For diseases of the blood forming system, which is located in the bone marrow, replacing the diseased bone marrow with new, healthy bone marrow is a potentially curative treatment.
The blood forming stem cells (hematopoietic stem cells) in the bone marrow have been found to be sufficient to re-build a new blood forming system. Hematopoietic stem cells can be obtained from bone marrow, peripheral blood or umbilical cord blood with peripheral blood now the most common source of stem cells for transplantation.
Allogeneic stem cell transplantation involves the use of donor stem cells. The donated stem cells can come from a related (family) or unrelated donor. A transplant is successful if the donor stem cells can restore normal bone marrow, thereby curing the patient’s disease.
The immune system is part of the blood system and all cells in the blood originate from the same hematopoietic stem cells. Because the immune system is part of the blood system, allogeneic transplantation means that the donor’s immune system is also transferred to the recipient. This can have the following effects:
• attack of the transplant by residual immune cells of the recipient (transplant/graft rejection); and
• immune reaction by the donor cells against the tissues of the recipient Graft-versus-Host-Disease (GVHD)
Prior to an allogeneic hematopoietic stem cell transplantation (HSCT), patients receive high doses of chemotherapy and sometimes radiation therapy. This treatment is referred to as a “conditioning regime”. Through this treatment the cancer cells are destroyed, but the treatment also damages and destroys the blood forming system in the bone marrow, including the patient’s immune system.
Consequently, HSCT has two main goals:
• to enable the formation of a new blood forming system and immune system using donated stem cells; and
• to provide the patient with mature immune cells present in the graft, some of which can immediately fight opportunistic infections and recognize remaining cancer cells that have survived even high doses of chemotherapy and kill them, helping to prevent disease relapse.
Although HSCT has the potential to cure patients with blood cancer, due to the current risks and limitations connected with HSCT it is only commonly used in patients who are at high risk of relapse, who do not respond fully to more traditional treatment, or who relapse.