Chimera? What's a Chimera?

The Chimera is a mythical creature, as described by Homer in the Iliad, as "a thing of immortal make, not human, lion-fronted and snake behind, a goat in the middle, and snorting out the breath of the terrible flame of bright fire".

Chimerism in scientific terms is when an animal has two or more different populations of genetically distinct cells that originated in different zygotes; if the different cells emerged from the same zygote, it is called a mosaicism. (From Wikipedia, see here for the whole article). A male tortoiseshell cat is a chimera.

Microchimerism is the presence of a small number of cells, genetically distinct from those of the host individual(also from Wiki). In some cases, fetal cells can find there way into the mother's circulation and remain there for as long as decades. This has also been found in some severely immunocompromised trauma patients who have received blood transfusions.

Even more startling is the case of a woman who was found to have two sets of genetically distinct cells. It was discovered when she and her family went for tissue typing because she needed a kidney transplant. From the New Scientist, November, 2003, this is a really interesting story on it, and explains it very well. As researchers have looked into it, genetic material can be transferred from mother to children and vice versa, or between twins in the womb. There is even speculation that a mother can receive cells from her fetus, carry them in her circulation for years and pass them on to future children that she carries, all without anyone having a reaction to what should be seen as a foreign invader by the body. Could they be blast cells? Like a stem cell transfusion? Even more perplexing is why this should happen.

I'd be lying if I said I really understood this. Chimerism first came to my attention this past fall when I was doing research on transplantation. I had never heard of it while working as a TC, but immediately it was clear to me why transplant researchers would be so intrigued by this. One of the biggest, (the biggest?) problem with transplant is rejection. The recipient's immune system sees the new tissue as an invader and tries to destroy it. Not only is it a problem with the life of the organ and the life expectancy of the patient, but also the the cost of the antirejection drugs, which can be $50,000 a year, are a big factor in people's financial ability to pay for a transplant. Imagine not having to take antirejection drugs. Imagine being able to extend the viability of a tranplanted organ. Some centers, like Northwestern Memorial in Chicago, among others, have been transplanting stem cells from living kidney donors into the recipients. The logistics of both stem cell and organ transplantation make this currently unfeasible for cadaveric organ donation.

Just when things can't get any weirder, comes this story from Down Undah. 15 year old Demi-Lee Brennan received a liver transplant. Nine months later, her doctors discovered that her blood type had changed to that of her donor's. More importantly, they found that stem cells from the donor liver had penetrated her bone marrow, changing her immune system and allowing her to come off anti rejection drugs. While transplant surgeons have known about stem cells "floating about" in the recipient's body, this is the first time anyone has, in effect, received a stem cell transplant and changed their immune system. If they can find a way to replicate this, it could change the field of organ donation.

Imagine taking on your donor's DNA. Makes that new Jessica Alba movie seem downright tame.