Illustrated State-of-the-Art Capsules of the ISTH 2022 Congress

Ejaife O. Agbani BPharm, MSc, PhD

Josefin Ahnström PhD

The initiation phase of coagulation is regulated by tissue factor pathway inhibitor (TFPI), which efficiently reduces/delays thrombin generation. In contrast to the endothelium-bound TFPIβ, which in itself is an effective regulator of coagulation, the soluble form, TFPIα, is completely dependent on cofactors (compare the top and bottom figures for the absence and presence of cofactors, respectively).¹ Protein S enhances the anticoagulant properties of TFPIα through a direct protein-protein interaction, involving specific amino acid residues in TFPIα Kunitz 3 and protein S laminin G-type 1.^{1, 2} This interaction enables TFPIα to interact with and inhibit membrane-bound factor Xa more efficiently. A splice variant of factor V (FV), FV-short, regulates TFPIα levels through a high-affinity interaction, likely resulting in an increased half-life, as well as functioning as a synergistic TFPIα cofactor, together with protein S.^{1, 3}

Robert Ahrends PhD

For references, see Peng et al.^{4, 5}; Holinstat⁶

Raza Alikhan MD, FRCP, FRCPath

Alice Assinger PhD

Zsuzsa Bagoly MD, PhD

Currently, there are two proven reperfusion strategies for the opening of the occluded vessel in patients with acute ischemic stroke (AIS): intravenous thrombolysis using recombinant tissue-type plasminogen activator (rt-PA; alteplase) or tenecteplase, and mechanical thrombectomy.⁷ Both therapies must be delivered within a rapid time frame in selected patients; moreover, mechanical thrombectomy is eligible in only a fraction of patients with large-artery occlusion. Despite the unquestionable effectiveness of rt-PA as first-line treatment of AIS, successful reperfusion is achieved in only ≈30% to 40% of patients, while ≈6% to 8% of patients develop intracranial hemorrhage as a side effect. As of today, outcomes cannot be foreseen at the initiation of therapy, and this remains one of the greatest challenges of AIS treatment.⁸ Due to the short time frame before treatment, few studies are available on hemostasis tests, and several methodological issues are raised.^{8, 9} Nevertheless, some assays show promising results and need to be further investigated and validated in large populations.

Alessandra Balduini MD

Circulating platelets are specialized cells produced by megakaryocytes through the formation of proplatelets. Leading studies point to the bone marrow niche as the core of hematopoietic stem cell (HSC) differentiation, revealing interesting and complex environmental factors for consideration. Megakaryocytes take cues from the physicochemical bone marrow microenvironment, which includes contact with extracellular matrix components, interaction with endothelium, and contact with the turbulent flow generated by the blood circulation into the sinusoid lumen.¹⁰ Germinal and acquired mutation in HSCs may manifest in altered megakaryocyte maturation, proliferation, and platelet production. Alterations of the whole hematopoietic niche may also occur, highlighting the central role of megakaryocytes in the control of the physiological bone marrow homeostasis. Tissue-engineering approaches have been developed to create a functional mimic of the native tissue.^{11, 12} Reproducing the thrombopoietic environment is instrumental to gaining new insight into its activity and answering the growing demand for human platelets for fundamental studies and clinical applications in transfusion medicine. (ECM, extracellular matrix; MK, megakaryocyte; iPSC, induced pluripotent stem cell; MSC, mesenchymal stem cell; PPF, proplatelet formation; PLT, platelet; TPO, thrombopoietin; TGFβ, transforming growth factor-β).

The figure was created with BioRender.com

Elena Barbon PhD

Adeno-associated viral vectors have emerged as one of the most promising gene delivery platforms for in vivo liver-directed gene therapy for the treatment of blood coagulation disorders, particularly hemophilia A (HA) and B (HB). The ongoing clinical trials are based on adeno-associated virus (AAV)-mediated hepatocyte-targeted expression of coagulation factor VIII or IX for HA and HB, respectively.¹³ Nevertheless, there is increasing interest in the possibility of improving the targeting of different cell types. To this aim, AAV capsid engineering represents an attractive strategy to enhance cell-specific transduction.^{14, 15} This may represent a promising approach in coagulation diseases such as HA or von Willebrand disease, where expressing the therapeutic protein from its natural biosynthetic site, that is, endothelial cells, could be beneficial to guarantee an optimized protein biosynthesis, secretion, and activity.

Christopher D. Barrett MD

For references, see Barrett et al.^{16, 17}; ATTACC Investigators et al.¹⁸

Paul Batty MBBS, PhD

Jorge David Aivazoglou Carneiro MD, PhD

For references, see Yuki et al.¹⁹; Gustine and Jones²⁰; Diamond and Kanneganti²¹

Wee Shian Chan MD, MSc, FRCP

For references, see Konstantinides et al.²²; van der Pol et al.²³; Righini et al.²⁴

Moniek de Maat PhD

Kerstin de Wit MBChB, MD, MSc

Patients with acute pulmonary embolism (PE) can be classified as having low-risk PE (75% of cases), intermediate-risk PE (20% of cases), or high-risk PE (5% of cases). Low-risk patients can be assessed for outpatient treatment with rapid follow-up. Intermediate-risk patients are admitted for monitoring, and high-risk patients require immediate reperfusion therapy.²²

PESI, pulmonary embolism severity index; VTE, venous thromboembolism.

Cécile Denis PhD

FVIII, factor VIII; VWD, von Willebrand disease; VWF, von Willebrand factor.

Martin H. Ellis MD

Polycythemia vera, essential thrombocythemia, and primary myelofibrosis (termed myeloproliferative neoplasms [MPNs]) are clonal diseases that may result in fatal end-stage bone marrow fibrosis or acute leukemia. During the long natural history of these diseases, thrombosis is an important complication.

The median age at diagnosis of the MPNs is >60 years; however, 20% of patients are <40 years old when diagnosed, making MPNs in pregnancy a relevant clinical issue. Indeed, the estimated incidence of MPNs among pregnant women in the United Kingdom is 3.2 in 100 000 pregnancies per year.²⁵

The risk of maternal (venous or arterial thrombosis or hemorrhage), or placenta-related (fetal loss or preeclampsia/eclampsia) complications is higher in pregnancies in patients with MPNs than in the general population. Meta-analysis data demonstrate that 28.7% pregnancies are lost among these patients and that an additional 9.6% of women experience an adverse outcome such as thrombosis, preeclampsia, eclampsia, placental abruption, or intrauterine growth retardation.²⁶

Treatment is based on observational data and expert opinion and includes aspirin, low-molecular-weight heparin, and interferon-α. Prospective studies are needed to better define appropriate treatment for these patients.

Renee Eslick BMed, FRACP, FRCPA

IVIg, intravenous immunoglobulin; ITP, immune thrombocytopenia; PET, preeclampsia.

For reference, see Eslick et al.²⁷

Hongxia Fu PhD

Blood protein von Willebrand factor (VWF; green) is essential in thrombosis and hemostasis. Force induced by blood flow is an important regulator to mediate VWF hemostatic function. Dynamic single-molecule imaging in a microfluidic system reveals that a two-step conformational transition induces VWF activation mechanism.^{28, 29} First, VWF elongates from compact to linear form. Second, a tension above 21 pN (dark green shading) induces VWF transition to a state with high affinity for platelet glycoprotein Ibα (GPIbα; magenta). For clarity, platelets are not shown. GPIbα dissociates rapidly when tension is under 21 pN. This mechanism allows VWF to be activated by hydrodynamic force at sites of hemorrhage but avoid thrombus formation downstream. (Single-molecule fluorescence images were reproduced from Fu et al.,²⁸ Nature Communications, 2017 with modifications and shared via a Creative Commons 4.0 license.)

Catherine P. M. Hayward MD, PhD

Benoit Ho-Tin-Noé PhD

The introduction of mechanical thrombectomy has considerably increased the rate of successful recanalization in large-vessel occlusion stroke. From a basic research standpoint, it has enabled the analysis of stroke thrombi and a better understanding of the mechanisms underlying resistance to intravenous thrombolysis. Therefore, although the efficacy of mechanical thrombectomy could have caused the end of thrombolysis, it has instead opened the way for its rethinking and improvement, through the development of innovative thrombolytic strategies.

Frederikus A. Klok MD, PhD

Riten Kumar MD, MSc

ACR, American College of Rheumatology; ACTION, Advanced Cardiac Therapies Improving Outcomes Network; anti-FXa, anti–factor Xa; CAA, coronary artery aneurysm; CVL, central venous line; DOAC, direct oral anticoagulant; DTI, direct thrombin inhibitor; ECG, electrocardiogram; LMWH, low-molecular-weight heparin; LV, left ventricular; MA, maximum amplitude; TEG, thromboelastogram; UFH, unfractionated heparin; ULN, upper limit of normal; VKA, vitamin K antagonist; VTE, venous thromboembolism

For references, see Goldenberg et al.³⁰; Henderson et al.³¹; Bansal et al.³²

Karin Leiderman PhD

Rustem I. Litvinov MD, PhD

Contraction (retraction) of blood clots and thrombi is clinically important since it can affect their obstructiveness, permeability, susceptibility to fibrinolysis, and the propensity to rupture (embologenicity). The rate and extent of clot contraction in vitro and in vivo can be enhanced or inhibited by altered platelet functionality as well as pathological cellular and molecular composition of the blood.³³ Clot contraction is reduced in the blood of patients with various (pro)thrombotic conditions, due to continuous platelet activation followed by their exhaustion and reduced contractility.³⁴ Impaired clot contraction is a significant but understudied and underappreciated pathogenic factor that can influence the course and outcomes of thrombotic disorders.³⁵

Nigel Mackman PhD

The majority of studies on the mechanisms of cancer-associated thrombosis (CAT) have focused on pancreatic cancer and brain cancer because they have the highest rates of venous thromboembolism (VTE). There are several pathways that contribute to venous thrombosis in pancreatic and brain cancer. Human studies have identified circulating biomarkers that are associated with VTE. Mouse cancer models are used to directly examine the role of a pathway in venous thrombosis. Taken together, these studies suggest that tissue factor–positive extracellular vesicles (EVs) directly and indirectly (via activation of platelets) enhance venous thrombosis in both pancreatic and brain cancer.^{36, 37} Neutrophils form neutrophil extracellular traps that can enhance venous thrombosis in pancreatic cancer.³⁶ Plasminogen activator inhibitor-1 contributes to venous thrombosis in pancreatic cancer by inhibiting fibrinolysis.³⁸ In brain cancer, podoplanin-positive EVs activate platelets and enhance venous thrombosis.³⁷ Targeting these pathways may reduce CAT.

Image created with BioRender.com

Zoe McQuilten MBBS, PhD

Matthew D. Neal MD

Following trauma and hemorrhage, the systemic signals of hypoxia, danger-associated molecular patterns and cytokines produce endothelial injury in addition to direct endothelial disruption.³⁹ The platelet is the sentinel responder to injury, orchestrating the complex processes of hemostasis and coagulation through interaction with damaged endothelium.⁴⁰ This State of the Art will review the adaptive and maladaptive responses of platelets and endothelium to bleeding with a focus on key mediators including von Willebrand factor, the metalloprotease ADAMTS-13,⁴¹ neutrophil extracellular trap release, and the role of endothelial and platelet extracellular vesicles in cell signaling.

William A. E. Parker MB, PhD, MRCP

Drugs for control of modifiable risk factors in the prevention of stroke. Modified with permission from Parker et al. 2020 under Creative Commons CC-BY-NC license.⁴² CKD, chronic kidney disease; DAPT, dual antiplatelet therapy; DATT, dual antithrombotic therapy; GLP-1, glucagonlike peptide 1; HbA_1c, glycated hemoglobin; LDL-C, low-density lipoprotein cholesterol; LEAD, lower-extremity arterial disease; NOAC, non–vitamin-K antagonist oral anticoagulant; OAC, oral anticoagulant; PCSK9, proprotein convertase subtilisin/kexin type 9; RASi, renin-angiotensinsystem inhibitor; SAPT, single antiplatelet therapy; T2DM, type 2 diabetes mellitus; VKA, vitamin K antagonist.

Roger J. S. Preston PhD

Most coagulation factor cell signaling is mediated by protease-activated receptors (PARs), a unique family of G-protein–coupled receptors whose activation is triggered by receptor proteolysis. Cell signaling output arising from PAR activation is shaped by both the activating protease and the specific cellular contexts in which receptor activation has occurred. Although not yet fully understood, intra- and extracellular regulatory determinants of PAR signaling diversity include protease-specific cleavage sites, association with protease coreceptors, receptor oligomerization, and recruitment of distinct signaling intermediates. These mechanisms combine to enable coagulation proteases to initiate a spectrum of downstream signaling pathways in multiple cell types.

Julie Rayes PhD

• S100A8/A9 is a damage-associated molecular pattern with prothrombotic and proinflammatory properties.
• S100A8/A9 levels are increased in the plasma of patients with COVID-19.
• Deposition of S100A8/A9 on the vessel wall in patients with COVID-19 correlates with thrombotic complications.
• S100A8/A9 induces the formation of procoagulant platelets and amplifies fibrin generation.
• Glycoprotein Ibα is the main receptor for S100A8/A9 on platelets, with a supporting role for CD36.

Alireza R. Rezaie PhD

Lara N. Roberts MBBS, MD Res

Created with BioRender.com

VTE, venous thromboembolism.

For references, see Roberts et al.⁴³; NHS Digital⁴⁴

Bianca Rocca MD, PhD

Variability in the response to fixed-dose antiplatelet drugs can be increased by primary (essential thrombocythemia) or secondary (reactive inflammatory states) pathological conditions associated with increased platelet turnover and count, that is, pharmacodynamic (PD) variability, as well as by high-degree obesity, that is, pharmacokinetic (PK) variability.

Understanding PK- and/or PD-related mechanisms of variability is central to design personalized regimens and to optimize the risk/benefit balance of common antiplatelet drugs by increasing the daily frequency or the once-daily dosing.

Moreover, in silico PK/PD modeling and in silico trials can be instrumental in designing clinical trials, developing antithrombotic therapy, and generating new personalized drug regimens.

Finally, in silico PK/PD modelling can be essential in rare diseases and “special” populations such as extreme ages (elderly and children), extreme body sizes (underweight and high-degree obesity), severe comorbidities (severe kidney and liver dysfunction), which are either underrepresented or excluded from cardiovascular randomized trials.

A, acetylsalicylic acid; MK, megakaryocyte; od, once daily; P, placebo; TXB₂, thromboxane B₂.

For references, see Tosetto et al.⁴⁵; Rocca et al.⁴⁶; Petrucci et al.⁴⁷

Susan Shapiro MD, PhD

Deborah M. Siegal MD, MSc, FRCPC

For reference, see Xu and Siegal⁴⁸

Lirlândia P. Sousa PhD

The plasminogen/plasmin (Plg/Pla) system is associated with a variety of biological activities beyond the classical dissolution of fibrin clots, including cell migration, tissue repair, and inflammation. Inflammation is an evolutionarily conserved response that guarantees the maintenance of tissue homeostasis through resolution—an active process mediated by molecules named proresolving mediators.⁴⁹ Despite the classical view of the Plg/Pla system on inflammation, emerging studies from our group^{50, 51} and others have revealed its anti-inflammatory and proresolving actions that includes reduction of proinflammatory mediators (1) enhanced neutrophil apoptosis (2), and efferocytosis (3); promotion of nonphlogistic recruitment of mononuclear cells (4); induction of macrophage reprogramming toward resolving phenotypes (5); and induction of expression of the proresolving mediator annexin A1 (6). We have now identified a novel proresolving feature of Plg/Pla during sepsis that is the regulation of neutrophil extracellular trap release (7). Red question marks indicate specific processes that Pla-protease activity need to be covered.

Katsue Suzuki-Inoue MD, PhD

Tahira Zafar MB, DCP, FRCPath

For references, see Ali⁵²; Butt⁵³; UN.⁵⁴

Jiaxi Zhou PhD

EC, endothelial cell; HPC, hematopoietic progenitor cell; HSC, hematopoietic stem cell; MK, megakaryocyte; NEU, neutrophil; PLT, platelet.

For references, see Wang et al.⁵⁵; Liu et al.^{56, 57}