Intercellular Adhesion Molecule-3 Binding of Integrin αLβ2 Requires Both Extension and Opening of the Integrin Headpiece1

The leukocyte-restricted integrin αLβ2 is required in immune processes such as leukocyte adhesion, migration, and immune synapse formation. Activation of αLβ2 by conformational changes promotes αLβ2 binding to its ligands, ICAMs. It was reported that different affinity states of αLβ2 are required for binding ICAM-1 and ICAM-3. Recently, the bent, extended with a closed headpiece, and extended with open headpiece conformations of αLβ2, was reported. To address the overall conformational requirements of αLβ2 that allow selective binding of these ICAMs, we examined the adhesion properties of these αLβ2 conformers. αLβ2 with different conformations were generated by mutations, and verified by using a panel of reporter mAbs that detect αLβ2 extension, hybrid domain movement, or I-like domain activation. We report a marked difference between extended αLβ2 with closed and open headpieces in their adhesive properties to ICAM-1 and ICAM-3. Our data show that the extension of αLβ2 alone is sufficient to mediate ICAM-1 adhesion. By contrast, an extended αLβ2 with an open headpiece is required for ICAM-3 adhesion.