Regulation of microvascular thromboembolism in vivo.

Atherothrombosis and embolization are main causes of morbidity and mortality in the Western world. To optimize treatment, better understanding of the factors involved in thromboembolism in vivo is needed. The course and outcome of a thromboembolic process are determined by the local balance between anti and prothrombotic factors.

Monoclonal antibody IAC-1 is specific for activated alpha2beta1 and binds to amino acids 199 to 201 of the integrin alpha2 I-domain.

In this study we describe the first monoclonal antibody, integrin activated conformation-1 (IAC-1), to recognize the active form of the platelet-collagen receptor, the integrin alpha(2)beta(1). IAC-1 has the following properties: (1) IAC-1 fails to bind to resting platelets but readily interacts with platelets stimulated by the glycoprotein VI-specific agonist, convulxin, and by other agonists; (2) similar concentration response relationships for binding of IAC-1 and soluble collagen were observed in convulxin-stimulated platelets; (3) the epitope for IAC-1 is T199Y200K201, which is located at the opposite site of the metal ion-dependent adhesion site in a region not involved in the I-domain "shifts" that occur upon ligand binding; (4) IAC-1 strongly binds to recombinant alpha(2) I-domain, therefore suggesting that the neo-epitope appears to be exposed by an "unmasking" of I-domain-covering regions upon activation; (5) IAC-1 binds to platelets during adhesion to collagen under shear conditions, demonstrating activation of alpha(2)beta(1); (6) as IAC-1 does not interfere with platelet-collagen binding, it defines a new class of antibodies that is distinct from those belonging to the "cation- and ligand-induced binding sites" (CLIBSs) and the "ligand mimetic" group. These characteristics make IAC-1 a very powerful tool to study alpha(2)beta(1) activation under dynamic and physiologically relevant conditions.