Function-blocking Integrin αvβ6 Monoclonal Antibodies DISTINCT LIGAND-MIMETIC AND NONLIGAND-MIMETIC CLASSES

Abstract We have generated a panel of potent, selective monoclonal antibodies that bind human and mouse αvβ6 integrin with high affinity (up to 15 pm). A subset of these antibodies blocked the binding of αvβ6 to the transforming growth factor-β1 latency-associated peptide with IC50 values as low as 18 pm, and prevented the subsequent αvβ6-mediated activation of transforming growth factor-β1. The antibodies also inhibited αvβ6 binding to fibronectin. The blocking antibodies form two biochemical classes. One class, exemplified by the ligand-mimetic antibody 6.8G6, bound to the integrin in a divalent cation-dependent manner, contained an RGD motif or a related sequence in CDR3 of the heavy chain, was blocked by RGD-containing peptides, and was internalized by αvβ6-expressing cells. Despite containing an RGD sequence, 6.8G6 was specific for αvβ6 and showed no cross-reactivity with the RGD-binding integrins αvβ3, αvβ8,or αIIbβ3. The nonligand-mimetic blocking antibodies, exemplified by 6.3G9, were cation-independent, were not blocked by RGD-containing peptides, were not internalized, and did not contain RGD or related sequences. These two classes of antibody were unable to bind simultaneously to αvβ6, suggesting that they may bind overlapping epitopes. The “ligand-mimetic” antibodies are the first to be described for αvβ6 and resemble those described for αIIbβ3. We also report for the first time the relative abilities of divalent cations to promote αvβ6 binding to latency-associated peptide and to the ligand-mimetic antibodies. These antibodies should provide valuable tools to study the ligand-receptor interactions of αvβ6 as well as the role of αvβ6 in vivo.