Atomic Basis for the Species-specific Inhibition of αV Integrins by Monoclonal Antibody 17E6 Is Revealed by the Crystal Structure of αV β3 Ectodomain-17E6 Fab Complex

Background: 17E6, a primate-specific mouse mAb that inhibits V integrins, is in phase II trials for treating cancer. Results: We determined crystal structure of the V3-17E6 Fab complex, revealing the molecular basis of 17E6 specificity and function. Conclusion: 17E6 is an allosteric inhibitor of fibronectin-integrin interaction. Significance: The defined 17E6 epitope may help in developing novel therapeutics targeting related regions in other integrins. The function-blocking, non-RGD-containing, and primate-specific mouse monoclonal antibody 17E6 binds the V subfamily of integrins. 17E6 is currently in phase II clinical trials for treating cancer. To elucidate the structural basis of recognition and the molecular mechanism of inhibition, we crystallized V3 ectodomain in complex with the Fab fragment of 17E6. Protein crystals grew in presence of the activating cation Mn2+. The integrin in the complex and in solution assumed the genuflected conformation. 17E6 Fab bound exclusively to the Propeller domain of the V subunit. At the core of V-Fab interface were interactions involving Propeller residues Lys-203 and Gln-145, with the latter accounting for primate specificity. The Propeller residue Asp-150, which normally coordinates Arg of the ligand Arg-Gly-Asp motif, formed contacts with Arg-54 of the Fab that were expected to reduce soluble FN10 binding to cellular V3 complexed with 17E6. This was confirmed in direct binding studies, suggesting that 17E6 is an allosteric inhibitor of V integrins.