Overview of Pediatric Bone Marrow Transplantation

Prof. Chan Ka Wah MD *
MD Anderson Cancer Center

History of Pediatric Bone Marrow Transplant

The idea of using bone marrow to treat blood disease is not new. In the early 1900, patients were given marrow orally. Obviously this did not meet with success and interest faded. After the Second World War, this treatment strategy was re-evaluated due to the development of nuclear weapon and generators.

Early attempts to transplant bone marrow into humans via blood vessels were met with mixed success. While a few leukemia patients had transient improvement of their disease, most transplants were rejected. Other patients developed a clinical syndrome of skin rash, diarrhea, and jaundice, which was mostly fatal. The enthusiasm waned in the 60's.

Around this time, the genetics of graft rejection and tissue (called HLA) matching was described. For the first time, physicians were able to match and select the most suitable marrow donor for the patient. The first successful bone marrow transplant of the modern era was performed in 1968, when an infant born without an intact immune system received a transplant from a sibling. This patient remains healthy 35 years later. Similar success was soon reported for patients with severe aplastic anemia and acute leukemia. By 1986, more than 200 transplant centers worldwide were performing 5,000 transplants annually.

Around this time, it was discovered that stem cells (which produces subsequent generations of blood-forming cells) move freely between the bone marrow and the blood stream. This exodus of stem cells into the circulation is most evident during the recovery phase after chemotherapy. It can be also enhanced by the administration of proteins called colony-stimulating factors (G-CSF). By the middle of the 1990's, many transplant centers were using stem cells collected from peripheral blood instead of, or in addition to, bone marrow. Such a procedure is called peripheral blood stem cell transplantation.

The latest player in the field is placental blood. Like bone marrow, blood collected from the placenta (also known as umbilical cord blood) is rich in stem cells. In 1988, researchers in France transplanted placental blood collected from a newborn child to the baby's sibling - a child with Fanconi anemia. Since then successful cord blood transplants have performed on children with leukemia, severe aplastic anemia, and a number of other blood disorders. Due to the small number of stem cells in a placental blood collection, cord blood transplantation is currently only a suitable option only for children and, possibly, small adults. Research is underway to determine if cord blood cells can be expanded so that more adults can benefit from this therapy.

The majority of patients who could have benefited from a transplant do not have a brother or sister with matching marrow type. In 1973, the first unrelated marrow donor (matched for HLA tissue type) transplant was performed in New York. The success of this procedure led to the establishment of marrow donor registries around the world. In 1986, the National Marrow Donor Program in the US was created. Currently there is information on over 7 millions volunteer donors worldwide. Similarly over 20 umbilical cord blood banks are collecting and preserving placental blood from newborn infants. There is international cooperation to facilitate the identification, testing, and procurement of stem cells to be used for transplantation.

Types of Stem Cell Transplant

The bone marrow is a spongy tissue found inside bones. It contains stem cells, which divide and generate different kinds of blood cells in the circulation. The white blood cells fight infection; the red blood cells carry oxygen to body tissues; and the platelets control bleeding.

Diseases affecting the stem cells result in bone marrow malfunctioning: either by producing too many defective / immature blood cells (e.g. thalassemia and leukemia), or too few blood cells (apalstic anemia). When these conditions are diagnosed, replacement of the patient's marrow with stem cells from a healthy donor may lead to a cure. This type of transplant is called an allogeneic stem cell transplant. A stem cell transplant may also be part of the treatment for patients with solid tumor cancers. In these situations, some of the patient's own stem cell can be collected and frozen. This allows a higher dose of chemotherapy to be given to kill the tumors, and the patient can be "rescued" by reinfusion of the previously stored cells. The use of the patient's own, otherwise normal, marrow or blood stem cells for treatment is called autologous stem cell transplantation.

How does Stem Cell Transplant Work?

Prior to transplant, the patient's diseased bone marrow has to be destroyed. This usually involves the use of a combination of chemotherapy drugs, or by a combination of chemotherapy and irradiation, given in very high doses. At the same time the patient's immune system is also eliminated to prevent rejection. This phase of treatment is called the preparative or conditioning regimen and typically lasts five to ten days.

The procedure to collect stem cells differs according to nature of the cells used for transplantation. Bone marrow is harvested in a hospital operating theatre. The donor is put under general anesthesia and a needle is inserted into the hipbone. Over several skin puncture sites repeated bone punctures are performed. A small amount of bone marrow is extracted each time and the total quantity of marrow is pooled, filtered, and transported to the patient's room for immediate infusion. If peripheral blood stem cells are to be used, the donor will receive three to five days of G-CSF to mobilize stem cells to move out of the bone marrow into the blood stream. A needle or special tubing is placed in a large vein either in the donor's arm or the groin. Blood flows into a computer-control machine called a cell separator, which will remove the stem cells and return the rest of the blood cells back to the donor.

On the day of transplant, usually one to three days after the completion of conditioning, stem cells are passed into the patient through a catheter, much like a transfusion. Typically the infusion takes 30 minutes to two hours to complete.

Success Rate: Is It Worth All the Side Effects?

The question parents most frequently asked is, " What is the success rate of the transplant center in treating my child's disease?" It should be stressed that success rate quoted by centers is meaningless unless it is properly interpreted.

Many factors influence the success of a stem cell transplant. The most important element is the patient's status of disease at the time of transplant. Leukemia that is not under good control by conventional chemotherapy is very difficult to cure by a stem cell transplant. There is a higher chance of leukemia recurrence in these cases. When there is progressive disease a transplant will not help and should not be done. The health condition of the patient is also critical for the success of the transplant. Active infection, poor function of any organ system, and general debilitation are usually associated with a higher complication rate due to the effect of conditioning. Finally the donor type affects the transplant outcome. Autologous and matched sibling donor transplants have a lower complication rate whereas mismatched and unrelated donor transplants are generally more dangerous.

What Complications Should I be Aware of?

1. Graft-Versus-Host Disease (GVHD)

This condition is triggered by the T cells of the donor, which are a type of white blood cells that recognize and attack the body cells of another individual (that of the recipient). The larger the differences in the genetic makeup between donor and recipient, the higher the likelihood of GVHD. On the average 20 to 70% of transplant recipients may develop this problem, and as expected the incidence is higher when stem cells from an unrelated or mismatched donor are used.

The acute form of GVHD usually appears during the first few weeks after engraftment. The earliest sign is often a skin rash on the patient's face and hands. It may spread to other parts of the body into general redness, similar to a sunburn, with peeling and blistering of the skin. The stomach and the bowel may also be affected, causing cramping, nausea, and watery diarrhea. Jaundice (yellow color of the skin and eyes) is a sign the liver is also involved. Depending on the number of organs affected and their severity, acute GVHD may be mild and transient or it may be severe and life threatening.

The chronic form of GVHD develops several months after the transplant. It may evolve from the acute form or it may present independently. Patients usually experience skin problems such as a dry itchy rash, color changes, and tightness. Dryness or burning of the mouth and eyes, liver abnormalities and weight loss are also common complaints. Less frequently, patients may suffer from skin scarring and contractures, loss of hair and nails, and swallowing and breathing difficulties.

Both acute and chronic GVHD are treated by medications that suppress the overactive T cells. Both the diagnosis and its treatment weaken the patient's immune system, so there is an increased risk of opportunistic infections.

2. Infections

Transplant patients are prone to infections during all phases of the procedure. With high dose chemotherapy and/or radiation during the preparative phase, the bone marrow is destroyed and the white blood cell count becomes very low. Antibody-producing cells are depleted. The skin and the lining of the mouth and intestine, the body's other first line of defense, are also damaged. The patient is almost certain to have fever in the first two weeks after transplant. Most of these episodes are responsive to antibiotics, suggesting the infections are bacterial in origin. Occasionally fungi may be the cause of the fever, especially after prolonged antibiotic treatment.

Even after the white cell count recovers, usually in two to four weeks' time, the function of the patient's immune system remained subnormal. The risk of unusual infections caused by virus, protozoa, parasite and mycobacterium remain significant. It takes 6 to 12 months or more for the body defense to function at 100% efficiency. Recovery is delayed by GVHD and its treatment.

Extra precaution is necessary to prevent infection after transplant. Good hand washing and avoiding contact with others who are ill are of paramount importance. Close surveillance and prophylactic antibiotics may also be helpful to manage the reactivation of some viruses dormant in the patient's body.

Summary

* Dr. Chan Ka Wah is the professor of Pediatrics Patient Care, MD Anderson Cancer Center. His clinical interests are on bone marrow and stem cell transplantation for children and adolescents with cancer and blood disorders; placental cord blood transplantation.