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16 p.-5 fig.-3 tab.-1 il. cent., BACKGROUND: The mechanisms underlying early atherosclerotic plaque formation are not completely understood. Moreover, plasma biomarkers of subclinical atherosclerosis are lacking., OBJECTIVES: The purpose of this study was to analyze the temporal and topologically resolved protein changes taking place in human aortas with early atherosclerosis to find new potential diagnostic and/or therapeutic targets., METHODS: The protein composition of healthy aortas (media layer) or with early atheroma (fatty streak and fibrolipidic, media and intima layers) was analyzed by deep quantitative multiplexed proteomics. Further analysis was performed by Western blot, immunohistochemistry, real-time polymerase chain reaction, and enzyme-linked immunosorbent assay. Plasma levels of complement C5 were analyzed in relation to the presence of generalized (>2 plaques) or incipient (0 to 2 plaques) subclinical atherosclerosis in 2 independent clinical cohorts (PESA [Progression of Early Subclinical Atherosclerosis] [n=360] and NEFRONA [National Observatory of Atherosclerosis in Nephrology] [n=394])., RESULTS: Proteins involved in lipid transport, complement system, immunoglobulin superfamily, and hemostasis are increased in early plaques. Components from the complement activation pathway were predominantly increased in the intima of fibrolipidic plaques. Among them, increased C5 protein levels were further confirmed by Western blot, enzyme-linked immunosorbent assay and immunohistochemistry, and associated with in situ complement activation. Plasma C5 was significantly increased in individuals with generalized subclinical atherosclerosis in both PESA and NEFRONA cohorts, independently of risk factors. Moreover, in the PESA study, C5 plasma levels positively correlated with global plaque volume and coronary calcification., CONCLUSIONS: Activation of the complement system is a major alteration in early atherosclerotic plaques and is reflected by increased C5 plasma levels, which have promising value as a novel circulating biomarker of subclinicalatherosclerosis., This study was funded by the Spanish MINECO(SAF2016-80843-R, BIO2015-67580-P, and PGC2018-097019-B-I00), CAM (Complemento II-CM, S2017/BMD-3673),Fondo de Investigaciones Sanitarias ISCiii-FEDER (Biobancos RD09/0076/00101) and PRB3 (IPT17/0019, ProteoRed), and FEDER “Una manera de hacer Europa.” CIBERCV is an Instituto de Salud Carlos III project. The work leading to these results has also received funding from “la Caixa” Banking Foundation under the project code HR17-00247. The PESA study is cofunded equally by the Centro Nacional de Investigaciones Cardiovasculares (CNIC), and Banco Santander, Madrid, Spain. The study also receives funding from the Instituto de Salud Carlos III (PI15/02019) and the European Regional Development Fund (ERDF) “Una manera de hacer Europa.”