The origin of post-injury neointimal cells in the rat balloon injury model

The origin of post-injury neointimal cells is still a matter of debate. This study aims to determine the anatomic source of neointimal cells in one of the most important animal models for the study of vascular stenosis in response to injury, the rat balloon injury model. Chimeric rats were generated by rescuing lethally irradiated animals with green fluorescent protein (GFP)(+) bone marrow (BM) cells from transgenic rats. Neointimal formation was induced in the right iliac artery of these animals using a balloon angioplasty catheter. Injured and non-injured contra-lateral arteries were harvested at 7, 14, and 30 days post-surgery. BM-derived monocytes/macrophages (CD68(+) GFP(+)) were abundant in the media and adventitia of injured vessels harvested at 7 days as determined by immunofluorescence and confocal microscopy. The number of GFP(+) cells declined in the vascular wall with time. Post-injury neointimal cells were mostly GFP(-)/smooth muscle actin (SMA)(+), which indicated that those cells originated in the recipient. Only a few neointimal cells seemed to come from circulating progenitors (GFP(+) SMA(+), 2.34% +/- 1.61). The vascular origin of cells in the neointima was further confirmed by transplanting injured GFP arteries into wild-type recipients. In these grafts, 94.23 +/- 0.44% of medial and 92.95 +/- 19.34% of neointimal cells were GFP(+) SMA(+). Finally, we tested the capacity of vascular smooth muscle cells (VSMC) to migrate through the vascular wall using a novel in vivo assay. As expected, VSMC migrated and populated the neointima only in response to injury. Our results suggest that neointimal cells in the rat balloon injury model mostly derive from pre-existing vascular cells and that only a small population of those cells come from BM-derived progenitors.