Understanding Stem Cell Transplant:
Hematologic and Oncologic Diseases
The Blood Forming System
To help you better understand the process of Hematopoietic Stem Cell Transplant (HSCT) , it is important for you to know a little about the human blood forming system. Blood is composed of many different types of cells, each with a very special function. These cells are formed in the spongy center of the large bones of the body, called bone marrow.
Stem cells are the parent cells present in your bone marrow. These cells have the ability to divide into the various types of blood cells as well as form new stem cells. These stem cells are what we need to transplant patients.
There are three different types of blood cells that are produced by the stem cells: Red Blood Cells (RBC), White Blood Cells (WBC), and Platelets.
- The function of RBCs is to carry oxygen from your lungs to the rest of your body.
- The function of Platelets is to form a clot, which stops bleeding.
- The function of WBCs is to defend the body against infection. There are several different kinds of WBC, and each has a very important role in the immune, or infection fighting, system.
Some examples of the types of WBCs include:
- Neutrophils: Neutrophils provide the first line of defense in bacterial infections, attacking and destroying bacteria. Neutrophils are sometimes called bands and segs.
- Monocytes: Monocytes are important WBC’s because they remove damaged and destroyed cells from circulation.
- Lymphocytes: Lymphocytes are key members of the immune system. There are two different types of lymphocytes:
- T-lymphocytes: T-cells directly attack and destroy infectious agents or cells not recognized as belonging to the patient. These cells are very important in fighting viruses. T-lymphocytes are cells that mediate graft rejection and graft versus host disease (GVHD) following stem cell transplantation. We will discuss them both in greater detail later in the booklet.
- B-lymphocytes: B-cells, with the help of T-cells, produce antibodies to specific antigens, which are usually proteins on the surface of bacteria. Once B-cells for a specific antigen are produced, they survive for a very long time (memory cells) and are able to respond to future exposures to that specific antigen (bacteria) much more quickly.
Why Would We Want To Replace or Transplant Such a Complex System?
Transplantation for oncologic diseases is performed for three basic reasons: (1) to eliminate residual cancer cells with high doses of chemo- and/or radiation-therapy followed by stem cell rescue, (2) immune therapy to help control cancer after transplant, and (3) to replace sick or damaged marrow due to disease or therapy.
Transplants are done for several different reasons. However, the actual transplant procedure is similar for all.
Types of Transplants
There are two basic types of transplant, autologous and allogeneic. However, choices in the source of stem cells for HSC (Bone Marrow, PBSC, Cord Blood), the intensity of the conditioning regimen (ablative versus non-ablative) prior to HSC, and immunosuppressive medications post-HSC complicate this simplistic typing of transplants. The type of transplant a person receives depends on several different factors, including the type and course of the disease, availability of suitable donors, and the patient’s overall health.
In an autologous transplant, the patient's own stem cells are used. Autologous transplants are performed predominantly on patients with solid tumors. However, this procedure may be used in select patients with leukemia. In autologous transplants, stem cells are harvested when the patient is free from signs of disease or when the cells can be processed to remove or “purge” any remaining cancer cells. The stem cells are collected from the bone marrow or from the peripheral blood. Once the necessary volume of cells is collected, they are frozen and stored until the patient is ready for transplant. See Sources of Stem Cells (link to section further down in text) for more information on the collection process.
In an allogeneic transplant, the stem cells are donated by someone other than the patient. The stem cells can come from a sibling (brother/sister), a parent, or from an unrelated donor. The patient and donor are matched through a special blood test called Human Leukocyte Antigen (HLA) typing. HLA antigens are a unique group of proteins found on the surface of every cell in your body. These antigens serve to identify each cell as self (like a name tag). Cells expressing HLA different from your own will be attacked and destroyed. HLA matching of the donor and recipient (patient) as closely as possible is important to reduce the possibility of graft rejection and graft versus host disease (GVHD). Further details regarding GVHD will be discussed later.
Close family members (usually brothers or sisters) are the best choices for allogeneic hematopoietic stem cell donors. There is a one-in-four chance that one sibling will match another sibling because your HLA antigens are inherited from each of your parents. If no family member is a match, a search for an unrelated donor may be started. There are international registries of potential volunteer hematopoietic stem cell donors available to identify a suitable donor in most cases.
Sources of Stem Cells
There are three different sources of hematopoietic stem cells. The stem cell source used for a given transplant depends upon the underlying disease, the type of transplant ( allogeneic or autologous), and size of the patient.
Bone marrow can be used for either autologous or allogeneic transplants. In allogeneic transplants, the bone marrow is collected when they are free of disease. The bone marrow is harvested the day of transplant, transported to the Transplant Center, and given to the patient that same day. Bone marrow is harvested or collected from the large bones of the hip. More specifically, these are the bones to the right and left of the tailbone. The harvest is done in the Operating Room under general anesthesia. More specific information on the process will be given to you and your family in the event a family member is the bone marrow donor.
Peripheral Blood Stem Cells (PBSC)
Peripheral blood stem cells can be used for either autologous or allogeneic transplants. PBSC are collected from the circulating blood. The patient is connected to a special machine by a catheter in a large vein. The machine filters out the stem cells and returns the rest of the blood back to the patient. In order to obtain a sufficient number of stem cells for transplant, the donor may be given a medication to increase the number of stem cells in his/her blood. The collection process may be completed in one day or may need to be done over several days. The cells may be frozen and stored until the patient is ready for transplant.
Umbilical Cord Blood
Umbilical cord blood is a rich source of hematopoietic stem cells and has been proven to be a feasible alternative to bone marrow when used in unrelated transplants. Cord blood is the blood that remains in the umbilical cord and the placenta that is normally discarded after the baby is born. Because it is collected after the birth of a healthy baby, there is no harm or risk to either the mother or the baby. Considered a source of adult stem cells, the use of these cells from cord blood eliminates the ethical concerns surrounding more controversial sources of stem cells.
The St. Louis Cord Blood Bank, in cooperation with community medical professionals and donor families, provides a worldwide resource for recipients in need of stem cell transplants. These products are used to treat malignant bone marrow diseases like leukemia and lymphoma, congenital disorders like Fanconi’s anemia, Wiskott Aldrich syndrome and Severe Combined Immune Deficiency (the boy in the bubble syndrome), and non-malignant diseases such as Severe Aplastic Anemia, thalassemia and sickle cell anemia. By collecting cord blood from donors of different ethnic backgrounds, a larger number of patients will benefit.
Because cord blood stem cells are typed, frozen and stored at the St. Louis Cord Blood Bank, they are ready for use when the patient needs them. Although umbilical cord blood is a rich source of hematopoietic stem cells, the volume collected is small compared with either bone marrow or PBSC (peripheral blood stem cell). For this reason, the use of cord blood was previously restricted to use in children. But new studies are showing that these cell dose limitations can be overcome and have expanded the use of cord blood stem cells into adults.
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