Renal transplantation is currently the preferred treatment for patients with end-stage renal failure [1]. To address the widening gap between the growing need for renal transplantation and the availability of organs from brain-dead donors, the use of kidneys from donors after circulatory death has increased and become the major source of transplants in several countries [2]. However, during deceased donor renal transplantation, prolonged ischemic time from donor harvest to kidney reperfusion in the recipients resulted in higher incidences of delayed graft function (DGF) and acute rejection, which has adverse effects on grafts outcomes [3–6]. For renal allografts, DGF is primarily a consequence of pre-transplant injury and immune responses after reperfusion, whereas acute rejection is related to T cell clonal expansion and differentiation of effector cells during donor kidney injury [7]. Hence, the optimal therapy to prevent both DGF and acute rejection for renal transplantation using deceased donors should be able to repair renal structure injury and suppress immune response simultaneously. However, no current induction therapies possessed such capacity.

Mesenchymal stem cells (MSCs) present significant anti-inflammatory, tissue repair, and immune-modulatory properties, and could be used in a novel cell-based approach in renal transplantation. Effects of MSCs have been explored in several preclinical models of acute kidney injury (AKI) [8]. MSCs could attenuate the process of interstitial fibrosis/tubular atrophy, reduce macrophage infiltration and inflammatory cytokine expression, and increase anti-inflammatory factors in a rat kidney allograft model [9, 10]. Although initial trials showed safety and feasibility of MSCs treatment in renal transplantation, few data on its capacity to prevent both DGF and acute rejection have been reported, and the disadvantages associated with uncertainty of influencing factors, timing, dosage, route of administration, and frequency of treatment had negative effects on the clinical application of MSCs [11–14].

Thus, we conducted a multicenter randomized controlled trial of MSCs to clarify the clinical value of allogeneic MSCs, i.e., human umbilical-cord-derived MSCs (UC-MSCs), as induction therapy to prevent both DGF and acute rejection in deceased donor renal transplantation.

To the best of our knowledge, this is the first multicenter randomized paired trial that showed the safety and feasibility of UC-MSCs infusion in patients undergoing kidney transplantation. We found that systemic administration of fresh UC-MSCs as induction therapy could achieve a substantial reduction in the incidence of DGF, although no significant difference was found between the MSC and non-MSC groups. Additionally, we also demonstrated that the administration of UC-MSCs via the renal artery during operation is not feasible because no injected MSCs were observed in the renal allograft.

Mesenchymal stem cells not only can limit inflammatory responses but also have the potential to induce antifibrotic activity and tissue regeneration [23–25]. Multiple studies using animal models have attempted to prove the beneficial effects of MSC on tissue injury and inflammation and presented promising results; however, few studies that assess the safety and efficacy of MSCs clinically have been conducted. In a recent study, patients administered with autologous MSCs had a lower incidence of acute rejection and opportunistic infection and better eGFR at 1 year postoperatively [12]. Although recent MSC trials in renal transplantation presented evidence on the benefits of MSC, no studies on the simultaneous prevention of DGF and acute rejection by MSCs in deceased donor renal transplantation exist. Additionally, most studies used autologous MSCs, which possess several key disadvantages due to lack of donor selection and availability “off-the-shelf” for clinical use without the delay required for expansion [26]. Moreover, numerous different challenges in the use of MSCs in clinical trials exist. Firstly, because of the absence of updated and adequate good manufacturing practice guidelines for MSC-based therapy for the kidneys, MSCs may be derived from different materials, which may in turn exert a different influence on clinical trial results [27]. Secondly, no studies involving a large dose of MSCs have been conducted, and in most studies, the dose and frequency of MSC administration are empirically chosen. Studies on renal transplantation typically involved a dose ranging from 0.58 × 10⁶ to 5.0 × 10⁶ MSCs per kilogram body weight; however, in graft-versus-host disease, a higher dose of 9.0 × 10⁶ MSCs was administered [25]. The route of MSC administration is another aspect that should be considered in renal treatment. In most clinical studies, intravenous infusion administration was performed; however, cells administered intravenously could also be attracted to lung tissues, thereby affecting the accurate homing of the cells to the renal tissues [28]. Hence, specific delivery of MCSs needs to be performed. It must be noted that MSC-based therapy is currently being developed. Nevertheless, this therapy would be considered a new approach for the treatment of kidney disease. To obtain better outcomes, randomized and controlled multicenter clinical trials are necessary [29].

Consequently, we performed this multicenter randomized paired trial of MSCs to clarify the clinical value of UC-MSCs as induction therapy to prevent both DGF and acute rejection in deceased donor grafts. The ultimate goal of this approach was to achieve low DGF and acute rejection incidences as well as long-term transplant survival. Moreover, confirming the safety and efficacy of human UC-MSCs could expand the source of MSCs. Potential risks of MSCs in solid organ transplantation include direct toxicity, malignancies, and risks for excessive immunosuppression and immunogenicity [30, 31]. In previous studies, some opportunistic infections including BKV nephropathy and cytomegalovirus infections were observed because of immunosuppression related to MSCs [12, 13]. Additionally, due to a short follow-up period and the inclusion of ill patients with poor prognosis, it was difficult to find an association between new malignancies and the use of MSC therapy [32]. In our study, MSCs infusion was safe and well tolerated during the perioperative and follow-up periods with no adverse events, which could be attributed to the adequate good manufacturing practice guideline for MSCs in our center. Moreover, the single dose of 2 × 10⁶ UC-MSCs per kilogram body weight was insufficient to induce direct toxicity and excessive immune suppression and immunogenicity. Risk of malignancies could be evaluated with a longer follow-up period.

In our study, administration of UC-MSCs could not decrease the incidence of DGF and acute rejection. We speculate that the timing and frequency of UC-MSC administration limited the effect on the renal allografts. The timing of MSC administration has been studied in some ischemic disease models [33, 34]. In a previous study, MSCs used immediately or 24 h post ischemia/reperfusion (I/R) induced significant renoprotection [35]. In a sheep model, MSCs treatment for renal I/R showed that treatment with MSCs at 15 days post-I/R was inferior to immediate treatment [36]. A recent meta-analysis also revealed that MSCs administration 1 day after injury yielded greater therapeutic value than that within 24 h of injury [37]. In our study, three disadvantageous processes possibly limited the positive effect of UC-MSCs administered before the operation. First, the inflammation environment in the early injured kidney was unfavorable for MSCs survival within the tissue, thereby decreasing the stem cell effect. Second, the insufficient expression of the homing adhesion molecules ICAM-1 and VCAM-1 prevented MSCs integration into the injured kidney tissue [38]. Third, upregulated pro-inflammatory markers in the I/R kidney tissue, such as TNF-α and IL-1β, prevented the effect of MSCs.

Moreover, we performed a multiprobe FISH assay to detect cell location in a kidney biopsy sample and showed that no UC-MSCs were found in the female recipient’s biopsy sample, which is consistent with aforementioned explanations on the effects of MSCs administration. Additionally, only one dose of UC-MSCs was administered during the operation, which was possibly insufficient to have an effect on renal allograft outcome. Hence, several infusions may be more preferable than one infusion [10]. In a previous clinical trial, a high dose, ranging from 150 to 300 million MSCs, was used because of the low survival and engraftment of MSCs after transplantation. However, this could result in transfusion reactions such as allergic reaction, fever, hypotension, and infection [39]. Thus, we suggested that increasing MSCs administration frequency, such as twice per week over the course of 2 weeks, may provide a sufficient number of cells and prevent risks of transfusion reaction.

In our study, graft and recipient survivals and the incidence of postoperative complications were comparable between the two groups. Postoperative renal function recovery could be understood through changes in glomerular filtration rate (GFR) curves. We compared the GFR curves postoperatively between the MSC and non-MSC groups; no significant difference was found, indicating similar renal function recovery between the groups.

Our results have some promising significance for future work with UC-MSCs therapy in renal transplantation. However, several limitations of this study must be considered. Firstly, information about the long-term outcome of UC-MSCs therapy in renal transplantation was not available in our study because of the limited follow-up time. Secondly, we failed to fully demonstrate the efficacy of the treatment due to the small sample size and the single dose of cell infusion. Moreover, the small sample size in this study makes full assessment of the differences in DGF incidence difficult, although a lower incidence was found in the MSCs group.

Mesenchymal stem cell-based cell therapy to prevent complications associated with renal transplantation is considered a new horizon. Our pilot study suggests that adequate administration timing and frequency of UC-MSCs possibly have an important effect on graft and recipient outcomes. Furthermore, direct administration of UC-MSCs via the renal artery is insufficient because of the observed absence of injected MSCs in renal tissues. Nevertheless, it must be noted that the therapy is still at the preliminary stages of development. Further studies are needed to fully establish the administration of MSCs as a novel induction therapy in renal transplantation.