Most transplanted tissues are seen by the recipient as foreign and are attacked by the immune system, but certain parts of the body do not mount attacks against foreign tissue because doing so would be self-destructive. Michael Young, Ph.D., lead author of the study and an assistant scientist at Schepens Eye Research Institute and assistant professor at Harvard Medical School, suspected that “neural stem cells” might be one of these tissues (immune privileged).

He and his colleagues therefore took brain stem cells from green mice (mice in which the gene for green protein found in jellyfish has been inserted), and placed them under the kidney capsule (a non-immune privileged area) in normal non-green mice. The team found that none of the mice rejected the stem cells.

They concluded that these neural stems are invisible to the immune system. To determine if the cells possessed the antigens that most other tissues had, the team took other brain cells (not stem cells) from the green mice and implanted them in the normal non-green mice.

Based on their results, the team concluded that the brain stem cells did possess antigens, but unless the recipient was primed or pre-immunized, the antigens were not visible to the immune system of the recipient and not rejected.