Inflammatory Monocyte Counts Determine Venous Blood Clot Formation and Resolution

Background: Monocytes are thought to be involved in venous thrombosis but the role of individual monocyte subpopulations on thrombus formation, clot inflammation, and degradation is an important unresolved issue. We investigate the role of inflammatory Ly6C hi monocytes in deep vein thrombosis and their potential therapeutic impact. Methods: Frequencies and compositions of blood monocytes were analyzed by flow cytometry in CCR2 −/− (C-C chemokine receptor type 2) and wild-type mice of different ages and after treatment with the NR4A1 (nuclear receptor group 4 family A member 1, Nur77) agonist CnsB (cytosporone B). TF (tissue factor) sufficient and deficient Ly6C hi monocytes were adoptively transferred into aged CCR2 − /− mice. Thrombus formation and size were followed by ultrasound over a 3-week period after surgical reduction of blood flow (stenosis) in the inferior vena cava. Results: Reduced numbers of peripheral monocytes in aged (>30 w) CCR2 −/− mice are accompanied by reduced thrombus formation after inferior vena cava ligation. Reducing the number of inflammatory Ly6C hi monocytes in wild-type mice by CsnB treatment before ligation, similarly suspends clotting, while later treatment (d1 or d4) reduces thrombus growth and accelerates resolution. We describe how changes in inflammatory monocyte numbers affect the gradual differentiation of monocytes in thrombi and show that only tissue factor-competent Ly6C hi monocytes restore thrombosis in aged CCR2 − /− mice. Conclusions: We conclude that the number of inflammatory Ly6C hi monocytes controls deep vein thrombosis formation, growth, and resolution and can be therapeutically manipulated with a NR4A1 agonist at all disease stages.