Evidence for direct transfer of tissue factor from monocytes to platelets in whole blood

Cardiovascular Disease and Thrombosis in HIV Infection

HIV infection has transitioned from an acute, fatal disease to a chronic one managed by antiretroviral therapy. Thus, the aging population of people living with HIV (PLWH) continues to expand. HIV infection results in a dysregulated immune system, wherein CD4+ T cells are depleted, particularly in the gastrointestinal tract, disrupting the gut epithelial barrier. Long-term HIV infection is associated with chronic inflammation through potentially direct mechanisms caused by viral replication or exposure to viral proteins and indirect mechanisms resulting from increased translocation of microbial products from the intestine or exposure to antiretroviral therapy. Chronic inflammation (as marked by IL [interleukin]-6 and CRP [C-reactive protein]) in PLWH promotes endothelial cell dysfunction and atherosclerosis. PLWH show significantly increased rates of cardiovascular disease, such as myocardial infarction (risk ratio, 1.79 [95% CI, 1.54-2.08]) and stroke (risk ratio, 2.56 [95% CI, 1.43-4.61]). In addition, PLWH have increased levels of the coagulation biomarker D-dimer and have a two to ten-fold increased risk of venous thromboembolism compared with the general population. Several small clinical trials analyzed the effect of different antithrombotic agents on platelet activation, coagulation, inflammation, and immune cell activation. Although some markers for coagulation were reduced, most agents failed to reduce inflammatory markers in PLWH. More studies are needed to understand the underlying mechanisms driving inflammation in PLWH to create better therapies for lowering chronic inflammation in PLWH. Such therapies can potentially reduce atherosclerosis, cardiovascular disease, and thrombosis rates in PLWH and thus overall mortality in this population.

Interaction of von Willebrand factor with blood cells in flow models: a systematic review

The presence of blood flow influences the interaction between von Willebrand factor (VWF) and blood cells, affecting characteristics of forming blood clots. The interactions between coagulation and inflammation have mainly been studied in thrombosis models, but it remains unclear whether these interactions might also play a role in reduced bleeding in patients with bleeding disorders. In this systematic review, we provide an overview of the literature investigating the interactions between VWF and blood cells in flow models. For article selection, a systematic search was performed in Embase, Medline-Ovid, Cochrane Library, Web of Science databases, and Google Scholar. After selection, 24 articles were included. These articles describe direct or platelet-dependent interactions between VWF and neutrophils, monocytes, erythrocytes, or lymphocytes under different flow conditions. Almost all the described interactions required the presence of activated platelets. Only erythrocytes, monocytes, and natural killer cells were capable of directly binding the VWF multimers. Overall, interactions between VWF and blood cells mainly occurred in the presence of platelets. Because of the large variation in study design and used flow rates, further research is necessary to compare the results between studies and draw firm conclusions on when and under what conditions these interactions can occur. After our findings, many questions remained unanswered. This review might provide a starting point for future research. Extended knowledge on the influence of blood flow on VWF and blood cell interactions can contribute to improved understanding of the variation in bleeding in patients with bleeding disorders.

Mechanisms and biomarkers of cancer-associated thrombosis

Cancer-associated thrombosis is a leading cause of non-cancer death in cancer patients and is comprised of both arterial and venous thromboembolism (VTE). There are multiple risk factors for developing VTE, including cancer type, stage, treatment, and other medical comorbidities, which suggests that the etiology of thrombosis is multifactorial. While cancer-associated thrombosis can be treated with anticoagulation, benefits of therapy must be balanced with the increased bleeding risks seen in patients with cancer. Although risk models exist for primary and recurrent VTE, additional predictors are needed to improve model performance and discrimination of high-risk patients. This review will outline the diverse mechanisms driving thrombosis in cancer patients, as well as provide an overview of biomarkers studied in thrombosis risk and important considerations when selecting candidate biomarkers.

Diurnal variations in tissue factor and tissue factor pathway inhibitor concentrations in relation to on-treatment platelet reactivity: an analysis of patients with acute myocardial infarction

The aim of the study was to evaluate diurnal changes of tissue factor (TF) and tissue factor pathway inhibitor (TFPI) concentrations in relation to on-treatment platelet reactivity. The study group included 51 patients with acute myocardial infarction (AMI) treated with primary percutaneous coronary intervention and dual antiplatelet therapy. TF and TFPI concentrations were assessed using enzyme-linked immunosorbent assay kits. We found a significant increase of TF concentration in clopidogrel-resistant, but not clopidogrel-sensitive, patients at 10.00 a.m. (410.66 pg/mL) in comparison with 6.00 a.m. (250.99 pg/mL), 14.00 p.m. (255.12 pg/mL) and 19.00 p.m. (267.58 pg/mL). Moreover, TF concentration at 10.00 a.m. was 30% higher in clopidogrel-resistant than clopidogrel-sensitive patients (p = .043). We failed to demonstrate diurnal variation in TFPI concentration in clopidogrel-resistant patients. However, TFPI concentration in clopidogrel-sensitive patients was significantly higher at 10.00 a.m. as compared with other sampling points (p < .05). We observed a marked elevation in TF concentration at 10.00 a.m. only in aspirin-resistant patients and a significant increase in TFPI concentration at 10 a.m. only in aspirin-sensitive patients. Our findings suggest the presence of diurnal variations in TF and TFPI concentrations in AMI patients, with the highest thrombotic risk in patients with high on-treatment platelet reactivity in the midmorning.

Tissue Factor: An Essential Mediator of Hemostasis and Trigger of Thrombosis

Tissue factor (TF) is the high-affinity receptor and cofactor for factor (F)VII/VIIa. The TF-FVIIa complex is the primary initiator of blood coagulation and plays an essential role in hemostasis. TF is expressed on perivascular cells and epithelial cells at organ and body surfaces where it forms a hemostatic barrier. TF also provides additional hemostatic protection to vital organs, such as the brain, lung, and heart. Under pathological conditions, TF can trigger both arterial and venous thrombosis. For instance, atherosclerotic plaques contain high levels of TF on macrophage foam cells and microvesicles that drives thrombus formation after plaque rupture. In sepsis, inducible TF expression on monocytes leads to disseminated intravascular coagulation. In cancer patients, tumors release TF-positive microvesicles into the circulation that may contribute to venous thrombosis. TF also has nonhemostatic roles. For instance, TF-dependent activation of the coagulation cascade generates coagulation proteases, such as FVIIa, FXa, and thrombin, which induce signaling in a variety of cells by cleavage of protease-activated receptors. This review will focus on the roles of TF in protective hemostasis and pathological thrombosis.

Increased number of tissue factor protein expressing thrombocytes in canine idiopathic immune mediated hemolytic anemia

Dogs suffering from canine idiopathic immune mediated hemolytic anemia (cIIMHA) are at great risk of dying particularly in the first two weeks after the diagnosis is made. This high mortality risk may be associated with the development of thromboembolism (TE) and/or disseminated intravascular coagulation (DIC) resulting in organ failure. The exact mechanism of the development of TE and/or DIC in cIIMHA is still undetermined. Therefore, this study investigates the presence of tissue factor (TF) in thrombocytes of dogs suffering from cIIMHA, using OptiPrep™ for the isolation of blood cells and immunocytochemistry (ICC) to visualize TF on thrombocytes. The normalised TF quantity, acquired with 'colour deconvolution' (ImageJ plug in), revealed that in cIIMHA dogs the fraction TF positive thrombocytes was statistically significant higher (P < 0.001; mean 0.79; n = 7) compared to the fraction TF positive thrombocytes of the healthy dogs (mean 0.43; n = 9). We further have indications that the fraction of TF positive thrombocytes decreases with time and therapy, but that the progression rate differs individually. Since cIIMHA dogs have more thrombocytes that are TF-positive compared to healthy dogs, this may explain the increased risk to develop TE and DIC. Furthermore, it seems that the number of TF-positive thrombocytes in cIIMHA dogs remains high during the first two weeks of the disease, the time when the animals are at greatest health risk.

Different Potential of Extracellular Vesicles to Support Thrombin Generation: Contributions of Phosphatidylserine, Tissue Factor, and Cellular Origin

Cells release diverse types of vesicles constitutively or in response to proliferation, injury, inflammation, or stress. Extracellular vesicles (EVs) are crucial in intercellular communication, and there is emerging evidence for their roles in inflammation, cancer, and thrombosis. We investigated the thrombogenicity of platelet-derived EVs, which constitute the majority of circulating EVs in human blood, and assessed the contributions of phosphatidylserine and tissue factor exposure on thrombin generation. Addition of platelet EVs to vesicle-free human plasma induced thrombin generation in a dose-dependent manner, which was efficiently inhibited by annexin V, but not by anti-tissue factor antibodies, indicating that it was primarily due to the exposure of phosphatidylserine on platelet EVs. Platelet EVs exhibited higher thrombogenicity than EVs from unstimulated monocytic THP-1 cells, but blockade of contact activation significantly reduced thrombin generation by platelet EVs. Stimulation of monocytic cells with lipopolysaccharide enhanced their thrombogenicity both in the presence and in the absence of contact activation, and thrombin generation was efficiently blocked by anti-tissue factor antibodies. Our study provides evidence that irrespective of their cellular origin, EVs support the propagation of coagulation via the exposure of phosphatidylserine, while the expression of functional tissue factor on EVs appears to be limited to pathological conditions.

Synergies of phosphatidylserine and protein disulfide isomerase in tissue factor activation

Tissue factor (TF), the cellular receptor and cofactor for factor VII/VIIa, initiates haemostasis and thrombosis. Initial tissue distribution studies suggested that TF was sequestered from the circulation and only present at perivascular sites. However, there is now clear evidence that TF also exists as a blood-borne form with critical contributions not only to arterial thrombosis following plaque rupture and to venous thrombosis following endothelial perturbation, but also to various other clotting abnormalities associated with trauma, infection, or cancer. Because thrombin generation, fibrin deposition, and platelet aggregation in the contexts of haemostasis, thrombosis, and pathogen defence frequently occur without TF de novo synthesis, considerable efforts are still directed to understanding the molecular events underlying the conversion of predominantly non-coagulant or cryptic TF on the surface of haematopoietic cells to a highly procoagulant molecule following cellular injury or stimulation. This article will review some of the still controversial mechanisms implicated in cellular TF activation or decryption with particular focus on the coordinated effects of outer leaflet phosphatidylserine exposure and thiol-disulfide exchange pathways involving protein disulfide isomerase (PDI). In this regard, our recent findings of ATP-triggered stimulation of the purinergic P2X7 receptor on myeloid and smooth muscle cells resulting in potent TF activation and shedding of procoagulant microparticles as well as of rapid monocyte TF decryption following antithymocyte globulin-dependent membrane complement fixation have delineated specific PDI-dependent pathways of cellular TF activation and thus illustrated additional and novel links in the coupling of inflammation and coagulation.

Tumor-derived tissue factor-positive microparticles and venous thrombosis in cancer patients

Patients with cancer have an increased risk for venous thrombosis. Interestingly, different cancer types have different rates of thrombosis, with pancreatic cancer having one of the highest rates. However, the mechanisms responsible for the increase in venous thrombosis in patients with cancer are not understood. Tissue factor (TF) is a transmembrane receptor and primary initiator of blood coagulation. Tumor cells express TF and spontaneously release TF-positive microparticles (MPs) into the blood. MPs are small membrane vesicles that are highly procoagulant. It has been proposed that these circulating tumor-derived, TF-positive MPs may explain the increased rates of venous thrombosis seen in patients with cancer. In animal models, increased levels of tumor-derived, TF-positive MPs are associated with activation of coagulation. Moreover, these MPs bind to sites of vascular injury and enhance thrombosis. We and others have found that patients with cancer have elevated levels of circulating TF-positive MPs. These MPs are derived from tumors because they express tumor markers and are decreased by tumor resection. Importantly, several studies have shown that increased levels of TF-positive MPs correlate with venous thrombosis in patients with cancer. Taken together, these results suggest that TF-positive MPs may be a useful biomarker to identify patients with cancer who are at high risk for thrombosis.

Tissue factor structure and function

Tissue factor (TF) is an integral membrane protein that is essential to life. It is a component of the factor VIIa-TF complex enzyme and plays a primary role in both normal hemostasis and thrombosis. With a vascular injury, TF becomes exposed to blood and binds plasma factor VIIa, and the resulting complex initiates a series of enzymatic reactions leading to clot formation and vascular sealing. Many cells, both healthy, and tumor cells, produce detectable amounts of TF, especially when they are stimulated by various agents. Despite the relative simplicity and small size of TF, there are numerous contradictory reports about the synthesis and presentation of TF on blood cells and circulation in normal blood either on microparticles or as a soluble protein. Another subject of controversy is related to the structure/function of TF. It has been almost commonly accepted that cell-surface-associated TF has low (if any) activity, that is, is "encrypted" and requires specific conditions/reagents to become active, that is, "decrypted." However there is a lack of agreement related to the mechanism and processes leading to alterations in TF function. In this paper TF structure, presentation, and function, and controversies concerning these features are discussed.