Hematopoietic Stem Cell Transplantation

Hematopoietic (blood-forming) stem cell transplantation, also known as bone marrow transplantation, is a procedure to introduce healthy hematopoietic stem cells to patients whose bone marrow cells are destroyed by the disease, aggressive therapy, or otherwise non-functioning.

When infused intravenously, hematopoietic stem cells travel to the bone marrow, developing into healthy blood cells.

Hematopoietic bone marrow transplantation may be a life-saving treatment for patients with certain malignant and non-malignant diseases; however, it is associated with some serious risks. This article reviews the types of the procedure, its practical implementation, and the associated risks.

What is bone marrow, and why may it fail?

The bone marrow, a spongy tissue found in the center of most bones, is rich in stem cells. These stem cells give rise to the three types of blood cells:

• Red blood cells distribute oxygen through the body.
• White blood cells that fight infection.
• Platelets that help blood clotting.

Certain diseases may affect the bone marrow. The amount of normal hematopoietic stem cells may decrease because of the cancer invasion, destruction by autoimmune processes, or as a side effect of cancer treatment.

Gene mutations may lead to the defective development and non-functionality of blood cells. Transplantation of healthy hematopoietic stem cells may provide a chance to eradicate the disease or prolong survival in patients with these conditions.

It is used in patients with blood cancers, certain solid tumors, primary immunodeficiency syndromes, aplastic anemia, hereditary anemias, and certain autoimmune disorders.

Types of hematopoietic stem cell transplantation

Depending on the origin of hematopoietic stem cells, hematopoietic stem cell transplantation is classified as autologous, which uses the patient’s hematopoietic stem cells, and allogeneic, which uses the hematopoietic stem cells from another person.

In autologous hematopoietic stem cell transplantation, which is performed in patients with blood cancers, the patient’s bone marrow cells are collected and saved during remission from the disease. With this method, cancer treatment modalities can be administered at a toxic intensity to normal bone marrow cells while effectively destroying most cancer cells. Autologous transplantation rarely results in a complete cancer cure but significantly prolongs a patient’s survival without disease progression.

In other diseases, another person’s hematopoietic stem cells are transplanted. A donor may be found among the patient’s relatives or in bone marrow registers. If these options are unsuitable, umbilical cord stem cells may be used as an allogeneic source of hematopoietic stem cells.

A full match between the donor’s and recipient’s Human Leukocyte Antigens (HLA) is usually sought to prevent complications. However, the developments in pre-and post-transplant care have led to the acceptance of a half-matched hematopoietic stem cell transplantation.

How are hematopoietic stem cells collected and stored?

There are two ways to collect hematopoietic stem cells:

1. Harvesting cells directly from bone marrow.
2. Collecting stem cells from venous blood after mobilization.

The donor receives general or regional anesthesia if direct bone marrow harvesting is selected. The pelvic bone is punctured using a thick needle at several sites, and bone marrow is drawn.

The procedure lasts for about an hour. The collected bone marrow is placed in a sterile plastic bag with a substance to prevent clotting.

Up to five days before hematopoietic stem cells are collected from venous blood, a donor receives medicines that mobilize hematopoietic stem cells from bone marrow. Apheresis begins when the blood level of stem cells reaches a target.

A thin plastic tube is placed in a donor’s vein, most often in the region between the neck and the chest, under local anesthesia. After connecting to the apheresis machine, the fraction of blood that contains the stem cells is collected into plastic bags, and the rest of the blood returns to the donor.

The procedure itself is painless and usually lasts for up to 4 hours.

If not infused into the patients immediately, hematopoietic stem cells must be frozen and stored under up to -196°C.

How is hematopoietic stem cell transplantation done?

Up to 10 days before hematopoietic stem cell transplant, the patient undergoes the so-called “conditioning” phase, during which the therapy to eradicate the remaining cancer cells, facilitate donor cell engraftment, and prevent transplant rejection via suppressing the immune system is administered.

Similarly to blood transfusions, hematopoietic stem cells are infused through a catheter into the veins. This procedure lasts for approximately 30 minutes.

The hematopoietic stem cells then travel to the bone marrow, where they engraft and begin generating healthy blood cells. Engraftment may take two to four weeks.

During this period, the patients are extremely susceptible to various infections. Therefore, they must stay in the hospital, limit their contacts, and follow enhanced personal hygiene and a safe diet.

Frequent assessments of the body temperature, physical state, fluid balance, and blood cell counts are performed. Blood transfusions and medications might be given to prevent infections.

After successful engraftment and in the absence of complications, the patients can usually return home. However, the whole immune system reorganization process may last up to a year.

During this period, the patients must be extremely vigilant to the signs of infection (fever, cough, shortness of breath, diarrhea) or other side effects and attend regular check-ups.

Is it safe for a healthy person to donate hematopoietic stem cells?

Hematopoietic stem cell donation is generally safe for healthy donors, whether by direct bone marrow harvesting or by collection from peripheral blood.

The donors who undergo direct harvesting of the bone marrow may experience the risks associated with anesthesia (e.g., allergic reaction), mild pain, tenderness, or bruising around the puncture sites, fatigue, headache, and mild weakness.

The donors who select the stem cell collection from the venous blood may experience flu-like symptoms (fever, chills, headache, bone and muscle pains) caused by the medications given before the procedure.

During the procedure, they may feel tingling around the mouth, fingers, or toes, bruising at the needle site, or chills. Platelet count may temporarily decrease after the procedure.

All the side effects usually subside within days.

Risks of hematopoietic stem cell transplantation for the patient

Hematopoietic stem cell transplantation is a complex procedure associated with some serious risks. Aggressive chemotherapy or radiotherapy administered in the conditioning phase may cause:

Long-term effects may include infertility and the development of other cancers.

Before the engraftment of stem cells, the patients are at high risk of infections. Patients may also experience fatigue, general weakness, and bleeding.

A very rare but potentially lethal complication of allogeneic bone marrow transplantation is graft failure, a condition when transplantation does not result in sufficient production of normal blood cells.

Even after the full engraftment, the success of the transplantation may not be durable, with underlying disease coming back in some patients.

After allogeneic hematopoietic stem cell transplantation, the donor's transplanted immune cells may attack the recipient's healthy cells.

Within the first 90 days of transplantation, this manifests as:

When this condition develops within 90-600 days after transplantation, its symptoms include:

Overall, the risks of the procedure depend on the recipients’ age, health state, the nature of the underlying disease, the quality of the transplant, and some features of the donor).

Although associated with some serious risks, hematopoietic stem cell transplantation gives a chance to cure some life-threatening diseases completely. A decision to perform the procedure is based on the goals of treatment and the assessment of the individual clinical situation of a patient.