Our study was designed to investigate the surface molecules involved in the adhesion and cytotoxicity of activated human monocytes on resting and IL-1-stimulated endothelial cells (EC). Monocytes, exposed to the prototypic activating stimuli IFN-gamma and LPS, showed increased binding to resting and IL-1-treated EC. Activated monocytes were cytotoxic for resting and IL-1-treated EC in a 24- to 48-h [3H]TdR release assay. Anti-CD18 mAb significantly inhibited binding of monocytes on EC: in particular they caused 59 and 22% inhibition of adhesion of activated monocytes to resting and IL-1-stimulated EC, respectively. Anti-VLA4 mAb had little or no effect on binding when used alone, but combined use with anti-CD18 revealed an important role for this adhesion pathway: in particular, VLA4-dependent adhesion accounted for 40% of the binding of activated monocytes on IL-1-treated EC. Anti-CD18 mAb caused similar inhibition (77 and 81%) of the cytotoxicity of activated monocytes on resting and IL-1-treated EC in spite of the fact that this pathway accounted for only 22% of binding to activated EC. Moreover, anti-VLA4 mAb, alone or in combination with anti-CD18, had no effect on cytotoxicity. These results suggest that adhesion of activated monocytes to activated EC involves the CD18- and VLA4-dependent pathways, but that the former is dominant for the expression of cytotoxicity. Thus, in the ensemble of adhesion molecules available for interaction between endothelium and activated monocytes, the hierarchy of their importance may vary for different functions.

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