SUMMARY Background Factors associated with post-thrombotic syndrome are known clinically, but the underlying cellular processes at the vein wall are not well-delineated. Prior work suggests that vein wall damage does not correlate with thrombus resolution, but rather with plasminogen activator-1 (PAI-1) and matrix metalloproteinase (MMP) activity. Objective We hypothesized that PAI-1 would confer post venous thrombosis (VT) vein wall protection via a Vitronectin (Vn) dependent mechanism. Methods A stasis model of VT was used with harvest over 2 weeks, in wild type (WT), Vn−/−, and PAI-1 overexpressing mice (PAI-1 Tg). Results PAI-1 Tg mice had larger VT at 6 and 14 days, compared to controls, but Vn−/−mice had no alteration of VT resolution. Gene deletion of Vn resulted in increased, rather than expected decrease in circulating PAI-1 activity. While both Vn−/− and PAI-1 Tg had attenuated intimal fibrosis, PAI-1 Tg had significantly less vein wall collagen and a compensatory increase in collagen III gene expression. Both Vn−/− and PAI-1 Tg vein wall had less monocyte chemotactic factor-1, and fewer macrophages (F4/80), with significantly less MMP-2 activity and decreased TIMP-1 antigen. Ex vivo assessment of TGFβ mediated fibrotic response showed that PAI-1 Tg vein walls had increased profibrotic gene expression (collagen I, III, MMP-2 and α-SMA) as compared with controls, opposite of the in vivo response. Conclusions The absence of Vn increases circulating PAI-1, which positively modulates vein wall fibrosis in a dose-dependent manner. Translationally, PAI-1 elevation may decrease vein wall damage after DVT, perhaps by decreasing macrophage-mediated activities.

Background Treatment with low-molecular-weight heparin (LMWH) favorably alters the vein wall response to deep venous thrombosis (DVT), although the mechanisms remain unclear. Previous studies have suggested that LMWH alters the levels of circulating plasminogen activator inhibitor 1 (PAI-1), a known mediator of fibrosis, and may improve endogenous fibrinolysis. We hypothesized that LMWH favorably alters the vein wall response by binding of PAI-1 and acceleration of fibrinolysis. Methods Wild-type and PAI-1 −/− mice underwent treatment with LMWH after induction of occlusive DVT. Vein wall and plasma were harvested and analyzed by enzyme-linked immunosorbent assay, zymography, real-time polymerase chain reaction, and immunohistochemistry. Results Wild-type mice treated with LMWH exhibited diminished vein wall fibrosis (0.6 ± 0.6 vs 1.4 ± 0.2; P < .01; n = 5) and elevation of circulating PAI-1 (1776 ± 342 vs 567 ± 104 ρg/mL; P < .01; n = 5) compared with untreated controls after occlusive DVT. PAI-1−/− mice treated with LMWH were not similarly protected from fibrosis, despite improved thrombus resolution. Treatment with LMWH was associated with decreased intrathrombus interleukin-lβ (68.6 ± 31.0 vs 223.4 ± 28.9 ρg/mg total protein; P < .01; n = 5) but did not alter inflammatory cell recruitment to the vein wall. PAI-1 −/− mice exhibited significantly elevated intrathrombus (257.2 ± 51.5 vs 4.3 ± 3.8 ρg/mg total protein; n = 5) and vein wall interleukin-13 (187.2 ± 57.6 vs 9.9 ± 1.1 ρg/mg total protein; P < .05; n = 5) as well as vein wall F4/80 positively staining monocytes (53 ± 11 vs 16 ± 2 cells/5 high-power fields; P < .05; n = 4). Conclusions LMWH did not accelerate venous thrombosis resolution but did protect against vein wall fibrosis in a PAI-1-dependent manner in an occlusive DVT model. Lack of PAI-1 correlated with accelerated venous thrombosis resolution but no protection from fibrosis. PAI-1 inhibition as a treatment strategy for DVT is likely to accelerate clearance of the thrombus but may come at the expense of increased vein wall fibrosis. Clinical Relevance The pathophysiologic mechanism of post-thrombotic syndrome is not well understood clinically or experimentally. In this study, we evaluated the effect of the prominent fibrinolytic mechanism, plasminogen activator inhibitor 1 (PAI-1), and low-molecular-weight heparin (LMWH) on vein wall injury after thrombosis. We show here that LMWH is protective from vein wall fibrosis, but this is abrogated in PAI-1-deleted mice. This is also correlated with monocyte vein wall influx. These data support the clinical observation that LMWH may be protective from post-thrombotic vein wall injury in a PAI-1-dependent manner.

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