Chimerism After Sickle Cell Transplant: When It Means Cure

Chimerism after a sickle cell transplant is the percentage of the patient's blood-forming cells that come from the donor. Above 95% is full donor chimerism; 5 to 95% is mixed chimerism; below 5% means the graft has failed. A sickle cell transplant does not need full chimerism to cure: a stable mixed chimerism that stops falling, even at 25% or lower, is enough to end the symptoms.

That counter-intuitive point was among the most instructive of the masterclass given in September 2026 by Dr. Arun Singh Danewa, Senior Consultant – Pediatric-Haemato-Oncology & Bone Marrow Transplant (Unit II), Artemis Hospitals, Gurugram, India. This article follows the transplant from the day the donor cells are infused to the point at which a patient can be called cured. How the donor is chosen and the patient is prepared is covered in the article on HLA matching and conditioning; the full journey is in the complete guide to the sickle cell bone marrow transplant process.

Engraftment: the first sign the transplant is working

Engraftment is the moment the donor's stem cells start producing blood in the patient's marrow. It takes 12 to 14 days after the infusion. Neutrophils arrive first, and neutrophil engraftment is declared when the neutrophil count stays above 500 per microlitre for three consecutive days. Platelets follow two or three days later, then haemoglobin.

A rising blood count shows that cells are being produced, but not whose cells they are. For that, Dr. Danewa relies on a more precise test.

Chimerism testing after a sickle cell transplant

A chimerism test measures the proportion of blood-forming cells that are of donor origin. It is a more accurate measure than the blood count of whether a bone marrow transplant has succeeded. Dr. Danewa's team tests chimerism at fixed time points: day 30, day 60, day 90, six months and one year. Testing to one year is usually sufficient.

Full, mixed and failed chimerism

By the textbook definitions:

At day 30, the target is above 95%.

Why mixed chimerism can still mean cure in sickle cell disease

In sickle cell disease the target is not 95%; it is stability. Dr. Danewa's central point was that a chimerism of 50% at six months that is no longer falling is a stable mixed chimerism: the body has accepted that share of donor cells, and that is good enough. A chimerism test ten years after a sickle cell transplant may show only about 50% donor cells in a patient who has no symptoms at all.

How low can the level go? Published reports suggest that as little as 12% donor chimerism can be sufficient. In Dr. Danewa's own experience, patients with 25% donor cells at one, two or five years after transplant are free of sickle symptoms. He grounds that threshold in a comparison with sickle cell trait: carriers live with a substantial minority of sickle haemoglobin in their blood and usually have no symptoms. In practical terms, he regards a stable chimerism of roughly 12 to 25% or more as compatible with a symptom-free life.

Thalassaemia behaves in a similar way: stable mixed chimerism with as few as 25% donor cells has kept patients free of transfusions for years, because donor red cells survive longer than the patient's own. In sickle cell disease, any stable level between about 25% and 95% is acceptable.

Why early levels still matter

The tolerance of mixed chimerism applies to the long-term plateau, not to the early weeks. A chimerism of 70% at day 30 often falls quickly, sometimes to 5%, as the patient's immune system rejects the donor cells. That is why the team wants a high, stable level at first: it may then decline somewhat and settle on a plateau, which can occur at any time. The aim is a level that is not falling every month or every year.

What to do when chimerism is falling

When chimerism at day 30 is below target, for example 75 or 80%, Dr. Danewa's team works through a defined sequence.

1. Reduce immunosuppression. Ciclosporin and tacrolimus, given to prevent graft-versus-host disease, also suppress the donor cells. The first step is to withdraw them and recheck chimerism after two weeks. If it rises, the drugs are resumed at a lower dose.
2. Donor lymphocyte infusion (DLI). If chimerism keeps falling despite withdrawal, the team gives a calculated dose of the donor's lymphocytes, collected from the donor's blood. Sometimes stem cells are used.
3. Second transplant. If chimerism still falls below 5%, a second transplant is the only remedy for graft failure.

Donors who travel for the transplant may have gone home by the time DLI is needed. To prepare for this rare scenario, the team sometimes sets aside and cryopreserves a portion of the donor's cells for DLI at the time of harvest; otherwise the donor may need to return.

Why bone marrow grafts are rejected

Graft rejection has two immunological mechanisms.

• Antibody-mediated rejection. A patient who has formed donor-specific antibodies against the donor's HLA, usually through repeated transfusion, will reject the stem cells immediately if the antibodies are not first lowered by desensitisation. Neutrophil engraftment never occurs. This is why the donor-specific antibody test must be negative before a half-match transplant.
• Cell-mediated rejection. If the conditioning chemotherapy is too weak to clear the patient's own marrow, the surviving recipient cells can attack and reject the donor graft.

The defences follow directly: a negative donor-specific antibody test and a conditioning regimen strong enough to eliminate the recipient's cells.

How often a sickle cell transplant graft fails

Graft failure is uncommon, and its likelihood follows the donor hierarchy. With a fully matched sibling donor, graft failure occurs in only about 3 to 5% of transplants in Dr. Danewa's account, and graft-versus-host disease is minimal and manageable. With matched unrelated and half-match donors the risk is somewhat higher, around 5 to 8%, provided the team uses good conditioning, monitors ciclosporin and tacrolimus levels tightly, tests chimerism on schedule and intervenes early. The skill of the transplant team, in his view, lies in staying current with guidelines and acting on these measurements promptly.

When can a second transplant be done?

For a benign disease such as sickle cell disease or thalassaemia, Dr. Danewa advises waiting about six months to a year after graft failure before a second transplant, to let the body recover from the first. Sickle cell disease is not immediately life-threatening, alternatives exist, and a transplant is a demanding process. Some centres retransplant within about three months, and there is no absolute rule; patients travelling from other countries often prefer not to wait. For leukaemia, by contrast, a second transplant must be done immediately.

The same donor can be used again. Because a donor can give twice in a lifetime, Dr. Danewa still prefers to recollect from a fully matched sibling rather than switch to a half-match donor. In one case he described, a boy whose first graft from his mother failed returned two years later for a second transplant from a matched unrelated donor.

Complications after engraftment

Engraftment is not the end of the risk. Dr. Danewa named three groups of complications that demand vigilance after the patient leaves the transplant unit.

• Graft-versus-host disease (GVHD). Even matched donor cells do not belong to the patient's body and can react against it. The graft is the donor's cells and the host is the patient. He calls GVHD one of a transplant physician's nightmares. A parent who joined the session described her son's course after a full-match transplant: chimerism of 100% at days 30, 60 and 90, then GVHD of the eyes and skin after returning home, treated with additional drugs on review. He is now back at school.
• Infections. The new immune system starts from zero and matures over months. The first 30, 60 and 100 days are the crucial periods for viral, bacterial and fungal infection. Dr. Danewa mentioned a boy whose otherwise smooth transplant was complicated by cytomegalovirus (CMV) reactivation; he now plays football.
• Other complications, including transplant-associated thrombotic microangiopathy (TA-TMA), damage to the small blood vessels, and veno-occlusive disease of the liver (VOD), a blockage of the liver's small veins.

This is why patients stay close to the transplant centre for about 100 days, with frequent outpatient reviews and blood tests, before returning home with a follow-up plan.

What "cure" means after a sickle cell transplant

A successful transplant ends sickling completely. When the donor cells are present in sufficient, stable numbers, Dr. Danewa described the effect as all or none: there are no further sickling episodes, not milder or fewer ones.

He defines cure in time as well as in chimerism. After one year, if chimerism is satisfactory, the chance of graft failure is below 5% and the patient is almost cured; after two to three years, cure is almost certain. Nearly all medication stops at about one year, and the patient is revaccinated, because the transplant drugs erase the immune memory of earlier vaccines.

Damage that does not reverse

A transplant prevents new damage; it does not undo old damage. A patient left bedridden or wheelchair-bound by a stroke before the transplant will remain so. Dr. Danewa has transplanted such patients and counsels families clearly in advance: the aim is to stop further harm. This is the clearest argument for transplanting before the organs are affected.

Children of a cured patient

A transplant replaces the blood-forming system, not the patient's genes. The reproductive organs still carry the sickle (SS) genes, so a cured patient passes one sickle gene to every child. With a partner who has normal haemoglobin, every child will be a carrier (AS). With a partner who is a carrier, each child has a chance of sickle cell disease. Dr. Danewa therefore advises that patients cured by transplant, and their partners, be tested for sickle cell status before planning children.

Life after the one-year mark

The endpoint of the pathway is a return to normal life with only routine follow-up. In practice this means three milestones: a stable chimerism, the withdrawal of almost all medication around one year, and a fresh course of childhood vaccines. The parent who described her son's transplant summarised it plainly: most drugs stopped, vaccinations restarted and the child back at school.

More questions from the session on chimerism, donation and inheritance are collected on the Live Discussion page for this masterclass. For long-term follow-up after the patient returns home, see also Dr. Dharma Choudhary's article on sickle cell transplant aftercare.