BACKGROUND: The contribution of inflammation to initiation and progression of atherosclerosis is now well established, Colchicine, a microtubule inhibitor with anti-inflammatory properties, represents a potentially useful agent as it led to a reduction of cardiovascular events among patients with recent acute coronary syndrome and chronic coronary disease. However, its exact mechanisms of action remain unclear. Here, we describe how colchicine impacts plaque leukocyte recruitment to reduce plaque inflammation.

METHODS&RESULTS: In our study, we treated mice with early atherosclerosis (Apoe^-/- mice on a high cholesterol diet for 8 weeks) with colchicine (0.25mg/kg bodyweight) once daily over four weeks. Using flow cytometry and immunohistochemistry, we found that colchicine reduced numbers of plaque neutrophils (vehicle: 4478+667; colchicine: 2689+350.1 neutrophils/aorta; mean+SEM, p=0.0239), monocytes (vehicle: 3298+450.7, colchicine: 2126+353.6 Ly6c^highmonocytes/aorta; mean+SEM, p=0.0374) and macrophages (vehicle: 16856+2604; colchicine: 10655+1252 macrophages/aorta; mean+SEM, p=0.0401) shifting the overall plaque character toward a less inflammatory phenotype. To explore the mechanism of the observed reduction in plaque leukocyte numbers, we performed adoptive transfer experiments demonstrating that colchicine dampens recruitment of leukocytes from blood to plaques (vehicle: 218.9+27.75, colchicine 97.33+13.66 GFP⁺cells/aorta; mean+SEM, p=0.0015). Specifically, colchicine alters the recruitment profile of blood monocytes and neutrophils, whereas plaque endothelial cells remain rather unaffected. In line, mice treated with colchicine after myocardial infarction, showed reduced inflammatory leukocyte accumulation in the vessel wall (vehicle: 1486+146.1, colchicine: 862+157.0 neutrophils/aorta; mean+SEM, p=0.0105 // vehicle: 7376+1190, colchicine: 4422+695.4 macrophages/aorta; mean+SEM, p=0.0381 // vehicle: 1203+169.7, colchicine:454.5+51.76 Ly6c^highmonocytes/aorta; mean+SEM, p=0.0020). Moreover, we examined the direct effects of colchicine on macrophage maturation and proliferation using bone marrow derived macrophages in cell culture experiments. Our experiment revealed that colchicine has neither an effect on differentiation of monocytes to macrophages nor on macrophage proliferation.

CONCLUSION: Our data reveal new mechanistic insights into how colchicine beneficially alters the progression of atherosclerosis. Colchicine prevents expansion of plaque inflammatory leukocytes through lowering recruitment of blood myeloid cells to plaques. Our findings may inform future anti-inflammatory interventions in patients at risk.