Monocytic microparticles promote atherogenesis by modulating inflammatory cells in mice

Microparticles (MP) are generated during a vast number of biological processes such as inflammation, cell activation and apoptosis. Increasing evidence points towards an important role of MP as intercellular messengers of biological information. During atherogenesis, monocytes infiltrate the vascular wall and foster inflammation, accompanied by the release of monocytic MP (mono-MP). To date, only little is known about the biological function of mono-MP in the vascular wall. Here, we investigated the role of mono-MP during atherogenesis. Mono-MP were generated by starvation of THP-1 monocytes and isolated by ultracentrifugation. To investigate whether mono-MP influence atherogenesis, ApoE-/- mice were fed a high-fat, cholesterol-rich diet for 8 weeks and simultaneously treated with mono-MP or vehicle twice a week. Mice treated with mono-MP showed significantly increased monocyte and T-cell infiltration into the vessel wall, as assessed by Moma-2 and CD3 staining, and enhanced plaque formation, as assessed by oil-red-O staining. However, atherosclerotic plaque composition was not influenced by mono-MP application. In vitro, incubation of mono-MP with murine macrophages and endothelial cells resulted in the uptake of calcein-labelled mono-MP. Mono-MP uptake initiated the generation of intracellular reactive oxygen species. Murine macrophages pre-treated with mono-MP showed significantly enhanced expression of CCR2, migration to MCP-1 and increased release of pro-inflammatory interleukin-6. Co-incubation of mono-MP with endothelial cells resulted in significantly increased expression of ICAM-1, as assessed by RT-PCR and ELISA. Mono-MP act as paracrine messengers that intensify inflammation during atherogenesis by stimulating vascular-bound and inflammatory cells in their vicinity.