Characterization of molecular interactions between intercellular adhesion molecule-1 and leukocyte function-...

The ability of soluble ICAM-1 (sICAM-1) to inhibit ICAM-1/LFA-1 adhesion events has been reported previously by numerous investigators. sICAM-1 has been demonstrated to inhibit various in vitro assays at concentrations ranging from 2 nM to greater than 40 microM. Given the hypothesis that circulating ICAM-1 modulates immune functions, the ability of sICAM-1 to inhibit cellular functions may have significant ramifications. Considering the potential clinical importance of the interaction between ICAM-1 and its receptor, LFA-1, it is necessary to understand this receptor-ligand interaction at a molecular level. In this study, direct binding experiments were utilized to determine the affinity between biotinylated monomeric sICAM-1 and immobilized LFA-1 (approximately 130 nM). Competitive binding experiments with unlabeled sICAM-1 and a truncated form of sICAM-1 (D1D2) yielded similar affinities. The specificity of this interaction was characterized using mAbs directed against sICAM-1 or LFA-1. This assay system was extended to include multimeric species using nonblocking mAbs directed against domains D4 and D5 of sICAM-1. Dimerizing sICAM-1 with a mAb alphaD4 or alphaD5 increased the affinity for immobilized LFA-1 by two orders of magnitude (approximately 4 nM), an effect presumably due to avidity. These results indicate that while the monomeric sICAM-1/LFA-1 interaction may involve only a moderate binding affinity, multimeric ICAM-1 present on a cell surface may bind cell surface-immobilized LFA-1 with very high avidity. These sICAM-1/LFA-1 molecular assays should be useful in defining the efficacy of potential antagonists.