Acute promyelocytic leukemia (APL) is a subtype of acute myeloid leukemia that is associated with disseminated intravascular coagulation and fatal bleeding. Although chemotherapy and differentiation therapies are quite effective, APL is still associated with a high incidence of early death (4-26%) that typically occurs within 30 days of diagnosis. Notably, severe bleeding, particularly intracranial bleeding, is the leading cause of the early deaths in APL. The precise mechanisms of APL-induced bleeding are unknown. However, observational studies indicate that it is characterized by thrombocytopenia, hyperfibrinolysis and coagulation activation. The goal of the proposal is to determine the mechanisms of bleeding in APL. We have recently established two new mouse models of APL (xenograft and allograft models) that reproduce many of the hemostatic abnormalities of APL patients. Podoplanin (PDPN) activates platelets via C-type lectin-like receptor 2 (CLEC-2) on platelets. Blast cells from APL patients express high levels of PDPN suggesting a role of PDPN in thrombocytopenia. Consistently, we observed high plasma PDPN levels in our allograft model. Increased levels of plasmin- antiplasmin complexes (PAP) and D-dimer are observed in APL patients. We observed high levels of PAP and D-dimer in our mouse models of APL. The S100A10 (S100)/annexin A2 (AA2) and the urokinase-type plasminogen activator (uPA)/ uPA receptor (uPAR) complexes are expressed on APL cells and may play a role in hyperfibrinolysis. APL patients have high levels of thrombin-antithrombin complex (TAT). We observed high levels of TAT in our mouse models of APL. Peripheral blood mononuclear cells (PBMC), which includes blast cells, from APL patients have high levels of tissue factor (TF) activity. Importantly, we recently found that inhibition of APL cell-derived TF reduced TAT levels in the xenograft model. Similarly, inhibition of both APL cell- derived and host cell-derived TF reduced TAT levels in the allograft model. To date, no studies have evaluated the roles of the PDPN-CLEC-2 pathway, the S100/AA2/tPA and uPA/uPAR pathways in the coagulopathy and bleeding in APL using mouse models. In addition, our study is the only study that shows the role of TF in the activation of coagulation in mouse models of APL. Our central hypothesis is that bleeding in APL patients is driven by simultaneous reduction of platelets and activation of fibrinolysis, and coagulation which collectively impose a catastrophic failure of the hemostatic system. We will test this hypothesis by determining the individual contributions of the PDPN-CLEC-2 pathway, the S100/AA2/tPA and the uPA/uPAR pathways, and TF to the coagulopathy and bleeding in mouse models of APL. Our study will provide a better understanding of mechanisms of APL-associated coagulopathy and bleeding. We will also have explored new treatments for APL- associated bleeding and APL itself. In addition, our study may find new biomarkers to identify A...