Blood and Marrow Stem Cell Transplantation
Stem cell transplantation from marrow was introduced approximately 35 years ago and is now standard therapy for selected patients with leukemia, lymphoma and myeloma. There are two major types of stem cell transplants: syngeneic and allogeneic. Syngeneic transplant describes the use of an identical twin as donor. An allogeneic transplant uses blood or marrow stem cells from a normal donor, usually a brother or sister with the same tissue type. If a sibling is not available, a search of the National Marrow Donor Program registry of tissue-typed volunteers could be made for a matched unrelated donor. In special instances, especially in young children, mismatched donors may be used, for example a parent.
Autologous transplantation uses the patient’s own marrow stem cells and is technically not transplantation since another person is not the donor. The technique is important, however. The blood or marrow stem cells are collected while the patient is in remission, and it may be treated with chemotherapy agents or monoclonal antibodies to decrease the presence of contaminating tumor cells before being given back. The stem cells are frozen and administered later in the course of the disease if intensive chemotherapy and/or radiotherapy is required for subsequent treatment.
The technique of harvesting stem cells from blood and cord blood has made transplantation available for more patients. Blood and cord blood transplants differ from marrow transplants principally in the source of the cells collected for transplant. Stem cells not only reside in the marrow but also circulate in the blood. Because blood, as well as marrow, is a source of stem cells for transplantation, these cells can be harvested from the blood of a donor, frozen and stored, and later transplanted to the patient. To ensure there will be enough blood stem cells for successful transplantation, donors of blood stem cells require special treatment to mobilize sufficient stem cells from marrow into their blood before cells are harvested.
Stem cells circulate in large numbers in fetal blood also and can be recovered from umbilical cord and placental blood after childbirth. The harvesting, freezing and storing of cord blood has provided another source of stem cells for transplantation, especially for children. The numbers of stem cells in cord blood are often insufficient for the needs of larger adult patients.
Cord blood stem cell transplantation provides an additional donor pool and the opportunity for greater racial diversity in the blood supply because of collection efforts in hospitals where children of underrepresented ethnic backgrounds are born.
“Non-ablative” allogeneic stem cell transplantation is the term applied to a technique of allogeneic transplant that uses lower doses of chemotherapy and/or radiotherapy to prepare the recipient to receive the donor’s stem cells. This still experimental approach greatly lessens the early toxicity of transplantation and has extended the age at which recipients with leukemia or lymphoma can have a transplant. It has been made possible by more effective immunosuppressive drugs that are capable of preventing rejection of the donor’s cells without full intensity treatment of the patient’s immune system. Over time the donor’s cells take hold and the patient’s leukemia or lymphoma is attacked and suppressed by donor lymphocytes that form from the donor stem cells. This “graft versus leukemia or lymphoma effect” suppresses (cures) the malignancy and is a prolonged (indefinite) form of immunotherapy. In standard stem cell transplantation, ablation of the recipient’s blood-cell-forming and immune cells was the price that had to be paid to eradicate the leukemia or lymphoma and permit the donor’s cells to be accepted by the temporarily immunodeficient recipient. “Ablation” referred to wiping-out the recipient’s cancer and immune system. In non-ablative transplantation, the recipient’s blood cell and immune system are preserved, making the procedure far more tolerable.