Stem cell transplant
Harvested stem cells can revitalize your immune system.
Your blood cells perform a variety
of functions that are critical to life,
carrying oxygen and other essential
elements to your tissues, removing
toxins, controlling bleeding, and fighting disease as an essential part
of your immune system.
Bone marrow — the spongy tissue
inside certain bones — is where new
blood cells are born. Your bone
marrow produces hematopoietic,
or blood-forming, multipotent stem cells,
which divide into red cells, white
cells, and platelets. As these new cells
mature, they leave the marrow and
circulate through the blood stream,
along with a small number of the
immature stem cells.
If your bone marrow is damaged,
it may not be able to produce enough
stem cells to create the healthy blood
cells you need to support your body
and fight off infection. This can happen if you develop a disease that
originates in your blood or bone
marrow, such as leukemia. In addition,
because chemotherapy and
radiation
target rapidly dividing cancer cells,
these treatments may destroy stem
cells, which also reproduce quickly,
and the bone marrow itself.
Creating a new immune system
In the 1970s, scientists developed a
breakthrough procedure known as a
bone marrow transplant (BMT) to
treat hematologic, or blood, cancers.
In this treatment, doctors administer high doses of chemotherapy to
eliminate cancerous bone marrow and
replace it with healthy bone marrow.
Researchers have since developed
the technology to remove stem cells
from the blood stream rather than
having to extract bone marrow
containing the cells. In a process
known as a stem cell transplant (SCT),
doctors administer radiation or
chemotherapy to eradicate cancerous
bone marrow. Then, healthy stem
cells are infused into the blood where
they can begin replenishing the bone
marrow with new, healthy blood and
immune cells. This process is known
as engraftment.
Stem cell transplant was originally
developed to treat blood cancers.
Today doctors sometimes use the
procedure to repair bone marrow
and revitalize the immune systems
of people with solid tumors who
have undergone high doses of
chemotherapy. Used this way, SCT
is sometimes called stem cell rescue,
and it offers hope to people whose
cancer might be cured with aggressive
conventional therapy.
A good match
There are two main types of stem
cell transplants:
- Autologous transplants use your
own stem cells, which are removed
and sometimes treated to destroy
any remaining cancerous cells
before they're returned to your
body after you receive high-dose
chemotherapy.
- Allogeneic transplants use stem cells
from a volunteer donor, often a
member of your immediate family.
The success of an allogeneic transplant
often depends on how closely
the donor's stem cells match your
own. Scientists have developed a process
called HLA typing to match your
genetic material and the proteins
on your cells to those of a potential
donor. Not surprisingly, siblings are
often the closest matches.
Finding a close match is important
because if the transplanted immune
cells recognize your body as foreign,
they may attack it, resulting in
graft-versus-host disease,
or GVHD, a common side effect of allogeneic SCT.
Symptoms of GVHD may be relatively
mild, such as skin sensitivity, but it
can also cause organ damage that can
be life-threatening. Chronic GVHD
can be managed with immunosuppressive
drugs.
However, one side effect of GVHD
is actually beneficial for some people
with cancer. When the transplanted
immune cells attack the host's body,
they also attack whatever cancer cells
may remain after treatment. Known as
the graft-versus-tumor (GVT) effect, this
reaction may be curative in the case of
slow-growing blood cancers and may
reduce the risk of tumor recurrence.
Mini-transplants
Some facilities are investigating
ways to make SCT milder and less
toxic. One promising new method —
called a non-myeloablative or
mini-transplant — uses lower and
less-toxic doses of chemotherapy. The
lower dose destroys some, but not
all, of the bone marrow. Some cancer
cells may also survive the reduced
chemo, but mini-transplants can
trigger the GVT effect — where newly
transplanted cells recognize and
attack whatever cancer cells survived
conventional therapy.