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

EWS-FLI1 low Ewing sarcoma cells demonstrate decreased susceptibility to T-cell-mediated tumor cell apoptosis

Metastatic and relapsed Ewing sarcoma typically afflicts the adolescent population and is largely fatal. These bone tumors are most commonly driven by the fusion oncoprotein EWS-FLI1. Ewing tumors demonstrate significant intra-tumoral heterogeneity, and individual tumor cells can express highly variable and dynamic levels of EWS-FLI1. Recent studies revealed that the EWS-FLI1 oncoprotein level (high versus low expression) greatly influences the behavior of Ewing tumor cells. As compared to cells with high EWS-FLI1, Ewing cells in the EWS-FLI1 low state demonstrate an increased propensity for metastasis. In light of these observations, we sought to determine how tumor cell EWS-FLI1 level influences the anti-tumor cell immune response. Since ICAM-1, which can promote tumor cell/T-cell interaction and T-cell activation, is highly expressed on EWS-FLI1 low cells, we hypothesized that EWS-FLI1 low cells would be more susceptible to T-cell mediated tumor cell apoptosis as compared to cells with high EWS-FLI1. Unexpectedly, we found that EWS-FLI1 low cells are more resistant to T-cell mediated apoptosis than EWS-FLI1 high cells. We investigated the potential mechanisms by which EWS-FLI1 level might influence the T-cell anti-tumor response, and discovered that low EWS-FLI1 expression results in upregulation of PD-L1 and PD-L2, both important ligands for the PD-1 immune checkpoint receptor on T-cells. We demonstrated that blocking PD-1 results in a greater increase of T-cell mediated killing of EWS-FLI1 low tumor cells as compared to cells with higher EWS-FLI1 expression. Our studies suggest that Ewing cells in the EWS-FLI1 low expression state may serve as a niche of tumor immune-evasion.

Donor lymphocyte infusion and methotrexate for immune recovery after T-cell depleted haploidentical transplantation

CD34+ cell selection minimizes graft-versus-host disease (GVHD) after haploidentical donor stem cell transplant but is associated with slow immune recovery and infections. We report a Phase I/II study of prophylactic donor lymphocyte infusion (DLI) followed by methotrexate (MTX) GVHD prophylaxis after CD34-selected haploidentical donor transplant. A prophylactic DLI was given between day +30 and +42. Rituximab was given with DLI for the last 10 patients. The goal of the study was to determine a DLI dose that would result in a CD4+ cell count > 100/µL at Day +120 in ≥ 66% of patients with ≤ 33% grade II-III, ≤ 17% grade III, and no grade IV acute GVHD by Day +180. Thirty-five patients with malignant (n = 25) or nonmalignant disease (n = 10) were treated after CD34-selected haploidentical donor peripheral blood stem cell transplant. The DLI dose of 5 × 10⁴ /kg met the CD4/GVHD goal with 67% of patients having CD4+ cells > 100/µL and 11% grade II-IV acute GVHD. The cumulative incidence of chronic GVHD was 16%. Fatal viral and fungal infections occurred in 11%. The 2 year estimated overall survival was 69% and the relapse rate was 14% for patients in remission at transplant. There was no effect of NK alloreactivity on relapse. Nine of ten patients at the target DLI dose cohort of 5 × 10⁴ /kg are alive with median follow-up of 18 mos (range 6-29). Delayed prophylactic DLI and MTX was associated with promising outcomes at the target DLI dose. This trial was registered at clinicaltrials.gov, # NCT01027702.

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

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

A novel reduced-intensity conditioning regimen for unrelated umbilical cord blood transplantation in children with nonmalignant diseases

Reduced-intensity conditioning (RIC) regimens have the potential to decrease transplantation-related morbidity and mortality. However, engraftment failure has been prohibitively high after RIC unrelated umbilical cord blood transplantation (UCBT) in chemotherapy-naïve children with nonmalignant diseases (NMD). Twenty-two children with a median age of 2.8 years, many with severe comorbidities and prior viral infections, were enrolled in a novel RIC protocol consisting of hydroxyurea, alemtuzumab, fludarabine, melphalan, and thiotepa followed by single UCBT. Patients underwent transplantation for inherited metabolic disorders (n = 8), primary immunodeficiencies (n = 9), hemoglobinopathies (n = 4) and Diamond Blackfan anemia (n = 1). Most umbilical cord blood (UCB) units were HLA-mismatched with median infused total nucleated cell dose of 7.9 × 10(7)/kg. No serious organ toxicities were attributable to the regimen. The cumulative incidence of neutrophil engraftment was 86.4% (95% confidence interval [CI], 65% to 100%) in a median of 20 days, with the majority sustaining > 95% donor chimerism at 1 year. Cumulative incidence of acute graft-versus-host disease (GVHD) grades II to IV and III to IV by day 180 was 27.3% (95% CI, 8.7% to 45.9%) and 13.6% (95 CI, 0% to 27.6%), respectively. Cumulative incidence of extensive chronic GVHD was 9.1% (95% CI, 0% to 20.8%). The primary causes of death were viral infections (n = 3), acute GVHD (n = 1) and transfusion reaction (n = 1). One-year overall and event-free survivals were 77.3% (95% CI, 53.7% to 89.8%) and 68.2% (95% CI, 44.6% to 83.4%) with 31 months median follow-up. This is the first RIC protocol demonstrating durable UCB engraftment in children with NMD. Future risk-based modifications of this regimen could decrease the incidence of viral infections. (www.clinicaltrials.gov/NCT00744692).

Vascular endothelial growth factor-A enhances indoleamine 2,3-dioxygenase expression by dendritic cells and subsequently impacts lymphocyte proliferation

Dendritic cells (DCs) are antigen (Ag)-presenting cells that activate and stimulate effective immune responses by T cells, but can also act as negative regulators of these responses and thus play important roles in immune regulation. Pro-angiogenic vascular endothelial growth factor (VEGF) has been shown to cause defective DC differentiation and maturation. Previous studies have demonstrated that the addition of VEGF to DC cultures renders these cells weak stimulators of Ag-specific T cells due to the inhibitory effects mediated by VEGF receptor 1 (VEGFR1) and/or VEGFR2 signalling. As the enzyme indoleamine 2,3-dioxygenase (IDO) is recognised as an important negative regulator of immune responses, this study aimed to investigate whether VEGF affects the expression of IDO by DCs and whether VEGF-matured DCs acquire a suppressor phenotype. Our results are the first to demonstrate that VEGF increases the expression and activity of IDO in DCs, which has a suppressive effect on Ag-specific and mitogen-stimulated lymphocyte proliferation. These mechanisms have broad implications for the study of immunological responses and tolerance under conditions as diverse as cancer, graft rejection and autoimmunity.

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.

T-lymphocyte recovery and function after cord blood transplantation

The Szabolcs laboratory is focused on understanding the biology of donor-derived cellular immunity in recipients of allogeneic hematopoietic cell transplantation that can be translated into new immunotherapy strategies. To this end, we are focused on developing novel laboratory approaches to analyze and augment immune recovery for high risk patient cohorts without increasing graft-versus-host disease. Much of our work has focused on unrelated cord blood transplantation as the dominant clinical scenario and laboratory model. Our overarching goal is to minimize transplant-related mortality and morbidity and render HLA-mismatched unrelated cord blood transplant, a widely accepted safe cellular therapy. Donor leukocyte infusions in the allogeneic hematopoietic transplant setting can provide a clinically relevant boost of immunity to reduce opportunistic infections and to increase graft-versus-leukemia activity. Our laboratory has a major focus toward ex vivo expansion of cord blood T cells with anti-apoptotic cytokines and CD3/CD28 co-stimulatory beads. Expanded lymphocytes lack alloreactivity against recipient and other allogeneic cells indicating a favorable safety profile from graft-versus-host disease. Nevertheless, expanded T cells can be primed subsequently against lymphoid and myeloid leukemia cells to generate tumor-specific cytotoxic T cells. These findings offer a major step in fulfilling critical biological requirements to quickly generate a cellular product ex vivo, using a negligible fraction of a cord blood graft that provides a flexible adoptive immunotherapy platform for both children and adults.

The immunobiology of cord blood transplantation

Despite significant recent advances in the applicability and outcome following unrelated cord blood transplantation (UCBT), infections remain a major cause of mortality associated with poor immune recovery in the first 6 months after UCBT. Enhanced immune reconstitution not only could improve survival by reduced transplant related mortality, but may also favorably impact on relapse incidence by improved graft-versus-leukemia effects. This review will summarize our current understanding of the biology of immune recovery post-UCBT with an emphasis on adaptive T cell dependent immunity. New efforts to boost immunity will be also highlighted including our own laboratory, where ex vivo T cell expansion is pursued towards adoptive immunotherapy.

Interleukin-7 permits Th1/Tc1 maturation and promotes ex vivo expansion of cord blood T cells: a critical step toward adoptive immunotherapy after cord blood transplantation

Donor leukocyte infusions (DLI) in the allogeneic hematopoietic transplant setting can provide a clinically relevant boost of immunity to reduce opportunistic infections and to increase graft-versus-leukemia activity. Despite significant advances in applicability, DLI has not been available for single-unit recipients of unrelated cord blood transplant. Ex vivo expansion of cord blood T cells can be achieved with interleukin (IL)-2 and CD3/CD28 costimulatory beads. However, significant apoptosis occurs in proliferating T cells, diminishing the yield and skewing the CD4/CD8 ratio in the T-cell population, jeopardizing the potential efficacy of DLI. In this study, we show that interleukin (IL)-7 not only reduces apoptosis of activated T lymphocytes and enhances their proliferation but also promotes functional maturation, leading to secretion of IFN-gamma and other key cytokines. Recognizing that infused T lymphocytes will need to meet microbial antigens in secondary lymphoid organs to generate effectors, we also show that expansion with IL-7 promotes the preservation of a polyclonal broad T-cell receptor repertoire and a surface phenotype that favors lymph node homing. Expanded lymphocytes lack alloreactivity against recipient and other allogeneic cells, indicating a favorable safety profile from graft-versus-host disease. Nevertheless, expanded T cells can be primed subsequently against lymphoid and myeloid leukemia cells to generate tumor-specific cytotoxic T cells. Taken together, our findings offer a major step in fulfilling critical numerical and biological requirements to quickly generate a DLI product ex vivo using a negligible fraction of a cord blood graft that provides a flexible adoptive immunotherapy platform for both children and adults.

Unrelated umbilical cord blood transplantation and immune reconstitution

This review highlights the unique features of immune reconstitution following unrelated cord blood transplantation (UCBT) that lead to heightened risk of infection-related mortality in the early post-UCBT period. There is no evidence that innate immunity is uniquely compromised after UCBT, but the development of antigen-specific cellular immunity is affected by numerical and qualitative deficits, primarily within the first 100 days. Nevertheless, beyond the first few months after UCBT there is no evidence for reduced graft-versus-leukemia (GVL) or anti-viral immunity compared to other hematopoietic cell therapy (HCT) modalities. Novel cellular therapies that are about to enter the clinical setting in the form of natural killer (NK) cell and T-cell therapies in the form of donor lymphocyte infusion (DLI) are also discussed.

Immune reconstitution in children after unrelated cord blood transplantation

There is a great need to learn more about the biology of immune recovery after UCBT. Fundamental gaps in knowledge remain regarding the biology and kinetics of developing antigen-specific protective immunity and understanding the impact of recipient age and immunosuppressive agents. However, there is also realistic hope that clinical translation of new immunotherapy strategies could enhance immune competence after UCBT either by having an impact on the thymic-independent early period or by fostering thymic recovery.

Protocols for hematopoietic stem cell expansion from umbilical cord blood

The reconstitution of adult stem cells may be a promising source for the regeneration of damaged tissues and for the reconstitution of organ dysfunction. However, there are two major limitations to the use of such cells: they are rare, and only a few types exist that can easily be isolated without harming the patient. The best studied and most widely used stem cells are of the hematopoietic lineage. Pioneering work on hematopoietic stem cell (HSC) transplantation was done in the early 1970s by ED. Thomas and colleagues. Since then HSCs have been used in allogenic and autologous transplantation settings to reconstitute blood formation after high-dose chemotherapy for various blood disorders. The cells can be easily harvested from donors, but the cell number is limited, especially when the HSCs originate from umbilical cord blood (UCB). It would be desirable to set up an ex vivo strategy to expand HSCs in order to overcome the cell dose limit, whereby the expansion would favor cell proliferation over cell differentiation. This review provides an overview of the various existing HSC expansion strategies-focusing particularly on stem cells derived from UCB-of the parameters that might affect the outcome, and of the difficulties that may occur when trying to expand such cells.

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.

Ex vivo expansion and Th1/Tc1 maturation of umbilical cord blood T cells by CD3/CD28 costimulation

One major limitation of UCBT is the lack of donor cells available for posttransplantation donor leukocyte infusions (DLI) to boost immunity or induce graft-versus-leukemia (GVL) activity. Starting from a approximately 5% fraction of a UCB graft, we report the feasibility and biological characteristics of ex vivo expansion of frozen/thawed CB T cells by anti-CD3 and anti-CD28 antibody-coated Dynal beads in the presence of interleukin (IL)-2. We postulated that while undergoing expansion, UCB T cells may mature toward a Th1/Tc1 phenotype and acquire the potential for cytotoxicity. Whereas an almost 2-log expansion also led to the acquisition of IL-12Ralpha and an increase in Th1 characteristics, postexpansion lymphocytes produced less interferon-gamma, tumor necrosis factor-alpha, and granzyme B; stored almost no perforin; and lacked cytotoxicity against allogeneic targets. Collectively, these suggest relative safety from acute/hyperacute graft-versus-host disease. CD8(+) T cells expanded preferentially, whereas a higher rate of apoptosis in CD4(+) T cells also promoted an inverted CD4/CD8 ratio. Most expanded T cells retained expression of CD27, CD28, and L-selectin but down-regulated CCR-7. In summary, UCB T cell proliferation sustained by CD3/CD28 co-stimulatory beads and IL-2 can lead to clinically relevant doses of DLI from a very small fraction of the UCB graft, although future strategies to reduce apoptosis may enhance their clinical potential.

Immune reconstitution after unrelated cord blood transplantation

Over the past years unrelated cord blood transplant (UCBT) has emerged as an effective alternative to unrelated donor blood and marrow transplantation. However, despite several advantages, its success is limited by the high incidence of opportunistic infections (OI), most of which are viral. Infection-related mortality is the primary cause of death after UCBT with most deaths occurring in the first 3-6 months post transplant. For several months, until recovery of the thymus is restored to support de novo T cell generation, protective antiviral immunity depends on the activity of post-thymic T cells infused within the cord blood (CB) grafts. However, almost all CB T cells are antigen inexperienced (naïve) lymphocytes that have been functionally altered by placental factors to protect pregnancy. CB T cells need to undergo in vivo priming, Th1/Tc1 maturation, and peripheral expansion before they can afford immunologic protection. This article provides an overview of what is currently known regarding the reconstitution of adaptive immunity following UCBT including our own data from prospective analyses of pediatric cohorts. Remarkable immunophenotypic changes are notable already in the first 2-3 weeks post-UCBT. These changes result from apparent 'homeostatic' peripheral T cell expansion in the lymphopenic environment. While we can identify patient- and graft-specific predictive factors, the concordant emergence of T cell subsets displaying the phenotype of Th1/Tc1 cytotoxic effector cells can be statistically linked to those UCBT recipients who will subsequently develop viral and other opportunistic infections. Antigen presenting dendritic cell reconstitution may also reflect alterations in immunocompetence due to OI and/or GVHD.

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.

Evaluation of CD34+ - and Lin- -selected cells from peripheral blood stem cell grafts of patients with lymphoma during differentiation in culture ex vivo using a cDNA microarray technique

OBJECTIVE

Hematopoietic stem cells (enriched in fraction of CD34+ cells) have the ability to regenerate hematopoiesis in all of its lineages, and this potential is clinically used in transplanting bone marrow or peripheral blood stem cells. Our objective was to assemble a suitable method for evaluating gene expression in enriched populations of hematopoietic stem cells. We compared biologic properties of cells cultured ex vivo obtained using two different ways of immunomagnetic separation (positive selection of CD34+ cells and negative selection of Lin- cells) by means of a cDNA microarray technique.

METHODS

CD34+ and Lin- cells were enriched from peripheral blood stem cell (PBSCs) grafts of patients with non-Hodgkin's lymphoma. Isolated cells were in the presence of cytokine PBSCs, Flt-3 ligand, interleukin-3, interleukin-6, and granulocyte colony-stimulating factor. At days 0, 4, 6, 8, 10, 12, and 14 cells were harvested and analyzed by cDNA microarrays. Total cell expansion, CD34+, colony-forming unit for granulocyte-macrophage and megakaryocytes expansion, vitality, and phenotype of cells were also analyzed.

RESULTS

cDNA microarray analysis of cultured hematopoietic cells proved equivalence of the two enrichment methods for PBSC samples and helped us characterize differentiating cells cultured ex vivo.

CONCLUSION

Our methodologic approach is helpful in characterizing cultured hematopoietic cells cultured ex vivo, but it is also suitable for more general purposes. Equivalence of CD34+ and Lin- selection methods from PBSC samples proved by cDNA microarray may have an implication for graft manipulation in an experimental setting of hematopoietic transplantation. Total cell expansion and colony formation and phenotype from CD34+ selected and from Lin- samples were comparable.

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.

In vitro priming and expansion of cytomegalovirus-specific Th1 and Tc1 T cells from naive cord blood lymphocytes

Adoptive transfer of CMV-specific cytotoxic T cells (CTLs) expanded in vitro from memory donor T cells can reduce the incidence of CMV disease in allogeneic transplant recipients. However, this approach has been unavailable in the cord blood (CB) transplantation setting because CB T cells are antigen naive and biased toward Th2/Tc2 function. We developed a protocol to in vitro prime and expand CMV-specific CTLs from CB. T cells were primed with cytokines to trigger skewing toward Th1/Tc1 lineage before encountering monocyte and CD34+ progenitor-derived dendritic cells loaded with CMV antigen and its immune complex. CMV-pulsed cultures expanded significantly more over 4 to 6 weeks than CMV cultures despite identical cytokine milieu. T cells isolated from CMV+ cultures showed a preferential expansion of CD45RA-/RO+/CD27+ T cells compared to CMV- cultures. CMV-specific IFN-gamma- and TNF-alpha-producing CD4+ (Th1) and CD8+ (Tc1) T cells were enriched after 3 to 4 weeks and CMV-specific cytotoxicity developed 1 to 2 weeks later.

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.