Types of Bone marrow transplantation
In a bone marrow transplant, the diseased bone marrow of the patient is destroyed and healthy stem cells are transplanted into the patient’s blood-stream. After a successful transplant, the new stem cells in the blood migrates to the bone marrow of the large bones, engrafts and restores production of normal blood cells.
Based on the source of stem cells, there are mainly two type of bone marrow transplantation procedures:
In this, the patient’s own bone marrow cells are used for the transplantation. An autologous BMT is possible when the disease affecting the bone marrow is either in remission or the diseases being treated does not affect the bone marrow cells (such as Hodgkin’s disease, non-Hodgkin’s lymphoma, breast cancer, ovarian cancer, and brain tumors). The cells from the bone marrow are extracted from the patient before the transplant – prior to eliminating the bone morrow to purge the lingering malignant cells (if the disease affects the bone marrow).
There are some advantages associated with this type of transplant. As patient’s own cells are used, there is no risk of tissue incompatibility and rejection of the transplant. However, it is prerequisite for a successful Autologous BMT that the patient have healthy bone marrow cells.
In this transplantation, the patient’s bone marrow cells are quickly processed and infused back after the conditioning regime. Autologous bone marrow transplantation is also a part of the treatment plan for some cancer patients who undergo chemotherapy and/or radiotherapy. This is because these therapies might damage the bone marrow and lead to health problems afterwards.
For autologous transplant, the stem cells harvested from the bone marrow or blood are processed, frozen and stored for the future use. After chemotherapy and/or radiotherapy, when the bone marrow gets obliterated, autologous transplantation is performed to infuse the healthy blood forming cells.
Allogeneic BMT involves the use of stem cells derived a healthy donor who is tissue-compatible with the recipient. The donor for BMT can be a relative or any other person who has compatible tissue type, which is a prerequisite of the allogeneic transplantation. In many cases, patient have a sibling whose bone marrow is a perfect match. It is referred to as syngeneic BMT when the donor is an identical twin. In cases where the patient has no matched sibling, a donor can be found in one of the international bone marrow donor registries.
A haploidentical (half- matched donor) or mis-matched transplant may be considered. Umbilical cord transplant is another option in which the cord blood, rich in stem cells, is used for replacing the damaged stem cells in a patient. A mother may choose to preserve the umbilical cord after the baby is born. It is frozen and stored for future use.
In an allogeneic BMT, gentic compatibility is important to lower the risk of rejection and other complications. This means that new stem cells harvested from the donor must match the genetic makeup of the patient’s as much as possible. For this, a special blood test, known as HLA testing, is done to determine whether the donor candidate’s bone marrow matches the patient’s.
Human leukocyte antigen (HLA) is a protein (marker) found on most cells in our body. The best transplant outcomes are considered when the donor’s HLA markers closely matches the HLA of the patient. HLA match is important as it helps reduce the risk for complications after transplant. If the donor is not a good genetic match, the healthy transplant cells will attack the patient’s body, perceiving it as foreign.
This condition is referred to as graft-versus-host disease (GVHD). It can be a life-threatening complication post transplant. Alternatively, the patient’s immune system may recognize the new transplanted stem cells as foreign and destroy them, a condition known as graft rejection. It can cause some severe symptoms. Therefore, it is necessary to have matched donors for allogenic bone marrow transplants.
Usually, the first candidates tested for a matching donor are from patient’s family, usually a brother or sister. This is because children inherit HLA markers from the parents. Each sibling has a 25% chance of completely matching the genetic markers, if they have the same parents. However, the parents and children always have exactly half match of HLA markers. Some patients are considered for a transplant from a half-matched donor, it is known as haploidentical transplant.