A mother’s immune system leaves the fetus, with its alien DNA, alone (Image: Stefanie Sudek/Getty Images)

It is one of the greatest mysteries of pregnancy: how a fetus avoids being attacked by its mother’s immune system. Now a study in mice has taken us a step closer to an answer.

The mammalian body usually responds to foreign cells or tissue by releasing chemicals that promote inflammation and summon immune cells called T-cells to destroy the invaders. But during pregnancy, something turns off this process. This means that despite containing genetic material from the father, the embryo can implant in its mother’s uterus and grow there safely without triggering an immune response.

Adrian Erlebacher at the New York University School of Medicine and his colleagues used mouse embryos engineered to express a specific protein that would enable them to watch how immune cells reacted to it as the embryo began to implant.

Gene silencers

Erlebacher’s team found that cells of the decidua, the lining of the uterus where the embryo implants, did not produce the usual chemical signals that would trigger inflammation and summon T-cells to the site. Closer inspection of these decidual cells revealed changes to the structure of DNA around the genes that usually produce these chemical beacons, resulting in them being silenced.


“The T-cells can’t go into the decidua and we think this is a major reason why the fetus doesn’t get attacked,” says Erlebacher.

The next steps are to figure out how implantation of the embryo triggers this gene silencing, and to see whether the same process occurs during human pregnancy. Assuming it does, Erlebacher believes a similar process might explain how cancer manages to evade the immune system.

Journal reference: Science, DOI: 10.1126/science.1220030