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

HLA-DPB1 Reactive T Cell Receptors for Adoptive Immunotherapy in Allogeneic Stem Cell Transplantation

HLA-DPB1 antigens are mismatched in about 80% of allogeneic hematopoietic stem cell transplantations from HLA 10/10 matched unrelated donors and were shown to be associated with a decreased risk of leukemia relapse. We recently developed a reliable in vitro method to generate HLA-DPB1 mismatch-reactive CD4 T-cell clones from allogeneic donors. Here, we isolated HLA-DPB1 specific T cell receptors (TCR DP) and used them either as wild-type or genetically optimized receptors to analyze in detail the reactivity of transduced CD4 and CD8 T cells toward primary AML blasts. While both CD4 and CD8 T cells showed strong AML reactivity in vitro, only CD4 T cells were able to effectively eliminate leukemia blasts in AML engrafted NOD/SCID/IL2Rγc^−/− (NSG) mice. Further analysis showed that optimized TCR DP and under some conditions wild-type TCR DP also mediated reactivity to non-hematopoietic cells like fibroblasts or tumor cell lines after HLA-DP upregulation. In conclusion, T cells engineered with selected allo-HLA-DPB1 specific TCRs might be powerful off-the-shelf reagents in allogeneic T-cell therapy of leukemia. However, because of frequent (common) cross-reactivity to non-hematopoietic cells with optimized TCR DP T cells, safety mechanisms are mandatory.

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.

Antiviral T-cell therapy

Serious viral infections are a common cause of morbidity and mortality after allogeneic stem cell transplantation. They occur in the majority of allograft recipients and are fatal in 17–20%. These severe infections may be prolonged or recurrent and add substantially to the cost, both human and financial, of the procedure. Many features of allogeneic stem cell transplantation contribute to this high rate of viral disease. The cytotoxic and immunosuppressive drugs administered pretransplant to eliminate the host hematopoietic/immune system and any associated malignancy, the delay in recapitulating immune ontogeny post‐transplant, the immunosuppressive drugs given to prevent graft versus host disease (GvHD), and the effects of GvHD itself, all serve to make stem cell transplant recipients vulnerable to disease from endogenous (latent) and exogenous (community) viruses, and to be incapable of controlling them as quickly and effectively as most normal individuals.

Boost and loss of immune responses against tumor-associated antigens in the course of pregnancy as a model for allogeneic immunotherapy

Donor-derived immunity against tumor-associated antigens (TAAs) may exert selective antileukemic activity reprieving the allogeneic recipient from graft-versus-host disease. As TAAs are highly expressed in placental tissues we hypothesized that pregnancy could drive respective immunity in healthy individuals. Thus, we investigated the frequency and level of immune responses against clinically relevant TAAs in 114 blood donors and 44 women during their first pregnancy. Quantitative reverse-transcription polymerase chain reaction was employed to detect low levels of interferon-γ after primary peptide stimulation of CD8(+) T lymphocytes. In blood donors, primary immune responses of low and/or high avidity were found against WT1 (15%), MUC1 (14%), PRAME (7%), and HER2/neu (5%) and exerted killing functions against leukemic cells. Men had higher responses than women, likely due to gonadal cancer-testis-antigen expression. Interestingly, a history of prior delivery was not associated with increased responses, whereas the strongest responses during pregnancy were found in early trimesters to disappear after delivery. This boost and loss of TAA-specific immunity suggests that virtually every donor harbors the potential to mount antileukemic immune responses in a recipient. However, in the absence of the driving target and a permissive environment, they are short-lived and thus require supplemental strategies such as vaccination or immunomodulation to facilitate their persistence.

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.

Depletion of naive T cells using clinical grade magnetic CD45RA beads: a new approach for GVHD prophylaxis

Depletion of naive T cells from donor leukapheresis products (LPs) aims at the reduction of alloreactivity, while preserving memory T-cell reactivity (for example, to pathogens). This study established the immunomagnetic depletion procedure under clean room conditions using CD45RA beads and analyzed LPs of six donors for cell composition and functional immune responses. CD45RA depletion resulted in 3.4-4.7 log (median 4.4) reduction of CD45RA(+) T cells, thereby eliminating naive and late effector T cells. B cells were also completely removed, whereas significant proportions of NK cells, monocytes and granulocytes persisted. CD45RA-depleted LPs contained effector and central memory CD4(+) and CD8(+) T cells that showed sustained IFN-γ secretion to CMV, EBV, Aspergillus and Candida Ags. Alloreactivity was measured in MLRs between donors with complete HLA-mismatch. Alloreactive CD8(+) T cells were strongly reduced (median >1-log) upon CD45RA depletion, whereas alloreactive CD4(+) T cells persisted in significant numbers. In conclusion, clinical grade depletion of CD45RA(+) naive T cells from donor LPs is feasible and highly efficient. The depleted products show sustained CD4(+) and CD8(+) T-cell reactivity to pathogens and effectively reduced CD8-mediated alloreactivity. Prophylactic and preemptive infusions after allogeneic SCT may improve T-cell reconstitution and pathogen-specific immunosurveillance, along with lower risk of inducing GVHD.

Immunotherapeutic approaches to improve graft-versus-tumor effect and reduce graft-versus-host disease

The therapeutic efficacy of allogeneic stem cell transplantation is mainly based on the alloreactive immune response of the graft against the host. However, the graft-versus-host process can be viewed as a double-edged sword since it is responsible for both the beneficial graft-versus-tumor effect and the deleterious graft-versus-host disease. During the last two decades, intensive research has been focused on the development of novel immunotherapeutic methods aimed to dissociate graft-versus-host disease from graft-versus-tumor effect. A brief description of these efforts is discussed in this review.

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.

IL-21-treated naive CD45RA+ CD8+ T cells represent a reliable source for producing leukemia-reactive cytotoxic T lymphocytes with high proliferative potential and early differentiation phenotype

Clinical tumor remissions after adoptive T-cell therapy are frequently not durable due to limited survival and homing of transfused tumor-reactive T cells, what can be mainly attributed to the long-term culture necessary for in vitro expansion. Here, we introduce an approach allowing the reliable in vitro generation of leukemia-reactive cytotoxic T lymphocytes (CTLs) from naive CD8+ T cells of healthy donors, leading to high cell numbers within a relatively short culture period. The protocol includes the stimulation of purified CD45RA+ CD8+ T cells with primary acute myeloid leukemia blasts of patient origin in HLA-class I-matched allogeneic mixed lymphocyte-leukemia cultures. The procedure allowed the isolation of a large diversity of HLA-A/-B/-C-restricted leukemia-reactive CTL clones and oligoclonal lines. CTLs showed reactivity to either leukemia blasts exclusively, or to leukemia blasts as well as patient-derived B lymphoblastoid-cell lines (LCLs). In contrast, LCLs of donor origin were not lysed. This reactivity pattern suggested that CTLs recognized leukemia-associated antigens or hematopoietic minor histocompatibility antigens. Consistent with this hypothesis, most CTLs did not react with patient-derived fibroblasts. The efficiency of the protocol could be further increased by addition of interleukin-21 during primary in vitro stimulation. Most importantly, leukemia-reactive CTLs retained the expression of early T-cell differentiation markers CD27, CD28, CD62L and CD127 for several weeks during culture. The effective in vitro expansion of leukemia-reactive CD8+ CTLs from naive CD45RA+ precursors of healthy donors can accelerate the molecular definition of candidate leukemia antigens and might be of potential use for the development of adoptive CTL therapy in leukemia.

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

IMPORTANCE OF THE FIELD

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

AREAS COVERED IN THIS REVIEW

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

WHAT THE READER WILL GAIN

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

TAKE HOME MESSAGE

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

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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