gp100/pmel 17 is a murine tumor rejection antigen: induction of "self"-reactive, tumoricidal T cells using high-affinity, altered peptide ligand

The stress kinase GCN2 does not mediate suppression of antitumor T cell responses by tryptophan catabolism in experimental melanomas

Tryptophan metabolism is a key process that shapes the immunosuppressive tumor microenvironment. The two rate-limiting enzymes that mediate tryptophan depletion, indoleamine-2,3-dioxygenase (IDO) and tryptophan-2,3-dioxygenase (TDO), have moved into the focus of research and inhibitors targeting IDO and TDO have entered clinical trials. Local tryptophan depletion is generally viewed as the crucial immunosuppressive mechanism. In T cells, the kinase general control non-derepressible 2 (GCN2) has been identified as a molecular sensor of tryptophan deprivation. GCN2 activation by tryptophan depletion induces apoptosis and mitigates T cell proliferation. Here, we investigated whether GCN2 attenuates tumor rejection in experimental B16 melanoma using T cell-specific Gcn2 knockout mice. Our data demonstrate that GCN2 in T cells did not affect immunity to B16 tumors even when animals were treated with antibodies targeting cytotoxic T lymphocyte antigen-4 (CTLA4). GCN2-deficient gp100 TCR-transgenic T cells were equally effective as wild-type pmel T cells against gp100-expressing B16 melanomas after adoptive transfer and gp100 peptide vaccination. Even augmentation of tumoral tryptophan metabolism in B16 tumors by lentiviral overexpression of Tdo did not differentially affect GCN2-proficient vs. GCN2-deficient T cells in vivo. Importantly, GCN2 target genes were not upregulated in tumor-infiltrating T cells. MALDI-TOF MS imaging of B16 melanomas demonstrated maintenance of intratumoral tryptophan levels despite high tryptophan turnover, which prohibits a drop in tryptophan sufficient to activate GCN2 in tumor-infiltrating T cells. In conclusion, our results do not suggest that suppression of antitumor immune responses by tryptophan metabolism is driven by local tryptophan depletion and subsequent GCN2-mediated T cell anergy.

Optimized tumor cryptic peptides: the basis for universal neo-antigen-like tumor vaccines

The very impressive clinical results recently obtained in cancer patients treated with immune response checkpoint inhibitors boosted the interest in immunotherapy as a therapeutic choice in cancer treatment. However, these inhibitors require a pre-existing tumor specific immune response and the presence of tumor infiltrating T cells to be efficient. This immune response can be triggered by cancer vaccines. One of the main issues in tumor vaccination is the choice of the right antigen to target. All vaccines tested to date targeted tumor associated antigens (TAA) that are self-antigens and failed to show a clinical efficacy because of the immune self-tolerance to TAA. A new class of tumor antigens has recently been described, the neo-antigens that are created by point mutations of tumor expressing proteins and are recognized by the immune system as non-self. Neo-antigens exhibit two main properties: they are not involved in the immune self-tolerance process and are immunogenic. However, the majority of the neo-antigens are patient specific and their use as cancer vaccines requires their previous identification in each patient individualy that can be done only in highly specialized research centers. It is therefore evident that neo-antigens cannot be used for patient vaccination worldwide. This raises the question of whether we can find neo-antigen like vaccines, which would not be patient specific. In this review we show that optimized cryptic peptides from TAA are neo-antigen like peptides. Optimized cryptic peptides are recognized by the immune system as non-self because they target self-cryptic peptides that escape self-tolerance; in addition they are strongly immunogenic because their sequence is modified in order to enhance their affinity for the HLA molecule. The first vaccine based on the optimized cryptic peptide approach, Vx-001, which targets the widely expressed tumor antigen telomerase reverse transcriptase (TERT), has completed a large phase I clinical study and is currently being tested in a randomized phase II trial in non-small cell lung cancer (NSCLC) patients.

Immunotherapy Expands and Maintains the Function of High-Affinity Tumor-Infiltrating CD8 T Cells In Situ

Cancer cells harbor high-affinity tumor-associated Ags capable of eliciting potent antitumor T cell responses, yet detecting these polyclonal T cells is challenging. Therefore, surrogate markers of T cell activation such as CD69, CD44, and programmed death-1 (PD-1) have been used. We report in this study that in mice, expression of activation markers including PD-1 is insufficient in the tumor microenvironment to identify tumor Ag-specific T cells. Using the Nur77GFP T cell affinity reporter mouse, we highlight that PD-1 expression can be induced independent of TCR ligation within the tumor. Given this, we characterized the utility of the Nur77GFP model system in elucidating mechanisms of action of immunotherapies independent of PD-1 expression. Coexpression of Nur77GFP and OX40 identifies a polyclonal population of high-affinity tumor-associated Ag-specific CD8(+) T cells, which produce more IFN-γ in situ than OX40 negative and doubles in quantity with anti-OX40 and anti-CTLA4 mAb therapy but not with anti-PD-1 or programmed death ligand-1. Moreover, expansion of these high-affinity CD8 T cells prolongs survival of tumor-bearing animals. Upon chronic stimulation in tumors and after adoptive cell therapy, CD8 TCR signaling and Nur77GFP induction is impaired, and tumors progress. However, this can be reversed and overall survival significantly enhanced after adoptive cell therapy with agonist OX40 immunotherapy. Therefore, we propose that OX40 agonist immunotherapy can maintain functional TCR signaling of chronically stimulated tumor-resident CD8 T cells, thereby increasing the frequency of cytotoxic, high-affinity, tumor-associated Ag-specific cells.

Ret mouse very large tumors (VLTs) display altered ratios of infiltrating memory to naive T cells: Roles in tumor expansion

Melanoma is an aggressive skin cancer, however it is immunogenic. The size of the primary tumor is associated with the nodal metastases. Our goals were to characterize melanoma-associated antigens (MAAs) and tumor-infiltrating T-lymphocytes (TILs) subsets in the few very large tumors (VLTs) developing in ret transgenic mice of melanoma. Tumors >700mg (VLTs) were investigated for MAAs and subsets of TILs. Immunohistochemistry and flow cytometry-based studies were performed to determine the infiltration patterns of T-lymphocytes in VLTs. It was observed that zinc fixative restores the antigenicity of the cell-surface markers of lymphocyte subpopulations without the need of antigen retrieval, whereas formalin-based fixative fails to restore the antigenicity in the presence of antigen retrieval in the immunohistochemistry. VLTs from ret mice express MAAs, such as Tyrosinase, TRP-1, TRP-2 and gp-100. The mean±standard deviation (S.D.) T-cell infiltration per 400 times-high power field in VLTs; CD4(+) (2.33±1.3), CD8(+) (2.00±1.0), and CD4(+) Foxp3(+) (2.5±0.5) regulatory T cells infiltration was exclusively restricted to the tumor stroma. Moreover, our flow cytometry-based data reveal that % mean±S.D. naive CD3(+) CD4(+) T cell infiltration (32.8±4.0%) was significantly larger than effector (25.8±2.8%, p<0.01) and central memory cells (16.1±3.7%, p<0.001) in VLTs. Similarly, between CD3(+) CD8(+) T cells, naive cells infiltrate (57.7±2.3%) in a significantly larger frequency than effector (5.0±0.4%, p<0.0001) and central memory cell (4.8±1.7%, p<0.0001) subsets. These results suggest that the VLTs from ret mice display lowered infiltration ratios between memory and naive T cells, which could be associated with the relatively large growth of VLTs.

Superior Immunologic and Therapeutic Efficacy of a Xenogeneic Genetic Cancer Vaccine Targeting Carcinoembryonic Human Antigen

We have generated a xenogeneic vaccine against human carcinoembryonic antigen (hCEACAM-5 or commonly hCEA) using as immunogen rhesus CEA (rhCEA). RhCEA cDNA was codon-usage optimized (rhCEAopt) and delivered by sequential DNA electro-gene-transfer (DNA-EGT) and adenoviral (Ad) vector. RhCEAopt was capable to break tolerance to CEA in hCEA transgenic mice and immune responses were detected against epitopes distributed over the entire length of the protein. Xenovaccination with rhCEA resulted in the activation of CD4+ T-cell responses in addition to self-reactive CD8+ T-cells, the development of high-titer antibodies against hCEA, and significant antitumor effects upon challenge with hCEA+ tumor cells. The superior activity of rhCEAopt compared with hCEAopt was confirmed in hCEA/HHD double-transgenic mice, where potent CD8+ T-cell responses against specific human HLA A*0201 hCEA epitopes were detected. Our data show that xenogeneic gene-based vaccination with rhCEA is a viable approach to break tolerance against CEA, thus suggesting further development in the clinical setting.

Inflammatory Signals Regulate IL-15 in Response to Lymphodepletion

Induction of lymphopenia has been exploited therapeutically to improve immune responses to cancer therapies and vaccinations. Whereas IL-15 has well-established roles in stimulating lymphocyte responses after lymphodepletion, the mechanisms regulating these IL-15 responses are unclear. We report that cell surface IL-15 expression is upregulated during lymphopenia induced by total body irradiation (TBI), cyclophosphamide, or Thy1 Ab-mediated T cell depletion, as well as in RAG(-/-) mice; interestingly, the cellular profile of surface IL-15 expression is distinct in each model. In contrast, soluble IL-15 (sIL-15) complexes are upregulated only after TBI or αThy1 Ab. Analysis of cell-specific IL-15Rα conditional knockout mice revealed that macrophages and dendritic cells are important sources of sIL-15 complexes after TBI but provide minimal contribution in response to Thy1 Ab treatment. Unlike with TBI, induction of sIL-15 complexes by αThy1 Ab is sustained and only partially dependent on type I IFNs. The stimulator of IFN genes pathway was discovered to be a potent inducer of sIL-15 complexes and was required for optimal production of sIL-15 complexes in response to Ab-mediated T cell depletion and TBI, suggesting products of cell death drive production of sIL-15 complexes after lymphodepletion. Lastly, we provide evidence that IL-15 induced by inflammatory signals in response to lymphodepletion drives lymphocyte responses, as memory CD8 T cells proliferated in an IL-15-dependent manner. Overall, these studies demonstrate that the form in which IL-15 is expressed, its kinetics and cellular sources, and the inflammatory signals involved are differentially dictated by the manner in which lymphopenia is induced.

Adoptive Transfer of CD8+ T Cells Generated from Induced Pluripotent Stem Cells Triggers Regressions of Large Tumors Along with Immunological Memory

Current approaches to adoptive T-cell therapy are limited by the difficulty of obtaining sufficient numbers of T cells against targeted antigens with useful in vivo characteristics. Theoretically, this limitation could be overcome by using induced pluripotent stem cells (iPSC) that could provide an unlimited source of autologous T cells. However, the therapeutic efficacy of iPSC-derived regenerated T cells remains to be demonstrated. Here, we report the first successful reprogramming of T-cell receptor (TCR) transgenic CD8(+) T cells into pluripotency. As part of the work, we established a syngeneic mouse model for evaluating in vitro and in vivo antitumor reactivity of regenerated T cells from iPSCs bearing a rearranged TCR of known antigen specificity. Stably TCR retained T-cell-derived iPSCs differentiated into CD4(+)CD8(+) T cells that expressed CD3 and the desired TCR in vitro Stimulation of iPSC-derived CD4(+)CD8(+) T cells with the cognate antigen in the presence of IL7 and IL15 followed by expansion with IL2, IL7, and IL15 generated large numbers of less-differentiated CD8(+) T cells with antigen-specific potent cytokine production and cytolytic capacity. Furthermore, adoptively transferred iPSC-derived CD8(+) T cells escaped immune rejection, mediated effective regression of large tumors, improved survival, and established antigen-specific immunological memory. Our findings illustrate the translational potential of iPSCs to provide an unlimited number of phenotypically defined, functional, and expandable autologous antigen-specific T cells with the characteristics needed to enable in vivo effectiveness. Cancer Res; 76(12); 3473-83. ©2016 AACR.

Targeting endothelial connexin40 inhibits tumor growth by reducing angiogenesis and improving vessel perfusion

Endothelial connexin40 (Cx40) contributes to regulate the structure and function of vessels. We have examined whether the protein also modulates the altered growth of vessels in tumor models established in control mice (WT), mice lacking Cx40 (Cx40-/-), and mice expressing the protein solely in endothelial cells (Tie2-Cx40). Tumoral angiogenesis and growth were reduced, whereas vessel perfusion, smooth muscle cell (SMC) coverage and animal survival were increased in Cx40-/- but not Tie2-Cx40 mice, revealing a critical involvement of endothelial Cx40 in transformed tissues independently of the hypertensive status of Cx40-/- mice. As a result, Cx40-/- mice bearing tumors survived significantly longer than corresponding controls, including after a cytotoxic administration. Comparable observations were made in WT mice injected with a peptide targeting Cx40, supporting the Cx40 involvement. This involvement was further confirmed in the absence of Cx40 or by peptide-inhibition of this connexin in aorta-sprouting, matrigel plug and SMC migration assays, and associated with a decreased expression of the phosphorylated form of endothelial nitric oxide synthase. The data identify Cx40 as a potential novel target in cancer treatment.

A multi-antigen vaccine in combination with an immunotoxin targeting tumor-associated fibroblast for treating murine melanoma

A therapeutically effective cancer vaccine must generate potent antitumor immune responses and be able to overcome tolerance mechanisms mediated by the progressing tumor itself. Previous studies showed that glycoprotein 100 (gp100), tyrosinase-related protein 1 (TRP1), and tyrosinase-related protein 2 (TRP2) are promising immunogens for melanoma immunotherapy. In this study, we administered these three melanoma-associated antigens via lentiviral vectors (termed LV-3Ag) and found that this multi-antigen vaccine strategy markedly increased functional T-cell infiltration into tumors and generated protective and therapeutic antitumor immunity. We also engineered a novel immunotoxin, αFAP-PE38, capable of targeting fibroblast activation protein (FAP)-expressing fibroblasts within the tumor stroma. When combined with αFAP-PE38, LV-3Ag exhibited greatly enhanced antitumor effects on tumor growth in an established B16 melanoma model. The mechanism of action underlying this combination treatment likely modulates the immune suppressive tumor microenvironment and, consequently, activates cytotoxic CD8⁺ T cells capable of specifically recognizing and destroying tumor cells. Taken together, these results provide a strong rationale for combining an immunotoxin with cancer vaccines for the treatment of patients with advanced cancer.

Clinically feasible approaches to potentiating cancer cell-based immunotherapies

The immune system exerts both tumor-destructive and tumor-protective functions. Mature dendritic cells (DCs), classically activated macrophages (M1), granulocytes, B lymphocytes, aβ and ɣδ T lymphocytes, natural killer T (NKT) cells, and natural killer (NK) cells may be implicated in antitumor immunoprotection. Conversely, tolerogenic DCs, alternatively activated macrophages (M2), myeloid-derived suppressor cells (MDSCs), and regulatory T (Tregs) and B cells (Bregs) are capable of suppressing antitumor immune responses. Anti-cancer vaccination is a useful strategy to elicit antitumor immune responses, while overcoming immunosuppressive mechanisms. Whole tumor cells or lysates derived thereof hold more promise as cancer vaccines than individual tumor-associated antigens (TAAs), because vaccinal cells can elicit immune responses to multiple TAAs. Cancer cell-based vaccines can be autologous, allogeneic or xenogeneic. Clinical use of xenogeneic vaccines is advantageous in that they can be most effective in breaking the preexisting immune tolerance to TAAs. To potentiate immunotherapy, vaccinations can be combined with other modalities that target different immune pathways. These modalities include 1) genetic or chemical modification of cell-based vaccines; 2) cross-priming TAAs to T cells by engaging dendritic cells; 3) T-cell adoptive therapy; 4) stimulation of cytotoxic inflammation by non-specific immunomodulators, toll-like receptor (TLR) agonists, cytokines, chemokines or hormones; 5) reduction of immunosuppression and/or stimulation of antitumor effector cells using antibodies, small molecules; and 6) various cytoreductive modalities. The authors envisage that combined immunotherapeutic strategies will allow for substantial improvements in clinical outcomes in the near future.

Multiple-purpose immunotherapy for cancer

Anti-cancer vaccination is a useful strategy to elicit antitumor immune responses, while overcoming immunosuppressive mechanisms. Whole tumor cells or lysates derived thereof hold more promise as cancer vaccines than individual tumor-associated antigens (TAAs), because vaccinal cells can elicit immune responses to multiple TAAs. Cancer cell-based vaccines can be autologous, allogeneic or xenogeneic. Clinical use of xenogeneic vaccines is advantageous in that they can be most effective in breaking the preexisting immune tolerance to TAAs. An attractive protocol would be to combine vaccinations with immunostimulating and/or immunosuppression-blocking modalities. It is reasonable to anticipate that combined immunotherapeutic strategies will allow for substantial improvements in clinical outcomes in the near future.

Oncolytic adenoviruses coated with MHC-I tumor epitopes increase the antitumor immunity and efficacy against melanoma

The stimulation of the immune system using oncolytic adenoviruses (OAds) has attracted significant interest and several studies suggested that OAds immunogenicity might be important for their efficacy. Therefore, we developed a versatile and rapid system to adsorb tumor-specific major histocompatibility complex class I (MHC-I) peptides onto the viral surface to drive the immune response toward the tumor epitopes. By studying the model epitope SIINFEKL, we demonstrated that the peptide-coated OAd (PeptiCRAd) retains its infectivity and the cross presentation of the modified-exogenous epitope on MHC-I is not hindered. We then showed that the SIINFEKL-targeting PeptiCRAd achieves a superior antitumor efficacy and increases the percentage of antitumor CD8⁺ T cells and mature epitope-specific dendritic cells in vivo. PeptiCRAds loaded with clinically relevant tumor epitopes derived from tyrosinase-related protein 2 (TRP-2) and human gp100 could reduce the growth of primary-treated tumors and secondary-untreated melanomas, promoting the expansion of antigen-specific T-cell populations. Finally, we tested PeptiCRAd in humanized mice bearing human melanomas. In this model, a PeptiCRAd targeting the human melanoma-associated antigen A1 (MAGE-A1) and expressing granulocyte and macrophage colony-stimulating factor (GM-CSF) was able to eradicate established tumors and increased the human MAGE-A1-specific CD8⁺ T cell population. Herein, we show that the immunogenicity of OAds plays a key role in their efficacy and it can be exploited to direct the immune response system toward exogenous tumor epitopes. This versatile and rapid system overcomes the immunodominance of the virus and elicits a tumor-specific immune response, making PeptiCRAd a promising approach for clinical testing.

Systemic Tolerance Mediated by Melanoma Brain Tumors Is Reversible by Radiotherapy and Vaccination

PURPOSE

Immune responses to antigens originating in the central nervous system (CNS) are generally attenuated, as collateral damage can have devastating consequences. The significance of this finding for the efficacy of tumor-targeted immunotherapies is largely unknown.

EXPERIMENTAL DESIGN

The B16 murine melanoma model was used to compare cytotoxic responses against established tumors in the CNS and in the periphery. Cytokine analysis of tissues from brain tumor-bearing mice detected elevated TGFβ secretion from microglia and in the serum and TGFβ signaling blockade reversed tolerance of tumor antigen-directed CD8 T cells. In addition, a treatment regimen using focal radiation therapy and recombinant Listeria monocytogenes was evaluated for immunologic activity and efficacy in this model.

RESULTS

CNS melanomas were more tolerogenic than equivalently progressed tumors outside the CNS as antigen-specific CD8 T cells were deleted and exhibited impaired cytotoxicity. Tumor-bearing mice had elevated serum levels of TGFβ; however, blocking TGFβ signaling with a small-molecule inhibitor or a monoclonal antibody did not improve survival. Conversely, tumor antigen-specific vaccination in combination with focal radiation therapy reversed tolerance and improved survival. This treatment regimen was associated with increased polyfunctionality of CD8 T cells, elevated T effector to T regulatory cell ratios, and decreased TGFβ secretion from microglia.

CONCLUSIONS

These data suggest that CNS tumors may impair systemic antitumor immunity and consequently accelerate cancer progression locally as well as outside the CNS, whereas antitumor immunity may be restored by combining vaccination with radiation therapy. These findings are hypothesis-generating and warrant further study in contemporary melanoma models as well as human trials.

Anti-Tumor Effects after Adoptive Transfer of IL-12 Transposon-Modified Murine Splenocytes in the OT-I-Melanoma Mouse Model

Adoptive transfer of gene modified T cells provides possible immunotherapy for patients with cancers refractory to other treatments. We have previously used the non-viral piggyBac transposon system to gene modify human T cells for potential immunotherapy. However, these previous studies utilized adoptive transfer of modified human T cells to target cancer xenografts in highly immunodeficient (NOD-SCID) mice that do not recapitulate an intact immune system. Currently, only viral vectors have shown efficacy in permanently gene-modifying mouse T cells for immunotherapy applications. Therefore, we sought to determine if piggyBac could effectively gene modify mouse T cells to target cancer cells in a mouse cancer model. We first demonstrated that we could gene modify cells to express murine interleukin-12 (p35/p40 mIL-12), a transgene with proven efficacy in melanoma immunotherapy. The OT-I melanoma mouse model provides a well-established T cell mediated immune response to ovalbumin (OVA) positive B16 melanoma cells. B16/OVA melanoma cells were implanted in wild type C57Bl6 mice. Mouse splenocytes were isolated from C57Bl6 OT-I mice and were gene modified using piggyBac to express luciferase. Adoptive transfer of luciferase-modified OT-I splenocytes demonstrated homing to B16/OVA melanoma tumors in vivo. We next gene-modified OT-I cells to express mIL-12. Adoptive transfer of mIL-12-modified mouse OT-I splenocytes delayed B16/OVA melanoma tumor growth in vivo compared to control OT-I splenocytes and improved mouse survival. Our results demonstrate that the piggyBac transposon system can be used to gene modify splenocytes and mouse T cells for evaluating adoptive immunotherapy strategies in immunocompetent mouse tumor models that may more directly mimic immunotherapy applications in humans.

Co-potentiation of antigen recognition: A mechanism to boost weak T cell responses and provide immunotherapy in vivo

Adaptive immunity is mediated by antigen receptors that can induce weak or strong immune responses depending on the nature of the antigen that is bound. In T lymphocytes, antigen recognition triggers signal transduction by clustering T cell receptor (TCR)/CD3 multiprotein complexes. In addition, it hypothesized that biophysical changes induced in TCR/CD3 that accompany receptor engagement may contribute to signal intensity. Nonclustering monovalent TCR/CD3 engagement is functionally inert despite the fact that it may induce changes in conformational arrangement or in the flexibility of receptor subunits. We report that the intrinsically inert monovalent engagement of TCR/CD3 can specifically enhance physiologic T cell responses to weak antigens in vitro and in vivo without stimulating antigen-unengaged T cells and without interrupting T cell responses to strong antigens, an effect that we term as "co-potentiation." We identified Mono-7D6-Fab, which biophysically altered TCR/CD3 when bound and functionally enhanced immune reactivity to several weak antigens in vitro, including a gp100-derived peptide associated with melanoma. In vivo, Mono-7D6-Fab induced T cell antigen-dependent therapeutic responses against melanoma lung metastases, an effect that synergized with other anti-melanoma immunotherapies to significantly improve outcome and survival. We conclude that Mono-7D6-Fab directly co-potentiated TCR/CD3 engagement by weak antigens and that such concept can be translated into an immunotherapeutic design. The co-potentiation principle may be applicable to other receptors that could be regulated by otherwise inert compounds whose latent potency is only invoked in concert with specific physiologic ligands.

Immunotherapy for Lewis lung carcinoma utilizing dendritic cells infected with CK19 gene recombinant adenoviral vectors

Dendritic cells (DCs) as 'professional' antigen-presenting cells (APCs) initiate and regulate immune responses to various antigens. DC-based vaccines have become a promising modality in cancer immunotherapy. Cytokeratin 19 (CK19) protein is expressed at high levels in lung cancer and many other tumor cells, suggesting CK19 as a potential tumor-specific target for cancer immune therapy. We constructed a recombinant adenoviral vector containing the CK19 gene (rAd-CK19). DCs transfected with rAd-CK19 were used to vaccinate C57BL/6 mice bearing xenografts derived from Lewis lung carcinoma (LLC) cells. The transfected DCs gave rise to potent CK19-specific cytotoxic T lymphocytes (CTLs) capable of lysing LLC cells. Mice immunized with the rAd-CK19-DCs exhibited significantly attenuated tumor growth (including tumor volume and weight) when compared to the tumor growth of mice immunized with rAd-c DCs or DCs during the 24-day observation period (P<0.05). The results revealed that the mice vaccinated with the rAd-CK19-DCs exhibited a potent protective and therapeutic antitumor immunity to LLC cells in the subcutaneous model along with an inhibitive effect on tumor growth compared to the mice vaccinated with the rAd-c DCs or DCs alone. The present study proposes a meaningful mode of action utilizing rAd-CK19 DCs in lung cancer immunotherapy.

Inhibitory receptors as targets for cancer immunotherapy

Inhibitory receptors expressed on T cells control immune responses while limiting autoimmunity. However, tumors can hijack these "checkpoints" for protection from immune attack. Tumor-specific T cells that exhibit an exhausted, unresponsive phenotype express high levels of inhibitory receptors including CTLA4, PD1, and LAG3, among others. Intratumoral regulatory T cells promote immunosuppression and also express multiple inhibitory receptors. Overcoming this inhibitory receptor-mediated immune tolerance has thus been a major focus of recent cancer immunotherapeutic developments. Here, we review how boosting the host's immune system by blocking inhibitory receptor signaling with antagonistic mAbs restores the capacity of T cells to drive durable antitumor immune responses. Clinical trials targeting the CTLA4 and PD1 pathways have shown durable effects in multiple tumor types. Many combinatorial therapies are currently being investigated with encouraging results that highlight enhanced antitumor immunogenicity and improved patient survival. Finally, we will discuss the ongoing identification and dissection of novel T-cell inhibitory receptor pathways, which could lead to the development of new combinatorial therapeutic approaches.

Local and systemic effect of transfection-reagent formulated DNA vectors on equine melanoma

Background

Equine melanoma has a high incidence in grey horses. Xenogenic DNA vaccination may represent a promising therapeutic approach against equine melanoma as it successfully induced an immunological response in other species suffering from melanoma and in healthy horses. In a clinical study, twenty-seven, grey, melanoma-bearing, horses were assigned to three groups (n = 9) and vaccinated on days 1, 22, and 78 with DNA vectors encoding for equine (eq) IL-12 and IL-18 alone or in combination with either human glycoprotein (hgp) 100 or human tyrosinase (htyr). Horses were vaccinated intramuscularly, and one selected melanoma was locally treated by intradermal peritumoral injection. Prior to each injection and on day 120, the sizes of up to nine melanoma lesions per horse were measured by caliper and ultrasound. Specific serum antibodies against hgp100 and htyr were measured using cell based flow-cytometric assays. An Analysis of Variance (ANOVA) for repeated measurements was performed to identify statistically significant influences on the relative tumor volume. For post-hoc testing a Tukey-Kramer Multiple-Comparison Test was performed to compare the relative volumes on the different examination days. An ANOVA for repeated measurements was performed to analyse changes in body temperature over time. A one-way ANOVA was used to evaluate differences in body temperature between the groups. A p–value < 0.05 was considered significant for all statistical tests applied.

Results

In all groups, the relative tumor volume decreased significantly to 79.1 ± 26.91% by day 120 (p < 0.0001, Tukey-Kramer Multiple-Comparison Test). Affiliation to treatment group, local treatment and examination modality had no significant influence on the results (ANOVA for repeated measurements). Neither a cellular nor a humoral immune response directed against htyr or hgp100 was detected. Horses had an increased body temperature on the day after vaccination.

Conclusions

This is the first clinical report on a systemic effect against equine melanoma following treatment with DNA vectors encoding eqIL12 and eqIL18 and formulated with a transfection reagent. Addition of DNA vectors encoding hgp100 respectively htyr did not potentiate this effect.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-015-0422-9) contains supplementary material, which is available to authorized users.

mRNA-transfected Dendritic Cells Expressing Polypeptides That Link MHC-I Presentation to Constitutive TLR4 Activation Confer Tumor Immunity

Recently, we have developed a novel genetic platform for improving dendritic cell (DC) induction of peptide-specific CD8 T cells, based on membrane-anchored β2-microglobulin (β2m) linked to a selected antigenic peptide at its N-terminus and to the cytosolic domain of toll-like receptor (TLR)4 C-terminally. In vitro transcribed mRNA transfection of antigen presenting cells resulted in polypeptides that efficiently coupled peptide presentation to cellular activation. In the present study, we evaluated the immunogenicity of such constructs in mRNA-transfected immature murine bone marrow-derived DCs. We show that the encoded peptide β2m-TLR4 products were expressed at the cell surface up to 72 hours and stimulated the maturation of DCs. In vivo, these DCs prompted efficient peptide-specific T-cell activation and target cell killing which were superior to those induced by peptide-loaded, LPS-stimulated DCs. This superiority was also evident in the ability to protect mice from tumor progression following the administration of B16F10.9 melanoma cells and to inhibit the development of pre-established B16F10.9 tumors. Our results provide evidence that the products of two recombinant genes encoding for peptide-hβ2m-TLR4 and peptide-hβ2m-K(b) expressed from exogenous mRNA can cooperate to couple Major Histocompatibility Complex (MHC-I) peptide presentation to TLR-mediated signaling, offering a safe, economical and highly versatile genetic platform for a novel category of CTL-inducing vaccines.

Insertion of exogenous epitopes in the E3-19K of oncolytic adenoviruses to enhance TAP-independent presentation and immunogenicity

Oncolytic adenoviruses can promote immune responses against tumors by expressing and/or displaying tumor-associated antigens. However, the strong immunodominance of viral antigens mask responses against tumor epitopes. In addition, defects in major histocompatibility complex class I antigen presentation pathway such as the downregulation of the transporter-associated with antigen processing (TAP) are frequently associated with immune evasion of tumor cells. To promote the immunogenicity of exogenous epitopes in the context of an oncolytic adenovirus, we have taken advantage of the ER localization of the viral protein E3-19K. We have inserted tumor-associated epitopes after the N-terminal signal sequence for membrane insertion of this protein and flanked them with linkers cleavable by the protease furin to facilitate their TAP-independent presentation. This strategy allowed an enhanced presentation of the exogenous epitopes in TAP-deficient tumor cells in vitro and the generation of higher specific immune responses in vivo that were able to significantly control tumor growth.

Adenovirus Improves the Efficacy of Adoptive T-cell Therapy by Recruiting Immune Cells to and Promoting Their Activity at the Tumor

Despite the rapid progress in the development of novel adoptive T-cell therapies, the clinical benefits in treatment of established tumors have remained modest. Several immune evasion mechanisms hinder T-cell entry into tumors and their activity within the tumor. Of note, oncolytic adenoviruses are intrinsically immunogenic due to inherent pathogen-associated molecular patterns. Here, we studied the capacity of adenovirus to overcome resistance of chicken ovalbumin-expressing B16.OVA murine melanoma tumors to adoptive ovalbumin-specific CD8(+) T-cell (OT-I) therapy. Following intraperitoneal transfer of polyclonally activated OT-I lymphocytes, control of tumor growth was superior in mice given intratumoral adenovirus compared with control mice, even in the absence of oncolytic virus replication. Preexisting antiviral immunity against serotype 5 did not hinder the therapeutic efficacy of the combination treatment. Intratumoral adenovirus injection was associated with an increase in proinflammatory cytokines, CD45(+) leukocytes, CD8(+) lymphocytes, and F4/80(+) macrophages, suggesting enhanced tumor immunogenicity. The proinflammatory effects of adenovirus on the tumor microenvironment led to expression of costimulatory signals on CD11c(+) antigen-presenting cells and subsequent activation of T cells, thus breaking the tumor-induced peripheral tolerance. An increased number of CD8(+) T cells specific for endogenous tumor antigens TRP-2 and gp100 was detected in combination-treated mice, indicating epitope spreading. Moreover, the majority of virus/T-cell-treated mice rejected the challenge of parental B16.F10 tumors, suggesting that systemic antitumor immunity was induced. In summary, we provide proof-of-mechanism data on combining adoptive T-cell therapy and adenovirotherapy for the treatment of cancer.

Local and systemic effect of transfection-reagent formulated DNA vectors on equine melanoma

Background

Equine melanoma has a high incidence in grey horses. Xenogenic DNA vaccination may represent a promising therapeutic approach against equine melanoma as it successfully induced an immunological response in other species suffering from melanoma and in healthy horses. In a clinical study, twenty-seven, grey, melanoma-bearing, horses were assigned to three groups (n = 9) and vaccinated on days 1, 22, and 78 with DNA vectors encoding for equine (eq) IL-12 and IL-18 alone or in combination with either human glycoprotein (hgp) 100 or human tyrosinase (htyr). Horses were vaccinated intramuscularly, and one selected melanoma was locally treated by intradermal peritumoral injection. Prior to each injection and on day 120, the sizes of up to nine melanoma lesions per horse were measured by caliper and ultrasound. Specific serum antibodies against hgp100 and htyr were measured using cell based flow-cytometric assays. An Analysis of Variance (ANOVA) for repeated measurements was performed to identify statistically significant influences on the relative tumor volume. For post-hoc testing a Tukey-Kramer Multiple-Comparison Test was performed to compare the relative volumes on the different examination days. An ANOVA for repeated measurements was performed to analyse changes in body temperature over time. A one-way ANOVA was used to evaluate differences in body temperature between the groups. A p–value < 0.05 was considered significant for all statistical tests applied.

Results

In all groups, the relative tumor volume decreased significantly to 79.1 ± 26.91% by day 120 (p < 0.0001, Tukey-Kramer Multiple-Comparison Test). Affiliation to treatment group, local treatment and examination modality had no significant influence on the results (ANOVA for repeated measurements). Neither a cellular nor a humoral immune response directed against htyr or hgp100 was detected. Horses had an increased body temperature on the day after vaccination.

Conclusions

This is the first clinical report on a systemic effect against equine melanoma following treatment with DNA vectors encoding eqIL12 and eqIL18 and formulated with a transfection reagent. Addition of DNA vectors encoding hgp100 respectively htyr did not potentiate this effect.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-015-0414-9) contains supplementary material, which is available to authorized users.

Trial watch: Naked and vectored DNA-based anticancer vaccines

One type of anticancer vaccine relies on the administration of DNA constructs encoding one or multiple tumor-associated antigens (TAAs). The ultimate objective of these preparations, which can be naked or vectored by non-pathogenic viruses, bacteria or yeast cells, is to drive the synthesis of TAAs in the context of an immunostimulatory milieu, resulting in the (re-)elicitation of a tumor-targeting immune response. In spite of encouraging preclinical results, the clinical efficacy of DNA-based vaccines employed as standalone immunotherapeutic interventions in cancer patients appears to be limited. Thus, efforts are currently being devoted to the development of combinatorial regimens that allow DNA-based anticancer vaccines to elicit clinically relevant immune responses. Here, we discuss recent advances in the preclinical and clinical development of this therapeutic paradigm.

Engineering α-fetoprotein-based gene vaccines to prevent and treat hepatocellular carcinoma: review and future prospects

Activation of a patient's immune system offers an attractive approach to prevent and treat hepatocellular carcinoma (HCC). However, the antitumor efficacy of current HCC vaccines was weak owing to insufficient immune activation of targeting self/tumor antigens. We recently found that epitope-optimized α-fetoprotein effectively activated CD8 T cells and generated potent antitumor effects in the carcinogen-induced autochthonous HCC mouse model. We predict that the same antigen engineering approach of epitope-optimization will enable us to develop effective human vaccines to prevent HCC recurrence after liver resection. The engineered human HCC vaccines may also allow us to identify high-affinity T-cell receptors and antibodies that can be used to reprogram T cells to treat HCC tumors via adoptive transfer.

Transgene-derived overexpression of miR-17-92 in CD8+ T-cells confers enhanced cytotoxic activity

MicroRNAs (miRs) play important roles in regulation of a variety of cell functions, including immune responses. We have previously demonstrated that miR-17-92 expression in T-cells enhances Th1 phenotype and provides a long-term protection against glioblastoma when co-expressed as a transgene in T-cells along with a chimeric antigen receptor. To further elucidate the function of miR-17-92 in tumor antigen-specific CD8(+) T-cells, we generated transgenic (Tg) mice in which CD8(+) T-cells overexpress transgene-derived miR-17-92 under the lck promoter as well as T-cell receptor specific for human gp10025-33 (Pmel-1) (miR-17-92/Pmel-Tg). CD8(+) T-cells from miR-17-92/Pmel-Tg mice demonstrated enhanced interferon (IFN)-γ production and cytotoxicity in response to the cognate antigen compared with those from control Pmel-Tg mice without the transgene for miR-17-92. In addition, miR-17-92/Pmel-Tg mouse-derived CD8(+)CD44(+) T-cells demonstrated increased frequencies of cells with memory phenotypes and IFN-γ production. We also found that miR-17-92/Pmel-Tg-derived CD8(+) T-cells expressed decreased levels of transforming growth factor (TGF)-β type II receptor (TGFBR2) on their surface, thereby resisting against suppressive effects of TGF-β1. Our findings suggest that engineering of tumor antigen-specific CD8(+) T-cells to express miR-17-92 may improve the potency of cancer immunotherapy.

Mouse model for pre-clinical study of human cancer immunotherapy

This unit describes protocols for developing tumors in mice, including subcutaneous growth, pulmonary metastases of B16 melanoma, and spontaneous melanoma in B-Raf V600E/PTEN deletion transgenic mouse models. Two immunization methods to prevent B16 tumor growth are described using B16.GM-CSF and recombinant vaccinia virus. A therapeutic approach is also included that uses adoptive transfer of tumor antigen-specific T cells. Methods including CTL induction, isolation, testing, and genetic modification of mouse T cells for adoptive transfer by using retrovirus-expressing genes of interest are provided. Additional sections, including growing B16 melanoma, enumerating pulmonary metastases, tumor imaging technique, and use of recombinant viruses for vaccination, are discussed together with safety concerns.

Simvastatin prevents and reverses depigmentation in a mouse model of vitiligo

Vitiligo is a common autoimmune disease of the skin that results in disfiguring white spots. There are no FDA-approved treatments, and current treatments are time-consuming, expensive, and have low efficacy. We sought to identify new treatments for vitiligo, and first considered repurposed medications because of the availability of safety data and expedited regulatory approval. We previously reported that the IFN-γ-induced chemokine CXCL10 is expressed in lesional skin from vitiligo patients, and that it is critical for the progression and maintenance of depigmentation in our mouse model of vitiligo. We hypothesized that targeting IFN-γ signaling might be an effective new treatment strategy. STAT1 activation is required for IFN-γ signaling and recent studies revealed that simvastatin, an FDA-approved cholesterol-lowering medication, inhibited STAT1 activation in vitro. Therefore, we hypothesized that simvastatin may be an effective treatment for vitiligo. We found that simvastatin both prevented and reversed depigmentation in our mouse model of vitiligo, and reduced the number of infiltrating autoreactive CD8⁺ T cells in the skin. Treatment of melanocyte-specific, CD8⁺ T cells in vitro decreased proliferation and IFN-γ production, suggesting additional effects of simvastatin directly on T cells. Based on these data, simvastatin may be a safe, targeted treatment option for patients with vitiligo.

Challenges in the development of a survivin vaccine (SurVaxM) for malignant glioma

There is growing interest in immunotherapy for malignant gliomas. This interest stems from a number of immunological observations, together with the failure of conventional therapeutic agents to produce broad and clinically meaningful improvements in survival and quality of life. The challenges faced in translating laboratory-based immunological observations to Phase I and II clinical trials for immunotherapy of gliomas are substantial. Nevertheless, as our understanding of the effects of active specific vaccination in glioma patients grows, results support optimism that such methods may eventually prove useful as an adjunctive treatment for these cancers. This paper highlights a number of barriers encountered in the translational development of a survivin-targeted peptide vaccine (SurVaxM) for patients with malignant gliomas.

Peptide Dose and/or Structure in Vaccines as a Determinant of T Cell Responses

While T cells recognise the complex of peptide and major histocompatibility complex (MHC) at the cell surface, changes in the dose and/or structure of the peptide component can have profound effects on T cell activation and function. In addition, the repertoire of T cells capable of responding to any given peptide is variable, but broader than a single clone. Consequently, peptide parameters that affect the interaction between T cells and peptide/MHC have been shown to select particular T cell clones for expansion and this impacts on clearance of disease. T cells with high functional avidity are selected on low doses of peptide, while low avidity T cells are favoured in high peptide concentrations. Altering the structure of the peptide ligand can also influence the selection and function of peptide-specific T cell clones. In this review, we will explore the evidence that the choice of peptide dose or the structure of the peptide are critical parameters in an effective vaccine designed to activate T cells.

Radiation-induced autophagy potentiates immunotherapy of cancer via up-regulation of mannose 6-phosphate receptor on tumor cells in mice

There is a significant body of evidence demonstrating that radiation therapy (XRT) enhances the effect of immune therapy. However, the precise mechanisms by which XRT potentiates the immunotherapy of cancer remain elusive. Here, we report that XRT potentiates the effect of immune therapy via induction of autophagy and resultant trafficking of mannose-6-phopsphate receptor (MPR) to the cell surface. Irradiation of different tumor cells caused substantial up-regulation of MPR on the cell surface in vitro and in vivo. Down-regulation of MPR in tumor cells with shRNA completely abrogated the combined effect of XRT and immunotherapy (CTLA4 antibody) in B16F10-bearing mice without changes in the tumor-specific responses of T cells. Radiation-induced MPR up-regulation was the result of redistribution of the receptor to the cell surface. This effect was caused by autophagy with redirection of MPR to autophagosomes in a clathrin-dependent manner. In autophagosomes, MPR lost its natural ligands, which resulted in subsequent trafficking of empty receptor(s) back to the surface. Together, our data demonstrated a novel mechanism by which XRT can enhance the effect of immunotherapy and the molecular mechanism of this process.

Hapten-induced contact hypersensitivity, autoimmune reactions, and tumor regression: plausibility of mediating antitumor immunity

Haptens are small molecule irritants that bind to proteins and elicit an immune response. Haptens have been commonly used to study allergic contact dermatitis (ACD) using animal contact hypersensitivity (CHS) models. However, extensive research into contact hypersensitivity has offered a confusing and intriguing mechanism of allergic reactions occurring in the skin. The abilities of haptens to induce such reactions have been frequently utilized to study the mechanisms of inflammatory bowel disease (IBD) to induce autoimmune-like responses such as autoimmune hemolytic anemia and to elicit viral wart and tumor regression. Hapten-induced tumor regression has been studied since the mid-1900s and relies on four major concepts: (1) ex vivo haptenation, (2) in situ haptenation, (3) epifocal hapten application, and (4) antigen-hapten conjugate injection. Each of these approaches elicits unique responses in mice and humans. The present review attempts to provide a critical appraisal of the hapten-mediated tumor treatments and offers insights for future development of the field.

Identification of the genomic insertion site of Pmel-1 TCR α and β transgenes by next-generation sequencing

The pmel-1 T cell receptor transgenic mouse has been extensively employed as an ideal model system to study the mechanisms of tumor immunology, CD8⁺ T cell differentiation, autoimmunity and adoptive immunotherapy. The ‘zygosity’ of the transgene affects the transgene expression levels and may compromise optimal breeding scheme design. However, the integration sites for the pmel-1 mouse have remained uncharacterized. This is also true for many other commonly used transgenic mice created before the modern era of rapid and inexpensive next-generation sequencing. Here, we show that whole genome sequencing can be used to determine the exact pmel-1 genomic integration site, even with relatively ‘shallow’ (8X) coverage. The results were used to develop a validated polymerase chain reaction-based genotyping assay. For the first time, we provide a quick and convenient polymerase chain reaction method to determine the dosage of pmel-1 transgene for this freely and publically available mouse resource. We also demonstrate that next-generation sequencing provides a feasible approach for mapping foreign DNA integration sites, even when information of the original vector sequences is only partially known.

Xenogeneic therapeutic cancer vaccines as breakers of immune tolerance for clinical application: to use or not to use?

Accumulation of firm evidence that clinically apparent cancer develops only when malignant cells manage to escape immunosurveillance led to the introduction of tumor immunotherapy strategies aiming to reprogramm the cancer-dysbalanced antitumor immunity and restore its capacity to control tumor growth. There are several immunotherapeutical strategies, among which specific active immunotherapy or therapeutic cancer vaccination is one of the most promising. It targets dendritic cells (DCs) which have a unique ability of inducing naive and central memory T cell-mediated immune response in the most efficient manner. DCs can be therapeutically targeted either in vivo/in situ or by ex vivo manipulations followed by their re-injection back into the same patient. The majority of current DC targeting strategies are based on autologous or allogeneic tumor-associated antigens (TAAs) which possess various degrees of inherent tolerogenic potential. Therefore still limited efficacy of various tumor immunotherapy approaches may be attributed, among various other mechanisms, to the insufficient immunogenicity of self-protein-derived TAAs. Based on such an idea, the use of homologous xenogeneic antigens, derived from different species was suggested to overcome the natural immune tolerance to self TAAs. Xenoantigens are supposed to differ sufficiently from self antigens to a degree that renders them immunogenic, but at the same time preserves an optimal homology range with self proteins still allowing xenoantigens to induce cross-reactive T cells. Here we discuss the concept of xenogeneic vaccination, describe the cons and pros of autologous/allogeneic versus xenogeneic therapeutic cancer vaccines, present the results of various pre-clinical and several clinical studies and highlight the future perspectives of integrating xenovaccination into rapidly developing tumor immunotherapy regimens.

The rationale of vectored gene-fusion vaccines against cancer: evolving strategies and latest evidence

The development of vaccines that target tumor antigens in cancer has proven difficult. A major reason for this is that T cells specific for tumor self-antigens and neoantigens are eliminated or inactivated through mechanisms of tolerance. Antigen fusion strategies which increase the ability of vaccines to stimulate T cells that have escaped tolerance mechanisms, may have a particular potential as immunotherapies. This review highlights antigen fusion strategies that have been successful in stimulating the induction of T-cell immunity against cancer and counteracting tumor-associated tolerance. In preclinical studies, these strategies have shown to improve the potency of vectored vaccines through fusion of tumor antigen to proteins or protein domains that increase CD4+ T-cell help, CD8+ T-cell responses or both the CD4+ and CD8+ T-cell responses. However, in clinical trials such strategies seem to be less efficient when provided as a DNA vaccine. The first clinical trial using a viral vectored fusion-gene vaccine is expected to be tested as a partner in a heterologous prime-boost regimen directed against cervical cancer.

Epitope-optimized alpha-fetoprotein genetic vaccines prevent carcinogen-induced murine autochthonous hepatocellular carcinoma

Immunization with effective cancer vaccines can offer a much needed adjuvant therapy to fill the treatment gap after liver resection to prevent relapse of hepatocellular carcinoma (HCC). However, current HCC cancer vaccines are mostly based on native shared-self/tumor antigens that are only able to induce weak immune responses. In this study we investigated whether the HCC-associated self/tumor antigen of alpha-fetoprotein (AFP) could be engineered to create an effective vaccine to break immune tolerance and potently activate CD8 T cells to prevent clinically relevant carcinogen-induced autochthonous HCC in mice. We found that the approach of computer-guided methodical epitope-optimization created a highly immunogenic AFP and that immunization with lentivector expressing the epitope-optimized AFP, but not wild-type AFP, potently activated CD8 T cells. Critically, the activated CD8 T cells not only cross-recognized short synthetic wild-type AFP peptides, but also recognized and killed tumor cells expressing wild-type AFP protein. Immunization with lentivector expressing optimized AFP, but not native AFP, completely protected mice from tumor challenge and reduced the incidence of carcinogen-induced autochthonous HCC. In addition, prime-boost immunization with the optimized AFP significantly increased the frequency of AFP-specific memory CD8 T cells in the liver that were highly effective against emerging HCC tumor cells, further enhancing the tumor prevention of carcinogen-induced autochthonous HCC.

CONCLUSIONS

Epitope-optimization is required to break immune tolerance and potently activate AFP-specific CD8 T cells, generating effective antitumor effect to prevent clinically relevant carcinogen-induced autochthonous HCC in mice. Our study provides a practical roadmap to develop effective human HCC vaccines that may result in an improved outcome compared to the current HCC vaccines based on wild-type AFP.

Direct T cell activation via CD40 ligand generates high avidity CD8+ T cells capable of breaking immunological tolerance for the control of tumors

CD40 and CD40 ligand (CD40L) are costimulatory molecules that play a pivotal role in the proinflammatory immune response. Primarily expressed by activated CD4⁺ T cells, CD40L binds to CD40 on antigen presenting cells (APCs), thereby inducing APC activation. APCs, in turn, prime cytotoxic T lymphocytes (CTLs). Here, two tumor-associated antigen (TAA) animal models, p53-based and GP100-based, were utilized to examine the ability of CD40-CD40L to improve antigen-specific CTL-mediated antitumor immune responses. Although p53 and GP100 are self-antigens that generate low affinity antigen-specific CD8⁺ T cells, studies have shown that their functional avidity can be improved with CD40L-expressing APCs. Therefore, in the current study, we immunized mice with a DNA construct encoding a TAA in conjunction with another construct encoding CD40L via intramuscular injection followed by electroporation. We observed a significant increase in the antigen-specific CTL-mediated immune responses as well as the potent antitumor effects in both models. Antibody depletion experiments demonstrated that CD8⁺ T cells play a crucial role in eliciting antitumor effects in vaccinated mice. Furthermore, we showed that in vitro stimulation with irradiated tumor cells expressing both TAA and CD40L improved the functional avidity of antigen-specific CD8⁺ T cells. Thus, our data show that vaccination with TAA/CD40L DNA can induce potent antitumor effects against TAA-expressing tumors through the generation of better functioning antigen-specific CD8⁺ T cells. Our study serves as an important foundation for future clinical translation.

Oxidized lipids block antigen cross-presentation by dendritic cells in cancer

Cross-presentation is one of the main features of dendritic cells (DCs), which is critically important for the development of spontaneous and therapy-inducible antitumor immune responses. Patients, at early stages of cancer, have normal presence of DCs. However, the difficulties in the development of antitumor responses in patients with low tumor burden raised the question of the mechanisms of DC dysfunction. In this study, we found that, in differentiated DCs, tumor-derived factors blocked the cross-presentation of exogenous Ags without inhibiting the Ag presentation of endogenous protein or peptides. This effect was caused by intracellular accumulation of different types of oxidized neutral lipids: triglycerides, cholesterol esters, and fatty acids. In contrast, the accumulation of nonoxidized lipids did not affect cross-presentation. Oxidized lipids blocked cross-presentation by reducing the expression of peptide-MHC class I complexes on the cell surface. Thus, this study suggests the novel role of oxidized lipids in the regulation of cross-presentation.

DNA vaccines encoding altered peptide ligands for SSX2 enhance epitope-specific CD8+ T-cell immune responses

Plasmid DNA serves as a simple and easily modifiable form of antigen delivery for vaccines. The USDA approval of DNA vaccines for several non-human diseases underscores the potential of this type of antigen delivery method as a cost-effective approach for the treatment or prevention of human diseases, including cancer. However, while DNA vaccines have demonstrated safety and immunological effect in early phase clinical trials, they have not consistently elicited robust anti-tumor responses. Hence many recent efforts have sought to increase the immunological efficacy of DNA vaccines, and we have specifically evaluated several target antigens encoded by DNA vaccine as treatments for human prostate cancer. In particular, we have focused on SSX2 as one potential target antigen, given its frequent expression in metastatic prostate cancer. We have previously identified two peptides, p41-49 and p103-111, as HLA-A2-restricted SSX2-specific epitopes. In the present study we sought to determine whether the efficacy of a DNA vaccine could be enhanced by an altered peptide ligand (APL) strategy wherein modifications were made to anchor residues of these epitopes to enhance or ablate their binding to HLA-A2. A DNA vaccine encoding APL modified to increase epitope binding elicited robust peptide-specific CD8+ T cells producing Th1 cytokines specific for each epitope. Ablation of one epitope in a DNA vaccine did not enhance immune responses to the other epitope. These results demonstrate that APL encoded by a DNA vaccine can be used to elicit increased numbers of antigen-specific T cells specific for multiple epitopes simultaneously, and suggest this could be a general approach to improve the immunogenicity of DNA vaccines encoding tumor antigens.

Plasmid DNA vaccines against cancer: cytotoxic T-lymphocyte induction against tumor antigens

In recent years, a number of tumor vaccination strategies have been developed. Most of these rely on the identification of tumor antigens that can be recognized by the immune system. DNA vaccination represents one such approach for the induction of both humoral and cellular immune responses against tumor antigens. Studies in animal models have demonstrated the feasibility of utilizing DNA vaccination to elicit protective antitumor immune responses. However, most tumor antigens expressed by cancer cells in humans are weakly immunogenic, and therefore require the development of strategies to potentiate DNA vaccine efficacy in the clinical setting. This review focuses on recent advances in understanding of the immunology of DNA vaccines, as well as strategies used to increase DNA vaccine potency with respect to cytotoxic T-lymphocyte activity.

Understanding the biology of ex vivo-expanded CD8 T cells for adoptive cell therapy: role of CD62L

CD62L governs the circulation of CD8(+) T cells between lymph nodes and peripheral tissues, whereby the expression of CD62L by CD8(+) T cells promotes their recirculation through lymph nodes. As such, CD62L participates in the fate of adoptively transferred CD8(+) T cells and may control their effectiveness for cancer immunotherapy, including settings in which host preconditioning results in the acute lymphopenia-induced proliferation of the transferred cells. Indeed, previous studies correlated CD62L expression by donor CD8(+) cells with the success rate of adoptive cell therapy (ACT). Here, we analyzed the functions and fate of ex vivo-activated, tumor-specific CD62L(-/-) CD8(+) T cells in a mouse melanoma model for ACT. Unexpectedly, we observed that CD62L(-/-) CD8(+) T cells were functionally indistinguishable from CD62L(+/+) CD8(+) T cells, i.e., both greatly expanded in cyclophosphamide preconditioned animals, controlled subcutaneously and hematogenously spreading tumors, and generated anti-tumor-specific CD8(+) T cell memory. Moreover, even in hosts with rudimentary secondary lymphoid organs (LT(-/-) animals), CD8(+) T cells with and without CD62L expanded equivalently to those adoptively transferred into wild-type animals. These results put into question the utility of CD62L as a predictive biomarker for the efficacy of ex vivo-expanded T cells after ACT in lymphopenic conditions and also offer new insights into the homing, engraftment, and memory generation of adoptively transferred ex vivo-activated CD8(+) T cells.

Trial Watch: Peptide vaccines in cancer therapy

Throughout the past 3 decades, along with the recognition that the immune system not only influences oncogenesis and tumor progression, but also determines how established neoplastic lesions respond therapy, renovated enthusiasm has gathered around the possibility of using vaccines as anticancer agents. Such an enthusiasm quickly tempered when it became clear that anticancer vaccines would have to be devised as therapeutic, rather than prophylactic, measures, and that malignant cells often fail to elicit (or actively suppress) innate and adaptive immune responses. Nonetheless, accumulating evidence indicates that a variety of anticancer vaccines, including cell-based, DNA-based, and purified component-based preparations, are capable of circumventing the poorly immunogenic and highly immunosuppressive nature of most tumors and elicit (at least under some circumstances) therapeutically relevant immune responses. Great efforts are currently being devoted to the identification of strategies that may provide anticancer vaccines with the capacity of breaking immunological tolerance and eliciting tumor-associated antigen-specific immunity in a majority of patients. In this sense, promising results have been obtained by combining anticancer vaccines with a relatively varied panels of adjuvants, including multiple immunostimulatory cytokines, Toll-like receptor agonists as well as inhibitors of immune checkpoints. One year ago, in the December issue of OncoImmunology, we discussed the biological mechanisms that underlie the antineoplastic effects of peptide-based vaccines and presented an abundant literature demonstrating the prominent clinical potential of such an approach. Here, we review the latest developments in this exciting area of research, focusing on high-profile studies that have been published during the last 13 mo and clinical trials launched in the same period to evaluate purified peptides or full-length proteins as therapeutic anticancer agents.

Going Pro to enhance T-cell immunogenicity: easy as π?

MHC class I molecules bind intracellular oligopeptides and present them on the cell surface for CD8(+) T-cell activation and recognition. Strong peptide/MHC class I (pMHC) interactions typically induce the best CD8(+) T-cell responses; however, many immunotherapeutic tumor-specific peptides bind MHC with low affinity. To overcome this, immunologists can carefully alter peptides for enhanced MHC affinity but often at the cost of decreased T-cell recognition. A new report published in this issue of the European Journal of Immunology [Eur. J. Immunol. 2013. 43:3051-3060] shows that the substitution of proline at the third residue (p3P) of a common tumor peptide increases pMHC affinity and complex stability while enhancing T-cell receptor recognition. X-ray crystallography indicates that stability is generated through newly introduced CH-π bonding between p3P and a conserved residue (Y159) in the MHC heavy chain. This finding highlights a previously unappreciated role for CH-π bonding in MHC peptide binding, and importantly, arms immunologists with a novel and possibly general approach for increasing pMHC stability without compromising T-cell recognition.

Improving antigenic peptide vaccines for cancer immunotherapy using a dominant tumor-specific T cell receptor

Vaccines that incorporate peptide mimics of tumor antigens, or mimotope vaccines, are commonly used in cancer immunotherapy and function by eliciting increased numbers of T cells that cross-react with the native tumor antigen. Unfortunately, they often elicit T cells that do not cross-react with or that have low affinity for the tumor antigen. Using a high affinity tumor-specific T cell clone, we identified a panel of mimotope vaccines for the dominant peptide antigen from a mouse colon tumor that elicits a range of tumor protection following vaccination. The TCR from this high affinity T cell clone was rarely identified in ex vivo evaluation of tumor-specific T cells elicited by mimotope vaccination. Conversely, a low affinity clone found in the tumor and following immunization was frequently identified. Using peptide libraries, we determined if this frequently identified TCR improved the discovery of efficacious mimotopes. We demonstrated that the representative TCR identified more protective mimotopes than the high affinity TCR. These results suggest that targeting a dominant fraction of tumor-specific T cells generates potent immunity and that consideration of the available T cell repertoire is necessary for targeted T cell therapy. These results have important implications when optimizing mimotope vaccines for cancer immunotherapy.

Comparison of vaccine-induced effector CD8 T cell responses directed against self- and non-self-tumor antigens: implications for cancer immunotherapy

It is generally accepted that CD8 T cells play a major role in tumor control, yet vaccination aimed at eliciting potent CD8 T cell responses are rarely efficient in clinical trials. To try and understand why this is so, we have generated potent adenoviral vectors encoding the endogenous tumor Ags (TA) tyrosinase-related protein-2 (TRP-2) and glycoprotein 100 (GP100) tethered to the invariant chain (Ii). Using these vectors, we sought to characterize the self-TA-specific CD8 T cell response and compare it to that induced against non-self-Ags expressed from a similar vector platform. Prophylactic vaccination with adenoviral vectors expressing either TRP-2 (Ad-Ii-TRP-2) or GP100 (Ad-Ii-GP100) had little or no effect on the growth of s.c. B16 melanomas, and only Ad-Ii-TRP-2 was able to induce a marginal reduction of B16 lung metastasis. In contrast, vaccination with a similar vector construct expressing a foreign (viral) TA induced efficient tumor control. Analyzing the self-TA-specific CD8 T cells, we observed that these could be activated to produce IFN-γ and TNF-α. In addition, surface expression of phenotypic markers and inhibitory receptors, as well as in vivo cytotoxicity and degranulation capacity matched that of non-self-Ag-specific CD8 T cells. However, the CD8 T cells specific for self-TAs had a lower functional avidity, and this impacted on their in vivo performance. On the basis of these results and a low expression of the targeted TA epitopes on the tumor cells, we suggest that low avidity of the self-TA-specific CD8 T cells may represent a major obstacle for efficient immunotherapy of cancer.

Proline substitution independently enhances H-2D(b) complex stabilization and TCR recognition of melanoma-associated peptides

The immunogenicity of H-2D(b) (D(b)) restricted epitopes can be significantly increased by substituting peptide position 3 to a proline (p3P). The p3P modification enhances MHC stability without altering the conformation of the modified epitope allowing for T-cell cross-reactivity with the native peptide. The present study reveals how specific interactions between p3P and the highly conserved MHC heavy chain residue Y159 increase the stability of D(b) in complex with an optimized version of the melanoma-associated epitope gp10025-33 . Furthermore, the p3P modification directly increased the affinity of the D(b)/gp10025-33 -specific T-cell receptor (TCR) pMel. Surprisingly, the enhanced TCR binding was independent from the observed increased stability of the optimized D(b)/gp10025-33 complex and from the interactions formed between p3P and Y159, indicating a direct effect of the p3P modification on TCR recognition.

Src-kinase inhibitors sensitize human cells of myeloid origin to Toll-like-receptor-induced interleukin 12 synthesis

Src-kinase inhibitors hold great potential as targeted therapy against malignant cells. However, such inhibitors may also affect nonmalignant cells and cause pronounced off-target effects. We investigated the role of the dual kinase inhibitor dasatinib on human myeloid cells. Dasatinib is clinically used for the treatment of bcr/abl⁺ leukemias because it blocks the mutated tyrosine kinase abl. To understand its effect on the development of antigen-specific T-cell responses, we assessed antigen-specific priming of human, naïve T cells. In surprising contrast to the direct inhibition of T-cell activation by dasatinib, pretreatment of maturing dendritic cells (DCs) with dasatinib strongly enhanced their stimulatory activity. This effect strictly depended on the activating DC stimulus and led to enhanced interleukin 12 (IL-12) production and T-cell responses of higher functional avidity. Src-kinase inhibitors, and not conventional tyrosine kinase inhibitors, increased IL-12 production in several cell types of myeloid origin, such as monocytes and classical or nonclassical DCs. Interestingly, only human cells, but not mouse or macaques DCs, were affected. These data highlight the potential immunostimulatory capacity of a group of novel drugs, src-kinase inhibitors, thereby opening new opportunities for chemoimmunotherapy. These data also provide evidence for a regulatory role of src kinases in the activation of myeloid cells.