Biologists discover solution to problem limiting development of human stem cell therapies

PUBLIC RELEASE DATE:

2-Jan-2014

Contact: Kim McDonald kmcdonald@ucsd.edu 858-534-7572 University of California - San Diego

Biologists at UC San Diego have discovered an effective strategy that could prevent the human immune system from rejecting the grafts derived from human embryonic stem cells, a major problem now limiting the development of human stem cell therapies. Their discovery may also provide scientists with a better understanding of how tumors evade the human immune system when they spread throughout the body.

The achievement, published in a paper in this week's early online edition of the journal Cell Stem Cell by a collaboration that included scientists from China, was enabled by the development of "humanized" laboratory mice that contained a functional human immune system capable of mounting a vigorous immune rejection of foreign cells derived from human embryonic stem cells.

Because human embryonic stem cells are different from our own body's cells, or "allogenic," a normally functioning human immune system will attack these foreign cells. One way to reduce the body's "allogenic immune response" is to suppress the immune system with immunosuppressant drugs.

"For organ transplantation to save patients with terminal diseases that has been quite successful," says Yang Xu, a professor of biology who headed the team of researchers that included Ananda Goldrath, an associate biology professor at UC San Diego. "But for stem cell therapies, the long term use of toxic immunosuppressant drugs for patients who are being treated for chronic diseases like Parkinson's disease or diabetes pose serious health problems."

Researchers had long been searching for a human immunity relevant model that would allow them to develop strategies to implant allogenic cells derived from embryonic stem cells safely. "The problem is that we only had data from mouse immune system and those are not usually translatable in humans, because human and mouse immune systems are quite different," explains Xu. "So what we decided to do was to optimize the humanized mouse that carries a functional human immune system."

To do that, the biologists took immune deficient laboratory mice and grafted into their bodies human fetal thymus tissues and hematopoietic stem cells derived from fetal liver of the same human donor. "That reconstituted in these mice a normally functioning human immune system that effectively rejects cells derived human embryonic stem cells," says Xu. With these "humanized" mouse models, the biologists then tested a variety of immune suppressing molecules alone or in combination and discovered one combination that worked perfectly to protect cells derived from human embryonic stem cells from immune rejection.

That combination was CTLA4-lg, an FDA-approved drug for treating rheumatoid arthritis that suppresses T-cells responsible for immune rejection, and a protein called PD-L1 known to be important for inducing immune tolerance in tumors. The researchers discovered that the combination of these two molecules allowed the allogeneic cells to survive in humanized mice without triggering an immune rejection.

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Biologists discover solution to problem limiting development of human stem cell therapies