Histocompatibility testing for clinical bone marrow transplantation and prospects for identification of donors other than HLA genotypically identical siblings

Modern approaches to HLA-haploidentical blood or marrow transplantation

Allogeneic blood or bone-marrow transplantation (alloBMT) is a potentially curative treatment for a variety of haematological malignancies and nonmalignant diseases. Historically, human leukocyte antigen (HLA)-matched siblings have been the preferred source of donor cells owing to superior outcomes compared with alloBMT using other donors. Although only approximately one-third of patients have an HLA-matched sibling, nearly all patients have HLA-haploidentical related donors. Early studies using HLA-haploidentical alloBMT resulted in unacceptably high rates of graft rejection and graft-versus-host disease (GVHD), leading to high nonrelapse mortality and consequently poor survival. Several novel approaches to HLA-haploidentical alloBMT have yielded encouraging results with high rates of successful engraftment, effective GVHD control and favourable outcomes. In fact, outcomes of several retrospective comparative studies seem similar to those seen using other allograft sources, including those of HLA-matched-sibling alloBMT. In this Review, we provide an overview of the three most-developed approaches to HLA-haploidentical alloBMT: T-cell depletion with 'megadose' CD34(+) cells; granulocyte colony-stimulating factor-primed allografts combined with intensive pharmacological immunosuppression, including antithymocyte globulin; and high-dose, post-transplantation cyclophosphamide. We review the preclinical and biological data supporting each approach, results from major clinical studies, and completed or ongoing clinical studies comparing these approaches with other alloBMT platforms.

HLA compatibility for patients with thalassemia: implications for bone marrow transplantation

In order to evaluate the possibility of a patient with thalassemia finding an HLA-identical sibling donor, we performed an analysis of HLA antigens in families of thalassemic patients. The pattern distribution was not significantly different from the expected ratio 25:50:25. When the siblings were subdivided according to the age of the patients (under or over 5 years), the above pattern remained unmodified for both the age groups. The average size of the 129 thalassemic families was 2.4. Thus, taking into account that thalassemic patients have an average of 1.4 siblings and that the HLA genotype distribution is expected as such, HLA-matched siblings are available for 33% of the patients. Because an additional 8.5% of the patients were found HLA-phenotypically identical to one parent, the chance for a patient with thalassemia to find a suitable donor for bone marrow transplantation would be increased to 41.5%. Our preliminary data cannot be extrapolated to the overall Italian thalassemic population; however, it can be inferred that for a patient with thalassemia, the chance of finding a suitable donor for bone marrow transplantation is not reduced.

Mixed lymphocyte reactions for individuals with phenotypic identity for specific HLA-B,DR determinants: the role of linkage disequilibrium and of specific DR and other class II determinants

Although many patients who might benefit from therapeutic bone marrow transplantation lack HLA identical sibling donors, results from many centers now indicate that transplants involving donors other than identical siblings have been successful in a substantial number of cases. Most of these cases were selected because cells from the patient and donor were compatible in mixed lymphocyte culture. We have previously shown that the prediction of mixed lymphocyte culture nonreactivity by HLA-B,DR matching is far more successful if the matched donors shared antigen combinations known to possess significant positive linkage disequilibrium. We now also show that cells from donors with unrelated haplotypes having the specific DR determinants DR1, DR2, and DR3 are more likely than cells from donors with other haplotypes to be mutually compatible in mixed lymphocyte culture. However, even cells from donors with haplotypes with the highest levels of positive linkage disequilibrium frequently show significant mutual stimulation which can, in selected family studies, be attributed to determinants like SB that map between HLA-D/DR and GLO.