Patients with cancer often incur bone marrow damage, resulting in the destruction of stem cells. Stem cell transplants are used to replenish lost or damaged cells that have been affected by cancer and depending on where the stem cells come from these, the procedure may be a bone marrow transplant (BMT), peripheral blood stem cell transplant, or a cord blood transplant.
Typically, in a stem cell transplant, physicians administer high doses of chemotherapy, occasionally in conjunction with radiation therapy, to kill all cancer cells. This is known as myeloablative therapy.
Here are the two main types of transplants, as outline by the American Cancer Society:
Autologous stem cell transplants
In an autologous stem cell transplant, the patient serves as their own donor. Auto means self, therefore this procedure means harvesting your own stem cells from either your blood or bone marrow, then freezing them for preservation. Following high-dose chemo and radiation therapy, the frozen cells are thawed and returned to the (self) donor. Autologous transplants are sometimes used for testicular cancer and brain tumors, but are mainly utilized to treat leukemia, lymphoma, and multiple myeloma. For the latter, autologous stem cell procedures offers patients a chance for achieving sustained remission. “One advantage of autologous stem cell transplant is that you’re getting your own cells back. When you get your own stem cells back, you don’t have to worry about them (called the engrafted cells or the “graft”) being rejected by your body,” says the American Cancer Society.
Despite the benefits, as with all procedures, there are risks involved, including graft failure – which occurs when the transplanted stem cells don’t go into bone marrow fail to properly produce blood cells. “A possible disadvantage of an autologous transplant is that cancer cells might be collected along with the stem cells and then later put back into your body,” the ACS says, adding that another disadvantage of a autologous stem cell transplants “is that your immune system is the same as it was before your transplant. This means the cancer cells were able to escape attack from your immune system before, and may be able to do so again.”
But how exactly do physicians prevent any residual cancer cells from being transplanted with healthy cells? In a process known as purging, stem cells are treated before being infused back into the patient’s blood. Although purging carries its benefits, a potential downside, according to the ACS, is that normal cells may be lost during this process, which in turn could lead to unsafe levels of white blood cells as your body takes longer to produce normal blood cells. Cancer centers will also sometimes use in vivo purging, which involves not treating the stem cells, and instead administering anti-cancer drugs to patients post-transplant. The ACS notes, however, that the “need to remove cancer cells from transplanted stem cells or transplant patients and the best way to do it continues to be researched.”
Allogeneic Stem Cell Transplants
Whereas autologous procedures infuse stem cells from your own body, allogeneic stem cell transplants use cells from a donor with a very similar tissue type (in many cases a relative, usually a sibling). In cases where the ideal donor is not a relative, physicians may opt to perform a matched unrelated donor (MUD) transplant, which as the ACS notes, “are usually riskier than those with a relative who is a good match.”
Allogeneic transplants comprise of the same process as autologous stem cell transplants where stem cells are harvested, frozen, and subsequently thawed and put back following high-dose chemo and/or radiation therapy. In some cases, the procedures involve the infusion of blood extracted from the placenta and umbilical cord of a newborn because the cord contains a high number of stem cells that quickly multiple. “By 2017, an estimated 700,000 units (batches) of cord blood had been donated for public use. And, even more have been collected for private use. In some studies, the risk of a cancer not going away or coming back after a cord blood transplant was less than after an unrelated donor transplant,” writes the ACS.
A benefit of an allogeneic transplant is that donor stem cells create their own immune cells, which may eliminate any residual cancer cells that remain after high-dose treatment, which is known as the graft-versus-cancer effect. Moreover, because the donor stem cells are free of cancer, donors can be asked to donate stem cells or white blood cells multiple times.
As with autologous stem cell procedures, this donor dependent transplant also carries risk. The transplant, or graft, might be destroyed by the patient’s body before reaching the bone marrow. Allogeneic stem cell transplants also augment the risk of graft-versus-host-disease, where cells from the donor attack healthy cells in the recipient’s body. Furthermore, despite the healthy cells being tested before transplant, allogeneic procedures still carry a certain risk of infections because, as the ACS writes, your “immune system is held in check (suppressed) by medicines called immunosuppressive drugs. Such infections can cause serious problems and even death.”
The Search for a Donor Match
Because there’s a plethora of human leukocyte antigen (HLA) combinations, which are inherited from both parents, finding an exact donor match can often be an arduous task. The search usually starts at siblings, and there’s a 25% chance of a sibling being a perfect match. In the event that a sibling does not match, the search moves onto extended family (and parents) who are less likely to match.
The ACS writes: “As unlikely as it seems, it’s possible to find a good match with a stranger. To help with this process, the team will use transplant registries, like those listed here. Registries serve as matchmakers between patients and volunteer donors. They can search for and access millions of possible donors and hundreds of thousands of cord blood units.”