Physical confinement in alginate cryogels determines macrophage polarization to a M2 phenotype by regulating a STAT-related mRNA transcription pathway

The immunologic response is considered to play a pivotal role in the application of biomaterial implants, and intrinsic properties of biomaterials can significantly modulate the anti-inflammatory effects. However, how physical confinement influences M2 polarization of macrophages and the relevant mechanisms have not been clearly elucidated. In this study, pore size and porosity in cryogels can be mediated by utilizing alginates with different viscosities. Cryogels of small pore size and low porosity can restrict M2 polarization of macrophages in vitro, judging from cell morphology, secretion of cytokines and expression of key M2-related genes. In comparison, cryogels of large pore size or high porosity can induce M2 polarization in vivo, resulting in the anti-inflammation effects. High-throughput RNA-seq analysis demonstrates that the mRNA surveillance pathway is key in the polarization process, and four primary transcription factors (PPAR-γ, STAT6, NF-κB, and STAT1) participate probably by competition in DNA binding to regulate M2-related gene expression. This study confirms that enough physical space inside is necessary to promote M2 polarization for the anti-inflammatory performance, which can be applied widely in the fields of tissue engineering and regenerative medicine.