One of my current research projects involves studying the genetic component of the autoimmune disease Rheumatoid Arthritis. In this disease, the body’s immune system attacks its own tissues, usually around the joints, causing moderate to severe pain, and breakdown of the nearby bone. Rheumatoid Arthritis affects approximately 1% of the population. But, for reasons that we do not yet understand, the symptoms go away during pregnancy, in about 50% of women (in other patients the symptoms stay the same or worsen). While conducting background research for this project, I came across an amazing feature of human pregnancy: microchimerism.
What does microchimerism mean?
Let’s start with the biggest part of that word, chimera. In greek mythology, a chimera is a creature made of three different animals (a lion, a snake and a goat). In biology, the word chimera, or chimerism, can be used to refer to the existence of cells from genetically distinct individuals found in one person. So, microchimerism refers to the existence of a small set of cells of one genetic type intermixed with a large set of cells of another genetic type.
Let's think about development for a minute.
My two year old shares about half of her DNA with her father, and about half with me. And, because we have raised her, she's picking up our habits, good and bad. She smiles like her daddy, loves sharing snacks with our dog, and furrows her brow to glare at me when she’s upset (exactly like a million pictures of me growing up). I do everything I can to keep her a safe, while still allowing her the freedom to explore her world. It turns out that I’ve been doing this since she was a tiny, unrecognizable embryo.
Okay, you had a baby, but how did you become a chimera?
When I was pregnant, my body provided nourishment and protection. A portion of that protection came in the form of my immune system providing defense against infections. During pregnancy, some of my immune cells actually passed through the placenta, and became incorporated into my daughter’s developing body. This means that although most of the human cells in her body are her own (a unique combination of DNA from her father and from me), incredibly, there are some cells in her body that are actually mine. Even more extraordinary is that this sharing of immune cells is a two-way street. Some of the immune cells from her growing body passed back into mine. A subset of these cells became incorporated into my body, and will continue to replicate, sometimes for decades (Gammill and Nelson have a great review here, if you want to learn more).
|Immune cells cross the plancetal boundary and|
become incorporated among genetically dissimilar
cells where they continue to replicate
Whoa. So, you are both chimeras?
Yes. In fact, this exchange of cells occurs during all human pregnancies, with both male and female embryos. This means that some women have a small number of cells with male DNA from their sons, and men have some cells with female DNA from their mothers. So, we are all born as chimeras. Scientists are just now starting to understand how this exchange of cells affects our biology. My current interest in this topic is that fetal microchimerism might be responsible for some of the alleviation of symptoms during pregnancy for patients with Rheumatoid Arthritis.
It is also just mind-blowingly awesome.
It amazes me to realize that, right now, some of my daughter’s cells are incorporated into my body. There is, literally, a physical part of her with me wherever I go. And, similarly, a few of my cells are incorporated into her. The scientist in me is awestruck. But, as any parent will tell you, it doesn't change how I feel. Regardless of our genetic relationship, I will always be a part of my daughter, and she will be a part of me.