A paper appeared in the journal PLoS One by Liu and others that showed that heart muscle cells made from induced pluripotent stem cells were rejected by the immune system of mice. The way induced pluripotent stem cells (iPSCs) are made introduces mutations, many of which are harmless. However, mutations that alter the cell surface proteins of iPSC derivatives can cause the immune system of the host to attack and destroy any transplanted cells.
Are adult cells made from iPSC recognized by the immune system? Are the mouse experiments merely an anomaly of the mouse system?
Dr. Jun Takahashi of Kyoto University’s Center for iPS Cell Research and Application and his research group have examined how monkeys respond to implanted derivatives of iPSCs. They made iPSCs from monkey cells taken from the inside of the mouth. Then Takahashi and his group made midbrain-specific neurons from them and transplanted them back into the monkeys. Only a minimal immune response against these cells was observed. However if a monkey received midbrain neurons made from another animal’s cells, then a robust immune response followed.
Therefore, in non-human primates, iPSC derivatives are not rejected by the immune system of the host.
Takahashi said of this experiment, “These findings give a rationale to start autologous transplantation – at least of neural cells – in clinical situations.” Takahashi’s last statement is critically important – “At least of neural cells.” The brain is an immunologically privileged organ that normally does not have immune cells lurking in its midst. The heart, however, is constantly under immunological surveillance. Therefore, even though this experiment shows that IPSC derivatives are not rejected in non-human primates under these circumstances, there might be circumstances under which they are rejected.
Since there are ways to screen iPSCs and their derivatives for mutations that might sensitize the immune system to the host, such screenings could almost certainly decrease the rate of immunological rejection. Such screening were not done in either this experiment or in the experiments of Liu and others.