Alterations in macrophage phenotypes in experimental venous thrombosis

Macrophages are involved in venous thrombus (VT) resolution and vein wall remodeling. This study was undertaken to identify variations in macrophage phenotypes in thrombi and vein wall in multiple models of VT to clarify the natural history of macrophage polarization in clearance of VT. We also sought to demonstrate the feasibility of macrophage phenotyping in human VT.

Established murine models of VT were used to mimic the clinical spectrum of human VT (stasis and nonstasis models). Vein wall and thrombi were isolated at acute (2 days) or chronic (6-21 days) time points and analyzed by Bio-Plex assay (Bio-Rad, Carlsbad, Calif) for cytokines (interleukin [IL]-1β, IL-6, IL-10, IL-12), by immunohistochemistry for "M1-like" (IL-12) or "M2-like" (arginase 1 [Arg-1]) markers, and by histology for intimal thickness and collagen content (Sirius red staining). Bone marrow was harvested from animals 2 days after undergoing sham, stasis, or nonstasis surgery. Macrophages were skewed toward M1 using lipopolysaccharide, and RNA analysis was done for inflammatory cytokine genes (IL-1β, IL-12). Human blood samples were similarly analyzed with reverse transcription polymerase chain reaction for macrophage polarization markers (CD206, inducible nitric oxide synthase, CCR2) and thrombi with immunohistochemistry (inducible nitric oxide synthase, Arg-1).

Stasis (chronic) and nonstasis (acute and chronic) thrombi were characterized by a predominance in anti-inflammatory (M2) macrophages (n = 4-5/group; P < .05). Larger thrombi were found in the stasis model at both time points (n = 3; P < .01), correlating with decreased intrathrombus inflammatory (M1) cytokines (IL-1β, P = .03; IL-12, P = .17; n = 4) and diminished inflammatory response of bone marrow-derived macrophages to lipopolysaccharide (IL-1β, P = .03; IL-12, P = .04; n = 4) compared with nonstasis model. Anti-inflammatory (M2 [Arg-1]) macrophage cell counts were elevated in the post-thrombotic vein wall of stasis mice compared with nonstasis mice (acute: n = 4, P < .05; chronic: n = 5, P < .01), consistent with increased intimal thickness (P < .01; n = 4-6) and collagen deposition chronically (P = .005; n = 12). M2-like thrombi (Arg-1, P < .05; n = 4-7) and circulating markers (CD206, P < .05; n = 9-17) decreased over time in human VT.

Experimental VT is characterized by an anti-inflammatory predominant macrophage phenotype, possibly impairing thrombus resolution, and is model dependent. Altering the M1/M2 macrophage balance may accelerate thrombus resolution and allow the development of translatable novel therapies to treat VT and to prevent post-thrombotic syndrome.