Background: Red blood cells (RBCs) are important constituents of venous clots and contribute to thrombus size and stability. However, it remains unclear whether and how RBCs affect the thrombus. Transglutaminase (TG)2), a protein with similar activity to factor XIIIa, may influence thrombus characteristics. Objectives: We explored whether RBC-derived TG2 influences thrombus characteristics using a novel approach involving TG2 knockout (KO) RBCs. Methods: Inhibitors and TG2 KO RBCs were used in clotting assays. In vitro–generated clots were analyzed using advanced microscopy techniques to quantify the fibrin network. Whole blood clotting kinetics were assessed by thrombin generation assay and thromboelastography. Vesiculation was assessed using microparticle flow cytometry. Results: Our study showed clots formed with TG2 KO RBCs or upon TG2 inhibition displayed fewer and thinner fibrin fibers at the clot surface. Inhibition of TG2 further revealed a redistribution of fibrin away from the clot surface toward deeper regions. Functionally, TG2 inhibition accelerated thrombin generation and clot formation, as shown by shortened lag time in thrombin generation assay and faster kinetics in thromboelastography. A normal fibrin density was observed in clots treated with the TG2 inhibitor after addition of RBC-derived extracellular vesicles (EVs). Moreover, TG2 inhibition increased EV formation, associated with more phosphatidylserine exposure on RBC membranes. Conclusion: TG2 activity within RBCs plays a specific role in modulating EV formation, which in turn influences the fibrin structure and spatial distribution within blood clots. Absence of TG2 activity leads to more EV formation, promoting faster thrombin generation and clot formation, suggesting a regulatory role for TG2 in coagulation kinetics.

Abstract – Introduction: Arterial thrombosis is a multifaceted process characterized by platelet aggregation and fibrin deposition, leading to the occlusion of blood vessels. It plays a central role in cardiovascular conditions such as myocardial infarction and ischemic stroke. Gaining insight into the mechanisms underlying arterial thrombosis is essential for developing effective treatments aimed at preventing thrombotic events and reducing associated health burdens. In vitro and ex vivo models serve as critical tools for investigating the pathophysiology of arterial thrombosis by providing controlled environments to study thrombus formation and characteristics. This systematic review provides a comprehensive overview of in vitro and ex vivo flow-based models used to study arterial thrombosis, classifying them by scale (macro vs. micro) and evaluating their design principles, physiological relevance, and experimental utility. Methods: A systematic search of Medline, Embase, and Web of Science was conducted using broad and specific terms related to arterial thrombosis models incorporating flow or shear stress. Articles were screened by two independent reviewers. Studies were included if they described in vitro or ex vivo models with dynamic flow; models limited to static or venous conditions or in vivo studies were excluded. In total, 82 studies met the inclusion criteria. Results: Macro-scale models can mimic complex flow patterns in larger arterial conditions and enable the formation of thrombi comparable in size to clinical specimens. Microfluidic models allow precise control over shear conditions and geometry with minimal blood volumes and are suitable for high-resolution imaging and customization, including endothelialization and patient-specific designs. While, both model types present limitations in replicating complex in vivo hemodynamics, standardization, and scalability, they offer valuable, controllable platforms for mechanistic studies and drug testing in arterial thrombosis. Conclusions: While no single model fully recapitulates the in vivo environment, ongoing innovations, particularly in microfabrication and model standardization, continue to improve physiological relevance and clinical translatability.

The single most common microbe causing cardiovascular infections is Staphylococcus aureus (S. aureus). S. aureus produces coagulase that converts fibrinogen to fibrin, which is incorporated into biofilms. This process aids in adherence to intravascular structures, defense against the host immune system, and resistance to antimicrobial treatment. Despite its significance, fibrin formation in S. aureus biofilms remains poorly understood. Therefore, this study aimed to elucidate the early development of cardiovascular biofilms. Clinically isolated coagulase-positive S. aureus and coagulase-negative Staphylococcus lugdunensis (S. lugdunensis) from patients with cardiovascular infections, and a coagulase mutant S. aureus Δcoa, were grown in tryptic soy broth (TSB), Iscove's Modified Dulbecco's Medium (IMDM), and pooled human plasma, with or without porcine heart valves. Bacterial growth, metabolic activity, and bacterial fibrinogen utilization were measured over 24 h at 37 °C. Time-lapse confocal microscopy was used to visualize and track biofilm development. S. aureus exhibited more growth in TSB and human plasma than S. lugdunensis and S. aureus Δcoa, but showed similar growth in IMDM after 24 h. Peak metabolic activity for all isolates was highest in TSB and lowest in human plasma. The presence of porcine valves caused strain-dependent alterations in time to peak metabolic activity. Confocal imaging revealed fibrin-based biofilm development exclusively in the coagulase-producing S. aureus strains. Between 2 and 6 h of biofilm development, 74.9 % (p = 0.034) of the fibrinogen from the medium was converted to fibrin. Variations in fibrin network porosity and density were observed among different coagulase-producing S. aureus strains. Fibrin formation is mediated by S. aureus coagulase and first strands occurred within 3 h for clinical strains after exposure to human plasma. This study stresses the importance of experimental design given the bacterial changes due to different media and substrates and provides insights into the early pathogenesis of S. aureus cardiovascular biofilms.

The fibrin network is one of the main components of thrombi. Altered fibrin network properties are known to influence the development and progression of thrombotic disorders, at least partly through effects on the mechanical stability of fibrin. Most studies investigating the role of fibrin in thrombus properties prepare clots under static conditions, missing the influence of blood flow which is present in vivo. In this study, plasma clots in the presence and absence of flow were prepared inside a Chandler loop. Recitrated plasma from healthy donors were spun at 0 and 30 RPM. The clot structure was characterized using scanning electron microscopy and confocal microscopy and correlated with the stiffness measured by unconfined compression testing. We quantified fibrin fiber density, pore size, and fiber thickness and bulk stiffness at low and high strain values. Clots formed under flow had thinner fibrin fibers, smaller pores, and a denser fibrin network with higher stiffness values compared to clots formed in absence of flow. Our findings indicate that fluid flow is an essential factor to consider when developing physiologically relevant in vitro thrombus models used in researching thrombectomy outcomes or risk of embolization. Graphical Abstract: [Figure not available: see fulltext.].

Thrombus computed tomography (CT) imaging characteristics may correspond with thrombus mechanical properties and thus predict thrombectomy success. The impact of red blood cell (RBC) content on these properties (imaging and mechanics) has been widely studied. However, the additional effect of platelets has not been considered. The objective of the current study was to examine the individual and combined effects of blood clot RBC and platelet content on resultant CT imaging and mechanical characteristics. Human blood clot analogues were prepared from a combination of preselected RBC volumes and platelet concentrations to decouple their contributions. The resulting clot RBC content (%) and platelet content (%) were determined using Martius Scarlet Blue and CD42b staining, respectively. Non-contrast and contrast-enhanced CT (NCCT and CECT) scans were performed to measure the clot densities. CECT density increase was taken as a proxy for clinical perviousness. Unconfined compressive mechanics were analysed by performing 10 cycles of 80% strain. RBC content is the major determinant of clot NCCT density. However, additional consideration of the platelet content improves the association. CECT density increase is influenced by clot platelet and not RBC content. Platelet content is the dominant component driving clot stiffness, especially at high strains. Both RBC and platelet content contribute to the clot’s viscoelastic and plastic compressive properties. The current in vitro results suggest that CT density is reflective of RBC content and subsequent clot viscoelasticity and plasticity, and that perviousness reflects the clot’s platelet content and subsequent stiffness. However, these indications should be confirmed in a clinical stroke cohort.

Fibrous networks are essential structural components of biological and engineered materials. Accordingly, many approaches have been developed to quantify their structural properties, which define their material properties. However, a comprehensive overview and comparison of methods is lacking. Therefore, we systematically searched for automated tools quantifying network characteristics in confocal, stimulated emission depletion (STED) or scanning electron microscopy (SEM) images and compared these tools by applying them to fibrin, a prototypical fibrous network in thrombi. Structural properties of fibrin such as fiber diameter and alignment are clinically relevant, since they influence the risk of thrombosis. Based on a systematic comparison of the automated tools with each other, manual measurements, and simulated networks, we provide guidance to choose appropriate tools for fibrous network quantification depending on imaging modality and structural parameter. These tools are often able to reliably measure relative changes in network characteristics, but absolute numbers should be interpreted with care. Statement of significance: Structural properties of fibrous networks define material properties of many biological and engineered materials. Many methods exist to automatically quantify structural properties, but an overview and comparison is lacking. In this work, we systematically searched for all publicly available automated analysis tools that can quantify structural properties of fibrous networks. Next, we compared them by applying them to microscopy images of fibrin networks. We also benchmarked the automated tools against manual measurements or synthetic images. As a result, we give advice on which automated analysis tools to use for specific structural properties. We anticipate that researchers from a large variety of fields, ranging from thrombosis and hemostasis to cancer research, and materials science, can benefit from our work.

Endovascular thrombectomy procedures are significantly influenced by the mechanical response of thrombi to the multi-axial loading imposed during retrieval. Compression tests are commonly used to determine compressive ex vivo thrombus and clot analogue stiffness. However, there is a shortage of data in tension. This study compares the tensile and compressive response of clot analogues made from the blood of healthy human donors in a range of compositions. Citrated whole blood was collected from six healthy human donors. Contracted and non-contracted fibrin clots, whole blood clots and clots reconstructed with a range of red blood cell (RBC) volumetric concentrations (5–80%) were prepared under static conditions. Both uniaxial tension and unconfined compression tests were performed using custom-built setups. Approximately linear nominal stress–strain profiles were found under tension, while strong strain-stiffening profiles were observed under compression. Low- and high-strain stiffness values were acquired by applying a linear fit to the initial and final 10% of the nominal stress–strain curves. Tensile stiffness values were approximately 15 times higher than low-strain compressive stiffness and 40 times lower than high-strain compressive stiffness values. Tensile stiffness decreased with an increasing RBC volume in the blood mixture. In contrast, high-strain compressive stiffness values increased from 0 to 10%, followed by a decrease from 20 to 80% RBC volumes. Furthermore, inter-donor differences were observed with up to 50% variation in the stiffness of whole blood clot analogues prepared in the same manner between healthy human donors.