Haematopoietic Stem Cell Transplantation

The answer first

Haematopoietic stem cell transplantation (HSCT) is the infusion of haematopoietic stem cells after high-dose chemotherapy (and sometimes radiotherapy) to rescue marrow function and, in allogeneic transplantation, to generate a new immune system that fights the underlying disease. It is offered for diseases where escalating chemotherapy doses can overcome resistance (autologous) or where a new immune system can eliminate residual disease (allogeneic). Two bedside rules change outcome: [1]

1. Autologous versus allogeneic is the single most important distinction. An autologous transplant uses the patient's own previously collected stem cells — there is no graft-versus-host disease (GVHD), transplant-related mortality is 1 to 2 per cent, and the graft-versus-leukaemia (GVL) effect is absent. An allogeneic transplant uses a donor's cells — there is a real risk of GVHD, mortality is 10 to 30 per cent, but the GVL effect can cure disease that chemotherapy alone cannot [8].
2. Complications follow a predictable timeline. The first 30 days are dominated by mucositis, neutropenic fever, veno-occlusive disease (sinusoidal obstruction syndrome) and haemorrhagic cystitis. Days 30 to 100 are dominated by acute GVHD, CMV reactivation and opportunistic infection. Beyond day 100 the patient enters the chronic phase, defined by chronic GVHD, secondary malignancy and endocrine late effects.

For autologous transplant the conditioning backbone is disease-specific: melphalan 200 mg/m2 for myeloma, BEAM (carmustine, etoposide, cytarabine, melphalan) for lymphoma. For allogeneic transplant the conditioning choice trades toxicity against relapse: myeloablative regimens are total body irradiation (TBI) plus cyclophosphamide, or busulfan plus cyclophosphamide or fludarabine [10]; reduced-intensity regimens are fludarabine plus lower-dose busulfan or melphalan, relying more on the GVL effect and suitable for older or comorbid patients.

Engraftment is defined as an absolute neutrophil count over 0.5 times 10 to the 9 per litre for three consecutive days, with platelet recovery over 20, typically 10 to 20 days after infusion. Engraftment failure, infection, GVHD and recurrence of the original disease are the dominant concerns in the first year. Long-term, the transplant survivor becomes a chronic-disease patient with endocrine, bone, malignancy and psychosocial needs. [1]

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Classification

Autologous versus allogeneic — the foundational distinction

DWE high-yield: If a vignette describes a patient with multiple myeloma or relapsed lymphoma receiving high-dose melphalan or BEAM followed by reinfusion of their own cells, that is autologous — there is no GVHD. If a vignette describes a donor graft, immunosuppression, rash or diarrhoea in the weeks after, that is allogeneic — and GVHD is the complication to manage. [1]

Allogeneic donor sources

The donor source shapes GVHD, engraftment speed and access, and the choice is dictated by HLA matching, urgency and patient comorbidity. [1]

DWE conceptual: Why does post-transplant cyclophosphamide make haploidentical transplantation safe when it was historically a graveyard of GVHD? Cyclophosphamide given on days 3 and 4 after transplant selectively deletes the rapidly proliferating alloreactive T-cells that cause GVHD, while sparing the resting regulatory T-cells (Tregs) and the haematopoietic stem cells, both of which express the detoxifying enzyme aldehyde dehydrogenase. The result is tolerance across the HLA barrier, dramatically reduced GVHD and graft failure, and a donor for nearly every patient [8].

Conditioning intensity

Conditioning is the chemotherapy (plus or minus radiotherapy) given immediately before the stem cell infusion. Its jobs are to ablate the recipient marrow, to suppress host immunity sufficiently to prevent graft rejection (allogeneic only), and to deliver anti-tumour cytoreduction. [1]

DCE long-case trap: Conditioning intensity is not the same as curative intent. A reduced-intensity regimen is chosen when the patient's comorbidity or age precludes myeloablative conditioning, not because the disease is less aggressive. The GVL effect compensates for reduced cytoreduction, but in chemosensitive aggressive disease (high-risk AML, advanced lymphoma) the relapse risk is higher with RIC. The registrar's job is to align conditioning with the patient's disease biology, comorbidity (HCT-specific comorbidity index) and goals. [1]

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Pathophysiology

Mobilisation, harvest and the stem cell niche

Haematopoietic stem cells normally live in the marrow, tethered to osteoblasts and stromal cells by the SDF-1/CXCR4 axis. Mobilisation forces these cells into the peripheral blood so they can be collected by apheresis. [1]

• G-CSF (filgrastim, 10 micrograms/kg per day subcutaneously) expands the CD34-positive pool and downregulates the adhesion molecules that tether stem cells, releasing them into the circulation.
• Plerixafor (240 micrograms/kg subcutaneously) is a CXCR4 antagonist that blocks the SDF-1 anchor and enhances egress. The plerixafor trials in multiple myeloma and non-Hodgkin lymphoma showed that adding plerixafor to G-CSF significantly increased the proportion of patients who reached the target collection and proceeded to autograft [5][6]. It is reserved for predicted or actual poor mobilisers (older age, extensive prior chemotherapy, prior radiation, certain lymphoma subtypes).
• Bone marrow harvest under general anaesthetic is an alternative when peripheral mobilisation fails or is inappropriate (some aplastic anaemia or inherited disease protocols), but peripheral blood has largely replaced it because engraftment is faster.

Apheresis collects the mononuclear cell fraction, and the CD34-positive cell count is the surrogate for the stem cell dose. The minimum target is 2 million CD34-positive cells per kilogram; the ideal is over 5 million per kilogram, which speeds engraftment and lowers graft failure. [1]

Conditioning and the marrow niche

Conditioning ablates the recipient marrow, creating physical and cytokine space for the graft. In allogeneic transplant, conditioning also provides host immunosuppression so the new immune system is not rejected. Myeloablative regimens cause irreversible infertility and prolonged cytopenia; reduced-intensity regimens preserve some host immunity and the graft gradually replaces (mixed chimerism) over weeks to months. [1]

Engraftment and chimerism

Infused CD34-positive cells home to the marrow via the SDF-1/CXCR4 axis, seed the niche, and proliferate and differentiate. Neutrophil engraftment is declared when the absolute neutrophil count exceeds 0.5 times 10 to the 9 per litre for three consecutive days. Platelet engraftment is a sustained count over 20 times 10 to the 9 per litre unsupported by transfusion. Both typically occur 10 to 20 days post-infusion, faster with peripheral blood grafts and G-CSF mobilisation, slower with cord blood. [1]

Chimerism analysis — short tandem repeat (STR) PCR quantifying the proportion of donor versus recipient cells in the marrow or blood — confirms engraftment and detects graft rejection, relapse or mixed chimerism. Falling donor chimerism prompts consideration of donor lymphocyte infusion (to push the GVL effect) or, conversely, reassessment for relapse. [1]

The graft-versus-leukaemia effect

The GVL effect is the immune-mediated recognition and killing of residual malignant cells by donor T-cells and NK cells. It is why relapse is lower after allogeneic than autologous transplant, why T-cell depletion (to prevent GVHD) paradoxically raises relapse risk, and why donor lymphocyte infusion (DLI) can reinduce remission in a patient who relapses after allogeneic transplant. It is most powerful in chronic phase CML, less so in acute leukaemia and many lymphomas. The two faces of donor immunity — GVL (beneficial) and GVHD (harmful) — are inseparable in practice and are the central tension of allogeneic transplantation. [1]

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Clinical presentation — the complication timeline

The complications of HSCT follow a predictable calendar. The patient in front of you is best understood by asking "what day is it?" — the day count after the stem cell infusion. [1]

Early phase (0 to 30 days)

Mid phase (30 to 100 days) — acute GVHD and viral reactivation

Late phase (beyond day 100) — chronic GVHD and the late effects

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Differential diagnosis — distinguishing the transplant complications

The registrar on the transplant unit is constantly asked "is this GVHD, infection, drug toxicity, or relapse?" A structured approach to the differential saves time and lives. [1]

Jaundice in the first three weeks — is it veno-occlusive disease?

DWE discriminator: The modified Seattle criteria for veno-occlusive disease require bilirubin over 34 micromol per litre plus two of hepatomegaly, ascites and weight gain of at least 5 per cent, occurring within 21 days of transplant. The Baltimore criteria are stricter, requiring bilirubin over 34 plus at least two of hepatomegaly, ascites, weight gain of at least 5 per cent. Calculate these criteria daily in the first three weeks — early defibrotide changes outcome [4].

Diarrhoea after transplant — is it acute GVHD?

Rash after transplant — is it acute GVHD?

DWE high-yield: Acute GVHD rash characteristically involves the palms, soles and ears — areas often spared by ordinary drug rashes. A registrar who examines the palms and soles in a post-transplant patient with a rash is thinking correctly. [1]

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Investigations

The pre-transplant workup

Monitoring during and after transplant

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Management

Autologous transplant — the procedure

1. Mobilisation — G-CSF 10 micrograms/kg per day subcutaneously, with plerixafor 240 micrograms/kg added for predicted or actual poor mobilisers.
2. Collection by apheresis — target at least 2, and ideally over 5, million CD34-positive cells per kilogram. The product is cryopreserved.
3. Conditioning — disease-specific. Melphalan 200 mg/m2 intravenously for myeloma; BEAM (carmustine, etoposide, cytarabine, melphalan) for lymphoma. [1]4. Infusion of the thawed cryopreserved graft on day 0.
4. Supportive care through neutropenia — antimicrobial prophylaxis, transfusion, mucositis care, neutropenic fever pathway.
5. Engraftment at 10 to 14 days; discharge once engrafted, afebrile, orally tolerant and safe. [1]

Allogeneic transplant — the procedure

1. Donor selection — HLA-matched sibling first; otherwise matched unrelated donor (registry), haploidentical family donor (with post-transplant cyclophosphamide), or cord blood.
2. Conditioning — myeloablative (TBI plus cyclophosphamide, or busulfan plus fludarabine), reduced-intensity (fludarabine plus melphalan or busulfan), or non-myeloablative.
3. GVHD prophylaxis — calcineurin inhibitor (tacrolimus or cyclosporine) plus short-course methotrexate (days 1, 3, 6, 11) for matched donor; post-transplant cyclophosphamide 50 mg/kg on days 3 and 4 (with a calcineurin inhibitor) for haploidentical transplant; antithymocyte globulin added for unrelated donor in many protocols. [1]4. Infusion of fresh or cryopreserved graft on day 0.
4. Engraftment at 12 to 20 days (slower with cord blood); chimerism at day 28.
5. Long-term immunosuppression taper and surveillance for GVHD, infection, relapse and late effects. [1]

Neutropenic fever

The door-to-antibiotic time is a measurable survival determinant. On any fever in a neutropenic patient: [1]

• Cultures from each lumen of any central line and peripherally, plus urine and sputum.
• Empiric antipseudomonal beta-lactam — piperacillin-tazobactam 4.5 g intravenously, ceftazidime, or meropenem — within one hour. [1]- Add vancomycin for suspected line infection, haemodynamic instability, mucositis, or known colonization with resistant organisms.
• Assess for septic shock — early IV fluids, vasopressors and ICU as required.
• Do not delay antibiotics for imaging, review or results. [1]

Venous-occlusive disease (sinusoidal obstruction syndrome)

• Recognise early: calculate the modified Seattle or Baltimore criteria daily in the first three weeks.
• Supportive care: fluid and sodium restriction, analgesia, correction of coagulopathy (carefully — over-transfusion worsens volume overload), renal and respiratory support, paracentesis for tense ascites.
• Defibrotide 6.25 mg/kg intravenously every six hours for at least 21 days — the only agent with proven benefit in severe SOS with multiorgan failure. The phase 3 trial showed improved day 100 survival (38 versus 25 per cent against historical controls) [4].

Acute GVHD — grading and treatment

The Glucksberg-Przepiorka system stages each organ and combines them into an overall grade [9]:

First-line for grade II or higher acute GVHD is methylprednisolone 1 to 2 mg/kg per day, tapered on response. Second-line for steroid-refractory disease (progression after three days, or no response by day seven to 14) is ruxolitinib 5 to 10 mg twice daily, supported by the REACH1 (phase 2) and REACH2 (phase 3) trials [3][1]. REACH2 showed an overall response rate at day 28 of 62 per cent with ruxolitinib versus 39 per cent with best available therapy. Extracorporeal photopheresis is a steroid-sparing option, especially for skin and gut disease. Prophylaxis throughout is a calcineurin inhibitor (tacrolimus or cyclosporine) plus methotrexate or mycophenolate; for haploidentical transplant, post-transplant cyclophosphamide.

Chronic GVHD — diagnosis and treatment

Chronic GVHD is diagnosed by NIH criteria: diagnostic manifestations (poikiloderma, lichen sclerosis, joint stiffness, bronchiolitis obliterans) alone suffice, or distinctive manifestations confirmed on biopsy (lichenoid oral lesions, sicca, oesophageal web). It is staged by organ (0 to 3 across eight organs) into mild, moderate or severe global severity. [1]

• Mild — topical corticosteroid, topical calcineurin inhibitor; continued prophylactic immunosuppression.
• Moderate to severe — systemic corticosteroid (prednisolone 1 mg/kg per day) plus a calcineurin inhibitor; wean as tolerated.
• Steroid-refractory — ruxolitinib 10 mg twice daily (REACH3 showed an overall response rate at week 24 of 50 per cent versus 26 per cent with best available therapy) [2], extracorporeal photopheresis, ibrutinib, belumosudil, or abatacept for selected cases.

Opportunistic infection prophylaxis

CMV reactivation — preemptive therapy

• Monitor quantitative whole blood CMV PCR weekly from engraftment to day 100, and longer in high-risk patients (seropositive recipient, T-cell depletion, haploidentical, steroid-treated GVHD).
• A rising PCR or a threshold viral load triggers preemptive therapy — valganciclovir 900 mg twice daily for 14 to 21 days then maintenance once daily until one or two negative PCRs, or intravenous ganciclovir, or foscarnet for ganciclovir resistance or myelosuppression.
• Continue PCR monitoring during and after therapy, with secondary prophylaxis in high-risk patients.
• Symptomatic CMV disease (colitis, pneumonitis, retinitis, marrow suppression) is treated as disease, not as reactivation — higher threshold and longer therapy. [1]

Post-transplant lymphoproliferative disorder

EBV-driven, occurs in T-cell depleted, haploidentical, or ATG-containing transplants and in patients on heavy immunosuppression. Monitor EBV viral load in high-risk patients. The diagnosis is made by imaging for masses, tissue biopsy showing a lymphoid proliferation, flow cytometry and EBV-encoded RNA in situ hybridisation. [1]

Management: first, reduce immunosuppression as far as the underlying disease and GVHD risk permit. Second, rituximab 375 mg/m2 weekly for four doses. Cytotoxic chemotherapy (such as CHOP) is reserved for refractory or aggressive CD20-negative disease. [1]

Late effects — the survivorship clinic

The long-term transplant survivor is a chronic-disease patient. The survivorship clinic addresses: [1]

• Infection prophylaxis — wean off at six months if no chronic GVHD; continue indefinitely in chronic GVHD or ongoing immunosuppression.
• Vaccination — revaccinate from six months (influenza annually, pneumococcal, Haemophilus influenzae, hepatitis B, tetanus, polio); live vaccines (measles-mumps-rubella, varicella) only when off immunosuppression and at least two years post-transplant.
• Endocrine — annual thyroid function; gonadal hormone replacement for premature ovarian or testicular failure; growth monitoring in children.
• Bone density — baseline and serial DEXA; bisphosphonate for osteoporosis; consider avascular necrosis surveillance in patients on prolonged steroids.
• Secondary malignancy — annual skin examination; thyroid ultrasound; breast cancer screening from eight years post-transplant or age 40 (whichever first) for patients who received chest radiation; oral cavity examination.
• Cardiovascular — aggressive risk-factor modification; consider the transplant survivor as a high-risk patient.
• Psychosocial and sexual health — anxiety, depression, return to work, sexual function, relationships, fertility — address explicitly at each visit. [1]

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Long-term outcomes and prognosis

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Common exam traps and high-yield discriminators

• Autologous has no GVHD. A vignette that asks about a rash or diarrhoea after an autologous transplant is pointing to drug, infection or engraftment syndrome — not GVHD. This is a guaranteed MCQ trap.
• The 100-day boundary. Acute GVHD is within 100 days; chronic GVHD is after 100 days. A vignette describing a scleroderma-like rash at day 140 is chronic GVHD, not acute.
• CMV is managed by surveillance and preemptive therapy, not universal prophylaxis (although letermovir prophylaxis in seropositive recipients is increasingly standard). A rising PCR triggers valganciclovir or foscarnet before symptomatic disease.
• Veno-occlusive disease triad — weight gain, tender hepatomegaly, rising bilirubin within 21 days of conditioning. The treatment is defibrotide. A registrar who attributes the picture to "sepsis" or "drug" without applying the criteria misses the window.
• Post-transplant cyclophosphamide is given on days 3 and 4 after a haploidentical transplant — not before. It selectively deletes alloreactive T-cells while sparing regulatory T-cells. This is exam law and a common MCQ discriminator.
• PJP prophylaxis with co-trimoxazole continues for at least six months and longer with chronic GVHD or ongoing immunosuppression. Stopping at the standard time in a chronic GVHD patient is a common error.
• The GVL effect is why allogeneic can cure what autologous cannot. Donor lymphocyte infusion exploits the GVL effect to reinduce remission in relapsed disease. A registrar who understands GVL understands why allogeneic transplant is curative despite its toxicity. [1]- The conditioning regimens to memorise. Myeloablative: TBI plus cyclophosphamide, or busulfan plus cyclophosphamide or fludarabine. Reduced-intensity: fludarabine plus melphalan or busulfan. Autologous myeloma: melphalan 200. Autologous lymphoma: BEAM. Do not confuse them.
• Engraftment thresholds. Neutrophils over 0.5 for three consecutive days. Platelets over 20 unsupported. Typically 10 to 20 days. A guaranteed MCQ.
• The survivorship plan is the long-case differentiator. The candidate who addresses infection prophylaxis, vaccination, endocrine replacement, bone density, malignancy surveillance and psychosocial health demonstrates the integrated care the DCE long case rewards. [1]

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Regional guideline anchoring

• ANZ primary: HSANZ (Haematology Society of Australia and New Zealand) clinical practice guidelines; eviQ protocols for mobilisation, conditioning, GVHD prophylaxis and supportive care.
• UK secondary: BSH (British Society for Haematology) stem cell transplantation guidelines; EBMT (European Society for Blood and Marrow Transplantation) handbook.
• US tertiary: NCCN guidelines for haematopoietic growth factors, prevention of infections, and transplant indications; ASBMT (now ASTCT) consensus.
• Global: WHO and EBMT activity surveys; CIBMTR (Center for International Blood and Marrow Transplant Research) registry outcomes. [1]

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References and further reading

EBMT Handbook of Haematopoietic Stem Cell Transplantation; HSANZ and eviQ clinical protocols; NCCN Clinical Practice Guidelines in Oncology (Hematopoietic Growth Factors; Prevention of Infections); ASH Education Program — Stem Cell Transplantation; CIBMTR registry outcome reports. [1]

References

1. [1]Zeiser R, von Bubnoff N, Butler J, et al. Ruxolitinib for Glucocorticoid-Refractory Acute Graft-versus-Host Disease N Engl J Med, 2020.PMID 32320566
2. [2]Zeiser R, Lee SJ, Padmanabhan S, et al. Ruxolitinib for Glucocorticoid-Refractory Chronic Graft-versus-Host Disease N Engl J Med, 2021.PMID 34260836
3. [3]Jagasia M, Perales MA, Schroeder MA, et al. Ruxolitinib for the treatment of steroid-refractory acute GVHD (REACH1): a multicenter, open-label phase 2 trial Blood, 2020.PMID 32160294
4. [4]Richardson PG, Riches ML, Kass SL, et al. Phase 3 trial of defibrotide for the treatment of severe veno-occlusive disease and multi-organ failure Blood, 2016.PMID 26825712
5. [5]DiPersio JF, Stadtmauer EA, Nademanee A, et al. Plerixafor and G-CSF versus placebo and G-CSF to mobilize hematopoietic stem cells for autologous stem cell transplantation in patients with multiple myeloma Blood, 2009.PMID 19363221
6. [6]DiPersio JF, Micallef IN, Stiff PJ, et al. Phase III prospective randomized double-blind placebo-controlled trial of plerixafor plus granulocyte colony-stimulating factor compared with placebo plus granulocyte colony-stimulating factor for autologous stem-cell mobilization and transplantation for patients with non-Hodgkin's lymphoma J Clin Oncol, 2009.PMID 19720922
7. [7]Ullmann AJ, Lipton JH, Vesole DH, et al. Posaconazole or fluconazole for prophylaxis in severe graft-versus-host disease N Engl J Med, 2007.PMID 17251530
8. [8]McCurdy SR, Luznik L How we perform haploidentical stem cell transplantation with posttransplant cyclophosphamide Blood, 2019.PMID 31751485
9. [9]Przepiorka D, Weisdorf D, Martin P, et al. 1994 Consensus Conference on Acute GVHD Grading Bone Marrow Transplant, 1995.PMID 7581076
10. [10]Russell JA, Tran HT, Quinlan D, et al. Once-daily intravenous busulfan given with fludarabine as conditioning for allogeneic stem cell transplantation: study of pharmacokinetics and early clinical outcomes Biol Blood Marrow Transplant, 2002.PMID 12374451