I had a post written up covering the last couple days while we were waiting for results, but now that I have them, those details seem less important somehow.

First the Chimerism test results, then I’ll try to explain them.

Zoe’s test came back at “Greater than 98%”, which is as high a reading as this particular test offers. That’s good news. The best news, really.

Also important is the line on the report that reads, “no recipient cells detected”. This means that she has none of her old immune system left whatsoever. In short, Zoe’s test results were the best she could get on this test, and the best news we’ve had… well, ever, I suppose.

What does all of this mean?

Zoe “has” (we can’t say she’s cured yet but we’re well on our way) a hereditary condition/disease called HLH which caused her immune system to malfunction and attack her body, as if her body were an infection. The inflammation from this attack caused persistent fevers, enlarged organs, and without treatment and subsequent cure would have resulted in her death in a matter of weeks.

To cure Familial HLH Zoe had to get a new immune system via either a stem cell transplant or bone marrow transplant, otherwise the disease would continue to re-activate. Eventually the drugs used to control it would become ineffective, leaving her defenseless.

In order to get a new immune system, the old one has to be wiped out via a series of drugs (chemotherapy) that destroy the old bone marrow (which produces the white and other blood cells), so that a new one could be transfused. When a transplant takes place, there is a chance that both the old cells and the new cells will co-exist for a time, until either the new cells establish “dominance” as it were, or the old cells reject the new ones.

This rejection is the greatest risk for these kids. If it occurs, they have to go through the process of transplant again until they can establish a new functioning immune system. They can live with a mixed immune system, however, so long as the “dominant” portion are the new cells. Specifically, the old lymphocytes must be destroyed, if they continue to be produced in the marrow they could potentially begin attacking the body again.

This mix of old and new is called Chimerism. The word comes from the Chimera, a creature of Greek myth composed of multiple types of creatures. The test itself tells us what percentage of the old system still exists, and what percentage is new. We want as much as possible to be new, and in Zoe’s case, it appears that it is all new. She doesn’t appear to have any Chimerism, she simply has an entirely new immune system.

I am told that kids can do just fine with as little as 20% new cells, again assuming that the trouble-making lymphocytes are gone and that portion of the immune system is governed by the new cells. We had been optimistic, hoping for results greater than 50%, with room for growth toward that 100%. As time goes on after transplant, it’s expected in kids with a mix of old and new that one of the systems will continue to establish itself, and that they could eventually end up with 100% new cells, despite starting off with 50 or 75% at day +30. This is all part of the process.

Dr. P mentioned that he had been privately expecting and hoping for a 90/10 split for Zoe, due to the reduced intensity regimen and the fact that it can sometimes only partially wipe out the old cells. He would have been happy with that result. He was very pleased indeed with this result. Zoe’s RIC was on the harsher end of RIC, but I now firmly believe we got the best of both worlds.

Zoe was able to receive a lighter dose of the chemotherapy, potentially preserving her fertility and protecting her from the harsher side effects, while still seeing the “right” amount of destruction to her old immune system, leaving her able to accept the new cells successfully.

We will be eternally grateful to Duke University Health for the successful transplant, and to Wake Forest University Medical Center for her early diagnosis and life saving treatment. I now know which basketball team I’ll be rooting for for the rest of my life.

We don’t yet know all the ramifications of this for us or for Zoe. We know we will still need to maintain long term care for her, protect her from infection this next year through isolation, and watch her for GVH. Nothing in medicine is 100%, there is still a chance that there could be GVH or other issues, but we believe the chance much lower than it would be with a more mixed Chimerism.

Our next goal is to reach 60 days, after which the risks to Zoe for infection or GVH should be drastically lower than they will be these next 30 days. The worst is now firmly behind us and I hope that very soon we will be able to say that Zoe is cured. After this year has passed, our family should be able to return to our “new normal”.