Background: 

Recent studies claim that circulating bone marrow derived progenitor cells play an important role during the course of neointima formation by differentiating into smooth muscle cells (SMC). Only little information is available about the timepoints of accumulation, differentiation and the long term contribution of these cells compared to media derived SMC in the neointima.

Methods and Results: 

After irradiation with 9.5 Gy mice were reconstituted with bone marrow cells from enhanced green fluorescent protein (eGFP) transgenic mice. FACS analysis of chimeric mice revealed that 95% of circulating mononuclear cells expressed eGFP 12 weeks after transplantation. Wire induced dilatation of the mouse femoral artery was performed, and vessels were harvested after 3 days, 1, 2, 3, 4, 6 and 16 weeks (n=8 animals per timepoint, transplantation/time course experiments were performed in duplicate). Neointima/media ratio and number of bone marrow derived (eGFP+) cells were quantified. eGFP+ cells were further analysed for the expression of a-smooth muscle actin (aSMA) using immuno fluorescence imaging and deconvolution analysis of high resolution z-axis image stacks. Three days after dilatation only a few remaining resident SMC were detected in the medial layer, and the denuded/injured luminal surface was lined with thrombocytes. At one to two weeks an increasing accumulation of thrombocytes interspersed with eGFP+ leucocytes was observed. After three weeks a peak in the recruitment of mononuclear cells was detected, and at the same time SMA expressing cells started to accumulate in the neointima. Further characterization revealed a high proliferative index of neointima aSMA positive cells, as determined by PCNA expression. Interestingly, following careful analysis throughout the complete lesion range, the expression of aSMA in GFP+ cells occurred to be a very rare event. Moreover, some cells positive for eGFP and aSMA were also observed to express monocytic lineage markers like MoMa or CD11b, suggesting that eGFP/aSMA expressing cells may in part originate from a monocytic subpopulation. Furthermore, the number of eGFP+ cells in the neointima constantly declined at later timepoints, so that hardly any eGFP+ cells and no eGFP/aSMA expressing cells could be detected in the neointima 16 weeks after dilatation.

Conclusions: 

These data provide evidence that transdifferentiation of bone marrow derived progenitor cells into SMC lineages seems to be a relatively rare event. Moreover, the contribution of bone marrow derived cells to the cellular compartment of the neointimal lesion is limited to a temporary time period of the inflammatory response to the vascular injury.