Enhanced transgene expression in quiescent and activated human CD8+ T cells

Аpplication of massive parallel reporter analysis in biotechnology and medicine

The development and functioning of an organism relies on tissue-specific gene programs. Genome regulatory elements play a key role in the regulation of such programs, and disruptions in their function can lead to the development of various pathologies, including cancers, malformations and autoimmune diseases. The emergence of high-throughput genomic studies has led to massively parallel reporter analysis (MPRA) methods, which allow the functional verification and identification of regulatory elements on a genome-wide scale. Initially MPRA was used as a tool to investigate fundamental aspects of epigenetics, but the approach also has great potential for clinical and practical biotechnology. Currently, MPRA is used for validation of clinically significant mutations, identification of tissue-specific regulatory elements, search for the most promising loci for transgene integration, and is an indispensable tool for creating highly efficient expression systems, the range of application of which extends from approaches for protein development and design of next-generation therapeutic antibody superproducers to gene therapy. In this review, the main principles and areas of practical application of high-throughput reporter assays will be discussed.

Stable and reproducible transgene expression independent of proliferative or differentiated state using BAC TG-EMBED

Reproducible and stable transgene expression is an important goal in both basic research and biotechnology, with each application demanding a range of transgene expression. Problems in achieving stable transgene expression include multi-copy transgene silencing, chromosome-position effects, and loss of expression during long-term culture, induced cell quiescence, and/or cell differentiation. Previously, we described the “BAC TG-EMBED” method for copy-number dependent, chromosome position-independent expression of embedded transgenes within a BAC containing ~170 kb of the mouse Dhfr locus. Here we demonstrate wider applicability of the method by identifying a BAC and promoter combination that drives reproducible, copy-number dependent, position-independent transgene expression even after induced quiescence and/or cell differentiation into multiple cell types. Using a GAPDH BAC containing ~200 kb of the human GAPDH gene locus and a 1.2 kb human UBC promoter, we achieved stable GFP-ZeoR reporter expression in mouse NIH 3T3 cells after low-serum induced cell cycle arrest or differentiation into adipocytes. More notably, GFP-ZeoR expression remained stable and copy-number dependent even after differentiation of mouse ESCs into several distinct lineages. These results highlight the potential use of BAC TG-EMBED as an expression platform for high-level but stable, long-term expression of transgene independent of cell proliferative or differentiated state.

Driving gene-engineered T cell immunotherapy of cancer

Chimeric antigen receptor (CAR) gene-engineered T cell therapy holds the potential to make a meaningful difference in the lives of patients with terminal cancers. For decades, cancer therapy was based on biophysical parameters, with surgical resection to debulk, followed by radiation and chemotherapy to target the rapidly growing tumor cells, while mostly sparing quiescent normal tissues. One breakthrough occurred with allogeneic bone-marrow transplant for patients with leukemia, which provided a sometimes curative therapy. The field of adoptive cell therapy for solid tumors was established with the discovery that tumor-infiltrating lymphocytes could be expanded and used to treat and even cure patients with metastatic melanoma. Tumor-specific T-cell receptors (TCRs) were identified and engineered into patient peripheral blood lymphocytes, which were also found to treat tumors. However, these were limited by patient HLA-restriction. Close behind came generation of CAR, combining the exquisite recognition of an antibody with the effector function of a T cell. The advent of CD19-targeted CARs for treating patients with multiple forms of advanced B-cell malignancies met with great success, with up to 95% response rates. Applying CAR treatment to solid tumors, however, has just begun, but already certain factors have been made clear: the tumor target is of utmost importance for clinicians to do no harm; and solid tumors respond differently to CAR therapy compared with hematologic ones. Here we review the state of clinical gene-engineered T cell immunotherapy, its successes, challenges, and future.

Identification and selective expansion of functionally superior T cells expressing chimeric antigen receptors

BACKGROUND

T cells expressing chimeric antigen receptors (CARs) have shown exciting promise in cancer therapy, particularly in the treatment of B-cell malignancies. However, optimization of CAR-T cell production remains a trial-and-error exercise due to a lack of phenotypic benchmarks that are clearly predictive of anti-tumor functionality. A close examination of the dynamic changes experienced by CAR-T cells upon stimulation can improve understanding of CAR-T-cell biology and identify potential points for optimization in the production of highly functional T cells.

METHODS

Primary human T cells expressing a second-generation, anti-CD19 CAR were systematically examined for changes in phenotypic and functional responses to antigen exposure over time. Multi-color flow cytometry was performed to quantify dynamic changes in CAR-T cell viability, proliferation, as well as expression of various activation and exhaustion markers in response to varied antigen stimulation conditions.

RESULTS

Stimulated CAR-T cells consistently bifurcate into two distinct subpopulations, only one of which (CAR(hi)/CD25(+)) exhibit anti-tumor functions. The use of central memory T cells as the starting population and the resilience-but not antigen density-of antigen-presenting cells used to expand CAR-T cells were identified as critical parameters that augment the production of functionally superior T cells. We further demonstrate that the CAR(hi)/CD25(+) subpopulation upregulates PD-1 but is resistant to PD-L1-induced dysfunction.

CONCLUSIONS

CAR-T cells expanded ex vivo for adoptive T-cell therapy undergo dynamic phenotypic changes during the expansion process and result in two distinct populations with dramatically different functional capacities. Significant and sustained CD25 and CAR expression upregulation is predictive of robust anti-tumor functionality in antigen-stimulated T cells, despite their correlation with persistent PD-1 upregulation. The functionally superior subpopulation can be selectively augmented by careful calibration of antigen stimulation and the enrichment of central memory T-cell type.

Re-adapting T cells for cancer therapy: from mouse models to clinical trials

Adoptive T-cell therapy involves the isolation, expansion, and reinfusion of T lymphocytes with a defined specificity and function as a means to eradicate cancer. Our research has focused on specifying the requirements for tumor eradication with antigen-specific T cells and T cells transduced to express a defined T-cell receptor (TCR) in mouse models and then translating these strategies to clinical trials. Our design of T-cell-based therapy for cancer has reflected efforts to identify the obstacles that limit sustained effector T-cell activity in mice and humans, design approaches to enhance T-cell persistence, develop methods to increase TCR affinity/T-cell functional avidity, and pursue strategies to overcome tolerance and immunosuppression. With the advent of genetic engineering, a highly functional population of T cells can now be rapidly generated and tailored for the targeted malignancy. Preclinical studies in faithful and informative mouse models, in concert with knowledge gained from analyses of successes and limitations in clinical trials, are shaping how we continue to develop, refine, and broaden the applicability of this approach for cancer therapy.

A Good Manufacturing Practice procedure to engineer donor virus-specific T cells into potent anti-leukemic effector cells

A sequential, two-step procedure in which T-cell-depleted allogeneic stem cell transplantation is followed by treatment with donor lymphocyte infusion at 6 months can significantly reduce the risk and severity of graft-versus-host disease, with postponed induction of the beneficial graft-versus-leukemia effect. However, patients with high-risk leukemia have a substantial risk of relapse early after transplantation, at a time when administration of donor lymphocytes has a high likelihood of resulting in graft-versus-host disease, disturbing a favorable balance between the graft-versus-leukemia effect and graft-versus-host disease. New therapeutic modalities are, therefore, required to allow early administration of T cells capable of exerting a graft-versus-leukemia effect without causing graft-versus-host disease. Here we describe the isolation of virus-specific T cells using Streptamer-based isolation technology and subsequent transfer of the minor histocompatibility antigen HA-1-specific T-cell receptor using retroviral vectors. Isolation of virus-specific T cells and subsequent transduction with HA-1-T-cell receptor resulted in rapid in vitro generation of highly pure, dual-specific T cells with potent anti-leukemic reactivity. Due to the short production procedure of only 10-14 days and the defined specificity of the T cells, administration of virus-specific T cells transduced with the HA-1-T-cell receptor as early as 8 weeks after allogeneic stem cell transplantation is feasible. (This clinical trial is registered at www.clinicaltrialsregister.eu as EudraCT number 2010-024625-20).

Genetic modification of lymphocytes by retrovirus-based vectors

The genetic modification of lymphocytes is an important topic in the emerging field of gene therapy. Many clinical trials targeting immunodeficiency syndromes or cancer have shown therapeutic benefit; further applications address inflammatory and infectious disorders. Retroviral vector development requires a detailed understanding of the interactions with the host. Most researchers have used simple gammaretroviral vectors to modify lymphocytes, either directly or via hematopoietic stem and progenitor cells. Lentiviral, spumaviral (foamyviral) and alpharetroviral vectors were designed to reduce the necessity for cell stimulation and to utilize potentially safer integration properties. Novel surface modifications (pseudotyping) and transgenes, built using synthetic components, expand the retroviral toolbox, altogether promising increased specificity and potency. Product consistency will be an important criterion for routine clinical use.

Adoptive therapy using antigen-specific T-cell clones

T-cell therapy involves the ex vivo isolation and expansion of antigen-specific T cells for adoptive transfer. The use of T-cell clones represents one embodiment of this approach and provides a uniform population of effector cells, so that parameters contributing to an effective response can be rigorously evaluated. T cells of defined specificity, phenotype, and function are isolated and expanded; when infused into patients, these intrinsic factors can be considered in light of extrinsic factors such as the type of conditioning regimen, cytokine support, and immunomodulatory reagents. In this chapter, 2 topics related to the use of antigen-specific T-cell clones are discussed: first, advances enabling the isolation and expansion of antigen-specific T-cell clones for human trials of adoptive therapy, and second, a contextual framework of advantages and limitations in which the use of adoptively transferred T-cell clones can be judiciously applied as a means to dissect the requirements for effective therapy.

Suicide gene therapy for graft-versus-host disease

In allogeneic hematopoietic stem cell transplantation, donor-derived T cells are key players for early immune reconstitution and efficient engraftment, as well as the graft-versus-leukemia and graft-versus-infection effects. However, a severe and quite common life-threatening complication is the development of graft-versus-host disease, during which the alloreactive donor T cells attack the host. Controlling graft-versus-host disease while preserving the benefits of graft-versus-leukemia still constitutes a challenge. A promising approach for the control of graft-versus-host disease is suicide gene therapy, which involves the ex vivo genetic modification of donor T cells with a suicide gene that allows for the selective elimination of the cells in vivo if graft-versus-host disease occurs. This article presents an overview of such approaches with special reference to lessons learned from previous clinical experiences, as well as a discussion of critical factors in suicide gene therapy.

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.

Nanoparticles for retinal gene therapy

Ocular gene therapy is becoming a well-established field. Viral gene therapies for the treatment of Leber's congentinal amaurosis (LCA) are in clinical trials, and many other gene therapy approaches are being rapidly developed for application to diverse ophthalmic pathologies. Of late, development of non-viral gene therapies has been an area of intense focus and one technology, polymer-compacted DNA nanoparticles, is especially promising. However, development of pharmaceutically and clinically viable therapeutics depends not only on having an effective and safe vector but also on a practical treatment strategy. Inherited retinal pathologies are caused by mutations in over 220 genes, some of which contain over 200 individual disease-causing mutations, which are individually very rare. This review will focus on both the progress and future of nanoparticles and also on what will be required to make them relevant ocular pharmaceutics.

The hidden (and lazy) TCR

In this issue of Blood, Burns and colleagues analyzed samples from 18 patients with melanoma, treated with T lymphocytes gene-modified to express a tumor-specific TCR. Results show persistence of transduced cells, but early shutdown of TCR gene expression. Transgene down-regulation was not caused by epigenetic silencing and could be reversed by T-cell activation.

Expression-independent gene trap vectors for random and targeted mutagenesis in embryonic stem cells

Promoterless gene trap vectors have been widely used for high-efficiency gene targeting and random mutagenesis in embryonic stem (ES) cells. Unfortunately, such vectors are only effective for genes expressed in ES cells and this has prompted the development of expression-independent vectors. These polyadenylation (poly A) trap vectors employ a splice donor to capture an endogenous gene's polyadenylation sequence and provide transcript stability. However, the spectrum of mutations generated by these vectors appears largely restricted to the last intron of target loci due to nonsense-mediated mRNA decay (NMD) making them unsuitable for gene targeting applications. Here, we present novel poly A trap vectors that overcome the effect of NMD and also employ RNA instability sequences to improve splicing efficiency. The set of random insertions generated with these vectors show a significantly reduced insertional bias and the vectors can be targeted directly to a 5' intron. We also show that this relative positional independence is linked to the human beta-actin promoter and is most likely a result of its transcriptional activity in ES cells. Taken together our data indicate that these vectors are an effective tool for insertional mutagenesis that can be used for either gene trapping or gene targeting.

Lack of specific gamma-retroviral vector long terminal repeat promoter silencing in patients receiving genetically engineered lymphocytes and activation upon lymphocyte restimulation

Retroviral transduction of tumor antigen-specific T-cell receptor (TCR) genes into lymphocytes redirects T cells to lyse tumors. Furthermore, adoptive transfer of these lymphocytes has mediated objective responses in patients with metastatic cancer. From 2004 to 2006, more than 40 patients were treated with autologous gene-modified lymphocytes expressing a melanoma antigen-specific TCR at the National Cancer Institute. Eighteen such patients were analyzed for persistence and gene expression in vivo. In addition, the impact of epigenetic silencing and of lymphocyte restimulation was studied. Although gene-modified lymphocytes persisted in vivo, the shutdown of TCR transgene expression was observed. Bisulfite sequencing analysis and ex vivo DNA methyltransferase inhibition demonstrated that the decrease in gene expression did not result from DNA methylation. Surprisingly, down-regulation of vector-driven transgene transcriptional activity was not vector specific but mimicked that of endogenous genes. The decrease in TCR transgene expression, however, was reversed upon lymphocyte stimulation. These data demonstrate a lack of gamma-retroviral promoter-specific gene silencing in adoptively transferred human lymphocytes and support that transgene expression is largely affected by global cellular mechanisms. The use of immunomodulatory adjuvants, eg, vaccination or cytokine therapy, for in vivo T-cell activation may help overcome this metabolic quiescence and thus augment cellular immunotherapy-based cancer therapy.

Chimeric receptors containing CD137 signal transduction domains mediate enhanced survival of T cells and increased antileukemic efficacy in vivo

Persistence of T cells engineered with chimeric antigen receptors (CARs) has been a major barrier to use of these cells for molecularly targeted adoptive immunotherapy. To address this issue, we created a series of CARs that contain the T cell receptor-zeta (TCR-zeta) signal transduction domain with the CD28 and/or CD137 (4-1BB) intracellular domains in tandem. After short-term expansion, primary human T cells were subjected to lentiviral gene transfer, resulting in large numbers of cells with >85% CAR expression. In an immunodeficient mouse xenograft model of primary human pre-B-cell acute lymphoblastic leukemia, human T cells expressing anti-CD19 CARs containing CD137 exhibited the greatest antileukemic efficacy and prolonged (>6 months) survival in vivo, and were significantly more effective than cells expressing CARs containing TCR-zeta alone or CD28-zeta signaling receptors. We uncovered a previously unrecognized, antigen-independent effect of CARs expressing the CD137 cytoplasmic domain that likely contributes to the enhanced antileukemic efficacy and survival in tumor bearing mice. Furthermore, our studies revealed significant discrepancies between in vitro and in vivo surrogate measures of CAR efficacy. Together these results suggest that incorporation of the CD137 signaling domain in CARs should improve the persistence of CARs in the hematologic malignancies and hence maximize their antitumor activity.

Adoptive immunotherapy of disseminated leukemia with TCR-transduced, CD8+ T cells expressing a known endogenous TCR

Adoptive T-cell immunotherapy has shown promise in the treatment of human malignancies, but the challenge of isolating T cells with high avidity for tumor antigens in each patient has limited application of this approach. The transfer into T cells of T-cell receptor (TCR) genes encoding high-affinity TCRs recognizing defined tumor-associated antigens can potentially circumvent this obstacle. Using a well-characterized murine model of adoptive T-cell immunotherapy for widely disseminated leukemia, we demonstrate that TCR gene-modified T cells can cure mice of disseminated tumor. One goal of such adoptive therapy is to establish a persistent memory response to prevent recurrence; however, long-term function of transferred TCR-transduced T cells is limited due to reduced expression of the introduced TCR in vivo in quiescent resting T cells. However, by introducing the TCR into a cell with a known endogenous specificity, activation of these T cells by stimulation through the endogenous TCR can be used to increase expression of the introduced TCR, potentially providing a strategy to increase the total number of tumor-reactive T cells in the host and restore more potent antitumor activity.

Harnessing endogenous miR-181a to segregate transgenic antigen receptor expression in developing versus post-thymic T cells in murine hematopoietic chimeras

MicroRNAs (miRNAs) are small, noncoding RNAs that regulate gene expression by targeting complementary sequences, referred to as miRNA recognition elements (MREs), typically located in the 3' untranslated region of mRNAs. miR-181a is highly expressed in developing thymocytes and markedly downregulated in post-thymic T cells. We investigated whether endogenous miR-181a can be harnessed to segregate expression of chimeric antigen receptors (CARs) and TCRs between developing and mature T cells. Lentiviral-encoded antigen receptors were tagged with a miR-181a-specific MRE and transduced into mouse BM cells that were used to generate hematopoietic chimeras. Expression of a CAR specific for human CD19 (hCD19) was selectively suppressed in late double-negative and double-positive thymocytes, coinciding with the peak in endogenous miR-181a expression. Receptor expression was fully restored in post-thymic resting and activated T cells, affording protection against a subsequent challenge with hCD19+ tumors. Hematopoietic mouse chimeras engrafted with a conalbumin-specific TCR prone to thymic clonal deletion acquired peptide-specific T cell responsiveness only when the vector-encoded TCR transcript was similarly engineered to be subject to regulation by miR-181a. These results demonstrate the potential of miRNA-regulated transgene expression in stem cell-based therapies, including cancer immunotherapy.

Antigen sensitivity of CD22-specific chimeric TCR is modulated by target epitope distance from the cell membrane

We have targeted CD22 as a novel tumor-associated Ag for recognition by human CTL genetically modified to express chimeric TCR (cTCR) recognizing this surface molecule. CD22-specific cTCR targeting different epitopes of the CD22 molecule promoted efficient lysis of target cells expressing high levels of CD22 with a maximum lytic potential that appeared to decrease as the distance of the target epitope from the target cell membrane increased. Targeting membrane-distal CD22 epitopes with cTCR(+) CTL revealed defects in both degranulation and lytic granule targeting. CD22-specific cTCR(+) CTL exhibited lower levels of maximum lysis and lower Ag sensitivity than CTL targeting CD20, which has a shorter extracellular domain than CD22. This diminished sensitivity was not a result of reduced avidity of Ag engagement, but instead reflected weaker signaling per triggered cTCR molecule when targeting membrane-distal epitopes of CD22. Both of these parameters were restored by targeting a ligand expressing the same epitope, but constructed as a truncated CD22 molecule to approximate the length of a TCR:peptide-MHC complex. The reduced sensitivity of CD22-specific cTCR(+) CTL for Ag-induced triggering of effector functions has potential therapeutic applications, because such cells selectively lysed B cell lymphoma lines expressing high levels of CD22, but demonstrated minimal activity against autologous normal B cells, which express lower levels of CD22. Thus, our results demonstrate that cTCR signal strength, and consequently Ag sensitivity, can be modulated by differential choice of target epitopes with respect to distance from the cell membrane, allowing discrimination between targets with disparate Ag density.

An inducible caspase 9 safety switch can halt cell therapy-induced autoimmune disease

Transfer of either allogeneic or genetically modified T cells as a therapy for malignancies can be accompanied by T cell-mediated tissue destruction. The introduction of an efficient "safety switch" can potentially be used to control the survival of adoptively transferred cell populations and as such reduce the risk of severe graft-vs-host disease. In this study, we have tested the value of an inducible caspase 9-based safety switch to halt an ongoing immune attack in a murine model for cell therapy-induced type I diabetes. The data obtained in this model indicate that self-reactive T cells expressing this conditional safety switch show unimpaired lymphopenia- and vaccine-induced proliferation and effector function in vivo, but can be specifically and rapidly eliminated upon triggering. These data provide strong support for the evaluation of this conditional safety switch in clinical trials of adoptive cell therapy.

Development of optimal bicistronic lentiviral vectors facilitates high-level TCR gene expression and robust tumor cell recognition

In human gene therapy applications, lentiviral vectors may have advantages over gamma-retroviral vectors in several areas, including the ability to transduce nondividing cells, resistance to gene silencing and a potentially safer integration site profile. However, unlike gamma-retroviral vectors it has been problematic to drive the expression of multiple genes efficiently and coordinately with approaches such as internal ribosome entry sites or dual promoters. Using different 2A peptides, lentiviral vectors expressing two-gene T-cell receptors directed against the melanoma differentiation antigens gp100 and MART-1 were constructed. We demonstrated that addition of amino-acid spacer sequences (GSG or SGSG) before the 2A sequence is a prerequisite for efficient synthesis of biologically active T-cell receptors and that addition of a furin cleavage site followed by a V5 peptide tag yielded optimal T-cell receptor gene expression. Furthermore, we determined that the furin cleavage site was recognized in lymphocytes and accounted for removal of residual 2A peptides at the post-translational level with an efficiency of 20-30%, which could not be increased by addition of multiple furin cleavage sites. The novel bicistronic lentiviral vector developed herein afforded robust anti-melanoma activities to engineered peripheral blood lymphocytes, including cytokine secretion, cell proliferation and lytic activity. Such optimal vectors may have immediate applications in cancer gene therapy.

T-cell receptor gene therapy of established tumors in a murine melanoma model

Adoptive cell transfer therapy using tumor-infiltrating lymphocytes for patients with metastatic melanoma has demonstrated significant objective response rates. One major limitation of these current therapies is the frequent inability to isolate tumor-reactive lymphocytes for treatment. Genetic engineering of peripheral blood lymphocytes with retroviral vectors encoding tumor antigen-specific T-cell receptors (TCRs) bypasses this restriction. To evaluate the efficacy of TCR gene therapy, a murine treatment model was developed. A retroviral vector was constructed encoding the pmel-1 TCR genes targeting the B16 melanoma antigen, gp100. Transduction of C57BL/6 lymphocytes resulted in efficient pmel-1 TCR expression. Lymphocytes transduced with this retrovirus specifically recognized gp100-pulsed target cells as measured by interferon-gamma secretion assays. Upon transfer into B16 tumor-bearing mice, the genetically engineered lymphocytes significantly slowed tumor development. The effectiveness of tumor treatment was directly correlated with the number of TCR-engineered T cells administered. These results demonstrated that TCR gene therapy targeting a native tumor antigen significantly delayed the growth of established tumors. When C57BL/6 lymphocytes were added to antigen-reactive pmel-1 T cells, a reduction in the ability of pmel-1 T cell to treat B16 melanomas was seen, suggesting that untransduced cells may be deleterious to TCR gene therapy. This model may be a powerful tool for evaluating future TCR gene transfer-based strategies.

Selecting highly affine and well-expressed TCRs for gene therapy of melanoma

A recent phase 1 trial has demonstrated that the generation of tumor-reactive T lymphocytes by transfer of specific T-cell receptor (TCR) genes into autologous lymphocytes is feasible. However, compared with results obtained by infusion of tumor-infiltrating lymphocytes, the response rate observed in this first TCR gene therapy trial is low. One strategy that is likely to enhance the success rate of TCR gene therapy is the use of tumor-reactive TCRs with a higher capacity for tumor cell recognition. We therefore sought to develop standardized procedures for the selection of well-expressed, high-affinity, and safe human TCRs. Here we show that TCR surface expression can be improved by modification of TCR alpha and beta sequences and that such improvement has a marked effect on the in vivo function of TCR gene-modified T cells. From a panel of human, melanoma-reactive TCRs we subsequently selected the TCR with the highest affinity. Furthermore, a generally applicable assay was used to assess the lack of alloreactivity of this TCR against a large series of common human leukocyte antigen alleles. The procedures described in this study should be of general value for the selection of well- and stably expressed, high-affinity, and safe human TCRs for subsequent clinical testing.

Generation of potent and stable human CD4+ T regulatory cells by activation-independent expression of FOXP3

Therapies based on enhancing the numbers and/or function of T regulatory cells (Tregs) represent one of the most promising approaches to restoring tolerance in many immune-mediated diseases. Several groups have investigated whether human Tregs suitable for cellular therapy can be obtained by in vitro expansion, in vitro conversion of conventional T cells into Tregs, or gene transfer of the FOXP3 transcription factor. To date, however, none of these approaches has resulted in a homogeneous and stable population of cells that is as potently suppressive as ex vivo Tregs. We developed a lentivirus-based strategy to ectopically express high levels of FOXP3 that do not fluctuate with the state of T-cell activation. This method consistently results in the development of suppressive cells that are as potent as Tregs and can be propagated as a homogeneous population. Moreover, using this system, both naïve and memory CD4(+) T cells can be efficiently converted into Tregs. To date, this is the most efficient and reliable protocol for generating large numbers of suppressive CD4(+) Tregs, which can be used for further biological study and developed for antigen-specific cellular therapy applications.

Facilitating matched pairing and expression of TCR chains introduced into human T cells

Adoptive transfer of T lymphocytes is a promising treatment for a variety of malignancies but often not feasible due to difficulties generating T cells that are reactive with the targeted antigen from patients. To facilitate rapid generation of cells for therapy, T cells can be programmed with genes encoding the alpha and beta chains of an antigen-specific T-cell receptor (TCR). However, such exogenous alpha and beta chains can potentially assemble as pairs not only with each other but also with endogenous TCR alpha and beta chains, thereby generating alphabetaTCR pairs of unknown specificity as well as reducing the number of exogenous matched alphabetaTCR pairs at the cell surface. We demonstrate that introducing cysteines into the constant region of the alpha and beta chains can promote preferential pairing with each other, increase total surface expression of the introduced TCR chains, and reduce mismatching with endogenous TCR chains. This approach should improve both the efficacy and safety of ongoing efforts to use TCR transfer as a strategy to generate tumor-reactive T cells.

A self-inactivating retrovector incorporating the IL-2 promoter for activation-induced transgene expression in genetically engineered T-cells

Background

T-cell activation leads to signaling pathways that ultimately result in induction of gene transcription from the interleukin-2 (IL-2) promoter. We hypothesized that the IL-2 promoter or its synthetic derivatives can lead to T-cell specific, activation-induced transgene expression. Our objective was to develop a retroviral vector for stable and activation-induced transgene expression in T-lymphocytes.

Results

First, we compared the transcriptional potency of the full-length IL-2 promoter with that of a synthetic promoter composed of 3 repeats of the Nuclear Factor of Activated T-Cells (NFAT) element following activation of transfected Jurkat T-cells expressing the large SV40 T antigen (Jurkat TAg). Although the NFAT3 promoter resulted in a stronger induction of luciferase reporter expression post stimulation, the basal levels of the IL-2 promoter-driven reporter expression were much lower indicating that the IL-2 promoter can serve as a more stringent activation-dependent promoter in T-cells. Based on this data, we generated a self-inactivating retroviral vector with the full-length human IL-2 promoter, namely SINIL-2pr that incorporated the enhanced green fluorescent protein (EGFP) fused to herpes simplex virus thymidine kinase as a reporter/suicide "bifunctional" gene. Subsequently, Vesicular Stomatitis Virus-G Protein pseudotyped retroparticles were generated for SINIL-2pr and used to transduce the Jurkat T-cell line and the ZAP-70-deficient P116 cell line. Flow cytometry analysis showed that EGFP expression was markedly enhanced post co-stimulation of the gene-modified cells with 1 μM ionomycin and 10 ng/ml phorbol 12-myristate 13-acetate (PMA). This activation-induced expression was abrogated when the cells were pretreated with 300 nM cyclosporin A.

Conclusion

These results demonstrate that the SINIL-2pr retrovector leads to activation-inducible transgene expression in Jurkat T-cell lines. We propose that this design can be potentially exploited in several cellular immunotherapy applications.

CD8(+) T cells armed with retrovirally transduced IFN-gamma

Interferon-gamma (IFN-gamma) is considered a key cytokine involved in the preventive and defensive responses of T cells against infectious pathogens and tumors. Therefore, the transgenic expression of IFN-gamma in specific T cells appears to be an obvious therapeutic possibility. To directly examine whether IFN-gamma production can be increased in T cells, we introduced an IFN-gamma encoding cDNA into IFN-gamma(-/-) and IFN-gamma(+/+) CD8(+) effector populations by retroviral transduction. Here, we show that CD8 T cells can be equipped with IFN-gamma that increases their capacity to secrete the cytokine. Despite constitutive retroviral IFN-gamma mRNA transcription, translation and secretion of IFN-gamma protein was tightly regulated and only observed in activated T cells. Neither proliferation nor cytolytic activity of CTL was affected by IFN-gamma transduction. Importantly, CD8(+) T cells retrovirally transduced with IFN-gamma exhibit augmented tumor suppressive capacity upon adoptive transfer into IFN-gamma(-/-) mice. Thus, T cells can be readily armed with IFN-gamma without risking immunopathology by dysregulated production of this highly potent proinflammatory cytokine.

Gene expression in the cell cycle of human T lymphocytes: I. Predicted gene and protein networks

The key genes involved in the cell cycle of human T lymphocytes were identified by iterative searches of gene-related databases, as derived also from DNA microarray experimentation, revealing and predicting interactions between those genes, assigning scores to each of the genes according to numbers of interaction for each gene weighted by significance of each interaction, and finally applying several types of clustering algorithms to genes basing on the assigned scores. All clustering algorithms applied, both hierarchical and K-means, invariably selected the same six "leader" genes involved in controlling the cell cycle of human T lymphocytes. Relations of the six genes to experimental data describing switching between stages of cell cycle of human T lymphocytes are discussed.