Allosteric regulation of fibronectin binding by the anti-β1 integrin antibody TS2/16.

The monoclonal antibody (mAb) TS2/16 stabilizes the active conformation of β1 integrin, enhancing its adhesive capacity on the cell surface. However, the molecular mechanism by which TS2/16 modulates integrin affinity for extracellular ligands remains unclear. Using endogenous full-length α5β1 integrin purified from human placenta, we determined the structure of integrin α5β1 with fibronectin up to 2.61-Å resolution in the absence of TS2/16, capturing the active form without its aid, and performed comparative B-factor-based analysis and CABS-Flex simulation with and without TS2/16. Despite no global conformational differences, we found that TS2/16 interacts with α2 helix of the integrin β1 subunit and contacts the C-terminus of α3 helix, leading to a localized decrease in B factor. This interaction allosterically alters the dynamics of α2-α3 loop despite not being in direct contact with TS2/16. Notably, this loop directly engages fibronectin, and its dynamic change underlies the enhanced ligand-binding affinity and explains increased cell adhesion observed with TS2/16. These findings reveal an allosteric mechanism of integrin regulation by TS2/16 and offer insights for the rational design of therapeutic antibodies targeting integrin-mediated adhesion in pathological contexts such as inflammation and cancer.