Introduction

 

Definition

Hematopoietic stem cell transplantation (HSCT) involves the intravenous infusion of autologous or allogeneic stem cells collected from bone marrow, peripheral blood, or umbilical cord blood to reestablish hematopoietic function in patients with damaged or defective bone marrow or immune systems. HSCT is used throughout this article as a general term covering transplantation of progenitor/stem cells from any source (eg, bone marrow, peripheral blood, cord blood).

Background

Although some investigators have traced the origin of HSCT to the end of the 19th century, when patients were given bone marrow orally as a treatment for hematologic disorders, a more realistic starting point is a 1939 report of a patient who received 18 mL of intravenous marrow from his brother as a treatment for aplastic anemia.

Significant advances in transplantation science did not occur until the 1950s. At that time, 2 major preclinical developments advanced the field. The first was the observation that mice could be protected from the lethal effects of whole-body irradiation by exteriorizing and shielding the spleen or by using intravenous marrow infusion. The second was the identification and unraveling of transplantation antigens (ie, the human leukocyte antigen [HLA] system in humans).

Further clinical advances, such as the understanding and development of chemotherapeutic agents, progress in blood banking and transfusion sciences, and the availability of new potent antibiotic agents, led to the first successful transplantations in the late 1960s. Few fields in medicine better illustrate the effective use of translational science into the clinical arena, a fact that culminated in the awarding of the Nobel Prize in medicine to Joseph E. Murray and E. Donnall Thomas for their discoveries concerning organ and cell transplantation in the treatment of human diseases.

HSCT was undoubtedly one of the most important medical advances in the second half of the 20th century. Worldwide, approximately 30,000-40,000 transplantations are performed yearly, and the number continues to increase by 10-20% each year. More than 20,000 people have now survived 5 years or longer after HSCT.

Diseases treated by HSCT

Interpretation of the results of trials of bone marrow transplantation is always complicated by the problem of patient selection. The efficacy of transplantation can be underestimated if only patients with the worst prognoses are studied, or it can be overestimated if only those with the best prognoses are studied. HSCT has led to the cure of diverse forms of cancer, bone marrow failure, hereditary metabolic disorders, and severe congenital immunodeficiencies that would otherwise have been fatal.

The indications for HSCT vary according to disease categories and are influenced by factors such as cytogenetic abnormalities, response to prior therapy, patient age and performance status, disease status (remission vs relapse), disease-specific prognostic factors, and, most importantly, availability of a suitable graft source.

Hematopoietic stem-cell transplantation is used to treat the following conditions:

• Autologous transplantation
  • Multiple myeloma
  • Non-Hodgkin lymphoma
  • Hodgkin disease
  • Acute myeloid leukemia
  • Neuroblastoma
  • Germ cell tumors
  • Autoimmune disorders – Systemic lupus erythematosus (SLE), systemic sclerosis
  • Amyloidosis
• Allogeneic transplantation
  • Acute myeloid leukemia
  • Acute lymphoblastic leukemia
  • Chronic myeloid leukemia
  • Chronic lymphocytic leukemia
  • Myeloproliferative disorders
  • Myelodysplastic syndromes
  • Multiple myeloma
  • Non-Hodgkin lymphoma
  • Hodgkin disease
  • Aplastic anemia
  • Pure red cell aplasia
  • Paroxysmal nocturnal hemoglobinuria
  • Fanconi anemia
  • Thalassemia major
  • Sickle cell anemia
  • Severe combined immunodeficiency (SCID)
  • Wiskott-Aldrich syndrome
  • Hemophagocytic lymphohistiocytosis (HLH)
  • Inborn errors of metabolism (eg, mucopolysaccharidosis,

    Gaucher disease, metachromatic leukodystrophies and adrenoleukodystrophies)

• The summary of indications for SCT in some specific disorders is as follows:
  • Acute myeloid leukemia: Allogeneic HSCT is the treatment of choice for all children with acute myeloid leukemia (AML) with HLA-matched sibling in their first complete remission (CR1), and in adults this is reserved for those with high-risk features in their CR1. In adults with standard or good risk features, SCT is reserved for their second complete remission (CR2).  HSCT is the only curative option for patients with primary refractory or relapsed AML.
  • Acute lymphoid leukemia: SCT indications in adults with acute lymphoid leukemia are similar to those for persons with AML. 
  • Chronic myeloid leukemia: Ever since imatinib was introduced for the treatment of chronic myeloid leukemia (CML), the practice of recommending transplantation to eligible patients with CML has been changed. Currently, all newly diagnosed chronic phase CML patients younger than 20 years with HLA-identical donor or MUD and those younger than 40 years with HLA-identical sibling donor should be considered for HSCT. HSCT should also be considered for those with high-risk features and imatinib failure.
  • Myelodysplastic syndrome: Allogeneic HSCT should be considered for patients with myelodysplastic syndrome who are younger than 60 years and who have an HLA-matched sibling donor.
  • Chronic lymphocytic leukemia: Autologous and allogeneic HSCT have been used with success in young patients with chronic lymphocytic leukemia.
  • Non-Hodgkin lymphoma: A combination of high-dose chemotherapy and autologous or allogeneic HSCT has produced complete remissions in patients with relapsed disease and in those who do not achieve complete remission with primary therapy.
  • Hodgkin lymphoma: High-dose chemotherapy and autologous HSCT are the treatments of choice for patients with poor prognoses and early relapse after initial chemotherapy or induction failure (ie, resistant disease), second relapse after conventional treatment for first relapse, or generalized systemic relapse after initial chemotherapy.
  • Multiple myeloma: Although autologous HSCT does not produce a cure, event-free survival rates and overall survival rates are prolonged approximately 1 year compared with survival rates achieved by chemotherapy. Autologous HSCT is associated with a much lower mortality rate than allogeneic HSCT. Trials from France have shown the advantage of double autologous HSCT (tandem transplants) over single transplantation.
  • Breast cancer: HSCT in breast canceracu patients is controversial.
  • Testicular cancer: The achievement of disease-free survival in a minority of patients with severe disease suggests much better results if performed earlier.
  • Thalassemia: An 80% disease-free survival rate is achieved after allogeneic HSCT.
  • Sickle cell anemia: Sickle cell anemia is potentially curable with allogeneic HSCT.
  • Genetic disorders:  Many genetic immunologic or hematopoietic disorders are potentially curable with allogeneic HSCT.

HSCT in patients with AIDS has not been shown to be very encouraging. However, transfecting the hematopoietic stem cells with retroviral vectors that cleave the RNA of the HIV virus is in the experimental stage.     

Patient education

For excellent patient education resources, visit eMedicine's Cancer and Tumors Center. Also, see eMedicine's patient education article Ovarian Cancer.

 

Graft Sources and Donor Selection

 

Graft sources

Hematopoietic stem cells can be collected for clinical use from several sources, including bone marrow, peripheral blood, umbilical cord blood, and, rarely, fetal liver. The donor sources include cells obtained from another person (sibling or unrelated donor), termed allogeneic transplant; an identical twin, termed syngeneic transplant; or the patient, termed autologous transplant.

Autologous transplantation

Generally, candidates for autologous transplantation have no demonstrable malignancy in the blood or bone marrow. Whereas treatment-related morbidity and mortality rates are lowest with autografts, the major problem is tumor relapse. This finding relates to the absence of a graft versus tumor effect (ie, immunologic attack on the tumor by immunocompetent T cells and natural killer cells in the donor graft) and the reinfusion of occult tumor in the graft.

Allogeneic transplantation

Patients older than 50 years experience higher transplant-related morbidity and mortality rates with allogeneic grafts; this effect relates to the need for continuing immunosuppression after the transplantation to prevent the development of graft versus host disease (GVHD). Allogeneic transplants are associated with lower relapse rates compared with autologous transplants because of the graft versus tumor effect.

 

Donor selection for HSCT

Transplant donors must be in generally good health without other comorbid conditions and in general should have the same qualification as a blood donor. The donor must have a performance status that permits safe collection of cells, be able to tolerate anesthesia (general or regional), and have adequate cardiac, pulmonary, hepatic, and renal function.

Pediatric donors are used only for autologous collection or donation to siblings. Donors with ongoing malignancies or a history of a malignant condition other than minor skin cancers (eg, basal cell carcinomas) are generally excluded from further consideration.

The following studies are routinely performed on hematopoietic cell donors:

 

• History and physical examination
• CBC count and platelet count
• Serum creatinine, electrolyte, and liver function studies
• Serologic studies for cytomegalovirus, herpes viruses, HIV RNA, anti-HIV antibodies, hepatitis B and C viruses (including HCV NAT), HTLV-I/II and VDRL (In autologous donations CMV and VDRL testing is not required.)
• ABO blood typing
• HLA typing
• Chest radiography
• Electrocardiography

Identical twin donors

Rarely, patients who are candidates for HSCT have an identical twin who can serve as a donor. These patients do not require posttransplantation immunosuppressive therapy and do not develop graft versus host disease (GVHD), although they are at a higher risk of relapse of the underlying malignant disease compared with similar HLA-matched but nonidentical sibling donors. The reason for this apparent disparity is uncertain but is presumably related to the ability of the donor lymphocytes to recognize the recipient tumor cells (ie, a graft vs leukemia reaction). Interestingly, survival rates are similar with the two types of transplantations because the increased frequency of leukemia relapse with identical twin donors is counterbalanced by lower treatment-related mortality rates.

Matched related donors

Related donors are usually siblings because they have the opportunity to inherit the same HLA genes located on chromosome 6. A given sibling has a 25% chance of being HLA-matched at the A, B, and DRB1 loci (6-antigen match, because each complex is inherited from each parent and expressed codominantly). ABO red cell antigens are not expressed on stem cells.

Although hemolysis, delayed erythropoietic engraftment, and pure RBC aplasia may complicate ABO-incompatible transplantation after either bone marrow transplantation or peripheral blood progenitor cell (PBPC) transplantation, these complications are uncommon; therefore, ABO incompatibility is not a barrier to successful nonmyeloablative transplantation. HLA-matched sibling donors are generally the preferred donor source for an allogeneic transplantation. Whether matched related donors will be anything other than siblings is unlikely unless the parents happen to have very common haplotypes or intermarrying among families has occurred such that first cousins might be fully HLA-matched.

Matched unrelated donors

In 1986, the National Marrow Donor Program was established as a repository for HLA-typing information so that unrelated donors and recipients could be matched. At present, more than 3 million donors are registered in the National Marrow Donor Program data bank, all of whom have undergone HLA-A and HLA-B serologic typing.

Because the donor and recipient are not related, serologic typing alone does not ensure that the individuals share the same HLA genes. This is evident clinically by the higher risk of GVHD in recipients of unrelated donor grafts. DNA-based techniques have been developed that permit molecular typing, which has demonstrated that only 55% of serologically identical donor and recipient pairs (ie, antigen-matched) are highly matched by molecular typing (ie, allele-matched). Patients who are truly highly matched appear to have better outcomes. As a result, most transplantation centers now require complete serologic and molecular matching at the class II region before using a donor for a given transplantation procedure.

Mismatched related donors

Although most centers require a complete match at the HLA-A, HLA-B, and HLA-DRB1 loci for an individual to be used as a transplant donor, some centers consider the use of single antigen–mismatched siblings. As expected, transplants from single antigen–mismatched siblings are associated with a higher risk of GVHD, although the overall survival rate may not differ from that observed with fully matched siblings.

Haploidentical donors

Other transplantation centers are exploring the use of donors who are only haploidentical and are therefore mismatched at all 3 loci. Although encouraging data have been obtained, this approach is being explored only at centers with expertise in this area. These grafts must be manipulated in vitro to reduce the number of immunocompetent T cells to lessen the likelihood and severity of GVHD.

One potential advantage of using haploidentical donors is that multiple individuals are usually available who could serve as potential donors within a given family, including parents, siblings, and children. Another advantage is an equal availability of donors for all ethnic and racial groups in contrast to matched unrelated donors. In haploidentical transplantations, mismatching of maternal antigens, rather than paternal antigens, seems to be better tolerated, presumably because of exposure to maternal HLA antigens during the prenatal and perinatal period.

Umbilical cord blood donors

Transplantation of umbilical cord blood was successfully performed for the first time in 1988 to treat a boy with Fanconi anemia; the donor, the boy's newborn sister, was a perfect HLA match for her brother. Relatively high numbers of hematopoietic stem cells with superior proliferative capacity compared with hematopoietic stem cells from marrow and blood in adults are present in umbilical cord blood collected at the time of delivery. These cells can be processed and cryopreserved in cord blood banks.

Owing to the relative immaturity of the immune system in cord samples, stem cells from this source allow the crossing of immunologic barriers that would otherwise be prohibitive. As a result, the degree of tolerable HLA disparity is much greater in cord blood transplants. A match of 3-4 out of the 6 HLA-A, HLA-B and HLA-DRB1 antigens is sufficient for transplantation. For the same reason, the degree and severity of GVHD are also low with cord blood transplants. 

The advantages of cord blood transplant include the fact that it is readily available, carries less risk of transmission of blood-borne infections, and is transplantable across HLA barriers with diminished risk of GVHD compared to similarly mismatched stem cells from peripheral blood or bone marrow.

A major limitation is the relatively small volume obtained from cord blood collections, which makes using this approach difficult for transplantation in adults as the small volume results in delayed engraftment and increased risk of infections and mortality. The median time to neutrophil recovery after cord blood transplantation is 4 weeks, in contrast to 8-12 days after peripheral blood progenitor cell transplantation. To overcome this, pooled or sequential cord blood transplantation is practiced at some centers and has shown encouraging results; surprisingly, by day 100 after transplantation, cells from one cord blood have predominated, as documented by chimerism analysis. The possibility of expanding the cord blood stem cells in vitro is an area of active investigation.

The AmericanAcademy of Pediatrics has recently released a document with an intention to provide information to guide physicians in responding to parents questions about cord blood donation and banking. Directed donation of cord blood should be encouraged when there is a specific diagnosis of a disease within a family known to be amenable to stem cell transplantation.

In summary, multiple options are currently available for selecting donor and stem cell sources. The clinical problem, overall health of the donor and recipient, infectious disease history, clinical approach of the transplantation center, and other factors are important when deciding which type of donor is selected. Most transplantation centers prefer an identical twin donor in those rare instances when one is available; in other patients, an HLA-matched sibling donor is preferred for allogeneic transplantation when one is available. If an HLA-matched sibling donor is not available, then searching for unrelated matched donors at the National Marrow Donor Program (NMDP) Web site is the next most suitable option. Cord blood is being increasingly used in the pediatric setting when a suitable donor is not available either due to the small family size or the patient status as an ethnic minority. (Ethnic minorities represent only a small proportion of the NMDP donors.)

 

Procurement of Stem Cells

Bone marrow

Bone marrow harvesting has become a relatively routine procedure. Bone marrow is generally aspirated from the posterior iliac crests while the donor is under either regional or general anesthesia. Additional bone marrow can be obtained from the anterior iliac crest; however, the amounts available are relatively limited, and the marrow from this site is generally used only for diagnostic purposes.

Because only a small percentage of the total bone marrow is removed during a harvesting procedure, peripheral blood leukocyte counts do not significantly decrease. However, bone marrow is a highly vascular organ, and resultant blood loss can be substantial following the harvest, depending on the volume removed. Guidelines established by the National Marrow Donor Program limit the volume of bone marrow removed to 15 mL/kg of donor weight. A dose of 1 X 10⁸ and 2 X 10⁸ marrow mononuclear cells per kilogram are required to establish engraftment in autologous and allogeneic marrow transplants respectively. Complications with bone marrow harvesting are rare and involve anesthetic, infectious, and bleeding problems.

Peripheral blood

Hematopoietic stem cells circulate in blood, albeit in very low concentrations and can be identified and quantified using flow cytometry (cells express the CD34 antigen). Administration of recombinant hematopoietic growth factors (ie, granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor) to patients or donors down-regulates the adhesion molecules on the CD34 cells and releases them into the peripheral blood, which can be collected by apheresis procedure after 4 days of granulocyte colony-stimulating factor (G-CSF) (Filgrastim or Lenograstim) administration.

This concept of mobilization of stem cells by cytokines has led to the widespread adoption of peripheral blood progenitor cell collection by apheresis for hematopoietic stem cell transplantation. The dose of G-CSF used for mobilization is 10 mcg/kg/d. But in autologous donors who are heavily pretreated, doses of up to 40 mcg/kg/d can be given. Clinical trials have shown that mobilization with G-CSF is better compared to granulocyte-macrophage colony-stimulating factor (GM-CSF).

G-CSF is a cytokine and commonly leads to side effects like bone pains, malaise, headaches, chills, and sometimes fever. Filgrastim induces a hypercoagulable state, and, rarely, it could cause vascular thrombosis. G-CSF could exacerbate autoimmune disease, and some cases of ophthalmologic events have been reported in healthy donors. However, so far there has been no evidence in favor of development of myelodysplasia or hematologic malignancies following G-CSF administration. GM-CSF leads to the same somatic complaints as G-CSF; in addition, it can cause abnormal findings on liver function tests (LFT), fluid retention, serositis, and  “first dose reaction,” characterized by hypoxia and hypotension within 3 hours of administration.

Apheresis instruments, similar to those used for collecting platelet concentrates from volunteer donors for transplantation, are used in an ambulatory setting. Using this technique, during a 3- to 4-hour period, approximately 1 log more hematopoietic progenitors can be collected than in a bone marrow harvest. Furthermore, the need for general anesthesia and a sterile operating room is eliminated. This process, known as mobilization of stem cells, may be enhanced in patients destined for autografts in whom exposure to cytotoxic drugs (eg, cyclophosphamide) plus hematopoietic growth factor therapy causes the release of 10 times as many (or more) primitive hematopoietic progenitors into the circulation than the recombinant cytokines alone.

The dose required for engraftment is 1-2 X 10⁶ CD34⁺ cells/kg body weight for both autologous and allogeneic transplants. Higher doses would result in better engraftment, but doses in the range of 8 X 10⁶ are associated with increased risk of extensive GVHD. The use of peripheral blood rather than bone marrow as a source of hematopoietic stem cells results in the collection of more CD34⁺ progenitor cells and faster marrow recovery (8-10 days for neutrophil and 10-12 days for platelet recovery).

Stem cells can be collected in far larger amounts from apheresis collections, and peripheral blood stem cell transplantation (PBSCT) is associated with higher graft versus tumor (GVT) or graft versus leukemia (GVL) effect and results in decreased relapse rates. This is also associated with rapid engraftment (as higher number of committed progenitor cells are collected), which translates into decreased mortality and early hospital discharges. However, the disadvantage with PBSCT is the increased incidence of GVHD due to higher T-cell load. Bone marrow transplantation is considered to be superior to PBSCT for nonmalignant conditions (hemoglobinopathies), where rapid engraftment is not crucial and GVT effect is not required.

 

Manipulation of Stem Cell Grafts

These techniques often require sophisticated laboratories and highly trained technical personnel, and often they are quite expensive.

ABO-incompatible allogeneic transplants

Removal of isoagglutinins or RBCs from the donor graft prevents hemolysis in the recipient.

T-cell depletion in the allogeneic transplantation setting

Immunocompetent donor T cells may be removed using a variety of methods to reduce or eliminate the possibility for the development of GVHD. Although such a strategy is often effective in lowering the morbidity and mortality associated with GVHD, removal of these accessory cells may be associated with an increase in engraftment failure in up to 10% of transplantations (loss of T-helper cells, which facilitate engraftment). Furthermore, T-cell depletion results in higher relapse rates compared with T-cell–replete grafts because the removal of cytotoxic T cells eliminates the potential for a graft versus malignancy effect.

In vitro purging in autografts

Tumor cells can be detected by using sophisticated means such as tumor clonogenic assays, flow cytometry, or polymerase chain reaction. Tumor cell removal methods include chemical and immunologic methods, positive selection of CD34⁺, and negative selection.  

With regard to chemical and immunologic methods, 4-hydroperoxycyclophosphamide or mafosfamide (cyclophosphamide derivatives) chemotherapy in vitro kills residual autologous tumor cells; normal stem cells are injured, resulting in slower or incomplete engraftment. For positive selection of CD34⁺ cells, commercial instruments can be used to remove the desired cells by using solid-phase anti-CD34 monoclonal antibodies. With negative selection, anticancer monoclonal antibodies can be used to remove tumor cells, leaving stem cells in the graft.

 

Preparative Regimens for Hematopoietic Stem Cell Transplantation

The preparative or conditioning regimen is a critical element in HSCT. The purpose of the preparative regimen is to provide immunosuppression sufficient to prevent rejection of the transplanted graft and to eradicate the disease for which the transplantation is being performed. These goals have traditionally been achieved by delivering maximally tolerated doses of multiple chemotherapeutic agents with nonoverlapping toxicities, with or without radiation.

Recently, several novel approaches have been evaluated in an attempt to minimize toxicity. For example, nonmyeloablative preparative regimens have been used to induce a state of mixed chimerism (defined as the concurrent presence of donor and recipient hematopoietic cells); this can be followed by cellular therapy via the administration of donor lymphocyte infusions. Another alternative is targeted therapy in the form of radiolabeled monoclonal antibodies.

Prior to HSCT, patients are usually given extremely high doses of chemotherapy (with or without radiation) to eliminate the malignancy. Infusion of hematopoietic cells (eg, autologous or allogeneic HSCT) circumvents the problem of prolonged myelosuppression, permitting escalation to considerably higher dose levels. However, marrow recovery still takes weeks and requires sophisticated supportive care until the effects of such therapy have lessened.

In addition to myelotoxicity, common toxicities of preparative regimens include mucositis, nausea, vomiting, alopecia, diarrhea, rash, peripheral neuropathies, and pulmonary and hepatic toxicity. Infertility is almost universal when using myeloablative regimens. This problem can be addressed with sperm cryopreservation for male patients, assuming they have adequate sperm number and function. Oocyte cryopreservation in female patients has generally been unsuccessful. Long-term complications following total body irradiation (TBI) include asymptomatic alterations in pulmonary function, cataracts, sicca syndrome, hypothyroidism, and thyroiditis.

Myeloablative preparative regimens

Myeloablative regimens can be classified as radiation- or non–radiation-containing regimens. These regimens were developed by escalating the dose of either radiation or a particular drug to the maximally tolerated dose. Drugs with nonoverlapping toxicities were used in an effort to avoid synergistic injury to a particular organ.

Radiation-containing regimens

TBI has been the mainstay of preparative regimens since the inception of HSCT. TBI-based regimens typically fractionate the radiation and administer the total dose over several days (termed fractionated TBI [FTBI]), which helps decrease toxicity and increase tolerability. Partial lung shielding is included in an effort to reduce the potential for irreversible lung injury. The maximally tolerated dose of TBI is approximately 1500 cGy. Higher doses produce excessive nonhematologic toxicity, primarily to the lungs, but also to other organs including the heart.

Commonly used radiation-containing preparative regimens are as follows:

• FTBI at 1200 cGy and cyclophosphamide at 120 mg/kg
• FTBI at 1320 cGy and etoposide at 60 mg/kg
• FTBI at 1320 cGy, etoposide at 60 mg/kg, and cyclophosphamide at 60 mg/kg
• FTBI at 1200 cGy and melphalan at 200 mg/kg
• FTBI at 1200 cGy, etoposide at 60 mg/kg, and cyclophosphamide (for autologous) at 100 mg/kg

Non–radiation-containing preparative regimens

Regimens have been developed in which TBI is replaced with additional chemotherapeutic agents. These approaches have primarily been developed for autologous transplantation, but they have also been used in the allogeneic setting. The primary advantage of regimens that lack TBI is reduced toxicity. Additionally, the cost is lower, the regimen is easier to administer, and radiation can still be given to sites of prior disease following transplantation.

Commonly used non–radiation-containing preparative regimens are as follows:

• Busulfan at 16 mg/kg and cyclophosphamide at 120 mg/kg
• Busulfan at 16 mg/m² and etoposide at 60 mg/m²
• Cyclophosphamide at 6-7.2 g/m², carmustine at 300-500 mg/m², and etoposide at 600-2400 mg/m²
• Cyclophosphamide at 7.2 g/m², carmustine at 600 mg/m², etoposide at 1200 mg/m², and cisplatin at 150 mg/m²
• Carmustine at 300 mg/m², etoposide at 400-800 mg/m², cytarabine at 800-1600 mg/m², and melphalan at 140 mg/m²

Multiple cycles of high-dose chemotherapy

With the introduction of mobilized peripheral blood progenitor cell transplantation (PBPC) collection techniques, collection of doses of stem cells that far exceed the thresholds required for engraftment can be readily achieved. This permits an approach consisting of multiple cycles of high-dose chemotherapy followed by the infusion of PBPCs after each cycle in an attempt to increase the intensity of anticancer therapy beyond that achievable with a standard autologous transplantation. Each cycle is at or near myeloablative levels of chemotherapy, followed by rescue with previously collected PBPCs. At present, little evidence indicates that this approach increases the long-term disease-free survival rate over that achieved with standard transplantation.

Radiolabeled monoclonal antibodies

The maximally tolerated dose of TBI is approximately 1500 cGy. Randomized trials comparing 1220- and 1575-cGy doses found that the higher dose was associated with a lower relapse rate but no improvement in overall survival rates because of a higher rate of complications. These observations suggest that improvements in disease-free survival rates might be attained if the radiation dose could be increased without excessive toxicity.

One approach to achieving this goal has been the administration of monoclonal antibodies radiolabeled with high-energy–emitting radioisotopes. This would permit targeting of the radiation dose to the tumor cells and marrow with a potential reduction in dose to other organs, such as the liver, lungs, and kidneys. One such monoclonal antibody is directed against CD45, which is highly expressed in hematopoietic cells, thereby allowing targeting to the marrow space.

Nonmyeloablative preparative regimens

For patients with leukemia, an important contributing factor is a graft versus tumor effect mediated by the donor cells. This effect requires the engraftment of donor-type immunocompetent cells, which does not necessarily require a toxic myeloablative preparative regimen. As a result, the possibility of achieving mixed chimerism, defined as the concurrent presence of donor and recipient hematopoietic cells, using nonmyeloablative regimens (ie, minitransplants) has begun to be explored.

This approach, which relies more on donor cellular immune effects and less on the cytotoxic effects of the preparative regimen to control the underlying disease, has been successfully applied in murine, canine, and porcine models of transplantation. In humans, it may permit transplantation in older, high-risk, and/or heavily pretreated patients.

This approach permits transplantation in older patients, patients at high risk due to comorbidities, and heavily pretreated patients. The major use of minitransplants is in treating patients with immunologically responsive disorders like myeloma. The advantage with this kind of transplantation is that patients not engrafting still have an autologous recovery due to nonmyeloablative preparatory regimen. However, most nonmyeloablative transplants require donor lymphocyte infusions (DLI) for maximum GVT effect with attendant risk of increased GVHD.

 

Infusion of Stem Cells and Engraftment

Infusion of stem cells

The infusion of either bone marrow or PBPCs is a relatively simple process that is performed at the bedside. The bone marrow product is generally used fresh and is infused through a central vein over a period of several hours. Autologous products are almost always cryopreserved. They are thawed at the bedside and infused rapidly over a period of several minutes. The hematopoietic stem cells engraft within the bone marrow cavity by hominglike mechanisms that have not yet been fully elucidated. Vascular cell adhesion molecule-1, heparan sulphate, and stromal cell–derived factor-1 and its receptor (CXCR4) appear to play roles in this process.

Minimal toxicity has been observed in most cases. ABO-mismatched bone marrow infusions occasionally could lead to hemolytic reactions. Dimethylsulfoxide (DMSO) used for cryopreservation of stem cells may give rise to facial flushing, tickling sensation in the throat, and strong taste in the mouth (the taste of garlic). Rarely, it could cause bradycardia, abdominal pain, encephalopathy/ seizures, and renal failure. To avoid the risk of encephalopathy, which occurs with doses above 2 g/kg/d of DMSO, stem cell infusions exceeding 500 mL are infused over 2 days and the rate of infusion is limited to 20 mL/min.

 

Complications and Specific Therapies

Mucositis

Mucositis is the most common short-term complication of myeloablative preparatory regimens (common with etoposide-containing regimens) and methotrexate used to prevent GVHD. Oropharyngeal mucositis is a painful condition and may require intubation when it involves the supraglottic area. Intestinal mucositis results in nausea, abdominal cramping, and diarrhea and requires total parenteral nutrition (TPN) to maintain caloric requirements.

 

Topical pain medications and systemic opioids are required to manage pain adequately. A recombinant human keratinocyte growth factor, palifermin reduces the incidence of mucositis after autologous transplant. Amifostine reduces the risk of mucositis following radiation and alkylating agents.

 

Prolonged and severe pancytopenia

Severe (<500 per microliter but often <100 per microliter) and prolonged (up to 4 wk) neutropenia is common after transplantation and invariably requires the use of empiric broad-spectrum antimicrobials until recovery of the neutrophils.

Empiric antifungal therapy with amphotericin B, fluconazole, or other agents is often administered if unexplained fever persists despite the use of broad-spectrum antibacterials. Antiviral therapy is usually given as prophylaxis (acyclovir for autografts; ganciclovir for allografts). Serious infections (eg, pneumonia, bacteremia, fungemia, viremia) may occur in up to 50% of patients after transplantation, more frequently with matched unrelated than with autografts and sibling-matched allografts, and are the major contributors to the mortality associated with these procedures. Recombinant hematopoietic growth factors (eg, filgrastim 5-10 mcg/kg/d SC, sargramostim 250 mcg/m2/d SC) started 24-72 hours after stem cell infusion reduce the time to blood neutrophil recovery.

Severe thrombocytopenia requires prophylactic transfusions for platelet counts less than 10,000/µL, but for bleeding episodes or surgical procedures, the target may exceed 50,000/µL. Bleeding may occur despite platelet transfusions as a result of visceral organ injury from chemotherapy (eg, gastritis, pneumonitis), acute GVHD or infection (eg, adenovirus-induced hematuria due to cystitis). Severe anemia requires frequent RBC transfusions; recombinant erythropoietin (30,000-60,000 U/wk SC) is sometimes used after stem cell infusion to enhance erythroid recovery.

Infections

Many factors predispose to the development of infections in a transplant patient. Damage to the mucosal surfaces and skin from preparatory regimens and catheters, neutropenia and immunodeficiency from immunosuppressive medication, GVHD and T-cell depletion of the graft all contribute to this.

 

Early after transplantation (0-30 d) mucosal and skin injury and neutropenia contribute to infections with aerobic bacteria (particularly coagulase-negative Staphylococcus species and Viridans streptococci, gram-negative bacilli), Candida species and herpes simplex. Colonizing yeasts invade the mucosa and cause systemic mycotic infections in 10-15% of patients.

One to 3 months after transplantation, T-cell dysfunction, hypogammaglobulinemia, and acute GVHD predispose to infections with encapsulated bacteria (pneumococcus, Haemophilus influenzae), viruses (eg, cytomegalovirus [CMV]), and Pneumocystis carinii, molds (eg, Aspergillus, Zygomycetes), and Candida species.

Between 3 and 12 months after transplantation, slow T-cell reconstitution and chronic GVHD predispose to infections with encapsulated bacteria, CMV, Pneumocystis carinii infection, and herpes zoster virus.

CMV, Epstein-Barr virus (EBV), and hepatitis viruses are particularly important. CMV-positive patients carry a higher peritransplant mortality rate than do CMV-negative patients. Many patients, even though they have received a CMV-negative graft, become seropositive with time. Posttransplant patients are monitored for CMV infection and are treated both prophylactically and preemptively to prevent pneumonitis and CMV viremia. However, CMV infections still occur late posttransplant and carry high mortality. EBV causes posttransplant lymphoproliferative disorder (PTLD) in HSCT patients and is more common in patients receiving T-cell depleted grafts and intensive treatment for GVHD.

Viral hepatitis is the third most common cause of liver disease in transplant patients. (Liver GVHD and drug-induced hepatotoxicity are the first and second most common causes of liver disease in HSCT patients.) In patients with prior exposure to hepatitis B virus (HBV), the impaired cellular immunity in the first 3-6 months after transplantation can cause reactivation of latent virus and lead to fulminant hepatic failure. HBsAg positive patients should begin prophylactic antiviral therapy with nucleoside analogues before chemotherapy and continue for at least 3 months after chemotherapy. Hepatitis B surface antigen (HBsAg)negative patients should receive HBV vaccination before HSCT.

Unlike HBV, infection with hepatitis C virus (HCV) appears to have little impact on the short-term survival after HSCT. However, in the long-term, it is a risk factor for hepatic venoocclusive disease and GVHD. In the long-term survivors with HCV, fibrosis progresses rapidly in the presence of HSCT and leads to cirrhosis, decompensation, and malignancy; liver-related mortality is the third leading cause of late deaths after transplant. Therefore selected long-term survivors who have been off immunosuppression for more than 6 months and who have no evidence of GVHD should be considered for therapy with pegylated interferon and ribavirin.

Infection prophylaxis for HSCT patients

All patients are kept in high-efficiency particulate air (HEPA)-filtered, positive-air-pressure sealed rooms, and strict hand hygiene is practiced. Patients who received autograft are usually managed on an outpatient basis as they have a brief period of neutropenia and fungal infections. Most patients receive antibacterial prophylaxis with fluoroquinolone; antifungal prophylaxis is given with fluconazole or amphotericin B or voriconazole until day 75-100, posttransplant.

Herpes simplex-positive patients receive acyclovir prophylaxis (5 mg/kg IV q12h). CMV seronegative patients receive immunosuppression with ganciclovir and intravenous immunoglobulin (CytoGam) and CMV-negative blood products. All patients should receive Pneumocystis prophylaxis with trimethoprim/sulfamethoxazole double strength tablet twice weekly or pentamidine 300 mg once monthly for 1 year posttransplant. Patients with GVHD on immunosuppression should be on prophylaxis for Pneumocystis carinii and fungal infections for 1 month after discontinuation of immunosuppression; they should also receive prophylaxis with penicillin, erythromycin, or extended spectrum fluoroquinolones for pneumococcal bacteremia.

Patients with documented hypogammaglobulinemia receive intravenous immunoglobulin. Patients with acute GVHD should receive gut decontamination with metronidazole or fluoroquinolones. As mentioned above, all HBsAg-negative patients should receive HBV vaccine before HSCT.

Graft versus host disease

GVHD occurs when immunocompetent T cells and natural killer cells in the donor graft recognize host antigens as foreign targets and mediate a reaction. GVHD occurs very frequently in the allograft setting. On the other hand, by several mechanisms, GVHD rarely occurs in autologous and syngeneic settings. The disease may cause significant morbidity and mortality and has been divided into acute and chronic forms.

Acute GVHD

Acute GVHD involves skin, mucosal surfaces, gut, and liver. It starts as a erythematous macular skin rash, and as it progresses, blistering of the skin similar to severe burns, severe abdominal pain, profound diarrhea, and hyperbilirubinemia develop. Acute GVHD is graded as per Glucksberg criteria. Stage I disease is confined to skin and is mild; stage II-IV have systemic involvement. Stage III and IV acute GVHD carry grave prognosis.

Risk factors for acute GVHD include HLA-mismatched graft, MUD (matched unrelated donor) graft, graft from a parous female donor, and advanced patient age. Pathogenesis of acute GVHD is considered to be a cytokine storm. Cytokines tumor-necrosis factor (TNF) and interleukin (IL)-1 released due to damage to host tissues due to preparative regimen provoke increased major histocompatability complex (MHC) expression and amplify the recognition of minor HLA differences by the donor T cells, which proliferate and release cytokines. These cytokines recruit more T cells and macrophages, which in turn release TNF and IL-1 setting up a vicious circle of inflammation and tissue damage with the clinical manifestations of acute GVHD.

Prophylaxis of GVHD is more successful than treatment. This can be achieved either by T-cell depletion of the graft or by using immunosuppressive agents against donor cytotoxic lymphocytes. T-cell depletion results in significant reduction in GVHD but is accompanied by increased risk of engraftment failure and rate of relapse due to the loss of GVT effect. A common regimen used to prevent acute GVHD consists of cyclosporine or tacrolimus along with a few days of methotrexate. Both cyclosporine and tacrolimus are associated with renal toxicity and methotrexate with severe mucositis. Sirolimus (Rapamycin) and mycophenolate mofetil are alternatives with lower toxicity. Other measures to decrease acute GVHD include gut decontamination with metronidazole, intravenous immunoglobulin, and use of a less intense preparative regimen.

Treatment of acute GVHD remains disappointing and consists of high-dose steroids and antithymocyte globulin (ATG). Experimental therapies with monoclonal antibodies to TNF (infliximab) or IL-2, extracorporeal photopheresis using apheresis machines, and drugs like pentostatin are being offered to the patients. Mortality rate is increased in patients who do not respond to treatment due to the increased risk of infection (particularly invasive fungal infections) and chronic GVHD.

Chronic GVHD

Approximately 40-80% of long-term survivors of HCT experience this complication. The incidence of chronic GVHD is increasing as increasing number of transplants are being done in older patients. Other risk factors include peripheral blood stem cell transplants, mismatched or unrelated donors, second transplant, and donor leukocyte infusions (DLIs). The greatest risk for chronic GVHD is acute GVHD.

Chronic GVHD develops 2-12 months after HSCT and involves skin, eyes, mouth, liver, fascia, and almost any organ in the body. Patients with chronic GVHD present with chronic lichenoid skin changes, dryness of the eyes and mouth, and lichenoid skin changes in the oral mucosa, with ulceration and oral pain. Impaired range of motion occurs from fibrosis of the dermis and fascia. Hyperbilirubinemia and elevated alkaline phosphatase can occur. Although the clinical presentation of chronic GVHD mostly resembles scleroderma, it can mimic any other autoimmune disease.

The pathogenesis of chronic GVHD is not well studied, as most of the patients with this complication are at home and a good animal model for chronic GVHD is lacking. Some believe that it represents the consequence of an old acute GVHD, while others believe that it results from dysfunctional immunologic recovery after transplant.

Chronic GVHD is treated with steroids alone or in combination with cyclosporine. Other therapies under evaluation include extracorporeal phototherapy, pentostatin, acitretin, psoralen plus ultraviolet A (PUVA) therapy, thalidomide, and TBI.

A major cause of death in chronic GVHD is infection from profound immunodeficiency associated with the disease. All patients require prophylaxis against encapsulated organisms and patients with frequent infections and low immunoglobulin levels should receive intravenous immunoglobulin replacement.

 

Graft failure

Primary graft failure results from failure to establish hematologic engraftment after transplantation, and late graft failure results from loss of an established graft. Graft failure is associated with increased risk of infection and peritransplant mortality.

It occurs in approximately 1-5% of sibling-matched allografts versus 10-15% of matched unrelated donor grafts. The greater the degree of HLA mismatch, the greater the risk of graft rejection. Other risk factors include aplastic anemia diagnosis, T-cell depletion of the donor graft (loss of helper T cells, which help in engraftment), infusion of lower number of hematopoietic stem cells (as in cord blood transplants), nonmyeloablative transplants, GVHD, splenomegaly, and use of methotrexate, mycophenolate mofetil, antithymocyte globulin, and ganciclovir.

Poor graft function after transplant can be improved by growth factors like G-CSF, GM-CSF, and erythropoietin. In case of graft failure, a second stem cell infusion can be useful.

Pulmonary complications (interstitial pneumonitis)

In patients receiving allografts, interstitial pneumonitis is frequently a fatal syndrome often caused by viral infections (eg, cytomegalovirus infection) and is characterized by fever, infiltrates, hypoxemia, and acute respiratory distress syndrome. However, the prevalence has been lowered, owing to the use of anti-infective prophylaxis and selection of CMV negative blood products (either leukoreduced blood or blood drawn from  CMV-seronegative donors). Treatment with ganciclovir or foscarnet plus intravenous immunoglobulin is often effective. A diffuse alveolar hemorrhage is sometimes observed in the autograft setting. Lung injury can also be due to TBI or pulmonary toxins (carmustine/ methotrexate).

Hepatic venoocclusive disease

Hepatic venoocclusive disease, more accurately termed sinusoidal obstruction syndrome, is one of the very common and potentially lethal complication after SCT. It occurs in 10-60% of patients receiving SCT and accounts for 50% of deaths after SCT. Clinically, it is characterized by weight gain and fluid retention, tender hepatomegaly, jaundice, and ascites and can progress to fulminant hepatic failure, respiratory failure, and renal failure. It usually starts 8-10 days after starting the preparatory regimen. Risk factors include prior hepatocellular damage, high levels of busulphan, TBI of more than 10-12 Gy, heavy pretreatment prior to SCT, and presence of C282Y allele of hemochromatosis gene.

Pathologically, it is characterized by damage to the sinusoidal endothelium, which sloughs off and causes obstruction to the hepatic circulation leading to centrilobular hepatic injury and portal hypertension. Perivenular fibrosis and cholestasis also occur. Elevated levels of TNF-alpha precede the development of venoocclusive disease.

Prevention is the best approach to venoocclusive disease. Substitution of fludarabine for cyclophosphamide and use of nonmyeloablative regimens decrease the incidence of venoocclusive disease. Ursodiol (ursodeoxycholic acid) given in doses of 12 mg/kg in 2 divided doses starting a day before the preparatory regimen is shown to reduce the risk of both venoocclusive disease and grade III and IV GVHD.

Treatment is largely supportive. Recombinant tissue plasminogen activator (tPA) and heparin use has been associated with significant risk of hemorrhage. Defibrotide, a polydeoxyribonucleotide, has been used in clinical trials with complete resolution of venoocclusive disease in 36% cases with no significant hemorrhage. It has antithrombotic and fibrinolytic activities.

Late-onset problems

Patients undergoing HSCT have an increased risk of malignancy, most often occurring many years following the transplantation procedure. Secondary acute leukemias, solid tumors, and myelodysplastic syndromes have been described. These conditions are disease- and regimen-dependent; onset is months or years after transplantation, with increased prevalence after TBI.

Late-onset infections can occur months after transplantation. These infections usually occur after allograft procedures in association with GVHD or GVHD therapy. Occasionally, they occur in autograft procedures after posttransplantation immunotherapy. Vaccinations are strongly recommended (ie, pneumococcus, Haemophilus influenzae b, hepatitis B, poliovirus, diphtheria/tetanus, influenza).

 

Future Directions

• Improved patient selection: Patient selection can be improved by identifying and treating malignancies at high risk for recurrence using newer prognostic guides (eg, International Age-Adjusted Index in non-Hodgkin lymphoma, International Performance Scoring in myelodysplastic syndrome, Hasenclever-Diehl Classification in Hodgkin disease, poor-risk cytogenetics in leukemia) and by excluding high-risk patients or those unlikely to benefit from transplantation.
• Improved preparative regimens: Improving preparative regimens would reduce treatment-related morbidity, mortality, and relapse rates. This can be achieved by incorporating selective agents (monoclonal antibodies such as rituximab [anti-B cell], Campath-1H [anti-CD52], antihematopoietic tissue, and gemtuzumab ozogamicin anti-CD33 [anti-AML]).
• Incorporation of new-generation hematopoietic growth factors: One agent may include FLT3-ligand thrombopoietin. The new-generation hematopoietic growth factors can be used to enhance the collection of hematopoietic stem cells.
• Improved GVHD prevention and therapy: Agents such as monoclonal antibodies and blocking peptides and procedures such as suicide gene insertion may help eliminate GVHD effector cells. These strategies are on the horizon.
• Ex vivo expansion of hematopoietic progenitor cells for enhanced hematopoietic recovery: This concept is being investigated. Cytokine- and stromal cell–mediated expansion are current examples.
• Embryonic stem cells may become a source of hematopoietic stem cells; bioengineering of embryonic stem cells might eliminate the need for HLA typing and search for a HLA-matched donor and even the necessity for a preparatory regimen.
• Gene therapy research involving modification of autologous cord blood stem cells for the treatment of childhood genetic disorders, which is at an experimental stage at present, may prove to be of value in future.
• The pluripotent stem cells of the cord blood have been shown to have the potential to differentiate into cardiac, neurologic, pancreatic, and skin tissues in vitro. Extensive research is taking place to explore the potential therapeutic benefit of cord blood in various conditions.

Keywords

HSCT, stem cell transplantation, SCT, bone marrow transplantation, BMT, peripheral blood progenitor cell transplantation, PBPCT, fractionated total body irradiation, FTBI, allogenic transplant, syngeneic transplant, autologous transplant, nonmyeloablative transplants, non-myeloablative transplants, minitransplants, mini-transplants, matched unrelated donor, MUD, graft-versus-host disease, GVHD, National Marrow Donor Program, NMDP, veno-occlusive disease, venoocclusive disease, VOD, graft-versus-tumor effect, graft-versus-leukemia effect, GVT effect, GVL effect