MSCs Isolated From Patients With Ischemic Vascular Disease Have Normal Angiogenic Potential

Abstract

Critical limb ischemia (CLI) is a severe form of peripheral arterial diseases characterized by chronic ischemic rest pain, ulcers, or gangrene attributed to inadequate blood flow. A considerable percentage (40%) of CLI patients—so-called no-option patients—are not ideal for surgical revascularization, and amputation becomes the required treatment. Stem cell therapies using bone marrow–derived mesenchymal stem cells (BM-MSCs) have been explored as a potent alternative treatment for revascularization in CLI patients.1 The therapeutic benefit of this approach is due to the secretion of paracrine factors that may have anti-inflammatory, immunomodulatory, and proangiogenic effects. Although other patient-derived cell types have been shown to be dysfunctional with impaired angiogenic capacity,2,3 little is known about the neovascularization capacity of patient-derived BM-MSCs. In this issue of Molecular Therapy, Gremmels et al. report that the neovascularization capacity of BM-MSCs from CLI patients is similar to that of BM-MSCs from a healthy control population, in a murine model of hind-limb ischemia.4 The study, which is well designed and has high clinical applicability, suggests that autologous transplantation of BM-MSCs in patients with CLI will not be attenuated by disease-induced cellular dysfunction. Risk factors for CLI include smoking, dyslipidemia, and diabetes mellitus; risk-factor modification, such as smoking cessation, blood pressure control, good glycemic control, and reduction of lipid levels, can have a positive effect on the disease. Although such conservative therapy can benefit many patients, the severe nature of the disease may lead to consideration of operative intervention, including revascularization or amputation. Nevertheless, patients who undergo amputation still have a very poor prognosis. For instance, almost half of patients receiving a diabetes-related amputation will not be alive in five years.5 Thus, new modalities of treatments are urgently required for limb salvage in such patients. Stem cell therapies, with cells from a variety of sources, have shown promise. Cell types have included bone marrow–derived mononuclear cells (MNCs), endothelial progenitor cells (EPCs), and mesenchymal stem cells (MSCs).1,6 Autologous-transplantation approaches have been used in the majority of studies, and the issue of disease-induced cell dysfunction is important in this context. Patient-derived MNCs and EPCs have been shown to be dysfunctional and to have little angiogenic potential.2,3 Whether this applies to culture-expanded, BM-MSCs in patients with ischemic disorders is unknown, and the current report has shed important light on this question. Gremmels et al. compared MSCs derived from CLI patients enrolled in a clinical trial of bone marrow–derived MNCs to cells derived from healthy controls using gene expression profiling and functional assays for differentiation, senescence, and angiogenic capacity both in vitro and in vivo. Results from gene-transcription analysis between CLI-MSCs and healthy MSCs showed no differential gene expression. Although adipogenic and osteogenic differentiation was similar in both groups, the authors observed a significant decrease in the chondrogenic differentiation of CLI-MSCs, which may have been due to an associated cellular senescence. This particular finding is not significant for the current application, because the effect of MSCs delivered in the CLI patients is paracrine in nature. However, this could have a major impact in the case of autologous transplantation of MSCs for cartilage regeneration in diseased patients. On the other hand, donor age has previously been associated with increased senescence and reduced differentiation potential,7 and an overrepresentation of older individuals in the CLI group might have contributed to this finding. However, further research is necessary to clarify whether the impaired differentiation capacity is due to an age- or disease-related factor. Interestingly, the ability of secreted factors from CLI-MSCs or healthy MSCs to promote endothelial proliferation and tube formation was comparable and appeared unrelated to MSC senescence. But more importantly, Gremmels et al. were able to show in an in vivo murine model a similar capacity of CLI- and healthy BM-MSCs to restore blood perfusion after induction of ischemia by ligation of the femoral artery. Findings from this study suggest that autologous transplantation approaches to the treatment of CLI patients using BM-MSCs will not be impeded by disease-induced cellular dysfunction. It remains unclear whether allogeneic or autologous MSC transplantation is the optimal approach in patients with CLI. So far, most clinical trials of MSCs in CLI have used autologous cells.8,9,10 However, autologous therapies may have disadvantages. For instance, CLI patients normally comprise an older population who already suffer from severe pain and/or other diabetic-related complications, and who may not be well suited to an invasive harvesting procedure. Moreover, although this report shows that the cells are similar and do not have an impairment of angiogenic capacity, age-related changes (e.g., decrease in colony forming–unit fibroblast numbers and proliferation potential and increase in senescence7) in BM-MSCs from aged CLI patients may affect the ability of the cells to expand in culture, leading to a delay in the administration of the therapeutic. However, the cells isolated for this study were from passage 3 cultures, and it is estimated that this would yield 200 × 106 cells. From a practical point of view, an allogeneic “off the shelf” cell therapy would be a preferable option. Results from a study by Gupta et al., one of the first randomized, double-blinded, placebo-controlled multicenter phase I/II trials using allogeneic MSCs, support the allogeneic transplantation approach. Those authors were able to demonstrate safety of using allogeneic BM-MSCs in CLI patients and positive trends toward improvement in parameters such as ankle pressure and ankle-brachial index.11 Many reports have supported the notion that MSCs have hypoimmunogenic and anti-inflammatory/immunosuppressive properties, and therefore major histocompatibility complex (MHC)-mismatched MSCs have been proposed as “universal donors.”12 However, recent reports suggest that MHC-mismatched MSCs trigger a donor-specific cellular (T cell) and humoral (B cell/antibody) response in vivo, which may limit its use as a universal therapy.13,14 Analysis of MSC persistence after transplantation has shown a rapid clearance of cells after injection. However, Gremmels et al. and others15 have shown that cells are detected even 3 weeks or later after transplantation. This suggests that MSCs may not be subjected to an immediate and aggressive immunological rejection. However, whether specific immune responses are generated against MSC alloantigens soon after injection, as well as how long MSCs must persist in vivo to have beneficial effects, is not yet clear. An additional potential concern lies in the capability of these remaining cells to engraft and differentiate and become more immunogenic.14 For instance, Huang et al. reported upregulation of MHC complexes in locally administered allo-MSCs and a consequent T-cell and antibody response, which limited the long-term benefit of allo-MSCs for cardiac function.16 Another fundamental question that must be addressed in the context of allogeneic transplantation is whether preexisting antidonor antibody and memory T-cell response after allo-MSC transplantation would be detrimental in patients receiving a second dose of allo-MSCs (or multiple doses) or a future organ transplant. It remains to be determined whether the capacity of allo-MSCs to induce anti-donor immune responses might be prevented by coadministration of cells with conventional immunosuppressive drugs. Autologous and allogeneic transplantation of MSCs have pros and cons, and the optimal approach is not clear at the current time. MSC therapy for CLI is still in an early phase of development, and many fundamental issues regarding the safety and efficacy of autologous and allogeneic MSC transplantation need to be addressed in carefully performed clinical trials. Nevertheless, the current study is a significant advance that suggests that autologous transplantation of MSCs will not be hampered as a result of disease-induced cell dysfunction.