There are different kinds of donor for stem cell transplantation. The best matched donors as siblings. The ultimate such donor is an identical twin, which is called syngeneic transplantation. On the other end of the spectrum is unrelated donor. Not everyone has siblings.
Related haploidentical BMT is an alternative method for expanding the potential pool of stem cell donors; any patient shares one HLA haplotype with each biologic parent or child and siblings or half-siblings have a 50% chance of being haploidentical and 50% chance of not being haploidentical. The more differences between the patient and the donor , they higher is the incidence of graft rejection and severe graft vs. host disease. The disadvantage of the haploidentical approach has been the high incidence of graft rejection and severe GVHD. In order to prevent severe GVHD that would naturally follow, a variety of strategies to modify T cell response are being explored.
Continued research is needed to better define preferred conditioning regimens, methods and degree of T-cell depletion, and optimal CD34+ cell dose in the allograft. Although haploidentical transplantation is a potentially curative option for AML patients lacking a suitable sibling or unrelated donor, experts believe that at this time it should be performed only in experienced centers, within the context of clinical trials, especially in patients with poor-risk AML in CR1.
One emerging matter of dealing with this problem is post transplant treatment with Cytoxan. Others affect T cells.
A review paper by Alshemmari(2012) from Kuwait concludes:”Haploidentical transplants offer a potential cure for patients in need of allogeneic transplantation when an HLA-matched either related or unrelated donor is not available. It is a reasonable option for patients who do need urgent transplantation or also for non-Caucasian patients, in whom a chance of finding an unrelated matched donor is still low.” It remains questionable to me whether it represents the standard of care in the USA. NCCN approach to haploidentical transplant is not clear. NCCN does accept alternative donors, but it defines them as “including umbilical and matched unrelated donors”. It is not clear that NCCN considered haploidentical transplantation in this wording, since haploidentical transplantation is a very new approach.
The information that is coming out on haploidentical transplant is very encouraging, and in some ways revolutionary. It holds a promise of having a donor for any patient who needs a stem cell transplant, with results that may be identical or better than with sibling or umbilical cord stem cell transplantation. Recent Francophone guideline, recommends it as being equivalent to HLA matched transplantation.
Paolo Di Bartolomeo et al, Haploidentical, unmanipulated, G-CSF primed bone marrow transplantation for patients with high-risk hematological malignancies Blood November 19, 2012 blood-2012-08-453399
Salem Alshemmari et al, Bone Marrow ResearchHaploidentical Hematopoietic Stem-Cell Transplantation in Adults ,Volume 2011 (2011)
S Tuve al, Haploidentical bone marrow transplantation with post-grafting cyclophosphamide: multicenter experience with an alternative salvage strategy, Leukemia 25, 880-883 (May 2011)
R A Brodsky, L Luznik, J Bolaños-Meade, M S Leffell3, R J Jones and E J Fuchs Reduced intensity HLA-haploidentical BMT with post transplantation cyclophosphamide in nonmalignant hematologic diseases Bone Marrow Transplantation (2008) 42, 523–527
Uptodate 2017, HLA-Hapolidentical stem cell transplants
Nguyen et al, Haploidentical hematopoietic stem cell transplantation: Guidelines from the Francophone society of marrow transplantation and cellular therapy (SFGM-TC). Bull Cancer.
Read the Layperson version here.