Pentraxins PTX3 and CRP recruit C1q to cholesterol crystals and co-localize with the terminal complement complex in human atherosclerotic plaques

Background: The pentraxins PTX3 and C-reactive protein (CRP) are acute-phase proteins associated with increased risk of cardiovascular events and are known to interact with the complement system. Cholesterol crystals (CC) are found in atherosclerotic plaques, where inflammation is a part of the initial process leading to atherosclerosis. CC induce complement activation but the role of the pentraxins PTX3, CRP and serum amyloid P component (SAP) in CC mediated inflammation remains unknown. Materials and methods: Binding and interaction of recombinant and native PTX3, CRP or SAP with the complement pattern recognition molecule C1q on CC was investigated in vitro by flow cytometry and fluorescence microscopy. The role of C1q in phagocytosis of CC was investigated by flow cytometry using hirudin whole blood. Furthermore, co-localization of PTX3, CRP, and SAP with complement activation product C5b-9 was investigated in vivo inhumanatherosclerotic plaques by immunofluorescence and confocal microscopy. Results and conclusions: In the present study we report a novel link between the presence of pentraxins in atherosclerotic lesions and the known complement activation and inflammatory response induced by CC. Recombinant and native PTX3, CRP and SAP bound to CC in a concentration dependent manner. PTX3 and CRP interacted with the complement pattern recognition molecule C1q on CC by increasing the binding of both purified C1q and C1q in plasma. Furthermore, CRP binding to CC increased C1q mediate complement activation and deposition of C5b-9. In a phagocytic assay using whole blood we showed that phagocytosis of CC is complement dependent and initiated by C1q mediated activation. The pathophysiological relevance of the in vitro observations was examined in vivo in human atherosclerotic plaques. PTX3, CRP, and SAP were found in atherosclerotic plaques and were located mainly in the cholesterol-rich necrotic core where they show partial colocalization with complement activation product C5b-9. In conclusion, this study identifies interaction between PTX3 or CRP and C1q on CC as a new mechanism in the CC induced inflammation, which may be highly important in the pathophysiology of atherosclerosis.