Early Macrophage Infiltration Improves Fat Graft Survival by Inducing Angiogenesis and Hematopoietic Stem Cell Recruitment

Background: Fat grafting is a popular soft-tissue filler method; however, its results are variable and technique-dependent. Macrophages are present in fat grafts and closely associated with tissue regeneration. The authors hypothesized that activation/depletion of early macrophages in transferred fat improves/impairs fat graft survival. Methods: Mouse inguinal fat (approximately 150 mg) was transferred autologously. Fat grafting was first performed without other manipulations to obtain baseline information. Then, liposome-encapsulated clodronate and macrophage-colony stimulating factor were used in a mouse fat grafting model for local macrophage depletion or activation. The authors examined the graft stromal vascular fraction by fluorescence-activated cell sorting at 1, 2, 4, and 12 weeks after transplantation in manipulation and control groups. Graft weight, vascularization, and secreted factors were also compared. Results: Early depletion of macrophages resulted in incompetent angiogenesis, feeble Sca-1(+)/CD45(+) stem cell recruitment, and eventually a poor retention rate (34 +/- 6 mg versus control 84 +/- 15 mg; p = 0.006), whereas up-regulated macrophages allowed better angiogenesis and survival (117 +/- 12 mg versus control, 84 +/- 15 mg; p = 0.043). Conclusions: In fat grafting, macrophages and their polarization initiated changes in the levels of dominant secreted factors and influenced blood-derived stem cell infiltration, indicating that macrophages were crucial for tissue revascularization. The macrophage manipulation models described here show that graft macrophage number can profoundly influence graft survival.