MSCs for clinical applications are classified as advanced therapy medicinal products (ATMP) and should be manufactured in full compliance with Good Manufacturing Practice guidelines to ensure quality and reproducibility. Despite these demands, protocols are not uniform yet. MSCs may be derived from different starting materials (mainly BM, AT, or cord blood), and different media might be used for culturing the cells. MSCs require an approved and well-defined panel of assays to be released for clinical use. Because MSCs need to be cultured under Good Manufacturing Practice conditions, it is a costly therapy. Centralization of MSC production (with sharing of procedures and protocols) at specialized laboratories might reduce costs and promote standardization of the MSC product. Once the efficacy of MSC therapy has become clear, a cost-effect analysis is of major importance.
In renal transplantation, so far the published studies use BM-derived cells. The BM contains only a very low percentage (0.001%-0.01%) of primary MSCs, and therefore expansion of MSCs ex vivo is necessary. To culture and differentiate MSCs, an optimal combination of several factors, including cytokines, growth factors, and serum supplements is essential. Clinical trials so far have mainly used fetal calf serum–expanded cells; however, studies are currently investigating animal serum-free culture conditions.72 These animal-free additives include platelet lysate/platelet-rich plasma, cytokines and growth factors and human serum. The culture process takes approximately 4 to 6 weeks, which makes applications for acute indications a logistic challenge. Currently, more efficient methods for manufacturing of clinical MSCs is being developed, including bioreactor systems, which are specifically suited for MSC expansion.
Until now, no large-dose finding studies are performed, and in most studies, the dose and frequency of MSCs is chosen empiric. A potency assay which can guide us in the best dosage and frequency of administration would be of interest, but is a challenge to develop. Doses of MSC are chosen above what is considered the minimal effective dose and below a potential toxic dose. The studies in renal transplantation administered 0.5 × 106 to 5.0 × 106 MSCs per kilogram, however, in other studies, including GvHD higher amounts (0.4 to 9.0 × 106) were administered.73 The frequency of 2 infusions might be preferable compared with 1 infusion. A long-lasting response was hardly observed in patients with steroid-resistant GvHD who received 1 infusion, whereas most responders had 2 or more infusions.74 So far, intravenous infusion has been the route of administration for most clinical studies, including in renal transplantation and has been shown to be safe. MSCs can also be injected directly into the kidney or underneath the kidney capsula with the advantage of direct homing and no entrapment in the lungs. However, the development of embolisms could be one of the side effects.
It is of importance that an optimal concurrent immunosuppressive regimen is chosen in which drugs have no negative impact on MSC function and vice versa.75 Different in vitro studies75 and a few animal studies have elucidated the interaction of MSCs with concurrent immunosuppression.76-79 The combination of MSCs with mycophenolate mofetil (MMF) was shown to significantly prolong survival of allografts compared with MSCs alone.77 Of interest, it was also found that MSC monotherapy inhibited acute graft rejection and in combination with rapamycin induced donor-specific allograft tolerance. In tolerant recipients, MSCs migrated to the transplanted heart and various lymphoid organs, and a high frequency of Tol-DCs and CD4+CD25+Foxp3+ T cells was found.78 The results support the clinical applicability of MSCs in combination with a mammalian target of rapamycin inhibitor or MMF in clinical transplantation.
A few studies have focused on the role of MSCs in the early induction phase in renal transplant recipients. In a pilot study of safety and clinical feasibility, autologous MSC infusion was shown to be feasible, allowing an increase of Treg cells in the peripheral blood and control of memory CD8+ T-cell function.80 In these patients, as seen in the experimental studies, timing of the infusion seemed of major importance, because MSC therapy early after transplantation negatively affected kidney graft function, which was not the case when MSCs were given before transplantation.81 In a trial among 159 patients undergoing renal transplantation, the use of autologous MSCs compared with anti-IL-2 receptor antibody induction therapy resulted in lower incidence of acute rejection, decreased risk of opportunistic infection, and better estimated renal function at 1 year.82 In a study by Mudrabettu et al,83 in living kidney transplantation, safety and feasibility was demonstrated in 4 patients who were treated with autologous BM MSCs 1 day before and 1 month after transplantation. In this study, no early or late kidney dysfunction or viral infections were reported.
In a phase 1 clinical study, we tested safety and feasibility of autologous MSC therapy in HLA-DR mismatched patients with subclinical rejection in their renal biopsy at 4 or 24 weeks after renal transplantation.84 MSCs were infused in 6 patients, and the therapy was feasible and well tolerated without adverse events related to the treatment itself. In addition, initial results suggested immunosuppression after MSC therapy. The MSC-treated patients demonstrated a profound reduction in donor-specific peripheral blood mononuclear cells (PBMC) proliferation 12 weeks after MSC infusion, whereas the response to third-party PBMCs was more variable. Three patients developed opportunistic viral infections, which was likely to be related to the MSC treatment. In 2 patients with allograft rejection, the infiltrate had disappeared after the MSC infusion (Figure 2). In a follow-up study of 70 renal transplant recipients, the hypothesis that MSCs in combination with everolimus can facilitate tacrolimus withdrawal and reduce fibrosis is currently tested.85
Allogeneic MSCs offer the advantage of immediate availability for clinical use which is of major importance in indications where treatment is needed without delay, such as allograft rejection or CNI toxicity. However, allogeneic MSCs could possibly elicit an antidonor immunoresponse, which may increase the incidence of rejection and impact the allograft survival in the long term. Within the transplant setting, there are conflicting preclinical results regarding allogeneic MSCs. Some preclinical studies showed an accelerated graft rejection after allogeneic MSC administration,86-89 whereas others showed that allogeneic MSCs promote graft survival.77,78,87 One clinical study investigated allogeneic donor-derived MSCs combined with low-dose tacrolimus in 6 renal transplant recipients. They could give a 50% reduction of tacrolimus dose in the MSC group; however, long-term data have not been reported, and it was not determined whether HLA-specific antibodies were produced.90
In another study, donor-derived BM MSCs combined with a sparing dose of tacrolimus (0.04-0.05 mg/kg per day) were administered to 16 de novo living-related kidney transplant recipients; 16 other patients received a standard dose of tacrolimus (0.07-0.08 mg/kg per day). The safety of MSC infusion, acute rejection, graft function, graft survival, and patient survival were evaluated over 24 months or longer after kidney transplantation. All patients survived and had stable renal function at the 24-month follow-up. The combination of low-dose tacrolimus and MSCs was as effective as standard-dose tacrolimus in maintaining graft survival at least 2 years after transplantation.91 A recent study has analyzed the immunosuppressive effects of MSC (1.5-3.0 × 106/kg) infusion, planned 3 to 5 days after transplantation in 5 patients, who were prospectively screened for anti-HLA antibodies at months 1, 3, and 6. So far, 2 patients developed anti-HLA antibodies against shared kidney/MSC mismatch, and 1 patient developed 2 specific antibodies against MSC at month 6. All antibodies were anti-HLA class I except for 1.92 In our current clinical study (the Neptune study), allogeneic MSCs are infused at a timepoint where immunosuppression is lowered and the kidney is at increased risk for developing immune-mediated injury.88 Primary endpoints of this study include allograft rejection and graft loss. Secondary end points, all measured before and after MSC infusions, include de novo HLA antibody development and extensive immune monitoring.
To minimize the chance of development of antidonor immune responses and thus rejection, selected allogeneic MSCs are given in which MSCs have no similarity with the HLA mismatches between the kidney graft and the recipient and the recipient should have no HLA antibodies directed to the MSCs. In addition, in another clinical study, third-party allogeneic MSC will be tested as a strategy to induce tolerance in kidney transplant recipients (clinical trials, NCT02565459). It is expected that these studies will provide insight into the optimal strategy and risks of allogeneic MSCs in the transplant setting.
Because transplant recipients have already an increased risk of (opportunistic) infections and malignancies due to the concomitant immunotherapy, it is difficult to determine additional risks of MSC infusions. All clinical studies should be ethically approved and both serious adverse events as well as suspected unexpected serious adverse reactions should be carefully recorded and reported to the proper authorities. Careful assessment of both short- and long-term effects of MSC treatment is of major importance. For MSC treatment a scoring system, the so-called MiSOT-1 score was designed to evaluate safety of intravenous and intraportal infusion stem cell products after liver transplantations93 with the aim to identify very serious adverse events. There is no uniform score yet, which also identifies less severe adverse events and which can be used to assess safety in cell-based trials in renal transplantation.
Important potential risks in solid organ transplant recipients include direct toxicity related to MSC infusion, malignancies, risks for overimmunosuppression and immunogenicity, as also extensively reviewed elsewhere.45,94,95 To date, no direct toxicity related to the infusion itself or immediate adverse effects have been describedin the numerous clinical trials with MSCs for clinical indications.6 In humans, quite a large cohort of patients is exposed to MSC therapy, and no new malignancies have been reported so far. However, most clinical trials have a short follow-up period, and the inclusion of ill patients with poor prognosis could have biased the outcomes as well. In transplant recipients, it will be anyway a difficult task to link the development of malignancies to the use of MSC therapy, because there is already an increased risk for malignancies in this population. Because MSCs are particularly known for their immunosuppressive functions, a potential side effect of MSC therapy would be overimmunosuppression. In the study of Tan et al,82 a significant decrease in opportunistic infections was seen with MSC induction. However, the majority of patients in this study (151 of 154) had a cytomegalovirus (CMV) negative serological status, probably explaining the low incidence of CMV infections. In our phase I trial in renal transplant recipients, 3 of 6 patients developed opportunistic virus infections after MSC infusions that are typically associated with too much immunosuppression, including BK nephropathy and CMV infections.84 Clinical trials in the context of GvHD or hematopoietic stem cell transplantation also showed a trend to more infections after MSC therapy.96,97 Clearly, frequent and accurate monitoring of infectious complications remains essential in cell based therapy.
It is important to realize that many topics still need to be addressed. There is, for example, little known about the influencing factors, timing, dosage, route of administration, and frequency of MSC administration in renal recipients. These issues are challenging to study in a clinical setting because of the regulatory, logistic, and financial issues, where large numbers of included patients are mostly not available.
In the setting of renal transplantation, a number of different endpoints are used to measure clinical efficacy and to obtain mechanistic insight. The criterion standard clinical endpoint in renal transplantation is graft failure and death. However, substantial improvements in 1-year patient and kidney survival have shifted the endpoint to biopsy-proven acute rejection (BPAR), which is identified as a marker for chronic rejection and long-term graft survival. Declining rates of rejection have further shifted the endpoint to alternative surrogate markers, including renal function, histological findings, and immunological (bio)markers to measure efficacy and to provide mechanistic insight. Hence, it is important to evaluate short-term endpoint markers, which can predict potentially poorer long-term graft survival and BPAR. However, at the same time, this is a great challenge, because most surrogate endpoints are not adequate validated and only partly predict long-term graft survival.
According to the different indication of MSC therapy, the focus of the various endpoints might differ. For example, when assessing IRI, short-term endpoints, such as renal function and BPAR, will provide the most insight in efficacy. On the other hand, when assessing chronic allograft injury, the endpoints will be more focused on long-term outcomes and will include patient and graft survivals, histology, and renal function. Other endpoints, including immune monitoring and biomarker studies, will apply for all studies, although the focus on the markers chosen may vary between trials. Therefore, it is of importance to establish the rationale first, and the most relevant efficacy endpoints should then be determined accordingly.
Both gene expression profiling and detailed subset analysis of immune cells have identified potential fingerprints of immunoregulation. Interestingly, in these studies, not only T-cell subsets (CD4, CD8, effector, memory or Treg cells) but also other lineages, such as B cell, monocytes, and natural killer cells showed to be of importance. In the study of Perico et al,80 a progressive increase of the percentage of CD4+CD25highFoxP3+CD127− Treg cells and a marked inhibition of memory CD45 RO+RA−CD8+ T-cell expansion was observed posttransplant after MSC treatment. In the study by Mudrabettu et al,83 MSC infusion was also associated with expansion of Treg cells, and a reduced ex vivo proliferation of T cells was found in response to polyclonal stimuli. In our study, no differences in immune profiles were observed which might be due to a different timing of infusion and a short follow-up of immune monitoring.84 In addition, only a small number of patients were included in the different trials.
Recently, the ONE study consortium has developed an immune monitoring strategy to compare the efficacy of different cell therapies, including procedures for whole blood leukocyte subset profiling by flow cytometry. This standardized method, which includes 6 panels to analyze the immune response, facilitates fair and meaningful comparisons between trials, such as MSC therapy.98 Moreover, functional assays, such as the mixed lymphocyte reaction and measurements of different cytokines, should be performed to analyze donor-specific lymphocyte proliferation after MSC treatment. In general, sharing of procedures and protocols for safety and efficacy endpoints will allow for more reliable comparisons between the different clinical trials.
Preclinical and early clinical results of MSCs in solid organ transplantation seem promising. However, to further promote wider application of MSC in renal transplantation, it is of importance to increase the understanding of the mechanism of action, to develop tools to identify eligible patients, and to determine efficacy. Results from the currently enrolled studies are awaited in the near future and will provide us with additional information regarding the risks and benefits of MSCs. In addition, we currently face challenges with clinical implementation of cell therapy which include uncertainties of technological, financial, regulatory, and strategic nature. Identifying opportunities to address these challenges may accelerate the development and increase the impact of novel MSC therapy in the transplant setting.
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