A child having an allogeneic transplant (using someone else’s bone marrow or stem cells) needs a donor who is matched by tissue typing. Those having an autologous transplant (using their own cells) do not need a donor. If your child needs a donor, it’s important for you to understand how tissue matching works, how to find a matched donor, and what happens if no matched donor can be found. Today, lack of a donor is rarely a limitation to bone marrow transplantation.
What Does Matching Mean?
It’s important to closely match the tissue of a donor with the tissue of a transplant recipient to decrease the chance that the recipient’s body will reject the transplanted cells (called host-versus-graft disease) or that the transplanted cells will attack the recipient’s tissues (called graft-versus-host disease, GVHD).
To match donors and recipients, doctors conduct tests to determine and compare their human leukocyte antigen (HLA) tissue type. Basically, HLA is a “self” marker on every cell. The immune system uses this marker to recognize which cells belong to your child’s body and which do not.
You can think of the HLA tissue type as a kind of zip code. Your child’s cells have a distinctive zip code, and you want to find a donor who has a very similar zip code—if not identical, then at least in an adjoining neighborhood or city.
More technically, the HLA tissue type, or haplotype, is a set of closely linked genes that are inherited together. Children get one HLA haplotype from their mother and one HLA haplotype from their father. Tissue-typing tests look at either four or five points on each HLA haplotype, for a total of eight or 10 points. To be considered a good match for a transplant, the donor and recipient normally must match each other on at least seven of eight or nine of 10 points. A donor who matches your child on all of the tested points (eight of eight or 10 of 10) may be called “optimally matched.” A donor who matches your child on all but one point (seven of eight or nine of 10) may be called “minimally mismatched.”
Learn more about HLA typing.
How Do We Find a Matched Donor?
Doctors will want to look first for a potential donor within your family. Then they can expand the search, if needed.
Check the Family First
The first choice for any child having a transplant is to use a donor who is related and therefore much more likely to be closely matched. Time can be of the essence when a transplant is necessary—and relatives who are a match can often be enlisted to help quickly. Thus, the first step in the search for a donor is to test any relatives who are willing to donate.
If your child’s sibling is found to be a matched donor, our Child Life specialist, social worker, and nurses will help you explain the procedure to your child in a way that is appropriate for their age and maturity. Some children who act as donors for a brother or sister are happy and proud to do so, while others may have mixed feelings or be too young to completely understand what is happening. If the sibling donor is age 12 or younger, bone marrow is harvested on the day of the transplant at Seattle Children’s. For sibling donors older than age 12, the transplant may involve either bone marrow harvesting or mobilization and collection of peripheral blood stem cells.
Expanding the Search to Registries
If your child does not have a relative who is a close match, Seattle Cancer Care Alliance (SCCA) doctors will search a volunteer donor registry for a matched unrelated donor. The Be The Match Registry, the largest and most diverse marrow registry, now has more than 10.5 million potential donors. By partnering with other registries, Be The Match gives doctors and patients access to a total of more than 22.5 million donors around the world. The search typically takes weeks or months to complete.
Fred Hutch, a founding organization of SCCA, actually started the first registry, which later grew to become the National Marrow Donor Program, which operates the Be The Match Registry. Now the Fred Hutch Bone Marrow Transplant Program at SCCA has more experience doing transplants with matched unrelated donors than any transplant center in the world. For many children treated here, the survival rate with matched unrelated donors is nearly equivalent to that with matched related donors.
What If We Can’t Find a Donor?
About a third of children cannot find a suitable matched donor either within their family or in the donor registry. For these children, researchers at the Hutch, Seattle Children’s, and SCCA have developed two potentially lifesaving options as new sources of donor cells: half-matched (haploidentical) family members and umbilical cord blood.
Today, because of new sources of stem cells—and new methods for making these cells work safely and effectively—almost all patients who need a bone marrow transplant will have a donor. A study in the New England Journal of Medicine in 2014 verified that lack of a donor is no longer a major limitation to transplants. According to the study, more than 99 percent of white European-Americans and 95 percent of African-Americans who need a transplant will have a suitable match.1
Half-Matched (Haploidentical) Family Members
Haploidentical transplants use cells from a relative whose tissue is not considered a suitable match according to usual tissue-typing standards. Haploidentical donors have one haplotype in common with the recipient, so they are a half-match. A haploidentical match may be the best option for patients without a more closely matched donor, especially those who urgently need a transplant. Parents and children are haploidentical, and some siblings are haploidentical, so if a child needs a transplant, the child’s parents or partially matched siblings may be able to donate cells.
Umbilical Cord Blood
Cord blood transplants use stem cells from the blood that remains in the umbilical cord when a mother and newborn are separated. Because the infant’s immune system is immature, a cord blood transplant can succeed even if there’s a greater disparity between the infant’s tissue and the recipient’s tissue than would be allowed in a bone marrow or peripheral blood stem cell transplant.
Changing the World of Transplantation
Haploidentical and cord blood options have changed the world of transplantation, greatly improving the odds of finding suitable donors for children who need a transplant. The key to success has been development of new techniques to overcome hurdles with these alternative sources of stem cells.
With haploidentical transplants, the main hurdle was GVHD, which results when the donor’s immune system attacks the recipient’s tissue. One preventive therapy developed at SCCA involves use of high doses of cyclophosphamide and immunosuppression to kill certain of the donor immune cells that would otherwise cause GVHD. This has been extremely successful and is now standard practice in many transplant centers.
With cord blood transplants, the main hurdle was how long the stem cells took to repopulate and engraft in the bone marrow because cord blood yields a relatively small number of stem cells. Two solutions to this problem were developed at SCCA. In the first, now routinely used at SCCA, doctors combine two units of cord blood (from different donors). The other new option being developed involves expanding the number of stem cells in the lab before transplantation. Doctors use a growth protein to boost the stem cell population 150-fold, which leads to speedier engraftment after transplantation.
With these and other special methods developed at SCCA, children who cannot find matched donors are now achieving transplant outcomes very similar to outcomes in children with matched donors. These special methods open up the option of a potentially lifesaving transplant to many more children. To find out more about these special methods and clinical studies with even newer techniques, talk to your child’s SCCA doctor.
Becoming a Bone Marrow Donor
If you are related to someone who needs a donor and who plans to receive a transplant at SCCA and you would like to be tested as a possible match, we will coordinate this as part of the patient’s transplant process.
If you are a close-enough match and will be the donor, you will come to SCCA with the patient to meet with the medical team and review your role as a donor and the donation process. You will be able to have all of your questions answered at this time. You will also have a health evaluation and will sign consent forms. You can expect to be at SCCA for three full days to complete these steps. Then you will be able to leave while the patient undergoes conditioning. To learn more about the donor’s role, read about the transplant process.
Anonymous donation: If you would like to register to be an anonymous donor for someone who doesn’t have a matched relative available to donate, or if you would like to donate cord blood when your baby is born, you can do so through the National Marrow Donor Program.
Learn how bone marrow and stem cells are collected.