Allogeneic Stem Cell Transplantation

A comprehensive guide to donor-based bone marrow transplant — indications, HLA matching, conditioning, GvHD, and long-term outcomes

Key Points

What Is Allogeneic Stem Cell Transplantation?

Allogeneic hematopoietic stem cell transplantation (allo-HSCT), commonly referred to as a bone marrow transplant, is a potentially curative procedure in which a patient's diseased bone marrow is replaced by hematopoietic stem cells from a healthy donor. Unlike autologous transplantation (where the patient's own cells are used), allogeneic transplantation provides two distinct therapeutic benefits: it replaces the patient's marrow with normally functioning donor hematopoiesis, and it delivers a potent immunological anti-tumor effect through donor immune cells — the graft-versus-leukemia (GvL)…

Indications and Patient Selection

Allogeneic HSCT is indicated when the expected benefit — cure or durable disease control — outweighs the risk of treatment-related mortality (TRM). Appropriate patient selection requires rigorous assessment of disease status, patient performance status, organ function, and available donors. **Hematologic malignancies with established allo-HSCT indications:** - **Acute myeloid leukemia (AML):** Standard allo-HSCT in first complete remission (CR1) for intermediate and adverse-risk disease per ELN 2022 classification. High-risk features mandating transplant: TP53 mutation, complex karyotype,…

Donor Selection and HLA Matching

The ideal donor is an HLA-identical sibling — the probability of any given sibling being a match is 25%, meaning roughly 30% of patients will have an HLA-matched sibling. For the remaining majority, alternative donor strategies are employed. **HLA system and matching:** The human leukocyte antigen (HLA) system is the most polymorphic gene system in the human genome, encoded on chromosome 6p21. HLA molecules present peptide antigens to T cells; HLA disparities between donor and recipient drive both GvHD and GvL. High-resolution molecular HLA typing at 10 allele-level loci (HLA-A, B, C, DRB1,…

Conditioning Regimens

The conditioning regimen serves two purposes: (1) eradication of residual malignant cells, and (2) immunosuppression of the host immune system to prevent graft rejection. Conditioning is classified by intensity: **Myeloablative conditioning (MAC):** Achieves complete ablation of host hematopoiesis — the patient cannot recover marrow function without the graft. Delivers maximum anti-tumor efficacy but carries the highest regimen-related toxicity. Standard MAC regimens: - **BuCy (busulfan + cyclophosphamide):** Busulfan 3.2 mg/kg/day IV × 4 days (or targeted to AUC 900–1350 μmol·min) +…

Engraftment, Chimerism, and the Post-Transplant Course

**Stem cell infusion (Day 0):** Fresh or thawed cryopreserved graft is infused through a central venous catheter (typically a Hickman or PICC line) like a blood transfusion. The stem cells home to the bone marrow through a CXCR4-CXCL12 gradient. No immediate symptoms are expected, though DMSO in cryopreserved products can cause nausea, garlic odor, or rarely cardiovascular events during infusion. **Engraftment:** Neutrophil engraftment (ANC >500 cells/μL for 3 consecutive days) typically occurs by Day +14–21 for PBSC grafts and Day +21–28 for BM grafts. Platelet engraftment (>20,000…

Graft-versus-Host Disease (GvHD)

GvHD is the hallmark immunological complication of allogeneic HSCT and the leading cause of non-relapse morbidity and mortality. It occurs when donor T lymphocytes recognize host tissues as foreign and mount an immune attack. GvHD and GvL (the therapeutic benefit) are mechanistically linked — strategies that prevent GvHD often also reduce GvL, increasing relapse risk. This fundamental tension defines much of the complexity of allo-HSCT management. **Acute GvHD (aGvHD):** Classically defined as occurring within 100 days of transplant (though late acute GvHD can occur beyond Day +100 after…

The Graft-versus-Leukemia Effect

The graft-versus-leukemia (GvL) effect — the immunological attack of donor T cells (and NK cells) against residual host leukemia cells — is the fundamental mechanism by which allogeneic HSCT achieves cure in many diseases. Evidence for GvL comes from multiple sources: 1. **Relapse rates are higher with T-cell-depleted grafts:** Removing T cells to prevent GvHD simultaneously removes the GvL effect, resulting in higher relapse rates. This observation, made in the 1980s, established T cells as the effectors of GvL. 2. **DLI induces remission after relapse post-transplant:** Infusing additional…

Infectious Complications

Infection is a major cause of morbidity and mortality after allo-HSCT, occurring across a predictable temporal framework dictated by the phases of immune reconstitution: **Pre-engraftment (Day 0 to Day +30):** Profound neutropenia creates maximal risk for bacterial infections (gram-negatives including Pseudomonas, gram-positives from central lines and gut mucosal barrier damage) and fungal infections (Candida, Aspergillus). Prophylaxis: fluoroquinolone (levofloxacin or ciprofloxacin), fluconazole (Candida coverage) or mold-active antifungal (voriconazole, posaconazole, micafungin) in…

Long-Term Outcomes and Late Effects

Long-term outcomes after allo-HSCT have improved substantially over the past two decades, with 5-year overall survival now exceeding 50% for many indications at high-volume centers. However, survivors face a unique burden of late effects that requires structured long-term follow-up: **Secondary malignancies:** Risk is significantly elevated compared to the general population — from prior chemotherapy/radiation, chronic immunosuppression, and chronic GvHD. EBV-related PTLD (post-transplant lymphoproliferative disorder), MDS/AML (therapy-related, from conditioning), and skin squamous cell…