Desialylation is a mechanism of Fc-independent platelet clearance and a therapeutic target in immune thrombocytopenia

Immunological platelet transfusion refractoriness: current insights from mechanisms to therapeutics

Although there have been tremendous improvements in the production and storage of platelets, platelet transfusion refractoriness (PTR) remains a serious clinical issue that may lead to various severe adverse events. The burden of supplying platelets is worsened by rising market demand and limited donor pools of compatible platelets. Antibodies against platelet antigens are known to activate platelets through FcγR-dependent or complement-activated channels, thereby rapidly eliminating foreign platelets. Recently, other mechanisms of platelet clearance have been reported. The current treatment strategy for PTR is to select appropriate and compatible platelets; however, this necessitates a sizable donor pool and technical assistance for costly testing. Consolidation of these mechanisms should be of critical significance in providing insight to establish novel therapeutics to target immunological platelet refractoriness. Therefore, the purposes of this review were to explore the modulation of the immune system over the activation and elimination of allogeneic platelets and to summarize the development of alternative approaches for treating and avoiding alloimmunization to human leukocyte antigen or human platelet antigen in PTR.

Plasma concentration of thrombopoietin in dogs with immune thrombocytopenia

BACKGROUND

Immune thrombocytopenia (ITP) is a common cause of severe thrombocytopenia in dogs. The pathogenesis of nonassociative, primary ITP (pITP) appears complex, with ill-defined thrombopoietic response.

OBJECTIVES

Develop an immunoassay to measure plasma canine thrombopoietin (TPO) concentration and characterize TPO concentrations in dogs with pITP.

ANIMALS

Forty-one healthy dogs, 8 dogs in an induced ITP model (3 control, 5 ITP), and 58 pITP dogs.

METHODS

Recombinant canine TPO (rcTPO) was purchased and its identity confirmed by mass spectrometry. Monoclonal antibodies were raised to rcTPO and used to configure a sandwich ELISA using streptavidin-biotin detection. Assay performance, coefficients of variability, and healthy dog plasma TPO reference interval (RI) were determined, followed by assay of ITP samples.

RESULTS

Assay dynamic range was 15 pg/mL (lower limit of detection) to 1000 pg/mL TPO, with limit of quantitation of 62 pg/mL. Plasma TPO RI was 0 to 158 pg/mL, with plasma TPO <62 pg/mL for 35/41 healthy dogs. All dogs with induced ITP developed marked increases in plasma TPO concentration. Peak values ranged from 515 to >6000 pg/mL. In contrast, only 2/58 pITP dogs had TPO values above RI.

CONCLUSIONS AND CLINICAL IMPORTANCE

Plasma TPO concentration is paradoxically low at diagnosis for most dogs with pITP. This finding suggests that ineffective thrombopoiesis contributes to thrombocytopenia in pITP dogs and supports evaluating TPO receptor agonist treatment as used for pITP in humans. The TPO assay provides a new tool to study thrombopoiesis in pITP and other thrombocytopenic syndromes in dogs.

Novel treatments for immune thrombocytopenia: targeting platelet autoantibodies

INTRODUCTION

Immune thrombocytopenia (ITP) is an acquired autoimmune disorder characterized by low platelets and an increased risk of bleeding. Platelet autoantibodies target major platelet glycoproteins and cause Fc-mediated platelet destruction in the spleen and reticuloendothelial systems. As mechanisms of disease, platelet autoantibodies are important therapeutic targets. Neonatal Fc receptor (FcRn) antagonists are a new class of therapeutics that reduce the half-life of immunoglobulin G including pathogenic platelet autoantibodies. Spleen tyrosine kinase (Syk) inhibitors interfere with Fc-mediated platelet clearance. Bruton's tyrosine kinase (BTK) inhibitors and B-cell activating factor (BAFF) inhibitors reduce antibody production. The efficacy of these targeted therapies provides new support for the role of platelet autoantibodies in pathogenesis of ITP even these antibodies can be difficult to detect.

AREAS COVERED

This review includes an in-depth exploration of the pathophysiologic mechanisms of ITP, focusing on autoantibodies. Treatments outlined in this review include a) FcRn antagonists, b) complement inhibitors, c) B-cell directed therapies such as BTK inhibitors, and anti-BAFF agents, d) Syk inhibitors, e) plasma-cell directed therapies, and f) novel cellular therapeutic products.

EXPERT OPINION

Platelet autoantibodies are often elusive in ITP, yet novel treatments targeting this pathway reinforce their role in the pathogenesis of this autoimmune platelet disorder.

ACVIM consensus statement on the treatment of immune thrombocytopenia in dogs and cats

Management of immune thrombocytopenia (ITP) in dogs and cats is evolving, but there are no evidence-based guidelines to assist clinicians with treatment decisions. Likewise, the overall goals for treatment of ITP have not been established. Immunosuppressive doses of glucocorticoids are the first line treatment, but optimal treatment regimens beyond glucocorticoids remain uncertain. Additional options include secondary immunosuppressive drugs such as azathioprine, modified cyclosporine, and mycophenolate mofetil, usually selected based on clinician preference. Vincristine, human IV immunoglobulin (hIVIg), and transfusion of platelet or red blood cell-containing products are often used in more severe cases. Splenectomy and thrombopoietin receptor agonists are usually reserved for refractory cases, but when and in which patient these modalities should be employed is under debate. To develop evidence-based guidelines for individualized treatment of ITP patients, we asked 20 Population Intervention Comparison Outcome (PICO) format questions. These were addressed by 17 evidence evaluators using a literature pool of 288 articles identified by a structured search strategy. Evidence evaluators, using panel-designed templates and data extraction tools, summarized evidence and created guideline recommendations. These were integrated by treatment domain chairs and then refined by iterative Delphi survey review to reach consensus on the final guidelines. In addition, 19 non-PICO questions covering scenarios in which evidence was lacking or of low quality were answered by expert opinion using iterative Delphi surveys with panelist integration and refinement. Commentary was solicited from multiple relevant professional organizations before finalizing the consensus. The rigorous consensus process identified few comparative treatment studies, highlighting many areas of ITP treatment requiring additional studies. This statement is a companion manuscript to the ACVIM Consensus Statement on the Diagnosis of Immune Thrombocytopenia in Dogs and Cats.

[Pathophysiology and Diagnostics of Immune Thrombocytopenia]

Immune thrombocytopenia (ITP) is due to autoantibodies against platelet surface antigens. ITP is considered as either primary, with no clear etiology, or as secondary ITP (drug-induced; underlying diseases). Autoantibodies lead both to loss of platelets in the spleen and/or liver but simultaneously reduce their production. Contrary to other disorders with thrombocytopenia, ITP has reduced levels of thrombopoetin. ITP remains a diagnosis of exclusion. A single defining laboratory test does not exist. Glycoprotein-specific antibodies can be detected in only about 50% of cases. Ruling out EDTA-induced pseudo thrombocytopenia is of particular relevance. Secondary causes of thrombocytopenia should be excluded through medical history (especially medication history), physical examination and possibly bone-marrow puncture.

ACVIM consensus statement on the diagnosis of immune thrombocytopenia in dogs and cats

Immune thrombocytopenia (ITP) is the most common acquired primary hemostatic disorder in dogs. Immune thrombocytopenia less commonly affects cats but is an important cause of mortality and treatment-associated morbidity in both species. Immune thrombocytopenia remains a diagnosis of exclusion for which diagnostic guidelines are lacking. Primary, or non-associative, ITP refers to autoimmune platelet destruction. Secondary, or associative, ITP arises in response to an underlying disease trigger. However, evidence for which comorbidities serve as ITP triggers has not been systematically evaluated. To identify key diagnostic steps for ITP and important comorbidities associated with secondary ITP, we developed 12 Population Evaluation/Exposure Comparison Outcome (PECO) format questions. These questions were addressed by evidence evaluators utilizing a literature pool of 287 articles identified by the panelists using a structured search strategy. Evidence evaluators, using panel-designed templates and data extraction tools, summarized evidence and created guideline recommendations that then were integrated by diagnosis and comorbidity domain chairs. The revised PECO responses underwent a Delphi survey process to reach consensus on final guidelines. A combination of panel expertise and PECO responses were employed to develop algorithms for diagnosis of ITP in dogs and cats, which also underwent 4 iterations of Delphi review. Comorbidity evidence evaluators employed an integrated measure of evidence (IME) tool to determine evidence quality for each comorbidity; IME values combined with evidence summaries for each comorbidity were integrated to develop ITP screening recommendations, which also were subjected to Delphi review. Commentary was solicited from multiple relevant professional organizations before finalizing the consensus. The final consensus statement provides clinical guidelines for the diagnosis of, and underlying disease screening for, ITP in dogs and cats. The systematic consensus process identified numerous knowledge gaps that should guide future studies. This statement is a companion manuscript to the ACVIM Consensus Statement on the Treatment of Immune Thrombocytopenia.

GPIbα CAAR T cells function like a Trojan horse to eliminate autoreactive B cells to treat immune thrombocytopenia

Breakthrough treatment for refractory and relapsed immune thrombocytopenia (ITP) patients is urgently needed. Autoantibody- mediated platelet clearance and megakaryocyte dysfunction are important pathogenic mediators of ITP. Glycoprotein (GP) Ibα is a significant autoantigen found in ITP patients and is associated with poor response to standard immunosuppressive treatments. Here, we engineered human T cells to express a chimeric autoantibody receptor (CAAR) with GPIbα constructed into the ligand-binding domain fused to the CD8 transmembrane domain and CD3ζ-4-1BB signaling domains. We performed cytotoxicity assays to assess GPIbα CAAR T-cell selective cytolysis of cells expressing anti-GPIbα B-cell receptors in vitro. Furthermore, we demonstrated the potential of GPIbα CAAR T cells to persist and precisely eliminate GPIbα-specific B cells in vivo. In summary, we present a proof of concept for CAAR T-cell therapy to eradicate autoimmune B cells while sparing healthy B cells with GPIbα CAAR T cells that function like a Trojan horse. GPIbα CAAR T-cell therapy is a promising treatment for refractory and relapsed ITP patients.

Platelets retain function and can be stored following disruption of human leucocyte antigens

BACKGROUND AND OBJECTIVES

Antibodies to human leucocyte antigen (HLA) Class-I antigens can lead to refractoriness to platelet transfusion. Although this can be overcome by transfusion of HLA-compatible platelets, they are not always available. Disruption of HLA antigens on platelets by acid treatment may be a suitable alternative when no other components are available. The aim of this study was to assess the effect of HLA disruption and subsequent storage of platelet components.

MATERIALS AND METHODS

Platelet components were treated with 0.9% saline or citric acid solution (pH 3.0), and then stored until expiry (Day 7). HLA and platelet glycoprotein expression, platelet viability, activation and sialylation were measured by flow cytometry. Release of soluble factors was measured by ELISA and metabolism by biochemistry analyser. Reactivity to patient anti-sera containing anti-HLA antibodies was measured using platelet immunofluorescence tests (PIFTs) and monoclonal antibody immobilization of platelet antigen (MAIPA) assays. Platelet function was measured using aggregometry and thromboelastography (TEG).

RESULTS

Acid treatment reduced detection of HLA Class-I on platelets by 75%, with significant reductions in reactivity to patient anti-sera. Acid treatment reduced platelet content and viability, increased platelet activation and accelerated metabolism. Glycan cleavage was increased by acid treatment. Treatment reduced platelet activation following agonist stimulation by ADP and TRAP-6, but platelets remained functional, displaying increased aggregation response and reduced time to clot formation by TEG.

CONCLUSION

Although HLA disruption had some detrimental effects, acid-treated platelets remained functional, retaining their capacity to respond to agonists and form clots, and with further development could be used to support refractory patients.

Treatment of Immune Thrombocytopenia: Contextualization from a Historical Perspective

Immune thrombocytopenia (ITP) is an autoimmune disease characterized by an isolated decrease in platelet count and an increased risk of bleeding. The pathogenesis is complex, affecting multiple components of the immune system and causing both peripheral destruction of platelets and inadequate production in the bone marrow. In this article, we review the treatment of ITP from a historical perspective, discussing first line and second line treatments, and management of refractory disease.

The role of platelet desialylation as a biomarker in primary immune thrombocytopenia: mechanisms and therapeutic perspectives

Primary immune thrombocytopenia (ITP) is an acquired autoimmune disorder characterized by the destruction of platelets. Although it was long believed that the critical role of autoantibodies in platelet destruction, primarily through the Fc-dependent platelet clearance pathway, recent findings indicate that the significance of the Fc-independent platelet clearance pathway mediated by hepatocytes, thus shedding light on a previously obscure aspect of ITP pathogenesis. Within this context, the desialylation of platelets has emerged as a pivotal biochemical marker. Consequently, targeting platelet desialylation emerges as a novel therapeutic strategy in the pathogenesis of ITP. Notably, prevailing research has largely focused on antiplatelet antibodies and the glycosylation-associated mechanisms of platelet clearance, while comprehensive analysis of platelet desialylation remains scant. In response, we retrospectively discuss the historical progression, inducing factors, generation process, and molecular regulatory mechanisms underlying platelet desialylation in ITP pathogenesis. By systematically evaluating the most recent research findings, we contribute to a comprehensive understanding of the intricate processes involved. Moreover, our manuscript delves into the potential application of desialylation regulatory strategies in ITP therapy, heralding novel therapeutic avenues. In conclusion, this manuscript not only fills a critical void in existing literature but also paves the way for future research by establishing a systematic theoretical framework. By inspiring new research ideas and offering insights into the development of new therapeutic strategies and targeted drugs, our study is poised to significantly advance the clinical management of ITP.

Immune thrombocytopenia: Pathophysiology and impacts of Romiplostim treatment

Immune thrombocytopenia (ITP) is an autoimmune bleeding disease caused by immune-mediated platelet destruction and decreased platelet production. ITP is characterized by an isolated thrombocytopenia (<100 × 109/L) and increased risk of bleeding. The disease has a complex pathophysiology wherein immune tolerance breakdown leads to platelet and megakaryocyte destruction. Therapeutics such as corticosteroids, intravenous immunoglobulins (IVIg), rituximab, and thrombopoietin receptor agonists (TPO-RAs) aim to increase platelet counts to prevent hemorrhage and increase quality of life. TPO-RAs act via stimulation of TPO receptors on megakaryocytes to directly stimulate platelet production. Romiplostim is a TPO-RA that has become a mainstay in the treatment of ITP. Treatment significantly increases megakaryocyte maturation and growth leading to improved platelet production and it has recently been shown to have additional immunomodulatory effects in treated patients. This review will highlight the complex pathophysiology of ITP and discuss the usage of Romiplostim in ITP and its ability to potentially immunomodulate autoimmunity.

A Novel Anti-CD38 Monoclonal Antibody for Treating Immune Thrombocytopenia

BACKGROUND

Immune thrombocytopenia (ITP) is an autoimmune disease characterized by autoantibody-mediated platelet destruction. Treatment with CM313, a novel anti-CD38 monoclonal antibody, can result in targeted clearance of CD38-positive cells, including plasma cells.

METHODS

We conducted a phase 1-2, open-label study to evaluate the safety and efficacy of CM313 in adult patients with ITP. CM313 was administered intravenously at a dose of 16 mg per kilogram of body weight every week for 8 weeks, followed by a 16-week follow-up period. The primary outcomes were adverse events and documentation of two or more consecutive platelet counts of at least 50×109 per liter within 8 weeks after the first dose of CM313. The status of peripheral-blood immune cells in patients and changes in the mononuclear phagocytic system in passive mouse models of ITP receiving anti-CD38 therapy were monitored.

RESULTS

Of the 22 patients included in the study, 21 (95%) had two consecutive platelet counts of at least 50×109 per liter during the treatment period, with a median cumulative response duration of 23 weeks (interquartile range, 17 to 24). The median time to the first platelet count of at least 50×109 per liter was 1 week (range, 1 to 3). The most common adverse events that occurred during the study were infusion-related reaction (in 32% of the patients) and upper respiratory tract infection (in 32%). After CD38-targeted therapy, the percentage of CD56dimCD16+ natural killer cells, the expression of CD32b on monocytes in peripheral blood, and the number of macrophages in the spleen of the passive mouse models of ITP all decreased.

CONCLUSIONS

In this study, anti-CD38 targeted therapy rapidly boosted platelet levels by inhibiting antibody-dependent cell-mediated cytotoxicity on platelets, maintained long-term efficacy by clearing plasma cells, and was associated with mainly low-grade toxic effects. (Funded by the Chinese Academy of Medical Sciences Innovation Fund for Medical Sciences and others; ClinicalTrials.gov number, NCT05694767).

Apolipoprotein A-IV polymorphisms Q360H and T347S attenuate its endogenous inhibition of thrombosis

Platelets are small anucleate cells that play a key role in thrombosis and hemostasis. Our group previously identified apolipoprotein A-IV (apoA-IV) as an endogenous inhibitor of thrombosis by competitive blockade of the αIIbβ3 integrin on platelets. ApoA-IV inhibition of platelets was dependent on the N-terminal D5/D13 residues, and enhanced with absence of the C-terminus, suggesting it sterically hinders its N-terminal platelet binding site. The C-terminus is also the site of common apoA-IV polymorphisms apoA-IV-1a (T347S) and apoA-IV-2 (Q360H). Interestingly, both are linked with an increased risk of cardiovascular disease, however, the underlying mechanism remains unclear. Here, we generated recombinant apoA-IV and found that the Q360H or T347S polymorphisms dampened its inhibition of platelet aggregation in human platelet-rich plasma and gel-filtered platelets, reduced its inhibition of platelet spreading, and its inhibition of P-selectin on activated platelets. Using an ex vivo thrombosis assay, we found that Q360H and T347S attenuated its inhibition of thrombosis at both high (1800s-1) and low (300s-1) shear rates. We then demonstrate a conserved monomer-dimer distribution among apoA-IV WT, Q360H, and T347S and use protein structure modelling software to show Q360H and T347S enhance C-terminal steric hindrance over the N-terminal platelet-binding site. These data provide critical insight into increased cardiovascular risk for individuals with Q360H or T347S polymorphisms.

Sialic acids on T cells are crucial for their maintenance and survival

Sialic acids are found as terminal sugars on glycan structures on cellular surfaces. T cells carry these sialoglycans abundantly, and they are thought to serve multiple functions in cell adhesion, cell migration, and protection from complement attack. We studied the role of sialoglycans on T cells in a mouse model with a T cell-specific deletion of cytidine monophosphate-sialic acid synthase (CMAS), the enzyme that is crucial for the synthesis of sialoglycans. These mice showed a T-cell deficiency in peripheral lymphoid organs. Many T cells with an undeleted Cmas allele were found in the periphery, suggesting that they escaped the Cre-mediated deletion. The remaining peripheral T cells of T cell-specific Cmas KO mice had a memory-like phenotype. Additional depletion of the complement factor C3 could not rescue the phenotype, showing that the T-cell defect was not caused by a host complement activity. Cmas-deficient T cells showed a high level of activated caspase 3, indicating an ongoing apoptosis. In bone marrow chimeric cellular transfer experiments, we observed a strong competitive disadvantage of Cmas-deficient T cells compared to wild-type T cells. These results show that sialoglycans on the surface of T cells are crucial for T-cell survival and maintenance. This function has not been recognized before and is similar to the function of sialoglycans on B cells.

An extensive database analysis demonstrates significant increase in platelet quantity in unselected hospitalized patients following treatment with oseltamivir

Not available.

Rituximab alleviates pediatric systemic lupus erythematosus associated refractory immune thrombocytopenia: a case-based review

A complication of pediatric systemic lupus erythematosus (pSLE) is immune thrombocytopenia (ITP). Although corticosteroids and immunoglobulins are frequently used as preliminary treatments, some patients do not respond to them. Rituximab has been reported to be safe and effective in the treatment of pSLE complicated with refractory ITP. Research is currently underway to determine the optimal rituximab dose for these individuals. We report a case of a child with SLE-associated ITP (SLE-ITP) who was successfully treated with rituximab. Rituximab is likely the most viable therapeutic option for refractory SLE-ITP. Furthermore, a comprehensive review of the relevant literature was performed and a concise overview of the pathogenesis and available treatment modalities for pediatric patients diagnosed with SLE and concurrent ITP was provided.

Salvianolic acid B inhibits thrombosis and directly blocks the thrombin catalytic site

Background

Salvianolic acid B (SAB) is a major component of Salvia miltiorrhiza root (Danshen), widely used in East/Southeast Asia for centuries to treat cardiovascular diseases. Danshen depside salt, 85% of which is made up of SAB, is approved in China to treat chronic angina. Although clinical observations suggest that Danshen extracts inhibited arterial and venous thrombosis, the exact mechanism has not been adequately elucidated.

Objective

To delineate the antithrombotic mechanisms of SAB.

Methods

We applied platelet aggregation and coagulation assays, perfusion chambers, and intravital microscopy models. The inhibition kinetics and binding affinity of SAB to thrombin are measured by thrombin enzymatic assays, intrinsic fluorescence spectrophotometry, and isothermal titration calorimetry. We used molecular in silico docking models to predict the interactions of SAB with thrombin.

Results

SAB dose-dependently inhibited platelet activation and aggregation induced by thrombin. SAB also reduced platelet aggregation induced by adenosine diphosphate and collagen. SAB attenuated blood coagulation by modifying fibrin network structures and significantly decreased thrombus formation in mouse cremaster arterioles and perfusion chambers. The direct SAB-thrombin interaction was confirmed by enzymatic assays, intrinsic fluorescence spectrophotometry, and isothermal titration calorimetry. Interestingly, SAB shares key structural similarities with the trisubstituted benzimidazole class of thrombin inhibitors, such as dabigatran. Molecular docking models predicted the binding of SAB to the thrombin active site.

Conclusion

Our data established SAB as the first herb-derived direct thrombin catalytic site inhibitor, suppressing thrombosis through both thrombin-dependent and thrombin-independent pathways. Purified SAB may be a cost-effective agent for treating arterial and deep vein thrombosis.

Pathophysiology, Clinical Manifestations and Diagnosis of Immune Thrombocytopenia: Contextualization from a Historical Perspective

Immune thrombocytopenia (ITP) is an autoimmune disease characterized by an isolated decrease in the platelet count and an increased risk of bleeding. The pathogenesis is complex, affecting multiple components of the immune system and causing both peripheral destruction of platelets and impaired central megakaryopoiesis and platelet production in the bone marrow. Here, we intend to contextualize the current knowledge on the pathophysiology, terminology, epidemiology, clinical manifestations, diagnosis, and prognosis of ITP from a historical perspective and the first references to the never-stopping garnering of knowledge about this entity. We highlight the necessity to better understand ITP in order to be able to provide ITP patients with personalized treatment options, improving disease prognosis and reducing the incidence or frequency of refractoriness.

Desialylation by neuraminidases in platelets, kiss of death or bittersweet?

PURPOSE OF REVIEW

Loss of surface sialic acid by neuraminidases is known as 'desialylation'. Platelets are desialylated in bacterial or viral infections, during storage, senescence, various mutations, platelet auto antibodies, hemostasis and shear stress. In this review the recent literature on the different sialic acid capped glycan structures will be covered as well as platelet desialylation in inherited glycan disorders and induced by external neuraminidases.

RECENT FINDINGS

Neuraminidases are released from platelet intracellular stores and translocated to the platelet surface. Apart from clearance, loss of surface sialic acid by neuraminidases ('desialylation') affects platelet signaling including ligand binding and their procoagulant function. Platelets are also desialylated in infections, various mutations, presence of platelet auto antibodies.

SUMMARY

Since platelet desialylation occurs in various healthy and pathological conditions, measuring desialylation might be a new diagnostic tool.

Translating mechanisms into therapeutic strategies for immune thrombocytopenia (ITP): Lessons from clinical trials

Immune thrombocytopenia (ITP) is an autoimmune disorder that causes a significant reduction in peripheral blood platelet count. Fortunately, due to an increased understanding of ITP, there have been significant improvements in the diagnosis and treatment of these patients. Over the past decade, there have been a variety of proven therapeutic options available for ITP patients, including intravenous immunoglobulins (IVIG), Rituximab, corticosteroids, and thrombopoietin receptor agonists (TPO-RAs). Although the effectiveness of current therapies in treating more than two-thirds of patients, still some patients do not respond well to conventional therapies or fail to achieve long-term remission. Recently, a significant advancement has been made in identifying various mechanisms involved in the pathogenesis of ITP, leading to the development of novel treatments targeting these pathways. It seems that new agents that target plasma cells, Bruton tyrosine kinase, FcRn, platelet desialylation, splenic tyrosine kinase, and classical complement pathways are opening new ways to treat ITP. In this study, we reviewed the pathophysiology of ITP and summarized updates in this population's management and treatment options. We also took a closer look at the 315 ongoing trials to investigate their progress status and compare the effectiveness of interventions. May our comprehensive view of ongoing clinical trials serve as a guiding beacon, illuminating the path towards future trials of different drugs in the treatment of ITP patients.

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.

Successful treatment with oseltamivir phosphate in children with ITP who failed first-line therapy: a case series report

Immune thrombocytopenia (ITP) is a common bleeding disorder in children. First-line medicines (glucocorticoids and immunoglobulin) may not be effective for some children, endangering their lives, posing challenges for healthcare facilities, and leading to an unfavorable prognosis. As a sialidase inhibitor, oseltamivir phosphate can reduce the destruction of platelets in liver macrophages by inhibiting the sialylation of platelets, and finally achieve the purpose of increasing platelet count. In this paper, three cases of children with ITP who failed first-line therapy and were cured by oral administration of oseltamivir phosphate granules were reported. The mechanism of action of oseltamivir phosphate granules was clarified.

Improving the chances of response to splenectomy in immune thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder characterized by isolated thrombocytopenia. Its pathogenesis is complex relying in large part on destruction of platelets recognized by autoantibodies within the spleen. However, other mechanisms, such as platelet desialylation, may play a role in platelet reduction by accelerating their clearance in the liver. In their study, Mendoza and colleagues reported on platelet scintigraphy performed in 51 ITP patients, showing a response in 87.5% when the sequestration occurred in the spleen versus 45% in case of non-splenic destruction. Platelet desialylation was also measured after splenectomy and found to be higher in non-responder patients. These latter results, while requiring confirmation prior to splenectomy, support platelet desialylation may also be a potential biomarker of non-response to splenectomy. Commentary on: Mendoza et al. Study of platelet kinetics in immune thrombocytopenia to predict splenectomy response. Br J Haematol 2024;204:315-323.

The many facets of immune-mediated thrombocytopenia: Principles of immunobiology and immunotherapy

Immune thrombocytopenia (ITP) is a rare autoimmune condition, due to peripheral platelet destruction through antibody-dependent cellular phagocytosis, complement-dependent cytotoxicity, cytotoxic T lymphocyte-mediated cytotoxicity, and megakaryopoiesis alteration. This condition may be idiopathic or triggered by drugs, vaccines, infections, cancers, autoimmune disorders and systemic diseases. Recent advances in our understanding of ITP immunobiology support the idea that other forms of thrombocytopenia, for instance, occurring after immunotherapy or cellular therapies, may share a common pathophysiology with possible therapeutic implications. If a decent pipeline of old and new agents is currently deployed for classical ITP, in other more complex immune-mediated thrombocytopenic disorders, clinical management is less harmonized and would deserve further prospective investigations. Here, we seek to provide a fresh overview of pathophysiology and current therapeutical algorithms for adult patients affected by this disorder with specific insights into poorly codified scenarios, including refractory ITP and post-immunotherapy/cellular therapy immune-mediated thrombocytopenia.

Platelet lifespan and mechanisms for clearance

PURPOSE OF REVIEW

Activated or aged platelets are removed from circulation under (patho)physiologic conditions, the exact mechanism of platelet clearance under such conditions remains unclear and are currently being investigated. This review focuses on recent findings and controversies regarding platelet clearance and the disruption of platelet life cycle.

RECENT FINDINGS

The platelet life span is determined by glycosylation of platelet surface receptors with sialic acid. Recently, it was shown that platelet activation and granule release leads to desialylation of glycans and accelerated clearance of platelets under pathological conditions. This phenomenon was demonstrated to be a main reason for thrombocytopenia being a complication in several infections and immune disorders.

SUMMARY

Although we have recently gained some insight into how aged platelets are cleared from circulation, we are still not seeing the full picture. Further investigations of the platelet clearance pathways under pathophysiologic conditions are needed as well as studies to unravel the connection between platelet clearance and platelet production.

Reference guide for the diagnosis of adult primary immune thrombocytopenia, 2023 edition

Primary immune thrombocytopenia (ITP) is an autoimmune disorder characterized by isolated thrombocytopenia due to accelerated platelet destruction and impaired platelet production. Diagnosis of ITP is still challenging because ITP has been diagnosed by exclusion. Exclusion of thrombocytopenia due to bone marrow failure is especially important in Japan because of high prevalence of aplastic anemia compared to Western countries. Hence, we propose a new diagnostic criteria involving the measurement of plasma thrombopoietin (TPO) levels and percentage of immature platelet fraction (RP% or IPF%); 1) isolated thrombocytopenia with no morphological evidence of dysplasia in any blood cell type in a blood smear, 2) normal or slightly increased plasma TPO level (< cutoff), 3) elevated RP% or IPF% (> upper limit of normal), and 4) absence of other conditions that potentially cause thrombocytopenia including secondary ITP. A diagnosis of ITP is made if conditions 1-4 are all met. Cases in which criterion 2 or 3 is not met or unavailable are defined as "possible ITP," and diagnosis of ITP can be made mainly by typical clinical course. These new criteria enable us to clearly differentiate ITP from aplastic anemia and other forms of hypoplastic thrombocytopenia and can be highly useful in clinical practice for avoiding unnecessary bone marrow examination as well as for appropriate selection of treatments.

Study of platelet kinetics in immune thrombocytopenia to predict splenectomy response

Despite the efficacy of splenectomy for chronic immune thrombocytopenia (ITP), its considerable failure rate and its possible related complications prove the need for further research into potential predictors of response. The platelet sequestration site determined by 111 In-labelled autologous platelet scintigraphy has been proposed to predict splenectomy outcome, but without standardisation in clinical practice. Here, we conducted a single-centre study by analysing a cohort of splenectomised patients with ITP in whom 111 In-scintigraphy was performed at La Paz University Hospital in Madrid to evaluate the predictive value of the platelet kinetic studies. We also studied other factors that could impact the splenectomy outcome, such as patient and platelet characteristics. A total of 51 patients were splenectomised, and 82.3% responded. The splenic sequestration pattern predicted a higher rate of complete response up to 12 months after splenectomy (p = 0.005), with 90% sensitivity and 77% specificity. Neither age, comorbidities, therapy lines nor previous response to them showed any association with response. Results from the platelet characteristics analysis revealed a significant loss of sialic acid in platelets from the non-responding patients compared with those who maintained a response (p = 0.0017). Our findings highlight the value of splenic sequestration as an independent predictor of splenectomy response.

Decoding the role of platelets in tumour metastasis: enigmatic accomplices and intricate targets for anticancer treatments

The canonical role of platelets as central players in cardiovascular disease by way of their fundamental role in mediating thrombosis and haemostasis is well appreciated. However, there is now a large body of experimental evidence demonstrating that platelets are also pivotal in various physiological and pathophysiological processes other than maintaining haemostasis. Foremost amongst these is the emerging data highlighting the key role of platelets in driving cancer growth, metastasis and modulating the tumour microenvironment. As such, there is significant interest in targeting platelets therapeutically for the treatment of cancer. Therefore, the purpose of this review is to provide an overview of how platelets contribute to the cancer landscape and why platelets present as valuable targets for the development of novel cancer diagnosis tools and therapeutics.

Antiplatelet strategies: past, present, and future

Antiplatelet therapy plays a critical role in the prevention and treatment of major cardiovascular diseases triggered by thrombosis. Since the 1900s, significant progress in reducing morbidity and death caused by cardiovascular diseases has been made. However, despite the development and approval of drugs that specifically target the platelet, including inhibitors for cycloxygenase-1, P2Y12 receptor, integrin αIIbβ3, phosphodiesterases, and protease-activated receptor 1, the risk of recurrent thrombotic events remains high, and the increased risk of bleeding is a major concern. Scientific advances in our understanding of the role of platelets in haemostasis and thrombosis have revealed novel targets, such as protease-activated receptor 4 (PAR4), glycoprotein Ib (GPIb)-V-IX complex, glycoprotein VI, and 12-lipoxygenase. The antithrombotic effects and safety of the pharmacologic inhibition of these targets are currently under investigation in clinical studies. This review provides an overview of drugs in early development to target the platelet and those in current use in clinical practice. Furthermore, it describes the emerging drug targets being developed and studied to reduce platelet activity and outlines potential novel therapeutic targets in the platelet.

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.

Platelets interact with CD169+ macrophages and cDC1 and enhance liposome-induced CD8+ T cell responses

Historically platelets are mostly known for their crucial contribution to hemostasis, but there is growing understanding of their role in inflammation and immunity. The immunomodulatory role of platelets entails interaction with pathogens, but also with immune cells including macrophages and dendritic cells (DCs), to activate adaptive immune responses. In our previous work, we have demonstrated that splenic CD169+ macrophages scavenge liposomes and collaborate with conventional type 1 DCs (cDC1) to induce expansion of CD8+ T cells. Here, we show that platelets associate with liposomes and bind to DNGR-1/Clec9a and CD169/Siglec-1 receptors in vitro. In addition, platelets interacted with splenic CD169+ macrophages and cDC1 and further increased liposome internalization by cDC1. Most importantly, platelet depletion prior to liposomal immunization resulted in significantly diminished antigen-specific CD8+ T cell responses, but not germinal center B cell responses. Previously, complement C3 was shown to be essential for platelet-mediated CD8+ T cell activation during bacterial infection. However, after liposomal vaccination CD8+ T cell priming was not dependent on complement C3. While DCs from platelet-deficient mice exhibited unaltered maturation status, they did express lower levels of CCR7. In addition, in the absence of platelets, CCL5 plasma levels were significantly reduced. Overall, our findings demonstrate that platelets engage in a cross-talk with CD169+ macrophages and cDC1 and emphasize the importance of platelets in induction of CD8+ T cell responses in the context of liposomal vaccination.

New insights into the glycobiology of immune thrombocytopenia

PURPOSE OF REVIEW

The platelet surface harbors a lush forest of glycans (carbohydrate polymers) attached to membrane proteins and lipids. Accumulating evidence suggests that these glycans may be relevant to the pathophysiology of immune thrombocytopenia (ITP). Here, we critically evaluate data that point to a possible role for loss of sialic acid in driving platelet clearance in ITP, comment on the potential use of neuraminidase inhibitors for treatment of ITP, and highlight open questions in this area.

RECENT FINDINGS

Multiple lines of evidence suggest a role for loss of platelet sialic acid in the pathophysiology of thrombocytopenia. Recent work has tested the hypothesis that neuraminidase-mediated cleavage of platelet sialic acid may trigger clearance of platelets in ITP. Some clinical evidence supports efficacy of the viral neuraminidase inhibitor oseltamivir in ITP, which is surprising given its lack of activity against human neuraminidases.

SUMMARY

Further study of platelet glycobiology in ITP is necessary to fill key knowledge gaps. A deeper understanding of the roles of platelet glycans in ITP pathophysiology will help to guide development of novel therapies.

Desialylated Platelet Clearance in the Liver is a Novel Mechanism of Systemic Immunosuppression

Platelets are small, versatile blood cells that are critical for hemostasis/thrombosis. Local platelet accumulation is a known contributor to proinflammation in various disease states. However, the anti-inflammatory/immunosuppressive potential of platelets has been poorly explored. Here, we uncovered, unexpectedly, desialylated platelets (dPLTs) down-regulated immune responses against both platelet-associated and -independent antigen challenges. Utilizing multispectral photoacoustic tomography, we tracked dPLT trafficking to gut vasculature and an exclusive Kupffer cell-mediated dPLT clearance in the liver, a process that we identified to be synergistically dependent on platelet glycoprotein Ibα and hepatic Ashwell-Morell receptor. Mechanistically, Kupffer cell clearance of dPLT potentiated a systemic immunosuppressive state with increased anti-inflammatory cytokines and circulating CD4+ regulatory T cells, abolishable by Kupffer cell depletion. Last, in a clinically relevant model of hemophilia A, presensitization with dPLT attenuated anti-factor VIII antibody production after factor VIII ( infusion. As platelet desialylation commonly occurs in daily-aged and activated platelets, these findings open new avenues toward understanding immune homeostasis and potentiate the therapeutic potential of dPLT and engineered dPLT transfusions in controlling autoimmune and alloimmune diseases.

B cells and antibodies in refractory immune thrombocytopenia

Immune thrombocytopenia (ITP) is an acquired bleeding disorder mediated by pathogenic autoantibodies secreted by plasma cells (PCs) in many patients. In refractory ITP patients, the persistence of splenic and bone marrow autoreactive long-lived PCs (LLPCs) may explain primary failure of rituximab and splenectomy respectively. The reactivation of autoreactive memory B cells generating new autoreactive PCs contributes to relapses after initial response to rituximab. Emerging strategies targeting B cells and PCs aim to prevent the settlement of splenic LLPCs with the combination of anti-BAFF and rituximab, to deplete autoreactive PCs with anti-CD38 antibodies, and to induce deeper B-cell depletion in tissues with novel anti-CD20 monoclonal antibodies and anti-CD19 therapies. Alternative strategies, focused on controlling autoantibody mediated effects, have also been developed, including SYK and BTK inhibitors, complement inhibitors, FcRn blockers and inhibitors of platelet desialylation.

Early immunomodulation in immune thrombocytopenia-A report of the ICIS meeting in Lenzerheide, Switzerland 2022

The only way to prevent immune thrombocytopenia (ITP) from becoming refractory would be to restore tolerance to platelets at an early phase of the disease. Numerous immune alterations probably accumulate in chronic ITP; thus, the chances of cure decrease significantly with time. Currently, sustained remission off treatment (SROT) is a clinical definition describing patients who can discontinue their ITP treatment without risk and maintain a state of remission. Different treatment strategies are presently being evaluated with the goal of attaining SROT, mostly combining drugs targeting the innate and/or the adaptive immune system, the inflammation state, so as increasing the platelet load. In this sense, thrombopoietin receptor agonists (TPO-RAs) have shown promising results if used as upfront treatment. TPO-RAs seem to exhibit immunomodulation and immune tolerance properties, increasing not only the platelet antigen mass but also increasing the transforming growth factor-β concentration, and stimulating regulatory T and B lymphocytes. However, more immunological studies are needed to establish accurate molecular alterations in ITP that are potentially reversed with treatments.

Pathogenesis of refractory ITP: Overview

A subset of individuals with 'primary' or 'idiopathic' immune thrombocytopenia (ITP) who fail to respond to conventional first- and second-line agents or who lose responsiveness are considered to have 'refractory' disease (rITP), placing them at increased risk of bleeding and complications of intensive treatment. However, the criteria used to define the refractory state vary among studies, which complicates research and clinical investigation. Moreover, it is unclear whether rITP is simply 'more severe' ITP, or if there are specific pathogenic pathways that are more likely to result in refractory disease, and whether the presence or development of rITP can be established or anticipated based on these differences. This paper reviews potential biological features that may be associated with rITP, including genetic and epigenetic risk factors, dysregulation of T cells and cytokine networks, antibody affinity and specificity, activation of complement, impaired platelet production and alterations in platelet viability and clearance. These findings indicate the need for longitudinal studies using novel clinically available methodologies to identify and monitor pathogenic T cells, platelet antibodies and other clues to the development of refractory disease.

Markers of refractory primary immune thrombocytopenia

Refractory immune thrombocytopenia (ITP) is a challenging disease that can be defined by refractoriness to second-line treatments. In this review, we list and comment available evidence about clinical and biological factors associated with refractoriness to splenectomy, thrombopoietin receptor agonists (TPO-RAs), rituximab and fostamatinib, as well as those associated with multirefractory ITP (active disease with failure of rituximab, TPO-RAs and splenectomy).

Complement in immune thrombocytopenia (ITP): The role of complement in refractory ITP

Immune thrombocytopenia (ITP) is a disorder characterized by low platelets due to increased clearance and decreased platelet production. While ITP has been characterized as an acquired disorder of the adaptive immune system, the resulting platelet autoantibodies provide ancillary links to the innate immune system via antibody interaction with the complement system. Most autoantibodies in patients with ITP are of the IgG1 subclass, which can be potent activators of the classical complement pathway. Antibody-coated platelets can initiate complement activation via the classical pathway leading to both direct platelet destruction and enhanced clearance of C3b-coated platelets by complement receptors. Similar autoantibody interactions with bone marrow megakaryocytes can also result in complement injury and ineffective thrombopoiesis. The development of novel therapeutic complement inhibitors has revived interest in the role of complement in autoantibody-mediated disorders, such as ITP. A recent early-phase clinical trial of a classical complement pathway inhibitor has demonstrated efficacy in a subset of ITP patients refractory to conventional immune modulation. In this review, we will analyse the role of complement in refractory ITP.

Perivascular niche cells sense thrombocytopenia and activate hematopoietic stem cells in an IL-1 dependent manner

Hematopoietic stem cells (HSCs) residing in specialized niches in the bone marrow are responsible for the balanced output of multiple short-lived blood cell lineages in steady-state and in response to different challenges. However, feedback mechanisms by which HSCs, through their niches, sense acute losses of specific blood cell lineages remain to be established. While all HSCs replenish platelets, previous studies have shown that a large fraction of HSCs are molecularly primed for the megakaryocyte-platelet lineage and are rapidly recruited into proliferation upon platelet depletion. Platelets normally turnover in an activation-dependent manner, herein mimicked by antibodies inducing platelet activation and depletion. Antibody-mediated platelet activation upregulates expression of Interleukin-1 (IL-1) in platelets, and in bone marrow extracellular fluid in vivo. Genetic experiments demonstrate that rather than IL-1 directly activating HSCs, activation of bone marrow Lepr+ perivascular niche cells expressing IL-1 receptor is critical for the optimal activation of quiescent HSCs upon platelet activation and depletion. These findings identify a feedback mechanism by which activation-induced depletion of a mature blood cell lineage leads to a niche-dependent activation of HSCs to reinstate its homeostasis.

Current knowledge of thrombocytopenia in sepsis and COVID-19

Thrombocytopenia, characterized by a decrease in platelet count, is commonly observed in sepsis and COVID-19. In sepsis, thrombocytopenia can result from various mechanisms, including impaired platelet production in the bone marrow, accelerated platelet destruction due to increased inflammation, sequestration of platelets in the spleen, immune-mediated platelet destruction, or dysregulated host responses. Similarly, thrombocytopenia has been reported in COVID-19 patients, but the immune-related mechanisms underlying this association remain unclear. Notably, interventions targeting thrombocytopenia have shown potential for improving outcomes in both sepsis and COVID-19 patients. Understanding these mechanisms is crucial for developing effective treatments.

Protection of β2GPI Deficient Mice from Thrombosis Reflects a Defect in PAR3-facilitated Platelet Activation

Background

Antibodies to β2-glycoprotein I (β2GPI) cause thrombosis in antiphospholipid syndrome, however the role of β2GPI itself in regulation of coagulation pathways in vivo is not well understood.

Methods

We developed β2GPI-deficient mice (Apoh -/- ) by deleting exon 2 and 3 of Apoh using CRISPR/Cas9 and compared the propensity of wild-type (WT) and Apoh -/- mice to develop thrombosis using rose bengal and FeCl 3 -induced carotid thrombosis, laser-induced cremaster arteriolar injury, and inferior vena cava (IVC) stasis models. We also compared tail bleeding times and assessed platelet activation in WT and Apoh -/- mice in the absence and presence of exogenous β2GPI.

Results

Compared to WT littermates, Apoh -/- mice demonstrated a prolonged time to occlusion of the carotid artery after exposure to rose bengal or FeCl 3 , and reduced platelet and fibrin accumulation in cremasteric arterioles after laser injury. Similarly, significantly smaller thrombi were retrieved from the IVC of Apoh -/- mice 48 hours after IVC occlusion. The activated partial thromboplastin time (aPTT) and prothrombin time, as well as aPTT reagent- and tissue factor-induced thrombin generation times using plasma from Apoh -/- and WT mice revealed no differences. However, we observed significant prolongation of tail bleeding in Apoh -/- mice, and reduced P-selectin expression and binding of fibrinogen to the activated α2bβ3 integrin on platelets from these mice after stimulation with low thrombin concentrations; these changes were reversed by exogenous β2GPI. An antibody to PAR3 blocked thrombin-induced activation of WT, but not Apoh -/- platelets, as well as the ability of β2GPI to restore the activation response of Apoh -/- platelets to thrombin. β2GPI deficiency did not affect platelet activation by a PAR4-activator peptide, or ADP.

Conclusions

In mice, β2GPI may mediate procoagulant activity by enhancing the ability of PAR3 to present thrombin to PAR4, promoting platelet activation at low thrombin concentrations.

Key Points

β2GPI deficient mice are protected from experimental arterial, venous, and microvascular thrombosis.β2GPI deficient mice display prolonged tail bleeding times and reduced PAR3-facilitated platelet activation by low concentrations of thrombin.

Characterization of the human platelet N- and O-glycome upon storage using tandem mass spectrometry

Changes in surface glycan determinants, specifically sialic acid loss, determine platelet life span. The gradual loss of stored platelet quality is a complex process that fundamentally involves carbohydrate structures. Here, we applied lipophilic extraction and glycan release protocols to sequentially profile N- and O-linked glycans in freshly isolated and 7-day room temperature-stored platelet concentrates. Analytical methods including matrix assisted laser desorption/ionization time-of-flight mass spectrometry, tandem mass spectrometry, and liquid chromatography were used to obtain structural details of selected glycans and terminal epitopes. The fresh platelet repertoire of surface structures revealed diverse N-glycans, including high mannose structures, complex glycans with polylactosamine repeats, and glycans presenting blood group epitopes. The O-glycan repertoire largely comprised sialylated and fucosylated core-1 and core-2 structures. For both N- and O-linked glycans, we observed a loss in sialylated epitopes with a reciprocal increase in neutral structures as well as increased neuraminidase activity after platelet storage at room temperature. The data indicate that loss of sialylated glycans is associated with diminished platelet quality and untimely removal of platelets after storage.

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.

Vagus nerve stimulation primes platelets and reduces bleeding in hemophilia A male mice

Deficiency of coagulation factor VIII in hemophilia A disrupts clotting and prolongs bleeding. While the current mainstay of therapy is infusion of factor VIII concentrates, inhibitor antibodies often render these ineffective. Because preclinical evidence shows electrical vagus nerve stimulation accelerates clotting to reduce hemorrhage without precipitating systemic thrombosis, we reasoned it might reduce bleeding in hemophilia A. Using two different male murine hemorrhage and thrombosis models, we show vagus nerve stimulation bypasses the factor VIII deficiency of hemophilia A to decrease bleeding and accelerate clotting. Vagus nerve stimulation targets acetylcholine-producing T lymphocytes in spleen and α7 nicotinic acetylcholine receptors (α7nAChR) on platelets to increase calcium uptake and enhance alpha granule release. Splenectomy or genetic deletion of T cells or α7nAChR abolishes vagal control of platelet activation, thrombus formation, and bleeding in male mice. Vagus nerve stimulation warrants clinical study as a therapy for coagulation disorders and surgical or traumatic bleeding.

IVIg treatment increases thrombin activation of platelets and thrombin generation in paediatric patients with immune thrombocytopenia

Clinical manifestations and laboratory parameters of haemostasis were investigated in 23 children with newly diagnosed immune thrombocytopenia (ITP) before and after intravenous immunoglobulin (IVIg) treatment. ITP patients with platelet counts of less than 20 × 109 /L and mild bleeding symptoms, graded by a standardized bleeding score (BS), were compared with healthy children with normal platelet counts and children with chemotherapy-related thrombocytopenia. Markers of platelet activation and platelet apoptosis in the absence and presence of platelet activators were analysed by flow cytometry; thrombin generation in plasma was determined. ITP patients at diagnosis presented with increased proportions of platelets expressing CD62P and CD63 and activated caspases, and with decreased thrombin generation. Thrombin-induced activation of platelets was reduced in ITP compared with controls, while increased proportions of platelets with activated caspases were observed. Children with a higher BS had lower proportions of CD62P-expressing platelets compared with those with a lower BS. IVIg treatment increased the number of reticulated platelets, the platelet count to more than 20 × 109 /L and improved bleeding in all patients. Decreased thrombin-induced platelet activation, as well as thrombin generation, were ameliorated. Our results indicate that IVIg treatment helps to counteract diminished platelet function and coagulation in children with newly diagnosed ITP.

Predictor Factors for Chronicity in Immune Thrombocytopenic Purpura in Children

(1) Background: Immune thrombocytopenia (ITP) is an acute autoimmune blood disorder that is the main cause of thrombocytopenia in children. It is characterized by a decrease in platelets below 100 × 10⁹/L, and limited evolution with severe complications such as intracranial hemorrhage. The chronic form is defined by the persistence of thrombocytopenia more than 12 months after diagnosis. (2) Methods: We performed a retrospective study over a period of 10 years (1 January 2011-31 December 2020) at the Emergency Clinical Hospital for Children "Sf. Maria", Iasi. The aim of the study was to describe the clinical characteristics and to determine the prognostic factors in immune thrombocytopenia in children. (3) Results: In this study we included 271 children with ITP, comprising 123 females (45.4%) and 148 males (54.6%). The remission rate was higher in males, being 68.9% compared to 56.1% in females. Children with ITP under 9 years of age had a higher remission rate. Children with a platelet count > 10 × 10⁹/L at diagnosis had a higher likelihood-of-remission rate compared to patients who presented initial platelet count below this value. (4) Conclusions: The risk factors highly suggestive for chronicity are: age at diagnosis, female sex, and the number of platelets at the onset of the disease.

Structure of the immunoregulatory sialidase NEU1

Sialic acids linked to glycoproteins and glycolipids are important mediators of cell and protein recognition events. These sugar residues are removed by neuraminidases (sialidases). Neuraminidase-1 (sialidase-1 or NEU1) is a ubiquitously expressed mammalian sialidase located in lysosomes and on the cell membrane. Because of its modulation of multiple signaling processes, it is a potential therapeutic target for cancers and immune disorders. Genetic defects in NEU1 or in its protective protein cathepsin A (PPCA, CTSA) cause the lysosomal storage diseases sialidosis and galactosialidosis. To further our understanding of this enzyme's function at the molecular level, we determined the three-dimensional structure of murine NEU1. The enzyme oligomerizes through two self-association interfaces and displays a wide substrate-binding cavity. A catalytic loop adopts an inactive conformation. We propose a mechanism of activation involving a conformational change in this loop upon binding to its protective protein. These findings may facilitate the development of selective inhibitor and agonist therapies.

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.