Interactions of granulocytes with the lungs.

Under normal conditions, there is a sizeable pool of marginated granulocytes in the lung circulation which is in dynamic equilibrium with the circulating granulocyte pool. The number of granulocytes in the lungs' microcirculation may depend on pulmonary blood flow or biochemical interactions between granulocytes and pulmonary vascular endothelium, or both. There is some evidence that normal lung function may be affected by granulocytes. Several acute and chronic diseases may result, at least in part, from interactions of granulocytes with the lungs. Acute diffuse lung injury (adult respiratory distress syndrome) is characterized by diffuse pulmonary inflammation, and, in animal models, some of the lung dysfunction depends on the presence of granulocytes. Bronchoconstriction and airway hyperreactivity, characteristic of asthma, may be influenced by granulocyte-generated products of arachidonic acid. Granulocyte-derived proteases and oxidants may contribute to the pathogenesis of pulmonary emphysema and may affect connective tissue synthesis in interstitial pulmonary fibrosis. There is some evidence suggesting a connection between granulocytes and chronic pulmonary hypertension. The fact that some interventions which cause pulmonary leukostasis do not cause severe, persistent lung injury indicates that as yet unknown factors may determine whether interactions of granulocytes with the lungs are benign or pathological. Such factors could include the generation of humoral substances, and metabolites of arachidonic acid are particularly interesting in this regard. Research related to interactions of granulocytes with the lungs suggests strongly that such interactions are integral to the pathogenesis of several lung diseases. Understanding those diseases will require further basic studies of granulocyte behavior and the modes of communication between cells intrinsic to the lung and granulocytes.