The US researchers have discovered a class of immune cells that plays a role in miscarriage, which affects about a quarter of pregnancies.

Researchers at the University of California-San Francisco found that the recently discovered subset of cells known as extrathymic Aire-expressing cells in the immune system may prevent the mother's immune system from attacking the placenta and foetus. The researchers showed that the subset of cells were twice as likely to miscarry, and in many of these pregnancies foetal growth was severely restricted.

"When you're pregnant, the immune system is seeing the placenta for the first time in decades -- not since the mother made a placenta when she herself was a foetus," said Eva Gillis-Buck, from UCSF. "Our research suggests that this subset of immune cells is carrying out a sort of 'secondary education' -- sometimes many years after the better-known population of the educator cells have carried out the primary education in the thymus teaching T cells not to attack the foetus, the placenta and other tissues involved in pregnancy," she added. The findings are published in the journal Science Immunology.

The immune system has to be educated not to attack one's own tissues and organs to prevent autoimmune disease. But pregnancy presents a unique challenge since the foetus expresses proteins found in the placenta as well as proteins whose genetics are distinct from the mother. "It was a conceptual leap to link Aire-expressing cells, which are critical for preventing autoimmune disease, to pregnancy," said Tippi Mackenzie, Professor of Surgery at UCSF's Center for Maternal Foetal Precision Medicine.

In the thymus, Aire-expressing cells begin interacting with other immune cells very early in life to teach them what not to attack. The thymus begins to shrink and is nearly gone by adulthood, by which time most immune cells have been educated. But as the thymus shrinks, the population of eTACs in lymph nodes and the spleen expands, the researchers explained. The study suggests a healthy pregnancy may depend on having these cells around, they added.