If mouse studies were transferable to humans, we'd have cured every disease thousands of times. That is the big reason why you shouldn't accept scaremongering about the chemical of the week in the New York Times, or claims about Miracle Vegetables in the Washington Post.

Stem cell therapy is all the rage, with suspect companies sprouting up like supplement stores, claiming to be a benefit for this and that. Often all they have are mouse studies and FDA disclaimers on their side. That's not to say mouse studies are not valuable, they eliminate a lot of bad products, and in some instances mouse models are good analogues of humans, like in  HIV infection, but a new paper reveals what chemists have long known: When it comes to the immune system rats are not little people, even "humanized" mice which have been engineered to have a human, rather than a murine, immune system. 

These animals have been used for decades to study things like the immune response to the transplantation of pancreatic islet cells for diabetes and skin grafts for burn victims. But unlike what would occur in a human patient, the humanized mice are unable to robustly reject the transplantation of genetically mismatched human stem cells. As a result, they can't be used to study the immunosuppressive drugs that patients will likely require after transplant. The researchers conclude that the humanized mouse model is not suitable for studying the human immune response to transplanted stem cells or cells derived from them.

"In an ideal situation, these humanized mice would reject foreign stem cells just as a human patient would," said Joseph Wu, MD, PhD, director of Stanford University School of Medicine's Cardiovascular Institute and professor of cardiovascular medicine and of radiology. "We could then test a variety of immunosuppressive drugs to learn which might work best in patients, or to screen for new drugs that could inhibit this rejection. We can't do that with these animals."

The researchers write in Cell Reports that they were studying pluripotent stem cells, which can become any tissue in the body. They tested the animals' immune response to human embryonic stem cells, which are naturally pluripotent, and to induced pluripotent stem cells. Although iPS cells can be made from a patient's own tissues, future clinical applications will likely rely on pre-screened, FDA-approved banks of stem cell-derived products developed for specific clinical situations, such as heart muscle cells to repair tissue damaged by a heart attack, or endothelial cells to stimulate new blood vessel growth. Unlike patient-specific iPS cells, these cells would be reliable and immediately available for clinical use. But because they won't genetically match each patient, it's likely that they would be rejected without giving the recipients immunosuppressive drugs. 

The authors found that two varieties of humanized mice were unable to completely reject unrelated human embryonic stem cells or iPS cells, despite the fact that some human immune cells homed to and were active in the transplanted stem cell grafts. In some cases, the cells not only thrived, but grew rapidly to form cancers called teratomas. In contrast, mice with unaltered immune systems quickly dispatched both forms of human pluripotent stem cells.

The researchers obtained similar results when they transplanted endothelial cells derived from the pluripotent stem cells.

A new mouse model

To understand more about what was happening, they created a new mouse model similar to the humanized mice. Instead of reconstituting the animals' nonexistent immune systems with human cells, however, they used immune and bone marrow cells from a different strain of mice. They then performed the same set of experiments again.

Unlike the humanized mice, these new mice robustly rejected human pluripotent stem cells as well as mouse stem cells from a genetically mismatched strain of mice. In other words, their newly acquired immune systems appeared to be in much better working order.

Although more research needs to be done to identify the cause of the discrepancy between the two types of animals, the researchers speculate it may have something to do with the complexity of the immune system and the need to further optimize the humanized mouse model to perhaps include other types of cells or signaling molecules. In the meantime, they are warning other researchers of potential pitfalls in using this model to screen for immunosuppressive drugs that could be effective after human stem cell transplants.