Macrophage functional polarization (M1/M2) in response to varying fiber and pore dimensions of electrospun scaffolds

In this study, we investigated the effect of fiber and pore size of an electrospun scaffold on the polarization of mouse bone marrow-derived macrophages (BMMΦs) towards regenerative (M2) or inflammatory (M1) phenotypes. BMMΦs were seeded on Polydioxanone (PDO) scaffolds electrospun from varying polymer concentrations (60, 100, and 140 mg/ml). Higher polymer concentrations yielded larger diameter fibers with larger pore sizes and porosity. BMMΦ cultured on these scaffolds showed a correlation between increasing fiber/pore size and increased expression of the M2 marker Arginase 1 (Arg1), along with decreased expression of the M1 marker inducible nitric oxide synthase (iNOS). Secretion of the angiogenic cytokines VEGF, TGF-β1 and bFGF was higher among cultures employing larger fiber/pore size scaffolds (140 mg/ml). Using a 3D in vitro angiogenesis bead assay, we have demonstrated that the M2-like profile of BMMΦ induced by the 140 mg/ml is functional. Furthermore, our results show that the pore size of a scaffold is a more critical regulator of the BMMΦ polarization compared to the fiber diameter. The study also shows a potential role for MyD88 in regulating M1 BMMΦ signaling on the large vs. small fiber/pore size PDO scaffold. These data are instructive for the rationale design of implantable prosthetics designed to promote in situ regeneration.