Granulocyte-macrophage colony-stimulating factor (GM-CSF) causes upregulation of neutrophil surface CD11b/CD18 expression, and enhances the adhesion of neutrophils to cultured human endothelial cells in vitro. Systemic administration of GM-CSF results in a rapid, transient decrease in circulating phagocyte numbers. Using a nonhuman primate model (Cynomolgus), we provide histologic evidence that this transient leukopenia is associated with the margination of neutrophils in the pulmonary microcirculation. In four animals receiving 2 to 15 micrograms/kg recombinant human GM-CSF (rhGM-CSF), light microscopic sections of lung contained 36 +/- 8, 17 +/- 7, 21 +/- 6, and 15 +/- 8 (mean +/- SD, n = 20) neutrophils within a graticule grid, as compared with two control animals receiving saline injections whose lung sections contained 2.1 +/- 1.6 and 3.1 +/- 2.1 (mean +/- SD, n = 20) neutrophils within the same grid. Scanning electron microscopy shows activated leukocytes adherent to pulmonary vascular endothelium, but no morphologic evidence of endothelial damage, and no migration of cells into the extravascular space. Margination is associated with an increase in surface expression of CD11b/CD18 on circulating phagocytes, which could contribute to the adhesion to capillary endothelial cells, but CD11b/CD18 levels remain elevated even when demargination is complete. In vitro, monoclonal antibodies (MoAbs) to CD18 and CD11b were able to inhibit neutrophil aggregation and adhesion to endothelium. FMLP-induced neutrophil aggregation was inhibited by 39.8% +/- 11.5% and 44.8% +/- 12.3%, respectively, by MoAbs to CD18 and CD11b (P less than .0005, n = 4 for both); a similar effect was demonstrated on TPA-induced aggregation. MoAb CD18 reduced the adhesion of unstimulated neutrophils to endothelium by 44% (P less than .01, n = 7), and inhibited the amount of GM-CSF-stimulated adhesion by 74% (P less than .001, n = 7), while MoAb to CD11b produced a reduction of unstimulated neutrophil adhesion by 30%, and of GM-CSF-stimulated adhesion by 40% (P less than .01, n = 5, for both). However, when administered in vivo, MoAb CD18 produced only a small, albeit significant, amelioration of GM-CSF-induced margination in vivo, while MoAb CD11b was without effect. These results show that GM-CSF-induced transient leukopenia is associated with enhanced neutrophil adherence to pulmonary vascular endothelium, but suggest that the beta 2 leukocyte integrins CD11/CD18 play only a minor role in this process.

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