13 years ago, scientists succeeded in creating stem cells – inducted pluripotent stem cells (IPSCs) – from mature skin cells. Those cells showed tremendous promise in the field of regenerative medicine. IPSCs can be differentiated into any cell type in the body. If introduced into the body at the site where tissue needed to be regenerated, the stem cells could differentiate into the types of cell required.
The mature skin cells are harvested from a patient, reprogrammed into IPSCs in the lab, and are then reintroduced into the same patient. IPSCs should therefore not be attacked by the immune system, but that is not always the case.
When IPSCs are reintroduced, the stem cells are often rejected. Consequently, they have not lived up to their promise. What is not well understood is why the immune system attacks the cells, but new research from scientists at the University of California San Francisco’s Transplant and Stem Cell Immunology Lab has shed light on why these cells are often rejected.
In collaboration with researchers at Stanford University and the Laboratory of Transplantation Genomics at the National Heart, Lung, and Blood Institute (NHLBI), the researchers showed that when IPSCs are generated from mature skin cells, mutations can occur in mitochondrial DNA.
Mitochondria are small structures inside cells that are responsible for providing energy to the cells. As such they are often referred to as the powerhouses of cells. Without mitochondria, cells would not have any energy and biological processes could not occur. Mitochondria are unique for another reason. They contain their own genome. That genome is around 3,000 base pairs shorter than nuclear DNA.
“In cells that do a lot of work, like heart muscle cells, up to a third of the cell’s protein-producing mRNA molecules are mitochondrial in origin,” explained Sonja Schrepfer, MD, Ph.D., senior author of the study. “This means that the burden of a single mitochondrial mutation may be tremendous. You don’t end up with just a few proteins that can potentially provoke an immune response—you end up with thousands.”
To demonstrate the immune response to mutations in mitochondrial DNA, the researchers created hybrid stem cells with nuclear DNA from one mouse and mitochondrial DNA from another. The cells were then transplanted into a mouse with identical nuclear DNA, but with a single base change in two protein coding genes. Immune cells were harvested two days after transplantation and were exposed to mitochondrial protein fragments.
The proteins produced by the mitochondrial genes with the base pair mutation produced an immune response, but that did not happen with other proteins. The study was then repeated on human cells taken from liver and kidney transplant patients. The researchers found that mutations in mitochondrial DNA produced proteins that also triggered an immune response. All that was required for the immune response to be triggered was a single mutation in mitochondrial DNA.
The problem with converting skin cells into IPSCs is the process is highly mutagenic. “Under normal physiological conditions, mitochondrial DNA is 10 to 20 times more susceptible to mutation than nuclear DNA. Transforming adult cells into stem cells is a harsh process, so we expected mutation rates to be just as high or higher,” said Tobias Deuse, MD, lead author of the study.
While nuclear DNA has repair mechanisms that correct mutations, those mechanisms are not present in mitochondrial DNA. The body relies on the immune system to find and destroy cells that are producing unfamiliar mitochondrial proteins. Since IPSCs are created in the lab and not in the body, the immune system is not present to kill the cells with the mutations.
This mechanism of rejection could be leveraged to develop new immunosuppressive agents for use alongside IPSCs. The research also suggests IPSCs will have to be screened more carefully to ensure they are not rejected when introduced back into patients.
The study is detailed in the paper – De novo mutations in mitochondrial DNA of iPSCs produce immunogenic neoepitopes in mice and humans – which was recently published in the journal Nature Biotechnology. DOI: 10.1038/s41587-019-0227-7