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

Self-specific CD8+ T cells maintain a semi-naive state following lymphopenia-induced proliferation

Upon transfer into T cell-deficient hosts, naive CD8(+) T cells typically undergo lymphopenia-induced proliferation (LIP, also called homeostatic proliferation) and develop the phenotypic and functional characteristics of memory CD8(+) T cells. However, the capacity of T cells with self-peptide/MHC specificity to respond in this way has not been intensively studied. We examined pmel-1 TCR transgenic CD8(+) T cells that are specific for an epitope from gp100, a protein expressed by melanoma cells and normal melanocytes. Despite their self-specificity, naive pmel-1 cells were inefficient at LIP in typical lymphopenic hosts. In CD132 (common gamma-chain)-deficient hosts, pmel-1 CD8(+) T cells underwent extensive proliferation, but, surprisingly, the majority of these cells retained certain naive phenotypic traits (CD44(low), CD122(low)) rather than acquiring the expected central-memory phenotype. Following LIP, pmel-1 T cells acquired the capacity to control B16F10 tumor growth, but only in common gamma-chain-deficient host mice. Together, these data suggest that LIP does not always favor expansion of self-specific CD8 T cells and that sustained extensive lymphopenia is required for such cells to exhibit tumor control.

Dendritic cell recovery post-lymphodepletion: a potential mechanism for anti-cancer adoptive T cell therapy and vaccination

Adoptive transfer of autologous tumor-reactive T cells holds promise as a cancer immunotherapy. In this approach, T cells are harvested from a tumor-bearing host, expanded in vitro and infused back to the same host. Conditioning of the recipient host with a lymphodepletion regimen of chemotherapy or radiotherapy before adoptive T cell transfer has been shown to substantially improve survival and anti-tumor responses of the transferred cells. These effects are further enhanced when the adoptive T cell transfer is followed by vaccination with tumor antigens in combination with a potent immune adjuvant. Although significant progress has been made toward an understanding of the reasons underlying the beneficial effects of lymphodepletion to T cell adoptive therapy, the precise mechanisms remain poorly understood. Recent studies, including ours, would indicate a more central role for antigen presenting cells, in particular dendritic cells. Unraveling the exact role of these important cells in mediation of the beneficial effects of lymphodepletion could provide novel pathways toward the rational design of more effective anti-cancer immunotherapy. This article focuses on how the frequency, phenotype, and functions of dendritic cells are altered during the lymphopenic and recovery phases post-induction of lymphodepletion, and how they affect the anti-tumor responses of adoptively transferred T cells.

Peptide vaccines prevent tumor growth by activating T cells that respond to native tumor antigens

Peptide vaccines enhance the response of T cells toward tumor antigens and represent a strategy to augment antigen-independent immunotherapies of cancer. However, peptide vaccines that include native tumor antigens rarely prevent tumor growth. We have assembled a set of peptide variants for a mouse-colon tumor model to determine how to improve T-cell responses. These peptides have similar affinity for MHC molecules, but differ in the affinity of the peptide-MHC/T-cell receptor interaction with a tumor-specific T-cell clone. We systematically demonstrated that effective antitumor responses are generated after vaccination with variant peptides that stimulate the largest proportion of endogenous T cells specific for the native tumor antigen. Importantly, we found some variant peptides that strongly stimulated a specific T-cell clone in vitro, but elicited fewer tumor-specific T cells in vivo, and were not protective. The T cells expanded by the effective vaccines responded to the wild-type antigen by making cytokines and killing target cells, whereas most of the T cells expanded by the ineffective vaccines only responded to the peptide variants. We conclude that peptide-variant vaccines are most effective when the peptides react with a large responsive part of the tumor-specific T-cell repertoire.

TCR mimic monoclonal antibody targets a specific peptide/HLA class I complex and significantly impedes tumor growth in vivo using breast cancer models

Our laboratory has developed a process for generating mAbs with selectivity to unique peptides in the context of MHC molecules. Recently, we reported that RL4B, an mAb that we have called a TCR mimic (TCRm) because it recognizes peptide in the context of MHC, has cytotoxic activity in vitro and prevented growth of tumor cells in a prophylactic setting. When presented in the context of HLA-A2, RL4B TCRm recognizes the peptide GVLPALPQV derived from human chorionic gonadotropin (hCG)-beta. In this study, we show that RL4B TCRm has strong binding affinity for the GVLPALPQV peptide/HLA-A2 epitope and fine binding specificity for cells that express endogenous hCGbeta Ag and HLA-A2. In addition, suppression of tumor growth with RL4B TCRm was observed in orthotopic models for breast cancer. Using two aggressive human tumor cell lines, MDA-MB-231 and MCF-7, we provide evidence that RL4B TCRm significantly retards tumor growth, supporting a possible role for TCRm agents in therapeutic settings. Moreover, tumors in mice responded to RL4B TCRm therapy in a dose-dependent manner, eliminating tumors at the highest dose. RL4B TCRm strongly detects the hCGbeta peptide/HLA-A2 epitope in human primary breast tumor tissue, but does not react or reacts weakly with normal breast tissue from the same patient. These results further illustrate the selective nature of TCRm Abs and the clinical relevance of the GVLPALPQV peptide/HLA-A2 epitope expression in tumor cells, because they provide the first evidence that Abs that mimic the TCR can be used to markedly reduce and suppress tumor growth.

Increased intensity lymphodepletion enhances tumor treatment efficacy of adoptively transferred tumor-specific T cells

Lymphodepletion before adoptive cell transfer (ACT)-based immunotherapies can enhance anti-tumor responses by augmenting innate immunity, by increasing access to homeostatic cytokines, and by depressing the numbers of regulatory T cells and myeloid-derived suppressor cells. Although it is clear that high-dose total body irradiation given together with hematopoietic stem cell (HSC) transplantation effectively enhances ACT, the relationship between the intensity of lymphodepletion and tumor treatment efficacy has not been systematically studied. Using the pmel-1 mouse model of self/tumor-reactive CD8 T cells, we observed a strong correlation between the intensity of the conditioning regimen and the efficacy of ACT-based treatments using linear regression analysis. This was the case for preparative total body irradiation administered either as a single dose (R=0.97, P<0.001) or in fractionated doses (R=0.94, P<0.001). Increased amounts of preparative total body irradiation were directly correlated with progressively more favorable ratios of transferred tumor-reactive CD8 T cells toward endogenous cells with the potential for inhibitory activity including: CD4 cells (potentially T regulatory cells); Gr1 cells (which are capable of functioning as myeloid-derived suppressor cells); and endogenous CD8 and natural killer 1.1 cells (that can act as "sinks" for homeostatic cytokines in the postablative setting). With increasing ablation, we also observed elevated lipopolysaccharide levels in the sera and heightened levels of systemic inflammatory cytokines. Thus, increased intensity lymphodepletion triggers enhanced tumor treatment efficacy and the benefits of high-dose total body irradiation must be titrated against its risks.

Vaccination with tumor cells pulsed with foreign peptide induces immunity to the tumor itself

EMT-6 mammary carcinoma and B16 melanoma (B16M) cells are lethal and barely immunogenic in syngeneic BALB/c and C57BL/6 mice, respectively. We show that mice vaccinated with tumor cells pulsed with a MHC class I-restricted peptide develop a T cell response, not only to the peptide, but also to the unpulsed tumor. These mice display protective immunity against the unpulsed tumor, and their T cells adoptively transfer tumor-specific protection to immunodeficient SCID mice. Our data have implications for cancer vaccine strategies. Grafting a single well-defined foreign peptide on tumor cells might suffice to trigger anti-tumor immunity.

Design of agonistic altered peptides for the robust induction of CTL directed towards H-2Db in complex with the melanoma-associated epitope gp100

Immunogenicity of tumor-associated antigens (TAA) is often weak because many TAA are autoantigens for which the T-cell repertoire is sculpted by tolerance mechanisms. Substitutions at main anchor positions to increase the complementarity between the peptide and the MHC class I (MHC-I) binding cleft constitute a common procedure to improve binding capacity and immunogenicity of TAA. However, such alterations are tailored for each MHC-I allele and may recruit different CTL specificities through conformational changes in the targeted peptides. Comparative analysis of substituted melanoma-differentiation antigen gp100 in complex with H-2D(b) revealed that combined introduction of glycine and proline residues at the nonanchor positions 2 and 3, respectively, resulted in an agonistic altered peptide with dramatically enhanced binding affinity, stability, and immunogenicity of this TAA. Peptide vaccination using the p2Gp3P-altered peptide version of gp100 induced high frequencies of melanoma-specific CTL in the endogenous CD8+ repertoire. Crystal structure analysis of MHC/peptide complexes revealed that the conformation of the modified p2Gp3P-peptide was similar to the wild-type peptide, and indicated that this mimotope was stabilized through interactions between peptide residue p3P and the tyrosine residue Y159 that is conserved among most known MHC-I molecules throughout mammalian species. Our results may provide an alternative approach to enhance MHC stabilization capacity and immunogenicity of low-affinity peptides for induction of robust tumor-specific CTL.

Quantitating T cell cross-reactivity for unrelated peptide antigens

Quantitating the frequency of T cell cross-reactivity to unrelated peptides is essential to understanding T cell responses in infectious and autoimmune diseases. Here we used 15 mouse or human CD8+ T cell clones (11 antiviral, 4 anti-self) in conjunction with a large library of defined synthetic peptides to examine nearly 30,000 TCR-peptide MHC class I interactions for cross-reactions. We identified a single cross-reaction consisting of an anti-self TCR recognizing a poxvirus peptide at relatively low sensitivity. We failed to identify any cross-reactions between the synthetic peptides in the panel and polyclonal CD8+ T cells raised to viral or alloantigens. These findings provide the best estimate to date of the frequency of T cell cross-reactivity to unrelated peptides ( approximately 1/30,000), explaining why cross-reactions between unrelated pathogens are infrequently encountered and providing a critical parameter for understanding the scope of self-tolerance.

Harnessing the physiology of lymphopenia to support adoptive immunotherapy in lymphoreplete hosts

Lymphopenia enhances the effectiveness of adoptive immunotherapy by facilitating expansion of transferred T cells but also limits the T-cell repertoire available to mediate immune responses and, in humans, is associated with chronic immune dysfunction. Previous studies concluded that lymphopenia augments adoptive immunotherapy by diminishing Tregs and increasing homeostatic cytokines. We sought to determine whether targeted therapies that replicate the physiology of lymphopenia in lymphoreplete hosts could provide a similarly supportive milieu. Pmel-1 T cells were transferred to B16-bearing lymphopenic versus lymphoreplete mice receiving alphaCD25 and/or recombinant human interleukin-7. Although CD25-based Treg depletion was inefficient because of peripheral expansion of CD4+CD25-FOXP3+ cells, outcomes were better in alphaCD25-treated lymphoreplete hosts than in lymphopenic hosts, and adoptive immunotherapy was most effective in lymphoreplete hosts receiving alphaCD25 plus recombinant human interleukin-7. Lymphopenic hosts supported increased proliferation of adoptively transferred antigen-specific T cells, but cells transferred to lymphoreplete recipients receiving targeted therapies showed superior function. Further, determinant spreading was substantial in lymphoreplete hosts but absent in lymphopenic hosts. These results demonstrate that targeted therapies delivered to mimic the "physiology of lymphopenia" enhance the efficacy of adoptive immunotherapy in lymphoreplete hosts and provide a potentially superior alternative to the induction of lymphopenia.

Vesicular stomatitis virus as a novel cancer vaccine vector to prime antitumor immunity amenable to rapid boosting with adenovirus

Vesicular stomatitis virus (VSV) has proven to be an effective vaccine vector for immunization against viral infection, but its potential to induce an immune response to a self-tumor antigen has not been investigated. We constructed a recombinant VSV expressing human dopachrome tautomerase (hDCT) and evaluated its immunogenicity in a murine melanoma model. Intranasal delivery of VSV-hDCT activated both CD4(+) and CD8(+) DCT-specific T-cell responses. The magnitude of these responses could be significantly increased by booster immunization with recombinant adenovirus (Ad)-hDCT, which led to enhanced efficacy against B16-F10 melanoma in both prophylactic and therapeutic settings. Notably, the interval of VSV/Ad heterologous vaccination could be shortened to as few as 4 days, making it a potential regimen to rapidly expand antigen-specific effector cells. Furthermore, VSV-hDCT could increase DCT-specific T-cell responses primed by Ad-hDCT, suggesting VSV is efficient for both priming and boosting of the immune response against a self-tumor antigen.

IL-7 adjuvant treatment enhances long-term tumor antigen-specific CD8+ T-cell responses after immunization with recombinant lentivector

Immunization with recombinant lentivector elicits higher frequencies of tumor antigen-specific memory CD8+ T cells than peptide-based vaccines. This finding correlates with our observation that, upon recombinant lentivector immunization, a higher fraction of antigen-specific effector CD8+ T cells does not down-regulate the expression of the survival/memory marker interleukin-7 receptor α chain (IL-7Rα). Here we show that, surprisingly, higher expression of IL-7Rα on recombinant lentivector-induced effector CD8+ T cells does not result in the up-regulation of survival molecules, such as Bcl-2. We thus hypothesized that physiologic levels of IL-7 might be limiting in vivo for delivering survival signals to the expanding population of effector cells. To test this hypothesis, we administered recombinant IL-7 during the effector phase of the response. We observed an up-regulation of Bcl-2 and a strong expansion of antigen-specific effector CD8+ T cells, and of naive CD8+ T cells. Strikingly, IL-7 treatment elicited also a significant increase in the number of antigen-specific memory CD8+ T cells in recombinant lentivector-immunized mice, but not in peptide-immunized mice. Altogether, these data show that IL-7 adjuvant treatment can enhance long-term antigen-specific CD8+ T-cell responses. However, its efficacy depends on the expression of IL-7Rα at the surface of effector CD8+ T cells.

Multiple antigen-targeted immunotherapy with alpha-galactosylceramide-loaded and genetically engineered dendritic cells derived from embryonic stem cells

Numerous tumor-associated antigens (TAA) have been identified and their use in immunotherapy is considered to be promising. For TAA-based immunotherapy to be broadly applied as standard anticancer medicine, methods for active immunization should be improved. In the present study, we demonstrated the efficacy of multiple TAA-targeted dendritic cell (DC) vaccines and also the additive effects of loading alpha-galactosylceramide to DC using mouse melanoma models. On the basis of previously established methods to generate DC from mouse embryonic stem cells (ES-DC), 4 kinds of genetically modified ES-DC, which expressed the melanoma-associated antigens, glypican-3, secreted protein acidic and rich in cysteine, tyrosinase-related protein-2, or gp100 were generated. Anticancer effects elicited by immunization with the ES-DC were assessed in preventive and also therapeutic settings in the models of peritoneal dissemination and spontaneous metastasis to lymph node and lung. The in vivo transfer of a mixture of 3 kinds of TAA-expressing ES-DC protected the recipient mice from melanoma cells more effectively than the transfer of ES-DC expressing single TAA, thus demonstrating the advantage of multiple as compared with single TAA-targeted immunotherapy. Loading ES-DC with alpha-galactosylceramide further enhanced the anticancer effects, suggesting that excellent synergic effects of TAA-specific cytotoxic T lymphocytes and natural killer T cells against metastatic melanoma can be achieved by using genetically modified ES-DC. With the aid of advancing technologies related to pluripotent stem cells, induced pluripotent stem cells, and ES cells, clinical application of DC highly potent in eliciting anticancer immunity will be realized in the near future.

IFN-alpha enhances peptide vaccine-induced CD8+ T cell numbers, effector function, and antitumor activity

Type I IFNs, including IFN-alpha, enhance Ag presentation and promote the expansion, survival, and effector function of CD8(+) CTL during viral infection. Because these are ideal characteristics for a vaccine adjuvant, we examined the efficacy and mechanism of exogenous IFN-alpha as an adjuvant for antimelanoma peptide vaccination. We studied the expansion of pmel-1 transgenic CD8(+) T cells specific for the gp100 melanocyte differentiation Ag after vaccination of mice with gp100(25-33) peptide in IFA. IFN-alpha synergized with peptide vaccination in a dose-dependent manner by boosting relative and absolute numbers of gp100-specific T cells that suppressed B16 melanoma growth. IFN-alpha dramatically increased the accumulation of gp100-specific, IFN-gamma-secreting, CD8(+) T cells in the tumor through reduced apoptosis and enhanced proliferation of Ag-specific CD8(+) T cells. IFN-alpha treatment also greatly increased the long-term maintenance of pmel-1 CD8(+) T cells with an effector memory phenotype, a process that required expression of IFN-alpha receptor on the T cells and IL-15 in the host. These results demonstrate the efficacy of IFN-alpha as an adjuvant for peptide vaccination, give insight into its mechanism of action, and provide a rationale for clinical trials in which vaccination is combined with standard-of-care IFN-alpha therapy for melanoma.

Progress and challenges in the vaccine-based treatment of head and neck cancers

Head and neck (HN) cancer represents one of the most challenging diseases because the mortality remains high despite advances in early diagnosis and treatment. Although vaccine-based approaches for the treatment of advanced squamous cell carcinoma of the head and neck have achieved limited clinical success, advances in cancer immunology provide a strong foundation and powerful new tools to guide current attempts to develop effective cancer vaccines. This article reviews what has to be rather what has been done in the field for the development of future vaccines in HN tumours.

Lentivector immunization stimulates potent CD8 T cell responses against melanoma self-antigen tyrosinase-related protein 1 and generates antitumor immunity in mice

Recombinant lentivector immunization has been demonstrated to induce potent CD8 T cell responses in vivo. In this study, we investigated whether lentivector delivering a self/tumor Ag, tyrosinase related protein 1 (TRP1), could stimulate effective antitumor T cell responses. We found that immunization with lentivector expressing mutated TRP1 Ag elicited potent CD8 T cell responses against multiple TRP1 epitopes. Importantly, the activated CD8 T cells effectively recognize wild-type TRP1 epitopes. At peak times, as many as 10% of CD8 T cells were effector cells against TRP1 Ag. These cells killed wild-type TRP1 peptide-pulsed target cells in vivo and produced IFN-gamma after ex vivo stimulation. The CD8 T cell responses were long-lasting (3-4 wk). Immunized mice were protected from B16 tumor cell challenge. In a therapeutic setting, lentivector immunization induced potent CD8 T cell responses in tumor bearing mice. The number of infiltrating T cells and the ratio of CD8/CD4 were dramatically increased in the tumors of immunized mice. The tumor-infiltrating CD8 T cells were functional and produced IFN-gamma. The potent CD8 T cell responses stimulated by lentivector immunization eliminated small 3-day s.c. B16 tumors and strongly inhibited the growth of more established 5-day tumors. These studies demonstrate that genetic immunization with lentivector expressing mutated self/tumor Ag can generate potent CD8 T cell immune responses and antitumor immunity that prevent and inhibit B16 tumor growth, suggesting that lentivector immunization has the potential for tumor immunotherapy and immune prevention.

CTL activation using the natural low-affinity epitope 222-229 from tyrosinase-related protein 1 leads to tumor rejection

Vaccine strategies for cancer immunotherapy have focused on peptide ligands with high affinity for MHC class I. Largely, these vaccines have not been therapeutic. We have examined the peptide specificity of a strongly protective T-cell response that eradicates established B16 melanoma and find that the recognized epitope is generated by a low-affinity MHC class I ligand from tyrosinase-related protein 1 (TRP1). Cytotoxic T-cell responses are induced against TRP1(222-229) by several vaccination schemes using a Toll-like receptor agonist, T regulatory cell depletion, or the immune modulator B7-DCXAb to drive immunity. TRP1(222) CTL are generated from multiple antigen sources, including antigens expressed by tumors growing in situ, tumor cell lysates, and peptide vaccines. The key finding in this study is that protection from freshly implanted or established B16 tumors is primarily mediated by TRP1(222)-specific CTL and not by CTL specific for more traditional melanoma antigens such as TRP2 or gp100. This finding challenges the assumption that the optimal peptide antigens for cancer vaccines are high-affinity MHC ligands. We propose that when administered appropriately, native low-affinity MHC ligands are optimal inducers of immunotherapeutic CTL.

Self-antigen-specific CD8+ T cell precursor frequency determines the quality of the antitumor immune response

A primary goal of cancer immunotherapy is to improve the naturally occurring, but weak, immune response to tumors. Ineffective responses to cancer vaccines may be caused, in part, by low numbers of self-reactive lymphocytes surviving negative selection. Here, we estimated the frequency of CD8⁺ T cells recognizing a self-antigen to be <0.0001% (∼1 in 1 million CD8⁺ T cells), which is so low as to preclude a strong immune response in some mice. Supplementing this repertoire with naive antigen-specific cells increased vaccine-elicited tumor immunity and autoimmunity, but a threshold was reached whereby the transfer of increased numbers of antigen-specific cells impaired functional benefit, most likely because of intraclonal competition in the irradiated host. We show that cells primed at precursor frequencies below this competitive threshold proliferate more, acquire polyfunctionality, and eradicate tumors more effectively. This work demonstrates the functional relevance of CD8⁺ T cell precursor frequency to tumor immunity and autoimmunity. Transferring optimized numbers of naive tumor-specific T cells, followed by in vivo activation, is a new approach that can be applied to human cancer immunotherapy. Further, precursor frequency as an isolated variable can be exploited to augment efficacy of clinical vaccine strategies designed to activate any antigen-specific CD8⁺ T cells.

Dendritic cell-mediated-immunization with xenogenic PrP and adenoviral vectors breaks tolerance and prolongs mice survival against experimental scrapie

In prion diseases, PrP^c, a widely expressed protein, is transformed into a pathogenic form called PrP^Sc, which is in itself infectious. Antibodies directed against PrP^c have been shown to inhibit PrP^c to PrP^Sc conversion in vitro and protect in vivo from disease. Other effectors with potential to eliminate PrPSc-producing cells are cytotoxic T cells directed against PrP-derived peptides but their ability to protect or to induce deleterious autoimmune reactions is not known. The natural tolerance to PrP^c makes difficult to raise efficient adaptive responses. To break tolerance, adenovirus (Ad) encoding human PrP (hPrP) or control Ad were administered to wild-type mice by direct injection or by transfer of Ad-transduced dendritic cells (DCs). Control Ad-transduced DCs from Tg650 mice overexpressing hPrP were also used for immunization. DC-mediated but not direct administration of AdhPrP elicited antibodies that bound to murine native PrP^c. Frequencies of PrP-specific IFNγ-secreting T cells were low and in vivo lytic activity only targeted cells strongly expressing hPrP. Immunohistochemical analysis revealed that CD3⁺ T cell infiltration was similar in the brain of vaccinated and unvaccinated 139A-infected mice suggesting the absence of autoimmune reactions. Early splenic PrP^Sc replication was strongly inhibited ten weeks post infection and mean survival time prolonged from 209 days in untreated 139A-infected mice to 246 days in mice vaccinated with DCs expressing the hPrP. The efficacy appeared to be associated with antibody but not with cytotoxic cell-mediated PrP-specific responses.

Effective immunotherapy against murine gliomas using type 1 polarizing dendritic cells--significant roles of CXCL10

In an attempt to develop effective vaccines against central nervous system (CNS) tumors, we evaluated the ability of vaccines with standard dendritic cells (DC) versus type 1 polarizing DCs (DC1) to induce glioma-specific type 1 CTLs with CNS tumor-relevant homing properties and the mechanism of their action. C57BL/6 mouse-derived bone marrow cells were cultured with mouse granulocyte/macrophage colony-stimulating factor (GM-CSF) for 6 days, and CD11c(+) cells were subsequently cultured with GM-CSF, rmIFN-gamma, rmIFN-alpha, rmIL-4, and polyinosinic-polycytidylic acid stabilized by lysine and carboxymethylcellulose for 24 hours to generate DC1s. In analogy to their human counterparts, mouse DC1s exhibited surface marker profiles of mature DCs and produced high levels of IL-12 and CXCL10. Importantly for their application as cancer vaccines, such DC1s stably retained their type 1 phenotype even when exposed to type 2-promoting or regulatory T cell (Treg)-promoting environments. Consistently, mouse DC1s induced antigen-specific type 1 CTLs more efficiently than nonpolarized DCs in vitro. DC1s given s.c. migrated into draining lymph nodes, induced antigen-specific CTLs, and suppressed Treg accumulation. In addition, s.c. immunization with DC1s loaded with glioma-associated antigen (GAA)-derived CTL epitope peptides prolonged the survival of CNS GL261 glioma-bearing mice, which was associated with efficient CNS glioma homing of antigen-specific CTLs. Intratumoral injections of GAA peptide-loaded DC1s further enhanced the anti-CNS glioma effects of DC1-based s.c. immunization. Interestingly, the antitumor functions were abrogated with CXCL10(-/-) mouse-derived DC1s. Collectively, these findings show the anti-CNS glioma effects of DC1-based therapy and a novel role of CXCL10 in the immunologic and therapeutic activity of DC-based cancer vaccines.

The good, the bad and the ugly: how altered peptide ligands modulate immunity

BACKGROUND

The basis of T cell immune responses is the specific recognition of an immunogenic peptide epitope by a T cell receptor. Peptide alterations of such T cell epitopes with single or few amino acid variations can have drastic effects on the outcome of this recognition. These altered peptide ligands can act as modulators of immune responses as they are capable of downregulating or upregulating responses.

OBJECTIVE/METHODS

We review how altered peptide ligands can have 'good' 'bad' and 'ugly' outcomes in treating diseases.

RESULTS/CONCLUSION

Altered peptide ligands have been used as immunotherapeutics in autoimmune (and allergic) diseases, infectious diseases and cancer. In the next five years we anticipate seeing a number of altered peptide ligands in clinical trials, progressing from contradictory classifications of good, bad or ugly, to the exciting outcome of 'useful'.

In vivo functional efficacy of tumor-specific T cells expanded using HLA-Ig based artificial antigen presenting cells (aAPC)

Adoptive immunotherapy for treatment of cancers and infectious diseases is often hampered by a high degree of variability in the final T cell product and in the limited in vivo function and survival of ex vivo expanded antigen-specific cytotoxic T cells (CTL). This has stimulated interest in development of standardized artificial antigen presenting cells (aAPC) to reliably expand antigen specific CTL. However, for successful immunotherapy the aAPC ex vivo generated CTL must have anti-tumor activity in vivo. Here, we demonstrate that HLA-Ig based aAPC stimulated tumor-specific CTL from human peripheral blood T lymphocytes showed robust expansion and functional activity in a human/SCID mouse melanoma model. HLA-Ig based aAPC expanded CTL were detected in the peripheral blood up to 15 days after transfer. Non-invasive bioluminescence imaging of tumor bearing mice demonstrated antigen dependent localization of transferred CTL to the tumor site. Moreover, adoptive transfer of HLA-Ig based aAPC generated CTL inhibited the tumor growth both in prevention and treatment modes of therapy and was comparable to that achieved by dendritic cell expanded CTL. Thus, our data demonstrate potential therapeutic in vivo activity of HLA-Ig based aAPC expanded CTL to control tumor growth.

Behavior of immune players in the tumor microenvironment

PURPOSE OF REVIEW

Tumors recruit various immune cells with seemingly contrasting functions. Yet, the precise role of these cells in situ remains vastly unknown. This review presents a new discovery effort that employs intravital imaging to study immune players directly in tissues.

RECENT FINDINGS

Cytotoxic T lymphocytes (CTLs) that recognize cognate antigenic peptide can infiltrate tumors from the periphery to the center, and physically engage and eliminate antigen-presenting tumor cells. Nevertheless, the reported kinetics for tumor cell killing by CTLs in vivo is surprisingly low as it takes several hours for one CTL to eliminate one tumor cell. Also, T regulatory (Treg) cells can create a suppressive milieu that restricts the release of CTL cytotoxic granules, which protects tumor cells from being killed. CTLs may be further subverted during lengthy interactions with tumor-associated macrophages (TAMs). Finally, TAMs can directly facilitate tumor invasion by recruiting tumor cells nearby vessels and promoting their intravasation.

SUMMARY

Intravital imaging has started to uncover tumor-related immune events as they unfold in vivo. The technology should be exploited in the coming years to dissect further the tumor microenvironment and to define therapeutics that augment antitumor immunity.

Improved tumor immunity using anti-tyrosinase related protein-1 monoclonal antibody combined with DNA vaccines in murine melanoma

Passive immunization with monoclonal antibody TA99 targeting melanoma differentiation antigen tyrosinase-related protein-1 (Tyrp1; gp75) and active immunization with plasmid DNA encoding altered Tyrp1 both mediate tumor immunity in the B16 murine melanoma model. We report here that TA99 enhances Tyrp1 DNA vaccination in the treatment of B16 lung metastases, an effect mediated by immunologic mechanisms as Tyrp1 has no known role in regulating tumor growth. TA99 is shown to increase induction of anti-Tyrp1 CD8+T-cell responses to DNA vaccination against Tyrp1 as assessed by IFN-gamma ELISPOT assays. Immunohistochemistry studies reveal that TA99 localizes rapidly and specifically to B16 lung nodules. Augmentation of T-cell responses is dependent on the presence of tumor as well as on activating Fc receptors. Furthermore, TA99 enhances DNA vaccination against a distinct melanoma antigen, gp100(pmel17/silver locus), improving antitumor efficacy, augmenting systemic CD8+ T-cell responses to gp100, and increasing CD8+ T-cell infiltration at the tumor site. Epitope spreading was observed, with CD8+ T-cell responses generated to Tyrp1 peptide in mice receiving gp100 DNA vaccination in the presence of TA99. Finally, we show that TA99 improves therapeutic efficacy of DNA vaccination combined with adoptive T-cell transfer in treatment of established subcutaneous B16 melanoma. In conclusion, TA99 enhances DNA vaccination against both the target antigen Tyrp1 and a distinct melanoma antigen gp100 in an Fc receptor-dependent mechanism, consistent with enhanced cross-presentation of tumor-derived antigen. Monoclonal antibodies should be tested as vaccine adjuvants in the treatment of cancer.

Local IL-21 promotes the therapeutic activity of effector T cells by decreasing regulatory T cells within the tumor microenvironment

The eradication of tumors by the immune system depends on the generation of antigen-specific T cells which can migrate to sites of tumor growth and maintain their effector functions despite local tumor-derived T-cell inhibitory factors. Interleukin-21 (IL-21) is an IL-2-related cytokine that has shown limited evidence of antitumor activity in murine models and early phase clinical trials. Effect of local IL-21 on T-cell responses within the tumor microenvironment, however, has not been extensively evaluated. Thus, we developed a stably transfected IL-21-secreting B16 melanoma cell line to test the effects of local IL-21 on endogenous and adoptively transferred T-cell responses. Tumors expressing IL-21 exhibited delayed growth in vivo, which was associated with an increase in activated systemic effector and memory CD8(+) T-cell responses. Local IL-21 also enhanced the therapeutic effects of adoptively transferred gp100-specific T cells and was synergistic with IL-2. The effect was also associated with an increased proliferation of local CD8(+) T cells and decreased accumulation of regulatory CD4(+)FOXP3(+) T cells within the tumor microenvironment. These data suggest that local IL-21 enhances endogenous and adoptively transferred T-cell immunity through increased effector CD8(+) T cells and decreased CD4(+) regulatory T cells in the tumor microenvironment.

Avidity maturation of memory CD8 T cells is limited by self-antigen expression

Immune tolerance to self-antigens is a complex process that utilizes multiple mechanisms working in concert to maintain homeostasis and prevent autoimmunity. We developed a system that revealed a population of self-specific CD8 T cells within the endogenous T cell repertoire. Immunization of ovalbumin (OVA)-expressing transgenic mice with recombinant viruses expressing OVA-peptide variants induced self-reactive T cells in vivo that matured into memory T cells able to respond to secondary infection. However, whereas the avidity of memory cells in normal mice increased dramatically with repeated immunizations, avidity maturation was limited for self-specific CD8 T cells. Despite decreased avidity, such memory cells afforded protection against infection, but did not induce overt autoimmunity. Further, up-regulation of self-antigen expression in dendritic cells using an inducible system promoted programmed death-1 expression, but not clonal expansion of preexisting memory cells. Thus, the self-reactive T cell repertoire is controlled by overlapping mechanisms influenced by antigen dose.

Tumor therapy in mice via antigen targeting to a novel, DC-restricted C-type lectin

The mouse CD8alpha+ DC subset excels at cross-presentation of antigen, which can elicit robust CTL responses. A receptor allowing specific antigen targeting to this subset and its equivalent in humans would therefore be useful for the induction of antitumor CTLs. Here, we have characterized a C-type lectin of the NK cell receptor group that we named DC, NK lectin group receptor-1 (DNGR-1). DNGR-1 was found to be expressed in mice at high levels by CD8+ DCs and at low levels by plasmacytoid DCs but not by other hematopoietic cells. Human DNGR-1 was also restricted in expression to a small subset of blood DCs that bear similarities to mouse CD8alpha+ DCs. The selective expression pattern and observed endocytic activity of DNGR-1 suggested that it could be used for antigen targeting to DCs. Consistent with this notion, antigen epitopes covalently coupled to an antibody specific for mouse DNGR-1 were selectively cross-presented by CD8alpha+ DCs in vivo and, when given with adjuvants, induced potent CTL responses. When the antigens corresponded to tumor-expressed peptides, treatment with the antibody conjugate and adjuvant could prevent development or mediate eradication of B16 melanoma lung pseudometastases. We conclude that DNGR-1 is a novel, highly specific marker of mouse and human DC subsets that can be exploited for CTL cross-priming and tumor therapy.

Engagement of Toll-like receptor-2 on cytotoxic T-lymphocytes occurs in vivo and augments antitumor activity

Toll-like receptors (TLRs) are among the fundamental molecules that alert the immune system to the presence of an infection by recognizing pathogen-associated molecules. Much of our understanding regarding TLR function stems from the study of innate immune cells. Recent studies by several groups, including ours, have shown that TLRs can function as costimulatory receptors for antigen-specific T cells, resulting in enhanced T-cell survival and increased expression of effector molecules. We report that the ligation of the TLR1/2 heterodimer on OT-1 cytotoxic T-lymphocytes (CTL) but not TLR2(-/-)OT-1 T cells increased cytolytic activity in vitro and in vivo. On the basis of these data, we tested the hypothesis that TLR1/2 stimulation on CTLs would enhance antitumor activity in a therapeutic model of B16-Ova melanoma. Adoptive OT-1 T-cell transfer into wild-type and MyD88(-/-) mice, followed by injection with TLR1/2 ligand, resulted in a synergistic antitumor effect, which correlated with the induction of CD8 T cells specific to various tumor antigens. In contrast, mice receiving TLR2(-/-)OT-1 T cells and TLR1/2 ligand showed minimal therapeutic efficacy. These findings emphasize the physiological significance of TLR2 engagement on CTLs and could make possible new approaches for the development of effective immunotherapies by manipulating TLR signaling within CTLs.

Effective tumor treatment targeting a melanoma/melanocyte-associated antigen triggers severe ocular autoimmunity

Nonmutated tissue differentiation antigens expressed by tumors are attractive targets for cancer immunotherapy, but the consequences of a highly effective antitumor immune response on self-tissue have not been fully characterized. We found that the infusion of ex vivo expanded adoptively transferred melanoma/melanocyte-specific CD8+ T cells that mediated robust tumor killing also induced autoimmune destruction of melanocytes in the eye. This severe autoimmunity was associated with the up-regulation of MHC class I molecules in the eye and high levels of IFN-gamma derived from both adoptively transferred CD8+ T cells and host cells. Furthermore, ocular autoimmunity required the presence of the IFN-gamma receptor on target tissues. Data compiled from >200 eyes and tumors in 10 independently performed experiments revealed a highly significant correlation (P < 0.0001) between the efficacy of tumor immunotherapy and the severity of ocular autoimmunity. Administration of high doses of steroids locally mitigated ocular autoimmunity without impairing the antitumor effect. These findings have particular importance for immunotherapies directed against self-antigens and highlight the need for targeting unique tumor antigens not expressed in critical tissues.

Modeling the CD8+ T effector to memory transition in adoptive T-cell antitumor immunotherapy

Adoptive T-cell therapy with CD8(+) CTLs is often characterized by poor persistence of the transferred T cells and limited effector responses. Improved persistence and therapeutic efficacy have been noted when antigen-activated CD8(+) T cells express properties of memory cells. The current study was undertaken to more precisely characterize the development of memory-like CD8(+) T cells from short-term CTLs in vitro and upon transfer in vivo, including their antitumor activity. Ovalbumin (OVA)-specific OT-I CTLs acquired phenotypic and functional properties of memory cells 2 to 3 days later either by lowering the concentration of antigen to a level that does not support primary responses and providing a survival signal through transgenic Bcl-2 in vitro or simply by transferring early day 3 CTLs to antigen-free lymphoid-replete mice. In lymphoid-replete mice, established OVA-expressing E.G7 tumor was rejected by short-term CTLs that simultaneously acquired memory-like properties in secondary lymphoid tissues, where tumor antigen level remained low. Collectively, these data indicate that CTLs readily converted to memory-like cells upon lowering antigen to a concentration that selectively supports memory responses and suggest that such conversion predicts successful adoptive immunotherapy.

An immunotherapy approach with dendritic cells genetically modified to express the tumor-associated antigen, HER2

Dendritic cells (DC), genetically modified to express ovalbumin by the retroviral vector GCDNsap, can elicit stronger anti-tumor immunity than those loaded with the peptides. To assess the clinical feasibility of the strategy, such DC were prepared by differentiation of hematopoietic progenitor cells transduced with the human epidermal growth factor receptor 2 (HER2). When inoculated in mice, the DC primed both HER2-specific cytotoxic T lymphocytes and type 1 T helper lymphocytes, resulting in production of HER2-specific antibody. Of importance is that the antibody mediated antibody-dependent cellular cytotoxicity and opsonization. The potent anti-tumor effects were also confirmed by results of experiments using HER2-transgenic mice. Inoculation of HER2-transduced DC resulted in longer disease-free survival of treated mice that showed significant reduction of primary and metastatic tumors. Interestingly, footpad inoculation resulted in stronger anti-tumor effects compared to subcutaneous administration and induced higher levels of the HER2-specific antibody, suggesting that an important role of humoral immunity in anti-tumor effects for malignancies with membrane-type tumor-associated antigens (TAA). Taken together, vaccination of the TAA-transduced DC may represent a promising form of therapy for breast cancers expressing HER2.

Inhibition of STAT3 promotes the efficacy of adoptive transfer therapy using type-1 CTLs by modulation of the immunological microenvironment in a murine intracranial glioma

A variety of cancers, including malignant gliomas, show aberrant activation of STAT3, which plays a pivotal role in negative regulation of antitumor immunity. We hypothesized that inhibition of STAT3 signals would improve the efficacy of T cell adoptive transfer therapy by reversal of STAT3-induced immunosuppression in a murine GL261 intracranial glioma model. In vitro treatment of GL261 cells with JSI-124, a STAT3 inhibitor, reversed highly phosphorylated status of STAT3. Systemic i.p. administration of JSI-124 in glioma-bearing immunocompetent mice, but not athymic mice, resulted in prolonged survival, suggesting a role of adaptive immunity in the antitumor effect. Furthermore, JSI-124 promoted maturation of tumor-infiltrating CD11c(+) dendritic cells and activation of tumor-conditioned cytotoxic T cells, enhanced dendritic cells and GL261 production of CXCL-10, a critical chemokine for attraction of Tc1 cells. When i.p. JSI-124 administration was combined with i.v. transfer of Pmel-I mouse-derived type-1 CTLs (Tc1), glioma-bearing mice exhibited prolonged survival compared with i.p. JSI-124 or i.v. Tc1 therapy alone. Flow cytometric analyses of brain infiltrating lymphocytes revealed that JSI-124-treatment enhanced the tumor-homing of i.v. transferred Tc1 cells in a CXCL-10-dependent fashion. Systemic JSI-124 administration also up-regulated serum IL-15 levels, and promoted the persistence of transferred Tc1 in the host. These data suggest that systemic inhibition of STAT3 signaling can reverse the suppressive immunological environment of intracranial tumor bearing mice both systemically and locally, thereby promoting the efficacy of adoptive transfer therapy with Tc1.

Mechanisms of immunization against cancer using chimeric antigens

Successful approaches to tumor immunotherapy must overcome the physiological state of tolerance of the immune system to self-tumor antigens. Immunization with appropriate variants of syngeneic antigens can achieve this. However, improvements in vaccine design are needed for efficient cancer immunotherapy. Here we explore nine different chimeric vaccine designs, in which the antigen of interest is expressed as an in-frame fusion with polypeptides that impact antigen processing or presentation. In DNA immunization experiments in mice, three of nine fusions elevated relevant CD8(+) T-cell responses and tumor protection relative to an unfused melanoma antigen. These fusions were: Escherichia coli outer membrane protein A (OmpA), Pseudomonas aeruginosa exotoxin A, and VP22 protein of herpes simplex virus-1. The gains of immunogenicity conferred by the latter two are independent of epitope presentation by major histocompatibility complex class II (MHC II). This finding has positive implications for immunotherapy in individuals with CD4(+) T-cell deficiencies. We present evidence that antigen instability is not a sine qua non condition for immunogenicity. Experiments using two additional melanoma antigens identified different optimal fusion partners, thereby indicating that the benefits of fusion vectors remain antigen specific. Therefore large fusion vector panels such as those presented here can provide information to promote the successful advancement of gene-based vaccines.

Priming of naive CD8+ T cells in the presence of IL-12 selectively enhances the survival of CD8+CD62Lhi cells and results in superior anti-tumor activity in a tolerogenic murine model

During the antigen-dependant activation process several subsets CD8+ T cells appear with different phenotypic and functional characteristics. Recent studies indicate that the state of T cell differentiation radically affects their ability to effectively respond to tumor challenge, with early effector CD8+ T (CD62Lhigh) cells having better anti-tumor activity. Thus strategies aimed at optimizing the generation of such subpopulations could significantly enhance the effectiveness of adoptive cell therapy (ACT) for cancer. In this study, we show that priming of naïve CD8+ T cells in the presence of IL-12 selectively rescued early CD8+ CD62L(hi) from activation induced cell death and resulted in the increased accumulation of this subset of CD8+ T cells. Furthermore, we demonstrated that IL-12 directly modulated the expression of CD62L on activated CD8+ T cells. When used for ACT, naïve CD8+ T cells primed in vitro in the presence of IL-12 showed superior anti-tumor activity toward B16 melanoma. Importantly, using the Pmel-1 model, priming pmel-1 cells in vitro with IL-12 reduced the state of functional tolerance associated with the non-mutated "self" tumor antigen gp100, as demonstrated by significant tumor responses in the absence of vaccination. Together, our results suggest that in vitro conditioning of naïve CD8+ T cells with IL-12 prior to ACT could significantly enhance their anti-tumor activity.

CD62L (L-selectin) down-regulation does not affect memory T cell distribution but failure to shed compromises anti-viral immunity

The down-regulation of CD62L that accompanies T lymphocyte activation is thought to redirect cells away from lymph nodes to sites of infection. In this study, CD62L was maintained on Ag-activated T cells and their distribution, and ability to clear pathogen from peripheral sites determined. CD62L was down-regulated on Ag-specific CD8 T cells in lungs of C57BL/6 mice but maintained in CD62L transgenic mice at day 8 after influenza infection. However, the numbers of influenza-specific CD8 T cells recruited were similar in CD62L transgenic and C57BL/6 mice. Memory CD8 T cell numbers in the lungs and noninvolved organs 100 days after primary infection were similar in CD62L transgenic and C57BL/6 mice, despite differing CD62L expression. Transgenic mice expressing wild-type CD62L cleared a recombinant vaccinia virus expressing an influenza-derived CD8 T cell epitope as efficiently as C57BL/6 mice. However, transgenic mice expressing a protease resistant mutant of CD62L showed significantly delayed viral clearance, despite normal CTL generation and the presence of CD107a and IFN-gamma expressing influenza-specific CD8 T cells. These results demonstrate that CD62L down-regulation is not required for CD8 memory cells to home to sites of infection. However, their ability to clear virus is significantly compromised if CD62L shedding is abrogated.

Leukocyte infiltration and tumor cell plasticity are parameters of aggressiveness in primary cutaneous melanoma

Various clinical and experimental observations detected an immunological host defense in cutaneous melanoma. In order to investigate the prognostic value of leukocyte effector mechanisms, we examined the presence of different subsets of leukocytes in tumor samples of 58 patients diagnosed with primary cutaneous melanoma. The presence of T lymphocytes, cytotoxic T lymphocytes, B lymphocytes, CD16+ cells and macrophages was correlated to Breslow depth. A significantly higher amount of several subsets of leukocytes was found in samples with a more progressed tumor stage and survival analysis demonstrated that a higher amount of T lymphocytes and CD16+ cells was associated with a short survival. The amount of FOXP3+ regulatory T lymphocytes did not correlate with survival, nevertheless, it correlated with the amount of total infiltrate. In contrast, analysis of the expression of CD69, a marker for activated lymphocytes, demonstrated that patients with a higher amount of CD69+ lymphocytes had a better survival. In addition, a new parameter for aggressiveness of melanoma, tumor cell plasticity [i.e., the presence of periodic acid Schiff's (PAS) reagent positive loops], also predicted short survival and a trend of a higher amount of tumor infiltrating leukocytes in tumors with PAS positive loops was observed. These findings demonstrate that leukocyte infiltration and the presence of PAS loops is a sign of tumor aggressiveness and may have prognostic value.

A comparison and critical analysis of preclinical anticancer vaccination strategies

Anticancer vaccines have been extensively studied in animal models and in clinical trials. While vaccination can lead to tumor protection in numerous murine models, objective tumor regressions after anticancer vaccination in clinical trials have been rare. B16 is a poorly immunogenic murine melanoma that has been extensively used in anticancer vaccination experiments. Because B16 has been widely used, different vaccination strategies can be compared. We reviewed the results obtained when B16 was treated with five common vaccine types: recombinant viral vaccines, DNA vaccines, dendritic cell vaccines, whole-tumor vaccines, and peptide vaccines. We also reviewed the results obtained when B16 was treated with vaccines combined with adoptive transfer of tumor antigen-specific T cells. We found several characteristics of vaccination regimens that were associated with antitumor efficacy. Many vaccines that incorporated xenogeneic antigens exhibited more potent anticancer activity than vaccines that were identical except that they incorporated the syngeneic version of the same antigen. Interleukin-2 enhanced the antitumor efficacy of several vaccines. Finally, several effective regimens generated large numbers of tumor antigen-specific CD8(+) T cells. Identification of vaccine characteristics that are associated with antitumor efficacy may aid in the development of more effective anticancer vaccination strategies.

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

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

DAP10 deficiency breaks the immune tolerance against transplantable syngeneic melanoma

DAP10, an activating adaptor protein, associates with the NKG2D protein to form a multisubunit receptor complex that is expressed in lymphoid and myeloid cells. The ligands for NKG2D-DAP10 receptor are expressed in both normal and tumor cells, suggesting distinct roles for this receptor in autoimmunity and cancer. In this study, we report that constitutive DAP10 activating signaling is part of regulatory mechanisms that control immunity against tumors. Mice lacking DAP10 (DAP10KO), showed enhanced immunity against melanoma malignancies due to hyperactive functioning of NK1.1+CD3+ NKT cells. DAP10 deficiency resulted in substantially increased NKT cell functions, including cytokine production and cytotoxicity, leading to efficient killing of melanoma tumors. Moreover, the antitumor phenotype of DAP10KO mice correlated with impaired activation status of CD4+CD25+ T regulatory cells (Tregs). Upon activation, DAP10KO Tregs maintained higher levels of IL-2 and produced significantly lower amounts of IL-10 and IFN-gamma cytokines when compared with wild-type Tregs. Our data suggest that DAP10 signaling is involved in adjusting the activation threshold and generation of NKT cells and Tregs to avoid autoreactivity, but also modulates antitumor mechanisms.

Immunogenicity of the carcinoembryonic antigen derived peptide 694 in HLA-A2 healthy donors and colorectal carcinoma patients

Carcinoembryonic antigen (CEACAM5) is commonly overexpressed in human colon cancer. Several antigenic peptides recognized by cytolytic CD8+ T-cells have been identified and used in colon cancer phase-I vaccination clinical trials. The HLA-A*0201-binding CEA(694-702) peptide was recently isolated from acid eluted MHC-I associated peptides from a human colon tumor cell line. However, the immunogenicity of this peptide in humans remains unknown. We found that the peptide CEA(694-702) binds weakly to HLA-A*0201 molecules and is ineffective at inducing specific CD8+ T-cell responses in healthy donors. Immunogenic-altered peptide ligands with increased affinity for HLA-A*0201 were identified. Importantly, the elicited cytolytic T lymphocyte (CTL) lines and clones cross-reacted with the wild-type CEA(694-702) peptide. Tumor cells expressing CEA were recognized in a peptide and HLA-A*0201 restricted fashion, but high-CEA expression levels appear to be required for CTL recognition. Finally, CEA-specific T-cell precursors could be readily expanded by in vitro stimulation of peripheral blood mononuclear cell (PBMC) from colon cancer patients with altered CEA peptide. However, the CEA-specific CD8+ T-cell clones derived from cancer patients revealed low-functional avidity and impaired tumor-cell recognition. Together, using T-cells to demonstrate the processing and presentation of the peptide CEA694-702, we were able to corroborate its presentation by tumor cells. However, the low avidity of the specific CTLs generated from cancer patients as well as the high-antigen expression levels required for CTL recognition pose serious concerns for the use of CEA694-702 in cancer immunotherapy.

GM-CSF-secreting cancer immunotherapies: preclinical analysis of the mechanism of action

Granulocyte-macrophage colony-stimulating factor (GM-CSF)-secreting tumor cell immunotherapies have demonstrated long-lasting, and specific anti-tumor immune responses in animal models. The studies reported here specifically evaluate two aspects of the immune response generated by such immunotherapies: the persistence of irradiated tumor cells at the immunization site, and the breadth of the immune response elicited to tumor associated antigens (TAA) derived from the immunotherapy. To further define the mechanism of GM-CSF-secreting cancer immunotherapies, immunohistochemistry studies were performed using the B16F10 melanoma tumor model. In contrast to previous reports, our data revealed that the irradiated tumor cells persisted and secreted high levels of GM-CSF at the injection site for more than 21 days. Furthermore, dense infiltrates of dendritic cells were observed only in mice treated with GM-CSF-secreting B16F10 cells, and not in mice treated with unmodified B16F10 cells with or without concurrent injection of rGM-CSF. In addition, histological studies also revealed enhanced neutrophil and CD4+ T cell infiltration, as well as the presence of apoptotic cells, at the injection site of mice treated with GM-CSF-secreting tumor cells. To evaluate the scope of the immune response generated by GM-CSF-secreting cancer immunotherapies, several related B16 melanoma tumor cell subclones that exist as a result of genetic drift in the original cell line were used to challenge mice previously immunized with GM-CSF-secreting B16F10 cells. These studies revealed that GM-CSF-secreting cancer immunotherapies elicit T cell responses that effectively control growth of related but antigenically distinct tumors. Taken together, these studies provide important new insights into the mechanism of action of this promising novel cancer immunotherapy.

Survivin-derived peptide epitopes and their role for induction of antitumor immunity in hematological malignancies

The immune system's ability to detect and destroy tumor cells offers an attractive approach to broaden the spectrum of cancer therapies. Survivin, a member of the apoptosis inhibitor protein family, is a tumor antigen, overexpressed in human cancers giving rise to peptides eliciting spontaneous CD8+ and CD4+ responses. Due to its dual function, blockade of apoptosis and regulation of cell division, survivin is directly associated with tumor survival and therefore regarded as an ideal target structure for immunotherapeutic approaches. Strong evidence that survivin acts as a T-cell activating antigen has been collected in recent years and the first clinical trials using survivin-based vaccines aim to prove its therapeutic efficacy in the clinic. We focus on the role of survivin in hematological malignancies, including a list of survivin-derived peptides eliciting potent immune responses.

Toll-like receptors in tumor immunotherapy

Lymphodepletion with chemotherapeutic agents or total body irradiation (TBI) before adoptive transfer of tumor-specific T cells is a critical advancement in the treatment of patients with melanoma. More than 50% of patients that are refractory to other treatments experience an objective or curative response with this approach. Emerging data indicate that the key mechanisms underlying how TBI augments the functions of adoptively transferred T cells include (a) the depletion of regulatory T cells (T(reg)) and myeloid-derived suppressor cells that limit the function and proliferation of adoptively transferred cells; (b) the removal of immune cells that act as "sinks" for homeostatic cytokines, whose levels increase after lymphodepletion; and (c) the activation of the innate immune system via Toll-like receptor 4 signaling, which is engaged by microbial lipopolysaccharide that translocated across the radiation-injured gut. Here, we review these mechanisms and focus on the effect of Toll-like receptor agonists in adoptive immunotherapy. We also discuss alternate regimens to chemotherapy or TBI, which might be used to safely treat patients with advanced disease and promote tumor regression.

Design and development of synthetic peptide vaccines: past, present and future

Synthetic peptide vaccines aiming at the induction of a protective CD8(+) T-cell response against infectious or malignant diseases are widely used in the clinic but, despite their success in animal models, they do not yet live up to their promise in humans. This review assesses the development of synthetic peptide vaccines, weighs it against the immunological concepts that have emerged, and identifies the key issues that play a role in the failure or success of a synthetic peptide vaccine. The current state-of-the-art peptide vaccine is a complete synthetic inflammatory product that is ingested by professional antigen-presenting cells and stimulates both CD4(+) and CD8(+) T cells.

Mobilizing the low-avidity T cell repertoire to kill tumors

Optimally, T cells destroy infected and transformed cells of the host. To be effective the T cell repertoire must have a sufficiently diverse number of T cell receptors (TCRs) to recognize the abundance of foreign and tumor antigens presented by MHC molecules. The T cell repertoire must also not be reactive toward self-antigens on healthy cells to prevent autoimmunity. Unlike antigens derived from pathogens, most tumor-associated antigens (TAA) are also self-antigens. Therefore, central and peripheral tolerance mechanisms delete or inhibit tumor-reactive T cells. Although there are T cells within the peripheral repertoire that recognize TAA, these T cells are not sufficient to prevent growth of clinically relevant tumors. We will discuss how this dysfunction results, in part, from the low functional avidity of T cells for tumor, or antigen presenting cells (APC) displaying TAA. We discuss the limitations of these low-avidity tumor-reactive T cells and review current immunotherapies aimed at enhancing the avidity and antitumor activity of the tumor-specific T cell repertoire.

Elevated frequencies of self-reactive CD8+ T cells following immunization with a xenoantigen are due to the presence of a heteroclitic CD4+ T-cell helper epitope

Immunization of mice with human dopachrome tautomerase (hDCT) provides greater protection against melanoma than immunization with the murine homologue (mDCT). We mapped the CD8(+) and CD4(+) T-cell epitopes in both proteins to better understand the mechanisms of the enhanced protection. The dominant CD8(+) T-cell epitopes were fully conserved between both proteins, yet immunization with hDCT produced frequencies of CD8(+) T cells that were 5- to 10-fold higher than immunization with mDCT. This difference was not intrinsic to the two proteins because comparable frequencies of CD8(+) T cells were elicited by both antigens in DCT-deficient mice. Strikingly, only hDCT elicited a significant level of specific CD4(+) T cells in wild-type (WT) mice. The murine protein was not devoid of CD4(+) T-cell epitopes because immunization of DCT-deficient mice with mDCT resulted in robust CD4(+) T-cell immunity directed against two epitopes that were not identified in WT mice. These results suggested that the reduced immunogenicity of mDCT in WT mice may be a function of insufficient CD4(+) T-cell help. To address this possibility, the dominant CD4(+) T-cell epitope from hDCT was introduced into mDCT. Immunization with the mutated mDCT evoked CD8(+) T-cell frequencies and protective immunity comparable with hDCT. These results reveal a novel mechanism by which xenoantigens overcome tolerance. Our data also suggest that immunologic tolerance is more stringent for CD4(+) T cells than CD8(+) T cells, providing a mechanism of peripheral tolerance where autoreactive CD8(+) T cells fail to be activated due to a lack of autoreactive CD4(+) T cells specific for the same antigen.

Induction of postsurgical tumor immunity and T-cell memory by a poorly immunogenic tumor

The generation of protective CD8 T-cell memory against tumor-expressed self-antigens is an important but elusive goal of cancer immunotherapy. The possibility that a progressive, poorly immunogenic tumor can induce T-cell memory against self-antigens has not previously been studied. Herein, we report that growth of the poorly immunogenic B16 melanoma in the absence of regulatory T cells (T(reg)) generates CD8 T-cell responses that develop into functional memory after the tumor has been surgically excised. Tumor-primed memory T cells recognized melanocyte differentiation antigens TRP-2/DCT and gp100 and persisted for as long as 5 months following surgical tumor excision. Phenotypic analysis showed that these cells develop into both central and effector memory T-cell subsets, which produce IFN-gamma and interleukin-2 on reencounter with antigen. Most importantly, tumor-primed memory T cells mediated the rejection of intradermal and systemically disseminated challenge tumors given 30 to 60 days following surgery. Tumor-excised mice also developed autoimmune vitiligo, showing that T(reg) cells prevent tissue-specific autoimmunity in tumor-bearing hosts. This study establishes that T(reg) depletion in tumor-bearing hosts drives the natural development of protective T-cell memory. Generating such responses may aid in the clinical management of tumor recurrence and metastasis following surgery.

Development of an antigen-presenting cell-targeted DNA vaccine against melanoma by mannosylated liposomes

As part of our research involving the targeted delivery of plasmid DNA (pDNA) to antigen-presenting cells (APCs), we developed mannosylated cationic liposomes: N-[1-(2,3-dioleyloxy)propyl]-N,N,N-trimethylammonium chloride (DOTMA)/cholesten-5-yloxy-N-(4-((1-imino-2-D-thiomannosyl-ethyl)amino)butyl)formamide (Man-C4-Chol)/Chol (Man liposomes). In this study, we used melanoma-associated antigen expressing pDNA; pUb-M and Man liposomes to create a novel APC-targeted DNA vaccine against melanoma and examined its potency by measuring the Ub-M mRNA expression in splenic dendritic cells and macrophages, the cytotoxic T lymphocyte (CTL) activity against melanoma B16BL6 cells and the melanoma B16BL6-specific anti-tumor effect after intraperitoneal (i.p.) administration. We verified that Man lipoplex induces significantly higher pUb-M gene transfection into dendritic cells and macrophages than unmodified lipoplex and naked DNA and it also strongly induces CTL activity against melanoma, inhibits its growth and prolongs the survival after tumor challenge compared with unmodified liposomes and the standard method (naked pDNA, intramuscular (i.m.)). These results demonstrate that Man liposomes are a potent APCs-targeted vector that induce strong immunopotency of DNA vaccine against melanoma.

Hydrodynamic Limb Vein Delivery of a Xenogeneic DNA Cancer Vaccine Effectively Induces Antitumor Immunity

Tumor-associated antigens (TAA) are typically poorly immunogenic "self" antigens. An effective strategy to break tolerance and induce antitumor immunity is by genetic vaccination, employing the orthologous TAA-sequence from a different species. We recently developed a clinically relevant approach for intravascular hydrodynamic limb vein (HLV) delivery of nucleic acids to skeletal muscle. Using the human gp100 xenogeneic TAA in the murine B16 melanoma model, we show that genetic vaccination of mice by HLV plasmid DNA delivery was highly effective at breaking tolerance against the homologous murine gp100 (mgp100) TAA and induced prophylactic antitumor protection. HLV vaccination resulted in an anti-hgp100 humoral and cellular response, with 4-5% of CD8(+) T cells being gp100(25-33)-epitope-specific. Vaccinated animals demonstrated in vivo cytolytic activity against human and mgp100(25-33) peptide-pulsed targets. Antitumor immunity could be adoptively transferred by splenocytes from human gp100-vaccinated animals. Furthermore, a durable antitumor memory response was established as approximately 3% of CD8(+) T cells were gp100(25-33) antigen-specific in mice 6 months after vaccination. Following a single HLV human gp100 DNA boost, this level increased to approximately 17% and protected animals from subsequent B16 tumor rechallenge. Our results warrant further consideration of HLV as a clinically relevant method for cancer gene therapy.