Transplants between humans are easily attacked by the recipient’s immune defense system and can therefore stop functioning and be rejected. But there is one occasion when such tissue transfers succeed and no rejection takes place-during normal pregnancy.
The fetus, a separate individual, can be seen as a transplant that runs the risk of being rejected, but is accepted and not attacked by the immune system of the pregnant woman.
This is a phenomenon that has long stumped scientists. If the reasons for this lack of rejection during normal pregnancy could be understood, it would be a tremendous boon, not only when it comes to certain infertility problems but also for facilitating transplants and enhancing our knowledge of how the immune defense system works.
In a pioneering study, now being published in the leading publication Journal of Immunology, Associate Professor Lucia Mincheva-Nilsson and her associates at the Section for Clinical Immunology at Umeå University have shown for the first time that an entirely new mechanism is involved in this acceptance of tissue during pregnancy.
The surface cells of the placenta that belong to the fetus express and secrete proteins (MICA and MICB) into the blood of the pregnant woman. These proteins inhibit her white blood corpuscles, so-called NK cells and cytotoxic T lymphocytes, which would normally attack the fetus. The attack is thus averted, and the mother’s immune system is fooled, which enables the pregnancy to continue in a normal manner.
This is a discovery that is of great significance in the understanding of the immunological mechanisms in normal pregnancy, but it can also help us understand and find solutions to rejection problems in transplants. Also, certain tumor cells can sometimes produce these molecules and avoid being detected by the immune system by using the same mechanism.