Stanford study successful in transplant of mismatched stem cells, tissue in mice

Dr. Irv Weissman at Stanford University

A transplant can be a lifesaving procedure for many people across the United States. In fact, according to the Health Resources & Services Administration, 36,528 transplants were performed in 2018. However, as of January 2019, the number of men, women, and children on the national transplant waiting list is over 113,000, with 20 people dying each day waiting for a transplant and a new person being added to the list every 10 minutes.

Before considering a transplant, there needs to be an immunological match between the donated tissue and/or blood stem cells and the recipient. To put it simply, a “match” indicates that the donor’s cells will not be marked by the recipient’s immune cells as foreign and begin to attack it, a process known as graft-versus-host disease. Unfortunately, these matches can be challenging to find, particularly for some ethnic minorities. Often times, immunosuppression drugs are also needed in order to prevent the foreign cells from being attacked by the body’s immune system. Additionally, chemotherapy and radiation are often needed as well.

Fortunately, a CIRM-funded study at Stanford has shown some promising results towards addressing the issue of matching donor cells and recipient. Dr. Irv Weissman and his colleagues at Stanford have found a way to prepare mice for a transplant of blood stem cells, even when donor and recipient are an immunological mismatch. Their method involved using a combination of six specific antibodies and does not require ongoing immunosuppression.

The combination of antibodies did this by eliminating several types of immune cells in the animals’ bone marrow, which allowed blood stem cells to engraft and begin producing blood and immune cells without the need for continued immunosuppression. The blood stem cells used were haploidentical, which, to put it simply, is what naturally occurs between parent and child, or between about half of all siblings. 

Additional experiments also showed that the mice treated with the six antibodies could also accept completely mismatched purified blood stem cells, such as those that might be obtained from an embryonic stem cell line. 

The results established in this mouse model could one day lay the foundation necessary to utilize this approach in humans after conducting clinical trials. The idea would be that a patient that needs a transplanted organ could first undergo a safe, gentle transplant with blood stem cells derived in the laboratory from embryonic stem cells. The same embryonic stem cells could also then be used to generate an organ that would be fully accepted by the recipient without requiring the need for long-term treatment with drugs to suppress the immune system. 

In a news release, Dr. Weissman is quoted as saying,

“With support by the California Institute for Regenerative Medicine, we’ve been able to make important advances in human embryonic stem cell research. In the past, these stem cell transplants have required a complete match to avoid rejection and reduce the chance of graft-versus-host disease. But in a family with four siblings the odds of having a sibling who matches the patient this closely are only one in four. Now we’ve shown in mice that a ‘half match,’ which occurs between parents and children or in two of every four siblings, works without the need for radiation, chemotherapy or ongoing immunosuppression. This may open up the possibility of transplant for nearly everyone who needs it. Additionally, the immune tolerance we’re able to induce should in the future allow the co-transplantation of [blood] stem cells and tissues, such as insulin-producing cells or even organs generated from the same embryonic stem cell line.”

The full results to this study were published in Cell Stem Cell.

Study Identifies Safer Stem Cell Therapies

To reject or not reject, that is the question facing the human immune system when new tissue or cells are transplanted into the body.

Stem cell-therapy promises hope for many debilitating diseases that currently have no cures. However, the issue of immune rejection has prompted scientists to carefully consider how to develop safe stem cell therapies that will be tolerated by the human immune system.

Before the dawn of induced pluripotent stem cells (iPSCs), embryonic stem cells (ESCs) were suggested as a potential source for transplantable cells and tissue. However, ESCs run into a couple of issues, including their origin, and the fact that ESC-derived cells likely would be rejected when transplanted into most areas of a human due to differences in genetic backgrounds.

The discovery of iPSCs in the early 2000’s gave new hope to the field of stem cell therapy. By generating donor cells and tissue from a patient’s own iPSCs, transplanting those cells/tissue back into the same individual shouldn’t – at least theoretically – cause an immune reaction. This type of transplantation is called “autologous” meaning that the stem cell-derived cells have the same genetic background as the person.

Unfortunately, scientists have run up against a roadblock in iPSC-derived stem cell therapy. They discovered that even cells derived from a patient’s own iPSCs can cause an immune reaction when transplanted into that patient. The answers as to why this occurs remained largely unanswered until recently.

In a paper published last week in Cell Stem Cell, scientists from the University of California, San Diego (UCSD) reported that different mature cell types derived from human iPSCs have varying immunogenic effects (the ability to cause an immune reaction) when transplanted into “humanized” mice that have a human immune system. This study along with the research conducted to generate the humanized mice was funded by CIRM grants (here, here).

In this study, retinal pigment epithelial cells (RPE) and skeletal muscle cells (SMC) derived from human iPSCs were transplanted into humanized mice. RPEs were tolerated by the immune system while SMCs were rejected. (Adapted from Zhao et al. 2015)

Scientists took normal mice and replaced their immune system with a human one. They then took human iPSCs generated from the same human tissue used to generate the humanized mice and transplanted different cell types derived from the iPSCs cells into these mice.

Because they were introducing cells derived from the same source of human tissue that the mouse’s immune system was derived from, in theory, the mice should not reject the transplant. However, they found that many of the transplants did indeed cause an immune reaction.

Interestingly, they found that certain mature cell types derived from human iPSCs created a substantial immune reaction while other cell types did not. The authors focused on two specific cell types, smooth muscle cells (SMC) and retinal pigment epithelial cells (RPE), to get a closer look at what was going on.

iPSC-derived smooth muscle cells created a large immune response when transplanted into humanized mice. However, when they transplanted iPSC-derived retinal epithelial cells (found in the retina of the eye), they didn’t see the same immune reaction. As a control, they transplanted RPE cells made from human ESCs, and as expected, they saw an immune response to the foreign ESC-derived RPE cells.


iPSC derived RPE cells (green) do not cause an immune reaction (red) after transplantation into humanized mice while H9 embryonic stem cell derived RPE cells do. (Zhao et al. 2015)

When they looked further to determine why the humanized mice rejected the muscle cells but accepted the retinal cells, they found that SMCs had a different gene expression profile and higher expression of immunogenic molecules. The iPSC-derived RPE cells had low expression of these same immunogenic molecules, which is why they were well tolerated in the humanized mice.

Results from this study suggest that some cell types generated from human iPSCs are safer for transplantation than others, an issue which can be addressed by improving the differentiation techniques used to produce mature cells from iPSCs. This study also suggests that iPSC-derived RPE cells could be a safe and promising stem cell therapy for the treatment of eye disorders such as age-related macular degeneration (AMD). AMD is a degenerative eye disease that can cause vision impairment or blindness and usually affects older people over the age of 50. Currently there is no treatment for AMD, a disease that affects approximately 50 million people around the world. (However there is a human iPSC clinical trial for AMD out of the RIKEN Center for Developmental Biology in Japan that has treated one patient but is currently on hold due to safety issues.)

The senior author on this study, Dr. Yang Xu, commented on the importance of this study in relation to AMD in a UCSD press release:

Dr. Yang Xu

Dr. Yang Xu

Immune rejection is a major challenge for stem cell therapy. Our finding of the lack of immune rejection of human iPSC-derived retinal pigment epithelium cells supports the feasibility of using these cells for treating macular degeneration. However, the inflammatory environment associated with macular degeneration could be an additional hurdle to be overcome for the stem cell therapy to be successful.

Xu makes an important point by acknowledging that iPSC-derived RPE cells aren’t a sure bet for curing AMD just yet. More research needs to be done to address other issues that occur during AMD in order for this type of stem cell therapy to be successful.


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What a Difference Differentiation can Make: a Little Change can Reduce the Risk of Rejection

No one likes to be rejected. It hurts. But while rejection is something most of us experience at least a couple of times in our life, researchers at Stanford have found a way to reduce the risk of rejection, at least when it comes to one form of stem cells. Reporting in the latest issue of Nature Communications they have found that turning iPS or induced pluripotent stem cells into other, more specialized kinds of cells, can reduce the risk the immune system will attack them.

Our immune systems are things of beauty. They hunt out invaders like viruses and when they spot something that shouldn’t be there, they attack it. It’s a critical part of our body’s way of fighting off disease and staying healthy.

However, the immune system is not perfect. Researchers have known for some time that when you take skin from an individual and turn it into an iPS cell – one that is capable of turning into any other cell in the body – and then transplant that cell back into the same individual their immune system often attacks it. So far, the only individuals this has been done with are mice, but we assume the same would happen in people.

So Joseph Wu and his CIRM-funded Stanford team decided to see what would happen if they took those same iPS cells and, before transplanting them back into the individual they came from, turned them into a more specialized form of cell. The results were encouraging: the immune system didn’t wage an attack.

Dr. Joseph Wu, Stanford University School of Medicine

Dr. Joseph Wu, Stanford University School of Medicine

Researchers were not sure why the body would attack something that was created from its own tissue, but speculated that turning ordinary skin cells into iPS cells created a kind of cell that the immune system hadn’t seen before, or at least hadn’t seen since it since it was an embryo.

In the Stanford news release, Wu said this finding could be really important in helping avoid rejection in organ or other tissue transplants:

“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.” 

The team first transplanted some iPS cells into genetically identical recipient mice. The transplants were rejected and within 42 days there were no signs that any cells had survived.

Then they took the same kind of iPS cell and differentiated, or ‘re-programmed,’ them so that they turned into endothelial cells, the kind found in the inner lining of blood vessels. Then they transplanted those cells into the mice. At the same time they took some of the mice’s own endothelial cells out, and transplanted them back into genetically identical mice to see how they would compare. Both sets of cells, the iPS-turned-into endothelial and the endothelial cells, survived for at least 63 days after transplantation.

When the researchers repeated the experiment and examined the areas where the cells had been transplanted, they found much greater signs of immune system activity in the mice that were given iPS cells compared to the mice who got iPS cells that had been turned into endothelial cells, and the mice that just got endothelial cells.

For Wu, the bottom line was simple:

“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.” 

 We work closely with Joseph Wu and his team on a number of other different projects, most focusing on heart disease.

kevin mccormack