Immune cells surveil aberrantly sialylated O-glycans on megakaryocytes to regulate platelet count

Sialic acid in the regulation of blood cell production, differentiation and turnover

Sialic acid is a unique sugar moiety that resides in the distal and most accessible position of the glycans on mammalian cell surface and extracellular glycoproteins and glycolipids. The potential for sialic acid to obscure underlying structures has long been postulated, but the means by which such structural changes directly affect biological processes continues to be elucidated. Here, we appraise the growing body of literature detailing the importance of sialic acid for the generation, differentiation, function and death of haematopoietic cells. We conclude that sialylation is a critical post-translational modification utilized in haematopoiesis to meet the dynamic needs of the organism by enforcing rapid changes in availability of lineage-specific cell types. Though long thought to be generated only cell-autonomously within the intracellular ER-Golgi secretory apparatus, emerging data also demonstrate previously unexpected diversity in the mechanisms of sialylation. Emphasis is afforded to the mechanism of extrinsic sialylation, whereby extracellular enzymes remodel cell surface and extracellular glycans, supported by charged sugar donor molecules from activated platelets.

Plasmacytoid dendritic cells control homeostasis of megakaryopoiesis

Platelet homeostasis is essential for vascular integrity and immune defence1,2. Although the process of platelet formation by fragmenting megakaryocytes (MKs; thrombopoiesis) has been extensively studied, the cellular and molecular mechanisms required to constantly replenish the pool of MKs by their progenitor cells (megakaryopoiesis) remains unclear3,4. Here we use intravital imaging to track the cellular dynamics of megakaryopoiesis over days. We identify plasmacytoid dendritic cells (pDCs) as homeostatic sensors that monitor the bone marrow for apoptotic MKs and deliver IFNα to the MK niche triggering local on-demand proliferation and maturation of MK progenitors. This pDC-dependent feedback loop is crucial for MK and platelet homeostasis at steady state and under stress. pDCs are best known for their ability to function as vigilant detectors of viral infection5. We show that virus-induced activation of pDCs interferes with their function as homeostatic sensors of megakaryopoiesis. Consequently, activation of pDCs by SARS-CoV-2 leads to excessive megakaryopoiesis. Together, we identify a pDC-dependent homeostatic circuit that involves innate immune sensing and demand-adapted release of inflammatory mediators to maintain homeostasis of the megakaryocytic lineage.

The role of plasmacytoid dendritic cells (pDCs) in immunity during viral infections and beyond

Type I and III interferons (IFNs) are essential for antiviral immunity and act through two different but complimentary pathways. First, IFNs activate intracellular antimicrobial programs by triggering the upregulation of a broad repertoire of viral restriction factors. Second, IFNs activate innate and adaptive immunity. Dysregulation of IFN production can lead to severe immune system dysfunction. It is thus crucial to identify and characterize the cellular sources of IFNs, their effects, and their regulation to promote their beneficial effects and limit their detrimental effects, which can depend on the nature of the infected or diseased tissues, as we will discuss. Plasmacytoid dendritic cells (pDCs) can produce large amounts of all IFN subtypes during viral infection. pDCs are resistant to infection by many different viruses, thus inhibiting the immune evasion mechanisms of viruses that target IFN production or their downstream responses. Therefore, pDCs are considered essential for the control of viral infections and the establishment of protective immunity. A thorough bibliographical survey showed that, in most viral infections, despite being major IFN producers, pDCs are actually dispensable for host resistance, which is achieved by multiple IFN sources depending on the tissue. Moreover, primary innate and adaptive antiviral immune responses are only transiently affected in the absence of pDCs. More surprisingly, pDCs and their IFNs can be detrimental in some viral infections or autoimmune diseases. This makes the conservation of pDCs during vertebrate evolution an enigma and thus raises outstanding questions about their role not only in viral infections but also in other diseases and under physiological conditions.

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.

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.

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.

Inside-to-outside and back to the future of megakaryopoiesis

A State of the Art lecture titled "Megakaryocytes and different thrombopoietic environments" was presented at the ISTH Congress in 2022. Circulating platelets are specialized cells produced by megakaryocytes. Leading studies point to the bone marrow niche as the core of hematopoietic stem cell differentiation, revealing interesting and complex environmental factors for consideration. Megakaryocytes take cues from the physiochemical bone marrow microenvironment, which includes cell-cell interactions, contact with extracellular matrix components, and flow generated by blood circulation in the sinusoidal lumen. Germinal and acquired mutations in hematopoietic stem cells may manifest in altered megakaryocyte maturation, proliferation, and platelet production. Diseased megakaryopoiesis may also cause modifications of the entire hematopoietic niche, highlighting the central role of megakaryocytes in the control of physiologic bone marrow homeostasis. Tissue-engineering approaches have been developed to translate knowledge from in vivo (inside) to functional mimics of native tissue ex vivo (outside). Reproducing the thrombopoietic environment is instrumental to gain new insight into its activity and answering the growing demand for human platelets for fundamental studies and clinical applications. In this review, we discuss the major achievements on this topic, and finally, we summarize relevant new data presented during the 2022 ISTH Congress that pave the road to the future of megakaryopoiesis.

Comprehensive analysis of platelet glycoprotein Ibα ectodomain glycosylation

BACKGROUND

Platelet glycoprotein (GP) Ibα is the major ligand-binding subunit of the GPIb-IX-V complex that binds von Willebrand factor. GPIbα is heavily glycosylated, and its glycans have been proposed to play key roles in platelet clearance, von Willebrand factor binding, and as target antigens in immune thrombocytopenia syndromes. Despite its importance in platelet biology, the glycosylation profile of GPIbα is not well characterized.

OBJECTIVES

The aim of this study was to comprehensively analyze GPIbα amino acid sites of glycosylation (glycosites) and glycan structures.

METHODS

GPIbα ectodomain that was recombinantly expressed or that was purified from human platelets was analyzed by Western blot, mass spectrometry glycomics, and mass spectrometry glycopeptide analysis to define glycosites and the structures of the attached glycans.

RESULTS

We identified a diverse repertoire of N- and O-glycans, including sialoglycans, Tn antigen, T antigen, and ABO(H) blood group antigens. In the analysis of the recombinant protein, we identified 62 unique O-glycosites. In the analysis of the endogenous protein purified from platelets, we identified 48 unique O-glycosites and 1 N-glycosite. The GPIbα mucin domain is densely O-glycosylated. Glycosites are also located within the macroglycopeptide domain and mechanosensory domain.

CONCLUSIONS

This comprehensive analysis of GPIbα glycosylation lays the foundation for further studies to determine the functional and structural roles of GPIbα glycans.

Inherited Thrombocytopenia Caused by Variants in Crucial Genes for Glycosylation

Protein glycosylation, including sialylation, involves complex and frequent post-translational modifications, which play a critical role in different biological processes. The conjugation of carbohydrate residues to specific molecules and receptors is critical for normal hematopoiesis, as it favors the proliferation and clearance of hematopoietic precursors. Through this mechanism, the circulating platelet count is controlled by the appropriate platelet production by megakaryocytes, and the kinetics of platelet clearance. Platelets have a half-life in blood ranging from 8 to 11 days, after which they lose the final sialic acid and are recognized by receptors in the liver and eliminated from the bloodstream. This favors the transduction of thrombopoietin, which induces megakaryopoiesis to produce new platelets. More than two hundred enzymes are responsible for proper glycosylation and sialylation. In recent years, novel disorders of glycosylation caused by molecular variants in multiple genes have been described. The phenotype of the patients with genetic alterations in GNE, SLC35A1, GALE and B4GALT is consistent with syndromic manifestations, severe inherited thrombocytopenia, and hemorrhagic complications.

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.

Clinical impact of glycans in platelet and megakaryocyte biology

Humans produce and remove 1011 platelets daily to maintain a steady-state platelet count. The tight regulation of platelet production and removal from the blood circulation prevents anomalies in both processes from resulting in reduced or increased platelet count, often associated with the risk of bleeding or overt thrombus formation, respectively. This review focuses on the role of glycans, also known as carbohydrates or oligosaccharides, including N- and O-glycans, proteoglycans, and glycosaminoglycans, in human and mouse platelet and megakaryocyte physiology. Based on recent clinical observations and mouse models, we focused on the pathologic aspects of glycan biosynthesis and degradation and their effects on platelet numbers and megakaryocyte function.

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.