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Endothelial cells form a monolayer, which lines blood vessels. They are crucially involved in maintaining blood fluidity and providing controlled vascular hemostasis at sites of injury. Thereby endothelial cells facilitate multiple mechanisms, including both procoagulant and anticoagulant, which must be kept in balance. Under physiological conditions, endothelial cells constitute a nonadhesive surface preventing activation of platelets and the coagulation cascade. Multiple fibrinolytic and antithrombotic properties act on their cell surface contributing to the maintenance of blood fluidity. These include platelet inhibition, the heparin-antithrombin III system, tissue factor pathway inhibition, thrombomodulin/protein C system, and fibrinolytic qualities. At sites of vascular damage, platelets react immediately by adhering to the exposed extracellular matrix, followed by platelet-platelet interactions to form a clot that effectively seals the injured vessel wall to prevent excessive blood loss. For solid thrombus formation, functional platelets are essential. In this process, endothelial cells serve as a support surface for formation of procoagulant complexes and clotting. This review gives an overview about the central role of the endothelium as a dynamic lining which controls the complex interplay of the coagulation system with the surrounding cells.

Veno-venous extracorporeal membrane oxygenation (vvECMO) has become a routine treatment for severe lung failure in specialized centers. Spontaneous bleeding complications, however, are observed in 30–60% of patients during vvECMO treatment. Bleeding increases mortality by factors 2–3. Anticoagulation in combination with several acquired bleeding disorders caused by the mechanical pump and the foreign layer of the extracorporeal system contribute to the risk of bleeding. In this review, the mechanisms of the underlying pathologies and the route from diagnosis to treatment are described.

Purpose Real-world data and study data regarding therapy with Emicizumab in pediatric cohorts with haemophilia A is scarce. Especially, data on previously untreated pediatric patients (PUPs) and minimally treated patients (MTPs) are missing. Methods Thirteen pediatric patients with haemophilia A and treatment with Emicizumab were retrospectively evaluated for Annual Bleeding Rates (ABR) pre-and post-Emicizumab treatment. Safety data and data on management of minor surgery as well as laboratory results were collected. Additionally, we describe the clinical features of two PUPs and one MTP that are included in our cohort. Results Median age at initiation of Emicizumab was 5.3 (range: 0.26–17.5) years, three patients were younger than one year at initiation of treatment with Emicizumab. Median follow-up time on Emicizumab was 23.8 (range: 0.7–40) months. Total ABR ( p  = 0.009) as well as spontaneous ( p  = 0.018), traumatic ( p  = 0.018), and joint ( p  = 0.027) ABR reduced significantly post-Emicizumab transition. Safety profile was favourable as only one local site reaction occurred; no cessation of treatment was necessary. Surgery was successfully performed in three patients receiving rFVlla pre- and post-surgery. Emicizumab trough levels showed a median of 43.2 μg/ml (range: 23.9–56.8) after three doses of 3 mg/kg and 51.9 μg/ml (range: 30.4–75) at first follow-up with 1.5 mg/kg. Conclusion Emicizumab is safe and efficient in pediatric patients with and without inhibitors. More data on larger multicenter cohorts and especially on PUPs/MTPs are still needed.

Septins are conserved cytoskeletal GTP-binding proteins identified in almost all eukaryotes except higher plants. Mammalian septins comprise 13 family members with either ubiquitous or organ- and tissue-specific expression patterns. They form filamentous oligomers and complexes with other proteins to serve as diffusions barrier and/or multi-molecular scaffolds to function in a physiologically regulated manner. Diverse septins are highly expressed in endothelial cells and platelets, which play an important role in hemostasis, a process to prevent blood loss after vascular injury. Endothelial septins are involved in cellular processes such as exocytosis and in processes concerning organismal level, like angiogenesis. Septins are additionally found in endothelial cell-cell junctions where their presence is required to maintain the integrity of the barrier function of vascular endothelial monolayers. In platelets, septins are important for activation, degranulation, adhesion, and aggregation. They have been identified as mediators of distinct platelet functions and being essential in primary and secondary hemostatic processes. Septin-knockout mouse studies show the relevance of septins in several aspects of hemostasis. This is in line with reports that dysregulation of septins is clinically relevant in human bleeding disorders. The precise function of septins in the biology of endothelial cells and platelets remains poorly understood. The following mini-review highlights the current knowledge about the role of septin cytoskeleton in regulating critical functions in these two cell types.

Complement activation is a relevant driver in the pathomechanisms of vasculitis. The involved proteins in the interaction between endothelia, complement, and platelets in these conditions are only partially understood. Thrombospondin-1 (TSP-1), found in platelet α-granules and released from activated endothelial cells, interacts with factor H (FH) and vWF. However, to our knowledge, direct regulatory interaction with the complement cascade has not yet been described. Our study shows that TSP-1 is a potent, FH-independent inhibitor of the alternative complement pathway. TSP-1 binds to complement proteins and inhibits cleavage of C3 and C5 and the formation of the membrane attack complex. We validated complement-regulatory function in blood samples from patients with primary complement defects. The physiological relevance of TSP-1 was demonstrated in patients with antineutrophil cytoplasmic antibody-associated vasculitis (AAV) by significantly enhanced TSP-1 staining in glomerular lesions and increased complement activity and NETosis after TSP-1 deficiency in an in vitro and in vivo model of AAV. The complement-inhibiting function of TSP-1 represents an important mechanism in the interaction of endothelia and complement. In particular, the interplay between released TSP-1 and the complement system locally, especially on surfaces, influences the balance between complement activation and inhibition and may be relevant in various vascular diseases.

During the last decade the use of ventricular assist devices (VADs) for patients with severe heart failure has increased tremendously. However, flow disturbances, mainly high shear induced by the device is associated with bleeding complications. Shear stress-induced changes in VWF conformation are associated with a loss of high molecular weight multimers (HMW) of VWF and an increased risk of bleeding. This phenomenon and its cause will be elaborated and reviewed in the following.

Primary blood coagulation and wound sealing are orchestrated by von Willebrand factor (VWF), a large multimeric glycoprotein. Upon release of arginine vasopressin (AVP), VWF containing high molecular weight multimers is secreted. By measuring copeptin, the C-terminal part of the AVP prohormone, we recently found strongly increased AVP levels in children with febrile seizures (FS) as compared to children with fever but without seizures. It is unknown if increased AVP levels in FS are of any biological function. Therefore, our a priori hypothesis was that children with FS have increased VWF parameters in parallel with higher AVP levels. We conducted a prospective, cross-sectional study of children aged between 6 months and 5 years. Children that presented at our emergency department with fever or a recent FS (within four hours) were evaluated to be included to the study. We measured serum copeptin and VWF parameters, including analyses of VWF:Antigen (WVF:Ag), VWF:collagen binding activity (VWF:CB) and VWF multimers in children with FS, febrile infections without seizures and additionally, in a non-febrile control group. We included 54 children in our study, 30 with FS, 10 in the febrile control group, and 14 in the non-febrile control group. Serum copeptin levels were significantly higher in children with FS (median [IQR] 24.73 pmol/l [13.65-68.65]) compared to the febrile control group (5.66 pmol/l [4.15-8.07], p = 0.002) and the non-febrile control group (4.78 pmol/l [3.33-5.3], p<0.001). VWF:CB levels were also significantly higher in children with FS (VWF:CB 2.29 U/ml [1.88-2.97]) as compared to the febrile (VWF:CB 1.41 U/ml [1.27-1.93], p = 0.048) and the non-febrile control group (VWF:CB 1.15 U/ml [0.98-1.21], p<0.001). VWF:Ag tended to be higher in children with FS compared to both control groups. Multivariate regression analysis revealed FS and copeptin as major determinants of VWF:CB. Our results suggest that increased secretion of AVP in children with FS is associated with higher plasma levels of VWF parameters. Especially VWF:CB may serve as additional biomarker in the diagnosis of FS.

Wiskott–Aldrich syndrome (WAS) is caused by mutations in the WAS gene and is characterized by immunodeficiency, eczema and microthrombocytopenia. The molecular link between WAS mutations and microthrombocytopenia is unknown. Profilin1 (Pfn1) is a key actin-regulating protein that, besides actin, interacts with phosphoinositides and multiple proline-rich proteins, including the WAS protein (WASp)/WASp-interacting protein (WIP) complex. Here we report that mice with a megakaryocyte/platelet-specific Pfn1 deficiency display microthrombocytopenia due to accelerated turnover of platelets and premature platelet release into the bone marrow. Both Pfn1-null mouse platelets and platelets isolated from WAS patients contained abnormally organized and hyperstable microtubules. These results reveal an unexpected function of Pfn1 as a regulator of microtubule organization and point to a previously unrecognized mechanism underlying the platelet formation defect in WAS patients. Patients with mutations in the gene encoding the cytoskeleton regulator WAS have platelet defects. Here the authors show that the WAS-binding protein, Profilin1, is essential for platelet formation in mice, and that its deficiency reproduces the bleeding disorder of patients with WAS mutations.

Hermansky-Pudlak syndrome (HPS), a rare heterogeneous autosomal recessive disorder, is characterized by oculocutaneous albinism (OCA) and a bleeding diathesis due to a defect regarding melanosomes and platelet delta (δ)-granule secretion. Interestingly, patients with HPS type 2 (HPS-2) or HPS type 10 (HPS-10) present additionally with an immunological defect. We investigated three patients (IP1, IP2, and IP3) who suffer from a bleeding diathesis. Platelet aggregometry showed impaired platelet function and flow cytometry revealed a severely reduced platelet CD63 expression hinting to either a defect of platelet delta granule secretion or a decreased number of delta granules in these patients. However, only IP3 presents with an apparent OCA. We performed panel sequencing and identified a homozygous deletion of exon 6 in DTNBP1 for IP3 . Western analysis confirmed the absence of the encoded protein dysbindin confirming the diagnosis of HPS-7. Interestingly, this patient reported additionally recurrent bacterial infections. Analysis of lymphocyte cytotoxicity showed a slightly reduced NK-degranulation previously documented in a more severe form in patients with HPS-2 or HPS-10. IP1 is carrier of two compound heterozygous variants in the HPS3 gene (c.65C > G and c.1193G > A). A homozygous variant in HPS5 (c.760G > T) was identified in IP2. The novel missense variants were classified as VUS (variant of uncertain significance) according to ACMG guidelines. For IP1 with the compound heterozygous variants in HPS3 a specialized ophthalmological examination showed ocular albinism. HPS3 and HPS5 encode subunits of the BLOC-2 complex and patients with HPS-3 or HPS-5 are known to present with variable/mild hypopigmentation.

Hemophilia A (HA) and hemophilia B (HB) are X-linked-bleeding disorders caused by deficiency of clotting factors VIII and IX, while von Willebrand disease (vWD) type 3 involves the lack of von Willebrand factor and FVIII. Chronic joint damage from recurrent bleeding is a serious complication. The aim was to investigate the association of prophylactical treatment, severity of the disease and joint outcome. In this retrospective, single-center study we evaluated joint health in 41 patients with HA, HB, and vWD type 3 who visited our outpatient clinic since 2000 using Magnetic resonance imaging (MRI) and applied the International Prophylaxis Study Group (IPSG) score. A total of 246 MRI images (knees, elbows, ankles) were analyzed in relation to disease severity, genetics, inhibitor-formation, and therapy. Of 41 patients, 28 (68%) had severe HA or HB, 10 (24%) moderate, one (2%) mild, and two (5%) were vWD patients. 19 patients with severe HA/HB received primary prophylaxis. Inhibitors developed in 7 patients (17%), most of them had loss-of-function mutations. We observed hemophilic arthropathy in 7/39 (18%) hemophilia patients (all with severe HA/HB). Only one of the 19 patients receiving early prophylaxis developed arthropathy, in the context of inhibitor development. Minor changes (IPSG score 1-5) were observed in 20% of joints while 74% of joints showed no alterations (IPSG score 0). Only 6% of joints showed hemophilic arthropathy (IPSG score ≥ 8) with ankle joints most frequently affected (10%). Among vWD-patients, one exhibited minor changes; the other had no detectable joint damage despite vWF-inhibitor presence. This study shows that the IPSG score is a suitable tool for assessing joint health in patients with hemophilia and vWD. Reduced joint damage was associated with early diagnosis, consistent prophylaxis, and therapy adherence.