Mechanisms of anti-GPIbα antibody-induced thrombocytopenia in mice

Course of immune thrombocytopenia according to the site of platelet destruction identified by indium-111 platelet scintigraphy

In primary immune thrombocytopenia (ITP), predictors of disease evolution and treatment response are needed. Data based on the site of platelet destruction are scarce. We performed a retrospective single-centre study of adult patients with primary ITP undergoing at least one Indium-111 platelet scintigraphy (IPS) between 2009 and 2018. Thirty-three patients had isolated hepatic platelet destruction (H-group), and 97 isolated splenic destruction (S-group). Median age at diagnosis (p < 0.001), proportion of associated cardiovascular (p < 0.001), organ-specific autoimmune diseases (p = 0.02), dependence on steroids (p = 0.003) and failure to rituximab (p = 0.01) were higher and relapse more frequent (p = 0.03) in H-group compared to non-splenectomized patients in S-group. Splenectomy was only performed in patients from S-group (as patients with hepatic sequestration are not splenectomized in our centre): 79% were in relapse-free remission at the end of a median 3.4-year post-IPS follow-up, 16% relapsed. In multivariate analyses, only a history of organ-specific autoimmune or inflammatory disease was significantly associated with hepatic sequestration (OR = 4.3, 95% CI = 1.2-15, p = 0.02). Patients with isolated hepatic sequestration were older, had more cardiovascular events and organ-specific autoimmune diseases, greater dependence on steroids, more relapses and a decreased response rate to rituximab suggesting an increased refractoriness to immunomodulatory therapies. Patients with isolated splenic sequestration responded well to splenectomy.

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.

The role of protein kinase C and the glycoprotein Ibα cytoplasmic tail in anti-glycoprotein Ibα antibody-induced platelet apoptosis and thrombocytopenia

INTRODUCTION

Immune thrombocytopenia (ITP) is an autoimmune disease characterized by low platelet counts. ITP patients with anti-platelet glycoprotein (GP) Ibα (a subunit of GPIb-IX-V complex) autoantibodies, which induce Fc-independent signaling and platelet clearance, are refractory to conventional treatment. Protein kinase C (PKC) is activated by the binding of the ligand von Willebrand factor (VWF) to GPIbα and regulates VWF-GPIbα-induced platelet activation. However, the role of PKC in anti-GPIbα antibody-induced thrombocytopenia remains unknown.

MATERIALS AND METHODS

The anti-GPIbα antibody-induced PKC activation and its underlying mechanisms were first detected by Western blot, and then the effects of PKC inhibitors, PKC knockout, or GPIbα C-terminal removal on anti-GPIbα antibody-induced platelet apoptosis, activation, aggregation, and clearance were investigated by flow cytometry, platelet aggregometry, and platelet posttransfusion, respectively. Meanwhile, platelet retention and co-localization with macrophages in the liver were detected by spinning disc intravital confocal microscopy.

RESULTS

Anti-GPIbα antibody-induced PKC activation depends on GPIbα clustering and phosphoinositide 3-kinase (PI3K) activation and results in Akt phosphorylation. Pharmacologic inhibition or genetic ablation of PKC suppresses anti-GPIbα antibody-induced platelet apoptosis and activation. Moreover, the GPIbα cytoplasmic tail is required for antibody-induced PKC activation, platelet apoptosis, and activation. Inhibition or ablation of PKC and deletion of the GPIbα cytoplasmic tail protect platelets from clearance in vivo.

CONCLUSIONS

Our study indicates the important role of PKC and the GPIbα cytoplasmic tail in anti-GPIbα antibody-mediated platelet signaling and clearance and suggests a novel therapeutic target for ITP and other thrombocytopenic diseases.

Engineering supramolecular peptide nanofibers for in vivo platelet-hitchhiking beyond ligand-receptor recognition

Ex vivo or in vivo cell-hitchhiking has emerged as a potential means for efficient drug delivery and various disease therapies. However, many challenges remain, such as the complicated engineering process and dependence on ligand-receptor interaction. Here, we present a simple in vivo platelet-hitchhiking strategy based on self-assembling peptides without ligand modification. The engineered peptide nanofibers can hitchhike ultrafast (<5 s) and efficiently on both resting and activated platelets in a receptor-independent and species-independent manner. Mechanistic studies showed that unique secondary structure of nanofibers, which lead to surface exposure of hydrophobic and hydrogen bond-forming groups, might primarily contribute to the selective and efficient platelet-hitchhiking behavior. After intravenous injection, these peptide nanofibers hitchhiked in situ on circulating platelets and achieved almost 20-fold lung accumulation. Our study provides not only a different paradigm of in vivo platelet-hitchhiking beyond ligand-receptor recognition but also a potential strategy for lung-targeted drug delivery and pulmonary disease therapy.

Desialylation and Apoptosis in Immune Thrombocytopenia: Implications for Pathogenesis and Treatment

Immune thrombocytopenia (ITP) is an autoimmune disease in which platelet autoantibodies play a significant role in its pathogenesis. Regulatory T cell dysfunction and T cell-mediated cytotoxicity also contribute to thrombocytopenia. Current therapies are directed towards immune suppression and modulation as well as stimulation of platelet production with thrombopoietin receptor agonists. Additional mechanisms of the pathogenesis of ITP have been suggested by recent experimental data. One of these processes, known as desialylation, involves antibody-induced removal of terminal sialic acid residues on platelet surface glycoproteins, leading to hepatic platelet uptake and thrombocytopenia. Apoptosis, or programmed platelet death, may also contribute to the pathogenesis of ITP. The extent of the impact of desialylation and apoptosis on ITP, the relative proportion of patients affected, and the role of antibody specificity are still the subject of investigation. This review will discuss both historical and new evidence of the influence of desialylation and apoptosis in the pathogenesis of ITP, with an emphasis on the clinical implications of these developments. Further understanding of both platelet desialylation and apoptosis might change current clinical practice and improve patient outcomes.

RBCs regulate platelet function and hemostasis under shear conditions through biophysical and biochemical means

ABSTRACT

Red blood cells (RBCs) have been hypothesized to support hemostasis by facilitating platelet margination and releasing platelet-activating factors such as adenosine 5'-diphosphate (ADP). Significant knowledge gaps remain regarding how RBCs influence platelet function, especially in (patho)physiologically relevant hemodynamic conditions. Here, we present results showing how RBCs affect platelet function and hemostasis in conditions of anemia, thrombocytopenia, and pancytopenia and how the biochemical and biophysical properties of RBCs regulate platelet function at the blood and vessel wall interface and in the fluid phase under flow conditions. We found that RBCs promoted platelet deposition to collagen under flow conditions in moderate (50 × 103/μL) but not severe (10 × 103/μL) thrombocytopenia in vitro. Reduction in hematocrit by 45% increased bleeding in mice with hemolytic anemia. In contrast, bleeding diathesis was observed in mice with a 90% but not with a 60% reduction in platelet counts. RBC transfusion improved hemostasis by enhancing fibrin clot formation at the site of vascular injury in mice with severe pancytopenia induced by total body irradiation. Altering membrane deformability changed the ability of RBCs to promote shear-induced platelet aggregation. RBC-derived ADP contributed to platelet activation and aggregation in vitro under pathologically high shear stresses, as observed in patients supported by left ventricular assist devices. These findings demonstrate that RBCs support platelet function and hemostasis through multiple mechanisms, both at the blood and vessel wall interface and in the fluidic phase of circulation.

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.

Circulating platelets modulate oligodendrocyte progenitor cell differentiation during remyelination

Revealing unknown cues that regulate oligodendrocyte progenitor cell (OPC) function in remyelination is important to optimise the development of regenerative therapies for multiple sclerosis (MS). Platelets are present in chronic non-remyelinated lesions of MS and an increase in circulating platelets has been described in experimental autoimmune encephalomyelitis (EAE) mice, an animal model for MS. However, the contribution of platelets to remyelination remains unexplored. Here we show platelet aggregation in proximity to OPCs in areas of experimental demyelination. Partial depletion of circulating platelets impaired OPC differentiation and remyelination, without altering blood-brain barrier stability and neuroinflammation. Transient exposure to platelets enhanced OPC differentiation in vitro, whereas sustained exposure suppressed this effect. In a mouse model of thrombocytosis (Calr+/-), there was a sustained increase in platelet aggregation together with a reduction of newly-generated oligodendrocytes following toxin-induced demyelination. These findings reveal a complex bimodal contribution of platelet to remyelination and provide insights into remyelination failure in MS.

High dimensional proteomic mapping of bone marrow immune characteristics in immune thrombocytopenia

To investigate the role of co-stimulatory and co-inhibitory molecules on immune tolerance in immune thrombocytopenia (ITP), this study mapped the immune cell heterogeneity in the bone marrow of ITP at the single-cell level using Cytometry by Time of Flight (CyTOF). Thirty-six patients with ITP and nine healthy volunteers were enrolled in the study. As soluble immunomodulatory molecules, more sCD25 and sGalectin-9 were detected in ITP patients. On the cell surface, co-stimulatory molecules like ICOS and HVEM were observed to be upregulated in mainly central memory and effector T cells. In contrast, co-inhibitory molecules such as CTLA-4 were significantly reduced in Th1 and Th17 cell subsets. Taking a platelet count of 30×109 L-1 as the cutoff value, ITP patients with high and low platelet counts showed different T cell immune profiles. Antigen-presenting cells such as monocytes and B cells may regulate the activation of T cells through CTLA-4/CD86 and HVEM/BTLA interactions, respectively, and participate in the pathogenesis of ITP. In conclusion, the proteomic and soluble molecular profiles brought insight into the interaction and modulation of immune cells in the bone marrow of ITP. They may offer novel targets to develop personalized immunotherapies.

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).

The use of belimumab on patients with both systemic lupus erythematosus and immune thrombocytopenia: A retrospective cohort study

OBJECTIVE

The objective of this study is to provide a description of a group of retrospective cohort outcomes in patients with systemic lupus erythematosus (SLE) complicated with immune thrombocytopenia (ITP) receiving belimumab.

METHODS

This study reports on the treatment of 10 female patients (mean age 34.3 ± 14.0 years, mean weight 58.7 ± 18.2 kg) with both SLE and ITP who received belimumab in addition to basic drug therapy. The belimumab treatment regimen consisted of a dosage of 10 mg/kg, with an initial infusion every 2 weeks for the first 3 doses, followed by an infusion every 4 weeks.

RESULTS

Ten patients were included in the study. The overall response rate of thrombocytopenia was 90% after treatment. The parameters such as platelet count, lymphocyte count, erythrocyte count, hemoglobin, dsDNA, C3, and C4 were significantly improved (p < .05). The SLE Disease Activity Index (SLEDAI), British Islet lupus Assessment Group 2004 (BILAG-2004), and Physician Global assessment (PGA) scores were significantly decreased (p < .05). There were no significant differences in glutamic pyruvic transaminase (ALT), glutamic oxaloacetic transaminase (AST), and serum creatinine (Scr) before and after treatment (p > .05).

CONCLUSION

Belimumab shows promising clinical outcomes in the treatment on patients with both SLE and ITP. Further studies are needed to validate these findings in larger patient populations and compare the efficacy of belimumab with other treatments for SLE complicated with ITP. Long-term response rates and adverse events associated with belimumab treatment also warrant further investigation.

Platelet ageing: A review

Platelet ageing is an area of research which has gained much interest in recent years. Newly formed platelets, often referred to as reticulated platelets, young platelets or immature platelets, are defined as RNA-enriched and have long been thought to be hyper-reactive. This latter view is largely rooted in associations and observations in patient groups with shortened platelet half-lives who often present with increased proportions of newly formed platelets. Evidence from such groups suggests that an increased proportion of newly formed platelets is associated with an increased risk of thrombotic events and a reduced effectiveness of standard anti-platelet therapies. Whilst research has highlighted the existence of platelet subpopulations based on function, size and age within patient groups, the common intrinsic changes which occur as platelets age within the circulation are only just being explored. By understanding the changes that occur during the natural ageing processes of platelets, we may be able to identify the triggers for alterations in platelet life span and platelet reactivity. Here we review research on platelet ageing in the context of health and disease, paying particular attention to the experimental approaches taken and the robustness of conclusions that can be drawn.

Serotonin reduction in post-acute sequelae of viral infection

Post-acute sequelae of COVID-19 (PASC, "Long COVID") pose a significant global health challenge. The pathophysiology is unknown, and no effective treatments have been found to date. Several hypotheses have been formulated to explain the etiology of PASC, including viral persistence, chronic inflammation, hypercoagulability, and autonomic dysfunction. Here, we propose a mechanism that links all four hypotheses in a single pathway and provides actionable insights for therapeutic interventions. We find that PASC are associated with serotonin reduction. Viral infection and type I interferon-driven inflammation reduce serotonin through three mechanisms: diminished intestinal absorption of the serotonin precursor tryptophan; platelet hyperactivation and thrombocytopenia, which impacts serotonin storage; and enhanced MAO-mediated serotonin turnover. Peripheral serotonin reduction, in turn, impedes the activity of the vagus nerve and thereby impairs hippocampal responses and memory. These findings provide a possible explanation for neurocognitive symptoms associated with viral persistence in Long COVID, which may extend to other post-viral syndromes.

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.

Platelets mediate acute hepatic microcirculatory injury in a protease-activated-receptor-4-dependent manner after extended liver resection

BACKGROUND

Small-for-size syndrome (SFSS) is a major complication following extended liver resection. The role of platelets in the early development of SFSS remains to be cleared. We aimed to investigate the impact of platelets and PAR-4, a receptor for platelet activation, on the acute phase microcirculatory injury after liver resection by in vivo microscopy analyzing the changes in leukocyte recruitment, platelet-neutrophil interaction, and microthrombosis-induced perfusion failure.

METHODS

Sixty-percent partial hepatectomy (PH) models using C57BL/6 mice receiving platelet depletion with anti-GPIbα, PAR-4 blockade with tcY-NH2, or vehicle treatment with saline were used. Sham-operated animals served as controls. Epifluorescence microscopic analysis was performed 2 h after PH to quantify the leukocyte recruitment and microcirculatory changes. Sinusoidal neutrophil recruitment, platelet-neutrophil interaction, and microthrombosis were evaluated using two-photon microscopy. ICAM-1 expression and liver liver injury were assessed in tissue/blood samples.

RESULTS

The increments of leukocyte recruitment in post-sinusoidal venules and sinusoidal perfusion failure, the upregulation of ICAM-1 expression, and the deterioration of liver function 2 h after 60% PH were alleviated in the absence of platelets or by PAR-4 blockade. Intensified platelet-neutrophil interaction and microthrombosis in sinusoids were observed 2 h after 60% PH, which significantly attenuated after PAR-4 blockade.

CONCLUSION

Platelets play a critical role in acute liver injury after extended liver resection within 2 h. The deactivation of platelets via PAR-4 blockade ameliorated liver function deterioration by suppressing early leukocyte recruitment, platelet-neutrophil interaction, and microthrombosis in hepatic sinusoids.

Immune-mediated platelet depletion augments Alzheimer's disease neuropathological hallmarks in APP-PS1 mice

In Alzheimer's disease (AD), platelets become dysfunctional and might contribute to amyloid beta deposition. Here, we depleted platelets in one-year-old APP Swedish PS1 dE9 (APP-PS1) transgenic mice for five days, using intraperitoneal injections of an anti-CD42b antibody, and assessed changes in cerebral amyloidosis, plaque-associated neuritic dystrophy and gliosis. In APP-PS1 female mice, platelet depletion shifted amyloid plaque size distribution towards bigger plaques and increased neuritic dystrophy in the hippocampus. In platelet-depleted females, plaque-associated Iba1+ microglia had lower amounts of fibrillar amyloid beta cargo and GFAP+ astrocytic processes showed a higher overlap with thioflavin S+ amyloid plaques. In contrast to the popular hypothesis that platelets foster plaque pathology, our data suggest that platelets might limit plaque growth and attenuate plaque-related neuritic dystrophy at advanced stages of amyloid plaque pathology in APP-PS1 female mice. Whether the changes in amyloid plaque pathology are due to a direct effect on amyloid beta deposition or are a consequence of altered glial function needs to be further elucidated.

Chemically modified in-vitro-transcribed mRNA encoding thrombopoietin stimulates thrombopoiesis in mice

The use of messenger RNA (mRNA) enables the transient production of therapeutic proteins with stable and predictable translational kinetics and without the risk of insertional mutagenesis. Recent findings highlight the enormous potential of mRNA-based therapeutics. Here, we describe the synthesis of chemically modified thrombopoietin (TPO) mRNA through in vitro transcription and in vivo delivery via lipid nanoparticles (LNPs). After delivery of TPO mRNA in mice, compared with normal physiological values, plasma TPO protein levels increased over 1000-fold in a dose-dependent manner. Moreover, through a single intravenous dose of TPO mRNA-loaded LNPs, both reticulated and total platelet count increased significantly in mice, demonstrating that TPO protein derived from the exogenous mRNA was able to maintain normal activity. Submicrogram quantity of N1-methylpseudouridine-modified TPO mRNA showed a similar effect in promoting thrombopoiesis as that by the TPO receptor agonist romiplostim. In addition, a therapeutic value was established in anti-GPIbα (CD42b) antibody-induced thrombocytopenia mouse models that showed a fast recovery of platelet count. Our study demonstrated chemically modified in-vitro-transcribed TPO mRNA as a potentially safe therapeutic intervention to stimulate thrombopoiesis.

ADAP restraint of STAT1 signaling regulates macrophage phagocytosis in immune thrombocytopenia

Heightened platelet phagocytosis by macrophages accompanied by an increase in IFN-γ play key roles in the etiology of immune thrombocytopenia (ITP); however, it remains elusive how macrophage-mediated platelet clearance is regulated in ITP. Here, we report that adhesion and degranulation-protein adaptor protein (ADAP) restrains platelet phagocytosis by macrophages in ITP via modulation of signal transducer and activator of transcription 1 (STAT1)-FcγR signaling. We show that ITP was associated with the underexpression of ADAP in splenic macrophages. Furthermore, macrophages from Adap-/- mice exhibited elevated platelet phagocytosis and upregulated proinflammatory signaling, and thrombocytopenia in Adap-/- mice was mitigated by the depletion of macrophages. Mechanistically, ADAP interacted and competed with STAT1 binding to importin α5. ADAP deficiency potentiated STAT1 nuclear entry, leading to a selective enhancement of FcγRI/IV transcription in macrophages. Moreover, pharmacological inhibition of STAT1 or disruption of the STAT1-importin α5 interaction relieved thrombocytopenia in Adap-/- mice. Thus, our findings not only reveal a critical role for ADAP as an intracellular immune checkpoint for shaping macrophage phagocytosis in ITP but also identify the ADAP-STAT1-importin α5 module as a promising therapeutic target in the treatment of ITP.

Murine models of glycoprotein Ib-IX

The utility of mouse models to dissect the molecular basis of hemostasis and thrombosis is now well established. The anucleate properties of circulating blood platelet and their specialized release from mature megakaryocytes makes the use of in vivo models all the more informative and powerful. Indeed, they are powerful but there do exist limitations. Here, we review the contributions of mouse models to the pathogenesis of the Bernard-Soulier syndrome, their use in platelet-specific gene expression, the recent development of mice expressing both human GPIb-IX and human von Willebrand factor (VWF), and finally the use of GPIb-IX mouse models to examine the impact of platelet biology beyond clotting. The humanization of the receptor and ligand axis is likely to be a major advancement in the characterization of therapeutics in the complex pathogenesis that drives thrombosis. When appropriate, we highlight some limitations of each mouse model, but this is not to minimize the contributions these models to the field. Rather, the limitations are meant to provide context for any direct application to the important mechanisms supporting human primary hemostasis and thrombosis.

The GPIb-IX complex on platelets: insight into its novel physiological functions affecting immune surveillance, hepatic thrombopoietin generation, platelet clearance and its relevance for cancer development and metastasis

The glycoprotein (GP) Ib-IX complex is a platelet receptor that mediates the initial interaction with subendothelial von Willebrand factor (VWF) causing platelet arrest at sites of vascular injury even under conditions of high shear. GPIb-IX dysfunction or deficiency is the reason for the rare but severe Bernard-Soulier syndrome (BSS), a congenital bleeding disorder. Although knowledge on GPIb-IX structure, its basic functions, ligands, and intracellular signaling cascades have been well established, several advances in GPIb-IX biology have been made in the recent years. Thus, two mechanosensitive domains and a trigger sequence in GPIb were characterized and its role as a thrombin receptor was deciphered. Furthermore, it became clear that GPIb-IX is involved in the regulation of platelet production, clearance and thrombopoietin secretion. GPIb is deemed to contribute to liver cancer development and metastasis. This review recapitulates these novel findings highlighting GPIb-IX in its multiple functions as a key for immune regulation, host defense, and liver cancer development.

Targeted SERPIN (TaSER): A dual-action antithrombotic agent that targets platelets for SERPIN delivery

BACKGROUND

Occlusive thrombi are not homogeneous in composition. The core of a thrombus is rich in activated platelets and fibrin while the outer shell contains resting platelets. This core is inaccessible to plasma proteins. We produced a fusion protein (targeted SERPIN-TaSER), consisting of a function-blocking VH H against glycoprotein Ibα (GPIbα) and a thrombin-inhibiting serine protease inhibitor (SERPIN; α1-antitrypsin 355 AIAR358 ) to interfere with platelet-driven thrombin formation.

AIM

To evaluate the antithrombotic properties of TaSER.

METHODS

Besides TaSER, we generated three analogous control variants with either a wild-type antitrypsin subunit, a non-targeting control VH H, or their combination. We investigated TaSER and controls in protease activity assays, (platelet-dependent) thrombin generation assays, and by western blotting. The effects of TaSER on platelet activation and von Willebrand factor (VWF) binding were studied by fluorescence-activated cell sorting, in agglutination studies, and in ATP secretion experiments. We studied the influence of TaSER in whole blood (1) on platelet adhesion on VWF, (2) aggregate formation on collagen, and (3) thrombus formation (after recalcification) on collagen and tissue factor.

RESULTS

TaSER binds platelets and inhibits thrombin activity on the platelet surface. It blocks VWF binding and disassembles platelet agglutinates. TaSER delays tissue factor-triggered thrombin generation and ATP secretion in platelet-rich plasma in a targeted manner. In flow studies, TaSER interferes with platelet adhesion and aggregate formation due to GPIbα blockade and limits thrombus formation due to targeted inhibition of platelet-dependent thrombin activity.

CONCLUSION

The synergy between the individual properties of TaSER makes it a highly effective antithrombotic agent with possible clinical implications.

Bone marrow remodelling supports hematopoiesis in response to immune thrombocytopenia progression

Sustained ITP activates and increases the number of functional LT-HSCs.

The remodeled ITP bone marrow enhances hematopoiesis.

Immune thrombocytopenia (ITP) is an acquired autoimmune condition characterized by both reduced platelet production and the destruction of functionally normal platelets by sustained attack from the immune system. However, the effect of prolonged ITP on the more immature hematopoietic progenitors remains an open area of investigation. By using a murine in vivo model of extended ITP, we revealed that ITP progression drives considerable progenitor expansion and bone marrow (BM) remodeling. Single-cell assays using Lin^–Sca1⁺c-Kit⁺CD48^–CD150⁺ long-term hematopoietic stem cells (LT-HSCs) revealed elevated LT-HSC activation and proliferation in vitro. However, the increased activation did not come at the expense of LT-HSC functionality as measured by in vivo serial transplantations. ITP progression was associated with considerable BM vasodilation and angiogenesis, as well as a twofold increase in the local production of CXCL12, a cytokine essential for LT-HSC function and BM homing expressed at high levels by LepR⁺ BM stromal cells. This was associated with a 1.5-fold increase in LepR⁺ BM stromal cells and a 5.5-fold improvement in progenitor homing to the BM. The increase in stromal cells was transient and reverted back to baseline after platelet count returned to normal, but the vasculature changes in the BM persisted. Together, our data demonstrate that LT-HSCs expand in response to ITP and that LT-HSC functionality during sustained hematopoietic stress is maintained through an adapting BM microenvironment.

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.

Immune Checkpoint-Related Gene Polymorphisms Are Associated With Primary Immune Thrombocytopenia

Cancer immunotherapy by immune checkpoint blockade has been effective in the treatment of certain tumors. However, the association between immune checkpoints and autoimmune diseases remains elusive and requires urgent investigation. Primary immune thrombocytopenia (ITP), characterized by reduced platelet count and a consequent increased risk of bleeding, is an autoimmune disorder with a hyper-activated T cell response. Here, we investigated the contribution of immune checkpoint-related single-nucleotide polymorphisms (SNPs), including CD28, ICOS, PD1, TNFSF4, DNAM1, TIM3, CTLA4, and LAG3 to the susceptibility and therapeutic effects of ITP. In this case-control study, 307 ITP patients and 295 age-matched healthy participants were recruited. We used the MassARRAY system for genotyping immune checkpoint-related SNPs. Our results revealed that rs1980422 in CD28 was associated with an increased risk of ITP after false discovery rate correction (codominant, CT vs. TT, OR = 1.788, 95% CI = 1.178-2.713, p = 0.006). In addition, CD28 expression at both the mRNA and protein levels was significantly higher in patients with CT than in those with the TT genotype (p = 0.028 and p = 0.001, respectively). Furthermore, the T allele of PD1 rs36084323 was a risk factor for ITP severity and the T allele of DNAM1 rs763361 for corticosteroid-resistance. In contrast, the T allele of LAG3 rs870849 was a protective factor for ITP severity, and the T allele of ICOS rs6726035 was protective against corticosteroid-resistance. The TT/CT genotypes of PD1 rs36084323 also showed an 8.889-fold increase in the risk of developing refractory ITP. This study indicates that immune checkpoint-related SNPs, especially CD28 rs1980422, may be genetic factors associated with the development and treatment of ITP patients. Our results shed new light on prognosis prediction, disease severity, and discovering new therapeutic targets.