Bone marrow transplant, more broadly called hematopoietic stem cell transplantation (HSCT), is a specialised treatment in which healthy blood-forming stem cells are infused into a patient to restore or replace the blood and immune system.

Although the term "bone marrow transplant" is widely used, stem cells can be collected from the bloodstream, bone marrow, or umbilical cord blood. Today, many transplants use stem cells collected from peripheral blood.

Stem cell transplantation may be used to treat selected blood cancers, bone marrow disorders, inherited blood disorders, and other serious diseases. The choice of transplant depends on the disease, treatment history, patient's overall condition, and whether a suitable donor is available.

What Is a Bone Marrow Transplant?

The bone marrow is responsible for producing blood-forming stem cells that develop into:

• Red blood cells

• White blood cells

• Platelets

During transplantation, patients may first receive conditioning treatment, usually chemotherapy with or without radiation, depending on the transplant approach and disease.

The stem cells are then infused through a vein. They travel to the bone marrow and begin producing new blood cells.

There are two broad categories:

• Autologous transplant, using the patient's own stem cells

• Allogeneic transplant, using stem cells from another person

Allogeneic transplantation can involve a matched sibling, matched unrelated donor, or haploidentical donor.

Autologous Stem Cell Transplant

In an autologous transplant, the patient's own blood-forming stem cells are collected before high-dose treatment and stored until they are needed.

After conditioning treatment, the stored stem cells are returned to the patient.

How Does an Autologous Transplant Work?

The process generally involves:

1. Evaluation and treatment planning

2. Stem cell mobilisation

3. Stem cell collection through apheresis

4. Cryopreservation and storage of the cells

5. Conditioning or high-dose therapy

6. Stem cell infusion

7. Blood-count recovery and monitoring

Because the patient's own cells are used, there is no donor-recipient HLA matching requirement. However, there remains a small risk that cancer cells may be present in the collected product.

Autologous transplantation is used for selected conditions, including certain lymphomas and multiple myeloma.

Allogeneic Stem Cell Transplant

In an allogeneic transplant, the stem cells come from another individual.

The donor's immune system can provide an additional anti-cancer effect in some diseases. This is known as the graft-versus-leukemia or graft-versus-tumor effect.

However, allogeneic transplantation also introduces risks related to immune incompatibility, including graft-versus-host disease (GVHD). Donor selection and HLA matching are therefore important parts of transplant planning.

Matched Sibling Donor Transplant

A matched sibling donor is a brother or sister whose HLA characteristics are sufficiently compatible with the recipient.

HLA, or human leukocyte antigen, markers help the immune system distinguish the body's own cells from foreign cells.

A sibling has approximately a 25% chance of being a full HLA match in the usual inheritance pattern.

When an appropriate matched sibling is available, this may be considered as a donor option for patients who require an allogeneic transplant.

The specific suitability of the donor depends on detailed HLA testing and other donor and recipient factors.

Matched Unrelated Donor Transplant

If a suitable matched sibling is not available, doctors may search for a compatible unrelated donor through donor registries.

A matched unrelated donor (MUD) is an unrelated volunteer whose HLA characteristics closely match those of the recipient.

High-resolution HLA testing is used to assess donor compatibility. The degree of matching can be described using different HLA loci, such as 8/8, 10/10, or 12/12, depending on the matching system being used. (Cancer.gov)

An unrelated donor search can take time, so the urgency of the patient's disease and availability of alternative donors are important considerations.

Haploidentical Stem Cell Transplant

A haploidentical, or "half-matched," transplant uses a donor who shares approximately half of the recipient's HLA characteristics.

Potential haploidentical donors can include:

• Parent

• Child

• Sibling

• Other close family members

Haploidentical transplantation has expanded donor availability for patients who do not have a suitable matched sibling or matched unrelated donor. Modern approaches to preventing and managing complications have made haploidentical transplantation an established option in many transplant settings.

Specialised strategies, including post-transplant cyclophosphamide and other graft-manipulation approaches, may be used to control immune reactions after haploidentical transplantation.

Comparing the Major Transplant Types

The choice between these approaches is not based on donor matching alone. The underlying disease, disease status, urgency, previous treatments, patient factors, donor availability, and transplant-centre experience are also considered. (Cancer.gov)

How Is a Stem Cell Donor Selected?

For an allogeneic transplant, donor selection involves detailed HLA testing.

Doctors may consider:

• HLA compatibility

• Donor availability

• Patient and donor characteristics

• Presence of donor-specific antibodies

• Donor health

• Disease-specific considerations

• Urgency of transplantation

A donor who is a good HLA match is not necessarily the same blood group as the recipient. HLA matching and blood-group matching are different considerations. (EBMT)

What Conditions May Require a Bone Marrow Transplant?

Stem cell transplantation may be considered for selected patients with conditions such as:

• Acute myeloid leukemia (AML)

• Acute lymphoblastic leukemia (ALL)

• Myelodysplastic syndromes (MDS)

• Certain lymphomas

• Multiple myeloma

• Severe aplastic anemia

• Myelofibrosis

• Certain inherited blood disorders

• Selected immune and metabolic disorders

The indication for transplantation varies considerably between diseases and individual patients.

What Happens Before a Transplant?

Before transplantation, the patient undergoes a detailed evaluation.

This may include:

• Blood tests

• Bone marrow assessment

• Imaging where appropriate

• Heart and lung evaluation

• Infection screening

• Organ-function assessment

• HLA and donor testing

• Assessment of previous treatments

The transplant team uses this information to determine the appropriate transplant strategy and conditioning regimen.

Conditioning Treatment

Conditioning treatment is given before the stem cell infusion.

It may involve chemotherapy, radiation therapy, or a combination, depending on the disease and transplant protocol.

The goals can include:

• Destroying abnormal cells

• Suppressing the patient's immune system

• Creating space in the bone marrow

• Allowing donor stem cells to establish themselves

The intensity of conditioning varies between patients.

Stem Cell Infusion and Engraftment

The stem cells are generally infused through a central venous catheter.

Following infusion, the cells migrate to the bone marrow and begin producing new blood cells.

The establishment of new blood-cell production is called engraftment.

During this period, blood counts may remain very low, and patients may require close monitoring, infection prevention, transfusions, and other supportive treatment.

Possible Complications

Bone marrow transplantation is a complex treatment and can have significant complications.

These may include:

• Infections

• Low blood counts

• Bleeding

• Mucositis

• Organ complications

• Graft failure

• Disease relapse

• Infertility

• Graft-versus-host disease after allogeneic transplantation

GVHD occurs when donor immune cells recognise the recipient's tissues as foreign and attack them. It can affect organs including the skin, liver, and gastrointestinal tract.

The risk and type of complications vary according to the transplant type and individual patient factors.

Recovery After Bone Marrow Transplant

Recovery continues well beyond the initial hospitalisation.

Patients may require:

• Regular blood tests

• Infection monitoring

• Immunosuppressive medicines after allogeneic transplantation

• Blood or platelet transfusions when required

• Nutritional support

• Vaccinations at appropriate intervals

• Monitoring for GVHD and other complications

• Long-term follow-up

Immune recovery can take considerable time, particularly following an allogeneic transplant.

Is Bone Marrow Transplant a Cure?

For some diseases, stem cell transplantation can provide long-term disease control and may have curative potential.

However, transplantation does not guarantee that the disease will not return.

The expected outcome depends on the underlying disease, disease status before transplant, genetic and molecular characteristics, transplant type, donor factors, age, overall health, and other clinical considerations.

Which Donor Is Suitable for a Patient?

There is no single donor type that is appropriate for every patient.

A transplant team may consider a matched sibling, matched unrelated donor, or haploidentical family donor depending on the clinical situation and donor availability.

Advances in HLA typing, donor selection, GVHD prevention, infection management, and supportive care have expanded the number of patients who can be considered for allogeneic transplantation.

Conclusion

Bone marrow transplantation, or hematopoietic stem cell transplantation, is an important treatment option for selected blood cancers, bone marrow disorders, and other serious diseases.

Transplants may use the patient's own stem cells or cells from a donor. Allogeneic donor options include matched siblings, matched unrelated donors, and haploidentical family members.

The transplant approach is carefully individualised based on the disease, treatment history, patient's condition, donor availability, HLA compatibility, and the risks and benefits of each option.

A specialised transplant team can assess whether transplantation is appropriate and determine the most suitable donor and treatment strategy for an individual patient.