Box 1. Great expectations

Millions of people around the world happily tolerate foreign tissues for months on end. No scientific trick here: simply pregnancy. Researchers are trying to understand how pregnant women tolerate their fetus in the hope of understanding tolerance, or even preventing miscarriage.

One such team is headed by Harry Moore at the University of Sheffield, UK. He and his team are studying a particular kind of blood-vessel cell found in the placenta. This secretes a protein known as HLA-G, which seems to help prevent the mother's T cells from attacking her fetus.

Recently, researchers found that a type of immune cell called a monocyte can also make HLA-G. Intriguingly, these cells infiltrate newly transplanted tissue. The higher the levels of HLA-G expressed, the greater the chance that the transplant will be accepted. This finding, says Moore, suggests that HLA-G may naturally help to protect some tissue grafts, although he stresses that no one knows how the process works.

Moore's group hopes that studies of this kind of blood-vessel cell may reveal more about how an embryo normally implants and why some mothers' immune systems do reject their fetuses. In the longer term, says Moore, it might be possible to develop the cells for transplant therapy in heart disease. The cells might also be given with other therapeutic tissues, with the aim of making the recipient tolerate both types of cell.