Stem Cell Therapy

Methods of Harvesting and Storing Stem Cells

• Bone marrow originally represented the only source of hematopoietic progenitor cells for transplantation.
• Peripheral blood hematopoietic stem cells are widely utilized and are harvested after being mobilized into the peripheral circulation by cytokines alone (such as granulocyte colony-stimulating factor), a combination of cytokines and cytotoxic agents, or a CXCR4 antagonist.
• Umbilical cord blood serves as an additional source of hematopoietic progenitors, offering the advantage of immediate availability of cells that are cryopreserved and stored in banks.

Principles of Stem Cell Therapy

Types of Hematopoietic Stem Cell Transplantation

• Hematopoietic stem cell transplantation (HSCT) utilizes either allogeneic (donor-derived) or autologous (patient-derived) stem cells to cure both malignant and nonmalignant disorders.
• Autologous transplantation serves as a rescue strategy, administered after delivering otherwise lethal doses of chemotherapy, with or without radiotherapy, to treat malignancies such as relapsed lymphoma or selected solid tumors.
• Allogeneic transplantation is utilized to treat children with hematologic malignancies (e.g., leukemia, myelodysplastic syndromes) as well as genetic diseases of blood cells, inherited metabolic diseases, and bone marrow failure.

Sources of Hematopoietic Stem Cells

• Bone marrow originally represented the sole source of hematopoietic progenitor cells used for transplantation.
• Peripheral blood hematopoietic stem cells are now widely utilized and are harvested after being mobilized into the peripheral circulation.
• Umbilical cord blood serves as an additional, immediately available source of hematopoietic progenitors.

Preparative (Conditioning) Regimens

• Protocols for allogeneic HSCT begin with a preparative conditioning regimen that utilizes chemotherapy, sometimes combined with irradiation.
• The primary principle of the preparative regimen is to eliminate the patient's existing hematopoietic system and suppress the recipient's immune system, specifically T cells, to prevent graft rejection.
• In patients with malignancies, the conditioning regimen additionally serves to significantly reduce the overall tumor burden before the intravenous infusion of donor hematopoietic cells.
• Reduced-intensity conditioning regimens are frequently employed in pediatric patients; these regimens are primarily immunosuppressive and aim to induce a state of reduced immune competence in the recipient to avoid rejecting the donor cells, without being fully myeloablative.

Immunologic Principles and the Graft-Versus-Leukemia Effect

• The immunology of HSCT is unique among transplants because the infused graft contains not only stem cells but also mature blood cells of donor origin, including T cells, B cells, natural killer cells, and dendritic cells.
• These mature donor cells repopulate the recipient’s lymphohematopoietic system and establish a new immune system.
• The new immune system provides a critical graft-versus-leukemia (GVL) effect, where immunocompetent donor cells help eliminate residual leukemia cells that survived the conditioning regimen.
• The GVL effect is primarily T-cell–mediated, utilizing alloreactions directed against histocompatibility antigens displayed on the recipient's leukemia cells.
• Because some of these histocompatibility antigens are also present on normal recipient tissues (particularly the skin, gastrointestinal tract, and liver), alloreactive donor immune cells may attack these tissues, causing acute or chronic graft-versus-host disease (GVHD),.

Histocompatibility and Donor Selection

• The success of allogeneic HSCT relies heavily on minimizing the diversity between the donor and recipient in major and minor histocompatibility antigens.
• Human leukocyte antigens (HLA) must be matched, including major histocompatibility complex (MHC) class I molecules (HLA-A, HLA-B, HLA-C) that present peptides to CD8+ T cells, and MHC class II molecules (HLA-DR, HLA-DQ, HLA-DP) that present peptides to CD4+ T cells.
• Disparities in HLA alleles between the donor and recipient serve as independent risk factors for the development of both acute and chronic GVHD.
• The traditionally preferred donor for HSCT is an HLA-identical sibling, as any pair of siblings has a 25% chance of being HLA identical.
• When an HLA-identical sibling is unavailable, alternative options include matched unrelated volunteer donors, full-haplotype mismatched family members, and unrelated umbilical cord blood donors.

Indications of Stem Cell Therapy

Malignant Disorders

• Acute lymphoblastic leukemia (ALL) in patients in their first complete remission with high-risk features, or in second or subsequent remissions.
• Acute myeloid leukemia (AML) in the first complete remission for high-risk patients, or in advanced disease phases.
• Philadelphia chromosome-positive chronic myelogenous leukemia (CML), particularly for patients with a poor response or intolerance to tyrosine kinase inhibitors.
• Juvenile myelomonocytic leukemia (JMML) and myelodysplastic syndromes.
• Autologous stem cell transplantation is indicated for relapsed Hodgkin and non-Hodgkin lymphoma, stage IV or relapsed neuroblastoma, very high-risk Ewing sarcoma, and high-risk central nervous system tumors.

Non-Malignant Disorders

• Primary immunodeficiency diseases, including severe combined immunodeficiency (SCID), Wiskott-Aldrich syndrome, leukocyte adhesion deficiency, hyper-IgM syndrome, and chronic granulomatous disease.
• Severe acquired aplastic anemia and inherited bone marrow failure syndromes such as Fanconi anemia, dyskeratosis congenita, Shwachman-Diamond syndrome, and Diamond-Blackfan anemia.
• Hemoglobinopathies, specifically thalassemia major and severe sickle cell disease.
• Inherited metabolic diseases, including mucopolysaccharidosis type 1 (Hurler syndrome) and adrenoleukodystrophy, to prevent irreversible neurologic damage.

Patient Preparation (Conditioning Regimen)

• Patient preparation involves a conditioning regimen that utilizes chemotherapy, sometimes in conjunction with irradiation (such as total body irradiation).
• The primary goals of the preparative regimen are to eliminate the patient's existing hematopoietic system, suppress the recipient's immune system to prevent graft rejection, and significantly reduce the tumor burden in patients with malignancies.
• Reduced-intensity conditioning regimens are frequently employed in pediatric patients; these are primarily immunosuppressive rather than myeloablative, aiming to induce reduced immune competence to avoid rejection of donor cells while minimizing toxicity.
• Preparative regimens are tailored to specific diseases; for example, patients with Fanconi anemia require reduced doses of cyclophosphamide and minimal radiation due to extreme sensitivity to DNA crosslinking agents.

Potential Complications of Stem Cell Therapy

Early Complications

• Acute Graft-Versus-Host Disease (GVHD): Occurs within the first 2 to 8 weeks when alloreactive donor T cells attack recipient tissues, manifesting as an erythematous maculopapular rash, vomiting, diarrhea, and liver disease with elevated bilirubin and transaminases.
• Infections: Profound immunodeficiency leads to pre-engraftment susceptibility to bacterial sepsis (e.g., enteric gram-negative bacilli) and invasive fungal diseases (e.g., Candida, Aspergillus). Post-engraftment risks include severe viral infections like Cytomegalovirus (CMV), Epstein-Barr virus (associated with post-transplant lymphoproliferative disease), and Adenovirus.
• Graft Failure: Can be primary (failure to achieve a neutrophil count of 0.5 × 10^9/L) or secondary (loss of peripheral counts post-engraftment), caused by an inadequate stem cell dose, viral infections, or immune-mediated rejection by surviving host T cells.
• Venoocclusive Disease (VOD): Also known as sinusoidal obstruction syndrome, this presents within 30 days with hepatomegaly, jaundice, weight gain, and ascites due to conditioning-induced endothelial damage.

Late Complications

• Chronic GVHD: A disorder of immune regulation developing >3 months post-transplant, characterized by systemic autoimmune-like symptoms such as scleroderma, sicca syndrome, arthritis, and bronchiolitis obliterans.
• Endocrine Dysfunction: Includes growth impairment due to growth hormone deficiency, hypothyroidism, primary ovarian or testicular failure, delayed puberty, and a high risk of permanent infertility.
• Cardiovascular Effects: Increased risk of metabolic syndrome, dyslipidemia, hypertension, and cardiomyopathy, particularly following total body irradiation or pre-transplant anthracycline exposure.
• Secondary Malignancies: Elevated risk for myelodysplastic syndromes, secondary leukemias, thyroid carcinoma, brain tumors, and epithelial cancers.
• Other Toxicities: Restrictive pulmonary disease, renal toxicity, leukoencephalopathy, neurocognitive deficits, cataracts, and dental abnormalities.