Depletion of Alloreactive T Cells by a Specific Anti–Interleukin-2 Receptor p55 Chain Immunotoxin Does Not Impair In Vitro Antileukemia and Antiviral Activity

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

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

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

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

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

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

Depletion of Alloreactive T-Cells by Anti-CD137-Saporin Immunotoxin

Depletion of alloreactive T-lymphocytes from allogeneic bone marrow tansplants may prevent graft-versus-host disease (GVHD) without impairing donor cell engraftment, immunity, and the graft-versus-leukemia (GVL) effect. Alloreactive T-cells may be identified by their expression, upon activation, of CD137, a costimulatory receptor and putative surrogate marker for antigen-specific effector T-cells. In this context, we tested the use of anti-CD137-saporin immunotoxin to selectively deplete mouse and human alloreactive T-cells. Anti-CD137 antibodies were internalized by cells within 4 h of binding to the cell surface CD137, and anti-CD137-saporin immunotoxin effectively killed polyclonally activated T-cells or antigen-stimulated T-cells. Transfer of donor T-cells after allodepletion with anti-CD137-saporin immunotoxin failed to induce any evident expression of GVHD; however, a significant GVL effect was observed. Targeting of CD137 with an immunotoxin was also effective in killing polyclonally activated or alloreactive human T-cells. Our results indicate that anti-CD137-saporin immunotoxin may be used to deplete alloreactive T-cells prior to bone marrow transplantation and thereby prevent GVHD and the relapse of leukemia.

iCaspase 9 Suicide Gene System

Although cellular therapies may be effective in cancer treatment, their potential for expansion, damage of normal organs, and malignant transformation is a source of concern. The ability to conditionally eliminate aberrant cells in vivo would ameliorate these concerns and broaden the application of cellular therapy. We devised an inducible T-cell safety switch that can be stably and efficiently expressed in human T cells without impairing phenotype, function, or antigen specificity. This system is based on the fusion of human caspase 9 to a modified human FK-binding protein, allowing conditional dimerization using a small-molecule drug. When exposed to a synthetic dimerizing drug, the inducible caspase 9 (iC9) becomes activated and leads to the rapid apoptosis of cells expressing this construct. We have demonstrated the clinical feasibility and efficacy of this approach after haploidentical hematopoietic stem cell transplant (haplo-HSCT). A single dose of a small-molecule drug (AP1903) eliminated more than 90 % of the modified T cells within 30 min after administration and symptoms resolved without recurrence. This system has the potential to broaden the clinical applications of cellular therapy.

Accelerating immune reconstitution after hematopoietic stem cell transplantation

Viral infections remain a significant cause of morbidity and mortality after hematopoietic stem cell transplantation. Pharmacologic agents are effective against some pathogens, but they are costly and can be associated with significant toxicities. Thus, many groups have investigated adoptive T-cell transfer as a means of hastening immune reconstitution and preventing and treating viral infections. This review discusses the immunotherapeutic strategies that have been explored.

Optimization of methodology for production of CD25/CD71 allodepleted donor T cells for clinical use

BACKGROUND AIMS

Immunotherapy with allodepleted donor T cells improves immunity after T cell-depleted hematopoietic stem cell transplantation. We developed a methodology for selective depletion of alloreactive T cells after activation with host antigen-presenting cells by targeting T cells up-regulating CD25 and CD71. Combined depletion of these cells yields a pool of allodepleted donor T cells with antiviral properties with minimal capacity to cause graft-versus-host disease.

METHODS

Mature dendritic cells were irradiated and used to stimulate donor peripheral blood mononuclear cells for 4 days. The co-culture was stained with anti-CD71-biotin followed by CliniMACS CD25 and Anti-Biotin Reagents (Miltenyi Biotec GmbH; Bergisch Gladbach, Germany) before depletion on the CliniMACS Plus (Miltenyi Biotec GmbH). Residual alloreactivity was tested by flow cytometry, a secondary mixed lymphocyte reaction and limiting dilution analysis, and specific anti-viral immunity with pentamer staining. The large-scale protocol was tested under current good manufacturing practice conditions in five donor-recipient pairs of human leukocyte antigen-matched volunteer donors.

RESULTS

We developed a closed-system methodology using cell differentiation bags for cell culture and the COBE2991 Cell Processor (CaridianBCT, Lakewood, CO, USA). We also validated an anti-CD71-biotin generated for ex vivo clinical use. In five large-scale runs, the depleted fraction demonstrated excellent viability (99.9%), minimal residual expression of CD3/CD25 and CD3/CD71 (<0.2%) and passed tests for Mycoplasma, endotoxin, bacterial and fungal sterility. In secondary mixed lymphocyte reaction assays, the median response to host after allodepletion was 0%, whereas responses to third-party peripheral blood mononuclear cells were preserved (median, 105%; range 37%-350%). Limiting dilution analysis assays also demonstrated a reduction in response to host (median, -1.11 log) with preservation of third-party responses, and testing with human leukocyte antigen-restricted pentamers showed that populations of Epstein-Barr virus-specific and cytomegalovirus-specific CD8(+) T cells were retained after depletion.

CONCLUSIONS

We optimized a protocol for the combined immunomagnetic depletion of alloreactive CD25/CD71 T cells under current good manufacturing practice conditions and tested the efficacy in five donor-recipient pairs.

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

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

The humanized anti-HLA-DR moAb, IMMU-114, depletes APCs and reduces alloreactive T cells: implications for preventing GVHD

In contrast to the conventional immunosuppressive agents and nonselective T-cell-depleting antibodies, selective depletion of donor alloreactive T cells and/or host APCs, particularly DCs, represents a novel approach that can effectively control GVHD with less or no impairment of T-cell-mediated antiviral and GVL immunity. Here we report that IMMU-114, a humanized anti-human leukocyte antigen-DR (HLA-DR) moAb, efficiently depleted human PBMCs of all APCs, including B cells, monocytes, myeloid DC type-1 (mDC1), mDC2 and plasmacytoid DCs (pDCs). Early and late apoptosis of mDC1, mDC2 and pDCs, and late apoptosis of all APC subsets, were increased by IMMU-114 treatment. Although IMMU-114 had little, if any, effect on the survival and apoptosis of non-B lymphocytes (>80% of which are T cells and ∼1-2% of T cells express HLA-DR), it selectively inhibited the proliferation of purified HLA-DR(+) T cells rather than HLA-DR(-) T cells. As a consequence, IMMU-114 treatment resulted in suppressed T-cell proliferation and reduced CD25(+) alloreactive T cells in allogeneic MLRs. Given the critical roles of APCs and alloreactive T cells in the pathogenesis of GVHD, these results suggest that IMMU-114 may have therapeutic potential against GVHD.

Clinical evaluation of cellular immunotherapy in acute myeloid leukaemia

Immunotherapy is currently under active investigation as an adjuvant therapy to improve the overall survival of patients with acute myeloid leukaemia (AML) by eliminating residual leukaemic cells following standard therapy. The graft-versus-leukaemia effect observed following allogeneic haematopoietic stem cell transplantation has already demonstrated the significant role of immune cells in controlling AML, paving the way to further exploitation of this effect in optimized immunotherapy protocols. In this review, we discuss the current state of cellular immunotherapy as adjuvant therapy for AML, with a particular focus on new strategies and recently published results of preclinical and clinical studies. Therapeutic vaccines that are being tested in AML include whole tumour cells as an autologous source of multiple leukaemia-associated antigens (LAA) and autologous dendritic cells loaded with LAA as effective antigen-presenting cells. Furthermore, adoptive transfer of cytotoxic T cells or natural killer cells is under active investigation. Results from phase I and II trials are promising and support further investigation into the potential of cellular immunotherapeutic strategies to prevent or fight relapse in AML patients.

How I treat adenovirus in hematopoietic stem cell transplant recipients

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

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.

Depletion of Host Reactive T Cells by Photodynamic Cell Purging and Prevention of Graft Versus Host Disease

Graft versus Host Disease (GVHD) is the principal cause of morbidity and mortality in patients undergoing allogeneic stem cell transplant. T cell depletion has been recognized as a method of reducing the incidence of GVHD in allogeneic transplants. Until recently, most T cell depletion methods were non-selective in reducing lymphocytes. Rhodamine purging is one method, which selectively reduces alloreactive T cells preventing GVHD. We review here the methods of non-selective and selective T cell depletion, particularly the newer method of photodynamic purging utilizing rhodamine.

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.

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.

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.

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

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

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

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

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.

Advances in the understanding of acute graft-versus-host disease

Allogeneic stem cell transplantation (SCT) remains the definitive immunotherapy for malignancy. However, morbidity and mortality due to graft-vs.-host disease (GVHD) remains the major barrier to its advancement. Emerging experimental data highlights the immuno-modulatory roles of diverse cell populations in GVHD, including regulatory T cells, natural killer (NK) cells, NK T cells, gammadelta T cells, and antigen presenting cells (APC). Knowledge of the pathophysiology of GVHD has driven the investigation of new rational strategies to both prevent severe GVHD and treat steroid-refractory GVHD. Novel cytokine inhibitors, immune-suppressant agents known to preserve or even promote regulatory T-cell function and the depletion of specific alloreactive T-cell sub-populations all promise significant advances in the near future. As our knowledge and therapeutic options expand, the ability to limit GVHD whilst preserving anti-microbial and tumour responses becomes a realistic prospect.

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.

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.

Immunotoxins for targeted cancer therapy

Immunotoxins are proteins that contain a toxin along with an antibody or growth factor that binds specifically to target cells. Nearly all protein toxins work by enzymatically inhibiting protein synthesis. For the immunotoxin to work, it must bind to and be internalized by the target cells, and the enzymatic fragment of the toxin must translocate to the cytosol. Once in the cytosol, 1 molecule is capable of killing a cell, making immunotoxins some of the most potent killing agents. Various plant and bacterial toxins have been genetically fused or chemically conjugated to ligands that bind to cancer cells. Among the most active clinically are those that bind to hematologic tumors. At present, only 1 agent, which contains human interleukin-2 and truncated diphtheria toxin, is approved for use in cutaneous T-cell lymphoma. Another, containing an anti-CD22 Fv and truncated Pseudomonas exotoxin, has induced complete remissions in a high proportion of cases of hairy-cell leukemia. Refinement of existing immunotoxins and development of new immunotoxins are underway to improve the treatment of cancer.

Longitudinal analysis of T-cell receptor variable beta chain repertoire in patients with acute graft-versus-host disease after allogeneic stem cell transplantation

T-cell receptor variable beta chain (TCRBV) repertoire spectratyping involves the estimation of CDR3 length distributions for monitoring T-cell receptor diversity and has proven useful for analyses of immune reconstitution and T-cell clonal expansions in graft-versus-host disease (GVHD) and graft-versus-leukemia after allogeneic stem cell transplantation. We performed a longitudinal spectratype analysis of 23 TCRBV families in 28 patients who underwent allogeneic T cell-depleted peripheral blood stem cell transplantation. Sixteen patients subsequently developed acute GVHD. We recently developed statistical methods that bring increased power and flexibility to spectratype analysis and allow us to analyze TCRBV repertoire development under appropriately complex statistical models. Applying these methods, we found that patients with acute GVHD demonstrated TCRBV repertoire development statistically distinct from that repertoire development in patients without GVHD. Specifically, GVHD patients showed spectratypes indicative of lower diversity and greater deviation from the spectratypes expected in healthy individuals at intermediate times. Most individual TCRBV subfamilies had spectratypes statistically distinguishable between GVHD and non-GVHD patients at 6 months after transplantation. These results suggest that the T-cell receptor repertoire perturbations associated with acute GVHD are widely spread throughout the TCRBV families.

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.

T-Cell Immunotherapy for Adenoviral Infections of Stem-Cell Transplant Recipients

Human adenoviruses are ubiquitous lytic DNA viruses that can be divided into 51 different serotypes, grouped from A to F on the basis of genome size, composition, homology, and organization. Adenovirus infections, although frequent, are rarely fatal in immunocompetent individuals, due to potent innate and adaptive immune responses. By contrast, adenoviruses are a significant cause of morbidity and mortality in immunosuppressed individuals, for whom there are limited treatment options. Since antiviral drugs have variable efficacy in the treatment of severe adenovirus disease, iatrogenic reconstitution with in vitro expanded virus-specific cytotoxic T lymphocytes (CTLs) is an attractive option for prophylaxis and treatment, particularly because the endogenous recovery of adenovirus-specific T cells has proved important in controlling infection in vivo. Thus, we have characterized human T-cell responses to adenovirus in vitro and explored the potential of adoptive T-cell immunotherapy as a prophylactic or therapeutic strategy for adenovirus infections posttransplant.

Improving immune reconstitution while preventing GvHD in allogeneic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) is the treatment of choice for many hematologic malignancies and inherited disorders of the hematopoietic system. Ex vivo T-cell depletion (TCD) of the graft and post-transplantation immunosuppression efficiently prevents the development of GvHD in no- MHC-identical settings. However, the consequence of these non-specific strategies is a long-lasting immunodeficiency associated with increased incidence of disease relapse, graft rejection and reactivation of viral infections. Donor lymphocyte infusion, which is used for treating leukemic relapse after allogeneic HSCT, can result in severe GvHD. Several strategies are being optimized specifically to inactivate anti-host T cells while preserving anti-leukemic or anti-microbial immunocompetence. Based on the ex vivo or in vivo elimination of anti-host T cells, or on the modulation of their anti-host activity, these approaches, which have been explored extensively in pre-clinical studies and tested in some preliminary clinical trials, are discussed in this paper.

Selective depletion strategies in allogeneic stem cell transplantation

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

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

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

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

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

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

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

Immune restoration following hematopoietic stem cell transplantation: an evolving target

Hematopoietic stem cell transplantation (HSCT) is the definitive cure for many malignant and nonmalignant diseases. However, delays in immune reconstitution (IR) following HSCT significantly limit the success of transplantation and increase the risk for infection and disease relapse in the transplant recipient. Therefore, ways to measure and to manipulate immune recovery following HSCT are emerging and their success depends directly upon an enhanced understanding for the underlying mechanisms responsible for reconstituted immunity and hematopoiesis. Recent discoveries in the activation, function, and regulation of dendritic cell (DC), natural killer (NK) cell, and T-lymphocyte subtypes have been critical in developing immunotherapies used to prevent graft-versus-host disease and to enhance graft-versus-leukemia. For example, regulatory T cells that induce tolerance and NK receptor-tumor ligand disparities that result in tumor lysis are being used to minimize GVHD and tumor burden, respectively. Furthermore, expansion and modulation of immune effector cells are being used to augment hematopoietic and immune recovery and to decrease transplant-related toxicity in the transplant recipient. Specifically, DC expansion and incorporation into antitumor and anti-microbial vaccines is fast approaching application into clinical trials. This paper will review our current understanding for IR following HSCT and the novel ways in which to restore immune function and decrease transplant-related toxicity in the transplant recipient.

Recombinant immunotoxins for the treatment of haematological malignancies

Recombinant immunotoxins are fusion proteins which contain a ligand derived from the immune system fused to a toxin. The protein toxin is truncated to delete its binding domain, allowing selective ligand-directed binding. Growth factor fusion toxins are often considered immunotoxins. One of these molecules, containing the truncated diphtheria toxin and human IL-2 (Ontak), Ligand Pharmaceuticals), has been approved for the treatment of cutaneous T-cell lymphoma. Recombinant immunotoxins have also been produced containing the variable domains (Fv fragment) of monoclonal antibodies fused to toxins. These agents are relatively versatile with respect to the range of antigens possible. Several of these recombinant immunotoxins have showed clinical effectiveness in Phase I testing against haematological malignancies. One of these molecules, BL22, targets CD22 on hairy-cell leukaemia and has enabled patients to achieve complete remissions despite previous treatment and resistance to chemotherapy.

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

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

“Pruning” of Alloreactive CD4+T Cells Using 5- (and 6-)Carboxyfluorescein Diacetate Succinimidyl Ester Prolongs Skin Allograft Survival

Removal of alloreactive cells by either thymic deletion or deletion/anergy in the periphery is regarded as crucial to the development of tolerance. Dyes, such as CFSE, that allow monitoring of cell division suggest that in vitro proliferation could be a used as a way of "pruning" alloreactive cells while retaining a normal immune repertoire with retention of memory to previously encountered pathogens. This would overcome the problems occurring as a result of therapies that use massive depletion of T cells to allow acceptance of organ transplants or bone marrow grafts. We therefore used a skin graft model of CD4-mediated T cell rejection across a major H-2 mismatch (C57BL/6 (H-2(b)) to BALB/c (H-2(d)) mice) to evaluate whether nondividing CD4(+) T cells derived from a mixed lymphocyte culture would exhibit tolerance to a skin graft from the initial stimulator strain. We demonstrate that selective removal of dividing alloreactive CD4(+) T cells resulted in marked specific prolongation of allogeneic skin graft survival, and that the nondividing CD4(+) T cells retained a broad TCR repertoire and the ability to maintain memory. This novel way of depleting alloreactive T cells may serve as a useful strategy in combination with other mechanisms to achieve transplant tolerance.

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

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

Ex vivo generation and expansion of anti-tumor cytotoxic T-cell lines derived from patients or their HLA-identical sibling

Successful ex-vivo priming and long-term maintenance of anti-tumor cytotoxic T-cell (CTL) lines are preliminary conditions for their use in approaches of adoptive immunotherapy for patients with cancer. We describe the results of a novel procedure for generating in vitro anti-tumor CTL using CD8-enriched peripheral blood mononuclear cells (PBMC) and dendritic cells (DC), pulsed with irradiated tumor cells (TC) as source of tumor antigen. Eight patients were enrolled in our study: 4 sarcoma, 2 renal cell carcinoma, 1 ovarian carcinoma and 1 breast carcinoma. Ten anti-tumor CTL-lines cytotoxic towards patient TC were generated. Five CTL-lines were obtained using both DC and PBMC from the patients (autologous setting). For 5 CTL-lines, DC derived from an HLA-identical sibling were employed (allogeneic setting): patients or siblings PBMC were used to generate CTL-lines in 2 and 3 cases, respectively,. After tumor-specific rounds of stimulation, followed by antigen-independent cycle of expansion, CTL-lines obtained in both autologous and allogeneic setting showed an expansion of the absolute number of cultured cells. In 6 of 10 CTL-lines, the majority of effector cells (>70%) were CD3+/CD8+, while in the remaining 4, 40-70% of effector cells were CD3+/CD4+. Both CD8+ and CD4+ T cells displayed anti-tumor cytotoxic activity. Spectratyping analysis of the TCR-Vbeta subfamilies revealed a preferential expansion of oligoclonal populations in 18 of 24Vbeta subfamily. Altogether these results demonstrate that our experimental approach is suitable for efficiently generating and expanding anti-solid tumor CTL to be used for adoptive immunotherapy.

Adoptive immunotherapy for posttransplantation viral infections

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

Graft-versus-leukemia reactions in allogeneic chimeras

There is a strong graft-versus-leukemia (GVL) effect of allogeneic stem cell transplantation (SCT) due to elimination of tumor cells by alloimmune effector lymphocytes. When leukemia relapses after allogeneic SCT, donor lymphocyte transfusions (DLTs) can induce sustained remissions in some patients. This review summarizes the current status on clinical use of DLT, the basis of GVL reactions, problems associated with this therapy, and new strategies to improve DLT. Several multicenter surveys demonstrated that the GVL effect of DLT is most effective in chronic myelogenous leukemia (CML), whereas it is less pronounced in acute leukemia and myeloma. Cytokine stimulation to induce differentiation of myeloid progenitor cells or to up-regulate costimulatory molecules on tumor cells may improve the efficacy of DLT. Infections and graft-versus-host disease (GVHD) are major complications of DLT. Control of GVHD may be improved using suicide gene-modified T cells for DLT, allowing T-cell elimination if severe GVHD develops. Hopefully, in the future, GVL effect can be separated from GVHD through adoptive transfer of selected T cells that recognize leukemia-specific antigens or minor histocompatibility antigens, which are expressed predominantly on hematopoietic cells, thereby precluding attack of normal tissues. In patients with leukemia and lymphomas with fast progression, tumor growth may outpace development of effector T cells. Here it may be preferable to select stem cell transplant donors with HLA-mismatches that allow alloreactive natural killer cells, which appear early after transplantation, to retain their cytolytic function. New approaches for adoptive immune therapy of leukemia, which promise a better prognosis for these patients, are being developed.

Ex vivo depletion of alloreactive cells based on CFSE dye dilution, activation antigen selection, and dendritic cell stimulation

Eliminating alloreactive cells from T-cell populations would enable the transfer of immune function to patients who receive stem cell transplants. However, high-efficiency depletion has proved difficult to achieve. We sought to develop ex vivo approaches for the maximal depletion of alloreactive CD4+ T cells. Using a flow cytometric cell sorting approach after mixed lymphocyte reaction (MLR) culture, we have found that sorted CFSEbright (5-(and-6)-carboxyfluorescein diacetate succinmidyl ester) (nondivided) and activation antigen-negative cells are markedly depleted of alloreactivity. With HLA-mismatched peripheral blood mononuclear cell (PBMC) stimulators we have consistently attained (90%-95%) depletion of alloreactivity. Importantly, when purified matured monocyte-derived dendritic cells (DCs) are used as stimulators, a 100-fold (99%) reduction in alloreactivity was attained, resulting in abrogation of the secondary MLR. Significantly, the CFSEbright CD25- cells recovered from these cultures retained general immunoreactivity, including responses to Candida and cytomegalovirus (CMV) antigens. In addition, a CFSE-based approach was tested and found to be sufficient for graft-versus-host disease (GVHD) prevention in vivo, in a major histocompatibility complex (MHC) class II disparate murine model. This efficient approach to selectively deplete mature alloantigen-specific T cells may permit enhanced immune reconstitution without GVHD. (Blood. 2004;103:1158-1165)

Comparative in vitro study of the immunomodulatory activity of humanized and chimeric anti-CD25 monoclonal antibodies

Humanized or chimeric monoclonal antibodies (MoAbs) directed against the interleukin-2 (IL-2) receptor alpha-chain, CD25, are promising immunosuppressive agents due to improved pharmacokinetic profiles and less toxicity. These MoAbs have been used effectively in preventing and/or treating rejection in solid organ transplantation and are currently under investigation for prevention/treatment of graft-versus-host disease (GvHD) in stem cell transplantation. We analysed the in vitro activities of the chimeric anti-CD25 MoAb basiliximab and the humanized anti-CD25 MoAb daclizumab in various test systems for alloimmune response and T cell activation in comparison to cyclosporin A (CsA) and prednisolone. Anti-CD3- and alloantigen-induced T cell proliferation were decreased significantly by the anti-CD25 MoAbs in a dose-dependent fashion. At a concentration of 10 ng/ml daclizumab and CsA synergistically decreased T cell proliferation of mixed lymphocyte cultures, whereas basiliximab showed only subadditive activity. Simultaneous addition of the anti-CD25 MoAbs and prednisolone did not result in combined activity. Addition of exogenous IL-2 completely overcame the inhibitory effect on T cell proliferation of both anti-CD25 MoAbs, but not that of CsA and prednisolone. Anti-CD25 MoAbs inhibited the generation of antigen-specific cytotoxic T lymphocytes in a limiting dilution assay, whereas they showed no effect on the cytolytic activity of established antigen-specific T cell clones. This in vitro study demonstrates strong immunosuppressive activity by both chimeric and humanized MoAbs against CD25. The combined activity with CsA justifies their early use for prevention rather than treatment of GvHD.

The role of haploidentical stem cell transplantation in the management of children with haematological disorders

The broader application of stem cell transplantation (SCT) for paediatric diseases has been limited by a lack of human leucocyte antigen (HLA)-matched donors. Virtually all children, however have at least one haploidentical parent who could serve as a donor. Such a donor is immediately available and the considerable costs of additional HLA typing, registry and banking expenditures that are necessary to procure an unrelated donor, could be reduced. Recent technological advances appear to have overcome the historical problems of graft rejection and severe graft versus host disease in the haploidentical setting, and in the latest studies the overall survival for children undergoing haploidentical SCT for leukaemia is now comparable with that following unrelated donor bone marrow or cord blood transplantation. Post-transplant infectious complications and leukaemia relapse remain the most important barriers yet to overcome, and new directions in the use of adoptive cellular immunity appear to be promising in this respect. Haploidentical SCT is now a viable option for those children who do not have an HLA compatible sibling or fully matched unrelated donor. The relative merits of a haploidentical family donor versus mismatched unrelated bone marrow or cord blood donation needs to be assessed in prospective, randomized clinical trials.

Humanized anti-CD25 monoclonal antibody for prophylaxis of graft-vs-host disease (GVHD) in haploidentical bone marrow transplantation without ex vivo T-cell depletion

OBJECTIVE

To investigate the effects of a novel anti-IL-2 receptor (CD25) monoclonal antibody, basiliximab, on graft-vs-host disease (GVHD) and engraftment in haploidentical bone marrow transplantation (BMT).

MATERIALS AND METHODS

Thirteen consecutive high-risk leukemia patients (age 9-41) underwent haploidentical BMT with G-CSF-primed marrow as stem cells without ex vivo T-cell depletion. Basiliximab, along with a combination of cyclosporine (CSA), methotrexate (MTX), and mycophenolate mofetil (MMF), was used for GVHD prophylaxis. Immunophenotyping, limited-dilution assay, and colony-forming assays were used to measure the effect of basiliximab on the subsets of lymphocytes, cytotoxic T-lymphocyte precursors (CTLp), and hematopoietic cells.

RESULTS

All patients established successful trilineage engraftment with full donor chimerism. No patients developed grade II-IV acute GVHD. Patients who survived more than 12 months and were free of relapse showed limited chronic skin GVHD. Ten of 13 patients are currently alive with a Karnofsky performance score of 100% at median follow-up of 17 months (range 12-24 months). Basiliximab significantly decreased alloreactive CTLp by 10-fold to 100-fold in limiting-dilution assays. It had no effect on hematopoietic stem and progenitor cells as determined by in vitro colony-forming assays.

CONCLUSION

The addition of basiliximab to CSA, MMF, and MTX as GVHD prophylaxis effectively reduced severe lethal GVHD in haploidentical BMT. It is possible to selectively eliminate or reduce the number of alloreactive T cells with anti-CD25 antibody, which results in prevention of or a reduction in the severity of GVHD.