Allelic HLA Matching and Pair Origin Are Favorable Prognostic Factors for Unrelated Hematopoietic Stem Cell Transplantation in Neoplastic Hematologic Diseases: An Italian Analysis by the Gruppo Italiano Trapianto di Cellule Staminali e Terapie Cellulari, Italian Bone Marrow Donor Registry, and Associazione Italiana di Immunogenetica e Biologia dei Trapianti

Reduced human leukocyte antigen mismatching is associated with more favourable outcomes after unrelated donor haematopoietic stem cell transplantation

The patient-donor human leukocyte antigen (HLA) match remains the most important prognostic factor for successful unrelated donor haematopoietic stem cell transplantation (UD-HSCT). This single-centre study comprised 125 adult patients with malignant haematological diseases undergoing their first UD-HSCT. The primary goal of this study was to validate the impact of HLA matching on HSCT outcomes, specifically at the HLA-DPB1 and HLA-DRB3/4/5 loci. A multivariable Cox regression analysis with a backward selection algorithm was employed to assess the associations of selected prognostic factors with outcomes after UD-HSCT. Any HLA locus mismatch was found to be associated with an increased incidence of grade II-IV acute graft versus host disease (aGvHD) at 100 days (p = .031; hazard ratio [HR] 1.935) and 6 months (p = .004; HR 2.284) after HSCT. The results of the following analyses also confirmed the strong impact of HLA-DPB1-only mismatch on the incidence of grade II-IV aGvHD at 100-day (p = .006; HR 2.642) as well as at 6-month (p = .007; HR 2.401) time periods. The HLA-DPB1-only mismatch was also shown to be statistically significantly associated with lower relapse incidence (p = .034; HR 0.333). The impact of the HLA-DRB3/4/5 mismatch on outcomes was inconclusive, though the two and more HLA-DPB1 + DRB3/4/5-only mismatches showed a trend towards worse outcomes than a single mismatch. Based on our findings and those of more comprehensive studies, the extended HLA loci typing of patients and donors is suggested to avoid unexpected HLA mismatches during the UD selection.

[Unrelated donor selection for allogeneic hematopoietic stem cell transplantation: Guidelines from the Francophone Society of Bone Marrow Transplantation and Cellular Therapy (SFGM-TC)]

The selection of a donor is an essential element in allogeneic hematopoietic stem cell transplantation. In the absence of an HLA-matched related donor, the selection of an unrelated donor is considered, and is currently the most common type of allogenic donor used in practice. Many criteria are considered for the selection when multiple donors are available, particularly in case of partial match. The aim of this workshop is to assist in the selection of an unrelated donor, in keeping with recent data from the literature.

Human Leucocyte Antigen System and Selection of Unrelated Hematopoietic Stem Cell Donors: Impact of Patient-Donor (Mis)matching and New Challenges with the Current Technologies

The selection of hematopoietic stem cell donors for allogeneic transplantation (allo-HSCT) is mainly driven by human leucocyte antigen (HLA) matching between patient and donor, with HLA-identical matched siblings being the preferred choice in most situations. Although other clinical and demographical variables matter, especially, donor age, which is unequivocally associated with better transplant outcomes, the histocompatibility criteria have a central role in the search for the best donor, particularly in the setting of unrelated allo-HSCT where HLA disparities between patient and donor are frequent. The present review is focused on the role of HLA incompatibilities on patient outcome according to the most recent literature, in an attempt to guide transplant physicians and search coordinators during the process of adult unrelated-donor selection. The technological progresses in HLA typing, i.e., with next-generation sequencing (NGS), now allow disclosing a growing number of HLA incompatibilities associated with a heterogeneous and sometimes unknown spectrum of clinical severity. Their immunogenic characteristics, i.e., their position inside or outside the antigen recognition domain (ARD), their permissiveness, their intronic or exonic nature and even the expected expression of the HLA loci where those mismatches occur, will be presented and discussed here, integrating the advances in the immunobiology of transplantation with survival and toxicity outcomes reported in the most relevant studies, within the perspective of improving donor selection in the current practice.

Availability of HLA-allele-matched unrelated donors and registry size: Estimation from haplotype frequency in the Italian population

In Italy, an HLA-matched unrelated donor is currently the primary donor when a HLA matched sibling is not found for allogeneic haematopoietic stem cell transplantation (HSCT). Better outcomes for transplantation require optimal matching between donor and recipient at least at the HLA-A, -B, -C, and -DRB1 loci; therefore, the availability of HLA-matched unrelated donors is important. The enormous HLA polymorphism has always necessitated registries with a large number of individuals in order to be able to provide well-matched donors to a substantial percentage of patients. In order to increase the efficiency of the Italian Bone Marrow Donor Registry (IBMDR) in providing Italian patients with a suitable donor, the probability of finding an HLA-A, -B, -C, and -DRB1 allele-matched (8/8) or a single mismatch unrelated donor (7/8) was estimated in this study according to IBMDR size. Using a biostatistical approach based on HLA haplotype frequencies of more than 100,000 Italian donors enrolled in the IBMDR and HLA-typed at high-resolution level, the probability of finding an 8/8 HLA-matched donor was 23.8%; 33.4%; and 41.4% in simulated registry sizes of 200,000; 500,000; and 1,000,000 donors; respectively. More than 2 million recruited donors are needed to increase the likelihood of identifying an HLA 8/8 matched donor for 50% of Italian patients. If one single mismatch at HLA I class loci was accepted, the probability of finding a 7/8 HLA-matched donor was 62.8%; 73.7%; and 80.3% in 200,000 donors; 500,000; and 1,000,000 donors; respectively. Using the regional haplotype frequencies of IBMDR donors, the probability of recruiting a donor with a new HLA phenotype, in the different Italian regions, was also calculated. Our findings are highly relevant in estimating the optimal size of the national registry, in planning a cost-effective strategy for donor recruitment in Italy, and determining the regional priority setting of recruitment activity in order to increase the phenotypic variability of IBMDR as well as its efficiency.