Conditional superagonist CTL ligands for the promotion of tumor-specific CTL responses

Antigenic variability: Obstacles on the road to vaccines against traditionally difficult targets

Despite the impressive impact of vaccines on public health, the success of vaccines targeting many important pathogens and cancers has to date been limited. The burden of infectious diseases today is mainly caused by antigenically variable pathogens (AVPs), which escape immune responses induced by prior infection or vaccination through changes in molecular structures recognized by antibodies or T cells. Extensive genetic and antigenic variability is the major obstacle for the development of new or improved vaccines against "difficult" targets. Alternative, qualitatively new approaches leading to the generation of disease- and patient-specific vaccine immunogens that incorporate complex permanently changing epitope landscapes of intended targets accompanied by appropriate immunomodulators are urgently needed. In this review, we highlight some of the most critical common issues related to the development of vaccines against many pathogens and cancers that escape protective immune responses owing to antigenic variation, and discuss recent efforts to overcome the obstacles by applying alternative approaches for the rational design of new types of immunogens.

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.

Enhanced anti-colon cancer immune responses with modified eEF2-derived peptides

Eukaryotic elongation factor-2 (eEF2) is overexpressed in many human cancers and is an attractive target for cancer immunotherapy. The eEF2 derived polypeptides have been shown to be able to induce cytotoxic T lymphocytes from healthy donor. Here, we demonstrate the evidence indicating that modification of a segment of peptides from wild type eEF2-derived immunogenic peptides is able to further enhance its capacity of inducing antigen-specific cytotoxic T lymphocytes (CTLs) against colon cancer cells. Using peptide-MHC binding algorithms, potential HLA-A2.1-restricted epitopes capable of inducing specific CD8(+) CTLs were identified. By analyzing HLA-A2.1 affinity and immunogenicity, we further identified one novel immunogenic peptide, P739-747 (RLMEPIYLV), that elicited specific CTL responses in HLA-A2.1/K(b) transgenic mice and culture with peripheral blood lymphocytes from colon cancer patients. Furthermore, replacing certain amino acids (at positions 1, 3, 7) within the P739-747 sequence improved the immunogenicity against eEF2. Several analogs containing the auxiliary HLA-A*0201 anchor residues were able to stably bind to HLA-A*0201 and enhance CTL responses compared with the native sequence; two of them showed increased anti-tumor effects during the adoptive immunotherapy in vivo. Thus, these results support that modified immunogenic analogs are promising candidates for peptide-based cancer vaccination and immunotherapy.

Variable epitope library carrying heavily mutated survivin-derived CTL epitope variants as a new class of efficient vaccine immunogen tested in a mouse model of breast cancer

The antigenic variability of tumor cells leading to dynamic changes in cancer epitope landscape along with escape from immune surveillance by down-regulating tumor antigen expression/presentation and immune tolerance are major obstacles for the design of effective vaccines. We have developed a novel concept for immunogen construction based on introduction of massive mutations within the epitopes targeting antigenically variable pathogens and diseases. Previously, we showed that these immunogens carrying large combinatorial libraries of mutated epitope variants, termed as variable epitope libraries (VELs), induce potent, broad and long lasting CD8+IFN-γ+ T-cell response as well as HIV-neutralizing antibodies. In this proof-of-concept study, we tested immunogenic properties and anti-tumor effects of the VELs bearing survivin-derived CTL epitope (GWEPDDNPI) variants in an aggressive metastatic mouse 4T1 breast tumor model. The constructed VELs had complexities of 10,500 and 8,000 individual members, generated as combinatorial M13 phage display and synthetic peptide libraries, respectively, with structural composition GWXPXDXPI, where X is any of 20 natural amino acids. Statistically significant tumor growth inhibition was observed in BALB/c mice immunized with the VELs in both prophylactic and therapeutic settings. Vaccinated mice developed epitope-specific spleen cell and CD8+ IFN-γ+ T-cell responses that recognize more than 50% of the panel of 87 mutated epitope variants, as demonstrated in T-cell proliferation assays and FACS analysis. These data indicate the feasibility of the application of this new class of immunogens based on VEL concept as an alternative approach for the development of molecular vaccines against cancer.

HLA-F and MHC-I open conformers cooperate in a MHC-I antigen cross-presentation pathway

Peptides that are presented by MHC class I (MHC-I) are processed from two potential sources, as follows: newly synthesized endogenous proteins for direct presentation on the surface of most nucleated cells and exogenous proteins for cross-presentation typically by professional APCs. In this study, we present data that implicate the nonclassical HLA-F and open conformers of MHC-I expressed on activated cells in a pathway for the presentation of exogenous proteins by MHC-I. This pathway is distinguished from the conventional endogenous pathway by its independence from TAP and tapasin and its sensitivity to inhibitors of lysosomal enzymes, and further distinguished by its dependence on MHC-I allotype-specific epitope recognition for Ag uptake. Thus, our data from in vitro experiments collectively support a previously unrecognized model of Ag cross-presentation mediated by HLA-F and MHC-I open conformers on activated lymphocytes and monocytes, which may significantly contribute to the regulation of immune system functions and the immune defense.

Amino-terminal extended peptide single-chain trimers are potent synthetic agonists for memory human CD8+ T cells

Upon Ag exposure, most memory T cells undergo restimulation-induced cell death. In this article, we describe a novel synthetic agonist, an N-terminal extended decamer peptide expressed as a single-chain trimer, the amino-terminal extended peptide MHC class I single-chain trimer (AT-SCT), which preferentially promotes the growth of memory human CD8(+) T cells with minimal restimulation-induced cell death. Using CMV pp65 and melanoma gp100 Ags, we observe the in vitro numerical expansion of a clonally diverse polyfunctional population of Ag-specific CD8(+) T cells from healthy individuals and vaccinated melanoma patients, respectively. Memory CD8(+) T cells stimulated with AT-SCT presented on MHC class I/II-null cells show reduced cytokine production, slower kinetics of TCR downregulation, and decreased cell death compared with native nonamer MHC class I single-chain trimer (SCT)-activated T cells. However, both ERK phosphorylation and cell cycle kinetics are identical in AT-SCT- and SCT-activated T cells. Probing of SCT and AT-SCT peptide-MHC complexes using fluorochrome-conjugated TCR multimers suggests that nonamer- and decamer-linked peptides may be anchored differently to the HLA-A2 peptide-binding groove. Our findings demonstrate that modified peptide-MHC structures, such as AT-SCT, can be engineered as T cell agonists to promote the growth and expansion of memory human CD8(+) T cells.

Identification of novel myeloma-specific XBP1 peptides able to generate cytotoxic T lymphocytes: a potential therapeutic application in multiple myeloma

The purpose of these studies was to identify HLA-A2⁺ immunogenic peptides derived from XBP1 antigens to induce a multiple myeloma (MM)-specific immune response. Six native peptides from non-spliced XBP1 antigen and three native peptides from spliced XBP1 antigen were selected and evaluated for their HLA-A2 specificity. Among them, XBP1_184–192, XBP1 SP_196–204 and XBP1 SP_367–375 peptides showed the highest level of binding affinity, but not stability to HLA-A2 molecules. Novel heteroclitic XBP1 peptides, YISPWILAV or YLFPQLISV, demonstrated a significant improvement in HLA-A2 stability from their native XBP1_184–192 or XBP1 SP_367–375 peptide, respectively. Cytotoxic T lymphocytes generated by repeated stimulation of CD3⁺ T cells with each HLA-A2-specific heteroclitic peptide showed an increased percentage of CD8⁺ (cytotoxic) and CD69⁺/CD45RO⁺ (activated memory) T cells and a lower percentage of CD4⁺ (helper) and CD45RA⁺/CCR7⁺ (naïve) T cells, which were distinct from the control T cells. Functionally, the CTLs demonstrated MM-specific and HLA-A2-restricted proliferation, IFN-γ secretion and cytotoxic acivity in response to MM cell lines and importantly, cytotoxicty against primary MM cells. These data demonstrate the distinct immunogenic characteristics of unique heteroclitic XBP1 peptides which induce MM-specific CTLs and highlights their potential application for immunotherapy to treat the patients with MM or its pre-malignant condition.