In vivo CTL induction with point-substituted ovalbumin peptides: immunogenicity correlates with peptide-induced MHC class I stability

In Silico-Guided Sequence Modification of Epitopes in Cancer Vaccine Development

Discovery of tumor antigenic epitopes is important for cancer vaccine development. Such epitopes can be designed and modified to become more antigenic and immunogenic in order to overcome immunosuppression towards the native tumor antigen. In silico-guided modification of epitope sequences allows predictive discrimination of those that may be potentially immunogenic. Therefore, only candidates predicted with high antigenicity will be selected, constructed, and tested in the lab. Here, we described the employment of in silico tools using a multiparametric approach to assess both potential T-cell epitopes (MHC class I/II binding) and B-cell epitopes (hydrophilicity, surface accessibility, antigenicity, and linear epitope). A scoring and ranking system based on these parameters was developed to shortlist potential mimotope candidates for further development as peptide cancer vaccines.

On Peptides and Altered Peptide Ligands: From Origin, Mode of Action and Design to Clinical Application (Immunotherapy)

T lymphocytes equipped with clonotypic T cell antigen receptors (TCR) recognize immunogenic peptides only when presented in the context of their own major histocompatibility complex (MHC) molecules. Peptide loading to MHC molecules occurs in intracellular compartments (ER for class I and MIIC for class II molecules) and relies on the interaction of the respective peptides and peptide binding pockets on MHC molecules. Those peptide residues not engaged in MHC binding point towards the TCR screening for possible peptide MHC complex binding partners. Natural or intentional modification of both MHC binding registers and TCR interacting residues of peptides - leading to the formation of altered peptide ligands (APLs) - might alter the way peptides interact with TCRs and hence influence subsequent T cell activation events, and consequently T cell effector functions. This review article summarizes how APLs were detected and first described, current concepts of how APLs modify T cellular signaling, which biological mechanisms might force the generation of APLs in vivo, and how peptides and APLs might be used for the benefit of patients suffering from allergic or autoimmune diseases.

Pan-Specific Prediction of Peptide-MHC Class I Complex Stability, a Correlate of T Cell Immunogenicity

Binding of peptides to MHC class I (MHC-I) molecules is the most selective event in the processing and presentation of Ags to CTL, and insights into the mechanisms that govern peptide-MHC-I binding should facilitate our understanding of CTL biology. Peptide-MHC-I interactions have traditionally been quantified by the strength of the interaction, that is, the binding affinity, yet it has been shown that the stability of the peptide-MHC-I complex is a better correlate of immunogenicity compared with binding affinity. In this study, we have experimentally analyzed peptide-MHC-I complex stability of a large panel of human MHC-I allotypes and generated a body of data sufficient to develop a neural network-based pan-specific predictor of peptide-MHC-I complex stability. Integrating the neural network predictors of peptide-MHC-I complex stability with state-of-the-art predictors of peptide-MHC-I binding is shown to significantly improve the prediction of CTL epitopes. The method is publicly available at http://www.cbs.dtu.dk/services/NetMHCstabpan.

The TLR4 agonist fibronectin extra domain A is cryptic, exposed by elastase-2; use in a fibrin matrix cancer vaccine

Fibronectin (FN) is an extracellular matrix (ECM) protein including numerous fibronectin type III (FNIII) repeats with different functions. The alternatively spliced FN variant containing the extra domain A (FNIII EDA), located between FNIII 11 and FNIII 12, is expressed in sites of injury, chronic inflammation, and solid tumors. Although its function is not well understood, FNIII EDA is known to agonize Toll-like receptor 4 (TLR4). Here, by producing various FN fragments containing FNIII EDA, we found that FNIII EDA's immunological activity depends upon its local intramolecular context within the FN chain. N-terminal extension of the isolated FNIII EDA with its neighboring FNIII repeats (FNIII 9-10-11) enhanced its activity in agonizing TLR4, while C-terminal extension with the native FNIII 12-13-14 heparin-binding domain abrogated it. In addition, we reveal that an elastase 2 cleavage site is present between FNIII EDA and FNIII 12. Activity of the C-terminally extended FNIII EDA could be restored after cleavage of the FNIII 12-13-14 domain by elastase 2. FN being naturally bound to the ECM, we immobilized FNIII EDA-containing FN fragments within a fibrin matrix model along with antigenic peptides. Such matrices were shown to stimulate cytotoxic CD8⁺ T cell responses in two murine cancer models.

Functional avidity-driven activation-induced cell death shapes CTL immunodominance

Immunodominance is a complex phenomenon that relies on a mere numerical concept, while being potentially influenced at every step of the immune response. We investigated the mechanisms leading to the establishment of CTL immunodominance in a retroviral model and found that the previously defined subdominant Env-specific CD8(+) T cells are endowed with an unexpectedly higher functional avidity than is the immunodominant Gag-recognizing counterpart. This high avidity, along with the Env Ag overload, results in a supraoptimal TCR engagement. The overstimulation makes Env-specific T lymphocytes more susceptible to apoptosis, thus hampering their expansion and leading to an unintentional "immune kamikazing." Therefore, Ag-dependent, hyperactivation-induced cell death can be regarded as a novel mechanism in the establishment of the immunodominance that restrains and opposes the expansion of high-avidity T cells in favor of lower-affinity populations.

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.

Prediction of HLA class I-restricted T-cell epitopes of islet autoantigen combined with binding and dissociation assays

Identification of cognate peptides recognized by human leucocyte antigen (HLA)/T cell receptor (TCR) complex provides insight into the pathogenic process of type 1 diabetes (T1D). We hypothesize that HLA-binding assays alone are inadequate metrics for the affinity of peptides. Zinc transporter-8 (ZnT8) has emerged in recent years as a novel, major, human autoantigen. Therefore, we aim to identify the HLA-A2-restricted ZnT8 epitopes using both binding and dissociation assays. HLA class I peptide affinity algorithms were used to predict candidate ZnT8 peptides that bind to HLA-A2. We analyzed 15 reported epitopes of seven β-cell candidate autoantigens and eight predicted candidate ZnT8 peptides using binding and dissociation assays. Using IFN-γ ELISpot assay, we tested peripheral blood mononuclear cells (PBMCs) from recent-onset T1D patients and healthy controls for reactivity to seven reported epitopes and eight candidate ZnT8 peptides directly ex vivo. We found five of seven recently reported epitopes in Chinese T1D patients. Of the eight predicted ZnT8 peptides, ZnT8(153-161) had a strong binding affinity and the lowest dissociation rate to HLA-A*0201. We identified it as a novel HLA-A*0201-restricted T-cell epitope in three of eight T1D patients. We conclude that ZnT8(153-161) is a novel HLA-A*0201-restricted T-cell epitope. We did not observe a significant correlation (P = 0.3, R = - 0.5) between cytotoxic T cell (CTL) response and peptide/HLA*0201 complex stability. However, selection of peptides based on affinity and their dissociation rate may be helpful for the identification of candidate CTL epitopes. Thus, we can minimize the number of experiments for the identification of T-cell epitopes from interesting antigens.

Antigen storage compartments in mature dendritic cells facilitate prolonged cytotoxic T lymphocyte cross-priming capacity

Dendritic cells (DCs) are crucial for priming of naive CD8(+) T lymphocytes to exogenous antigens, so-called "cross-priming." We report that exogenous protein antigen can be conserved for several days in mature DCs, coinciding with strong cytotoxic T lymphocyte cross-priming potency in vivo. After MHC class I peptide elution, protein antigen-derived peptide presentation is efficiently restored, indicating the presence of an intracellular antigen depot. We characterized this depot as a lysosome-like organelle, distinct from MHC class II compartments and recently described early endosomal compartments that allow acute antigen presentation in MHC class I. The storage compartments we report here facilitate continuous supply of MHC class I ligands. This mechanism ensures sustained cross-presentation by DCs, despite the short-lived expression of MHC class I-peptide complexes at the cell surface.

Superior induction of anti-tumor CTL immunity by extended peptide vaccines involves prolonged, DC-focused antigen presentation

Anti-tumor vaccines consisting of extended CTL peptides in combination with CpG-ODN were shown to be superior to those comprising minimal CTL epitopes and CpG-ODN, in that they elicit stronger effector CTL responses with greater tumoricidal potential. We now demonstrate that this improved performance is primarily due to the focusing of CTL epitope presentation onto activated DC in the inflamed lymph nodes draining the vaccination site. In the case of vaccination with minimal peptides, additional APC including T and B cells are also loaded with CTL epitopes. Our data suggest that circulation of these peptide-loaded lymphocytes leads to epitope presentation in non-inflamed lymphoid organs distal from the vaccination site, in the absence of potent costimulatory signals required for efficient CTL priming. The resulting blend of pro-immunogenic and tolerogenic signals, which results in suboptimal activation of the CTL response, is avoided by vaccinating with extended CTL peptides. An additional advantage of extended CTL peptide vaccines is an increased duration of in vivo epitope presentation.

Tapasin shapes immunodominance hierarchies according to the kinetic stability of peptide-MHC class I complexes

Peptide loading of MHC class I molecules involves multiple cofactors including tapasin. We showed previously in vitro that tapasin edits the peptide repertoire by favoring the binding of peptides with slow dissociation rates. Here, using tapasin-deficient mice and a DNA vaccine that primes directly, we confirm that tapasin establishes hierarchical responses in vivo according to peptide-MHC stability. In contrast, this hierarchy is lost when the peptides are cross-presented via an alternative DNA vaccine. By regulating transgene expression, we found that the dominant response modifier was antigen persistence. Our findings reveal strategies for activating T cells against low-affinity peptides, of potential importance for patients with repertoires narrowed by deletional tolerance.

Vaccination of fiber-modified adenovirus-transfected dendritic cells to express HER-2/neu stimulates efficient HER-2/neu-specific humoral and CTL responses and reduces breast carcinogenesis in transgenic mice

HER-2/neu transgene-modified dendritic cell (DC)-based vaccines are potent at eliciting HER-2/neu-specific antitumor immunity. In this study, we constructed a recombinant adenovirus (RGD)AdVneu with fiber gene modified by RGD insertion into the viral knob's H1 loop. We transfected DCs with (RGD)AdVneu, and assessed/compared HER-2/neu-specific humoral and cytotoxic T lymphocyte (CTL) responses and antitumor immunity derived from the original AdVneu-transfected DCs (DCneu1) and (RGD)AdVneu-transfected DCs (DCneu2). We demonstrated that DCneu2 displayed increased HER-2/neu expression by 8.3-fold compared to DCneu1. We also demonstrated that DCneu2 vaccination induced stronger HER-2/neu-specific humoral and CTL immune responses than DCneu1 vaccination. DCneu2 vaccination protected all the mice from HER-2/neu-expressing Tg1-1 tumor cell challenge in wild-type FVB/NJ mice, compared to a partial protection in DCneu1-immunized mice. In addition, DCneu2 vaccination also significantly delayed tumor growth than DCneu1 immunization (P<0.05) in Tg FVBneuN mice. Three immunizations of DCneu2 starting at the mouse age of 2 months also significantly delayed breast cancer development in Tg mice compared to DCneu2 vaccine (P<0.05). Importantly, DCneu2 vaccine reduced breast carcinogenesis by 9% in Tg mice with self HER-2/neu tolerance. Therefore, vaccination of fiber-modified adenovirus-transfected DCs to enhance expression of tumor antigens such as HER-2/neu is likely representative of a new direction in DC-based vaccine of breast cancer.

A deterministic model for the processing and presentation of bacteria-derived antigenic peptides

The amount and the dynamics of antigen supply to the cellular antigen processing and presentation machinery differ largely among diverse microbial antigens and various types of antigen presenting cells. The precise influence, however, of antigen supply on the antigen presentation pattern of cells is not known. Here, we provide a basic deterministic mathematical model of antigen processing and presentation of microbial antigens. The model predicts that different types of antigen presenting cells e.g. cells presenting or cross-presenting exogenous antigens, cells infected with replicating microbes, or cells in which microbial antigen synthesis is blocked after a certain period of time have inherently different antigen presentation patterns which are defined by the kinetics of antigen supply. The reevaluation of existing experimental data [Sijts, A.J., Pamer, E.G., 1997. Enhanced intracellular dissociation of major histocompatibility complex class I-associated peptides: a mechanism for optimizing the spectrum of cell surface-presented cytotoxic T lymphocyte epitopes. J. Exp. Med. 185, 1403-1411] describing the processing and presentation of two antigenic peptides derived from the p60 proteins of the facultatively intracellular bacterium Listeria monocytogenes shows that p60 proteins accumulating intracellularly during bacterial infection of cells play no measurable role as substrate for the cytosolic antigen presentation pathway.

CD8+ CTL priming by exact peptide epitopes in incomplete Freund's adjuvant induces a vanishing CTL response, whereas long peptides induce sustained CTL reactivity

Therapeutic vaccination trials, in which patients with cancer were vaccinated with minimal CTL peptide in oil-in-water formulations, have met with limited success. Many of these studies were based on the promising data of mice studies, showing that vaccination with a short synthetic peptide in IFA results in protective CD8(+) T cell immunity. By use of the highly immunogenic OVA CTL peptide in IFA as a model peptide-based vaccine, we investigated why minimal CTL peptide vaccines in IFA performed so inadequately to allow full optimization of peptide vaccination. Injection of the minimal MHC class I-binding OVA(257-264) peptide in IFA transiently activated CD8(+) effector T cells, which eventually failed to undergo secondary expansion or to kill target cells, as a result of a sustained and systemic presentation of the CTL peptides gradually leaking out of the IFA depot without systemic danger signals. Complementation of this vaccine with the MHC class II-binding Th peptide (OVA(323-339)) restored both secondary expansion and in vivo effector functions of CD8(+) T cells. Simply extending the CTL peptide to a length of 30 aa also preserved these CD8(+) T cell functions, independent of T cell help, because the longer CTL peptide was predominantly presented in the locally inflamed draining lymph node. Importantly, these functional differences were reproduced in two additional model Ag systems. Our data clearly show why priming of CTL with minimal peptide epitopes in IFA is suboptimal, and demonstrate that the use of longer versions of these CTL peptide epitopes ensures the induction of sustained effector CD8(+) T cell reactivity in vivo.

Disulfide bond engineering to trap peptides in the MHC class I binding groove

Immunodominant peptides in CD8 T cell responses to pathogens and tumors are not always tight binders to MHC class I molecules. Furthermore, antigenic peptides that bind weakly to the MHC can be problematic when designing vaccines to elicit CD8 T cells in vivo or for the production of MHC multimers for enumerating pathogen-specific T cells in vitro. Thus, to enhance peptide binding to MHC class I, we have engineered a disulfide bond to trap antigenic peptides into the binding groove of murine MHC class I molecules expressed as single-chain trimers or SCTs. These SCTs with disulfide traps, termed dtSCTs, oxidized properly in the endoplasmic reticulum, transited to the cell surface, and were recognized by T cells. Introducing a disulfide trap created remarkably tenacious MHC/peptide complexes because the peptide moiety of the dtSCT was not displaced by high-affinity competitor peptides, even when relatively weak binding peptides were incorporated into the dtSCT. This technology promises to be useful for DNA vaccination to elicit CD8 T cells, in vivo study of CD8 T cell development, and construction of multivalent MHC/peptide reagents for the enumeration and tracking of T cells-particularly when the antigenic peptide has relatively weak affinity for the MHC.

Cytotoxic T cells generated against heteroclitic peptides kill primary tumor cells independent of the binding affinity of the native tumor antigen peptide

Heteroclitic peptide modifications increase immunogenicity, allowing generation of cytotoxic T lymphocytes (CTLs) against weakly immunogenic tumor-associated antigens (TAAs). A critical issue is whether T cells generated against heteroclitic peptides retain the ability to recognize and kill tumor cells expressing the original weak TAAs, and whether there is a lower threshold of binding affinity of the native peptides, below which such CTLs can still kill primary tumor cells. To examine this we used a model examining the ability of native and heteroclitic immunoglobulin (Ig)-derived peptides to generate CTLs that can kill chronic lymphocytic leukemia (CLL) cells. We demonstrate that CTLs generated against heteroclitic peptides have enhanced killing of CD40-activated B cells pulsed with either heteroclitic (P < .001) or native peptide (P = .04) and primary CLL cells (P = .01). The novel finding reported here is that the rate-limiting factor appears to be the ability to generate CTLs and that once generated, CTL lysis of primary tumor cells is independent of the binding affinity of the native peptide. These findings have implications for vaccination strategies in malignancies and are currently being further examined in vivo in murine models.

Vaccination by genetically modified dendritic cells expressing a truncated neu oncogene prevents development of breast cancer in transgenic mice

Dendritic cells (DCs) are powerful antigen-presenting cells that process antigens and present peptide epitopes in the context of the major histocompatibility complex molecules to generate immune responses. DCs are being studied as potential anticancer vaccines because of their ability to present antigens to naive T cells and to stimulate the expansion of antigen-specific T-cell populations. We investigated an antitumor vaccination using DCs modified by transfer of a nonsignaling neu oncogene, a homologue of human HER-2/neu, in a transgenic model of breast cancer. BALB-neuT mice develop breast cancers as a consequence of mammary gland-specific expression of an activated neu oncogene. We vaccinated BALB-neuT mice with bone marrow-derived DCs transduced with Ad.Neu, a recombinant adenovirus expressing a truncated neu oncoprotein. The vaccine stimulated the production of specific anti-neu antibodies, enhanced interferon-gamma expression by T cells, and prevented or delayed the onset of mammary carcinomas in the mice. Over 65% of vaccinated mice remained tumor free at 28 weeks of age, whereas all of the mice in the control groups developed tumors. When challenged with a neu-expressing breast cancer cell line, vaccinated tumor-free animals had delayed tumor growth compared with controls. The antitumor effect of the vaccine was specific for expression of neu. Studies showed that CD4+ T cells were required in order to generate antitumor immunity. Importantly, the effectiveness of the vaccine was not diminished by preexisting immunity to adenovirus, whereas the protection afforded by vaccination that used direct injection of Ad.Neu was markedly reduced in mice with anti-adenovirus antibody titers. DCs modified by recombinant adenoviruses expressing tumor-associated antigens may provide an effective antitumor vaccination strategy.

Phenotypic and functional maturation of tumor antigen-reactive CD8+ T lymphocytes in patients undergoing multiple course peptide vaccination

Successful immunotherapy with peptide vaccines depends on the in vivo generation of sufficient numbers of anti-tumor T cells with appropriate phenotypic and functional characteristics to mediate tumor destruction. Herein, we report the induction of high frequencies of circulating CD8+ T cells (4.8% to 38.1%) directed against the native gp100:209-217 peptide derived from the gp100 melanoma-melanocyte tumor antigen in five HLA-A*0201 patients at high risk of recurrence of melanoma after multiple courses of immunization with modified gp100:209-217(210M) peptide in IFA. Longitudinal peripheral blood mononuclear cell (PBMC) analysis revealed a phenotypic shift of native peptide-specific CD8+ T cells from an early effector to an effector memory (CD27- CD28- CD62L- CD45RO+) phenotype with repeated immunizations and functional maturation that correlated with gp100:209-217 peptide-specific T-cell precursor frequencies. Postimmunization PBMC exhibited direct ex vivo recognition of melanoma cell lines in ELISPOT analysis, showed lytic capability against peptide-pulsed target cells, and proliferated in response to native peptide stimulation. One year after final immunization, circulating vaccine-specific CD8+ T cells persisted in patients' PBMC with a maintained effector memory phenotype. The results herein demonstrate the efficacy of a multiple course peptide-immunization strategy for the generation of high frequencies of tumor antigen-specific T cells in vivo, and further show that continued peptide immunization results in the escalating generation of functionally mature, tumor-reactive effector memory CD8+ T lymphocytes.

Tapasin enhances MHC class I peptide presentation according to peptide half-life

Understanding how peptides are selected for presentation by MHC class I is crucial to vaccination strategies based on cytotoxic T lymphocyte priming. We have studied this selection of the MHC class I peptide repertoire in terms of the presentation of a series of individual peptides with a wide range of binding to MHC class I. This series was expressed as minigenes, and the presentation of each peptide variant was determined with the same MHC class I peptide-specific antibody. In wild-type cells, the hierarchy of presentation followed peptide half-life. This hierarchy broke down in cells lacking tapasin but not in cells lacking calreticulin or in cells lacking transporter associated with antigen processing-associated ERp57. We demonstrate a key role for tapasin in shaping the MHC class I peptide repertoire, as enhancement of presentation in the presence of tapasin correlated with peptide half-life.

Functional and Structural Characteristics of NY-ESO-1-related HLA A2-restricted Epitopes and the Design of a Novel Immunogenic Analogue

NY-ESO-1, a commonly expressed tumor antigen of the cancer-testis family, is expressed by a wide range of tumors but not found in normal adult somatic tissue, making it an ideal cancer vaccine candidate. Peptides derived from NY-ESO-1 have shown preclinical and clinical trial promise; however, biochemical features of these peptides have complicated their formulation and led to heterogeneous immune responses. We have taken a rational approach to engineer an HLA A2-restricted NY-ESO-1-derived T cell epitope with improved formulation and immunogenicity to the wild type peptide. To accomplish this, we have solved the x-ray crystallographic structures of HLA A2 complexed to NY-ESO (157-165) and two analogues of this peptide in which the C-terminal cysteine residue has been substituted to alanine or serine. Substitution of cysteine by serine maintained peptide conformation yet reduced complex stability, resulting in poor cytotoxic T lymphocyte recognition. Conversely, substitution with alanine maintained complex stability and cytotoxic T lymphocyte recognition. Based on the structures of the three HLA A2 complexes, we incorporated 2-aminoisobutyric acid, an isostereomer of cysteine, into the epitope. This analogue is impervious to oxidative damage, cysteinylation, and dimerization of the peptide epitope upon formulation that is characteristic of the wild type peptide. Therefore, this approach has yielded a potential therapeutic molecule that satiates the hydrophobic F pocket of HLA A2 and exhibited superior immunogenicity relative to the wild type peptide.

Development of HLA-A2402/K(b) transgenic mice

HLA-transgenic mice have been developed to facilitate studies of HLA-restricted cytotoxic responses, e.g., for the identification of immunodominant HLA-restricted CTL epitopes and the optimization of peptide or DNA vaccine constructs for human use. We have developed HLA-A2402/K(b)-transgenic mice expressing chimeric human (alpha1 and alpha2 domains of HLA-A2402) and mouse (alpha3, transmembrane and cytoplasmic domains of H-2K(b)) class I molecules. Immunization of these HLA-A2402/K(b)-transgenic mice with various known HLA-A24-restricted immunodominant cancer CTL epitope peptides derived from gp100, MAGE-1, MAGE-3, Her2/neu, CEA and TERT induced HLA-A24-restricted, peptide-specific CTLs. Using these transgenic mice, we identified a novel HLA-A24-restricted CTL epitope, PSA(152-160), encoded by human prostate-specific antigen. Staining with HLA tetramers showed that the cytotoxic activity induced by immunizing with PSA(152-160) in HLA-A2402/K(b) transgenic mice was HLA-A2402-restricted and CD8-dependent. Therefore, PSA(152-160) might be a candidate peptide for vaccination of HLA-A24(+) patients with prostate cancer. Our results suggest that HLA-A2402/K(b) transgenic mice might be useful in the search for HLA-A24-restricted CTL epitopes functioning as human cancer antigens and for the development of peptide-based cancer immunotherapy.

IL-12 administration leads to a transient depletion of T cells, B cells, and APCs and concomitant abrogation of the HLA-A2.1-restricted CTL response in transgenic mice

The injection of a mixture of bona fide T cell epitopes can lead to the occurrence of immunodominance, meaning that the immune response is focused on the recognition of a single epitope or a small portion of the epitopes injected. We have previously demonstrated that the administration of rIL-12 can counteract immunodominance in BALB/c mice. In this study, we show that the administration of rIL-12 to HLA-A2.1 transgenic mice (A2k(b) mice) abrogates specifically the immune response against HLA-A2.1-restricted HIV epitopes in the spleen. This lack of immune response is most probably due to a transient depletion of B cells, T cells, macrophages, and dendritic cells in this organ. Therefore, our study explains the mechanism of immunosuppression by rIL-12 in vivo.

Monoclonal and polyclonal humoral immune response to EC HER-2/NEU peptides with low similarity to the host's proteome

We are studying peptide immunogenicity as a function of the similarity level to the host's proteome. By using as a model the breast/prostate cancer-associated HER-2/neu antigen, we analyzed the monoclonal and polyclonal humoral immune responses against HER-2/neu peptide motifs not shared with the host proteome. We show here that (i) a mouse monoclonal antibody (MAb) raised against the extracellular domain (EC) of human HER-2/neu oncoprotein recognized a linear peptide motif endowed with low similarity level to the mouse proteome; (ii) likewise, human sera from breast/prostate cancer patients preferentially recognized peptide fragments from the EC of the HER-2/neu oncoprotein having sequences that are not present in the human proteome. Together with previous results obtained in other disease models (cervical cancer-associated HPV16 E7 oncoprotein and Pemphigus vulgaris auto-antigen desmoglein-3), the present data suggest that a low level of sequence similarity to the host's proteome might be an important factor in shaping the pool of B cell epitopes.

Processing of exogenous antigens for presentation by class I MHC molecules involves post-Golgi peptide exchange influenced by peptide-MHC complex stability and acidic pH

Vacuolar alternate class I MHC (MHC-I) Ag processing allows presentation of exogenous Ag by MHC-I molecules with binding of antigenic peptides to post-Golgi MHC-I molecules. We investigated the role of previously bound peptides and their dissociation in generating peptide-receptive MHC-I molecules. TAP1-knockout macrophages were incubated overnight with an initial exogenous peptide, producing a large cohort of peptide-K(b) complexes that could influence subsequent peptide dissociation/exchange. Initial incubation with FAPGNYPAL, KVVRFDKL, or RGYVYQGL enhanced rather than reduced subsequent binding and presentation of a readout peptide (SIINFEKL or FAPGNYPAL) to T cells. Thus, K(b) molecules may be stabilized by an initial (stabilizing) peptide, enhancing their ability to bind readout peptide and implicating peptide dissociation/exchange. In contrast, incubation with SIINFEKL as stabilizing peptide reduced presentation of readout peptide. SIINFEKL-K(b) complexes were more stable than other peptide-K(b) complexes, which may limit their contribution to peptide exchange. Stabilizing peptides (FAPGNYPAL, KVVRFDKL, or RGYVYQGL) enhanced alternate MHC-I processing of HB101.Crl-OVA (Escherichia coli expressing an OVA fusion protein), indicating that alternate MHC-I Ag processing involves peptide dissociation/exchange. Stabilizing peptide enhanced processing of HB101.Crl-OVA more than presentation of exogenous OVA peptide (SIINFEKL), suggesting that peptide dissociation/exchange may be enhanced in the acidic phagosomal processing environment. Furthermore, exposure of cells to acidic pH increased subsequent binding and presentation of readout peptide. Thus, peptide dissociation/exchange contributes to alternate MHC-I Ag processing and may be influenced by both stability of peptide-MHC-I complexes and pH.

Induction of a cytotoxic T-cell response to HIV-1 proteins with short synthetic peptides and human compatible adjuvants

The goal of this study was the induction of a strong CTL response against multiple CTL epitopes present in HIV proteins using short synthetic peptides. Four HLA-A2.1 restricted peptides (RT 476-484, p17 77-85, gp41 814-823, RT 956-964) that showed stable binding to the HLA-A2.1 molecule in an in vitro binding assay were able to elicit a strong specific immune response in HLA-A2.1 transgenic mice when injected with IFA or Montanide. The use of biodegradable microspheres (MS) as adjuvant was also successfully tested for all peptides. When the peptides were injected as a mixture the response was weaker as compared to individual injections of the peptides indicating the occurrence of immunodominance (ID). We are currently investigating whether ID can be overcome by a combined injection of peptide loaded MS with different release patterns. Taken together, it seems feasible to induce a specific CTL response in HLA-A2.1 transgenic mice against several HIV proteins using short synthetic peptides and human compatible adjuvants.

Bacterial CpG-DNA activates dendritic cells in vivo: T helper cell-independent cytotoxic T cell responses to soluble proteins

Receptors for conserved molecular patterns associated with microbial pathogens induce synthesis of co-stimulatory molecules and cytokines in immature dendritic cells (DC), as do antigen-reactive CD4 T helper cells via CD40 signaling. Once activated, antigen-presenting DC may activate CD8 T cell responses in a T helper cell-independent fashion. Using immunostimulatory CpG-oligonucleotides (ODN) mimicking bacterial CpG-DNA, we tested whether CpG-DNA bypasses the need for T helper cells in CTL responses towards proteins by directly activating antigen-presenting DC to transit into professional APC. We describe that immature DC in situ constitutively process soluble proteins and generate CD8 T cell determinants yet CD8 T cell responses remain abortive. Induction of primary antigen-specific CD8 cytotoxic T lymphocyte (CTL)-mediated responses becomes initiated in wild-type as well as T helper cell-deficient mice, provided soluble protein and CpG-ODN are draining into the same lymph node. Specifically we show that CpG-ODN trigger antigen-presenting immature DC within the draining lymph node to acutely up-regulate co-stimulatory molecules and produce IL-12. These results provide new insights for generating in vivo efficient CTL responses to soluble proteins which may influence vaccination strategies.

Evaluation of T-cell responses to peptides and lipopeptides with MHC class I binding motifs derived from the amino acid sequence of the 19-kDa lipoprotein of Mycobacterium tuberculosis

Cytotoxic T-lymphocyte (CTL) epitopes on the 19-kDa lipoprotein from Mycobacterium tuberculosis were identified by the use of lipopeptides and their cytokine profile studied. Selection of candidate CTL epitopes was based on synthetic peptides derived from the amino acid sequence of the 19-kDa lipoprotein showing major histocompatibility complex class I (MHC-I) binding motifs (H-2D(b) and H-2L(d)). Their ability to up-regulate and stabilize MHC-I molecules on the mouse lymphoma cell line RMA-S was studied. Similar studies were performed with peptides, in which the anchor amino acid of the H-2D(b) MHC-I motif was replaced by alanine. Three out of five peptides with H-2D(b) or H-2L(d) binding motifs and their corresponding lipopeptides as well, up-regulated and stabilized the H-2D(b) molecules on RMA-S cells. Replacement of the anchor amino acid residues of the H-2D(b) MHC-I motif by alanine revealed that the anchor amino acid asparagine at position 5, contributed more to binding of peptide to H-2D(b) molecules than leucine at position 11. The closely related lipopeptides LP19c and LP19d, in combination with incomplete Freund's adjuvant (IFA), induced CTL responses in C57BL/6 (H-2(b)) mice. These CTLs could recognize the naturally processed antigen, i.e. the 19-kDa antigen protein produced and processed by the EX-19 cell line. The capacity of the various lipopeptides to induce CTL correlated well with the ability of the (lipo)peptide to up-regulate and to stabilize H-2D(b) molecules. Lipopeptide LP19c primed spleen cells showed a T helper type one profile after in vitro stimulation with P19c and P19d 19 kDa peptides. The approach to characterize presumptive 19-kDa CTL epitopes might lead to selection of promising CTL epitopes, which can be applied in the development of subunit tuberculosis vaccines.

HPV type 16 protein E7 HLA-A2 binding peptides are immunogenic but not processed and presented

Human papillomaviruses (HPV) have been implicated in the etiology of cervical malignancies and a high percentage of cervical carcinoma cells express HPV-16 E6 and E7 oncoproteins. These proteins are attractive targets for cytolytic T lymphocyte (CTL) mediated immunotherapy. We screened peptides derived from the HPV-16 E7 protein for binding to HLA-A2 and tested their potential to induce specific CTL responses in chimeric HLA-A2/H2-Kb transgenic mice. From eight potential binding peptides four displayed binding and were tested for immunogenicity. CTL activity was tested using target cells pulsed with peptide or expressing E7 protein. While there was no CTL induction observed with the peptides 7-15, 66-74 and 82-90, CTL from mice immunized with 86-93 lysed targets presenting the peptide in the context of the HLA-A2/H2-Kb molecule or wild-type HLA-A2. In contrast, 86-93 induced CTL showed no cytolytic activity against cells expressing the protein E7 and vaccination with the E7 protein did not lead to cytotoxicity against targets pulsed with the 86-93 peptide. Therefore the peptide 86-93, which binds to HLA-A2, is able to induce CTL responses in context of HLA-A2, but the peptide appears not to be processed or presented by HPV type 16 infected cells.

Covalent Linkage to β2-Microglobulin Enhances the MHC Stability and Antigenicity of Suboptimal CTL Epitopes

Many CTL epitopes of clinical importance, particularly those derived from tumor Ags, display relatively poor MHC binding affinity and stability. Because in vivo immunogenicity, and thus the efficacy of peptide-based vaccines, is thought to be determined by MHC/peptide complex stability, there is a need to develop a simple strategy for enhancing the binding of suboptimal epitopes. Toward this goal, the ability to enhance suboptimal peptides through covalent linkage to β2-microglobulin (β2m) was explored. Two suboptimal variants of a high-affinity Db-restricted influenza nucleoprotein peptide were covalently linked, via a polypeptide spacer, to the amino terminus of human β2m and the recombinant fusion proteins expressed in Escherichia coli. When compared with their uncoupled counterparts, the β2m-linked epitopes display enhanced MHC stabilization and antigenicity. Thus, tethering epitopes to β2m provides a simple method for augmenting the biological activity of suboptimal peptides and could be useful in the design of peptide-based vaccines or immunotherapeutics.

Enhanced in vitro potency and in vivo immunogenicity of a CTL epitope from hepatitis C virus core protein following amino acid replacement at secondary HLA-A2.1 binding positions

Since the natural immune response to hepatitis C virus (HCV) is often unable to clear the infection, to enhance immunogenicity we studied substituted peptides from an HCV cytotoxic T lymphocyte (CTL) epitope (C7A2) from a conserved region of the HCV core protein (DLMGYIPLV) recognized by CTL lines from HLA-A2.1(+) HCV-infected patients and HLA-A2.1 transgenic mice. HLA-A2.1 binding, human and murine CTL recognition, and in vivo immunogenicity (using mice transgenic for human HLA-A2 in lieu of immunizing humans) were analyzed to define peptides with enhanced immunogenicity. Peptides substituted at position 1 showed enhanced HLA-A2 binding affinity, but paradoxically poorer immunogenicity. A peptide with Ala substituted at position 8 (8A) showed higher HLA-A2 binding affinity and CTL recognition and was a more potent in vivo immunogen in HLA-A2-transgenic mice, inducing higher CTL responses with higher avidity against native C7A2 than induced by C7A2 itself. These results suggest that peptide 8A is a more potent in vitro antigen and in vivo immunogen than C7A2 and may be useful as a vaccine component. They provide proof of principle that the strategy of epitope enhancement can enhance immunogenicity of a CTL epitope recognized by human CTL.

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

Many tumor-associated antigens are nonmutated, poorly immunogenic tissue differentiation antigens. Their weak immunogenicity may be due to "self"-tolerance. To induce autoreactive T cells, we studied immune responses to gp100/pmel 17, an antigen naturally expressed by both normal melanocytes and melanoma cells. Although a recombinant vaccinia virus (rVV) encoding the mouse homologue of gp100 was nonimmunogenic, immunization of normal C57BL/6 mice with the rVV encoding the human gp100 elicited a specific CD8(+) T cell response. These lymphocytes were cross-reactive with mgp100 in vitro and treated established B16 melanoma upon adoptive transfer. To understand the mechanism of the greater immunogenicity of the human version of gp100, we characterized a 9-amino acid (AA) epitope, restricted by H-2Db, that was recognized by the T cells. The ability to induce specific T cells with human but not mouse gp100 resulted from differences within the major histocompatibility complex (MHC) class I-restricted epitope and not from differences elsewhere in the molecule, as was evidenced by experiments in which mice were immunized with rVV containing minigenes encoding these epitopes. Although the human (hgp10025-33) and mouse (mgp10025-33) epitopes were homologous, differences in the three NH2-terminal AAs resulted in a 2-log increase in the ability of the human peptide to stabilize "empty" Db on RMA-S cells and a 3-log increase in its ability to trigger interferon gamma release by T cells. Thus, the fortuitous existence of a peptide homologue with significantly greater avidity for MHC class I resulted in the generation of self-reactive T cells. High-affinity, altered peptide ligands might be useful in the rational design of recombinant and synthetic vaccines that target tissue differentiation antigens expressed by tumors.

Identification of new cytotoxic T-cell epitopes on the 38-kilodalton lipoglycoprotein of Mycobacterium tuberculosis by using lipopeptides

Induction of cytotoxic T lymphocytes (CTLs) by vaccination has been shown to protect against bacterial, viral, and tumoral challenge. The aim of this study was to identify CTL epitopes on the 38-kDa lipoglycoprotein from Mycobacterium tuberculosis. The identification of these CTL epitopes was based on synthesizing peptides designed from the 38-kDa lipoglycoprotein, with known major histocompatibility complex class I (MHC-I) binding motifs (H-2Db), and studying their ability to up-regulate and stabilize MHC-I molecules on the mouse lymphoma cell line RMA-S. To improve the capacity of the identified peptides to induce CTL responses in mice, palmitic acid with a cysteine-serine-serine spacer amino acid sequence was attached to the amino terminus of the peptide. Two of five peptides with H-2Db binding motifs and their corresponding lipopeptides up-regulated and stabilized the H-2Db molecules on RMA-S cells. Both lipopeptides, in combination with incomplete Freund's adjuvant, induced CTL responses in C57BL/6 (H-2(b)) mice. Moreover, the lipopeptide induced stronger CTL responses than the peptide. The capacity of the various lipopeptides to induce CTL displayed a good relationship with the ability of the (lipo)peptide to up-regulate and to stabilize H-2Db molecules. The capacity of the peptides and lipopeptides to up-regulate and stabilize MHC-I expression can therefore be used to predict their potential to function as a CTL epitope. The newly identified CTL epitopes and their lipid derivatives provide us with important information for future M. tuberculosis vaccine design.

Self-veto mechanism of CD8+ cytotoxic effector T cells. Peptide-induced paralysis affects the peptide-MHC-recognizing cytotoxic T lymphocytes and is independent of Fas/Fas ligand interactions

The lytic activity of CD8+ cytotoxic T lymphocyte (CTL) cell lines or clones can be inhibited by addition of the peptide recognized by these cells. The mechanisms underlying this phenomenon are not fully understood. Here we have analyzed peptide-induced CTL paralysis using in vivo generated ovalbumin (OVA)-specific CTL. Lytic activity of OVA-specific CTL was inhibited by addition of the immunodominant OVA-peptide SIINFEKL in a dose-dependent manner. Paralysis was induced rapidly and binding of the peptide to MHC class I molecules was required. Using mixing experiments with CTL populations of different peptide specificities restricted to the same MHC class I molecule we identified a veto-like mechanism: the cytotoxic activity of the peptide-recognizing CTL was inhibited while the lytic activity of the peptide-presenting CTL was unaltered. Only CD8+ CTL but not CD4+ T cells or B+ cells induced paralysis. After removal of the peptide-presenting CTL by magnetic cell sorting, paralysis was maintained and paralyzed CTL showed no signs of apoptosis. Loss of cytotoxicity could be induced in CTL populations from Fas-deficient (lpr+/lpr+) or Fas ligand-deficient (gld+/gld+) mice and mixtures thereof, implying that Fas/Fas ligand interactions are not involved during induction of paralysis. Hence, peptide-induced paralysis of CTL is due to a self-veto mechanism rather than to mutual killing of CTL. These findings may have implications for in vivo immunization with peptides, viral escape and peripheral tolerance mechanisms.

Development of a Murine Mutant Ras CD8+ CTL Peptide Epitope Variant That Possesses Enhanced MHC Class I Binding and Immunogenic Properties

We recently identified a murine mutant Ras p21 CD8+ CTL epitope reflecting residues 4 to 12, containing the mutation of Gly to Val at codon 12, that bound weakly to H-2Kd in vitro and generated a weak primary CTL response in immunized BALB/c mice. Here, we explored the hypothesis that specific modifications to the Ras4–12 peptide sequence can improve MHC binding, leading to enhanced immunogenicity without altering immune specificity. We synthesized Ras4–12 peptides in which Val at residue 12 was replaced with the more dominant H-2Kd C-terminus anchor residue Leu or Ile. In functional H-2Kd binding assays, Ras4–12(L12 or I12) peptide variants competed more effectively than the Ras4–12(V12) peptide. Ras4–12(L12 or I12) peptide variants enhanced both in vitro cytotoxicity and proliferation responses of anti-Ras4–12 CTL compared with the mutant Ras4–12(V12) peptide. Additionally, the Ras4–12(L12) peptide variant induced a quantitatively greater T cell response in vivo compared with that produced by Ras4–12(V12) as determined by IFN-γ production. Mice immunized with Ras4–12(L12) peptide elicited CD8+ CTL activity specific for target cells presenting the Ras4–12(V12) epitope exogenously and endogenously. Moreover, both anti-Ras4–12(V12)-derived and anti-Ras4–12(L12)-derived CTL lines were similar insofar as their TCR usage and amino acid contact residues in the Ras4–12(V12) peptide. These experiments demonstrate that modifications can be introduced in tumor-specific peptide epitopes to enhance both in vitro and in vivo immunogenicity. The design of oncogene-specific peptide epitope variants as immunogens may accelerate the generation of anti-tumor T cell responses for cancer immunotherapy.

Anthrax toxin-mediated delivery in vivo and in vitro of a cytotoxic T-lymphocyte epitope from ovalbumin

We reported earlier that a nontoxic form of anthrax toxin was capable of delivering a cytotoxic T-lymphocyte (CTL) epitope in vivo, such that a specific CTL response was primed against the epitope. The epitope, of bacterial origin, was fused to an N-terminal fragment (LFn) from the lethal-factor component of the toxin, and the fusion protein was injected, together with the protective antigen (PA) component, into BALB/c mice. Here we report that PA plus LFn is capable of delivering a different epitope--OVA(257-264) from ovalbumin. Delivery was accomplished in a different mouse haplotype, H-2Kb and occurred in vitro as well as in vivo. An OVA(257-264)-specific CTL clone, GA-4, recognized EL-4 cells treated in vitro with PA plus as little as 30 fmol of the LFn-OVA(257-264) fusion protein. PA mutants attenuated in toxin self-assembly or translocation were inactive, implying that the role of PA in epitope delivery is the same as that in toxin action. Also, we showed that OVA(257-264)-specific CTL could be induced to proliferate by incubation with splenocytes treated with PA plus LFn-OVA(257-264). These findings imply that PA-LFn may serve as a general delivery vehicle for CTL epitopes in vivo and as a safe, efficient tool for the ex vivo expansion of patient-derived CTL for use in adoptive immunotherapy.

Contribution of peptide backbone atoms to binding of an antigenic peptide to class I major histocompatibility complex molecule

Antigenic peptides are thought to bind to class I major histocompatibility complex (MHC) molecules through three modes of interaction: van der Waals interaction and, to a lesser extent, hydrogen bonding of anchor side chain atoms to residues comprising the binding pockets of the MHC molecule; hydrogen bonding of N- and C-termini to residues at the ends of the binding groove; and hydrogen bonding of peptide backbone atoms to residues lining the binding groove. To dissect the relative contribution of each of these interactions to class I MHC-peptide stability, a retro inverso (RI) analog of VSV-8. an H-2Kb restricted cytotoxic T lymphocyte (CTL) epitope and terminally modified variants of both VSV-8 and RI VSV-8 were synthesized and their ability to target H-2Kb bearing cells for CTL mediated lysis was compared. None of RI VSV-8 analogs elicited lysis of target cells by CTL specific for VSV-8 nor did they appear to compete with the native peptide for binding to H-2Kb. In contrast, terminally modified VSV-8 peptides elicited target lysis. These findings suggest that side chain topochemistry of the peptide is insufficient for stable peptide binding to H-2Kb; rather, hydrogen bonding of the peptide backbone atoms to H-2Kb side chain atoms appears to play a major role in the stability of the complex. Computer modeling confirmed that none of the RI analogs participate in the extensive hydrogen bonding network between the peptide backbone and the MHC molecule seen in the native structure.

Enhanced immunogenicity of HIV-1 vaccine construct by modification of the native peptide sequence

Viral proteins are not naturally selected for high affinity major histocompatibility complex (MHC) binding sequences; indeed, if there is any selection, it is likely to be negative in nature. Thus, one should be able to increase viral peptide binding to MHC in the rational design of synthetic peptide vaccines. The T1 helper peptide from the HIV-1 envelope protein was made more immunogenic for inducing T cell proliferation to the native sequence by replacing a residue that exerts an adverse influence on peptide binding to an MHC class II molecule. Mice immunized with vaccine constructs combining the more potent Th helper (Th) epitope with a cytotoxic T lymphocyte (CTL) determinant developed greatly enhanced CTL responses. Use of class II MHC-congenic mice confirmed that the enhancement of CTL response was due to class II-restricted help. Thus, enhanced T cell help is key for optimal induction of CTL, and, by modification of the native immunogen to increase binding to MHC, it is possible to develop second generation vaccine constructs that enhance both Th cell activation and CTL induction.

Immunobiology of human melanoma antigens MART-1 and gp100 and their use for immuno-gene therapy

Two genes encoding human melanoma antigens MART-1 and gp100 recognized by HLA-A2 restricted melanoma reactive CTL derived from tumor infiltrating lymphocytes (TIL) were isolated by cDNA expression cloning methods. Multiple unmutated self peptides were identified as T cell epitopes in these melanocyte/melanoma specific proteins (2 from MART-1 and 5 from gp100). Most of these melanoma epitopes contain non-dominant anchor amino acids at the primary anchor positions and have intermediate binding affinity to HLA-A2.1. Melanoma reactive CTL were efficiently induced from PBL and TIL of patients by in vitro stimulation with PBMC pulsed with these epitopes. There is a significant correlation between vitiligo development and clinical response to IL2 based immunotherapy, suggesting that autoreactive T cells are involved in melanoma regression in vivo. These results have implications for understanding the nature of tumor antigens recognized by T cells and for the development of new cancer immunotherapies.

Immunodominance of cytotoxic T lymphocyte epitopes co-injected in vivo and modulation by interleukin-12

Immunodominance (ID) of T cell epitopes is a well-documented phenomenon that might have profound significance in the evolution of T cell responses to pathogens, tumors, autoantigens and vaccines. With the intention of developing vaccines composed of several cytotoxic T cell (CTL) epitopes, we injected mice with peptide mixtures containing two to five CTL epitopes and observed clear patterns of ID. In a first case, ID strictly correlated with the competitor activity of the individual peptides for H-2Kd, whereas in a second case, the absence of correlation between ID and competitor activity, binding affinity, half-life of the peptides in serum, induction of proliferation in vitro and the individual immunogenicity of the peptides, suggested to us that ID of co-injected CTL epitopes can be determined both at the peptide level (binding affinity to H-2Kd) and at the T cell level. This hypothesis is supported by our finding that interleukin-12 strongly modulates ID when it is not correlated with MHC binding.

Melanoma-specific CD4+ T cells recognize nonmutated HLA-DR-restricted tyrosinase epitopes

Tyrosinase was the first melanoma-associated antigen shown to be recognized by CD4+ T cells. In this study, we have identified two HLA-DRB1*0401-restricted peptides recognized by these T cells: Ty 56-70 and Ty 448-462. As with many of the MHC class I-restricted melanoma epitopes, both are nonmutated self peptides that have intermediate and weak MHC binding affinities, respectively. Mutated and truncated versions of these peptides were used to define their MHC binding anchor residues. Anchor residues were then modified to derive peptides with increased MHC binding affinities and T cell stimulatory properties. Ty 56-70 and Ty 448-462 enhance the list of immunogenic HLA-A2-, A24-, and B44-restricted tyrosinase peptides already described. Thus, tyrosinase provides a model for anti-melanoma vaccines in which a single molecule can generate multivalent immunization incorporating both CD4+ and CD8+ T cell responses.

Identification of H-2Kb binding and immunogenic peptides from human papilloma virus tumour antigens E6 and E7

Peptides can be used to induce MHC class I restricted cytotoxic T cells (CTL) through in vivo immunization. This approach may enable the development of peptide vaccination schemes for immunization against viral infection in humans. Human papillomavirus (HPV) is one of a few viruses associated with human cancer and the development of an anti-cancer vaccine seems possible. As a model approach, we searched the E6 and E7 proteins of the human papillomavirus type 16 for possible murine MHC class I restricted peptide epitopes. We utilized the mouse H2-Kb peptide binding motif which consists of phenylalanine or tyrosine at position five and leucine at the carboxy-terminus with the modification that leucine could be replaced by other aliphatic but non-aromatic amino acids. Four peptide sequences from E6 and two from E7 were selected. These peptides were tested for their ability to bind and stabilize Kb and for their immunogenicity in vivo. It was shown that one peptide from E6, E6.1 (50-57), bound Kb, but was not able to prime mice in vivo. In contrast, the two selected E7 peptides E7.1 (21-28) and E7.2 (48-55) bound Kb and were immunogenic in vivo. The peptide induced CTL lysed syngeneic EL-4 cells transfected with the open reading frame of E7 but not vector only transfectants. This implies that both peptides were naturally processed and presented by Kb on the surface of target cells. MHC class I peptide binding motifs therefore appear to be an effective and useful tool to predict peptide epitopes of proteins associated with cancer.