Cells from your mother (and maybe grandmother) are hiding among yours

Did you know that your body houses human cells from other individuals? From your mother, from the children you bore, perhaps even from your distant ancestors or from a twin whose existence you did not know? Interview with French science journalist Lise Barnéoud, who wrote a book on a recent (and fascinating) field of research: “microchimerism”.

For a year, you investigated microchimerism, which focuses on human cells that come from other individuals. Does everyone have these kinds of cells? And in what proportion?

LB: Almost all the researchers I asked the question told me that yes, we all have microchimeric cells within us. On the other hand, the proportion is very variable: there are ranges from 0.000001% to more than 50%, for certain individuals. And even within an individual, it will depend on the organ considered. For example, we could imagine finding 0% microchimerism in the blood and having a pancreas made up of 10% microchimeric cells.

Who do these cells come from and how do they settle in us?

LB: The first moment when we recover all these cells is when we are in our mother’s womb. We imagine the placenta as a sort of border, but we have understood that many things can cross its membranes, including human cells. The first cells that will reach us are those of our mothers, but we can also recover the microchimeric cells of our mothers. The team of French researcher Nathalie Lambert showed in 2021 that certain infants have cells from their grandmother in their blood.

Could this transmission of cells even take place over several generations?

LB: It’s a hypothesis. In humans, only grandmaternal origin has been proven, but in mice, experiments show that it can be transmitted over 51 generations. In humans, 51 generations means at least 1000 years. But we must keep in mind the fact that, when we are in our mother’s womb, we not only receive the cells that come from the maternal organism: we also send our own cells there. Women therefore have another source of microchimerism: they carry within them cells from the embryos they have carried, whether they have given birth or not.

During pregnancy, the embryo can also receive, from its mother, cells from brothers, sisters and embryos that preceded it, but also cells from a non-identical, living or “vanishing” twin…

LB: Yes. In the book I tell the story of M^me McK who, in 1953, donated blood in England. Her red blood cells were both group O and group A. She was born without a twin, but it was hypothesized that she carried the red blood cells of a vanishing twin: an embryo fertilized at the same time as her , but which quickly disappeared, without anyone realizing it.

Microchimeric cells, where can they be found in our body?

LB: Really everywhere! Every time we look for them and take biopsies, after a while we find them, whether in the lungs, muscles, skin, heart and even the brain. And they actively integrate into our organs: they multiply there, communicate with their neighbors, and produce proteins which are not necessarily the same as those that our cells will produce.

Microchimeric cells, you write, can even make certain DNA tests lie…

LB: Yes. We know of around ten cases where microchimerism has taken hold in the gametes (eggs and spermatozoa)in both women and men. The case of American Karen Keegan has been studied in detail. Karen needed a kidney transplant, and potential donors were sought among her relatives. And it was discovered that two of her sons genetically speaking could not be her sons: they only had 25% of her DNA, as if Karen were their aunt. Karen has two different cell lines in her body. And one of them is probably that of a vanishing twin. The cells that this twin left her have partly lodged in her ovaries.

Are these cells harmful or beneficial?

LB: It’s never all black or white, neither in life nor in biology. In the early 1990s, we assumed that they were evil because of the view we had of the immune system. By digging, we found some clues which show that indeed, in certain cases, they can induce inflammation and perhaps help a tumor to develop. But from the moment we realized that we all have microchimeric cells, we also looked at whether they had positive effects. And we discovered that indeed, these cells can regenerate tissues. When they land in the skin, for example, they have the ability to help with healing.

Do they have a role to play? To what extent do they influence us?

LB: That remains the great mystery! In my opinion, they already have a role to play in the way we understand our immune system. And I think that indeed, we will discover that they can influence the fate of a tumor, the healing processes… but we are at the very beginning. Can such small numbers of cells be significant on the scale of the entire organism? Many researchers are still skeptical on this point, but others rightly point out that in biology, sometimes, even small quantities of elements can drastically change a trajectory.