Targeting β3 Integrin Psi Domain Inhibits Both Platelet Aggregation and Blood Coagulation: Two Birds with One Stone

Integrin αIIbβ3 plays key roles in thrombosis and hemostasis primarily through mediating platelet adhesion and aggregation. We recently reported that the active site of thiol-isomerase enzymes, CXXC motif, is expressed twice within the plexin-semaphorin-integrin (PSI) domain across all integrins and species, and the PSI domain of β3 integrin possesses endogenous thiol-isomerase activity, which may be a novel target for anti-thrombotic therapy (Blood, 2017).

We developed four mouse anti-mouse β3 integrin PSI domain monoclonal antibodies (mAbs). These mAbs cross-react with β3 PSI domains of human, mouse, pig, rat, and rabbit tested but not other regions of β3 integrin, other integrins or other thiol-isomerase enzymes. They inhibit the thiol-isomerase activity of β3 PSI domain, decrease platelet adhesion/aggregation and thrombosis without increasing bleeding. Interestingly, the inhibitory effect of these mAbs on thrombosis in vivo (no anti-coagulant) was 10-20 times greater than their inhibitory effect on platelet aggregation in anti-coagulated platelet-rich plasma in vitro. This motivated us to explore whether this PSI domain contributes to blood coagulation.

To asses blood clot formation and retraction, blood was incubated in non-stick tubes for two hours at 37°C in clot retraction assays. These assays showed less clot retraction and significantly lower dry clot weight in human and mouse whole blood treated with these anti-PSI mAbs compared to controls (p<0.05). As a visual representation, laser scanning confocal microscopy of platelet-rich plasma samples with labelled fibrinogen/fibrin showed that fibrin network formation of anti-PSI mAb treated clots was decreased. To measure blood coagulation in a global assay in the presence of platelets and natural blood constituents, thromboelastography (TEG) was performed. Blinded TEG showed that anti-PSI mAb-treated whole blood (human and mouse) and platelet rich plasma (human) decreased in coagulation compared to controls (p<0.05). We further compared our anti-PSI mAbs with other anti-β3 integrin mAbs that inhibit αIIbβ3 ligand binding and platelet aggregation much stronger than the anti-PSI mAbs. In this TEG assay we found that the anti-PSI mAbs were still significantly decreasing platelet coagulation parameters, much more than other inhibitory anti-β3 mAbs including M1 and JAN D1 as well as the Abciximab precursor 7E3 (p<0.05). This suggests the decreased coagulation may result from the anti-thiol-isomerase activity of the anti-PSI mAbs.

Disulfide bond exchanges are required for blood coagulation and thiol-isomerase enzymes play important roles in thrombosis. Given that blood coagulation is rapidly amplified on the platelet surface (i.e. cell-based coagulation), many coagulation factors are in close proximity to β3 integrin PSI domains (with thiol-isomerase activity). The study of interaction between β3 PSI domain and coagulation factors is currently being exploring.

This is the first evidence of an allosteric therapeutic target of β3 integrin. This work suggests that a novel interaction exists between platelets and the coagulation cascade that requires the β3 integrin PSI domain and its thiol-isomerase activity. Importantly, the PSI domain can be targeted to partially block both platelet aggregation and coagulation, resulting in less bleeding while still providing a strong overall anti-thrombotic effect.