BackgroundHuman mesenchymal stromal cells (hMSC) are multipotent cells with both regenerative and immunomodulatory activities making them an attractive tool for cellular therapy. In the last few years it has been shown that the beneficial effects of hMSC may be due to paracrine effects and, at least in part, mediated by extracellular vesicles (EV). EV have emerged as important mediators of cell-to-cell communication. Flow cytometry (FCM) is a routine technology used in most clinical laboratories and could be used as a methodology for hMSC-EV characterization. Although several reports have characterized EV by FCM, a specific panel and protocol for hMSC-derived EV is lacking. The main objective of our study was the characterization of hMSC-EV using a standard flow cytometer.MethodsHuman MSC from bone marrow of healthy donors, mesenchymal cell lines (HS-5 and hTERT) and a leukemic cell line (K562 cells) were used to obtain EV for FCM characterization. EV released from the different cell lines were isolated by ultracentrifugation and were characterized, using a multi-parametric analysis, in a conventional flow cytometer. EV characterization by transmission electron microscopy (TEM), western blot (WB) and Nano-particle tracking analysis (NTA) was also performed.ResultsEV membranes are constituted by the combination of specific cell surface molecules depending on their cell of origin, together with specific proteins like tetraspanins (e.g. CD63). We have characterized by FCM the EV released from BM-hMSC, that were defined as particles less than 0.9 μm, positive for the hMSC markers (CD90, CD44 and CD73) and negative for CD34 and CD45 (hematopoietic markers). In addition, hMSC-derived EV were also positive for CD63 and CD81, the two characteristic markers of EV. To validate our characterization strategy, EV from mesenchymal cell lines (hTERT/HS-5) were also studied, using the leukemia cell line (K562) as a negative control. EV released from mesenchymal cell lines displayed the same immunophenotypic profile as the EV from primary BM-hMSC, while the EV derived from K562 cells did not show hMSC markers. We further validated the panel using EV from hMSC transduced with GFP.Finally, EV derived from the different sources (hMSC, hTERT/HS-5 and K562) were also characterized by WB, TEM and NTA, demonstrating the expression by WB of the exosomal markers CD63 and CD81, as well as CD73 in those from MSC origin. EV morphology and size/concentration was confirmed by TEM and NTA, respectively.ConclusionWe described a strategy that allows the identification and characterization by flow cytometry of hMSC-derived EV that can be routinely used in most laboratories with a standard flow cytometry facility.Electronic supplementary materialThe online version of this article (doi:10.1186/s12964-015-0124-8) contains supplementary material, which is available to authorized users.

We evaluated the feasibility, safety, and efficacy of the administration of 4 sequential doses (intravenously administered on days 1, 4, 11, and 18) of cryopreserved bone marrow-derived mesenchymal stromal cells (MSC) expanded with platelet lysate and obtained from third-party donors as a second-line treatment for steroid-refractory acute graft-versus-host (aGVHD) disease in a series of 25 patients. All patients received at least 2 doses of MSC, whereas 21 received 3 doses and 18 received the initially planned 4 doses. Because of the achievement of partial response, 4 patients received additional doses of MSC. Median single cell dose administered was 1.1 × 10(6) MSC/kg of recipient body weight. There were no adverse events related to the MSC infusion in the 99 procedures performed, with the exception of a cardiac ischemic event that occurred twice in a patient with prior history of cardiac ischemia. Response to MSC at 60 days after the first dose was evaluable in 24 patients. Seventeen patients (71%) responded (11 complete and 6 partial responses), with a median time to response of 28 days after the first MSC dose, whereas 7 patients did not respond. In summary, we can conclude that sequential cryopreserved third-party MSC therapy administered on days 1, 4, 11, and 18 is a safe procedure for patients with steroid-refractory aGVHD. This strategy may provide a high rate of overall responses of aGVHD with a low toxicity profile.