that of inflammatory neutrophils in a number of assays commonly used as descriptors of neutrophil activation. The influence of neutrophil a ctivation on the accumulation of cells in the lungs was assessed by co mparing the lung sequestration of control neutrophils, isolated from p eripheral blood, with that of inflammatory neutrophils obtained from b ronchoalveolar lavage of inflamed rat lungs. Lung sequestration of neu trophils was defined as the fold increase in the ratio of neutrophils labelled with chromium-51 to red blood cells labelled with technetium- 99m in lung tissue compared with the same ratio in peripheral blood. R esults - Sequestration of peripheral blood neutrophils occurred in con trol rat lungs as shown by a 17.5 (2.1) fold increase in the ratio of neutrophils to red blood cells in the pulmonary circulation compared w ith the ratio of these cells in the peripheral circulation. When infla mmatory neutrophils, obtained by bronchoalveolar lavage from C parvum- treated animals, were injected into control rats, the increase was 90. 6 (11.0) fold. Induction of an inflammatory response in the lung tissu e of the recipient rat also caused an increase in the sequestration of control neutrophils compared with the same cells in control rat lungs which was, however, less marked than when inflammatory neutrophils we re used (34.7 (4.7) fold). The mean (SE) pressure developed on filtrat ion of inflammatory neutrophils in vitro through a millipore filter (7 .53 (0.2) cm H2O) was greater than that of peripheral blood neutrophil s (1.18 (0.2) cm H2O). Increased filtration pressure indicates a decre ase in cell deformability and suggests that this may be a contributory factor to the increased sequestration of inflammatory neutrophils in the pulmonary vasculature. Conclusions - This study shows that there i s sequestration of neutrophils in the pulmonary vasculature in normal rat lungs which increases in acute lung inflammation and when inflamma tory neutrophils are injected into control animals. In this model chan ges in the neutrophil, such as cell deformability, may have a more imp ortant role in inducing increased neutrophil sequestration than the in flammatory response in the lungs.