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BACKGROUND: Functional tests for the diagnosis of heparin-induced thrombocytopenia (HIT) exhibit variable performance. OBJECTIVES: We evaluated in a multicenter study whether 5B9, a monoclonal anti-PF4/heparin IgG mimicking human HIT antibodies, could be used as an internal quality control. METHODS: 5B9 was sent to 11 laboratories in seven countries, and six initial concentrations ranging from 10 to 400 μg/mL were tested by heparin-induced platelet activation assay (HIPA), serotonin release assay (SRA), platelet aggregation test (PAT), flow cytometry (FC), or heparin-induced multiple-electrode aggregometry (HIMEA). Each method was evaluated in three different laboratories using experimental procedures identical to those usually applied for the diagnosis of HIT by testing platelets from 10 different healthy donors. RESULTS: The procedures used varied among the laboratories, particularly when platelet-rich plasma and whole blood were used. Nevertheless, positive results were obtained with at least 100 μg/ml of 5B9 for most donors tested by all centers (except one) performing HIPA, SRA, or HIMEA. FC and PAT results were more heterogeneous. FC results from one center that used washed platelets preincubated with PF4 were positive with all donors at 50 µg/ml 5B9, but at least 200 μg/ml of 5B9 were required to activate cells with most donors tested using PAT. CONCLUSION: This study confirms that HIT functional tests are not well standardized and exhibit variable sensitivity for the detection of platelet-activating antibodies. However, 5B9 is a potentially useful tool to standardize functional tests, to select responding platelet donors, and consequently to improve the performance of these assays and comparability between laboratories.

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare yet serious adverse effect of adenoviral vector vaccines (AstraZeneca and Johnson & Johnson) against COVID-191. Anti-platelet factor 4 (PF4) antibodies are present in VITT patients2,3. Although the current view suggests that platelet activation by anti-PF4 antibodies is the cause of thrombosis there is as yet no direct evidence that the antibodies induce clot formation and thrombocytopenia (reduction in platelet counts) in VITT and the mechanisms involved remain unknown4. Here we show that VITT antibodies induce thrombosis and thrombocytopenia, and that thrombus formation is mediated by neutrophil extracellular traps (NETs). We found markers of NETosis, abundance of neutrophil/platelet aggregates and presence of neutrophils undergoing NETosis in patients with active VITT. VITT antibodies directly stimulate neutrophils to release NETs and induce thrombus formation containing abundant platelets, neutrophils, fibrin, extracellular DNA and citrullinated histone H3 using an in vitro blood flow microfluidic system. In transgenic mice expressing human PF4 and FcγRIIa, VITT antibodies lead to thrombosis, thrombocytopenia and formation of low density granulocytes. Pharmacological and genetic inhibition of NETosis prevents VITT-induced thrombosis in mice but not thrombocytopenia. In contrast, in vivo blockage of FcγRIIa abrogates both thrombosis and thrombocytopenia suggesting they are distinct processes. Our findings indicate that VITT antibodies activate cells via FcγRIIa and are responsible for thrombosis and thrombocytopenia. This study identifies NETosis as a pathogenic mechanism for thrombus formation in VITT. We anticipate our findings will motivate future development of NETosis and FcγRIIa inhibitors as potential specific therapies for VITT and consequently better patient outcomes.

Background: A previously unknown beta-coronavirus was discovered through the use of unbiased sequencing in samples from patients with pneumonia. The virus was named as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) by the international committee for the classification of viruses (ICTV). The disease caused by this virus was named as coronavirus disease 2019 (COVID-19). In addition to pulmonary manifestations, hematological changes such as lymphocytopenia, thrombocytopenia, and coagulation dysfunction can also be found in COVID-19 patients, and the mechanism is still unclear. Case data and methods: A total of 633 COVID-19 patients from Wuhan hospital of China were retrospectively analyzed. Clinical case data of all patients were collected, including gender, age, chronic underlying diseases, outcome, and blood laboratory test results. The hematological features of COVID-19 patients and the factors affecting their outcome were analyzed. Results: Of 633 patients with COVID-19, the median age was 62 years (interquartile range, IQR, 51.0-70.0) and 330 (52%) were men. Lymphocytopenia (lymphocyte count, 1.0 ×109 / L [IQR, 0.7-1.4]) occurred in 317/607 patients (52%), thrombocytopenia (platelet count Conclusion: Hematological changes are common in patients with COVID-19. The early stage of the disease is mainly characterized by lymphocytopenia, thrombocytopenia, and the late stage may be characterized by more severe lymphocytopenia, even neutrophils elevation, elevated C-reactive protein, and severe coagulation disorder. The pathogenesis may be mediated by a direct viral infection and/or indirect immunopathology. Disclosures No relevant conflicts of interest to declare.

Heparin-induced thrombocytopenia (HIT) is an immune reaction to heparin. It often causes severe thrombosis which may lead to limb gangrene and thrombosis-associated death. The concept of its pathogenesis has been evolving during the past five decades. Initially, HIT was thought to be caused by disseminated intravascular coagulation. Later it became clear that HIT was mediated by an immune mechanism whereby an IgG antibody induced platelet aggregation, release of procoagulant materials and consequently thrombus formation. The antigen comprises Platelet Factor 4 (PF4) and heparin which have a tendency to form ultralarge complexes. The HIT immune response has atypical features. IgG antibody appears early without IgM precedence and lasts transiently. One explanation is that there is prior priming by bacterial infection. Another unique characteristic is that it is processed as if it is a particulate antigen involving complement activation and B cells. Antigen-presenting cells/monocytes are also involved but the role of T cells is controversial. Recent advances have provided new insights into the underlying mechanisms of HIT-related thrombosis. Previously, platelets were believed to play a central role; their activation and consequently the induction of blood coagulation was the basis of the hypercoagulability in HIT. More recently, several studies have provided clear evidence that neutrophil and NETosis, monocytes and endothelial cells contribute significantly to the thrombosis in HIT. These new insights may result in development of better diagnostic laboratory assays and more effective treatments for HIT.

Background: Thrombospondin 1 (TSP-1) is an extracellular matrix protein that interacts with a wide array of ligands including cell receptors, growth factors, cytokines, and proteases to regulate various physiological and pathological processes. TSP-1 induces apoptosis of endothelial and cancer cells via its receptor CD36. This study was to investigate the effect of TSP-1 on apoptosis of human megakaryocytic leukemia cell line Meg-01 and its possible mechanism. Methods: The expression of CD36 antigen in Meg-01 cells was detected by flow cytometry and immunocytochemistry. Meg-01 cells were cultured for 48 hours with TSP-1 and CD36 antibody FA6-152 at different concentrations, to investigate the effect of TSP-1 on the growth of megakaryocytes. The early apoptosis and the activity of the apoptotic protease caspase-3 were detected by flow cytometry. Bone marrow cells of mice were cultured for CFU-MK and stained with acetylcholine to detect the differentiation of megakaryocytes. Results: CD36 antigen was detected on the surface of Meg-01 cells by flow cytometry and immunocytochemistry. TSP-1 (5 μg/mL) inhibited the proliferation of Meg-01 cells, but not M-07e cells (CD36 -). After the addition of CD36 antibody FA6-152 (5, 10 and 25 μg/mL), the inhibitory effect of TSP-1 was significantly reduced. TSP-1 (2.5, 5 and 7.5 μg/mL) exerted a pro-apoptotic effect by increasing the expression of Annexin V (P Conclusion: TSP-1 could inhibit the proliferation of Megakaryocyte cell line Meg-01 cells, and induce it`s apoptosis. TSP-1 may induce apoptosis of Meg-01 cells via CD36 or caspase-3, which provides a potential new drug choice for clinical treatment of megakaryocytic leukemia. Disclosures No relevant conflicts of interest to declare.

Background: Angelica Polysaccharide (APS) is from the root of Radix Angelicae Sinensis (Danggui). Danggui has been used for centuries to treat blood-deficiency related diseases. The hematopoietic effect of Danggui may be related to its constituent, polysaccharide. The effects of angelica polysaccharide on cryopreservation of platelets and megakaryocytes have not been well studied. This study focused on anti-apoptotic effect of APS and TPO on cryopreservation of platelets and megakaryocytes and provided new methods for prolonging the preservation time of platelets in vitro. Methods: The expression of platelet membrane glycoprotein CD41 and CD61, as well as the platelet apoptotic rate, Caspase 3 expression and mitochondrial membrane potential (MMP) were detected by flow cytometry; the anti-apoptotic mechanism of APS by PI3K /AKT signaling pathway was analyzed by Western blot assay. CFU assays were used to determine the effects of APS on megakaryocytic progenitor cells. Analyses of Annexin V, Caspase-3, and Mitochondrial Membrane Potential were conducted in megakaryocytic cell line M-07e. The effects of APS on cells treated with Ly294002, PI3K inhibitor and the effect of APS on the p-AKT were also studied. Results: The platelets were divided into 4 group: control group (4 ℃ stored platelets), APS group (APS-treated platelets stored at 4 ℃), LY294002 group (LY294002-treated platelets stored at 4 ℃) and LY294002+APS group (LY294002+APS treated platelets stored at 4 ℃). The apoptotic rate of platelets in LY294002 group was obviously increased. Compared with control group, the expression of CD41 and CD61 gradually decreased along with the enhancement of LY294002 concentrations (r=-0.953). The apoptotic rate of platelets in LY294002 group was enhanced significantly (P Conclusion: APS, like TPO, has an anti-apoptotic effect on the cryopreserved platelets and megakaryocytes through activating PI3K/AKT, decreasing the expression of Caspase-3 and inhibiting the reduction of MMP. Disclosures No relevant conflicts of interest to declare.

Introduction Tanshinone IIA, an active component of Danshen (Salvia miltiorrhiza), has been used for centuries to treat hypercoagulation-related diseases, which attributed to its anti-platelet and anti-inflammatory effects. However, the role of Tanshinone IIA in megakaryocytes, the precursor of platelet within the bone marrow, remains unclear. Therefore, the present study established a rabbit model with immune vasculitis to examine the effect of Tanshinone IIA on megakaryopoiesis and to identify the underlying mechanism(s). Methods Immune vasculitis was established in rabbits (3-4 weeks old) by two intravenous injection of 10% bovine serum albumin (2.5 ml/kg) at two-week interval. Those rabbits were randomly treated with Tanshinone IIA (5 mg/kg/d, 7 d, iv) or aspirin (100 mg/kg/d, 7 d, ig). Megakaryocyte count and CFU-MK formation were measured by Wright's and AChE staining, respectively. Human megakaryotic cell lines Meg-01 and CHRF-288-11 were used to examine the effect of Tanshinone IIA on apoptosis by Annexin V-FITC/PI, mitochondrial membrane potential/JC-1 and Caspase-3 activity assays using flow cytometry. ResultsIn rabbits with immune vasculitis, the platelet count, platelet aggregation and the serum levels of inflammatory cytokines IL-1β, TNF-α and IL-6 were significantly increased when compared to their healthy controls. After 7 days of Tanshinone IIA treatment, all these parameters were significantly reduced, with the inhibitions comparable to those caused by aspirin. In addition, the number of megakaryocytes and the formation of CFU-MK were also statistically increased in rabbits with immune vasculitis, which could be significantly reduced by Tanshinone IIA. In vitro, Tanshinone IIA (1, 3, 10 and 30 μg/ml) also significantly inhibited the formation of CFU-MK of bone marrow cells of BALB/c mice (6-10 weeks) in a dose-dependent manner. In human megakaryocytic cell line Meg-01, Tanshinone ⅡA (10 μg/ml, 72 h) induced apoptosis; both early and late apoptotic rates were significantly increased. In another human megakaryocytic cell line CHRF-288-11, Tanshinone ⅡA (10 μg/ml, 72 h) statistically increased the proportion of depolarized cells, from 9.70% to 14.13%, according to mitochondrial membrane potential using JC-1 assay. The expression of active Caspase-3 in CHRF-288-11 was also significantly increased by Tanshinone ⅡA (10 μg/ml, 72 h) from 5.25% to 15.86%. Conclusion The present study shows that Tanshinone IIA ameliorates immune vasculitis by inhibiting megakaryopoiesis and inducing apoptosis of megakaryocytes, which might explain the anti-platelet and anti-inflammatory effects of Tanshinone IIA. Disclosures No relevant conflicts of interest to declare.

Background: Essential Thrombocythemia (ET) is characterized by persistently elevated platelet counts in the context of a normal red cell mass. However, the molecular mechanism of ET is still under investigation. Our previous studies demonstrated the presence of functional PDGF receptors (PDGFR) on megakaryocytes and their ability to mediate hematopoiesis and megakaryopoiesis (Ye et al, Haematologica, 2010). The role of PDGF-BB on megakaryocytic progenitors (CD41+, CD34+ cells) and its mechanisms on ET will be further studied in this project. Methods: ELISA, CFU assay, immunofluorescence microscope, flow cytometry and NOD/SCID mice were used in this study. Results: Bone marrow plasma levels of PDGF-BB in ET patients (n=18) and normal control (n=10) were tested and found an increased PDGF-BB levels in ET patients (2071.2±124.8 pg/ml), compared with normal control (1382.5±128.3pg/ml) (P=0.002). In vitro experiment, PDGF-BB promoted the ex vivo expansion of human hematopoietic stem (CD34 +) and progenitor (CD41 + CD61 +) cells. More significantly, PDGF enhanced the engraftment of human CD45 + cells and their myeloid subsets (CD33 +, CD14 + cells) in NOD/SCID mice. PDGF-BB stimulated megakaryopoiesis via PDGFR and its signalling. It also showed a direct stimulatory effect of PDGF-BB on c-Fos, GATA-1 and NF-E2 expressions in megakaryocytes. We speculate that these transcription factors might be involved in the signal transduction of PDGF-BB on the regulation of megakaryopoiesis. PDGF-BB also enhanced platelet recovery in mice model with radiation-induced thrombocytopenia. Studies showed that PDGF, like TPO, significantly promoted platelet recovery and the formation of CFU-MK in this irradiated-mouse. An increased number of hematopoietic stem/progenitor cells and a reduction of apoptosis were found in the bone marrow histology sections. In the CHRF apoptotic model, PDGF-BB had a similar anti-apoptotic effect as TPO on megakaryocytes. We also demonstrated that PDGF-BB activated the p -Akt , p-Jak2 and p-Stat3 expression, while addition of imatinib mesylate reduced p-Akt, p-Jak2 and p-Stat3 expression in CHRF cells. Conclusion: Our findings suggested that PDGF-BB is likely to be mediated via PDGF receptors with subsequent activation of the Akt and Jak2/ Stat3 pathways in megakaryopoiesis. These studies provide a possible explanation that PDGF-BB and its signaling may be involved in the molecular mechanism of essential thrombocythemia. Disclosures No relevant conflicts of interest to declare.

Β2-Glycoprotein I (β2GPI) is an important anti-thrombotic protein and is the major auto-antigen in the antiphospholipid syndrome (APS). The clinical relevance of nitrosative stress in post translational modification of β2GPI was examined.The effects of nitrated (n)β2GPI on its anti-thrombotic properties and its plasma levels in primary and secondary APS were determined with appropriate clinical control groups. β2-glycoprotein I was nitrated at tyrosines 218, 275 and 309. β2-glycoprotein I binds to lipid peroxidation modified products through Domains IV and V. Nitrated β2GPI loses this binding (p

Background: Thrombopoietin (TPO) is a hematopoietic growth factor that regulates the production of platelets and stimulates production and differentiation. The expression of TPO and TPO receptor (c-mpl) in the central nervous system (CNS) has been identified. However, the role of TPO in neural cells and endothelial cells were not clear. Methods: C17.2 and human umbilical vein endothelial (HUVEC) cells were treated with CoCl2, TPO, or TPO + CoCl2. TPO was added into the culture medium 48 h before CoCl2 treatment. The cell viability and apoptosis of each group were tested by Cell Counter Kit 8 (CCK-8) assay and flow cytometry. The expression of Caspase-3 and mitochondrial membrane potential (MMP) were then determined by flow cytometry with Caspase-3-PE and JC-1. The effect of TPO in the PI3K/AKT pathway was detected by using Western blot. Results: TPO has a dose-dependent effect on the growth of C17.2 cells. LY-294002 pretreatment suppressed the TPO-induced AKT activation and abolished the prosurvival effect of TPO. Via the Bcl-2/BAX signaling pathway, TPO exerted an anti-apoptotic effect by suppressing mitochondria membrane potentials. We also investigated the protective effect of TPO on human endothelial cells. The cell viability of HUVECs decreased gradually with the enhancement of CoCl2 at a gradient of chemical concentrations (r= -0.997). CoCl2 dramatically increased apoptosis of HUVECs, whereas pre-treatment with TPO rescued cell apoptosis induced by CoCl2 (P Conclusion: TPO has a protective effect against apoptosis of neural cells and endothelial cells through activating the PI3K/AKT pathway, thus decreasing the expression of apoptosis protease Caspase-3 and inhibiting the reduction of MMP. Disclosures No relevant conflicts of interest to declare.

Background The notable discrepancy between platelet count and bleeding manifestations in immune thrombocytopenia (ITP) patients with acquired Glanzmann thrombasthenia (GT) has been described. Objectives We aimed to examine the mechanisms responsible for thrombocytopenia and the bleeding phenotype in a patient with acquired GT. Patient, methods, and results A patient with primary ITP underwent splenectomy due to steroid intolerance. Despite platelet count normalization, bleeding continued. Platelet aggregometry was abnormal with all agonists except for ristocetin. Flow cytometry demonstrated the presence of antiplatelet antibody, which caused dose-dependent inhibition of fibrinogen and PAC-1 binding, induction of neuraminidase-1 expression as well as platelet desialylation in donor platelets. Indirect monoclonal antibody immobilization of platelet specific antigen assay (MAIPA) confirmed specificity to αIIb β3 only, corroborated by binding on Chinese hamster ovary (CHO) cells expressing human glycoprotein αIIb β3 but not GP Ib/IX. Both desialylation and neuraminidase expression were observed with plasma adsorbed on Ib/IX CHO cells and with the immunoglobulin G (IgG) fraction. Desialylation was inhibited in the presence of anti-Fc-gamma receptor IIa (FcγRIIa) antibody. A nonobese diabetic/severe combined immunodeficient ITP murine model was established, which showed rapid hepatic donor platelet clearance in the presence of patient IgG. Treatment of mice with the neuraminidase inhibitor oseltamivir significantly reduced antibody-induced platelet destruction. Conclusions We report the first case of a patient with acquired GT due to ITP with FcγRIIa mediated platelet desialylation, independent of platelet activation. Treatment with neuraminidase inhibitor may prevent platelet clearance by anti-αIIb β3 antibodies.

Heparin-induced thrombocytopenia/thrombosis (HIT) is a serious immune reaction to heparins, characterized by thrombocytopenia and often severe thrombosis with high morbidity and mortality. HIT is mediated by IgG antibodies against heparin/platelet factor 4 antigenic complexes. These complexes are thought to activate platelets leading to thrombocytopenia and thrombosis. Here we show that HIT immune complexes induce NETosis via interaction with FcγRIIa on neutrophils and through neutrophil-platelet association. HIT immune complexes induce formation of thrombi containing neutrophils, extracellular DNA, citrullinated histone H3 and platelets in a microfluidics system and in vivo, while neutrophil depletion abolishes thrombus formation. Absence of PAD4 or PAD4 inhibition with GSK484 abrogates thrombus formation but not thrombocytopenia, suggesting they are induced by separate mechanisms. NETs markers and neutrophils undergoing NETosis are present in HIT patients. Our findings demonstrating the involvement of NETosis in thrombosis will modify the current concept of HIT pathogenesis and may lead to new therapeutic strategies., The pathogenesis of heparin-induced thrombocytopenia and thrombosis (HIT) is mediated by heparin-reactive autoantibodies binding to platelets (thrombocytes). Here the authors show neutrophil activation and NETosis are elevated in patients with HIT, and are essential for thrombosis in HIT mouse models.

Background: Platelets are produced by megakaryocytes and are primarily regulated by thrombopoietin (TPO). In the conditions of inflammation or stress, increased 5-HT and other inflammatory cytokines may affect liver and bone marrow to increase TPO and platelet production (Yang et al., Stem Cells, 2007; 2014). Mature megakaryocytes are fragmented in pulmonary circulation to generate more platelet intermediate proplatelets, then entered the circulation. We hypothesized that in patients with cerebral infarction, the secretion of inflammatory cytokines increases under stress or inflammatory conditions, increasing circulating TPO levels, followed by an increase in platelet production, and an increase in the number of platelet intermediate proplatelets. At the same time, platelets also have varying degrees of activation. Therefore, the increase in the number of proplatelets in the circulation and the increase in TPO levels may be the new pathological mechanism of cerebral infarction. Methods: The plasma levels of TPO were detected by ELISA. The in vivo proplatelets from patients were stained with CD41-FITC, which was further confirmed under a fluorescence microscope and flow cytometry. The in vitro proplatelets were observed in a culture system by CD41-FITC staining under a fluorescence microscope and flow cytometry detection. Results: Our clinical data demonstrated that patients with acute inflammation states, plasma TPO levels were significantly higher compared with healthy subjects (181.1 ± 35.38 vs 96.1 ± 9.7 pg/ml, p Conclusions: The study demonstrated that increased numbers of proplatelets in circulation and elevated plasma TPO levels may be novel pathological mechanisms of cerebral infarction Disclosures No relevant conflicts of interest to declare.

Background: Our previous work confirmed that serotonin (5-HT) promotes the proliferation of hemopoietic stem cells and megakaryocytes (Yang M et al, Stem Cells, 2007; 2014). However, the mechanisms remain indefinite. Methods: Q-PCR, Flow Cytometry, Western Blot, or Immunofluorescence microscope were used in the receptor and TPO study. MTT/CCK-8, Proplatelet assay, and Flow Cytometry were also used in cell proliferation and apoptosis study. The relationship between 5-HT and TPO was studied in a traumatic stress mice model. Results: In-vitro study, there was a stimulating effect of 5-HT on proplatelet formation in human bone marrow megakaryocytes. Human BM MK progenitors cultured in serum-free medium with either 5-HT (200nM) or TPO (100 ng/ml) had more proplatelet bearing MKs than the control group (5-HT (12.3 ± 5.0)% vs. Control (6.2 ± 3.5)%, P=0.025; TPO (15.6 ± 2.5)% vs. Control, P=0.04; n=4). The 5-HT treatment group showed more mature and more in the final stage MK cells as compared to the TPO group. 5-HT2A, 2B, 2C receptors were detected in the surface of megakaryocytes. The effect of 5-HT on proplatelet formation in MK cells was via 5-HT2 receptors and this effect was reduced by 5-HT2 receptor inhibitor ketanserin. 5-HT acted on cytoskeleton reorganization in MKs via 5-HT2 receptors and ERK1/2 pathway. Using an immunofluorescence microscope with F-actin specific binder rhodamine-phalloidin staining, the polymerized actin level was lower in the control group than the 5-HT group and actin distributed diffusely throughout the cytoplasm. In contrast, the polymerization actin level was higher in the 5-HT group. Adding ketanserin and ERK1/2 inhibitor PD98059 to 5-HT treatment, the fluorescence intensity was correspondingly reduced. Our data also demonstrated that ERK1/2 was activated in MKs treated with 5-HT for 30 minutes. In a traumatic stress mice model, both of 5-HT and TPO were increased, but the increasing of TPO is posterior to 5-HT. After added LX1606, the synthesis inhibitor of 5-HT, 5-HT was reduced markedly, as well as TPO. The expression of TPO mRNA and the production of TPO protein were increased as compared with the control in this model. Conclusions: This study suggests that 5-HT promotes thrombopoiesis from two aspects: one is the direct effect on megakaryocytes. 5-HT could promote the proplatelet formation from megakaryocytes. The second is the indirect effect by promoting the production of TPO, which is a paracrine secretion to influence thrombopoiesis. Disclosures No relevant conflicts of interest to declare.

Platelet production occurs principally in the bone marrow in a process known as thrombopoiesis. During thrombopoiesis, hematopoietic progenitor cells differentiate to form platelet precursors called megakaryocytes, which terminally differentiate to release platelets from long cytoplasmic processes termed proplatelets. Megakaryocytes are rare cells confined to the bone marrow and are therefore difficult to harvest in sufficient numbers for laboratory use. Efficient production of human megakaryocytes can be achieved in vitro by culturing CD34(+) cells under suitable conditions. The protocol detailed here describes isolation of CD34(+) cells by magnetic cell sorting from umbilical cord blood samples. The necessary steps to produce highly pure, mature megakaryocytes under serum-free conditions are described. Details of phenotypic analysis of megakaryocyte differentiation and determination of proplatelet formation and platelet production are also provided. Effectors that influence megakaryocyte differentiation and/or proplatelet formation, such as anti-platelet antibodies or thrombopoietin mimetics, can be added to cultured cells to examine biological function.

Promising reports of combination immunosuppression with high-dose dexamethasone and rituximab for the treatment of primary immune thrombocytopenia (ITP) have recently emerged. They suggest a potential to further optimize the efficacy of therapy. We investigate the use of a novel combination of conventional therapies in ITP given over 4 weeks. From 2011 to 2014, 20 patients were prospectively enrolled onto a single-arm phase 2b study to describe the safety, efficacy, and tolerability of oral dexamethasone 40 mg for days 1 to 4, oral cyclosporine 2.5 to 3 mg/kg daily for day 1 to 28, and intravenous low-dose rituximab 100 mg for days 7, 14, 21, and 28. There were no therapy-related serious adverse side effects, 6-month response rate was 60%, and treatment was well tolerated. Responders enjoyed relapse-free survivals of 92% and 76%, respectively, at 12 and 24 months. This study highlights the possibility of achieving an enduring remission from 4 weeks of therapy. This study is registered at www.anzctr.org.au (#ANZCTRN12611000015943).

Heme oxygenase-1 is critical for iron recycling during red blood cell turnover, whereas its impact on steady-state erythropoiesis and red blood cell lifespan is not known. We show here that in 8- to 14-week old mice, heme oxygenase-1 deficiency adversely affects steady-state erythropoiesis in the bone marrow. This is manifested by a decrease in Ter-119+-erythroid cells, abnormal adhesion molecule expression on macrophages and erythroid cells, and a greatly diminished ability to form erythroblastic islands. Compared with wild-type animals, red blood cell size and hemoglobin content are decreased, while the number of circulating red blood cells is increased in heme oxygenase-1 deficient mice, overall leading to microcytic anemia. Heme oxygenase-1 deficiency increases oxidative stress in circulating red blood cells and greatly decreases the frequency of macrophages expressing the phosphatidylserine receptor Tim4 in bone marrow, spleen and liver. Heme oxygenase-1 deficiency increases spleen weight and Ter119+-erythroid cells in the spleen, although α4β1-integrin expression by these cells and splenic macrophages positive for vascular cell adhesion molecule 1 are both decreased. Red blood cell lifespan is prolonged in heme oxygenase-1 deficient mice compared with wild-type mice. Our findings suggest that while macrophages and relevant receptors required for red blood cell formation and removal are substantially depleted in heme oxygenase-1 deficient mice, the extent of anemia in these mice may be ameliorated by the prolonged lifespan of their oxidatively stressed erythrocytes.

Primary immune thrombocytopenia is an autoimmune disorder characterized by increased platelet destruction and insufficient platelet production without another identified underlying disorder. Splenectomy may alter responsiveness to treatment and/or increase the risk of thrombosis, infection, and pulmonary hypertension. The analysis herein evaluated the safety and efficacy of the thrombopoietin receptor agonist romiplostim in splenectomized and nonsplenectomized adults with primary immune thrombocytopenia. Data were pooled across 13 completed clinical studies in adults with immune thrombocytopenia from 2002-2014. Adverse event rates were adjusted for time of exposure. Results were considered different when 95% confidence intervals were non-overlapping. Safety was analyzed for 1111 patients (395 splenectomized; 716 nonsplenectomized) who received romiplostim or control (placebo or standard of care). At baseline, splenectomized patients had a longer median duration of immune thrombocytopenia and a lower median platelet count, as well as a higher proportion with >3 prior immune thrombocytopenia treatments versus nonsplenectomized patients. In each treatment group, splenectomized patients used rescue medications more often than nonsplenectomized patients. Platelet response rates (≥50×109/L) for romiplostim were 82% (310/376) for splenectomized and 91% (592/648) for nonsplenectomized patients (P

Background: Previous studies have shown that the vascular niche was damaged in patients with immune thrombocytopenia (ITP). It is mainly manifested that the migration and tube-formation ability of bone marrow endothelial progenitor cells (BM-EPCs) were significantly reduced. Endothelial cells as an important part of the vascular niche play an important role in regulation of megakaryocytes and platelet hematopoiesis. Our previous studies suggested that thrombopoietin (TPO) has a protective effect on human endothelial cells. This study aim to investigate whether TPO has a protective effect in EPCs of ITP patients and to explore its mechanism. Methods: BM-EPCs were isolated from ITP patients, and divided into two groups: TPO-treated group and TPO-free (control group). CCK8 was used to explore whether TPO has proliferative effect on BM-EPCs. BM-EPCs was identified by flow cytometry with co-expression of mouse anti-humanCD133, mouse anti-human CD34 and mouse anti-human vascular endothelial growth factor receptor (CD309) monoclonal antibodies. Moreover, the number of EPCs was evaluated with DiL-Ac-LDL uptake and FITC-UEA -I binding assay. Furthermore, the function of BM-EPCs was studied by using tube-formation and migration experiments. Results: The ITP patient-derived EPCs in TPO-treated group exhibited longer fusiform and elliptical shape after 5 days and cobblestone-like morphology after 7-10 days. The result of CCK-8 showed TPO could promote the proliferation of EPCs and the optional drug concentration is 100 ng/ml. Much higher expression of CD34/CD133/CD309 in TPO-treated group was detected by flow cytometry. The number of cells with immunofluorescence double staining in TPO group was significantly higher than of control group (n=4, p Conclusion: Our studies indicates that TPO may exert megakaryopoiesis effect in ITP patients via protection of BM-EPCs. This may provide a supplemental explanation of how recombinant TPO and TPO receptor antagonist acts on ITP treatment. Keywords: thrombopoietin; vascular niche; endothelial progenitor cell, megakaryopoiesis; endothelial cell Disclosures No relevant conflicts of interest to declare.

Background: Huangqi (the dried root of Radix Astragali) is a notable Chinese herb that has been used to promote hematopoiesis for centuries. Astragalus polysaccharide (ASPS), a bioactive constituent extracted from Radix Astragali, may play an essential role for hematopoiesis. The objective of this study is to investigate the hematopoietic effects of ASPS in mouse models and explore underlying mechanism, which provides scientific basis for future applications. Methods: An irradiation-induced myelosuppressive mouse model and colony-forming unit (CFU) assays were used to determine the in vivo hematopoietic effects of ASPS. The anti-apoptotic effects of ASPS were also examined by using annexin V, caspase-3, and JC-1 assays with flow cytometry in HL-60 cells. Results: In the myelosuppressed mouse model, ASPS enhanced the recovery of platelet and red blood cell at day 21, and white blood cell at day 14. ASPS also promoted the CFU formation of all lineages (MK, GM, E, GEMM and F). Morphological study of bone marrow revealed that as compared to the control group, tri-lineage hematopoiesis was preserved in the ASPS- and TPO-treated group. Overall cellularity (MTC/area) of ASPS-treated group was similar to that of TPO. Furthermore, in-vitro study showed that 100 μg/ml of ASPS had maximum effect on CFU formation. ASPS also reduced apoptosis via inhibiting mitochondrial/Caspase-3 signal pathway. Conclusions: ASPS may promote hematopoiesis in irradiated myelosuppressive mice by enhancing hematopoietic proliferation and inhibiting apoptosis of hematopoietic cells. Disclosures No relevant conflicts of interest to declare.

Thrombocytopenia is a common clinical condition and requires more effective treatments. Our previous original work confirmed that 5-HT contributes to the expansion of cord blood stem/progenitor cells (CD34+/CD41+CD61+) and haematopoietic reconstitution in NOD/SCID mice, and through 5-HT2R and Erk1/2 pathway, 5-HT remodel F-actin organization, promoting megakaryocytes to pro-platelet formation (Stem Cells, 2007; 2014). It is suggested that there is a subtype of haematopoietic stem cells (CD34+5HT2R+) with the potential to differentiate into megakaryocytes (CD41+5HT2R+). We studied the specific functions of cells through subsequent experiments, including receptor expression and signaling pathways. Results showed that: (1) The presence of 5-HT2 receptors on CD34+ and CD41+ cells; (2) 5-HT activated the PI3K/AKT pathway and the Wnt/β-catenin pathway via 5-HT2R in CD34+ and CD41+ cells, with anti-apoptotic and pro-proliferative functions; (3) These CD34+5HT2R+ differentiated CD41+5HT2R+ cells, 5-HT acted on 5-HT2R and Erk1/2 pathways, and promoted the formation of pro-platelets by F-actin reorganization; (4) CD34+5HT2R+ haematopoietic stem cells implanted in NOD/SCID mice reconstituted blood function, especially on thrombopoiesis. This study provides a more scientific basis for haematopoiesis and drug development, especially for the treatment of thrombocytopenia. Disclosures No relevant conflicts of interest to declare.

Thrombopoietin (TPO) is a growth factor for the megakaryocytic lineage. The expression of TPO and TPO receptor (c-mpl) in the central nervous system (CNS) and the role of TPO in neural cells and brain damage models were investigated. Our results showed the expression of TPO in human cerebral hemisphere, cerebellum, cerebrospinal fluid and blood plasma. We found that TPO had a protective effect in hypoxic-ischemic rat model, as indicated by the increased ipsilateral brain weight and neuron density in a neonatal rat model of hypoxic-ischemic brain damage. Recoveries of sensorimotor functions and histopathology were observed in these animals that received TPO. In addition, TPO could promote C17.2 cells proliferation by activating PI3K/Akt signaling pathway, and the proliferation could be reduced to nearly basal level by the pre-treatment with LY 294002. The phosphorylation of AKT, which is a hallmark of activation of each molecule was significantly enhanced after the treatment with TPO in the cells, peaking at 30 min after stimulation with TPO. TPO was also found to have an anti-apoptotic effect which mediated via Bcl-2/BAX and suppressing the mitochondrial membrane potential. Results showed the increased level of Bcl-2 and decreased level of BAX were in the time-dependence manner (0, 5, 15, 30 and 60 mins) in these cells. In addition, the mitochondrial membrane potential was significantly decreased by adding 100 ng/ml TPO. Our results indicated that TPO have neural protective effects. Disclosures No relevant conflicts of interest to declare.

Parkinson's disease (PD) is a multicentred neurodegenerative disorder characterised by the accumulation and aggregation of alpha-synuclein (α-syn) in several parts of the central nervous system. However, it is well established that PD can generate symptoms of constipation and other gastrointestinal problems and α-syn containing lesions have been identified in intestinal nerve cells. In this study, we show that α-syn can be taken up and accumulate in primary human foetal enteric neurons from the gastrointestinal tract and can be transferred between foetal enteric neurons. Impaired proteosomal/lysosomal degradation can promote the uptake and accumulation of α-syn in enteric neurons. Enteric neurons exposed to α-syn can also lead to impaired mitochondrial complex I activity, reduced mitochondrial function, and NAD(+) depletion culminating in cell death via energy restriction. These findings demonstrate neuron-to-neuron transmission of α-syn in enteric neurons, providing renewed evidence for Braak's hypothesis and the aetiology of PD.

Thrombocytopenia is caused by immune reactions elicited by diverse drugs in clinical practice. The activity of the drug-dependent antibodies produces a marked decrease in blood platelets and a risk of serious bleeding. Understanding of the cellular mechanisms that drive drug-induced thrombocytopenia has advanced recently but there is still a need for improved laboratory tests and treatment options. This article provides an overview of the different types of drug-induced thrombocytopenia, discusses potential pathologic mechanisms, and considers diagnostic methods and treatment options.

Signaling pathways play a critical role in the maintenance of cellular structure and function. These pathways can act together with synergistic or antagonistic outcome. Cooperative and integrative cellular communication networks, otherwise known as crosstalk can amplify signaling cascades. Here, we focus on receptor crosstalk in the context of cardiovascular pathologies, mainly involving the epidermal growth factor receptor (EGFR), a critical mediator of multiple receptor pathways in normal physiological and pathophysiological processes. Considerable experimental evidence suggests that the uncontrolled expression of EGFR contributes to tumorigenesis through inhibition of apoptosis, angiogenesis, anchorage-independent growth and tumor-associated inflammation. Abnormal activation of the intrinsic tyrosine kinase of EGFR through mutation or overexpression is observed in various human cancer types. On the other hand, the role of EGFR in vascular biology is not well understood. In cardiovascular pathologies, such as atherosclerosis and restenosis, vascular smooth muscle cells (SMCs) migrate and proliferate, contributing to neointima formation, whilst apoptosis may cause plaque instability. EGFR can be transactivated by numerous pathologic stimuli that regulate SMC behaviour. This review describes our current understanding of the role of EGFR in SMC biology and pathology.

The prometastatic role of platelets has long been recognized with proposed mechanisms of action including shielding tumor cells from natural killer (NK) cell destruction and aiding endothelial attachment and extravasation of tumor cells with platelet P-selectin being implicated in these processes. However, many aspects of the prometastatic function of platelets remain unclear. In this study, we used mouse models of metastatic breast cancer and melanoma to investigate the platelet effect, focusing on organ specificity, the relationship with NK cells and the relative importance of platelet-derived versus endothelial-derived P-selectin. We found that platelets promote lung metastasis in the absence of NK cells in both acute and spontaneous metastasis models. In addition, the prometastatic action of platelets was found to be organ specific, clearly enhancing lung metastasis but not affecting B16F1 liver metastasis, in fact, liver metastasis was enhanced in the absence of platelets. Furthermore, the profound antimetastatic activity of NK cells was equally effective in the presence or absence of platelets and chronologically distinct from the prometastatic role of platelets. Finally, it was shown that endothelial-derived P-selectin is just as important as platelet-derived P-selectin in promoting lung metastasis and also plays an important role in liver metastasis. Taken together, our findings help clarify the roles of platelets, NK cells and P-selectin in metastasis, and they identify P-selectin as an attractive therapeutic target for preventing metastasis in multiple organs. Cancer Res; 72(18); 4662–71. ©2012 AACR.

Patients with primary immune thrombocytopenia (ITP) from the Asia-Pacific region often exhibit characteristics distinct from those of patients from the West. Moreover, as the region itself is heterogeneous, the ITP landscape among individual Asia-Pacific countries can be diverse. The recently released international consensus report on ITP places new emphasis on ITP, but does not address the unique ITP landscape in the Asia-Pacific region, which is home to 60% of the world’s population. In an attempt to characterize how the ITP landscape differs between the West and the Asia-Pacific region, an expert panel with representatives from Northeast Asia, Southeast Asia, and Australia was convened. Important differences were identified between the guidance provided in the international consensus report and experience in the Asia-Pacific region, namely diagnostic practices, incidence and approach to ITP secondary to H. pylori infection, systemic lupus erythematosus-related ITP, the use of bone marrow examination, initial treatment strategies, and the role of splenectomy, rituximab, and thrombopoietin receptor agonists.

The development of immune cytopenias is a well-recognized side effect of many drugs. Quinine- and quinidine-dependent antibodies are classic examples of drug-induced effects that cause severe, life-threatening thrombocytopenia. Whereas the effects of drug-dependent antibodies on platelets have been well documented, their effects on megakaryocyte (Mk) biology are still unclear. We analyzed sera from several quinine-induced thrombocytopenia (QITP) patients on highly pure Mks (98% glycoprotein IIb-positive [GPIIb+]; 92% GPIX+) derived from human CD34+ cells cultured with human thrombopoietin. We demonstrate by flow cytometry and confocal microscopy that QITP IgGs bind Mks efficiently in the presence of quinine. Incubation of day-4 Mks with QITP sera or purified IgG resulted in induction of apoptosis, a significant decrease in cell viability, and an increase in cell death. Furthermore, QITP sera preferentially reduced the number of late GPIX+/GPIbα+ Mks and the number of receptors per cell in the surviving population. Ploidy distribution, lobularity, and average cell size of Mks remained unchanged after treatment. In addition, treated Mks showed a marked decrease in their proplatelet production capacity, suggesting that drug-dependent antibodies hinder platelet production. Therefore, QITP antibodies considerably reduce the proplatelet production capabilities of Mks despite undetectable effects on DNA content, morphology, and cell size.

Introduction The pathogenesis of heparin-induced thrombocytopenia (HIT) primarily involves anti-platelet factor(PF)4/heparin antibodies engaging FcγRIIa receptors on platelets to trigger a deleterious cascade of activation, thrombosis, and thrombocytopenia. Recent work with murine monoclonal antibodies has shed light on epitopic determinants that potentiate platelet activation (pathogenic KKO vs non-pathogenic RTO antibodies), but these mechanisms may not fully reflect the gamut of polyspecific antibodies seen in human clinical practice. We examined the development and characteristics of anti-PF4/heparin antibodies in patients undergoing cardiac surgery. Methods After obtaining informed consent from patients undergoing cardiac bypass surgery, we performed ELISA using: (1) commercial Diagnostica Stago Asserachrom IgG kit (Cat.Nr 00624); or (2) in-house ELISA with platelet derived native(n)PF4. ELISA performed as per manufacturer instructions and published methods. Bacterial genomic nucleic acids (NA) were extracted from overnight culture of BL21(DE3)pLysS-wtPF4 using Promega kit Wizard® SV Genomic DNA Purification System (#A2361, Promega). Cross linking assays were performed as previously described using Bis(sulfosuccinimidyl)suberate (BS3) obtained commercially (#21585, Thermo Scientific Pierce). Optical density was normalised with controls to assist interassay comparability. All samples were screened with the commercial kit day 7 post-operatively, or the nearest available serum between day 4-10. Positive samples were then examined longitudinally to chart the onset of antibody formation and their duration. Results Commercial ELISA identified 30/123 (24%) patients positive for anti-PF4/heparin antibodies following cardiac surgery, but there was no HIT observed in the complete cohort of 123 patients and therefore all 30 were false positive. Surprisingly, in-house ELISA using nPF4/heparin was negative for 22/30, but the addition of NA to PF4 rescued the OD of these to become positive again. In contrast, OD fell for all true HIT controls with the addition of NA, as they did for a subset of the false-positives 7/30 [Figure A]. Five patients had persistently raised OD despite the addition of NA, suggesting the presence of both NA/nPF4/heparin antibodies and nPF4/heparin antibodies. Overall, the incidence of false-positive anti-PF4/heparin antibodies was only 12/123 (10%) using the in-house ELISA method with nPF4. SRA was performed on all patients with nPF4/heparin antibodies, but they were all negative. Spiking nPF4 with increasing concentrations of NA caused reduction of tetramer formation and an increasing proportion of dimeric and monomeric forms seen on cross-linkage assays [Figures B and C]. Longitudinal studies demonstrated that all 13 patients positive by commercial ELISA with only low range absolute results (OD Conclusions Increasing amounts of NA inhibit PF4 tetramer formation, and reduce the ability for HIT antibodies to bind. Thus, NA may participate in the preferential presentation of PF4 monomers that are recognised by non-pathogenic NA/PF4 monomer/heparin antibodies, similar to RTO. There is a smaller subset of "true" non-pathogenic antibodies that bind to PF4 tetramers, and an even smaller number of patients who may possess both classes of non-pathogenic antibodies. We speculate that pre-formed antibodies to NA/PF4 monomers are responsible for the majority of false positive results from commercial ELISA kits, and these are unrelated to the pathogenic anti-PF4/heparin antibodies of HIT. Figure Caption A: Representative example of five patients with increasing ELISA OD in the presence of NA (NA1-5), two patients with OD that fell in the presence of NA (TFP1-2), and known HIT serum with reduced OD in the presence of NA. B: Reduction in PF4 tetramer formation and increasing PF4 monomers with increasing NA concentration. Lanes from left-right: control PF4 without BS3, and then increasing concentration of NA from 0, 5, 30 and 45ng/μL with BS3. C: Percentage of PF4 as tetramers or monomers with increasing concentration of NA. Disclosures No relevant conflicts of interest to declare.

Adverse drug effects are common in clinical practice and often have a negative impact on patient safety. Heparin and heparin-derived drugs may induce an immune reaction, termed heparin-induced thrombocytopenia (HIT). HIT is mediated by IgG antibodies with specificity for heparin/platelet factor 4 (PF4) antigenic complexes. HIT is a hypercoagulable state which often causes severe and extensive thrombosis that results in high morbidity and mortality. The prevailing view is that these immune complexes activate platelets via FcγRIIa receptors leading to thrombocytopenia and thrombosis. Neutrophil extracellular traps (NETs) are DNA-containing structures released by neutrophils that are increasingly being reported in patients with infection and thrombosis associated with various autoimmune and non-immune disorders. Here we show that HIT immune complexes directly activate neutrophils via FcγRIIa and induce NETs formation. In addition, NETosis is also induced by activated platelets/neutrophil interactions mediated by P-selecting and PSGL-1. Ex vivo reconstitution of the HIT condition in a microfluidics system demonstrated the formation of thrombi rich in platelets, neutrophils, extracellular DNA and citrullinated histone 3 (Figure 1A, left panels). Neutrophil depletion abolished thrombus formation. Conversely neutrophil reconstitution restored thrombus deposition (Figure 1A, middle panels and right panels). Moreover, neutrophils alone treated with HIT IgG plus heparin formed thrombi containing extracellular DNA networks and citrullinated histone 3 on P-selectin coated channels (Figure 1B). Establishment of HIT in hFcγRIIa+/hPF4+ transgenic mice (HIT mice) using HIT patient's IgG or the HIT-like monoclonal antibody KKO recapitulated the hallmarks of NETosis: citrullinated histone 3, cell free DNA and MPO were detected in plasma and the presence of neutrophils, extracellular DNA and citrullinated histone 3 was found in lung thrombi. Low density neutrophils were also present in HIT mice treated with HIT IgG plus heparin but not in animals treated with control IgG. Treatment of HIT mice with DNase I or NETs formation inhibition with the PAD4 inhibitor GSK484 led to a dramatic decrease in thrombosis. This was corroborated by deletion of PAD4 in HIT mice. No thrombi were detected in hFcγRIIa+/hPF4+/PAD4-/- mice treated with HIT IgG and heparin (Figure 1C), indicating that NETs formation is required for thrombosis. Unlike thrombosis, thrombocytopenia was not affected by the absence of NETs formation, suggesting that these are separable processes. However, they are FcγRIIa-mediated mechanisms as anti-FcγRIIa antibodies abolished both processes. Analyses of sera from HIT patients revealed the presence of NETs markers and a significant proportion of neutrophils from patients with active HIT were undergoing NETosis. Our observations demonstrate that NETs formation is present in HIT and that it is essential for the development of thrombosis. Thrombocytopenia is not affected by the absence of NETosis. These findings suggest a new concept of the pathogenesis of thrombosis in HIT and as such are of clinical significance. Figure 1. Figure 1. Disclosures No relevant conflicts of interest to declare.

Previously published guidelines for the diagnosis and management of primary immune thrombocytopenia (ITP) require updating largely due to the introduction of new classes of therapeutic agents, and a greater understanding of the disease pathophysiology. However, treatment-related decisions still remain principally dependent on clinical expertise or patient preference rather than high-quality clinical trial evidence. This consensus document aims to report on new data and provide consensus-based recommendations relating to diagnosis and treatment of ITP in adults, in children, and during pregnancy. The inclusion of summary tables within this document, supported by information tables in the online appendices, is intended to aid in clinical decision making.

Thrombocytopenia following heparin administration can be associated with an immune reaction, now referred to as heparin-induced thrombocytopenia . HIT is essentially a prothrombotic disorder mediated by an IgG antiplatelet factor 4/heparin antibody, which induces platelet, endothelial cell, monocyte, and other cellular activation, leading to thrombin generation and thrombotic complications. Indeed, HIT can also be regarded as a serious adverse drug effect. Importantly, HIT can be a life-threatening and limb-threatening condition frequently associated with characteristically severe and extensive thromboembolism rather than with bleeding. This article provides an overview of HIT, with an emphasis on the clinical diagnosis and management.

Diagnosis and management of immune thrombocytopenic purpura (ITP) remain largely dependent on clinical expertise and observations more than on evidence derived from clinical trials of high scientific quality. One major obstacle to the implementation of such studies and in producing reliable meta-analyses of existing data is a lack of consensus on standardized critical definitions, outcome criteria, and terminology. Moreover, the demand for comparative clinical trials has dramatically increased since the introduction of new classes of therapeutic agents, such as thrombopoietin receptor agonists, and innovative treatment modalities, such as anti-CD 20 antibodies. To overcome the present heterogeneity, an International Working Group of recognized expert clinicians convened a 2-day structured meeting (the Vicenza Consensus Conference) to define standard terminology and definitions for primary ITP and its different phases and criteria for the grading of severity, and clinically meaningful outcomes and response. These consensus criteria and definitions could be used by investigational clinical trials or cohort studies. Adoption of these recommendations would serve to improve communication among investigators, to enhance comparability among clinical trials, to facilitate meta-analyses and development of therapeutic guidelines, and to provide a standardized framework for regulatory agencies.

Hematological changes in patients with Severe Acute Respiratory Syndrome (SARS) are common and frequently include thrombocytopenia. Using a ELISA method, we found an increase in thrombopoietin (TPO) levels in the plasma of convalesced SARS patients (290 ± 53 pg/ml) and active SARS patients (251 ± 23 pg/ml) comparing to that from normal control patients (228 ± 17 pg/ml). In addition, the plasma from active SARS patients had an inhibitory effect on CFU-MK formation, which could be neutralized by anti-TGF-β antibodies. In the experiment to determine whether SARS-CoV can directly infect hematopoietic stem cells and megakaryocytic cells, incubation of the cells with SARS-CoV did not show active infection. Our findings of increased TPO levels in the plasma of SARS patients provide a possible explanation for the genesis of thrombocytosis, which frequently develops from thrombocytopenia in SARS patients.

Interleukin-16 (IL-16) induces the chemotaxis and activation of mast cells (MCs) and other cell types. While it has been concluded that CD4 is the primary IL-16 receptor on T cells, at least one other IL-16 receptor exists. We now show that the IL-16–responsive human MC line HMC-1 lacks CD4, and that the IL-16–mediated chemotactic and Ca2+ mobilization responses of this cell can be blocked by anti-CD9 monoclonal antibodies (mAbs) but not by mAbs directed against CD4 or other tetraspanins. Anti-CD9 mAbs also inhibited the IL-16–mediated activation of nontransformed human cord blood–derived MCs and mouse bone marrow–derived MCs by 50% to 60%. The chemotactic response of HMC-1 cells to IL-16, as well as the binding of the cytokine to the cell's plasma membrane, was inhibited by CD9-specific antisense oligonucleotides. CD9 is therefore essential for the IL-16–mediated chemotaxis and activation of the HMC-1 cell line. In support of this conclusion, IL-16 bound to CD9-expressing CHO cell transfectants. The ability of wortmannin and xestopongin C to inhibit the IL-16–mediated chemotactic response of these cells suggests that the cytokine activates a phosphatidylinositol 3-kinase (PI3K)/inositol trisphosphate–dependent signaling pathway in MCs. This is the first report of a tetraspanin that plays a prominent role in a cytokine-mediated chemotactic response of human MCs.

Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disorder in which platelets coated with mainly antibodies against platelet GPIIb/IIIa and GPIb/IX are destroyed in the spleen. Recent evidence suggests that platelets are also destroyed by cytotoxic T cells. The diagnosis is made by exclusion for other causes of thrombocytopenia. As routine blood counts are becoming more available, many mild cases of ITP (platelets30 x 10(9) L(-1)) are being diagnosed and they usually do not require treatment. In patients with platelet counts persistently30 x 10(9) L(-1), treatment with corticosteroids, and/or intravenous immunoglobulin (IVIG) or anti-D may be required. The primary goal of treatment is to maintain the platelet count at a safe level with minimal side effects. After 3-6 months, if spontaneous remission has not occurred and if the side effects are significant, splenectomy is recommended. This is the single most effective treatment of ITP. The refractory patients who fails splenectomy and subsequently first- and second-line therapies, is a management dilemma. Therapeutic options are limited, available treatments potentially toxic and the chances of sustained response low. Observation with no active treatment is a reasonable option. With the increased availability of the thrombopoietic agents in the future, there may be a good prospect of keeping the platelet counts of these refractory patients at a safe long-term level with one of these drugs.

In this issue of Blood, Jaax and colleagues show that heparin-PF4 antibodies cross-reacted with nucleic acid (NA)–PF4 complexes and induced platelet activation, suggesting that NA-PF4 can potentially cause a heparin-induced thrombocytopenia (HIT)–like prothrombotic disorder.

Human platelet survival studies have been hampered due to the lack of a suitable animal model. Transfusion of human platelets into immune competent animals leads to the rapid destruction of these platelets by naturally occurring xenoantibodies. The NOD/SCID mouse lacks T and B cells and therefore lack natural antibodies that could destroy infused human platelets [1]. Because of this property, human platelets given to the mouse intravenously circulate for several days, permitting the model to be used for testing the ability of human antibodies to cause platelet destruction in vivo [2–7]. Preliminary studies have demonstrated the usefulness of NOD/SCID mouse model to monitor the survival and immune destruction of human platelets. However, differences exist between the research groups regarding the method of PLT injection, the amount and route of antibody injection and the preparation of blood samples collected from the animal making the results poorly comparable. Basically, in all laboratories resting human platelets are injected intravenously into the mouse circulation, where they can, in the absence of platelet-reactive antibodies, circulate up to 48h [8]. After estimating a baseline value (100%) of human platelets, platelet-reactive antibodies (with or without drug administration) can be infused. The impact of these antibodies on the survival of human platelets can then be analyzed by taking blood samples over time from the mouse [8]. Methodological details that require attention in this model include: platelet preparation and resuspension in plasma or ‘synthetic plasma’, the concentrations and volume of applied analytes (platelet, antibody or drug), the route of platelet injection (retro-orbital injection or tail vein injection) and antibody injection (intravenous, intra-peritoneal). The method of data capture, including time points of blood sampling and subsequent sample preparation for analysis, percentage of circulating human platelets and software details should also be reported in detail. Additional steps required for answering the scientific questions, for example platelet preincubation with a drug of interest or an antibody in pooled plasma or ‘synthetic plasma’, should also be reported [2, 9]. Surprisingly, application procedures and the amount of injected platelets and antibodies have only been loosely defined and standardization should be carried out in order to improve the reproducibility of the procedures and to enable reliable comparison of the results. This report is not didactic in relation to how to measure the survival of human platelets using the NOD/SCID mouse model. Its purpose is to suggest standardized procedures and define variables that should be considered when presenting methodology in published reports. The presented procedures were introduced and discussed during the meetings of the Subcommittee of Platelet Immunology of the Scientific and Standardization Committee Liverpool 2012 and Milwaukee 2014. Suggestions were introduced to the SSC members and the presented recommendations had unanimous agreement. Adopting these recommendations will be of advantage for investigators and laboratories to reduce imprecision and harmonize results and will allow other laboratories to readily reproduce reported methods and findings and interpret results appropriately.

Primary immune thrombocytopenia is an autoimmune disease mediated by antiplatelet autoantibodies that cause platelet destruction and suppression of platelet production. In vitro effects of autoantibodies on megakaryocyte production and maturation have been reported recently. However, the impact of these autoantibodies on crucial megakaryocyte functions, proplatelet formation and subsequent platelet release, has not been evaluated. We examined the effects of serum and IgG from 19 patients with immune thrombocytopenia using day 8 or 9 megakaryocytes (66.3 ± 10.6% CD41(+)), derived from cord blood hematopoietic stem cells (CD34(+)). The number of proplatelet-bearing megakaryocytes, the number of platelets released in the culture, total megakaryocyte numbers, ploidy pattern and caspase activation were measured at various times after treatment. After 5 days of treatment the number of proplatelet-bearing megakaryocytes was significantly decreased by 13 immune thrombocytopenia autoantibodies relative to the control group (P

To maintain hemostasis under shear conditions, there must be an interaction between the platelet glycoprotein (GP) Ib-IX receptor and the plasma ligand von Willebrand factor (vWf). In platelet-type von Willebrand disease (Pt-vWD), hemostasis is compromised. Two mutations in the GPIbα polypeptide chain have been identified in these patients—a glycine-233 to valine change and a methionine-239 to valine change. For this investigation, these mutant proteins have been expressed in a Chinese hamster ovary cell model system. Ligand-binding studies were performed at various concentrations of ristocetin, and adhesion assays were performed under flow conditions. The Pt-vWD mutations resulted in a gain-of-function receptor. vWf binding was increased at all concentrations of ristocetin examined, and adhesion on a vWf matrix was enhanced in terms of cell tethering, slower rolling velocity, and decreased detachment with increasing shear rate. Two other mutations were also introduced into the GPIbα chain. One mutation, encompassing both the Pt-vWD mutations, created an increase in the hydrophobicity of this region. The second mutation, involving a valine-234 to glycine change, decreased the hydrophobicity of this region. Both mutations also resulted in a gain-of-function receptor, with the double mutation producing a hyperreactive receptor for vWf. These data further support the hypothesis that ligand binding is regulated by conformational changes in the amino-terminal region of GPIbα, thereby influencing the stability of the GPIbα–vWf interaction.

Objective: The infusion of bone marrow cells into the damaged brain has been proposed as a new clinical practice for this disorder. Alternatively, hematopoietic growth factors may have a direct role on neural protection or have a mobilizing effect on bone marrow stem/progenitor cells to circulation for brain repair. Based on our previous findings, there are many similarities between megakaryocytes and neurons on functions and antigen expression such as neural marker MAP2, GFAP and Tau, 5-HT2A, 2B and 2C receptors (Stem Cells, 2014). Thrombopoietin (TPO) is a growth factor for megakaryocytic lineage. We postulate that TPO may play a role on neural protection or regeneration. The effect of TPO on nervous system has not been well investigated. Methods: To validate this hypothesis, we investigated the expression and role of TPO/TPO receptors in neural cells and a neonatal rat model of hypoxic-ischemic (HIE) brain damage. Results: To investigate the effect of TPO on in-vivo neural protection, a neonatal rat model of HIE brain damage was established. Brain injury was measured by the percentage weight reduction of the ipsilateral cerebral hemisphere as compared to the contralateral hemisphere. There was significantly less brain atrophy in TPO treated animals (12.0±1.2% and 11.5±1.0%) when compared with the saline control (21.0±1.6% and 24.4±2.2%) at 7 and 28 days post-operation (P Conclusions: Our study provided the evidences showing the expression of TPO and TPO receptor (c-mpl) in neural cells and this effect may be mediated by c-mpl and Akt signaling. More importantly, our observation further demonstrated the functional role of TPO on neural protection in a rat model. These findings point to the possibility of a new strategy for treating brain damage by hematopoietic growth factors. Disclosures Yang: National Natural Science Foundation of China: Other: National Natural Science Foundation of China(81270580).

Objective: Thrombospondin-1 (TSP-1) is a multi-modular glycoprotein synthesized by megakaryocytes, endothelial cells, smooth muscle cells, fibroblasts and some tumor cells, found in the platelet a-granules and extracellular matrix (ECM) of numerous tissues. TSP-1 has been identified as one of the few naturally occurring anti-angiogenic agents. It inhibited endothelial cells in vitro by the activation of the CD36-p59fyn-Caspase-3-p38MAPK apoptotic cascade. TSP-1 induced in vitro apoptosis of micro-vascular endothelial cells as well as tumor cells. The objectives of our study were to investigate the in vitro effects of TSP-1 on the apoptosis of megakaryocytic leukemia cells and the possible mechanism involving CD36. Methods: We investigated CD36 expression and the effect of TSP-1 on megakaryocytic leukemia cells (Meg-01 and CHRF-288-11), with and without thrombopoietin (TPO), and with and without blocking TSP-1 binding with receptor CD36 on megakaryocytic leukemia cells. The determination of apoptotic markers were used the Annexin V-FITC/Propidium Iodide (PI) and Caspase-3 detection kit by flow cytometry.Results: Our data showed that TSP-1 induced a dose dependent growth inhibition in human CFU-MK assays and significantly counteracted the mitogenic effect from TPO. Moreover, the growth suppression induced by TSP-1 was correlated with CD36 expression in megakaryocytic leukemia cells, where growth inhibition was demonstrated in CD36 positive (Meg-01 and CHRF-288-11) but not in CD36 negative (K562) cells. More importantly, the inhibitory effect of TSP-1 on both human CFU-MK and Meg-01 cells was partially but significantly reversed by the addition of FA6-152 (anti-CD36), a functionally blocking antibody which blocks the access of TSP-1 to CD36 receptor, suggesting that the TSP-1 induced inhibition of megakaryocytopoiesis is probably mediated in part by the binding of TSP-1 to CD36 expressed on the megakaryocytic progenitors. Thus, our findings represent the demonstration that TSP-1 inhibits in vitro megakaryocytopoiesis via interaction with CD36. Our results further demonstrated that TSP-1 induced apoptosis in CD36 positive cells CHRF-288-11 and Meg-01, but not CD36 negative K562 cells, at a dose-dependent manner as demonstrated by DNA Annexin V and propidium iodide (PI) stainings. The addition of anti-CD36 antibody FA6-152 or TPO significantly nullified the effects of TSP-1. TSP-1-mediated apoptosis was consistently associated with the up-regulation of active Caspase-3. Responses of CD36 positive primary AML samples (n=3) to TSP-1 and FA6-152 were similar with those of leukemia cell lines. CD36 negative AML cells appeared less susceptible to TSP-1 and FA6-152.Conclusions: Our data provided evidence that TSP-1 exerted direct apoptotic effects on megakaryocytic leukemia cells and could be developed as an adjunct to conventional therapy, particularly for leukemia cells that express CD36 or other TSP-1 receptors. Thus, our findings represent the demonstration that TSP-1 induces apoptosis in megakaryocytic leukemia cells via CD36 and Caspase-3. Disclosures Yang: National Natural Science Foundation of China: Other: National Natural Science Foundation of China(81270580).

Approximates 80% of essential thrombocythemia (ET) patients carrying JAK2, CALR, MPL mutations. However, there is still 20% of ET patients are negative for all three mutations. Our previous studies have demonstrated that platelet-derived growth factor (PDGF) and its receptor (PDGFR) has a potential effect in the regulation of hematopoiesis and megakaryopoiesis. Therefore, we speculated that PDGF/PDGFR may plays an important role in megakaryocyte/platelet related disease, such as ET. In this study, we found an increase of PDGF-BB and PDGFR expression in ET patients comparing to the normal persons. We then used the adjusted dosage of PDGF-BB (1.5ug/kg/d) to establish an ET mimic mouse model. We found that PDGF-BB can enhance platelet production in vivo significantly, while PDGFR inhibitor imatinib can block this effect. Bone marrow histology data also confirmed the megakaryopoiesis effect of PDGF, as shown by an increased number of megakaryocytes in PDGF-treated mouse. The addition of imatinib can block the effect, evident by a decrease number of megakaryocytes and an increase of apoptosis. Furthermore, we demonstrated that PDGF-BB activated theP-JAK2, the P-STAT3 and the P-AKT expression in the megakaryocytic cell line CHRF-288, while addition of imatinib can reduce the phosphorylation level of all three genes. Our results suggested that the overexpression of PDGF-BB and PDGFR may be important in the pathogenesis of ET, especially in the JAK2-mutation negative ET. The elevated PDGF-BB activates PDGFR with subsequently activation of the JAK2/ STAT3 and PI3K/AKT pathway. Imatinib may have an effect on treating ET via blocking PDGFR and the following JAK2/STAT3 and PI3K/AKT pathway. This study suggests that PDGF/PDGFR may be involved in the pathogenesis of essential thrombocythemia, and the expression of PDGF-BB and PDGFR is potential to be a diagnostic index of ET. Blockage of PDGFR by imatinib may be a new therapeutic target for ET. Figure 1 Effect of PDGF-BB on Jak2/Stat3 and PI3K/Akt pathway of megakaryocytes. Figure 1. Effect of PDGF-BB on Jak2/Stat3 and PI3K/Akt pathway of megakaryocytes. Figure 2 The platelet numbers and the megakaryocytes of each group mice. Figure 2. The platelet numbers and the megakaryocytes of each group mice. Figure 3 Potential mechanisms for imatinib in treating essential thrombocythemia. Figure 3. Potential mechanisms for imatinib in treating essential thrombocythemia. Disclosures No relevant conflicts of interest to declare.

Objective: Thrombocytopenia is a common clinical problem in patients with cancer or bone marrow transplantation. Currently it is mainly managed by platelet transfusion. Repeated platelet transfusions are associated with the risks of transfusion reactions/alloimmunisation and may lead to platelet refractoriness. Infusion of ex vivo expanded megakaryocytic (MK) progenitor cells is other strategy for the treatment of thrombocytopenia. This study aimed to establish efficient conditions for the expansion of the MK progenitors from enriched CD34(+) cells of umbilical cord blood. Methods: This study investigated the effect of flt-3 ligang (FL), stem cell factor (SCF) and platelet-derived growth factor (PDGF) in combination with other megakaryocyte-promoting cytokines such as thrombopoietin (TPO) on the differentiation and proliferation of megakaryocytic progenitors. As an early acting growth factor, FL may promote the ex vivo expansion of hematopoietic stem and progenitor cells. We compared the effects of FL and SCF in combination with other megakaryocyte-promoting cytokines in megakaryocytic progenitors. Results: In liquid cultures of enriched CD34+ cells from human umbilical cord blood for 14 days, FL plus TPO, interleukin-3 (IL-3), and IL-6 promoted the expansion of nucleated cells, CD34+ cells, CD34+ CD38- cells, and megakaryocyte colony-forming units (CFU-MK) by 300 +/- 115-, 23.8 +/- 11.3-, 33.9 +/- 28.6-, and 584 +/- 220-fold, respectively. Replacing FL with SCF significantly decreased the yield of all cell types. While one human acute lymphoblastic leukemia sample expressed high levels of flt-3 receptor, the four megakaryocytic cell lines (Meg-01, CHRF-288-11, M-07e, and Dami) did not show any positive expression. Our data suggest that the effect of FL in augmenting the expansion of MK progenitors might be due to the early action of FL at the pluripotent stem cell stage. Our results also demonstrated that TPO alone produced a high proportion of CD61(+)CD41(+) cells but a low total cell count and high cell death, resulting in an inferior expansion. The addition of in IL-1 beta, FL and to a lesser extent IL-3 improved the expansion outcome. The treatment groups with three to five cytokines produced efficient expansions of CFU-MK up to 400-fold with the highest yield observed in the presence of TPO, IL-1 beta, IL-3, IL-6 and FL. CD34(+) cells were expanded by five to 22-fold. PDGF improved the expansion of all cell types with CD61(+)CD41(+) cells, CFU-MK and CD34(+) cells increased by 101%, 134% and 70%, respectively. More significantly, PDGF enhanced the engraftment of human CD45+ cells and their myeloid subsets (CD33+, CD14+ cells) in NOD/SCID mice. Conclusions: This study showed that the present cytokine combination and expansion conditions provide an effective and potentially useful system for the clinical expansion of cord blood for bone marrow transplantation (BMT). PDGF might be a suitable growth factor to improve the ex vivo expansion of MK progenitors for clinical applications. Disclosures Yang: National Natural Science Foundation of China: Other: National Natural Science Foundation of China(81270580).

Objective: Chinese herbs have been reported to be effective and safe as a treatment for thrombocytopenia. Sanqi, Radix Notoginseng, is the dried roots of Panax notoginseng (Burk.) F. H. Chen (Araliaceae). It has been prescribed in several Chinese formulas including "Yunnan Bai Yao", which is used for treatment of trauma and bleeding due to internal and external injury. Its main constituents are ginsenosides (a kind of saponins), as well as notoginsenosides (only rich in Notoginseng species). Although Sanqi is a well-known haemostatic drug, its effects and mechanisms on megakaryocyte/platelet production have not been well studied. The objective of this study was to compare the effect of a purified notoginsenoside R1 (NR1) and thrombopoietin (TPO) on thrombopoiesis in irradiated mice. Methods: NR1 (2.5 mg) and TPO (0.25 ug) were dissolved in distilled water and given by intra-peritoneal injection daily for 14 days starting from the day after radiotherapy. Peripheral blood platelets, white blood cells (WBC), and red blood cells (RBC) were analyzed from NR1, TPO, and vehicle control groups on day 0, 7 and 14. On day 14, the mice were sacrificed and bone marrow cells were harvested for CFU-MK, CFU-GM, BFU-E and CFU-F (fibroblastoid) assays. Results: Our results showed that NR1 enhanced the recovery of platelets, WBC, and RBC count. Moreover, NR1 also promoted the CFU-F (12 + 0.8 vs 20 + 0.4 colonies/2 x 106 cells, p=0.003), CFU-MK (22 + 1.8 vs 26 + 3.6 colonies/2 x 105 cells, p=0.025), CFU-GM (26 + 5.0 vs 38 + 4.2 colonies/2 x 105 cells, p=0.002), and BFU-E (12 + 2.6 vs 18 + 1.8 colonies/2 x 105 cells, p=0.003) formation. Similar results were obtained in TPO-treated group. In in-vitro study, we further analyzed the effect of NR1 (0-50mM) on mouse CFU-MK formation using a plasma clot colony assay. The results showed that NR1 (20 mM) enhanced TPO (50 ng/ml)-induced CFU-MK formation (18 + 2.2 vs 30 + 6.0 colonies/2 x 105 cells, p=0.022, n=6). Furthermore, the effect of NR1 (5-50 mM) on the growth of bone marrow stromal cells was also investigated using CFU-F assay. NR1 (50mM) had a promoting effect on CFU-F growth (18 + 3.6 vs 24 + 1.8 colonies/2 x 106 cells, p=0.04, n=6). Our studies showed that NR1 enhances thrombopoiesis in vivo and the growth of bone marrow stromal cells as well as megakaryocytes in vitro. Therefore, we speculate that the thrombopoietic activity of NR1 may be mediated via promoting the progenitor of platelet, megakaryocytes, and bone marrow stromal cells. Consequently, we tested the anti-apoptotic effect of NR1 using megakaryocytic cell line M-07e. Data demonstrated that nutrient deprivation significantly increased cell death (n = 6, control vs. normal), and NR1 significantly reduced apoptosis and total cell death (n = 6, NR1 vs. control). TPO treatment also significantly reduced M-07e cell death (n = 6, TPO vs. control). Data also demonstrated that nutrient deprivation increased caspase-3 expression (n = 6, normal vs. control), and NR1 (NR1 vs. control) or TPO (TPO vs. control) treatment significantly reduced caspase-3 expression. M-07e cells were stained with JC-1 reagent. Control cultures had increased proportion of cells containing JC-1 monomers, the percentage of R2 populations are significant higher in the normal, NR1 and TPO treated groups comparing to those of the control group (n = 6). R1, a transitional cell subset containing both monomers and aggregates, was not significantly affected by these reagents. R2: a cell subset showing mitochondria membrane damage. Conclusions: Here we reported that the effect of NR1 is comparable with TPO on the production of platelets in irradiated mice. Therefore, the effect of NR1 on thrombopoiesis may be mediated via anti-apoptosis on megakaryocytic cells. Disclosures Yang: National Natural Science Foundation of China: Other: National Natural Science Foundation of China(81270580).

Immune heparin-induced thrombocytopenia (HIT) is associated with antibodies directed against a complex of platelet factor 4 (PF4) and heparin. We were able to affinity purify anti-PF4–heparin IgG (HIT IgG) from the plasma of 2 patients with HIT. Under conditions that were more physiological and sensitive than those in previous studies, we observed that this HIT IgG caused platelet aggregation on the addition of heparin. Platelets activated with HIT IgG increased their release and surface expression of PF4. We quantitated, for the first time, the binding of affinity-purified HIT iodine 125–IgG to platelets as they activated in a plasma milieu. Binding of the HIT IgG was dependent on heparin and required some degree of platelet activation. Blocking the platelet FcγRII with the monoclonal antibody IV.3 did not prevent HIT IgG binding to activated platelets. We concluded that anti-PF4–heparin IgG is the component in these HIT plasmas that induces platelet aggregation. The Fab region of HIT IgG binds to PF4–heparin on the surface of activated platelets. We propose that only then does the Fc portion of the bound IgG further activate the same or adjacent platelets through the Fc receptor. Our data support a dynamic model of platelet activation in which released PF4 enhances further antibody binding and more release.

Thrombopoietin (TPO), the specific cytokine that regulates platelet production, is expressed in human bone marrow (BM), kidney, and liver. There appears to be no regulation of TPO in the kidney and liver, but TPO messenger RNA (mRNA) expression can be modulated in the stromal cells of the BM. In this study, we used primary human BM stromal cells as a model to study the regulation of TPO mRNA expression in response to various platelet -granular proteins. We showed that platelet-derived growth factor (PDGF) BB and fibroblast growth factor (FGF) 2 stimulated TPO mRNA expression in both a dose-dependent and time-dependent manner. The addition of 50 ng/mL of PDGF and 20 ng/mL of FGF resulted in maximal induction of TPO mRNA expression in 4 hours. We also found that platelet factor 4 (PF4), thrombospondin (TSP), and transforming growth factor-beta (TGF-β) are negative modulators of megakaryocytopoiesis. We observed suppression in TPO mRNA expression with 1 μg/mL of both PF4 and TSP and 50 ng/mL of TGF-β, with maximal suppression occurring 4 hours after the addition of these proteins. Finally, the addition of whole-platelet lysate produced a dose-dependent inhibition of TPO expression. On the basis of these findings, we propose that the platelet -granular proteins studied may regulate TPO gene expression in BM stromal cells by means of a feedback mechanism.

The drug-dependent antibody of a patient with rifampicin-induced thrombocytopenia was characterized using the antigen-capture enzyme-linked immunosorbent assay (MAIPA assay), flow cytometry, and immunoprecipitation. The antibody was found to bind glycoprotein (GP) Ib-IX but not GPIIb-IIIa because (1) it immunoprecipitated drug-dependently the former but not the latter glycoprotein complex and (2) the MAIPA assay showed strong rifampicin-dependent antibody binding when anti-GPIb-IX monoclonal antibodies (mAbs) (AK2 and FMC25) but not anti-GPIIb-IIIa mAbs (AP2, SZ21, and SZ22) were used to capture the antigen. The antibody binding site was further localized to the GPIX subunit of the GPIb-IX complex because flow cytometric analysis revealed drug-dependent antibody binding to L cells transfected with human GPIbβ and GPIX complementary DNA (L βIX cells) but not with human GPIb and GPIbβ complementary DNA (L β cells). Finally, in the MAIPA assay, the rifampicin-dependent antibody almost completely cross-blocked the binding of the anti-GPIX mAb (SZ1) to platelets. Similar cross-blocking of SZ1binding to platelets by the quinine-dependent antibodies was also observed. This finding not only confirms that the epitope of the rifampicin-dependent antibody is on GPIX but it is also identical to or located in close proximity to that of the quinine-dependent antibody and SZ1. Further characterization of the epitopes of these antibodies may have important implications for a general understanding of the mechanism of drug-induced thrombocytopenia.

A qualitative platelet defect was demonstrated in a patient with heparin-induced thrombocytopenia. His platelets showed impaired aggregation to collagen and arachidonate and absent second wave aggregation to adenosine diphosphate and epinephrine but normal response to ristocetin. These aggregation abnormalities were similar to those reported in idiopathic thrombocytopenia and systemic lupus erythematosus. This platelet function defect may contribute to haemorrhagic complications which appear to be common in heparin-induced thrombocytopenia.