A stem cell transplant is a treatment that involves infusing hematopoietic (blood-forming) stem cells from a donor into a recipient. The collection of infused stem cells is called the stem cell “graft.”
Hematopoietic stem cells produce the different cells that make up blood: white blood cells to fight infection, red blood cells to move oxygen through the body, and platelets that help control bleeding. Hematopoietic stem cells reside in the bone marrow, where they reproduce themselves (self-renewal) and mature into circulating blood cells. In fact, our bodies are constantly making new stem cells in order to replace old and dying circulating blood cells.
If the body begins making unhealthy cells or not enough healthy cells, a stem cell transplant may be necessary. The most common reasons why a child might need a stem cell transplant are if he or she has:
- Certain types of cancer, such as leukemia and lymphoma;
- Certain blood-related diseases, such as sickle cell disease and aplastic anemia;
- Inherited immune system disorders, such as severe combined immune deficiency; or
- Metabolic disorders, such as Hurler syndrome
Are There Different Types of Stem Cell Transplants?
Patients can receive their own (autologous) or another person’s (allogeneic) hematopoietic stem cells. In an allogeneic transplant, the patient receives stem cells from a family member (parent or sibling) or unrelated donor who shares the same group of proteins expressed on the surface of white blood cells called human leukocyte antigen (HLA). These stem cells are collected from the donor’s bloodstream or bone marrow. A third source of stem cells is umbilical cord blood collected from the umbilical cord after a baby is born.
In an autologous transplant, the patient receives his or her own hematopoietic stem cells, which have been previously collected from his or her bloodstream or bone marrow. Autologous transplantation is usually reserved for patients with certain types of cancers like brain tumors and neuroblastoma.
How Is a Stem Cell Donor Selected?
Donors are selected based on the similarity of their human leukocyte antigen (HLA) to that of the patient. HLA markers are inherited in such a way that a child receives half of his or her complete set of HLA proteins from each parent. Therefore, related family members, particularly siblings with the same parents, are often tested as potential donors for patients needing a stem cell transplant.
A “matched” donor shares the same HLA proteins tested with the recipient, while an “unmatched” donor has one or two “HLA mismatches” different from his or her recipient. If a HLA-matched related donor is unavailable, then an unrelated donor search occurs using the National Marrow Donor Program (NMDP) Registry.
Usually, HLA matching between donor and recipient increases the chances that the donor stem cells will engraft or find their way into the recipient’s bone marrow, where they will produce the circulating blood cells. Also, use of HLA-matched stem cells reduces risks of complications associated with stem cell transplant, including graft-versus-host disease, an immune process in which healthy donor cells attack tissues in the recipient’s body.
Aside from HLA matching, other considerations for using a particular donor include age, gender, infection exposures, and overall donor health. Sometimes, a matched related or unrelated donor cannot be found.
When Can a Patient Receive a Stem Cell Transplant?
Before receiving a stem cell transplant, it is important to make sure the patient’s body is healthy enough for a transplant. Also, if the patient has cancer, then it is important that his or her cancer is either undetectable (in remission) or is at the lowest level possible (minimal residual disease) after receiving chemotherapy and surgery, if the patient has a solid tumor like a brain tumor.
The evaluation process that a potential transplant recipient undergoes to assess organ function and malignant disease response prior to transplant is called the “pre-transplant work-up.” Transplant coordinators work with patients and families in identifying a donor and setting up the necessary testing needed to determine if a patient is ready to receive a transplant.
When Should a Patient Be Referred for Transplant?
Patients with a known illness requiring a stem cell transplant should be referred as soon as possible.
At Nationwide Children’s Hospital, researchers and physicians are performing clinical trials to improve the success and reduce the risks associated with unmatched stem cell transplants. Using certain medications and techniques to change the content of the stem cell graft may help to improve outcomes when an unmatched transplant is necessary.