Molecular comparison of soluble intercellular adhesion molecule (sICAM)-1 and sICAM-3 binding to lymphocyte function-associated antigen-1.

The interactions of intercellular adhesion molecules-1 and -3 (ICAM-1 and ICAM-3) with lymphocyte function-associated antigen-1 (LFA-1) have been characterized and compared on the molecular and cellular level. Enzyme-linked immunosorbent-based molecular assays have been utilized to calculate the binding affinities of soluble ICAM-1 (sICAM-1) and soluble ICAM-3 (sICAM-3) for LFA-1. Consistent with previously published data, we found that sICAM-1 binds to LFA-1 with an affinity of approximately 60 nM. In contrast, sICAM-3 binds to LFA-1 with an affinity approximately 9 times weaker ( approximately 550 nM). Both sICAM-1 and sICAM-3 require divalent cations for binding. Specifically, both Mg2+ and Mn2+ support high affinity adhesion, although interestingly, high concentrations of Ca2+ decrease the affinity of each molecule for LFA-1 substantially. Furthermore, a panel of anti-LFA-1 monoclonal antibodies were characterized for their ability to block sICAM-1 and sICAM-3/LFA-1 interactions in molecular and cellular assays to help distinguish binding sites on LFA-1 for both molecules. Finally, molecular and cellular competition experiments demonstrate that sICAM-1 and sICAM-3 compete with each other for binding to LFA-1. The above data demonstrate that sICAM-1 and sICAM-3 share a common binding site or an overlapping binding site on LFA-1 and that the apparent differences in binding sites can be attributed to different affinities of sICAM-1 and sICAM-3 for LFA-1.