Packaging of supplemented urokinase into alpha-granules of in vitro-grown megakaryocytes for targeted nascent clot lysis.

Fibrinolytics delivered into the general circulation lack selectivity for nascent thrombi, reducing efficacy and increasing the risk of bleeding. Urokinase-type plasminogen activator (uPA) transgenically expressed within murine platelets provided targeted thromboprophylaxis without causing bleeding, but is clinically infeasible. Recent advances in generating megakaryocytes prompted us to develop a potentially clinically relevant means to produce "anti-thrombotic" platelets from CD34+ hematopoietic stem cell-derived in vitro-grown megakaryocytes. CD34+-megakaryocytes internalize and store in -granules single-chain uPA (scuPA) and a plasmin-resistant thrombin-activatable variant (uPAT). Both uPAs co-localized with internalized factor V (FV), fibrinogen and plasminogen, low-density lipoprotein receptor-related protein 1 (LRP1), and interferon-induced transmembrane protein 3, but not with endogenous von Willebrand factor (VWF). Endocytosis of uPA by CD34+-megakaryocytes was mediated, in part, via LRP1 and IIb3. scuPA-containing megakaryocytes degraded endocytosed intragranular FV, but not endogenous VWF in the presence of internalized plasminogen, whereas uPAT-megakaryocytes did not significantly degrade either protein. We used a carotid-artery injury model in NOD-scid IL2rnull (NSG) mice homozygous for VWFR1326H (a mutation switching binding VWF specificity from mouse to human glycoprotein Ib) to test whether platelets derived from scuPA- or uPAT-megakaryocytes would prevent thrombus formation. NSG/VWFR1326H mice exhibited a lower thrombotic burden after carotid artery injury compared to NSG mice unless infused with human platelets or megakaryocytes, whereas intravenous injection of uPA-megakaryocytes generated sufficient uPA-containing human platelets to lyse nascent thrombi. These studies describe the use of in vitro-generated megakaryocytes as a potential platform for delivering uPA or other ectopic proteins within platelet -granules to sites of vascular injury.