Decreased leukocyte adhesion with anti-CD18 monoclonal antibodies is mediated by receptor internalization.

Adhesion of polymorphonuclear leukocytes (PMNs) to endothelial cells is mediated partially by CD11/CD18 integrins. The purpose of this study was to define (1) the response of PMNs to anti-CD18 monoclonal antibody binding, and (2) the mechanism responsible for anti-CD18 monoclonal antibody-mediated decreases in PMN adhesion to endothelial cells. Canine PMN O2- production, myeloperoxidase, and lysozyme release in response to the anti-CD18 monoclonal antibody IB4 were measured by standard assays. To examine endocytosis of CD18 receptors, PMNs incubated with IB4 and a fluorescein isothiocyanate secondary antibody were analyzed by flow cytometry. Treatment of PMNs with IB4 did not stimulate O2- production or degranulation but decreased adhesion of 51Cr-labeled PMNs to ex vivo canine aorta. Incubation of PMNs at 25 degrees C resulted in a decrease in fluorescence intensity that was not affected by NaN3 or vanadate but was blocked by NaF, 4 degrees C, and bafilomycin, which prevents endosomal acidification. Treatment with an antifluorescein antibody decreased the fluorescence intensity in NaF and 4 degrees C, but not in bafilomycin-treated neutrophils. IB4 decreases PMN-endothelial cell adhesion but does not stimulate neutrophil oxidative metabolism or degranulation. These data suggest that reduced adhesion may be the result of internalization of the CD18/IB4 complex. Anti-CD18 monoclonal antibodies may be useful in preventing PMN adhesion without the potentially deleterious effects of cell activation.