Systemic application of sirolimus prevents neointima formation not via a direct anti-proliferative effect but via its anti-inflammatory properties.

BACKGROUND

Systemic treatment with sirolimus, as used for immunosuppression in transplant patients, results in markedly low rates of in-stent restenosis. Since the underlying mechanisms remain obscure, we aimed to determine the molecular and cellular effects of systemic sirolimus treatment on vascular remodeling processes.

METHODS AND

RESULTS

Systemic sirolimus treatment significantly reduced smooth muscle cell (SMC) proliferation 14days after wire-induced injury and neointima formation 28days after injury in C57BL/6 mice, while simultaneously impairing re-endothelialization. Interestingly, in vitro, sirolimus had no direct effect on the proliferation of SMC or endothelial cells (EC) at serum concentrations observed after systemic application. In contrast, sirolimus reduced the adhesion of leukocytes (CD45+) and bone marrow-derived progenitor cells (CD34+) to activated EC by down-regulating the adhesion molecules ICAM-1 and VCAM-1. In addition, sirolimus treatment also significantly reduced the upregulation of ICAM-1 and VCAM-1 and the recruitment of monocytic cells (MOMA-2+) in neointimal lesions in vivo.

CONCLUSION

Our findings show that systemic sirolimus treatment effectively prevents SMC and EC proliferation in vivo without directly affecting these cells. Instead, sirolimus prevents neointima formation and re-endothelialization by attenuating the inflammatory response after injury with secondary effects on SMC and EC proliferation. Thus, despite a similar net effect, the mechanisms of systemic sirolimus treatment are largely different from the local effects achieved after application of sirolimus-eluting stents.