Platelet mTOR Is a Regulator of Sterile Immunothrombosis

Immunothrombosis entails a tight interplay between thrombotic and inflammatory pathways and plays a pathological role in ischemic stroke and venous thrombosis. mTOR (mechanistic target of rapamycin) is a canonical serine/threonine kinase and is involved in platelet signaling and thrombus stabilization in vitro. Activation of platelet mTOR is upregulated in aging and inflammatory disorders. However, its role in vivo is poorly understood.

We used mice specifically lacking mTOR in platelets and assessed platelet activation and platelet-leukocyte interactions in response to platelet agonists. In addition, we examined the role of platelet mTOR in models of hemostasis, thrombosis, inflammatory bleeding, and sterile immunothrombosis, including ischemic stroke and venous thrombosis.

Platelets lacking mTOR had a small activation defect and had lower procoagulant potential. In the absence of mTOR, activated platelets interacted less with monocytes and neutrophils. In addition, platelet mTOR regulated platelet-mediated neutrophil activation. Platelet cytoplasmic calcium flux was similar in the presence and absence of mTOR, whereas clot retraction was reduced in the absence of platelet mTOR, suggesting a role for mTOR in selectively regulating Rac1 (Ras-related C3 botulinum toxin substrate 1)-dependent pathways involved in sustained platelet function. In vivo, the absence of platelet mTOR did not impact hemostasis, inflammatory bleeding in the lung and skin, or FeCl₃-induced arterial thrombosis. In contrast, platelet-specific mTOR knockout mice were protected from ischemic stroke brain injury and stasis-induced venous thrombosis due to reduced thrombosis and inflammation.

Platelet mTOR is a critical mediator of sterile immunothrombosis, although it is dispensable for hemostasis in mice. The immunothrombotic-specific effects of mTOR make it an attractive therapeutic target with a good safety profile.