Oncogene-targeting T cells reject large tumors while oncogene inactivation selects escape variants in mouse models of cancer

Adoptive tumor infiltrating lymphocyte transfer as personalized immunotherapy

Cancer is a leading cause of death worldwide and, despite new targeted therapies and immunotherapies, a large group of patients fail to respond to therapy or progress after initial response, which brings the need for additional treatment options. Manipulating the immune system using a variety of approaches has been explored for the past years with successful results. Sustained progress has been made to understand the T cell-mediated anti-tumor responses counteracting the tumorigenesis process. The T-lymphocyte pool, especially its capacity for antigen-directed cytotoxicity, has become a central focus for engaging the immune system in defeating cancer. The adoptive cell transfer of autologous tumor-infiltrating lymphocytes has been used in humans for over 30 years to treat metastatic melanoma. In this review, we provide a brief history of ACT-TIL and discuss the current state of ACT-TIL clinical development in solid tumors. We also discuss how key advances in understanding genetic intratumor heterogeneity, to accurately identify neoantigens, and new strategies designed to overcome T-cell exhaustion and tumor immunosuppression have improved the efficacy of the TIL-therapy infusion. Characteristics of the TIL products will be discussed, as well as new strategies, including the selective expansion of specific fractions from the cell product or the genetic manipulation of T cells for improving the in-vivo survival and functionality. In summary, this review outlines the potential of ACT-TIL as a personalized approach for epithelial tumors and continued discoveries are making it increasingly more effective against other types of cancers.

Investigating T Cell Immunity in Cancer: Achievements and Prospects

T cells play a key role in tumour surveillance, both identifying and eliminating transformed cells. However, as tumours become established they form their own suppressive microenvironments capable of shutting down T cell function, and allowing tumours to persist and grow. To further understand the tumour microenvironment, including the interplay between different immune cells and their role in anti-tumour immune responses, a number of studies from mouse models to clinical trials have been performed. In this review, we examine mechanisms utilized by tumour cells to reduce their visibility to CD8⁺ Cytotoxic T lymphocytes (CTL), as well as therapeutic strategies trialled to overcome these tumour-evasion mechanisms. Next, we summarize recent advances in approaches to enhance CAR T cell activity and persistence over the past 10 years, including bispecific CAR T cell design and early evidence of efficacy. Lastly, we examine mechanisms of T cell infiltration and tumour regression, and discuss the strengths and weaknesses of different strategies to investigate T cell function in murine tumour models.

Long-term in vitro expansion ensures increased yield of central memory T cells as perspective for manufacturing challenges

Adoptive T cell therapy (ATT) has revolutionized the treatment of cancer patients. A sufficient number of functional T cells are indispensable for ATT efficacy; however, several ATT dropouts have been reported due to T cell expansion failure or lack of T cell persistence in vivo. With the aim of providing ATT also to those patients experiencing insufficient T cell manufacturing via standard protocol, we evaluated if minimally manipulative prolongation of in vitro expansion (long-term [LT] >3 weeks with IL-7 and IL-15 cytokines) could result in enhanced T cell yield with preserved T cell functionality. The extended expansion resulted in a 39-fold increase of murine CD8⁺ T central memory cells (Tcm). LT expanded CD8⁺ and CD4⁺ Tcm cells retained a gene expression profile related to Tcm and T memory stem cells (Tscm). In vivo transfer of LT expanded Tcm revealed persistence and antitumor capacity. We confirmed our in vitro findings on human T cells, on healthy donors and diffuse large B cell lymphoma patients, undergoing salvage therapy. Our study demonstrates the feasibility of an extended T cell expansion as a practicable alternative for patients with insufficient numbers of T cells after the standard manufacturing process thereby increasing ATT accessibility.

CD28 Co-Stimulus Achieves Superior CAR T Cell Effector Function against Solid Tumors Than 4-1BB Co-Stimulus

Spacer or co-stimulatory components in chimeric antigen receptor (CAR) design influence CAR T cell effector function. Few preclinical mouse models optimally support CAR candidate pre-selection for clinical development. Here we use a model in which murine CAR T cells can be exploited with human tumor xenografts. This mouse-in-mouse approach avoids limitations caused by species-specific factors crucial for CAR T cell survival, trafficking and function. We compared trafficking, expansion and tumor control for T cells expressing different CAR construct designs targeting two antigens (L1CAM or HER2), structurally identical except for spacer (long or short) or co-stimulatory (4-1BB or CD28) domains to be evaluated. Using monoclonal, murine-derived L1CAM-specific CAR T cells in Rag-/- mice harboring established xenografted tumors from a human neuroblastoma cell line revealed a clear superiority in CAR T cell trafficking using CD28 co-stimulation. L1CAM-targeting short spacer-CD28/ζ CAR T cells expanded the most at the tumor site and induced initial tumor regression. Treating patient-derived neuroblastoma xenografts with human L1CAM-targeting CAR T cells confirmed the superiority of CD28 co-stimulus. CD28 superiority was also demonstrated with HER2-specific CAR T cells (targeting ovarian carcinoma xenografts). Our findings encourage incorporating CD28 signaling into CAR design for adoptive T cell treatment of solid tumors.

Longitudinal imaging of T cell-based immunotherapy with multi-spectral, multi-scale optoacoustic tomography

Most imaging studies of immunotherapy have focused on tracking labeled T cell biodistribution in vivo for understanding trafficking and homing parameters and predicting therapeutic efficacy by the presence of transferred T cells at or in the tumour mass. Conversely, we investigate here a novel concept for longitudinally elucidating anatomical and pathophysiological changes of solid tumours after adoptive T cell transfer in a preclinical set up, using previously unexplored in-tandem macroscopic and mesoscopic optoacoustic (photoacoustic) imaging. We show non-invasive in vivo observations of vessel collapse during tumour rejection across entire tumours and observe for the first time longitudinal tumour rejection in a label-free manner based on optical absorption changes in the tumour mass due to cellular decline. We complement these observations with high resolution episcopic fluorescence imaging of T cell biodistribution using optimized T cell labeling based on two near-infrared dyes targeting the cell membrane and the cytoplasm. We discuss how optoacoustic macroscopy and mesoscopy offer unique contrast and immunotherapy insights, allowing label-free and longitudinal observations of tumour therapy. The results demonstrate optoacoustic imaging as an invaluable tool in understanding and optimizing T cell therapy.

Systemic clinical tumor regressions and potentiation of PD1 blockade with in situ vaccination

Indolent non-Hodgkin's lymphomas (iNHLs) are incurable with standard therapy and are poorly responsive to checkpoint blockade. Although lymphoma cells are efficiently killed by primed T cells, in vivo priming of anti-lymphoma T cells has been elusive. Here, we demonstrate that lymphoma cells can directly prime T cells, but in vivo immunity still requires cross-presentation. To address this, we developed an in situ vaccine (ISV), combining Flt3L, radiotherapy, and a TLR3 agonist, which recruited, antigen-loaded and activated intratumoral, cross-presenting dendritic cells (DCs). ISV induced anti-tumor CD8⁺ T cell responses and systemic (abscopal) cancer remission in patients with advanced stage iNHL in an ongoing trial ( NCT01976585 ). Non-responding patients developed a population of PD1⁺CD8⁺ T cells after ISV, and murine tumors became newly responsive to PD1 blockade, prompting a follow-up trial of the combined therapy. Our data substantiate that recruiting and activating intratumoral, cross-priming DCs is achievable and critical to anti-tumor T cell responses and PD1-blockade efficacy.

Effective NY-ESO-1-specific MHC II-restricted T cell receptors from antigen-negative hosts enhance tumor regression

Adoptive transfer of T cell receptor-engineered (TCR-engineered) T cells is a promising approach in cancer therapy but needs improvement for more effective treatment of solid tumors. While most clinical approaches have focused on CD8+ T cells, the importance of CD4+ T cells in mediating tumor regression has become apparent. Regarding shared (self) tumor antigens, it is unclear whether the human CD4+ T cell repertoire has been shaped by tolerance mechanisms and lacks highly functional TCRs suitable for therapy. Here, TCRs against the tumor-associated antigen NY-ESO-1 were isolated either from human CD4+ T cells or from mice that express a diverse human TCR repertoire with HLA-DRA/DRB1*0401 restriction and are NY-ESO-1 negative. NY-ESO-1-reactive TCRs from the mice showed superior recognition of tumor cells and higher functional activity compared with TCRs from humans. We identified a candidate TCR, TCR-3598_2, which was expressed in CD4+ T cells and caused tumor regression in combination with NY-ESO-1-redirected CD8+ T cells in a mouse model of adoptive T cell therapy. These data suggest that MHC II-restricted TCRs against NY-ESO-1 from a nontolerant nonhuman host are of optimal affinity and that the combined use of MHC I- and II-restricted TCRs against NY-ESO-1 can make adoptive T cell therapy more effective.

In situ administration of cytokine combinations induces tumor regression in mice

Background

Recent advances in cancer immunotherapy suggest a possibility of harnessing the immune system to defeat malignant tumors, but the complex immunosuppressive microenvironment confines the therapeutic benefits to a minority of patients with solid tumors.

Methods

A lentivector-based inducible system was established to evaluate the therapeutic effect of cytokines in established tumors. Intratumoral injection of certain cytokine combination in syngeneic tumor models was conducted to assess the therapeutic potentials.

Findings

Doxycycline (Dox)-induced local expression of cytokine combinations exhibites a strong synergistic effect, leading to complete regression of tumors. Notably, IL12 + GMCSF+IL2 expression induces eradication of tumors in all mice tolerated with this treatment, including those bearing large tumors of ~15 mm in diameter, and generates intensive systemic antitumor immunity. Other combinations with similar immune regulatory roles also induce tumor elimination in most of mice. Moreover, intratumoral injection of chitosan/IL12 + GMCSF+IL2 solution induces a complete response in all the tested syngeneic tumor models, regardless of various tumor immunograms.

Interpretation

Administration of certain cytokine combinations in tumor microenvironment induces a strong synergistic antitumor response, including the recruitment of large amount of immune cells and the generation of systemic antitumor immunity. It provides a versatile method for the immunotherapy of intractable malignant neoplasms.

Fund

There is no external funding supporting this study.

Bladder cancer-associated cancer-testis antigen-derived long peptides encompassing both CTL and promiscuous HLA class II-restricted Th cell epitopes induced CD4+ T cells expressing converged T-cell receptor genes in vitro

DEP domain containing 1 (DEPDC1) and M-phase phosphoprotein 1 (MPHOSPH1) are human cancer testis antigens that are frequently overexpressed in urinary bladder cancer. In a phase I/II clinical trial, a DEPDC1- and MPHOSPH1-derived short peptide vaccine demonstrated promising efficacy in preventing bladder cancer recurrence. Here, we aimed to identify long peptides (LPs) derived from DEPDC1 and MPHOSPH1 that induced both T-helper (Th) cells and tumor-reactive cytotoxic T lymphocytes (CTLs). Stimulation of peripheral blood mononuclear cells (PBMCs) from healthy donors with the synthetic DEPDC1- and MPHOSPH1-LPs predicted to bind to promiscuous human leukocyte antigen (HLA) class II molecules by a computer algorithm induced specific CD4⁺ T cells as revealed by interferon-γ enzyme-linked immunospot assays. Three of six LPs encompassed HLA-A2- or -A24-restricted CTL epitopes or both, and all six LPs stimulated DEPDC1- or MPHOSPH1-specific Th cells restricted by promiscuous and frequently observed HLA class II molecules in the Japanese population. Some LPs are naturally processed from the proteins in DCs, and the capacity of these LPs to cross-prime CTLs was confirmed in vivo using HLA-A2 or -A24 transgenic mice. The LP-specific and HLA class II-restricted T-cell responses were also observed in PBMCs from patients with bladder cancer. Repeated stimulation of PBMCs with DEPDC1-LPs and MPHOSPH1-LPs yielded clonal Th cells expressing specific T-cell receptor (TCR)-α and β genes. These DEPDC1- or MPHOSPH1-derived LPs may have applications in immunotherapy in patients with bladder cancer, and the TCR genes identified may be useful for monitoring of Th cells specific to LPs in vivo.

A Transgenic Dual-Luciferase Reporter Mouse for Longitudinal and Functional Monitoring of T Cells In Vivo

Adoptive T-cell therapy (ATT) efficacy is limited when targeting large solid tumors. The evaluation of ATT outcomes using accessory treatment would greatly benefit from an in vivo monitoring tool, allowing the detection of functional parameters of transferred T cells. Here, we generated transgenic bioluminescence imaging of T cells (BLITC) mice expressing an NFAT-dependent click-beetle luciferase and a constitutive Renilla luciferase, which supports concomitant in vivo analysis of migration and activation of T cells. Rapid transferability of our system to preestablished tumor models was demonstrated in the SV40-large T antigen model via both crossbreeding of BLITC mice into a T-cell receptor (TCR)-transgenic background and TCR transduction of BLITC T cells. We observed rapid tumor infiltration of BLITC CD8⁺ T cells followed by a burst-like activation that mirrored rejection kinetics. Using the BLITC reporter in the clinically relevant H-Y model, we performed female to male transfers and detected H-Y-specific alloreactivity (graft-versus-host disease) in vivo In an H-Y solid tumor model, we found migration of adoptively transferred H-Y TCR-transgenic CD4⁺ T cells into the tumor, marked by transient activation. This suggests a rapid inactivation of infiltrating T cells by the tumor microenvironment, as confirmed by their expression of inhibitory receptors. In summary, the BLITC reporter system facilitates analysis of therapeutic parameters for ATT, is rapidly transferable to models of interest not restricted to tumor research, and is suitable for rapid screening of TCR clones for tumor rejection kinetics, as well as off-target effects. Cancer Immunol Res; 6(1); 110-20. ©2018 AACR.

The immune system prevents recurrence of transplanted but not autochthonous antigenic tumors after oncogene inactivation therapy

Targeted oncogene inactivation by small molecule inhibitors can be very effective but tumor recurrence is a frequent problem in the clinic. Therapy by inactivation of the cancer-driving oncogene in transplanted tumors was shown to be augmented in the presence of T cells. However, these experiments did not take into account the long-term, usually tolerogenic, interaction of de novo malignancies with the immune system. Here, we employed mice, in which SV40 large T (Tag) and firefly luciferase (Luc) as fusion protein (TagLuc) could be regulated with the Tet-on system and upon activation resulted in tumors after a long latency. TagLuc inactivation induced profound tumor regression, demonstrating sustained oncogene addiction. While tumor relapse after TagLuc inactivation was prevented in immunocompetent mice bearing transplanted tumors, autochthonous tumors relapsed or recurred after therapy discontinuation indicating that the immune system that coevolved with the malignancy over an extended period of time lost the potency to mount an efficient anti-tumor immune response. By contrast, adoptively transferred CD8⁺ T cells targeting the cancer-driving oncogene eradicated recurrent autochthonous tumors, highlighting a suitable therapy option in a clinically relevant model.

Tumour ischaemia by interferon-γ resembles physiological blood vessel regression

The relative contribution of the effector molecules produced by T cells to tumour rejection is unclear, but interferon-γ (IFNγ) is critical in most of the analysed models. Although IFNγ can impede tumour growth by acting directly on cancer cells, it must also act on the tumour stroma for effective rejection of large, established tumours. However, which stroma cells respond to IFNγ and by which mechanism IFNγ contributes to tumour rejection through stromal targeting have remained unknown. Here we use a model of IFNγ induction and an IFNγ-GFP fusion protein in large, vascularized tumours growing in mice that express the IFNγ receptor exclusively in defined cell types. Responsiveness to IFNγ by myeloid cells and other haematopoietic cells, including T cells or fibroblasts, was not sufficient for IFNγ-induced tumour regression, whereas responsiveness of endothelial cells to IFNγ was necessary and sufficient. Intravital microscopy revealed IFNγ-induced regression of the tumour vasculature, resulting in arrest of blood flow and subsequent collapse of tumours, similar to non-haemorrhagic necrosis in ischaemia and unlike haemorrhagic necrosis induced by tumour necrosis factor. The early events of IFNγ-induced tumour ischaemia resemble non-apoptotic blood vessel regression during development, wound healing or IFNγ-mediated, pregnancy-induced remodelling of uterine arteries. A better mechanistic understanding of how solid tumours are rejected may aid the design of more effective protocols for adoptive T-cell therapy.

Fusions of Tumor-derived Endothelial Cells with Dendritic Cells Induces Antitumor Immunity

To explore dendritic cells/tumor-derived endothelial cells (DC/EC) fusion cells are potent stimulators of T cells to impact tumor progression. ECs were isolated from mice hepatoma cell line (H22) Xenograft, and dendritic cells were isolated from bone marrow of BALB/c mice, then the isolated ECs were cultured and detected the endothelial surface expression of CD105 by flow cytometry. The endothelial characteristics of ECs were detected by tube formation assay and Dil-Ac-LDL uptake assay. After the fusion with polyethylene glycol (PEG), we used DCs, ECs, DCs mixed ECs as the control groups, DC/EC fusion cells as the experimental group, Secretion of IFN-α and IFN-γ was evaluated, T lymphocyte proliferation and cytotoxic T lymphocytes (CTL) were detected in vitro. In vivo, T lymphocyte induced by five groups was injected to detect the effect of tumor progression. Purified ECs (CD105⁺) took the function of endothelial cells, then successfully fused with DCs. The DC/EC fusion cells were functional in stimulating the proliferation of T cells, which produced IFN-α and IFN-γ. In vivo, T cells stimulated by DC/EC fusion cells effectively repressed tumor growth. The fusion cells, which was capable of stimulating T cells, is indispensable for antitumor immunity.

Preventing tumor escape by targeting a post-proteasomal trimming independent epitope

Blankenstein and colleagues describe a novel strategy to avoid tumor escape from adoptive T cell therapy.

Adoptive T cell therapy (ATT) can achieve regression of large tumors in mice and humans; however, tumors frequently recur. High target peptide-major histocompatibility complex-I (pMHC) affinity and T cell receptor (TCR)-pMHC affinity are thought to be critical to preventing relapse. Here, we show that targeting two epitopes of the same antigen in the same cancer cells via monospecific T cells, which have similar pMHC and pMHC-TCR affinity, results in eradication of large, established tumors when targeting the apparently subdominant but not the dominant epitope. Only the escape but not the rejection epitope required postproteasomal trimming, which was regulated by IFN-γ, allowing IFN-γ–unresponsive cancer variants to evade. The data describe a novel immune escape mechanism and better define suitable target epitopes for ATT.

Immune-mediated mechanisms influencing the efficacy of anticancer therapies

Conventional anticancer therapies, such as chemotherapy, radiotherapy, and targeted therapy, are designed to kill cancer cells. However, the efficacy of anticancer therapies is not only determined by their direct effects on cancer cells but also by off-target effects within the host immune system. Cytotoxic treatment regimens elicit several changes in immune-related parameters including the composition, phenotype, and function of immune cells. Here we discuss the impact of innate and adaptive immune cells on the success of anticancer therapy. In this context we examine the opportunities to exploit host immune responses to boost tumor clearing, and highlight the challenges facing the treatment of advanced metastatic disease.

Methods to study primary tumor cells and residual tumor cells in mouse models of oncogene dependence

The studies of oncogene dependence are aimed to understand an unfortunate and puzzling aspect of targeted anticancer treatments-their progression to drug resistance. Drug resistance develops from a pool of cells that survive the original treatment, called minimal residual disease. Mouse models based on tetracycline-dependent expression of transgenic oncogenes are used to imitate targeted oncogene blockade and to reproduce minimal residual disease in humans. Here we describe a novel method for generating oncogene-dependent mammary tumors using somatic transfer of transactivator-containing retroviruses into transgenic mice with tetracycline-dependent oncogenes and a method for measuring continuous mitotic activity in epithelial cells in real time.

Permissive expansion and homing of adoptively transferred T cells in tumor-bearing hosts

Activated T cells expressing endogenous or transduced TCRs are two cell types currently used in clinical adoptive T-cell therapy. The ability of these cells to recognize their antigen, expand and traffic to the tumor site are the initial steps necessary for successful therapy. In this study, we used in vivo bioluminescent imaging (BLI) of Renilla luciferase (RLuc) expressing T cells to evaluate the ability of adoptively transferred T cells to survive, expand and home to tumor site in vivo. Using this method, termed RT-Rack (Rluc T cell tracking), we followed T-cell response against tumors in vivo. Expansion and homing of adoptively transferred T cells were antigen dependent, but independent of the host immune status. Moreover, we successfully detected T-cell response to small and large tumors, including autochthonous liver tumors. The adoptively transferred T cells were not ignorant or excluded in a partially tolerant host, which expressed low level of the target in the periphery. Using T cell receptor (TCR)-engineered T cells, we showed the ability of these cells to respond in tumor-bearing hosts by expanding and homing to the tumor site. In all these models, the host immune status, the nature of the tumor or of the antigen, the tumor size and the presence of the targeted antigen in the periphery did not prevent the adoptively transferred T cells from responding by expanding and homing to the tumor. However, T cells had higher expression of the inhibitory receptor PD1 and reduced functional activity when a self-antigen was targeted.

Functional expression cloning identifies COX-2 as a suppressor of antigen-specific cancer immunity

The efficacy of immune surveillance and antigen-specific cancer immunotherapy equally depends on the activation of a sustained immune response targeting cancer antigens and the susceptibility of cancer cells to immune effector mechanisms. Using functional expression cloning and T-cell receptor (TCR) transgenic mice, we have identified cyclooxygenase 2/prostaglandin-endoperoxide synthase 2 (COX-2) as resistance factor against the cytotoxicity induced by activated, antigen-specific T cells. Expressing COX-2, but not a catalytically inactive COX-2 mutant, increased the clonogenic survival of E1A-transformed murine cancer cells when cocultured with lymphocytes from St42Rag2^−/− mice harboring a transgenic TCR directed against an E1A epitope. COX-2 expressing tumors established in immune-deficient mice were less susceptible to adoptive immunotherapy with TCR transgenic lymphocytes in vivo. Also, immune surveillance of COX-2-positive tumor cells in TCR transgenic mice was less efficient. The growth of murine MC-GP tumors, which show high endogenous COX-2 expression, in immunocompetent mice was effectively suppressed by treatment with a selective COX-2 inhibitor, celecoxib. Mechanistically, COX-2 expression blunted the interferon-gamma release of antigen-specific T cells exposed to their respective cellular targets, and increased the expression of interleukin-4 and indoleamine 2,3-dioxygenase by tumor cells. Addition of interferon-gamma sensitized COX-2 expressing cancer cells to tumor suppression by antigen-specific T cells. In conclusion, COX-2, which is frequently induced in colorectal cancer, contributes to immune evasion and resistance to antigen-specific cancer immunotherapy by local suppression of T-cell effector functions.

Efficacy of CAR T-cell therapy in large tumors relies upon stromal targeting by IFNγ

Adoptive T-cell therapy using chimeric antigen receptor-modified T cells (CAR-T therapy) has shown dramatic efficacy in patients with circulating lymphoma. However, eradication of solid tumors with CAR-T therapy has not been reported yet to be efficacious. In solid tumors, stroma destruction, due to MHC-restricted cross-presentation of tumor antigens to T cells, may be essential. However, CAR-Ts recognize antigens in an MHC-independent manner on cancer cells but not stroma cells. In this report, we show how CAR-Ts can be engineered to eradicate large established tumors with provision of a suitable CD28 costimulatory signal. In an HER2-dependent tumor model, tumor rejection by HER2-specific CAR-Ts was associated with sustained influx and proliferation of the adoptively transferred T cells. Interestingly, tumor rejection did not involve natural killer cells but was associated instead with a marked increase in the level of M1 macrophages and a requirement for IFNγ receptor expression on tumor stroma cells. Our results argue that CAR-T therapy is capable of eradicating solid tumors through a combination of antigen-independent stroma destruction and antigen-specific tumor cell targeting.

Oncogene withdrawal engages the immune system to induce sustained cancer regression

The targeted inactivation of a single oncogene can induce dramatic tumor regression, suggesting that cancers are “oncogene addicted.” Tumor regression following oncogene inactivation has been thought to be a consequence of restoration of normal physiological programs that induce proliferative arrest, apoptosis, differentiation, and cellular senescence. However, recent observations illustrate that oncogene addiction is highly dependent upon the host immune cells. In particular, CD4⁺ helper T cells were shown to be essential to the mechanism by which MYC or BCR-ABL inactivation elicits “oncogene withdrawal.” Hence, immune mediators contribute in multiple ways to the pathogenesis, prevention, and treatment of cancer, including mechanisms of tumor initiation, progression, and surveillance, but also oncogene inactivation-mediated tumor regression. Data from both the bench and the bedside illustrates that the inactivation of a driver oncogene can induce activation of the immune system that appears to be essential for sustained tumor regression.

An essential role for the immune system in the mechanism of tumor regression following targeted oncogene inactivation

Tumors are genetically complex and can have a multitude of mutations. Consequently, it is surprising that the suppression of a single oncogene can result in rapid and sustained tumor regression, illustrating the concept that cancers are often "oncogene addicted." The mechanism of oncogene addiction has been presumed to be largely cell autonomous as a consequence of the restoration of normal physiological programs that induce proliferative arrest, apoptosis, differentiation, and/or cellular senescence. Interestingly, it has recently become apparent that upon oncogene inactivation, the immune response is critical in mediating the phenotypic consequences of oncogene addiction. In particular, CD4(+) T cells have been suggested to be essential to the remodeling of the tumor microenvironment, including the shutdown of host angiogenesis and the induction of cellular senescence in the tumor. However, adaptive and innate immune cells are likely involved. Thus, the effectors of the immune system are involved not only in tumor initiation, tumor progression, and immunosurveillance, but also in the mechanism of tumor regression upon targeted oncogene inactivation. Hence, oncogene inactivation may be an effective therapeutic approach because it both reverses the neoplastic state within a cancer cell and reactivates the host immune response that remodels the tumor microenvironment.

CD8+ T-cell responses rapidly select for antigen-negative tumor cells in the prostate

Stimulation of patients' immune systems for the treatment of solid tumors is an emerging therapeutic paradigm. The use of enriched autologous T cells for adoptive cell therapy or vaccination with antigen-loaded dendritic cells have shown clinical efficacy in melanoma and prostate cancer, respectively. However, the long-term effects of immune responses on selection and outgrowth of antigen-negative tumor cells in specific tumor types must be determined to understand and achieve long-term therapeutic effects. In this study, we have investigated the expression of a tumor-specific antigen in situ after treatment with tumor-specific CD8(+) T cells in an autochthonous mouse model of prostate cancer. After T-cell treatment, aggregates of dead antigen-positive tumor cells were concentrated in the lumen of the prostate gland and were eventually eliminated from the prostate tissue. Despite the elimination of antigen-positive tumor cells, prostate tumor continued to grow in T-cell-treated mice. Interestingly, the remaining tumor cells were antigen negative and downregulated MHC class I expression. These results show that CD8(+) T cells are effective in eliminating antigen-bearing prostate tumor cells but they also can select for the outgrowth of antigen-negative tumor cells. These findings provide insights into the requirements for an effective cancer immunotherapy within the prostate that not only induces potent immune responses but also avoids selection and outgrowth of antigen-negative tumor cells.

TCR-Engineered T Cells Meet New Challenges to Treat Solid Tumors: Choice of Antigen, T Cell Fitness, and Sensitization of Tumor Milieu

Adoptive transfer of T cells gene-engineered with antigen-specific T cell receptors (TCRs) has proven its feasibility and therapeutic potential in the treatment of malignant tumors. To ensure further clinical development of TCR gene therapy, it is necessary to target immunogenic epitopes that are related to oncogenesis and selectively expressed by tumor tissue, and implement strategies that result in optimal T cell fitness. In addition, in particular for the treatment of solid tumors, it is equally necessary to include strategies that counteract the immune-suppressive nature of the tumor micro-environment. Here, we will provide an overview of the current status of TCR gene therapy, and redefine the following three challenges of improvement: “choice of target antigen”; “fitness of T cells”; and “sensitization of tumor milieu.” We will categorize and discuss potential strategies to address each of these challenges, and argue that advancement of clinical TCR gene therapy critically depends on developments toward each of the three challenges.

Targeting c-MYC with T-cells

Over-expression of the proto-oncogene c-MYC is frequently observed in a variety of tumors and is a hallmark of Burkitt´s lymphoma. The fact that many tumors are oncogene-addicted to c-MYC, renders c-MYC a powerful target for anti-tumor therapy. Using a xenogenic vaccination strategy by immunizing C57BL/6 mice with human c-MYC protein or non-homologous peptides, we show that the human c-MYC protein, despite its high homology between mouse and man, contains several immunogenic epitopes presented in the context of murine H2^b haplotype. We identified an MHC class II-restricted CD4⁺ T-cell epitope and therein an MHC class I-restricted CD8⁺ T-cell epitope (SSPQGSPEPL) that, after prime/boost immunization, protected up to 25% of mice against a lethal lymphoma challenge. Lymphoma-rejecting animals contained MHC multimer-binding CD8⁺ cell within the peripheral blood and displayed in vivo cytolytic activity with specificity for SSPQGSPEPL. Taken together these data suggest that oncogenic c-MYC can be targeted with specific T-cells.

T cell immunotherapy for melanoma from bedside to bench to barn and back: how conceptual advances in experimental mouse models can be translated into clinical benefit for patients

A solid scientific basis now supports the concept that cytotoxic T lymphocytes can specifically recognize and destroy melanoma cells. Over the last decades, clinicians and basic scientists have joined forces to advance our concepts of melanoma immunobiology. This has catalyzed the rational development of therapeutic approaches to enforce melanoma-specific T cell responses. Preclinical studies in experimental mouse models paved the way for their successful translation into clinical benefit for patients with metastatic melanoma. A more thorough understanding of how melanomas develop resistance to T cell immunotherapy is necessary to extend this success. This requires a continued interdisciplinary effort of melanoma biologists and immunologists that closely connects clinical observations with in vitro investigations and appropriate in vivo mouse models: From bedside to bench to barn and back.

Relapse or eradication of cancer is predicted by peptide-major histocompatibility complex affinity

Cancers often relapse after adoptive therapy, even though specific T cells kill cells from the same cancer efficiently in vitro. We found that tumor eradication by T cells required high affinities of the targeted peptides for major histocompatibility complex (MHC) class I. Affinities of at least 10 nM were required for relapse-free regression. Only high-affinity peptide-MHC interactions led to efficient cross-presentation of antigen, thereby stimulating cognate T cells to secrete cytokines. These findings highlight the importance of targeting peptides with high affinity for MHC class I when designing T cell-based immunotherapy.

Noncanonical roles of the immune system in eliciting oncogene addiction

Cancer is highly complex. The magnitude of this complexity makes it highly surprising that even the brief suppression of an oncogene can sometimes result in rapid and sustained tumor regression, illustrating that cancers can be 'oncogene addicted' [1-10]. The essential implication is that oncogenes may not only fuel the initiation of tumorigenesis, but in some cases must be excessively activated to maintain a neoplastic state [11]. Oncogene suppression acutely restores normal physiological programs that effectively overrides secondary genetic events and a cancer collapses [12,13]. Oncogene addiction is the description of the dramatic and sustained regression of some cancers upon the specific inactivation of a single oncogene [1-13,14(••),15,16(••)], that can occur through tumor intrinsic [1,2,4,12], but also host immune mechanisms [17-23]. Notably, oncogene inactivation elicits a host immune response that involves specific immune effectors and cytokines that facilitate a remodeling of the tumor microenvironment including the shut down of angiogenesis and the induction of cellular senescence of tumor cells [16(••)]. Hence, immune effectors are not only critically involved in tumor prevention, initiation [17-19], and progression [20], but also appear to be essential to tumor regression upon oncogene inactivation [21,22(••),23(••)]. Understanding how the inactivation of an oncogene elicits a systemic signal in the host that prompts a deconstruction of a tumor could have important implications. The combination of oncogene-targeted therapy together with immunomodulatory therapy may be ideal for the development of both robust tumor intrinsic and immunological responses, effectively leading to sustained tumor regression.

Genetically engineered mouse models of cancer reveal new insights about the antitumor immune response

Cancer is a complex disease that can originate in virtually all the tissues of the body, and tumors progress through many different stages during their development. While genetic mutations in the emerging cancer cells drive this disease, it has become increasingly clear that cancer development is strongly influenced by the surrounding microenvironment. Cells of the immune system are critical components of this extrinsic network of cancer regulators, contributing significantly to the microenvironment of most cancers and either promoting or inhibiting the initiation and progression of this disease. Genetically engineered mouse (GEM) mouse models of spontaneous cancer are starting to shape our understanding of how antitumor T cells may act to prevent or inhibit cancer progression in some settings and not others. Lessons learned from investigating spontaneous mouse cancer models have important implications for directing clinical efforts that attempt to direct a cancer patient's immune system to eradicate their disease.

Fas expression by tumor stroma is required for cancer eradication

The contribution of molecules such as perforin, IFN-γ (IFNγ), and particularly Fas ligand (FasL) by transferred CD8(+) effector T (T(E)) cells to rejection of large, established tumors is incompletely understood. Efficient attack against large tumors carrying a surrogate tumor antigen (mimicking a "passenger" mutation) by T(E) cells requires action of IFNγ on tumor stroma cells to avoid selection of antigen-loss variants. Because "cancer-driving" antigens (CDAs) are rarely counterselected, IFNγ may be expected to be dispensable in elimination of cancers by targeting a CDA. Here, initial regression of large, established tumors required neither IFNγ, FasL, nor perforin by transferred CD8(+) T(E) cells targeting Simian Virus (SV) 40 large T as CDA. However, cytotoxic T(E) cells lacking IFNγ or FasL could not prevent relapse despite retention of the rejection antigen by the cancer cells. Complete tumor rejection required IFNγ-regulated Fas by the tumor stroma. Therefore, T(E) cells lacking IFNγ or FasL cannot prevent progression of antigenic cancer because the tumor stroma escapes destruction if its Fas expression is down-regulated.

Virus-induced hepatocellular carcinomas cause antigen-specific local tolerance

T cell surveillance is often effective against virus-associated tumors because of their high immunogenicity. It is not clear why surveillance occasionally fails, particularly against hepatitis B virus- or hepatitis C virus-associated hepatocellular carcinoma (HCC). We established a transgenic murine model of virus-induced HCC by hepatocyte-specific adenovirus-induced activation of the oncogenic SV40 large T antigen (TAg). Adenovirus infection induced cytotoxic T lymphocytes (CTLs) targeted against the virus and TAg, leading to clearance of the infected cells. Despite the presence of functional, antigen-specific T cells, a few virus-infected cells escaped immune clearance and progressed to HCC. These cells expressed TAg at levels similar to HCC isolated from neonatal TAg-tolerant mice, suggesting that CTL clearance does not select for cells with low immunogenicity. Virus-infected mice revealed significantly greater T cell infiltration in early-stage HCC compared with that in late-stage HCC, demonstrating progressive local immune suppression through inefficient T cell infiltration. Programmed cell death protein-1 (PD-1) and its ligand PD-L1 were expressed in all TAg-specific CD8+ T cells and HCC, respectively, which contributed to local tumor-antigen-specific tolerance. Thus, we have developed a model of virus-induced HCC that may allow for a better understanding of human HCC.

Molecular pathways: comparing the effects of drugs and T cells to effectively target oncogenes

Mutant cancer-driving oncogenes are the best therapeutic targets, both with drugs like small-molecule inhibitors (SMI) and adoptive T-cell therapy (ATT), the most effective form of immunotherapy. Cancer cell survival often depends on oncogenes, which implies that they are homogeneously expressed by all cancer cells and are difficult to select against. Mutant oncogene-directed therapy is relatively selective, as it targets preferentially the oncogene-expressing cancer cells. Both SMI and ATT can be highly effective in relevant preclinical models as well as selected clinical situations, and both share the risk of therapy resistance, facilitated by the frequent genetic instability of cancer cells. Recently, both therapies were compared in the same experimental model targeting the same oncogene. It showed that the oncogene-inactivating drug selected resistant clones, leading eventually to tumor relapse, whereas ATT eradicated large established tumors completely. The mode of tumor destruction likely explained the different outcome with only ATT destroying the tumor vasculature. Elucidating the cellular and molecular mechanisms responsible for tumor regression and relapse will define optimal conditions for the clinic. We argue that the ideal conditions of ATT in the experimental cancer model can be translated to individuals with cancer.

Trial Watch: Adoptive cell transfer immunotherapy

During the last two decades, several approaches for the activation of the immune system against cancer have been developed. These include rather unselective maneuvers such as the systemic administration of immunostimulatory agents (e.g., interleukin-2) as well as targeted interventions, encompassing highly specific monoclonal antibodies, vaccines and cell-based therapies. Among the latter, adoptive cell transfer (ACT) involves the selection of autologous lymphocytes with antitumor activity, their expansion/activation ex vivo, and their reinfusion into the patient, often in the context of lymphodepleting regimens (to minimize endogenous immunosuppression). Such autologous cells can be isolated from tumor-infiltrating lymphocytes or generated by manipulating circulating lymphocytes for the expression of tumor-specific T-cell receptors. In addition, autologous lymphocytes can be genetically engineered to prolong their in vivo persistence, to boost antitumor responses and/or to minimize side effects. ACT has recently been shown to be associated with a consistent rate of durable regressions in melanoma and renal cell carcinoma patients and holds great promises in several other oncological settings. In this Trial Watch, we will briefly review the scientific rationale behind ACT and discuss the progress of recent clinical trials evaluating the safety and effectiveness of adoptive cell transfer as an anticancer therapy.

Adoptive immunotherapy for cancer: harnessing the T cell response

Immunotherapy based on the adoptive transfer of naturally occurring or gene-engineered T cells can mediate tumour regression in patients with metastatic cancer. Here, we discuss progress in the use of adoptively transferred T cells, focusing on how they can mediate tumour cell eradication. Recent advances include more accurate targeting of antigens expressed by tumours and the associated vasculature, and the successful use of gene engineering to re-target T cells before their transfer into the patient. We also describe how new research has helped to identify the particular T cell subsets that can most effectively promote tumour eradication.

Spleen cells from young but not old immunized mice eradicate large established cancers

PURPOSE

Solid tumors that have grown two weeks or longer in mice and have diameters larger than 1 cm are histologically indistinguishable from autochthonous human cancers. When experimental tumors reach this clinically relevant size, they are usually refractory to most immunotherapies but may be destroyed by adoptive T-cell transfer. However, TCR-transgenic T cells and/or tumor cells overexpressing antigens are frequently used in these experiments. Here we studied the requirements for destroying clinical size, unmanipulated 8101 tumors by adoptive cell therapy.

EXPERIMENTAL DESIGN

8101 arose in an old mouse after chronic exposure to UV light. A cancer line was established, which was never serially transplanted. The immunodominant CD8(+) T cell-recognized antigen of this tumor is caused by a somatic tumor-specific mutation in the RNA helicase p68. 8101 tumors were treated with spleen cells from young naive, or young and old immunized mice to ascertain the characteristics of immune cells that lead to rejection.

RESULTS

Here we show that the mutant p68 peptide has an exceptionally high affinity to the presenting MHC class I molecule K(b) and that spleen cells from immunized young syngeneic mice adoptively transferred to Rag(-/-) or cancer-suppressed euthymic mice eradicate 8101 tumors larger than 1 cm in average diameter and established for several weeks. Spleen cells from naive young mice or from old and boosted (reimmunized) mice were ineffective.

CONCLUSIONS

Relapse-free destruction of large and long-established tumors expressing a genuine very high-affinity tumor-specific antigen can be achieved by using adoptive transfer of lymphocytes from immunized young individuals.

Depot formation of doxycycline impairs Tet-regulated gene expression in vivo

The tetracycline (Tet) system is widely used for regulation of gene expression in vitro and in vivo. We constructed C57BL/6 transgenic mice (rtTA-CM2) with strong and ubiquitous reverse transactivator (rtTA2(S)-M2) gene expression. rtTA-CM2 mice were crossed to Tet-responsive reporter mice (LC-1) conditionally expressing the firefly luciferase (FLuc) gene under control of a Tet-responsive element, which allowed sensitive quantification of the transactivator activity by bioluminescent imaging. Following doxycycline (dox) application, up to 10(5)-fold increase in BL signal was measured. rtTA activity was inducible in most analyzed organs. After dox withdrawal the BL signal decreased significantly but did not disappear completely, most likely due to a dox depot formation in vivo. The residual dox was sufficient to partly down-regulate a Tet-off controlled oncogene in a tumor transplantation experiment, resulting in reduced tumor growth. rtTA-CM2 mice may be a useful tool to analyze the function of genes in various organs but also reveal that down-regulation of gene expression is not complete.