Background: Smooth muscle cells (SMC) together with expansion of extracellular matrix are the main driver of neointima formation and restenosis development following vascular injury and intervention. In animal models, endothelial progenitor cells (EPC) accelerate endothelial regeneration and reduce neointima formation of arterial injury, however EPC-capture stents with the aim to limit in-stent-restenosis are not superior compared to conventional stents in regard to target vessel failure. Here we examined the influence of EPC on SMC in crucial steps of neointima formation including proliferation, migration and phenotype switch of SMC.

Methods and Results: EPC, their conditioned medium containing paracrine factors as well as EPC-derived microparticles induce proliferation of SMC with a twofold increase while reducing their apoptosis. In transwell membrane experiments and scratch assays EPC stimulate migration of SMC and accelerate recovery from scratch-induced injury. Treatment of SMC with conditioned medium or microparticles from EPC triggered transformation of SMC towards a synthetic phenotype with decreased expression of smooth muscle-myosin heavy chain, smooth muscle-α-actin, calponin and h-caldesmon. However, co-cultivation of EPC and SMC enabling direct cell-cell contacts results in unaltered expression of contractile phenotype markers and inhibited the transformative effect of cholesterol-loading of SMC with abolished induction of macrophage-related proteins CD68 and Mac-2. Under flow conditions, adhesion of EPC to injured SMC is reduced by blocking CXCR2 and CXCR4. Interaction of EPC with SMC modulates their secretome and synergistically increase the release of selected chemokines. Following carotid wire injury in athymic mice, injection of EPC results not only in in accelerated reendothelialization and reduced neointima formation but in altered cellular composition of the neointima with augmented accumulation of SMC.

Conclusion: EPC stimulate proliferation and migration of SMC and increase their neointimal accumulation following vascular injury. Furthermore, EPC context-dependently modify the SMC phenotype with protection from the transformative effect of cholesterol-loading when direct cell-cell contact is established.