Recent progress in the understanding of
thrombus formation has suggested an important role for
glycoprotein (GP) VI in this process. To clarify the exact role in detail, it is necessary to use specific, high affinity inhibitory
antibodies. However, possibly due to the conserved structure of GPVI among species, it has been difficult to obtain potent
antibodies. In this study, we developed highly potent anti-human GPVI
monoclonal antibodies using GPVI knockout mice for immunization.
Fab fragments of these
antibodies, named OM1 and OM2, potently inhibit
collagen-induced platelet aggregation. The IC(50) values for OM1 and OM2 are 0.6+/-0.05 and 1.7+/-0.5 microg/mL, respectively, showing potency greater than, or equal to that of
abciximab (1.7+/-0.3 microg/mL), an anti-GPIIb/IIIa antibody.
Fab fragments of OM1 and OM2 also potently inhibit
collagen-induced
ATP release,
thromboxane A(2) formation, and platelet adhesion to immobilized
collagen under static and flow conditions. Interestingly, platelet aggregation induced with
collagen-related peptide was potently inhibited by OM2 but not OM1, indicating that OM1 recognizes an
epitope that is different from
collagen-related peptide-binding site on GPVI. These results suggest that OM1 and OM2 may be useful tools to understand the role of GPVI in
thrombus formation. Furthermore, these
antibodies have the potential to be developed as a new class of therapeutic tool.