Abstract Abstract 264 Background and objective: ADAMTS13 (A Disintegrin And Metalloprotease with Thrombospondin type I repeats-13) cleaves von Willebrand factor (VWF), a large multimeric protein that plays an important role in thrombus formation by binding to platelets following vascular injury. Epidemiological studies suggest that elevated VWF levels and reduced ADAMTS13 activity in the plasma are risk factors for myocardial infarction. It remains unknown, however, whether the ADAMTS13-VWF axis plays a causal role in the pathophysiology of myocardial infarction. We tested the hypothesis that ADAMTS13 reduces VWF-mediated acute myocardial ischemia/reperfusion (I/R) injury in mice. Methods: Myocardial infarction was induced in male mice (8–10 weeks of age) by ligating the left anterior descending coronary artery for 30 minutes followed by 23.5 hours of reperfusion. The extent of myocardium damage was evaluated by measuring infarct size (%) in 2 mm serial sections stained with 2% triphenyl-2, 3, 4-tetrazolium-chloride. Neutrophil infiltration and myocyte apoptosis in the left ventricular area was quantified by immunohistochemistry and TUNEL staining respectively. Results: Adamts13 -/- mice exhibited significantly increased infarct size (22.2 % ± 1.1 %, P <.01) compared with WT mice (16.9 % ± 1.2 %, P<0.05). Plasma levels of cardiac troponin T (cTnT), an index of myocyte injury, were significantly higher in Adamts13−/− mice compared with WT mice (P <0.01). Adamts13+/− mice, which have a 50% reduction in ADAMTS13 activity, had similar sized infarcts (16.6 ± 1.3%) and cTnT levels compared to those in WT mice. Larger infarcts in the Adamts13−/− mice were concordant with increased neutrophil infiltration and myocyte apoptosis compared with WT mice. Because VWF remains the only known substrate of ADAMTS13 in multiple experimental models, we hypothesized that ADAMTS13 reduces myocardial injury through its proteolytic effect on hyper adhesive ULVWF and /or VWF. Vwf−/− mice exhibited significantly reduced infarct size, neutrophil infiltration, and myocyte apoptosis compared with WT mice, suggesting a detrimental role for VWF in myocardial I/R injury. VWF-deficient mice have a defect in regulation of endothelial P-selectin due to the loss of Weibel-Palade body formation. To confirm that exacerbated myocardial I/R injury in the setting of ADAMTS13 deficiency is dependent on VWF rather than P-selectin, we compared WT and Adamts13−/− mice treated with anti-VWF inhibitory antibodies. Treating WT or Adamts13−/− mice with neutralizing antibodies to VWF prior to myocardial I/R injury significantly reduced infarct size compared with control Ig-treated mice, suggesting that exacerbated myocardial I/R injury observed in Adamts13−/− mice is entirely VWF-dependent. Finally, myocardial I/R injury in Adamts13−/−/Vwf−/− mice was similar to that in Vwf−/− mice, suggesting that the exacerbated myocardial I/R injury observed in the setting of ADAMTS13 deficiency is VWF-dependent. Conclusion: These findings reveal a new role for anti-thrombotic enzyme ADAMTS13 in reducing VWF-mediated myocardial ischemia/reperfusion injury. Disclosures: Lentz: Novo Nordisk A/S: Consultancy, Investigator Other.