Selective depletion of donor alloreactive T cells without loss of antiviral or antileukemic responses

The promise of 4-1BB (CD137)-mediated immunomodulation and the immunotherapy of cancer

The continuing efforts in biomedical research to develop new therapies for cancer are entering an exciting new phase. Research over the past two to three decades has yielded a much more detailed understanding of the complexities of the cellular and molecular interactions involved in the generation and regulation of immune responses. We are also gaining insights into the mechanisms by which tumors evade or escape immune recognition and by which they become resistant to various existing chemotherapeutic and/or radiotherapeutic strategies. A clear conclusion that can be drawn from these studies is that effective treatments of cancer will become much more multifaceted and will include immunotherapeutic approaches. The identification and molecular cloning of genes encoding the receptors and ligands that play crucial roles in the generation and regulation of immune responses provides exciting new opportunities to induce and enhance effective endogenous immune responses to cancer. In this regard, the genes that comprise the tumor necrosis factor and tumor necrosis factor receptor superfamilies show particular promise. One receptor:ligand pair (4-1BB/CD137 and 4-1BBL/CD137L) is emerging as a target with important potential in its ability to enhance the generation of effective tumor-specific immune responses in situ. The results of the studies cited in this review highlight the potentials of 4-1BB-mediated immunotherapy.

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.

Use of CD137 to study the full repertoire of CD8+ T cells without the need to know epitope specificities

CD137 (4-1BB) is a member of the TNFR-family with costimulatory function, triggering prosurvival signals in activated T-cells. Upregulation of CD137 upon stimulation allows identifying and isolating live, human antigen-specific CD8+ T-cells of all phenotypes, and therefore provides a comprehensive detection method. Furthermore responses against antigen mixtures can be easily detected, enabling antigen discovery in a stepwise deconvoluting approach. In this article, we will discuss various aspects of this methodology, including potential pitfalls as well as a variety of applications, as illustrated by examples from our laboratory.

Adenoviruses in immunocompromised hosts

SUMMARY

The number of patients with acquired immunodeficiency has grown steadily as a result of both a larger number of patients receiving solid organ and hematopoietic stem cell transplants and their longer survival times. The use of newer, more potent immunosuppressive regimens has increased the frequency of severe adenovirus infections. Human adenoviruses are a large group of viruses, represented by at least 52 serotypes with various genotypes divided into genomic clusters, and these may cause a broad variety of clinical manifestations. The development of molecular methods has increased the sensitivity and rapidity of adenovirus infection diagnosis. The implementation of PCR assays has significantly contributed to the identification of patients with disseminated adenovirus disease. More recently, the development of real-time PCR assays has permitted virus quantification and patient follow-up. There is no treatment for adenovirus with demonstrated efficacy, although cidofovir is widely used. Sensitive diagnostic tests for adenovirus can contribute to the early diagnosis and successful treatment of life-threatening adenovirus infections, especially in complex immunocompromised patients. The development of improved adenovirus therapy still remains a challenge. Adenovirus genetic diversity should be considered for diagnosis, typing, and therapeutic interventions.

Induction of alloanergy in human donor T cells without loss of pathogen or tumor immunity

BACKGROUND

Human leukocyte antigen (HLA)-mismatched allogeneic hematopoietic stem cell transplantation (HSCT) is limited by acute graft-versus-host disease (aGvHD). Nonselective T-cell depletion effectively prevents severe aGvHD but profoundly impairs donor-derived immune reconstitution, increasing infection and disease relapse. The strategy of induction of alloantigen-specific hyporesponsiveness ("alloanergization") in donor bone marrow by allostimulation with costimulatory blockade before haploidentical transplantation has demonstrated early promise in reducing severe aGvHD. However, the differential effect of alloanergization on CD4+ and CD8+ donor T-cell subsets and the degree to which beneficial pathogen- and tumor-immune responses are retained have not been extensively examined.

METHODS

We used an in vitro model of alloanergization by allostimulation of human donor T cells with irradiated unrelated recipient peripheral blood mononuclear cells and costimulatory blockade with humanized monoclonal anti-B7.1 and B7.2 antibodies. Residual alloresponses were assessed by proliferation (thymidine uptake, carboxyfluorescein diacetate succinimidyl ester dye dilution) and cytotoxicity assays. Retention of human herpes virus and tumor-associated antigen (TAA)-specific immunity was measured with HLA-class I-restricted pentamers, intracellular cytokine secretion, and CD107a assay using 5-color flow cytometry.

RESULTS

Alloanergization of HLA-mismatched donor T cells efficiently and selectively abrogated recipient-specific alloproliferation in both CD4+ and CD8+ cells while preserving functional CD4+ and CD8+ immune responses to clinically important human herpes viruses and to the TAA WT1.

CONCLUSIONS

Retention of pathogen- and TAA-specific immunity after alloanergization demonstrates that this methodology, which is simple to apply, has potential to improve immune reconstitution while limiting alloreactivity after HLA-mismatched hematopoietic stem cell transplantation, and deserves additional evaluation in further human clinical trials.

Prophylaxis of acute GVHD: manipulate the graft or the environment?

Graft-versus-host disease (GVHD) is the immune response of donor T lymphocytes responding to the recipient's alloantigens. The cellular and cytokine mechanisms driving GVHD are now well defined and have led to several prophylactic approaches. Selective allodepletion techniques promise to prevent GVHD without causing immune deficiency provoked by global T-cell depletion. Targeted dosing of other (non-T-cells) cells in the graft - such as CD34+ progenitors, regulatory T cells, natural killer cells and mesenchymal stromal cells - can also lead to transplants designed to retain immune capability without causing GVHD. Immunosuppressive drugs such as methotrexate, cyclosporine and anti-lymphocyte antibodies are the mainstay in the prevention of GVHD and can be used in conjunction with engineered grafts to eliminate GVHD. In future it is anticipated that further refinements in targeting the elimination or suppression of the GVHD reacting T cells should be selective enough to preserve the important graft-versus-leukemia effect which contributes to the cure of malignant diseases by allogeneic 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.

Outcome of alloanergized haploidentical bone marrow transplantation after ex vivo costimulatory blockade: results of 2 phase 1 studies

We report the outcomes of 24 patients with high-risk hematologic malignancies or bone marrow failure (BMF) who received haploidentical bone marrow transplantation (BMT) after ex vivo induction of alloantigen-specific anergy in donor T cells by allostimulation in the presence of costimulatory blockade. Ninety-five percent of evaluable patients engrafted and achieved full donor chimerism. Despite receiving a median T-cell dose of 29 x10(6)/kg, only 5 of 21 evaluable patients developed grade C (n = 4) or D (n = 1) acute graft-versus-host disease (GVHD), with only one attributable death. Twelve patients died from treatment-related mortality (TRM). Patients reconstituted T-cell subsets and immunoglobulin levels rapidly with evidence of in vivo expansion of pathogen-specific T cells in the early posttransplantation period. Five patients reactivated cytomegalovirus (CMV), only one of whom required extended antiviral treatment. No deaths were attributable to CMV or other viral infections. Only 1 of 12 evaluable patients developed chronic GVHD. Eight patients survive disease-free with normal performance scores (median follow-up, 7 years). Thus, despite significant early TRM, ex vivo alloanergization can support administration of large numbers of haploidentical donor T cells, resulting in rapid immune reconstitution with very few viral infections. Surviving patients have excellent performance status and a low rate of chronic GVHD.

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.

Cancer immunotherapy: will expanding knowledge lead to success in pediatric oncology?

The past 25 years have seen an increase in our understanding of immunology and further expansion in the clinical use of immunotherapeutic modalities. How immunotherapy will be integrated with chemotherapy, radiation, and surgery remains to be established. Although there have been successes in the field of immunotherapy, they have been inconsistent, and it is hoped that increased understanding of the basic principles of immunology will improve the consistency of beneficial effects. In this article, we briefly provide a general overview of our current understanding of the immune system, with a focus on concepts in tumor immunology, followed by a discussion of how these concepts are being used in the clinic.

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.

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.

Experimental and clinical approaches for optimization of the graft-versus-leukemia effect

The goal of allogeneic (allo)-hematopoietic stem-cell transplantation (HSCT) in the treatment of hematologic malignancies is to harness the graft-versus-leukemia (GVL) effect, while minimizing the risk of graft-versus-host disease (GVHD). Allo-HSCT research has focused on the GVL target antigens and effector mechanisms, and on potential approaches to exploit GVL independently of GVHD. Donor lymphocyte infusion (DLI) achieves the most powerful anti-leukemic responses, and this approach is often used in combination with nonmyeloablative transplant regimens to optimize GVL and reduce GVHD. Serial, dose-escalating, and CD8(+) T-cell-depleted DLI have been introduced into clinical practice, while other variants of DLI have so far been explored only in animal models. The role of naturally occurring regulatory T cells in transplantation tolerance is being increasingly acknowledged, and murine studies indicate the potential ability of T cells to regulate GVHD while maintaining GVL. Experimental and clinical studies have demonstrated the importance of host-type chimerism, particularly for antigen-presenting cells, in determining the occurrence of DLI-induced GVL. Murine studies could assist in the development of clinical strategies targeted at antigen-presenting cells. Clinical studies exploiting natural killer-cell-mediated antitumor reactivity in the context of killer inhibitory receptor-ligand-mismatched allo-HSCT have provided promising results.

Inducible caspase 9 suicide gene to improve the safety of allodepleted T cells after haploidentical stem cell transplantation

Addback of donor T cells following T cell-depleted stem cell transplantation (SCT) can accelerate immune reconstitution and be effective against relapsed malignancy. After haploidentical SCT, a high risk of graft-versus-host disease (GVHD) essentially precludes this option, unless the T cells are first depleted of alloreactive precursor cells. Even then, the risks of severe GVHD remain significant. To increase the safety of the approach and thereby permit administration of larger T cell doses, we used a suicide gene, inducible caspase 9 (iCasp9), to transduce allodepleted T cells, permitting their destruction should administration have adverse effects. We made a retroviral vector encoding iCasp9 and a selectable marker (truncated CD19). Even after allodepletion (using anti-CD25 immunotoxin), donor T cells could be efficiently transduced, expanded, and subsequently enriched by CD19 immunomagnetic selection to >90% purity. These engineered cells retained antiviral specificity and functionality, and contained a subset with regulatory phenotype and function. Activating iCasp9 with a small-molecule dimerizer rapidly produced >90% apoptosis. Although transgene expression was downregulated in quiescent T cells, iCasp9 remained an efficient suicide gene, as expression was rapidly upregulated in activated (alloreactive) T cells. We have demonstrated the clinical feasibility of this approach after haploidentical transplantation by scaling up production using clinical grade materials.

In vitro methotrexate as a practical approach to selective allodepletion

Graft-versus-host disease (GVHD) is a major cause of transplant-related morbidity and mortality in recipients of allogeneic hematopoietic stem cell transplantation. As GVHD is mediated predominantly by alloreactive donor T cells, selective allodepletion from the graft may alleviate GVHD, whereas potentially maintaining other advantages conferred by donor T cells, such as graft survival, antiviral immunity, and graft-versus-leukemia effect. In this study, we evaluated the ability of methotrexate, a clinically approved antimetabolite drug, to deplete alloreactive T cells in HLA-mismatched mixed lymphocyte reactions (MLR). We observed that methotrexate could inhibit the proliferation of alloreactive T cells in primary in vitro MLR. On reexposure of methotrexate-treated cells to the same allostimulus, a significant reduction in the alloreactive immune response was observed, whereas responses to third-party allostimuli and viral antigens were preserved. Thus, our results provide preclinical evidence that in vitro methotrexate treatment results in specific allodepletion and may be used as an effective agent for preventing GVHD.

Assay for monitoring in vitro selective depletion strategies in allogeneic stem cell transplantation

BACKGROUND

GvHD is a serious and potentially life-threatening side-effect of allogeneic BMT, caused by alloreactive cells attacking normal host cells. A number of different approaches have been attempted to remove allo-activated cells from the graft prior to transplantation. When developing such assays, there is a need to control for unwanted removal of cells, as well as depletion efficiency related to activation kinetics.

METHODS

The specific activation induced by the superantigens SEB and TSST-1 of T cells with defined Vbeta chains was utilized to follow activation of bystander cells and the kinetics of specific cellular activation by flow cytometry.

RESULTS

The activation marker CD69 was up-regulated on bystander T cells, and was only transiently highly expressed on the specific T cells, making this marker unreliable for removal of alloreactive cells. In contrast, CD25 was found only on specifically activated T cells and was stably expressed over several days. However, it was not detected on all specific cells until day 6. Likewise, proliferation occurred only in T cells expressing the expected Vbeta chains, with all activated cells having undergone at least one cell cycle by day 4.

DISCUSSION

In conclusion, our assay demonstrates that only temporary bystander activation occurs when polyclonally activating T cells by SEB or TSST-1, and that CD25, but not CD69, can be used for removal of specifically activated cells. Furthermore, this assay is useful for monitoring methods aiming at specific removal of cycling cells.

Improving outcome of allogeneic stem cell transplantation by immunomodulation of the early post-transplant environment

There has been great progress in understanding the alloresponse and the process of immune recovery after stem cell transplantation. Here, we highlight ways in which transplant outcome is determined by unique immunological features of the early post-transplant period that modulate the growth and function of the grafted donor T cells and stem cells. Better understanding of these early events and more detailed knowledge of the phenotype and function of transplanted donor cells facilitate strategies to optimize immune recovery, prevent graft-versus-host disease (GVHD) and boost immunity to viruses and leukemia. Approaches that optimize CD34 cell dose, techniques to remove GVHD-reacting T cells by T cell subset selection, suicide gene insertion or selective allodepletion, and the adoptive transfer of antigen-specific T cells have reached the stage of clinical trials. Furthermore, murine transplant experiments indicate ways to prevent GVHD while preserving immune function by depletion of naïve cells, T cytotoxic 1 and T helper 1 cells, or by enrichment of regulatory T cells. Many of these approaches appear feasible in clinical transplantation and have yielded promising initial results, but proof that the goal of controlled selective immune reconstitution can be achieved is still awaited.

Distinct orientation of the alloreactive monoclonal CD8 T cell activation program by three different peptide/MHC complexes

We have characterized three different programs of activation for alloreactive CD8 T cells expressing the BM3.3 TCR, their elicitation depending on the characteristics of the stimulating peptide/MHC complex. The high-affinity interaction between the TCR and the K(b)-associated endogenous peptide pBM1 (INFDFNTI) induced a complete differentiation program into effector cells correlated with sustained ERK activation. The K(bm8) variant elicited a partial activation program with delayed T cell proliferation, poor CTL activity and undetectable ERK phosphorylation; this resulted from a low-avidity interaction of TCR BM3.3 with a newly identified endogenous peptide, pBM8 (SQYYYNSL). Interestingly, mismatched pBM1/K(bm8) complexes induced a split response in BM3.3 T cells, with total reconstitution of T cell proliferation but defective generation of CTL activity that was correlated with strong but shortened ERK phosphorylation. Crystal structures highlight the molecular basis for the higher stability of pBM8/K(bm8) compared to pBM1/K(bm8) complexes that exist in two conformers. This study illustrates the importance of the stability of both peptide/MHC and peptide/MHC-TCR interactions for induction of sustained signaling required to induce optimal CTL effector functions. Subtle allelic structural variations, amplified by peptide selection, may thus orient distinct outcomes of alloreactive TCR-based therapies.

Ex Vivo Soluble Fas Ligand Treatment of Donor Cells to Selectively Reduce Murine Acute Graft Versus Host Disease

BACKGROUND

Allogeneic bone marrow transplantation (BMT) and donor lymphocyte infusion (DLI) provide valuable treatments for a range of diseases. However, the therapeutic utility of BMT and DLI is reduced by the high incidence of graft-versus-host disease (GvHD) mediated by activated donor T lymphocytes directed against recipient alloantigens.

METHODS

Using mouse models, we developed and evaluated a strategy to selectively enhance activation-induced cell death (AICD) of anti-recipient T cells within transplant donor cell populations, with the goal of reducing GvHD. Responder T lymphocytes were incubated ex vivo with irradiated allogenic stimulator cells in a mixed lymphocyte reaction (MLR) in the presence of soluble Fas ligand (sFasL) to induce AICD in alloreactive cells.

RESULTS

This ex vivo sFasL treatment reduced proliferation to the allogeneic stimulator cells in vitro and abrogated acute GvHD capacity in vivo. In contrast, the secondary immune responsiveness of the ex vivo sFasL-treated responder T cells to an unrelated model antigen was preserved. Furthermore, upon adoptive transfer in a DLI model, ex vivo sFasL-treated T cells were able to reject a model tumor. Finally, ex vivo sFasL treatment of bone marrow cells did not reduce their hematopoietic engraftment capacity.

CONCLUSIONS

Thus, ex vivo treatment with sFasL appears to have potential for translation to clinical cell processing of BMT allografts and DLI infusions.

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.

Membrane-bound CD95 ligand expressed on human antigen-presenting cells prevents alloantigen-specific T cell response without impairment of viral and third-party T cell immunity

Genetically modified antigen-presenting cells (APC) represent an attractive strategy for in vitro immunomodulation. In the human system, APC expressing HLA-A1 and a membrane-bound form of CD95L (m-CD95L) were used for selective depletion of HLA-A1-specific T cells. In short-term assays, m-CD95L-expressing APC-induced apoptosis in activated T cells and the constitutive presence of m-CD95L and HLA-A1 expressing APC in long-term T cell cultures prevented the expansion of CD4(+) and CD8(+) HLA-A1-specific T cells and the development of HLA-A1-specific cytotoxicity. However, immunity towards third party, viral and bacterial antigens was maintained and T cells spared from depletion could be induced to develop cytotoxicity towards unrelated antigens. Interestingly, inhibition of HLA-A1-specific T cell response absolutely requires the coexpression of m-CD95L and HLA-A1 antigen on the same APC. Thus, m-CD95L expressing APC might be used in clinical settings to obtain tolerance induction in allogeneic transplantation systems or autoimmune diseases.

EBV-Specific CD4+ T Cell Clones Exhibit Vigorous Allogeneic Responses

Alloreactive T cells play a key role in mediating graft-vs-host disease and allograft rejection, and recent data suggest that most T cell alloreactivity resides within the CD4 T cell subset. Particularly, T cell responses to herpesvirus can shape the alloreactive repertoire and influence transplantation outcomes. In this study, we describe six distinct EBV-specific CD4(+) T cell clones that cross-reacted with EBV-transformed lymphoblastoid cell lines (LCLs), dendritic cells, and endothelial cells expressing MHC class II alleles commonly found in the population. Allorecognition showed exquisite MHC specificity. These CD4(+) T cell clones efficiently killed dendritic cells or LCLs expressing the cross-reactive allogeneic MHC class II molecules, whereas they did not kill autologous LCLs. Endothelial cells expressing the proper allogeneic MHC molecules were poorly killed, but they induced high-level TNF-alpha production by the EBV-specific CD4(+) T cell clones. As already proposed, the strong alloreactivity toward LCLs suggest that these cells could be used for selective depletion of alloreactive T cells.

Adenovirus infections in transplant recipients

Adenoviruses are increasingly recognized as contributors to morbidity and mortality among stem cell and solid-organ transplant recipients. Clinical presentations range from asymptomatic viremia to respiratory and gastrointestinal disease, hemorrhagic cystitis, and severe disseminated illness. The limited clinical data available support the use of cidofovir for many of these illnesses. Prospective studies are needed to better understand the pathogenesis of and therapeutic options for adenoviral infections in this patient population.

Autologous immune strategies to reduce the risk of leukemic relapse: Consideration for IL-15

The graft-versus-leukemia effect following allogeneic stem cell transplantation (SCT) reduces the incidence of leukemic relapse and establishes that effector cells can eliminate or at least contain resistant leukemic stem cells. Natural killer cells also appear to play a role in directly lowering the rate of relapse following allogeneic SCT in patients with acute myeloid leukemia. To date, however, effective prevention of leukemic relapse by autologous immune effector cells has not been demonstrated. This article examines some of the challenges that limit autologous antileukemia immunity as well as some possible immunotherapeutic approaches that may help control leukemic relapse following autologous SCT.

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.

Adoptive T‐cell transfer in cancer immunotherapy

Adoptive T-cell therapy has definite clinical benefit in relapsed leukaemia after allogeneic transplant and in Epstein-Barr virus-associated post-transplant lymphoproliferative disease. However, the majority of tumour targets are weakly immunogenic self-antigens and success has been limited in part by inadequate persistence and expansion of transferred T cells and by tumour-evasion strategies. Adoptive immunotherapy presents the opportunity to activate, expand and genetically modify T cells outside the tolerising environment of the host and a number of strategies to optimize the cellular product, including gene modification and modulation of the host environment, in particular by lymphodepletion, have been developed.

Adoptive immunotherapy with allodepleted donor T-cells improves immune reconstitution after haploidentical stem cell transplantation

Poor T lymphocyte reconstitution limits the use of haploidentical stem cell transplantation (SCT) because it results in a high mortality from viral infections. One approach to overcome this problem is to infuse donor T cells from which alloreactive lymphocytes have been selectively depleted, but the immunologic benefit of this approach is unknown. We have used an anti-CD25 immunotoxin to deplete alloreactive lymphocytes and have compared immune reconstitution after allodepleted donor T cells were infused at 2 dose levels into recipients of T-cell-depleted haploidentical SCT. Eight patients were treated at 10(4) cells/kg/dose, and 8 patients received 10(5) cells/kg/dose. Patients receiving 10(5) cells/kg/dose showed significantly improved T-cell recovery at 3, 4, and 5 months after SCT compared with those receiving 10(4) cells/kg/dose (P < .05). Accelerated T-cell recovery occurred as a result of expansion of the effector memory (CD45RA(-)CCR-7(-)) population (P < .05), suggesting that protective T-cell responses are likely to be long lived. T-cell-receptor signal joint excision circles (TRECs) were not detected in reconstituting T cells in dose-level 2 patients, indicating they are likely to be derived from the infused allodepleted cells. Spectratyping of the T cells at 4 months demonstrated a polyclonal Vbeta repertoire. Using tetramer and enzyme-linked immunospot (ELISPOT) assays, we have observed cytomegalovirus (CMV)- and Epstein-Barr virus (EBV)-specific responses in 4 of 6 evaluable patients at dose level 2 as early as 2 to 4 months after transplantation, whereas such responses were not observed until 6 to 12 months in dose-level 1 patients. The incidence of significant acute (2 of 16) and chronic graft-versus-host disease (GVHD; 2 of 15) was low. These data demonstrate that allodepleted donor T cells can be safely used to improve T-cell recovery after haploidentical SCT and may broaden the applicability of this approach.

Depletion of alloreactive T cells via CD69: implications on antiviral, antileukemic and immunoregulatory T lymphocytes

Selective depletion of alloreactive T cells from stem-cell allografts should abrogate graft-versus-host disease while preserving beneficial T cell specificities to facilitate engraftment and immune reconstitution. We therefore explored a refined immunomagnetic separation strategy to effectively deplete alloreactive donor lymphocytes expressing the activation antigen CD69 upon stimulation, and examined the retainment of antiviral, antileukemic, and immunoregulatory T cells. In addition to the CD69high T cell fraction, our studies retrieved two T cell subsets based on residual CD69 expression. Whereas, truly CD69(neg) cells were devoid of detectable alloresponses to original stimulators, CD69-low (CD69low)-expressing T cells elicited significant residual alloreactivity upon restimulation. In interferon-gamma enzyme linked immunospot assays, anti-cytomegalovirus and anti-Epstein-Barr virus responses were preserved at significant numbers among CD69neg T lymphocytes. Accordingly, T cells recognizing the leukemia-associated Wilm's tumor-1 antigen were still detectable in the CD69neg subset. However, antiviral and antileukemic specificities were also consistently found within CD69low T cells, suggesting that memory-type donor T cells were partially captured due to residual CD69 expression. Finally, CD4+CD25+ Foxp3+ immunoregulatory T cells did not upregulate CD69 upon allogeneic stimulation. Our data suggest that CD69-mediated removal of alloreactivity can result in efficient allodepletion, but may partially affect the persistence of antiviral and antileukemic donor memory specificities captured among CD69low-expressing lymphocytes.

Graft-versus-Leukemia and Graft-versus-Host Reactions after Donor Lymphocyte Infusion Are Initiated by Host-Type Antigen-Presenting Cells and Regulated by Regulatory T Cells in Early and Long-Term Chimeras

Regulatory T (T(reg)) cells and host antigen-presenting cells (APCs) have been implicated in graft-versus-host disease (GVHD) and the graft-versus-leukemia (GVL) effect after donor lymphocyte infusion (DLI), but their relative contributions remain unclear in early versus long-term complete donor or mixed murine allogeneic hematopoietic stem cell (HSC) chimeras. We have previously demonstrated that donor HSC-derived Thy1(+) T(reg) cells, consisting primarily of CD4(+)CD25(+) cells, play an important role in the suppression of graft-versus-host (GVH) reactivity when DLI is given to complete donor chimeras 28 days after HSC transplantation. Data presented here demonstrate that protection against GVHD exerted by Thy1(+) T(reg) cells is less evident with time and eventually is not required in long-term complete donor chimeras because of an absence of host-type APCs to activate alloreactive T cells. Lethal GVHD was observed when Thy1(+) T(reg) cells were depleted from complete donor chimeras given by DLI at day 28, 35, or 42; however, T(reg) cell depletion and DLI at day 70 no longer induced GVHD-associated mortality. Moreover, the failure of DLI to induce GVHD with T(reg) depletion correlated with a loss of the DLI-induced GVL effect in long-term (day 100) complete donor chimeras. In contrast to the results from complete donor chimeras, GVL reactivity in day 100 mixed chimeras was robust after DLI. Loss of a DLI-induced GVL effect in long-term complete donor chimeras was attributed to the absence of host APCs because the infusion of exogenous host-type dendritic cells partially restored both DLI-induced GVL and GVH reactions in day 100 complete donor chimeras. The GVL and GVH reactions restored by infusion of host dendritic cells in day 100 complete donor chimeras were at least partially regulated by T(reg) cells because transient depletion of CD25(+) cells increased both the GVL effect and the severity of GVHD after DLI. Taken together, these data suggest that T(reg) cells can regulate DLI-induced GVL and GVH reactions in both early and long-term complete donor chimeras, and a state of mixed chimerism is superior to complete donor chimerism because host-type APCs facilitate a DLI-induced GVL effect without severe GVHD.

Inter-operator variation in ELISPOT analysis of measles virus-specific IFN-gamma-secreting T cells

The ELISPOT assay is a highly sensitive technique used for the detection of individual cytokine releasing cells. We have developed an IFN-gamma ELISPOT assay utilizing unfractionated frozen peripheral blood mononuclear cells (PBMC) to quantify the frequency of measles virus (MV)-specific IFN-gamma-secreting T cells in 117 healthy children who had been previously immunized with two doses of the measles-mumps-rubella vaccine. We have also estimated the variability associated with the quantification of ELISPOT plates and compared the number of MV-specific IFN-gamma-secreting T cells for each subject as determined by two different operators of an ELISPOT reader. The median frequency of MV-specific IFN-gamma-producing memory T cells detected by this assay was 0.005 % and 0.01 % as determined by an in-house and commercial operator, respectively. Although we found a significant correlation (r = 0.83, p<0.0001) between the number of spots counted by the commercial and in-house operators of an ELISPOT reader, the median number of spots counted by the commercial operator was twice the number of spots counted by an in-house operator (p<0.001). This demonstrates the importance of using a common ELISPOT reader and operator, among other parameters, to quantify the number of spots when a large volume of plates are being scanned and analyzed.

Shared biology of GVHD and GVT effects: Potential methods of separation

The difficult separation of clinical graft-versus-tumor (GVT) effects from graft-versus-host disease (GVHD) reflects their shared biology. Experimental approaches to mediate GVT effects while limiting GVHD include: (1) allograft T cell depletion followed by immune enhancement; (2) modulation of T cell dose or T cell subset composition; (3) donor lymphocyte infusion; (4) reduced-intensity host preparation; (5) modulation of Th1/Th2 and Tc1/Tc2 cell balance; (6) cytokine therapy or neutralization; (7) T regulatory cell therapy; (8) co-stimulatory pathway modulation; (9) chemokine pathway modulation; (10) induction of antigen-specific T cells; (11) alloreactive NK cell therapy; and (12) targeted pharmaceutical inhibition of proteosome, mammalian target of rapamycin, and histone deacetylase pathways. Clearly, a multitude of approaches exist that hold promise for separating GVT effects from GVHD. Future success in this endeavor will require a strong commitment towards translational research and continued advances in cell, vaccine, cytokine, monoclonal antibody, and targeted molecular therapy.

Remodeling specific immunity by use of MHC tetramers: demonstration in a graft-versus-host disease model

Major histocompatibility complex (MHC) molecules carrying selected peptides will bind specifically to their cognate T-cell receptor on individual clones of reactive T cells. Fluorescently labeled, tetrameric MHC-peptide complexes have been widely used to detect and quantitate antigen-specific T-cell populations via flow cytometry. We hypothesized that such MHC-peptide tetramers could also be used to selectively deplete unique reactive T-cell populations, while leaving the remaining T-cell repertoire and immune response intact. In this report, we successfully demonstrate that a tetramer-based depletion of T cells can be achieved in a murine model of allogeneic bone marrow transplantation. Depletion of a specific alloreactive population of donor splenocytes (< 0.5% of CD8+ T cells) prior to transplantation significantly decreased morbidity and mortality from graft-versus-host disease. There was no early regrowth of the antigen-specific T cells in the recipient and in vivo T-cell proliferation was greatly reduced as well. Survival was increased more than 3-fold over controls, yet the inherent antitumor activity of the transplant was retained. This method also provides the proof-of-concept for similar strategies to selectively remove other unwanted T-cell clones, which could result in novel therapies for certain autoimmune disorders, T-cell malignancies, and solid organ graft rejection.

Reconstitution of adaptive and innate immunity following allogeneic hematopoietic stem cell transplantation in humans

Allogeneic hematopoietic stem cell transplantation is a potentially curative treatment modality for a number of hematologic malignancies, as well as inherited immunodeficiencies and hemoglobinopathies, and may also have a role in selected acquired autoimmune disorders. The complete or near-complete ablation of host immunity and subsequent establishment of donor-derived immunity that is required for successful engraftment and long-term outcomes provide a major obstacle to such transplantation approaches. A delicate balance exists between the need for the reconstituted donor-derived immunity to provide both protection against pathogenic challenges and graft-versus-malignancy activity, and the potentially harmful expansion of alloreactive T-cell clones mediating GvHD. The search for interventions that would allow more rapid and selective reconstitution of beneficial immune specificities continues to be informed by the development of new tools enabling a more precise dissection of the kinetics of reconstituting populations. This review summarizes more recent data on immune reconstitution following allogeneic transplantation in humans.

Graft-versus-Host Disease after Allogeneic Stem Cell Transplantation: Evolving Concepts and Novel Therapies Including Photopheresis

Graft-versus-host disease (GVHD) remains a source of significant morbidity and mortality in the setting of allogeneic stem cell transplantation. Improving outcomes in stem cell transplant recipients will require additional therapeutic modalities for GVHD, especially for those patients who fail to respond to initial therapy with steroids. This article provides an introduction to accompanying articles in this issue of Biology of Blood and Marrow Transplantation concerning the current state of GVHD management with an emphasis on the role of extracorporeal photopheresis as a therapeutic modality. Topics discussed include challenges associated with the design of clinical studies of GVHD therapies, the efficacy of and proposed mechanisms of photopheresis in the setting of GVHD, and our emerging understanding of regulatory T cell biology as it relates to allogeneic stem cell transplantation.

CD4+CD25+ Regulatory T Cells in Hematopoietic Stem Cell Transplantation

Allogeneic hematopoietic stem cell transplantation (SCT) is a well-established treatment modality for malignant and nonmalignant hematologic diseases. High-dose radio- and/or chemotherapy eradicate the hematopoietic system of the patient and induce sufficient immunosuppression to enable donor stem cell engraftment. The replacement of the recipient's immune system with that of the donor significantly contributes to the success of this treatment, since donor immune cells facilitate stem cell engraftment, provide protection from infections, and eliminate residual malignant or nonmalignant host hematopoiesis, thereby protecting from disease relapse in patients transplanted for leukemia or lymphoma (graft-versus-leukemia effect, GVL). Mediators of these beneficial effects are mature T cells within the stem cell graft. However, donor T cells can also attack host tissues and induce a life-threatening syndrome called graft-versus-host disease (GVHD). The challenge of allogeneic SCT is to find a balance between beneficial and harmful T cell effects, which at present is only insufficiently achieved by the use of immunosuppressive drugs. In the future, it might be possible to replace or support such medications by using the intrinsic regulatory capacity of the transplanted immune system, as represented by T cell subpopulations with suppressive activity, such as CD4+ CD25+ regulatory T (Treg) cells. In various mouse model systems, these cells have been shown to suppress GVHD while preserving the GVL effect. As the characterization of their human counterparts is rapidly progressing, their application in allogeneic SCT might soon be explored in clinical trials.

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.

A novel artemisinin derivative, 3-(12-beta-artemisininoxy) phenoxyl succinic acid (SM735), mediates immunosuppressive effects in vitro and in vivo

AIM

To study the immunosuppressive activity of SM735 {[3-(12-beta-artemisininoxy)] phenoxyl succinic acid}, a synthetic artemisinin derivative with nonsteroidal anti-inflammatory drug structure, with the aim of finding potential immunosuppressive agents.

METHODS

Concanavalin A (ConA), lipopolysaccharide (LPS), and mixed lymphocyte reaction (MLR), were used to induce the proliferation of splenocytes, and [3H]-thymidine incorporation was used to evaluate the proliferation of splenocytes. Cytokine production was promoted with ConA, LPS, or PMA plus ionomycin, and was detected with the enzyme-linked immunosorbent assay. Dinitrofluorobenzene (DNFB) and sheep red blood cells (SRBC) were used to induce delayed-type hypersensitivity and quantitative hemolysis of SRBC (QHS) mouse models, as criteria for the evaluation of in vivo immune activity.

RESULTS

SM735 strongly inhibited the proliferation of splenocytes induced by ConA, LPS, or MLR, with IC(50) values of 0.33 micromol/L, 0.27 micromol/L, and 0.51 micromol/L, respectively. When compared with a CC(50) value of 53.1 micromol/L, SM735 had a favorable safety range. SM735 dose-dependently inhibited proinflammatory cytokine production [including interleukins (IL)-12, interferon (IFN)-gamma and IL-6] induced by LPS or PMA plus ionomycin. Upon ConA stimulation, SM735 suppressed IFN-gamma in a dose-dependent manner, but did not affect IL-2 secretion. SM735 also strongly suppressed both T-cell-mediated delayed-type hypersensitivity (DTH) and B-cell-mediated QHS reactions.

CONCLUSION

SM735 had strong immunosuppressive activity in vitro and in vivo, suggesting a potential role for SM735 as an immunosuppressive agent, and established the groundwork for further research on SM735.

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.

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

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

Improved outcome for children with disseminated adenoviral infection following allogeneic stem cell transplantation

Adenovirus (AdV) infections are a frequent cause of morbidity and mortality following allogeneic stem cell transplantation (SCT), and disseminated infection is associated with high mortality, particularly in paediatric SCT. Here, we describe an approach to reduce mortality from adenoviraemia by combining prospective monitoring for the occurrence of adenoviraemia using a sensitive polymerase chain reaction method, early antiviral therapy and prompt withdrawal of immunosuppression. A total of 155 consecutive paediatric SCT procedures were prospectively monitored, of which 113 (73%) transplants involved donors other than matched siblings and 126 (83%) employed T-cell depletion. Adenoviraemia was detected in 26/155 (17%) transplants and developed exclusively in patients who had received T-cell-depleted grafts. Withdrawal of immunosuppression coupled with early antiviral therapy led to resolution of adenoviraemia in 19/26 (81%) patients with only five patients succumbing to disseminate AdV infection. Survival from adenoviraemia was associated with lymphocyte recovery to above 0.3x10(9)/l. Mortality was closely linked with the absence of lymphocyte recovery because of profound T-cell depletion of the graft with CD34+ magnetic-activated cell sorting. Mortality from disseminated AdV infection was 5/26 (19%) in this study, which is significantly lower than previously reported.

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.

Add-back of allodepleted donor T cells to improve immune reconstitution after haplo-identical stem cell transplantation

Poor immune reconstitution after haplo-identical stem cell transplantation results in high mortality from viral infections and relapse. One approach to overcome this problem is to deplete alloreactive cells selectively by deleting T cells activated by recipient stimulators, using an immunotoxin directed against the activation marker CD25. However, the degree of depletion of alloreactive cells is variable following stimulation with recipient PBMC, and this can result in GvHD. We have shown that using recipient EBV-transformed LCL as stimulators to activate donor alloreactive T cells results in more consistent depletion of in vitro alloreactivity while preserving T-cell responses to viral and potential myeloid tumor Ag. Based on these data, we have embarked on a phase I clinical dose escalation study of add-back of allo-LCL-depleted donor T cells in the haplo-identical setting, to determine if the allodepletion we achieve to allow infusion of sufficient T cells to restore useful antiviral/anti-leukemic responses without causing GvHD. Fifteen patients have so far been treated. The incidence of significant acute or chronic GvHD has been low (2/15), as has mortality from infection (1/15). Preliminary data show accelerated immune reconstitution in dose level 2 patients. Infused allodepleted donor T cells appear able to expand significantly in the face of viral reactivations, and doses as low as 3 x 10(5)/kg may be sufficient to confer useful antiviral immunity in this setting. At a median follow-up of 19.5 months, nine of 15 patients are alive and disease-free. Five patients have relapsed, all of whom have died.

Selective elimination of alloreactivity from immunotherapeutic T cells by photodynamic cell purging and memory T-cell sorting

Allogeneic stem cell transplantation (alloSCT), especially in the mismatched setting, carries a high risk of life-threatening GvHD because of activation of donor T cells by Ag present on host cells. Removal of mature donor T cells can prevent GvHD but leads to delayed immune reconstitution, and an increased incidence of opportunistic infections and disease relapse. These findings demonstrate the vital role of donor T cells in providing graft-versus-tumor (GvT) and anti-pathogen effects as well as facilitating immune reconstitution. It has been well documented that GvHD can be separated from GvT effects, making it possible potentially to eliminate GvHD while preserving the immunotherapeutic benefits of donor T cells. Over the past decade, major attempts have been made to reduce GvHD incidence without loss of GvT effect, especially in the haplo-identical setting. Novel techniques to deplete host-reactive donor T cells selectively have been explored. This review focuses on the use of the photodynamic cell purging (PDP) process and of sorting memory T cells for the selective elimination of alloreactivity. Minimizing the threat of GvHD while maximizing the beneficial GvT effect would broaden the scope and effectiveness of alloSCT.