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SummaryDeep vein thrombosis (DVT) can give rise to chronic debilitating complications, which are expensive to treat. Anticoagulation, the standard therapy for DVT, prevents propagation, but does not remove the existing thrombus, which undergoes slow natural resolution. Alternative forms of treatment that accelerate resolution may arise from a better understanding of the cellular and molecular pathways that regulate the natural resolution of thrombi. This review will outline our current understanding of the mechanisms of thrombus resolution and the role of neovascularisation in this process. Novel experimental treatments that may one day find clinical use are also discussed. The process of thrombus resolution resembles wound healing. The mainly monocytic inflammatory infiltrate, which develops, is associated with the appearance of vascular channels. These cells may drive resolution by encouraging angiogenesis, which contributes to restoration of the vein lumen. Significant numbers of bone marrow-derived progenitor cells have also been found in naturally resolving thrombi, but their precise phenotype and their role in thrombus recanalisation is unclear. Enhanced thrombus neovascularisation and rapid vein recanalisation have been achieved in experimental models with proangiogenic agents. Recent reports of the role of bone marrow-derived progenitor cells in the revascularisation of ischaemic tissues suggest that it may be possible to obtain the same effect by delivering pluripotent or lineage specific stem cells into thrombus. These cells could contribute to thrombus recanalisation by expressing a variety of proangiogenic cytokines or by lining the new vessels that appear within the thrombus.

SummaryCurrent biochemical markers of thrombosis, such as d-dimer, are of little value in demonstrating the presence of thrombus postoperatively, as their levels are elevated by surgery. Thrombosis involves adhesive interactions between the endothelium, platelets and leukocytes. The aim of the study was to determine which of a panel of haemostatic and adhesion factors are altered by the presence of thrombus, but not by surgery. These factors were measured in 20 patients with established spontaneous DVTs, 13 patients having hip replacement surgery and 28 control patients. Circulating levels of P-selectin, VCAM-1 and tissue factor were found to be increased when thrombus was present (p

SummaryVascular endothelial growth factor (VEGF) is a regulator of physiological and pathological angiogenesis and is found in naturally resolving experimental venous thrombi, where it may also regulate recanalisation. In this study VEGF protein was injected into venous thrombi to determine if this enhanced recanalisation and organisation. A rat model of inferior vena cava (IVC) thrombosis was used. Thrombi were formed in 3 groups (n = 3 per group). 10 μl 125I-VEGF was directly injected into thrombus thirty minutes after induction. Three hours, 1 day and 6 days later thrombus, IVC, and other tissues were harvested. 125I-VEGF was mostly distributed in the thrombus and the IVC, with smaller amounts in other tissues. Thrombi were formed in a further 4 groups (n = 6 per group).Thirty minutes after induction control solution or 1 ng, 10 ng or 100 ng recombinant human VEGF165 was injected directly into the thrombus. Lumen recanalisation, thrombus organisation and monocyte content were measured on digitised sections by image analysis. In animals treated with 10 ng of VEGF there was a greater area of lumen recanalisation [mean 5492 pixels, standard error of mean (sem) 922] compared to controls (mean 2974, sem 385) (P = 0.005). There was a significant increase in the organisation score in all treated animals (1 ng: mean 70, sem 1.7, P = 0.0025; 10 ng: mean 70, sem 2.0, P = 0.0042; 100 ng: mean 72, sem 1.9, P = 0.0003) compared to controls (mean 63, sem 1.7). The monocyte content was lower in the animals treated with 1 ng VEGF (mean 3.8% of thrombus area, sem 0.3%) compared to controls (mean 5.5%, sem 0.4%) (P = 0.0008).The proportion of monocytes migrating to the centre of the thrombus increased in a dose-related manner. VEGF may prove to be of use in the treatment of venous thrombosis.

Infusing monocytes that have been stimulated to produce fibrinolytic activators and factors that regulate cell proliferation, migration and maturation, might enhance venous thrombus resolution. The aim of this study was to determine the time course of infused monocyte recruitment into venous thrombus in an appropriate model of this disease.Thrombus was induced in the inferior vena cava of male Wistar rats using reduced flow (80-90% stenosis). The vessel wall was examined at 1hr by scanning electron microscopy. Resolving thrombi with surrounding vena cava were obtained at 1, 7, 14 and 21 days after induction (n = 8). Sections, taken at 0.5 mm intervals (10-15 sections per thrombus), were stained using haematoxylin, Martius Scarlet Blue and antibodies against monocytes, platelets and fibrin. Sections from human venous thrombi (n = 4) were similarly stained. The area occupied by monocytes (in relative pixel units, RPU) was determined using computer aided image analysis. Peripheral rat blood monocytes were extracted, fluorescently labelled and injected intravenously into 7 rats prior to thrombus induction, Vena cava with thrombus was harvested 1 h, 2, 3, 4, 7, 14 and 25 days after induction and their fluorescence measured. The fluorescent content of the caval wall and thrombus was analysed in greater detail at 2 and 25 days after thrombus induction (n = 4 at each time interval).Experimental thrombi were structurally similar to human thrombus and resolved within 14-21 days. Scanning electron microscopy showed minimal endothelial damage at 1 h with signs of early thrombus formation (platelet, red cell leukocyte and fibrin deposition). Neutrophils were the predominant leukocyte in the thrombus at 1 day, with monocytes making up only 0.3% (0.04% sem) of the area of the thrombus. There was a steady increase in thrombus monocyte content and by 21 days the percentage area of thrombus covered by monocytes had increased by over 35 fold to 11.5% (2.3% sem) (p0.001). Initially, monocytes appeared around the edge of the thrombus and became more evenly distributed through the thrombus as resolution progressed. Labelled monocytes could be found in the circulation up to 1 week after infusion. The fluorescent content (RPU) of the thrombus increased over 25 days (mean RPU At 2 days 0.012, sem 0.005; mean RPU at 25 days 1.062, sem 0.252, p = 0.008). The number of labelled monocytes in the vessel wall peaked at 2 days and decreased thereafter.The structure of thrombi produced by this model was comparable to that of human venous thrombi. Endogenous and injected monocytes migrated into the thrombus during natural resolution, possibly via the vein wall. Monocyte targeting could therefore be used to develop novel treatments for venous thrombosis, with the aim of reducing post-thrombotic complications.