ALBERT

Description: Apoptosis, or programmed cell death, is the physiologic mechanism that serves for controlled deletion of unwanted cells. Apoptosis was initially attributed exclusively to nucleated cells but over the past decade it has been recognized that apoptosis also occurs in anucleated cytoplasts and platelets. In this study, using flow cytometry we analyzed in human platelets three critical manifestations of mitochondrial, cytoplasmic and plasma membrane apoptosis, mitochondrial inner transmembrane potential (Δψm) depolarization, caspase-3 activation and phosphatidylserine (PS) externalization, respectively. We found that these hallmarks of apoptosis can be induced in human platelet suspension by diverse stimuli, including human α-thrombin (1, 10, 100 nM), calcium ionophore A23187 (3, 5, 10 μM), high shear stresses generated by cone-and-plate viscometer (120, 200, 390 dyn/cm2) and prolonged storage of platelet concentrates in blood banking conditions at 22°C for 6 and 13 days. We also demonstrated that these apoptotic markers can be induced in mouse platelets in vivo in a murine model of immune thrombocytopenia caused by injection of anti-glycoprotein (GP) IIb (rat anti-mouse GPIIb, MWReg30) antibody. Other manifestations of apoptosis were detected in human platelets, including expression of proapoptotic members of Bcl-2 family proteins (Bax and Bak) induced by thrombin, and platelet shrinkage and shedding of microparticles induced by high shear stresses. In addition to apoptosis in fluid-phase platelets, apoptosis was also revealed by confocal fluorescent microscopy in adherent human platelets and thrombi-like platelet aggregates deposited on thrombogenic immobilized human vascular collagen types I and III, as detected by PS exposure and shedding of PS-exposed microparticles. Taken together, these data suggest that platelet apoptosis is a phenomenon that can be triggered by a wide diversity of chemical and physical stimuli using different mechanisms mediated by thrombin-, collagen- and integrin GPIIbIIIa-receptors, mechanoreceptors and Ca2+-overloading. These stimuli trigger platelet apoptosis by impacting on several intracellular apoptotic targets, including shifting the balance between Bcl-2 regulatory proteins in a proapoptotic direction, depolarizing the inner mitochondrial membrane, activating the executioner caspase-3, stimulating aberrant PS exposure on the platelet surface and, eventually, resulting in ‘terminal’ stages of platelet apoptosis, such as platelet shrinkage and shedding of PS-exposed microparticles resembling apoptotic bodies. Platelet apoptosis can be induced both in fluid-phase and adherent platelets and thrombi-like platelet aggregates. These data also indicate that natural PL agonists thrombin and subendothelial vascular collagens and hemodynamic shear forces, can be involved not only in the processes of hemostasis, thrombosis and blood coagulation but also can trigger platelet death via apoptosis. Platelet apoptosis may contribute to the pathophysiology of thrombocytopenia in diseases associated with enhanced thrombin generation, such as sepsis and disseminated intravascular coagulation, as well as in autoimmune and alloimmune thrombocytopenias.

Description: Abstract 3197 Background: Thrombosis and cardiovascular diseases (CVDs) result from blood vessel occlusion by inappropriately activated platelets. They are the leading causes of morbidity and mortality worldwide. Anthocyanins are major phytochemicals abundant in plant food and have been shown to play a protective role against CVDs. Our previous studies have demonstrated that anthocyanins are antioxidative and prevent inflammation (J Biol Chem. 2005; 280:36792-01; Arterioscler Thromb Vasc Biol. 2007; 27:519-24), which may indirectly affect platelet function. It has also been reported that anthocyanins affect platelet activities in whole blood and platelet rich plasma (PRP). However, the direct effects of anthocyanins on platelet function and thrombus formation have not been studied. Methods: Here we investigated the effects of anthocyanins on thrombosis using purified platelets as well as several thrombosis models in vitro and in vivo. Cyaniding-3-gulucoside (Cy-3-g) and delphinidin-3-glucoside (Dp-3-g), the two predominantly bioactive compounds of anthocyanin preparations, were prepared from Polyphenol AS Company in Norway. Purified gel-filtered platelets and PRP from healthy human volunteers and C57BL/6J mice were incubated at 37°C for 10 minutes with different concentrations (0.5μM, 5μM and 50μM) of Cy-3-g, Dp-3-g or PBS buffer as a control. Platelet aggregation was assessed by aggregometry using 5μM ADP, 10μg/ml collagen, or 100μM thrombin receptor activating peptide (TRAP; AYPGKF) as agonists. Platelet adhesion and aggregation were assessed in response to an immobilized collagen matrix in an ex vivo perfusion chamber at both high (1800 s-1) and low (600 s-1) shear rates. The expression of activated GPIIbIIIa was determined via PAC-1 monoclonal antibody in flow cytometry. Lastly, the effects of anthocyanins on thrombus formation in C57BL/6J mice were assessed using a FeCl3-induced intravital microscopy thrombosis model. Results: Both Cy-3-g and Dp-3-g significantly inhibited platelet aggregation induced by collagen and TRAP in gel-filtered platelets, and inhibited aggregation induced by ADP, TRAP and collagen in human and mouse PRP. These inhibitory functions were observed at Cy-3-g and Dp-3-g doses as low as 0.5μM. Cy-3-g and Dp-3-g also reduced the surface expression of activated GPIIbIIIa on resting human platelets in a dose-dependent manner. These compounds also markedly reduced platelet adhesion and aggregation in perfusion chamber assays at both low and high shear rates. Using intravital microscopy, we further demonstrated that Cy-3-g and Dp-3-g decreased platelet deposition, destabilized thrombi, and prolonged the time required for thrombus formation and vessel occlusion. Conclusions: our data clearly demonstrated for the first time that anthocyanin compounds directly inhibited platelet activation, adhesion and aggregation, as well as attenuated thrombus growth at both arterial and veinous shear stresses. These effects on platelets likely contribute to the protective effects of anthocyanins against thrombosis and CVDs. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 484 Background: Plasma fibronectin (pFn) is an abundant protein in the blood. It has long been suspected that pFn plays a role in thrombosis/hemostasis, but this has remained controversial. Our previous study using pFn deficient mice demonstrated that pFn supports thrombosis (PNAS. 2003; 100: 2415-9). Unexpectedly, depletion of pFn in fibrinogen (Fg) and von Willebrand factor (VWF) double deficient (Fg/VWF−/−) mice enhanced, rather than abolished, platelet aggregation and thrombus formation, revealing a functional switch of pFn in the presence and absence of Fg and VWF (Blood. 2009;113:1809-17). However, the mechanism that controls this switch is not known. Furthermore, the hemostatic function of pFn in VW disease (VWD) or afibrinogenemia is unclear. Methods: To address these questions, we bred pFn conditional knockout mice with VWF−/− or Fg−/− mice, establishing 2 new strains of mice: Fg/pFn−/− and VWF/pFn−/−. We also extended our studies of pFn in the triple knockout (TKO, Fg/VWF/pFn−/−) mice. PolyI-polyC was injected into Cre+ and Cre- mice, which resulted in the depletion of plasma pFn (〉98%) and platelet pFn (〉80%) in Cre+ mice but not in Cre- littermate controls. Aggregometry, a perfusion chamber system, thromboelastography (TEG), tail vein bleeding assay and intravital microscopy were used to study these mice. Results: We first observed a significantly higher mortality in TKO (25%, P

Description: Abstract 3686 In the International Consensus Report (Rodeghiero et al. Blood 2009;113:2386-2393), platelet transfusions are only recommended in patients with auto-immune thrombocytopenia (ITP) as an adjunctive therapy for life-threatening bleeding. However, the clinical evidence of post-transfusion increments in ITP suggests that in some cases of ITP, platelet transfusions may be a useful prophylactic therapy. To understand the importance of platelet transfusion in this setting, we utilized our murine model of ITP that demonstrates both antibody- and CTL-mediated thrombocytopenia. BALB/c CD61 knockout (KO) mice were immunized by transfusions of three different platelet populations that were either single antigen positive (e.g. CD61+ or MHC class I+) or double antigen positive (CD61+/MHC class I+). Splenocytes from the immune KO mice were either not depleted (ND) or depleted of CD19+ B cells and then transferred into SCID mice and the development of thrombocytopenia and bleeding diathesis was monitored. When ND splenocytes from MHC class I+ immune KO mice were transferred, no thrombocytopenia or bleeding was observed. In contrast, transfer of ND splenocytes from CD61+ immune mice induced a significant thrombocytopenia with a bleeding mortality of 40% within 3 weeks post-transfer; depletion of B cells was used to analyze CD8+ T cell- mediated thrombocytopenia and these depleted splenocytes also induced thrombocytopenia and bleeding. In addition, compared with healthy mice, the bone marrow of the transferred mice showed elevated numbers of megakaryocytes with some of them appearing to be apoptotic. On the other hand, when splenocytes from the double immunized (CD61+/MHC class I+) mice were transferred, the CD8+ T cell- mediated thrombocytopenia and bleeding mortality was not observed. In addition to preventing the thrombocytopenia and bleeding, histological analysis of the bone marrow showed normal numbers of megakaryocytes. These results suggest that both antibody- and CD8+ T cell-mediated immune thrombocytopenia exist and that when anti-CD61 specific CD8+ T cells interacted with allogeneic MHC antigens on platelets they were prevented from inducing CD8+ T cell-mediated thrombocytopenia. Thus, allogeneic MHC platelet transfusions may be responsible for inhibiting CD8+ T cell-mediated thrombocytopenia and may be a therapeutic option for those patients who suffer from cell-mediated ITP. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 3351 Transfusion related acute lung injury (TRALI) is a serious complication of transfusion. The pathogenesis of TRALI is not fully understood but previous findings have suggested that platelet depletion can protect mice in a two-hit model of TRALI (Looney et al J Clin Invest 119:3450, 2009). To further understand the role of platelets in preventing antibody-mediated TRALI, two mouse models of immune thrombocytopenia (ITP) were utilized. In the passive ITP model, SCID mice were injected with a monoclonal anti-platelet antibody (MWReg30) intraperitoneally (ip, 18 h before TRALI induction) or intravenously (iv, 2 h before TRALI induction). In the active ITP model, SCID mice were transferred with splenocytes from anti-CD61 immune GPIIIa-knockout mice and thrombocytopenia occurred within 2 weeks post transfer (Chow et al Blood 115;1247, 2010). TRALI induction was performed by injecting the various thrombocytopenic SCID mice with a murine monoclonal MHC class I antibody (mAb, 34-1 -2s) iv and several parameters were observed for up to 2 h post antibody injection. In control, non-thrombocytopenic SCID mice, 34-1 -2s injection caused severe systemic shock as noted by reduced rectal temperatures which was associated with significant lung damage and mortality (45%) within 1 hour of 34-1 -2s infusion as previously shown (Fung et al. Blood DOI 10.1182/blood-2010-05-284570). In contrast, while SCID mice depleted of platelets by the passive ip route had systemic shock, lung damage and a 60% mortality rate, those mice made thrombocytopenic by the iv route were completely protected from mortality. On the other hand, in the active ITP model, where the induced thrombocytopenia is associated with a proinflammatory anti-platelet immune response, no mortality was observed in those mice made thrombocytopenic by antibody-mediated immune mechanisms whereas 80% of mice rendered thrombocytopenic by CD8+ T cell-mediated immunity were dead within 1 hr post 34-1 -2s infusion. These results suggest that thrombocytopenia in itself does not protect against antibody-mediated TRALI severity but the nature of the thrombocytopenia induction (e.g. acute passive iv infusion or active ITP immune transfer) is important. In fact, depending on the inflammatory milieu associated with the thrombocytopenia, platelets may actually increase the severity of TRALI. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 4671 TTP results from deficiency of ADAMTS13 that leads to accumulation of high molecular weight multimers (HMWM) of von Willebrand factor (VWF) and thrombosis in the microvasculature. Previous studies have shown that HMWM VWF purified from blood group O individuals were cleaved faster by ADAMTS13 compared to HMWM VWF from non-O blood group individuals. We hypothesized that blood group O patients have lower prevalence of TTP and/or less severe form of TTP. We conducted a retrospective chart review of all TTP patients treated at our institution from 1993 to 2010. The patients were identified from the review of plasmapheresis treatment records. Any patients with missing charts were excluded. The diagnosis of TTP was made on the basis of laboratory evidence of microangiopathic hemolytic anemia and thrombocytopenia and neurologic and/or renal abnormalities, following exclusion of alternative diagnoses. ADAMTS13 levels were determined by collagen binding assay in select patients. 94 patients were included in the study. The patient characteristics were as follows: 32 males versus 62 females, mean age 45.4 years. 37/94 (39%) patients had ADAMTS13 analyzed with the following results: low ADAMTS13 (21 patients), normal ADAMTS13 (15 patients), equivocal 1. Presentation laboratory investigations were available for 91 patients and were as follows: median platelet count 16×109/L (min 3, max 274) and median hemoglobin 89 g/L (min 48, max 152). At presentation, 19/91 (20.9%) patients had fever (〉38C) and 43/91 (47%) had neurological abnormalities (ranging in severity from headache to seizures and decreased level of consciousness). The blood group distribution in the TTP cohort was blood group O 41 patients, non-O 48 patients, data not available 5 patients. All patients received plasmapheresis with cryosupernatant plasma (CSP) or FFP, with some also receiving steroids and other therapies. The patients received a median of 12 (min 1, max 67) plasmapheresis treatments. Overall mortality rate was 22%. The other outcomes were as follows: death at 6 months follow-up 19%; death at 6 months or relapse 20%, death during treatment or permanent dialysis or permanent neurological disability 21%. The prevalence of blood group O versus non-O in our TTP cohort (n=89) was not significantly different from the prevalence of blood group O versus non-O in the general patient population of our institution over 2000–2010 (n=24,852; p=0.71). As illustrated below, in univariate analysis, TTP patients with blood group O did not have significantly different outcomes compared to TTP patients with non-O group blood. Outcomes Group O patients Non-group O patients p-values Death at 6 months 6/41 11/48 0.32 Death at 6 months or relapse 13/41 20/48 0.16 Death during treatment/permanent disability/permanent dialysis dependence 7/41 10/48 0.38 Conclusion: The prevalence of blood group O vs. non-O was not significantly different between the TTP cohort and our general patient population. Although there appeared to be a trend towards worse outcomes in patients with non-O blood group, there were no statistically significant differences in the outcomes in the patients with blood group O vs. non-O. The outcomes assessed in our study included death at 6 months, death at 6 months or relapse, and death during treatment/permanent neurological disability/permanent dialysis dependence. Disclosures: No relevant conflicts of interest to declare.

Description: Background:Immune thrombocytopenia (ITP) is a common bleeding disorder caused primarily by autoantibodies against platelet GPIIbIIIa (70-80%) and/or GPIb-complex (20-40%). Current theory suggests antibody-mediated platelet destruction occurs in the spleen, via macrophages through Fc-FcγR interactions. However, evidence from us and others demonstrated that anti-GPIbα, but not anti-GPIIbIIIa, can induce thrombocytopenia via an Fc-independent pathway, which is resistant to intravenous IgG (IVIG) therapy in murine ITP models (Blood 2006) and subsequent IVIG studies in human ITP patients, including our recent large patient cohort study (JTH 2014). This suggests that binding of anti-GPIbα antibodies may induce platelet clearance through a presently unidentified mechanism different than that of anti-GPIIbIIIa. Methods: We developed unique mouse anti-mouse monoclonal antibodies (mAbs) in GPIIIa-/- or GPIba-/- mice, which also recognize GPIbα and GPIIbIIIa of different species including human. Flow cytometry, immunofluorescence, and western blotting were used to evaluate whether these mAbs induced platelet activation, neuraminidase-1 translocation and desialylation of the heavily glycosylated GPIbα in the presence of sialidase inhibitor N-acetyl-2,3-dehydro-2-deoxy neuraminic acid (DANA). These experiments were repeated with human platelets and human ITP patient plasma. We further investigated the effects of anti-GPIbα antibodies on platelet activation, desialylation and clearance in vivo; BALB/c mice were injected with anti-GPIbα or anti-GPIIbIIIa mAbs and following, platelet activation and desialylation were measured by flow cytometry. Hepatocytic Ashwell-Morell receptor (AMR) mediated anti-GPIbα platelet clearance in the liver was examined using immunohistochemistry or blocking the AMR with asialofetuin in both wild-type and macrophage depleted mice. Therapeutic administration of DANA in a murine ITP model assessed the significance of Fc-independent anti-GPIbα mediated platelet clearance in ITP. Results and Discussion: We found that anti-GPIbα, but not anti-GPIIbIIIa antibodies, induced significant P-selectin expression, JON/A binding, neuraminidase-1 translocation and desialylation in murine platelets. Interestingly, certain human platelets were activated (P-selectin expression) and desialylated in the presence of both anti-GPIbα and anti-GPIIbIIIa mAbs or ITP patient plasma. However, we demonstrate that the anti-GPIIbIIIa antibody mediated platelet effects are dependent on the FcγRIIa present exclusively on human platelets as FcγRII blocker IV.3 completely attenuated the response. In contrast, IV.3 had little effect on anti-GPIbα mediated platelet activation or desialylation. Anti-GPIbα Fab fragments and platelet signal pathway inhibitors demonstrate that anti-GPIbα mediated platelet activation and desialylation are consequences of GPIbα cross linking and are reinforced by a positive feedback loop. In vivo, we found significant increases in P-selectin and desialylation in anti-GPIbα injected mice, independent of IgG subclass. A significant role for the hepatic AMR in the clearance of deglycosylated platelets was observed; particularly in macrophage depleted mice whereby, although anti-GPIIbIIIa mediated platelet clearance was completely attenuated, anti-GPIbα mediated platelets clearance still occurred, but was completely rescued with asialofetuin. Immunohistochemistry revealed significant co-localization of anti-GPIbα opsonized platelets with AMR. These suggest the AMR is the dominant Fc-independent anti-GPIbα mediated platelet clearance pathway in the absence of macrophages. Remarkably, sialidase inhibitor DANA ameliorated anti-GPIbα mediated thrombocytopenia in mice. Thus, we demonstrate for the first time that anti-GPIbα antibodies induce platelet activation leading to GPIbα desialyation and platelet clearance via a novel Fc-independent pathway: the hepatic AMR. Our data also suggested that some anti-GPIIbIIIa autoantibodies in human patients may also induce platelet activation and desialylation via the platelet FcR signaling pathway. These findings may lead to novel therapeutic regimens including designating desialylation as a potential diagnostic biomarker and therapeutic target in the treatment of both anti-GPIIbIIIa and anti-GPIbα mediated and/or refractory ITP. Disclosures No relevant conflicts of interest to declare.

Description: Integrin αIIbβ3 plays key roles in thrombosis and hemostasis primarily through mediating platelet adhesion and aggregation. We recently reported that the active site of thiol-isomerase enzymes, CXXC motif, is expressed twice within the plexin-semaphorin-integrin (PSI) domain across all integrins and species, and the PSI domain of β3 integrin possesses endogenous thiol-isomerase activity, which may be a novel target for anti-thrombotic therapy (Blood, 2017). We developed four mouse anti-mouse β3 integrin PSI domain monoclonal antibodies (mAbs). These mAbs cross-react with β3 PSI domains of human, mouse, pig, rat, and rabbit tested but not other regions of β3 integrin, other integrins or other thiol-isomerase enzymes. They inhibit the thiol-isomerase activity of β3 PSI domain, decrease platelet adhesion/aggregation and thrombosis without increasing bleeding. Interestingly, the inhibitory effect of these mAbs on thrombosis in vivo (no anti-coagulant) was 10-20 times greater than their inhibitory effect on platelet aggregation in anti-coagulated platelet-rich plasma in vitro. This motivated us to explore whether this PSI domain contributes to blood coagulation. To asses blood clot formation and retraction, blood was incubated in non-stick tubes for two hours at 37°C in clot retraction assays. These assays showed less clot retraction and significantly lower dry clot weight in human and mouse whole blood treated with these anti-PSI mAbs compared to controls (p

Description: Antithrombin (AT) exhibits anti-inflammatory properties that reduce mortality in sepsis models. We examined the effects of AT on leukocyte-endothelial interactions in sepsis by using intravital microscopy to measure leukocyte rolling and firm adhesion in post-capillary venules of the cremaster muscle in live mice. Human AT (0.25U g−1 i.v. of Thrombate III, Bayer Corporation) or vehicle was infused prior to proinflammatory stimuli. We evaluated venules with diameters between 20 and 50 μm and shear rates between 200 and 800 s−1. The average diameter and shear rate was not different between treatment groups. The number of leukocytes rolling past a defined vessel point was expressed as leukocyte rolling flux, which normalizes for leukocyte count and flow rate. The number of firmly adherent leukocytes (stationary for 30 s) was normalized for the area of vessel wall analyzed. We evaluated C57BL/6 mice in three inflammatory models. When inflammation was caused by surgical isolation of the cremaster, AT decreased leukocyte rolling flux (28±2 vs. 21±2%, P=0.02) but not the number of firmly adherent leukocytes (41±5 vs. 51±7 per μm2, NS). In contrast, AT did not effect leukocyte rolling flux when inflammation was induced by tumor necrosis factor alpha (0.5 μg intrascrotally). Thus in this model, either pro- and anti-inflammatory effects are balanced or leukocyte rolling involves other interactions that are AT insensitive. Similarly, AT did not affect leukocyte rolling flux induced by an injection of lipopolysaccharide (LPS at 1 μg i.p.) 8 h before observation. However, in this model, AT did inhibit the number of firmly adherent leukocytes (72±6 vs. 39±6 per μm2, P

Description: Apoptosis, or programmed cell death, is appreciated as the main physiologic mechanism that regulates cell life-span and serves for controlled deletion of unwanted cells. Since its discovery in 1972, apoptosis was long attributed exclusively to nucleate cells. It took more than 20 years to recognize apoptosis in enucleated cells cytoplasts and anucleate platelets. During the following years, apoptosis has been demonstrated in platelets treated with natural and artificial agonists, in platelet concentrates aged during storage under standard blood banking conditions, and in animal models of suppressed thrombopoiesis and thrombocytopenia. Other studies documented that mechanical forces (shear stresses) stimulate platelet activation and signaling in the absence of exogenous chemical stimuli. We analysed whether shear stresses can trigger platelet apoptosis, a question that has not yet been studied. Using a cone-and-plate viscometer (CAP-2000, Brookfield Engineering Labs, Inc., Middleboro, MA), we exposed human platelet-rich plasma to different shear stresses, ranging from physiologic arterial and arterioles levels (10–44 dynes/cm2) to pathologic high levels (117–388 dynes/cm2) occurring in stenosed coronary, peripheral or cerebral arteries. We found that pathologic shear stresses induce not only platelet activation (P-selectin upregulation and GPIb-alpha downregulation) but also trigger apoptosis events, including mitochondrial transmembrane potential depolarization, caspase 3 activation, phosphatidylserine exposure, and platelet shrinkage and fragmentation into microparticles, whereas physiologic shear stresses are not effective. Platelets subjected to pathologic shear stresses are characterized by impaired platelet function as shown by the absence of ADP-induced platelet aggregation. Apoptosis changes were also induced by the treatment of platelets with calcium ionophore A23187 (10 μM) and thrombin (1 U/mL). Thus, in the present work, we have demonstrated that platelet apoptosis can be induced by chemical stimuli and by mechanical rheological forces (pathologic high shear stresses). Most of shear-induced activation and apoptosis events occur inside of the platelet, including translocation of CD62 from alpha-granules to the platelet surface, depolarization of mitochondrial inner membrane potential, activation of cytosolic enzyme caspase 3, and translocation of phosphatidylserine from the inner to the outer plasma membrane leaflet. These data suggest that the effects of shear stress on platelet activation and apoptosis are mediated by mechanoreceptor(s) that transmit activation and apoptosis signals to the cell interior. The platelet paradigm of apoptosis induced by chemical agonists and shear stresses suggests that apoptotic cytoplasmic machinery may function without nuclear participation.