Involvement of intraplaque hemorrhage in atherothrombosis evolution via neutrophil protease enrichment

The pathological remodeling of the arterial wall in atherosclerosis involves protease activities, which play a major role in complications via plaque rupture. Circulating leukocytes and particularly neutrophils have been shown to be an independent predictor of recurrent ischemic events. However, neutrophils are poorly documented within atherosclerotic plaques. We hypothesized that intraplaque hemorrhage could convey neutrophils into the lesion, spreading into the necrotic core, thus participating in its protease enrichment. One hundred human carotid endarterectomy specimens were dissected into culprit-stenosing plaques (CPs) and adjacent noncomplicated plaques. Half of CPs exhibited hemorrhage, which was confirmed by the release of hemoglobin. Pro- and active forms of matrix metalloproteinase-9 (MMP-9) were increased in media conditioned by hemorrhagic plaques. Higher levels of lipocalin neutrophil gelatinase-associated lipocalin (NGAL)/MMP-9 complexes, specifically released by neutrophils, were also found in conditioned media from plaques with hemorrhage. Immunohistochemical analysis of the corresponding carotid samples showed that neutrophil markers such as elastase, NGAL/MMP-9, CD66b, and proteinase 3 colocalized with blood constituents (i.e., hemoglobin, plasminogen). All markers of neutrophil degranulation were positively correlated in CP-conditioned media (alpha1-antitrypsin/elastase complexes, myeloperoxidase, and alpha-defensins), and higher levels came from CPs containing intraplaque hemorrhages. Addition of an elastase inhibitor at the time of incubation led to a decrease in the proMMP-9 activation in CPs, suggesting cross-talk between proteases released by neutrophils. Finally, we found that neovessels observed at the interface between cap and core exhibit an activated endothelium, which may favor leukocyte diapedesis. Our study thus provides evidence for the involvement of neutrophils in plaque vulnerability.