Commentary on: A Systematic Review on Extracellular Vesicles-Enriched Fat Grafting: A Shifting Paradigm

The outstanding systematic review on the application of extracellular vesicles (EVs) in fat grafting shed a promising light on the different strategies adopted to improve fat retention following grafting. The role of mesenchymal stem cell–derived extracellular vesicles (MSC-EVs), otherwise known as “exosomes” in fat graft, has emerged overwhelmingly in the very recent years, fundamentally as straightforward proposals based on in vitro and experimental animal models.^1,2 In the very recent years, EVs have earned encouraging success in several clinical fields, acting not only as drug vectors but even as concurrent elements in modulating host physiology and participating in the complex cell-to-cell signaling.^3,4 Due to their high variability, a possible question may arise whether handling exosomes might result into a tricky and burdensome practice. The contribution published in this journal gives encouraging perspectives about EVs utilization in fat grafting. EVs represent a highly complex machinery by which stem cells tune and organize their survival and differentiation in a widely expressing repertoire of different signals.⁵ On the other hand, EV application encourages many researchers in considering their possible role in clinics. Some authors addressed several reports forwarding the promoting role of EV-enriched fat grafting in improving fat graft retention rates.^1,6-11 Many of these reports should better consider a crucial step in fat grafting, that is, the “amount and way” by which fat is transferred, a hallmark that unfortunately is missing in the excellent aforementioned systematic review. The recent International Expert Panel Consensus of Fat Grafting in the Breast, for example, reported that the majority of panelists experienced a fat intake/adipocyte survival as high as 30%.¹² The introduction of fat cells should take into account the recipient’s micro-environment, and the ability of adipocytes to be induced in forming cross-talk interactions with the host’s milieu is closely dependent on the way fat graft is performed.¹³ More than introducing EVs, a fundamental issue should be highlighting the interplay between grafted EV-enriched fat and the recipient’s microenvironment. Whereas contrasting opinions were recently reported about the utilization of filtration/centrifugation rather than gravity sedimentation in harvesting and treating fat immediately before grafting,^12-16 a technique employed more for practical than scientific reasons,¹⁵ fat graft retention, necrosis, and inflammation still represent the major burden of concerns regarding fat grafting.¹² Rose et al¹⁶ demonstrated that sedimentation technique preserves intact adipocytes and nucleated cells, thus allowing to rescue a correct ratio of pre-adipocytes to mature adipocytes, which might yield a promising outcome in fat retention rates. Even a modest decantation approach (1300 rpm for 5 minutes) showed a higher retention rate than Coleman’s centrifugation at 3000 rpm for 3 minutes.¹⁷ It is tempting to speculate that relatively softer procedures in fat harvesting and the utilization of 1- to 2-mm cannulas for autologous reinjection by most of surgeons¹² should contribute to enriching fat grafts from micro-vesicles and exosomes, which may be lost or damaged in a much harder filtration/centrifugation method. This issue might be expanded in a further debate to ensuring the successful application of EVs in fat grafting. Moreover, in our modest opinion, these items should be taken much more seriously in clinics rather than in experimental models.

As the authors attempted in their published contribution,¹⁸ a sound meta-analysis should elucidate if EV enrichment in fat grafting could be considered a reliable, straightforward tool for aesthetic medicine and plastic surgery, along with the increasing conviction that EVs may represent an amazing novelty. Despite that initially, the role of EVs in clinics may appear anecdotal, the interest in their application is widely acknowledged even in fields beyond aesthetic medicine.^19,20

The systematic review on the utilization of MSC-EVs in fat grafting we are debating herein reports a huge deal of studies performed on experimental models. This should indicate that evidence from clinical trials is yet poorly represented and encourage researchers to improve this field. From a whole collection of 1727 contributions in the field, a bulk of 62 full-length papers was retrieved, and 7 articles, dealing with a total of 294 mice, were selected, probably too scanty to forward a sound conclusion about the clinical role of MSC-EVs in human fat grafting. The studies included in the systematic review^1,6-11 merit to be elucidated further in this Commentary.

The research by He Huang and colleagues⁶ reported that EVs from bone marrow-derived stem cells (BMSC-EVs) of Wistar rats improved the retention of fat grafting from humans by liposuction to female nude mice (BALB/c-nu, age: 6 weeks; weight: 14-17 g). In this experimental paper, the authors utilized EVs from rat BMSC characterized by the positive expression of CD90, CD29, CD81, and CD63, and negative for endothelial and hematopoietic cell markers (ie, CD31 and CD45), so reporting only EVs coming from bone marrow stem populations. No information about how fat was harvested and re-injected was fully addressed despite the fact mice were being discussed. The authors transplanted fat grafts withdrawn from 10 female patients by liposuction, calculating the amounts of fat via their method,²¹ into BMSC-EV–pretreated nude mice.⁶ The paper by Huang and colleagues⁶ might therefore erroneously suggest a patchwork-like investigation, made of different experimental models, which would be difficult to be included in a debate on EVs in clinics. However, this is the scientific, straightforward frontier of EVs in fat grafting. On the contrary, Han et al⁸ employed EVs or exosomes from adipose-tissue derived stem cells (ADSC), but their experimental model was, again, the mouse (26-week-old female BALB/c nude mice). Yudi Han and colleagues⁸ also reported that hypoxic ADSC may produce exosomes able to increase a VEGF-mediated angiogenesis, thus assessing a role of EVs in enhancing micro-vasculation as recently conducted by Huang and colleagues⁶ on human umbilical vein endothelial cells. In the study by Yudi Han and colleagues,⁸ ADSC-EVs from humans were utilized on nude mice as fat graft recipients. Moreover, the contribution by Shan Mou et al⁹ which was highly debated,^22-24 also employed fat from humans for grafting into mice (male BALB/c-nu nude mice, 6 weeks old, weighing 16-18 g).⁹ One of the leading comments on the paper from Mou et al²⁴ is that in vitro experimental attempts are difficult to consider as pivotal studies to forward a clinical decision about the reliability of EVs in fat grafting. This circumstance can be observed also considering further studies included in the aforementioned systematic survey. When considering the study by Chen et al,¹⁰ many comments arose in the literature.^25-29 Criticisms on the papers by Mou et al²⁴ and Chen et al¹⁰ emerged because of the burdensome experimental process employed to demonstrate that EVs can in fact ameliorate fat retention on grafting, probably inducing angiogenesis, a promise forwarded also by more recent papers, again on animal models of fat grafting.^1,11

Finally, EVs represent an intriguing opportunity for fat grafting, but further insightful clues, particularly from clinics, are mandatory. This continues to represent a big concern for investigators, and in this sense the preliminary evidence earned from experimental models might be considered with particular positivity. The exosome dynamics and physiology are a highly complex machinery driving the way in which adipocytes communicate with each other and with their tissue microenvironments.³⁰ Therefore, in this sense, the local host’s microenvironment should be better targeted to elucidate how EVs work once they are transferred in the EV-enriched fat grafts in the recipient. Particular attention should be devoted to what is occurring within the complex microenvironment (resident adipose tissue/transferred fat) more than emphasizing the role of EV transfer.¹³

Furthermore, even the content of exosomes is crucial to forecast the proper suitability of MSC-EVs in improving fat grafting in aesthetic medicine, particularly because EVs transfer particular micro-RNAs, which may be critical in adjusting and deploying different signaling systems and codes able to finely tune the subtle balance between fat cells and their grafted micro-environment.^13,30 EV biology is amazing, but its suitability surely needs to be better defined for fat grafting in humans. Probably the authors solicited EVs as an amazing opportunity, though more insights are needed. Fundamentally, the systematic review, which appears to encourage further investigation on the issue, should highlight better that despite the burdensome complexity of EV application in fat grafting, because EVs are complex systems of vesicles endowed with a huge panoply of different mediators and miRNAs, they may be perfectly suited for humans. However, research is still progressing in this field. Research in EVs represents an intriguing novelty in the field and more novel investigations, based on clinical trials, are required to elucidate the role of EVs in fat grafting.

Disclosures

The authors declared no potential conflicts of interest with respect to the research, authorship, and publication of this article.

Funding

The authors received no financial support for the research, authorship, and publication of this article.

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