What are the steps involved in a hematopoietic stem cell transplantation (bone marrow transplant)?

Bone Marrow Transplant Process

The bone marrow transplant (hematopoietic stem cell transplantation) process involves five sequential phases: donor selection and matching, stem cell mobilization and collection, conditioning (preparative) regimen, stem cell infusion, and post-transplant recovery with monitoring.

Phase 1: Donor Selection and HLA Matching

The first critical step is identifying an appropriate donor through high-resolution HLA typing:

• HLA-matched sibling donors are the gold standard and should be prioritized first 1, 2
• If no matched sibling exists, proceed to matched unrelated donors (MUD) requiring 9-10/10 HLA loci matches for HLA-A, B, C, DRB1, and DQ 3
• Haploidentical (half-matched) family donors are acceptable alternatives when no matched donor is available 3
• Consider donor age and CMV serostatus when selecting between multiple potential donors, as younger donors with favorable CMV matching show better outcomes 2
• Umbilical cord blood should only be used when no other suitable donor exists, as outcomes are generally inferior 1, 2

Phase 2: Stem Cell Mobilization and Collection

For autologous transplants, the patient's own stem cells must be mobilized and harvested:

• Mobilization regimen: Cyclophosphamide 3-4 g/m² plus G-CSF (filgrastim) 5 mg/kg/day for 5 days is the standard approach 3, 4
• Avoid melphalan and nitrosoureas before collection as they damage stem cells 4
• Target cell dose: Minimum 2×10⁶ CD34+ cells/kg per planned transplant 4
• Collection method: Leukapheresis typically begins on day 4-5 of G-CSF when CD34+ cells are circulating 5, 6
• For allogeneic transplants, bone marrow is the preferred source for non-malignant diseases to reduce GVHD risk, while peripheral blood is preferred for malignancies 1, 7

Phase 3: Conditioning (Preparative) Regimen

Administered before stem cell infusion to eradicate disease and create space for donor cells:

Myeloablative Conditioning (MAC)

• Standard regimen: Busulfan (intravenous formulation preferred) plus cyclophosphamide plus ATG 1, 3
• For thalassemia: Busulfan 9.6 mg/kg IV (days -8 to -6) + Cyclophosphamide 3.6 g/m² (days -5, -4) 3
• For sickle cell disease: IV Busulfan + Cyclophosphamide 200 mg/kg + ATG 1
• Melphalan 200 mg/m² is the gold standard for multiple myeloma 4

Reduced Intensity Conditioning (RIC)

• Used for older patients or those with comorbidities
• Typical regimen: Fludarabine + melphalan + thiotepa + ATG or alemtuzumab 1
• No clear superiority over MAC in most diseases, but may reduce non-relapse mortality 2

Common pitfall: Oral busulfan has unpredictable absorption; always use intravenous formulation when available 1

Phase 4: Stem Cell Infusion (Day 0)

The actual transplant is straightforward:

• Stem cells are infused intravenously like a blood transfusion on Day 0
• For the two-step approach: CD3+ donor lymphocyte infusion on day -6, cyclophosphamide on days -3 and -2 for tolerance induction, then CD34-selected stem cells on day 0 8
• No special procedures required during infusion beyond standard IV access
• Cells naturally home to bone marrow and begin engraftment

Phase 5: Post-Transplant Management

GVHD Prophylaxis (Allogeneic Only)

• Standard regimen: Cyclosporine A + methotrexate (IV on days +1, +3, +6, +11) 1
• Add ATG or alemtuzumab for unrelated or mismatched donors 1, 3
• Alemtuzumab reduces GVHD but increases viral complications 1

Supportive Care During Neutropenia

• Antibacterial prophylaxis: Fluoroquinolones are standard 9
• Antifungal prophylaxis: Required from day 10 of neutropenia when fungal infections become prevalent 9
• G-CSF support: Filgrastim or sargramostim accelerates neutrophil recovery post-transplant 5, 7
• Expect neutropenia lasting 10-21 days depending on stem cell source and dose

Monitoring and Follow-up

• Chimerism testing: Regular monitoring to assess donor cell engraftment, though mixed chimerism doesn't necessarily impact survival in non-malignant diseases 1
• Disease-specific evaluation: Post-transplant care requires collaboration between transplant specialists and disease experts (e.g., hematologists for thalassemia/sickle cell) 1
• Monitor for acute GVHD (days 0-100) and chronic GVHD (after day 100)

Timeline Summary

• Days -10 to -3: Conditioning regimen
• Day 0: Stem cell infusion
• Days +1 to +21: Neutropenic period with intensive supportive care
• Days +10 to +21: Expected engraftment
• Days 0 to +100: Acute GVHD risk period
• Beyond day +100: Chronic GVHD monitoring, immune reconstitution (takes up to 2 years) 9, 10

Critical caveat: The entire process requires care at an experienced transplant center with appropriate volume and expertise, as outcomes are significantly better at high-volume centers 10, 7.