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Adoptive T cell therapies can produce objective clinical responses in patients with hematologic and solid malignancies. Genetic manipulation of T lymphocytes has been proposed as a means of increasing the potency and range of this anti-tumor activity. We now review how coupling expression of transgenic receptors with countermeasures against potent tumor immune evasion strategies is proving highly effective in pre-clinical models and describe how these approaches are being evaluated in human subjects.
Infusion of antigen-specific cytotoxic T lymphocytes (CTLs) has proven safe and apparently effective for the prophylaxis and treatment of viral infections such as cytomegalovirus (CMV) [1–3], adenovirus (Adv) , and Epstein-Barr virus (EBV) [4, 5] that arise in immunocompromised patients. This therapeutic approach has also been extended to the treatment of cancer and has shown some success in patients with melanoma  and EBV-associated malignancies such as Hodgkin’s lymphoma [7, 8] and nasopharyngeal carcinoma [9, 10]. This form of “personalized medicine” is now being explored in both early- and late-stage clinical trials. Recent improvements in molecular biology have increased the potential applications and effectiveness of this therapeutic approach by allowing the genetic modification of T cells using genes which confer properties such as new antigen specificity (αβT cell receptors – αβTCRs  or chimeric antigen receptors – CARs ), improved homing to tumor sites , increased resistance to tumor immune evasion strategies [14, 15], and the ability to up- or down- regulate proliferation of the infused cells [16, 17]. This review evaluates recent improvements in T cell engineering and describes their current clinical impact and their future potential as cancer therapeutics.
The development of an anti-tumor T cell mediated immune response is a multi step process in which tumor-associated antigens (TAAs) expressed by the tumor cells are processed and presented by professional antigen presenting cells (APCs) to circulating T cells, which become activated. The quality and potency of this T cell response will depend on the nature of the antigen presented, the functionality of the APC itself and the cytokines and co-stimulatory interactions that occur in the T cell/APC microenvironment . Once activated, TAA-specific effector T cells can migrate to the tumor site and target malignant cells expressing the cognate antigen epitope displayed in the context of a major histocompatibility complex (MHC) molecule .
The potency and protective abilities of endogenous tumor-specific T cells are evident from studies of immunosuppressed individuals with dysfunctional or absent T cells (such as HIV-infected patients or transplant recipients), who are at increased risk of developing a range of malignances such as non-Hodgkin’s lymphoma (NHL) , Kaposi’s sarcoma, and invasive cervical cancer . The importance of the immune system for prevention and control of tumors in immunocompetent individuals, however, is much more controversial . When tumors arise in otherwise normal subjects, the conclusion is that specific T cells were either not induced in vivo, or were unable to destroy the malignant cells. Many of the antigens expressed by tumor cells are not unique to cancer cells and are instead expressed in normal tissues . Consequently, the immune system may have antigen specific T cells that recognize the tumors, but that are rendered unresponsive or tolerant . Efforts to break this unresponsiveness by aggressive vaccination have often failed . An alternative approach is to adoptively transfer non-tolerant T cells that have been expanded in vitro in an environment that is capable of breaking their tolerant state and inducing potent effector cells. For example, EBV-specific CTLs can be generated ex vivo from EBV-specific T cell precursors circulating in the peripheral blood of individuals with EBV+ malignancies [7, 8]. Once returned to the patients, these effector T cells can effectively eradicate tumors that had been resistant to their endogenous precursors, even though both share the same antigenic specificity [7, 8]. Similarly, autologous tumor-infiltrating T lymphocytes (TIL) obtained from melanoma patients, expanded ex vivo and infused, have produced clinical response in about 50% of treated individuals [6, 24].
Breaking tolerance alone, however, is often insufficient for CTLs to be able to exert potent in vivo anti-tumor effector function. As illustrated in Fig. (1), malignancies have evolved a number of evasion strategies to mitigate tumor-directed T cell reactivity. These include (i) the secretion of immunosuppressive molecules like TGF-β, interleukin (IL)-10, and IL-13, which inhibit effector T cells , (ii) modulation of MHC and costimulatory molecules to prevent antigen-specific T cell recognition , (iii) recruitment of regulatory T cells (Tregs) which inhibit effector T cells by direct cell-to-cell contact or by the production of soluble factors [26–29], (iv) constitutive expression of the tryptophan-depleting enzyme, indoleamine 2,3-dioxygenase (IDO) that prevents lymphocyte proliferation , (v) expression of Fas ligand which can induce premature apoptosis of activated T cells through Fas-Fas ligand interaction  and, (vi) expression of inhibitory cell surface molecules such as PD-L1 (program death ligand), which interacts with PD-1 expressed on activated T cells, and induces T cell exhaustion [32, 33].
Genetic modifications have been devised to act as countermeasures to these immune evasion mechanisms, thereby improving the effectiveness of tumor-specific T cells in vivo.
Effective genetic modification of T cells requires systems that produce adequate gene delivery and adequate expression of functional transgenes. While the level and duration of expression required for a given transgene depends on the specific function required, in general, the sustained modification of highly proliferative cells such as the T lymphocytes calls for an efficient integrating vector system that produces at least moderately high expression. The great majority of studies to date have therefore used retroviral vectors (Moloney or Lentivirus based) [5, 34–38] but the more recent development of integrating plasmids has renewed interest in the use of non viral approaches.
Though widely used in clinical studies, viral vectors are expensive to produce and test, and there is often a requirement for onerous and prolonged follow up of treated patients that further adds costs and complexity. Non viral vectors (eg. DNA plasmids) though cheaper to produce and test, require physical (e.g. electroporation) or chemical (e.g. liposomes) approaches to mediate gene transfer [39–41]. Until recently, DNA plasmids also integrated with extremely low efficiency, and so were associated with rapid disappearance in highly proliferating T cells. The discovery of transposon-based gene delivery systems overcomes this limitation by combining transposons and transposases in a plasmid with the gene of interest. Systems such as Sleeping Beauty [42, 43] and Piggybac  are currently used to gene modify T lymphocytes.
Gene transfer can be used to (i) improve the intrinsic biological activity of T cells (e.g. confer tumor specificity, enhance cytokine production or improve T cell migration), (ii) provide resistance to the immunosuppressive tumor microenvironment, (iii) enhance the safety of adoptively transferred T cells (e.g. addition of suicide genes), or (iv) enable T cell marking or imaging studies.
Because most TAA are either “self” antigens or “naïve” targets for the immune system, the generation of tumor-reactive T cells from cancer patients is often difficult or impossible, even under optimal conditions of ex vivo culture [45, 46]. To overcome this problem, investigators have genetically modified T cells to allow them to recognize antigens expressed by tumor cells. The two most common approaches are (a) expression of CARs that recognize tumors through single-chain variable fragments (scFv) isolated from specific antibodies  and (b) gene modification with α and β TCR chains cloned from TAA-specific T cell clones with high antigen avidity .
CARs are artificial receptors directed against antigens expressed on the cell surface of tumor cells, but with reduced or absent expression on normal tissues, thereby minimizing “off target” effects. First generation CARs are composed of the single-chain variable fragment (scFv) obtained from a specific antibody linked, via an extracellular spacer, with a transmembrane region and the intracytoplasmatic domain of the CD3ζ  (Fig. 2). The endodomain is responsible for transmitting the signal from the chimeric receptor and triggering phosphorylation of ITAM motifs, and the subsequent induction of T cell cytotoxic activity. CARs can combine the antigen specificity of an antibody with the lytic capacity of T cells and mediate antigen recognition in an MHC-unrestricted fashion. They also extend the range of antigens that can be recognized by T cells to include carbohydrates and glycolipids [49–51]. The overall efficacy of CAR-modified T cells against tumor target cells is the result of a complex interaction that includes the avidity of the scFv, the level of expression of CAR molecules on T cells, the density of antigen expressed on the target cells, and the position of the epitope on the target molecule in respect to the length and flexibility of the extracellular spacer sequence incorporated in the CAR.
CAR-expressing T cells can specifically recognize and respond to soluble, immobilized and/or tumor antigens expressed on target cells, and to date a range of CARs targeting a variety of surface molecules expressed by many solid tumors and hematological malignancies have been developed [52–61] (Table 1). While preclinical studies demonstrate that T cells expressing CARs can eliminate tumors in murine models [56, 61, 62], clinical trials with “first generation” CARs, containing only the CD3ζ signaling domain, were disappointing [39, 63, 64]. Following engagement with tumor antigens, the CD3ζ endodomain provides insufficient co-stimulatory signaling to the T cells to produce activation, proliferation and cytokine release, while the tumor cells themselves lack sufficient costimulatory signals to accomplish this through the endogenous counter-receptors on CAR-modified T cells . This led to the development of “second generation” CARs, in which additional intracellular co-stimulatory endodomains are added to enhance their function. These endodomains are derived from costimulatory molecules such as CD28, ICOS, CD134 and CD137 [56, 66–70], and T cells expressing these endodomains produce a broad array of cytokines such as IL-2 and TNF-α upon chimeric receptor engagement. These cytokines enhance the proliferation and persistence of genetically modified T cells after recognition of their CAR specific target. More recently “third generation” CARs, characterized by the presence of two costimulatory molecules in addition to the CD3ζ, have been developed [70, 71]. Carpenito and colleagues formally compared the potency of a CAR linked to a range of different endodomains and found that the simultaneous expression of CD137 (41BB) and CD28 endomains was more potent than either endodomain alone . It remains to be demonstrated whether the “superphysiological” activation of T cells also increases the toxicity of CAR-modified T cells.
Factors other than inadequate expression of costimulatory endodomains can result in poor in vivo persistence of CAR-modified T cells. Since most CARs are derived from murine antibodies, the chimera itself can induce an immune response in vivo following adoptive transfer. Lamers and colleagues reported the development of anti-scFv antibodies in three patients treated with T cells expressing a CAIX-specific CAR . Similarly, Kershaw and colleagues observed a CAR-specific antibody response in a patient treated with T cells modified with a CAR directed against the ovarian-associated α-folate receptor (α-FR) . Although this phenomenon has not been observed for all patients treated with CARs, the future use of humanized single chains may reduce the risk of premature deletion of T cells due to immune responses.
CAR-modified T cells may cause significant in vivo toxicities, such as the “on target antigen but off target tissue” toxicity. Lamers et al. reported the development of cholestasis following the infusion of T cells modified with a CAR targeting the carbonic anhydrase as treatment for renal cell carcinoma as biliary epithelial cells also express carbonic anhydrase . Brentjens et al. recently reported renal and respiratory failure in a patient with B-cell chronic lymphocytic leukemia treated with a single dose of 3×107/kg of T cells modified with a second generation CAR targeting the CD19 molecule. This serious adverse event occurred when T cells were infused after high doses of cyclophosphamide to induce host lympohodepletion and may have been due to a cytokine storm or to rapid tumor lysis .
T cells can be genetically modified to express transgenic α and β TCR chains, which have been isolated from T cell clones specific for tumor-associated antigens including MART-1, MAGE-3, MDM2, and WT-1 [11, 74, 75]. Both chains can be accommodated in a single retroviral or lentiviral vector, and transfer of the tumor-specific αβTCR to autologous mitogen-activated T cells allows rapid production of large numbers of tumor peptide-specific T cells. These gene modified T cells acquire MHC-restricted cytotoxic activity against tumor cells expressing the specific epitope. In most examples to date, polyclonal T cells are manipulated to express a second, transgenic, αβTCR in addition to their native αβTCR [11, 74]. The presence of two pairs of receptors in a single cell leads to unwanted cross pairing between endogenous α and transgenic β chains (and vice versa), reducing the affinity of the transgenic receptor for the desired target antigen and potentially creating new and unwanted reactivities . Investigators have manipulated the sequences of the transgenic chains to favor their linkage, and have also tried introducing the receptors into TCR negative effector lymphocytes [76, 77]. At present it is unclear how valuable these approaches will be.
T cells expressing a transgenic αβTCR have been studied in clinical trials of patients with melanoma. Investigators at the National Cancer Institute demonstrated that adoptive transfer of polyclonal T cells transduced with a retroviral vector encoding the αβTCR chains specific for an HLA-A2 restricted MART-1 peptide induced objective clinical response in 13% of the patients . In an effort to increase the frequency of significant tumor responses, the same investigators infused T cells that expressed higher affinity transgenic αβTCRs specific for MART-1 or gp100 peptides . Disappointingly, this strategy only modestly increased the tumor response rate but significantly exacerbated toxicity. The majority of patients developed skin rush, uveitis and hearing loss, attributed to an immune attack on normal melanocytes present in the skin, eye and inner ear . Hence potent T lymphocytes can be generated by appropriate genetic modification, but toxicity may become increasingly evident as potency increases, particularly if the targeted antigen is not unique to the malignant cells.
Though antigen specificity can be readily conferred using genetic modification, once infused, the tumor-specific T cells must expand in vivo and/or persist until all malignant cells have been eliminated. T cell proliferation requires continued antigenic stimulation, either via direct interaction with tumor cells or through professional APCs that cross-present tumor antigens, and the presence of appropriate cytokines. Moreover, following in vivo expansion, a proportion of the tumor-specific T cells should ideally enter the memory T cell compartment to provide long-term protection. To prevent these events, tumors have developed potent strategies to avoid effective antigen presentation, inhibit T cell proliferation, and prevent T cell entry into the memory compartment . As a consequence, both preclinical and clinical studies have shown that transferred cells have limited capacity to expand and persist in vivo. Several countermeasures have been developed against these unwanted tumor effects on immune persistence. These include: i) genetic modification of T cells to improve proliferation and survival, ii) the infusion of T cells with memory-type characteristics , and iii) genetic modification of infused cells to withstand the tumor microenvironment (Fig. 3).
Cytokines such as IL-2 promote T effector cell expansion and survival, and systemic administration can support adoptive T cell therapies [6, 24]. However, prolonged administration of recombinant IL-2 can also cause significant toxicity [6, 24] and expand Tregs which antagonize the beneficial effects of the cytokine on T effector cells . We and others have explored alternative methods to expand T cells in vivo, by genetically modifying T cells with retroviral vectors encoding the cytokines IL-2 or IL-15 to produce cells which are cytokine self-sufficient and self-sustaining [16, 81, 82]. Extensive characterization demonstrated that these cytokine-transgenic cells conserved their antigen specificity, phenotype and function and also retained their dependence on antigenic stimulation for continued expansion. Importantly, genetically modified T cells had enhanced persistence and superior anti-tumor activity in vivo compared to unmodified cells .
T cell growth and survival can also be increased by engineering cells to respond to cytokines in the tumor environment, or which can be safely administered as a recombinant protein, but which do not normally induce T cell growth . We investigated this option by genetically restoring the ability of activated antigen-specific CTLs to respond to IL-7, a cytokine that has been safely administered to human subjects without apparent enhancement of Treg cell number and function [83–85]. Unfortunately, IL-7 has only limited activity on antigen-specific T cells, since IL-7Rα expression is rapidly downregulated by naïve T cells once they have received antigenic stimulation . By forcing the expression of IL-7Rα in antigen-specific T cells, we selectively expanded them in vitro and in vivo in response to IL-7, without inducing unwanted proliferation of other T cell subsets . Clinically, the IL-7 required to support expansion of the transgenic receptor-expressing CTLs may derive either from the physiological release of the cytokine in a lymphodepleted host or from the administration of the recombinant protein.
T cell survival can also be increased by overexpressing the human telomerase reverse transcriptase (hTERT) gene . Although this approach greatly increases the number of population doublings of transduced T cells by preventing telomere erosion, it may also favor genomic instability and limit its safety and hence clinical application. An alternative means of increasing the persistence of infused T cells is to modify them with anti-apoptotic genes, such as Bcl-2 and Bcl-xL, which increase T cell resistance to death and IL-2 cytokine withdrawal .
T cell proliferation and survival require antigenic stimulation by APCs expressing co-stimulatory molecules. Many tumor cells lack expression of co-stimulatory molecules, such as CD80 and CD86, and substantially reduce the opportunity of T cells to successfully encounter the antigen in a suitably co-stimulatory environment [25, 88]. One way to circumvent this problem is to use CARs or transgenic αβTCRs to retarget virus-specific CTLs, rather than “random” T cells. Such virus-specific CTLs should receive optimal activation and co-stimulation following engagement of their native (virus specific) αβTCRs with viral latent antigens expressed by professional APCs, thus enabling superior anti-tumor activity mediated by their tumor reactive transgenic receptor. This hypothesis has been tested in a clinical trial performed at our institution, in which we compared the longevity of polyclonal activated T cells and polyclonal EBV-specific CTLs each expressing a CAR targeting the GD2 antigen, which is expressed on neuroblastoma [12, 60]. The EBV-specific CTLs remained detectable substantially longer (>18months) than their activated T cell counterparts (<1 month), suggesting the beneficial effects of co-stimulation of EBV-specific CTLs provided by APCs expressing EBV-latent antigens .
An alternative means for providing T cell co-stimulation is to force the expression of co-stimulatory ligands, such as CD80 and 4-1BBL, for the co-stimulatory receptors they already possess . Post-infusion, these molecules will insure there is “autocostimulation” and a bystander “transcostimulation”, which substitutes for the lack of appropriate co-stimulation from the tumor cells. Preclinical studies have shown that incorporation of these molecules in CAR-modified T cells enhances their ability to produce effector cytokines (IFNγ, IL-2, TNFα) and, in an in vivo mouse model, promotes tumor rejection in mice with large tumor burden .
In many of the settings in which adoptively transferred T cells will be used, patients are also receiving immunosuppressive and/or T cell inhibitory agents, which can adversely affect the persistence, functional capabilities and survival of adoptively-transferred T cells. For example, solid organ transplant recipients require continued administration of immunosuppressive drugs such as tacrolimus to prevent graft rejection. However, since these agents also impair endogenous T cell function, these patients are at high risk of developing virus-associated complications including EBV-post-transplant lymphomas . Although adoptive transfer of EBV-specific CTLs can still be effective in these patients, the cells do not persist long-term [91–93]. Strategies to make antigen-specific CTLs resistant to the effects of immunosuppressive drugs have been recently developed and include stable siRNA-mediated knock-down of the FK506-binding protein 12 (FKBP12) , the key protein that binds FK506, and overexpression of calcineurin mutants . Both strategies protect virus-specific CTLs from inhibitory effects whilst preserving their anti-viral activity.
Genetic modification of T cells can also be used to counteract the immune-inhibitory tumor microenvironment that can neutralize adoptively transferred antigen-specific CTLs. One of the most widely used tumor evasion strategies is local secretion of TGF-β by the tumor or its stromal elements . TGF-β is a multifunctional cytokine that, among other activities, limits T cell proliferation and effector function and induces tolerance . Antigen-specific CTLs modified to express a dominant-negative TGF-β receptor type II (dnTGF-β-RII) are resistant to the antiproliferative effects of TGF-β in vitro and in vivo , and expression prolongs their persistence and enhances tumor elimination in mice bearing TGF-β-expressing tumors [14, 97]. However, the gene modified T cells persisted only as long as the mice were restimulated with antigen, offering reassurance that the cells safely maintained their antigen dependence. A clinical trial to assess the safety and efficacy of dnTGF-β-RII-modified tumor-specific CTLs for the treatment of patients with relapsed/refractory Hodgkin’s lymphoma has recently opened at our institution.
Indoleamine 2,3-dioxygenase (IDO), is an enzyme catalyzing the initial step in the catabolism of tryptophan along the kynurenine pathway . T cells are particularly sensitive to tryptophan shortage, a property exploited by a number of tumors that secrete this enzyme either constitutively or in the presence of proinflammatory cytokines [30, 98, 99]. The inhibitory effects of IDO in T cells may be overcome by gene modification aimed to downregulate GCN2, a major component of IDO-mediated suppression. Munn and colleagues showed in pre-clinical studies that GCN2-knockout cells were refractory to IDO-induced anergy .
Fas ligand (FasL/CD95L) is a member of the tumor necrosis factor (TNF) superfamily and can trigger apoptosis through cross-linking of its receptor Fas (CD95). Many tumors express Fas ligand (FasL) as part of their tumor evasion arsenal, and researchers have suggested that this allows “immune privilege” because activated effector T cells express Fas on their cell surfaces and so are sensitive to Fas-mediated apoptosis, impairing T cell-mediated tumor rejection [101–103]. To overcome this, stable gene transfer of small interfering RNA (siRNA) designed to down regulate Fas can render T cells resistant to the apoptotic effects of FasL-expressing tumor cells .
At present, we do not know the best means of neutralizing the hostile tumor environment, and it is likely that a combination of the above countermeasures will need to be employed. Nonetheless, as we progressively accrue clinical data to add to the information provided by preclinical models, it should become possible to discern the optimal approach.
The interaction of secreted chemokines and chemokine receptors on the cell surface of the T cells plays a significant role in modulating the in vivo migration of T cells [104, 105]. Tumors can generate a chemokine milieu that significantly modifies the trafficking of Th1, Th2 cells and Tregs . Malignant Hodgkin’s lymphoma cells, for example, produce TARC and MDC which strongly attract Th2 and Tregs since these T cell subsets constitutively express CCR4, the receptor for both chemokines [106, 107]. By contrast, Th1 cells lack CCR4 and hence migrate poorly to HL tumor cells. Gene transfer offers the opportunity to re-shape the migration profile of Th1 cells so that they can exploit the tumor’s own escape mechanisms. We recently produced effector CD8+ CTL migration to TARC+ tumors by transgenic expression of the CCR4 receptor . This genetic modification enhances the anti-tumor effects of Th1 cells without apparent modification to their effector function . Similar approaches could be extended to other human malignancies in which a signature chemokine pattern can be identified .
The genetic approaches described above can enhance the expansion and persistence of T cells, change their homeostatic properties, alter their trafficking pattern and re-direct their antigen specificity to recognize specific antigens. Inevitably, these changes will be associated with an increased risk of direct toxicity and of unwanted proliferation, and even cell transformation due to insertional mutagenesis . To allow the rapid and complete elimination of infused cells, several groups have evaluated safety switches or suicide genes, which can be triggered should toxicity occur.
The best validated of the suicide genes is thymidine kinase from herpes simplex virus I (HSV-tk) [35, 110]. This enzyme phosphorylates the nontoxic prodrug ganciclovir, which then becomes phosphorylated by endogenous kinases to GCV-triphosphate, causing chain termination and single-strand breaks upon incorporation into DNA, thereby killing dividing cells. Several phase I-II studies have shown that Ganciclovir administration can safely eliminate transferred HSV-tk-modified cells in vivo [110, 111]. Unfortunately, the HSV-tk gene product is itself immunogenic and can induce reactive T cells which may prematurely eliminate the infused cells , compromising the persistence and hence efficacy of the transferred T cells.
More recently, inducible Fas , Fas-associated death domain–containing protein (FADD), and Caspase9 [114, 115] molecules have been considered as alternative non-immunogenic suicide genes. Each of these molecules can act as a suicide switch when fused with a FK-binding protein (FKBP) variant that binds a chemical inducer of dimerization (CID), AP1903, a synthetic drug that has proven safe in healthy volunteers . Administration of this small molecule results in cross-linking and activation of the proapoptotic target molecules. Up to 90% of T cells transduced with inducible Fas or FADD undergo apoptosis after exposure to CID . While promising, elimination of 90% of transduced cells may be insufficient to ensure safety of genetically modified cells in vivo. Moreover, death molecules that act upstream of most apoptosis inhibitors may be ineffective when tried in other cell types. Modification of the late-stage apoptosis pathway molecules such as caspase9 may be preferable . This suicide gene can be stably expressed in human T cells without compromising their functional and phenotypic characteristics [16, 114]. The cells are highly sensitive to the CID, which can induce apoptosis in up to 95% of transduced cells [16, 115]. This suicide gene strategy is currently being tested clinically at our institution as a means of eliminating adoptively-transferred polyclonal T cells after haploidentical transplantation.
Transgenic expression of the CD20 moleucle, which is normally expressed on B cells, has also been postulated as suicide gene for T cell therapies . This strategy relies on the clinical availability of a humanized anti-CD20 antibody (Rituximab) which is widely used to eliminate both normal and neoplastic B cells expressing the CD20 antigen. Thus, infusion of T cells expressing human CD20 and subsequent in vivo administration of Rituximab should efficiently eliminate the infused T cell population, although it will also eliminate normal B cells.
Reports of leukemia development caused by insertional mutagenesis using retroviral vectors for stem cell modification [109, 118], have spurred development of safer techniques for modifying cell DNA sequences, either by pre-determining the integration site of the transgene or by using modified vectors with a safer profile.
Zinc finger nucleases (ZFN) are artificial restriction enzymes generated by fusing a zinc finger DNA-binding domain to a DNA-cleavage domain. The underlying technology is known as gene targeting and is based on the cellular homologous recombination (HR) pathway, which has evolved to promote genetic recombination during meiosis and the repair of DNA double strand breaks. These ZFN can be customized to generate a DNA double stand break at a preselected site in the human genome and their development has paved the way for both knock-out and knock-in strategies in gene therapy. The therapeutic efficacy of this strategy is currently being evaluated in CD4+ T cells, in which disruption of the chemokine receptor 5 is expected to protect the T cells from infection with the HIV virus. Infected mice engrafted with ZFN-modified human CD4+ T cells had a lower load of HIV1 and higher CD4+ T cell counts than controls . Validation studies using this approach for therapeutic transgenes are in the planning stages. Researchers are also moving away from Moloney based retroviral vectors to lentivectors, which appear less prone to integrate near transcription start sites and may thereby reduce the risk of oncogenesis .
Gene marking was first used clinically as a means of determining the persistence of infused T cells in vivo, and to allow investigators to track their migration and association with adverse effect such as GvHD, rather than as a tool to confer additional biological characteristics to cells [5, 34, 37]. Initial trials included inclusion of integrants such as the neomycin resistance gene [5, 34] and non-functional proteins like the truncated nerve growth factor receptor . Expression of genes that extend this conceptual approach and enable direct T cell imaging in vivo are currently in development. For example, repetitive positron emission tomography and computed tomography (PET-CT) imaging of T cells genetically labeled to express the HSV1-tk reporter gene is possible in small and large animals using 2'-fluoro-2'-deoxy-1-β-D-arabinofuranosyl-5-iodouracil (FIAU), 9-(4-[18F]fluoro-3-(hydroxymethyl)butyl)guanine (18F-FHBG) or 2'-[18F]fluoro-5-ethyl-1-beta-D-arabinofuranosyluracil (18F-FEAU) radiotracer [121–123]. Recently, Yaghoubi et al.  reported that 18F-FHBG PET can detect HSV1-tk+ T lymphocytes infused loco-regionally into the site of tumor resection in a patient with grade IV glioblastoma multiforme indicating that these methodologies may be extensively clinically applicable in the future.
Although the administration of ex vivo activated and expanded primary and antigen-specific CTLs is associated with promising clinical results, there are several limitations to broader implementation. A major practical constraint is the current complexity of production. T cell proliferation in traditional cell cultureware is limited by requirements for nutrients and oxygen (O2), and by accumulation of waste products such as carbon dioxide (CO2) and lactic acid so that cultures must be fed and split regularly .
To improve cell output with minimal cell handling, a number of closed-system bioreactors have been explored. Mechanical rocking or stirring increases the availability of O2 in the culture, while media and nutrients can be exchanged by perfusion . We have evaluated a novel gaspermeable rapid expansion cultureware (G-Rex) developed by Wilson Wolf Manufacturing, in which O2 and CO2 are exchanged across a silicone membrane at the base of the flask . Gas exchange from below allows an increased depth of medium above, providing more nutrients and diluting waste. This system supports the expansion of antigen-specific CTLs, as well as genetically modified T cells, is scalable, GMP-compliant, and reduces the number of technician interventions approximately 4-fold while increasing the cell output by 3–20-fold compared with conventional methods (Fig. 4).
To tip the balance of power away from the tumor and in favor of adoptively-transferred T cells many groups have used genetic modification to improve target recognition, enhance T cell persistence and migration, and to increase T cell resistance to the inhibitory tumor microenvironment. Promising pre-clinical data are now being validated in current and future human clinical trials. Even if successful, these therapies will ultimately be limited by the cost and complexity of the cell manufacturing process and simplified and accelerated cell culture systems are now in development and should help to make T cell therapy more accessible.
This work was supported in part from Leukemia and Lymphoma Society Specialized Center of Research (SCOR; grant no. 7018) (M.K.B.), NIH PO1CA94237, NIH P50CA126752, Leukemia and Lymphoma Society Translational Research grants (G.D.) Doris Duke Charitable Foundation/Clinical Scientist development award (G.D.), the ASBMT New Investigator Award (J.F.V.) and the When Everyone Survives (J.F.V.).
All authors contributed in writing the manuscript. The authors declare no competing financial interests.