Background/Aims: The role of adventitial fibroblasts in the vascular inflammation observed in the adventitia of large vessels in numerous cardiovascular diseases remains unclear. Our objective was to explore the contribution of these cells to angiotensin II (Ang II)-induced aortic inflammation and adventitial expansion. Methods: Cytokine production by primary human aortic adventitial fibroblasts (AoAF) in tissue culture was detected using multiplex ELISA, and increases in cytokine mRNA following Ang II stimulation were quantitated by real-time PCR. The ability of AoAF-derived MCP-1 to attract monocytes was studied in vitro using Boyden assays, and the resulting effect of the monocyte-AoAF interaction on fibroblast proliferation was measured in vitro using proliferation and ³H-thymidine incorporation assays. Ang II-induced fibroblast proliferation was measured in vivo using aortic digestion of single cells followed by flow cytometric quantification of fibroblast numbers as well as fibroblast and PCNA immunostaining. The ability of monocytes to induce AoAF proliferation was demonstrated in vivo using CCR2^+/+ wild-type monocyte adoptive transfer into Ang II-stimulated CCR2-null mice which can produce MCP-1 but have cells lacking the MCP-1 receptor – CCR2. Results: AoAF constitutively secreted numerous proinflammatory cytokines, particularly IL-6 and MCP-1, whose gene expressions were further upregulated in response to Ang II stimulation. AoAF-derived MCP-1 was potent in recruiting THP-1 monocytes in vitro, and these monocytes stimulated AoAF proliferation based on a flow cytometric assessment of cell number and ³H-thymidine incorporation in tissue culture. In vivo, Ang II induced fibroblast proliferation, increased fibroblast and PCNA adventitial staining, and blunted inflammatory responses in the CCR2^–/– background. Injection of CCR2^+/+ monocytes into Ang II-treated CCR2^–/– mice restored adventitial thickening which resulted in increased fibrosis secondary to adventitial fibroblast proliferation. Conclusions: Our results suggest that Ang II-stimulates AoAF to recruit monocytes via fibroblast-derived MCP-1, and the recruited monocytes further activate fibroblast proliferation, adventitial thickening, and additional cytokine production. This fibroblast-monocyte amplification loop may critically mediate hallmarks of adventitial inflammation common to many cardiovascular diseases.

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