Mary Murphy

Metabolic Flexibility Permits Mesenchymal Stem Cell Survival in an Ischemic Environment

The application of mesenchymal stem cells (MSCs) for myocardial repair following ischemic injury is of strong interest, but current knowledge regarding the survival and retention of differentiation potency of stem cells under ischemic conditions is limited. The present study investigated the effects of ischemia and its components (hypoxia and glucose depletion) on MSC viability and multipotency. We demonstrate that MSCs have a profoundly greater capacity to survive under conditions of ischemia compared with cardiomyocytes, measured by detecting changes in cellular morphology, caspase activity and phosphatidylserine exposure. MSCs were also resistant to exposure to hypoxia (0.5% O2), as well as inhibition of mitochondrial respiration with 2,4‐dinitrophenol for 72 hours, indicating that in the absence of oxygen, MSCs can survive using anaerobic ATP production. Glucose deprivation (glucose‐free medium in combination with 2‐deoxyglucose) induced rapid death of MSCs. Depletion of cellular ATP occurred at a lower rate during glucose deprivation than during ischemia, suggesting that glycolysis has specific prosurvival functions, independent of energy production in MSCs. After exposure to hypoxic or ischemic conditions, MSCs retained the ability to differentiate into chondrocytes and adipocytes and, more importantly, retained cardiomyogenic potency. These results suggest that MSCs are characterized by metabolic flexibility, which enables them to survive under conditions of ischemic stress and retain their multipotent phenotype. These results highlight the potential utility of MSCs in the treatment of ischemic disease.

Immunogenicity of allogeneic mesenchymal stem cells

Mesenchymal stem cells (MSCs) inhibit proliferation of allogeneic T cells and express low levels of major histocompatibility complex class I (MHCI), MHCII and vascular adhesion molecule‐1 (VCAM‐1). We investigated whether their immunosuppressive properties and low immunophenotype protect allogeneic rat MSCs against cytotoxic lysis in vitro and result in a reduced immune response in vivo. Rat MSCs were partially protected against alloantigen‐specific cytotoxic T cells in vitro. However, after treatment with IFN‐γ and IL‐1β, MSCs upregulated MHCI, MHCII and VCAM‐1, and cytotoxic lysis was significantly increased. In vivo, allogeneic T cells but not allogeneic MSCs induced upregulation of the activation markers CD25 and CD71 as well as downregulation of CD62L on CD4+ T cells from recipient rats. However, intravenous injection of allo‐MSCs in rats led to the formation of alloantibodies with the capacity to facilitate complement‐mediated lysis, although IgM levels were markedly decreased compared with animals that received T cells. The allo‐MSC induced immune response was sufficient to lead to significantly reduced survival of subsequently injected allo‐MSCs. Interestingly, no increased immunogenicity of IFN‐γ stimulated allo‐MSCs was observed in vivo. Both the loss of protection against cytotoxic lysis under inflammatory conditions and the induction of complement‐activating antibodies will likely impact the utility of allogeneic MSCs for therapeutic applications.