Diversity and recognition efficiency of T cell responses to cancer

Peptide-based vaccine for cancer therapies

Different strategies based on peptides are available for cancer treatment, in particular to counter-act the progression of tumor growth and disease relapse. In the last decade, in the context of therapeutic strategies against cancer, peptide-based vaccines have been evaluated in different tumor models. The peptides selected for cancer vaccine development can be classified in two main type: tumor-associated antigens (TAAs) and tumor-specific antigens (TSAs), which are captured, internalized, processed and presented by antigen-presenting cells (APCs) to cell-mediated immunity. Peptides loaded onto MHC class I are recognized by a specific TCR of CD8+ T cells, which are activated to exert their cytotoxic activity against tumor cells presenting the same peptide-MHC-I complex. This process is defined as active immunotherapy as the host's immune system is either de novo activated or restimulated to mount an effective, tumor-specific immune reaction that may ultimately lead to tu-mor regression. However, while the preclinical data have frequently shown encouraging results, therapeutic cancer vaccines clinical trials, including those based on peptides have not provided satisfactory data to date. The limited efficacy of peptide-based cancer vaccines is the consequence of several factors, including the identification of specific target tumor antigens, the limited immunogenicity of peptides and the highly immunosuppressive tumor microenvironment (TME). An effective cancer vaccine can be developed only by addressing all such different aspects. The present review describes the state of the art for each of such factors.

T-cell repertoire diversity: friend or foe for protective antitumor response?

Profiling the T-Cell Receptor (TCR) repertoire is establishing as a potent approach to investigate autologous and treatment-induced antitumor immune response. Technical and computational breakthroughs, including high throughput next-generation sequencing (NGS) approaches and spatial transcriptomics, are providing unprecedented insight into the mechanisms underlying antitumor immunity. A precise spatiotemporal variation of T-cell repertoire, which dynamically mirrors the functional state of the evolving host-cancer interaction, allows the tracking of the T-cell populations at play, and may identify the key cells responsible for tumor eradication, the evaluation of minimal residual disease and the identification of biomarkers of response to immunotherapy. In this review we will discuss the relationship between global metrics characterizing the TCR repertoire such as T-cell clonality and diversity and the resultant functional responses. In particular, we will explore how specific TCR repertoires in cancer patients can be predictive of prognosis or response to therapy and in particular how a given TCR re-arrangement, following immunotherapy, can predict a specific clinical outcome. Finally, we will examine current improvements in terms of T-cell sequencing, discussing advantages and challenges of current methodologies.

Position-Scanning Peptide Libraries as Particle Immunogens for Improving CD8+ T-Cell Responses

Short peptides reflecting major histocompatibility complex (MHC) class I (MHC-I) epitopes frequently lack sufficient immunogenicity to induce robust antigen (Ag)-specific CD8+ T cell responses. In the current work, it is demonstrated that position-scanning peptide libraries themselves can serve as improved immunogens, inducing Ag-specific CD8+ T cells with greater frequency and function than the wild-type epitope. The approach involves displaying the entire position-scanning library onto immunogenic nanoliposomes. Each library contains the MHC-I epitope with a single randomized position. When a recently identified MHC-I epitope in the glycoprotein gp70 envelope protein of murine leukemia virus (MuLV) is assessed, only one of the eight positional libraries tested, randomized at amino acid position 5 (Pos5), shows enhanced induction of Ag-specific CD8+ T cells. A second MHC-I epitope from gp70 is assessed in the same manner and shows, in contrast, multiple positional libraries (Pos1, Pos3, Pos5, and Pos8) as well as the library mixture give rise to enhanced CD8+ T cell responses. The library mixture Pos1-3-5-8 induces a more diverse epitope-specific T-cell repertoire with superior antitumor efficacy compared to an established single mutation mimotope (AH1-A5). These data show that positional peptide libraries can serve as immunogens for improving CD8+ T-cell responses against endogenously expressed MHC-I epitopes.

A minimal model of T cell avidity may identify subtherapeutic vaccine schedules

T cells protect the body from cancer by recognising tumour-associated antigens. Recognising these antigens depends on multiple factors, one of which is T cell avidity, i.e., the total interaction strength between a T cell and a cancer cell. While both high- and low-avidity T cells can kill cancer cells, durable anti-cancer immune responses require the selection of high-avidity T cells. Previous experimentation with anti-cancer vaccines, however, has shown that most vaccines elicit low-avidity T cells. Optimising vaccine schedules may remedy this by preferentially selecting high-avidity T cells. Here, we use mathematical modelling to develop a simple, phenomenological model of avidity selection that may identify vaccine schedules that disproportionately favour low-avidity T cells. We calibrate our model to our prior, more complex model, and then validate it against several experimental data sets. We find that the sensitivity of the model's parameters change with vaccine dosage, which allows us to use a patient's data and clinical history to screen for suitable vaccine strategies.

Mature Dendritic Cells May Promote High-Avidity Tuning of Vaccine T Cell Responses

Therapeutic vaccines can elicit tumor-specific cytotoxic T lymphocytes (CTLs), but durable reductions in tumor burden require vaccines that stimulate high-avidity CTLs. Recent advances in immunotherapy responses have led to renewed interest in vaccine approaches, including dendritic cell vaccine strategies. However, dendritic cell requirements for vaccines that generate potent anti-tumor T-cell responses are unclear. Here we use mathematical modeling to show that, counterintuitively, increasing levels of immature dendritic cells may lead to selective expansion of high-avidity CTLs. This finding is in contrast with traditional dendritic cell vaccine approaches that have sought to harness ex vivo generated mature dendritic cells. We show that the injection of vaccine antigens in the context of increased numbers of immature dendritic cells results in a decreased overall peptide:MHC complex load that favors high-avidity CTL activation and expansion. Overall, our results provide a firm basis for further development of this approach, both alone and in combination with other immunotherapies such as checkpoint blockade.

Systematic evaluation of immune regulation and modulation

Cancer immunotherapies are showing promising clinical results in a variety of malignancies. Monitoring the immune as well as the tumor response following these therapies has led to significant advancements in the field. Moreover, the identification and assessment of both predictive and prognostic biomarkers has become a key component to advancing these therapies. Thus, it is critical to develop systematic approaches to monitor the immune response and to interpret the data obtained from these assays. In order to address these issues and make recommendations to the field, the Society for Immunotherapy of Cancer reconvened the Immune Biomarkers Task Force. As a part of this Task Force, Working Group 3 (WG3) consisting of multidisciplinary experts from industry, academia, and government focused on the systematic assessment of immune regulation and modulation. In this review, the tumor microenvironment, microbiome, bone marrow, and adoptively transferred T cells will be used as examples to discuss the type and timing of sample collection. In addition, potential types of measurements, assays, and analyses will be discussed for each sample. Specifically, these recommendations will focus on the unique collection and assay requirements for the analysis of various samples as well as the high-throughput assays to evaluate potential biomarkers.

Heterogeneity assessment of functional T cell avidity

The potency of cellular immune responses strongly depends on T cell avidity to antigen. Yet, functional avidity measurements are rarely performed in patients, mainly due to the technical challenges of characterizing heterogeneous T cells. The mean functional T cell avidity can be determined by the IFN-γ Elispot assay, with titrated amounts of peptide. Using this assay, we developed a method revealing the heterogeneity of functional avidity, represented by the steepness/hillslope of the peptide titration curve, documented by proof of principle experiments and mathematical modeling. Our data show that not only natural polyclonal CD8 T cell populations from cancer patients, but also monoclonal T cells differ strongly in their heterogeneity of functional avidity. Interestingly, clones and polyclonal cells displayed comparable ranges of heterogeneity. We conclude that besides the mean functional avidity, it is feasible and useful to determine its heterogeneity (hillslope) for characterizing T cell responses in basic research and patient investigation.

Establishment of specific cytotoxic T lymphocyte culture system and its inhibitory effect on ovarian cancer

The present study aimed to establish a novel method for efficiently inducing cytotoxic T lymphocytes (CTLs) in vitro, in order to develop an immune-based therapy for suppressing and killing ovarian cancer cells with a high safety and efficacy. Peripheral blood mononuclear cells (PBMCs) were stimulated with CpG oligodeoxynucleotide (CpGODN) and ginsenoside Rg1, which were united as an immune adjuvant, and human epidermal growth factor receptor 2 (HER2/neu) antigen peptide, in order to establish a specific CTL culture system in vitro. Chromosome karyotype analysis, growth curve construction and flow cytometric analysis of immune phenotypes, including cluster of differentiation (CD)3, CD4 and CD8, were performed to characterize the stimulated PBMCs in vitro. Subsequently, SKOV3 ovarian cancer cells were treated with the specific CTL culture system in vitro, and MTT assays were performed to test the inhibitory and lethal effects of the CTLs on SKOV3 cells. The number of CTLs was significantly increased from day 7 of stimulation with the specific mixture (CpGODN, ginsenoside Rg1 and HER2/neu) (P<0.01), and plateaued on day 19. Following activation, the number of CD3⁺, CD3⁺CD4⁺ and CD3⁺CD8⁺ cells was significantly increased (P<0.01). The lymphocyte karyotype did not change following exposure to antigen. After treatment with the specific CTL system, the number of SKOV3 cells in the experimental group was significantly reduced compared with that in the control group (P<0.01). The results of the present study suggested that two novel immune adjuvants, CpGODN and ginsenoside Rg1, could be combined with the HER2/neu antigen peptide to establish a specific CTL culture system in vitro. This system demonstrated a high antigen specificity, safety and proliferative ability, and exerted significant lethal and inhibitory effects on SKOV3 cells in vitro.

Immunodynamics: a cancer immunotherapy trials network review of immune monitoring in immuno-oncology clinical trials

The efficacy of PD-1/PD-L1 targeted therapies in addition to anti-CTLA-4 solidifies immunotherapy as a modality to add to the anticancer arsenal. Despite raising the bar of clinical efficacy, immunologically targeted agents raise new challenges to conventional drug development paradigms by highlighting the limited relevance of assessing standard pharmacokinetics (PK) and pharmacodynamics (PD). Specifically, systemic and intratumoral immune effects have not consistently correlated with standard relationships between systemic dose, toxicity, and efficacy for cytotoxic therapies. Hence, PK and PD paradigms remain inadequate to guide the selection of doses and schedules, both starting and recommended Phase 2 for immunotherapies. The promise of harnessing the immune response against cancer must also be considered in light of unique and potentially serious toxicities. Refining immune endpoints to better inform clinical trial design represents a high priority challenge. The Cancer Immunotherapy Trials Network investigators review the immunodynamic effects of specific classes of immunotherapeutic agents to focus immune assessment modalities and sites, both systemic and importantly intratumoral, which are critical to the success of the rapidly growing field of immuno-oncology.

CRISPR-Cas9 mediated efficient PD-1 disruption on human primary T cells from cancer patients

Strategies that enhance the function of T cells are critical for immunotherapy. One negative regulator of T-cell activity is ligand PD-L1, which is expressed on dentritic cells (DCs) or some tumor cells, and functions through binding of programmed death-1 (PD-1) receptor on activated T cells. Here we described for the first time a non-viral mediated approach to reprogram primary human T cells by disruption of PD-1. We showed that the gene knockout of PD-1 by electroporation of plasmids encoding sgRNA and Cas9 was technically feasible. The disruption of inhibitory checkpoint gene PD-1 resulted in significant reduction of PD-1 expression but didn't affect the viability of primary human T cells during the prolonged in vitro culture. Cellular immune response of the gene modified T cells was characterized by up-regulated IFN-γ production and enhanced cytotoxicity. These results suggest that we have demonstrated an approach for efficient checkpoint inhibitor disruption in T cells, providing a new strategy for targeting checkpoint inhibitors, which could potentialy be useful to improve the efficacy of T-cell based adoptive therapies.

TCRβ repertoire of CD4+ and CD8+ T cells is distinct in richness, distribution, and CDR3 amino acid composition

The TCR repertoire serves as a reservoir of TCRs for recognizing all potential pathogens. Two major types of T cells, CD4(+) and CD8(+), that use the same genetic elements and process to generate a functional TCR differ in their recognition of peptide bound to MHC class II and I, respectively. However, it is currently unclear to what extent the TCR repertoire of CD4(+) and CD8(+) T cells is different. Here, we report a comparative analysis of the TCRβ repertoires of CD4(+) and CD8(+) T cells by use of a 5' rapid amplification of cDNA ends-PCR-sequencing method. We found that TCRβ richness of CD4(+) T cells ranges from 1.2 to 9.8 × 10(4) and is approximately 5 times greater, on average, than that of CD8(+) T cells in each study subject. Furthermore, there was little overlap in TCRβ sequences between CD4(+) (0.3%) and CD8(+) (1.3%) T cells. Further analysis showed that CD4(+) and CD8(+) T cells exhibited distinct preferences for certain amino acids in the CDR3, and this was confirmed further by a support vector machine classifier, suggesting that there are distinct and discernible differences between TCRβ CDR3 in CD4(+) and CD8(+) T cells. Finally, we identified 5-12% of the unique TCRβs that share an identical CDR3 with different variable genes. Together, our findings reveal the distinct features of the TCRβ repertoire between CD4(+) and CD8(+) T cells and could potentially be used to evaluate the competency of T cell immunity.

Clonal Deletion Prunes but Does Not Eliminate Self-Specific αβ CD8(+) T Lymphocytes

It has long been thought that clonal deletion efficiently removes almost all self-specific T cells from the peripheral repertoire. We found that self-peptide MHC-specific CD8(+) T cells in the blood of healthy humans were present in frequencies similar to those specific for non-self antigens. For the Y chromosome-encoded SMCY antigen, self-specific T cells exhibited only a 3-fold lower average frequency in males versus females and were anergic with respect to peptide activation, although this inhibition could be overcome by a stronger stimulus. We conclude that clonal deletion prunes but does not eliminate self-specific T cells and suggest that to do so would create holes in the repertoire that pathogens could readily exploit. In support of this hypothesis, we detected T cells specific for all 20 amino acid variants at the p5 position of a hepatitis C virus epitope in a random group of blood donors.

Generation of cytotoxic T lymphocytes specific for native or modified peptides derived from the epidermal growth factor receptor pathway substrate 8 antigen

The ideal tumor antigen for the development of a cancer immunotherapy is one that is expressed only in tumor cells. The epidermal growth factor receptor pathway substrate 8 gene (Eps8) might be an effective antigen for cancer immunotherapy as it is overexpressed in a variety of cancer cells but not in normal tissues. In this study, the potential utility of an Eps8-derived immunotherapy was tested in vitro and in vivo. Three computer-based algorithms were used to design eight Eps8 native epitopes with potentially high binding affinity to the HLA-A2.1 molecule, which is found at a high frequency in the Chinese population. Of these eight, three peptides with a moderate affinity to the HLA-A2.1 molecule were modified at anchor residue positions to achieve stronger immunogenicity. These four modified peptides displayed stronger binding affinity to HLA-A2.1 molecules on T2 cells and a lower dissociation rate. In functional assays with human PBMCs in vitro and in HLA-A2.1/K(b) transgenic mice in vivo, CTLs primed by each native and modified peptide secreted IFN-γ and were toxic to cancer cells from a variety of tissue types in an HLA-A2.1-restricted and Eps8-specific manner. p101-109-2L and p276-284-1Y9V were superior to other modified and native epitopes both in vitro and in vivo. These results indicate that employing the native and modified epitopes identified here in Eps8-based immunotherapy for HLA-A2.1 positive cancer patients may result in efficient anticancer immune responses for diverse tumor types.

Antigen-specific inhibition of high-avidity T cell target lysis by low-avidity T cells via trogocytosis

Current vaccine conditions predominantly elicit low-avidity cytotoxic T lymphocytes (CTLs), which are non-tumor-cytolytic but indistinguishable by tetramer staining or enzyme-linked immunospot from high-avidity CTLs. Using CTL clones of high or low avidity for melanoma antigens, we show that low-avidity CTLs can inhibit tumor lysis by high-avidity CTLs in an antigen-specific manner. This phenomenon operates in vivo: high-avidity CTLs control tumor growth in animals but not in combination with low-avidity CTLs specific for the same antigen. The mechanism involves stripping of specific peptide-major histocompatibility complexes (pMHCs) via trogocytosis by low-avidity melanoma-specific CTLs without degranulation, leading to insufficient levels of specific pMHC on target cell surface to trigger lysis by high-avidity CTLs. As such, peptide repertoire on the cell surface is dynamic and continually shaped by interactions with T cells. These results describe immune regulation by low-avidity T cells and have implications for vaccine design.

Paradigm Shift in Dendritic Cell-Based Immunotherapy: From in vitro Generated Monocyte-Derived DCs to Naturally Circulating DC Subsets

Dendritic cell (DC)-based immunotherapy employs the patients’ immune system to fight neoplastic lesions spread over the entire body. This makes it an important therapy option for patients suffering from metastatic melanoma, which is often resistant to chemotherapy. However, conventional cellular vaccination approaches, based on monocyte-derived DCs (moDCs), only achieved modest response rates despite continued optimization of various vaccination parameters. In addition, the generation of moDCs requires extensive ex vivo culturing conceivably hampering the immunogenicity of the vaccine. Recent studies, thus, focused on vaccines that make use of primary DCs. Though rare in the blood, these naturally circulating DCs can be readily isolated and activated thereby circumventing lengthy ex vivo culture periods. The first clinical trials not only showed increased survival rates but also the induction of diversified anti-cancer immune responses. Upcoming treatment paradigms aim to include several primary DC subsets in a single vaccine as pre-clinical studies identified synergistic effects between various antigen-presenting cells.

Vaccination: role in metastatic melanoma

Based on the poor impact on overall survival obtained by systemic chemotherapy in metastatic melanoma and the identification of many melanoma antigens recognized by T cells, in the last decade many efforts have been devoted to the development of active specific immunotherapy as a promising systemic treatment for this neoplastic disease. A number of Phase I-II clinical trials have been performed with different vaccination approaches that included whole tumor cells, antigen peptides, antigen-pulsed dendritic cells, recombinant viruses, plasmids or naked DNA, and heat-shock proteins. Despite some promising immunological and clinical results obtained in these studies, melanoma-specific vaccines have altogether failed to prove their efficacy in the few large Phase III randomized clinical trials performed. Nonetheless, the possibility of activating the human immune system to recognize and destroy tumor cells remains a challenging investigative field, considering that the new knowledge of the intricate cellular and molecular mechanisms that regulate the immune function and tumor-host interactions may allow the development of new clinically relevant melanoma vaccination strategies.

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.

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

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

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.

Modulation of tumor immunity by soluble and membrane-bound molecules at the immunological synapse

To circumvent pathology caused by infectious microbes and tumor growth, the host immune system must constantly clear harmful microorganisms and potentially malignant transformed cells. This task is accomplished in part by T-cells, which can directly kill infected or tumorigenic cells. A crucial event determining the recognition and elimination of detrimental cells is antigen recognition by the T cell receptor (TCR) expressed on the surface of T cells. Upon binding of the TCR to cognate peptide-MHC complexes presented on the surface of antigen presenting cells (APCs), a specialized supramolecular structure known as the immunological synapse (IS) assembles at the T cell-APC interface. Such a structure involves massive redistribution of membrane proteins, including TCR/pMHC complexes, modulatory receptor pairs, and adhesion molecules. Furthermore, assembly of the immunological synapse leads to intracellular events that modulate and define the magnitude and characteristics of the T cell response. Here, we discuss recent literature on the regulation and assembly of IS and the mechanisms evolved by tumors to modulate its function to escape T cell cytotoxicity, as well as novel strategies targeting the IS for therapy.

Modification of a tumor antigen determinant to improve peptide/MHC stability is associated with increased immunogenicity and cross-priming a larger fraction of CD8+ T cells

Altered peptide ligands (APLs) with enhanced binding to MHC class I can increase the CD8(+) T cell response to native Ags, including tumor Ags. In this study, we investigate the influence of peptide-MHC (pMHC) stability on recruitment of tumor Ag-specific CD8(+) T cells through cross-priming. Among the four known H-2(b)-restricted CD8(+) T cell determinants within SV40 large tumor Ag (TAg), the site V determinant ((489)QGINNLDNL(497)) forms relatively low-stability pMHC and is characteristically immunorecessive. Absence of detectable site V-specific CD8(+) T cells following immunization with wild-type TAg is due in part to inefficient cross-priming. We mutated nonanchor residues within the TAg site V determinant that increased pMHC stability but preserved recognition by both TCR-transgenic and polyclonal endogenous T cells. Using a novel approach to quantify the fraction of naive T cells triggered through cross-priming in vivo, we show that immunization with TAg variants expressing higher-stability determinants increased the fraction of site V-specific T cells cross-primed and effectively overcame the immunorecessive phenotype. In addition, using MHC class I tetramer-based enrichment, we demonstrate for the first time, to our knowledge, that endogenous site V-specific T cells are primed following wild-type TAg immunization despite their low initial frequency, but that the magnitude of T cell accumulation is enhanced following immunization with a site V variant TAg. Our results demonstrate that site V APLs cross-prime a higher fraction of available T cells, providing a potential mechanism for high-stability APLs to enhance immunogenicity and accumulation of T cells specific for the native determinant.

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.

Mutated Ras-transfected, EBV-transformed lymphoblastoid cell lines as a model tumor vaccine for boosting T-cell responses against pancreatic cancer: a pilot trial

Genetically modified lymphoblastoid cell lines (LCL) have been shown to be an attractive alternative source of antigen-presenting cells for cancer vaccination in vitro. We tested their application in patients with pancreatic cancer in a phase I clinical trial. As a model tumor antigen, we selected the point-mutated (codon 12) Ki-Ras p21 oncogene (muRas) frequently (∼85%) present in pancreatic adenocarcinoma. Autologous LCLs were established in vitro by spontaneous outgrowth from peripheral blood lymphocytes of seven pancreatic carcinoma patients and were genetically modified with an episomal Epstein-Barr virus (EBV)-based expression vector to express muRas (muRas-LCL). Weekly vaccinations with subcutaneous injection of 5×10(6) muRas-LCL were done. In six of seven patients, therapeutic vaccination elicited a T-cell response with an increase in the frequency of muRas-specific precursor cytotoxic T lymphocytes in the peripheral blood and positive delayed-type hypersensitivity reactions at the injection site. Besides local reactions and flu-like symptoms, there were no signs of toxicity and no acute EBV infection, onset of EBV-associated lymphoma, or other severe complications. A clinical response (stable disease) was observed for a short time period (2-4 months) in four of seven patients (57%), mostly in earlier tumor stages. Our results indicate that LCL presenting genetically modified antigen represent a valuable and easily available tool for in vivo autologous tumor vaccination. LCL can be transfected with any known tumor antigen and therefore should be further clinically investigated.

TCR hypervariable regions expressed by T cells that respond to effective tumor vaccines

A major goal of immunotherapy for cancer is the activation of T cell responses against tumor-associated antigens (TAAs). One important strategy for improving antitumor immunity is vaccination with peptide variants of TAAs. Understanding the mechanisms underlying the expansion of T cells that respond to the native tumor antigen is an important step in developing effective peptide-variant vaccines. Using an immunogenic mouse colon cancer model, we compare the binding properties and the TCR genes expressed by T cells elicited by peptide variants that elicit variable antitumor immunity directly ex vivo. The steady-state affinity of the natural tumor antigen for the T cells responding to effective peptide vaccines was higher relative to ineffective peptides, consistent with their improved function. Ex vivo analysis showed that T cells responding to the effective peptides expressed a CDR3β motif, which was also shared by T cells responding to the natural antigen and not those responding to the less effective peptide vaccines. Importantly, these data demonstrate that peptide vaccines can expand T cells that naturally respond to tumor antigens, resulting in more effective antitumor immunity. Future immunotherapies may require similar stringent analysis of the responding T cells to select optimal peptides as vaccine candidates.

Cytotoxic T lymphocyte responses against melanocytes and melanoma

Background

Vitiligo is a common toxicity associated with immunotherapy for melanoma. Cytotoxic T lymphocytes (CTLs) against melanoma commonly target melanoma-associated antigens (MAAs) which are also expressed by melanocytes. To uncouple vitiligo from melanoma destruction, it is important to understand if CTLs can respond against melanoma and melanocytes at different levels.

Methods

To understand the dichotomous role of MAA-specific CTL, we characterized the functional reactivities of established CTL clones directed to MAAs against melanoma and melanocyte cell lines.

Results

CTL clones generated from melanoma patients were capable of eliciting MHC-restricted, MAA-specific lysis against melanocyte cell lines as well as melanoma cells. Among the tested HLA-A*0201-restricted CTL clones, melanocytes evoked equal to slightly higher degranulation and cytolytic responses as compared to melanoma cells. Moreover, MAA-specific T cells from vaccinated patients responded directly ex vivo to melanoma and melanocytes. Melanoma cells express slightly higher levels of MART-1 and gp100 than melanocytes as measured by quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) and immunohistochemistry.

Conclusions

Our data suggest that CTLs respond to melanoma and melanocytes equally in vitro and directly ex vivo.

Identification of a novel HLA-A2-restricted cytotoxic T lymphocyte epitope from cancer-testis antigen PLAC1 in breast cancer

Identification of cytotoxic T lymphocyte (CTL) epitopes from tumor antigens is essential for the development of peptide vaccines against tumor immunotherapy. Among all the tumor antigens, the caner-testis (CT) antigens are the most widely studied and promising targets. PLAC1 (placenta-specific 1, CT92) was considered as a novel member of caner-testis antigen, which expressed in a wide range of human malignancies, most frequently in breast cancer. In this study, three native peptides and their analogues derived from PLAC1 were predicted by T cell epitope prediction programs including SYFPEITHI, BIMAS and NetCTL 1.2. Binding affinity and stability assays in T2 cells showed that two native peptides, p28 and p31, and their analogues (p28-1Y9 V, p31-1Y2L) had more potent binding activity towards HLA-A*0201 molecule. In ELISPOT assay, the CTLs induced by these four peptides could release IFN-γ. The CTLs induced by these four peptides from the peripheral blood mononuclear cells (PBMCs) of HLA-A*02+ healthy donor could lyse MCF-7 breast cancer cells (HLA-A*0201+, PLAC1+) in vitro. When immunized in HLA-A2.1/Kb transgenic mice, the peptide p28 could induce the most potent peptide-specific CTLs among these peptides. Therefore, our results indicated that the peptide p28 (VLCSIDWFM) could serve as a novel candidate epitope for the development of peptide vaccines against PLAC1-positive breast cancer.

The magnitude of CD4(+) T-cell activation rather than TCR diversity determines the outcome of Leishmania infection in mice

CD4(+) T cells play a critical role in determining the disease outcome in murine cutaneous leishmaniasis, and selective usage of T-cell receptor (TCR) is implied in promoting Leishmania major infection. However, little information is available on TCR usage in Leishmania-specific, IFN-γ-producing CD4(+) T cells. In this study, we investigated the TCR diversity and activation of CD4(+) T cells in a nonhealing model associated with L. amazonensis (La) infection and a self-healing model associated with L. braziliensis (Lb) infection. While marked expansion in the absolute number of several subsets was observed in Lb-infected mice, the percentages of TCR Vβ(+) CD4(+) -cell subsets were comparable in draining LN- and lesion-derived T cells in two infection models. We found that multiple TCR Vβ CD4(+) T cells contributed collectively and comparably to IFN-γ production and that the overall levels of IFN-γ production positively correlated with the control of Lb infection. Moreover, pre-infection with Lb parasites provided cross-protection against secondary La infection, owing to an enhanced magnitude of T-cell activation and IFN-γ production. Collectively, this study suggests that the magnitude of CD4(+) T-cell activation, rather than the TCR diversity, is the major determining factor for the outcome of Leishmania infection.

Wild-type and modified gp100 peptide-pulsed dendritic cell vaccination of advanced melanoma patients can lead to long-term clinical responses independent of the peptide used

Dendritic cell (DC)-based immunotherapy is explored worldwide in cancer patients. Several strategies have been employed to load DC with antigen, including peptide loading. To increase immunogenicity of peptides, major histocompatibility complex (MHC) class I binding affinity and stability of peptide-MHC complexes at the cell surface may be improved by modification of the amino acid sequence. In this study, we compared the capacity of DC loaded with wild-type versus modified gp100 peptides with higher binding affinities to induce an immune and clinical response in advanced melanoma patients. Metastatic HLA-A2.1(+) melanoma patients were vaccinated intravenously (on average 25 × 10(6) DC) and intradermally (on average 11 × 10(6) DC) with mature DC loaded with keyhole limpet hemocyanin (KLH) together with tyrosinase peptide and either wild-type (15 patients) or modified (12 patients) gp100 peptides. All vaccinated patients showed a pronounced proliferative T cell or humoral response against KLH. Gp100-specific T cell responses were monitored in post-treatment delayed type hypersensitivity (DTH) skin biopsies by tetramer and functional analysis. Antigen-specific T cells were found in 2 of 15 patients vaccinated with wild-type gp100-loaded DC, versus 1 of 12 patients vaccinated with modified peptide-loaded DC. These three patients also had the best clinical response, with long-term (>8 years) complete responses in two patients, one in each group. We conclude that vaccination with peptide-loaded DC can result in long-term clinical responses in a minority of metastatic melanoma patients, and that the use of modified as compared to wild-type gp100 peptides for DC loading does not result in a relevant enhanced immune responses.

Profile of a serial killer: cellular and molecular approaches to study individual cytotoxic T-cells following therapeutic vaccination

T-cell vaccination may prevent or treat cancer and infectious diseases, but further progress is required to increase clinical efficacy. Step-by-step improvements of T-cell vaccination in phase I/II clinical studies combined with very detailed analysis of T-cell responses at the single cell level are the strategy of choice for the identification of the most promising vaccine candidates for testing in subsequent large-scale phase III clinical trials. Major aims are to fully identify the most efficient T-cells in anticancer therapy, to characterize their TCRs, and to pinpoint the mechanisms of T-cell recruitment and function in well-defined clinical situations. Here we discuss novel strategies for the assessment of human T-cell responses, revealing in part unprecedented insight into T-cell biology and novel structural principles that govern TCR-pMHC recognition. Together, the described approaches advance our knowledge of T-cell mediated-protection from human diseases.

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.

Immunoinformatics and modeling perspective of T cell epitope-based cancer immunotherapy: a holistic picture

Cancer immunotherapy is fast gaining global attention with its unique position as a potential therapy showing promise in cancer prevention and cure. It utilizes the natural system of immunity as opposed to chemotherapy and radiotherapy that utilize chemical drugs and radiation, respectively. Cancer immunotherapy essentially involves treatment and/or prevention with vaccines in the form of peptide vaccines (T and B cell epitopes), DNA vaccines and vaccination using whole tumor cells, dendritic cells, viral vectors, antibodies and adoptive transfer of T cells to harness the body's own immune system towards the targeting of cancer cells for destruction. Given the time, cost and labor involved in the vaccine discovery and development, researchers have evinced interest in the novel field of immunoinformatics to cut down the escalation of these critical resources. Immunoinformatics is a relatively new buzzword in the scientific circuit that is showing its potential and delivering on its promise in expediting the development of effective cancer immunotherapeutic agents. This review attempts to present a holistic picture of our race against cancer and time using the science and technology of immunoinformatics and molecular modeling in T cell epitope-based cancer immunotherapy. It also attempts to showcase some problem areas as well as novel ones waiting to be explored where development of novel immunoinformatics tools and simulations in the context of cancer immunotherapy would be highly welcome.

CD8+ T cell concentration determines their efficiency in killing cognate antigen-expressing syngeneic mammalian cells in vitro and in mouse tissues

We describe a quantitative model for assessing the cytolytic activity of antigen-specific CD8⁺ T cells in vitro and in vivo in which the concentration of antigen-specific CD8⁺ T cells determines the efficiency with which these cells kill cognate antigen–expressing melanoma cells in packed cell pellets, in three-dimensional collagen-fibrin gels in vitro, and in established melanomas in vivo. In combination with a clonogenic assay for melanoma cells, collagen-fibrin gels are 4,500–5,500-fold more sensitive than the packed cell pellet–type assays generally used to measure CD8⁺ T cell cytolytic activity. An equation previously used to describe neutrophil bactericidal activity in vitro and in vivo also describes antigen-specific CD8⁺ T cell–mediated cytolysis of cognate antigen-expressing melanoma cells in collagen-fibrin gels in vitro and in transplanted tumors in vivo. We have used this equation to calculate the critical concentration of antigen-specific CD8⁺ T cells, which is the concentration of these cells required to hold constant the concentration of a growing population of cognate antigen-expressing melanoma cells. It is ∼3.5 × 10⁵/ml collagen-fibrin gel in vitro and ∼3 × 10⁶/ml or /g melanoma for previously published studies of ex vivo–activated adoptively transferred tumor antigen–specific CD8⁺ T cell killing of cognate antigen–expressing melanoma cells in established tumors in vivo. The antigen-specific CD8⁺ T cell concentration required to kill 100% of 2 × 10⁷/ml cognate antigen-expressing melanoma cells in collagen fibrin gels is ≥10⁷/ml of gel.

Identification and modification of an HLA-A*0201-restricted cytotoxic T lymphocyte epitope from Ran antigen

Ran is considered to be a promising target for tumor-specific immunotherapy because its protein is exclusively expressed in tumor tissues, though its mRNA can be expressed in most normal tissues. In our study, we obtained four candidate wild-type epitopes designated Ran1, Ran2, Ran3, and Ran4, derived from the Ran antigen with the highest predicted affinity with MHC-I, indicated by affinity prediction plots and molecular dynamics simulation. However, in vitro affinity assays of these epitopes showed only a moderate affinity with MHC-I. Thus, we designed altered peptide ligands (APLs) derived from Ran wild-type epitopes with preferred primary and auxiliary HLA-A*0201 molecule anchor residue replacement. Of the eight tested peptides, the 1Y analog had the strongest binding-affinity and lowest-dissociation rate to HLA-A*0201. Additionally, we investigated the CTLs activities induced by Ran wild-type peptides and the APLs in human PBMCs and in HLA-A*0201/K(b) transgenic mice. Ran1 1Y was superior to other APLs and wild-type peptides in eliciting epitope-specific CTL immune responses both in vitro and in vivo. In summary, a wild-type epitope of the tumor-specific antigen Ran, expressed broadly in many tumors, was identified and designated Ran1. An APL of Ran1, Ran1 1Y, was further designed and verified in vitro and in vivo and found to elicit a stronger Ran-specific CTL response, indicating a potential anti-tumor application in the future.

Fine structural variations of alphabetaTCRs selected by vaccination with natural versus altered self-antigen in melanoma patients

Immunotherapy of cancer is often performed with altered "analog" peptide Ags optimized for HLA class I binding, resulting in enhanced immunogenicity, but the induced T cell responses require further evaluation. Recently, we demonstrated fine specificity differences and enhanced recognition of naturally presented Ag by T cells after vaccination with natural Melan-A/MART-1 peptide, as compared with analog peptide. In this study, we compared the TCR primary structures of 1489 HLA-A*0201/Melan-A(26-35)-specific CD8 T cells derived from both cohorts of patients. Although a strong preference for TRAV12-2 segment usage was present in nearly all patients, usage of particular TRAJ gene segments and CDR3alpha composition differed slightly after vaccination with natural vs analog peptide. Moreover, TCR beta-chain repertoires were broader after natural than analog peptide vaccination. In all patients, we observed a marked conservation of the CDR3beta amino acid composition with recurrent sequences centered on a glycyl-leucyl/valyl/alanyl-glycyl motif. In contrast to viral-specific TCR repertoires, such "public" motifs were primarily expressed by nondominant T cell clonotypes, which contrasted with "private" CDR3beta signatures frequently found in T cell clonotypes that dominated repertoires of individual patients. Interestingly, no differences in functional avidity were observed between public and private T cell clonotypes. Collectively, our data indicate that T cell repertoires generated against natural or analog Melan-A peptide exhibited slightly distinct but otherwise overlapping and structurally conserved TCR features, suggesting that the differences in binding affinity/avidity of TCRs toward pMHC observed in the two cohorts of patients are caused by subtle structural TCR variations.

Towards in silico design of epitope-based vaccines

BACKGROUND

Epitope-based vaccines (EVs) make use of immunogenic peptides (epitopes) to trigger an immune response. Due to their manifold advantages, EVs have recently been attracting growing interest. The success of an EV is determined by the choice of epitopes used as a basis. However, the experimental discovery of candidate epitopes is expensive in terms of time and money. Furthermore, for the final choice of epitopes various immunological requirements have to be considered.

METHODS

Numerous in silico approaches exist that can guide the design of EVs. In particular, computational methods for MHC binding prediction have already become standard tools in immunology. Apart from binding prediction and prediction of antigen processing, methods for epitope design and selection have been suggested. We review these in silico approaches for epitope discovery and selection along with their strengths and weaknesses. Finally, we discuss some of the obvious problems in the design of EVs.

CONCLUSION

State-of-the-art in silico approaches to MHC binding prediction yield high accuracies. However, a more thorough understanding of the underlying biological processes and significant amounts of experimental data will be required for the validation and improvement of in silico approaches to the remaining aspects of EV design.

Modified peptides in anti-cancer vaccines: are we eventually improving anti-tumour immunity?

The discovery of tumour antigens recognized by T cells and the features of immune responses directed against them has paved the way to a multitude of clinical studies aimed at boosting anti-tumour T cell immunity as a therapeutic tool for cancer patients. One of the different strategies explored to ameliorate the immunogenicity of tumour antigens in vaccine protocols is represented by the use of optimized peptides or altered peptide ligands, whose amino acid sequence has been modified for improving HLA binding or TCR interaction with respect to native epitopes. However, despite the promising results achieved with preclinical studies, the clinical efficacy of this approach has not yet met the expectations. Although multiple reasons could explain the relative failure of altered peptide ligands as more effective cancer vaccines, the possibility that T cells primed by modified tumour peptides might may be unable to effectively cross-recognize tumour cells has not been sufficiently addressed. Indeed, the introduction of conservative amino acid substitutions may still produce diverse and unpredictable changes in the HLA/peptide interface, with consequent modifications of the TCR repertoire that can interact with the complex. This could lead to the expansion of a broad array of T cells whose TCRs may not necessarily react with equivalent affinity with the original antigenic epitope. Considering the results presently achieved with this vaccine approach, and the emerging availability of alternative strategies for boosting anti-tumour immunity, the use of modified tumour peptides could be reconsidered.

Dendritic cell-based cancer immunotherapies

Because of their unique role in linking the innate and adaptive immune systems, dendritic cells (DCs) have been a logical focus for novel immunotherapies. However, strategies employing active immunization with ex vivo generated and antigen-pulsed DCs have shown limited efficacy in clinical trials. These past approaches did not take into account the complex interactions between cells of the innate immune system and DCs during DC maturation, antigen processing, and presentation to naïve T cells. By better understanding the natural sequence of events occurring in vivo during an effective immune response, we can tailor antitumor immunotherapeutic strategies to augment aspects of this response from the activation of innate immune cells to antigen uptake and DC maturation to priming of naïve T cells and, ultimately, to the establishment of antitumor immunity. Current DC vaccination strategies utilize a number of methods to recapitulate the cascade of events that culminate in a protective antitumor immune response.

Human dendritic cell subsets for vaccination

Protective immunity results from the interplay of antigen (Ag)-nonspecific innate immunity and Ag-specific adaptive immunity. The cells and molecules of the innate system employ non-clonal recognition pathways such as lectins and TLRs. B and T lymphocytes of the adaptive immune system employ clonal receptors recognizing Ag or peptides in a highly specific manner. An essential link between innate and adaptive immunity is provided by dendritic cells (DCs). As a component of the innate immune system, DC organize and transfer information from the outside world to the cells of the adaptive immune system. DC can induce such contrasting states as active immune responsiveness or immunological tolerance. Recent years have brought a wealth of information regarding DC biology and pathophysiology that shows the complexity of this cell system. Thus, presentation of antigen by immature (non-activated) DCs leads to tolerance, whereas mature, antigen-loaded DCs are geared towards the launching of antigen-specific immunity. Furthermore, DCs are composed of multiple subsets with distinct functions at the interface of the innate and adaptive immunity. Our increased understanding of DC pathophysiology will permit their rational manipulation for therapy such as vaccination to improve immunity.

Immunization with analog peptide in combination with CpG and montanide expands tumor antigen-specific CD8+ T cells in melanoma patients

Analog peptides represent a promising tool to further optimize peptide-based vaccines in promoting the expansion of tumor antigen-specific cytotoxic T lymphocytes. Here, we report the results of a pilot trial designed to study the immunogenicity of the analog peptide NY-ESO-1 157-165V in combination with CpG 7909/PF3512676 and Montanide ISA 720 in patients with stage III/IV NY-ESO-1-expressing melanoma. Eight patients were immunized either with Montanide and CpG (arm 1, 3 patients); Montanide and peptide NY-ESO-1 157-165V (arm 2, 2 patients); or with Montanide, CpG, and peptide NY-ESO-1 157-165V (arm 3, 3 patients). Only the 3 patients immunized with Montanide, CpG, and peptide NY-ESO-1 157-165V in arm 3 developed a rapid increase of effector-memory NY-ESO-1-specific CD8+ T cells, detectable ex vivo. The majority of these cells exhibited an intermediate/late-stage differentiated phenotype (CD28-). Our study further demonstrated that our vaccine approach stimulated spontaneous tumor-reactive NY-ESO-1-specific CD8+ T cells in 2 patients with advanced disease, but failed to prime tumor-reactive NY-ESO-1-specific T cells in 1 patient with no spontaneously tumor-induced CD8+ T-cell responses to NY-ESO-1. Collectively, our data support the capability of the analog peptide NY-ESO-1 157-165V in combination with CpG and Montanide to promote the expansion of NY-ESO-1-specific CD8+ T cells in patients with advanced cancer. They also suggest that the presence of tumor-induced NY-ESO-1-specific T cells of well-defined clonotypes is critical for the expansion of tumor-reactive NY-ESO-1-specific CD8+ T cells after peptide-based vaccine strategies.

T-cell responses associated with neonatal alloimmune thrombocytopenia: isolation of HPA-1a-specific, HLA-DRB3*0101-restricted CD4+ T cells

T-cell responses have been implicated in the development of HPA-1a-induced neonatal alloimmune thrombocytopenia (NAIT). However, HPA-1a-specific T cells have neither been isolated nor characterized. Here, we aimed to determine whether HPA-1a-specific T cells could be isolated from HPA-1a-immunized women. In the present study, peripheral blood mononuclear cells (PBMCs) from an HPA-1a-alloimmunized woman were cultured for weeks in the presence of HPA-1a peptide, labeled with CFSE, and assayed for antigen-specific proliferation. Individual proliferating cells were isolated by fluorescence-activated cell sorting and expanded in culture. Antigen specificity and HLA restriction were determined by cytokine secretion (enzyme-linked immunospot [ELISPOT]) and proliferation assays. Several CD3(+)CD4(+) T-cell clones were isolated that proliferated and secreted cytokines in response to HPA-1a peptide. Two of these clones have been established in long-term culture in our laboratory. Both of these recognize synthetic as well as naturally processed HPA-1a antigen, and the recognition is restricted by the MHC molecule HLA-DRB3*0101 that is strongly associated with NAIT. These HPA-1a-specific T-cell clones represent unambiguous evidence for the association of T-cell responses with NAIT, and they will serve as unique tools to elucidate the cellular immune response that may result in NAIT.

Functions of Anti-MAGE T-cells induced in melanoma patients under different vaccination modalities

Tumor regressions have been observed in a small proportion of melanoma patients vaccinated with a MAGE-A3 peptide presented by HLA-A1, administered as peptide, ALVAC canarypox virus containing a MAGE-A3 minigene, or peptide-pulsed dendritic cells (DC). There was a correlation between tumor regression and the detection of anti-MAGE-3.A1 CTL responses. These responses were monoclonal and often of a very low magnitude after vaccination with peptide or ALVAC, and usually polyclonal and of a higher magnitude after DC vaccination. These results suggested that, at least in some patients, surprisingly few anti-MAGE-3.A1 T-cells could initiate a tumor regression process. To understand the role of these T cells, we carried out a functional analysis of anti-MAGE-3.A1 CTL clones derived from vaccinated patients who displayed tumor regression. The functional avidities of these CTL clones, evaluated in lysis assays, were surprisingly low, suggesting that high avidity was not part of the putative capability of these CTL to trigger tumor rejection. Most anti-MAGE-3.A1 CTL clones obtained after DC vaccination, but not after peptide or ALVAC vaccination, produced interleukin 10. Transcript profiling confirmed these results and indicated that approximately 20 genes, including CD40L, prostaglandin D2 synthase, granzyme K, and granzyme H, were highly differentially expressed between the anti-MAGE-3.A1 CTL clones derived from patients vaccinated with either peptide-ALVAC or peptide-pulsed DC. These results indicate that the modality of vaccination with a tumor-specific antigen influences the differentiation pathway of the antivaccine CD8 T-cells, which may have an effect on their capacity to trigger a tumor rejection response.

CD8+ T cell efficacy in vaccination and disease

Much effort has been devoted to the design of vaccines that induce adaptive cellular immunity, in particular CD8+ T cells, which have a central role in the host response to viral infections and cancers. To date, however, the development of effective T cell vaccines remains elusive. This is due, in part, to the lack of clearly defined correlates of protection and the inherent difficulties that hinder full characterization of the determinants of successful T cell immunity in humans. Recent data from the disparate fields of infectious disease and tumor immunology have converged, with an emphasis on the functional attributes of individual antigen-specific T cell clonotypes, to provide a better understanding of CD8+ T cell efficacy. This new knowledge paves the way to the design of more effective T cell vaccines and highlights the importance of comprehensive immunomonitoring.

The immunologic constant of rejection

The complexity underlying a pathologic process does not necessarily require a complex explanation. The biology determining allograft or cancer rejection, autoimmunity or tissue damage during pathogen infections is complex; however, common patterns are emerging that lead to a common final outcome. For instance, tissue destruction occurs with resolution of the pathogenic process (cancer, infection) or tissue damage and organ failure (autoimmunity, allograft rejection). Observations in humans based on transcriptional profiling converge into what we call an 'immunologic constant of rejection' that characterizes such occurrences. This constant includes the coordinate activation of interferon-stimulated genes (ISGs) and immune effector functions (IEFs). Understanding this final effector pathway may suggest novel strategies for the induction or inhibition of tissue-specific destruction with therapeutic intent in cancer and other immune pathologies.

Distinct CD8+ T cell repertoires primed with agonist and native peptides derived from a tumor-associated antigen

Heteroclitic peptides are used to enhance the immunogenicity of tumor-associated Ags to break T cell tolerance to these self-proteins. One such altered peptide ligand (Cap1-6D) has been derived from an epitope in human carcinoembryonic Ag, CEA(605-613) (Cap1). Clinical responses have been seen in colon cancer patients receiving a tumor vaccine comprised of this altered peptide. Whether Cap1-6D serves as a T cell agonist for Cap1-specific T cells or induces different T cells is unknown. We, therefore, examined the T cell repertoires elicited by Cap1-6D and Cap1. Human CTL lines and clones were generated with either Cap1-6D peptide (6D-CTLs) or Cap1 peptide (Cap1-CTLs). The TCR Vbeta usage and functional avidity of the T cells induced in parallel against these target peptides were assessed. The predominant CTL repertoire induced by agonist Cap1-6D is limited to TCR Vbeta1-J2 with homogenous CDR3 lengths. In contrast, the majority of Cap1-CTLs use different Vbeta1 genes and also had diverse CDR3 lengths. 6D-CTLs produce IFN-gamma in response to Cap1-6D peptide with high avidity, but respond with lower avidity to the native Cap1 peptide when compared with the Cap1-CTLs. Nevertheless, 6D-CTLs could still lyse targets bearing the native epitope. Consistent with these functional results, 6D-CTLs possess TCRs that bind Cap-1 peptide/MHC tetramer with higher intensity than Cap1-CTLs but form less stable interactions with peptide/MHC as measured by tetramer decay. These results demonstrate that priming with this CEA-derived altered peptide ligand can induce distinct carcinoembryonic Ag-reactive T cells with different functional capacities.

Unmodified self antigen triggers human CD8 T cells with stronger tumor reactivity than altered antigen

Human cancer vaccines are often prepared with altered "analog" or "heteroclitic" antigens that have been optimized for HLA class I binding, resulting in enhanced immunogenicity. Here, we take advantage of CpG oligodeoxynucleotides as powerful vaccine adjuvants and demonstrate the induction of high T cell frequencies in melanoma patients, despite the use of natural (unmodified) tumor antigenic peptide. Compared with vaccination with analog peptide, natural peptide induced T cell frequencies that were approximately twofold lower. However, T cells showed superior tumor reactivity because of (i) increased functional avidity for natural antigen and (ii) enhancement of T cell activation and effector function. Thus, novel vaccine formulations comprising potent immune stimulators may allow to circumvent the need for modified antigens and can induce highly functional T cells with precise antigen specificity.

Development and dynamics of robust T-cell responses to CML under imatinib treatment

Novel molecular targeted therapies, such as imatinib for chronic myelogenous leukemia (CML), represent the first agents that inhibit cancer cells more than other dividing cells, such as immune cells. We hypothesize that imatinib may create a window in which the immune response is partially restored while apoptotic leukemic cells are present, thus rendering leukemic cells immunogenic as patients enter remission. To detect and quantify antileukemia immune responses in an antigen-unbiased way, we used cryopreserved autologous pretreatment blood samples (representing predominantly leukemic cells) as stimulators to detect antileukemia T-cell responses in CML patients in remission on imatinib. We studied patients over time to address the dynamics of such responses. Our data show that antileukemia T-cell responses develop in the majority of CML patients (9 of 14) in remission and that CD4(+) T cells producing tumor necrosis factor-alpha (median 17.6%) represent the major response over interferon-gamma. This confirms the immune system's ability to respond to leukemia under certain conditions. Such responses may be further amplified as a potential therapy that synergizes with imatinib for improved control of CML.

Programmed death-1 blockade enhances expansion and functional capacity of human melanoma antigen-specific CTLs

Negative co-stimulatory signaling mediated via cell surface programmed death (PD)-1 expression modulates T and B cell activation and is involved in maintaining peripheral tolerance. In this study, we examined the effects of a fully human PD-1-abrogating antibody on the in vitro expansion and function of human vaccine-induced CD8+ T cells (CTLs) specific for the melanoma-associated antigens glycoprotein 100 (gp100) and melanoma antigen recognized by T cells (MART)-1. PD-1 blockade during peptide stimulation augmented the absolute numbers of CD3+, CD4+, CD8+ and gp100/MART-1 MHC:peptide tetramer+ CTLs. This correlated with increased frequencies of IFN-gamma-secreting antigen-specific cells and augmented lysis of gp100+/MART-1+ melanoma targets. PD-1 blockade also increased the fraction of antigen-specific CTLs that recognized melanoma targets by degranulation, suggesting increased recognition efficiency for cognate peptide. The increased frequencies and absolute numbers of antigen-specific CTLs by PD-1 blockade resulted from augmented proliferation, not decreased apoptosis. Kinetic analysis of cytokine secretion demonstrated that PD-1 blockade increased both type-1 and type-2 cytokine accumulation in culture without any apparent skewing of the cytokine repertoire. These findings have implications for developing new cancer immunotherapy strategies.

Low TCR avidity and lack of tumor cell recognition in CD8(+) T cells primed with the CEA-analogue CAP1-6D peptide

The use of "altered peptide ligands" (APL), epitopes designed for exerting increased immunogenicity as compared with native determinants, represents nowadays one of the most utilized strategies for overcoming immune tolerance to self-antigens and boosting anti-tumor T cell-mediated immune responses. However, the actual ability of APL-primed T cells to cross-recognize natural epitopes expressed by tumor cells remains a crucial concern. In the present study, we show that CAP1-6D, a superagonist analogue of a carcinoembriyonic antigen (CEA)-derived HLA-A*0201-restricted epitope widely used in clinical setting, reproducibly promotes the generation of low-affinity CD8(+) T cells lacking the ability to recognized CEA-expressing colorectal carcinoma (CRC) cells. Short-term T cell cultures, obtained by priming peripheral blood mononuclear cells from HLA-A*0201(+) healthy donors or CRC patients with CAP1-6D, were indeed found to heterogeneously cross-react with saturating concentrations of the native peptide CAP1, but to fail constantly lysing or recognizing through IFN- gamma release CEA(+)CRC cells. Characterization of anti-CAP1-6D T cell avidity, gained through peptide titration, CD8-dependency assay, and staining with mutated tetramers (D227K/T228A), revealed that anti-CAP1-6D T cells exerted a differential interaction with the two CEA epitopes, i.e., displaying high affinity/CD8-independency toward the APL and low affinity/CD8-dependency toward the native CAP1 peptide. Our data demonstrate that the efficient detection of self-antigen expressed by tumors could be a feature of high avidity CD8-independent T cells, and underline the need for extensive analysis of tumor cross-recognition prior to any clinical usage of APL as anti-cancer vaccines.