Stem cell transplant is a very important medical procedure, used for the treatment of leukemia and other diseases (e.g. genetic diseases). (For more on stem cells, watch the TED Ed video: What are stem cells?) To treat leukemia, two major types of stem cell transplant are used: autologous transplant and allogeneic transplant. This video illustrates the procedure of autologous stem cell transplant.

In autologous stem cell transplant, healthy (non-leukemic) stem cells are collected from the patient, purified and frozen. The patient is then treated with ablative chemotherapy, a very high dose chemotherapy that is likely to destroy most bone marrow stem cells (to know more about chemotherapy, watch this video). After therapy, frozen stem cells are thawed and re-injected into the patient, in order to repopulate his blood and bone marrow. This kind of procedure does not raise compatibility problems, since cells are collected from the patient himself. However, stem cell preparations obtained after purification may be contaminated with leukemic cells, which may able to restart leukemia if injected into the patient.

The other kind of stem cell transplant used to treat leukemias is allogeneic stem cell transplant. In this case, stem cells are collected from a healthy donor, either a relative of the patient, or an unrelated person. This video illustrates the procedure of allogeneic stem cells transplant. In this case, donor and recipient have to be matched. This means that specific molecules present on the surface of their cells have to be identical, or at least similar. The reason why compatibility is required is that donor cells may recognize recipient cells as “foreign” if they are too different. As a result, the patient can develop a graft-versus-host disease, where recipient tissues are damaged by the immune system derived from the donor (to know more about graft-versus-host disease, check this web page).

The major compatibility proteins are encoded for by a set of genes called HLA (Human Leukocyte Antigen), located in chromosome 6. These are highly variable genes expressed in leukocytes (white blood cells), but some of them are also expressed in most other cells. Since they are inherited as a group within a single chromosome, the probability that two sibling share the same HLA genes is significant (have a look at this picture for more information). However, if the patient has no matching siblings, the probability of finding a matching unrelated donor is very small, due to the high variability of HLA genes within the human population. That’s why millions of possible donors have to be screened in order to find a compatible donor for a patient who has no matched relatives. Visit Be The Match to find out more about being a potential donor and consider joining the donor registry if you are able to.

Matching HLA genes does not always prevent the development of graft-versus-host disease, as other minor compatibility factors are also involved in the reaction. In some cases, however, chronic graft-versus-host disease may help control leukemia, because donor’s cells recognize leukemic cells as foreign and destroy them, checking or eradicating the tumor.

This beneficial reaction is called graft-versus-tumor effect (click here to read a scientific article about graft-versus-tumor effect in transplanted leukemic patients).

If you want to have more information about cancer and medicine, have a look at the websites of the authors of this video: biocomiche.it (Danilo Allegra) and graphicdna.it (Dania Puggioni)!

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