# Stem Cell Transplantation in Sickle Cell Disease

Allogeneic hematopoietic cell transplantation can cure sickle cell disease, but donor availability, conditioning toxicity, graft failure, and graft-versus-host disease determine whether its risks outweigh progressive disease. Donor selection and post-transplant lineage-specific chimerism are central management decisions.

**Clinical question:** Which patients with sickle cell disease should undergo allogeneic hematopoietic cell transplantation, with which donor, and how should graft function be monitored?

Updated: 2026-08-24T16:51:12.344669+00:00

## What matters in practice
- Allogeneic hematopoietic stem cell transplantation is an established curative treatment for sickle cell disease; matched-sibling transplantation has the strongest outcomes data. [5][7][19]
- Assess transplant candidacy early in symptomatic disease when an HLA-identical sibling donor is available; transplant outcomes are more favorable at younger ages, including age 12 years or younger in registry analysis. [10][15]
- For patients without a matched sibling donor, matched unrelated, sibling cord-blood, and haploidentical approaches expand access but require individualized risk assessment in an experienced transplant program; alternate-donor transplantation has historically been recommended within clinical trials. [15][19]
- After transplant, mixed chimerism alone is not an indication for intervention. In sickle cell disease, donor chimerism above 25% or hemoglobin S below 50% has been associated with freedom from recurrent disease manifestations. [18]
- Interpret chimerism by lineage when available: whole-blood mixed chimerism driven by T-cell chimerism after T-cell-depleted grafts may resolve without changing immunosuppression or administering donor lymphocyte infusion. [17]

## When to refer for transplant evaluation

Refer before cumulative sickle-related organ injury narrows the therapeutic window.

Treat allogeneic hematopoietic cell transplantation (HCT) as a curative option rather than escalation of acute-care management. Matched-sibling HCT is the best-established donor strategy for sickle cell disease (SCD), with reported overall survival of 95% and high event-free survival in children in a Lancet Haematology review. [5][7]

Initiate transplant discussion early for a symptomatic child or young adult with an HLA-identical sibling donor rather than waiting for additional irreversible complications. Expert recommendations cited in current reviews favor transplanting symptomatic young patients with an HLA-identical sibling as early as possible, preferably at preschool age; registry data also associate transplantation at age 12 years or younger with improved event-free survival. [10][15]

Frame the decision against the alternative of ongoing disease-modifying and supportive care. Hydroxyurea, L-glutamine, voxelotor, and crizanlizumab can reduce vaso-occlusive events or severe complications but are not curative; HCT introduces risks of conditioning-related mortality, infertility or gonadal failure, graft-versus-host disease (GVHD), and graft failure. [15][24]
- Refer patients with symptomatic SCD and a potential sibling donor for HLA typing and transplant-program assessment without using age alone as a reason to defer evaluation. [10][15]
- Discuss fertility and gonadal risk before conditioning, because infertility and gonadal failure are recognized HCT morbidities. [24]
- Use shared decision-making when considering HCT from either HLA-identical sibling or alternate donors; donor source and transplant-associated morbidity materially alter the benefit-risk balance. [22][24]

*Donor-source framework for allogeneic HCT in SCD. [7][15][19]*

| Donor branch | Clinical implication | Next decision |
| --- | --- | --- |
| HLA-identical sibling donor | Best-established allogeneic HCT strategy; bone marrow and sibling umbilical cord blood are recommended stem-cell sources in expert recommendations. [7][15] | Proceed to comprehensive transplant assessment and counseling, particularly in symptomatic younger patients. [10][15] |
| HLA-matched unrelated donor | An alternate option when no matched sibling is available; high-level matching at HLA-A, -B, -C, and -DRB1 is described as an 8/8 match. [15] | Review center-specific experience, conditioning approach, and risk of graft failure and GVHD. [15][19] |
| Haploidentical related donor | Expands access but protocol outcomes vary; pediatric graft failure has been reported at 15% to 25% and chronic GVHD up to 30% in an EBMT meeting report. [13][19] | Use an experienced SCD transplant program and protocol-specific counseling; evaluate trial availability. [15][19] |
| No suitable donor | A matched sibling donor is available for fewer than 10% of potential SCD allogeneic transplant recipients in one report. [9] | Continue disease-modifying management while revisiting unrelated-donor, haploidentical, and clinical-trial options at a transplant center. [15][19] |

## Pretransplant donor and risk assessment

The practical pretransplant objective is to define donor feasibility and identify risks that will alter conditioning or counseling.

Perform donor identification and high-resolution HLA assessment early. For unrelated donor selection, HLA-A, HLA-B, HLA-C, and HLA-DRB1 matching defines an 8/8 match in National Marrow Donor Program recommendations summarized in a current review. [15] A potential matched sibling donor should be evaluated alongside the recipient before a patient is labeled ineligible because few SCD candidates have access to this donor type. [9][15]

Document pre-existing HLA antibodies and red-cell alloimmunization before HCT. These immunologic exposures are specifically studied as pretransplant variables in SCD transplantation and should trigger coordinated transfusion-medicine and transplant planning rather than ad hoc peri-transplant transfusion decisions. [8]

Counsel explicitly that conditioning intensity and donor source trade donor-engraftment reliability against toxicity. Myeloablative matched-sibling HCT in children has excellent outcomes, whereas reduced-intensity approaches are associated with more mixed chimerism; haploidentical protocols have variable graft-failure and chronic-GVHD rates. [18][19][24]
- Obtain high-resolution HLA typing of the recipient and available related donors; if considering an unrelated donor, assess 8/8 HLA-A, -B, -C, and -DRB1 matching. [15]
- Capture red-cell antibody history and pre-existing HLA antibody status before donor selection and conditioning planning. [8]
- Offer fertility-focused counseling before conditioning because gonadal failure and infertility are recognized transplant risks. [24]
- Refer alternate-donor candidates to centers with SCD-specific transplant expertise; expert recommendations summarized in review literature reserve these approaches for experienced programs and historically for clinical-trial settings. [15]

*Pretransplant findings that change the HCT plan in SCD. [8][15][24]*

| Finding | Why it matters | Action |
| --- | --- | --- |
| HLA-identical sibling donor identified | This donor strategy has the strongest SCD HCT outcome data. [7][15] | Prioritize matched-sibling transplant counseling and formal candidacy assessment. [10][15] |
| No matched sibling donor | Unrelated, sibling cord-blood, and haploidentical donors are potential alternatives, with different graft-failure and GVHD risks. [13][15][19] | Use center-specific protocol review and clinical-trial assessment when appropriate. [15][19] |
| Pre-existing HLA antibodies or RBC alloimmunization | Both are relevant pretransplant immunohematologic variables in SCD HCT. [8] | Coordinate donor selection and transfusion support with transplant and transfusion-medicine teams. [8] |
| Concern about future fertility | Conditioning-associated infertility and gonadal failure are recognized morbidities. [24] | Complete reproductive-risk counseling before conditioning. [24] |

## Choose donor and conditioning strategy by engraftment risk

Do not treat all allogeneic HCT platforms as interchangeable.

For children with a matched sibling donor, myeloablative HCT remains the benchmark approach described as having excellent outcomes. [19] Bone marrow and HLA-identical sibling cord blood are the recommended stem-cell sources in expert recommendations summarized in review literature. [15] The expected benefit is durable donor erythropoiesis and elimination of SCD manifestations, but counseling must include acute transplant toxicity and later infertility, gonadal failure, GVHD, and graft failure. [18][24]

For an HLA-matched unrelated donor, distinguish fully matched from mismatched donor options. FDA advisory analyses in a different transplant context found outcomes with matched unrelated donors comparable to those with a matched donor strategy, while mismatched donors had frequent early events driven largely by second transplantation for graft failure; this reinforces avoiding extrapolation from matched-unrelated to mismatched-donor risk. [2] In SCD, use an 8/8 unrelated-donor match when pursuing this pathway and review conditioning and graft source within the transplant program. [15]

For haploidentical HCT, transplant protocol choice is a clinical determinant, not a technical detail. Published review describes a range of haploidentical protocols, and an EBMT meeting report notes markedly worse outcomes in participants younger than 18 years than in adults, with graft failure reported in 15% to 25% and chronic GVHD up to 30% in pediatric recipients. [13][19] These estimates should be presented as protocol- and cohort-specific, rather than as universal rates for every haploidentical platform.
- Prefer matched sibling bone marrow or HLA-identical sibling cord blood when a suitable sibling donor exists. [15]
- Separate matched-unrelated donor discussions from mismatched-donor discussions; graft failure is a key early risk in the latter group. [2]
- For haploidentical candidates, discuss graft failure, chronic GVHD, and the possibility of subsequent transplantation before consent. [13][19]

*Tradeoffs among allogeneic HCT approaches for SCD. [13][15][18][19][24]*

| Approach | Principal advantage | Dominant tradeoff to discuss |
| --- | --- | --- |
| Myeloablative HLA-identical sibling HCT | Excellent pediatric outcomes and established curative role. [7][19] | Conditioning-related morbidity, including infertility or gonadal failure, plus GVHD and graft-failure risk. [24] |
| Reduced-intensity HCT | May reduce conditioning intensity for selected patients. [18] | Mixed donor chimerism is more common; chimerism requires longitudinal interpretation rather than automatic intervention. [18] |
| 8/8 matched unrelated-donor HCT | Provides an option when no matched sibling is available. [15] | Requires stringent HLA matching and individualized assessment of transplant morbidity. [15][24] |
| Haploidentical HCT | Broadens donor availability among relatives. [19] | Pediatric cohorts have reported graft failure of 15% to 25% and chronic GVHD up to 30%. [13] |

## Monitor chimerism and hemoglobin S to detect clinically meaningful graft loss

Post-HCT surveillance should distinguish stable mixed chimerism from declining disease-controlling donor erythropoiesis.

Measure donor-recipient chimerism serially after HCT and pair its interpretation with hemoglobin S (HbS) measurement and clinical recurrence. In a retrospective SCD cohort with assessments at day 100 and at 1 and 2 years, recurrent disease was defined by vaso-occlusive crises, acute chest syndrome, stroke, and/or HbS greater than 50%. [18] This makes HbS trajectory and recurrence of SCD events clinically relevant alongside the percentage of donor cells.

Do not intervene for mixed chimerism solely because it is present. In the same cohort, donor chimerism greater than 25% or HbS less than 50% was associated with freedom from SCD symptoms; the lowest donor chimerism documented without symptomatic disease was 26%. [18] By contrast, donor chimerism below 10% was defined as graft failure in that analysis. [18]

Request lineage-specific chimerism rather than relying only on whole-blood results when a T-cell-depleted graft has been used. Whole-blood mixed chimerism driven by the T-cell lineage was not associated with adverse outcomes in a pediatric study and resolved without immunosuppression adjustment or donor lymphocyte infusion; intervention decisions should be based on lineage-specific results. [17]
- At each scheduled chimerism assessment, review donor percentage, HbS percentage, and interval vaso-occlusive events, acute chest syndrome, or stroke. [18]
- Treat a falling donor-chimerism trend toward less than 10% as possible graft failure, particularly if HbS rises above 50% or SCD manifestations recur. [18]
- Do not change immunosuppression or administer donor lymphocyte infusion for isolated whole-blood mixed chimerism without lineage-specific interpretation and evidence of clinically relevant graft loss. [17]
- Use lower donor chimerism thresholds cautiously when the patient has donor sickle trait, because HbS interpretation depends on donor hemoglobin genotype; this distinction is part of transplant-specific hematology assessment. [18]

*Post-HCT interpretation of donor chimerism and HbS in SCD. [17][18]*

| Post-HCT result | Interpretation | Next action |
| --- | --- | --- |
| Mixed donor chimerism with donor chimerism >25% or HbS <50% | Associated with freedom from SCD symptoms in retrospective follow-up; mixed chimerism itself does not require intervention. [18] | Continue serial chimerism, HbS, and clinical-event monitoring. [18] |
| Whole-blood mixed chimerism driven by T-cell lineage after T-cell-depleted graft | May resolve without adverse outcomes and without intervention. [17] | Obtain or review lineage-specific chimerism before altering immunosuppression or considering donor lymphocyte infusion. [17] |
| Donor chimerism <10% | Defined as graft failure in the SCD cohort. [18] | Urgently reassess graft status, HbS, and recurrent SCD manifestations in the transplant program. [18] |
| HbS >50% with vaso-occlusive crisis, acute chest syndrome, or stroke | Meets the study definition of recurrent disease. [18] | Evaluate for declining graft function or graft failure and reinstitute disease-directed management while transplant options are reassessed. [18] |

## Conduct a donor-specific shared decision

A curative intent does not eliminate uncertainty about late toxicity, donor access, or alternate-donor durability.

The counseling conversation should compare the patient’s current disease burden and expected trajectory with a one-time transplant risk profile. HCT can cure SCD and has been reported to improve quality of life in high-risk patients, but it may cause mortality, GVHD, infertility, gonadal failure, and graft failure. [5][24] The appropriate decision is therefore individualized rather than based solely on whether a donor can be found.

For a matched sibling donor, explain that this approach has the clearest evidence base and is generally favored early in symptomatic pediatric disease. [7][10][15] For patients without a matched sibling, explicitly describe that alternate donor strategies may provide access to cure but have more uncertain and protocol-dependent risk, including pediatric haploidentical graft failure and chronic GVHD. [13][15][19]

Continue longitudinal reassessment if HCT is deferred. Donor availability, transplant-platform experience, and the balance between progressive SCD complications and treatment-related toxicity can change over time; a prior decision not to transplant should not preclude renewed transplant review. [15][19][22]
- Document whether the decision reflects matched-sibling eligibility, alternate-donor risk, fertility concerns, transplant morbidity, or the patient’s informed preference. [15][22][24]
- When an alternate-donor strategy is considered, discuss trial participation and center-specific experience as part of consent. [15][19]
- If HCT is deferred, maintain disease-modifying treatment and revisit transplant eligibility as disease burden or donor options evolve. [15]

*Questions that should be resolved before proceeding with SCD HCT. [10][13][15][18][24]*

| Decision question | Clinical consequence |
| --- | --- |
| Is an HLA-identical sibling donor available? | A yes answer supports the best-established curative HCT pathway; absence of such a donor shifts the discussion to alternate donor platforms. [7][15] |
| Is the patient symptomatic and still early enough to benefit from prevention of further injury? | Earlier transplantation, including at age 12 years or younger in registry analysis, is associated with better event-free survival. [10] |
| Can the patient accept conditioning-associated fertility and GVHD risks? | Infertility, gonadal failure, GVHD, mortality, and graft failure must be weighed against the anticipated benefit of cure. [24] |
| If mixed chimerism develops, what result would trigger action? | Use donor chimerism, HbS, clinical recurrence, and lineage-specific analyses rather than mixed chimerism alone. [17][18] |

## References
1. Blood Products Advisory Committee Curriculum Vitae ... — www.fda.gov — https://www.fda.gov/media/142756/download
2. Cellular, Tissue, and Gene Therapies Advisory Committee June ... — www.fda.gov — https://www.fda.gov/media/160149/download?_sp=e15b6e1c-0e44-43da-a082-f0101c495b84
3. Cellular, Tissue, and Gene Therapies Advisory Committee ... — www.fda.gov — https://www.fda.gov/media/159009/download
4. university of minnesota health sciences cv updated — www.fda.gov — https://www.fda.gov/media/181788/download
5. Sickle cell disease in children: an update of the evidence in low — adc.bmj.com — https://adc.bmj.com/content/early/2022/09/07/archdischild-2021-323633?versioned=true
6. Beyond hematopoietic stem cell transplantation — gh.bmj.com — https://gh.bmj.com/content/11/5/e022716
7. The role of haematopoietic stem cell transplantation for ... — www.thelancet.com — https://www.thelancet.com/journals/lanhae/article/PIIS2352-3026(20)30283-0/abstract
8. The impact of pre-existing HLA and red blood cell ... — www.thelancet.com — https://www.thelancet.com/journals/eclinm/article/PIIS2589-5370(20)30176-0/fulltext
9. Effect of increased dose of total body irradiation on graft failure ... — www.thelancet.com — https://www.thelancet.com/article/S2352302619300316/pdf
10. Effect of donor type and conditioning regimen intensity on ... — www.thelancet.com — https://www.thelancet.com/journals/lanhae/article/PIIS2352-3026(19)30154-1/abstract
11. Indications for haematopoietic cell transplantation and CAR-T for haematological diseases, solid tumours and immune disorders: 2025 EBMT practice recommendations — www.nature.com — https://www.nature.com/articles/s41409-025-02701-3
12. The 2023 EBMT report on hematopoietic cell transplantation and cellular therapies. Increased use of allogeneic HCT for myeloid malignancies and of CAR-T at the expense of autologous HCT | Bone Marrow Transplantation — www.nature.com — https://www.nature.com/articles/s41409-025-02524-2
13. The 52nd Annual Meeting of the European Society for Blood and Marrow Transplantation: Physicians – Poster Session (P001-P972) | Bone Marrow Transplantation — www.nature.com — https://www.nature.com/articles/s41409-026-02885-2
14. The 50th Annual Meeting of the European Society for Blood and Marrow Transplantation: Physicians - Oral Session (O009-O154) | Bone Marrow Transplantation — www.nature.com — https://www.nature.com/articles/s41409-024-02347-7
15. CURATIVE TREATMENT OPTIONS IN SICKLE CELL DISEASE - ScienceDirect — www.sciencedirect.com — https://www.sciencedirect.com/science/article/pii/S2531137925024812
16. Induction of Chimerism in Rhesus Macaques through Stem ... — www.sciencedirect.com — https://www.sciencedirect.com/science/article/pii/S1600613522145788
17. Donor-Host Lineage-Specific Chimerism Monitoring and Analysis in Pediatric Patients Following Allogeneic Stem Cell Transplantation: Influence of Pretransplantation Variables and Correlation with Post-Transplantation Outcomes - ScienceDirect — www.sciencedirect.com — https://www.sciencedirect.com/science/article/pii/S2666636721009398
18. Relationship between Mixed Donor–Recipient Chimerism and Disease Recurrence after Hematopoietic Cell Transplantation for Sickle Cell Disease - ScienceDirect — www.sciencedirect.com — https://www.sciencedirect.com/science/article/pii/S1083879117306936
19. The range of haploidentical transplant protocols in sickle cell ... — ashpublications.org — https://ashpublications.org/hematology/article/2023/1/532/506442/The-range-of-haploidentical-transplant-protocols
20. How I manage iron overload in the hematopoietic cell ... — ashpublications.org — https://ashpublications.org/blood/article/145/4/372/516079/How-I-manage-iron-overload-in-the-hematopoietic
21. The Effect of Allogeneic Hematopoietic Stem Cell ... — ashpublications.org — https://ashpublications.org/blood/article/142/Supplement%201/2364/504069/The-Effect-of-Allogeneic-Hematopoietic-Stem-Cell
22. Informed shared decision-making or rigid eligibility and ... — ashpublications.org — https://ashpublications.org/bloodadvances/article/10/3/608/547809/Informed-shared-decision-making-or-rigid
23. Novartis Page 3 of 119 EU Safety Risk Management Plan ... — www.ema.europa.eu — https://www.ema.europa.eu/en/documents/rmp-summary/exjade-epar-risk-management-plan_en.pdf
24. Xromi, INN-hydroxycarbamide - EMA — www.ema.europa.eu — https://www.ema.europa.eu/en/documents/variation-report/xromi-h-c-004837-ii-0019-epar-assessment-report-variation_en.pdf

## Editorial note

Prepared from cited clinical literature using Astra's research workflow. Verify recommendations against current guidance and patient-specific factors.