Ly6C Lo Monocyte/Macrophages are Essential for Thrombus Resolution in a Murine Model of Venous Thrombosis

Venous thrombosis (VT) resolution is a complex process, resembling sterile wound healing. Infiltrating blood-derived monocyte/macrophages (Mo/M phi s) are essential for the regulation of inflammation in tissue repair. These cells differentiate into inflammatory (CD11b (+) Ly6C (Hi) ) or proreparative (CD11b (+) Ly6C (Lo) ) subtypes. Previous studies have shown that infiltrating Mo/M phi s are important for VT resolution, but the precise roles of different Mo/M phi s subsets are not well understood. Utilizing murine models of stasis and stenosis inferior vena cava thrombosis in concert with a Mo/M phi depletion model (CD11b-diphtheria toxin receptor [DTR]-expressing mice), we examined the effect of Mo/M phi depletion on thrombogenesis and VT resolution. In the setting of an 80 to 90% reduction in circulating CD11b (+) Mo/M phi s, we demonstrated that Mo/M phi s are not essential for thrombogenesis, with no difference in thrombus size, neutrophil recruitment, or neutrophil extracellular traps found. Conversely, CD11b (+) Mo/M phi are essential for VT resolution. Diphtheria toxoid (DTx)-mediated depletion after thrombus creation depleted primarily CD11b (+) Ly6C (Lo) Mo/M phi s and resulted in larger thrombi. DTx-mediated depletion did not alter CD11b (+) Ly6C (Hi) Mo/M phi recruitment, suggesting a protective effect of CD11b (+) Ly6C (Lo) Mo/M phi s in VT resolution. Confirmatory Mo/M phi depletion with clodronate lysosomes showed a similar phenotype, with failure to resolve VT. Adoptive transfer of CD11b (+) Ly6C (Lo) Mo/M phi s into Mo/M phi-depleted mice reversed the phenotype, restoring normal thrombus resolution. These findings suggest that CD11b (+) Ly6C (Lo) Mo/M phi s are essential for normal VT resolution, consistent with the known proreparative function of this subset, and that further study of Mo/M phi subsets may identify targets for immunomodulation to accelerate and improve thrombosis resolution.