Mice lacking factor XII (fXII) or factor XI (fXI) are resistant to experimentallyinduced thrombosis, suggesting fXIIa activation of fXI contributes to thrombus formation in vivo. It is not clear whether this reaction has relevance for thrombosis in primates. In 2 carotid artery injury models (FeCl 3 and Rose Bengal/laser), fXII-deficient mice are more resistant to thrombosis than fXI-or factor IX (fIX)-deficient mice, raising the possibility that fXII and fXI function in distinct pathways. Antibody 14E11 binds fXI from a variety of mammals and interferes with fXI activation by fXIIa in vitro. In mice, 14E11 prevented arterial occlusion induced by FeCl 3 to a similar degree to total fXI deficiency. 14E11 also had a modest beneficial effect in a tissue factor-induced pulmonary embolism model, indicating fXI and fXII contribute to thrombus formation even when factor VIIa/tissue factor initiates thrombosis. In baboons, 14E11 reduced plateletrich thrombus growth in collagen-coated grafts inserted into an arteriovenous shunt. These data support the hypothesis that fXIIa-mediated fXI activation contributes to thrombus formation in rodents and primates. Since fXII deficiency does not impair hemostasis, targeted inhibition of fXI activation by fXIIa may be a useful antithrombotic strategy associated with a low risk of bleeding complications. (Blood. 2010;116(19):3981-3989) IntroductionInitiation of fibrin formation by contact activation requires proteolytic conversion of plasma factor XII (fXII) to the protease factor XIIa (fXIIa) on a surface. 1-3 FXIIa activates the next zymogen in the coagulation cascade, factor XI (fXI), to factor XIa (fXIa), which in turn converts factor IX (fIX) to factor IXa␤ (fIXa␤). This series of reactions, referred to as the intrinsic pathway of coagulation, drives thrombin generation and fibrin formation in the activated partial thromboplastin time (aPTT) assay used by clinical laboratories. A role for fIX in hemostasis is not in question, as its deficiency causes the severe bleeding disorder hemophilia B. However, the importance of the intrinsic pathway, as a whole, to clot formation and stability at a site of injury is probably limited, as fXII deficiency is not associated with abnormal bleeding, 1,2 and fXI-deficient patients have a variable hemorrhagic disorder with milder symptoms than hemophiliacs. 2,4 Current models of thrombin generation address these phenotypic differences by incorporating additional mechanisms for protease activation. Thus, fIX is activated by the factor VIIa/tissue factor complex in addition to fXIa, 3,5 while fXI can be activated by thrombin. 3,6 Mice lacking fXII, like their human counterparts, do not have a demonstrable bleeding abnormality, 7 supporting the premise that fXIIa activation of fXI is not required for hemostasis. 8 Given this, it was surprising to observe that mice lacking fXII 9 or fXI 10 were resistant to arterial thrombotic occlusion. While this suggested contact activation might play an important role in pathologic coagulation, if not hemostasis...

Obesity has become a pandemic, affecting both children and adults. We sought to determine the effect of obesity among 200 children who were prospectively enrolled on a multicenter cord blood transplant (CBT) trial. All patients received myeloablative preparative regimens. Children were classified into groups according to body mass index percentile. Normal weight was defined as body mass index between the 5th and 85th percentile (n=117), overweight between the 85th and 95th percentile (n=35) and obesity above 95th percentile (n=39) for age and gender. A total of 55 patients (27%) had AML, 113 patients (57%) had ALL and 32 patients (16%) had other malignant diseases. There was no evidence for a difference in all major characteristics among the groups. Time to neutrophil and platelet engraftment, TRM, risk of acute GVHD, disease-free survival and OS were not significantly different in overweight or obese patients compared with normal weight patients. There was a trend towards increased risk of chronic GVHD in obese patients (P=0.05) compared with normal weight patients. In conclusion, there is insufficient evidence from this sample that obesity has an effect on multiple outcomes after unrelated CBT in children with malignant diseases.

The most common etiology of pancytopenia in hospitalized children without cancer was infections. This differs from earlier reports in other countries, where megaloblastic anemia was found most often. Our review should provide guidance to the diagnoses which should be considered when evaluating a child with pancytopenia.

3336 Poster Board III-224 Engraftment and overall survival after umbilical cord blood transplant is highly dependent on the total nucleated cell count (TNC). Current standard post thaw processing includes a wash step to remove dimethyl sulfoxide (DMSO), lysed red cells and stroma. The contribution of the wash step to cell loss and ultimately the dose of cells available for transplant is not well described. To investigate the amount of cell loss after washing and its impact on major outcomes compared to pre-cryopreserved TNC, we analyzed data from 310 patients prospectively enrolled on a National Heart Lung Blood Institute (NHLBI) sponsored cord blood transplant study between 1999 and 2003. Dataset was obtained after signed agreement with the NHLBI and local IRB approval. There were 310 patients ≤18 years of age with malignant (N=217) or non-malignant (N=93) disease enrolled on this trial. Only single cord units were used. All cord blood units were thawed and washed using an identical process developed by Rubinstein et al. All patients received myeloablative preparative regimen with either total body irradiation or busulfan based regimens with cyclosporine and prednisone GVHD prophylaxis. All patients received anti-thymocyte globulin as part of their conditioning regimen. For the overall survival, Cox proportional hazard models were generated for pre-wash cell dose and post-wash cell dose separately and then combined in one model. All models included identical covariates. Total cell dose was modeled as a continuous variable with appropriate transformation using restricted cubic lines to account for non-linear relationships. For transplant related mortality (TRM) and neutrophil engraftment, competing risk analyses were used. These analyses were done with adjustment for age, gender, disease (malignant versus nonmalignant), performance status (<90 versus ≥90), HLA (3-4/6 versus 5-6/6 match), and CMV status. The median age was 4.59 years (range 0.04 – 17.90) with 188 (61%) male, 249 patients (80%) had a performance status of ≥90. 166 patients (54%) received a cord blood unit matched at 3/6 or 4/6 HLA antigens and 144 patients (46%) received a cord blood unit matched at 5/6 or 6/6 HLA antigens. The median pre-cryopreserved TNC per kg was 6.93 × 107/kg (range 1.5-80.9 × 107/kg). The median TNC recovery after thawing and washing (PTW) was 5.43 × 107/kg (range 1-31.6 × 107/kg). The average cell recovery was 89% after thawing and washing. Neutrophil engraftment was significantly associated with higher pre-cryopreserved (p=0,003) and PTW TNC infused (p=0.005); younger age (p=0.03), better HLA match (p=0.03). The risk of transplant related mortality was significantly higher among older patients (p=0.02), female patients (p=0.02) and those receiving 3-4/6 HLA matched cord units (p=0.02). Neither the pre-cryopreserved or PTW TNC were significant contributing factors. The risk of grade II-IV acute GVHD was significantly higher among older patients (p=0.04) and those receiving higher pre-cryopreserved TNC (p=0.02) but not higher PTW TNC (p=0.07). Overall survival was significantly better among younger patients (p=0.02), male recipients (p<0.001), patients with non-malignant diseases (p<0.001), patients with performance status >90 (p=0.04) and those receiving 5-6/6 HLA matched cord units (p=0.04). Pre-cryopreserved and PTW TNC did not influence overall survival. In conclusion, pre-cryopreserved and post thaw and wash TNC were equally predictive for major outcomes of unrelated cord blood transplant in children. Disclosures No relevant conflicts of interest to declare.