PB0382 - The antibody JAQ1 binds to both human and mouse GPVI but differently affect their function and in vivo depletion.

Background: Platelet glycoprotein (GP) VI is the major platelet collagen receptor and a promising antithrombotic target. This was first demonstrated in mice using the monoclonal antibody JAQ1, which inhibits completely collagen-related peptide (CRP) and collagen-induced platelet activation, aggregation and thrombus formation. Injection of JAQ1-IgG induces transient thrombocytopenia and depletion of GPVI from platelets, whereas its F(ab´)-fragment only blocks GPVI without affecting platelet count or GPVI expression in vivo.

Aims: The aim of this study was testing the effects of JAQ1 on human GPVI function in vitro and the immunodepletion of the receptor in newly generated hGPVI-knockin (hGPVItg/tg) mice in vivo.

Methods: We used flow cytometry, aggregometry, spreading assays and ex vivo flow adhesion studies to characterize the effect of JAQ1 on human platelets and hGPVItg/tg mouse platelets. The effects of JAQ1 treatment on hGPVI expression in vivo was analyzed in hGPVItg/tg mice.

Results: Flow cytometry revealed binding of JAQ1 to human GPVI, but not to other species, including rat, hamster, rabbit, swine, cat, dog, horse, and cow. In hGPVItg/tg mouse platelets, JAQ1 binding did not cause detectable activation unless cross-linked, comparable to the effect on wild-type platelets. Remarkably, in contrast to mouse GPVI, JAQ1 did not inhibit CRP or collagen binding to human GPVI. In vivo, JAQ1-treatment induce complete depletion of mGPVI in wild-type mice, but only partially downregulate the receptor in hGPVItg/tg mice, which was in both groups accompanied by a comparable transient thrombocytopenia.

Conclusion(s): The binding site of JAQ1 on GPVI is conserved between mouse and human, but differs in its functional significance. In vivo targeting of this site with JAQ1 IgG results in different immuno-depletion efficacies, indicating that epitope-specific effects could be involved in this process.