Clinical importance of HLA-DPB1 in haematopoietic cell transplantation

From clones to immunopeptidomes: New developments in the characterization of permissive HLA-DP mismatches in hematopoietic cell transplantation

Mismatching at the HLA-DPB1 locus occurs frequently in hematopoietic cell transplantation with unrelated donors. Despite this, HLA-DPB1 allelic mismatches have traditionally not been considered in patient-donor matching. A T-cell epitope (TCE) model for the functional assessment of permissive mismatches at this locus has nevertheless been adopted in clinical practice. While initially based on a hierarchical immunogenicity elucidated from allorecognition by T-cell clones isolated from a patient, newer developments in the understanding of this model's biological basis, including a central role for immunopeptidome divergence between mismatched allotypes, have prompted changes in the assignment of permissiveness, providing the opportunity for a more granular evaluation of graft-versus-host disease and relapse risks according to the nature and directionality of permissive mismatches. How these advances impact the assessment of permissiveness at HLA-DPB1 and potentially the intelligent selection of donors according to the main clinical goal for different patients is the subject of the present review.

Experimental Data on PIRCHE and T-Cell Reactivity: HLA-DPB1-Derived Peptides Identified by PIRCHE-I Show Binding to HLA-A*02:01 in vitro and T-Cell Activation in vivo

Introduction

Human leukocyte antigen (HLA)-DPB1 mismatches during hematopoietic stem cell transplantation (HSCT) with an unrelated donor result in an increased risk for the development of graft-versus-host disease (GvHD). The number of CD8+ T-cell epitopes available for indirect allorecognition as predicted by the PIRCHE algorithm has been shown to be associated with GvHD development. As a proof of principle, PIRCHE-I predictions for HLA-DPB1 mismatches were validated in vitro and in vivo.

Methods

PIRCHE-I analysis was performed to identify HLA-DPB1-derived peptides that could theoretically bind to HLA-A*02:01. PIRCHE-I predictions for HLA-DPB1 mismatches were validated in vitro by investigating binding affinities of HLA-DPB1-derived peptides to the HLA-A*02:01 in a competition-based binding assay. To investigate the capacity of HLA-DPB1-derived peptides to elicit a T-cell response in vivo, mice were immunized with these peptides. T-cell alloreactivity was subsequently evaluated using an interferon-gamma ELISpot assay.

Results

The PIRCHE-I algorithm identified five HLA-DPB1-derived peptides (RMCRHNYEL, YIYNREEFV, YIYNREELV, YIYNREEYA, and YIYNRQEYA) to be presented by HLA-A*02:01. Binding of these peptides to HLA-A*02:01 was confirmed in a competition-based peptide binding assay, all showing an IC50 value of 21 μm or lower. The peptides elicited an interferon-gamma response in vivo.

Conclusion

Our results indicate that the PIRCHE-I algorithm can identify potential immunogenic HLA-DPB1-derived peptides present in recipients of an HLA-DPB1-mismatched donor. These combined in vitro and in vivo observations strengthen the validity of the PIRCHE-I algorithm to identify HLA-DPB1 mismatch-related GvHD development upon HSCT.

[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.

Current donor selection strategies for allogeneic hematopoietic cell transplantation

Since the first allogeneic hematopoietic stem cell transplantation (HCT) was performed by Dr. E. Donnall Thomas in 1957, the field has advanced with new stem cell sources, immune suppressive regimens, and transplant protocols. Stem cells may be collected from bone marrow, peripheral or cord blood from an identical twin, a sibling, or a related or unrelated donor, which can be human leukocyte antigen (HLA) matched, mismatched, or haploidentical. Although HLA matching is one of the most important criteria for successful allogeneic HCT (allo-HCT) to minimize graft vs host disease (GVHD), prevent relapse, and improve overall survival, the novel immunosuppressive protocols for GVHD prophylaxis offered improved outcomes in haploidentical HCT (haplo-HCT), expanding donor availability for the majority of HCT candidates. These immunosuppressive protocols are currently being tested with the HLA-matched and mismatched donors to improve HCT outcomes further. In addition, fine-tuning the DPB1 mismatching and discovering the B leader genotype and mismatching may offer further optimization of donor selection and transplant outcomes. While the decision about a donor type largely depends on the patient's characteristics, disease status, and the transplant protocols utilized by an individual transplant center, there are general approaches to donor selection dictated by donor-recipient histocompatibility and the urgency for HCT. This review highlights recent advances in understanding critical factors in donor selection strategies for allo-HCT. It uses clinical vignettes to demonstrate the importance of making timely decisions for HCT candidates.

Tools for optimizing risk assessment in hematopoietic cell transplant - What can we get away with?

Unrelated allogeneic hematopoietic cell transplant (HCT) is a critical modality to treat hematologic malignancies. The current objective of donor selection is to match donor and recipient at the HLA (human leukocyte antigen) peptide-binding region which should lower the risk of graft-versus-host disease. However, depending on the patient's ethnicity/race, finding a matched donor is challenging, especially for HLA-DPB1 which is due to the weak linkage disequilibrium between HLA-DPB1 and the other HLA class II loci. Recent evidence, on the molecular level, has shown that certain HLA mismatches carry lower clinical risk. More specifically, there is an increasing understanding of polymorphisms of the innate and adaptive immune systems and their impact on transplant outcomes, allowing us to expand our "toolkit" for optimization of donor selection in HCT. Therefore, in this review we discuss matching strategies based on comparing donor and recipient polymorphisms that may influence innate and adaptive immune response genes in allorecognition and the role of single nucleotide polymorphisms in non-HLA genes that have the potential for providing additional tools to refine risk stratification.

Is there any impact of HLA-DPB1 disparity in 10/10 HLA-matched unrelated hematopoietic SCT? Results of a French multicentric retrospective study

We retrospectively analyzed the impact of HLA-DPB1 mismatches in a large cohort of 1342 French patients who underwent 10/10 HLA-matched unrelated HSCT. A significant impact of HLA-DPB1 allelic mismatches (2 vs 0) was observed in severe acute GVHD (aGVHDIII-IV) (risk ratio (RR)=1.73, confidence interval (CI) 95% 1.09-2.73, P=0.019) without impact on OS, TRM, relapse and chronic GVHD (cGVHD). According to the T-cell epitope 3 (TCE3)/TCE4 HLA-DPB1 disparity algorithm, 37.6% and 58.4% pairs had nonpermissive HLA-DPB1, respectively. TCE3 and TCE4 disparities had no statistical impact on OS, TRM, relapse, aGVHD and cGVHD. When TCE3/TCE4 disparities were analyzed in the graft-vs-host or host-vs-graft (HVG) direction, only a significant impact of TCE4 nonpermissive disparities in the HVG direction was observed on relapse (RR=1.34, CI 95% 1.00-1.80, P=0.048). In conclusion, this French retrospective study shows an adverse prognosis of HLA-DPB1 mismatches (2 vs 0) on severe aGVHD and of nonpermissive TCE4 HVG disparities on relapse after HLA-matched 10/10 unrelated HSCT.

NOD2 Polymorphisms and Their Impact on Haematopoietic Stem Cell Transplant Outcome

Haematopoietic stem cell transplantation (HSCT) is a valuable tool in the treatment of many haematological disorders. Advances in understanding HLA matching have improved prognoses. However, many recipients of well-matched HSCT develop posttransplant complications, and survival is far from absolute. The pursuit of novel genetic factors that may impact on HSCT outcome has resulted in the publication of many articles on a multitude of genes. Three NOD2 polymorphisms, identified as disease-associated variants in Crohn's disease, have recently been suggested as important candidate gene markers in the outcome of HSCT. It was originally postulated that as the clinical manifestation of inflammatory responses characteristic of several post-transplant complications was of notable similarity to those seen in Crohn's disease, it was possible that they shared a common cause. Since the publication of this first paper, numerous studies have attempted to replicate the results in different transplant settings. The data has varied considerably between studies, and as yet no consensus on the impact of NOD2 SNPs on HSCT outcome has been achieved. Here, we will review the existing literature, summarise current theories as to why the data differs, and suggest possible mechanisms by which the SNPs affect HSCT outcome.

Clinical significance of donor-recipient HLA matching on survival after myeloablative hematopoietic cell transplantation from unrelated donors

The application of unrelated donor hematopoietic cell transplantation can be expanded with the use of mismatched donors if human leukocyte antigen (HLA) disparity does not lead to increased morbidity and mortality. The rules that govern permissibility of HLA mismatches are not well defined. The International Histocompatibility Working Group in hematopoietic cell transplantation measured the risks associated with locus-specific disparity in 4796 patients transplanted for low, intermediate, or high-risk hematologic diseases. The permissibility of a given HLA mismatch is in part defined by the locus and by disease risk.

14thInternational HLA and Immunogenetics Workshop: report on hematopoietic cell transplantation

Deciphering the role of human leukocyte antigen (HLA), killer immunoglobulin like receptor, and immune response genes in a model as complex as unrelated donor hematopoietic cell transplantation is a challenge. The allelic diversity of these genes is shaped by the race and ethnicity of transplant donors and recipients. Coupled with the genetic polymorphism is the complexity of clinical phenotypes of transplant populations: donor and recipient demographic characteristics and the regimens used by transplant physicians to prepare patients for transplantation and to prevent and treat graft-vs-host disease (GVHD). Furthermore, GVHD is itself a complex disease shaped by both genes and 'environment'. How does one begin to deconstruct the genetic barrier to understand risk factors important to transplant outcome? To begin with, population-based studies, particularly retrospective ones, benefit from adequate sample sizes to measure genetic effects. The more homogeneous the population for variables that influence clinical endpoints, the higher the likelihood that a real genetic effect can be uncovered. Even so, the feasibility of studies can be hampered if genotype and clinical data are not both complete and precise. For studies of HLA, diversity of alleles and antigens contributed by ethnically different transplant populations is an asset, because not only can a broader range of HLA mismatches be studied but they provide the opportunity for side-by-side analyses that may yield clues as to why transplant outcomes differ between populations.