Relative efficacy of intravenous immunoglobulin G in ameliorating thrombocytopenia induced by antiplatelet GPIIbIIIa versus GPIbalpha antibodies

The effects of complement-independent, autoantibody-induced apoptosis of platelets in immune thrombocytopenia (ITP)

Autoantibodies that cause platelet apoptosis may play a role in the development of immune thrombocytopenia (ITP), specifically antibodies that target GPIIbIIIa and GPIbα. Our research aims to compare the impact of the antigen specificity of antiplatelet antibodies on normal platelets under conditions that do not rely on complement. Using a modified monoclonal antibody-specific immobilization of platelet antigen (MAIPA) assay, we detected the levels of autoantibodies against specific platelet membrane glycoproteins (GPIIb/IIIa, GPIb/IX) in the plasma of 36 patients diagnosed with chronic ITP. IgG was isolated and purified using a protein A agarose affinity chromatography column, and their concentrations were measured using spectrophotometry. We obtained normal platelets and treated them with the purified IgG anti-GPIIb/IIIa and/or anti-GPIb/IX antibodies, as well as an IgG-free buffer and healthy control IgG. Flow cytometry was used to analyze markers of apoptosis, including phosphatidylserine (PS) exposure, mitochondrial inner membrane potential (ΔΨm), and platelet particle formation. Our results indicate that ITP patients with GPIb/IX-specific autoantibodies can induce platelet apoptosis and platelet particle formation through complement-independent pathways, which are not associated with platelet activation, while GPIIb/IIIa-specific autoantibodies did not have this effect. This suggests that specific autoantibodies may serve as a valuable predictive tool to identify patients who could potentially benefit from complement-inhibiting therapy in the future.

Platelets mediate the clearance of senescent red blood cells by forming prophagocytic platelet-cell complexes

ABSTRACT

In humans, ∼0.1% to 0.3% of circulating red blood cells (RBCs) are present as platelet-RBC (P-RBC) complexes, and it is 1% to 2% in mice. Excessive P-RBC complexes are found in diseases that compromise RBC health (eg, sickle cell disease and malaria) and contribute to pathogenesis. However, the physiological role of P-RBC complexes in healthy blood is unknown. As a result of damage accumulated over their lifetime, RBCs nearing senescence exhibit physiological and molecular changes akin to those in platelet-binding RBCs in sickle cell disease and malaria. Therefore, we hypothesized that RBCs nearing senescence are targets for platelet binding and P-RBC formation. Confirming this hypothesis, pulse-chase labeling studies in mice revealed an approximately tenfold increase in P-RBC complexes in the most chronologically aged RBC population compared with younger cells. When reintroduced into mice, these complexes were selectively cleared from the bloodstream (in preference to platelet-free RBC) through the reticuloendothelial system and erythrophagocytes in the spleen. As a corollary, patients without a spleen had higher levels of complexes in their bloodstream. When the platelet supply was artificially reduced in mice, fewer RBC complexes were formed, fewer erythrophagocytes were generated, and more senescent RBCs remained in circulation. Similar imbalances in complex levels and senescent RBC burden were observed in humans with immune thrombocytopenia (ITP). These findings indicate that platelets are important for binding and clearing senescent RBCs, and disruptions in platelet count or complex formation and clearance may negatively affect RBC homeostasis and may contribute to the known risk of thrombosis in ITP and after splenectomy.

Anti-GPIb/IX autoantibodies are associated with poor response to dexamethasone combined with rituximab therapy in primary immune thrombocytopenia patients

This retrospective study aimed to evaluate whether anti-glycoproteins (GPs) autoantibodies can be used as predictors of response to high-dose dexamethasone combined with rituximab (DXM-RTX) in the treatment of primary immune thrombocytopenia (ITP) patients. One-hundred twenty-six ITP patients were included and retrospectively analyzed, 66.7% of anti-GPIb/IX and 65.9% of anti-GPIIb/IIIa autoantibodies. Results showed that overall response (OR) and complete response (CR) rates of patients without anti-GPIb/IX autoantibodies to DXM-RTX were significantly higher than those with anti-GPIb/IX autoantibodies at 4 weeks (OR: 73.8% vs. 47.6%, CR: 50.0% vs. 26.2%; P < 0.05) and 6 months (OR: 71.4% vs. 45.2%, CR: 42.9% vs. 25.0%; P < .05). Furthermore, patients with anti-GPIb/IX single-positivity exhibited higher resistance to DXM-RTX than patients with anti-GPIIb/IIIa single-positivity at 4 weeks (OR: 37.5% vs. 78.3%; P < .05) and 6 months (OR: 29.2% vs. 78.3%; P < .05). Multivariable logistic regression analysis revealed that anti-GPIb/IX autoantibodies and megakaryocytes were associated with the OR rate of patients at both 4 weeks and 6 months, and anti-GPIb/IX autoantibodies at 4 weeks represented the only significant factor affecting OR rate with DXM-RTX (F = 9.128, P  = .003). Therefore, platelet anti-GPIb/IX autoantibodies might predict poor response to DXM-RTX in ITP patients.

Chronic refractory immune thrombocytopenia in adolescents and young adults

Defining immune thrombocytopenia (ITP) in two age groups-children and adults-overlooks the specific clinical features and needs of adolescents and young adults (AYAS). We previously reported a high risk of chronic disease at 12 months (50%); however, data on the course of chronic ITP, the risk of refractoriness and treatment strategies in AYAS are limited. Data from patients aged 12-25 years with chronic primary ITP at 12 months were extracted from three large registries between 2004 and 2021. Clinical and laboratory data were evaluated until 48 months of follow-up (FU). Refractory ITP was defined as the administration of ≥3 different lines of therapy. A total of 427 AYAS (64% female) with chronic ITP were included. Overall, 7% and 14% were classified as 'refractory' at 12 and 48 months of FU respectively. The proportion of males was greater in the refractory group than in the non-refractory group (43% vs. 35%). AYAS with refractory disease displayed lower median platelet counts, more bleeding and a higher need for treatment at initial diagnosis and FU than non-refractory patients. This study reveals that refractory ITP is uncommon in AYAS; however, AYAS with refractory ITP display a high disease burden at all time points, including at initial diagnosis.

Rituximab resistance in ITP and beyond

The pathophysiology of immune thrombocytopenia (ITP) is complex and encompasses innate and adaptive immune responses, as well as megakaryocyte dysfunction. Rituximab is administered in relapsed cases and has the added benefit of inducing treatment-free remission in over 50% of patients. Nevertheless, the responses to this therapy are not long-lasting, and resistance development is frequent. B cells, T cells, and plasma cells play a role in developing resistance. To overcome this resistance, targeting these pathways through splenectomy and novel therapies that target FcγR pathway, FcRn, complement, B cells, plasma cells, and T cells can be useful. This review will summarize the pathogenetic mechanisms implicated in rituximab resistance and examine the potential therapeutic interventions to overcome it. This review will explore the efficacy of established therapies, as well as novel therapeutic approaches and agents currently in development.

SARS-CoV-2 RBD and Its Variants Can Induce Platelet Activation and Clearance: Implications for Antibody Therapy and Vaccinations against COVID-19

The COVID-19 pandemic caused by SARS-CoV-2 virus is an ongoing global health burden. Severe cases of COVID-19 and the rare cases of COVID-19 vaccine-induced-thrombotic-thrombocytopenia (VITT) are both associated with thrombosis and thrombocytopenia; however, the underlying mechanisms remain inadequately understood. Both infection and vaccination utilize the spike protein receptor-binding domain (RBD) of SARS-CoV-2. We found that intravenous injection of recombinant RBD caused significant platelet clearance in mice. Further investigation revealed the RBD could bind platelets, cause platelet activation, and potentiate platelet aggregation, which was exacerbated in the Delta and Kappa variants. The RBD-platelet interaction was partially dependent on the β3 integrin as binding was significantly reduced in β3-/- mice. Furthermore, RBD binding to human and mouse platelets was significantly reduced with related αIIbβ3 antagonists and mutation of the RGD (arginine-glycine-aspartate) integrin binding motif to RGE (arginine-glycine-glutamate). We developed anti-RBD polyclonal and several monoclonal antibodies (mAbs) and identified 4F2 and 4H12 for their potent dual inhibition of RBD-induced platelet activation, aggregation, and clearance in vivo, and SARS-CoV-2 infection and replication in Vero E6 cells. Our data show that the RBD can bind platelets partially though αIIbβ3 and induce platelet activation and clearance, which may contribute to thrombosis and thrombocytopenia observed in COVID-19 and VITT. Our newly developed mAbs 4F2 and 4H12 have potential not only for diagnosis of SARS-CoV-2 virus antigen but also importantly for therapy against COVID-19.

Novel Biomarkers for Diagnosis and Monitoring of Immune Thrombocytopenia

Lower-than-normal platelet counts are a hallmark of the acquired autoimmune illness known as immune thrombocytopenia, which can affect both adults and children. Immune thrombocytopenia patients' care has evolved significantly in recent years, but the disease's diagnosis has not, and it is still only clinically achievable with the elimination of other causes of thrombocytopenia. The lack of a valid biomarker or gold-standard diagnostic test, despite ongoing efforts to find one, adds to the high rate of disease misdiagnosis. However, in recent years, several studies have helped to elucidate a number of features of the disease's etiology, highlighting how the platelet loss is not only caused by an increase in peripheral platelet destruction but also involves a number of humoral and cellular immune system effectors. This made it possible to identify the role of immune-activating substances such cytokines and chemokines, complement, non-coding genetic material, the microbiome, and gene mutations. Furthermore, platelet and megakaryocyte immaturity indices have been emphasized as new disease markers, and prognostic signs and responses to particular types of therapy have been suggested. Our review's goal was to compile information from the literature on novel immune thrombocytopenia biomarkers, markers that will help us improve the management of these patients.

A bibliometric analysis of primary immune thrombocytopenia from 2011 to 2021

Primary immune thrombocytopenia (ITP) is an autoimmune disorder characterized by isolated thrombocytopenia. This bibliometric analysis was applied to identify the characteristics of global scientific output, the hotspots, and frontiers of ITP over the past 10 years. We retrieved publications from 2011 to 2021 from the Web of Science Core Collection (WoSCC). Bibliometrix package, VOSviewer, and Citespace were used to analyse and visualize the trend, distribution, and hotspots of research on ITP. Altogether, there were 2084 papers, written by 9080 authors from 410 organizations in 70 countries/regions, published in 456 journals with 37 160 co-cited references. In the last decades, the most productive journal was British Journal of Haematology, China was the most productive country. and the most cited journal was Blood. Shandong University was the most productive institution in the field of ITP. NEUNERT C, 2011, BLOOD, CHENG G, 2011, LANCET, and PATEL VL, 2012, BLOOD were the top three most cited documents. "Thrombopoietin receptor agonist", "regulatory T cell" and "sialic acid" were three hotspots of the last decade. And "immature platelet fraction", "Th17", and "fostamatinib" would be research frontiers in the feature. The present study provided a novel insight for future research directions and scientific decision-making.

Platelet Desialylation Is a Novel Mechanism and Therapeutic Target in Daboia siamensis and Agkistrodon halys Envenomation-Induced Thrombocytopenia

Venom-induced thrombocytopenia (VIT) is one of the most important hemotoxic effects of a snakebite, which is often associated with venom-induced consumptive coagulopathy (VICC). Refractory thrombocytopenia without significant coagulation abnormalities has also been reported after envenomation by some viperid snakes; however, the mechanisms are not well understood and therapeutic strategies are lacking. Here, we found that patients injured by Daboia siamensis or Agkistrodon halys snakes, who were resistant to standard antivenom treatment, had developed coagulopathy-independent thrombocytopenia. Venoms from these viperid snakes, rather than from the elapid snake (Bungarus multicinctus), induced platelet surface expression of neuraminidase-1 (NEU-1), and significantly increased the desialylation of the glycoproteins on human platelets. The desialylated platelets caused by viperid snake venoms were further internalized by macrophages, which resulted in reduced platelet numbers in peripheral blood. Importantly, neuraminidase inhibitor significantly decreased viper venom-induced platelet desialylation, therefore inhibiting platelet phagocytosis by macrophages, and alleviating venom-induced thrombocytopenia. Collectively, these findings support an important role for desialylated platelet clearance in the progression of viper envenomation-induced, coagulopathy-independent thrombocytopenia. Our study demonstrates that the neuraminidase inhibitor may be a potential therapy or adjuvant therapy to treat snakebite-induced thrombocytopenia.

Antiplatelet antibody predicts platelet desialylation and apoptosis in immune thrombocytopenia

Immune thrombocytopenia (ITP) is a bleeding disorder caused by dysregulated B- and T- cell functions, which lead to platelet destruction. A well-recognized mechanism of ITP pathogenesis involves anti-platelet and anti-megakaryocyte antibodies recognizing membrane glycoprotein (GP) complexes, mainly GPIb/IX and GPIIb/IIIa. In addition to the current view of phagocytosis of the opsonised platelets by splenic and hepatic macrophages via their Fc γ receptors, antibodyinduced platelet desialylation and apoptosis have also been reported to contribute to ITP pathogenesis. Nevertheless, the relationship between the specific thrombocytopenic mechanisms and various types of anti-platelet antibodies has not been established. In order to ascertain such association, we used sera from 61 ITP patients and assessed the capacity of anti-platelet antibodies to induce neuraminidase 1 (NEU1) surface expression, RCA-1 lectin binding and loss of mitochondrial inner membrane potential on donors' platelets. Sera from ITP patients with detectable antibodies caused significant platelet desialylation and apoptosis. Anti-GPIIb/IIIa antibodies appeared more capable of causing NEU1 surface translocation while anti-GPIb/IX complex antibodies resulted in a higher degree of platelet apoptosis. In ITP patients with anti-GPIIb/IIIa antibodies, both desialylation and apoptosis were dependent on FcγRIIa signaling rather than platelet activation. Finally, we confirmed in a murine model of ITP that destruction of human platelets induced by anti-GPIIb/IIIa antibodies can be prevented with the NEU1 inhibitor oseltamivir. A collaborative clinical trial is warranted to investigate the utility of oseltamivir in the treatment of ITP.

A Practical Perspective of the Hematologic Manifestations of Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a chronic inflammatory disease with an unknown etiology that can affect any organ or system of the human body. Hematological, renal, or central nervous system manifestations in these patients result in great morbidity because high doses of glucocorticoids, cytotoxic medications, or biological drugs are required to control these manifestations. It is noteworthy that hematological involvement predominates during the first years of the disease and tends to last over time, with the premise that it may be the initial manifestation of the disease. Clear examples of this are the cases of hemolytic anemia and immune thrombocytopenia that can be initially classified as idiopathic or primary to be later classified as secondary when associated with infections, medications, neoplasms, or autoimmune diseases. The spectrum of hematologic manifestations in SLE is very broad, including lymphopenia, anemia, thrombocytopenia, or pancytopenia. In some cases, lymphadenopathy and splenomegaly are also identified. The vast majority of these manifestations denote high disease activity. However, many of these alterations have a multifactorial cause that must be taken into account to adopt a more complete therapeutic approach. The objective of this review is to characterize in detail the hematological manifestations of SLE to offer clinicians a practical vision of its diagnosis and treatment.

Sialic acid and platelet count regulation: Implications in immune thrombocytopenia

Platelets are blood components that survive in circulation for 7 to 10 days in humans. Thus, platelet production by bone marrow (BM) megakaryocytes (MKs), and their removal from the blood circulation is precisely orchestrated to maintain an average platelet count. Abnormalities in both processes can result in thrombocytopenia (low platelet count) or thrombocytosis (high platelet count), often associated with the risk of bleeding or overt thrombus formation, respectively. Platelet glycans, particularly sialic acids, are indicators of platelet count. Loss of platelet sialic acids leads to platelet clearance. A State-of-the-Art lecture titled "Platelet and Megakaryocyte Glycobiology" was presented at the ISTH virtual congress 2021 to discuss (i) the loss of O-glycan sialic acid on BM MKs, revealing the Thomsen-Friedenreich (TF) antigen as a new concept of thrombocytopenia; herein, impaired thrombopoiesis is attributed to activation of immune cells with a plasmacytoid dendritic cell signature; and (ii) upregulation of antibodies against the TF antigen in pediatric patients with immune thrombocytopenia (ITP), positing that glycan alterations such as MK asialylation can lead to immune cell responses. Here, we discuss our findings alongside new data presented at the 2020 and 2021 ISTH congresses on the role of sialic acids and glycans in regulating platelet count. Desialylation is a prominent feature in thrombocytopenia, notably in ITP presentation. We compare similarities between ITP mediated with shear-stress and with storage-related asialylation. We also discuss genes involved in sialic acid synthesis leading to thrombocytopenia. Increased awareness in gene-regulating MK and platelet glycans is a giant leap to understanding the underpinning mechanisms of ITP and other forms of thrombocytopenia.

Early treatment of rituximab combined with eltrombopag for secondary thrombocytopenic purpura in rheumatoid arthritis: A case report

RATIONALE

Secondary immune thrombocytopenic purpura (ITP) is also known as acquired thrombocytopenic purpura, autoimmune disease is usually one of the important causes. There are few reports about treatment of refractory thrombocytopenic purpura in rheumatoid arthritis (RA). We report a case of refractory ITP in which changes in platelet-related markers with therapeutic agents are worthy of the attention of clinicians.

PATIENT CONCERNS

A 69-year-old woman admitted for ecchymosis on the neck and arms for 15 days presented to our hospital. She was diagnosed with RA 5 years ago.

DIAGNOSIS

The diagnosis met the American College of Rheumatology/European League Against Rheumatism 2010 classification criteria. The disease activity score 28 (DAS-28) was 4.6, indicating that the disease activity was moderate.

INTERVENTIONS

Treatment with first-line therapies and second-line treatment--eltrombopag (EPAG) were ineffective. Therefore, we performed rituximab combined with a low dose of EPAG.

OUTCOMES

The patient received 2 cycles of rituximab combined with EPAG, and reported no new petechiae on her buccal mucosa and limbs during follow-up.

LESSONS

This case suggests that early treatment of rituximab combined with EPAG is beneficial to patients with refractory ITP in RA. In terms of disease dynamic monitoring, immature platelet fraction (IPF) may be an auxiliary indicator for predicting efficacy, but its significance needs further study.

Viper venoms drive the macrophages and hepatocytes to sequester and clear platelets: novel mechanism and therapeutic strategy for venom-induced thrombocytopenia

Venomous snakebites cause clinical manifestations that range from local to systemic and are considered a significant global health challenge. Persistent or refractory thrombocytopenia has been frequently reported in snakebite patients, especially in cases caused by viperidae snakes. Viper envenomation-induced thrombocytopenia may persist in the absence of significant consumption coagulopathy even after the antivenom treatment, yet the mechanism remains largely unknown. Our study aims to investigate the mechanism and discover novel therapeutic targets for coagulopathy-independent thrombocytopenia caused by viper envenomation. Here we found that patients bitten by Protobothrops mucrosquamatus and Trimeresurus stejnegeri, rather than Naja. atra may develop antivenom-resistant and coagulopathy-independent thrombocytopenia. Crude venoms and the derived C-type lectin-like proteins from these vipers significantly increased platelet surface expression of neuraminidase and platelet desialylation, therefore led to platelet ingestion by both macrophages and hepatocytes in vitro, and drastically decreased peripheral platelet counts in vivo. Our study is the first to demonstrate that desialylation-mediated platelet clearance is a novel mechanism of viper envenomation-induced refractory thrombocytopenia and C-type lectin-like proteins derived from the viper venoms contribute to snake venom-induced thrombocytopenia. The results of this study suggest the inhibition of platelet desialylation as a novel therapeutic strategy against viper venom-induced refractory thrombocytopenia.

GPIb-IX-V and platelet clearance

Platelet adhesion to the site of vascular damage is a critical early step in hemostasis. The platelet glycoprotein (GP) Ib-IX-V plays a key role in this step via its interaction with immobilized von Willebrand Factor (VWF). In addition to its well-known role in adhesion, GPIb-IX-V is critical for platelets' survival in circulation and plays an important role in the regulation of platelet clearance. Several mechanisms of platelet clearance work in concert to maintain a normal platelet count and ensure that circulating platelets are functionally viable via removal of senescent or activated platelets. Furthermore, dysregulation of platelet clearance underlies several bleeding disorders. GPIb-IX-V is central to many physiological mechanisms of platelet clearance including clearance via glycan receptors, clearance of VWF-platelet complexes, and fast clearance of transfused platelets. GPIb-IX-V dependent clearance also underlies thrombocytopenia in several bleeding disorders, including von Willebrand disease (VWD) and immune thrombocytopenia. This review will cover physiological and pathological mechanisms of platelet clearance, focusing on the role of GPIb-IX-V.

Updated Understanding of Platelets in Thrombosis and Hemostasis: The Roles of Integrin PSI Domains and their Potential as Therapeutic Targets

Platelets are small blood cells known primarily for their ability to adhere and aggregate at injured vessels to arrest bleeding. However, when triggered under pathological conditions, the same adaptive mechanism of platelet adhesion and aggregation may cause thrombosis, a primary cause of heart attack and stroke. Over recent decades, research has made considerable progress in uncovering the intricate and dynamic interactions that regulate these processes. Integrins are heterodimeric cell surface receptors expressed on all metazoan cells that facilitate cell adhesion, movement, and signaling, to drive biological and pathological processes such as thrombosis and hemostasis. Recently, our group discovered that the plexin-semaphorin-integrin (PSI) domains of the integrin β subunits exert endogenous thiol isomerase activity derived from their two highly conserved CXXC active site motifs. Given the importance of redox reactions in integrin activation and its location in the knee region, this PSI domain activity may be critically involved in facilitating the interconversions between integrin conformations. Our monoclonal antibodies against the β3 PSI domain inhibited its thiol isomerase activity and proportionally attenuated fibrinogen binding and platelet aggregation. Notably, these antibodies inhibited thrombosis without significantly impairing hemostasis or causing platelet clearance. In this review, we will update mechanisms of thrombosis and hemostasis, including platelet versatilities and immune-mediated thrombocytopenia, discuss critical contributions of the newly discovered PSI domain thiol isomerase activity, and its potential as a novel target for anti-thrombotic therapies and beyond.

GPIbα is the driving force of hepatic thrombopoietin generation

Thrombopoietin (TPO), a glycoprotein hormone produced predominantly in the liver, plays important roles in the hematopoietic stem cell (HSC) niche, and is essential for megakaryopoiesis and platelet generation. Long-standing understanding proposes that TPO is constitutively produced by hepatocytes, and levels are fine-tuned through platelet and megakaryocyte internalization/degradation via the c-Mpl receptor. However, in immune thrombocytopenia (ITP) and several other diseases, TPO levels are inconsistent with this theory. Recent studies showed that platelets, besides their TPO clearance, can induce TPO production in the liver. Our group also accidentally discovered that platelet glycoprotein (GP) Ibα is required for platelet-mediated TPO generation, which is underscored in both GPIbα-/- mice and patients with Bernard-Soulier syndrome. This review will introduce platelet versatilities and several new findings in hemostasis and platelet consumption but focus on its roles in TPO regulation. The implications of these new discoveries in hematopoiesis and the HSC niche, particularly in ITP, will be discussed.

The molecular basis of immune-based platelet disorders

Platelets have a predominant role in haemostasis, the maintenance of blood volume and emerging roles as innate immune cells, in wound healing and in inflammatory responses. Platelets express receptors that are important for platelet adhesion, aggregation, participation in inflammatory responses, and for triggering degranulation and enhancing thrombin generation. They carry a cargo of granules bearing enzymes, adhesion molecules, growth factors and cytokines, and have the ability to generate reactive oxygen species. The platelet is at the frontline of a host of cellular responses to invading pathogens, injury, and infection. Perhaps because of this intrinsic responsibility of a platelet to rapidly respond to thrombotic, pathological and immunological factors as part of their infantry role; platelets are susceptible to targeted attack by the adaptive immune system. Such attacks are often transitory but result in aberrant platelet activation as well as significant loss of platelet numbers and platelet function, paradoxically leading to elevated risks of both thrombosis and bleeding. Here, we discuss the main molecular events underlying immune-based platelet disorders with specific focus on events occurring at the platelet surface leading to activation and clearance.

A multicenter, randomized phase III trial of hetrombopag: a novel thrombopoietin receptor agonist for the treatment of immune thrombocytopenia

Background

Hetrombopag, a novel thrombopoietin receptor agonist, has been found in phase I studies to increase platelet counts and reduce bleeding risks in adults with immune thrombocytopenia (ITP). This phase III study aimed to evaluate the efficacy and safety of hetrombopag in ITP patients.

Methods

Patients who had not responded to or had relapsed after previous treatment were treated with an initial dosage of once-daily 2.5 or 5 mg hetrombopag (defined as the HETROM-2.5 or HETROM-5 group) or with matching placebo in a randomized, double-blind, 10-week treatment period. Patients who received placebo and completed 10 weeks of treatment switched to receive eltrombopag, and patients treated with hetrombopag in the double-blind period continued hetrombopag during the following open-label 14-week treatment. The primary endpoint was the proportion of responders (defined as those achieving a platelet count of ≥ 50 × 10⁹/L) after 8 weeks of treatment.

Results

The primary endpoint was achieved by significantly more patients in the HETROM-2.5 (58.9%; odds ratio [OR] 25.97, 95% confidence interval [CI] 9.83–68.63; p < 0.0001) and HETROM-5 (64.3%; OR 32.81, 95% CI 12.39–86.87; p < 0.0001) group than in the Placebo group (5.9%). Hetrombopag was also superior to placebo in achieving a platelet response and in reducing the bleeding risk and use of rescue therapy throughout 8 weeks of treatment. The durable platelet response to hetrombopag was maintained throughout 24 weeks. The most common adverse events were upper respiratory tract infection (42.2%), urinary tract infection (17.1%), immune thrombocytopenic purpura (17.1%) and hematuria (15%) with 24-week hetrombopag treatment.

Conclusions

In ITP patients, hetrombopag is efficacious and well tolerated with a manageable safety profile.

Trial registration Clinical trials.gov NCT03222843, registered July 19, 2017, retrospectively registered.

Mechanisms of anti-GPIbα antibody-induced thrombocytopenia in mice

Immune thrombocytopenia (ITP) is an acquired bleeding disorder characterized by antibody-mediated platelet destruction. Different mechanisms have been suggested to explain accelerated platelet clearance and impaired thrombopoiesis, but the pathophysiology of ITP has yet to be fully delineated. In this study, we tested 2 mouse models of immune-mediated thrombocytopenia using the rat anti-mouse GPIbα monoclonal antibody 5A7, generated in our laboratory. After a single IV administration of high-dose (2 mg/kg) 5A7, opsonized platelets were rapidly cleared from the circulation into the spleen and liver; this was associated with rapid upregulation of thrombopoietin (TPO) messenger RNA. In contrast, subcutaneous administration of low-dose 5A7 (0.08-0.16 mg/kg) every 3 days gradually lowered the platelet count; in this case, opsonized platelets were observed only in the spleen, and TPO levels remained unaltered. Interestingly, in both models, the 5A7 antibody was found on the surface of, as well as internalized to, bone marrow megakaryocytes. Consequently, platelets generated in the chronic phase of repeated subcutaneous 5A7 administration model showed reduced GPIbα membrane expression on their surface. Our findings indicate that evaluation of platelet surface GPIbα relative to platelet size may be a useful marker to support the diagnosis of anti-GPIbα antibody-induced ITP.

Primary Immune Thrombocytopenia: Novel Insights into Pathophysiology and Disease Management

Immune thrombocytopenia (ITP) is an autoimmune disorder defined by a significantly reduced number of platelets in blood circulation. Due to low levels of platelets, ITP is associated with frequent bruising and bleeding. Current evidence suggests that low platelet counts in ITP are the result of multiple factors, including impaired thrombopoiesis and variations in immune response leading to platelet destruction during pathological conditions. Patient outcomes as well as clinic presentation of the disease have largely been shown to be case-specific, hinting towards ITP rather being a group of clinical conditions sharing common symptoms. The most frequent characteristics include dysfunction in primary haemostasis and loss of immune tolerance towards platelet as well as megakaryocyte antigens. This heterogeneity in patient population and characteristics make it challenging for the clinicians to choose appropriate therapeutic regimen. Therefore, it is vital to understand the pathomechanisms behind the disease and to consider various factors including patient age, platelet count levels, co-morbidities and patient preferences before initiating therapy. This review summarizes recent developments in the pathophysiology of ITP and provides a comprehensive overview of current therapeutic strategies as well as potential future drugs for the management of ITP.

Co-Inhibition of the Immunoproteasome Subunits LMP2 and LMP7 Ameliorates Immune Thrombocytopenia

The immunoproteasome, a special isoform of the 20S proteasome, is expressed when the cells receive an inflammatory signal. Immunoproteasome inhibition proved efficacy in the treatment of autoimmune diseases. However, the role of the immunoproteasome in the pathogenesis of immune thrombocytopenia (ITP) remains unknown. We found that the expression of the immunoproteasome catalytic subunit, large multifunctional protease 2 (LMP2), was significantly upregulated in peripheral blood mononuclear cells of active ITP patients compared to those of healthy controls. No significant differences in LMP7 expression were observed between patients and controls. ML604440, an specific LMP2 inhibitor, had no significant impact on the platelet count of ITP mice, while ONX-0914 (an inhibitor of both LMP2 and LMP7) increased the number of platelets. In vitro assays revealed that ONX-0914 decreased the expression of FcγRI in ITP mice and decreased that of FcγRIII in ITP patients, inhibited the activation of CD4+ T cells, and affected the differentiation of Th1 cells in patients with ITP. These results suggest that the inhibition of immunoproteasome is a potential therapeutic approach for ITP patients.

New Developments in the Pathophysiology and Management of Primary Immune Thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disease that is characterized by a significant reduction in the number of circulating platelets and frequently associated with bleeding. Although the pathogenesis of ITP is still not completely elucidated, it is largely recognized that the low platelet count observed in ITP patients is due to multiple alterations of the immune system leading to increased platelet destruction as well as impaired thrombopoiesis. The clinical manifestations and patients' response to different treatments are very heterogeneous suggesting that ITP is a group of disorders sharing common characteristics, namely, loss of immune tolerance toward platelet (and megakaryocyte) antigens and dysfunctional primary hemostasis. Management of ITP is challenging and requires intensive communication between patients and caregivers. The decision to initiate treatment should be based on the platelet count level, age of the patient, bleeding manifestation, and other factors that influence the bleeding risk in individual patients. In this review, we present recent data on the mechanisms that lead to platelet destruction in ITP with a particular focus on current findings concerning alterations of thrombopoiesis. In addition, we give an insight into the efficacy and safety of current therapies and management of ITP bleeding emergencies.

A novel method to detect autoantibodies against platelets in patients with immune thrombocytopenia

BACKGROUND AND OBJECTIVES

To examine anti-platelet autoantibodies in patients with immune thrombocytopenia (ITP) not only provides solid evidence for diagnosis, and also helps to select an individualized strategy for the treatment. The aim of this study is to develop a novel cell-based assay to detect autoantibodies in ITP patients.

METHODS/PATIENTS

The DNA sequences of human platelet membrane protein GPIbα, GPIbβ, GP IX, GPIIb and GPIIIa subunits were obtained from NCBI database and synthesized. The synthetic fragments were ligated into pcDNA 3.3 and constructed the recombinant plasmids and transfected into Chinese hamster ovary (CHO) cells to establish cell lines stable expressing GPIb-IX and/or GPIIb/IIIa complexes. One hundred and two ITP patients with different anti-platelet autoantibodies, 57 patients with other kinds of autoimmune diseases and 104 healthy control were selected to examine sensitivity, specificity and accuracy of this method.

RESULTS

CHO cells stable expressing GPIb-IX and/or GPIIb/IIIa proteins were established. The cells were fixed with 4% paraformaldehyde and stored at -80 ℃, more than 80% of the cells were still expressed target proteins after 180 days of storage. The concentrations of target antibody from 0.1 to 100 μg/ml were detectable by this method, and 10-50 μg/ml antibody binding to the CHO cells yielded higher distinguishable fluorescent intensities. Inter-assay and intra-assay coefficients of variation and receiver operating characteristic curve analysis showed that this method had relatively higher reproducibility and specificity. Compared with Flow Cytometric Immunobead Array, this method has relatively higher specificity (95.2%) and accuracy (90.8%) in detection of 102 ITP patients.

CONCLUSION

A novel cell-based assay to detect autoantibodies in ITP patients is established, which appears to be a promising method to diagnose ITP.

Evolution and Utility of Antiplatelet Autoantibody Testing in Patients with Immune Thrombocytopenia

To this day, immune thrombocytopenia (ITP) remains a clinical diagnosis made by exclusion of other causes for thrombocytopenia. Reliable detection of platelet autoantibodies would support the clinical diagnosis, but the lack of specificity and sensitivity of the available methods for platelet autoantibody testing limits their value in the diagnostic workup of thrombocytopenia. The introduction of methods for glycoprotein-specific autoantibody detection has improved the specificity of testing and is acceptable for ruling in ITP but not ruling it out as a diagnosis. The sensitivity of these assays varies widely, even between studies using comparable assays. A review of the relevant literature combined with our own laboratory's experience of testing large number of serum and platelet samples makes it clear that this variation can be explained by variations in the characteristics of the tests, including in the glycoprotein-specific monoclonal antibodies, the glycoproteins that are tested, the platelet numbers used in the assay and the cutoff levels for positive and negative results, as well as differences in the tested patient populations. In our opinion, further standardization and optimization of the direct autoantibody detection methods to increase sensitivity without compromising specificity seem possible but will still likely be insufficient to distinguish the often very weak specific autoantibody signals from background signals. Further developments of autoantibody detection methods will therefore be necessary to increase sensitivity to a level acceptable to provide laboratory confirmation of a diagnosis of ITP.

Conformation-Specific Blockade of αIIbβ3 by a Non-RGD Peptide to Inhibit Platelet Activation without Causing Significant Bleeding and Thrombocytopenia

Bleeding and thrombocytopenia to readministration are the most serious side effects of clinical integrin αIIbβ3 antagonists such as RGD-containing peptides. Here we show that a non-RGD peptide ZDPI, identified from skin secretions of Amolops loloensis, inhibited platelet aggregation induced by agonists, such as adenosine diphosphate, collagen, arachidonic acid, PAR1AP, and integrin αIIbβ3 allosteric activator, and reduces soluble fibrinogen binding to activated platelets without perturbing adhesion numbers on immobilized fibrinogen. Further study showed that ZDPI preferred to bind to the active conformation of integrin αIIbβ3, and thus inhibited c-Src-mediated integrin signaling transduction. In contrast to currently used clinical blockers of integrin αIIbβ3, which are all conformation-unspecific blockers, ZDPI conformation specifically binds to activated integrin αIIbβ3, subsequently suppressing platelet spreading. In vivo study revealed that ZDPI inhibited carotid arterial thrombosis with limited bleeding and thrombocytopenia. A non-RGD peptide which targets the active conformation of integrin αIIbβ3, such as ZDPI, might be an excellent candidate or template to develop antithrombotics without significant bleeding and thrombocytopenia side effects.

Gut Microbiome and Metabolome Were Altered and Strongly Associated With Platelet Count in Adult Patients With Primary Immune Thrombocytopenia

Gut microbiota has been implicated in the pathogenesis of many autoimmune diseases. This is still an area of active research given that the role of gut microbiota on the primary immune thrombocytopenia (ITP) remains unclear. In this study, fecal samples of 30 untreated adult primary ITP patients and 29 healthy controls (HCs) were used to investigate the gut microbial community and metabolite profiles. Our results show that fecal bacteria such as Blautia, Streptococcus, and Lactobacillus are enriched, whereas bacteria such as Bacteroides are depleted in ITP patients. Notably, fecal metabolites such as fatty acyls and glycerophospholipids are enriched and strongly correlate with discrepant gut microbiota. Furthermore, combinations of Weissella and Streptococcus anginosus, or Cer (t18:0/16:0), Cer (d18:1/17:0), and 13-hydroxyoctadecanoic acid could provide good diagnostic markers for ITP. Moreover, a strong negative correlation was found between platelet count and altered gut microbiota such as S. anginosus and gut metabolites such as Cer (t18:0/16:0) in ITP. In conclusion, dysbiosis of both gut microbiota and metabolome develops in ITP patients compared to HCs. Several ITP-altered gut bacteria and metabolites can be diagnostic biomarkers for ITP, and are highly correlated with platelet count, suggesting that they may also play a role in ITP pathogenesis.

Antiplatelet Antibodies Do Not Predict the Response to Intravenous Immunoglobulins during Immune Thrombocytopenia

Immune thrombocytopenia (ITP) is a rare autoimmune disease due to autoantibodies targeting platelet glycoproteins (GP). The mechanism of platelet destruction could differ depending on the specificity of antiplatelet antibodies: anti-GPIIb/IIIa antibodies lead to phagocytosis by splenic macrophages, in a Fcγ receptor (FcγR)-dependent manner while anti-GPIb/IX antibodies induce platelet desialylation leading to their destruction by hepatocytes after binding to the Ashwell-Morell receptor, in a FcγR-independent manner. Considering the FcγR-dependent mechanism of action of intravenous immunoglobulins (IVIg), we assumed that the response to IVIg could be less efficient in the presence of anti-GPIb/IX antibodies. We conducted a multicentric, retrospective study including all adult ITP patients treated with IVIg who had antiplatelet antibodies detected between January 2013 and October 2017. Among the 609 identified, 69 patients were included: 17 had anti-GPIb/IX antibodies and 33 had anti-GPIIb/IIIa antibodies. The response to IVIg was not different between the patients with or without anti-GPIb/IX (88.2% vs. 73.1%). The response to IVIg was better in the case of newly diagnosed ITP (odds ratio (OR) = 5.4 (1.2-24.7)) and in presence of anti-GPIIb/IIIa (OR = 4.82 (1.08-21.5)), while secondary ITP had a poor response (OR = 0.1 (0.02-0.64)). In clinical practice, the determination of antiplatelet antibodies is therefore of little value to predict the response to IVIg.

Identifying and treating refractory ITP: difficulty in diagnosis and role of combination treatment

Immune thrombocytopenia (ITP) is the most common acquired thrombocytopenia after chemotherapy-induced thrombocytopenia. Existing guidelines describe the management and treatment of most patients who, overall, do well, even if they present with chronic disease, and they are usually not at a high risk for bleeding; however, a small percentage of patients is refractory and difficult to manage. Patients classified as refractory have a diagnosis that is not really ITP or have disease that is difficult to manage. ITP is a diagnosis of exclusion; no specific tests exist to confirm the diagnosis. Response to treatment is the only affirmative confirmation of diagnosis. However, refractory patients do not respond to front-line or other treatments; thus, no confirmation of diagnosis exists. The first section of this review carefully evaluates the diagnostic considerations in patients with refractory ITP. The second section describes combination treatment for refractory cases of ITP. The reported combinations are divided into the era before thrombopoietin (TPO) and rituximab and the current era. Current therapy appears to have increased effectiveness. However, the definition of refractory, if it includes insufficient response to TPO agents, describes a group with more severe and difficult-to-treat disease. The biology of refractory ITP is largely unexplored and includes oligoclonality, lymphocyte pumps, and other possibilities. Newer treatments, especially rapamycin, fostamatinib, FcRn, and BTK inhibitors, may be useful components of future therapy given their mechanisms of action; however, TPO agents, notwithstanding failure as monotherapy, appear to be critical components. In summary, refractory ITP is a complicated entity in which a precise specific diagnosis is as important as the development of effective combination treatments.

Novel scientific approaches and future research directions in understanding ITP

Diagnosis of immune thrombocytopenia (ITP) and prediction of response to therapy remain significant and constant challenges in hematology. In patients who present with ITP, the platelet count is frequently used as a surrogate marker for disease severity, and so often determines the need for therapy. Although there is a clear link between thrombocytopenia and hemostasis, a direct correlation between the extent of thrombocytopenia and bleeding symptoms, especially at lower platelet counts is lacking. Thus, bleeding in ITP is heterogeneous, unpredictable, and nearly always based on a multitude of risk factors, beyond the platelet count. The development of an evidence-based, validated risk stratification model for ITP treatment is a major goal in the ITP community and this review discusses new laboratory approaches to evaluate the various pathobiologies of ITP that may inform such a model.

Murine models for studying treatment, prevention and pathogenesis of FNAIT

Maternal alloimmunization to paternally inherited antigens on fetal/neonatal platelets can cause fetal/neonatal alloimmune thrombocytopenia (FNAIT) after antibody-mediated removal of platelets from the fetal circulation. The complications vary from mild bleeding symptoms to severe intracranial hemorrhage and subsequent neurological impairment or death. Studies on in vivo mechanisms are challenging to measure directly in pregnant women, rendering murine models as valuable and attractive alternatives, despite some critical differences between mice and men affecting the translational value. Here we present and discuss, the different murine models that substantially have increased our knowledge and understanding of FNAIT pathogenesis - as well as pre-clinical evaluation of therapeutic and preventive strategies.

Immune thrombocytopenia (ITP): Pathophysiology update and diagnostic dilemmas

Immune thrombocytopenia (ITP) is a common autoimmune bleeding disorder. The understanding of ITP pathogenesis is rapidly evolving. We now recognize ITP as a complex and heterogeneous syndrome that results from a combination of humoral and cell-mediated attacks on platelets peripherally and megakaryocytes in the bone marrow. Autoantibody-mediated ITP also varies in the pathway used to clear platelets, which depends on the platelet glycoprotein being targeted. Moreover, ITP patients present with variable bleeding severities and treatment responses that do not closely correlate with platelet count. A gold standard diagnostic test for ITP is lacking, and biomarkers to assess disease severity are in their infancy. This review provides an update on the immunopathogenesis of ITP and summarizes currently available tests for ITP diagnosis, prediction of disease severity, and treatment responses. Given the heterogeneous pathogenesis and clinical presentation of ITP, we highlight the need for the development of diagnostic and prognostic tests that would allow for the individualized management of a complex disease.

Acquired Glanzmann thrombasthenia: From antibodies to anti-platelet drugs

In contrast to the inherited platelet disorder given by mutations in the ITGA2B and ITGB3 genes, mucocutaneous bleeding from a spontaneous inhibition of normally expressed αIIbβ3 characterizes acquired Glanzmann thrombasthenia (GT). Classically, it is associated with autoantibodies or paraproteins that block platelet aggregation without causing a fall in platelet count. However, inhibitory antibodies to αIIbβ3 are widely associated with primary immune thrombocytopenia (ITP), occur in secondary ITP associated with leukemia and related disorders, solid cancers and myeloma, other autoimmune diseases, following organ transplantation while cytoplasmic dysregulation of αIIbβ3 function features in myeloproliferative and myelodysplastic syndromes. Antibodies to αIIbβ3 occur during viral and bacterial infections, while drug-dependent antibodies reacting with αIIbβ3 are a special case. Direct induction of acquired GT is a feature of therapies that block platelets in coronary artery disease. This review looks at these conditions, emphasizing molecular mechanisms, therapy, patient management and future directions for research.

Low-dose chidamide restores immune tolerance in ITP in mice and humans

Increased macrophage phagocytosis of antibody-coated platelets, as well as decreased numbers and/or impaired function of CD4⁺CD25⁺Foxp3⁺ regulatory T (Treg) cells, has been shown to participate in the pathogenesis of immune thrombocytopenia (ITP). Low-dose histone deacetylase inhibitors (HDACi's) are anti-inflammatory and immunomodulatory agents that can enhance immunosuppression in graft-versus-host disease by increasing the number and function of Foxp3⁺ Treg cells, but it is unclear whether they have the potential to promote immune tolerance and platelet release in ITP. In this study, we performed in vitro and in vivo experiments and found that a low-dose HDACi (chidamide) alleviated thrombocytopenia in passive and active murine models of ITP. Further, low-dose HDACi's attenuated macrophage phagocytosis of antibody-coated platelets, stimulated the production of natural Foxp3⁺ Treg cells, promoted the peripheral conversion of T cells into Treg cells, and restored Treg cell suppression in vivo and in vitro. Finally, we confirmed that low-dose HDACi's could regulate CTLA4 expression in peripheral blood mononuclear cells through modulation of histone H3K27 acetylation. Low-dose HDACi treatment in ITP could be offset by blocking the effect of CTLA4. Therefore, we propose that low-dose chidamide administration has potential as a novel treatment for ITP in the clinic.

Platelet-specific antibodies and differences in their expression in childhood immune thrombocytopenic purpura predicts clinical progression

IMPORTANCE

Immune thrombocytopenic purpura (ITP) is the most common bleeding disorder in children. Despite the highly spontaneously remission, still almost 20% of cases progress into chronic or refractory ITP, which seriously affects children's quality of life. Currently there is no method to predict the initial stage of childhood ITP.

OBJECTIVES

To evaluate platelet-specific antibodies and compare differences in their expression in childhood ITP to predict clinical progression.

METHODS

This is a single-center prospective cohort study from April 2014 to October 2015. We enrolled children initially diagnosed as ITP. Anti-GPIIb/IIIa and GPIb/IX antibodies were assayed by enzyme-linked immunoadsorbent assay (ELISA) and patients were followed up for 1 year. We also analyzed the relationship between the expression of the platelet-specific antibodies GPIIb/IIIa and GPIb/IX and their respective clinical prognoses.

RESULTS

Overall, 134 cases were enrolled including 77 boys and 57 girls with a median age of 19 months (range: 1 to 159). Positive rates of anti-platelet antibodies were 79.8%. After a 1-year observation period, 84.3% were diagnosed as newly diagnosed ITP and 13.4% were diagnosed as chronic ITP. Patients with anti-GPIIb/IIIa antibody had a higher risk for newly diagnosed ITP compared with patients who were anti-GPIb/IX antibody positive only (93% vs 25%, P = 0.005; 87% vs 25%, P = 0.014, respectively). There were more anti-GPIb/IX antibody positive only cases, diagnosed as chronic ITP, compared with anti-GPIIb/IIIa antibody positive only cases and double GPIIb/IIIa and GPIb/IX antibody positive cases (75% vs 7%, P = 0.005; 75% vs 13%, P = 0.014, respectively). Interpretation.

INTERPRETATION

Patients with anti-GPIIb/IIIa antibody (either single or double) were predicted to have a good prognosis, whereas anti-GPIb/ IX antibody only predicted a poor prognosis. These results should be confirmed via a larger cohort multicenter study.

Multirefractory primary immune thrombocytopenia; targeting the decreased sialic acid content

Patients with multirefractory immune thrombocytopenia (ITP) have limited treatment options. Recent data suggest that specific anti-platelet antibodies may cause destruction of platelets by favoring platelet loss of sialic acid. In this multicenter study 35 patients with ITP, including 16 with multirefractory disease, were analyzed for antiplatelet-antibodies, thrombopoietin (TPO) levels, and platelet desialylation. In selected cases, responses to a novel treatment strategy using oseltamivir were tested. We found that antibodies against GPIbα were overrepresented in multirefractory patients compared to responders (n = 19). In contrast to conventional ITP patients, multirefractory patients exhibited a significant increased platelet activation state (granule secretion) and desialylation (RCA-1 binding) (p < 0.05), and a trend toward higher plasma TPO concentrations. The decreased sialic acid content seemed to be restricted to platelet glycoproteins, since other plasma proteins were not hypoglycosylated. A total of 10 patients with multirefractory ITP having remarkable loss of platelet terminal sialic acids were given oseltamivir phosphate. When the antiviral drug was combined with TPO receptor agonists (TPO-RAs) or with immunosuppressant drugs, platelet responses were observed in 66.7% of patients. All responding patients presented with antibodies reactive only against GPIbα. These findings suggest that desialylation may play a key pathogenic role in some multirefractory ITP patients, and provide diagnostic tools for the identification of such patients. Furthermore, we show that sialidase inhibitor treatment in combination with therapies that help to increase platelet production can induce sustained platelet responses in some patients with anti-GPIbα -mediated thrombocytopenia that have failed previous therapies.

Transplantation of bone-marrow-derived mesenchymal stem cells into a murine model of immune thrombocytopenia

: Several reports have demonstrated T regulatory cells may play an important role in the pathophysiology of immune thrombocytopenia (ITP). As the immunomodulator, bone-marrow-derived mesenchymal stem cells (MSCs) (BM-MSCs) regulate T regulatory cells and show therapeutic effects on autoimmune diseases. However, it is not clear how BM-MSCs affect ITP. In this study, we explored the specific effects of BM-MSCs on ITP in mice. Using a murine model of ITP, mice were randomly divided into three groups: normal control group, ITP control group and ITP and BM-MSCs group. Platelet (PLT) levels were monitored by an automatic blood cell counter, and T regulatory cells were analyzed by flow cytometry. Compared with the untreated ITP mice, the PLT level of the ITP mice was significantly increased after BM-MSCs treatment. In the BM-MSCs group, T regulatory cells were significantly decreased. These findings demonstrate that bone-marrow-derived MSCs are effective in improving PLT levels and reducing the T regulatory cells mediating proinflammatory response in ITP mice.

GPIbα is required for platelet-mediated hepatic thrombopoietin generation

Thrombopoietin (TPO), a hematopoietic growth factor produced predominantly by the liver, is essential for thrombopoiesis. Prevailing theory posits that circulating TPO levels are maintained through its clearance by platelets and megakaryocytes via surface c-Mpl receptor internalization. Interestingly, we found a two- to threefold decrease in circulating TPO in GPIbα^-/- mice compared with wild-type (WT) controls, which was consistent in GPIbα-deficient human Bernard-Soulier syndrome (BSS) patients. We showed that lower TPO levels in GPIbα-deficient conditions were not due to increased TPO clearance by GPIbα^-/- platelets but rather to decreased hepatic TPO mRNA transcription and production. We found that WT, but not GPIbα^-/-, platelet transfusions rescued hepatic TPO mRNA and circulating TPO levels in GPIbα^-/- mice. In vitro hepatocyte cocultures with platelets or GPIbα-coupled beads further confirm the disruption of platelet-mediated hepatic TPO generation in the absence of GPIbα. Treatment of GPIbα^-/- platelets with neuraminidase caused significant desialylation; however, strikingly, desialylated GPIbα^-/- platelets could not rescue impaired hepatic TPO production in vivo or in vitro, suggesting that GPIbα, independent of platelet desialylation, is a prerequisite for hepatic TPO generation. Additionally, impaired hepatic TPO production was recapitulated in interleukin-4/GPIbα-transgenic mice, as well as with antibodies targeting the extracellular portion of GPIbα, demonstrating that the N terminus of GPIbα is required for platelet-mediated hepatic TPO generation. These findings reveal a novel nonredundant regulatory role for platelets in hepatic TPO homeostasis, which improves our understanding of constitutive TPO regulation and has important implications in diseases related to GPIbα, such as BSS and auto- and alloimmune-mediated thrombocytopenias.

Fc engineering of anti-Nectin-2 antibody improved thrombocytopenic adverse event in monkey

Nectin-2 is a transmembrane glycoprotein which is involved in the process of Ca2+-independent cell-cell adhesion. In our previous study, we have demonstrated that Nectin-2 is over-expressed in breast and ovarian cancer tissues by using gene expression analysis and immunohistochemistry. Furthermore, we discovered multiple anti-Nectin-2 fully human monoclonal antibodies which inhibited tumor growth in in vivo subcutaneous xenograft models with antibody-dependent cellular cytotoxicity (ADCC) as the principal mechanism of action. In this report, we assessed the toxicity of Y-443, a fully human IgG1/kappa anti-Nectin-2 monoclonal antibody exhibiting strong in vitro ADCC and in vivo anti-tumor activity in cynomolgus monkeys (Macaca fascicularis (Cynos)). Unexpectedly, upon administration, Y-443 induced strong thrombocytopenia through Nectin-2 expressed on Cyno platelets, presumably followed by phagocytosis in the mononuclear phagocytic system. To mitigate the adverse safety profile, we mutated the Fc region of Y-443 to reduce the Fc binding activity to Fcγ receptor I, which is the primary receptor for phagocytosis on macrophages. Moreover, we further engineered the Fc through defucosylation to maintain ADCC activity. The resultant Fc engineered antibody, termed Y-634, demonstrated diminished thrombocytopenia in Cyno toxicological studies and maintained anti-tumor activity in a mouse xenograft model. These findings suggest that Y-634 may have a therapeutic potential for the treatment of Nectin-2 positive cancers, and moreover, Fc engineering is a potential mitigation strategy to ameliorate safety liabilities in antibody induced thrombocytopenia while maintaining antibody potency.

Mechanisms of platelet clearance and translation to improve platelet storage

Hundreds of billions of platelets are cleared daily from circulation via efficient and highly regulated mechanisms. These mechanisms may be stimulated by exogenous reagents or environmental changes to accelerate platelet clearance, leading to thrombocytopenia. The interplay between antiapoptotic Bcl-x_L and proapoptotic molecules Bax and Bak sets an internal clock for the platelet lifespan, and BH3-only proteins, mitochondrial permeabilization, and phosphatidylserine (PS) exposure may also contribute to apoptosis-induced platelet clearance. Binding of plasma von Willebrand factor or antibodies to the ligand-binding domain of glycoprotein Ibα (GPIbα) on platelets can activate GPIb-IX in a shear-dependent manner by inducing unfolding of the mechanosensory domain therein, and trigger downstream signaling in the platelet including desialylation and PS exposure. Deglycosylated platelets are recognized by the Ashwell-Morell receptor and potentially other scavenger receptors, and are rapidly cleared by hepatocytes and/or macrophages. Inhibitors of platelet clearance pathways, including inhibitors of GPIbα shedding, neuraminidases, and platelet signaling, are efficacious at preserving the viability of platelets during storage and improving their recovery and survival in vivo. Overall, common mechanisms of platelet clearance have begun to emerge, suggesting potential strategies to extend the shelf-life of platelets stored at room temperature or to enable refrigerated storage.

Fc-independent immune thrombocytopenia via mechanomolecular signaling in platelets

Immune thrombocytopenia (ITP) is a prevalent autoimmune disease characterized by autoantibody-induced platelet clearance. Some ITP patients are refractory to standard immunosuppressive treatments such as intravenous immunoglobulin (IVIg). These patients often have autoantibodies that target the ligand-binding domain (LBD) of glycoprotein Ibα (GPIbα), a major subunit of the platelet mechanoreceptor complex GPIb-IX. However, the molecular mechanism of this Fc-independent platelet clearance is not clear. Here, we report that many anti-LBD monoclonal antibodies such as 6B4, but not AK2, activated GPIb-IX in a shear-dependent manner and induced IVIg-resistant platelet clearance in mice. Single-molecule optical tweezer measurements of antibodies pulling on full-length GPIb-IX demonstrated that the unbinding force needed to dissociate 6B4 from the LBD far exceeds the force required to unfold the juxtamembrane mechanosensory domain (MSD) in GPIbα, unlike the AK2-LBD unbinding force. Binding of 6B4, not AK2, induced shear-dependent unfolding of the MSD on the platelet, as evidenced by increased exposure of a linear sequence therein. Imaging flow cytometry and aggregometry measurements of platelets and LBD-coated platelet-mimetic beads revealed that 6B4 can sustain crosslinking of platelets under shear, whereas 6B4 Fab and AK2 cannot. These results suggest a novel mechanism by which anti-LBD antibodies can exert a pulling force on GPIb-IX via platelet crosslinking, activating GPIb-IX by unfolding its MSD and inducing Fc-independent platelet clearance.

Flow cytometric immunobead assay for quantitative detection of platelet autoantibodies in immune thrombocytopenia patients

BACKGROUND

Platelet autoantibody detection is critical for immune thrombocytopenia (ITP) diagnosis and prognosis. Therefore, we aimed to establish a quantitative flow cytometric immunobead assay (FCIA) for ITP platelet autoantibodies evaluation.

METHODS

Capture microbeads coupled with anti-GPIX, -GPIb, -GPIIb, -GPIIIa and P-selectin antibodies were used to bind the platelet-bound autoantibodies complex generated from plasma samples of 250 ITP patients, 163 non-ITP patients and 243 healthy controls, a fluorescein isothiocyanate (FITC)-conjugated secondary antibody was the detector reagent and mean fluorescence intensity (MFI) signals were recorded by flow cytometry. Intra- and inter-assay variations of the quantitative FCIA assay were assessed. Comparisons of the specificity, sensitivity and accuracy between quantitative and qualitative FCIA or monoclonal antibody immobilization of platelet antigen (MAIPA) assay were performed. Finally, treatment process was monitored by our quantitative FCIA in 8 newly diagnosed ITPs.

RESULTS

The coefficient of variations (CV) of the quantitative FCIA assay were respectively 9.4, 3.8, 5.4, 5.1 and 5.8% for anti-GPIX, -GPIb, -GPIIIa, -GPIIb and -P-selectin autoantibodies. Elevated levels of autoantibodies against platelet glycoproteins GPIX, GPIb, GPIIIa, GPIIb and P-selectin were detected by our quantitative FCIA in ITP patients compared to non-ITP patients or healthy controls. The sensitivity, specificity and accuracy of our quantitative assay were respectively 73.13, 81.98 and 78.65% when combining all 5 autoantibodies, while the sensitivity, specificity and accuracy of MAIPA assay were respectively 41.46, 90.41 and 72.81%.

CONCLUSIONS

A quantitative FCIA assay was established. Reduced levels of platelet autoantibodies could be confirmed by our quantitative FCIA in ITP patients after corticosteroid treatment. Our quantitative assay is not only good for ITP diagnosis but also for ITP treatment monitoring.

Expression of CD markers' in immune thrombocytopenic purpura: prognostic approaches

Immune Thrombocytopenic Purpura (ITP) is a common autoimmune bleeding disorder characterized by a reduction in peripheral blood platelet counts. In this disease, autoantibodies (Auto-Abs) are produced against platelet GPIIb/GPIIIa by B cells, which require interaction with T cells. In this review, the importance of B and T lymphocytes in ITP prognosis has been studied. Relevant literature was identified by a PubMed search (1990-2016) of English-language papers using the terms B and T lymphocyte, platelet, CD markers and immune thrombocytopenic purpura. T and B lymphocytes are the main immune cells in the body. Defective function causes disrupted balance of different subgroups of lymphocytes, and abnormal expression of surface markers of these cells results in self-tolerance dysfunction, as well as induction of Auto-Abs against platelet glycoproteins (PG). Given the role of B and T cells in production of autoantibodies against PG, it can be stated that the detection of changes in CD markers' expression in these cells can be a good approach for assessing prognosis in ITP patients.

Phosphatidylserine-mediated platelet clearance by endothelium decreases platelet aggregates and procoagulant activity in sepsis

The mechanisms that eliminate activated platelets in inflammation-induced disseminated intravascular coagulation (DIC) in micro-capillary circulation are poorly understood. This study explored an alternate pathway for platelet disposal mediated by endothelial cells (ECs) through phosphatidylserine (PS) and examined the effect of platelet clearance on procoagulant activity (PCA) in sepsis. Platelets in septic patients demonstrated increased levels of surface activation markers and apoptotic vesicle formation, and also formed aggregates with leukocytes. Activated platelets adhered were and ultimately digested by ECs in vivo and in vitro. Blocking PS on platelets or αvβ3 integrin on ECs attenuated platelet clearance resulting in increased platelet count in a mouse model of sepsis. Furthermore, platelet removal by ECs resulted in a corresponding decrease in platelet-leukocyte complex formation and markedly reduced generation of factor Xa and thrombin on platelets. Pretreatment with lactadherin significantly increased phagocytosis of platelets by approximately 2-fold, diminished PCA by 70%, prolonged coagulation time, and attenuated fibrin formation by 50%. Our results suggest that PS-mediated clearance of activated platelets by the endothelium results in an anti-inflammatory, anticoagulant, and antithrombotic effect that contribute to maintaining platelet homeostasis during acute inflammation. These results suggest a new therapeutic target for impeding the development of DIC.

[A Study on the establishment of immune thrombocytopenia model induced by anti-platelet GPⅠbα antibodies]

Objective: To establish primary immune thrombocytopenia (ITP) animal model induced by anti-platelet membrane glycoprotein GPⅠbα antibodies AN51 and R300. Methods: Twenty guinea pigs (6-8 week) were divided into 4 groups. Five guinea pigs in each group were intravenously injected with different doses of AN51 (0.05, 0.1, 0.2 μg/g) and 0.2 μg/g IgG as control. The whole blood was collected from inner angular venous plexus. Platelets number was determined by an automated cell counter and Swiss-Jim method. Then, the similar protocol was used to establish ITP nude mice model by intraperitoneal injection of different concentrations of anti-platelet GPⅠbα antibody R300, respectively. Results: ①Five minutes after intravenous injection of AN51 at 0.05, 0.1 and 0.2 μg/g, the platelet counts of guinea pigs reduced about 0-5%, 50%-60% and 70%-80% compared to the control group, respectively. The difference was statistically significant (P<0.01) . ②Six hours after intraperitoneal injection of R300 at 0.05, 0.1, 0.2 μg/g, the platelet counts of nude mice decreased about 20%-30%, 60%-70% and 80%-90% compared to the control group, respectively. The difference was statistically significant (P<0.01) . The nude mice, injected 0.2 μg/g R300 once a day for 2 weeks, showed typical ITP clinical manifestations including large number of petechiaes or ecchymoses on limbs, head and abdomen. Conclusion: AN51 at 0.2 μg/g and R300 at 0.2 μg/g could establish stable ITP model in guinea pigs and nude mice respectively.

Pathogenesis and Therapeutic Mechanisms in Immune Thrombocytopenia (ITP)

Immune thrombocytopenia (ITP) is a complex autoimmune disease characterized by low platelet counts. The pathogenesis of ITP remains unclear although both antibody-mediated and/or T cell-mediated platelet destruction are key processes. In addition, impairment of T cells, cytokine imbalances, and the contribution of the bone marrow niche have now been recognized to be important. Treatment strategies are aimed at the restoration of platelet counts compatible with adequate hemostasis rather than achieving physiological platelet counts. The first line treatments focus on the inhibition of autoantibody production and platelet degradation, whereas second-line treatments include immunosuppressive drugs, such as Rituximab, and splenectomy. Finally, third-line treatments aim to stimulate platelet production by megakaryocytes. This review discusses the pathophysiology of ITP and how the different treatment modalities affect the pathogenic mechanisms.

Platelet desialylation correlates with efficacy of first-line therapies for immune thrombocytopenia

Immune thrombocytopenia (ITP) is a common autoimmune bleeding disorder. Despite considerable investigation, the pathogenesis of ITP remains incompletely understood, and for many patients, effective therapy is still unavailable. Using murine models and in vitro studies of human blood samples, we recently identified a novel Fc-independent platelet clearance pathway, whereby antibody-mediated desialylated platelets can be cleared in the liver via asialoglycoprotein receptors, leading to decreased response to standard first-line therapies targeting Fc-dependent platelet clearance. Here, we evaluated the significance of this finding in 61 ITP patients through correlation of levels of platelet desialylation with the efficacy of first-line therapies. We found that desialylation levels between different responses to treatment groups were statistically significant (p < 0.01). Importantly, correlation analysis indicated response to treatment and platelet desialylation were related (p < 0.01), whereby non-responders had significantly higher levels of platelet desialylation. Interestingly, we also found secondary ITP and certain non-ITP thrombocytopenias also exhibited significant platelet desialylation compared to healthy controls. These findings designate platelet desialylation as an important biomarker in determining response to standard treatment for ITP. Furthermore, we show for the first time platelet desialylation in other non-ITP thrombocytopenias, which may have important clinical implications and deserve further investigation.