Minors come of age: minor histocompatibility antigens and graft-versus-host disease

Donor Genetic Predisposition to High Interleukin-10 Production Appears Protective against Acute Graft-Versus-Host Disease

The persistence of graft-versus-host disease (GVHD) as the principal complication of allogeneic hematopoietic cell transplantation (HCT) demonstrates that HLA matching alone is insufficient to prevent alloreactivity. We performed molecular and functional characterization of 22 candidate cytokine genes for their potential to improve matching in 315 myeloablative, 10/10 HLA-matched donor−recipient pairs. Recipients of a graft carrying the -1082GG IL10 gene promoter region variant had a three-fold lower incidence of grade II−IV acute GVHD compared to IL10-1082AA graft recipients (SHR = 0.25, p = 0.005). This was most evident in matched unrelated donor (MUD) transplants, where the greatest alloreactivity is expected. IL10-1082GG transplants did not experience an increased incidence of relapse, and, consequently, overall survival was two-fold higher in IL10-1082GG MUD transplants (HR = 0.17, p = 0.023). Longitudinal post-transplant measurements demonstrated that -1082GG is a high-IL10-producing and -expressing genotype with attenuated CD8+ T-cell reconstitution. High post-transplant donor chimerism in T- and myeloid-cells (>95%) confirmed a predominant donor, rather than recipient, genotype effect on immune function and aGVHD. To date, this is the first study to report corroborating genome-to-cellular evidence for a non-HLA donor immunogenetic variant that appears to be protective against GVHD. The incorporation of IL10 variants in donor selection criteria and clinical-management decisions has the potential to improve patient outcomes.

Taking T-Cell Oncotherapy Off-the-Shelf

Banked allogeneic or 'off-the-shelf' (OTS) T cells from healthy human donors are being developed to address the limitations of autologous cell therapies. Potential challenges of OTS T cell therapies are associated with their allogeneic origin and the possibility of graft-versus-host disease (GvHD) and host-versus-graft immune reactions. While the risk of GvHD from OTS T cells has been proved to be manageable in clinical studies, approaches to prevent immune rejection of OTS cells are at an earlier stage of development. We provide an overview of strategies to generate OTS cell therapies and mitigate alloreactivity-associated adverse events, with a focus on recent advances for preventing immune rejection.

Current Status and Future Directions in Graft-Versus-Host Disease Prevention Following Allogeneic Blood and Marrow Transplantation in Adults

Graft-versus-host disease (GvHD) in its acute and chronic forms continues to represent a significant barrier to the success and wide-applicability of blood and marrow transplantation as a potentially curative treatment modality for a number of benign and malignant blood conditions. Presently, calcineurin inhibitor (CNI)-based regimens remain the most commonly used prevention strategy, although post-transplant cyclophosphamide is emerging as an alternative approach, and is providing a backbone for innovative CNI-free combinations. In this paper, we review the current strategies used for the prevention of GvHD, and highlight some of the developing and promising combinations.

T cell metabolism in graft-versus-host disease

Graft-versus-host disease (GVHD) is a major source of morbidity and mortality following allogeneic hematopoietic stem cell transplant (allo-HSCT), one of the most effective approaches to treat hematopoietic malignancies.¹ However, current prophylaxis regimens and treatments that reduce the detrimental effect of acute GVHD can be offset by increased incidence in opportunistic infections and relapse of the primary malignancy.² In addition, the majority of the approaches that inhibit T cell responses are non-specific, resulting in the inhibition of both alloreactive T cells and protective T cells from the donor. Therefore, there is an increase in the demand to develop novel approaches that selectively target alloreactive T cells. One potential means to address this issue is to take advantage of the unique metabolic profile of activated T cells.

Unexpected High Incidence of Human Herpesvirus-6 Encephalitis after Naive T Cell-Depleted Graft of Haploidentical Stem Cell Transplantation in Pediatric Patients

The CD45RA T cell depletion (TCD) method has been used to deplete naive T cells, preventing graft-versus-host disease (GVHD) but preserving memory cells, providing immediate functional T cells with anti-infection, antileukemia, and antirejection effects. We describe a series of 25 consecutive high-risk patients with leukemia who received haploidentical hematopoietic stem cell transplantation (haplo-HSCT) with CD45RA TCD. Each patient received 2 cell products: 1 created by CD34 positive selection and the other through CD45RA depletion from the CD34 negative fraction by a CliniMACS device. CD45RA-depleted haplo-HSCT was well tolerated, with rapid engraftment and low risk of severe acute GVHD and chronic GVHD. Although this treatment achieved a good control of viral reactivations, such as cytomegalovirus and adenovirus, we observed an unexpectedly high rate of limbic encephalitis due to human herpesvirus-6 (HHV-6; 8 cases). Characteristically, the infection appeared early in almost all patients, just after the engraftment. Although no patient died from encephalitis, 1 patient showed neuropsychological sequelae, and another experienced secondary graft failure just after the HHV-6 reactivation.

Engineering human peripheral blood stem cell grafts that are depleted of naïve T cells and retain functional pathogen-specific memory T cells

Graft-versus-host disease (GVHD) is a frequent major complication of allogeneic hematopoietic cell transplantation (HCT). Approaches that selectively deplete T cells that cause GVHD from allogeneic stem cell grafts and preserve T cells specific for pathogens may improve HCT outcomes. It has been hypothesized that the majority of T cells that can cause GVHD reside within the naïve T cell (TN) subset, and previous studies performed in mouse models and with human cells in vitro support this hypothesis. As a prelude to translating these findings to the clinic, we developed and evaluated a novel 2-step clinically compliant procedure for manipulating peripheral blood stem cells (PBSC) to remove TN, preserve CD34(+) hematopoietic stem cells, and provide for a fixed dose of memory T cells (TM) that includes T cells with specificity for common opportunistic pathogens encountered after HCT. Our studies demonstrate effective and reproducible performance of the immunomagnetic cell selection procedure for depleting TN. Moreover, after cell processing, the CD45RA-depleted PBSC products are enriched for CD4(+) and CD8(+) TM with a central memory phenotype and contain TM cells that are capable of proliferating and producing effector cytokines in response to opportunistic pathogens.

A polymorphism in the splice donor site of ZNF419 results in the novel renal cell carcinoma-associated minor histocompatibility antigen ZAPHIR

Nonmyeloablative allogeneic stem cell transplantation (SCT) can induce remission in patients with renal cell carcinoma (RCC), but this graft-versus-tumor (GVT) effect is often accompanied by graft-versus-host disease (GVHD). Here, we evaluated minor histocompatibility antigen (MiHA)-specific T cell responses in two patients with metastatic RCC who were treated with reduced-intensity conditioning SCT followed by donor lymphocyte infusion (DLI). One patient had stable disease and emergence of SMCY.A2-specific CD8+ T cells was observed after DLI with the potential of targeting SMCY-expressing RCC tumor cells. The second patient experienced partial regression of lung metastases from whom we isolated a MiHA-specific CTL clone with the capability of targeting RCC cell lines. Whole genome association scanning revealed that this CTL recognizes a novel HLA-B7-restricted MiHA, designated ZAPHIR, resulting from a polymorphism in the splice donor site of the ZNF419 gene. Tetramer analysis showed that emergence of ZAPHIR-specific CD8+ T cells in peripheral blood occurred in the absence of GVHD. Furthermore, the expression of ZAPHIR in solid tumor cell lines indicates the involvement of ZAPHIR-specific CD8+ T cell responses in selective GVT immunity. These findings illustrate that the ZNF419-encoded MiHA ZAPHIR is an attractive target for specific immunotherapy after allogeneic SCT.

Immunohematopoietic stem cell transplantation in Cape Town: a ten-year outcome analysis in adults

BACKGROUND AND OBJECTIVES

Immunohematopoietic stem cell transplantation has curative potential in selected hematologic disorders. Stem cell transplantation was introduced into South Africa in 1970 as a structured experimental and clinical program. In this report, we summarize the demography and outcome by disease category, gender, and type of procedure in patients older than 18 years of age who were seen from April 1995 to December 2002.

PATIENTS AND METHODS

This retrospective analysis included 247 individuals over 18 years of age for whom complete data were available. These patients received grafts mostly from peripheral blood with the appropriate stem cell population recovered by apheresis.

RESULTS

Patient ages ranged from 20 to 65 years with a median age of 42 years. There were 101 females and 146 males. There were no withdrawals and 63% survived to the end of the study. At 96 months of follow-up, a stable plateau was reached for each disease category. Median survival was 3.3 years (n=6, 14.6%) for acute lymphoblastic anemia, 3.1 years (n=44, 18%) for acute myeloid leukemia, 2.8 years (n=47, 19%) for chronic granulocytic leukemia, 2.8 years (n=71, 29%) for lymphoma, 1.5 years (n=23, 9%) for myeloma, 1.43 years (n=10, 4%) for aplasia, and 1.4 years (n=38, 15%) for a miscellaneous group comprising less than 10 examples each. Multivariate analysis showed that only diagnosis and age had a significant impact on survival, but these two variables might be interrelated. There was no significant difference in outcome by source of graft.

CONCLUSION

The results confirm that procedures carried out in a properly constituted and dedicated unit, which meets established criteria and strictly observes treatment protocols, generate results comparable to those in a First World referral center. Low rates of transplant-related mortality, rejection and graft-versus-host disease are confirmed, but the benefits cannot be extrapolated outside of academically oriented and supervised facilities.

One-antigen mismatched related versus HLA-matched unrelated donor hematopoietic stem cell transplantation in adults with acute leukemia: Center for International Blood and Marrow Transplant Research results in the era of molecular HLA typing

Approximately 13% of patients lacking an HLA-identical sibling have a one-antigen-mismatched related donor (MMRD). Historically, outcomes from the use of a one-antigen MMRD were considered equivalent to those from the use of a matched unrelated donor (UD). Recent improvements in UD stem cell transplantation (SCT) resulting from better molecular HLA matching justifies investigating whether UD should be preferred over MMRD in adult patients with acute leukemia. Here, we compared the outcomes of MMRD (n = 89) and HLA-A, -B, -C, and -DRB1 allele-matched UD (n = 700) SCT reported to the Center for International Blood and Marrow Transplant Research between 1995 and 2005. The patients underwent transplantation for acute myelogenous leukemia or acute lymphoblastic leukemia in first or second complete remission. Donor type was not associated with hematologic recovery. Univariate and multivariate comparisons of MMRD versus HLA-matched UD transplants showed no statistically significant differences in overall survival, disease-free survival, treatment-related mortality, relapse, or 100-day grade III-IV acute graft-versus-host disease (GVHD). MMRD SCT was associated with a lower rate of chronic GVHD at 1 year (35% vs 47%; P = .03), which was confirmed by multivariate analysis (relative risk, 0.58; 95% confidence interval, 0.39-0.85; P < .01). According to our data, HLA-matched UD and MMRD SCT are associated with comparable survival. Given that less chronic GVHD was observed in the MMRD transplantations, this option, when available, remains the first choice in patients with acute leukemia without an HLA-identical sibling in need of allogeneic SCT.

Therapy of relapsed leukemia after allogeneic hematopoietic cell transplantation with T cells specific for minor histocompatibility antigens

The adoptive transfer of donor T cells that recognize recipient minor histocompatibility antigens (mHAgs) is a potential strategy for preventing or treating leukemic relapse after allogeneic hematopoietic cell transplantation (HCT). A total of 7 patients with recurrent leukemia after major histocompatibility complex (MHC)-matched allogeneic HCT were treated with infusions of donor-derived, ex vivo-expanded CD8(+) cytotoxic T lymphocyte (CTL) clones specific for tissue-restricted recipient mHAgs. The safety of T-cell therapy, in vivo persistence of transferred CTLs, and disease response were assessed. Molecular characterization of the mHAgs recognized by CTL clones administered to 3 patients was performed to provide insight into the antileukemic activity and safety of T-cell therapy. Pulmonary toxicity of CTL infusion was seen in 3 patients, was severe in 1 patient, and correlated with the level of expression of the mHAg-encoding genes in lung tissue. Adoptively transferred CTLs persisted in the blood up to 21 days after infusion, and 5 patients achieved complete but transient remissions after therapy. The results of these studies illustrate the potential to selectively enhance graft-versus-leukemia activity by the adoptive transfer of mHAg-specific T-cell clones and the challenges for the broad application of this approach in allogeneic HCT. This study has been registered at http://clinicaltrials.gov as NCT00107354.

Multiple unit HLA-unmatched sex-mismatched umbilical cord blood transplantation for advanced hematological malignancy

We investigated the effect of multiple-unit umbilical cord blood (UCB) transplantation on engraftment in the setting of severe human leukocyte antigen (HLA) mismatch. Ten poor-risk adult patients with hematological malignancy received multiple unit, HLA-unmatched, sex-mismatched, unrelated UCB transplantation after a reduced intensity-conditioning regimen (RICR) with engraftment as the primary endpoint. The median age of the patients was 55 years with a range of 28-67. Patients received one unit of UCB per 10 kg of recipient body weight (5-7 units). The median number of nucleated cells and CD34(+) cells per kilogram of recipient body weight infused was 6.3 x 10(7) (range 3.8-10.0) (NC/kg) and 5.7 x 10(5) (range 1.1-11.9) (CD34/kg), respectively. Three patients expired before day 28 and were not evaluable for engraftment. Five of the remaining 7 patients showed increasing neutrophil counts. Fluorescent in situ hybridization (FISH) for the Y chromosome or HLA-typing showed only donor cells in the peripheral blood. After engraftment, HLA typing was done on 3 patients and their infused UCB units. All revealed the presence of a single HLA type concordant with one of the infused units. Moreover, the order of infusion did not influence which UCB unit engrafted. The engrafting UCB units were infused first or second in one case and fourth in the other two. One patient transplanted for refractory acute lymphoblastic leukemia (ALL) survives in continuous complete remission 4 years after transplant. He engrafted with one UCB unit, is fully hematologically reconstituted, has no evidence of graft-versus-host disease (GVHD), and takes no immunosuppressive medication. HLA typing reveals that the recipient and the engrafted cord blood match at only one HLA-B locus using conventional 6 antigen typing (A, B, and DR). Although engraftment was not accelerated, it did occur in the majority of evaluable patients. Long-term disease-free survivorship without debilitating GVHD is possible in patients with refractory hematological malignancy who receive unmatched multiple unit UCB.

HA-1 Mismatch Has Significant Effect in Chronic Allograft Nephropathy in Clinical Renal Transplantation

BACKGROUND

The minor histocompatibility antigen HA-1 occurs in two allelic forms: H and R. The HA-1(H) form presented in the context of HLA A2 can elicit specific cytotoxic lymphocyte (CTL) responses and can cause graft-versus-host disease in marrow transplants. However, its significance in solid organ transplants is unknown. We determined whether incompatibility of the HA-1 resulted in enhanced rejection and whether HA-1 specific CTLs were generated.

MATERIALS AND METHODS

HLA A2-matched donor/recipient pairs were selected and typed for HA-1 antigens by polymerase chain reaction. Nineteen of 81 pairs were mismatched for HA-1. Peripheral blood mononuclear leucocytes from five recipients, HLA A2 DR-matched with donors, were stimulated for 3 days with third-party donor, matched for HLA A2 DR but mismatched for HA-1. Cells were stained for surface markers, HA-1(H)-specific tetramer reagent, and analyzed by flow cytometry. Controls were unstimulated cells; PBML from two patients never exposed to HA-1(H); immunoglobulin G isotype-matched controls. For all patients, acute rejection rates posttransplant was ascertained. Long-term data was available for 36 patients.

RESULTS AND CONCLUSIONS

There was no difference in acute rejection rates between the HA-1-matched and -mismatched groups, but there was a significant difference in chronic rejection rates, evidenced by increased graft failures during the follow-up period (P = .0024). Lymphocytes from five HA-1-mismatched recipients were stimulated in vitro with cells from HLA-A2 and DR-matched but HA-1-mismatched surrogate donor. Though there seemed to be an excess of tetramer-positive cells, anti-HA-1-specific CTL responses were not conclusively elicited in vitro.

Mechanisms of minor histocompatibility antigen immunogenicity: the role of infinitesimal versus structurally profound polymorphisms

Minor histocompatibility antigens (mHAgs) are a diverse collection of major histocompatibility complex (MHC)-bound peptides that play a critical role in the induction of detrimental graft-versus-host disease (GVHD) or the development of beneficial graft-versustumor (GVT) effects after allogeneic hematopoietic stem cell transplantation. mHAgs are a consequence of allelic polymorphism that translates to disparity in MHC-presented peptide epitopes between transplant donor and recipient. This donor/recipient allelic disparity can range from infinitesimal amino side chain differences between MHC-presented peptides, to profound structural polymorphisms in genes and proteins that can nullify transcription or translation of one allelic variant and result in the complete abrogation of its presentation by MHC.

New directions in the genomics of allogeneic hematopoietic stem cell transplantation

Despite optimal supportive care and high-resolution HLA matching, complications such as GVHD and infection remain major barriers to the success of allogeneic HCT (allo-HCT). This has led to growing interest in the non-HLA genetic determinants of complications after allo-HCT. Most studies have examined genetic predictors of GVHD, relapse, and mortality and have focused on 3 main areas: minor histocompatibility antigen (miHAs), inflammatory mediators of GVHD, and more recently NK cell-mediated allorecognition. The genetic basis of other outcomes such as infection and drug toxicity are less well studied but are being actively investigated. High-throughput methodologies such as single nucleotide polymorphism arrays are enabling the study of hundreds of thousands of genetic markers throughout the genome and the interrogation of novel genetic variants such as copy number variations. These data offer the opportunity to better predict those at risk of complications and to identify novel targets for therapeutic intervention. This review examines the current data regarding the non-HLA genomics of allo-HCT and appraises the promises and pitfalls for integration of this new genetic information into clinical transplantation practice.

Advances in the understanding of acute graft-versus-host disease

Allogeneic stem cell transplantation (SCT) remains the definitive immunotherapy for malignancy. However, morbidity and mortality due to graft-vs.-host disease (GVHD) remains the major barrier to its advancement. Emerging experimental data highlights the immuno-modulatory roles of diverse cell populations in GVHD, including regulatory T cells, natural killer (NK) cells, NK T cells, gammadelta T cells, and antigen presenting cells (APC). Knowledge of the pathophysiology of GVHD has driven the investigation of new rational strategies to both prevent severe GVHD and treat steroid-refractory GVHD. Novel cytokine inhibitors, immune-suppressant agents known to preserve or even promote regulatory T-cell function and the depletion of specific alloreactive T-cell sub-populations all promise significant advances in the near future. As our knowledge and therapeutic options expand, the ability to limit GVHD whilst preserving anti-microbial and tumour responses becomes a realistic prospect.

Evaluation of alloreactivity in responder-stimulator pairs by determination of gamma interferon-producing cells and cytotoxic-T-lymphocyte precursor frequencies

We used the enzyme-linked immunospot (ELISPOT) assay and the cytotoxic-T-lymphocyte precursor frequency assay to evaluate alloreactivity in responder-stimulator pairs. High frequencies of responder cells among cells from HLA-mismatched pairs and low frequencies among cells from pairs of siblings with identical HLA types were detected by both assays. The ELISPOT assay thus illustrated the helper and cytotoxic-T-cell response to allogeneic HLA antigens.

Mapping of susceptibility and protective loci for acute GVHD in unrelated HLA-matched bone marrow transplantation donors and recipients using 155 microsatellite markers on chromosome 22

Despite matching donors and recipients for the human leukocyte antigens (HLAs) expressed by the major histocompatibility genomic region of the short arm of chromosome 6, several recipients still develop acute graft-versus-host disease (aGVHD) after bone marrow transplantation (BMT). This is possibly due to non-HLA gene polymorphisms, such as minor histocompatibility antigens (mHas) and genes coding for cytokines. However, a detailed genetic background for aGVHD has not yet been established. To find novel susceptibility and/or protective loci for aGVHD, a whole genome-wide association study of donors and recipients needs to be performed. As the first step to such a study, we retrospectively analyzed polymorphisms of 155 microsatellite markers spread across the long arm of chromosome 22 in 70 pairs of HLA-matched unrelated BMT donors and recipients. We performed individual typing and then compared the markers' allele frequencies (1) between all the aGVHD (grades III and IV GVHD) and GVHD-free (grade 0 GVHD) groups in donors and recipients and (2) between the aGVHD and aGVHD-free groups in donor/recipient pairs that were matched and mismatched for the microsatellite marker's allele. Screening of the microsatellite markers revealed five loci with a significant difference between the aGVHD and GVHD-free groups and revealed eight loci on chromosome 22, where the microsatellite allele mismatched markers were associated with aGVHD. This screening analysis suggests that several aGVHD-associated susceptible and protective loci exist on chromosome 22, which may encompass novel gene regions that need to be elucidated for their role in aGVHD.

Alloreactive T cell clonotype recruitment in a mixed lymphocyte reaction: Implications for graft engineering

OBJECTIVE

The selective elimination of alloreactive T cells from donor stem cell grafts prior to hematopoietic stem cell transplantation (HSCT) is an important goal in the prevention of graft-vs-host disease (GVHD). However, in HLA-identical donor-recipient pairs, it has proven difficult to identify alloreactive T cells using in vitro systems pretransplant due, in part, to their low frequency and a lack of methodological standardization. To better understand the alloresponse between HLA-identical related pairs, we characterized the alloreactive T cells generated in a mixed lymphocyte reaction (MLR) assay system.

METHODS

HSCT donor peripheral blood mononuclear cells (responder) were labeled with carboxyfluorescein diacetate, succinimidyl ester (CFSE) dye and cocultured with irradiated HSCT recipient cells (stimulator) in a one-way MLR. Alloreactive T cells were sorted by upregulation of activation markers (CD25 in most cases) and the responding clonotypes were defined by sequencing the complementarity region 3 (CDR3) of the T cell receptor beta-chain.

RESULTS

We show that the recruitment of alloreactive CD4(+) T cells is highly variable. Oligoclonal CD4(+) T-cell expansions in repeated MLRs performed in the same donor-recipient pair showed inconsistent recruitment of clonotypes. The recruitment of alloreactive CD8(+) T cells was more consistent in repeated assays, with the same clonotypes identified in the same donor-recipient pair performed under different conditions.

CONCLUSION

Taken together, our data show that even in culture conditions constrained to eliminate background proliferation, stochastic events and low precursor frequencies preclude reproducible elicitation of immunodominant T cell clonotypes with the potential to cause GVHD.

The PANE1 gene encodes a novel human minor histocompatibility antigen that is selectively expressed in B-lymphoid cells and B-CLL

Minor histocompatibility antigens (mHAg's) are peptides encoded by polymorphic genes that are presented by major histocompatibility complex (MHC) molecules and recognized by T cells in recipients of allogeneic hematopoietic cell transplants. Here we report that an alternative transcript of the proliferation-associated nuclear element 1 (PANE1) gene encodes a novel human leukocyte antigen (HLA)-A(*)0301-restricted mHAg that is selectively expressed in B-lymphoid cells. The antigenic peptide is entirely encoded within a unique exon not present in other PANE1 transcripts. Sequencing of PANE1 alleles in mHAg-positive and mHAg-negative cells demonstrates that differential T-cell recognition is due to a single nucleotide polymorphism within the variant exon that replaces an arginine codon with a translation termination codon. The PANE1 transcript that encodes the mHAg is expressed at high levels in resting CD19(+) B cells and B-lineage chronic lymphocytic leukemia (B-CLL) cells, and at significantly lower levels in activated B cells. Activation of B-CLL cells through CD40 ligand (CD40L) stimulation decreases expression of the mHAg-encoding PANE1 transcript and reciprocally increases expression of PANE1 transcripts lacking the mHAg-encoding exon. These studies suggest distinct roles for different PANE1 isoforms in resting compared with activated CD19(+) cells, and identify PANE1 as a potential therapeutic target in B-CLL.

Therapeutic and Diagnostic Applications of Minor Histocompatibility Antigen HA-1 and HA-2 Disparities in Allogeneic Hematopoietic Stem Cell Transplantation: A Survey of Different Populations

Minor histocompatibility antigens (mHags) HA-1 and HA-2 are encoded by biallelic loci, with immunogenic variants, HA-1H and HA-2V, which induce strong HLA-A2-restricted alloreactive T-cell responses, and nonimmunogenic counterparts, HA-1R and HA-2M, which represent functional null alleles that are poorly presented by HLA class I molecules. HA-1 and HA-2 are potential targets of selective graft-versus-leukemia and graft-versus-tumor reactivity after allogeneic hematopoietic stem cell transplantation (HSCT); however, these applications are restricted to a limited number of patients. Here, we show that a far more frequent application of HA-1 and HA-2 disparity relies on their use as markers for the state of host chimerism after allogeneic HSCT. We have determined allelic frequencies of 29.3% and 70.7% for HA-1H and HA-1R, respectively, and of 83.7% and 16.3% for HA-2V and HA-2M, respectively, in >200 healthy individuals from northern Italy. Similar frequencies were observed in nearly 100 patients affected by hematologic malignancies or solid tumors, thus showing that HA-1 and HA-2 variability are not associated with the presence of cancer. On the basis of these data, we predict that HA-1 and HA-2 can be used in 32.8% and 23.5% of Italian transplant patients, respectively, as markers for the state of host chimerism, whereas exploitation of disparity for these mHags for targeted immunotherapy will be possible in 10.7% and 1.1% of Italian patients, respectively. Retrospective HA-2 typing of bone marrow aspirates obtained from a patient during complete remission or recurrence of acute myeloid leukemia after haploidentical HSCT showed the feasibility of using HA-2 as a surrogate marker for disease monitoring. Because of an apparent north-south gradient for HA-1 allelic frequencies, with higher frequencies for the HA-1H variant reported in white populations from Southern Europe as compared with Northern Europe and North America, the diagnostic applicability of HA-1 disparity will be slightly more frequent in transplant patients from the north. Taken together, our data show that determination of HA-1 and HA-2 variability can be an important parameter for the selection of allogeneic stem cell donors, in particular for patients affected by hematologic malignancies without a tumor-specific molecular marker.

Antibody-mediated organ-allograft rejection

Recent studies show that alloantibodies mediate a substantial proportion of graft-rejection episodes, contributing to both early and late graft loss. Rejection that is caused by antibody is mediated by different mechanisms from rejection that is caused by T cells, thereby requiring other approaches to treatment and prevention. Antibody induces rejection acutely through the fixation of complement, resulting in tissue injury and coagulation. In addition, complement activation recruits macrophages and neutrophils, causing additional endothelial injury. Antibody and complement also induce gene expression by endothelial cells, which is thought to remodel arteries and basement membranes, leading to fixed and irreversible anatomical lesions that permanently compromise graft function.