ALBERT

Description: The role of the intrinsic coagulation pathway in acute myocardial infarction is poorly defined. Both coagulation factors XII (FXII) and XI (FXI) support experimental thrombus propagation in animals. Additionally, humans with FXI deficiency have a lower incidence of thrombosis and stroke, however no such association has been established for FXII. Curiously, the incidence of previously verified myocardial infarction (MI) among 96 surviving FXI deficient subjects that were interviewed in an epidemiologic study was found to be similar to or possibly even higher than the recorded incidence of MI in an age/sex matched dataset from morbidity/mortality statistics of the general Israeli population (Salomon et al. J Thromb Haemost. 2003;1:658). However, the outcome of these coronary events were not reported, except for the fact that all interviewed FXI subjects were alive at the time of the interview. To investigate the contribution of FXI activation by FXIIa in experimental MI, we used a standard mouse model of acute myocardial ischemia (AMI). To inhibit FXI in the mouse, we utilized our monoclonal antibody (14E11) that targets the Apple 2 domain of FXI, and has been shown in vitro to inhibit the activation of FXI by factor XIIa, while not significantly inhibiting activation of FXI by thrombin. To evaluate the efficacy of 14E11 in reducing ischemic injury in mice, the left coronary artery (LCA) of wildtype male mice was reversibly ligated for 40 min, and 14E11 (1 mg/kg; iv) or vehicle was infused during the last 15 min of occlusion. Occlusion was confirmed by sustained S-T elevation, regional cyanosis and wall motion abnormalities. Following occlusion, the ligature was removed and the heart reperfused for 2 hr. To delineate the area of risk and ischemia, the LCA was re-occluded at 2 hr post-reperfusion and fluorescent polymers infused into the apex of the heart. The heart was excised, cut into 1 mm thick transverse slices and photographed under UV light to identify the area at risk. Tissue sections were additionally stained with 2,3,5-triphenyltetrazolium chloride solution and infarcted areas evaluated via morphometric analysis. The area at risk was evaluated as the percent of total heart volume and infarct size was calculated as the percentage of area at risk. Our results indicated that the area of risk did not differ between treatment groups, however treatment with 14E11 reduced infarct volume by 33% (p

Description: Severe bacterial sepsis often leads to a systemic procoagulant and proinflammatory condition that can manifest as disseminated intravascular coagulation, septic shock, and multiple organ failure. Because activation of the contact proteases factor XII (FXII), prekallikrein, and factor XI (FXI) can trigger coagulation and inflammatory responses, the contact factors have been considered potential targets for the treatment of sepsis. However, the pathogenic role of contact activation in severe infections has not been well defined. We therefore investigated whether an anticoagulant antibody (14E11) that selectively inhibits prothrombotic FXI activation by activated FXII (FXIIa) modifies the course of bowel perforation-induced peritoneal sepsis in mice. Early anticoagulation with 14E11 suppressed systemic thrombin- antithrombin complex formation, IL-6, and TNF-α levels, and reduced platelet consumption in the circulation and deposition in the blood vessels. Treatment with 14E11 within 12 hours after bowel perforation significantly improved survival compared with vehicle treatment, and the saturating dose did not increase tail bleeding. These data suggest that severe polymicrobial abdominal infection induces prothrombotic FXI activation, to the detriment of the host. Systemic anticoagulation by inhibiting FXI activation or FXIIa procoagulant activity during sepsis may therefore limit the development of disseminated intravascular coagulation without increasing bleeding risks.

Description: IL-7 is an important cytokine for lymphocyte differentiation. Similar to what occurs in vivo, human CD19+ cells developing in human/murine xenogeneic cultures show differential expression of the IL-7 receptor α (IL-7Rα) chain (CD127). We now describe the relationship between CD127 expression/signaling and Ig gene rearrangement. In the present study, 〈 10% of CD19+CD127+ and CD19+CD127− populations had complete VDJH rearrangements. IGH locus conformation measurements by 3D FISH revealed that CD127+ and CD127− cells were less contracted than pediatric BM pro-B cells that actively rearrange the IGH locus. Complete IGH rearrangements in CD127+ and CD127− cells had smaller CDR3 lengths and fewer N-nucleotide insertions than pediatric BM B-lineage cells. Despite the paucity of VDJH rearrangements, microarray analysis indicated that CD127+ cells resembled large pre-B cells, which is consistent with their low level of Ig light-chain rearrangements. Unexpectedly, CD127− cells showed extensive Ig light-chain rearrangements in the absence of IGH rearrangements and resembled small pre-B cells. Neutralization of IL-7 in xenogeneic cultures led to an increase in Ig light-chain rearrangements in CD127+ cells, but no change in complete IGH rearrangements. We conclude that IL-7–mediated suppression of premature Ig light-chain rearrangement is the most definitive function yet described for IL-7 in human B-cell development.

Description: Clinical observations and laboratory evidence link bone marrow failure in myelodysplastic syndrome (MDS) to a T cell–mediated immune process that is responsive to immunosuppressive treatment (IST) in some patients. Previously, we showed that trisomy 8 MDS patients had clonally expanded CD8+ T-cell populations that recognized aneuploid hematopoietic progenitor cells (HPC). Furthermore, microarray analyses showed that Wilms tumor 1 (WT1) gene was overexpressed by trisomy 8 hematopoietic progenitor (CD34+) cells compared with CD34+ cells from healthy donors. Here, we show that WT1 mRNA expression is up-regulated in the bone marrow mononuclear cells of MDS patients with trisomy 8 relative to healthy controls and non–trisomy 8 MDS; WT1 protein levels were also significantly elevated. In addition, using a combination of physical and functional assays to detect the presence and reactivity of specific T cells, respectively, we demonstrate that IST-responsive MDS patients exhibit significant CD4+ and CD8+ T-cell responses directed against WT1. Finally, WT1-specific CD8+ T cells were present within expanded T-cell receptor Vβ subfamilies and inhibited hematopoiesis when added to autologous patient bone marrow cells in culture. Thus, our results suggest that WT1 is one of the antigens that triggers T cell–mediated myelosuppression in MDS.

Description: Abstract 2244 In the widely used activated partial thromboplastin time (aPTT) assay, fibrin formation is induced by a series of sequential activations of the plasma protease zymogens factor (f) XII, fXI, fIX, fX and prothrombin, in that order. Conversion of prothrombin to the protease α-thrombin results in fibrin formation. α-Thrombin also enhances its own generation through activation of the cofactors fV and fVIII. While the linear sequence of reactions in the aPTT implies that loss of any single protease should have a comparable deleterious effect on the system, it is recognized that complete deficiency of a protein near the start of the sequence (e.g. fXII or fXI) results in greater aPTT prolongation than deficiency of proteins further down the sequence (e.g. fIX). This implies that proteases activated early in the process have multiple plasma substrates. For example, fXIa was recently reported to activate fVIII and fV (JTH 8;1532:2010), in addition to its role in fIX activation. Here, we present evidence that fXIa contributes to α-thrombin generation in the absence of fIX through activation of fX and/or fV. We noted that an anti-fXI antibody (O1A6) prolonged the aPTT of plasma from a patient with severe hemophilia B (fIX antigen undetectable) or plasma immunodepleted of fIX. This observation held even when an anti-fIX antibody was added to neutralize potential traces of fIX. Addition of activated fXI (fXIa - 3 nM) directly to fIX-deficient recalcified plasmas induced clot formation, and the time to clot formation was prolonged by O1A6. To further exclude the possibility that traces of fIX were contributing to thrombin generation, we confirmed the results using plasma from mice with combined complete deficiencies of fXII, fXI, and fIX. We tested the capacity of fXIa to cleave/activate fX and fV, the protease zymogen and cofactor, respectively, immediately downstream of fIX in the coagulation cascade. FX, the zymogen of the protease fXa, is evolutionarily related to fIX. SDS-PAGE analysis confirmed that fXIa cleaves fX. FX cleaved by fXIa demonstrated fXa activity in a chromogenic substrate assay, and converted prothrombin to α-thrombin in the presence of fVa and phospholipid. As previously reported, fXIa readily cleaved fV. The cleavage pattern differed from that generated by α-thrombin, however, formation of the fVa light chain was clearly evident. In a plasma clotting assay designed to measure either fXa or fVa activity, fX or fV pre-incubated with fXIa significantly shortened the clotting time of fIX-deficient plasma, while fX or fV pre-incubated with vehicle did not. In thrombin generation assays, fXIa (1.25 to 15 nM) induced thrombin generation in fIX-deficient plasma supplemented with anti-fIX antibody in a concentration dependent manner. FXIa did not induce thrombin generation in plasma lacking fV, or in fIX-deficient plasma containing the fXa inhibitor apixaban. This indicates that fXIa is working at the level of fX/fV in this assay, and is not directly converting prothrombin to α-thrombin. A recombinant variant of fXIa lacking the major fIX-binding exosite (fXIaPKA3, J Biol Chem 1996;271:29023) demonstrated a marked defect, compared to wild type fXIa (fXIaWT), in its capacity to induce thrombin generation in normal plasma. However, in fIX-deficient plasma, fXIaPKA3 and fXIaWT are comparable in their ability to enhance thrombin generation, supporting the premise that fXIa is acting through activation of fX and/or fV in the absence of fIX. Previously, we observed that fXI deficient mice and fIX deficient mice are comparably resistant to carotid artery thrombosis induced by exposure of the vessel to ferric chloride, despite having very different propensities to bleed. The animals were uniformly resistant to thrombosis with 5% FeCl3, and some were resistant at 7.5% FeCl3. All experienced vessel occlusion with 10% FeCl3. This is consistent with fXIa contributing to thrombosis in this model through fIX activation. However, we observed that some mice with combined fIX and fXI deficiency were resistant to FeCl3 concentrations up to 12.5%, implying that fXIa was contributing to thrombosis in a fIX-independent manner, as well. These results are consistent with those from the in vitro assays described above, and support the hypothesis that fXIa contributes to thrombin generation through fIX-dependent and fIX-independent processes. Disclosures: Tucker: Aronora, LLC: Employment, Equity Ownership. Gruber:Aronora, LLC: Consultancy, Equity Ownership.

Description: Abstract 1106 The plasma protease factor XIIa (FXIIa) contributes to vascular occlusion in murine thrombosis models, at least partly through activation of factor XI (FXI). While there is good correlation between plasma FXI levels and thrombotic events in humans, the situation is not as clear for FXII (the precursor of FXIIa), suggesting fundamental differences in thrombus formation in mice and humans. To facilitate studies on the effects of FXII/XIIa on thrombus formation, we developed novel inhibitory antibodies to human FXII, designated 9A2 and 15H8, by immunizing FXII-deficient mice with human FXII. Using recombinant human FXII molecules that lack various domains, and chimeras in which specific domains in FXII are replaced with those from the related protein hepatocyte growth factor activator, we determined that 9A2 and 15H8 bind to the FXII/XIIa non-catalytic heavy chain at different sites. 9A2 binds on or near the EGF2 domain, while 15H8 binds to the fibronectin type I and/or kringle domain. These areas have been implicated in FXII binding to polyanionic surfaces. Saturating concentrations of 9A2 or 15H8 reduced FXII activity by 50% and 90%, respectively, in an aPTT assay using normal plasma, while combining the antibodies resulted in 〉95% inhibition. However, in assays in which clot formation was triggered by adding FXIIa directly to plasma, preincubation of FXIIa with either antibody did not prolong the clotting time. Furthermore, neither antibody had a strong effect in a chromogenic assay of FXI activation by FXIIa, indicating the antibodies interfere with the aPTT assay primarily by inhibiting FXII activation. FXII activation in the aPTT assay is initiated by addition of a polyanion such as silica to the plasma to induce contact activation. In vivo, polymers of inorganic phosphate (polyP) may serve a similar function. Contact activation is triggered in plasma when FXII bound to the polyanion is activated, probably by trace amounts of FXIIa or another protease present in the plasma. Once formed, FXIIa converts the zymogens prekallikrein and FXI to the proteases kallikrein and FXIa, both of which can activate additional FXIIa to amplify the process. In the presence of 9A2 or 15H8, activation of pure FXII in the presence of either silica or polyP was significantly reduced. Interestingly, the antibodies actually potentiated FXII activation by kallikrein or FXIa in the absence of a polyanion. Taken as a whole, these results suggest that binding of 9A2 or 15H8 to FXII results in conformational changes that make FXII a better substrate for kallikrein and FXIa, possibly by mimicking the effect of FXII binding to a polyanion, but that prevent activation of FXII by FXIIa (autoactivation), blunting the overall rate of activation. We tested the effects of 9A2 and 15H8 in a mouse model in which thrombotic occlusion of the carotid artery is induced by exposing the vessel to a 3.5% solution of ferric chloride. Wild type C57Bl/6 mice develop arterial occlusion within 5 to 10 minutes, while FXII-deficient mice are resistant to arterial occlusion. Infusion of human FXII into FXII-deficient mice restores the wild type phenotype. 15H8 prevented thrombus formation in mice reconstituted with human FXII, while 9A2 reduced the rate of thrombotic occlusion by 50%. In an ex vivo flow model, perfusion of human blood through collagen-coated tubes at a shear rate of 300 sec−1 results in tube occlusion by platelet and fibrin rich clot in ∼15 minutes. 15H8 effectively blocked fibrin formation and reduced platelet accumulation, preventing tube occlusion. 9A2 was also effective at preventing clot formation, but there was evidence of some fibrin accumulation over time. In summary, the monoclonal anti-human FXII IgGs 9A2 and 15H8 prevent thrombus formation in whole blood in vivo and ex vivo by interfering with FXII activation. Our data support the hypothesis that pharmacologic inhibition of FXII activation may have therapeutic utility in disorders that are driven or aggravated by the blood contact system. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 3362 Activation of coagulation factors (F) XII and XI support thrombogenesis through multiple pathways. FXII-deficient mice are more resistant to FeCl3-induced arterial occlusion than either FIX or FXI deficient mice, suggesting that the resistance of FXII-deficient mice to experimental thrombosis is not completely explained by the FXII-FXI-FIX pathway, suggesting the existence of a pathological FXII by-pass, in vivo. The APTT of FXII deficient plasma is longer than the APTT of FXI, FIX, or FX deficient plasmas. We found that addition of 150 nM activated FXII (FXIIa) decreased the recalcification time of FXI or FIX-deficient plasma by up to 85%. In a purified system FXIIa could activate prothrombin but not FX. Addition of rivaroxaban, a FXa inhibitor, to FXI or FIX-deficient plasma blocked the observed procoagulant effect of FXIIa, suggesting that FXIIa promotes the activation of FX independent of FXI or FIX, but the ability of FXIIa alone to induce coagulation is insufficient in plasma, in vitro. Addition of long polyphosphate (polyP), typically found in bacteria, but not short polyP, which is secreted by activated platelets, decreased the recalcification time of FXI or FIX-deficient plasma. The presence of either corn trypsin inhibitor (CTI), that inhibits FXIIa, or rivaroxaban blocked the procoagulant effect of long polyP, suggesting that the activation of FXII by long polyP promotes coagulation in an FXI- and FIX-independent manner. Addition of CTI or an antibody that inhibits FIX activation by FXIa, but not addition of an antibody that inhibits activation of FXI by FXIIa, increased the time of occlusive thrombus formation in recalcified human blood that was driven through collagen and tissue factor (TF)-coated capillary tubes, consistent with the thrombogenic roles of FXIIa and feedback activation of FXI. Only CTI inhibited the prothrombotic effect of long polyP, also suggesting that FXIIa could be thrombogenic independent of FXI and FIX. In summary, we propose that pathological FXII activation, e.g., by foreign surfaces or long polyP, is thrombogenic both in FXI/FIX-dependent and -independent manners. Provided that FXII has no significant physiological function in humans, our data support the hypothesis that inhibition of FXII activity or activation may have safe antithrombotic effects. Disclosures: Morrissey: No organization, but the speaker is co-inventor on pending patent applications on the medical uses of polyphosphate: Patents & Royalties.

Description: Key Points Coincident with major changes in cancer treatments, the occurrence of tAML has changed significantly with time. The risks for tAML should be weighed against the benefits of chemotherapy.

Description: Key Points Factor XII can contribute to thrombus formation in human and nonhuman primate blood. An antibody that blocks factor XII activation (15H8) produces an antithrombotic effect in a primate thrombosis model.

Description: Abstract 3220 Neutrophils play a vital role in innate immunity. Activated neutrophils can release proteolytic enzymes capable of neutralizing microbes and contributing importantly to host-defense. In severe sepsis, microbial components and pro-inflammatory cytokines can contribute to excess systemic neutrophil activation, resulting in tissue damage and organ failure. Thus, regulation of neutrophil activation and factor release is critical during pathologic conditions. Recent data indicate that components of the contact system modulate numerous inflammatory mediators during severe sepsis, but the exact role of the contact pathway in host-defense is not well understood. Inhibition of factor XII (FXII) in septic baboons reduces circulating neutrophil elastase (NE), a potent cytolytic enzyme that is increased during sepsis and implicated in organ failure. In vitro studies also indicate that both plasma kallikrein and FXIIa are capable of directly inducing NE release. While it is apparent that factors of the contact system interact with neutrophils, the molecular mechanisms by which these factors modulate neutrophil function have not been established. We therefore examined factor XI (FXI) neutrophil interactions and the cellular signaling pathways regulating FXIIa neutrophil stimulation. Human neutrophils were isolated from peripheral blood and resuspended in HBSS at a concentration of 0.5 ×106/ml. Cells were treated with FXI, FXIa, FXII, or FXIIa with or without fMLP (1 μM) stimulation, and the release of NE was assayed in the cell supernatants via ELISA. FXI, FXIa or FXII had no direct stimulatory effect on NE release compared to vehicle. While neither FXI nor FXII had any inhibitory effect on fMLP induced NE release, FXIa (10 μg/ml) modestly reduced fMLP-induced NE release by 20% (n=3). FXIIa (3, 10, 30 μg/ml) dose-dependently increased NE release in the presence of cytochalasin B (5 μg/ml), consistent with published data. To examine the mechanism by which FXIIa induces NE release, neutrophils were pretreated with signaling inhibitors and subsequently activated with FXIIa (30 μg/ml). Mammalian target of rapamycyin (mTOR) is a downstream serine/threonine kinase of the PI3K/AKT pathway that integrates signals from the microenvironment such as cytokines and growth factors. It is known that inhibition of mTORC2 abrogates neutrophil polarization and directed migration, thus we examined the role of rapamycin complex 1 and 2 (mTORC1/2) in mediating NE release. Pretreatment of cells with RAD001 (20 nM), an mTORC1 inhibitor had no effect on FXIIa-induced NE release, whereas the combined mTORC1/mTORC2 inhibitor, pp242 (100 nM) abrogated FXIIa-induced NE release, suggesting that components of the mTORC2 pathway contribute to NE release. Pretreatment with EHT 1864 (50 uM), a Rac inhibitor, significantly potentiated NE release induced by either fMLP or FXIIa, suggesting that Rac is also capable of modulating FXIIa signaling. Taken together, these results suggest that coagulation factors FXIa and FXIIa differentially modulate neutrophil function, and that the mTOR and Rac signaling pathways participate in FXIIa stimulated neutrophil activation. These data suggest that the contact pathway is involved in neutrophil stimulation through mTOR and Rac signaling, and thus modulating these pathways could be a potential therapeutic strategy for limiting excess neutrophil activation. Disclosures: Gruber: Aronora, LLC: Consultancy, Equity Ownership.