Venous thromboembolism (VTE) is a pathology encompassing deep vein thrombosis (DVT) and pulmonary embolism (PE) associated with high morbidity and mortality. Because patients often present after a thrombus has already formed, the mechanisms that drive DVT resolution are being investigated in search of treatment. Herein, we review the current literature, including the molecular mechanisms of fibrinolysis and collagenolysis, as well as the critical cellular roles of macrophages, neutrophils, and endothelial cells. We propose two general models for the operation of the immune system in the context of venous thrombosis. In early thrombus resolution, neutrophil influx stabilizes the tissue through NETosis. Meanwhile, macrophages and intact neutrophils recognize the extracellular DNA by the TLR9 receptor and induce fibrosis, a complimentary stabilization method. At later stages of resolution, pro-inflammatory macrophages police the thrombus for pathogens, a role supported by both T-cells and mast cells. Once they verify sterility, these macrophages transform into their pro-resolving phenotype. Endothelial cells both coat the stabilized thrombus, a necessary early step, and can undergo an endothelial-mesenchymal transition, which impedes DVT resolution. Several of these interactions hold promise for future therapy. already progressed past the point of anticoagulant efficacy [12,13]. Thus, there is potential clinical utility in investigating and controlling the process by which DVTs resolve in order to provide better care to these later-stage patients, with less bleeding risk.
Overview of Venous Thrombus Development in HumansThe time sequence of DVT development and resolution was correctly suggested as early as 1962, based on empirical stains that detected changes in the nanoscopic roughness of clotted material [14]. As Lendrum et. al. remarked, "we consider that the intracytoplasmic granules, which we think are engulfed fibrin, seen in pulmonary phagocytes and in the endothelium of arteries and veins containing thrombus, are better shown by this (methyl-scarlet-blue) method than by any of the others." In fresh clot (whether in deep veins or in diabetic kidneys), platelets and erythrocytes are initially linked in a fine mesh yielding a relatively smooth surface (stage 1). A mat of the medium-textured fibrin replaces the initial thrombus (stage 2), which is gradually replaced by much coarser collagen (stage 3). This staging of clot development quickly became the basis of a pathological guide to grade thrombi [15]. The current three-stage refinement of the DVT grading scheme indicates that the turnover of erythrocyte-rich to fibrin-rich clot in humans is complete approximately seven days after the initial thrombotic event [16], whereas fibrin deposition commences after one day [17]. Various leukocytes, initially neutrophils and later macrophages, are believed to control this progression of DVTs development from a mass of erythrocytes, to fibrin, and finally to collagen [18]. Infiltrating T-lymphocytes and macrophages conta...