T-cells, a type of white blood cell, are the cells that cause graft versus host disease (GvHD) after a haematopoietic stem cell transplant (HSCT, also known as bone marrow transplant or BMT). The T-cells are removed when there is a strong possibility that GvHD may occur after an HSCT. The removal process is called T-cell depletion.
Tissue matching and graft versus host disease (GvHD)
A possible side effect of an HSCT occurs when the donated cells do not recognise that they should be in the patient and try to reject the patient’s cells, resulting in GvHD. GvHD can be fatal. Doctors try to prevent it by choosing a donor who is an identical tissue type match, because an exact tissue match makes GvHD unlikely. They generally also prescribe medication (commonly cyclosporin) to patients, to further guard against GvHD. However, despite these interventions, GvHD may still occur. It is usually after about four months that the donated cells become tolerant of the patient and the risk of GvHD passes.
T-cell depletion is used when a good match cannot be found
Occasionally it is not possible to find a donor whose tissue type matches that of the recipient exactly, and so doctors may consider using a parent as a donor. Parents are generally a 50% tissue type match to their children. When a parent donor is used, it is highly likely that GvHD will occur. To prevent GvHD, it is possible to remove the T-cells. After the transplant, the patient produces his or her own T-cells that do not cause GvHD. This process takes around four months after HSCT.
Balancing the risks and benefits of T-cell depletion
T-cells are important for fighting viral infections, and so the drawback with T-cell depleted transplants is that patients are extremely vulnerable to viral infections until their own T-cells develop, which is about four months after HSCT, even though the risk of GvHD is reduced.
Outlook for the future
Historically the outcome of T-cell depleted transplants has been much worse than transplants using matched donors. Newly available techniques have been developed that remove the T-cells but leave behind cells that will fight viral infection. Results using these techniques are remarkable, with success rates of over 90%. Even more refined techniques are likely to become available in the future, meaning that there should almost always be an option for stem cell therapy for patients with CGD.