Increasingly, peripheral blood stem cells (PBSCs) are being collected for use in all types of transplants instead of bone marrow. This practice is generally more common in autologous transplants. PBSCs are harvested through a relatively simple procedure called apheresis, which is like a blood transfusion. The donor’s blood, including circulating stem cells, is collected from a large vein in the arm or through a temporary central venous line, depending on the hospital’s procedures and the donor’s physical condition.
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The blood is then run through a machine called a blood cell separator, which distinguishes the small fraction of stem cells in the blood from all other blood cells. The stem cells are collected and the rest of the cells are returned to the donor. Depending on the type of transplant and the timing of donation, the stem cells may be taken directly to the recipient for infusion or they may be treated and frozen for later use in the same method as bone marrow. Apheresis takes longer than bone marrow harvesting because there are fewer stem cells circulating in the bloodstream than there are in bone marrow. The procedure may be done either as an inpatient, which requires admission to the hospital, or as an outpatient. It usually takes two to four hours and may be repeated on several days.
A number of strategies are being explored to help “mobilize” stem cells – to induce them to leave the bone marrow and enter the bloodstream – in order to make apheresis more efficient. One method that is being increasingly used is pre-treatment with growth factors, which stimulate the production of blood cells. Two of the most-studied growth factors, colony-stimulating factors and interleukins, have been shown to increase the volume of stem cells in the blood. Patients may want to ask the doctors at the hospital about their use of growth factors for donors if considering a PBSC transplant.
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Allogeneic bone marrow/stem cell transplants (BMT/SCT), by definition, rely on the availability of a healthy donor whose bone marrow is biologically compatible with the recipient – what doctors refer to as an “HLA match.” HLAs, or human leukocyte antigens, are protein “markers” found on the surface of white blood cells. They serve as a kind of genetic fingerprint, enabling the body’s immune system to recognize the body’s own cells. HLA markers occur in pairs, with one part of each pair inherited from a mother and one from a father. Therefore, the search for a matched donor begins within a patient’s immediate family. There is a 25 percent (1 in 4) chance that any one brother or sister perfectly matches a sibling’s HLA type. The overall chances of having any sibling who is a match depend upon the number of siblings.
Potential donors need to be tissue-typed, which involves taking a series of blood tests to determine if the HLA “fingerprints” match. If a brother or sister doesn’t match, parents are screened for HLA compatibility, followed by the extended family of aunts, uncles, cousins, etc.
If a sibling or family match is not available, the transplant center should have procedures for finding an unrelated donor through national bone marrow registries, such as the National Marrow Donor Program. They will search a database of donors to identify a potential match. On average, the chances of finding an unrelated donor with a similar ethnic background are 60 to 70 percent. If someone is found, the registry contacts the potential donor with instructions about how to proceed. The identity of the donor is always kept confidential for a period of time following the transplant, after which point the patient and/or the donor are free to contact one another if both give written consent.
As successful as national registries have been in helping identify unrelated donors, many people in need of a BMT/SCT are unable to find a matched donor. To help fill this unmet demand, alternative sources of stem cells are being explored. For example, blood harvested from the umbilical cord of newborn babies (at no risk or pain to them) is a rich source of stem cells, and the use of cord blood in the BMT/SCT setting is increasing.
Related donors who are not exact HLA matches may also present an alternative for BMT/SCT candidates with no matched relatives. These “HLA-mismatched” procedures need to be discussed with the patient’s doctor. Such donors have not previously been a viable option because of the high risk of Graft vs. Host Disease (GVHD) and graft failure associated with such mismatches. Recent advances have helped make this an option in some cases.
The basic idea behind bone marrow/stem cell transplantation (BMT/SCT) is to allow high doses of chemotherapy and/or radiation therapy to kill rapidly dividing cells in the body to make room for new, healthy cells. Cancer cells, like other cells in the body, divide rapidly. Though these treatments are among the most effective weapons against many forms of cancer, they do not have precise aim and they cannot target only diseased cells. As a result, many normal rapidly dividing cells, including stem cells, are also destroyed during the treatment. Therefore, “rescue” with transplanted bone marrow or stem cells enables the patient to produce new blood cells to replace those destroyed during treatment.
There are two main types of bone marrow or stem cell transplants: allogeneic (a donor supplies the marrow or stem cells) and autologous (the patient’s own bone marrow or stem cells are used). A syngeneic transplant is a type of allogeneic transplant of marrow or stem cells from an identical twin. Several factors determine what type of BMT/SCT a patient should have, including the type of disease, age, overall health, and availability of a donor.
Click here to learn how you can receive a copy of The Bone Marrow Foundation’s medical and educational handbooks for allogeneic and autologous transplantation. Both books are also available in Spanish.