Tuning macrophage phenotype for enhancing patency rate and tissue regeneration of vascular grafts

Macrophages are primary immune cells that play a crucial role in tissue regeneration during the early stages of biomaterial implantation. They create a microenvironment that facilitates cell infiltration, angiogenesis, and tissue remodeling. In the field of vascular tissue engineering, numerous studies have been conducted to modulate the macrophage phenotype by designing various biomaterials, which in turn enhances the regenerative capacity and long-term patency of vascular grafts. However, the mechanism underlying the different phenotypes of macrophages involved in the tissue regeneration of vascular grafts remains unclear. In this study, vascular grafts loaded with various macrophage phenotypes were developed, and their effects were evaluated both in vivo and in vitro. The RAW 264.7 macrophages (M0) were initially treated with LPS or IL-4/IL-10 and polarized into M1 and M2 phenotypes. Subsequently, M0, M1, and M2 macrophages were seeded onto electrospun PCL scaffolds to obtain macrophage-loaded vascular grafts (PCL-M0, PCL-M1, and PCL-M2). As prepared vascular grafts were implanted into the mouse carotid artery for up to one month. The results indicate that the loading of M2 macrophages effectively enhances the patency rate and neotissue formation of vascular grafts. This is achieved through the development of a well-defined endothelium and smooth muscle layer. RNA sequencing was used to investigate the mechanisms of action of different macrophages on tissue regeneration. The study found that M1 macrophages inhibited tissue regeneration by mediating angiogenesis and chronic inflammation through upregulation of VEGFa, IL-1β, and IL-6 expression. In contrast, M2 macrophages regulate the immune microenvironment by upregulating the expression of IL-4 and TGF-β, thereby promoting tissue regeneration. In conclusion, our study demonstrates how different macrophage phenotypes contribute to the initial inflammatory microenvironment surrounding vascular grafts, thereby modulating the biological process of vascular remodeling. STATEMENT OF SIGNIFICANCE: Regulating the biophysical and biochemical characteristics of biomaterials can induce macrophage polarization and enhance vascular remodeling. In previous work, we fabricated a vascular graft with a macroporous structure that promoted macrophage infiltration and polarization into a pro-regenerative phenotype. To illustrate the mechanism, we established a new mouse model and evaluated the effects of different macrophages on vascular regeneration. The study revealed that tuning macrophage phenotype can impact the initial inflammatory microenvironment by secreting cytokines, which can increase the patency rate and regenerative capacity of vascular grafts. These findings provide essential theoretical support for the development of immunoregulatory scaffolds for vascular and other tissue regeneration.