BASICS OF TRANSPLANTATION
Hematopoietic stem cell transplantation (HSCT), which has historically been referred to as bone marrow transplantation (BMT), is the process of transfer of multipotent hematopoietic stem cells from a donor to a recipient for various indications. These cells will subsequently proliferate and differentiate to form the hematopoietic and immune system in the recipient. Over the last 50 years, major strides have been made in the field of HSCT, and currently, transplant is performed for multiple indications using a variety of donor sources. Due to wider utilization of this treatment worldwide and improvements in outcome over the past several decades, the current era has seen an increase in the number of HSCTs performed both for malignant as well as nonmalignant disorders.
GENERAL PRINCIPLES OF HEMATOPOIETIC STEM CELL TRANSPLANTATION
HSCTs can be categorized on the basis of several characteristics including donor type and source of stem cells.
HSCT can be divided into 2 broad categories based on the type of donor: allogeneic HSCT or autologous HSCT. As the name implies, allogeneic transplants are those where the hematopoietic cells infused into the patient originated in another person. This includes genetically identical human leukocyte antigen (HLA)-matched siblings or alternative donors who are not genetically identical.
There are 8 clinically relevant HLA genes, which are inherited in haplotypes of 4 genes on each copy of chromosome 6. Thus, the probability that 2 siblings have identical HLA genes is 25%. Given the typical family size, the majority of patients who require HSCT therefore will need a nonsibling donor. Common alternative donors include unrelated donors who are HLA matched to the recipient (matched unrelated donors), unrelated donors with a partial but acceptable match (mismatched unrelated donors), unrelated cord blood donated at birth to public banks, or related donors who are a half-HLA match (haplo-identical donors). Haplo-identical donors can be parents or siblings who share 1 haplotype with the recipient. National and international donor registries maintain an HLA database of more than 5 million volunteers who agreed to be HLA typed. Approximately 60% to 80% of patients can find an adequately matched donor through these registries if they are of European decent. The probability of finding a donor decreases significantly for patients of ethnic minorities because there are fewer minorities in the registries and there is greater HLA polymorphism in nonwhite ethnicities.
Hematopoietic stem cells that are capable of reconstituting the hematopoietic and immune systems can be derived from (1) bone marrow (BM), (2) peripheral blood after stimulation with growth factors (peripheral blood stem cells [PBSC]), or (3) umbilical cord blood (UCB). While bone marrow has been the traditional source of stem cells, the collection is time consuming, requires anesthesia, and has the highest complication rate. As a result, many adult donors prefer to donate PBSC, which are obtained ...