Alternatively Activated M2 Macrophages Improve Autologous Fat Graft Survival in a Mouse Model through Induction of Angiogenesis

Background: Variability in graft retention with subsequent undercorrection remains a significant limitation of autologous fat grafting. The authors evaluated whether graft retention in a mouse model could be improved via graft supplementation with alternatively activated M2 macrophages, cells known to play a critical role in tissue repair. Methods: Grafts from C57BL/6 mouse inguinal fat pads were supplemented with M2 macrophages generated by intraperitoneal Brewer's thioglycollate injection and in vitro culture. Grafts with saline or M2 macrophages were injected under recipient mouse scalps and assessed by serial micro-computed tomographic analysis. Explanted grafts underwent immunohistochemical and flow cytometric analyses. M2 culture supernatants were added to stromal vascular fraction adipose-derived stem cells to assess adipogenic gene expression induction. Results: One month after graft injection, no significant difference was noted between M2 macrophage-supplemented (105 +/- 7.0 mm(3)) and control graft volumes (72 +/- 22 mm(3)). By 3 months after injection, M2 macrophage-supplemented grafts remained stable, whereas controls experienced further volume loss (103 +/- 8 mm(3) versus 39.4 +/- 15 mm(3); p = 0.015). Presence of macrophages in supplemented grafts was confirmed by flow cytometry. M2 macrophage-supplemented grafts exhibited a 157 percent increase in vascular density compared with controls (p < 0.05). Induction of adipogenic C/EBP alpha gene expression was observed with M2 supernatants addition to stromal vascular fraction adipose-derived stem cells. Conclusions: M2 macrophages improve autologous fat graft volume retention by stimulating angiogenesis. These findings provide proof-of-principle for development of fat grafting techniques that harness reparative properties of M2 macrophages.