Impaired Contraction (Retraction) of Blood Clots Is a Possible Pathogenic Mechanism in Venous Thromboembolism

Introduction: Deep venous thrombosis of the lower limbs, alone or associated with pulmonary embolism, complicates many diseases and remains as one of the most challenging medical and social problems due to a relatively high incidence and life-threatening risk.Blood clots undergo volume shrinkage induced by the contractile forces of activated platelets and propagated through the clot volume due to platelet-fibrin interactions and elasticity of the fibrin network. This process is known as clot contraction (retraction) which remains one of the least studied steps of blood clotting. Moreover, this phenomenon of clot contraction has been shown to occur not only in vitro but also in in vivo, but its (patho)physiological importance remains unclear.

Aims: This work was designed to examine the potential pathogenic role of clot contraction in venous thromboembolism (VTE).

Methods: We investigated contraction of clots formed from the blood of 55 patients with VTE compared with those of 60 healthy donors. Patients with suspected pulmonary embolism underwent a CT scan of the chest with contrast, which was performed in 32 (58%) patients, of which 23 (71%) were diagnosed with pulmonary embolism. VTE patients were excluded from this study if they were given anticoagulants, thrombolytics or antiplatelet drugs at least 7 days prior to examination. Determination of the kinetics and the extent of clot contraction was carried out using an original method based on the optical detection of clot size over time with the Thrombodynamics Analyzer System (HemaCore, Russia). Functionality of isolated platelets from 11 patients with VTE and 11 control samples of healthy donors was assessed by expression of P-selectin (CD62p) and active integrin αIIbβ3 (determined by its fibrinogen-binding capacity) before and after activation with thrombin-receptor activation peptide (TRAP). Isolated platelets from 4 healthy donors and 4 patients with VTE were prepared for scanning electron microscopy.

Results: The main finding of this work was that the degree of clot contraction in blood from patients with VTE was significantly reduced (32±2%) when compared to that of healthy subjects (51±1%, p<0.0001). The average velocity of clot contraction in VTE patients (0.3±0.01%/sec) was also remarkably lower than in healthy donors (0.4±0.01%/sec, p<0.001). In addition, the contraction of clots from deep vein thrombosis patients associated with pulmonary embolism was significantly impaired compared to the isolated vein thrombosis (29±2% and 35±2%, respectively, p<0.05), suggesting that clot compaction could lead to a decrease of the risk of embolization. In patients with acute thrombosis lasting <21 days, the initiation time (lag time) of clot contraction was significantly suppressed compared to VTE patients with subacute thrombosis lasting >21 days (198±15 sec and 128±19 sec, respectively, p<0.05). The reduced ability of clots to contract correlated with adverse changes in the blood composition and pronounced platelet dysfunction. In VTE, platelets were initially activated as revealed by the frequently observed shape change and formation of filopodia in platelets from VTE patients (average 75%, n=4211), compared to the less common morphologically altered platelets from healthy subjects (average 41%, n=480, p<0.001). In contrast to the morphological changes, there was no significant difference in the background platelet activation assessed by the levels of P-selectin expression and αIIbβ3 activation in unstimulated cells. However, unlike in resting untreated platelets, in response to TRAP-induced stimulation, platelets from the blood of VTE patients had a significantly lower expression of P-selectin compared to TRAP-activated normal platelets (30±6.6% and 43±5.8%, respectively, p<0.05). TRAP-induced stimulation also resulted in a reduced fibrinogen-binding capacity compared to the TRAP-activated normal platelets (41±8% and 60±7%, respectively, p<0.05).

Conclusions: The results obtained suggest that contraction of clots and thrombi is an underappreciated pathogenic mechanism that may affect the course and outcome of VTE. The clinical importance of the reduced clot contraction as well as the diagnostic and prognostic value of the clot contraction assay in VTE is worth further exploration.

The work was supported by the Program for Competitive Growth at Kazan Federal University.