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Types of stem cell transplants
The types of stem cell transplants are based on who the stem cells come from. There are 3 basic types:
- Allogeneic transplants use stem cells from a donor.
- Syngeneic transplants use stem cells from a donor who is an identical twin.
- Autologous transplants use the recipient’s own stem cells.
An allogeneic transplant is also called an allograft. Stem cells are taken (harvested) from one person (donor) and given to another (recipient).
- This type of transplant is usually done with family members. The most common donor is a sibling (brother or sister). The donor must be between the ages of 17 and 50. People in this age range have the healthiest and the largest number of stem cells.
- Allogeneic transplant can also be done with an unrelated donor identified through the OneMatch Stem Cell and Marrow Network.
- A transplant using matched unrelated donors is sometimes called MUD.
- Canadian Blood Services (or Héma-Québec in Québec) has a bone marrow donor registry with a database of potential donors. They search the database to match donors and recipients. This Canadian registry is linked to many registries in other countries.
- Bone marrow is matched through a process called human leukocyte antigen (HLA) typing.
- Human leukocyte antigens are proteins found on the surface of blood and tissue cells.
- Stems cells are used from a donor whose HLA typing best matches the recipient’s.
An allogeneic transplant has higher complication rates than an autologous transplant. One of the most important risks is graft-versus-host disease (GVHD). GVHD is when the donated stem cells have an immune response to the recipient’s cells. The donated cells see the recipient’s cells as foreign and start to destroy them.
At the same time, allogeneic transplant is associated with the beneficial graft-versus-tumour (or graft-versus-leukemia) effect. In the graft-versus-tumour effect, the donor’s stem cells attack any cancer cells that are still in the recipient’s body. As a result, cancer is less likely to return after treatment.
A newer method for allogeneic transplant involves using stem cells collected from umbilical cord blood. This method is used mostly in transplants for children and small adults.
Compared to a standard allogeneic transplant, reduced-intensity transplant uses lower-dose chemotherapy, radiation therapy or both before the stem cell transplant. This approach kills some cancer cells and some bone marrow cells. This approach weakens the recipient’s immune system enough to allow donor cells to destroy any remaining cancer cells (graft-versus-tumour effect).
- This approach may be useful in older people, those with other health problems who may not tolerate a standard transplant or those who have already had a stem cell transplant.
- This procedure has only been used during the last 10 years.
Reduced-intensity allogeneic transplant may also be called non-myeloablative transplant or mini-transplant.
Syngeneic transplant is an allogeneic transplant between identical twins. Stem cells are taken from one identical twin and given to the other.
- Identical twins share all the same genes and tissue types. They have the same HLAs, so there is little chance of rejection and the transplant is usually accepted.
- This is an uncommon transplant because it is rare to have an identical twin.
An autologous transplant uses the recipient’s own stem cells. It is also called an autotransplant or autograft.
- Stem cells are harvested from the person’s own bone marrow or blood and are frozen (cryopreserved) for later use.
- Cancer cells may be harvested along with the stem cells, so the cells may be treated with chemotherapy to reduce the number of cancer cells present. This process is called purging.
- After the recipient has high-dose chemotherapy, radiation therapy or both, the stem cells are thawed and given back to them.
There is no risk of graft-versus-host disease (GVHD) with autologous transplant. There is a higher chance of relapse with this type of transplant.
A tandem transplant means that 2 autologous transplants are given in a row. The recipient is given 2 courses of high-dose chemotherapy. Each dose is followed by a stem cell transplant.
- All the stem cells needed for the transplant are collected before the first course of high-dose chemotherapy.
- Half of the stem cells are used for each transplant.
- The transplants are usually done within 6 months of each other. The second transplant is done after the person recovers from the first transplant.
Researchers are still trying to determine the best use of this type of transplant. It has been used for some people with multiple myeloma, Hodgkin lymphoma and non-Hodgkin lymphoma.
Stem cells can be taken (harvested) from bone marrow, peripheral blood or umbilical cord blood.
Bone marrow stem cells can be used for all types of transplants. Stem cells are collected from the bone marrow during a procedure called harvesting.
- A syringe is inserted through the skin into a bone. Bone marrow and blood are removed from the bone through the syringe. Repeated needle sticks are made until enough cells are collected for the transplant.
- Once the stem cells are collected, they are filtered to remove bone fragments and fat particles.
- The stem cells can be combined with a preservative and frozen (cryopreserved) to keep them alive until needed.
Peripheral blood stem cells can be used for all types of transplants. Stem cells are collected from the circulating (peripheral) blood.
- Harvesting is done by a process called apheresis or hemapheresis.
- Blood is removed from the donor through a large vein.
- The blood circulates through a machine that separates the stem cells from the rest of the blood cells.
- Blood and remaining cells are then returned to the donor’s bloodstream.
- The collection process is repeated until enough stem cells are collected.
- Stem cells can be frozen and stored until they are needed.
- Engraftment (when new stem cells start to appear) happens more quickly when peripheral stem cells are used than when bone marrow stem cells are used. As a result, bone marrow suppressionbone marrow suppressionA condition in which the bone marrow does not produce normal numbers of red blood cells, white blood cells and platelets. does not last as long as when stem cells are collected from the bone marrow.
Umbilical cord blood
Stem cells from umbilical cord blood are mostly used for transplants in children because there usually aren’t enough stem cells for adults.
- Stem cells are collected from blood in the umbilical cord of a newborn.
- The stem cells are processed, frozen and stored for future use by cord blood banks or programs.
Using stem cells from umbilical cord blood offers some benefits. Stem cells from cord blood rarely contain harmful viruses and infections. There is also a lower risk of graft-versus-host-disease (GVHD) when cord blood stem cells are used.
On the other hand, engraftment (when new stem cells start to appear) happens more slowly with stem cells from cord blood than with stem cells from bone marrow or adult blood. Cord blood stem cells may carry undetected genetic or congenital diseases.
Researchers are still studying cord blood to find out how long stem cells can be stored before they lose their effectiveness. They are also studying the long-term success rate of transplants using umbilical cord blood.