Minor histocompatibility antigens: allo target molecules for tumor-specific immunotherapy

Beyond the anti-PD-1/PD-L1 era: promising role of the BTLA/HVEM axis as a future target for cancer immunotherapy

Recent introduction of monoclonal antibodies targeting immune checkpoints to harness antitumor immunity has revolutionized the cancer treatment landscape. The therapeutic success of immune checkpoint blockade (ICB)-based therapies mainly relies on PD-1/PD-L1 and CTLA-4 blockade. However, the limited overall responses and lack of reliable predictive biomarkers of patient´s response are major pitfalls limiting immunotherapy success. Hence, this reflects the compelling need of unveiling novel targets for immunotherapy that allow to expand the spectrum of ICB-based strategies to achieve optimal therapeutic efficacy and benefit for cancer patients. This review thoroughly dissects current molecular and functional knowledge of BTLA/HVEM axis and the future perspectives to become a target for cancer immunotherapy. BTLA/HVEM dysregulation is commonly found and linked to poor prognosis in solid and hematological malignancies. Moreover, circulating BTLA has been revealed as a blood-based predictive biomarker of immunotherapy response in various cancers. On this basis, BTLA/HVEM axis emerges as a novel promising target for cancer immunotherapy. This prompted rapid development and clinical testing of the anti-BTLA blocking antibody Tifcemalimab/icatolimab as the first BTLA-targeted therapy in various ongoing phase I clinical trials with encouraging results on preliminary efficacy and safety profile as monotherapy and combined with other anti-PD-1/PD-L1 therapies. Nevertheless, it is anticipated that the intricate signaling network constituted by BTLA/HVEM/CD160/LIGHT involved in immune response regulation, tumor development and tumor microenvironment could limit therapeutic success. Therefore, in-depth functional characterization in different cancer settings is highly recommended for adequate design and implementation of BTLA-targeted therapies to guarantee the best clinical outcomes to benefit cancer patients.

Shared graft-versus-leukemia minor histocompatibility antigens in DISCOVeRY-BMT

T-cell responses to minor histocompatibility antigens (mHAs) mediate graft-versus-leukemia (GVL) effects and graft-versus-host disease (GVHD) in allogeneic hematopoietic cell transplantation. Therapies that boost T-cell responses improve allogeneic hematopoietic cell transplant (alloHCT) efficacy but are limited by concurrent increases in the incidence and severity of GVHD. mHAs with expression restricted to hematopoietic tissue (GVL mHAs) are attractive targets for driving GVL without causing GVHD. Prior work to identify mHAs has focused on a small set of mHAs or population-level single-nucleotide polymorphism-association studies. We report the discovery of a large set of novel GVL mHAs based on predicted immunogenicity, tissue expression, and degree of sharing among donor-recipient pairs (DRPs) in the DISCOVeRY-BMT data set of 3231 alloHCT DRPs. The total number of predicted mHAs varied by HLA allele, and the total number and number of each class of mHA significantly differed by recipient genomic ancestry group. From the pool of predicted mHAs, we identified the smallest sets of GVL mHAs needed to cover 100% of DRPs with a given HLA allele. We used mass spectrometry to search for high-population frequency mHAs for 3 common HLA alleles. We validated 24 predicted novel GVL mHAs that are found cumulatively within 98.8%, 60.7%, and 78.9% of DRPs within DISCOVeRY-BMT that express HLA-A∗02:01, HLA-B∗35:01, and HLA-C∗07:02, respectively. We confirmed the immunogenicity of an example novel mHA via T-cell coculture with peptide-pulsed dendritic cells. This work demonstrates that the identification of shared mHAs is a feasible and promising technique for expanding mHA-targeting immunotherapeutics.

60 Years Young: The Evolving Role of Allogeneic Hematopoietic Stem Cell Transplantation in Cancer Immunotherapy

The year 2020 marked the 30th anniversary of the Nobel Prize in Medicine awarded to E. Donnall Thomas for the development of allogeneic hematopoietic stem cell transplantation (allo-HSCT) to treat hematologic malignancies and other blood disorders. Dr. Thomas, "father of bone marrow transplantation," first developed and reported this technique in 1957, and in the ensuing decades, this seminal study has impacted fundamental work in hematology and cancer research, including advances in hematopoiesis, stem cell biology, tumor immunology, and T-cell biology. As the first example of cancer immunotherapy, understanding the mechanisms of antitumor biology associated with allo-HSCT has given rise to many of the principles used today in the development and implementation of novel transformative immunotherapies. Here we review the historical basis underpinning the development of allo-HSCT as well as advances in knowledge obtained by defining mechanisms of allo-HSCT activity. We review how these principles have been translated to novel immunotherapies currently utilized in clinical practice and describe potential future applications for allo-HSCT in cancer research and development of novel therapeutic strategies.

The Connection Between Minor H Antigens and Neoantigens and the Missing Link in Their Prediction

For hundreds of thousands of years, the human genome has extensively evolved, resulting in genetic variations in almost every gene. Immunological reflections of these genetic variations become clearly visible after an allogeneic stem cell transplantation (allo-SCT) as minor Histocompatibility (H) antigens. Minor H antigens are peptides cleaved from genetically encoded variable protein regions after which they are presented at the cell surface by HLA molecules. After allo-SCT with minor H antigen mismatches between donor and recipient, donor T cells recognize the minor H antigens of the recipient as foreign, evoking strong alloreactive immune responses. Studies in the late eighties have discovered that a subset of minor H antigens are encoded by hematopoietic system-specific genes. After allo-SCT, this subset is strictly expressed on the hematopoietic malignant cells and was therefore the first well-defined highly immunogenic group of tumor-specific antigens. In the last decade, neoantigens derived from genetic mutations in tumors have been identified as another group of immunogenic tumor-specific antigens. Therefore, hematopoietic minor H antigens and neoantigens are therapeutic equivalents. This review will connect our current knowledge about the immune biology and identification of minor H antigens and neoantigens leading to novel conclusions on their prediction.

H60: A Unique Murine Hematopoietic Cell-Restricted Minor Histocompatibility Antigen for Graft-versus-Leukemia Effect

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an important treatment for many types of hematological malignancies. Matching of donor and recipient for the major histocompatibility complex (MHC) improves the HSCT reconstitution, but donor-derived T cells reactive to non-MHC encoded minor histocompatibility antigens (MiHAs) can induce graft-versus-host disease (GVHD) while also being needed for graft-versus-leukemia (GVL) effects. MiHAs are allelically variant self-peptides presented conventionally on MHC molecules, but are alloantigenic in transplantation settings. Immunodominant MiHAs are most strongly associated with GVHD and GVL. There is need for mouse paradigms to understand these contradictory effects. H60 is a highly immunodominant mouse MiHA with hematopoietic cell-restricted expression. Immunodominance of H60 is tightly associated with its allelic nature (presence vs. absence of the transcripts), and the qualitative (TCR diversity) and quantitative (frequency) traits of the reactive T cells. The identity as a hematopoietic cell-restricted antigen (HRA) of H60 assists the appearance of the immunodominace in allo-HSCT circumstances, and generation of GVL effects without induction of serious GVHD after adoptive T cell transfer. Also it allows the low avidity T cells to escape thymic negative selection and exert GVL effect in the periphery, which is a previously unevaluated finding related to HRAs. In this review, we describe the molecular features and immunobiology in detail through which H60 selectively exerts its potent GVL effect. We further describe how lessons learned can be extrapolated to human allo-HCST.

Immunotherapy in myeloma: how far have we come?

The treatment of multiple myeloma (MM) has evolved substantially over the past decades, leading to a significantly improved outcome of MM patients. The introduction of high-dose therapy, especially, and autologous stem cell transplantation, as well as the development of new drugs, such as immunomodulatory drugs (IMiDs) and proteasome inhibitors have contributed to the improvement in survival. However, eventually most MM patients relapse, which indicates that there is a need for new agents and novel treatment strategies. Importantly, the long-term survival in a subset of MM patients after allogeneic stem cell transplantation illustrates the potential of immunotherapy in MM, but allogeneic stem cell transplantation is also associated with a high rate of treatment-related mortality. Recently, a better insight into several immune-evasion mechanisms, which contribute to tumor progression, has resulted in the development of active and well-tolerated novel forms of immunotherapy. These immunotherapeutic agents can be used as monotherapy, or, even more successfully, in combination with other established anti-MM agents to further improve depth and duration of response by preventing the outgrowth of resistant clones. This review will discuss the mechanisms used by MM cells to evade the immune system, and also provide an overview of currently approved immunotherapeutic drugs, such as IMiDs (e.g. lenalidomide and pomalidomide) and monoclonal antibodies that target cell surface antigens present on the MM cell (e.g. elotuzumab and daratumumab), as well as novel immunotherapies (e.g. chimeric antigen receptor T-cells, bispecific antibodies and checkpoint inhibitors) currently in clinical development in MM.

Efficacy of host-dendritic cell vaccinations with or without minor histocompatibility antigen loading, combined with donor lymphocyte infusion in multiple myeloma patients

Donor lymphocyte infusions (DLI) can induce durable remissions in multiple myeloma (MM) patients, but this occurs rather infrequently. As the graft-versus-tumor (GvT) effect of DLI depends on the presence of host-dendritic cells (DCs), we tested in a phase I/II trial whether the efficacy of DLI could be improved by simultaneous vaccination with host-DCs. We also analyzed the possibility of further improving the GvT effect by loading the DCs with peptides of mismatched hematopoietic cell-specific minor histocompatibility antigens (mHags). Fifteen MM patients not responding to a first DLI were included. Eleven patients could be treated with a second equivalent dose DLI combined with DC vaccinations, generated from host monocytes (moDC). For four patients, the DC products did not meet the quality criteria. In four of the treated patients the DCs were loaded with host mHag peptides. Toxicity was limited and no acute GvHD occurred. Most patients developed objective anti-host T-cell responses and in one patient a distinct mHag-specific T-cell response accompanied a temporary clinical response. These findings confirm that DLI combined with host-DC vaccination, either unloaded or loaded with mHag peptides, is feasible, safe and capable of inducing host-specific T-cell responses. The limited clinical effects may be improved by developing more immunogenic DC products or by combining this therapy with immune potentiating modalities like checkpoint inhibitors.

The double-edge role of B cells in mediating antitumor T-cell immunity: Pharmacological strategies for cancer immunotherapy

Emerging evidence reveals the controversial role of B cells in antitumor immunity, but the underlying mechanisms have to be explored. Three latest articles published in the issue 521 of Nature in 2015 reconfirmed the puzzling topic and put forward some explanations of how B cells regulate antitumor T-cell responses both positively and negatively. This paper attempts to demonstrate that different B-cell subpopulations have distinct immunological properties and that they are involved in either antitumor responses or immunosuppression. Recent studies supporting the positive and negative roles of B cells in tumor development were summarized comprehensively. Several specific B-cell subpopulations, such as IgG(+), IgA(+), IL-10(+), and regulatory B cells, were described in detail. The mechanisms underlying the controversial B-cell effects were mainly attributed to different B-cell subpopulations, different B-cell-derived cytokines, direct B cell-T cell interaction, different cancer categories, and different malignant stages, and the immunological interaction between B cells and T cells is mediated by dendritic cells. Promising B-cell-based antitumor strategies were proposed and novel B-cell regulators were summarized to present interesting therapeutic targets. Future investigations are needed to make sure that B-cell-based pharmacological strategies benefit cancer immunotherapy substantially.

Minor Antigen Vaccine-Sensitized DLI: In Vitro Responses Do Not Predict In Vivo Effects

Background

We reported on a pilot study of minor histocompatibility antigen vaccination using constructs expressing male-specific gene disparities of selected mouse CDNA on Y and sex determining region Y in the canine model. We performed reduced-intensity hematopoietic cell transplantation with female donors and male recipients, producing stable mixed donor-recipient hematopoietic chimeras. We then performed a vaccine series in three female transplant donors followed by donor lymphocyte infusion (DLI) into their respective mixed chimeras. One mixed chimera experienced a significant shift in the percentage of donor chimerism, but no response occurred in the other 2 recipients. We then hypothesized that inadequate donor sensitization was responsible for these results.

Methods

To test this hypothesis, we added 4 monthly booster vaccinations to 2 of the original hematopoietic cell transplantation donors, including the donor that drove the partial response, followed by a second DLI.

Results

Strong T cell responses were shown by ELISpot and confirmed by intracellular cytokine staining in both donors. A second DLI resulted in a further increase in donor chimerism in the same mixed chimera that experienced the previous increase, but no change in donor chimerism was again seen in the other recipient. Evaluation of RNA expression of the target antigens demonstrated that conversion occurred in the recipient that expressed both selected mouse CDNA on Y and sex determining region Y.

Conclusions

T cell responses against Y chromosome-encoded disparities were not necessarily sufficient to drive in vivo female antimale responses. Other factors including the presence of specific haplotypes or the heterogeneous expression of the target antigen may affect T cell responses against minor histocompatibility antigens. These results warrant future vaccine studies in a larger transplant cohort using epigenetic modulation of the recipient to promote target gene expression.

Minor histocompatibility Ags: identification strategies, clinical results and translational perspectives

Allogeneic stem cell transplantation (allo-SCT) and donor lymphocyte infusion are effective treatment modalities for various hematological malignancies. Their therapeutic effect, the graft-versus-tumor (GvT) effect, is based mainly on an alloimmune response of donor T cells directed at tumor cells, in which differences in the expression of minor histocompatibility Ags (mHags) on the cells of the patient and donor have a crucial role. However, these differences are also responsible for induction of sometimes detrimental GvHD. As relapse and development of GvHD pose major threats for a large proportion of allotransplanted patients, additional therapeutic strategies are required. To augment the GvT response without increasing the risk of GvHD, specific mHag-directed immunotherapeutic strategies have been developed. Over the past years, much effort has been put into the identification of therapeutically relevant mHags to enable these strategies for a substantial proportion of patients. Currently, the concept of mHag-directed immunotherapy is tested in clinical trials on feasibility, safety and efficacy. In this review, we will summarize the recent developments in mHag identification and the clinical data on mHag-specific immune responses and mHag-directed therapies in patients with hematological malignancies. Finally, we will outline the current challenges and future prospectives in the field.

Development of a Minor Histocompatibility Antigen Vaccine Regimen in the Canine Model of Hematopoietic Cell Transplantation

BACKGROUND

Minor histocompatibility antigen (miHA) vaccines have the potential to augment graft-versus-tumor effects without graft-versus-host disease (GVHD). We used mixed hematopoietic chimerism in the canine model of major histocompatibility complex-matched allogeneic hematopoietic cell transplantation as a platform to develop a miHA vaccination regimen.

METHODS

We engineered DNA plasmids and replication-deficient human adenovirus type 5 constructs encoding large sections of canine SMCY and the entire canine SRY gene.

RESULTS

Priming with replication-deficient human adenovirus type 5 constructs and boosting with ex vivo plasmid-transfected dendritic cells and cutaneous delivery of plasmids with a particle-mediated epidermal delivery device (PMED) in 2 female dogs induced antigen-specific T-cell responses. Similar responses were observed after a prime-boost vaccine regimen in three female hematopoietic cell transplantation donors. Subsequent donor lymphocyte infusion resulted in a significant change of chimerism in 1 of 3 male recipients without any signs of graft-versus-host disease. The change in chimerism in the recipient occurred in association with the development of CD4+ and CD8+ T-cell responses to the same peptide pools detected in the donor.

CONCLUSIONS

These studies describe the first in vivo response to miHA vaccination in a large, outbred animal model without using recipient cells to sensitize the donor. This model provides a platform for ongoing experiments designed to define optimal miHA targets and develop protocols to directly vaccinate the recipient.

Programmed death-1 checkpoint blockade in acute myeloid leukemia

INTRODUCTION

Immune checkpoints are regulatory pathways induced in activated T lymphocytes that regulate antigen responsiveness. These immune checkpoints are hijacked by tumors to promote dysfunction of anti-tumor effector cells and consequently of tumor escape from the host immune system.

AREAS COVERED

Programmed death-1/programmed death ligand (PD-1/PDL-1), a checkpoint pathway, has been extensively investigated in leukemia mouse models. Expression of PD-1 on the surface of activated immune cells and of its ligands, PD-L1 and PD-L2, on leukemic blasts has been documented. Clinical trials with PD-1 inhibitors in patients with hematological malignancies are ongoing with promising clinical responses.

EXPERT OPINION

Therapy of hematological cancers with antibodies blocking inhibitory receptors is expected to be highly clinically effective. Checkpoint inhibitory receptors and their ligands are co-expressed on hematopoietic cells found in the leukemic milieu. Several distinct immunological mechanisms are likely to be engaged by antibody-based checkpoint blockade. Co-expression of multiple inhibitory receptors on hematopoietic cells offers an opportunity for combining blocking antibodies to achieve more effective therapy. Up-regulation of receptor/ligand expression in the leukemic milieu may provide a blood marker predictive of response. Finally, chemotherapy-induced up-regulation of PD-1 on T cells after conventional leukemia therapy creates a solid rationale for application of checkpoint blockade as a follow-up therapy.

Allogeneic IgG combined with dendritic cell stimuli induce antitumour T-cell immunity

While cancers grow in their hosts and evade host immunity through immunoediting and immunosuppression^1–5, tumors are rarely transmissible between individuals. Much like transplanted allogeneic organs, allogeneic tumors are reliably rejected by host T cells, even when the tumor and host share the same major histocompatibility complex (MHC) alleles, the most potent determinants of transplant rejection^6–10. How such tumor-eradicating immunity is initiated remains unknown, though elucidating this process could provide a roadmap for inducing similar responses against naturally arising tumors. We found that allogeneic tumor rejection is initiated by naturally occurring tumor-binding IgG antibodies, which enable dendritic cells (DC) to internalize tumor antigens and subsequently activate tumor-reactive T cells. We exploited this mechanism to successfully treat autologous and autochthonous tumors. Either systemic administration of DC loaded with allogeneic IgG (alloIgG)-coated tumor cells or intratumoral injection of alloIgG in combination with DC stimuli induced potent T cell mediated anti-tumor immune responses, resulting in tumor eradication in mouse models of melanoma, pancreas, lung and breast cancer. Moreover, this strategy led to eradication of distant tumors and metastases, as well as the injected primary tumors. To assess the clinical relevance of these findings, we studied antibodies and cells from patients with lung cancer. T cells from these patients responded vigorously to autologous tumor antigens after culture with alloIgG-loaded DC, recapitulating our findings in mice. These results reveal that tumor-binding alloIgG can induce powerful anti-tumor immunity that can be exploited for cancer immunotherapy.

Nonmyeloablative transplantation for solid tumors: a new frontier for allogeneic immunotherapy

The failure of conventional chemotherapy to improve survival in a large percentage of patients with advanced solid tumors has prompted the development of alternative anticancer approaches. Conventional allogeneic hematopoietic stem cell transplantation (HSCT) relies on myeloablative conditioning to eradicate the underlying disease, as well as suppress the patient's immune response, allowing engraftment of the donor's lymphohematopoietic system. Such preparative regimens are frequently associated with serious hematologic and nonhematologic toxicities, resulting in substantial morbidity and mortality. A significant curative component of allogeneic HSCT is the immune-mediated graft-versus-tumor (GVT) effect. Nonmyeloablative preparative regimens were designed to suppress host immunity to allow for sufficient engraftment of the donor immune system for the subsequent generation of GVT effects. These relatively low-dose preparative regimens are generally well tolerated and are associated with a reduction in the risk of transplant-related mortality. Nonmyeloablative HSCT provides a safer platform to explore the efficacy of allogeneic HSCT in patients with solid tumors. Initial reports have demonstrated that GVT may occur against several different solid tumors, including renal cell carcinoma, ovarian cancer, breast cancer and others. Based on these preliminary encouraging results, further exploration of nonmyeloablative HSCT for solid tumors is clearly warranted. The development of strategies to decrease graft-versus-host disease while enhancing post-transplant antitumor immunity will hopefully be forthcoming in the near future.

Allogeneic stem cell transplantation for patients with metastatic renal cell carcinoma

Allogeneic stem cell transplantation and donor lymphocyte infusions are currently under clinical investigation as an innovative therapeutic option for patients with metastatic renal cell carcinoma. A variety of trials have proven the clinical efficacy of allogeneic stem cell transplantation using reduced-intensity conditioning protocols and donor lymphocyte infusions, as demonstrated by the induction of objective remissions in metastatic renal cell carcinoma patients. However, despite clinical remissions, reduced-intensity conditioning protocols and donor lymphocyte infusions were associated with a high treatment-related mortality rate of approximately 17%. The disproportion between clinical efficacy and treatment-related mortality may mainly be caused by the selection of patients that had often been heavily pretreated, with a large tumor burden and rapidly progressing tumors. The improvement of efficacy with the preservation of a powerful graft-versus-tumor effect while reducing the toxicity, is the major experimental and clinical challenge of allogeneic stem cell transplantation in the treatment of metastatic renal cancer and other solid tumors. Recently, there has been a revolutionary development of molecular-targeted agents in metastatic renal cancer. These inhibitors of angiogenesis and signal-transduction pathways have demonstrated clinical efficacy and significant survival prolongation in the first- and second-line settings, while causing moderate toxicity. Some of these agents have already been approved by the US FDA and will probably replace standard cytokines, such as interferon-alpha2 and interleukin-2, in metastatic renal cancer. In the context of these innovative clinical developments, allogeneic stem cell transplantation clearly has to be regarded an investigational clinical treatment approach. Therefore, patients should only be treated at centers that are experienced in clinical trials, and patient selection remains a critical factor for a successful transplant procedure.

Minor histocompatibility antigens and the maternal immune response to the fetus during pregnancy

The tolerance of the semiallogeneic fetus by the maternal immune system is an important area of research for understanding how the maternal and fetal systems interact during pregnancy to ensure a successful outcome. Several lines of research reveal that the maternal immune system can recognize and respond to fetal minor histocompatibility antigens during pregnancy. Reactions to these antigens arise because of allelic differences between the mother and fetus and have been shown more broadly to play an important role in mediating transplantation outcomes. This review outlines the discovery of minor histocompatibility antigens and their importance in solid organ and hematopoietic stem cell transplantations, maternal T-cell responses to minor histocompatibility antigens during pregnancy, expression of minor histocompatibility antigens in the human placenta, and the potential involvement of minor histocompatibility antigens in the development and manifestation of pregnancy complications.

B and T lymphocyte attenuator mediates inhibition of tumor-reactive CD8+ T cells in patients after allogeneic stem cell transplantation

Allogeneic stem cell transplantation (allo-SCT) can cure hematological malignancies by inducing alloreactive T cell responses targeting minor histocompatibility antigens (MiHA) expressed on malignant cells. Despite induction of robust MiHA-specific T cell responses and long-term persistence of alloreactive memory T cells specific for the tumor, often these T cells fail to respond efficiently to tumor relapse. Previously, we demonstrated the involvement of the coinhibitory receptor programmed death-1 (PD-1) in suppressing MiHA-specific CD8(+) T cell immunity. In this study, we investigated whether B and T lymphocyte attenuator (BTLA) plays a similar role in functional impairment of MiHA-specific T cells after allo-SCT. In addition to PD-1, we observed higher BTLA expression on MiHA-specific CD8(+) T cells compared with that of the total population of CD8(+) effector-memory T cells. In addition, BTLA's ligand, herpes virus entry mediator (HVEM), was found constitutively expressed by myeloid leukemia, B cell lymphoma, and multiple myeloma cells. Interference with the BTLA-HVEM pathway, using a BTLA blocking Ab, augmented proliferation of BTLA(+)PD-1(+) MiHA-specific CD8(+) T cells by HVEM-expressing dendritic cells. Notably, we demonstrated that blocking of BTLA or PD-1 enhanced ex vivo proliferation of MiHA-specific CD8(+) T cells in respectively 7 and 9 of 11 allo-SCT patients. Notably, in 3 of 11 patients, the effect of BTLA blockade was more prominent than that of PD-1 blockade. Furthermore, these expanded MiHA-specific CD8(+) T cells competently produced effector cytokines and degranulated upon Ag reencounter. Together, these results demonstrate that BTLA-HVEM interactions impair MiHA-specific T cell functionality, providing a rationale for interfering with BTLA signaling in post-stem cell transplantation therapies.

Anti-thymocyte globulin for conditioning in matched unrelated donor hematopoietic cell transplantation provides comparable outcomes to matched related donor recipients

Rabbit anti-thymocyte globulin (ATG) is used as prophylaxis against GVHD following allogeneic hematopoietic cell transplantation (HCT). At our institution, ATG is exclusively used in the conditioning of matched unrelated donor (URD) transplant recipients. A total of 50 URD HCT recipients who received ATG (ATG group) were retrospectively compared with 48 matched related donor (MRD) HCT recipients who did not receive ATG (no ATG group). There were no significant differences between the groups in rates of day 100 mortality, acute GVHD or relapse. Chronic GVHD incidence was significantly lower in the ATG group (P = 0.007). At a median follow-up of 36 months in the entire cohort, 50% patients are alive in the ATG group and 63% of the patients are alive in the no ATG group (P = 0.13). We conclude that the administration of ATG to patients undergoing URD HCT preserves the anti-leukemia benefit of the transplant, while reducing the risk of developing GVHD, resulting in OS rates that are comparable to MRD HCT recipients.

PD-1/PD-L1 interactions contribute to functional T-cell impairment in patients who relapse with cancer after allogeneic stem cell transplantation

Tumor relapses remain a serious problem after allogeneic stem cell transplantation (alloSCT), despite the long-term persistence of minor histocompatibility antigen (MiHA)-specific memory CD8(+) T cells specific for the tumor. We hypothesized that these memory T cells may lose their function over time in transplanted patients. Here, we offer functional and mechanistic support for this hypothesis, based on immune inhibition by programmed death-1 (PD-1) expressed on MiHA-specific CD8(+) T cells and the associated role of the PD-1 ligand PD-L1 on myeloid leukemia cells, especially under inflammatory conditions. PD-L1 was highly upregulated on immature human leukemic progenitor cells, whereas costimulatory molecules such as CD80 and CD86 were not expressed. Thus, immature leukemic progenitor cells seemed to evade the immune system by inhibiting T-cell function via the PD-1/PD-L1 pathway. Blocking PD-1 signaling using human antibodies led to elevated proliferation and IFN-γ production of MiHA-specific T cells cocultured with PD-L1-expressing leukemia cells. Moreover, patients with relapsed leukemia after initial MiHA-specific T-cell responses displayed high PD-L1 expression on CD34(+) leukemia cells and increased PD-1 levels on MiHA-specific CD8(+) T cells. Importantly, blocking PD-1/PD-L1 interactions augment proliferation of MiHA-specific CD8(+) memory T cells from relapsed patients. Taken together, our findings indicate that the PD-1/PD-L pathway can be hijacked as an immune escape mechanism in hematological malignancies. Furthermore, they suggest that blocking the PD-1 immune checkpoint offers an appealing immunotherapeutic strategy following alloSCT in patients with recurrent or relapsed disease.

On the use of donor-derived iNKT cells for adoptive immunotherapy to prevent leukemia recurrence in pediatric recipients of HLA haploidentical HSCT for hematological malignancies

T-cell-depleted hematopoietic stem cell transplantation from an HLA haploidentical relative (hHSCT) is a useful therapy for children with high-risk leukemia lacking suitable HLA-matched donors. The immune deficiency ensuing hHSCT renders patients susceptible to life-threatening infections and disease recurrence. Adoptive immunotherapy can restore/enhance early post-transplantation immunocompetence of hHSCT recipients. Efforts are directed to identify strategies for inducing graft-versus-leukemia (GVL) response, while avoiding graft-versus-host disease (GVHD) occurrence. CD1d-restricted invariant iNKT cells are innate-like, lipid-reactive T lymphocytes implicated in the control of innate and adaptive immunity. Preclinical data suggest that iNKT cells positively modulate both GVL response and GVHD. Our recent findings in a cohort of 22 children given hHSCT for different hematological malignancies show that failure to reconstitute iNKT cells after transplantation correlates with leukemia relapse. In this review, we will discuss potential new options for adoptively transferring donor-derived iNKT cells into hHSCT recipients in the early post-transplantation period to prevent disease recurrence.

A pilot study of allogeneic cellular therapy for patients with advanced hematologic malignancies

Allogeneic hematopoietic stem cell transplantation provides curative therapy for some patients with advanced hematologic malignancies. Disease response after allogeneic transplant is, at least in part, mediated by donor immune cells. In this report we describe a cellular therapy using haploidentical peripheral blood stem cells administered after very low dose total body irradiation (TBI) (100cGy). The donor cells were anticipated to be rejected, so no graft-versus-host (GVHD) prophylaxis was used. Patients with persistent disease beyond 8 weeks could be further treated with infusions of irradiated haploidentical donor cells. Of the 10 patients enrolled in the study, durable engraftment of allogeneic cells was seen in one patient. Two patients with resistant relapsed acute myelogenous leukemia (AML) had a disease response. Analysis of T cell reactivity from one patient who achieved a complete response but did not have durable engraftment of donor cells indicated that disease response was associated with the generation of host-derived anti-leukemic cytotoxic CD8+ T cells that reacted with an AML-associated proteinase 3 epitope. Results from this patient suggest that allogeneic therapy induced a host anti-tumor response associated with cytotoxic T cells reactive with a low affinity self-antigen.

Overlap between in vitro donor antihost and in vivo posttransplantation TCR Vbeta use: a new paradigm for designer allogeneic blood and marrow transplantation

Following allogeneic blood and marrow transplantation (BMT), mature donor T cells can enhance engraftment, counteract opportunistic infections, and mount graft-versus-tumor (GVT) responses, but at the risk of developing graft-versus-host disease (GVHD). With the aim of separating the beneficial effects of donor T cells from GVHD, one approach would be to selectively deplete subsets of alloreactive T cells in the hematopoietic cell inoculum. In this regard, TCR Vbeta repertoire analysis by CDR3-size spectratyping can be a powerful tool for the characterization of alloreactive T-cell responses. We investigated the potential of this spectratype approach by comparing the donor T-cell alloresponses generated in vitro against patient peripheral blood lymphocytes (PBLs) with those detected in vivo posttransplantation. The results indicated that for most Vbeta families that exhibited alloreactive CDR3-size skewing, there was a robust overlap between the in vitro antipatient and in vivo spectratype histograms. Thus, in vitro spectratype analysis may be useful for determining the alloreactive T-cell response involved in GVHD development and, thereby, could serve to guide select Vbeta family depletion for designer transplants to improve outcomes.

EpiToolKit--a web server for computational immunomics

Predicting the T-cell-mediated immune response is an important task in vaccine design and thus one of the key problems in computational immunomics. Various methods have been developed during the last decade and are available online. We present EpiToolKit, a web server that has been specifically designed to offer a problem-solving environment for computational immunomics. EpiToolKit offers a variety of different prediction methods for major histocompatibility complex class I and II ligands as well as minor histocompatibility antigens. These predictions are embedded in a user-friendly interface allowing refining, editing and constraining the searches conveniently. We illustrate the value of the approach with a set of novel tumor-associated peptides. EpiToolKit is available online at www.epitoolkit.org.

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.

Risk assessment in haematopoietic stem cell transplantation: GvHD prevention and treatment

Graft-versus-host disease (GvHD) is the major cause of transplant-related mortality and morbidity. As it is closely related to the major therapeutic principle, graft-versus-leukaemia (GvL) effect, risk assessment has to balance both risks depending on the pre-transplant status. This is clearly demonstrated when comparing the two major strategies for prevention of GvHD. While the majority of approaches aiming at T-cell depletion show efficacy in reducing acute and chronic GvHD and transplant-related mortality, T-cell depletion also affects graft-versus-leukaemia effects and thus results in a higher relapse rate. Thus, standard prophylaxis relying on calcineurin inhibitors frequently results in at least equivalent or even superior long-term disease-free survival, and the risk of relapse has to be considered when selecting regimens for prevention of GvHD. In addition to this general dilemma, drug-specific side-effects and risks have to be considered when selecting regimens for GvHD prevention and treatment.

Allogeneic bone marrow transplant for refractory mediastinal germ cell tumour: possible evidence of graft-versus-tumour effect

Mediastinal germ cell tumours (GCT) carry a poor prognosis, particularly after relapse. We have reviewed the published reports of current treatments and describe a case with a cure after an allogeneic bone marrow transplant, which is not reported in the current published data. We believe that GCT may be susceptible to a graft-versus-tumour effect and suggest that patients with relapsed GCT be considered for allogeneic transplantation.

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.

Successful immune reconstitution decreases leukemic relapse and improves survival in recipients of unrelated cord blood transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) is established therapy for selected patients with acute leukemia. After transplantation, antileukemic immune responses are believed to eliminate residual leukemia cells and decrease the likelihood of relapse. However, the clinical effect of successful antigen-specific immune reconstitution after HSCT on the likelihood of leukemic relapse and overall survival is not known. Pediatric recipients of unrelated cord blood transplants who underwent transplantation for acute leukemia were sequentially evaluated for their development of antigen-specific T-lymphocyte immunity to herpes viruses. The clinical effect of a positive antigen-specific response on relapse-free survival was determined. The presence of an antigen-specific response resulted in a relapse-free survival advantage (P = .0001), which was primarily due to a decrease in leukemic relapse (P = .003). Proportional hazards modeling for time to relapse and time to relapse or death defined 3 variables that were strongly associated with a poor outcome: female gender, poor remission status before transplantation, and negative antigen-specific T-lymphocyte proliferation. Notably neither acute nor chronic graft-versus-host disease had any effect on the incidence of leukemic relapse. Successful antigen-specific immune reconstitution after unrelated cord blood transplantation results in decreased leukemic relapse and improved overall survival.

Tetanus toxoid provides efficient T-cell help for the induction of HA-1(H) cytotoxic T cells

BACKGROUND

In vitro generation and expansion of leukemia-reactive T cells may improve the efficacy and specificity of cellular immunotherapy against hematologic malignancies in the context of allogeneic stem cell transplantation. Since the expression of minor histocompatibility antigen HA-1(H) is limited to hematopoietic cells, ex vivo generated HA-1(H)-specific CD8+ cytotoxic T lymphocytes (CTLs) can be used for adoptive immunotherapy.

STUDY DESIGN AND METHODS

Numerous studies have shown that primary CTL induction from naïve precursors requires professional antigen-presenting cells. Here, the feasibility of ex vivo induction of HA-1(H)-specific CD8+ CTLs is demonstrated from unfractionated peripheral blood mononuclear cells (PBMNCs) from healthy blood donors when CD4+ T-cell help is provided during primary stimulation. As a stimulus for the induction of T-cell help, tetanus toxoid (TT) was used.

RESULTS

After the second restimulation cycle, approximately 1 percent of CD8+ T cells stained positively with the HLA-A*0201/HA-1(H) pentamer. Positive T cells were further expanded more than 1000-fold by antigen-independent stimulation with anti-CD3/CD28 monoclonal antibodies. HA-1(H)-induced T cells showed the classical phenotype for CD8+ memory effector cells: the phenotype changed from a mixed CD45RA/RO phenotype to an activated phenotype characterized by high expression of CD45RO and no expression of CCR7. The generated T cells revealed a very potent CTL response, even at low E:T ratios.

CONCLUSION

This study demonstrates that TT provides a very potent and cost-effective tool for the in vitro induction of antigen-specific CTLs from precursor PBMNCs that can easily be adapted to GMP conditions for translational purposes.

The diallelic locus encoding the minor histocompatibility antigen HA-1 is evolutionarily conserved

The polymorphic minor histocompatibility antigen HA-1 induces powerful T-cell alloreactivities with important consequences for graft-vs-tumor activity and development of graft-vs-host disease in patients after human leukocyte antigen-matched stem-cell transplantation (SCT). In view of possible translational animal studies, we analyzed the evolutionary conservation of the diallelic HA-1 locus in four mammalian species. Our results show that rodents do not encode the HA-1(H) allele, neither show polymorphism in this position on the HA-1 gene. Contrariwise, the HA-1(H) allele is present in non-human primate species and dogs. Interestingly, both the HA-1(H) T-cell epitope and its non-immunogenic counterpart HA-1(R) are present in the latter species. Thus, the HA-1 allelic polymorphism is conserved in evolution in primates and dogs.

Generation of tumor-specific T-cell therapies

Antigen-specific tumor immunotherapy remains an attractive strategy for the treatment of malignancies. In this review we will discuss why, despite the identification of large numbers of T cell recognised tumor antigens, effective immunotherapy remains a formidable challenge. Effective strategies are needed to deal with the tolerogenic properties of many tumor antigens, and with the immunocompromised status of patients. We discuss different methods of generating tumor-specific T cells which are currently being evaluated in clinical practice, such as vaccination and adoptive transfer of tumor antigen-specific T cells. Finally, we shall discuss novel strategies in development, such as the adoptive transfer of T cell receptor (TCR) gene modified T cells to establish antigen-specific immunity in patients with leukemia and solid cancers. The transfer of validated high avidity TCRs, isolated from 'non-tolerant' repertoires or produced by in vitro affinity maturation, can serve to equip patient T cells with new anti-tumor specificities that are not naturally present in the autologous repertoire. TCR transfer into CD4(+) and CD8(+) T cells can serve to harness the function of both helper and cytotoxic T cells for tumor elimination and establishment of long-term tumor immunity.

Expanding the immunotherapeutic potential of minor histocompatibility antigens

Minor histocompatibility antigens (mHAgs) selectively expressed by cells or cell subsets of the hematopoietic system are targets of the T cell-mediated graft-versus-leukemia response that develops following allogeneic hematopoietic stem cell transplantation (HSCT) for the treatment of hematological malignancies. This observation has served as the rationale for utilizing mHAg-specific immunotherapy for the treatment of particular patients. However, at present, only a select and small number of patients could potentially benefit from mHAg-based immunotherapy. A report from de Rijke et al. in this issue of the JCI describes a new hematopoietic lineage-specific HLA-B7-restricted mHAg associated with remission of chronic myeloid leukemia. This result represents another example of an mHAg-mediated graft-versus-leukemia response, thereby expanding the number of patients eligible for mHAg-based immunotherapy in the setting of HSCT.

Possible involvement of allogeneic antigens recognised by donor-derived CD4 cytotoxic T cells in selective GVL effects after stem cell transplantation of patients with haematological malignancy

Cytotoxic T lymphocyte (CTL) lines specific for allogeneic antigens were generated by in vitro stimulation of donor-derived peripheral blood mononuclear cells obtained from patients who received human leucocyte antigen (HLA)-matched allogeneic haematopoietic stem cell transplantation (HSCT). One of the allogeneic antigen-specific CD4+ CTL lines, CTL-A, generated from a patient with T cell acute lymphoblastic leukaemia, recognised HLA-DPB1*0501-positive Epstein-Barr virus-immortalised human B cell line (EBV-B cells), phytohaemagglutinin blasts and leukaemia cells, but not interferon-gamma (IFN-gamma) treated HLA-DPB1*0501-positive fibroblasts, indicating that this CD4+ T-cell line recognised a minor histocompatibility antigen (mHa) that is preferentially expressed in haematopoietic cells in an HLA-DPB1*0501-restricted manner. The other CD4+ CTL line, CTL-B, generated from a patient with chronic myeloid leukaemia, recognised mismatched HLA-DQB1*0303 on EBV-B cells and phytohaemagglutinin (PHA) blasts. Interestingly, this CTL line did not recognise IFN-gamma-treated recipient's skin fibroblasts, as HLA-DQ was merely upregulated even after IFN-gamma stimulation in non-haematopoietic cells including fibroblasts, endothelial cells and hepatocytes. These results suggest that these CD4 positive CTLs, specific for mismatch HLA-DQ and mHa that are preferentially expressed on haematopoietic cells, may play an important role in induction of selective graft-versus-leukaemia effect without development of graft-versus-host disease after allogeneic HSCT.

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.

SNEP: SNP-derived Epitope Prediction program for minor H antigens

The single nucleotide polymorphism (SNP)-derived Epitope Prediction program (SNEP) is now available to the public. It predicts minor histocompatibility antigens (miHAgs), which are T-cell epitopes containing polymorphic spots, from proteins listed in the SWISS-PROT database. SNEP recognizes polymorphisms (termed VARIANT or CONFLICT in SWISS-PROT) and predicts potential T-cell epitopes within a chosen distance around the polymorphic residue. The prediction algorithm is based on the SYFPEITHI T-cell epitope prediction program. SNEP is able to search for proteins according to their accession numbers, sequence stretches or gene names, for example. The predictions are available for several human leucocyte antigen class I and class II allelic products, which allow for a rapid and precise evaluation of potential miHAgs within polymorphic antigens.

Multiplex Genotyping of Human Minor Histocompatibility Antigens

Minor histocompatibility antigens (mHAg) induce major histocompatibility complex-restricted, T cell-mediated immune responses that may contribute to increased risk of graft-versus-host disease and graft-versus-leukemia effects. Unlike human leukocyte antigen genes, mHAg are encoded by genetically and functionally unrelated genes located throughout the chromosome. The role of mHAg in stem cell transplantation and the population frequencies of mHAg alleles remain unknown due in part to the lack of suitable high throughput methods for genotyping these diverse genes. Here we describe the development and utility of a multiplexed Luminex assay for genotyping human mHAg, including HA-1, HA-2, HA-3, HA-8, HB-1, CD31(125), and CD31(563). The assay uses a multiplexed, allele-independent, gated amplification of mHAg genes followed by differential detection of allele-specific primer extension products using the MultiCode PLx system (EraGen Biosciences, Madison, WI). The alleles are interrogated using a multiplex allele-specific primer extension reaction using primers tagged with EraCodes. The products are hybridized to Luminex beads and the hybridization duplexes are detected using streptavidin-phycoerythrin. The assay resolved the mHAg genotypes of 259 Caucasian donors and provided population estimates of mHAg gene and phenotypic frequencies. All mHAg alleles evaluated in this study exhibited Hardy-Weinberg equilibrium, although some mHAg phenotypes were present in large majority of individuals tested (HA-2, HB-1). This assay will provide a valuable tool for determining mHAg frequencies in other ethnic populations, as well as for establishing the clinical importance of mHAg disparities in stem cell transplantation.

Immunotherapy in allogeneic hematopoietic stem cell transplantation--not just a case for effector cells

The concept that in allogeneic hematopoietic stem cell transplantation (alloHSCT) the immune system plays a prominent role in the control of leukemic disease is supported by the clinical observation that immunological effector mechanisms contribute to the elimination of leukemic blasts. The failure to induce prolonged remission after alloHSCT has led to resurgent interest in complementing concepts of immune modulation to improve the antileukemic reponse. While the general focus has been placed on manipulation of cytotoxic effector cell populations, we will explore the dual role of leukemia cells as both antigen-presenting and target cells and describe various vaccination strategies to facilitate a protective antileukemic immune response in this setting. In addition, we will introduce mesenchymal stem cells (MSC) as another cell population recently recognized for their immunomodulatory properties. The potential benefits and hazards of MSC-cotransplantation in alloHSCT with regard to the graft versus leukemia (GvL) and the graft versus host (GvH) response will be discussed.

Elimination of human leukemia cells in NOD/SCID mice by WT1-TCR gene-transduced human T cells

Cytotoxic T lymphocytes (CTLs) specific for an HLA-A2-presented peptide epitope of the Wilms tumor antigen-1 (WT1) can selectively kill immature human leukemia progenitor and stem cells in vitro. In this study we have used retroviral gene transfer to introduce a WT1-specific T-cell receptor (TCR) into T lymphocytes obtained from patients with leukemia and from healthy donors. TCR-transduced T cells kill leukemia cells in vitro and display WT1-specific cytokine production. Intravenous injection of TCR-transduced T cells into nonobese diabetic-severe combined immunodeficiency (NOD/SCID) mice harboring human leukemia cells resulted in leukemia elimination, whereas transfer of control T cells transduced with an irrelevant TCR was ineffective. The data suggest that adoptive immunotherapy with WT1-TCR gene-modified patient T cells should be considered for the treatment of leukemia.

Cytotoxic minor histocompatibility antigen HA-1–specific CD8+ effector memory T cells: artificial APCs pave the way for clinical application by potent primary in vitro induction

Induction of cytotoxic T lymphocytes (CTLs) for treatment of relapsed leukemia after allogeneic stem-cell transplantation is hindered by the laborious and time-consuming procedure of generating dendritic cells for antigen presentation. Artificial antigen-presenting cells (aAPCs) offer the advantage of being readily available in sufficient numbers, thus allowing for a highly standardized in vitro induction of CTLs. We generated aAPCs coated with anti-CD28 antibody (Ab) and either high-density (HD) or low-density (LD) major histocompatibility complex (MHC) class I molecules loaded with HA-1H, a nonapeptide derived from the hematopoiesis-restricted minor histocompatibility antigen HA-1. HD- and LD-aAPCs potently induced HA-1H–specific CD8+ CTLs from untouched CD8+ T cells of healthy donors. CTLs were subsequently purified by magnetic-activated cell sorting. HD- as well as LD-aAPC–induced CTLs exerted high HA-1H–specific cytotoxicity, resembled Tc1 effector memory cells, survived a long time in vitro, and were expanded by a factor varying between 8.2 × 104 and 51 × 104. The T-cell receptor (TCR) repertoire of HA-1H tetramer–positive CTLs was oligoclonal with a prominent usage of Vβ6. The TCR repertoire of tetramer-positive CTLs was distinct from and more restricted than that of tetramer-negative cells. These findings indicate that aAPCs are attractive tools for the ex vivo generation of HA-1H–specific CTLs suitable for immunotherapy of relapsed leukemia.

Detection of Microchimerism by Minor Histocompatibility Antigen HA-1 Allele-Specific Nested Polymerase Chain Reaction

Minor histocompatibility antigens (mHags) can induce T-cell reactivities with important consequences for the graft-versus-leukemia effect and the development of graft-versus-host disease in HLA-matched stem cell transplantation settings. Recently, mHag-specific T cells were also demonstrated in multiparous woman and in solid organ transplant recipients. Microchimeric cells have been detected in the latter settings. To study whether microchimerism is instrumental in the induction and/or maintenance of mHag T cells, we developed an HA-1 allele-specific nested polymerase chain reaction. To optimize and validate the reliability of this method at different levels of microchimerism, serial dilutions of HA-1(H) cells titrated into HA-1(R) cells were tested. We demonstrated that the HA-1(H) allele can be reliably and consistently detected at concentrations as low as 1:10(5) without losing specificity. The developed HA-1-specific nested polymerase chain reaction is an important tool that facilitates the detection of HA-1 microchimerism in various clinical specimens and that promotes investigation of the effects of microchimerism on induction of mHag-specific T cells in the various settings of immunization.

Is there a role for minimal residual disease levels in the treatment of ALL patients who receive allogeneic stem cells?

Relapse is the major complication after allogeneic stem cell transplantation (SCT) for acute lymphoblastic leukemia (ALL) in children. Since it has been possible to measure minimal residual disease (MRD) by real-time quantitative polymerase chain reaction, this parameter is used more frequently in the treatment of ALL. In this article, the role of MRD and chimerism in the treatment and monitoring of pediatric transplantation recipients is described. Pre-SCT MRD levels can predict the risk of relapse and can thus be used to adjust treatment. Post-SCT MRD levels and changes in chimerism can predict relapses as well, although not many treatment options are available today, except relying on a graft-versus-leukemia effect mediated by graft-versus-host disease. Finding new treatments will be the challenge for the near future.

Immunotherapy of cancer through targeting of minor histocompatibility antigens

Minor histocompatibility antigens are allogeneic targets of T-cell mediated graft-versus-tumour effects following allogeneic stem cell transplantation. Recent research has identified several minor histocompatibility antigens as tumour proteins and has also disclosed their unique properties in both the induction and the effector phase of graft-versus-tumour effects. Targeting tumour-specific minor histocompatibility antigens by adoptive immunotherapy will battle against tumour tolerance and evoke allo-immune responses, thereby enhancing graft-versus-tumour effects against leukaemia and solid tumours. Recently acquired knowledge of the role of donor immunisation status, new techniques in the generation of minor histocompatibility antigen-specific cytotoxic T lymphocytes in vitro, and innovative principles in vaccination will help to design clinical trials that exploit minor histocompatibility antigens in the immunotherapy of cancer.

Cord blood comprises antigen-experienced T cells specific for maternal minor histocompatibility antigen HA-1

Umbilical cord blood transplantation is applied as treatment for mainly pediatric patients with hematologic malignancies. The clinical results show a relatively low incidence of graft-versus-host disease and leukemia relapse. Since maternal cells traffic into the fetus during pregnancy, we questioned whether cord blood has the potential to generate cytotoxic T cells specific for the hematopoietic minor histocompatibility (H) antigen HA-1 that would support the graft-versus-leukemia effect. Here, we demonstrate the feasibility of ex vivo generation of minor H antigen HA-1-specific T cells from cord blood cells. Moreover, we observed pre-existing HA-1-specific T cells in cord blood samples. Both the circulating and the ex vivo-generated HA-1-specific T cells show specific and hematopoietic restricted lysis of human leukocyte antigen-A2pos/HA-1pos (HLA-A2pos/HA-1pos) target cells, including leukemic cells. The cord blood-derived HA-1-specific cytotoxic T cells are from child origin. Thus, the so-called naive cord blood can comprise cytotoxic T cells directed at the maternal minor H antigen HA-1. The apparent immunization status of cord blood may well contribute to the in vivo graft-versus-leukemia activity after transplantation. Moreover, since the fetus cannot be primed against Y chromosome-encoded minor H antigens, cord blood is an attractive stem cell source for male patients. (Blood. 2005;105:1823-1827)