Ultrastructural localization of secretory type II phospholipase A2 in atherosclerotic and nonatherosclerotic regions of human arteries

We recently reported on the immunolocalization of type II secretory nonpancreatic phospholipase A(2) (snpPLA(2)) in human atherosclerotic lesions. In the present study, we present data on the distribution and ultrastructural localization of snpPLA(2) in adjacent nonatherosclerotic and atherosclerotic regions of human arteries. Electron microscopy (EM) of immunogold labeling techniques with a monoclonal antibody was used to analyze arterial tissue. The human specimens analyzed were obtained from autopsy and surgery cases. The results with EM showed a stronger snpPLA(2) immunoreactivity in regions of arteries with atherosclerotic lesions than in regions without lesions from the same individual. snpPLA(2) immunoreactivity was stronger in the arterial intima of atherosclerotic than of nonatherosclerotic tissue. EM-immunogold examination revealed that the majority of snpPLA(2) was localized along the extracellular matrix, associated with collagen fibers and other extracellular matrix structures. Intracellular snpPLA(2) was observed in electron-dense vesicles in intimal cells. snpPLA(2) was also found in contact with large, extracellular lipid droplets. These results support the hypothesis that extracellular snpPLA(2) is localized at sites where it may hydrolyze phospholipids from lipoproteins and lipid aggregates retained in the extracellular matrix of the arterial wall. This may be a mechanism for in situ release of proinflammatory lipids, free fatty acids, and lysophosphatidylcholine in regions of apolipoprotein B accumulation, which are abundant in atherosclerotic lesions.