LAUSR

The aim of this study was to analyze the impact of the adventitial layer on vascular remodeling processes and to define the underlying cellular mechanisms. Morphometric analysis of human coronary arteries and of murine femoral arteries at several times following vascular intervention revealed a significant correlation of neointimal and adventitial thickening (R2=0.6845, P<0.001 for human samples; R2=0.6845, P<0.001 for human samples. Immunohistochemical staining for the proliferation marker Ki-67 was performed 7, 14, and 21 days following injury of the murine femoral artery. Formation of a neointimal lesion at 21 days was preceded by high adventitial proliferation rates at 7 and 14 days (85.00±6.041 Ki67+adventitial cells vs. 5.118±0.633 Ki-67+neointimal cells at 7d, P<0.0014; 28.80±5.240 Ki-67+adv. cells vs. 19.40±2.468 Ki-67+neoint. cells at 14d, P<0.006, n=17). Complete removal of the adventitial layer prevented neointima formation, attributing pivotal importance to the adventitial layer (luminal stenosis: 71.73±3.77% vs. 7.44±1.71%, n=5, P<0.0001). Re-transplantation of the aortic adventitia of ubiquitously GFP expressing C57BL/6-Tg (CAG-EGFP)1Osb/Jmice around the medial vascular layer of the femoral artery where the native adventitia has been removed completely restored neointima formation. Importantly, only very view GFP+cells were present in the neointimal layer, indicating that a direct contribution of adventitial cells to the neointimal lesion represents an extremely rare event. To investigate a potential paracrine effect of the activated adventitial layer, we explanted adventitial transplants 14 days following injury and transplantation and incubated the respective samples in serum-free media for 24 hours. BrdU incorporation assays and scratch wound assays revealed significantly increased proliferation and migration rates of human coronary artery SMCs in response to the supernatant of adventitial transplants compared to the supernatant of control samples. Further secretome analyses of the same adventitial supernatants identified predominantly interleukin (IL)-6 to trigger SMC proliferation and migration. Accordingly, serum-free media incubated with adventitial grafts of IL-6−/− mice prevented SMC proliferation and migration. Transplantation of the adventitia of IL-6−/− mice into C57BL/6J wild type mice was not sufficient to trigger neointima formation. Plain old balloon angioplasty, bare metal stent implantation, or drug-eluting stent implantation in swine coronary arteries and analysis for Ki-67+ cell counts supported the hypothesis in the large animal model and a more clinical setting. Acute vascular injury is followed by an expansion of cytokine-producing adventitial cells, whose paracrine function and especially whose release of IL-6 is essential for the subsequent induction of the proliferation and migration of local SMC and thus for neointima formation.