Analogues of CTL epitopes with improved MHC class-I binding capacity elicit anti-melanoma CTL recognizing the wild-type epitope

Enhanced antigen cross-presentation in human colorectal cancer-associated fibroblasts through upregulation of the lysosomal protease cathepsin S

Background

Cross-presentation of exogenous antigens in HLA-class I molecules by professional antigen presenting cells (APCs) is crucial for CD8+ T cell function. Recent murine studies show that several non-professional APCs, including cancer-associated fibroblasts (CAFs) also possess this capacity. Whether human CAFs are able to cross-present exogenous antigen, which molecular pathways are involved in this process and how this ultimately affects tumor-specific CD8+ T cell function is unknown.

Methods

In this study, we investigated the ability of human colorectal cancer (CRC)-derived CAFs to cross-present neoantigen-derived synthetic long peptides (SLPs), corresponding to tumor-derived mutant peptides, and how this affects tumor-specific T-cell function. Processing of the SLP was studied by targeting components of the cross-presentation machinery through CRISPR/Cas9 and siRNA-mediated genetic ablation to identify the key molecules involved in fibroblast-mediated cross-presentation. Multispectral flow cytometry and killing assays were performed to study the effect of fibroblast cross-presentation on T cell function.

Results

Here, we show that human CRC-derived CAFs display an enhanced capacity to cross-present neoantigen-derived SLPs when compared with normal colonic fibroblasts. Cross-presentation of antigens by fibroblasts involved the lysosomal protease cathepsin S. Cathepsin S expression by CAFs was detected in situ in human CRC tissue, was upregulated in ex vivo cultured CRC-derived CAFs and showed increased expression in normal fibroblasts after exposure to CRC-conditioned medium. Cognate interaction between CD8+ T cells and cross-presenting CAFs suppressed T cell function, reflected by decreased cytotoxicity, reduced activation (CD137) and increased exhaustion (TIM3, LAG3 and CD39) marker expression.

Conclusion

These data indicate that CAFs may directly suppress tumor-specific T cell function in an antigen-dependent fashion in human CRC.

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

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

Tumor rejection properties of gp100209-specific T cells correlate with T cell receptor binding affinity towards the wild type rather than anchor-modified antigen

Although there are exceptions and outliers, T cell functional responses generally correlate with the affinity of a TCR for a peptide/MHC complex. In one recently described outlier case, the most promising clinical candidate in a series of TCRs specific for the gp100₂₀₉ melanoma antigen bound with the weakest solution affinity and produced the least amount of cytokine in vitro. Hypotheses for this outlier behavior included unusual cytokine expression patterns arising from an atypical TCR binding geometry. Studying this instance in more detail, we found here that outlier behavior is attributable not to unusual cytokine patterns or TCR binding, but the use of a position 2 anchor-modified peptide variant in in vitro experiments instead of the wild type antigen that is present in vivo. Although the anchor-modified variant has been widely used in basic and clinical immunology as a surrogate for the wild type peptide, prior work has shown that TCRs can clearly distinguish between the two. We show that when this differential recognition is accounted for, the functional properties of gp100₂₀₉-specific TCRs track with their affinity towards the peptide/MHC complex. Beyond demonstrating the correlates with T cell function for a clinically relevant TCR, our results provide important considerations for selection of TCRs for immunotherapy and the use of modified peptides in immunology.

Peptide mimotopes alter T cell function in cancer and autoimmunity

T cells recognize and respond to self antigens in both cancer and autoimmunity. One strategy to influence this response is to incorporate amino acid substitutions into these T cell-specific epitopes. This strategy is being reconsidered now with the goal of increasing time to regression with checkpoint blockade therapies in cancer and antigen-specific immunotherapies in autoimmunity. We discuss how these amino acid substitutions change the interactions with the MHC class I or II molecule and the responding T cell repertoire. Amino acid substitutions in epitopes that are the most effective in therapies bind more strongly to T cell receptor and/or MHC molecules and cross-react with the same repertoire of T cells as the natural antigen.

Adjuvant dendritic cell vaccination induces tumor-specific immune responses in the majority of stage III melanoma patients

Purpose: To determine the effectiveness of adjuvant dendritic cell (DC) vaccination to induce tumor-specific immunological responses in stage III melanoma patients.

Experimental design: Retrospective analysis of stage III melanoma patients, vaccinated with autologous monocyte-derived DC loaded with tumor-associated antigens (TAA) gp100 and tyrosinase after radical lymph node dissection. Skin-test infiltrating lymphocytes (SKILs) obtained from delayed-type hypersensitivity skin-test biopsies were analyzed for the presence of TAA-specific CD8⁺ T cells by tetrameric MHC-peptide complexes and by functional TAA-specific T cell assays, defined by peptide-recognition (T2 cells) and/or tumor-recognition (BLM and/or MEL624) with specific production of Th1 cytokines and no Th2 cytokines.

Results: Ninety-seven patients were analyzed: 21 with stage IIIA, 34 with stage IIIB, and 42 had stage IIIC disease. Tetramer-positive CD8⁺ T cells were present in 68 patients (70%), and 24 of them showed a response against all 3 epitopes tested (gp100:154–162, gp100:280–288, and tyrosinase:369–377) at any point during vaccinations. A functional T cell response was found in 62 patients (64%). Rates of peptide-recognition of gp100:154–162, gp100:280–288, and tyrosinase:369–377 were 40%, 29%, and 45%, respectively. Median recurrence-free survival and distant metastasis-free survival of the whole study population were 23.0 mo and 36.8 mo, respectively.

Conclusions: DC vaccination induces a functional TAA-specific T cell response in the majority of stage III melanoma patients, indicating it is more effective in stage III than in stage IV melanoma patients. Furthermore, performing multiple cycles of vaccinations enhances the chance of a broader immune response.

Analogue peptides for the immunotherapy of human acute myeloid leukemia

The use of peptide vaccines, enhanced by adjuvants, has shown some efficacy in clinical trials. However, responses are often short-lived and rarely induce notable memory responses. The reason is that self-antigens have already been presented to the immune system as the tumor develops, leading to tolerance or some degree of host tumor cell destruction. To try to break tolerance against self-antigens, one of the methods employed has been to modify peptides at the anchor residues to enhance their ability to bind major histocompatibility complex molecules, extending their exposure to the T-cell receptor. These modified or analogue peptides have been investigated as stimulators of the immune system in patients with different cancers with variable but sometimes notable success. In this review we describe the background and recent developments in the use of analogue peptides for the immunotherapy of acute myeloid leukemia describing knowledge useful for the application of analogue peptide treatments for other malignancies.

Superagonism at G protein-coupled receptors and beyond

Ligands targeting GPCRs can be categorized according to their intrinsic efficacy to trigger a specific, receptor-mediated response. A ligand endowed with the same level of efficacy as the endogenous agonist can be classified as a full agonist, whereas a compound that displays greater efficacy, that is, higher receptor signalling output than the endogenous agonist, can be called a superagonist. Subsequent to GPCR activation, an intracellular signalling cascade is set in motion, which may generate substantial amplification of the signal. This may obscure superagonism in pharmacological assays and, therefore, the definition of superagonism necessitates a combination of operational approaches, reduction of spare receptors or estimation of receptor activation close to the receptor level to quantify relative agonist efficacies in a particular system. The first part of this review will compare GPCR superagonism with superagonism in the field of immunology, where this term is well established. In the second part, known GPCR superagonists will be reviewed. Then, the experimental and analytical challenges in the deconvolution of GPCR superagonism will be addressed. Finally, the potential benefit of superagonism is discussed. The molecular mechanisms behind GPCR superagonism are not completely understood. However, crystallography shows that agonist binding alone is not sufficient for a fully active receptor state and that binding of the G protein is at least equally important. Accordingly, the emerging number of reported superagonists implies that ligand-induced receptor conformations more active than the ones stabilized by the endogenous agonist are indeed feasible. Superagonists may have therapeutic potential when receptor function is impaired or to induce negative feedback mechanisms. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Favorable overall survival in stage III melanoma patients after adjuvant dendritic cell vaccination

Melanoma patients with regional metastatic disease are at high risk for recurrence and metastatic disease, despite radical lymph node dissection (RLND). We investigated the immunologic response and clinical outcome to adjuvant dendritic cell (DC) vaccination in melanoma patients with regional metastatic disease who underwent RLND with curative intent. In this retrospective study, 78 melanoma patients with regional lymph node metastasis who underwent RLND received autologous DCs loaded with gp100 and tyrosinase and were analyzed for functional tumor-specific T cell responses in skin-test infiltrating lymphocytes. The study shows that adjuvant DC vaccination in melanoma patients with regional lymph node metastasis is safe and induced functional tumor-specific T cell responses in 71% of the patients. The presence of functional tumor-specific T cells was correlated with a better 2-year overall survival (OS) rate. OS was significantly higher after adjuvant DC vaccination compared to 209 matched controls who underwent RLND without adjuvant DC vaccination, 63.6 mo vs. 31.0 mo (p = 0.018; hazard ratio 0.59; 95%CI 0.42-0.84). Five-year survival rate increased from 38% to 53% (p < 0.01). In summary, in melanoma patients with regional metastatic disease, who are at high risk for recurrence and metastatic disease after RLND, adjuvant DC vaccination is well tolerated. It induced functional tumor-specific immune responses in the majority of patients and these were related to clinical outcome. OS was significantly higher compared to matched controls. A randomized clinical trial is needed to prospectively validate the efficacy of DC vaccination in the adjuvant setting.

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.

Flanking residues are central to DO11.10 T cell hybridoma stimulation by ovalbumin 323-339

T cell activation requires formation of a tri-molecular interaction between a major histocompatibility complex (MHC), peptide, and T cell receptor. In a common model system, the ovalbumin epitope 323–339 binds the murine class II MHC, I-A^d, in at least three distinct registers. The DO11.10 T cell recognizes the least stable of these, as determined by peptide-MHC dissociation rates. Using exogenous peptides and peptide insertions into a carrier protein in combination with IL-2 secretion assays, we show that the alternate registers do not competitively inhibit display of the active register four. In contrast, this weakly binding register is stabilized by the presence of n-terminal flanking residues active in MHC binding. The DO11.10 hybridoma is sensitive to the presence of specific wild-type residues extending to at least the P-3 peptide position. Transfer of the P-4 to P-2 flanking residues to a hen egg lysozyme epitope also presented by I-A^d increases the activity of that epitope substantially. These results illustrate the inherent complexity in delineating the interaction of multiple registers based on traditional thermodynamic measurements and demonstrate the potential of flanking residue modification for increasing the activity of weakly bound epitopes. The latter technique represents an alternative to substitution of anchor residues within a weakly bound register, which we show can significantly decrease the activity of the epitope to a responding T cell.

Skin-test infiltrating lymphocytes early predict clinical outcome of dendritic cell-based vaccination in metastatic melanoma

The identification of responding patients early during treatment would improve the capability to develop effective new immunotherapies more rapidly. Here, we describe a bioassay that may link early T-cell-mediated immune responses to later clinical benefits. This bioassay rests upon the tenet of immunotherapy that tumor-specific effector T cells capable of invading peripheral tissue can recognize tumor antigens and exert cytotoxic functions there. To show its utility, we conducted a retrospective study of a large cohort of metastatic melanoma patients (n = 91) enrolled in dendritic cell (DC)-based vaccination protocols to examine a hypothesized correlation of posttreatment skin-infiltrating lymphocytes (SKIL) with overall survival (OS). Stringent immunologic criteria were defined to identify long-term survivors. The presence of tumor-associated antigen (TAA)-specific CD8(+) T cell populations within SKILs (criterion I) was highly predictive for long-term survival. Further restriction by selecting for the presence of TAA-specific CD8(+) T cells specifically recognizing tumor peptide (criterion II) was also associated with improved OS. Recognition of naturally processed antigen (criterion III) maximized the accuracy of the test, with a median OS of 24.1 versus 9.9 months (P = 0.001). Our results show that detailed characterization of SKILs can permit an accurate selection of metastatic melanoma patients who benefit most from DC-based vaccination. This simple and robust bioassay integrates multiple aspects of cellular functions that mediate effective immune responses, thereby offering an effective tool to rapidly identify patients who are responding to immunotherapy at an early stage of treatment.

Degenerate T-cell recognition of peptides on MHC molecules creates large holes in the T-cell repertoire

The cellular immune system screens peptides presented by host cells on MHC molecules to assess if the cells are infected. In this study we examined whether the presented peptides contain enough information for a proper self/nonself assessment by comparing the presented human (self) and bacterial or viral (nonself) peptides on a large number of MHC molecules. For all MHC molecules tested, only a small fraction of the presented nonself peptides from 174 species of bacteria and 1000 viral proteomes ([Formula: see text]0.2%) is shown to be identical to a presented self peptide. Next, we use available data on T-cell receptor-peptide-MHC interactions to estimate how well T-cells distinguish between similar peptides. The recognition of a peptide-MHC by the T-cell receptor is flexible, and as a result, about one-third of the presented nonself peptides is expected to be indistinguishable (by T-cells) from presented self peptides. This suggests that T-cells are expected to remain tolerant for a large fraction of the presented nonself peptides, which provides an explanation for the "holes in the T-cell repertoire" that are found for a large fraction of foreign epitopes. Additionally, this overlap with self increases the need for efficient self tolerance, as many self-similar nonself peptides could initiate an autoimmune response. Degenerate recognition of peptide-MHC-I complexes by T-cells thus creates large and potentially dangerous overlaps between self and nonself.

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.

Design of a HIV-1-derived HLA-B07.02-restricted polyepitope construct

OBJECTIVE

To design a vaccine construct containing various but conserved HIV-1-derived epitopes and generating broad CD8 T cell responses.

METHODS

HLA-B7 transgenic H-2KD KO transgenic mice were used to identify potential new HLA-B07.02-restricted HIV-1-derived epitopes. Immunological recognition of these epitopes was confirmed by IFN-gamma ELISpot assays with PBMCs from HLA-B*0702 HIV-1-infected individuals. For these peptides as well as others previously identified, the capacity to induce cross-reactive responses against their frequent allelic variants was evaluated in the mouse model. A set of epitopes inducing strong T cell responses against various and conserved regions of HIV-1 was selected. A DNA vaccine was designed to express them as a unique antigen with or without a three amino acid ARY extension flanking each epitope. The spectrum of CD8 T responses generated by polyepitope constructs was tested in HLA-B7 transgenic mice.

RESULTS

Five new epitopes were identified in accessory and regulatory HIV-1 proteins. Twelve HLA-B07.02-restricted epitopes were selected on the basis of their structural conservation and cross-reactive immunogenicity. The ARY N-terminal extension flanking each epitope markedly increases their affinity for TAP and the use of this flanking extension in polyepitope vaccine has a sizable advantage to induce CD8 T cell cytotoxic responses in mice following DNA immunization.

CONCLUSION

The HLA-B7 mouse model allows to rapidly identify various HIV-1-derived epitopes of vaccine interest. Grouped in a polyepitope construct designed to increase their processing, this vaccine may be suitable for inducing multiple and relevant HIV-1-specific CTL responses in humans.

Combined anti-CD40 conditioning and well-timed immunization prolongs CD8+ T cell accumulation and control of established brain tumors

Adoptive cell transfer has been shown to significantly reduce established tumors in both experimental models and cancer patients. Owing to the tolerogenic nature of cancer, approaches that lead to durable maintenance of functional T cells in tumor-bearing hosts are needed to maximize tumor regression. In this study, we investigated strategies to augment CD8+ T-cell (T-CD8)-mediated adoptive immunotherapy of mice bearing advanced-stage autochthonous brain tumors by targeting a weakly immunogenic epitope. We found that immunization enhanced the accumulation of adoptively transferred T-CD8 at the tumor site, but that the timing of immunization was critical for optimal T cell expansion. A more rapid accumulation of T-CD8 was achieved when mice were conditioned with agonist anti-CD40 antibody before adoptive transfer due to increased T cell activation against the endogenous tumor antigen. Both approaches led to an increase in the lifespan of SV11 mice due to decreased tumor progression. However, tumor-specific T-CD8 did not persist long term at the tumor site after administration of either regimen. Importantly, the combination of anti-CD40 conditioning followed by optimally timed immunization synergistically promoted long-term maintenance of T-CD8 in the brain and dramatically enhanced survival. A second round of combination immunotherapy resulted in a further increase in survival, suggesting long-term tumor sensitivity to CD8+ T-cell-based immunotherapy. These results demonstrate that even a weak antigen can be effectively targeted for control of established tumors using a combined adoptive transfer plus immune modulation approach and suggest that similar strategies may translate to clinical practice.

Enhancement of lytic activity of leukemic cells by CD8+ cytotoxic T lymphocytes generated against a WT1 peptide analogue

The Wilms tumor antigen 1 (WT1) antigen is over-expressed in human leukemias, making it an attractive target for immunotherapy. Most WT1-specific Cytotoxic T Lymphocytes (CTLs) described so far displayed low avidity, limiting its function. To improve the immunogenicity of CTL epitopes, we replaced the first-amino-acid of two known immunogenic WT1-peptides (126 and 187) with a tyrosine. This modification enhances 126Y analogue-binding ability, triggers significant number of IFN-gamma-producing T cells (P = 0.0003), induces CTL that cross-react with the wild-type peptide, exerts a significant lytic activity against peptide-loaded-targets (P = 0.0006) and HLA-A0201-matched-leukemic cells (P = 0.0014). These data support peptide modification as a feasible approach for the development of a leukemia-vaccine.

Positive and negative modulation of peptidases by pro-inflammatory cytokines

The capacity of pro-inflammatory cytokines to modulate proteolysis was analyzed by liquid chromatography using human fibroblasts as cell model and enzyme source, and the immunodominant epitope gp100(280-288) (YLEPGPVTA) as substrate. The measurements made after fibroblast pre-incubation with either IL-1, TNF, or IL-6 plus its soluble receptors have been compared with those made with un-stimulated fibroblasts. The results obtained suggest an uneven association of cytokine treatment with substrate degradation, and with a prevailingly positive - but also negative - association with release of smaller peptides and free amino acids. Data obtained by separately measuring these two groups of by-products indicate that, after IL-1 cell pre-treatment, the velocity of formation of both groups of by-products increased, resulting in a net increase of substrate degradation. After TNF and IL-6 pre-treatment, the increase of one group was compensated by a decrease of the other group; specifically, the compensation was only partial for TNF, and overall substrate hydrolysis increased. In the case of IL-6, the increase of free amino acids was almost exactly compensated by a reduction of peptidic by-products, resulting in a negligible increase of substrate hydrolysis. In addition, the existence of reaction time-related modifications in the apparent velocity of substrate degradation and formation of by-products, allows hypothesizing different effects of cytokines on the enzymes degrading the substrate with different time constants. Taken together, these data can be interpreted as indicating different, positive and negative, effects of the three cytokines on the individual enzymes expressed by fibroblasts and capable of degrading peptidic substrates.

Dendritic cell vaccination and immune monitoring

We exploited dendritic cells (DC) to vaccinate melanoma patients. We recently demonstrated a statistical significant correlation between favorable clinical outcome and the presence of vaccine-related tumor antigen-specific T cells in delayed type hypersensitivity (DTH) skin biopsies. However, favorable clinical outcome is only observed in a minority of the treated patients. Therefore, it is obvious that current DC-based protocols need to be improved. For this reason, we study in small proof of principle trials the fate, interactions and effectiveness of the injected DC.

Immunological validation of the EpitOptimizer program for streamlined design of heteroclitic epitopes

One strategy to generate T-cell responses to tumors is to alter subdominant epitopes through substitution of amino acids that are optimal anchors for specific MHC molecules, termed heteroclitic epitopes. This approach is manually error-prone and time-consuming. In here, we describe a computer-based algorithm (EpitOptimizer) for the streamlined design of heteroclitic epitopes. Analysis of two cancer-related proteins showed that EpitOptimizer-generated peptides have enhanced MHC-I binding compared with their wild-type counterparts; and were able to induce stronger CD8+ T-cell responses against their native epitope. These data demonstrate that this approach can serve as the basis of epitope-engineering against cancer and intracellular pathogens.

Secondary immunisation with high-dose heterologous peptide leads to CD8 T cell populations with reduced functional avidity

Expansion of high- or low-avidity CD8 T cells in vitro inversely correlates with the concentration of peptide ligand present during culture. In contrast, the selective enrichment of high- or low-avidity T cell populations in vivo using peptide immunisation is not well documented. In our study, a single immunisation with different doses of wild-type peptide or a variant peptide able to stimulate CTL responses cross-reactive with wild-type peptide failed to shift the average avidity of the responding CD8 T cell population specific to either peptide. However, in contrast to homologous prime-boost immunisation, heterologous prime-boost immunisation incorporating high doses of the second immunogen resulted in peptide-specific CD8 T cell populations polarized toward a low average functional avidity. These data suggest that sequential exposure to structurally related viral peptides could impair rather than promote anti-viral immunity by lowering the avidity of the responding CD8 T cell population. This study has implications for improving vaccine strategies against viruses and tumours and enhances our understanding of heterologous immunity during sequential viral infection.

Induction of antitumor immunity by CTL epitopes genetically linked to membrane-anchored beta2-microglobulin

Level and persistence of antigenic peptides presented by APCs on MHC class I (MHC-I) molecules influence the magnitude and quality of the ensuing CTL response. We recently demonstrated the unique immunological properties conferred on APCs by expressing beta2-microglobulin (beta2m) as an integral membrane protein. In this study, we explored membrane-anchored beta2m as a platform for cancer vaccines using as a model MO5, an OVA-expressing mouse B16 melanoma. We expressed in mouse RMA-S cells two H-2Kb binding peptides from MO5, OVA257-264, and TRP-2181-188, each genetically fused with the N terminus of membranal beta2m via a short linker. Specific Ab staining and T cell hybridoma activation confirmed that OVA257-264 was properly situated in the MHC-I binding groove. In vivo, transfectants expressing both peptides elicited stronger CTLs and conferred better protection against MO5 than peptide-saturated RMA-S cells. Cells expressing OVA257-264/beta2m were significantly superior to OVA257-264-charged cells in their ability to inhibit the growth of pre-established MO5 tumors. Our results highlight the immunotherapeutic potential of membranal beta2m as a universal scaffold for optimizing Ag presentation by MHC-I molecules.

Assessment of CD8 involvement in T cell clone avidity by direct measurement of HLA-A2/Mage3 complex density using a high-affinity TCR like monoclonal antibody

Peptide affinity for MHC molecules determines the number of MHC/peptide complexes stabilized at the cell surface in in vitro tests or in vaccination protocols. We isolated a high affinity monoclonal antibody specific for the HLA-A2/Mage3 complex that enables an equilibrium binding assay to be performed on T2 cell line loaded with a range of Mage3 peptides. Binding of Mage3 to the HLA-A2 molecule can be modeled by a standard receptor-ligand interaction characterized by an affinity constant. This model enables the measurement of the affinity of other immunogenic peptides for HLA-A2 by a competition test and the calculation of the density of complexes stabilized at the T2 cell surface for all peptide concentrations. Quantification of the HLA-A2/Mage3 complexes at target cell surfaces was used to estimate the number of complexes required to reach cytotoxicity ED50 of human T cell clones sorted from an unprimed repertoire. We confirm with this antibody the direct relationship between clone avidity and TCR affinity, and the moderate contribution of the CD8 co-receptor in the reinforcement of TCR-MHC/peptide contact. Nevertheless, CD8 plays a critical role in the amplification of the specific signal to establish an efficient T cell response at low specific complex densities found in physiological situations.

Cancer vaccine development

A new era involving the evaluation of recombinant cancer vaccines has begun with the concurrent emergence of insights and technologies in the fields of molecular biology and immunology. These advances include: The identification and cloning of an array of genes associated with the neoplastic process, such as oncogenes, suppressor genes, genes encoding oncofoetal antigens and tissue-lineage determinants. The development of a variety of viral and bacterial vectors to deliver and present gene products. The identification of numerous T-cell costimulatory molecules and an understanding of their mode of action. The cloning and analysis of the modes of action of an array of cytokines and other immunomodulatory molecules. More sophisticated knowledge of the mode(s) of antigen presentation and T-cell activation. One current challenge in cancer therapy is the delineation of strategies toward the rational design and implementation of recombinant vaccines that will be of therapeutic benefit to cancer patients and/or members of groups at high risk for specific neoplasias. Numerous concepts are emerging in this regard. The study of immunologic intervention using laboratory animal models demonstrates that no one approach will prevail for all cancer types or, perhaps, for the various stages of the neoplastic process of a given tumour type. The immunological role(s) of CD8+, CD4+, natural killer and other cell types, as well as the roles of antibodies, must all be taken into consideration. This article reviews some of the strategies currently undergoing evaluation toward the development of recombinant vaccines for several carcinoma types.

Structural and kinetic basis for heightened immunogenicity of T cell vaccines

Analogue peptides with enhanced binding affinity to major histocompatibility class (MHC) I molecules are currently being used in cancer patients to elicit stronger T cell responses. However, it remains unclear as to how alterations of anchor residues may affect T cell receptor (TCR) recognition. We correlate functional, thermodynamic, and structural parameters of TCR–peptide–MHC binding and demonstrate the effect of anchor residue modifications of the human histocompatibility leukocyte antigens (HLA)–A2 tumor epitope NY–ESO-1_157–165–SLLMWITQC on TCR recognition. The crystal structure of the wild-type peptide complexed with a specific TCR shows that TCR binding centers on two prominent, sequential, peptide sidechains, methionine–tryptophan. Cysteine-to-valine substitution at peptide position 9, while optimizing peptide binding to the MHC, repositions the peptide main chain and generates subtly enhanced interactions between the analogue peptide and the TCR. Binding analyses confirm tighter binding of the analogue peptide to HLA–A2 and improved soluble TCR binding. Recognition of analogue peptide stimulates faster polarization of lytic granules to the immunological synapse, reduces dependence on CD8 binding, and induces greater numbers of cross-reactive cytotoxic T lymphocyte to SLLMWITQC. These results provide important insights into heightened immunogenicity of analogue peptides and highlight the importance of incorporating structural data into the process of rational optimization of superagonist peptides for clinical trials.

Hydrolysis of the tumor-associated antigen epitope gp100(280-288) by membrane-associated and soluble enzymes expressed by immature and mature dendritic cells

The hydrolysis of the tumor-associated HLA-A2.1-restricted gp100(280-288) epitope by in vitro generated immature and mature dendritic cells (iDCs and mDCs) and by soluble supernatants prepared from these same cells, as well as the effect of the hydrolysis on in vitro immunorecognition, was studied by chromatographic and functional analyses. The results obtained indicate that exposure to iDCs induced a very rapid hydrolysis of the model peptide (half life, 62 s), resulting in complete loss of immunorecognition within 60 min. In the presence of mDCs, the hydrolysis kinetics were even faster (half life, 54 s), and the pattern of hydrolysis by-products was different from that observed for iDCs. Gp100(280-288) was also degraded in the presence of cell-free supernatants prepared both from iDCs and mDCS; in this case, degradation kinetics were slower, and the pattern of hydrolysis by-products was different from that observed in the presence of intact cells. The model epitope was degraded to non-immunogenic products by membrane and soluble enzymes expressed both by iDCs and by mDCs within periods of time that appear to be physiologically relevant. Development of antigenic formulations capable of protecting synthetic epitopes from these effects appears to represent a prerequisite for effective immunization procedures.

Cytotoxic activity of natural killer cells lacking killer-inhibitory receptors for self-HLA class I molecules against autologous hematopoietic stem cells in healthy individuals

Killer-inhibitory receptors (KIR) are receptors for self-HLA class I molecules, which are expressed on natural killer (NK) cells and small subsets of T-lymphocytes. KIR receptors that do not bind to self-HLA class I have been implicated in the pathogenesis of pure red-cell aplasia and other autoimmune diseases. However, NK cells whose inhibitory receptors lack any apparent self-ligand can also be found in healthy individuals. We therefore tested whether these NK cells are capable of exerting cytotoxic activity against autologous CD34(+) hematopoietic precursors. We detected NK cells whose sole inhibitory receptors were CD94/NKG2-A and that had no affinity for autologous HLA-C molecules. In vitro, such cells were able to kill autologous CD34(+) stem cells that expressed MHC class I antigen at a high level in about 50% of the cases of HLA-C group 2 donors. Two individual clones derived from this NK subpopulation were stimulated by autologous HLA-Cw5/6-positive stem cells, but not by allogeneic HLA-Cw7-positive stem cells. Our findings demonstrate the presence of potentially autoreactive natural killer cells in otherwise healthy individuals.

Human CTLs to Wild-Type and Enhanced Epitopes of a Novel Prostate and Breast Tumor-Associated Protein, TARP, Lyse Human Breast Cancer Cells

Vaccine therapy for prostate and breast cancer may have potential for treating these major causes of death in males and females, respectively. Critical to the development of tumor-specific vaccines is finding and characterizing novel antigens to be recognized by CD8(+) T cells. To define new CD8(+) T-cell tumor antigens, we determined two wild-type HLA-A2 epitopes from a recently found tumor-associated protein, TARP (T-cell receptor gamma alternate reading frame protein), expressed in prostate and breast cancer cells. We were also able to engineer epitope-enhanced peptides by sequence modifications. Both wild-type and enhanced epitopes induced peptide-specific CD8(+) T-cell responses in A2K(b) transgenic mice. In vitro restimulation of human CD8(+) T cells from a prostate cancer patient resulted in CD8(+) T cells reactive to the peptide epitopes that could lyse HLA-A2(+) human breast cancer cells (MCF-7) expressing TARP. Epitope-specific human CD8(+) T cells were also enumerated in patients' peripheral blood by tetramer staining. Our data suggest that HLA-A2-binding TARP epitopes and enhanced epitopes discovered in this study could be incorporated into a potential vaccine for both breast and prostate cancer.

Simultaneous prediction of binding capacity for multiple molecules of the HLA B44 supertype

We selected for study a set of B44-supertype molecules collectively represented in >40% of the individuals in all major ethnicities (B*1801, B*4001, B*4002, B*4402, B*4403, and B*4501). The peptide-binding specificity of each molecule was characterized using single amino acid substitution analogues and nonredundant peptide libraries. In all cases, only peptide ligands with glutamic acid in position 2 were preferred. At the C terminus, each allele was associated with a unique but broad pattern of preferences, but all molecules tolerated hydrophobic/aliphatic (leucine, isoleucine, valine, methionine), aromatic (tyrosine, phenylalanine, tryptophan), and small (alanine, glycine, threonine) residues. Secondary anchor motifs were also defined for all molecules. Together, these features were used to define a B44 supermotif and a novel algorithm for calculating degeneracy scores that can be used to predict B44-supertype degenerate binders. Approximately 90% of the peptides with a B44 supermotif degeneracy score of >10 bound at least three of the six B44-supertype molecules studied with high affinity. Finally, a number of peptides derived from hepatitis B and C viruses, HIV, and Plasmodium falciparum have been identified that have degenerate B44 supertype-binding capacity. Taken together, these findings have important implications for epitope-based approaches to vaccination, immunotherapy, and the monitoring of immune responses.

Degradation of the tumor antigen epitope gp100280–288 by fibroblast-associated enzymes abolishes specific immunorecognition

Degradation of the tumor antigen epitope gp100(280-288) (YLEPGPVTA) was investigated in the presence of cultured human fibroblasts, and acellular supernatants obtained from these cells; the possible effect of substrate degradation on in vitro immunorecognition was also addressed. In the presence of fibroblasts, gp100(280-288) was degraded to free amino acids with a half-life of less than 4 min; hydrolysis data support the hypothesis that substrate degradation was mainly caused by the activity of cell-expressed amino- and carboxypeptidases. Gp100(280-288) was also degraded in the presence of acellular supernatants: under these conditions, the hydrolysis pattern was similar to that observed in the presence of whole cells, but degradation kinetics was slower. As a result of these phenomena, immunorecognition of gp100(280-288)-specific cytotoxic T lymphocyte (CTL) clones was severely hampered, and was totally suppressed after 90 min. In conclusion, the high activity of fibroblast-expressed proteases, and the presence of wide-scope soluble enzymes, may explain, at least in part, the low activity of peptide-based antineoplastic vaccines, as well as the transient effectiveness of subcutaneously administered peptides in general.

A single heteroclitic epitope determines cancer immunity after xenogeneic DNA immunization against a tumor differentiation antigen

Successful active immunization against cancer requires induction of immunity against self or mutated self Ags. However, immunization against self Ags is difficult. Xenogeneic immunization with orthologous Ags induces cancer immunity. The present study evaluated the basis for immunity induced by active immunization against a melanoma differentiation Ag, gp100. Tumor rejection of melanoma was assessed after immunization with human gp100 (hgp100) DNA compared with mouse gp100 (mgp100). C57BL/6 mice immunized with xenogeneic full-length hgp100 DNA were protected against syngeneic melanoma challenge. In contrast, mice immunized with hgp100 DNA and given i.p. tolerizing doses of the hgp100 D(b)-restricted peptide, hgp100(25-33), were incapable of rejecting tumors. Furthermore, mice immunized with DNA constructs of hgp100 in which the hgp100(25-27) epitope was substituted with the weaker D(b)-binding epitope from mgp100 (mgp100(25-27)) or a mutated epitope unable to bind D(b) did not reject B16 melanoma. Mice immunized with a minigene construct of hgp100(25-33) rejected B16 melanoma, whereas mice immunized with the mgp100(25-33) minigene did not develop protective tumor immunity. In this model of xenogeneic DNA immunization, the presence of an hgp100 heteroclitic epitope with a higher affinity for MHC created by three amino acid (25 to 27) substitutions at predicted minor anchor residues was necessary and sufficient to induce protective tumor immunity in H-2(b) mice with melanoma.

Degradation of the immunogenic peptide gp100(280-288) by the monocyte-like U937 cell line

The possible degradation of the tumor antigen epitope gp100(280-288) (YLEPGPVTA) in the presence of the monocyte-like line U937, and the effect of degradation on the in vitro-measured immune recognition, were investigated by chromatographic techniques and immunological assays. Results indicate a rapid hydrolysis of the substrate in the presence of the model cells, which is consistent with the hypothesis that degradation of gp100(280-288) is caused by the activity of U937-expressed enzymes, specifically amino- and carboxypeptidases. On the other hand, these results do not support the involvement of other enzymes known to be expressed by U937 cells. From a functional point of view, these data indicate that the degradation of gp100(280-288) severely hampered recognition by specific CTL clones. The results obtained may provide a model for epitope degradation by the antigen-presenting cells found in defined anatomical compartments and may, at least in part, account for the low activity of peptide-based antineoplastic vaccines, as well as for the transience of the effects of subcutaneously administered peptides in general.

Peptide fine specificity of anti-glycoprotein 100 CTL is preserved following transfer of engineered TCR alpha beta genes into primary human T lymphocytes

TCR with known antitumor reactivity can be genetically introduced into primary human T lymphocytes and provide promising tools for immunogene therapy of tumors. We molecularly characterized two distinct TCRs specific for the same HLA-A2-restricted peptide derived from the melanocyte differentiation Ag gp100, yet exhibiting different stringencies in peptide requirements. The existence of these two distinct gp100-specific TCRs allowed us to study the preservation of peptide fine specificity of native TCRalphabeta when engineered for TCR gene transfer into human T lymphocytes. Retroviral transduction of primary human T lymphocytes with either one of the two sets of TCRalphabeta constructs enabled T lymphocytes to specifically kill and produce TNF-alpha when triggered by native gp100(pos)/HLA-A2(pos) tumor target cells as well as gp100 peptide-loaded HLA-A2(pos) tumor cells. Peptide titration studies revealed that the cytolytic efficiencies of the T lymphocyte transductants were in the same range as those of the parental CTL clones. Moreover, primary human T lymphocytes expressing either one of the two engineered gp100-specific TCRs show cytolytic activities in response to a large panel of peptide mutants that are identical with those of the parental CTL. The finding that two gp100-specific TCR, derived from two different CTL, can be functionally introduced into primary human T lymphocytes without loss of the Ag reactivity and peptide fine specificity, holds great promise for the application of TCR gene transfer in cancer immunotherapy.

Antigens derived from melanocyte differentiation proteins: self-tolerance, autoimmunity, and use for cancer immunotherapy

A large set of peptide antigens presented by class I major histocompatibility complex (MHC) molecules on human and murine melanomas and recognized by CD8+ T cells have been defined. These peptides represent attractive candidates for the development of therapeutic and/or prophylactic approaches to treat this cancer. However, the majority of the peptides that are presented by multiple tumors and recognized by T cells from multiple patients arise from proteins that are also expressed in normal melanocytes. It is expected that immune responses to such peptides will be compromised by self-tolerance or, alternatively, that stimulation of effective immune responses will be accompanied by autoimmune vitiligo. In this review, we describe a preclinical model to evaluate these issues and recent data to suggest that tolerance can be overcome to generate effective antitumor responses. This model also allows the rapid and systematic examination of parameters for the effective use of synthetic peptide vaccines.

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.

Amino acid modifications in the wild type sequence p53 232-240 overcome the poor immunogenicity of this self tumour epitope

A major limitation in antigen-specific cancer vaccines is that most of the tumour antigens that are potent candidates for broad applicability originate from self proteins. The peptides presented by tumour cells are derived from tissue-specific differentiation proteins, from proteins altered by genetic mutation or by non mutated proteins that are normally silent in most adult tissues. As a consequence, T-cell responses elicited against those antigens are rather weak. Several data showed that amino acid modifications could enhance the immunogenicity of such antigens by priming T-cells that have escaped central tolerance based on a poor avidity. In this regard, this strategy could be powerful for inducing immunity against tumours. The present report focuses on the murine wild type epitope p53 232-240 that is poorly immunogenic. It shows that substitution of the two cysteine residues by serine or amino butyric acid derivatives and substitution of the two methionine residues by norleucine residues resulted in enhanced stability of the MHC/peptide complex. The MHC binding affinity of analogue peptides was enhanced between 10 and 100 fold. They were also potent immunogens, stronger than was the original wild type epitope; T-cell responses were increased up to 50 times. Moreover, the effector T-cells elicited by three of these peptides cross reacted with the natural epitope. These observations have important implications for strategies that use the modified-peptide epitope.

Generation of CTL recognizing an HLA-A*0201-restricted epitope shared by MAGE-A1, -A2, -A3, -A4, -A6, -A10, and -A12 tumor antigens: implication in a broad-spectrum tumor immunotherapy

MAGE-A1, -A2, -A3, -A4, -A6, -A10, and -A12 are expressed in a significant proportion of primary and metastatic tumors of various histological types and are targets of tumor Ag-specific CTL. Individual MAGE-A expression varies from one tumor type to the other but, overall, the large majority of tumors expresses at least one MAGE-A Ag. Therefore, targeting epitopes shared by all MAGE-A Ags would be of interest in immunotherapy against a broad spectrum of cancers. In the present study, we describe a heteroclitic MAGE-A peptide (p248V9) that induces CTL in vivo in HLA-A*0201 transgenic HHD mice and in vitro in healthy donors. These CTL are able to recognize two low HLA-A*0201 affinity peptides differing at their C-terminal position and derived from MAGE-A2, -A3, -A4, -A6, -A10, and -A12 (p248G9) and MAGE-A1 (p248D9). Interestingly, p248V9-specific CTL respond to endogenous MAGE-A1, -A2, -A3, -A4, -A6, -A10, and -A12 in an HLA-A*0201-restricted manner and recognize human HLA-A*0201(+)MAGE-A(+) tumor cells of various histological origin. Therefore, this heteroclitic peptide may be considered as a potent candidate for a broad-spectrum tumor vaccination.

Induction of HLA-A2-restricted CTL responses by a tubular structure carrying human melanoma epitopes

Epitope-based vaccination strategies designed to induce strong tumor-specific CD8(+) T cell responses are being widely considered for cancer immunotherapy. Here, two recombinant tubular structures, NS1-Mela 1 and NS1-Mela 2, carrying, respectively two HLA-A2 epitopes derived from human melanoma antigens were constructed and their capability to induce CTL responses in vivo were studied in HLA-A2 transgenic mice. Strong CTL responses specific for GnT-V/NA 17-A and gp100 (154-162) epitopes were generated in HLA-A2 transgenic mice immunized by the construct NS1-Mela l carrying these two epitopes. The second construct NS1-Mela 2 carrying both Tyrosinase (369-377Da) and Melan-A/Mart-1 (27-35) epitopes induced a weak Tyrosinase-specific CTL response in mice but failed to induce specific CTL responses against the Melan-A/Mart-1 (27-35) epitope in the tested mice. Thus, recombinant tubular structures containing multiple tumoral epitopes may lead to new strategies for the induction of strong tumor-specific CTL responses in cancer patients.

Improved assessment of T-cell receptor (TCR) VB repertoire in clinical specimens: combination of TCR-CDR3 spectratyping with flow cytometry-based TCR VB frequency analysis

Antigen-specific T-cell responses may be described by combining three categories: (i) the specificity and effector functions of a T-cell population, (ii) the quantity of T-cell responses (i.e., the number of responding T cells within the CD4/CD8 population), and (iii) the "quality" of T cells (defined by the T-cell receptor [TCR] structure). Several methods to measure T-cell responses are now available including evaluation of T-cell precursors using limiting dilution, the enzyme-linked immunospot assay, ex vivo TCR variable (v)-segment analysis determined by flow cytometry, and TCR-CDR3 length analysis (spectratyping), as well as identification of peptide-specific T cells using major histocompatibility complex (MHC) class I tetramers containing appropriate peptides. Until now, only a limited set of MHC-peptide complexes have been available as tetramer complexes. We demonstrate that CD8(+) or CD4(+) T cells in patients with cancer can be molecularly defined using a combination of spectratyping (TCR structure and "molecular composition") plus the implementation of an antibody panel directed against 21 individual VB TCR chains ("quantity" of T-cell families). This approach is instrumental in defining and comparing the magnitudes of CD4(+) or CD8(+) T-cell responses over time in individual patients, in comparing the TCR VA and VB repertoire in different anatomic compartments, and in comparing the TCR VA-VB diversity with that in normal healthy controls. This method provides the means of objectively defining and comparing the TCR repertoire in patients undergoing vaccination protocols and underlines the necessity to calibrate the TCR-CDR3 analysis with a qualitative assessment of individual TCR VB families.

Majority of peptides binding HLA-A*0201 with high affinity crossreact with other A2-supertype molecules

The A*0201, A *0202, A*0203, A*0206, and A*6802 binding capacity of single amino acid substitution analogs of known A2-supertype binding peptides and of large nonredundant peptide libraries was measured. The results were utilized to rigorously define the peptide binding specificities of these A2-supertype molecules. Although each molecule was noted to have unique preferences, large overlaps in specificity were found. The presence of L, I, V, M, A, T, and Q residues in position 2, and L, I, V, M, A, and T residues at the C-terminus of peptide ligands were tolerated by all molecules. Likewise, whereas examination of secondary influences on peptide binding revealed allele specific preferences, shared features could also be identified. These shared features were utilized to define an A2-supermotif and were noted to correlate with crossreactivity. Over 70% of the peptides that bound A *0201 with high affinity were found to bind at least two other A2-supertype molecules. Because the A2-supertype molecules studied herein cover the variants most common in different major ethnicities, these findings have important implications for epitope-based approaches to vaccination, immunotherapy, and the monitoring of immune responses.

Cancer immunotherapy: insights from transgenic animal models

A wide range of strategies in cancer immunotherapy has been developed in the last decade, some of which are currently being used in clinical settings. The development of these immunotherapeutical strategies has been facilitated by the generation of relevant transgenic animal models. Since the different strategies in experimental immunotherapy of cancer each aim to activate different immune system components, a variety of transgenic animals have been generated either expressing tumor associated, HLA, oncogenic or immune effector cell molecule proteins. This review aims to discuss the existing transgenic mouse models generated to study and develop cancer immunotherapy strategies and the variable results obtained. The potential of the various transgenic animal models regarding the development of anti-cancer immunotherapeutical strategies is evaluated.

Structural features of peptide analogs of human histocompatibility leukocyte antigen class I epitopes that are more potent and immunogenic than wild-type peptide

Certain peptide analogs that carry substitutions at residues other than the main major histocompatibility complex anchors and are surprisingly much more antigenic than wild-type peptide (heteroclitic analogs). To date, it was unknown how frequently wild-type epitopes could be modified to obtain heteroclitic activity. In this study, we analyzed a large panel of analogs of two different human histocompatibility leukocyte antigen (HLA)-A2.1-restricted epitopes and found that heteroclitic analogs were associated with higher magnitude responses and increased (up to 10(7)-fold) sensitivity to antigen, and corresponded to conservative or semiconservative substitutions at odd-numbered positions in the middle of the peptide (positions 3, 5, or 7). These findings were validated by performing additional immunogenicity studies in murine and human systems with four additional epitopes. The biological relevance of heteroclitic analogs was underlined when predicted analogs of the p53.261 epitope was shown to induce cytotoxic T lymphocytes (CTLs) that recognize low concentrations of peptide (high avidity) in vivo and demonstrate in vitro antitumor recognition. Finally, in vitro immunization of human peripheral blood mononuclear cells with two heteroclitic analogs resulted in recruitment of more numerous CTLs which were associated with increased antigen sensitivity. In conclusion, heteroclitic analogs were identified in each of the six cases studied and structural features were defined which allow identification of such analogs. The strong CTL immunity elicited by heteroclitic epitopes suggest that they could be of significant value in vaccination against tolerant or weakly immunogenic tumor-associated and viral antigens.

Crystal structures of two closely related but antigenically distinct HLA-A2/melanocyte-melanoma tumor-antigen peptide complexes

We have determined high-resolution crystal structures of the complexes of HLA-A2 molecules with two modified immunodominant peptides from the melanoma tumor-associated protein Melan-A/Melanoma Ag recognized by T cells-1. The two peptides, a decamer and nonamer with overlapping sequences (ELAGIGILTV and ALGIGILTV), are modified in the second residue to increase their affinity for HLA-A2. The modified decamer is more immunogenic than the natural peptide and a candidate for peptide-based melanoma immunotherapy. The crystal structures at 1.8 and 2.15 A resolution define the differences in binding modes of the modified peptides, including different clusters of water molecules that appear to stabilize the peptide-HLA interaction. The structures suggest both how the wild-type peptides would bind and how three categories of cytotoxic T lymphocytes with differing fine specificity might recognize the two peptides.

Identification and design of p53-derived HLA-A2-binding peptides with increased CTL immunogenicity

The replacement of a suboptimal amino acid in a primary anchor position with an optimal residue improves human leucocyte antigen (HLA) binding and immunogenicity, while maintaining cytotoxic T lymphocyte (CTL) specificity. Using a neural network capable of performing quantitative predictions of peptide binding to HLA-A2 molecules, we identified three p53 protein-derived nonamer peptides with intermediate binding owing to suboptimal amino acids in the P2 anchor position. These peptides were synthesized along with the corresponding analogs, where the natural P2 residue had been replaced with the optimal leucine residue. All three modified peptides bound to and more efficiently stabilized HLA-A2 molecules than the corresponding nonmodified peptides. The HLA-A2 transgenic mice were used for immunization. Two of the epitopes were more immunogenic in their modified than in their natural versions. The CTLs raised against the modified peptides efficiently killed the target cells pulsed with the corresponding native peptide. In terms of sensitizing the targets cells for the CTL killing, the modified peptides were more efficient than native peptides. Finally, the CTLs induced by modified peptide killed HLA-A2 transgenic mouse fibrosarcoma cells transfected with human p53 DNA. The data suggest that modified self-peptides derived from overexpressed tumour-associated proteins can be used in vaccine development against cancer, and that quantitative predictions of HLA binding is of value in the rational selection and improvement of target epitopes recognized by CTLs.

A general strategy to enhance immunogenicity of low-affinity HLA-A2. 1-associated peptides: implication in the identification of cryptic tumor epitopes

Low-affinity MHC class I-associated cryptic epitopes derived from self proteins overexpressed in a wide variety of human tumors or derived from antigens of viruses exhibiting a high mutation rate, could be interesting candidates for tumor and virus immunotherapy, respectively. However, identification of low-affinity MHC-associated epitopes comes up against their poor immunogenicity. Here we describe an approach that enhances immunogenicity of nonimmunogenic low-affinity HLA-A2.1-binding peptides. It consists of modifying their sequence by introducing a tyrosine in the first position (P1Y). P1Y substitution enhances affinity of HLA-A2.1-associated peptides without altering their antigenic specificity. In fact, P1Y variants of ten nonimmunogenic low-affinity peptides exhibited a 2.3- to 55-fold higher binding affinity and/or stabilized the HLA-A2.1 for at least 2 h more than the corresponding native peptides. More importantly, P1Y variants efficiently triggered in vivo native peptide-specific CTL which also recognized the corresponding naturally processed epitope. The possibility for generating CTL against any low-affinity HLA-A2.1-associated peptide provides us with the necessary tool for the identification of cryptic tumor and virus epitopes which could be used for peptide-based immunotherapy.