ENDOTHELIAL-DEPENDENT MECHANISMS OF LEUKOCYTE ADHESION: ROLE OF MONOKINES

Abstract

Localized adhesion of peripheral blood leukocytes to the vessel wall is an essential component of inflammatory reactions. There is increasing experimental evidence that vascular endothelial cells play an active role in this process. Our laboratory has been especially interested in defining endothelial-dependent mechanisms of leukocyte adhesion, and the role of leukocyte products in their modulation. We have reported1 that purified natural human monocyte-derived interleukin 1 (IL-1) can act directly on cultured human endothelial cells (HEC) to dramatically increase the adhesiveness of their surfaces for human polymorphonuclear leukocytes (PMN), monocytes and the related cell lines HL-60 and U937. This effect was concentration-, time- (onset≅30 min; peak≅4h) , and protein/RNA-synthesis-requiring, and, in selective pretreatment/fixation experiments, was shown to be mediated primarily through the endothelial cell. To better define this inducible endothelial pro-adhesive mechanism, we have developed a series of murine monoclonal antibodies directed against monokine-stimulated HEC surfaces. One of these antibodies (H4/18) recognizes an endothelial cell surface structure which is induced by IL-1 (and certain other cytokines)2 in a similar fashion (kinetics, concentration - dependence, sensitivity to metabolic inhibitors) as the pro-adhesive surface change for leukocytes. H4/18 partially blocks HD-60 cell adhesion to monokine-treated HEC, and, in vivo, labels human vascular endothelium at sites of experimental delayed hypersensitivity reactions4. A second monoclonal antibody (H18/7)5 significantly blocks the adhesion of both HL-60 cells and PMN to monokine-treated HEC. Monoclonal antibodies H4/18 and H18/7 appear to recognize the same inducible surface structure as assessed by immunoprecipitation of extracts of metabolically labeled, monokine-stimulated HEC. We have designated this monokine-inducible, endothelial-leukocyte adhesion molecule "E-IAM 1". IL-1 treated HEC cultures (in contrast to sham-treated control cultures) generate a soluble leukocyte adhesion inhibitor (LAI)6,7. LAI acts on PMN to inhibit their adhesion to hyperadhesive endothelial monolayers as well as to serum-coated plastic surfaces, but does not inhibit PMN activation by chemotactic stimuli (LTB4, f-met-leu-phe). IAI appears to differentially inhibit adhesion of peripheral blood leukocytes, isolated from the same donor, to hyperadhesive HEC (PMN > monocytes; lymphocytes, no effect), and does not inhibit HL-60 cell-HEC adhesion. Endothelial production of IAI is time-dependent (peak 5-6 h.), and blocked by cycloheximide but not by aspirin. Preliminary characterization indicates that LAI is nonsedimentable (250,000 xg, 45 min), nondialyzable (>10 kD), stable to heat (80°C, 30 min) and acid (pH 2) and is precipitable by ammonium sulphate (60-80% saturation). Thus, this endothelial-derived inhibitory activity, which appears to be distinct from PGI2 or other cyclooxygenase products, blocks leukocyte adhesion without globally suppressing leukocyte function. Further characterization of the cellular and molecular mechanisms regulating the endothelial expression of E-LAM 1 and LAI should contribute to our understanding of the active role of the vascular wall in the inflammatory process.1. Bevilacqua et al. (1985); J. Clin. Invest.76:2003.2. Cotran et al. (1986); J. Exp. Med. 164:661.3. Bevilacqua et al. (1987); Fed. Proc. (in press).4. Wheeler et al. (1986); Fed. Proc. 45:1725.5. Wheeler et al. (1987); Fed. Proc. (in press).