Genomics of unrelated-donor hematopoietic cell transplantation

Non-HLA genomics: does it have a role in predicting haematopoietic stem cell transplantation outcome?

Haematopoietic stem cell transplantation (HSCT) remains the only cure for many haematological neoplasms; however, the mortality rate remains high, at around 30-80%. Complications after HSCT include relapse, graft-versus-host disease, graft rejection and infection. High-resolution HLA matching has improved survival in HSCT over recent years; however, GVHD still remains a serious complication. Single nucleotide polymorphisms (SNPS) within genes that are involved with an individual's capability to mount an immune response to infectious pathogens, residual leukaemia, alloantigens or genes involved in drug metabolism have been studied for their association with HSCT outcome. Indeed, over the last 15 years, several groups, including ourselves, have demonstrated that non-HLA gene polymorphisms can be predictive of HSCT outcome. Can genetic characteristics of the patient and donor be used in the future to tailor HSCT protocols and determine GVHD prophylaxis? This review summarizes some of the recent SNP association studies in HSCT and highlights some of the disparities therein, discussing the integral problems of performing genetic association studies on diseases with complex outcomes using heterogeneous cohorts. The review will comment on recent genomewide association studies (GWAS) and discuss their relevance in this field, and it will also comment on recent meta-analysis combining GWAS studies with other studies such as gene expression micro array data in the field of autoimmune disease and solid organ transplantation. It will mention possible novel candidate gene polymorphisms, for example SNPS in microRNAs. In addition, it will discuss some of the inherent problems associated with gene association studies including the GRIPs (genetic risk prediction studies) recommendations. In summary, this review will assess the usefulness of non-HLA genomic studies in HSCT with regard to predicting outcome and modifying therapy.

Manipulating the immune system for anti-tumor responses and transplant tolerance via mixed hematopoietic chimerism

SUMMARY

Stem cells (SCs) with varying potentiality have the capacity to repair injured tissues. While promising animal data have been obtained, allogeneic SCs and their progeny are subject to immune-mediated rejection. Here, we review the potential of hematopoietic stem cells (HSCs) to promote immune tolerance to allogeneic and xenogeneic organs and tissues, to reverse autoimmunity, and to be used optimally to cure hematologic malignancies. We also review the mechanisms by which hematopoietic cell transplantation (HCT) can promote anti-tumor responses and establish donor-specific transplantation tolerance. We discuss the barriers to clinical translation of animal studies and describe some recent studies indicating how they can be overcome. The recent achievements of durable mixed chimerism across human leukocyte antigen barriers without graft-versus-host disease and of organ allograft tolerance through combined kidney and bone marrow transplantation suggest that the potential of this approach for use in the treatment of many human diseases may ultimately be realized.

Hematopoietic transplantation from adult unrelated donors as treatment for acute myeloid leukemia

Transplantation from unrelated donors (URD) is increasingly being used as treatment for hematological malignancies, including acute myeloid leukemia (AML). This increase is the consequence of the availability of more than 11 million URD volunteers and the more efficient donor search process in the recent years. Median time to identify a suitable URD is now 2 months. More than 50% of Caucasian patients have an human leukocyte antigen (HLA)-allele donor match and a one-antigen or allele HLA-mismatched donor may also be acceptable. Complications of URD transplants are particularly frequent and severe, with long-term OS in the registries being 10-20% inferior to HLA-identical sibling transplantation. High resolution DNA techniques for donor and recipient HLA matching have contributed to the survival in experienced centres after unrelated donor SCT approaching that achieved with sibling donors. The introduction of reduced intensity conditioning (RIC) has extended URD transplants to elderly and/or debilitated patients with AML. With this approach, TRM decreases, although graft-versus-host disease-related morbidity and mortality remain a problem. Despite this complication, results after URD transplantation in this age group seem better than those achieved with chemotherapy and/or autologous transplantation. To confirm this possibility, prospective multicenter comparisons of URD transplants after RIC with other treatment options for elderly AML patients have recently been started.

Haematopoietic stem cell transplantation: can our genes predict clinical outcome?

Haematopoietic stem cell transplantation (HSCT) is currently the only curative treatment for many patients with malignant and non-malignant haematological diseases. The success of HSCT is greatly reduced by the development of complications, which include graft-versus-host disease (GVHD), relapse and infection. Human leukocyte antigen (HLA) matching of patients and donors is essential, but does not completely prevent these complications; non-HLA genes may also have an impact upon transplant outcome. Polymorphisms within genes that are associated with an individual's capability to mount an immune response to alloantigen and infectious pathogens and/or response to drugs (pharmacogenomics) are all currently being studied for their association with HSCT outcome. This review summarises the potential role of non-HLA polymorphisms in predicting HSCT outcome, from studies on retrospective transplant cohorts of HLA-identical siblings and matched unrelated donors. The clinical relevance and interpretation of non-HLA genetics, and how these could be used alongside clinical risk factors in HSCT, are also discussed.

Allogeneic hematopoietic transplantation and natural killer cell recognition of missing self

Although the optimal donor for allogeneic hematopoietic stem cell transplantation (HSCT) is a human leukocyte antigen-matched sibling, 75% of patients do not have a match, and alternatives are matched unrelated volunteers, unrelated umbilical cord blood units, and full-haplotype-mismatched family members. To cure leukemia, allogeneic HSCT relies on donor T cells in the allograft, which promote engraftment, eradicate malignant cells, and reconstitute immunity. Here, we focus on the open issues of rejection, graft-versus-host disease (GVHD), and infections and the benefits of natural killer (NK) cell alloreactivity and its underlying mechanisms. Donor-versus-recipient NK cell alloreactivity derives from a mismatch between inhibitory receptors for self-major histocompatibility complex (MHC) class I molecules on donor NK clones and the MHC class I ligands on recipient cells. These NK clones sense the missing expression of the self-MHC class I allele on the allogeneic targets and mediate alloreactions. HSCT from 'NK alloreactive' donors controls acute myeloid relapse without causing GVHD. We review the translation of NK cell recognition of missing self into the clinical practice of allogeneic hematopoietic transplantation and discuss how it has opened innovative perspectives in the cure of leukemia.

Allogeneic haematopoietic cell transplantation for chronic myelogenous leukaemia in the era of imatinib: a retrospective multicentre study

OBJECTIVE

To analyse the results of allogeneic haematopoietic cell transplantation (HCT) in patients with advanced stages of Philadelphia chromosome-positive chronic myelogenous leukaemia (CML) who had previously been treated with imatinib mesylate (IM).

METHODS

We analysed the outcome of 61 patients with CML who had received allogeneic HCT from sibling (n = 18) or unrelated (n = 43) donors after having been treated with IM. Forty-one patients had received IM because of accelerated or blast phase CML. Conditioning therapy contained standard doses of busulfan (n = 25) or total-body irradiation (n = 20) in conjunction with cyclophosphamide in the majority of cases. Sixteen patients received dose-reduced conditioning with fludarabine-based regimens.

RESULTS

The incidence of grades II-IV and III-IV graft-versus-host disease was 66% and 38% respectively. The probability of overall survival (OS), disease-free survival (DFS) and relapse at 18 months for the whole patient cohort were 37%, 33% and 24% respectively. The probability of non-relapse mortality (NRM) at 100 d and 12 months was 30% and 46% respectively. Univariate analysis showed that fludarabine-based conditioning therapy, age > or = 40 yr and >12 months interval between diagnosis and transplantation were associated with a significantly lower OS and DFS and a higher NRM.

CONCLUSION

These data suggest that although pretreatment with IM is not an independent negative prognostic factor, it cannot improve the dismal prognosis of CML patients at high risk for transplant-related mortality.

Immunogenomics of hematopoietic stem cell transplantation

Recipients of allogeneic hematopoietic stem cell transplantation (HSCT) incur the risk of graft-versus-host disease even when the donor is a sibling who shares the Major Histocompatibility Antigens. Therefore, even the perfect HLA match does not represent the optimal genetic match between donors and recipients in HSCT. In addition to the HLA complex other genetic systems operate and affect the outcome of HSCT. These include minor histocompatibility systems (Martin P. Applicability of matching for minor histocompatibility antigens in human bone marrow transplantation. In: Roopenian DC, Simpson E, editors. Minor histocompatibility antigens: From the laboratory to the clinic. Georgetown: Landis Bioscience; 2000. p. 97-103) (inducing bona fide allogeneic responses) as well as a series of functional polymorphisms in cytokines and chemokines and receptors genes (Transplantation 1997;64:553). Among the items affecting the outcome of HSCT the incidence and severity of infections have an important impact. Polymorphisms of genes controlling both arms of the immune responses to pathogens (innate versus cognate) are strong candidates for susceptibility factors to infection in allogeneic transplantation. These include the MHC alleles (HLA class I, class II, MIC) CD1, Toll and TLR genes MBP, MPO genes, ...). In addition to the NK alloreactivity induced by HLA class I epitopes mismatching (a common situation in HSCT) variations in the genotype of the KIR genes (Tissue Antigens 2001;57:358) may also be encountered between the donor and the recipient leading to potentially harmful or beneficial combinations. An integrated knowledge of the role and hierarchy of the most important genetic factors (MHC and non-MHC) will provide the rationale for a comprehensive matching in HSCT (Curr Opin Hematol 3 (1996) 416). This short review provides a panorama of this strategic issue for further development of HSCT.

Impact of HLA-A,B,C Allele Mismatches on Outcome after Unrelated Blood Stem Cell Transplantation in Whites

At our institution the selection of unrelated donors for hematopoietic stem cell transplantation (HSCT) relies on low resolution human leukocyte antigen (HLA)-A,B and high resolution HLA-DRB1,DQB1 DNA-based typing. To answer the question of whether routine high resolution HLA-A,B,C typing might improve HSCT outcome, 171 white "HLA-identical" donor/recipient pairs, as stated by our pretransplant tissue typing routine, were retyped for HLA-A,B,C using sequence based typing (SBT). The numbers of HLA-A,B,C allele mismatches detected by SBT were correlated to established clinical endpoints of HSCT outcome. We found 33.9% of the study transplants to be fully HLA-A,B,C matched, whereas 66.1 % exhibited one through four donor/recipient HLA-A,B,C allele mismatches. However, statistical analysis could not demonstrate an impact of the number of HLA-A,B,C allele mismatches on overall survival and other analyzed endpoints. Thus, our series of white donor/recipient pairs does not suggest the routine use of HLA-A,B,C SBT to improve HSCT outcome substantially.

High-resolution DNA melting curve analysis to establish HLA genotypic identity

High-resolution melting curve analysis is a closed-tube fluorescent technique that can be used for genotyping and heteroduplex detection after polymerase chain reaction. We applied this technique at the HLA-A locus and suggest that this method can be used as a rapid, inexpensive screen between siblings prior to living-related transplantation. At any locus, there are seven general cases of shared alleles among two individuals, ranging from identical homozygous genotypes (all alleles shared) to two heterozygous genotypes that share no alleles. We studied each case using previously typed cell lines to show that identity or non-identity can be determined in all cases by high-resolution melting curve analysis. HLA genotype identity is suggested when two individuals have the same melting curves. Identity is confirmed by comparing the melting curve of a 1:1 mixture with the individual melting curves. Non-identity at the amplified locus changes the heteroduplexes formed in the mixture compared with the original samples and alters the shape of the melting curve. The technique was tested on DNA from a 17-member CEPH family. High-resolution melting curve analysis revealed six different genotypes in the family. The genotype clustering was confirmed by sequence-based typing. Although this technique does not sequence or determine specific HLA alleles, it does rapidly establish identity at highly polymorphic HLA loci. The technique may also prove useful for confirmation of HLA genotypic identity between unrelated individuals prior to allogeneic hematopoietic stem-cell transplantation.

Immunobiology of Natural Killer Lymphocytes in Transplantation

Natural killer (NK) lymphocytes are powerful effector cells of the peripheral immune system. NK cell functions are controlled by the expression of a variety of cell surface receptors with either inhibitory or activating roles. The genetic and functional diversity of this repertoire of receptors and the role of human leukocyte antigen class I histocompatibility molecules as a major group of NK receptor ligands endows NK cells with an innate alloreactive capacity. Early studies of experimental bone marrow transplantation revealed an important role for NK cells in the rejection of allogeneic grafts and contributed significantly to our understanding of NK cell behavior. Both animal models and in vitro studies have since implicated NK cells as contributors to the pathology of clinical transplantation. However, recent clinical studies have demonstrated the potential benefits of exploiting NK cell alloreactivity in mismatched hematopoietic stem cell transplantation for particular types of acute leukemia. Future investigations of NK cell alloreactive functions will undoubtedly reveal additional roles and potential therapeutic applications of this fundamental cell type in clinical transplantation.

Hematopoietic stem cell transplantation: contrasting the outcome of transplantations from HLA-identical siblings, partially HLA-mismatched related donors, and HLA-matched unrelated donors

Allogeneic hematopoietic stem cell transplantation (HSCT) is a proven curative therapy for many hematologic malignancies. HSCT from HLA-identical sibling donors (ISDs) is still the golden standard. For the remaining 70% of the patients lacking an ISD, alternative (partially) HLA-matched family donors (MFDs) and HLA-matched unrelated donors (MUDs) are now widely accepted. However, it is presently unclear whether outcome after HSCT from an MFD or an MUD is superior. Thus, the classical clinical end points after HSCT from an ISD (n = 138), MFD (n = 86), and MUD (n = 101) were compared by means of univariate and multivariate statistical analyses. MFD transplantations with HLA class II (DRB1 +/- DQB1) mismatches in graft-versus-host (GVH) direction showed an increased risk of grades II to IV graft-versus-host disease, and MFD transplantations with more than a single HLA class I (A +/- B +/- C) mismatch in host-versus-graft (HVG) direction were associated with a higher risk of graft failure. However, no significant difference in overall survival was detectable among the 3 study groups after adjustment for the main predictors of transplantation outcome. Thus, for patients lacking an ISD, an already identified MFD with an HLA-DRB1 +/- DQB1 mismatch in GVH or a combined HLA-A +/- B +/- C mismatch in HVG direction should be accepted only in clinically urgent settings that leave no time to identify an MUD.

Natural killer receptor repertoires in transplantation

Natural killer (NK) lymphocytes are potent effector cells that are controlled by the expression of a variety of cell surface receptors with either inhibitory or activating functions. The genetic and functional diversity of this receptor repertoire and the role of HLA class I molecules as a major group of NK receptor ligands create an innate alloreactive capacity in this cell type. Both animal models and in vitro studies have implicated NK cells as contributors to the pathology of clinical transplantation. However, recent clinical studies have indicated the potential benefit of exploiting NK cell alloreactivity in mismatched haematopoietic stem cell transplantation. Further investigations of NK cell alloreactivity will undoubtedly reveal additional applications of this fundamental cell type in clinical transplantation.

Immunogenomics of hematopoïetic stem cell transplantation

Recipients of allogeneic hematopoïetic stem cell transplantation (HSCT) incur the risk of graft-versus-host disease (GVHD) even when the donor is a sibling who shares the major histocompatibility antigens. Therefore, even the perfect HLA match does not represent the optimal genetic match between donors and recipients in HSCT. In addition to the HLA complex other genetic systems operate and affect the outcome of HSCT. These include minor histocompatibility systems (inducing bona fide allogeneic responses) as well as a series of functional polymorphisms in cytokines and chemokines and receptors genes. Among the items affecting the outcome of HSCT the incidence and severity of infections have an important impact. Polymorphisms of genes controlling both arms of the immune responses to pathogens (innate vs. cognate) are strong candidates for susceptibility factors to infection in allogeneic transplantation. These include the MHC alleles (HLA class I, class II, MIC) CD1, Toll and TLR genes MBP, MPO genes.). In addition to the NK alloreactivity induced by HLA class I epitopes mismatching (a common situation in HSCT) variations in the genotype of the KIR genes may also be encountered between the donor and the recipient leading to potentially harmful or beneficial combinations. An integrated knowledge of the role and hierarchy of the most important genetic factors (MHC and non-MHC) will provide the rationale for a comprehensive matching in HSCT. This short review provides a panorama of this strategic issue for further development of HSCT.

Bone marrow transplantation: how important is CD34 cell dose in HLA-identical stem cell transplantation?

A recent analysis of Fred Hutchinson Cancer Research Center data has been undertaken to investigate the association of infused CD34 cell dose with various clinical outcomes after HLA-identical transplantation. Separate assessments for unrelated vs related donors and the use of bone marrow or mobilized G-CSF stimulated peripheral blood mononuclear cells (G-PBMC) have been incorporated. The three primary findings are: (1) Higher CD34 dose results in better neutrophil and platelet recovery in all settings. (2) Higher CD34 doses (8 x 10(6)/kg) are associated with the development of more chronic graft-versus-host disease when using related G-PBMC. (3) Higher CD34 dose is correlated with improved survival after unrelated donor bone marrow transplantation. These data suggest that the CD34 content of a graft can have a significant impact on clinical outcome after allogeneic transplantation, but defining an optimal dose is dependent on both the type of donor and the stem cell source.

HLA-identical stem cell transplantation: is there an optimal CD34 cell dose?

A review of the published literature, supplemented with a recent analysis of Fred Hutchinson data, has been undertaken to investigate the association of infused CD34 cell dose with various clinical outcomes after HLA-identical transplantation. Separate assessments for unrelated vs related donors and the use of bone marrow or mobilized G-PBMC have been incorporated. The three primary findings are: (1) higher CD34 dose results in better neutrophil and platelet recovery in all settings; (2) high CD34 doses (>8 x 10(6)/kg) are associated with the development of more chronic GVHD when using related G-PBMC; (3) higher CD34 dose is correlated with improved survival after bone marrow transplantation, especially with unrelated donors. This is not seen when using G-PBMC. The data suggest that the CD34 content of the graft can have a significant impact on clinical outcome after allogeneic transplantation, but optimal dose is dependent on both donor type and stem cell source.

Alloreactive killer cells: hindrance and help for haematopoietic transplants

Haematopoietic-cell transplantation is a treatment for leukaemia and lymphoma. To reduce the incidence of graft-versus-host disease (GVHD) caused by transplanted T cells, donors and recipients are HLA matched. For patients for whom a matched donor is not available, one option is transplantation from an HLA-mismatched relative who shares one HLA haplotype. This procedure is distinguished by the use of a stronger conditioning regimen for the patient and of a T-cell-depleted graft containing numerous stem cells. After transplantation, natural killer cells are prevalent, and they can include alloreactive cells that kill tumour cells and prevent GVHD. The alloreactions seem to be determined by the mismatched HLA class I ligands and their killer-cell immunoglobulin-like receptors.

Predictive parameters for in vivo alloreactivity

Despite optimal HLA matching and a negative serological crossmatch, confrontation with allogeneic cells by organ- or stem-cell transplantation or platelet transfusion, can lead to an alloimmune response resulting in graft rejection, graft vs. host disease or platelet refractoriness. It would be attractive to be able to select beforehand those donor/recipient combinations, that do not lead to a destructive alloimmune response and exclude transplantation or transfusion with donors, that induce a strong alloimmune reaction. Many predictive parameters have been identified on the basis of retrospective analysis of graft survival data, and the results of in vitro tests to measure T and B cell alloreactivity. However, most of these parameters have shown to be relevant when a statistical analysis is performed on the population level but do not have a direct impact for the individual patient. An exception is a molecularly based algorithm, called HLA matchmaker, which seems to predict which HLA mismatches do not lead to alloantibody formation in a particular individual. Prediction of T cell alloreactivity is more difficult and will need the development of additional in vitro tools or adaptation of the HLA matchmaker program. Although the direct clinical implication of NK cell mediated allorecognition is not clear yet, this may be a complicating factor when establishing solid parameters for the prediction of an alloimmune reaction in vivo.