Hypoxemia regulates effect of lipopolysaccharide on polymorphonuclear leukocyte CD11b/CD18 expression

Expression of the receptor CD11b/CD18 on the polymorphonuclear leukocyte (PMN) surface is important for several aspects of PMN function during endotoxemia. The mechanisms underlying regulation of CD11b/CD18 expression during hypoxemia and endotoxemia, however, are less clear. We investigated the effects of exposure of whole blood PMNs to lipopolysaccharide (LPS) during hypoxemia. During hypoxemia (10-30% O2 saturation), LPS reduced CD11b/CD18 expression. Both kinetic assays and experiments with microfilament stabilizers (phalloidin, cytochalasin B) demonstrated that this was most likely due to receptor shedding. Incubation of whole blood PMNs with an anti-CD14 monoclonal antibody (MEM18) largely prevented the LPS-induced reduction of CD11b/CD18 expression. Decreased CD11b/CD18 expression reduced PMN functional capability, as the binding of its ligand (erythrocytes opsonized with the 3rd component of complement Cbi) and intracellular H2O2 production were reduced. After exposure to LPS, N-formyl-methionyl-leucyl-phenylalanine could rapidly induce new CD11b/CD18 receptors to the cell surface, and this was not inhibited by actinomycin D or cycloheximide. After reoxygenation (> 90% O2 saturation), CD11b/CD18 expression was restored, and this was abrogated by exposure to cytochalasin B. Lipid A was able to reproduce the effects of LPS during hypoxemia and hypoxemia-reoxygenation but required a 10-fold greater concentration to do so. These results demonstrate that during hypoxemia an important pathophysiological property of LPS is to reduce CD11b/CD18 expression. This results in diminished PMN functional capability, which would contribute to the pathogenicity of LPS during acute infectious states.