The goal of regenerative medicine is to replace dead or damaged cells, tissues and even organs with living, properly functioning cells tissues and organs. However, this goal has a few genuine barriers that include tumor formation in the case of pluripotent stem cells, poor cell survival, or even immunological rejection of the transplanted cells before they can render any long-term benefits. Induced pluripotent stem cells (iPSCs), which are made from adult cells by a combination of genetic engineering and cell culture techniques, can be made from a patient’s own mature cells and the differentiated into almost any tissue in the adult body. However, research with mouse iPSCs has shown that even stem cells produced from the subject’s own tissues can be rejected by the subject’s own immune system.
Immune rejection of iPSCs is a legitimate concern, but research from the Stanford University School of Medicine has shown that differentiation of iPSCs into more mature cells before transplantation into mice allows them to be tolerated by the immune system.
Joseph Wu, MD, PhD, director of the Stanford Cardiovascular Institute, said, “Induced pluripotent stem cells have tremendous potential as a source for personalized cellular therapeutics for organ repair. This study shows that undifferentiated iPS cells are rejected by the immune system upon transplantation in the same recipient, but that fully differentiating these cells allows for acceptance and tolerance by the immune system without the need for immunosuppression.”
Wu is the senior author of this publication, which appeared online on May 30th in Nature Communications. Lead authorship of this paper is shared by Patricia Almeida, PhD, and Nigel Kooreman, MD, and assistant professor of medicine Everett Meyer, MD, PhD.
Several other studies have suggested that differentiation of iPSCs can reduce their tendency to activate the immune system after transplantation. However, this study of Wu and others is the first to closely examine, at the molecular and cellular level, how this works.
“We’ve demonstrated definitively that, once the cells are differentiated, the immune response to iPS-derived cells is indistinguishable from its response to unmodified tissue derived from elsewhere in the body,” said lead author Nigel Kooreman.
Pluripotent stem cells have the capacity to differentiate into any cell in the adult body. Of the two types of pluripotent stem cells, embryonic stem cells are made from embryos and iPSCs are made in the laboratory from existing adult cells (e.g., skin or blood). Induced pluripotent stem cells are easier to come by than embryonic stem cells, they match the genetic background of the person from whom they were obtained, and they are not as ethically dubious as embryonic stem cells. Thus, in theory, iPSCs are a good option for any physician who wants to make patient-specific stem cells for potential therapies.
Previous studies in mice have shown, however, that even genetically identicaliPSCs can trigger an immune response after transplantation. Thus, Wu and his colleagues have, for the past six years, been investigating how to use immunosuppressive medications to dampen the body’s response to both embryonic andiPSCs and render them more amenable for clinical use (see AS Lee, et al., J Biol Chem 2011 286(37):32697-704; Durruthy-Durruthy L, et al.,PLoS One, 2014 9(4):e94231 and others).
In this recent study, Kooreman and his co-lead authors decided to examine the immune response against transplanted stem cells. They first transplanted undifferentiated iPS cells into the leg muscles of genetically identical recipient mice. These grafts were rejected and no iPSCs were detected six weeks after transplantation.
Next, Wu and his co-workers differentiated the iPSCs into blood vessel-making endothelial cells that line the interior of the heart and blood vessels and then transplanted them into genetically-identical mice. Kooreman, Almeida, and Meyer then compared the acceptance by the immune system of these iPSC-derived endothelial cells with that of naturally occurring endothelial cells derived from the aortic lining of genetically-identical donor mice. To emphasize once again, all the transplanted cells were genetically identical to the mice in which they were injected. Unlike the undifferentiated iPS cells, both the iPS-derived endothelial cells and the aortic endothelial cells survived for at least nine weeks after transplantation.
Next, Wu and his group repeated the experiment, but they removed the grafts 15 days after transplantation. They observed immune cells called lymphocytes in all grafts, but these immune cells were much more prevalent in the grafts of undifferentiated iPS cells. When the lymphocytes that infiltrated the grafts of undifferentiated iPSCs were compared with those in the differentiated iPSC-derived grafts and the endothelial grafts, their gene expression profiles differed significantly. Those lymphocytes in the undifferentiated iPSC grafts expressed high levels of genes known to be involved in robust immune responses, but lymphocytes in both types of endothelial cell grafts expressed higher levels of genes known to be involved in dampening the immune response and inducing self-tolerance.
Finally, Wu and others directly examined a specific type of lymphocyte called a T cell. Grafts of undifferentiated iPS cells harbored large numbers of T cells that were largely homogeneous, which is characteristic of a robust immune response. Conversely, T cell from grafts of the two types of endothelial cells were more diverse, which suggests a more limited immune response which is typically associated with a phenomenon known as self-tolerance.
“The immune response to the iPS-derived endothelial cells and the aortic endothelial cells, and the longevity of the grafts, was very similar,” said Kooreman. “If we specifically look at the T cells, we see they’re also very similar and that they look much different from grafts that are rejected.”
Wu, who is also a professor of cardiovascular medicine and of radiology, said, “This study certainly makes us optimistic that differentiation — into any nonpluripotent cell type — will render iPS cells less recognizable to the immune system. We have more confidence that we can move toward clinical use of these cells in humans with less concern than we’ve previously had.”