Nanofiber arrangement regulates peripheral nerve regeneration through differential modulation of macrophage phenotypes.

Topographical cues presented by aligned nanofibers have been demonstrated to stimulate peripheral nerve regeneration across long gaps, but the underlying mechanisms remain incompletely elucidated. Because macrophages play a crucial role in peripheral nerve regeneration and can be phenotypically modulated by topographical cues, we hypothesized that aligned nanofibers might induce the development of macrophage phenotypes that facilitate the regeneration of peripheral nerves. Here, macrophages were seeded on aligned and random poly(l-lactic acid-co-ε-caprolactone) nanofibers and their morphology and phenotypes were compared. Aligned nanofibers drastically stimulated macrophage elongation along the nanofibers, and, more importantly, induced the development of a pro-healing macrophage phenotype (M2 type), whereas random nanofibers induced a proinflammatory phenotype (M1 type). Notably, the macrophages polarized by aligned nanofibers potently promoted the proliferation and migration of Schwann cells in vitro. Thus, we constructed nerve-guidance conduits by using aligned and random nanofibers and evaluated their effects on macrophage polarization and nerve regeneration in a rat sciatic nerve defect model. Our in vivo results showed that the ratio of pro-healing macrophages was again higher in the aligned-nanofiber group, and further that Schwann cell infiltration and axon numbers were 2.0- and 2.84-fold higher in the aligned group than in the random group, respectively. This study demonstrates that nanofiber arrangement differentially regulates macrophage activation and that nerve-guidance conduits constructed from aligned nanofibers markedly facilitate peripheral nerve regeneration at least partly by promoting the pro-healing phenotype in macrophages.