Photodepletion differentially affects CD4+ Tregs versus CD4+ effector T cells from patients with chronic graft-versus-host disease

Impact of Cryopreservation on Extracorporeal Photopheresis (ECP)-Treated Leukocyte Subsets

BACKGROUND

Extracorporeal photopheresis (ECP) is frequently utilized in the treatment of steroid-refractory acute and chronic graft-versus-host disease (GVHD). Although the mechanism of action is not fully understood, it has been postulated that its therapeutic effect is immunologic tolerance linked to the associated apoptosis of the treated cells. Despite significant advances in allogeneic hematopoietic stem cell transplantation (HSCT), prophylaxis and treatment of GVHD remain a challenge and major limitation associated with this therapy. Use of ECP is a valuable strategy; however, it is time, cost, resource intensive, and not readily accessible.

OBJECTIVE

In an effort to expand access to this therapy, we are investigating the use of cryopreserved ECP-treated cells. This will provide the ability to administer a significant proportion of the treatment at a facility closer to the patient's residence, thereby decreasing the number of visits to the primary treatment center with the goal of improving and expanding access to this therapy. Here we report the effects of cryopreservation on ECP-treated leukocytes.

STUDY DESIGN

Mononuclear cells were pheresed from human patients, ECP-treated, and collected for viability and apoptotic analysis. Cells were then cryopreserved at -80°C or -150°C for 1 week, 1 month, and 3 months. Following thaw, repeat viability and apoptosis studies were performed on the leukocytes.

RESULTS

WBC viability for freshly ECP-treated leukocytes was 84.5% ± 3.5 at 1 week, 87.3% ± 5.2 at 1 month, and 79.1% ± 1.1 at 3 months post thaw. Similar results were seen for cells frozen in cryovials. Leukocytes frozen the day after ECP treatment had 1 week and 1 month WBC viabilities of 84.0 ± 4.1 and 83.1 ± 2.1, respectively. Apoptotic potential was well preserved at 3 months, with cryopreserved ECP-treated lymphocytes being 19.2%, 44.5%, 75.5%, and 94.0% apoptotic after thaw on days 0, 1, 2, and 3 in culture, respectively.

CONCLUSIONS

ECP-treated leukocytes cryopreserved at -80°C or -150°C for 3 months remain viable and as capable of apoptosis as freshly treated cells. Cryopreservation of an ECP-product warrants further in vivo investigation as a strategy to facilitate access to this needed therapy.

Key Aspects of the Immunobiology of Haploidentical Hematopoietic Cell Transplantation

Hematopoietic stem cell transplantation from a haploidentical donor is increasingly used and has become a standard donor option for patients lacking an appropriately matched sibling or unrelated donor. Historically, prohibitive immunological barriers resulting from the high degree of HLA-mismatch included graft-vs.-host disease (GVHD) and graft failure. These were overcome with increasingly sophisticated strategies to manipulate the sensitive balance between donor and recipient immune cells. Three different approaches are currently in clinical use: (a) ex vivo T-cell depletion resulting in grafts with defined immune cell content (b) extensive immunosuppression with a T-cell replete graft consisting of G-CSF primed bone marrow and PBSC (GIAC) (c) T-cell replete grafts with post-transplant cyclophosphamide (PTCy). Intriguing studies have recently elucidated the immunologic mechanisms by which PTCy prevents GVHD. Each approach uniquely affects post-transplant immune reconstitution which is critical for the control of post-transplant infections and relapse. NK-cells play a key role in haplo-HCT since they do not mediate GVHD but can successfully mediate a graft-vs.-leukemia effect. This effect is in part regulated by KIR receptors that inhibit NK cell cytotoxic function when binding to the appropriate HLA-class I ligands. In the context of an HLA-class I mismatch in haplo-HCT, lack of inhibition can contribute to NK-cell alloreactivity leading to enhanced anti-leukemic effect. Emerging work reveals immune evasion phenomena such as copy-neutral loss of heterozygosity of the incompatible HLA alleles as one of the major mechanisms of relapse. Relapse and infectious complications remain the leading causes impacting overall survival and are central to scientific advances seeking to improve haplo-HCT. Given that haploidentical donors can typically be readily approached to collect additional stem- or immune cells for the recipient, haplo-HCT represents a unique platform for cell- and immune-based therapies aimed at further reducing relapse and infections. The rapid advancements in our understanding of the immunobiology of haplo-HCT are therefore poised to lead to iterative innovations resulting in further improvement of outcomes with this compelling transplant modality.

Allodepleted T-cell immunotherapy after haploidentical haematopoietic stem cell transplantation without severe acute graft-versus-host disease (GVHD) in the absence of GVHD prophylaxis

Graft‐versus‐host disease (GVHD) is a major cause of transplant‐related mortality (TRM) after allogeneic haematopoietic stem cell transplantation (HSCT) and presents a challenge in haploidentical HSCT. GVHD may be prevented by ex vivo graft T‐cell depletion or in vivo depletion of proliferating lymphocytes. However, both approaches pose significant risks, particularly infections and relapse, compromising survival. A photodepletion strategy to eliminate alloreactive T cells from mismatched donor lymphocyte infusions (enabling administration without immunosuppression), was used to develop ATIR101, an adjunctive therapy for use after haploidentical HSCT. In this phase I dose‐finding study, 19 adults (median age: 54 years) with high‐risk haematological malignancies were treated with T‐cell‐depleted human leucocyte antigen‐haploidentical myeloablative HSCT followed by ATIR101 at doses of 1 × 10⁴–5 × 10⁶ CD3⁺ cells/kg (median 31 days post‐transplant). No patient received post‐transplant immunosuppression or developed grade III/IV acute GVHD, demonstrating the feasibility of ATIR101 infusion for evaluation in two subsequent phase 2 studies. Additionally, we report long‐term follow ‐up of patients treated with ATIR101 in this study. At 1 year, all 9 patients receiving doses of 0·3–2 × 10⁶ CD3⁺ cells/kg ATIR101 remained free of serious infections and after more than 8 years, TRM was 0%, relapse‐related mortality was 33% and overall survival was 67% in these patients.

The European Society for Blood and Marrow Transplantation (EBMT) consensus recommendations for donor selection in haploidentical hematopoietic cell transplantation

The number of HLA-haploidentical hematopoietic cell transplants continues to increase worldwide due to recent improvements in outcomes, allowing more patients with hematological malignancies and non-malignant disorders to benefit from this procedure and have a chance to cure their disease. Despite these encouraging results, questions remain as multiple donors are usually available for transplantation, and choosing the best HLA-haploidentical donor for transplantation remains a challenge. Several approaches to haploidentical transplantation have been developed over time and, based on the graft received, can be grouped as follows: T-cell depleted haploidentical transplants, either complete or partial, or with T-cell replete grafts, performed with post-transplant cyclophosphamide-based graft-versus-host disease (GVHD) prophylaxis, or G-CSF-primed bone marrow graft and enhanced GVHD prophylaxis. Carefully selecting the donor can help optimize transplant outcomes for recipients of haploidentical donor transplants. Variables usually considered in the donor selection include presence of donor-specific antibodies in the recipient, donor age, donor/recipient gender and ABO combinations, and immunogenic variables, such as natural killer cell alloreactivity or KIR haplotype. Here we provide a comprehensive review of available evidence for selecting haploidentical donors for transplantation, and summarize the recommendations from the European Society for Blood and Marrow Transplantation (EBMT) on donor selection for different transplant platforms.

Proceedings From the Fourth Haploidentical Stem Cell Transplantation Symposium (HAPLO2016), San Diego, California, December 1, 2016

The resurgence of haploidentical stem cell transplantation (HaploSCT) over the last decade is one of the most important advances in the field of hematopoietic stem cell transplantation (HSCT). The modified platforms of T cell depletion either ex vivo (CD34+ cell selection, "megadoses" of purified CD34+ cells, or selective depletion of T cells) or newer platforms of in vivo depletion of T cells, with either post-transplantation high-dose cyclophosphamide or intensified immune suppression, have contributed to better outcomes, with survival similar to that in HLA-matched donor transplantation. Further efforts are underway to control viral reactivation using modified T cells, improve immunologic reconstitution, and decrease the relapse rate post-transplantation using donor-derived cellular therapy products, such as genetically modified donor lymphocytes and natural killer cells. Improvements in treatment-related mortality have allowed the extension of haploidentical donor transplants to patients with hemoglobinopathies, such as thalassemia and sickle cell disease, and the possible development of platforms for immunotherapy in solid tumors. Moreover, combining HSCT from a related donor with solid organ transplantation could allow early tapering of immunosuppression in recipients of solid organ transplants and hopefully prevent organ rejection in this setting. This symposium summarizes some of the most important recent advances in HaploSCT and provides a glimpse in the future of fast growing field.

The European Society for Blood and Marrow Transplantation (EBMT) Consensus Guidelines for the Detection and Treatment of Donor-specific Anti-HLA Antibodies (DSA) in Haploidentical Hematopoietic Cell Transplantation

Haploidentical donors are now increasingly considered for transplantation in the absence of HLA-matched donors or when an urgent transplant is needed. Donor-specific anti-HLA antibodies (DSA) have been recently recognized as an important barrier against successful engraftment of donor cells, which can affect transplant survival. DSA appear more prevalent in this type of transplant due to higher likelihood of alloimmunization of multiparous females against offspring's HLA antigens, and the degree of mismatch. Here we summarize the evidence for the role of DSA in the development of primary graft failure in haploidentical transplantation and provide consensus recommendations from the European Society for Blood and Marrow Transplant Group on testing, monitoring, and treatment of patients with DSA receiving haploidentical hematopoietic progenitor cell transplantation.

Mechanistic approaches for the prevention and treatment of chronic GVHD

Clinical outcomes for patients undergoing allogeneic hematopoietic stem cell transplantation continue to improve, but chronic graft-versus-host disease (GVHD) remains a common toxicity and major cause of nonrelapse morbidity and mortality. Treatment of chronic GVHD has previously relied primarily on corticosteroids and other broadly immune suppressive agents. However, conventional immune suppressive agents have limited clinical efficacy in chronic GVHD, and prolonged immune suppressive treatments result in additional toxicities that further limit clinical recovery from transplant and return to normal daily function. Recent advances in our understanding of the immune pathology of chronic GVHD offer the possibility that new therapeutic approaches can be directed in more precise ways to target specific immunologic mechanisms and pathways. In this review, we briefly summarize current standard treatment options and present new therapeutic approaches that are supported by preclinical studies and early-phase clinical trials suggesting that these approaches may have clinical utility for treatment or prevention of chronic GVHD. Further evaluation of these new therapeutic options in well-designed prospective multicenter trials are needed to identify the most effective new agents and improve outcomes for patients with chronic GVHD.

Novel approaches for preventing acute graft-versus-host disease after allogeneic hematopoietic stem cell transplantation

INTRODUCTION

Allogeneic hematopoietic stem cell transplantation (alloHSCT) offers potential curative treatment for a wide range of malignant and nonmalignant hematological disorders. However, its success may be limited by post-transplant acute graft-versus-host disease (aGVHD), a systemic syndrome in which donor's immune cells attack healthy tissues in the immunocompromised host. aGVHD is one of the main causes of morbidity and mortality after alloHSCT. Despite standard GVHD prophylaxis regimens, aGVHD still develops in approximately 40-60% of alloHSCT recipients.

AREAS COVERED

In this review, after a brief summary of current knowledge on the pathogenesis of aGVHD, the authors review the current combination of a calcineurin inhibitor with an antimetabolite with or without added anti-thymocyte globulin (ATG) and emerging strategies for GVHD prevention.

EXPERT OPINION

A new understanding of the involvement of cytokines, intracellular signaling pathways, epigenetics and immunoregulatory cells in GVHD pathogenesis will lead to new standards for aGVHD prophylaxis allowing better prevention of severe aGVHD without affecting graft-versus-tumor effects.

Specific removal of alloreactive T-cells to prevent GvHD in hemopoietic stem cell transplantation: rationale, strategies and perspectives

Hemopoietic stem cell transplantation (HSCT) is a standard procedure for treatment of malignant and non-malignant hematological diseases. HSCT donors include HLA-identical siblings, matched or mismatched unrelated donors and haploidentical related donors. Graft-versus-host disease (GvHD), mediated by donor alloreactive T-cells in the graft, can be triggered by minor histocompatibility antigens in HLA-identical pairs, by alleles at loci not considered for MUD-matching or by the mismatched haplotype in haplo-HSCT. Therefore, removal of donor T-cells, that contain the alloreactive precursors, is required, but T-cell depletion associates with opportunistic infections and with reduced graft-versus-leukemia effect. Selective T-cell depletion strategies have been introduced, like removal of αβ T-lymphocytes and of naive T-cells, two subsets including the alloreactive precursors, but the ultimate goal is specific removal of alloreactive T-cells. Here we review the different approaches to deplete alloreactive T-cells only and discuss pros and cons, specificity, efficiency and efficacy. Combinations of different methods and innovative approaches are also proposed for depleting specific alloreactive T-cells with high efficiency.

Modern approaches to HLA-haploidentical blood or marrow transplantation

Allogeneic blood or bone-marrow transplantation (alloBMT) is a potentially curative treatment for a variety of haematological malignancies and nonmalignant diseases. Historically, human leukocyte antigen (HLA)-matched siblings have been the preferred source of donor cells owing to superior outcomes compared with alloBMT using other donors. Although only approximately one-third of patients have an HLA-matched sibling, nearly all patients have HLA-haploidentical related donors. Early studies using HLA-haploidentical alloBMT resulted in unacceptably high rates of graft rejection and graft-versus-host disease (GVHD), leading to high nonrelapse mortality and consequently poor survival. Several novel approaches to HLA-haploidentical alloBMT have yielded encouraging results with high rates of successful engraftment, effective GVHD control and favourable outcomes. In fact, outcomes of several retrospective comparative studies seem similar to those seen using other allograft sources, including those of HLA-matched-sibling alloBMT. In this Review, we provide an overview of the three most-developed approaches to HLA-haploidentical alloBMT: T-cell depletion with 'megadose' CD34(+) cells; granulocyte colony-stimulating factor-primed allografts combined with intensive pharmacological immunosuppression, including antithymocyte globulin; and high-dose, post-transplantation cyclophosphamide. We review the preclinical and biological data supporting each approach, results from major clinical studies, and completed or ongoing clinical studies comparing these approaches with other alloBMT platforms.

Haploidentical Hematopoietic Stem Cell Transplantation as a Platform for Post-Transplantation Cellular Therapy

Haploidentical transplantation can extend the opportunity for transplantation to almost all patients who lack an HLA-matched donor. Advances in the field of haploidentical transplantation have led to a marked decrease in treatment-related mortality, allowing investigators to focus on developing rationale pre- and peri-remission therapies aimed at preventing disease relapse after transplantation. Because of widespread availability, low treatment-related mortality, and cost, haploidentical donors may become the preferred "alternative" donors for allogeneic hematopoietic stem cell transplantation. One of the major advantages of using a related donor is the possibility of collecting or generating additional cellular products from the same immediately available donor, which will not be rejected. Infusion of these cells in the peri-transplantation period, derived from the same immune system, is opening the possibility of markedly enhancing the antitumor effects of the graft and hastening immunologic reconstitution after transplantation.

Alternative Donor Transplantation for Acute Myeloid Leukemia

Allogeneic hematopoietic cell transplantation (allo-HCT) is a potentially curative therapy for adult patients with acute myeloid leukemia (AML), but its use for consolidation therapy after first remission with induction chemotherapy used to be limited to younger patients and those with suitable donors. The median age of AML diagnosis is in the late 60s. With the introduction of reduced-intensity conditioning (RIC), many older adults are now eligible to receive allo-HCT, including those who are medically less fit to receive myeloablative conditioning. Furthermore, AML patients commonly have no human leukocyte antigen (HLA)-identical or medically suitable sibling donor available to proceed with allo-HCT. Technical advances in donor matching, suppression of alloreactivity, and supportive care have made it possible to use alternative donors, such as unrelated umbilical cord blood (UCB) and partially HLA-matched related (haploidentical) donors. Outcomes after alternative donor allo-HCT are now approaching the outcomes observed for conventional allo-HCT with matched related and unrelated donors. Thus, with both UCB and haploidentical donors available, lack of donor should rarely be a limiting factor in offering an allo-HCT to adults with AML.

"No donor"? Consider a haploidentical transplant

Haploidentical stem cell transplantation (HaploSCT) is an attractive option for patients requiring a hematopoietic stem cell transplant who do not have an HLA-matched donor, because it is cheaper, can be performed faster, and may extend transplantation to virtually all patients in need. Significant advances have been made in the recent decade with dramatic improvement in treatment outcomes. Historically, overcoming the HLA-incompatibility barrier has been a significant limitation to the expansion of this form of transplant. While ex vivo T-cell depletion effectively prevented the development of acute GVHD, it was associated with a higher treatment-related mortality, in excess of 40% in some series, due to a significant delay in recovery of the adaptive immune system. Newer methods have successfully maintained the memory T cells in the graft and/or selectively depleted alloreactive T cells, and are associated with improved treatment outcomes. Post-transplant cyclophosphamide for GVHD prevention has proven very effective in controlling GVHD with lower incidence of infectious complications and treatment-related mortality-as low as 7% at 1 year-and has become the new standard in how this transplant is performed. Here, we reviewed the current experience with this approach and various other strategies employed to control alloreactivity in this setting, including selective depletion of T cells from the graft, as well as we discuss post-transplantation therapy to prevent disease relapse and improve immunologic reconstitution.

Antiviral T-cell therapy

Serious viral infections are a common cause of morbidity and mortality after allogeneic stem cell transplantation. They occur in the majority of allograft recipients and are fatal in 17–20%. These severe infections may be prolonged or recurrent and add substantially to the cost, both human and financial, of the procedure. Many features of allogeneic stem cell transplantation contribute to this high rate of viral disease. The cytotoxic and immunosuppressive drugs administered pretransplant to eliminate the host hematopoietic/immune system and any associated malignancy, the delay in recapitulating immune ontogeny post‐transplant, the immunosuppressive drugs given to prevent graft versus host disease (GvHD), and the effects of GvHD itself, all serve to make stem cell transplant recipients vulnerable to disease from endogenous (latent) and exogenous (community) viruses, and to be incapable of controlling them as quickly and effectively as most normal individuals.

Adoptive immunotherapy with genetically modified lymphocytes in allogeneic stem cell transplantation

Hematopoietic stem cell transplantation from a healthy donor (allo-HSCT) represents the most potent form of cellular adoptive immunotherapy to treat malignancies. In allo-HSCT, donor T cells are double edge-swords: highly potent against residual tumor cells, but potentially highly toxic, and responsible for graft versus host disease (GVHD), a major clinical complication of transplantation. Gene transfer technologies coupled with current knowledge on cancer immunology have generated a wide range of approaches aimed at fostering the immunological response to cancer cells, while avoiding or controlling GVHD. In this review, we discuss cell and gene therapy approaches currently tested in preclinical models and in clinical trials.

Modulation of lymphocyte subpopulations by extracorporeal photopheresis in patients with acute graft-versus-host disease or graft rejection

Extracorporeal photopheresis (ECP) constitutes a promising treatment for patients with steroid-refractory acute graft-versus-host disease (aGvHD) after allogeneic stem cell transplantation and for patients with graft rejection after solid organ transplantation (SOT). There is an increasing body of evidence that modulation of lymphocyte subsets might play a crucial role in the mechanism of action in ECP. We therefore analyzed immunological effects concomitantly with clinical findings in patients under ECP therapy using multicolor flow cytometry. In a patient with steroid-refractory aGvHD and a patient with progressive bronchiolitis obliterans syndrome (BOS) after double-lung transplantation, clinical responses to ECP therapy were paralleled by an increase of CD4 + CD25hiFoxP3 + regulatory T cells and a decrease of T(EMRA) (CD3 + CD8+ CD45RA+ CD62L+ effector memory T) cells as well as of natural killer (NK)T cells. In summary, immunomonitoring of T cell subsets can elucidate the mechanism of action in ECP.

Haploidentical transplantation for hematologic malignancies: where do we stand?

The fundamental obstacle to the successful application of partially HLA-mismatched related donor, or HLA-haploidentical stem cell transplantation, is the strength of the host and donor T-cell response to allogeneic HLA molecules, which results in increased incidences of graft failure, GVHD, and nonrelapse mortality. The holy grail of haplo-SCT is to mitigate host-versus-graft and graft-versus-host responses while preserving immune responses to infection and the patient's malignancy. Two strategies have been taken to achieve this goal. The first strategy is to supplement a T cell-depleted graft with pathogen-specific T cells or populations of T cells in which alloreactivity can be controlled. The second strategy is to eliminate alloreactive T cells selectively from a T cell-replete graft. Substantial progress has been made with both approaches so that the safety of haplo-SCT now approaches that of SCT using grafts of umbilical cord blood or from HLA-matched donors. In light of the rapid and near universal availability of HLA-haploidentical related donors, it should now be possible to identify and mobilize a donor for every patient referred for allogeneic SCT. Prospective comparisons between haploidentical SCT and unrelated donor SCT should be performed to identify the most efficacious approach to alternative donor transplantation.

Selective T-cell depletion for haplotype-mismatched allogeneic stem cell transplantation

Haplotype-mismatched transplantation offers a unique opportunity to treat patients without a suitable matched related or unrelated donor. Indeed, related haplo-donors are usually extremely motivated, immediately available, and can provide additional stem or immune cells when required, a most important feature in the context of high-risk malignancies. Immunomagneticallly selected CD34(+) stem cell grafts enable rapid and sustained trilineage engraftment. However, the associated delay in immune reconstitution results in significant risk for severe infectious complications and malignant relapse. The infusion of T lymphocytes selectively depleted of their anti-host reactive components represents a most interesting approach to accelerate post-transplant T-cell recovery. Such a strategy relies on ex vivo donor cell activation against host antigens and their selective elimination. Immunotoxins and magnetic beads could target antigens such as CD25 with impressive results. Photodepletion of alloreactive T cells represents an appealing alternative to both eliminate anti-host immune T cells and spare resting T cells to fight infections. Interestingly, regulatory T cells can be retained after such treatment, and have been found to transform non-regulatory into regulatory T cells, a finding that may be of utmost importance in both prevention and control of graft-versus-host disease (GVHD). Efforts to promote efficient antigen presentation and selective allodepletion promise to accelerate immune reconstitution without GVHD and to address the most crucial issues in haplo-mismatched and other types of transplants.

Haploidentical transplantation for hematologic malignancies: where do we stand?

The fundamental obstacle to the successful application of partially HLA-mismatched related donor, or HLA-haploidentical stem cell transplantation, is the strength of the host and donor T-cell response to allogeneic HLA molecules, which results in increased incidences of graft failure, GVHD, and nonrelapse mortality. The holy grail of haplo-SCT is to mitigate host-versus-graft and graft-versus-host responses while preserving immune responses to infection and the patient's malignancy. Two strategies have been taken to achieve this goal. The first strategy is to supplement a T cell–depleted graft with pathogen-specific T cells or populations of T cells in which alloreactivity can be controlled. The second strategy is to eliminate alloreactive T cells selectively from a T cell–replete graft. Substantial progress has been made with both approaches so that the safety of haplo-SCT now approaches that of SCT using grafts of umbilical cord blood or from HLA-matched donors. In light of the rapid and near universal availability of HLA-haploidentical related donors, it should now be possible to identify and mobilize a donor for every patient referred for allogeneic SCT. Prospective comparisons between haploidentical SCT and unrelated donor SCT should be performed to identify the most efficacious approach to alternative donor transplantation.

Advances in graft-versus-host disease biology and therapy

Allogeneic haematopoietic stem cell transplantation is used to treat a variety of disorders, but its efficacy is limited by the occurrence of graft-versus-host disease (GVHD). The past decade has brought impressive advances in our understanding of the role of stimulatory and suppressive elements of the adaptive and innate immune systems from both the donor and the host in GVHD pathogenesis. New insights from basic immunology, preclinical models and clinical studies have led to novel approaches for prevention and treatment. This Review highlights the recent advances in understanding the pathophysiology of GVHD and its treatment, with a focus on manipulations of the immune system that are amenable to clinical application.

Next-generation leukemia immunotherapy

Allogeneic hematopoietic cell transplantation led to the discovery of the allogeneic GVL effect, which remains the most convincing evidence that immune cells can cure cancer in humans. However, despite its great paradigmatic and clinical relevance, induction of GVL by conventional allogeneic hematopoietic cell transplantation remains a quite rudimentary form of leukemia immunotherapy. It is toxic and its efficacy is far from optimal. It is therefore sobering that since the discovery of the GVL effect 3 decades ago, the way GVL is induced and manipulated has practically not changed. Preclinical and clinical studies suggest that injection of T cells primed against a single Ag present on neoplastic cells could enhance the GVL effect without causing any GVHD. We therefore contend that Ag-targeted adoptive T-cell immunotherapy represents the future of leukemia immunotherapy, and we discuss the specific strategies that ought to be evaluated to reach this goal. Differences between these strategies hinge on 2 key elements: the nature of the target Ag and the type of Ag receptor expressed on T cells.

Regulatory T cells and IL-17-producing cells in graft-versus-host disease

Graft-versus-host disease (GvHD), a major complication following allogeneic hematopoietic stem cell transplantation, is mediated by donor-derived T cells. On activation with alloantigens expressed on host antigen-presenting cells, naive CD4+ T cells differentiate into T-helper cell subsets of effector T cells expressing distinct sets of transcriptional factors and cytokines. Classically, acute GvHD was suggested to be predominantly related to Th1 responses. However, we now face a completely different and complex scenario involving possible roles of newly identified Th17 cells as well as Tregs in GvHD. Accumulating data from experimental and clinical studies suggest that the fine balance between Th1, Th2, Th17 and Tregs after transplantation may be an important determinant of the severity, manifestation and tissue distribution of GvHD. Understanding the dynamic process of reciprocal differentiation of regulatory and T-helper cell subsets as well as their interactions will be important in establishing novel strategies for preventing and treating GvHD.

The pathophysiology of chronic graft-versus-host disease: the unveiling of an enigma

Chronic graft-versus-host disease (CGVHD) is one of the most significant complications of long-term survivors after allogeneic hematopoietic stem cell transplantation (allo-HSCT). CGVHD may have protean manifestations and can pose unique diagnostic and therapeutic challenges. New recommendations that emphasize the importance of qualitative differences, as opposed to time of onset after HSCT, are now being used to standardize the diagnosis and clinical assessment of CGVHD, but they require validation. During the past 3 decades, experimental studies and clinical observations have elucidated the mechanisms of acute GVHD, but its biology is much less well-understood. Experimental studies have generated at least 4 theories to explain the pathophysiology of CGVHD: (1) thymic damage and the defective negative selection of T cells, (2) regulatory T cell deficiencies, (3) auto-antibody production by aberrant B cells, and (4) the formation of profibrotic lesions. Mouse models have provided important insights into the pathophysiology of CGVHD, and these have helped improve clinical outcomes following allo-HSCT, but no animal model fully replicates all of the features of CGVHD in humans. In this article, recent clinical changes, the pathogenesis of CGHVD, the cellular and cytokine networks implicated in its pathogenesis, and the animal models used to devise strategies to prevent and treat CGVHD are reviewed.