Macrophages Undergo M1-to-M2 Transition in Adipose Tissue Regeneration in a Rat Tissue Engineering Model

Macrophages are involved in the full processes of tissue healing or regeneration and play an important role in the regeneration of a variety of tissues. Although recent evidence suggests the role of different macrophage phenotypes in adipose tissue expansion, metabolism, and remodeling, the spectrum of macrophage phenotype in the adipose tissue engineering field remains unknown. The present study established a rat model of adipose tissue regeneration using a tissue engineering chamber. Macrophage phenotypes were assessed during the regenerative process in the model. Neo-adipose tissue was generated 6 weeks after implantation. Macrophages were obvious in the chamber constructs 3 days after implantation, peaked at day 7, and significantly decreased thereafter. At day 3, macrophages were predominantly M1 macrophages (CCR7+), and there were few M2 macrophages (CD206+). At day 7, the percentage of M2 macrophages significantly increased and remained stable at day 14. M2 macrophages became the predominant macrophage population at 42 days. Enzyme-linked immunosorbent assay demonstrated transition of cytokines from pro-inflammatory to anti-inflammatory, which was consistent with the transition of macrophage phenotype from M1 to M2. These results showed distinct transition of macrophage phenotypes from a pro-inflammatory M1 phenotype to an anti-inflammatory M2 in adipose tissue regeneration in our tissue engineering model. This study provides new insight into macrophage phenotype transition in the regeneration of adipose tissue.