Maisonneuve-Rosemont Hospital Université de Montréal

Allogeneic transplant

What is an allogeneic transplant?

In an allogeneic transplant, or allotransplantation, the donor and recipient are two different people.

Allotransplantation is more complicated than an autologous transplant, because the donor and the recipient will have different gene pools. This is what is known as histocompatibility or, more simply, HLA compatibility. In order for this type of transplant to be performed, the donor and recipient must be HLA-compatible. Obviously, the chances are greater of finding an HLA-compatible donor within the recipient’s immediate family, since brothers and sisters share a similar genetic background. However, if none of the intended recipient’s siblings offer sufficient compatibility, doctors will turn to unrelated donors or cord blood banks.

Myeloablative (bone marrow-destroying) allogeneic transplants

A myeloablative transplant involves the use of intensive chemotherapy and/or radiotherapy to destroy a recipient’s hematopoietic cells before transplanting or injecting healthy stem cells from a donor. In this therapeutic strategy, high-dose chemotherapy will eradicate any remaining tumour cells, the blood system that is the cause of the patient’s disease, as well as the immune system. The patient’s immune system, which is unable to identify the abnormal cancer cells, is replaced by that of the donor, which does have the ability to fight against such cells. This approach helps stimulate the transplant’s response to the tumour (GVL).

Myeloablative transplants are indicated in cases of cancers or other diseases affecting the bone marrow, such as myelodysplasias, leukemias, myelodepression and myeloproliferative disorders.

Non-myeloablative allogeneic transplants

Non-myeloablative transplants do not involve intensive chemotherapy. Instead, immunosuppressors are given to patients prior to the transplant, in order to enable their bodies to accept or tolerate it. This helps avoid rejection, since those agents block the action of recipients’ lymphocytes and immune systems, which would normally take the transplant for an invading organism. Such a strategy provides patients’ bodies with additional means for fighting the disease on their own (the graft-versus-tumour effect), while at the same time avoiding the toxicity of intensive chemotherapy.

Non-myeloablative transplants are recommended for older patients, or for those who have other illnesses in addition to a blood-related disease (comorbidities), which makes high-dose chemotherapy risky. They are also recommended in cases in which the disease is painless or progressing slowly.

Complications related to allogeneic transplants

The main risks associated with allogeneic transplants are complications related to the toxicity of the conditioning regimen in the case of myeloablative transplants, the possibility of rejection, and the graft-versus-host reaction (GVH). The two types of allogeneic transplants also pose risks in terms of complications related to infections.

GVL: The great advantage of allogeneic transplants

The final result sought through an allogeneic transplant is the graft-versus-tumour or graft-versus-leukemia (GVL) reaction.

You will remember that the advantage of an allogeneic transplant is that it replaces a patient’s defective immune system with a healthy one from a donor.

What explains the graft-versus-tumour reaction? The white blood cells, mainly T lymphocytes, contained within the transplant have the capacity to recognize recipients’ remaining tumour cells and destroy them. Patients’ bodies can then fight their disease on their own. Furthermore, they will have this new immune system for the rest of their lives, which will considerably reduce the risk of relapse.

Success rate

A successful allogeneic transplant depends upon a number of factors, including the HLA compatibility of the donor and recipient, the disease concerned and the stage it is at, as well as the recipient’s overall health. It is therefore quite problematical to indicate the chances for a successful allogeneic transplant here. However, the attending physician can assess the procedure’s risks and benefits for each patient and the chances of survival without any recurrence of the disease.