Scientists from the University of California San Francisco, UCSF, used the CRISPR/Cas9 gene editing system to create the first pluripotent stem cells, functionally “invisible” to the immune system. This tremendous breakthrough in stem cell therapy can prevent graft rejection.
Currently, mature stem cells are used in regenerative medicine, individual for each patient. However, obtaining universal pluripotent stem cells is more efficient and promising.
“Scientists often tout the therapeutic potential of pluripotent stem cells, which can mature into any adult tissue, but the immune system has been a major impediment to safe and effective stem cell therapies”, – said Tobias Deuse, MD, Endowed Chair in Cardiac Surgery at UCSF and lead author of the new study, published Feb. 18 in the journal Nature Biotechnology.
The immune system is programmed to destroy anything it perceives as alien, protecting the body from infections and other harmful agents that can damage the body.
However, it also means that transplanted organs, tissues or cells are considered as a potentially dangerous foreign invasion, which invariably causes a strong immune response leading to transplant rejection. When this happens, the donor and the recipient are said to have a «histocompatibility mismatched».
“We can administer drugs that suppress immune activity and make rejection less likely. Unfortunately, these immunosuppressants leave patients more susceptible to infection and cancer”, – explained Professor of Surgery Sonja Schrepfer, MD, PhD, the study’s senior author and director of the UCSF Transplant and Stem Cell Immunobiology (TSI).
Previously, scientists believed that the problem of rejection in the field of stem cell transplantation would be solved with the help of induced pluripotent stem cells (iPSCs). They are created from fully mature cells, such as skin, blood, or fat cells, reprogrammed into a pluripotent state that allows them to develop into any of the many cells of tissues and organs.
When cells grown from iPSC are transplanted to the same patient, from which the original cells are obtained, theoretically, the body should perceive the transplanted cells as “its own” and not activate the immune response.
But in practice, the clinical use of iPSCs has proven difficult. For reasons unknown so far, the cells of many patients are not susceptible to reprogramming. In addition, individual production of iPSCs for each patient undergoing stem cell therapy is an expensive and lengthy process.
“There are many issues with iPSC technology, but the biggest hurdles are quality control and reproducibility. We don’t know what makes some cells amenable to reprogramming, but most scientists agree it can’t yet be reliably done”, – Deuse said. “Most approaches to individualized iPSC therapies have been abandoned because of this.”
Deuse and Schrepfer wondered if it was possible to circumvent these problems by creating “universal” iPSCs that could be applied to any patient who needed them. In their new article, they describe how, after a change in the activity of just three genes, iPSCs were able to avoid rejection after being transplanted into histocompatibility-mismatched recipients with fully functional immune systems.
Researchers first used CRISPR to remove two genes, which are necessary for the proper functioning of a family of proteins known as major histocompatibility complex (MHC) class I and II. MHC proteins (antigens) are located on the surface of almost all cells and provide recognition of foreign agents by the immune system.
Cells that are missing MHC genes don’t present these signals, because they are not recognized as foreign. However, cells lacking MHC proteins become targets of immune cells known as natural killer cells or NK cells.
Working with Dr. Lewis Lanier, a professor co-author, chair of UCSF’s Department of Microbiology and Immunology, and an expert in the signals that activate and inhibit NK cell activity – Schrepfer’s team found that CD47, a cell surface protein that acts as a “do not eat me” signal against immune cells called macrophages, also has a strong inhibitory effect on NK cells.
Assuming that CD47 might hold the key to completely shutting down rejection, the researchers placed the CD47 gene into a virus that delivered additional copies of the gene into mouse and human stem cells that killed MHC proteins.
It was the CD47 that turned out to be the missing link. After the researchers transplanted the modified stem cells to the histone-incompatible mice with a normally functioning immune system, rejection did not occur.
The scientists then transplanted similarly constructed human stem cells to so-called humanized mice — animals, whose immune systems have been replaced by components of the human immune system to simulate human immunity. In this case, there was also no rejection.
The researchers additionally created from the engineered stem cells various types of human heart cells, which were also transplanted to humanized mice. Transplanted cardiomyocytes safely settled down and even began to form rudimentary blood vessels and heart muscle, which gives hope that the stem cells created can ever be used to repair damage in heart diseases.
“Our technique solves the problem of rejection of stem cells and stem cell-derived tissues, and represents a major advance for the stem cell therapy field”, – Deuse said. “Our technique can benefit a wider range of people with production costs that are far lower than any individualized approach.