Stem Cell Transplants for Leukaemia

Stem cell transplants take place in a fashion similar to blood transfusions with the recipient having an intravenous line put into place through which the stem cells are delivered. The transplant can take anywhere from 1-5 hours depending on the volume of transplant material being administered. Stem cell treatment alongside chemotherapy will usually be delivered when the patients’ cancer is in remission in order to give them the best chance of successfully eradicating the cancer. Patients with leukaemia who are treated with chemotherapy and/or radiation with subsequent stem cell transplant will usually experience a graft-versus-tumor (GVT) effect. This effect is a critical part of the leukaemia stem cell treatment as it is the targeting, and destruction, by the donor’s white blood cells of any residual cancer cells.

Immune Reactions to Stem Cell Transplants

Whilst the graft-versus-tumor effect is desired, the development of graft-versus-host disease (GVHD) is not and can lead to significant complication as the allogeneic white blood cells begin to attack the host’s (recipient’s) other cells. A number of organs are then at risk of serious damage, including the liver, skin, and the intestines and it is important for GVHD to be quickly identified and addressed, usually through a succession of blood counts and tests and immunosuppressant drugs where necessary. To better aid the prevention of GVHD occurring great care is taken to find a good match between donor and recipient in stem cell treatment for leukaemia. This is done by testing the human-leukocyte-associated (HLA) antigens present on the surface of the stem cells of both donor and recipient through blood tests. These sets of surface proteins can be fully matched, such as in identical twins and a small number of other people, or a close match with a large portion of the HLA antigens corresponding between ‘host’ and ‘graft’. Only 25-35% of patients requiring transplant have a sibling with matching HLA antigens, making this type of transplant less common than one from an unrelated donor. Those from minority ethnic and racial groups are often encouraged to donate stem cells, from cord blood, peripheral blood, or bone marrow, as the chances of finding an HLA-matched donor increase when donor and recipient are of similar racial or ethnic backgrounds.

Autologous Stem Cell Transplant

It is possible for leukaemia patients to receive an autologous stem cell transplant where their own stem cells are harvesting, purged of cancer cells and then transplanted following chemotherapy. More stem cells are harvested in this process as the filtering of cancer cells also reduces the number of healthy cells available, making the use of donated stem cells generally preferable. The stem cells transfused into the patient migrate towards the bone marrow where they settle and begin to produce new white blood cells along with red blood cells and platelets, thereby restoring the patient’s immune system, allowing more oxygenated blood to be carried around the body, and reducing the risk of bleeding by facilitating proper clotting. This process is called engraftment and can take between two and four weeks after the transplantation with doctors monitoring the progress through regular blood counts to assess the levels of these key cells.

Stem Cell Treatment Success

Engraftment may take place relatively quickly in comparison to the restoration of a patient’s immune system, which can take months, or even years, to fully recover during which time the patient is vulnerable to infection. Patients receiving an autologous stem cell transplant for their cancer will usually recover immune function faster than those having an allogeneic or syngeneic transplant which can take 1-2yrs for complete recovery. Patients will usually have to be inoculated and vaccinated again for many diseases, even if they already had these vaccinations in childhood, as the chemotherapy often destroys those cells which maintain immune memory. A study by van Tilburg (et al, 2006) found that children treated for leukaemia who retained their protection against a number of disease varied considerably: between 17-98% for diphtheria, 27-82% for Bordetella pertussis, 20-98% for tetanus, 62-100% for poliomyelitis, 35-100% for haemophilus influenzae type B (HiB), 29-92% for mumps, 29-60% for measles, and 72-92% for rubella. The researchers found that most of the children responded to re-vaccination appearing to show that immunological recovery did occur but that the cytostatic (chemotherapy or radiation) therapy temporarily reduces levels of specific antibodies.

References

van Tilburg, C.M., Sanders, E.A.M., et al, Loss of antibodies and response to (re-)vaccination in children after treatment for acute lymphocytic leukemia: a systematic review, Leukemia (2006) 20, 1717–1722.