Transfer of specificity for human immunodeficiency virus type 1 into primary human T lymphocytes by introduction of T-cell receptor genes

Adoptive Cell Therapy for T-Cell Malignancies

T-cell malignancies are often aggressive and associated with poor prognoses. Adoptive cell therapy has recently shown promise as a new line of therapy for patients with hematological malignancies. However, there are currently challenges in applying adoptive cell therapy to T-cell malignancies. Various approaches have been examined in preclinical and clinical studies to overcome these obstacles. This review aims to provide an overview of the recent progress on adoptive cell therapy for T-cell malignancies. The benefits and drawbacks of different types of adoptive cell therapy are discussed. The potential advantages and current applications of innate immune cell-based adoptive cell therapy for T cell malignancies are emphasized.

Rapid identification of CMV-specific TCRs via reverse TCR cloning system based on bulk TCR repertoire data

Advances in next-generation sequencing (NGS) have improved the resolution of T-cell receptor (TCR) repertoire analysis, and recent single-cell sequencing has made it possible to obtain information about TCR pairs. In our previous study, cytomegalovirus (CMV) pp65-specific T-cell response restricted by a single human leukocyte antigen (HLA) class I allotype was observed in an individual. Therefore, to effectively clone an antigen-specific TCR from these T cells, we developed a TCR cloning system that does not require a single cell level. First, we established the improved Jurkat reporter cell line, which was TCRαβ double knock-out and expressed CD8αβ molecules. Furthermore, functional TCRs were directly obtained by reverse TCR cloning using unique CDR3-specific PCR primers after bulk TCR sequencing of activation marker-positive CD8 T cells by NGS. A total of 15 TCRα and 14 TCRβ strands were successfully amplified by PCR from cDNA of 4-1BB-positive CD8 T cells restricted by HLA-A*02:01, HLA-A*02:06, HLA-B*07:02, and HLA-B*40:06. The panels with combinations of TCRα and TCRβ genes were investigated using Jurkat reporter cell line and artificial antigen-presenting cells (APCs). In two TCR pairs restricted by HLA-A*02:01, one TCR pair by HLA-A*02:06, four TCR pairs by HLA-B*07:02, and one TCR pair by HLA-B*40:06, their specificity and affinity were confirmed. The TCR pair of A*02:01/1-1 showed alloreactivity to HLA-A*02:06. The one TCR pair showed a higher response to the naturally processed antigen than that of the peptide pool. This reverse TCR cloning system will not only provide functional information to TCR repertoire analysis by NGS but also help in the development of TCR-T therapy.

Engineering the Bridge between Innate and Adaptive Immunity for Cancer Immunotherapy: Focus on γδ T and NK Cells

Most studies on genetic engineering technologies for cancer immunotherapy based on allogeneic donors have focused on adaptive immunity. However, the main limitation of such approaches is that they can lead to severe graft-versus-host disease (GvHD). An alternative approach would bolster innate immunity by relying on the natural tropism of some subsets of the innate immune system, such as γδ T and natural killer (NK) cells, for the tumor microenvironment and their ability to kill in a major histocompatibility complex (MHC)-independent manner. γδ T and NK cells have the unique ability to bridge innate and adaptive immunity while responding to a broad range of tumors. Considering these properties, γδ T and NK cells represent ideal sources for developing allogeneic cell therapies. Recently, significant efforts have been made to exploit the intrinsic anti-tumor capacity of these cells for treating hematologic and solid malignancies using genetic engineering approaches such as chimeric antigen receptor (CAR) and T cell receptor (TCR). Here, we review over 30 studies on these two approaches that use γδ T and NK cells in adoptive cell therapy (ACT) for treating cancer. Based on those studies, we propose several promising strategies to optimize the clinical translation of these approaches.

Targeting cancers through TCR-peptide/MHC interactions

Adoptive T cell therapy has achieved dramatic success in a clinic, and the Food and Drug Administration approved two chimeric antigen receptor-engineered T cell (CAR-T) therapies that target hematological cancers in 2018. A significant issue faced by CAR-T therapies is the lack of tumor-specific biomarkers on the surfaces of solid tumor cells, which hampers the application of CAR-T therapies to solid tumors. Intracellular tumor-related antigens can be presented as peptides in the major histocompatibility complex (MHC) on the cell surface, which interact with the T cell receptors (TCR) on antigen-specific T cells to stimulate an anti-tumor response. Multiple immunotherapy strategies have been developed to eradicate tumor cells through targeting the TCR-peptide/MHC interactions. Here, we summarize the current status of TCR-based immunotherapy strategies, with particular focus on the TCR structure, activated signaling pathways, the effects and toxicity associated with TCR-based therapies in clinical trials, preclinical studies examining immune-mobilizing monoclonal TCRs against cancer (ImmTACs), and TCR-fusion molecules. We propose several TCR-based therapeutic strategies to achieve optimal clinical responses without the induction of autoimmune diseases.

T-cell-receptor cross-recognition and strategies to select safe T-cell receptors for clinical translation

Adoptive transfer of T-cell-receptor (TCR)-transduced T cells has shown promising results for cancer treatment, but has also produced severe immunotoxicities caused by on-target as well as off-target TCR recognition. Off-target toxicities are related to the ability of a single T cell to cross-recognize and respond to several different peptide–major histocompatibility complex (pMHC) antigens; a property that is essential for providing broad antigenic coverage despite a confined number of unique TCRs in the human body. However, this degeneracy makes it incredibly difficult to account for the range of targets that any TCR might recognize, which represents a major challenge for the clinical development of therapeutic TCRs. The prospect of using affinity-optimized TCRs has been impeded due to observations that affinity enhancement might alter the specificity of a TCR, thereby increasing the risk that it will cross-recognize endogenous tissue. Strategies for selecting safe TCRs for the clinic have included functional assessment after individual incubations with tissue-derived primary cells or with peptides substituted with single amino acids. However, these strategies have not been able to predict cross-recognition sufficiently, leading to fatal cross-reactivity in clinical trials. Novel technologies have emerged that enable extensive characterization of the exact interaction points of a TCR with pMHC, which provides a foundation from which to make predictions of the cross-recognition potential of individual TCRs. This review describes current advances in strategies for dissecting the molecular interaction points of TCRs, focusing on their potential as tools for predicting cross-recognition of TCRs in clinical development.

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T-cell-receptor (TCR) degeneracy plays a fundamental role in the capacity of our immune systems to recognize foreign antigens.

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TCR cross-reactivity provides an inherent risk in TCR–gene transfer cell therapies.

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Advances in description of TCR cross-recognition can guide the selection process for TCRs into clinical use.

Engineering antigen-specific NK cell lines against the melanoma-associated antigen tyrosinase via TCR gene transfer

Introduction of Chimeric Antigen Receptors to NK cells has so far been the main practical method for targeting NK cells to specific surface antigens. In contrast, T cell receptor (TCR) gene delivery can supply large populations of cytotoxic T-lymphocytes (CTL) targeted against intracellular antigens. However, a major barrier in the development of safe CTL-TCR therapies exists, wherein the mispairing of endogenous and genetically transferred TCR subunits leads to formation of TCRs with off-target specificity. To overcome this and enable specific intracellular antigen targeting, we have tested the use of NK cells for TCR gene transfer to human cells. Our results show that ectopic expression of TCR α/β chains, along with CD3 subunits, enables the functional expression of an antigen-specific TCR complex on NK cell lines NK-92 and YTS, demonstrated by using a TCR against the HLA-A2-restricted tyrosinase-derived melanoma epitope, Tyr_368-377 . Most importantly, the introduction of a TCR complex to NK cell lines enables MHC-restricted, antigen-specific killing of tumor cells both in vitro and in vivo. Targeting of NK cells via TCR gene delivery stands out as a novel tool in the field of adoptive immunotherapy which can also overcome the major hurdle of "mispairing" in TCR gene therapy.

Engineering Approaches in Human Gamma Delta T Cells for Cancer Immunotherapy

Sharing both innate and adaptive immune properties, γδT cells are attractive candidates for cellular engineering. As the cancer immunotherapy field becomes increasingly busy, orthogonal approaches are required to drive advancement. Engineering of alternative effector cell types such as γδT cells represents one such approach. γδT cells can be modified using many of the techniques used in αβT cell engineering, with the added advantage of innate-like tumor recognition and killing. Progress has been made in T-cell receptor transfer to and from γδT cells as well as in a number of chimeric antigen receptor-based strategies. As the cancer immunotherapy field moves beyond repetitive iteration of established constructs to more creative solutions, γδT cells may offer an attractive chassis to drive anti-tumor responses that are not only broader, but also possess a more favorable safety profile.

New approaches for the enhancement of chimeric antigen receptors for the treatment of HIV

HIV infection continues to be a life-long chronic disease in spite of the success of antiretroviral therapy (ART) in controlling viral replication and preventing disease progression. However, because of the high cost of treatment, severe side effects, and inefficiency in curing the disease with ART, there is a call for alternative therapies that will provide a functional cure for HIV. Cytotoxic T lymphocytes (CTLs) are vital in the control and clearance of viral infections and therefore immune-based therapies have attempted to engineer HIV-specific CTLs that would be able to clear the infection from the body. The development of chimeric antigen receptors (CARs) provides an opportunity to engineer superior HIV-specific CTLs that will be independent of the major histocompatibility complex for target recognition. A CD4-based CAR has been previously tested in clinical trials to test the antiviral efficacy of peripheral T cells armed with this CD4-based CAR. The results from these clinical trials showed the safety and feasibility of CAR T cell therapy for HIV infection; however, minimal antiviral efficacy was seen. In this review, we will discuss the various strategies being developed to enhance the therapeutic potency of anti-HIV CARs with the goal of generating superior antiviral responses that will lead to life-long HIV immunity and clearance of the virus from the body.

Genetic Modulation Therapy Through Stem Cell Transplantation for Human Immunodeficiency Virus 1 Infection

Highly active anti-retroviral treatment has changed the dimensions of the outcomes for patients suffering from human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS). However, HIV infection is still an ailment which is spreading throughout the world extensively. Given the confinements of the present restorative methodologies and the non-availability of any strategic vaccination against HIV, there is a squeezing need to build a therapeutic treatment.

Viral tropism for HIV includes CD4+ cells, macrophages, and microglial cells, and it is through binding with co-receptors C-C chemokine receptor type 5 (CCR5) and C-X-C chemokine receptor type 4 (CXCR4). While these cell types are present in all individuals, there are rare cases that stayed uninfected even after getting exposed to an overwhelming load of HIV. Research revealed a homozygous 32-base pair deletion (Δ32/Δ32) in CCR5. After careful consideration, a hypothesis was proposed a few years back that a cure for HIV disease is possible, through hematopoietic stem cells transplantation from a donor homozygous for the CCR5-Δ32 deletion.

Hematopoietic stem cell (HSC) based quality treatment may serve as a promising tool as these perpetual, self-renewing progenitor cells could be modified to oppose HIV infection. If done properly, the changed HSCs would offer the permanent creation of genetically modified cells that are resistant to HIV infection and/or have improved hostility to viral action which will eventually clear the contaminated cells.

The purpose of this review is to concentrate on two facets of HSC genetic treatment for potentially life-threatening HIV infection: building HIV-resistant cells and designing cells that can target HIV disease. These two strategic approaches can be the frontline of a quality treatment plan against HIV infection and, as an individual treatment or a combination thereof, has been proposed to possibly destroy HIV altogether.

TCR-based therapy for multiple myeloma and other B-cell malignancies targeting intracellular transcription factor BOB1

Immunotherapy for hematological malignancies or solid tumors by administration of monoclonal antibodies or T cells engineered to express chimeric antigen receptors or T-cell receptors (TCRs) has demonstrated clinical efficacy. However, antigen-loss tumor escape variants and the absence of currently targeted antigens on several malignancies hamper the widespread application of immunotherapy. We have isolated a TCR targeting a peptide of the intracellular B cell-specific transcription factor BOB1 presented in the context of HLA-B*07:02. TCR gene transfer installed BOB1 specificity and reactivity onto recipient T cells. TCR-transduced T cells efficiently lysed primary B-cell leukemia, mantle cell lymphoma, and multiple myeloma in vitro. We also observed recognition and lysis of healthy BOB1-expressing B cells. In addition, strong BOB1-specific proliferation could be demonstrated for TCR-modified T cells upon antigen encounter. Furthermore, clear in vivo antitumor reactivity was observed of BOB1-specific TCR-engineered T cells in a xenograft mouse model of established multiple myeloma. Absence of reactivity toward a broad panel of BOB1^- but HLA-B*07:02⁺ nonhematopoietic and hematopoietic cells indicated no off-target toxicity. Therefore, administration of BOB1-specific TCR-engineered T cells may provide novel cellular treatment options to patients with B-cell malignancies, including multiple myeloma.

Engineering HIV-Specific Immunity with Chimeric Antigen Receptors

HIV remains a highly important public health and clinical issue despite many recent advances in attempting to develop a cure, which has remained elusive for most people infected with HIV. HIV disease can be controlled with pharmacologic therapies; however, these treatments are expensive, may have severe side effects, and are not curative. Consequently, an improved means to control or eliminate HIV replication is needed. Cytotoxic T lymphocytes (CTLs) play a critical role in controlling viral replication and are an important part in the ability of the immune response to eradicate most viral infections. There are considerable efforts to enhance CTL responses in HIV-infected individuals in hopes of providing the immune response with armaments to more effectively control viral replication. In this review, we discuss some of these efforts and focus on the development of a gene therapy-based approach to engineer hematopoietic stem cells with an HIV-1-specific chimeric antigen receptor, which seeks to provide an inexhaustible source of HIV-1-specific immune cells that are MHC unrestricted and superior to natural antiviral T cell responses. These efforts provide the basis for further development of T cell functional enhancement to target and treat chronic HIV infection in hopes of eradicating the virus from the body.

Human CD8(+) T cells transduced with an additional receptor bispecific for both Mycobacterium tuberculosis and HIV-1 recognize both epitopes

Tuberculosis (TB) and human immunodeficiency virus type 1 (HIV-1) infection are closely intertwined, with one-quarter of TB/HIV coinfected deaths among people died of TB. Effector CD8(+) T cells play a crucial role in the control of Mycobacterium tuberculosis (MTB) and HIV-1 infection in coinfected patients. Adoptive transfer of a multitude of effector CD8(+) T cells is an appealing strategy to impose improved anti-MTB/HIV-1 activity onto coinfected individuals. Due to extensive existence of heterologous immunity, that is, T cells cross-reactive with peptides encoded by related or even very dissimilar pathogens, it is reasonable to find a single T cell receptor (TCR) recognizing both MTB and HIV-1 antigenic peptides. In this study, a single TCR specific for both MTB Ag85B199-207 peptide and HIV-1 Env120-128 peptide was screened out from peripheral blood mononuclear cells of a HLA-A*0201(+) healthy individual using complementarity determining region 3 spectratype analysis and transferred to primary CD8(+) T cells using a recombinant retroviral vector. The bispecificity of the TCR gene-modified CD8(+) T cells was demonstrated by elevated secretion of interferon-γ, tumour necrosis factor-α, granzyme B and specific cytolytic activity after antigen presentation of either Ag85B199-207 or Env120-128 by autologous dendritic cells. To the best of our knowledge, this study is the first report proposing to produce responses against two dissimilar antigenic peptides of MTB and HIV-1 simultaneously by transfecting CD8(+) T cells with a single TCR. Taken together, T cells transduced with the additional bispecific TCR might be a useful strategy in immunotherapy for MTB/HIV-1 coinfected individuals.

T cell engineering as therapy for cancer and HIV: our synthetic future

It is now well established that the immune system can control and eliminate cancer cells. Adoptive T cell transfer has the potential to overcome the significant limitations associated with vaccine-based strategies in patients who are often immune compromised. Application of the emerging discipline of synthetic biology to cancer, which combines elements of genetic engineering and molecular biology to create new biological structures with enhanced functionalities, is the subject of this overview. Various chimeric antigen receptor designs, manufacturing processes and study populations, among other variables, have been tested and reported in recent clinical trials. Many questions remain in the field of engineered T cells, but the encouraging response rates pave a wide road for future investigation into fields as diverse as cancer and chronic infections.

Single injection of CD8+ T lymphocytes derived from hematopoietic stem cells - Mathematical and numerical insights

Recently, hematopoietic stem cell (HSC) based therapy is being discussed as a possible treatment for HIV infection. The main advantage of this approach is that it limits the immune impairing effect of infection by introducing an independent influx of antigen-specific cytotoxic T lymphocytes (CTL). In this paper, we present a mathematical approach to predict the dynamics of HSC based therapy. We use a modification of a basic mathematical model for virus induced impairment of help to study how virus - immune system dynamics can be influenced by a single injection of CD8+ T lymphocytes derived from hematopoietic stem cells. Our mathematical and numerical results indicate that a single, large enough dose of genetically derived CTL may lead to restoration of the cellular immune response and result in long-term control of infection.

Cell-based gene therapy against HIV

The ability to integrate inside the host genome lays a strong foundation for HIV to play hide and seek with the host's immune surveillance mechanisms. Present anti-viral therapies, although successful in suppressing the virus to a certain level, fail to wipe it out completely. However, recent approaches in modifying stem cells and enabling them to give rise to potent/resistant T-cells against HIV holds immense hope for eradication of the virus from the host. In this review, we will briefly discuss previous landmark studies on engineering stem cells or T-cells that have been explored for therapeutic efficacy against HIV. We will also analyze potential benefits and pitfalls of some studies done recently and will share our opinion on emerging trends.

Expression of chimeric receptor CD4ζ by natural killer cells derived from human pluripotent stem cells improves in vitro activity but does not enhance suppression of HIV infection in vivo

Cell-based immunotherapy has been gaining interest as an improved means to treat human immunodeficiency virus (HIV)/AIDS. Human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) could become a potential resource. Our previous studies have shown hESC and iPSC-derived natural killer (NK) cells can inhibit HIV-infected targets in vitro. Here, we advance those studies by expressing a HIV chimeric receptor combining the extracellular portion of CD4 to the CD3ζ intracellular signaling chain. We hypothesized that expression of this CD4ζ receptor would more efficiently direct hESC- and iPSC-derived NK cells to target HIV-infected cells. In vitro studies showed the CD4ζ expressing hESC- and iPSC-NK cells inhibited HIV replication in CD4+ T-cells more efficiently than their unmodified counterparts. We then evaluated CD4ζ expressing hESC (CD4ζ-hESC)- and iPSC-NK cells in vivo anti-HIV activity using a humanized mouse model. We demonstrated significant suppression of HIV replication in mice treated with both CD4ζ-modified and -unmodified hESC-/iPSC-NK cells compared with control mice. However, we did not observe significantly increased efficacy of CD4ζ expression in suppression of HIV infection. These studies indicate that hESC/iPSC-based immunotherapy can be used as a unique resource to target HIV/AIDS.

Human MHC Class I-restricted high avidity CD4+ T cells generated by co-transfer of TCR and CD8 mediate efficient tumor rejection in vivo

In this study, we generated human MHC Class I-restricted CD4⁺ T cells specific for Epstein-Barr virus (EBV) and cytomegalovirus (CMV), two herpesviridae associated with lymphoma, nasopharyngeal carcinoma and medulloblastoma, respectively. Retroviral transfer of virus-specific, HLA-A2-restricted TCR-coding genes generated CD4⁺ T cells that recognized HLA-A2/peptide multimers and produced cytokines when stimulated with MHC Class II-deficient cells presenting the relevant viral peptides in the context of HLA-A2. Peptide titration revealed that CD4⁺ T cells had a 10-fold lower avidity than CD8⁺ T cells expressing the same TCR. The impaired avidity of CD4⁺ T cells was corrected by simultaneously transferring TCR- and CD8-coding genes. The CD8 co-receptor did not alter the cytokine signature of CD4⁺ T cells, which remained distinct from that of CD8⁺ T cells. Using the xenogeneic NOD/SCID mouse model, we demonstrated that human CD4⁺ T cells expressing a specific TCR and CD8 can confer efficient protection against the growth of tumors expressing the EBV or CMV antigens recognized by the TCR. In summary, we describe a robust approach for generating therapeutic CD4⁺ T cells capable of providing MHC Class I-restricted immunity against MHC Class II-negative tumors in vivo.

Adoptive immunotherapy with genetically modified lymphocytes in allogeneic stem cell transplantation

Hematopoietic stem cell transplantation from a healthy donor (allo-HSCT) represents the most potent form of cellular adoptive immunotherapy to treat malignancies. In allo-HSCT, donor T cells are double edge-swords: highly potent against residual tumor cells, but potentially highly toxic, and responsible for graft versus host disease (GVHD), a major clinical complication of transplantation. Gene transfer technologies coupled with current knowledge on cancer immunology have generated a wide range of approaches aimed at fostering the immunological response to cancer cells, while avoiding or controlling GVHD. In this review, we discuss cell and gene therapy approaches currently tested in preclinical models and in clinical trials.

RNAi-mediated TCR knockdown prevents autoimmunity in mice caused by mixed TCR dimers following TCR gene transfer

Genetically modified T cells that express a transduced T cell receptor (TCR) α/β heterodimer in addition to their endogenous TCR are used in clinical studies to treat cancer. These cells express two TCR-α and two TCR-β chains that do not only compete for CD3 proteins but also form potentially self-reactive mixed TCR dimers, composed of endogenous and transferred chains. To overcome these deficits, we developed an RNAi-TCR replacement vector that simultaneously silences the endogenous TCR and expresses an RNAi-resistant TCR. Transduction of the virus-specific P14 TCR without RNAi resulted in unequal P14 TCR-α and -β chain surface levels, indicating heterodimerization with endogenous TCR chains. Such unequal expression was also observed following TCR gene optimization. Equal surface levels of the introduced TCR chains were however achieved by silencing the endogenous TCR. Importantly, all mice that received cells transduced with the native or optimized P14 TCR developed lethal TCR gene transfer-induced graft-versus-host-disease (TI-GVHD) due to formation of mixed TCR dimers. In contrast, TI-GVHD was almost completely prevented when using the RNAi-TCR replacement vector. Our data demonstrate that RNAi-assisted TCR replacement reduces the formation of mixed TCR dimers, and thereby significantly reduces the risk of TI-GVHD in TCR gene therapy.

Immune escape from NY-ESO-1-specific T-cell therapy via loss of heterozygosity in the MHC

Adoptive immunotherapy of tumors with T cells specific for the cancer-testis antigen NY-ESO-1 has shown great promise in preclinical models and in early stage clinical trials. Tumor persistence or recurrence after NY-ESO-1-specific therapy occurs, however, and the mechanisms of recurrence remain poorly defined. In a murine xenograft model of NY-ESO-1⁺ multiple myeloma, we observed tumor recurrence after adoptive transfer of CD8⁺ T cells genetically redirected to the prototypic NY-ESO-1_157-165 peptide presented by HLA-A*02:01. Analysis of the myeloma cells that had escaped from T cell control revealed intact expression of NY-ESO-1 and B2M, but selective, complete loss of HLA-A*02:01 expression from the cell surface. Loss of heterozygosity in the Major Histocompatibility Complex (MHC) involving the HLA-A locus was identified in the tumor cells, and further analysis revealed selective loss of the allele encoding HLA-A*02:01. Although loss of heterozygosity involving the MHC has not been described in myeloma patients with persistent or recurrent disease after immune therapies such as allogeneic hematopoietic cell transplantation (HCT), it has been described in patients with acute myelogenous leukemia who relapsed after allogeneic HCT. These results suggest that MHC loss should be evaluated in patients with myeloma and other cancers who relapse after adoptive NY-ESO-1-specific T cell therapy.

Stem-cell-based gene therapy for HIV infection

Despite the enormous success of combined anti-retroviral therapy, HIV infection is still a lifelong disease and continues to spread rapidly worldwide. There is a pressing need to develop a treatment that will cure HIV infection. Recent progress in stem cell manipulation and advancements in humanized mouse models have allowed rapid developments of gene therapy for HIV treatment. In this review, we will discuss two aspects of HIV gene therapy using human hematopoietic stem cells. The first is to generate immune systems resistant to HIV infection while the second strategy involves enhancing anti-HIV immunity to eliminate HIV infected cells.

Extracellular domains of CD8α and CD8ß subunits are sufficient for HLA class I restricted helper functions of TCR-engineered CD4(+) T cells

By gene transfer of HLA-class I restricted T-cell receptors (TCRs) (HLA-I-TCR) into CD8⁺ as well as CD4⁺ T-cells, both effector T-cells as well as helper T-cells can be generated. Since most HLA-I-TCRs function best in the presence of the CD8 co-receptor, the CD8αß molecule has to be co-transferred into the CD4⁺ T-cells to engineer optimal helper T-cells. In this study, we set out to determine the minimal part of CD8αβ needed for optimal co-receptor function in HLA-I-TCR transduced CD4⁺ T-cells. For this purpose, we transduced human peripheral blood derived CD4⁺ T-cells with several HLA-class I restricted TCRs either with or without co-transfer of different CD8 subunits. We demonstrate that the co-transduced CD8αβ co-receptor in HLA-I-TCR transduced CD4⁺ T-cells behaves as an adhesion molecule, since for optimal antigen-specific HLA class I restricted CD4⁺ T-cell reactivity the extracellular domains of the CD8α and ß subunits are sufficient.

Current translational and clinical practices in hematopoietic cell and gene therapy

Clinical trials over the last 15 years have demonstrated that cell and gene therapies for cancer, monogenic and infectious disease are feasible and can lead to long-term benefit for patients. However, these trials have been limited to proof-of-principle and were conducted on modest numbers of patients or over long periods of time. In order for these studies to move towards standard practice and commercialization, scalable technologies for the isolation, ex vivo manipulation and delivery of these cells to patients must be developed. Additionally, regulatory strategies and clinical protocols for the collection, creation and delivery of cell products must be generated. In this article we review recent progress in hematopoietic cell and gene therapy, describe some of the current issues facing the field and discuss clinical, technical and regulatory approaches used to navigate the road to product development.

T cell-based gene therapy of cancer

Adoptive immunotherapy using gene engineered T cells is a promising and rapidly evolving field, and the ability to engineer T cells to manifest desired phenotypes and functions has become a practical reality. In this review, we describe and summarize current thought about gene engineering of T cells. We focus on the identified requirements for the successful application of T cell based immunotherapy and discuss gene-therapy based strategies that address these requirements and have the potential to enhance the successful implementation of this promising approach to treat cancer.

Probing the effector and suppressive functions of human T cell subsets using antigen-specific engineered T cell receptors

Activation of T cells through the engagement of the T cell receptors (TCRs) with specific peptide-MHC complexes on antigen presenting cells (APCs) is the major determinant for their proliferation, differentiation and display of effector functions. To assess the role of quantity and quality of peptide-MHC presentation in eliciting T cell activation and suppression functions, we genetically engineered human T cells with two TCRs that recognize HLA-A*0201-restricted peptides derived from either HIV or melanoma antigens. The engineered-TCRs are highly functional in both CD8(+) and CD4(+) T cells as assessed by the upregulation of activation markers, induction of cytokine secretion and cytotoxicity. We further demonstrated that engineered-TCRs can also be expressed on naïve human T cells, which are stimulated through APCs presenting specific peptides to induce T cell proliferation and acquire effector functions. Furthermore, regulatory T cells (Tregs) ectopically expressing the engineered-TCRs are activated in an antigen-specific fashion and suppress T cell proliferation. In this system, the inhibitory activity of peptide-stimulated Tregs require the presence of dendritic cells (DCs) in the culture, either as presenters or as bystander cells, pointing to a critical role for DCs in suppression by Tregs. In conclusion, the engineered-TCR system reported here advances our ability to understand the differentiation pathways of naïve T cells into antigen-specific effector cells and the role of antigen-specific signaling in Treg-mediated immune suppression.

Multi-cistronic vector encoding optimized safety switch for adoptive therapy with T-cell receptor-modified T cells

T-cell receptor (TCR) gene transfer is an attractive strategy to equip T cells with defined antigen-specific TCRs using short-term in vitro procedures to target both hematological malignancies and solid tumors. TCR gene transfer poses different safety issues that might warrant the inclusion of a suicide gene. High affinity TCRs may result in on-target toxicity, and off-target reactivity directed against healthy tissue can be observed due to mixed TCR dimers. Inclusion of a suicide gene as a safety switch may abrogate these unwanted toxicities. Human CD20 has been proposed as a nonimmunogenic suicide gene targeted by widely used clinical-grade anti-CD20 antibodies that can additionally function as a selection marker. However, transduction of T cells with a multi-cistronic vector encoding both TCR and CD20 resulted in poor coexpression. In this study, we demonstrated that codon optimization of TCR and CD20 resulted in profound coexpression of both the preferentially expressed antigen in melanoma (PRAME)-TCR and CD20, allowing selective as well as efficient elimination of these engineered T cells in vitro. These results demonstrate the great potential of codon optimized CD20 to be broadly used in clinical trials as a safety switch.

How (specific) would like your T-cells today? Generating T-cell therapeutic function through TCR-gene transfer

T-cells are central players in the immune response against both pathogens and cancer. Their specificity is solely dictated by the T-cell receptor (TCR) they clonally express. As such, the genetic modification of T lymphocytes using pathogen- or cancer-specific TCRs represents an appealing strategy to generate a desired immune response from peripheral blood lymphocytes. Moreover, notable objective clinical responses were observed in terminally ill cancer patients treated with TCR-gene modified cells in several clinical trials conducted recently. Nevertheless, several key aspects of this approach are the object of intensive research aimed at improving the reliability and efficacy of this strategy. Herein, we will survey recent studies in the field of TCR-gene transfer dealing with the improvement of this approach and its application for the treatment of malignant, autoimmune, and infectious diseases.

Magnetic-activated cell sorting of TCR-engineered T cells, using tCD34 as a gene marker, but not peptide-MHC multimers, results in significant numbers of functional CD4+ and CD8+ T cells

T cell-sorting technologies with peptide-MHC multimers or antibodies against gene markers enable enrichment of antigen-specific T cells and are expected to enhance the therapeutic efficacy of clinical T cell therapy. However, a direct comparison between sorting reagents for their ability to enrich T cells is lacking. Here, we compared the in vitro properties of primary human T cells gene-engineered with gp100(280-288)/HLA-A2-specific T cell receptor-αβ (TCRαβ) on magnetic-activated cell sorting (MACS) with various peptide-MHC multimers or an antibody against truncated CD34 (tCD34). With respect to peptide-MHC multimers, we observed that Streptamer(®), when compared with pentamers and tetramers, improved T cell yield as well as level and stability of enrichment, of TCR-engineered T cells (>65% of peptide-MHC-binding T cells, stable for at least 6 weeks). In agreement with these findings, Streptamer, the only detachable reagent, revealed significant T cell expansion in the first week after MACS. Sorting TCR and tCD34 gene-engineered T cells with CD34 monoclonal antibody (mAb) resulted in the most significant T cell yield and enrichment of T cells (>95% of tCD34 T cells, stable for at least 6 weeks). Notably, T cells sorted with CD34 mAb, when compared with Streptamer, bound about 2- to 3-fold less peptide-MHC but showed superior antigen-specific upregulated expression of CD107a and production of interferon (IFN)-γ. Multiparametric flow cytometry revealed that CD4(+) T cells, uniquely present in CD34 mAb-sorted T cells, contributed to enhanced IFN-γ production. Taken together, we postulate that CD34 mAb-based sorting of gene-marked T cells has benefits toward applications of T cell therapy, especially those that require CD4(+) T cells.

In vivo suppression of HIV by antigen specific T cells derived from engineered hematopoietic stem cells

The HIV-specific cytotoxic T lymphocyte (CTL) response is a critical component in controlling viral replication in vivo, but ultimately fails in its ability to eradicate the virus. Our intent in these studies is to develop ways to enhance and restore the HIV-specific CTL response to allow long-term viral suppression or viral clearance. In our approach, we sought to genetically manipulate human hematopoietic stem cells (HSCs) such that they differentiate into mature CTL that will kill HIV infected cells. To perform this, we molecularly cloned an HIV-specific T cell receptor (TCR) from CD8+ T cells that specifically targets an epitope of the HIV-1 Gag protein. This TCR was then used to genetically transduce HSCs. These HSCs were then introduced into a humanized mouse containing human fetal liver, fetal thymus, and hematopoietic progenitor cells, and were allowed to differentiate into mature human CD8+ CTL. We found human, HIV-specific CTL in multiple tissues in the mouse. Thus, genetic modification of human HSCs with a cloned TCR allows proper differentiation of the cells to occur in vivo, and these cells migrate to multiple anatomic sites, mimicking what is seen in humans. To determine if the presence of the transgenic, HIV-specific TCR has an effect on suppressing HIV replication, we infected with HIV-1 mice expressing the transgenic HIV-specific TCR and, separately, mice expressing a non-specific control TCR. We observed significant suppression of HIV replication in multiple organs in the mice expressing the HIV-specific TCR as compared to control, indicating that the presence of genetically modified HIV-specific CTL can form a functional antiviral response in vivo. These results strongly suggest that stem cell based gene therapy may be a feasible approach in the treatment of chronic viral infections and provide a foundation towards the development of this type of strategy.

There is a desperate need for the development of new therapeutic strategies to eradicate HIV infection. HIV actively subverts the potent natural immune responses against it, particularly cellular cytotoxic T lymphocyte (CTL) responses. The development of a therapy that allows long-lived immune self-containment of HIV and restoration of these CTL responses by the host would be ideal. Through genetic manipulation of human blood-forming stem cells, we introduced a molecule– an HIV-targeting T cell receptor (TCR)–that allowed the generation of functional HIV-specific CTLs following differentiation within human tissues in a humanized mouse model. To assess if these newly developed, HIV-specific CTLs can allow active suppression of HIV replication, we infected these mice with HIV. We found that the development of genetically modified, HIV-specific CTLs in these mice results in the presence of a functional antiviral CTL response in vivo that significantly lowers viral replication following HIV infection. These results have strong implications for the use of this technology to engineer the human immune response to combat viral infections and suggest that genetic engineering via HSCs may allow tailoring of the immune response to target and eradicate HIV.

Redirecting T cells to Ewing's sarcoma family of tumors by a chimeric NKG2D receptor expressed by lentiviral transduction or mRNA transfection

We explored the possibility to target Ewing's sarcoma family of tumors (ESFT) by redirecting T cells. To this aim, we considered NKG2D-ligands (NKG2D-Ls) as possible target antigens. Detailed analysis of the expression of MICA, MICB, ULBP-1, -2, and -3 in fourteen ESFT cell lines revealed consistent expression of at least one NKG2D-L. Thus, for redirecting T cells, we fused a CD3ζ/CD28-derived signaling domain to the ectodomain of NKG2D, however, opposite transmembrane orientation of this signaling domain and NKG2D required inverse orientation fusion of either of them. We hypothesized that the particularly located C-terminus of the NKG2D ectodomain should allow reengineering of the membrane anchoring from a native N-terminal to an artificial C-terminal linkage. Indeed, the resulting chimeric NKG2D receptor (chNKG2D) was functional and efficiently mediated ESFT cell death triggered by activated T cells. Notably, ESFT cells with even low NKG2D-L expression were killed by CD8(pos) and also CD4(pos) cells. Both, mRNA transfection and lentiviral transduction resulted in high level surface expression of chNKG2D. However, upon target-cell recognition receptor surface levels were maintained by tranfected RNA only during the first couple of hours after transfection. Later, target-cell contact resulted in strong and irreversible receptor down-modulation, whereas lentivirally mediated expression of chNKG2D remained constant under these conditions. Together, our study defines NKG2D-Ls as targets for a CAR-mediated T cell based immunotherapy of ESFT. A comparison of two different methods of gene transfer reveals strong differences in the susceptibility to ligand-induced receptor down-modulation with possible implications for the applicability of RNA transfection.

Muscle CARs and TcRs: turbo-charged technologies for the (T cell) masses

A central role for T cells in the control of cancer has been supported by both animal models and clinical observations. Accordingly, the development of potent anti-tumor T cell immunity has been a long-standing objective of immunotherapy. Emerging data from clinical trials that test T cell immune-modulatory agents and genetically engineered and re-targeted T cells have begun to realize the profound potential of T cell immunotherapy to target cancer. This review will focus on a description of recent conceptual and technological advances for the genetic engineering of T cells to enhance anti-tumor T cell immunity through the introduction of tumor-specific receptors, both Chimeric Antigen Receptors (CAR) and T cell receptors (TcR), as well as an overview of emerging data from ongoing clinical trials that highlight the potential of these approaches to effect dramatic and potent anti-tumor immunity.

Transduction of SIV-specific TCR genes into rhesus macaque CD8+ T cells conveys the ability to suppress SIV replication

BACKGROUND

The SIV/rhesus macaque model for HIV/AIDS is a powerful system for examining the contribution of T cells in the control of AIDS viruses. To better our understanding of CD8(+) T-cell control of SIV replication in CD4(+) T cells, we asked whether TCRs isolated from rhesus macaque CD8(+) T-cell clones that exhibited varying abilities to suppress SIV replication could convey their suppressive properties to CD8(+) T cells obtained from an uninfected/unvaccinated animal.

PRINCIPAL FINDINGS

We transferred SIV-specific TCR genes isolated from rhesus macaque CD8(+) T-cell clones with varying abilities to suppress SIV replication in vitro into CD8(+) T cells obtained from an uninfected animal by retroviral transduction. After sorting and expansion, transduced CD8(+) T-cell lines were obtained that specifically bound their cognate SIV tetramer. These cell lines displayed appropriate effector function and specificity, expressing intracellular IFNγ upon peptide stimulation. Importantly, the SIV suppression properties of the transduced cell lines mirrored those of the original TCR donor clones: cell lines expressing TCRs transferred from highly suppressive clones effectively reduced wild-type SIV replication, while expression of a non-suppressing TCR failed to reduce the spread of virus. However, all TCRs were able to suppress the replication of an SIV mutant that did not downregulate MHC-I, recapitulating the properties of their donor clones.

CONCLUSIONS

Our results show that antigen-specific SIV suppression can be transferred between allogenic T cells simply by TCR gene transfer. This advance provides a platform for examining the contributions of TCRs versus the intrinsic effector characteristics of T-cell clones in virus suppression. Additionally, this approach can be applied to develop non-human primate models to evaluate adoptive T-cell transfer therapy for AIDS and other diseases.

Human T cells expressing affinity-matured TCR display accelerated responses but fail to recognize low density of MHC-peptide antigen

We have tested whether affinity-matured TCRs that retain peptide specificity improve the ability of primary human CD8+ T cells to mount antigen-specific responses. We found that TCR affinity correlated with the speed of T-cell responses. High affinity TCR–antigen interactions rapidly initiated T-cell responses, but low affinity TCR/antigen interactions required longer time periods to elicit the same responses. Within the “natural” affinity range, increased TCR-to-antigen affinity correlated with improved ability of T cells to recognize low concentration of antigen. However, affinity-matured TCR with 700-fold enhanced affinity for MHC-to-antigen required 100-fold higher antigen-density to initiate T-cell responses than did wild-type TCR. Using modified peptides to reduce the affinity of TCR-to-antigen interaction, we demonstrate that affinity-matured TCRs are not defective, being superior to wild-type TCR in recognizing low concentration of modified peptides. These data indicate that enhancing TCR affinity can accelerate the speed of T-cell activation and reduce the ability to recognize low density of MHC-to-peptide antigen. We predict that future studies of the human T-cell repertoire will reveal 2 types of low avidity T cells: fast and slow responders, with high-affinity and low-affinity TCR, respectively.

CD3 limits the efficacy of TCR gene therapy in vivo

The function of T-cell receptor (TCR) gene modified T cells is dependent on efficient surface expression of the introduced TCR α/β heterodimer. We tested whether endogenous CD3 chains are rate-limiting for TCR expression and antigen-specific T-cell function. We show that co-transfer of CD3 and TCR genes into primary murine T cells enhanced TCR expression and antigen-specific T-cell function in vitro. Peptide titration experiments showed that T cells expressing introduced CD3 and TCR genes recognized lower concentration of antigen than T cells expressing TCR only. In vivo imaging revealed that TCR+CD3 gene modified T cells infiltrated tumors faster and in larger numbers, which resulted in more rapid tumor elimination compared with T cells modified by TCR only. After tumor clearance, TCR+CD3 engineered T cells persisted in larger numbers than TCR-only T cells and mounted a more effective memory response when rechallenged with antigen. The data demonstrate that provision of additional CD3 molecules is an effective strategy to enhance the avidity, anti-tumor activity and functional memory formation of TCR gene modified T cells in vivo.

T-cell receptor gene therapy: critical parameters for clinical success

T-cell receptor (TCR) gene therapy aims to induce immune reactivity against tumors by introducing genes encoding a tumor-reactive TCR into patient T cells. This approach has been extensively tested in preclinical mouse models, and initial clinical trials have demonstrated the feasibility and potential of TCR gene therapy as a cancer treatment. However, data obtained from preclinical and clinical studies suggest that both the therapeutic efficacy and the safety of TCR gene therapy can be and needs to be further enhanced. This review highlights those strategies that can be followed to develop TCR gene therapy into a clinically relevant treatment option for cancer patients.

Stem cell-based approaches to treating HIV infection

PURPOSE OF REVIEW

Stem cell-based strategies for treating HIV-infected individuals represent a novel approach toward reconstituting the ravaged immune system with the ultimate aim of clearing the virus from the body. Genetic modification of human hematopoietic stem cells to produce cells that are either resistant to infection, cells that produce lower amounts of infectious virus, or cells that specifically target the immune response against the virus are currently the dominant strategies under development. This review focuses on the understanding of stem cell-based approaches that are under investigation and the rationale behind such approaches.

RECENT FINDINGS

Significant progress has recently been made utilizing stem cell-based approaches to treat HIV infection. Ideally, a successful strategy would result in immune clearance of the virus from the body as well long-term restoration of overall immune responses to successfully combat everyday environmental antigens. Two recent clinical trails illustrate how new advances in stem cell-based approaches may propel this field forward to clinical reality: one that demonstrates that large-scale gene therapy trials can be performed in a conventional, reproducible manner; and one that demonstrates the utilization of a multipronged approach using lentiviral-based gene therapy vectors. These clinical trails serve as the foundation for the development of other technologies, discussed here, that are currently in preclinical development.

SUMMARY

Recent advances using stem cell-based approaches to treat HIV infection have provided the impetus for a renewed and expanded interest in the development of new cell-based strategies to treat HIV infection as well as a variety of other diseases.

Potential clinical applications for human pluripotent stem cell-derived blood components

The ability of human embryonic stem cells (hESCs) and induced pluripotent stem cells (iPSCs) to divide indefinitely without losing pluripotency and to theoretically differentiate into any cell type in the body makes them highly attractive cell sources for large scale regenerative medicine purposes. The current use of adult stem cell-derived products in hematologic intervention sets an important precedent and provides a guide for developing hESC/iPSC based therapies for the blood system. In this review, we highlight biological functions of mature cells of the blood, clinical conditions requiring the transfusion or stimulation of these cells, and the potential for hESC/iPSC-derivatives to serve as functional replacements. Many researchers have already been able to differentiate hESCs and/or iPSCs into specific mature blood cell types. For example, hESC-derived red blood cells and platelets are functional in tasks such as oxygen delivery and blood clotting, respectively and may be able to serve as substitutes for their donor-derived counterparts in emergencies. hESC-derived dendritic cells are functional in antigen-presentation and may be used as off-the-shelf vaccine therapies to stimulate antigen-specific immune responses against cancer cells. However, in vitro differentiation systems used to generate these cells will need further optimization before hESC/iPSC-derived blood components can be used clinically.

Exploiting T cells specific for human minor histocompatibility antigens for therapy of leukemia

Minor histocompatibility (H) antigens are major targets of a graft-versus-leukemia (GVL) effect mediated by donor CD8(+) and CD4(+) T cells following allogeneic hematopoietic cell transplantation (HCT) between human leukocyte antigen identical individuals. In the 15 years since the first molecular characterization of human minor H antigens, significant strides in minor H antigen discovery have been made as a consequence of advances in cellular, genetic and molecular techniques. Much has been learned about the mechanisms of minor H antigen immunogenicity, their expression on normal and malignant cells, and their role in GVL responses. T cells specific for minor H antigens expressed on leukemic cells, including leukemic stem cells, can be isolated and expanded in vitro and infused into allogeneic HCT recipients to augment the GVL effect to prevent and treat relapse. The first report of the adoptive transfer of minor H antigen-specific T-cell clones to patients with leukemic relapse in 2010 illustrates the potential for the manipulation of alloreactivity for therapeutic benefit. This review describes the recent developments in T-cell recognition of human minor H antigens, and efforts to translate these discoveries to reduce leukemia relapse after allogeneic HCT.

Exploiting T cells specific for human minor histocompatibility antigens for therapy of leukemia

Minor histocompatibility (H) antigens are major targets of a graft-versus-leukemia (GVL) effect mediated by donor CD8(+) and CD4(+) T cells following allogeneic hematopoietic cell transplantation (HCT) between human leukocyte antigen identical individuals. In the 15 years since the first molecular characterization of human minor H antigens, significant strides in minor H antigen discovery have been made as a consequence of advances in cellular, genetic and molecular techniques. Much has been learned about the mechanisms of minor H antigen immunogenicity, their expression on normal and malignant cells, and their role in GVL responses. T cells specific for minor H antigens expressed on leukemic cells, including leukemic stem cells, can be isolated and expanded in vitro and infused into allogeneic HCT recipients to augment the GVL effect to prevent and treat relapse. The first report of the adoptive transfer of minor H antigen-specific T-cell clones to patients with leukemic relapse in 2010 illustrates the potential for the manipulation of alloreactivity for therapeutic benefit. This review describes the recent developments in T-cell recognition of human minor H antigens, and efforts to translate these discoveries to reduce leukemia relapse after allogeneic HCT.

Immunotherapeutic strategies: the melanoma example

T-cell-based immunotherapy can be induced by nonspecific activation, by antigen-specific immunization, or by adoptive immunotherapy. In this review, progress in these areas is discussed as based on data from clinical trials for the treatment of metastatic melanoma. Nonspecific immunotherapy has been shown to result in low, but in some cases significant, levels of objective tumor responses, and is often associated with autoimmune reactions. Antigen-specific targeting of tumors via vaccination has only resulted in low to very low levels of objective responses, and these strategies seem to have most value when the T-cell repertoire is not affected by tolerance. Finally, adoptive immunotherapy can be applied by in vitro expansion of autologous lymphocytes that have escaped tolerance or by genetic transfer of allogeneic T-cell receptors (TCRs). Autologous adoptive T-cell transfer has resulted in a very high frequency of clinical responses when combined with chemotherapy and IL-2 administration in single-center studies. Although TCR gene transfer has, until now, only resulted in a low frequency of clinical responses, it does have a broader application potential, and optimization of this strategy is likely to improve its efficacy.

Stem cell-based anti-HIV gene therapy

Human stem cell-based therapeutic intervention strategies for treating HIV infection have recently undergone a renaissance as a major focus of investigation. Unlike most conventional antiviral therapies, genetically engineered hematopoietic stem cells possess the capacity for prolonged self-renewal that would continuously produce protected immune cells to fight against HIV. A successful strategy therefore has the potential to stably control and ultimately eradicate HIV from patients by a single or minimal treatment. Recent progress in the development of new technologies and clinical trials sets the stage for the current generation of gene therapy approaches to combat HIV infection. In this review, we will discuss two major approaches that are currently underway in the development of stem cell-based gene therapy to target HIV: one that focuses on the protection of cells from productive infection with HIV, and the other that focuses on targeting immune cells to directly combat HIV infection.

Generation of diffuse large B cell lymphoma-associated antigen-specific Vα6/Vβ13+T cells by TCR gene transfer

Background

Our previous study had amplified antigen-specific full-length TCR α and β genes of clonally expanded T cells in the peripheral blood (PB) of patients with diffuse large B-cell lymphoma (DLBCL). The transfer of T cell receptor (TCR) genes endows T cells with new antigen specificity. Therefore, the aim of this study is to generate diffuse large B cell lymphoma (DLBCL)-specific T cells by T cell receptor (TCR) gene transfer.

Materials and methods

Two different eukaryotic expression plasmids harboring TCR Vα6 and TCR Vβ13 genes specific for DLBCL-associated antigens were constructed and subsequently transferred into human T cells using Nucleofector™ technique. The expression of targeted genes in TCR gene-modified cells was detected by real-time PCR, and western blot using TCR Vβ antibody. The specific cytotoxicity of TCR gene-transferred T cells in vitro was estimated using a lactate dehydrogenase (LDH) release assay.

Results

Two different eukaryotic expression plasmids harboring TCR Vα6 and TCR Vβ13 genes specific for DLBCL-associated antigens were constructed and subsequently transferred into T cells from healthy donors. Specific anti-DLBCL cytotoxic T lymphocytes (CTL) could be induced by transduction of specific TCR gene to modify healthy T cells. The transgene cassette of TCR Vβ13-IRES-TCR Vα6 was superior to the other in the function of TCR-redirected T cells.

Conclusions

Specific anti-DLBCL cytotoxic T lymphocyte (CTL) could be inducted by transduction of specific TCR gene to modify healthy T cells.

Fine-tuning of T-cell receptor avidity to increase HIV epitope variant recognition by cytotoxic T lymphocytes

OBJECTIVE

T-cell receptor (TCR) gene therapy is an approach being considered for HIV-1, but epitope mutation is a significant barrier. We assessed whether HIV-specific TCR can be modified to have broader coverage of epitope variants by recombining polymorphisms between public clonotype TCR sequences.

DESIGN

Public clonotype TCRs recognizing the same epitope often differ by polymorphisms in their third complementarity determining regions (CDR3). We assessed whether novel combinations of such polymorphisms could improve TCR recognition of epitope variation.

METHODS

A TCR recognizing the HLA A*0201-restricted epitope SLYNTVATL (Gag 77-85, SL9) was engineered to have combinations of four polymorphisms in the CDR3 regions compared to another SL9-specific TCR. These novel TCRs were screened for functional avidities against SL9 epitope variants and abilities to mediate cytotoxic T-lymphocyte suppression of HIV-1 containing the same epitope variants.

RESULTS

The TCRs varied modestly in functional avidities for SL9 variants, due to alterations in affinity. This translated to differences in antiviral activities against HIV-1 when functional avidity changes crossed the previously defined threshold required for efficient recognition of HIV-1-infected cells. Higher avidity TCR mutants had generally broader recognition of SL9 variants.

CONCLUSION

These results indicate that rationally targeted increases in functional avidities can be utilized to maximize the antiviral breadth of transgenic TCRs. In contrast to previously reported random mutagenesis to markedly increase functional avidities, tuning through recombining naturally occurring polymorphisms may offer a more physiologic approach that minimizes the risk of deleterious TCR reactivities.

T cell receptor gene therapy for cancer

T cell-based adoptive immunotherapy has been shown to be a promising treatment for various types of cancer. However, adoptive T cell therapy currently requires the custom isolation and characterization of tumor-specific T cells from each patient-a process that can be not only difficult and time-consuming but also often fails to yield high-avidity T cells, which together have limited the broad application of this approach as a clinical treatment. Employing T cell receptor (TCR) gene therapy as a component of adoptive T cell therapy strategies can overcome many of these obstacles, allowing autologous T cells with a defined specificity to be generated in a much shorter time period. Initial studies using this approach have been hampered by a number of technical difficulties resulting in low TCR expression and acquisition of potentially problematic specificities due to mispairing of introduced TCR chains with endogenous TCR chains. The last several years have seen substantial progress in our understanding of the multiple facets of TCR gene therapy that will have to be properly orchestrated for this strategy to succeed. Here we outline the challenges of TCR gene therapy and the advances that have been made toward realizing the promise of this approach.

Combination of IL-21 and IL-15 enhances tumour-specific cytotoxicity and cytokine production of TCR-transduced primary T cells

IL-21, and to a lesser extent IL-15, inhibits differentiation of antigen-primed CD8 T cells and promotes their homeostasis and anti-tumour activity. Here, we investigated molecular mechanisms behind tumour-specific responses of primary murine T lymphocytes engineered to express a TCR directed against human gp100/HLA-A2 following short-term exposure to IL-15 and/or IL-21. We demonstrated that IL-15 + IL-21, and to a lesser extent IL-21, enhanced antigen-specific T-cell cytotoxicity, which was related to enhanced expression of granzymes A and B, and perforin 1. Furthermore, IL-15 + IL-21 synergistically enhanced release levels and kinetics of T-cell IFNgamma and IL-2, but not IL-10. Enhanced secretion of IFNgamma was accompanied by increased gene expression and cytosolic protein content, and was restricted to effector memory T cells. To summarize, we show that IL-15 + IL-21 improves antigen-specific responses of TCR-transduced effector T cells at multiple levels, which provides a rationale to treat T cells with a combination of these cytokines prior to their use in adoptive TCR gene therapy.

Mixed T cell receptor dimers harbor potentially harmful neoreactivity

Adoptive transfer of T cell receptor (TCR)-transduced T cells may be an attractive strategy to target both hematological malignancies and solid tumors. By introducing a TCR, large numbers of T cells with defined antigen (Ag) specificity can be obtained. However, by introduction of a TCR, mixed TCR dimers can be formed. Besides the decrease in TCR expression of the introduced and endogenous TCR, these mixed TCR dimers could harbor potentially harmful specificities. In this study, we demonstrate that introduction of TCRs resulted in formation of neoreactive mixed TCR dimers, composed of the introduced TCR chains pairing with either the endogenous TCR alpha or beta chain. Neoreactivities observed were HLA class I or class II restricted. Most neoreactive mixed TCR dimers were allo-HLA reactive; however, neoreactive mixed TCR dimers with autoreactive activity were also observed. We demonstrate that inclusion of an extra disulfide bond between the constant domains of the introduced TCR markedly reduced neoreactivity, whereas enhanced effectiveness of the introduced TCR was observed. In conclusion, TCR transfer results in the formation of neoreactive mixed TCR dimers with the potential to generate off-target effects, underlining the importance of searching for techniques to facilitate preferential pairing.