Background Inflammatory processes are a key cause of atherosclerosis and cardiovascular diseases. Complement system has been implicated but evidence is less clear. We tested whether atherosclerosis severity, plaque types, and vascular region are associated with mRNA expression of genes encoding proteins of the complement system and investigated the cell-specific expression of the most differentially expressed transcripts in plaque macrophages, fibroblasts, and endothelial, smooth muscle, Schwann, mast, plasma, T-, and B-cells.

Methods Total mRNA was isolated, and gene expression analyzed from 29 carotid, 15 abdominal aortic, and 24 femoral plaque samples, and 28 atherosclerosis-free control left internal thoracic artery samples of 95 patients, as well as from 97 whole blood and 97 peripheral mononuclear cell samples of 97 patients. Genome-wide transcriptomic analyses were done using RNA bead microarray platforms. Differential expression was compared between plaques with normal arteries, unstable with stable plaques, as well as CAD patients with CAD-free patients.

Results A total of 33 out of 90 (37%) transcripts of the complement system were differentially expressed in atherosclerotic plaques as compared to histologically normal arteries. In aortic, carotid, and femoral plaques 38, 36 and 29 transcripts, respectively, were differentially expressed, of which 25 were shared by the plaques of all arterial beds. Among the most interesting gene-level findings, we observed that transcripts of the integrin gene ITGB2 are highly upregulated mostly in macrophages and T cells while having top centrality in the network and top enriched genesets related to cell junctions.

Conclusions This is the first study exploring the association of local complement expression across multiple arterial beds and histologically diverse plaque samples. The local effect of complement-mediated inflammation is indicated in inflammatory plaque tissue but not in smooth muscle cell-dominated plaques. Taken together, gene expression of the complement system components in artery cells is associated with advanced atherosclerosis.

The authors have declared no competing interest.

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This study was supported with grants from the Competitive Research Funding of the Tampere University Hospital (Grant 9M048, 9N035, X51001 for T.L.), Emil Aaltonen Foundation (T.L.), Pirkanmaa Regional Fund of the Finnish Cultural Foundation, the Research Foundation of Orion Corporation, the Jenny and Antti Wihuri Foundation, Academy of Finland (grants 322098 and 104821), Finnish Foundation for Cardiovascular Research, Yrjä Jahnsson Foundation, European Union 7th Framework Program (grant 201668 for AtheroRemo), EU Horizon 2020 (grant 755320 for TAXINOMISIS and grant 848146 for To Aition)), Tampere University Hospital Supporting Foundation, and the Finnish Society of Clinical Chemistry.

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The Tampere Vascular Study (TVS) (ETL-code R99204) and Finnish Cardiovascular Study (FINCAVAS) (ETL-code R00153) were approved by the Ethics Committee of Tampere Hospital District. All studies were conducted in accordance with the Declaration of Helsinki, and all study subjects provided written informed consent.

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The dataset supporting the conclusions of this article were obtained from the Tampere Vascular study which comprises health related participant data. The use of data is restricted under the regulations on professional secrecy (Act on the Openness of Government Activities, 612/1999) and on sensitive personal data (Personal Data Act, 523/1999, implemented in the EU data protection directive 95/46/EC). Due to these restrictions, the data cannot be stored in public repositories or otherwise made publicly available. Data access may be permitted on a case-by-case basis upon request only. Data sharing outside the group is done in collaboration with the TVS group and requires a data-sharing agreement. Investigators can submit an expression of interest to the chairman of the publication committee (Prof Terho Lehtimäki, Tampere University, Finland).