Researchers have made key findings on how the fetus and placenta manage to avoid immune system rejection during pregnancy, with unexpected results being published by New York University scientists.

NYU School of Medicine researchers have made an important discovery that partially answers the long-standing question of why a mother’s immune system does not reject a developing fetus as foreign tissue.

Lead investigator Adrian Erlebacher, MD, PhD, associate professor of pathology and a member of the NYU Cancer Institute at NYU Langone Medical Center said: “Our manuscript addresses a fundamental question in the fields of transplantation immunology and reproductive biology, namely, how do the fetus and placenta, which express antigens that are disparate from the mother, avoid being rejected by the maternal immune system during pregnancy?

“What we found was completely unexpected at every level,” he added.

They found that embryo implantation sparks a process that ultimately turns off a key pathway required for the immune system to attack foreign bodies. Therefore, immune cells are not recruited to the site of implantation and therefore cannot harm the developing fetus.

When a woman becomes pregnant, genes that recruit immune cells to the sites of inflammation are turned off within the decidua. This means that T cells are unable to accumulate within the decidua and, as a result, cannot attack the fetus and placenta.

Dr. Erlebacher said: “These findings give insight into mechanisms of fetal-maternal immune tolerance, as well as reveal the epigenetic modification of chemokine genes within tissue stromal cells as a modality for limiting the trafficking of activated T cells.

“It turns out that the cells that typically secrete the chemoattractants to bring the T cells to sites of inflammation are inhibited from doing so in the context of the pregnant uterus. The decidua appears instead as a zone of relative immunological inactivity.”