Molecular Basis for Leukocyte Integrin α_Eβ₇ Adhesion to Epithelial (E)-Cadherin

Cadherins are expressed in tissue-restricted patterns and typically mediate homophilic adhesion. Cadherins also mediate lymphocyte adhesion, providing the opportunity for lymphocyte attachment to parenchymal cells. The best characterized example of lymphocyte adhesion to a tissue-specific cell adhesion molecule, as opposed to a vascular endothelial adhesion molecule, is the interaction between integrin α_Eβ₇ on intraepithelial lymphocytes and E-cadherin on epithelial cells. However, the molecular basis for an integrin–cadherin interaction is not well defined. Realization that the cadherin domain adopts a topology similar to the immunoglobulin (Ig) fold suggested that integrin recognition of E-cadherin might be similar to recognition of Ig superfamily ligands. Thus, we modeled domain 1 of human E-cadherin and studied the role of solvent-exposed loops that connect Ig-like core-forming β strands. Mutational analyses localized the integrin α_Eβ₇ recognition site to the top of domain 1 at the face formed by the BC and FG loops, a site distinct from the region recognized in intercellular adhesion molecule (ICAM)-1, -2, and -3, mucosal addressin cell adhesion molecule 1 (MAdCAM-1), vascular cell adhesion molecule 1 (VCAM-1), and fibronectin by their integrin ligands. Moreover, the integrin α_Eβ₇ binding site is distinct from the homophilic binding site on E-cadherin. These studies provide a conceptual basis for integrin–cadherin binding and extend the model that an Ig-like fold can serve as a scaffold for recognition.