Foudroyant cerebral venous (sinus) thrombosis triggered through CLEC-2 and GPIIb/IIIa dependent platelet activation

Immunothrombosis versus thrombo-inflammation: platelets in cerebrovascular complications

A State-of-the Art lecture titled "Thrombo-Neuroinflammatory Disease" was presented at the International Society on Thrombosis and Haemostasis Congress in 2023. First, we would like to advocate for discrimination between immunothrombosis and thrombo-inflammation, as immunothrombosis describes an overshooting inflammatory reaction that results in detrimental thrombotic activity. In contrast, thrombo-inflammation describes the interplay of platelets and coagulation with the immunovascular system, resulting in the recruitment of immune cells and loss of barrier function (hence, hallmarks of inflammation). Both processes can be observed in the brain, with cerebral venous thrombosis being a prime example of immunothrombosis, while infarct progression in response to ischemic stroke is a paradigmatic example of thrombo-inflammation. Here, we review the pathomechanisms underlying cerebral venous thrombosis and ischemic stroke from a platelet-centric perspective and discuss translational implications. Finally, we summarize relevant new data on this topic presented during the 2023 International Society on Thrombosis and Haemostasis Congress.

Antibodies against platelet factor 4 and the risk of cerebral venous sinus thrombosis in patients with vaccine-induced immune thrombotic thrombocytopenia

BACKGROUND

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare complication of adenoviral vector-based vaccines against SARS-CoV-2. This syndrome is caused by antibodies against platelet factor 4 (PF4; CXCL4) that lead to platelet activation and is characterized by thrombocytopenia and thrombosis in unusual locations, including cerebral venous sinus thrombosis (CVST). VITT can be classified based on anti-PF4 antibodies properties in vitro: those that require PF4 to activate platelets (PF4-dependent) and those that can activate platelets without additional PF4 (PF4-independent) in the serotonin release assay.

OBJECTIVES

We aim to characterize the relationship of VITT platelet-activating profiles with CVST.

METHODS

We conducted a retrospective cohort study involving patients with confirmed VITT who were tested between March and June 2021. Data were collected with an anonymized form and cases were identified as VITT with high clinical suspicion according to platelet activation assays. Anti-PF4 antibody binding regions on PF4 were further characterized with alanine scanning mutagenesis.

RESULTS

Of the patients with confirmed VITT (n = 39), 17 (43.6%) had PF4-dependent antibodies and 22 (56.4%) had PF4-independent antibodies. CVST occurred almost exclusively in PF4-independent patients (11 of 22 vs 1 of 17; P < .05). Additionally, PF4-independent antibodies bound to 2 distinct epitopes on PF4, the heparin-binding region and a site typical for heparin-induced thrombocytopenia antibodies, whereas PF4-dependent antibodies bound to only the heparin-binding region.

CONCLUSION

These findings suggest that VITT antibodies that cause PF4-independent platelet activation represent a unique subset of patients more likely to be associated with CVST, possibly due to the 2 different types of anti-PF4 antibodies.

Platelet-derived lipids promote insulin secretion of pancreatic β cells

Hyperreactive platelets are commonly observed in diabetic patients indicating a potential link between glucose homeostasis and platelet reactivity. This raises the possibility that platelets may play a role in the regulation of metabolism. Pancreatic β cells are the central regulators of systemic glucose homeostasis. Here, we show that factor(s) derived from β cells stimulate platelet activity and platelets selectively localize to the vascular endothelium of pancreatic islets. Both depletion of platelets and ablation of major platelet adhesion or activation pathways consistently resulted in impaired glucose tolerance and decreased circulating insulin levels. Furthermore, we found platelet-derived lipid classes to promote insulin secretion and identified 20-Hydroxyeicosatetraenoic acid (20-HETE) as the main factor promoting β cells function. Finally, we demonstrate that the levels of platelet-derived 20-HETE decline with age and that this parallels with reduced impact of platelets on β cell function. Our findings identify an unexpected function of platelets in the regulation of insulin secretion and glucose metabolism, which promotes metabolic fitness in young individuals.

Loss of zinc transporters ZIP1 and ZIP3 augments platelet reactivity in response to thrombin and accelerates thrombus formation in vivo

Zinc (Zn2+) is considered as important mediator of immune cell function, thrombosis and haemostasis. However, our understanding of the transport mechanisms that regulate Zn2+ homeostasis in platelets is limited. Zn2+ transporters, ZIPs and ZnTs, are widely expressed in eukaryotic cells. Using mice globally lacking ZIP1 and ZIP3 (ZIP1/3 DKO), our aim was to explore the potential role of these Zn2+ transporters in maintaining platelet Zn2+ homeostasis and in the regulation of platelet function. While ICP-MS measurements indicated unaltered overall Zn2+ concentrations in platelets of ZIP1/3 DKO mice, we observed a significantly increased content of FluoZin3-stainable free Zn2+, which, however, appears to be released less efficiently upon thrombin-stimulated platelet activation. On the functional level, ZIP1/3 DKO platelets exhibited a hyperactive response towards threshold concentrations of G protein-coupled receptor (GPCR) agonists, while immunoreceptor tyrosine-based activation motif (ITAM)-coupled receptor agonist signalling was unaffected. This resulted in enhanced platelet aggregation towards thrombin, bigger thrombus volume under flow ex vivo and faster in vivo thrombus formation in ZIP1/3 DKO mice. Molecularly, augmented GPCR responses were accompanied by enhanced Ca2+ and PKC, CamKII and ERK1/2 signalling. The current study thereby identifies ZIP1 and ZIP3 as important regulators for the maintenance of platelet Zn2+ homeostasis and function.

Illustrated State-of-the-Art Capsules of the ISTH 2023 Congress

This year's Congress of the International Society of Thrombosis and Haemostasis (ISTH) took place in person in Montréal, Canada, from June 24-28, 2023. The conference, held annually, highlighted cutting-edge advances in basic, translational, population and clinical sciences relevant to the Society. As for all ISTH congresses, we offered a special, congress-specific scientific theme; this year, the special theme was immunothrombosis. Certainly, over the last few years, COVID-19 infection and its related thrombotic and other complications have renewed interest in the concepts of thromboinflammation and immunothrombosis; namely, the relationship between inflammation, infection and clotting. Other main scientific themes of the Congress included Arterial Thromboembolism, Coagulation and Natural Anticoagulants, Diagnostics and Omics, Fibrinolysis and Proteolysis, Hemophilia and Rare Bleeding Disorders, Hemostatic System in Cancer, Inflammation and Immunity, Pediatrics, Platelet Disorders, von Willebrand Disease and Thrombotic Microangiopathies, Platelets and Megakaryocytes, Vascular Biology, Venous Thromboembolism and Women's Health. Among other sessions, the program included 28 State-of-the-Art (SOA) sessions with a total of 84 talks given by internationally recognized leaders in the field. SOA speakers were invited to prepare brief illustrated reviews of their talks that were peer reviewed and are included in this article. These illustrated capsules highlight the major scientific advances with potential to impact clinical practice. Readers are invited to take advantage of the excellent educational resource provided by these illustrated capsules. They are also encouraged to use the image in social media to draw attention to the high quality and impact of the science presented at the Congress.

Divalent nanobodies to platelet CLEC-2 can serve as agonists or antagonists

CLEC-2 is a target for a new class of antiplatelet agent. Clustering of CLEC-2 leads to phosphorylation of a cytosolic YxxL and binding of the tandem SH2 domains in Syk, crosslinking two receptors. We have raised 48 nanobodies to CLEC-2 and crosslinked the most potent of these to generate divalent and tetravalent nanobody ligands. Fluorescence correlation spectroscopy (FCS) was used to show that the multivalent nanobodies cluster CLEC-2 in the membrane and that clustering is reduced by inhibition of Syk. Strikingly, the tetravalent nanobody stimulated aggregation of human platelets, whereas the divalent nanobody was an antagonist. In contrast, in human CLEC-2 knock-in mouse platelets, the divalent nanobody stimulated aggregation. Mouse platelets express a higher level of CLEC-2 than human platelets. In line with this, the divalent nanobody was an agonist in high-expressing transfected DT40 cells and an antagonist in low-expressing cells. FCS, stepwise photobleaching and non-detergent membrane extraction show that CLEC-2 is a mixture of monomers and dimers, with the degree of dimerisation increasing with expression thereby favouring crosslinking of CLEC-2 dimers. These results identify ligand valency, receptor expression/dimerisation and Syk as variables that govern activation of CLEC-2 and suggest that divalent ligands should be considered as partial agonists.

Platelet glycoprotein V spatio-temporally controls fibrin formation

The activation of platelets and coagulation at vascular injury sites is crucial for haemostasis but can promote thrombosis and inflammation in vascular pathologies. Here, we delineate an unexpected spatio-temporal control mechanism of thrombin activity that is platelet orchestrated and locally limits excessive fibrin formation after initial haemostatic platelet deposition. During platelet activation, the abundant platelet glycoprotein (GP) V is cleaved by thrombin. We demonstrate with genetic and pharmacological approaches that thrombin-mediated shedding of GPV does not primarily regulate platelet activation in thrombus formation, but rather has a distinct function after platelet deposition and specifically limits thrombin-dependent generation of fibrin, a crucial mediator of vascular thrombo-inflammation. Genetic or pharmacologic defects in haemostatic platelet function are unexpectedly attenuated by specific blockade of GPV shedding, indicating that the spatio-temporal control of thrombin-dependent fibrin generation also represents a potential therapeutic target to improve haemostasis.

Podoplanin: A potential therapeutic target for thrombotic diseases

As a specific lymphatic marker and a key ligand of C-type lectin-like receptor 2 (CLEC-2), podoplanin (Pdpn) is involved in various physiological and pathological processes such as growth and development, respiration, blood coagulation, lymphangiogenesis, angiogenesis, and inflammation. Thrombotic diseases constitute a major cause of disability and mortality in adults, in which thrombosis and inflammation play a crucial role. Recently, increasing evidence demonstrates the distribution and function of this glycoprotein in thrombotic diseases such as atherosclerosis, ischemic stroke, venous thrombosis, ischemic-reperfusion injury (IRI) of kidney and liver, and myocardial infarction. Evidence showed that after ischemia, Pdpn can be acquired over time by a heterogeneous cell population, which may not express Pdpn in normal conditions. In this review, the research progresses in understanding the roles and mechanisms of podoplanin in thromobotic diseases are summarized. The challenges of podoplanin-targeted approaches for disease prognosis and preventions are also discussed.

State of the art in cerebral venous sinus thrombosis animal models

Cerebral venous sinus thrombosis (CVST) is an uncommon venous thromboembolic event accounting for less than 1% of strokes resulting in brain parenchymal injuries. Diagnosis and prognosis are still challenging due to highly variable clinical course and etiologies. Beyond thrombosis, different CVST-related parenchymal injuries may occur and include edema, ischemic strokes, and intra-cerebral hemorrhage (ICH; i.e., parenchymal/subdural hematomas, and subarachnoid hemorrhages), which are identified in 40%-60% of patients without clearly identified mechanisms. In this perspective, experimental animal models contribute to the understanding of initiation, propagation, and resolution of thrombosis, as well as brain-related damages. Last but not least, animal models may be useful to study new therapeutic approaches. In this review, we provide a comprehensive overview of CVST experimental models, focusing on their strengths, limits, and contribution to the current knowledge.

Targeting of a Conserved Epitope in Mouse and Human GPVI Differently Affects Receptor Function

Glycoprotein (GP) VI is the major platelet collagen receptor and a promising anti-thrombotic target. This was first demonstrated in mice using the rat monoclonal antibody JAQ1, which completely blocks the Collagen-Related Peptide (CRP)-binding site on mouse GPVI and efficiently inhibits mouse platelet adhesion, activation and aggregation on collagen. Here, we show for the first time that JAQ1 cross-reacts with human GPVI (huGPVI), but not with GPVI in other tested species, including rat, rabbit, guinea pig, swine, and dog. We further demonstrate that JAQ1 differently modulates mouse and human GPVI function. Similar to its effects on mouse GPVI (mGPVI), JAQ1 inhibits CRP-induced activation in human platelets, whereas, in stark contrast to mouse GPVI, it does not inhibit the adhesion, activation or aggregate formation of human platelets on collagen, but causes instead an increased response. This effect was also seen with platelets from newly generated human GPVI knockin mice (hGP6tg/tg). These results indicate that the binding of JAQ1 to a structurally conserved epitope in GPVI differently affects its function in human and mouse platelets.

Impaired microtubule dynamics contribute to microthrombocytopenia in RhoB-deficient mice

RhoB-deficient mice display microthrombocytopenia.

RhoB exerts nonredundant functions in the megakaryocyte lineage compared with RhoA and regulates microtubule dynamics.

Megakaryocytes are large cells in the bone marrow that give rise to blood platelets. Platelet biogenesis involves megakaryocyte maturation, the localization of the mature cells in close proximity to bone marrow sinusoids, and the formation of protrusions, which are elongated and shed within the circulation. Rho GTPases play important roles in platelet biogenesis and function. RhoA-deficient mice display macrothrombocytopenia and a striking mislocalization of megakaryocytes into bone marrow sinusoids and a specific defect in G-protein signaling in platelets. However, the role of the closely related protein RhoB in megakaryocytes or platelets remains unknown. In this study, we show that, in contrast to RhoA deficiency, genetic ablation of RhoB in mice results in microthrombocytopenia (decreased platelet count and size). RhoB-deficient platelets displayed mild functional defects predominantly upon induction of the collagen/glycoprotein VI pathway. Megakaryocyte maturation and localization within the bone marrow, as well as actin dynamics, were not affected in the absence of RhoB. However, in vitro–generated proplatelets revealed pronouncedly impaired microtubule organization. Furthermore, RhoB-deficient platelets and megakaryocytes displayed selective defects in microtubule dynamics/stability, correlating with reduced levels of acetylated α-tubulin. Our findings imply that the reduction of this tubulin posttranslational modification results in impaired microtubule dynamics, which might contribute to microthrombocytopenia in RhoB-deficient mice. Importantly, we demonstrate that RhoA and RhoB are localized differently and have selective, nonredundant functions in the megakaryocyte lineage.

Early recognition and treatment of pre-VITT syndrome after adenoviral vector-based SARS-CoV-2 vaccination may prevent from thrombotic complications: review of published cases and clinical pathway

Vaccine-induced immune thrombotic thrombocytopenia (VITT) is a rare but highly morbid complication after adenoviral vector-based SARS-CoV-2 vaccination. The pre-VITT syndrome is defined as vaccine-induced immune thrombocytopenia without thrombosis typically presenting with new-onset headache. This review aims to identify at-risk patients before complications such as cerebral venous sinus thrombosis occur. We review previously published reports of 19 patients (median age 35 years, range 23-74; 16 females) who met the diagnostic criteria for a pre-VITT syndrome. Seven patients progressed to VITT, 12 patients did not. Patients who experienced VITT received delayed treatment. The median interval between the onset of headache and VITT-treatment (i.e. anticoagulation, immune globulins, or corticosteroids) was 5 days (range 1-8 days) compared with 2 days (0-5 days) in those without subsequent VITT (P = 0.033). The interval from onset of headache to anticoagulation was longer in patients with VITT (median 7 vs. 2 days; range 3-9 vs. 0-7 days; P = 0.01). Anticoagulation was safe in all patients with a pre-VITT syndrome as no haemorrhagic complications occurred after anticoagulation was started despite low platelets. The transient decline of platelet count after admission was significantly more pronounced in patients who progressed to VITT (median 67 vs. 0 × 10³/µL; range 0-77 × 10³/µL vs. 0-10 × 10³/µL; P = 0.005). d-dimers did not differ between groups. Pre-VITT syndrome is a 'red flag' and allows to identify and preemptively treat patients at-risk of further progression to VITT. However, it must be distinguished from post-vaccination immune thrombocytopenia.