Transplanted Brain Stem Cells Don't Trigger Rejection

Scientists have shown for the first time that brain stem cells are invisible to a transplant recipient's immune system and do not trigger the system to reject them, which may eliminate the need for tissue typing and immunosuppressive drugs. The finding was published in the July 2003 issue of Stem Cells.

Certain sites in the body do not mount attacks against foreign tissue because it would be too self-destructive. These sites, known as "immune privileged,” include the brain, the eye, the digestive system, and the reproductive system. To test the suspicion that brain stem cells might be immune privileged, researchers took brain stem cells from mice in which the gene for green protein found in jellyfish had been inserted. These were placed in normal mice under the kidney capsule, the pouch in which the kidney is located, a site that always rejects transplanted tissue without close tissue typing or immunosuppressant drugs. After four weeks, the researchers examined the mice and found that the stem cells had not been rejected in any of the mice and had grown into normal tissue. They concluded that these neural stem cells must be invisible to the immune system.

To determine if the cells possessed the antigens most other tissues have, the researchers took other brain cells that were not stem cells from the green mice and implanted them in the normal mice. These cells were rejected, and when brain stem cells were then implanted in the normal mice, they were also rejected. This showed that brains stem cells do possess antigens but unless the recipient is primed or pre-immunized, the antigens are not visible to the immune system and not rejected.

"These findings are very exciting,” said lead author Michael Young, Ph.D., an assistant professor at Harvard Medical School (Boston, MA, USA). "Though we suspected brain stem cells might be protected in this way, this is the first documented evidence.”




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