Ferumoxytol-enhanced MRI assessment of venous Thrombus resolution and macrophage content in a murine deep vein thrombosis model

Background: Imaging evaluation of acute deep vein thrombosis (DVT) or post-thrombotic syndrome (PTS) in animal or clinical models is limited to anatomical assessment of the location and extent of thrombi. We hypothesize that Fe-MRI, used to evaluate macrophage content in other inflammatory diseases, can be useful to evaluate the thromboinflammatory features after DVT over time.

Methods: Nineteen wild-type CD-1 mice underwent surgical IVC ligation to induce DVT. Mice received either saline or 5 mg/kg of 14E11, a Factor XI inhibitor, before the procedure. Fe-MRI was performed on days 6-7 after ligation to evaluate thrombus volume, perfusion, and macrophage content via T₂-weighted images. Mice were euthanized at days 3-15 after surgery. The thrombi and adjacent vein walls were excised, weighed, formalin-fixed, and paraffin-embedded for immunohistological analysis. Specimens were stained with specific antibodies to evaluate macrophage content, collagen deposition, neovascularization, and recanalization. Significance was determined using the Mann-Whitney U or Student's t-test.

Results: After IVC-ligation in control mice, thrombus weights decreased by 59 % from day 3 to 15. Thrombus volumes peaked on day 5 before decreasing by 85 % by day 13. FXI inhibition led to reduced macrophage content in both thrombi (p = .008) and vein walls (p = .01), decreased thrombus volume (p = .03), and decreased thrombus mass (p = .01) compared to control mice. CCR2+ staining corroborated these findings, showing significantly reduced macrophage presence in the thrombi (p = .002) and vein wall (p = .002).

Conclusions: Fe-MRI T₂ relaxation times can be used to characterize and quantify post-thrombotic changes of perfusion, macrophage content, and thrombus volume over time in a surgical mouse model of venous thrombosis. This approach could lead to better quantification of in vivo inflammation correlating monocyte and macrophage content within resolving thrombi and veins and may serve as a useful tool for research and clinically in the evaluation of the post-thrombotic environment.