Countless Cells from Your Mother Remain in You, and We Now Understand Why

Understanding Maternal Microchimerism

Everyone born on this planet isn’t entirely made up of their own cells. In fact, a tiny fraction—about one in a million—comes from our mothers. This means that, surprisingly, each of us carries millions of cells that our immune systems might identify as foreign. Yet, in most cases, these cells coexist without causing immune issues.

Researchers have now uncovered the reason for this tolerance. It turns out that some maternal immune cells cross the placenta during pregnancy and help train the fetus’s immune system to accept these cells for life.

The exchange of cells between a mother and a fetus is known as microchimerism. This is a well-established phenomenon that has been understood for over 50 years. It works both ways: anyone who has been pregnant retains cells from their fetus, and every person retains cells from their mother.

This lingering presence of maternal cells presents an interesting challenge for immunologists, who generally believe that the immune system should react against foreign cells.

A team led by pediatric infectious disease expert Sing Sing Way at Cincinnati Children’s Hospital sought to delve deeper into how these maternal cells help regulate the immune response and contribute to the shaping of the fetus’s immune system.

To explore this, the researchers studied maternal microchimerism in mice. Using their earlier work as a basis, they bred mice with immune cells specifically designed to express certain surface markers. This approach allowed them to selectively eliminate those cells and observe how immune tolerance was affected.

What they found was intriguing. A small group of maternal immune cells, with characteristics akin to bone marrow myeloid cells and dendritic cells, remained present well beyond birth. These cells were closely linked to immune activity and the increase of regulatory T cells—those crucial components that signal to the immune system that everything is functioning normally.

To verify their findings, the researchers then removed these specific maternal cells from the offspring mice.

The outcomes were striking. The regulatory T cells vanished, and with them, the immune tolerance to maternal cells disappeared.

This suggests that maintaining lifelong tolerance to maternal microchimeric cells likely depends on a small group of maternal cells. If those are removed, it could lead to immune disarray. This also implies that immune tolerance is an ongoing process rather than a one-time event during pregnancy.

This is not only fascinating from a biological standpoint but also opens doors to exploring a range of diseases and conditions that microchimerism may impact.

According to Way, “The new tools we developed to study these cells will assist scientists in pinpointing their exact roles in various contexts, such as autoimmune diseases, cancer, and neurological disorders.”

He adds, “Microchimerism is increasingly connected with many health issues. This study lays the groundwork for further investigation into whether these rare cells might cause disease or are simply present in tissues affected by disease as part of the healing process.”