Mobilized peripheral blood is a product derived from circulating blood that has been specially treated to increase its concentration of hematopoietic stem cells (HSCs). These cells, the building blocks of the blood and immune system, are found in high numbers within the bone marrow. The process of mobilization stimulates these cells to move from the marrow into the peripheral blood, making them accessible for collection. This approach provides a rich source of stem cells without needing to access the bone marrow directly.
The Mobilization Process
The number of hematopoietic stem cells circulating in the bloodstream is naturally low, making direct collection inefficient. To prepare for a stem cell harvest, a process called mobilization is used to increase the quantity of these cells in the blood. This is achieved through a series of injections of a medication known as a granulocyte-colony stimulating factor (G-CSF), such as filgrastim.
Administering G-CSF prompts the bone marrow to increase its production of HSCs, causing an overflow of stem cells into the peripheral bloodstream. Donors or patients receive daily injections for about four to five days leading up to the collection. The process is monitored through daily blood tests that count the specific cells needed, known as CD34+ cells.
For some individuals, G-CSF alone may not be sufficient. In these cases, another drug called plerixafor can be administered in combination with G-CSF to enhance the effect. Plerixafor works by blocking a protein that helps anchor stem cells within the bone marrow, facilitating their release into the blood.
The medications used for mobilization can cause side effects. The most common are bone pain, particularly in the lower back and hips, and flu-like symptoms such as headaches and fatigue. These discomforts are a direct result of the increased activity within the bone marrow and subside once the G-CSF injections are stopped.
Collecting the Mobilized Cells
Once blood tests confirm a sufficient number of stem cells are in the bloodstream, the collection process, known as apheresis, can begin. This nonsurgical procedure is performed in an outpatient setting and is often compared to donating plasma or platelets. It involves drawing blood, separating specific components, and returning the rest to the body.
During apheresis, intravenous (IV) lines are placed in each arm. Blood is drawn from one arm and directed into a machine that centrifugally separates the blood components by weight. The machine isolates and collects the layer containing the hematopoietic stem cells, while the remaining components are returned to the individual through the IV line in the other arm.
This continuous cycle lasts for several hours per session, and it may take one to five days to collect the target number of stem cells. If arm veins are not adequate, a temporary central venous catheter may be placed in a larger vein. The procedure is well-tolerated, though some may experience temporary side effects like feeling cold, lightheadedness, or numbness around the lips.
A blood-thinning agent used during the process can cause a temporary drop in calcium levels, leading to muscle cramps in the hands. This can be managed with calcium supplements.
Medical Applications of Mobilized Blood
The primary purpose for collecting mobilized peripheral blood is for use in hematopoietic stem cell transplantation (HSCT). This procedure is designed to restore a person’s blood-forming and immune systems after they have been damaged by high-dose chemotherapy or radiation. The collected stem cells are infused into the patient, where they travel back to the bone marrow, engraft, and begin producing new, healthy blood cells.
There are two main types of transplants that utilize these cells. An autologous transplant involves collecting and storing a patient’s own stem cells. These cells are later reinfused after the patient undergoes intensive treatments for diseases like multiple myeloma or lymphoma, essentially “rescuing” the bone marrow from the therapy’s effects. The collected cells can be cryopreserved and stored for years.
The other type is an allogeneic transplant, which uses stem cells collected from a healthy, matched donor. This approach is necessary for treating diseases where the patient’s own bone marrow is unhealthy, such as in cases of leukemia or aplastic anemia. The success of this procedure depends on finding a donor with a compatible tissue type to minimize the risk of rejection.
Comparison to Bone Marrow Harvest
Mobilized peripheral blood collection is a more common source for stem cells than the traditional method of bone marrow harvest. A bone marrow harvest is a surgical procedure performed under general anesthesia, where marrow is extracted directly from the pelvic bones with special needles. In contrast, peripheral blood collection is a nonsurgical procedure performed in an outpatient setting.
From the recipient’s perspective, transplants using peripheral blood stem cells lead to faster engraftment. The new stem cells begin producing blood cells more quickly, reducing the time the patient has dangerously low blood counts and shortening hospital stays. This quicker recovery can lower the immediate risks of infection and bleeding.
Each source of stem cells carries a different profile of potential long-term complications. For allogeneic transplants, using peripheral blood stem cells has been associated with a higher incidence of chronic graft-versus-host disease (GVHD). GVHD occurs when the donor’s immune cells recognize the recipient’s body as foreign and attack it, a risk physicians consider when choosing a stem cell source.