What is HST (Hematopoietic Stem Cell Transplantation)?

Hematopoietic Stem Cell Transplantation (HST)

Hematopoietic Stem Cell Transplantation (HST) is the infusion of hematopoietic stem cells from a donor into a patient who has received chemotherapy, which is usually marrow-ablative, to treat various neoplastic diseases, hematologic disorders, immunodeficiency syndromes, congenital enzyme deficiencies, and autoimmune disorders. 1

Types of HST

HSTs are classified into two main categories based on the source of transplanted cells:

1. Allogeneic HST

• Source: Cells harvested from a donor other than the transplant recipient 1
• Donor types:
  • HLA-identical twin or matched sibling (most successful)
  • Unrelated donor (through registry organizations)
  • Mismatched family member
  • Umbilical cord blood (UCB)
• Primary indications:
  • Severe aplastic anemia (most effective treatment)
  • Chronic myelogenous leukemia (only curative therapy) 1
• Key considerations:
  • Higher risk of graft-versus-host disease (GVHD) with non-HLA-matched siblings
  • T-lymphocyte depletion reduces GVHD but increases risk of graft rejection, CMV infection, invasive fungal infection, and EBV-associated posttransplant lymphoproliferative disease 1

2. Autologous HST

• Source: Patient's own cells 1
• Similar procedure: Syngeneic transplants (HLA-identical twin as donor)
• Primary indications:
  • Patients requiring high-level or marrow-ablative chemotherapy with healthy bone marrow
  • When immunologic antitumor effect of allograft is not beneficial
  • Most commonly used for breast cancer, non-Hodgkin's lymphoma, and Hodgkin's disease 1
• Key advantage: No risk of chronic GVHD 1

Sources of Hematopoietic Stem Cells

1. Bone marrow: Traditional source, requires general anesthesia for collection 1
2. Peripheral blood: Increasingly common, especially for autologous transplants
   • Requires treatment with hematopoietic colony-stimulating factors (G-CSF or GM-CSF)
   • Eliminates need for general anesthesia 1
3. Umbilical cord blood (UCB): Newer source
   • Used primarily for allogeneic transplants in children
   • May allow greater degrees of histoincompatibility without graft rejection or GVHD 1

Clinical Applications

HST has become standard treatment for multiple conditions:

• Malignant diseases:

  • Leukemias
  • Lymphomas
  • Breast cancer
  • Other solid tumors 1, 2

• Non-malignant conditions:

  • Severe aplastic anemia
  • Hematologic disorders
  • Immunodeficiency syndromes
  • Congenital enzyme deficiencies 1

• Autoimmune disorders:

  • Multiple sclerosis (MS) - particularly relapsing forms refractory to disease-modifying therapy 1
  • Systemic lupus erythematosus
  • Neuromyelitis optica spectrum disorders (NMOSD)
  • Other autoimmune diseases 1, 3

Complications and Considerations

1. Graft-versus-host disease (GVHD):

   • Condition where donated cells attack recipient's cells
   • Major complication of allogeneic HST
   • Managed with immunosuppressive agents like cyclosporine or tacrolimus 1

2. Infections:

   • Increased risk of opportunistic infections due to immunosuppression
   • Requires prophylactic measures and monitoring 1, 2

3. Transplant-related mortality (TRM):

   • Risk must be weighed against disease-related mortality
   • Careful patient selection is essential 3

4. Fertility concerns:

   • Risk of premature ovarian failure
   • Fertility preservation should be considered before treatment 1

Recent Advances

1. Reduced-intensity conditioning regimens:

   • Lower toxicity alternatives to traditional myeloablative conditioning 2
   • Immunotherapy approaches for bone marrow conditioning 4

2. Improved donor selection:

   • Advanced HLA typing techniques
   • Expanded donor registries 1

3. Specialized protocols:

   • "Beijing Protocol" for haploidentical HST in China 1
   • Specific protocols for autoimmune diseases 1, 3

Key Considerations for Clinical Practice

• HST requires specialized multidisciplinary teams including hematology, neurology (for neurological indications), radiology, physiotherapy, and reproductive medicine 1
• Careful patient selection is critical to balance risks and benefits
• Long-term monitoring for disease recurrence and transplant-related complications is essential 2
• The number of HSTs performed continues to increase globally, with approximately 20,000 procedures performed annually in North America 1

HST represents a potentially curative option for many patients with otherwise fatal or severely debilitating conditions, though the procedure carries significant risks that must be carefully considered against potential benefits.