MPO is a white blood cell-derived inflammatory enzyme that measures disease activity from the luminal aspect of the arterial wall. Briefly, when the artery wall is damaged, or inflamed, MPO is released by invading white blood cells where it accumulates. MPO mediates the vascular inflammation that propagates plaque formation and activates protease cascades that are linked to plaque vulnerability. White blood cell activation in the bloodstream, in response to luminal injury of the artery wall including fissures, erosions or a degrading collagen cap, leads to MPO release in the bloodstream. This combination of detrimental effects demonstrates that MPO is actively involved in the progression of atherosclerosis.