Hematopoietic stem cell transplantation
Hematopoietic stem cell transplantation is the transplantation of multipotent hematopoietic stem cells, usually derived from bone marrow, peripheral blood, or umbilical cord blood. It may be autologous, allogeneic or syngeneic.
It is most often performed for patients with certain cancers of the blood or bone marrow, such as multiple myeloma or leukemia. In these cases, the recipient's immune system is usually destroyed with radiation or chemotherapy before the transplantation. Infection and graft-versus-host disease are major complications of allogeneic HSCT.
Hematopoietic stem cell transplantation remains a dangerous procedure with many possible complications; it is reserved for patients with life-threatening diseases. As survival following the procedure has increased, its use has expanded beyond cancer to autoimmune diseases and hereditary skeletal dysplasias; notably malignant infantile osteopetrosis and mucopolysaccharidosis.
Medical uses
Indications
Indications for stem cell transplantation are as follows:Malignant (cancerous)
- Acute myeloid leukemia
- Chronic myeloid leukemia
- Acute lymphoblastic leukemia
- Hodgkin lymphoma
- Non-Hodgkin lymphoma
- Neuroblastoma
- Ewing sarcoma
- Multiple myeloma
- Myelodysplastic syndromes
- Gliomas, other solid tumors
Non-malignant (non-cancerous)
- Thalassemia
- Sickle cell anemia
- Aplastic anemia
- Fanconi anemia
- Malignant infantile osteopetrosis
- Mucopolysaccharidosis
- Pyruvate kinase deficiency
- Immune deficiency syndromes
- Autoimmune diseases
Number of procedures
In 2006, a total of 50,417 first hematopoietic stem cell transplants were recorded worldwide, according to a global survey of 1,327 centers in 71 countries conducted by the Worldwide Network for Blood and Marrow Transplantation. Of these, 28,901 were autologous and 21,516 were allogeneic. The main indications for transplant were lymphoproliferative disorders and leukemias, and the majority took place in either Europe or the Americas.The Worldwide Network for Blood and Marrow Transplantation reported the millionth transplant to have been undertaken in December 2012.
In 2014, according to the World Marrow Donor Association, stem cell products provided for unrelated transplantation worldwide had increased to 20,604.
Graft types
Autologous
Autologous HSCT requires the extraction of hematopoietic stem cells from the patient and storage of the harvested cells in a freezer. The patient is then treated with high-dose chemotherapy with or without radiotherapy with the intention of eradicating the patient's malignant cell population at the cost of partial or complete bone marrow ablation. The patient's own stored stem cells are then transfused into his/her bloodstream, where they replace destroyed tissue and resume the patient's normal blood cell production. Autologous transplants have the advantage of lower risk of infection during the immune-compromised portion of the treatment since the recovery of immune function is rapid. Also, the incidence of patients experiencing rejection is very rare due to the donor and recipient being the same individual. These advantages have established autologous HSCT as one of the standard second-line treatments for such diseases as lymphoma.However, for other cancers such as acute myeloid leukemia, the reduced mortality of the autogenous relative to allogeneic HSCT may be outweighed by an increased likelihood of cancer relapse and related mortality, and therefore the allogeneic treatment may be preferred for those conditions.
Researchers have conducted small studies using non-myeloablative HSCT as a possible treatment for type I diabetes in children and adults. Results have been promising; however, it was premature to speculate whether these experiments will lead to effective treatments for diabetes.
Allogeneic
Allogeneic HSCT involves two people: the donor and the recipient. Allogeneic HSC donors must have a tissue type that matches the recipient. Matching is performed on the basis of variability at three or more loci of the HLA gene, and a perfect match at these loci is preferred. Even if there is a good match at these critical alleles, the recipient will require immunosuppressive medications to mitigate graft-versus-host disease. Allogeneic transplant donors may be related, syngeneic or unrelated. Unrelated donors may be found through a registry of bone marrow donors, such as the National Marrow Donor Program in the U.S. People who would like to be tested for a specific family member or friend without joining any of the bone marrow registry data banks may contact a private HLA testing laboratory and be tested with a blood test or mouth swab to see if they are a potential match. A "savior sibling" may be intentionally selected by preimplantation genetic diagnosis in order to match a child both regarding HLA type and being free of any obvious inheritable disorder. Allogeneic transplants are also performed using umbilical cord blood as the source of stem cells. In general, by transfusing healthy stem cells to the recipient's bloodstream to reform a healthy immune system, allogeneic HSCTs appear to improve chances for cure or long-term remission once the immediate transplant-related complications are resolved.A compatible donor is found by doing additional HLA-testing from the blood of potential donors. The HLA genes fall in two categories. In general, mismatches of the Type-I genes increase the risk of graft rejection. A mismatch of an HLA Type II gene increases the risk of graft-versus-host disease. In addition, a genetic mismatch as small as a single DNA base pair is significant so perfect matches require knowledge of the exact DNA sequence of these genes for both donor and recipient. Leading transplant centers currently perform testing for all five of these HLA genes before declaring that a donor and recipient are HLA-identical.
Race and ethnicity are known to play a major role in donor recruitment drives, as members of the same ethnic group are more likely to have matching genes, including the genes for HLA.
, there were at least two commercialized allogeneic cell therapies, Prochymal and Cartistem.
Bone marrow
In the case of a bone marrow transplant, the HSC are removed from a large bone of the donor, typically the pelvis, through a large needle that reaches the center of the bone. The technique is referred to as a bone marrow harvest and is performed under local or general anesthesia.Peripheral blood stem cells
Peripheral blood stem cells are now the most common source of stem cells for HSCT. They are collected from the blood through a process known as apheresis. The donor's blood is withdrawn through a sterile needle in one arm and passed through a machine that removes white blood cells. The red blood cells are returned to the donor. The peripheral stem cell yield is boosted with daily subcutaneous injections of granulocyte-colony stimulating factor, serving to mobilize stem cells from the donor's bone marrow into the peripheral circulation.Amniotic fluid
It is also possible to extract stem cells from amniotic fluid for both autologous or heterologous use at the time of childbirth.Umbilical cord blood
Umbilical cord blood is obtained when a mother donates her infant's umbilical cord and placenta after birth. Cord blood has a higher concentration of HSC than is normally found in adult blood. However, the small quantity of blood obtained from an umbilical cord makes it more suitable for transplantation into small children than into adults. Newer techniques using ex vivo expansion of cord blood units or the use of two cord blood units from different donors allow cord blood transplants to be used in adults.Cord blood can be harvested from the umbilical cord of a child being born after preimplantation genetic diagnosis for human leukocyte antigen matching in order to donate to an ill sibling requiring HSCT.
Storage of HSC
Unlike other organs, bone marrow cells can be frozen for prolonged periods without damaging too many cells. This is a necessity with autologous HSC because the cells must be harvested from the recipient months in advance of the transplant treatment. In the case of allogeneic transplants, fresh HSC are preferred in order to avoid cell loss that might occur during the freezing and thawing process. Allogeneic cord blood is stored frozen at a cord blood bank because it is only obtainable at the time of childbirth. To cryopreserve HSC, a preservative, dimethyl sulfoxide, must be added, and the cells must be cooled very slowly in a controlled-rate freezer to prevent osmotic cellular injury during ice crystal formation. HSC may be stored for years in a cryofreezer, which typically uses liquid nitrogen.Conditioning regimens
Myeloablative
The chemotherapy or irradiation given immediately prior to a transplant is called the conditioning regimen, the purpose of which is to help eradicate the patient's disease prior to the infusion of HSC and to suppress immune reactions. The bone marrow can be ablated with dose-levels that cause minimal injury to other tissues. In allogeneic transplants a combination of cyclophosphamide with total body irradiation is conventionally employed. This treatment also has an immunosuppressive effect that prevents rejection of the HSC by the recipient's immune system. The post-transplant prognosis often includes acute and chronic graft-versus-host disease that may be life-threatening. However, in certain leukemias this can coincide with protection against cancer relapse owing to the graft-versus-tumor effect. Autologous transplants may also use similar conditioning regimens, but many other chemotherapy combinations can be used depending on the type of disease.Non-myeloablative
A newer treatment approach, non-myeloablative allogeneic transplantation, also termed reduced-intensity conditioning, uses doses of chemotherapy and radiation too low to eradicate all the bone marrow cells of the recipient. Instead, non-myeloablative transplants run lower risks of serious infections and transplant-related mortality while relying upon the graft versus tumor effect to resist the inherent increased risk of cancer relapse. Also significantly, while requiring high doses of immunosuppressive agents in the early stages of treatment, these doses are less than for conventional transplants. This leads to a state of mixed chimerism early after transplant where both recipient and donor HSC coexist in the bone marrow space.Decreasing doses of immunosuppressive therapy then allow donor T-cells to eradicate the remaining recipient HSC and to induce the graft-versus-tumor effect. This effect is often accompanied by mild graft-versus-host disease, the appearance of which is often a surrogate marker for the emergence of the desirable graft versus tumor effect, and also serves as a signal to establish an appropriate dosage level for sustained treatment with low levels of immunosuppressive agents.
Because of their gentler conditioning regimens, these transplants are associated with a lower risk of transplant-related mortality and therefore allow patients who are considered too high-risk for conventional allogeneic HSCT to undergo potentially curative therapy for their disease. The optimal conditioning strategy for each disease and recipient has not been fully established, but RIC can be used in elderly patients unfit for myeloablative regimens, for whom a higher risk of cancer relapse may be acceptable.
Engraftment
After several weeks of growth in the bone marrow, expansion of HSC and their progeny is sufficient to normalize the blood cell counts and re-initiate the immune system. The offspring of donor-derived hematopoietic stem cells have been documented to populate many different organs of the recipient, including the heart, liver, and muscle, and these cells had been suggested to have the abilities of regenerating injured tissue in these organs. However, recent research has shown that such lineage infidelity does not occur as a normal phenomenon.Chimerism monitoring is a method to monitor the balance between the patient's own stem cells and the new stem cells from a donor. In case the patient's own stem cells are increasing in number post-treatment, this might be a sign the treatment did not work as intended.
Complications
HSCT is associated with a high treatment-related mortality in the recipient, which limits its use to conditions that are themselves life-threatening. Major complications include veno-occlusive disease, mucositis, infections, graft-versus-host disease, and the development of new malignancies.Infection
Bone marrow transplantation usually requires that the recipient's own bone marrow be destroyed. Prior to the administration of new cells, patients may go for several weeks without appreciable numbers of white blood cells to help fight infection. This puts a patient at high risk of infections, sepsis and septic shock, despite prophylactic antibiotics. However, antiviral medications, such as acyclovir and valacyclovir, are quite effective in prevention of HSCT-related outbreak of herpetic infection in seropositive patients. The immunosuppressive agents employed in allogeneic transplants for the prevention or treatment of graft-versus-host disease further increase the risk of opportunistic infection. Immunosuppressive drugs are given for a minimum of 6-months after a transplantation, or much longer if required for the treatment of graft-versus-host disease. Transplant patients lose their acquired immunity, for example immunity to childhood diseases such as measles or polio. For this reason transplant patients must be re-vaccinated with childhood vaccines once they are off immunosuppressive medications.Veno-occlusive disease
Severe liver injury can result from hepatic veno-occlusive disease, newly termed sinusoidal obstruction syndrome. Elevated levels of bilirubin, hepatomegaly, and fluid retention are clinical hallmarks of this condition. There is now a greater appreciation of the generalized cellular injury and obstruction in hepatic vein sinuses. Severe cases of SOS are associated with a high mortality rate. Anticoagulants or defibrotide may be effective in reducing the severity of VOD but may also increase bleeding complications. Ursodiol has been shown to help prevent VOD, presumably by facilitating the flow of bile.Mucositis
The injury of the mucosal lining of the mouth and throat is a common regimen-related toxicity following ablative HSCT regimens. It is usually not life-threatening but is very painful, and prevents eating and drinking. Mucositis is treated with pain medications plus intravenous infusions to prevent dehydration and malnutrition.Hemorrhagic cystitis
The mucosal lining of the bladder could also be involved in approximately 5 percent of the children undergoing HSCT. This causes hematuria, frequent urination, abdominal pain, and thrombocytopenia.Graft-versus-host disease
Graft-versus-host disease is an inflammatory disease that is unique to allogeneic transplantation. It is an attack by the "new" bone marrow's immune cells against the recipient's tissues. This can occur even if the donor and recipient are HLA-identical because the immune system can still recognize other differences between their tissues. It is aptly named graft-versus-host disease because bone marrow transplantation is the only transplant procedure in which the transplanted cells must accept the body rather than the body accepting the new cells.Acute graft-versus-host disease typically occurs in the first 3 months after transplantation and may involve the skin, intestine, or the liver. High-dose corticosteroids, such as prednisone, are a standard treatment; however, this immunosuppressive treatment often leads to deadly infections. Chronic graft-versus-host disease may also develop after allogeneic transplant. It is the major source of late treatment-related complications, although it less often results in death. In addition to inflammation, chronic graft-versus-host disease may lead to the development of fibrosis, or scar tissue, similar to scleroderma; it may cause functional disability and require prolonged immunosuppressive therapy.
Graft-versus-host disease is usually mediated by T cells, which react to foreign peptides presented on the MHC of the host.
Mesenchymal stromal cells might be useful in preventing and treating a GvHD. Therefore, Fisher et al. conducted a Cochrane review with randomised controlled trials in 2019 to measure the safety and efficacy of mesenchymal stromal cells for people suffering from a graft-versus-host disease after receiving a haematopoietic stem cell transplantation as treatment for their haematological disease. Fisher et al. included trials that used MSCs either for therapeutic or prophylactic reasons. The exact inclusion and exclusion criteria and information regarding the dose can be found in the original Cochrane review. In the therapeutic trials it was necessary that the study participants already suffered from a GvHD. Fisher et al. conducted one analysis: MSCs compared to control/ placebo for treating immune mediated inflammation post-transplantation and in autoimmunity: Mesenchymal stromal cells may reduce the all-cause mortality if they are used for a therapeutic reason. Moreover, the therapeutic use of MSCs may increase the complete response of acute and chronic GvHD, but the evidence is very uncertain. The evidence suggests that MSCs for prophylactic reason result in little to no difference in the all-cause mortality, in the relapse of malignant diseases and in the incidence of acute GvHD. The evidence suggests that MSCs for prophylactic reason reduce the incidence of chronic GvHD.
Graft-versus-tumor effect
or "graft versus leukemia" effect is the beneficial aspect of the graft-versus-host phenomenon. For example, HSCT patients with either acute, or in particular chronic, graft-versus-host disease after an allogeneic transplant tend to have a lower risk of cancer relapse. This is due to a therapeutic immune reaction of the grafted donor T lymphocytes against the diseased bone marrow of the recipient. This lower rate of relapse accounts for the increased success rate of allogeneic transplants, compared to transplants from identical twins, and indicates that allogeneic HSCT is a form of immunotherapy. GVT is the major benefit of transplants that do not employ the highest immunosuppressive regimens.Graft versus tumor is mainly beneficial in diseases with slow progress, e.g. chronic leukemia, low-grade lymphoma, and in some cases multiple myeloma. However, it is less effective in rapidly growing acute leukemias.
If cancer relapses after HSCT, another transplant can be performed, infusing the patient with a greater quantity of donor white blood cells.
Malignancies
Patients after HSCT are at a higher risk for oral carcinoma. Post-HSCT oral cancer may have more aggressive behavior with poorer prognosis, when compared to oral cancer in non-HSCT patients.A meta-analysis showed that the risk of secondary cancers such as bone cancer, head and neck cancers, and melanoma, with SIRs of 10.04, 6.35 and 3.52, respectively, was significantly increased after HSCT. So, diagnostic tests for these cancers should be included in the screening program of these patients for the prevention and early detection of these cancers..
Bleedings
Patients might suffer from a bleeding after receiving a stem cell transplantation. Therefore, Estcourt et al. conducted a Cochrane review with randomised controlled trials in 2012 to assess which use of platelet transfusions is the most effective one to prevent bleeding if people suffer from a haematological disorder and undergo a stem cell transplantation. A study participation was only possible if the patients did not have an active bleeding within the last 5 days and did not receive a previous platelet transfusion because of the chemotherapy or stem cell transplantation. The exact inclusion and exclusion criteria and details regarding the dose can be found in the original Cochrane review. Estcourt et al. conducted four analyses to answer their research question. In the first analysis they compared therapeutic/ non-prophylactic platelet transfusions to prophylactic platelet transfusions: The evidence suggests that therapeutic platelet transfusions result in little to no difference in the mortality secondary to bleeding. Furthermore, they may result in a slight reduction in the number of days on which a significant bleeding event occurred. The evidence suggests that therapeutic platelet transfusions result in a large increase in the number of patients with at least one significant bleeding event and they likely result in a large reduction in the number of platelet transfusions. In the second analysis, the review authors conducted a comparison of prophylactic platelet transfusions at threshold of 10.000 to a higher transfusion threshold : Prophylactic platelet transfusions at threshold of 10.000 may result in little to no difference in the mortality due to bleeding. These transfusions probably reduce the number of platelet transfusions per patient slightly. Prophylactic platelet transfusions at threshold of 10.000 probably increase the number of patients with at least one significant bleeding event and they likely result in a large increase in the number of days on which a significant bleeding event occurred. Prophylactic platelet transfusion with one dose schedules were compared to prophylactic platelet transfusions with another dose schedule in the third analysis: Prophylactic platelet transfusions at one dose schedule may result in little to no difference in the mortality secondary to bleeding if low dosage platelet transfusions are compared to standard dose platelet transfusions. Furthermore, the transfusions at one dose schedule probably result in little to no difference in the mortality secondary to bleeding if high dose platelet transfusions and standard dosage platelet transfusions are compared to each other. Prophylactic platelet transfusions with one dose schedule result in little to no difference in the number of participants with a significant bleeding event if low dosage platelet transfusions or high dosage platelet transfusions are compared to standard dose platelet transfusions. The last analysis was conducted to compare prophylactic platelet transfusions to platelet-poor plasma: The evidence is very uncertain about the effect of prophylactic platelet transfusion on mortality secondary to bleeding, the number of participants with a significant bleeding event and the number of platelet transfusions. Additionally, Estcourt et al. renewed the second analysis from their Cochrane review from 2012 in 2015 with randomised controlled trials and aimed to evaluate whether different platelet transfusion thresholds for the management of prophylactic platelet transfusions have an influence on the safety and efficacy for patients that suffer from a haematological disorder and receive a stem cell transplantation. Estcourt et al. conducted the following analysis: Prophylactic platelet transfusion at threshold of 10.000 compared to higher transfusion threshold : The evidence suggests that prophylactic platelet transfusions at threshold of 10.000 result in little to no different in the time to the first bleeding episode, the number of participants with WHO Grade 3 or 4 bleeding and clinically significant bleedings per participant. The evidence suggests that prophylactic platelet transfusions at threshold of 10.000 reduce the number of platelet transfusions per participants slightly. Moreover, the evidence suggests that these transfusions increase the number of participants with at least one significant bleeding event. Prophylactic platelet transfusions at threshold of 10.000 may result in a large increase in the mortality due to all causes. Apart from the time to the first bleeding, all endpoints are related to the first 30 days after the study entry.Prognosis
in HSCT varies widely dependent upon disease type, stage, stem cell source, HLA-matched status, and conditioning regimen. A transplant offers a chance for cure or long-term remission if the inherent complications of graft versus host disease, immunosuppressive treatments and the spectrum of opportunistic infections can be survived. In recent years, survival rates have been gradually improving across almost all populations and subpopulations receiving transplants.Mortality for allogeneic stem cell transplantation can be estimated using the prediction model created by Sorror et al., using the Hematopoietic Cell Transplantation-Specific Comorbidity Index. The HCT-CI was derived and validated by investigators at the Fred Hutchinson Cancer Research Center in the U.S. The HCT-CI modifies and adds to a well-validated comorbidity index, the Charlson Comorbidity Index The CCI was previously applied to patients undergoing allogeneic HCT, but appears to provide less survival prediction and discrimination than the HCT-CI scoring system.
Risks to donor
The risks of a complication depend on patient characteristics, health care providers and the apheresis procedure, and the colony-stimulating factor used. G-CSF drugs include filgrastim, and lenograstim.Drug risks
Filgrastim is typically dosed in the 10 microgram/kg level for 4–5 days during the harvesting of stem cells. The documented adverse effects of filgrastim include splenic rupture, acute respiratory distress syndrome, alveolar hemorrhage, and allergic reactions. In addition, platelet and hemoglobin levels dip post-procedure, not returning to normal until after one month.The question of whether geriatrics react the same as patients under 65 has not been sufficiently examined. Coagulation issues and inflammation of atherosclerotic plaques are known to occur as a result of G-CSF injection. G-CSF has also been described to induce genetic changes in agranulocytes of normal donors. There is no statistically significant evidence either for or against the hypothesis that myelodysplasia or acute myeloid leukaemia can be induced by G-CSF in susceptible individuals.
Access risks
Blood is drawn from a peripheral vein in a majority of patients, but a central line to the jugular, subclavian, and femoral veins may be used. Adverse reactions during apheresis were experienced in 20 percent of women and 8 percent of men, these adverse events primarily consisted of numbness/tingling, multiple line attempts, and nausea.Clinical observations
A study involving 2,408 donors indicated that bone pain as a result of filgrastim treatment is observed in 80 percent of donors. Donation is not recommended for those with a history of back pain. Other symptoms observed in more than 40 percent of donors include muscle pain, headache, fatigue, and difficulty sleeping. These symptoms all returned to baseline one month post-donation in the majority of patients.In one meta-study that incorporated data from 377 donors, 44 percent of patients reported having adverse side effects after peripheral blood HSCT. Side effects included pain prior to the collection procedure as a result of G-CSF injections, and post-procedural generalized skeletal pain, fatigue and reduced energy.
Severe reactions
A study that surveyed 2,408 donors found that serious adverse events occurred in 15 donors, although none of these events were fatal. Donors were not observed to have higher than normal rates of cancer with up to 4–8 years of follow up.One study based on a survey of medical teams covered approximately 24,000 peripheral blood HSCT cases between 1993 and 2005, and found a serious cardiovascular adverse reaction rate of about 1 in 1,500. This study reported a cardiovascular-related fatality risk within the first 30 days of HSCT of about 2 in 10,000.
History
, a French oncologist, performed the first European bone marrow transplant in November 1958 on five Yugoslavian nuclear workers whose own marrow had been damaged by irradiation caused by a criticality accident at the Vinča Nuclear Institute, but all of these transplants were rejected. Fortunately, the five treated were able to ultimately recover, perhaps in part due to the transplants. Mathé later pioneered the use of bone marrow transplants in the treatment of leukemia.Stem cell transplantation was pioneered using bone-marrow-derived stem cells by a team at the Fred Hutchinson Cancer Research Center from the 1950s through the 1970s led by E. Donnall Thomas, whose work was later recognized with a Nobel Prize in Physiology or Medicine. Thomas' work showed that bone marrow cells infused intravenously could repopulate the bone marrow and produce new blood cells. His work also reduced the likelihood of developing a life-threatening complication called graft-versus-host disease. Collaborating with University of Washington Professor Eloise Giblett, he discovered genetic markers that could confirm donor matches.
The first physician to perform a successful human bone marrow transplant on a disease other than cancer was Robert A. Good at the University of Minnesota in 1968.
In 1975, John Kersey, M.D., also of the University of Minnesota, performed the first successful bone marrow transplant to cure lymphoma. His patient, a 16-year-old-boy, is today the longest-living lymphoma transplant survivor.
Donor registration and recruitment
At the end of 2012, 20.2 million people had registered their willingness to be a bone marrow donor with one of the 67 registries from 49 countries participating in Bone Marrow Donors Worldwide. 17.9 million of these registered donors had been ABDR typed, allowing easy matching. A further 561,000 cord blood units had been received by one of 46 cord blood banks from 30 countries participating. The highest total number of bone marrow donors registered were those from the U.S., and the highest number per capita were those from Cyprus.Within the U.S., racial minority groups are the least likely to be registered and therefore the least likely to find a potentially life-saving match. In 1990, only six African-Americans were able to find a bone marrow match, and all six had common European genetic signatures.
Africans are more genetically diverse than people of European descent, which means that more registrations are needed to find a match. Bone marrow and cord blood banks exist in South Africa, and a new program is beginning in Nigeria. Many people belonging to different races are requested to donate as there is a shortage of donors in African, mixed race, Latino, aboriginal, and many other communities.
Two registries in the U.S. recruit unrelated allogeneic donors: NMDP or Be the Match, and the Gift of Life Marrow Registry.
Research
HIV
In 2007, a team of doctors in Berlin, Germany, including Gero Hütter, performed a stem cell transplant for leukemia patient Timothy Ray Brown, who was also HIV-positive. From 60 matching donors, they selected a CCR5|-Δ32 homozygous individual with two genetic copies of a rare variant of a cell surface receptor. This genetic trait confers resistance to HIV infection by blocking attachment of HIV to the cell. Roughly one in 1,000 people of European ancestry have this inherited mutation, but it is rarer in other populations. The transplant was repeated a year later after a leukemia relapse. Over three years after the initial transplant, and despite discontinuing antiretroviral therapy, researchers cannot detect HIV in the transplant recipient's blood or in various biopsies of his tissues. Levels of HIV-specific antibodies have also declined, leading to speculation that the patient may have been functionally cured of HIV. However, scientists emphasise that this is an unusual case. Potentially fatal transplant complications mean that the procedure could not be performed in others with HIV, even if sufficient numbers of suitable donors were found.In 2012, Daniel Kuritzkes reported results of two stem cell transplants in patients with HIV. They did not, however, use donors with the Δ32 deletion. After their transplant procedures, both were put on antiretroviral therapies, during which neither showed traces of HIV in their blood plasma and purified CD4+ T cells using a sensitive culture method. However, the virus was once again detected in both patients some time after the discontinuation of therapy.
In 2019, a British man became the second to be cleared of HIV after receiving a bone marrow transplant from a virus-resistant donor. This patient is being called "the London patient"