Reply: Supplementation with Extracellular Vesicles Derived from Adipose-Derived Stem Cells Increases Fat Graft Survival and Browning in Mice: A Cell-Free Approach to Construct Beige Fat from White Fat Grafting.

Abstract

Sir: We thank Dr. Laloze et al. for their kind and insightful letter regarding our recent article identifying the finding that supplementation with extracellular vesicles derived from adipose-derived stem cells increases fat graft survival and browning by which extracellular vesicle–polarized M2 macrophages secrete catecholamines to promote beige adipose regeneration.1 Laloze et al. describe the limits of the method we used to measure the survival rate of fat grafts, and suggested that the main tools used as the gold standard are imaging with magnetic resonance imaging or computed tomography with three-dimensional volumetric computer analysis. We agree with this view that intuitively evaluates the volume retention under noninvasive conditions, although we have provided evidence that adipose-derived stem cell extracellular vesicles promote the survival rate of fat grafts by pathologic detection. As suggested by Laloze et al., it is necessary to perform a comparative study between the different methods of lipofilling enrichment in large-animal models first, and then in humans to compare the differences between adipose-derived stem cell extracellular vesicles and adipose-derived stem cells in regulating the survival and outcome of fat grafting, and to clarify which methods present the best clinical result. The comparative study between adipose-derived stem cell extracellular vesicles and adipose-derived stem cells is of great interest to us. Indeed, we compared the effects of both adipose-derived stem cells and adipose-derived stem cell extracellular vesicles to fat grafting in a model of nude mice. We co-grafted 100 μg/ml adipose-derived stem cell extracellular vesicles and 5 × 105 adipose-derived stem cells with 0.3-ml purified liposuction into the scalps of nude mice. The volume retention rate of adipose-derived stem cell–treated grafts was approximately 20 percent higher than that treated by adipose-derived stem cell extracellular vesicles at postoperative week 12. However, it is difficult to obtain convincing evidence in this comparative experiment because of the difference in dose units between adipose-derived stem cells (number of cells) and adipose-derived stem cell extracellular vesicles (protein concentration). We only hypothesize that adipose-derived stem cell extracellular vesicles and other secretion groups derived from stem cells could replace stem cell therapy under the premise of appropriate doses and times in the models and that transplanted stem cells play only a paracrine role and not homing or differentiation. For the application of adipose-derived stem cell extracellular vesicles in fat grafting and other ischemic diseases, researchers should pay more attention to extracting extracellular vesicles with increased biological function by optimizing the microenvironment of cell culture. It is a feasible method for simulating the microenvironment of transplanted stem cells in vitro. Because transplanted stem cells participate in tissue regeneration almost only by means of paracrine rather than directional differentiation in fat grafting,2 we used hypoxia incubator and serum-free medium to simulate the ischemic and hypoxic microenvironment of transplanted adipose-derived stem cells in our previous study. Adipose-derived stem cell extracellular vesicles cultured under hypoxia possess a stronger ability to promote angiogenesis both in vitro and in a fat grafting mouse model compared with that cultured under normoxic conditions.3 In addition to hypoxic preconditioning, any treatment to simulate the microenvironment of transplanted stem cells, such as three-dimensional culture, oxidative stress stimulation and the intervention of a variety of cytokines may also alter the biological function of adipose-derived stem cell extracellular vesicles. The potential therapy of adipose-derived stem cell–derived exosomes in obesity and diabetes by means of promoting M2 polarization of macrophages was studied by Zhao et al.4 Intraperitoneal administration of adipose-derived stem cell–derived exosomes led to a resistance to sustained weight gain in mice fed a high-fat diet, and significantly inhibited the mass growth of both visceral and subcutaneous white fat pads. In addition, treatment with adipose-derived stem cell–derived exosomes decreased the levels of serum triglyceride and total cholesterol in serum of mice fed a high-fat diet, and showed significant improvement in glucose tolerance and insulin sensitivity even before the reduction in obesity progression. In our study, we also formed beige fat with increased oxygen consumption from the treatment of conditioned medium of adipose-derived stem cell extracellular vesicle–polarized M2 macrophages. Thus, we prudently hypothesize that M2 polarization of macrophages could be used as an auxiliary scheme for the therapy of obesity or type 2 diabetes, and adipose-derived stem cell extracellular vesicles could be used as the inducing factor of M2 polarization. However, targeted therapy for the secretory function of hypertrophic adipocytes should also be evaluated because the secretion of hypertrophic adipocytes is responsible for the accumulation of M1 macrophages.5,6 DISCLOSURE The authors have no financial interest to declare in relation to the content of this communication. The authors received no funding support for this work. Yang-Yan Yi, M.D.Department of Plastic SurgerySecond Affiliated Hospital of Nanchang UniversityNanchang, Jiangxi, People’s Republic of China