Objective—Protease nexin-1 (PN-1), a serpin constitutively expressed by vascular smooth muscle cells and endothelial cells, inhibits thrombin, plasminogen activators, and plasmin and can thus be expected to play a role in vascular biology. The present study addressed the question of PN-1 expression in human atherothrombosis.

Methods and Results—Immunohistochemistry and biochemical studies confirmed that PN-1 was expressed at a moderate level in the medial layer of normal human arteries and showed that PN-1 expression was increased in atherothrombotic lesions. In early noncomplicated plaques, PN-1 was associated with infiltrating mononuclear cells. A strong PN-1 signal was observed in advanced lesions, principally in intraplaque hemorrhage-related structures. Monocytes/macrophages and platelets were identified as the main sources of PN-1 within atherothrombotic material. Isolated human monocytes and platelets both expressed high levels of active PN-1, and monocyte PN-1 expression was upregulated, at both messenger and protein levels, in response to stimulation by lipopolysaccharides. In contrast, PN-1 expression was downregulated during their differentiation into macrophages which were shown to produce degraded forms of PN-1.

Conclusions—Platelets and monocytes/macrophages are a major source of PN-1 in human atherothrombotic plaques. PN-1 could thus represent a new actor in the evolution of atherosclerotic lesions.