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

Efficient lysis of rhabdomyosarcoma cells by cytokine-induced killer cells: implications for adoptive immunotherapy after allogeneic stem cell transplantation

BACKGROUND

Rhabdomyosarcoma is the most common soft tissue sarcoma in childhood and has a poor prognosis. Here we assessed the capability of ex vivo expanded cytokine-induced killer cells to lyse both alveolar and embryonic rhabdomyosarcoma cell lines and investigated the mechanisms involved.

DESIGN AND METHODS

Peripheral blood mononuclear cells from six healthy donors were used to generate and expand cytokine-induced killer cells. The phenotype and composition of these cells were determined by multiparameter flow cytometry, while their cytotoxic effect against rhabdomyosarcoma cells was evaluated by a europium release assay.

RESULTS

Cytokine-induced killer cells efficiently lysed cells from both rhabdomyosarcoma cell lines. Antibody-mediated masking of either NKG2D molecule on cytokine-induced killer cells or its ligands on rhabdomyosarcoma cells (major histocompatibility antigen related chain A and B and UL16 binding protein 2) diminished this effect by 50%, suggesting a major role for the NKG2D molecule in rhabdomyosarcoma cell killing. No effect was observed after blocking CD11a, CD3 or TCRalphabeta molecules on cytokine-induced killer cells or CD1d on rhabdomyosar-coma cells. Remarkably, cytokine-induced killer cells used tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) to activate caspase-3, as the main caspase responsible for the execution of apoptosis. Accordingly, blocking TRAIL receptors on embryonic rhabdomyosarcoma cell lines significantly reduced the anti-tumor effect of cytokine-induced killer cells. About 50% of T cells within the cytokine-induced killer population had an effector memory phenotype, 20% had a naïve phenotype and approximately 30% of the cells had a central memory phenotype. In addition, cytokine-induced killer cells expressed low levels of activation-induced markers CD69 and CD137 and demonstrated a low alloreactive potential.

CONCLUSIONS

Our data suggest that cytokine-induced killer cells may be used as a novel adoptive immunotherapy for the treatment of patients with rhabdomyosarcoma after allogeneic stem cell transplantation.

Functional characterization of alloreactive T cells identifies CD25 and CD71 as optimal targets for a clinically applicable allodepletion strategy

Immunotherapy with allodepleted donor T cells (ADTs) improves immunity after T cell–depleted stem cell transplantation, but infection/relapse remain problematic. To refine this approach, we characterized the expression of surface markers/cytokines on proliferating alloreactive T cells (ATs). CD25 was expressed on 83% of carboxyfluorescein diacetate succinimidyl esterdim ATs, confirming this as an excellent target for allodepletion. Seventy percent of CD25− ATs expressed CD71 (transferrin receptor), identifying this as a novel marker to target ATs persisting after CD25 depletion. Comparison of residual alloreactivity after combined CD25/71 versus CD25 immunomagnetic depletion showed enhanced depletion of alloreactivity to host with CD25/71 depletion in both secondary (2°) mixed lymphocyte reactions (P < .01) and interferon-γ enzyme-linked immunospot assays (P < .05) with no effect on third-party responses. In pentamer/interferon-γ enzyme-linked immunospot assays, antiviral responses to cytomegalovirus, Epstein-Barr virus, and adenovirus were preserved after CD25/71 allodepletion. CD25/71 ADTs can be redirected to recognize leukemic targets through lentiviral transfer of a chimeric anti-CD19ζ T-cell receptor. Finally, we have established conditions for clinically applicable CD25/71 allodepletion under European Union Good Manufacturing Practice conditions, resulting in highly effective, reproducible, and selective depletion of ATs (median residual alloreactivity to host in 2° mixed lymphocyte reaction of 0.39% vs third-party response of 62%, n = 5). This strategy enables further clinical studies of adoptive immunotherapy with larger doses of ADTs to enhance immune reconstitution after T cell-depleted stem cell transplantation.

Selective depletion of alloreactive T cells by targeted therapy of heat shock protein 90: a novel strategy for control of graft-versus-host disease

Graft-versus-host disease (GVHD) is a major cause of morbidity and mortality in patients with hematologic malignancies undergoing allogeneic hematopoietic stem cell transplantation. Current treatment of GVHD relies on immunosuppressive regimens, considerably increasing the incidence of opportunistic infections. As T cells mediate both GVHD as well as protection against viral infections and the malignant disease, strategies to selectively target host-reactive T cells without impairing pathogen- and disease-specific immunity are highly warranted. Activation of T cells is accompanied by increased expression of the chaperone heat shock protein of 90 kDa (Hsp90), which stabilizes several key signaling pathways crucial for T-cell activation. In this study, selective targeting of Hsp90 in activated T lymphocytes with pharmacologic inhibitors already applied successfully in anticancer therapy resulted in induction of apoptosis predominantly in activated cells. Moreover, if T cells were stimulated with allogeneic dendritic cells, alloreactive T cells were selectively eliminated. In contrast, third party reactions including antiviral T-cell immunity were quantitatively and functionally fully preserved. These data suggest that Hsp90 represents a novel target for selective depletion of alloreactive T cells, and provide the rationale for application of Hsp90 inhibitors as potential approach to selectively prevent and treat GVHD in hematopoietic stem cell transplantation recipients without impairing pathogen- and disease-specific T-cell immunity.

Tolerance induction by removal of alloreactive T cells: in-vivo and pruning strategies

PURPOSE OF REVIEW

Current depletion strategies used in clinical transplantation can prevent acute rejection of a transplanted organ; however, they are nonspecific and are limited by their efficacy or the side effects of wide ranging cellular depletion. This review will focus on strategies that prevent rejection of allografts using specific allodepletion of the T cells that mediate rejection.

RECENT FINDINGS

Strategies that use either in-vivo targeting of alloreactive T cells or ex-vivo manipulation to specifically reduce the alloreactive T-cell pool have been developed. The advantage of these approaches is that they are specific, by depleting cells that cause rejection while leaving the remaining immune system intact, thereby minimizing the detrimental complications associated with standard immunosuppression.

SUMMARY

Strategies to reduce the proportion of alloreactive T cells that initiate transplant rejection are emphasized. This factor has the specific advantage of leaving the remaining T-cell repertoire intact and may therefore be used in combination with other immunemodulating and tolerance strategies.

Induction of lethal graft-versus-host disease by anti-CD137 monoclonal antibody in mice prone to chronic graft-versus-host disease

Chronic graft-versus-host disease (cGVHD) is an increasingly frequent complication of allogeneic stem cell transplantation. We previously showed that anti-CD137 monoclonal antibody (mAb) can cure advanced cGVHD by inducing activation-induced cell death of donor T cells. In this study, we examined whether administration of anti-CD137 mAb can prevent the development of cGVHD after bone marrow transplantation (BMT) in mice conditioned with total body irradiation (TBI). We used the B10.D2-->Balb/c (H-2(d)) minor histocompatibility antigen-mismatched model, which reflects clinical and pathological symptoms of human cGVHD. A single injection of anti-CD137 mAb was administered immediately after BMT. In contrast to the results obtained from the curing model of cGVHD, anti-CD137 given simultaneously with BMT resulted in lethal GVHD. Histopathologic evaluation revealed inflammation and damage of target organs from acute GVHD (aGVHD) in anti-CD137-treated mice. Anti-CD137-induced lethal aGVHD required host cells, as well as irradiation and mature donor T cells. Apparently, anti-CD137 mAb rapidly induced activation of donor T cells and sustained their activation status under the inflammatory condition triggered by irradiation. When given on day 12 after irradiation and BMT, anti-CD137 mAb could still exacerbate GVHD, but when given on day 30, it could not. Our data demonstrate that anti-CD137 mAb can amplify inflammation induced by host preconditioning, subsequently resulting in lethal aGVHD; thus, alleviating irradiation-induced toxicity is critical to allow the use of anti-CD137 mAb as GVHD prophylaxis.

Selective reduction of graft-versus-host disease-mediating human T cells by ex vivo treatment with soluble Fas ligand

Previous work done in our laboratory, using mouse models, showed that soluble Fas ligand (sFasL) can efficiently delete donor anti-host T cells during their activation against irradiated host cells in MLCs. In the mouse models, this ex vivo sFasL treatment abrogated graft-vs-host disease (GVHD) while sparing donor T cells with antitumor reactivity. The present work was performed with human cells, to extend our work toward reduction of clinical GVHD. PBMC responders from a given individual (first party) were stimulated in vitro with irradiated PBMC stimulators from a second person (second party), in the presence of sFasL. In control MLCs without sFasL, alloreacting T cells began to up-regulate Fas (CD95) detectably and became sensitive to Fas-mediated apoptosis by as early as day 1-2. In MLCs containing sFasL, there were greatly reduced numbers of alloreacting CD3(+)CFSE(lo) cells, activation Ag-expressing CD4(hi) and CD8(hi) cells, IFN-gamma-producing CD4(+) and CD8(+) cells, and CD8(+)CD107a(+) CTLs. Furthermore, mice transplanted with the ex vivo sFasL/MLR-treated cells had prolonged time to fatal GVHD in an in vivo xenogeneic GVHD model. Responder cells harvested from primary MLCs containing sFasL had reduced proliferation in response to second party cells, but proliferated in response to CMV Ags, PHA, and third party cells. In addition, sFasL/MLR-treated cell populations contained influenza-specific T cells, CD4(+)FOXP3(+) T cells, and CD4(+)CD25(+) T cells. These data indicate that this ex vivo sFasL/MLR depletion of alloreacting human donor anti-host T cells was efficient and selective.

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.

The programmed death (PD)-1/PD-ligand 1 pathway regulates graft-versus-host-reactive CD8 T cells after liver transplantation

Acute graft-versus-host disease (aGVHD) is a life-threatening complication after solid-organ transplantation, which is mediated by host-reactive donor T cells emigrating from the allograft. We report on two liver transplant recipients who developed an almost complete donor chimerism in peripheral blood and bone marrow-infiltrating T cells during aGVHD. By analyzing these T cells directly ex vivo, we found that they died by apoptosis over time without evidence of rejection by host T cells. The host-versus-donor reactivity was selectively impaired, as anti-third-party and antiviral T cells were still detectable in the host repertoire. These findings support the acquired donor-specific allotolerance concept previously established in animal transplantation studies. We also observed that the resolution of aGVHD was not accompanied by an expansion of circulating immunosuppressive CD4/CD25/FoxP3-positive T cells. In fact, graft-versus-host-reactive T cells were controlled by an alternative negative regulatory pathway, executed by the programmed death (PD)-1 receptor and its ligand PD-L1. We found high PD-1 expression on donor CD4 and CD8 T cells. In addition, blocking PD-L1 on host-derived cells significantly enhanced alloreactivity by CD8 T cells in vitro. We suggest the interference with the PD-1/PD-L1 pathway as a therapeutic strategy to control graft-versus-host-reactive T cells in allograft recipients.

Prevention of acute graft-vs-host disease by a single low-dose cyclophosphamide injection following allogeneic bone marrow transplantation

OBJECTIVE

Previously, we documented that conditioning based on donor-specific cell transfusion (DST) and subsequent selective depletion of activated donor-reactive cells by cyclophosphamide (CY) facilitates alloengraftment in a murine transplantation model. Transplantation event represents a strong immunogenic stimulus for host-reactive donor T cells that induce graft-vs-host disease (GVHD). Therefore, in this study, we addressed the question of whether a single posttransplantation CY administration (CY2) can prevent acute GVHD-related mortality without compromising engraftment of allogeneic transplant.

MATERIALS AND METHODS

Splenocyte-enriched C57BL/6 bone marrow was transplanted to BALB/c recipients after mild irradiation, and conditioning with DST and 100 mg/kg CY. Following transplantation, recipients were left untreated or given on a specified day a single CY2 injection (50 mg/kg). All animals were monitored for survival, chimerism, and clinical signs of GVHD. Experimental mice that received BCL1 leukemia cells before transplantation were monitored for leukemia-related mortality as well.

RESULTS

Animals that received no CY2 after transplantation died of acute GVHD. A single low-dose CY2 treatment within the first 5 days after transplantation prevented mortality in most recipients. However, only CY2 administration on days +1 or +5 preserved chimerism. Most chimeras survived GVHD-free for >200 days. Prolonged persistence of host-reactive T cells in mice (CY2 on day +5) permitted a reduction to be made in engraftment-essential irradiation dose and preserved a strong graft-vs-leukemia effect of transplantation.

CONCLUSION

Acute GVHD can be prevented in mice by a single properly timed posttransplantation low-dose CY administration.

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.

CD137-guided isolation and expansion of antigen-specific CD8 cells for potential use in adoptive immunotherapy

The efficient isolation and ex vivo expansion of antigen-specific T cells are crucial for successful adoptive immunotherapy against uncontrollable infections and cancers. Several methods have been reported for this purpose, for example, employing MHC-multimeric complexes, interferon-gamma secretion, and antibodies specific for molecules expressed on T-cell surfaces, including CD25, CD69, CD107a, CD137, and CD154. Of the latter, CD137 has been shown to be one of the most promising targets since it is only expressed on CD8(+) T cells early after encountering antigen, while being almost undetectable on resting cells. However, detailed comparisons between CD137-based and other methods have not yet been conducted. In this study, we therefore compared three approaches (with CD137, CD107a, and tetramers) using HLA-A24-restricted CMV pp65 and EBV BRLF1 epitopes as model antigens. We found that the CD137-based isolation of antigen-stimulated CD8(+) T cells was comparable to tetramer-based sorting in terms of purity and superior to the other two methods in terms of subsequent cell expansion. The method was less applicable to CD4(+) T cells since their CD137 upregulation is not sufficiently high. Collectively, this approach is most likely to be optimal among the methods tested for the isolation and expansion of antigen-specific CD8(+) cells.

Hematopoietic SCT from partially HLA-mismatched (HLA-haploidentical) related donors

Hematopoietic SCT from a partially HLA-mismatched (HLA-haploidentical) first-degree relative offers the benefits of rapid and near universal donor availability but also the risks that result from traversing the HLA barrier; namely, graft failure, severe GVHD and prolonged immunodeficiency. Improvements over the last 10 years in conditioning regimens, graft engineering and pharmacological immunoprophylaxis of GVHD have substantially reduced the morbidity and mortality of HLA-haploidentical SCT. Highly immunosuppressive but nonmyeloablative conditioning extends the availability of HLA-haploidentical SCT to elderly hematologic malignancy patients lacking HLA-matched donors and permits recovery of autologous hematopoiesis in the event of graft failure. Current regimens for HLA-haploidentical SCT are associated with a 2-year non-relapse mortality of 20+/-5%, relapse of 35+/-15% and overall survival of 50+/-20%. Major developmental areas include harnessing natural killer cell alloreactivity to reduce the risk of disease relapse and improving immune reconstitution by delayed infusions of lymphocytes selectively depleted of alloreactive cells. Hematologic malignancy patients who lack suitably matched related or unrelated donors can now be treated with HLA-haploidentical related donor or unrelated umbilical cord blood SCT. Future clinical trials will assess the relative risks and benefits of these two graft sources.

Refinement of strategies for the development of a human cytomegalovirus dense body vaccine

Development of a vaccine against human cytomegalovirus (HCMV) infection has been identified as a high priority goal in biomedical research, yet no vaccine has been licensed until now. Recombinant subviral dense bodies (recDB) are a promising basis for the establishment of such a vaccine. In this article, strategies for the generation of recDB, based on recombination-mediated genetic engineering of the 230 kb HCMV DNA genome in E. coli are outlined. Analysis of viral mutants that were constructed in this process provided the proof-of-principle that heterologous antigens can be packaged into recDB and that these particles prime CD8 T cell responses against the recombinant antigen upon their application to HLA-A2 transgenic mice.

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.

Activation-induced expression of CD137 permits detection, isolation, and expansion of the full repertoire of CD8+ T cells responding to antigen without requiring knowledge of epitope specificities

CD137 is a member of the TNFR-family with costimulatory function. Here we show that it also has many favorable characteristics as a surrogate marker for antigen-specific activation of human CD8(+) T cells. Although undetectable on unstimulated CD8(+) T cells, it is uniformly up-regulated 24 hours after stimulation on virtually all responding cells regardless of differentiation stage or profile of cytokine secretion, which circumvents limitations of current surrogate markers for defining the repertoire of responding cells based on only individual functions. Antibody-labeled responding CD137(+) cells can be easily and efficiently isolated by flow sorting or magnetic beads to substantially enrich antigen-specific T cells. To test this approach for epitope discovery, we examined in vitro priming of naive T cells from healthy donors to Wilms tumor antigen 1 (WT1), a protein overexpressed in various malignancies. Two overlapping pentadecamers were identified as immunogenic, and further analysis defined WT1((286-293)) as the minimal amino acid sequence and HLA-Cw07 as the HLA restriction element. In conclusion, this approach appears to be an efficient and sensitive in vitro technique to rapidly identify and isolate antigen-specific CD8(+) T cells present at low frequencies and displaying heterogeneous functional profiles, and does not require prior knowledge of the specific epitopes recognized or the HLA-restricting elements.

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.

Superior antitumor in vitro responses of allogeneic matched sibling compared with autologous patient CD8+ T cells

Allogeneic cell therapy as a means to break immunotolerance to solid tumors is increasingly used for cancer treatment. To investigate cellular alloimmune responses in a human tumor model, primary cultures were established from renal cell carcinoma (RCC) tissues of 56 patients. In three patients with stable RCC line and human leukocyte antigen (HLA)-identical sibling donor available, allogeneic and autologous RCC reactivities were compared using mixed lymphocyte/tumor cell cultures (MLTC). Responding lymphocytes were exclusively CD8(+) T cells, whereas CD4(+) T cells or natural killer cells were never observed. Sibling MLTC populations showed higher proliferative and cytolytic antitumor responses compared with their autologous counterparts. The allo-MLTC responders originated from the CD8(+) CD62L(high)(+) peripheral blood subpopulation containing naive precursor and central memory T cells. Limiting dilution cloning failed to establish CTL clones from autologous MLTCs or tumor-infiltrating lymphocytes. In contrast, a broad panel of RCC-reactive CTL clones was expanded from each allogeneic MLTC. These sibling CTL clones either recognized exclusively the original RCC tumor line or cross-reacted with nonmalignant kidney cells of patient origin. A minority of CTL clones also recognized patient-derived hematopoietic cells or other allogeneic tumor targets. The MHC-restricting alleles for RCC-reactive sibling CTL clones included HLA-A2, HLA-A3, HLA-A11, HLA-A24, and HLA-B7. In one sibling donor-RCC pair, strongly proliferative CD3(+)CD16(+)CD57(+) CTL clones with non-HLA-restricted antitumor reactivity were established. Our results show superior tumor-reactive CD8 responses of matched allogeneic compared with autologous T cells. These data encourage the generation of antitumor T-cell products from HLA-identical siblings and their potential use in adoptive immunotherapy of metastatic RCC patients.