Optimized clinical-scale culture conditions for ex vivo selective depletion of host-reactive donor lymphocytes: a strategy for GvHD prophylaxis in allogeneic PBSC transplantation

Improving the safety of T-Cell therapies using an inducible caspase-9 gene

Adoptive transfer of T cells can be an effective anticancer treatment. However, uncontrolled or unpredictable immediate or persistent toxic effects are a source of concern. The ability to conditionally eliminate aberrant cells in vivo is therefore becoming a critical step for the successful translation of this approach to the clinic. We review the evolution of safety systems, focusing on a suicide switch that can be expressed stably and efficiently in human T cells without impairing phenotype, function, or antigen specificity. This system is based on the fusion of human caspase-9 to a modified human FK-binding protein, allowing conditional dimerization in the presence of an otherwise bio-inert small molecule drug. When exposed to the synthetic dimerizing drug, the inducible caspase-9 becomes activated, resulting in the rapid apoptosis of cells expressing this construct. We have illustrated the clinical feasibility and efficacy of this approach after haploidentical hematopoietic stem cell transplant. Here we review the benefits and limitations of the approach.

A Two-Step Haploidentical Versus a Two-Step Matched Related Allogeneic Myeloablative Peripheral Blood Stem Cell Transplantation

Haploidentical stem cell transplantation (SCT) offers a transplantation option to patients who lack an HLA-matched donor. We developed a 2-step approach to myeloablative allogeneic hematopoietic stem cell transplantation for patients with haploidentical or matched related (MR) donors. In this approach, the lymphoid and myeloid portions of the graft are administered in 2 separate steps to allow fixed T cell dosing. Cyclophosphamide is used for T cell tolerization. Given a uniform conditioning regimen, graft T cell dose, and graft-versus-host disease (GVHD) prophylaxis strategy, we compared immune reconstitution and clinical outcomes in patients undergoing 2-step haploidentical versus 2-step MR SCT. We retrospectively compared data on patients undergoing a 2-step haploidentical (n = 50) or MR (n = 27) peripheral blood SCT for high-risk hematological malignancies and aplastic anemia. Both groups received myeloablative total body irradiation conditioning. Immune reconstitution data included flow cytometric assessment of T cell subsets at day 28 and 90 after SCT. Both groups showed comparable early immune recovery in all assessed T cell subsets except for the median CD3/CD8 cell count, which was higher in the MR group at day 28 compared with that in the haploidentical group. The 3-year probability of overall survival was 70% in the haploidentical group and 71% in the MR group (P = .81), while the 3-year progression-free survival was 68% in the haploidentical group and 70% in the MR group (P = .97). The 3-year cumulative incidence of nonrelapse mortality was 10% in the haploidentical group and 4% in the MR group (P = .34). The 3-year cumulative incidence of relapse was 21% in the haploidentical group and 27% in the MR group (P = .93). The 100-day cumulative incidence of overall grades II to IV acute GVHD was higher in the haploidentical group compared with that in the MR group (40% versus 8%, P < .001), whereas the grades III and IV acute GVHD was not statistically different between both groups (haploidentical, 6%; MR, 4%; P = .49). The cumulative incidence of cytomegalovirus reactivation was also higher in the haploidentical group compared to the MR group (haploidentical, 68%; MR, 19%; P < .001). There were no deaths from GVHD in either group. Using an identical conditioning regimen, graft T cell dose, and GVHD prophylaxis strategy, comparable early immune recovery and clinical outcomes were observed in the 2-step haploidentical and MR SCT recipients.

Preventing stem cell transplantation-associated viral infections using T-cell therapy

Hematopoietic stem cell transplantation is the treatment of choice for many hematologic malignancies and genetic diseases. However, viral infections continue to account for substantial post-transplant morbidity and mortality. While antiviral drugs are available against some viruses, they are associated with significant side effects and are frequently ineffective. This review focuses on the immunotherapeutic strategies that have been used to prevent and treat infections over the past 20 years and outlines different refinements that have been introduced with the goal of moving this therapy beyond specialized academic centers.

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.

Achievement of early complete donor chimerism in CD25+-activated leukocytes is a strong predictor of the development of graft-versus-host-disease after stem cell transplantation

Chimerism dynamics in bone marrow, peripheral blood (PB), and T lymphocytes (TL) has been associated with the development of various complications after allogeneic stem-cell transplantation (allo-SCT). In the present study, the usefulness of chimerism monitoring in CD25(+)-activated leukocytes (AL), together with that in bone marrow, PB, and TL, for the anticipation of complications after allo-SCT, has been analyzed in 68 patients. In AL, we observed a slower dynamics toward complete chimerism (CC) than in PB (p = 0.042), while no significant differences were found between TL and PB (p = 0.12). Complete chimerism achievement in AL at day +30 has shown to be an independent risk factor for the development of grade II-IV acute graft-versus-host disease (aGvHD; hazard ratio [95% confidence interval]: 11.9 [1.5-91.7]; p = 0.017). Moreover, among patients achieving CC in TL and AL at different time-points after SCT (n = 17/68), the incidence of grade II-IV aGvHD was significantly higher in patients who achieved CC earlier in AL (5/5) than in those who achieved CC earlier in TL (1/11; p = 0.001). Therefore, achievement of early complete donor chimerism in CD25(+) AL is a strong predictor for the development of aGvHD. Prospective analysis of chimerism in AL could improve the post-SCT management of immunosuppressive therapy in transplanted patients.

The cyclin dependent kinase inhibitor (R)-roscovitine mediates selective suppression of alloreactive human T cells but preserves pathogen-specific and leukemia-specific effectors

Graft versus host disease (GvHD), mediated by donor T cells, remains the primary cause of non-relapse mortality after allogeneic hematopoietic stem cell transplantation and novel therapeutic approaches are required. Cdk2 is a critical node of signal integration and programming of T cell responses towards immunity versus anergy but is dispensable for hematopoiesis and thymocyte development. We examined the effects of pharmacologic Cdk2 inhibition on alloreactive human T cells. Inhibition of Cdk2 blocked expansion of alloreactive T cells upon culture with HLA-mismatched dendritic cells and prevented generation of IFN-γ-producing alloantigen-specific effectors. In contrast, Cdk2 inhibition preserved effectors specific for Wilms' tumor 1 (WT1) leukemia antigen and for CMV as determined by WT1-specific and CMV-specific pentamers. Cdk2 inhibition preserved Treg cells, which have the ability to prevent GvHD while maintaining GvL. Thus, Cdk inhibitors may improve allogeneic HSCT by reducing alloreactivity and GvHD without loss of pathogen-specific and leukemia-specific immunity.

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.

Clinical Implications of T Cell Receptor Repertoire Analysis after Allogeneic Stem Cell Transplantation

Stem cell transplantation (SCT) constitutes a major challenge to the immune system. Long-term impairment of immunity against various common infectious stimuli leads to increased susceptibility to infectious diseases; in contrast, an immune response against the recipient may cause the devastating graft-versus-host disease (GvHD). Recovery of the immune system (both qualitative and quantitative) after SCT is perhaps the most important factor in determining the clinical outcome. Consequently, immune reconstitution has been extensively studied using different approaches, including quantitative analysis of immune cells as well as their phenotypic characterization. Analysis of diversity and clonality is an important tool in determining competence of the immune system, assuming that a broad diversity assures efficient response to different stimuli and clonal dominance reflects ongoing, potentially relevant immune responses. Detailed analysis of the immune repertoire through the flow cytometric and molecular study of the T cell receptor repertoire has been applied to gain quantitative and qualitative insights about the T cell immune competence and responsiveness. After SCT, a contraction of the T cell pool and a reduction in T cell receptor diversity is clearly associated with clinical immunodeficiency. Reconstitution of the immune system is often characterized by dominance of oligoclonal T cell populations, reflecting specific antigen-driven immune responses. Detailed characterization of T lymphocytes by T cell receptor analysis is possible, and may lead to the identification of individual clones involved in specific immune reactions, such as alloresponses in GvHD, the closely related graft-versus-leukemia effect and opportunistic viral agents such as CMV or EBV.

Advances in adoptive immunotherapy to accelerate T-cellular immune reconstitution after HLA-incompatible hematopoietic stem cell transplantation

Although partially HLA-mismatched hematopoietic stem cell transplantation (HSCT) has become an important therapeutic option for children with primary immunodeficiencies, delayed reconstitution of the T-cell compartment remains a major clinical concern. Adoptive immunotherapies to provide recipients with a protective and diverse T-cell repertoire in the months following HSCT are warranted. In order to improve T-cell reconstitution after T-cell-depleted HSCT, different strategies are currently being studied. Some are based on administration of modified mature T cells (e.g., allodepleted T cells or pathogen-specific T cells). Others aim at accelerating de novo thymopoiesis from donor-derived hematopoietic stem cells in vivo via the administration of thymopoietic agents or the transfer of large numbers of T-cell precursors generated ex vivo. The present article will provide a brief summary of recent advances in the field of allodepletion and adoptive transfer of pathogen-specific T cells and a detailed discussion of strategies for enhancing thymopoiesis in vivo.

How I treat adenovirus in hematopoietic stem cell transplant recipients

Adenovirus (AdV) infections are very common in the general pediatric population. The delayed clearance in young persons imposes a threat to immunocompromised patients after hematopoietic stem cell transplantation (HSCT), who can reactivate the virus, resulting in life-threatening disseminated disease. Although a definitive cure requires adequate immune reconstitution, 2 approaches appear to be feasible and effective to improve the outcomes of AdV infections. Strict monitoring with AdV quantitative polymerase chain reaction followed by preemptive treatment with low-dose (1 mg/kg) cidofovir 3 times a week, is effective in most cases to bridge the severely immunocompromised period shortly after HSCT, with acceptable toxicity rates. For centers who have the access, AdV-specific cytotoxic T cells can be the other important cornerstone of anti-AdV therapy with promising results so far. Methods to positively influence the reconstitution of the immune system after HSCT and optimizing new and currently available cellular immunotherapies will make HSCT safer against the threat of AdV infection/reactivation and associated disease.

Challenges of T cell therapies for virus-associated diseases after hematopoietic stem cell transplantation

IMPORTANCE OF THE FIELD

Hematopoietic stem cell transplantation (HSCT) is the treatment of choice for many hematological malignancies and genetic disorders. The majority of patients do not have a human leukocyte antigen (HLA) identical sibling donor, and alternative stem cell sources include HLA-matched or mismatched unrelated donors and haploidentical related donors. However, alternative donor HSCT are associated with three major complications i) graft rejection; ii) graft-versus-host disease (GvHD); and iii) delayed immune reconstitution leading to viral infections and relapse.

AREAS COVERED IN THIS REVIEW

Graft rejection and the risk of GvHD can be significantly reduced by using intensive conditioning regimens, including in vivo T cell depletion as well as ex vivo T cell depletion of the graft. However, the benefits of removing alloreactive T cells from the graft are offset by the concomitant removal of T cells with anti-viral or anti-tumor activity as well as the profound delay in endogenous T cell recovery post-transplant. Thus, opportunistic infections, many of which are not amenable to conventional small-molecule therapeutics, are frequent in these patients and are associated with significant morbidity and high mortality rates. This review discusses current cell therapies to prevent or treat viral infections/reactivations post-transplant.

WHAT THE READER WILL GAIN

The reader will gain an understanding of the current state of cell therapy to prevent and treat viral infections post-HSCT, and will be introduced to preclinical studies designed to develop and validate new manufacturing procedures intended to improve therapeutic efficacy and reduce associated toxicities.

TAKE HOME MESSAGE

Reconstitution of HSCT recipients with antigen-specific T cells, produced either by allodepletion or in vitro reactivation, can offer an effective strategy to provide both immediate and long-term protection without harmful alloreactivity.

Selective depletion of alloreactive T lymphocytes using patient-derived nonhematopoietic stimulator cells in allograft engineering

BACKGROUND

Selective depletion of alloreactive T cells in vitro results in efficient graft-versus-host disease prophylaxis in allogeneic hematopoietic stem-cell transplantation, but it is accompanied by increased recurrence of leukemia. To spare donor T-cell-mediated graft-versus-leukemia immunity against hematopoiesis-restricted minor histocompatibility (minor-H) antigens, we explored the use of patient-derived nonhematopoietic antigen-presenting cells (APC) as allogeneic stimulators for selective allodepletion in leukemia-reactive donor T-cell lines.

METHODS

Primary keratinocytes, dermal fibroblasts, and bone marrow fibroblasts were generated from skin biopsies and diagnostic bone marrow aspirates of acute myeloid leukemia patients in vitro. Cell cultures were analyzed for expansion, phenotype, and immunostimulatory capacity in comparison with CD40-activated B cells as professional APC. In addition, nonhematopoietic APCs were used for selective allodepletion in vitro.

RESULTS

Patient-derived fibroblasts could be reliably expanded to large cell numbers, whereas keratinocytes had limited growth potential. Interferon-gamma-pretreated fibroblasts showed increased expression of human leukocyte antigen (HLA)-class I and II molecules, CD40, and CD54. Fibroblasts and CD40-activated B cells comparably stimulated HLA-A*0301-specific CD8 T cells after transient expression of HLA-A*0301 as a model alloantigen. Finally, fibroblasts could be effectively applied to selectively deplete alloreactivity within leukemia-reactive donor CD8 T-cell lines by targeting the activation-induced antigen CD137.

CONCLUSIONS

Primary fibroblasts can be efficiently used as allogeneic nonhematopoietic APC for selective depletion of donor T cells reactive to HLA and ubiquitously expressed minor-H antigen disparities in leukemia-stimulated CD8 T-cell lines. Therefore, harnessing alloreactivity to hematopoietic minor-H antigens in addition to leukemia-associated antigens might increase graft-versus-leukemia immunity of donor lymphocyte grafts in allogeneic hematopoietic stem-cell transplantation.

Cytotoxic T lymphocytes as immune-therapy in haematological practice

Viral infections are a significant cause of morbidity and mortality, particularly in pediatric allogeneic haematopoietic stem cell transplant recipients. Effective therapies are limited and often associated with significant side effects. Adoptive transfer of virus-reactive T cells offers a means of reconstituting antiviral immunity and this approach has been successfully used to prevent and treat cytomegalovirus, Epstein-Barr virus, and adenovirus infections in vivo. This review outlines the clinical trials that have been performed to date, and will describe future initiatives to (a) develop strategies that can increase the breadth of the viruses that can be targeted, and (b) simplify the process to extend this technology to more centers so that cellular therapy to reconstitute immunity can be more widely applied.

CD134-allodepletion allows selective elimination of alloreactive human T cells without loss of virus-specific and leukemia-specific effectors

Graft-versus-host disease (GVHD) remains a frequent and severe complication of allogeneic stem cell transplantation (SCT). One approach to reducing alloreactivity is to deplete the graft of alloreactive T cells. Global T cell depletion results in poor immune reconstitution with high mortality from viral infections and disease relapse. Therefore, an approach to selectively deplete alloreactive T cells without compromising other responses would be highly beneficial. We undertook studies to identify an inducible activation marker expressed on alloreactive effector T cells following culture with HLA-mismatched allostimulators. Compared to other markers, CD134 was superior because of its negative baseline expression and rapid upregulation after activation. Depletion of CD134(+) cells from responder populations dramatically reduced specific alloreactivity as determined by reduction of helper T cell precursor frequencies below the threshold predicting development of clinical GVHD while retaining responses to third-party alloantigens. CD134-allodepleted populations retained effectors specific for the Wilms' tumor (WT1) leukemia antigen as determined by WT1 specific pentamers, and CMV-specific effectors as determined by CMV-specific pentamers and CMV-specific ELISpot. Thus, use of CD134-allodepleted grafts may improve allogeneic SCT by reducing GVHD without loss of pathogen-specific and leukemia-specific immunity.

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.

T-cell therapy after hematopoietic stem cell transplantation

PURPOSE OF REVIEW

The separation of graft versus host disease from graft versus leukaemia reactivity and the reconstitution of immunity to infectious agents are the main goals of T-cell therapy after allogeneic hematopoietic stem cell transplantation. We describe how an improved understanding of T-cell mediated graft versus leukemia and of antiviral responses is providing effective approaches to T-cell immunotherapy.

RECENT FINDINGS

Over the past several years, researchers have developed strategies to eliminate alloreactive T cells from the graft, to expand naturally occurring regulatory T cells, and to select and expand antigen-specific T cells specific for tumor-associated or viral antigens. Incorporation of suicide genes allows the selective destruction of allodepleted or antigen-selected cells after infusion, further increasing the safety and potential applicability of these approaches.

SUMMARY

In this review we describe current strategies for adoptive T-cell immunotherapy after hematopoietic stem cell transplantation.

Administration of a CD25-directed immunotoxin, LMB-2, to patients with metastatic melanoma induces a selective partial reduction in regulatory T cells in vivo

CD25+ CD4+ T regulatory (Treg) cells regulate peripheral self tolerance and possess the ability to suppress antitumor responses, which may in part explain the poor clinical response of cancer patients undergoing active immunization protocols. We have previously shown that in vitro incubation of human PBMC with LMB-2, a CD25-directed immunotoxin, significantly reduced CD25+ FOXP3+ CD4+ Treg cells without impairing the function of the remaining lymphocytes. In the current study, eight patients with metastatic melanoma were treated with LMB-2 followed by MART-1 and gp100-specific peptide vaccination. LMB-2 administration resulted in a preferential, transient reduction in mean circulating CD25+ CD4+ T cell number, from 83 +/- 16 cells/microl to a nadir of 17 +/- 5 cells/microl, a 79.1% reduction. FOXP3 analysis revealed a less robust depletion with mean FOXP3+ CD4+ Treg cell number decreasing from 74 +/- 15 cells/microl to 36 +/- 8 cells/microl, a 51.4% reduction. FOXP3+ CD4+ Treg cells that survived LMB-2-mediated cytotoxicity expressed little or no CD25. Similar to the peripheral blood, immunohistochemical analysis showed a 68.9% mean reduction in FOXP3+ CD4+ Treg cell frequency in evaluable lesions. Despite inducing a reduction in Treg cell numbers in vivo, LMB-2 therapy did not augment the immune response to cancer vaccination and no patient experienced an objective response or autoimmunity. These data demonstrate the capacity of a CD25-directed immunotoxin to selectively mediate a transient partial reduction in circulating and tumor-infiltrating Treg cells in vivo, and suggest that more comprehensive Treg cell elimination may be required to bolster antitumor responses in patients with metastatic melanoma.

A clinical-scale selective allodepletion approach for the treatment of HLA-mismatched and matched donor-recipient pairs using expanded T lymphocytes as antigen-presenting cells and a TH9402-based photodepletion technique

Selective allodepletion is a strategy to eliminate host-reactive donor T cells from hematopoietic stem cell allografts to prevent graft-versus-host disease while conserving useful donor immune functions. To overcome fluctuations in activation-based surface marker expression and achieve a more consistent and effective allodepletion, we investigated a photodepletion process targeting activation-based changes in p-glycoprotein that result in an altered efflux of the photosensitizer TH9402. Expanded lymphocytes, generated using anti-CD3 and IL-2, were cocultured with responder cells from HLA-matched or -mismatched donors. Optimal results were achieved when cocultured cells were incubated with 7.5 muM TH9402, followed by dye extrusion and exposure to 5 Joule/cm(2) light energy at 5 x 10(6) cells/mL. In mismatched stimulator-responder pairs, the median reduction of alloreactivity was 474-fold (range, 43-fold to 864-fold) compared with the unmanipulated responder. Third-party responses were maintained with a median 1.4-fold (range, 0.9-fold to 3.3-fold) reduction. In matched pairs, alloreactive helper T-lymphocyte precursors were reduced to lower than 1:100 000, while third-party responses remained higher than 1:10 000. This establishes a clinical-scale process capable of highly efficient, reproducible, selective removal of alloreactive lymphocytes from lymphocyte transplant products performed under current Good Manufacturing Practice. This procedure is currently being investigated in a clinical trial of allotransplantation.

Reconstitution of FOXP3+ regulatory T cells (Tregs) after CD25-depleted allotransplantation in elderly patients and association with acute graft-versus-host disease

Selective depletion (SD) of host-reactive donor T cells from allogeneic stem-cell transplants (SCTs) using an anti-CD25 immunotoxin (IT) is a strategy to prevent acute graft-versus-host disease (aGvHD). There is concern that concurrent removal of regulatory T cells (T(regs)) with incomplete removal of alloactivated CD25(+) T cells could increase the risk of aGvHD. We therefore measured T(regs) in the blood of 16 patients receiving a T-cell-depleted allograft together with anti-CD25-IT-treated SD lymphocytes, in 13 of their HLA-identical donors, and in 10 SD products. T(regs) were characterized by intracellular staining for forkhead box protein 3 (FOXP3) and by quantitative reverse-transcription-polymerase chain reaction (qRT-PCR) for FOXP3 gene in CD4(+) cells. Patients received a median of 1.0 x 10(8)/kg SD T cells and a stem cell product containing a median of 0.25 x 10(4)/kg residual T cells. T(regs) reconstituted promptly after SCT and underwent further expansion. Of the CD4(+) T cells in SD products, 1.5% to 4.8% were CD25(-) T(regs). Acute GvHD (>or= grade II) was restricted to 5 patients whose donors had significantly (P = .019) fewer T(regs) compared with those without clinically significant aGvHD. These results suggest that rapid T(reg) reconstitution can occur following SD allografts, either from CD25(-) T(regs) escaping depletion, or from residual CD25(-) and CD25(+) T(regs) contained in the stem-cell product that expand after transplantation and may confer additional protection against GvHD.

T cell therapies following hematopoietic stem cell transplantation: surely there must be a better way than DLI?

Advances in the past few years have significantly improved adoptive immunotherapy strategies available following autologous and allogeneic hematopoietic stem cell transplantation (HSCT). Minimal residual disease, relapsed disease and viral infections remain a significant cause of mortality in patients undergoing HSCT. Novel therapies are critically needed to overcome these management dilemmas, while sparing the graft-versus-tumor effect and avoiding graft-versus-host disease. This review focuses on the T-cell strategies currently available to allay disease while minimizing toxicities in patients who have undergone HSCT.

Robust expansion of viral antigen-specific CD4+ and CD8+ T cells for adoptive T cell therapy using gene-modified activated T cells as antigen presenting cells

Cytomegalovirus (CMV) reactivation after stem cell transplantation can be treated with CMV-specific T cells, but current in vitro techniques using dendritic cells as antigen-presenting cells are time-consuming and expensive. To simplify the production of clinical grade CMV-specific T cells, we evaluated gene-modified activated T cells [antigen presenting T cells (T-APCs)] as a reliable and easily produced source of APCs to boost CD4+ and CD8+ T-cell responses against the immunodominant CMV antigen pp65. T-APCs expressing the full-length immunodominant CMV pp65 gene were used to stimulate the expansion of autologous T cells. After 10 to 14 days, the T cell lines were tested for antigen specificity by using the flow cytometric intracellular detection of interferon-gamma after stimulation for 6 hours with a pp65 peptide library of 15-mers, overlapping by 11 amino acids. Under optimal conditions, this technique induced a median 766-fold and a 652-fold expansion of pp65-specific CD4+ and CD8+ responder cells, respectively, in 15 T cell lines. In 13 of 15 T cell lines, over 10 antigen-specific CD4+ plus CD8+ T cells were generated starting with only 5x10 peripheral blood mononuclear cells, representing an over 3-log increase. These data indicate that T-APCs efficiently boost pp65-specific CD4+ and CD8+ T cell numbers to clinically useful levels. The approach has the advantage of using a single leukocyte collection from the donor to generate large numbers of CMV-specific T cells within a total 3-week culture period using only one stimulation of antigen.

Targeting the activation-induced antigen CD137 can selectively deplete alloreactive T cells from antileukemic and antitumor donor T-cell lines

In HLA-incompatible hematopoietic stem cell transplantation, alloreactive donor T cells recognizing recipient mismatch HLA cause severe graft-versus-host disease (GVHD). Strategies allowing the selective depletion of alloreactive T cells as well as the enhancement of graft-versus-malignancy immunity would be beneficial. We generated donor CD8 T-cell lines in vitro using allogeneic recipient cells mismatched at a single HLA class I allele or haplotype as stimulators. Recipient cells were obtained from acute myeloid leukemias, renal-cell carcinomas, and CD40L-induced B lymphoblasts. Resulting alloreactive T cells were activated by incubating day 21 T-cell cultures with HLA-mismatch transfected K562 cells or recipient-derived fibroblasts. Selective allodepletion (SAD) was subsequently performed by a newly developed immunomagnetic depletion approach targeting the tumor necrosis factor receptor molecule CD137 (4-1BB). Compared with other activation-induced antigens, CD137 showed a superior performance based on a consistently low baseline expression and a rapid up-regulation following alloantigen stimulation. In 15 different SAD experiments, the frequency of alloreactive CD8 T cells was reduced to a median of 9.5% compared with undepleted control populations. The allodepleted T-cell subsets maintained significant antitumor and antiviral CD8 responses. In vitro expansion of tumor-reactive T cells followed by CD137-mediated SAD might enhance the antitumor efficacy of T-cell allografts with lower risk of inducing GVHD.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Prophylaxis and treatment of acute graft-versus-host disease

Acute graft-versus-host disease (GVHD) remains a major obstacle to successful allogeneic hematopoietic stem cell transplantation (HSCT). The ability to prevent GVHD--the application of successful prophylaxis--is crucial as treatment when prophylaxis fails or remains suboptimal. A calcineurin inhibitor in combination with methotrexate is still the mainstream regimen for prophylaxis of GVHD. Despite a steady increase in the repertoire of available drugs, corticosteroids remain the first-line therapy for patients who fail prevention and develop GVHD. Pan T-cell depletion studies suggest that success in prophylaxis and treatment of GVHD will depend on whether GVHD can be prevented without losing anti-malignancy and anti-infectious effects. Better understanding of the allogeneic response that is responsible for GVHD will facilitate the development of such an approach.

A comparison of an anti-CD25 immunotoxin, Ontak and anti-CD25 microbeads for their ability to deplete alloreactive T cells in vitro

Ex vivo depletion of alloreactive CD25(+) T cells from a stem cell transplant (SCT) can reduce the incidence of graft-versus-host disease (GVHD) while preserving antimicrobial and perhaps antileukemia activity. However, the most effective methods for allodepleting T cells prior to transplant have not been determined. In this study, we have compared three agents that deplete CD25(+) activated, alloreactive T cells. These included Ontak (Denileukin Diftitox), an IL-2 fusion toxin, anti-CD25 microbeads (MACS), an anti-CD25 immunotoxin (IT) and a combination of the IT and MACS. Peripheral blood mononuclear cells (PBMCs) activated in a primary mixed lymphocyte reaction (MLR) were allodepleted using optimal amounts of each agent, and the cells were then analyzed by flow cytometry. The treated cells were examined both for remaining alloreactivity and for the preservation of third party reactivity by testing them in a secondary MLR. Our data demonstrate that both the anti-CD25 IT and the anti-CD25 MACS were equally effective in depleting CD4(+)CD25(+) cells and in sparing T cells that were reactive with third party cells. The anti-CD25 IT was, however, superior in depleting alloreactive CD8(+)CD25(+) cells. In contrast, Ontak did not eliminate alloreactive cells and the Ontak-treated cells retained significant reactivity against the original stimulator cells.

Selective depletion strategies in allogeneic stem cell transplantation

Despite improved prophylaxis and treatment, GvHD remains a major limitation to optimal allogeneic stem cell transplantation. Ex vivo selective depletion (SD) is a strategy to prevent GvHD, in which host-reactive donor lymphocytes are selectively eliminated from a PBSC allograft while useful donor immune function is preserved. The elimination of alloreactive and thereby GvHD-mediating T cells has been shown to be feasible in both pre-clinical and more recently clinical studies. However, SD techniques and the translational research needed for clinical application are still under development. Here we summarize and discuss the following aspects of the SD approach: selection of an appropriate allogeneic stimulator; the responder population; the alloresponse; methods for removal of alloreacting T cells; product testing; clinical considerations. Our review highlights the diversity of possible approaches and the need to develop different techniques for specific clinical applications.

Bone marrow transplantation and approaches to avoid graft-versus-host disease (GVHD)

Haematopoietic stem cell transplantation (HSCT) offers promise for the treatment of haematological and immune disorders, solid tumours, and as a tolerance inducing regimen for organ transplantation. Allogeneic HSCTs engraftment requires immunosuppression and the anti-tumour effects are dependent upon the immune effector cells that are contained within or generated from the donor graft. However, significant toxicities currently limit its efficacy. These problems include: (i) graft-versus-host disease (GVHD) in which donor T cells attack the recipient resulting in multi-organ attack and morbidity, (ii) a profound period of immune deficiency following HSCT, and (iii) donor graft rejection. Currently available methods to prevent or treat GVHD with systemic immunosuppression can lead to impaired immune recovery, increased opportunistic infections, and higher relapse rates. This review will provide an overview of GVHD pathophysiology and discuss the roles of various cells, pathways, and factors in the GVHD generation process and in the preservation of graft-versus-tumour effects. Variables that need to be taken into consideration in attempting to extrapolate preclinical results to the clinical paradigm will be highlighted.

Improved Outcome for Peripheral Blood Stem Cell Transplantation for Advanced Primary Myelodysplastic Syndrome

Stem cell transplantation for myelodysplastic syndrome (MDS) is characterized by high transplant-related mortality (TRM), especially in older patients and those with more advanced disease. Outcome after peripheral blood stem cell transplantation (PBSCT) may be superior to earlier results with bone marrow transplantation. Forty-three patients (aged 12-73 years; median, 49 years) received an HLA-identical sibling donor PBSCT. Twenty three patients aged < or =55 years without prohibitive comorbidity received myeloablative total body irradiation-based conditioning, followed by a T cell-depleted PBSCT and delayed add-back of donor lymphocytes. Older patients or those with comorbidities (n = 20) received reduced-intensity conditioning and an unmanipulated PBSCT. Thirty-seven (86%) had advanced disease (refractory anemia with excess blasts [n = 9], refractory anemia with excess blasts in transformation [n = 6], acute myelogenous leukemia [n = 13], or treatment-related MDS [n = 9]); 6 had low-risk MDS (refractory anemia or refractory anemia with ringed sideroblasts). The median follow-up was 18 months (range, 5-89 months). Actuarial probabilities of 3-year overall survival (OS), disease-free survival, relapse, and TRM were 64%, 59%, 26%, and 23%, respectively, for 34 primary MDS patients. The best results were in 19 patients younger than 50 years of age undergoing myeloablative PBSCT (actuarial probabilities of OS, disease-free survival, relapse, and TRM were 81%, 72%, 22%, and 7%, respectively). Although outcomes for all stages of primary MDS were improved, that for therapy-related MDS remained dismal, with 11% OS, because of a high relapse rate (89%).

Allogeneic Hematopoietic Cell Transplantation as Immunotherapy for Solid Tumors

Although myeloablative conditioning can cytoreduce or debulk malignancies, the curative antitumor effects of allogeneic hematopoietic stem cell transplantation (HCT) are mostly mediated by transplanted donor immune cells. A heightened awareness and appreciation of the immune-mediated anticancer effects that occur after allogeneic transplantation has led to the increasing use of reduced-intensity stem cell transplantation (RIST) approaches to treat advanced malignancies. The graft-versus-leukemia effects that occur against hematologic cancers after RIST have recently attracted oncologists to explore the therapeutic potential of allogeneic HCT for treatment-refractory solid tumors. Delayed tumor regression after RIST in a subset of patients with metastatic renal cell, breast, ovarian, pancreatic, and colon carcinoma has recently been reported, confirming the existence of a graft-versus-tumor effect in solid tumors. Advanced disease states, rapidly growing tumors, and accrual of patients with extremely short survival are factors that have been identified to limit the efficacy of allogeneic immunotherapy. This review discusses results of allogeneic HCT for solid tumors and the development of newer transplant strategies to optimize the potential of the graft-versus-tumor effect.

Elimination of alloreactive T cells using photodynamic therapy

GvHD, the most important cause of morbidity and mortality after allogeneic stem cell transplantation, depends primarily on the ability of a donor T-cell subset to react to immunogenic host Ag. Recently developed culture conditions and treatment strategies may bring us closer to the selective elimination of such alloreactive T cells, often considered the holy grail of transplantation. Among the various therapeutic modalities, photodynamic therapy (PDT) offers a biological and global approach to the eradication of unwanted allo-activated T cells by combining mitochondrial targeting, P-glycoprotein inhibition and reactive oxygen species production. Indeed, the high potency of PDT against malignant cells has been harnessed to exert selective and extensive elimination of alloreactive T-cell subsets mediating GvHD, while preserving resting T cells with the ability to reconstitute the immune system for GvL activity and prevent or suppress viruses and fungi. The present paper reviews the basis of the PDT strategy, and the methodology employed. In vitro and in vivo studies that formed the proof of principle as a basis for human studies to investigate the clinical potential of PDT in the context of GvHD will be presented together with insights into future clinical applications of this versatile treatment platform.

Selective depletion of alloreactive donor lymphocytes: a novel method to reduce the severity of graft-versus-host disease in older patients undergoing matched sibling donor stem cell transplantation

We have selectively depleted host-reactive donor T cells from peripheral blood stem cell (PBSC) transplant allografts ex vivo using an anti-CD25 immunotoxin. We report a clinical trial to decrease graft-versus-host disease (GVHD) in elderly patients receiving selectively depleted PBSC transplants from HLA-identical sibling donors. Sixteen patients (median age, 65 years [range, 51-73 years]), with advanced hematologic malignancies underwent transplantation following reduced-intensity conditioning with fludarabine and either cyclophosphamide (n = 5), melphalan (n = 5), or busulfan (n = 6). Cyclosporine was used as sole GVHD prophylaxis. The allograft contained a median of 4.5 x 10(6) CD34 cells/kg (range, 3.4-7.3 x 10(6) CD34 cells/kg) and 1.0 x 10(8)/kg (range, 0.2-1.5 x 10(8)/kg) selectively depleted T cells. Fifteen patients achieved sustained engraftment. The helper T-lymphocyte precursor (HTLp) frequency assay demonstrated successful (mean, 5-fold) depletion of host-reactive donor T cells, with conservation of third-party response in 9 of 11 cases tested. Actuarial rates of acute GVHD were 46% +/- 13% for grades II to IV and 12% +/- 8% for grades III to IV. These results suggest that allodepletion of donor cells ex vivo is clinically feasible in older patients and may reduce the rate of severe acute GVHD. Further studies with selectively depleted transplants to evaluate graft-versus-leukemia (GVL) and survival are warranted.

Superior depletion of alloreactive T cells from peripheral blood stem cell and umbilical cord blood grafts by the combined use of trimetrexate and interleukin-2 immunotoxin

Acute graft-versus-host disease, a major obstacle to the overall success of allogeneic hematopoietic stem cell transplantation, is primarily induced by a subset of donor T cells. Most strategies to prevent acute graft-versus-host disease target all T cells regardless of their specificity, and this leads to prolonged posttransplantation immunodeficiency. Selective depletion of alloreactive T cells could spare protective immunity and facilitate engraftment and graft-versus-leukemia effects. Recently described depletion strategies target activation markers such as CD25 that are expressed by alloreactive T cells. However, incomplete depletion may occur when a single surface epitope or pathway of apoptosis is targeted that may not be fully and concurrently expressed among all alloreactive cells. We now report on a novel strategy effective in both cord blood and peripheral blood stem cell alloreactive T cells that simultaneously induces 2 independent pathways of apoptosis after stimulation by recipient dendritic cells or Epstein-Barr virus-transformed B cells. First, we demonstrate that the folate antagonist trimetrexate selectively depletes proliferating alloreactive precursors in vitro in a dose- and time-dependent manner. Similarly, a second agent, denileukin diftitox, kills activated alloreactive T cells expressing CD25. Most importantly, these 2 agents can exert their effects in concert with superior efficacy while sparing resting bystander T cells, which remain available to mount antimicrobial or third-party responses.

The transition from bench to bedside: lessons learned in the creation of a new T-cell product for the clinic

While the technology of translational research is sometimes considered a poor relation to 'basic science', it has become a central focus of work to turn exciting new concepts into practical therapies. Here we use the example of selective T-cell depletion of allografted lymphocytes to illustrate the problems of scale-up, reproducibility, sterility, safety and regulatory concerns encountered during the bench to bedside process.

Innovation meets quality: moving cellular therapies at the National Institutes for Health from bench to bedside

The National Institutes of Health (NIH) Department of Transfusion Medicine has supported clinical investigation in cellular therapies over the past 20 years. This experience, which encompasses product development research, quality system design and product manufacturing for a wide range of early phase clinical trials, provides a firm basis for future work with novel stem cell therapies.

Functional assessment and specific depletion of alloreactive human T cells using flow cytometry

Human T-cell alloreactivity plays an important role in many disease processes, including the rejection of solid organ grafts and graft-versus-host disease (GVHD) following allogeneic stem cell transplantation. To develop a better understanding of the T cells involved in alloreactivity in humans, we developed a cytokine flow cytometry (CFC) assay that enabled us to characterize the phenotypic and functional characteristic of T cells responding to allogeneic stimuli. Using this approach, we determined that most T-cell alloreactivity resided within the CD4+ T-cell subset, as assessed by activation marker expression and the production of effector cytokines (eg, tumor necrosis factor α [TNF]α) implicated in human GVHD. Following prolonged stimulation in vitro using either allogeneic stimulator cells or viral antigens, we found that coexpression of activation markers within the CD4+ T-cell subset occurred exclusively within a subpopulation of T cells that significantly increased their surface expression of CD4. We then developed a simple sorting strategy that exploited these phenotypic characteristics to specifically deplete alloreactive T cells while retaining broad specificity for other stimuli, including viral antigens and third-party alloantigens. This approach also was applied to specifically enrich or deplete human virus-specific T cells.

Acute graft-versus-host disease: Pathophysiology, clinical manifestations, and management

Hematopoietic stem cell transplantation has evolved as a central treatment modality in the management of different hematologic malignancies. Despite adequate posttransplantation immunosuppressive therapy, acute graft-versus-host disease (GVHD) remains a major cause of morbidity and mortality in the hematopoietic stem cell transplantation setting, even in patients who receive human leukemic antigen (HLA) identical sibling grafts. Up to 30% of the recipients of stem cells or bone marrow transplantation from HLA-identical related donors and most patients who receive cells from other sources (matched-unrelated, non-HLA-identical siblings, cord blood) will develop > Grade 2 acute GVHD despite immunosuppressive prophylaxis. Thus, GVHD continues to be a major limitation to successful hematopoietic stem cell transplantation. In this review, the authors summarize the most current knowledge on the pathophysiology, clinical manifestations, and management of this potentially life-threatening transplantation complication.

Nonmyeloablative allogeneic immunotherapy for solid tumors

Over the past decade, considerable advances have been made in the field of allogeneic hematopoietic stem cell transplantation. Recognition that transplanted donor immune cells can cure patients with leukemia has led to the development of nonmyeloablative or "low-intensity" conditioning regimens, which have expanded the application of allogeneic transplantation to a growing number of hematological malignancies. The improved safety and preliminary success of this transplant approach have justified applying allogeneic immunotherapy to patients with treatment-refractory solid tumors.

Antiviral immunity and T-regulatory cell function are retained after selective alloreactive T-cell depletion in both the HLA-identical and HLA-mismatched settings

Nonselective T-cell depletion reduces the incidence of severe graft-versus-host disease after allogeneic hematopoietic stem cell transplantation, but the cost is delayed and disordered antigen-specific immune reconstitution and increased infection. We use a method of selective depletion of alloreactive T cells expressing the activation marker CD69 after coculture with stimulator cells in a modified or standard mixed lymphocyte reaction. The technique has been shown to reduce alloreactivity while retaining third-party responses in vitro and, in a mismatched murine model, led to donor T-cell engraftment with a virtual absence of graft-versus-host disease and increased survival. We show in a human HLA-mismatched and unrelated HLA-identical setting that this technique retains >80% of specific cellular antiviral activity by cytomegalovirus-tetramer analysis and cytomegalovirus/Epstein-Barr virus peptide-stimulated interferon-gamma ELISpot assay. Furthermore, CD4(+) CD25(+) T-regulatory cells are not removed by this method of selective allodepletion and retain their function in suppressing allogeneic proliferative responses. Preservation of antiviral cytotoxic T lymphocytes in selectively allodepleted stem cell grafts would lead to improved antiviral immunity after transplantation. The retention of immunosuppressive CD4(+) CD25(+) T-regulatory cells could lead to more ordered immune reconstitution and further suppress alloreactive responses after transplantation.

Cellular therapy services provided by blood centers and hospitals in the United States, 1999: an analysis from the Nationwide Blood Collection and Utilization Survey

BACKGROUND

The 2000 Nationwide Blood Collection and Utilization Survey was designed to assess cellular therapy product services in US blood centers and hospitals.

STUDY DESIGN AND METHODS

Questionnaires were returned by 2,040 institutions. Data were analyzed for 30 quantitative variables related to cellular therapy product activities.

RESULTS

269 institutions, including 231 (12.2%) of the hospitals, 37 (25.9%) of the blood centers, and one cryobank, performed HPC services. Collected PBSC (20,517) and cord blood products (12,628) far exceeded bone marrow (1,572), lymphocytes (578), and cultured cells (344). PBPC collections dropped 36.5 percent since the 1997 survey. Cord blood accounted for 35.4 percent of collections and 39.5 percent of products processed, but only 1.9 percent of infusions.

CONCLUSIONS

Most cellular therapy services in hospitals and blood centers were HPC-related. The dramatic drop in PBPC collections since 1997 reflects the decline in autologous PBPC transplantation for breast cancer. Cord blood's high collection-to-infusion ratio demonstrates a substantial resource expenditure for banking a product for future clinical needs. Lymphocytes and cultured cell products contributed minimally to activities in this survey, but will likely increase in the future. Data from additional academic and commercial manufacturers of cellular therapy products should be included in future surveys.

Adoptive immunotherapy for posttransplantation viral infections

Viral diseases are a major cause of morbidity and mortality after hemopoietic stem cell transplantation. Because viral complications in these patients are clearly associated with the lack of recovery of virus-specific cellular immune responses, reconstitution of the host with in vitro expanded cytotoxic T lymphocytes is a potential approach to prevent and treat these diseases. Initial clinical studies of cytomegalovirus and Epstein-Barr virus in human stem cell transplant patients have shown that adoptively transferred donor-derived virus-specific T cells may restore protective immunity and control established infections. Preclinical studies are evaluating this approach for other viruses while strategies for generating T cells specific for multiple viruses to provide broader protection are being evaluated in clinical trials. The use of genetically modified T cells or the use of newer suicide genes may result in improved safety and efficacy.

CD25 expression on donor CD4+ or CD8+ T cells is associated with an increased risk for graft-versus-host disease after HLA-identical stem cell transplantation in humans

Graft-versus-host disease (GVHD) occurs in an unpredictable fashion after 30% to 50% of matched-related transplantations. The presence of increased frequencies of CD4(+)CD25(+) regulatory T cells in donor grafts has been shown to ameliorate GVHD after allogeneic transplantation in murine models. To determine whether a similar relationship exists in humans, we quantitated the coexpression of CD25 on CD4(+) and CD8(+) T cells within 60 donor grafts infused into matched siblings and examined GVHD incidence in the respective recipients. Recipients in whom GVHD developed received donor grafts containing significantly higher frequencies of CD4(+) T cells coexpressing CD25 than those who did not (median, 9.26% vs 2.22%; P =.004). Frequencies of donor graft CD8(+) T cells coexpressing CD25 were also higher (0.65% vs 0.14%; P =.002). Furthermore, transplant recipients who received grafts containing fewer CD4(+)CD25(+) and CD8(+)CD25(+) T cells were less likely to acquire acute GVHD, even though these donor-recipient pairs were similar to others with respect to relevant clinical variables. These data suggest that the coexpression of CD4 and CD25 may be insufficient to identify regulatory T cells in humans and that increased frequencies and numbers of CD25(+) T cells in donor grafts is associated with GVHD in transplant recipients.

New advances in acute graft-versus-host disease prophylaxis

Immunocompetent donor T cells in Allogeneic Haematopoietic Stem Cell grafts mediate acute Graft versus Host Disease (GvHD), still a major cause of recipient morbidity and mortality post transplant. Despite the advent of high resolution HLA-typing and matching at HLA loci, acute GvHD remains a significant problem, even in HLA matched siblings, due primarily to minor histocompatability antigen mismatches. Treatment of GvHD remains ineffective and highly immunosuppressive and the challenge to find effective methods of prevention continues. Non selective removal of donor T cells from the graft has been proven to be effective in preventing GvHD but the beneficial effects of donor T cells, namely effective immune reconstitution and anti tumour activity, are lost. This review considers mechanisms by which acute GvHD may be prevented in the context of the current model of GvHD immunopathogenesis, with a special emphasis on the recent techniques of selective removal or destruction of donor allogeneic T cells that have been described.

Cyclosporine is required to prevent severe acute GVHD following T-cell-depleted peripheral blood stem cell transplantation

Reduced immunosuppression may improve immune recovery and increase the graft-versus-leukemia effect after allogeneic stem cell transplantation. Furthermore, the requirement for post-transplant immunosuppression following extensive T-cell depletion remains unclear. We therefore evaluated the role of cyclosporine (CSA) in recipients of HLA-identical T-cell-depleted peripheral blood stem cell transplants (PBSCT), followed by donor lymphocyte infusions (DLIs) scheduled on days +45 and +100. Before day+45, successive cohorts of patients received decreasing amounts of CSA: standard-dose (SD) CSA, low-dose (LD) CSA, or no CSA until day+45. LD CSA was as effective as SD CSA in preventing acute graft-versus-host disease (GVHD). However, moderate-to-severe acute GVHD was significantly more frequent before the day +45 DLI in patients receiving no CSA (33.3 vs 12.7%, P=0.036, including the only four grade III-IV cases). As a result of higher rates of early acute GVHD, more patients in the 'no CSA' group failed to receive any DLI (30.7 vs 7.1%, P=0.01). Overall, there was no difference in the incidence of acute GVHD, as patients receiving CSA developed more GVHD after DLI. Similarly, no significant differences were found in chronic GVHD, transplant-related mortality, or survival. These results define a role for CSA in preventing GVHD at low T-cell doses following PBSCT.

Immunotherapy of Hematologic Malignancy

Over the past few years, improved understanding of the molecular basis of interactions between antigen presenting cells and effector cells and advances in informatics have both led to the identification of many candidate antigens that are targets for immunotherapy. However, while immunotherapy has successfully eradicated relapsed hematologic malignancy after allogeneic transplant as well as virally induced tumors, limitations have been identified in extending immunotherapy to a wider range of hematologic malignancies. This review provides an overview of three immunotherapy strategies and how they may be improved.

In Section I, Dr. Stevenson reviews the clinical experience with genetic vaccines delivered through naked DNA alone or viral vectors, which are showing promise in clinical trials in lymphoma and myeloma patients. She describes efforts to manipulate constructs genetically to enhance immunogenicity and to add additional elements to generate a more sustained immune response.

In Section II, Dr. Molldrem describes clinical experience with peptide vaccines, with a particular focus on myeloid tissue-restricted proteins as GVL target antigens in CML and AML. Proteinase 3 and other azurophil granule proteins may be particularly good targets for both autologous and allogeneic T-cell responses. The potency of peptide vaccines may potentially be increased by genetically modifying peptides to enhance T-cell receptor affinity.

Finally, in Section III, Dr. Heslop reviews clinical experience with adoptive immunotherapy with T cells. Transferred T cells have clinical benefit in treating relapsed malignancy post transplant, and Epstein-Barr virus associated tumors. However, T cells have been less successful in treating other hematologic malignancies due to inadequate persistence or expansion of adoptively transferred cells and the presence of tumor evasion mechanisms. An improved understanding of the interactions of antigen presenting cells with T cells should optimize efforts to manufacture effector T cells, while manipulation of lymphocyte homeostasis in vivo and development of gene therapy approaches may enhance the persistence and function of adoptively transferred T cells.

New Developments in Allotransplant Immunology

After allogeneic stem cell transplantation, the establishment of the donor’s immune system in an antigenically distinct recipient confers a therapeutic graft-versus-malignancy effect, but also causes graft-versus-host disease (GVHD) and protracted immune dysfunction. In the last decade, a molecular-level description of alloimmune interactions and the process of immune recovery leading to tolerance has emerged. Here, new developments in understanding alloresponses, genetic factors that modify them, and strategies to control immune reconstitution are described.

In Section I, Dr. John Barrett and colleagues describe the cellular and molecular basis of the alloresponse and the mechanisms underlying the three major outcomes of engraftment, GVHD and the graft-versus-leukemia (GVL) effect. Increasing knowledge of leukemia-restricted antigens suggests ways to separate GVHD and GVL. Recent findings highlight a central role of hematopoietic-derived antigen-presenting cells in the initiation of GVHD and distinct properties of natural killer (NK) cell alloreactivity in engraftment and GVL that are of therapeutic importance. Finally, a detailed map of cellular immune recovery post-transplant is emerging which highlights the importance of post-thymic lymphocytes in determining outcome in the critical first few months following stem cell transplantation. Factors that modify immune reconstitution include immunosuppression, GVHD, the cytokine milieu and poorly-defined homeostatic mechanisms which encourage irregular T cell expansions driven by immunodominant T cell–antigen interactions.

In Section II, Prof. Anne Dickinson and colleagues describe genetic polymorphisms outside the human leukocyte antigen (HLA) system that determine the nature of immune reconstitution after allogeneic stem cell transplantation (SCT) and thereby affect transplant outcomethrough GVHD, GVL, and transplant-related mortality. Polymorphisms in cytokine gene promotors and other less characterized genes affect the cytokine milieu of the recipient and the immune reactivity of the donor. Some cytokine gene polymorphisms are significantly associated with transplant outcome. Other non-HLA genes strongly affecting alloresponses code for minor histocompatibility antigens (mHA). Differences between donor and recipient mHA cause GVHD or GVL reactions or graft rejection. Both cytokine gene polymorphisms (CGP) and mHA differences resulting on donor-recipient incompatibilities can be jointly assessed in the skin explant assay as a functional way to select the most suitable donor or the best transplant approach for the recipient.

In Section III, Dr. Nelson Chao describes non-pharmaceutical techniques to control immune reconstitution post-transplant. T cells stimulated by host alloantigens can be distinguished from resting T cells by the expression of a variety of activation markers (IL-2 receptor, FAS, CD69, CD71) and by an increased photosensitivity to rhodamine dyes. These differences form the basis for eliminating GVHD-reactive T cells in vitro while conserving GVL and anti-viral immunity. Other attempts to control immune reactions post-transplant include the insertion of suicide genes into the transplanted T cells for effective termination of GVHD reactions, the removal of CD62 ligand expressing cells, and the modulation of T cell reactivity by favoring Th2, Tc2 lymphocyte subset expansion. These technologies could eliminate GVHD while preserving T cell responses to leukemia and reactivating viruses.