Low-avidity self-specific T cells display a pronounced expansion defect that can be overcome by altered peptide ligands

Identification of Highly Cross-Reactive Mimotopes for a Public T Cell Response in Murine Melanoma

While immune checkpoint blockade results in durable responses for some patients, many others have not experienced such benefits. These treatments rely upon reinvigorating specific T cell-antigen interactions. However, it is often unknown what antigens are being recognized by T cells or how to potently induce antigen-specific responses in a broadly applicable manner. Here, we characterized the CD8⁺ T cell response to a murine model of melanoma following combination immunotherapy to determine the basis of tumor recognition. Sequencing of tumor-infiltrating T cells revealed a repertoire of highly homologous TCR sequences that were particularly expanded in treated mice and which recognized an antigen from an endogenous retrovirus. While vaccination against this peptide failed to raise a protective T cell response in vivo, engineered antigen mimotopes induced a significant expansion of CD8⁺ T cells cross-reactive to the original antigen. Vaccination with mimotopes resulted in killing of antigen-loaded cells in vivo yet showed modest survival benefit in a prophylactic vaccine paradigm. Together, this work demonstrates the identification of a dominant tumor-associated antigen and generation of mimotopes which can induce robust functional T cell responses that are cross-reactive to the endogenous antigen across multiple individuals.

An In Vivo Screen to Identify Short Peptide Mimotopes with Enhanced Antitumor Immunogenicity

Tumor-associated self-antigens are potential cancer vaccine targets but suffer from limited immunogenicity. There are examples of mutated, short self-peptides inducing epitope-specific CD8⁺ T cells more efficiently than the wild-type epitope, but current approaches cannot yet reliably identify such epitopes, which are referred to as enhanced mimotopes ("e-mimotopes"). Here, we present a generalized strategy to develop e-mimotopes, using the tyrosinase-related protein 2 (Trp2) peptide Trp2180-188, which is a murine major histocompatibility complex class I (MHC-I) epitope, as a test case. Using a vaccine adjuvant that induces peptide particle formation and strong cellular responses with nanogram antigen doses, a two-step method systematically identified e-mimotope candidates with murine immunization. First, position-scanning peptide micro libraries were generated in which each position of the wild-type epitope sequence was randomized. Randomization of only one specific residue of the Trp2 epitope increased antitumor immunogenicity. Second, all 20 amino acids were individually substituted and tested at that position, enabling the identification of two e-mimotopes with single amino-acid mutations. Despite similar MHC-I affinity compared to the wild-type epitope, e-mimotope immunization elicited improved Trp2-specific cytotoxic T-cell phenotypes and improved T-cell receptor affinity for both the e-mimotopes and the native epitope, resulting in better outcomes in multiple prophylactic and therapeutic tumor models. The screening method was also applied to other targets with other murine MHC-I restriction elements, including epitopes within glycoprotein 70 and Wilms' Tumor Gene 1, to identify additional e-mimotopes with enhanced potency.

Murine CD8 T-cell functional avidity is stable in vivo but not in vitro: Independence from homologous prime/boost time interval and antigen density

It is known that for achieving high affinity antibody responses, vaccines must be optimized for antigen dose/density, and the prime/boost interval should be at least 4 weeks. Similar knowledge is lacking for generating high avidity T‐cell responses. The functional avidity (FA) of T cells, describing responsiveness to peptide, is associated with the quality of effector function and the protective capacity in vivo. Despite its importance, the FA is rarely determined in T‐cell vaccination studies. We addressed the question whether different time intervals for short‐term homologous vaccinations impact the FA of CD8 T‐cell responses. Four‐week instead of 2‐week intervals between priming and boosting with potent subunit vaccines in C57BL/6 mice did not improve FA. Equally, similar FA was observed after vaccination with virus‐like particles displaying low versus high antigen densities. Interestingly, FA was stable in vivo but not in vitro, depending on the antigen dose and the time interval since T‐cell activation, as observed in murine monoclonal T cells. Our findings suggest dynamic in vivo modulation for equal FA. We conclude that low antigen density vaccines or a minimal 4‐week prime/boost interval are not crucial for the T‐cell's FA, in contrast to antibody responses.

Peptide vaccines for hematological malignancies: a missed promise?

Despite the crucial aid that newly developed target therapies are providing to chemotherapy and stem cell transplant, the cure for many hematological malignancies is still an unmet need. Although available therapies are able to induce an effective debulking of the tumor, most of the time, an insidious minimal residual disease survives current treatments and it is responsible for an immediate or delayed relapse. Peptide-derived antitumor vaccines have been developed with the idea that an artificially "educated" immune system may exert an active specific antitumor response able to control and ultimately eradicate underlying post-treatment residual disease. This review will summarize current knowledge of peptide vaccines for hematological malignancies, trying to analyze promises and pitfalls of a safe and intelligent tool that after many years from its first appearance has not yet established its potential role as alternative immune mediated therapeutic approach for hematopoietic tumors.

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.

Improving T cell responses to modified peptides in tumor vaccines

Immune recognition and elimination of cancerous cells is the primary goal of cancer immunotherapy. However, obstacles including immune tolerance and tumor-induced immunosuppression often limit beneficial immune responses. Vaccination is one proposed intervention that may help to overcome these issues and is an active area of study in cancer immunotherapy. Immunizing with tumor antigenic peptides is a promising, straight-forward vaccine strategy hypothesized to boost preexisting antitumor immunity. However, tumor antigens are often weak T cell agonists, attributable to several mechanisms, including immune self-tolerance and poor immunogenicity of self-derived tumor peptides. One strategy for overcoming these mechanisms is vaccination with mimotopes, or peptide mimics of tumor antigens, which alter the antigen presentation and/or T cell activation to increase the expansion of tumor-specific T cells. Evaluation of mimotope vaccine strategies has revealed that even subtle alterations in peptide sequence can dramatically alter antigen presentation and T cell receptor recognition. Most of this research has been performed using T cell clones, which may not be accurate representations of the naturally occurring antitumor response. The relationship between clones generated after mimotope vaccination and the polyclonal T cell repertoire is unclear. Our work with mimotopes in a mouse model of colon carcinoma has revealed important insights into these issues. We propose that the identification of mimotopes based on stimulation of the naturally responding T cell repertoire will dramatically improve the efficacy of mimotope vaccination.

Augmenting antitumor T-cell responses to mimotope vaccination by boosting with native tumor antigens

Vaccination with antigens expressed by tumors is one strategy for stimulating enhanced T-cell responses against tumors. However, these peptide vaccines rarely result in efficient expansion of tumor-specific T cells or responses that protect against tumor growth. Mimotopes, or peptide mimics of tumor antigens, elicit increased numbers of T cells that crossreact with the native tumor antigen, resulting in potent antitumor responses. Unfortunately, mimotopes may also elicit cells that do not crossreact or have low affinity for tumor antigen. We previously showed that one such mimotope of the dominant MHC class I tumor antigen of a mouse colon carcinoma cell line stimulates a tumor-specific T-cell clone and elicits antigen-specific cells in vivo, yet protects poorly against tumor growth. We hypothesized that boosting the mimotope vaccine with the native tumor antigen would focus the T-cell response elicited by the mimotope toward high affinity, tumor-specific T cells. We show that priming T cells with the mimotope, followed by a native tumor-antigen boost, improves tumor immunity compared with T cells elicited by the same prime with a mimotope boost. Our data suggest that the improved tumor immunity results from the expansion of mimotope-elicited tumor-specific T cells that have increased avidity for the tumor antigen. The enhanced T cells are phenotypically distinct and enriched for T-cell receptors previously correlated with improved antitumor immunity. These results suggest that incorporation of native antigen into clinical mimotope vaccine regimens may improve the efficacy of antitumor T-cell responses.

Elevated tumor-associated antigen expression suppresses variant peptide vaccine responses

Variant peptide vaccines are used clinically to expand T cells that cross-react with tumor-associated Ags (TAA). To investigate the effects of elevated endogenous TAA expression on variant peptide-induced responses, we used the GP70 TAA model. Although young BALB/c mice display T cell tolerance to the TAA GP70(423-431) (AH1), expression of GP70 and suppression of AH1-specific responses increases with age. We hypothesized that as TAA expression increases, the AH1 cross-reactivity of variant peptide-elicited T cell responses diminishes. Controlling for immunosenescence, we showed that elevated GP70 expression suppressed AH1 cross-reactive responses elicited by two AH1 peptide variants. A variant that elicited almost exclusively AH1 cross-reactive T cells in young mice elicited few or no T cells in aging mice with Ab-detectable GP70 expression. In contrast, a variant that elicited a less AH1 cross-reactive T cell response in young mice successfully expanded AH1 cross-reactive T cells in all aging mice tested. However, these T cells bound the AH1/MHC complex with a relatively short half-life and responded poorly to ex vivo stimulation with the AH1 peptide. Variant peptide vaccine responses were also suppressed when AH1 peptide is administered tolerogenically to young mice before vaccination. Analyses of variant-specific precursor T cells from naive mice with Ab-detectable GP70 expression determined that these T cells expressed PD-1 and had downregulated IL-7Rα expression, suggesting they were anergic or undergoing deletion. Although variant peptide vaccines were less effective as TAA expression increases, data presented in this article also suggest that complementary immunotherapies may induce the expansion of T cells with functional TAA recognition.

Translating tumor antigens into cancer vaccines

Vaccines represent a strategic successful tool used to prevent or contain diseases with high morbidity and/or mortality. However, while vaccines have proven to be effective in combating pathogenic microorganisms, based on the immune recognition of these foreign antigens, vaccines aimed at inducing effective antitumor activity are still unsatisfactory. Nevertheless, the effectiveness of the two licensed cancer-preventive vaccines targeting tumor-associated viral agents (anti-HBV [hepatitis B virus], to prevent HBV-associated hepatocellular carcinoma, and anti-HPV [human papillomavirus], to prevent HPV-associated cervical carcinoma), along with the recent FDA approval of sipuleucel-T (for the therapeutic treatment of prostate cancer), represents a significant advancement in the field of cancer vaccines and a boost for new studies in the field. Specific active immunotherapies based on anticancer vaccines represent, indeed, a field in continuous evolution and expansion. Significant improvements may result from the selection of the appropriate tumor-specific target antigen (to overcome the peripheral immune tolerance) and/or the development of immunization strategies effective at inducing a protective immune response. This review aims to describe the vast spectrum of tumor antigens and strategies to develop cancer vaccines.

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.

B7-H1 restricts neuroantigen-specific T cell responses and confines inflammatory CNS damage: implications for the lesion pathogenesis of multiple sclerosis

The co-inhibitory B7-homologue 1 (B7-H1/PD-L1) influences adaptive immune responses and has been proposed to contribute to the mechanisms maintaining peripheral tolerance and limiting inflammatory damage in parenchymal organs. To understand the B7-H1/PD1 pathway in CNS inflammation, we analyzed adaptive immune responses in myelin oligodendrocyte glycoprotein (MOG)(35-55)-induced EAE and assessed the expression of B7-H1 in human CNS tissue. B7-H1(-/-) mice exhibited an accelerated disease onset and significantly exacerbated EAE severity, although absence of B7-H1 had no influence on MOG antibody production. Peripheral MOG-specific IFN-gamma/IL-17 T cell responses occurred earlier and enhanced in B7-H1(-/-) mice, but ceased more rapidly. In the CNS, however, significantly higher numbers of activated neuroantigen-specific T cells persisted during all stages of EAE. Experiments showing a direct inhibitory role of APC-derived B7-H1 on the activation of MOG-specific effector cells support the assumption that parenchymal B7-H1 is pivotal for delineating T cell fate in the target organ. Compatible with this concept, our data investigating human brain tissue specimens show a strong up-regulation of B7-H1 in lesions of multiple sclerosis. Our findings demonstrate the critical importance of B7-H1 as an immune-inhibitory molecule capable of down-regulating T cell responses thus contributing to the confinement of immunopathological damage.

Clonal expansions of pathogenic CD8+ effector cells in the CNS of myelin mutant mice

Tissue damage in the CNS is critically influenced by the adaptive immune system. Primary oligodendrocyte damage (by overexpression of PLP) leads to low-grade inflammation of high pathological impact, which is mediated by CD8+ T cells. To yield further insight into pathogenesis and nature of immune responses in myelin mutated mice, we here apply a detailed immunological characterization of CD8+ T cells in PLP-transgenic and aged wild type mice. We provide evidence that T effector cells accumulate in the CNS of PLP-transgenic and wild-type mice and show a higher level of activation in mutant mice, indicated by surface markers and clonal expansions, as demonstrated by T cell receptor CDR3-spectratype analysis. Vbeta-Jbeta similarities suggest specificity against a common antigen, albeit we could not find specific responses against myelin-antigen-derived peptides. The association of primary oligodendrocyte damage with secondary expansions of pathogenic cells underlines the role of adaptive immune reactions in neurodegenerative and neuroinflammatory diseases.

Enhanced Levels of Costimulation Lead to Reduced Effector/Memory CD8+ T Cell Functionality

The role of different levels of costimulation in conjunction with signal 1 in the activation of memory CD8+ T cells remains elusive. In this study, we demonstrate, in a mouse model with the influenza nucleoprotein epitope NP68, that mouse early memory (effector/memory) CD8+ T cells that were generated with high levels of costimulation have reduced CTL functionality compared with those that were generated with low levels of costimulation. This reduction is associated with increased phosphorylation of the negative regulatory site 292 on Zap70 and a decrease in granzyme B levels. Furthermore, we show that enhanced costimulation reduces proliferation and cytokine production of effector/memory CD8+ T cells in response to intermediate and weak TCR stimulation, in contrast to previously described positive effects of costimulation on naive CD8+ T cells. This effect is associated with the expression of ICAM-1 on APCs. Together, our results indicate that enhanced costimulation can lead to reduced functionality in effector/memory CD8+ T cells. This compromised effector function of effector/memory CD8+ T cells in response to high levels of costimulation can have important implications for designing immunotherapeutic strategies to enhance immune responses.

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.

Use of multiple peptides containing T cell epitopes is a feasible approach for peptide-based immunotherapy in Can f 1 allergy

We have previously shown that the major dog allergen Can f 1 contains seven T cell epitope regions, none of which was preferentially recognized. To identify the immune characteristics of Can f 1 epitopes and to verify their suitability for peptide-based allergen immunotherapy, short-term T cell lines were generated with epitope-containing peptides from peripheral blood mononuclear cells of Can f 1 skinprick test-positive allergic and healthy control subjects. The lines were examined for their proliferative capacity and cytokine production upon stimulation with the allergen peptide, a homologous peptide from human tear lipocalin (TL) and Can f 1 and TL proteins. Can f 1 peptides induced proliferation of T cells and gave rise to T cell lines with comparable efficiencies. In particular, the T cell lines of allergic subjects induced with p33-48 and p107-122 favoured the production of interferon-gamma and interleukin-10, respectively. A greater number of Can f 1-specific T cell lines were generated from allergic than from healthy individuals. Two p107-122-induced Can f 1-specific T cell lines also reacted to a homologous peptide of human TL. Our results suggest that several T cell epitope-containing peptides should be used in combination for specific immunotherapy in Can f 1 allergy.

Relating TCR-peptide-MHC affinity to immunogenicity for the design of tumor vaccines

One approach to enhancing the T cell response to tumors is vaccination with mimotopes, mimics of tumor epitopes. While mimotopes can stimulate proliferation of T cells that recognize tumor-associated antigens (TAAs), this expansion does not always correlate with control of tumor growth. We hypothesized that vaccination with mimotopes of optimal affinity in this interaction will improve antitumor immunity. Using a combinatorial peptide library and a cytotoxic T lymphocyte clone that recognizes a TAA, we identified a panel of mimotopes that, when complexed with MHC, bound the TAA-specific TCR with a range of affinities. As expected, in vitro assays showed that the affinity of the TCR-peptide-MHC (TCR-pMHC) interaction correlated with activity of the T cell clone. However, only vaccination with mimotopes in the intermediate-affinity range elicited functional T cells and provided protection against tumor growth in vivo. Vaccination with mimotopes with the highest-affinity TCR-pMHC interactions elicited TAA-specific T cells to the tumor, but did not control tumor growth at any of the peptide concentrations tested. Further analysis of these T cells showed functional defects in response to the TAA. Thus, stimulation of an antitumor response by mimotopes may be optimal with peptides that increase but do not maximize the affinity of the TCR-pMHC interaction.

A modified tyrosinase-related protein 2 epitope generates high-affinity tumor-specific T cells but does not mediate therapeutic efficacy in an intradermal tumor model

The generation of tumor-specific T cells is hampered by the presentation of poorly immunogenic tumor-specific epitopes by the tumor. Here, we demonstrate that, although CD8+ T cells specific for the self/tumor Ag tyrosinase-related protein 2 (TRP2) are readily detected in tumor-bearing hosts, vaccination of either tumor-bearing or naive mice with an epitope derived from TRP2 fails to generate significant numbers of tetramer-staining TRP2-specific T cells or antitumor immunity. We identified an altered peptide epitope, called deltaV, which elicits T cell responses that are cross-reactive to the wild-type TRP2 epitope. Immunization with deltaV generates T cells with increased affinity for TRP2 compared with immunization with the wild-type TRP2 epitope, although TRP2 immunization often generates a greater number of TRP2-specific T cells based on intracellular IFN-gamma analysis. Despite generating higher affinity responses, deltaV immunization alone fails to provide any greater therapeutic efficacy against tumor growth than TRP2 immunization. This lack of tumor protection is most likely a result of both the deletion of high affinity and functional tolerance induction of lower affinity TRP2-specific T cells. Our data contribute to a growing literature demonstrating the ability of variant peptide epitopes to generate higher affinity T cell responses against tumor-specific Ags. However, consistent with most clinical data, simple generation of higher affinity T cells is insufficient to mediate tumor immunity.

Accumulation of CD8+T Cells in Advanced-Stage Tumors and Delay of Disease Progression following Secondary Immunization against an Immunorecessive Epitope

Self-reactive T cells that survive the process of positive and negative selection during thymocyte development represent potential effector cells against tumors that express these same self-Ags. We have previously shown that CD8+ T lymphocytes (T(CD8)) specific for an immunorecessive epitope, designated epitope V, from the SV40 large T Ag (Tag) escape thymic deletion in line SV11 Tag-transgenic mice. In contrast, these mice are tolerant to the three most dominant Tag epitopes. The majority of the residual epitope V-specific T(CD8) have a low avidity for the target epitope, but a prime/boost regimen can expand higher avidity clones in vivo. Whether higher avidity T(CD8) targeting this epitope are affected by Tag-expressing tumors in the periphery or can be recruited for control of tumor progression remains unknown. In the current study, we determined the fate of naive TCR-transgenic T(CD8) specific for Tag epitope V (TCR-V cells) following transfer into SV11 mice bearing advanced-stage choroid plexus tumors. The results indicate that TCR-V cells are rapidly triggered by the endogenous Tag and acquire effector function, but fail to accumulate within the tumors. Primary immunization enhanced TCR-V cell frequency in the periphery and promoted entry into the brain, but a subsequent booster immunization caused a dramatic accumulation of TCR-V T cells within the tumors and inhibited tumor progression. These results indicate that epitope V provides a target for CD8+ T cells against spontaneous tumors in vivo, and suggests that epitopes with similar properties can be harnessed for tumor immunotherapy.

Immunological and antitumor effects of IL-23 as a cancer vaccine adjuvant

The promising, but modest, clinical results of many human cancer vaccines indicate a need for vaccine adjuvants that can increase both the quantity and the quality of vaccine-induced, tumor-specific T cells. In this study we tested the immunological and antitumor effects of the proinflammatory cytokine, IL-23, in gp100 peptide vaccine therapy of established murine melanoma. Neither systemic nor local IL-23 alone had any impact on tumor growth or tumor-specific T cell numbers. Upon specific vaccination, however, systemic IL-23 greatly increased the relative and absolute numbers of vaccine-induced CD8(+) T cells and enhanced their effector function at the tumor site. Although IL-23 specifically increased IFN-gamma production by tumor-specific T cells, IFN-gamma itself was not a primary mediator of the vaccine adjuvant effect. The IL-23-induced antitumor effect and accompanying reversible weight loss were both partially mediated by TNF-alpha. In contrast, local expression of IL-23 at the tumor site maintained antitumor activity in the absence of weight loss. Under these conditions, it was also clear that enhanced effector function of vaccine-induced CD8(+) T cells, rather than increased T cell number, is a primary mechanism underlying the antitumor effect of IL-23. Collectively, these results suggest that IL-23 is a potent vaccine adjuvant for the induction of therapeutic, tumor-specific CD8(+) T cell responses.

Identification of Cross-Reactive Peptides Using Combinatorial Libraries Circumvents Tolerance against Her-2/neu-Immunodominant Epitope

The majority of the currently defined tumor-associated Ags are often overexpressed products of normal cellular genes. Therefore, tolerance deletes high-affinity T cells directed against the TAAs, leaving only a low-affinity repertoire. We have demonstrated previously that the T cell repertoire against the immunodominant p773-782 A2.1-Her-2/neu-restricted peptide has low affinity in A2xneu mice (Her-2/neu mice crossed with A2.1/Kb mice), compared with A2xFVB mice (A2.1/Kb crossed with FVB-wild-type mice). Immunizations with this peptide have a minor impact in preventing tumor growth in A2xneu mice. Therefore, attempts to expand these responses may be of little clinical value. We hypothesized that if not all possible cross-reactive peptides (CPs) are naturally processed and presented, the possibility exists that T cells against these CPs persist in the repertoire and can be used to induce antitumor responses with higher avidity against native epitopes present on the tumor cells. We have used the positional scanning synthetic peptide combinatorial library methodology to screen the p773-782 T cell clone. The screening data identified potential amino acids that can be substituted in the primary sequences of the p773-782 peptide. The designed CPs induce CTL responses of higher affinity in A2xneu mice compared with the native p773-783 peptide. These CTLs recognize A2+-Her-2/neu(+) tumors with high efficiency. Moreover, multiple immunizations with CPs significantly prolonged the survival of tumor-bearing A2xneu mice. These results have demonstrated that it was possible to circumvent tolerance with the identification of CPs and that these peptides could be of significant clinical value.

CDR3 sequence preference of TCRBV8S2+ T cells within the CNS does not reflect single amino acid dependent avidity expansion

To investigate the influence of antigen and restricting MHC class II molecule on the T cell repertoire, we varied the peptide source by immunizing either with myelin basic protein (MBP) (rat)63-88 or MBP(GUINEA PIG (GP))63-88, which differ in the core region of the peptide binding site at position 79 by a single exchange of threonine (T) to serine (S) and by altering the MHC by immunizing MHC congenic LEW (RT1(1)) and LEW.1W (RT1u) rats. In both MHC haplotypes both peptides lead to oligoclonal dominance of TCRBV8S2 expressing T cells within the central nervous system (CNS) as assessed by complementary determining region 3 (CDR3) spectratyping. In contrast cytofluorometric analysis indicated that only MBP(GP)63-88 in context with the RT1(l) haplotype of the LEW rat lead to strong expansions of TCRBV8S2 expressing T cells within the CNS. Importantly, the small conservative change from S to T at position 79 within MBP63-88 had a strong influence both on the encephalitogenic potential of the peptide and on the number of TCRBV8S2+ T cells infiltrating the CNS. These results demonstrate that even minor changes in only one side chain of an amino acid within an (auto)antigen can dramatically alter TCR avidity for certain MHC class II/peptide complexes.

Recruitment of latent pools of high-avidity CD8(+) T cells to the antitumor immune response

A major barrier to successful antitumor vaccination is tolerance of high-avidity T cells specific to tumor antigens. In keeping with this notion, HER-2/neu (neu)-targeted vaccines, which raise strong CD8⁺ T cell responses to a dominant peptide (RNEU_420-429) in WT FVB/N mice and protect them from a neu-expressing tumor challenge, fail to do so in MMTV-neu (neu-N) transgenic mice. However, treatment of neu-N mice with vaccine and cyclophosphamide-containing chemotherapy resulted in tumor protection in a proportion of mice. This effect was specifically abrogated by the transfer of neu-N–derived CD4⁺CD25⁺ T cells. RNEU_420-429-specific CD8⁺ T cells were identified only in neu-N mice given vaccine and cyclophosphamide chemotherapy which rejected tumor challenge. Tetramer-binding studies demonstrated that cyclophosphamide pretreatment allowed the activation of high-avidity RNEU_420-429-specific CD8⁺ T cells comparable to those generated from vaccinated FVB/N mice. Cyclophosphamide seemed to inhibit regulatory T (T reg) cells by selectively depleting the cycling population of CD4⁺CD25⁺ T cells in neu-N mice. These findings demonstrate that neu-N mice possess latent pools of high-avidity neu-specific CD8⁺ T cells that can be recruited to produce an effective antitumor response if T reg cells are blocked or removed by using approaches such as administration of cyclophosphamide before vaccination.

Breaking tolerance in cancer immunotherapy: time to ACT

The discovery of defined tumor antigens and their application in therapeutic cancer vaccines has not yet resulted in a successful therapy for cancer patients. Recent data suggest that this might be because most current clinical immunotherapeutic strategies rely on a tolerized tumor-reactive T-cell repertoire, resulting in a weak T-cell response that cannot induce tumor regression in the face of a multitude of normal and tumor-induced immunoregulatory mechanisms. New insights from animal models and clinical trials suggest a rationale for combination approaches in which the ineffective endogenous anti-tumor immune response is enhanced through a combination of adoptive cell transfer (ACT), specific vaccination and cytokine help for the reliable induction of a robust anti-tumor immune response and tumor regression.

A Novel Bispecific Tetravalent Antibody Fusion Protein to Target Costimulatory Activity for T-cell Activation to Tumor Cells Overexpressing ErbB2/HER2

Persistent activation of T-lymphocytes requires two signals: one is initiated by T-cell receptor binding to antigenic peptide presented by MHC molecules. In addition, binding of the B7 family members CD80 or CD86 on professional antigen presenting cells to CD28 on T cells is considered to provide an important costimulatory signal. Activation without costimulation induces T-cell unresponsiveness or anergy. To selectively localize costimulatory activity to the surface of tumor cells and enhance activation of tumor-specific T cells, we have developed a novel molecular design for bispecific costimulatory proteins with antibody-like structure. Within a single polypeptide chain we have assembled the IgV-like, CD28-binding domain of human CD86 (CD86(111)) together with hinge, CH2 and CH3 domains of human IgG1, and the scFv(FRP5) antibody fragment which recognizes the ErbB2 (HER2) protooncogene present at high levels on the surface of many human tumor cells. Upon expression in the yeast Pichia pastoris, the resulting CD86(111)-IgG-scFv(FRP5) protein could be purified as a homodimeric, tetravalent molecule from culture supernatants using single-step affinity chromatography. Bispecific binding of the molecule to ErbB2 on the surface of tumor cells and to the B7 counter receptor CTLA-4 was demonstrated by FACS analysis. Potent costimulatory activity of chimeric CD86(111)-IgG-scFv(FRP5) was confirmed by its ability to stimulate the proliferation of primary human lymphocytes pre-activated by low concentrations of anti-CD3 antibody. Our results suggest that such multivalent soluble proteins which combine specific targeting to tumor cells with costimulatory activity may become useful tools to elicit and/or improve T-cell mediated, tumor-specific immune responses.

A Conjugate of a Tumor-Targeting Ligand and a T Cell Costimulatory Antibody To Treat Brain Tumors

T cell immunotherapy is a potential strategy for the treatment of brain tumors because it offers a high degree of specificity, the ability to extravasate into solid tumors, and the potential for eliciting a long-term protective immune response. Various approaches have been developed to overcome T cell immune tolerance to cancer, including the use of cytokines and bispecific antibodies. T cell stimulation with the proinflammatory cytokine IL-12 can elicit antitumor immunity. T cell activation can be increased using bispecific antibodies against activating molecules on the surface of T cells and a tumor antigen. We studied the effects of systemic IL-12 administration in combination with a conjugate of an anti-CD28 antibody and a ligand for the folate receptor. The high affinity folate receptor is expressed on endogenously arising choroid plexus tumors of SV11 mice, which are transgenic for large T antigen under the control of the SV40 promoter. SV11 mice are immunocompetent, yet immunologically tolerant to large T antigen expressed by choroid plexus tumors. MRI analysis showed that the administration of IL-12 and anti-CD28 Fab/folate significantly slowed tumor growth. Proliferating CD8(+) T cells were found in choroid plexus tumors of treated animals. Treatment of animals with IL-12 + anti-CD28 Fab/folate prolonged survival compared to IL-12 alone. Cytokine treatment combined with tumor-targeted costimulation may be a useful adjunct treatment.

Changes to peptide structure, not concentration, contribute to expansion of the lowest avidity cytotoxic T lymphocytes

The efficient in vitro expansion of antigen-specific CD8+ cytotoxic T lymphocytes (CTL) for use in adoptive immunotherapy represents an important clinical goal. Furthermore, the avidity of expanded CTL populations often correlates closely with clinical outcome. In our study, high-avidity CTL lines could be expanded ex vivo from an antigen-primed animal using low peptide concentration, and intermediate peptide concentrations favored the generation of lower avidity CTL. Further increases in peptide concentration during culture inhibited the expansion of all peptide-specific CD8+ cells. In contrast, a single amino acid variant peptide efficiently generated functional CTL populations at high or low peptide concentration, which responded to wild-type epitope with the lowest average avidity seen in this study. We propose that for some peptides, the efficient generation of low-avidity CTL responses will be favored by stimulation with altered peptide rather than high concentrations of wild-type epitope. In addition, some variant peptides designed to have improved binding to major histocompatibility complex class I may reduce rather than enhance the functional avidity for the wild-type peptide of ex vivo-expanded CTL. These observations are relevant to in vitro expansion of CTL for immunotherapy and strategies to elicit regulatory or therapeutic immunity to neo-self-antigen when central tolerance has eliminated high-avidity, cognate T cells.

The impact of self-tolerance on the polyclonal CD8+ T cell repertoire

TCRs possess considerable cross-reactivity toward structurally related Ags. Because the signaling threshold for negative selection is lower than that required for activation of mature T cells, the question arises as to which extent thymic deletion of self-specific T cells affects T cell responsiveness toward foreign peptides. In this study we show, in three different mouse models systems, that the polyclonal CD8(+) T cell repertoire has a marked ability to react against the majority of Ags related to self despite self-tolerance, even in cases where self and foreign differ only marginally at a single TCR-contact residue. Thus, while individual T cells are markedly cross-reactive, the ability to distinguish between closely related Ags is introduced at the polyclonal T cell level.

HLA-B35-restricted immune responses against survivin in cancer patients

Two HLA-A2 restricted epitopes have recently been identified from the broadly expressed tumor antigen survivin, and several vaccination trials in cancer patients based on these survivin-derived peptides have been initiated. Consequently, there is a crucial need for the identification of survivin epitopes restricted to other HLA-molecules in order to extend the proportion of patients that can enter these ongoing clinical trials. In the present study, we characterized 2 survivin-derived epitopes, which are restricted to HLA-B35. Specific T-cell reactivity against these survivin-derived epitopes was found in the peripheral blood from patients with different B-cell malignancies and melanoma. Substitution of the C-terminal anchor residue of the survivin-derived peptides improved the recognition by tumor-infiltrating lymphocytes from melanoma patients. Furthermore, we demonstrated spontaneous cytotoxic T-cell responses to survivin in a primary melanoma lesion. The characterization of these epitopes allows more patients can be included in the ongoing peptide-based survivin vaccination trials against cancer.

Immune tolerance to myelin proteins

Multiple sclerosis (MS) is a demyelinating disorder of the central nervous system. It is believed to be an autoimmune disease arising from a breakdown of immune tolerance in T cells specific for myelin antigens. The heterogeneity in clinical signs and pathology observed in MS patients suggests a complex pathogenesis in which the specificity of the pathogenic T cells and the tolerance mechanisms that are compromised vary among individual patients. In this review, we summarize some of the features of the diverse immune pathology observed in MS and the animal models used to study this disease. We then describe the current state of knowledge regarding the expression of the major myelin protein antigens believed to be targeted in MS and the mechanisms of immune tolerance that operate on T cells that recognize these antigens.

Induction of Tumor-Reactive CTL by C-Side Chain Variants of the CTL Epitope HER-2/neu Protooncogene (369-377) Selected by Molecular Modeling of the Peptide: HLA-A2 Complex

To design side chain variants for modulation of immunogenicity, we modeled the complex of the HLA-A2 molecule with an immunodominant peptide, E75, from the HER-2/neu protooncogene protein recognized by CTL. We identified the side chain orientation of E75. We modified E75 at the central Ser(5) (E75 wild-type), which points upward, by removing successively the HO (variant S5A) and the CH2-OH (variant S5G). Replacement of the OH with an aminopropyl (CH2)3-NH3 (variant S5K) maintained a similar upward orientation of the side chain. S5A and S5G were stronger stimulators while S5K was a weaker stimulator than E75 for induction of lytic function, indicating that the OH group and its extension hindered TCR activation. S5K-CTL survived longer than did CTL induced by E75 and the variants S5A and S5G, which became apoptotic after restimulation with the inducer. S5K-CTL also recognized E75 endogenously presented by the tumor by IFN-gamma production and specific cytolysis. S5K-CTL expanded at stimulation with E75 or with E75 plus agonistic anti-Fas mAb. Compared with S5K-CTL that had been restimulated with the inducer S5K, S5K-CTL stimulated with wild-type E75 expressed higher levels of E75(+) TCR and BCL-2. Activation of human tumor-reactive CTL by weaker agonists than the nominal Ag, followed by expansion with the nominal Ag, is a novel approach to antitumor CTL development. Fine tuning of activation of tumor-reactive CTL by weak agonists, designed by molecular modeling, may circumvent cell death or tolerization induced by tumor Ag, and thus, may provide a novel approach to the rational design of human cancer vaccines.

Can the low-avidity self-specific T cell repertoire be exploited for tumor rejection?

Can self-specific T cells that have escaped intrathymic deletion be exploited to generate antitumor immunity? To determine whether antitumor immunity to a self-Ag for which central tolerance exists can be generated, a mouse model is used in which a fragment of the influenza nucleoprotein (NP) is expressed as a transgene under the control of the H-2K promoter in C57BL/10 mice (B10NP mice). In these mice an oligoclonal population of NP-specific T cells escapes thymic and peripheral deletion and can be activated upon immunization. The main hallmark of these self-specific CD8(+) T cells is diminished avidity for the pertinent MHC/peptide complex. We show in this study that intranasal infection with influenza virus can stimulate low-avidity NP-specific T cells to recognize and destroy NP-expressing microtumors in the lung, but not NP-expressing tumors growing s.c. Only a memory NP-specific CD8(+) T cell response can suppress the growth of an s.c. growing NP-expressing tumor. This delay in tumor growth is associated with a dramatic increase in the number of circulating NP-specific CD8(+) T cells. In addition, cultured memory NP-specific T cells require approximately 100-fold less Ag to induce NP-specific lysis than primary T cells, consistent with the observation that memory T cells have an increased avidity due to affinity maturation. Finally, during an NP-specific memory response, substantial numbers of low-avidity NP-specific T cells can be recovered from s.c. growing tumors. Together, these findings indicate that, when only a low-avidity repertoire is available to generate antitumor immunity, the best strategy may be to enhance memory responses.

Circumventing T-cell tolerance to tumour antigens

During past decades, many attempts have been made to induce or enhance tumour-specific T-cell immunity in cancer patients by vaccination. However, it has become apparent that in a large number of cases the naturally occurring tumour-specific T-cell repertoire is of low affinity and therefore inefficient in mediating tumour rejection. Because of the potential therapeutic value of high affinity TCRs with tumour/lineage specificities, we set out to develop a number of new technologies that can be used to create improved tumour-specific T-cell immunity. These strategies entail: (i) the efficient expansion of low affinity T cells specific for self antigens through the use of variant peptides with improved TCR-binding characteristics; (ii) a retroviral library-based technology to improve the affinity of (self-specific) T-cell receptors in vitro, and (iii) proof of principle for the feasibility of TCR gene transfer as a means to generate T-cell populations with a desired antigen-specificity in vivo. Collectively this toolbox should allow us to create improved T-cell receptors for human tumour antigens, which can subsequently be used to impose tumour-reactivity on to peripheral T cells.