Pore size-mediated macrophage M1-to-M2 transition influences new vessel formation within the compartment of a scaffold

Increasing evidence indicates that material pore size can affect both macrophage (Mφ) polarization and new vessel formation within the compartment of a transplant. Using genipin cross-linked collagen/chitosan (Col-Ch) scaffolds with average pore sizes of 160 and 360 μm (Col-Ch-160 and Col-Ch-360, respectively) as material vascularization models, this study examined the influence of pore size-mediated M1-to-M2 transition of Mφs penetrating the scaffold on subsequent angiogenesis and vascularization. Following seeding in scaffolds, Mφs (M0 or M1) on both scaffolds tended to exhibit the M1 phenotype at day 1 and the M2 phenotype at day 3 and day 7. Col-Ch-360 exhibited a greater potential to encourage M1-to-M2 transition and to promote Mφ secretion of anti-inflammatory and pro-angiogenic cytokines. When supernatants derived from Mφ-scaffold cultures were used to incubate human umbilical vein endothelial cells (HUVECs), cells incubated with Col-Ch-360-derived supernatants displayed a great angiogenic response in terms of tube formation and cell migration. Compared with Col-Ch-160, subcutaneous transplantation of Col-Ch-360 scaffolds resulted in more blood vessel formation, along with more VEGF+ cells and fewer pro-inflammatory (M1) Mφs across the pores of the material. Our data suggest that compared to Col-Ch-160, Col-Ch-360 scaffolds promote angiogenesis in vitro and vascularization in vivo, most likely because Mφs residing in materials with larger pores undergo a higher degree of M1-to-M2 transition.