Antiproliferative activity and phenotypic modification induced by selected Peruvian medicinal plants on human hepatocellular carcinoma Hep3B cells

ETHNOPHARMACOLOGICAL RELEVANCE

The high incidence of human hepatocellular carcinoma (HCC) in Peru and the wide use of medicinal plants in this country led us to study the activity against HCC cells in vitro of somes species used locally against liver and digestive disorders.

MATERIALS AND METHODS

Ethnopharmacological survey: Medicinal plant species with a strong convergence of use for liver and digestive diseases were collected fresh in the wild or on markets, in two places of Peru: Chiclayo (Lambayeque department, Chiclayo province) and Huaraz (Ancash department, Huaraz province). Altogether 51 species were collected and 61 ethanol extracts were prepared to be tested. Biological assessment: All extracts were first assessed against the HCC cell line Hep3B according a 3-step multi-parametric phenotypic assay. It included 1) the evaluation of phenotypic changes on cells by light microscopy, 2) the measurement of the antiproliferative activity and 3) the analysis of the cytoskeleton and mitosis by immunofluorescence. Best extracts were further assessed against other HCC cell lines HepG2, PLC/PRF/5 and SNU-182 and their toxicity measured in vitro on primary human hepatocytes.

RESULTS

Ethnopharmacological survey: Some of the species collected had a high reputation spreading over the surveyed locations for treating liver problems, i.e. Baccharis genistelloides, Bejaria aestuans, Centaurium pulchellum, Desmodium molliculum, Dipsacus fullonum, Equisetum bogotense, Gentianella spp., Krameria lapacea, Otholobium spp., Schkuhria pinnata, Taraxacum officinale. Hep3B evaluation: Fourteen extracts from 13 species (Achyrocline alata, Ambrosia arborescens, Baccharis latifolia, Hypericum laricifolium, Krameria lappacea, Niphidium crassifolium, Ophryosporus chilca, Orthrosanthus chimboracensis, Otholobium pubescens, Passiflora ligularis, Perezia coerulescens, Perezia multiflora and Schkuhria pinnata) showed a significant antiproliferative activity against Hep3B cells (IC50≤ 50µg/mL). This was associated with a lack of toxicity on primary human hepatocytes in vitro. Immunofluorescence experiments on Hep3B cells showed that crude extracts of Schkuhria pinnata and Orthrosanthus chimboracensis could block Hep3B cells in mitosis with an original phenotype. Crude extracts of Perezia coerulescens, Perezia multiflora, Achyrocline alata, Ophryosporus chilca, Otholobium pubescens and Hypericum laricifolium could modify the overall microtubule cytoskeletal dynamics of Hep3B cells in interphase by an original mechanism.

CONCLUSIONS

Our method allowed us to select 9 extracts which displayed antiproliferative activities associated with original cellular phenotypes on Hep3B cells, regarding known microtubule-targeting drugs. Both chemical and cellular studies are ongoing in order to elucidate natural compounds and cellular mechanisms responsible of the activities described.

Abstract 3204: Screening of extracts from ethnopharmacologically selected peruvian plants in human hepatocarcinoma cell line Hep3B

Liver cancer, for which hepatocellular carcinoma (HCC) represents the most frequent primitive form, is ranked sixth and third in terms of incidence and death respectively in the world. Its survival rate at 5 years is low (less than 10 %). There is no effective treatment to date, surgical approaches excepted or sorafenib with a gain in survival of 3 months. In view of this limitation of therapeutic alternatives, the search and identification of new molecules of natural origin remain an important issue. In this study, the pharmacological properties of 63 extracts, prepared from Peruvian plants selected upon ethnopharmacological investigations conducted in Peru, were analyzed.

The extracts were tested in an in vitro phenotypic screening approach on the human hepatocellular carcinoma cell line, Hep3B, following a 3-step cell analysis:

1- the first step consisted in the determination of a screening score which was defined according to 3 criteria : APE, MIT and OMM. APE refers to the Anti-Proliferative Effect of the extract, MIT to the ability of the extract to modify the MITotic events (by increasing or decreasing the number of cells in mitosis), and OMM refers to the ability of an extract to induce Original Morphological Modifications to the treated cells. The score values ranged from 1 to 27 and a score value of 12 was defined as the threshold. Twenty five extracts exhibited a score value > 12 and were selected for further analysis.

2- the second step consisted in the determination of the cytotoxic effect of the 25 extracts selected from step 1, measured after a 48 hrs incubation time. Of these extracts, 11 showed IC50 values > 50 µg/ml, 10 with IC50 between 15 and 50 µg/ml and 4 with IC50 < 15 µg/ml The 14 extracts which exhibited IC50 values lower than 50 µg/ml were selected for the final step.

3- the third step consisted in studying the extracts inducing original phenotypic changes. This led to the selection of 4 extracts : 2 extracts of different families (Iridaceae and Asteraceae) induced blocking Hep3B cells in prometaphase while 2 other extracts, from the same genus (Asteraceae) induced cytoskeletal reorganization. This latter effect was original since it occurred on cells in interphase and not on mitosis as it can be seen with the classical tubulin inhibitors such as colchicine, vinblastine or paclitaxel.

Interestingly, the plants from which come the 4 extracts inducing original phenotypes have been little studied so far either chemically or pharmacologically, suggesting that they may be a source of new molecules of therapeutic interest and/or with original structures.

Citation Format: Jean Edouard Gairin, Cedric Lavergne, Maelle Carraz, Valérie Jullian, Geneviève Bourdy, Michel Wright. Screening of extracts from ethnopharmacologically selected peruvian plants in human hepatocarcinoma cell line Hep3B. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3204. doi:10.1158/1538-7445.AM2014-3204

Melanoma chemotherapy leads to the selection of ABCB5-expressing cells

Metastatic melanoma is the most aggressive skin cancer. Recently, phenotypically distinct subpopulations of tumor cells were identified. Among them, ABCB5-expressing cells were proposed to display an enhanced tumorigenicity with stem cell-like properties. In addition, ABCB5⁺ cells are thought to participate to chemoresistance through a potential efflux function of ABCB5. Nevertheless, the fate of these cells upon drugs that are used in melanoma chemotherapy remains to be clarified. Here we explored the effect of anti-melanoma treatments on the ABCB5-expressing cells. Using a melanoma xenograft model (WM266-4), we observed in vivo that ABCB5-expressing cells are enriched after a temozolomide treatment that induces a significant tumor regression. These results were further confirmed in a preliminary study conducted on clinical samples from patients that received dacarbazine. In vitro, we showed that ABCB5-expressing cells selectively survive when exposed to dacarbazine, the reference treatment of metastatic melanoma, but also to vemurafenib, a new inhibitor of the mutated kinase V600E BRAF and other various chemotherapeutic drugs. Our results show that anti-melanoma chemotherapy might participate to the chemoresistance acquisition by selecting tumor cell subpopulations expressing ABCB5. This is of particular importance in understanding the relapses observed after anti-melanoma treatments and reinforces the interest of ABCB5 and ABCB5-expressing cells as potential therapeutic targets in melanoma.

Abstract LB-295: Identification of the first class of proteasome inhibitors that impair 26S proteasome assembly in vitro

The Ubiquitin-Proteasome System (UPS) is the main non-lysosomial proteolytic machinery in the eukaryotic cells. Prior to their degradation proteins are adressed to the catalytic 26S proteasome complex composed of a 20S “core” that contains proteolytic activities, assembled with one or two 19S regulatory particles that bind proteins to be degraded. Due to its central role of in regulating cell cycle progression and signal transduction the UPS has been proposed as a suitable target for antineoplastic strategies. To date only catalytic inhibitors of proteasome, targeting proteolytic activities of the 20S core particle, have been characterized and/or have emerged as potent anticancer drugs (1).

However, the capacity of the UPS to degrade proteins not only depends of the catalytic activities of the 20S core, but also of the 26S proteasome assembly, a dynamic and tightly controlled cellular process. Thus, compounds impairing 26S proteasome assembly could represent a novel class of proteasome inhibitors.

With the aim of identifying such molecules we first set up a robust biochemical screening assay allowing us to follow 26S proteasome assembly in vitro. First, 26S proteasome was purified from Hela cells using affinity chromatography followed by gel filtration. Then, experimental conditions were determined for in vitro 26S disassembly into its constitutive sub-complexes (19S+20S) ands its re-assembly as an active and functional 26S entity. The differential CTL-activity between 26S and 20S proteasomes (2) was then used as the readout to follow the reassembly of 26S in vitro, prior to be confirmed using in gel overlay experiments and western-blotting. Using this biochemical assay:

1) we screened naturals compounds endowed with antiproliferative properties and described as inhibitors of the ubiquitine-proteasome pathway (3).

2) we identified structurally-related natural molecules, epidithiodioxopiperazine and diketodithiopiperazine derivatives, as the first class of molecules inhibiting proteasome assembly without significantly affecting neither 26S nor 20S catalytic activity in vitro.

Our research efforts now concentrate in showing that: 1) these molecules inhibit proteasome assembly in cells, and that 2) this inhibition can be correlated with their antiproliferative effects.

References

Sterz J. et al. Extert Opin. Invest. Drugs, 2008, 17

Rivett AJ et al., Experimental Gerontology, 2002, 37

Vandenberghe I. et al., Biochem. Pharmacol., 2008, 76

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-295.

Concepts and ways to amplify the antitumor immune response

In this chapter, a detailed description of how the innate and adaptive immune responses interact with malignant cells is presented. In addition, we discuss how developing tumors establish themselves, and how they benefit on one hand and organize their defense against the immune system on the other hand. New data from three tumor model systems in mice are discussed; in particular, the intricate interactions between the immune cells and the tumor cells are highlighted. With the present data and knowledge, we conclude that a first prerequisite for the combat against tumors is the activation of the innate immune system via external danger signals or damage signals and internal danger signals. The second prerequisite for efficient tumor cell eradication is combined therapeutic approaches of physical, chemical, pharmacological, and immunological origin. Finally, we propose new ways for further investigation of the relationship linking tumor cells and our defense system. It appears mandatory to understand how the malignant cells render the adaptive immune cells tolerant instead of turning them into aggressive effectors and memory cells. Perhaps, the most important thing, for immunologists and clinicians, to understand is that tumor cells must not be viewed just as antigens but much more.

Characterization and differential expression of a newly identified phosphorylated isoform of the human 20S proteasome beta7 subunit in tumor vs. normal cell lines

The search of new pharmacological targets with original mechanism of action within the ubiquitin-proteasome pathway is still a goal to be reached in oncopharmacology. Modification by phosphorylation/dephosphorylation has been found to be involved in cancer and to regulate functional activity of proteasome. Until now, phosphorylated forms of alpha subunits of the 20S human proteasome have been mostly reported. Here, we have rationally designed a polyclonal antibody specifically directed against a phosphorylated peptide sequence bearing the beta7 subunit Ser249 residue of the human 20S proteasome. This anti-beta7 phosphoSer249 antibody appeared to be a probe of choice to detect the presence of a phosphorylated isoform of the beta7 subunit of the human 20S proteasome using mono or two-dimensional gel electrophoresis. PhosphoSer249 was sensitive to acid phosphatase treatment of native 20S proteasome. Dephosphorylation affected the peptidylglutamyl-peptide hydrolyzing activity whereas the chymotrypsin-like and trypsin-like activities remained unchanged. A comparative analysis between human normal and tumor cells showed a differential expression of the phosphoSer249 beta7 isoform with a significantly lower detection in the proteasome isolated from tumor cells, suggesting its possible use as a biomarker.

In vivo major histocompatibility complex class I (MHCI) expression on MHCIlow tumor cells is regulated by gammadelta T and NK cells during the early steps of tumor growth

Cell surface expression of MHC class I molecules by tumor cells is determinant in the interplay between tumor cells and the immune system. Nevertheless, the mechanisms which regulate MHCI expression on tumor cells are not clear. We previously showed that immune innate cells from the spleen can regulate MHCI expression on MHCI(low) tumor cells. Here, using the murine model of B16 melanoma, we demonstrate that the MHCI status of tumor cells in vivo is regulated by the microenvironment. In subcutaneous grafts, induction of MHCI molecules on tumor cells is concomitant to the recruitment of lymphocytes and relies on an IFNgamma-mediated mechanism. gammadelta T and NK cells are essential to this regulation. A small proportion of tumor-infiltrating NK cells and gammadelta T cells were found to produce IFNgamma, suggesting a possible direct participation to the MHCI increase on the tumor cells upon tumor cell recognition. Depletion of gammadelta T cells increases the tumor growth rate, confirming their anti-tumoral role in our model. Taken together, our results demonstrate that in vivo, NK and gammadelta T cells play a dual role during the early growth of MHCI(low) tumor cells. In addition to controlling the growth of tumor cells, they contribute to modifying the immunogenic profile of residual tumor cells.

Pharmacological screening of bryophyte extracts that inhibit growth and induce abnormal phenotypes in human HeLa cancer cells

Antitumor activities of substances from natural sources apart from vascular plants and micro-organisms have been poorly investigated. Here we report on a pharmacological screening of a bryophyte extract library using a phenotypic cell-based assay revealing microtubules, centrosomes and DNA. Among the 219 moss extracts tested, we identified 41 extracts acting on cell division with various combinations of significant effects on interphasic and mitotic cells. Seven extracts were further studied using a cell viability assay, cell cycle analysis and the phenotypic assay. Three distinct pharmacological patterns were identified including two unusual phenotypes.

[Bryophytes, a potent source of drugs for tomorrow's medicine?]

Although secondary plant metabolites provided numerous leads for the development of a wide array of therapeutic drugs, the discovery of new drugs with novel structures has declined in the past few years. Indeed higher plants have a similar evolutionary history and so produce similar metabolites. Search for novel sources of new therapeutic compounds within unexplored parts of biodiversity is thus an attractive challenge. Bryophytes, a group of small terrestrial plants remain relatively untouched in the drug discovery process whereas some have been used as medicinal plants. Studies of their secondary metabolites are recent but reveal original compounds, some of which not synthesized by higher plants. However investigations often meet difficulties during harvest or isolation of active compounds. In consequence, small quantities of substances obtained may be the main reason for the lack of biological tests. Strategies to overcome those troubles may exist and then lead to innovative medicinal applications.

Capture of membrane components via trogocytosis occurs in vivo during both dendritic cells and target cells encounter by CD8(+) T cells

Cytotoxic T lymphocytes recently stimulated by antigen-presenting cells (APC) display major histocompatibility class (MHC) I and II molecules inherited from APC. We have previously reported that, in vitro, transfer of MHC molecules and several other proteins occurs through trogocytosis, i.e. the active acquisition of target cell membrane fragments by T lymphocytes. Here, using the model of viral antigen LCMVgp33-41 recognition in transgenic P14 mice, we show that CD8(+) T cells perform trogocytosis in vivo, as detected by the capture of biotin- or fluorescence-labeled components of the APC surface. Trogocytosis occurs during interactions of CD8(+) T cells with at least two kinds of cells: target cells and dendritic cells (DC). In lymph nodes, CD8(+) T cells having performed trogocytosis with DC express the CD69 activation marker indicating that trogocytosis detects recently activated cells. Taken together, our findings suggest that trogocytosis may be a new in vivo marker of the recent interaction between a CD8(+) T cell and its cellular partners or targets.

Allosteric Changes in the TCR/CD3 Structure Upon Interaction With Extra- or Intra-cellular Ligands

T lymphocytes are activated by the interaction between the T-cell antigen receptor (TCR) and peptides presented by major histocompatibility complex (MHC) molecules. The avidity of this TCR-pMHC interaction is very low. Therefore, several hypotheses have been put forward to explain how T cells become specifically activated despite this handicap: conformational change model, aggregation model, kinetic segregation model, sequential interaction model and permissive geometry model. In the present paper, we conducted experiments to distinguish between the TCR-aggregation model and the TCR-conformational change model. The results obtained using a TCR capture ELISA with Brij 98-solubilized TCR molecules from normal or activated T cells showed that the ligand-TCR interaction causes structural changes in the CD3 epsilon cytoplasmic tail as well as in the extracellular TCR beta FG loop region. Size-fractionation experiments with Brij 98-solubilized TCR/CD3/co-receptor complexes from naïve or activated CD4(+) or CD8(+) T cells demonstrated that such complexes are found as either dimers or tetramers. No monomers or multimers were detected. We propose that: (1) ligand-TCR interaction results in conformational changes in the CD3 epsilon cytoplasmic tail leading to T-cell activation; (2) CD3 epsilon cytoplasmic tail interaction with intracellular proteins may dissociate pMHC and co-receptors (CD4 or CD8) from TCR/CD3 complexes, thus leading to the arrest of T-cell activation; and (3) T-cell activation appears to occur among dimers or tetramers of TCR/CD3/co-receptor complexes interacting with self and non-self (foreign) peptide-MHC complexes.

Interactions between CD8alphabeta and the TCRalphabeta/CD3-receptor complex

CD8+ T cells recognize antigenic peptides bound to major histocompatibility complex (MHC) class I molecules on normal antigen-presenting cells (APC), as well as on virus-infected cells or tumour cells (pMHC). At least two receptor types participate in recognition of these complexes: T-cell receptor (TCR) alphabeta heterodimers and CD8alphabeta molecules. The former molecules react with antigenic peptide and variable regions of MHC class I molecules, whereas the latter molecules react with constant alpha3 regions of MHC class I molecules. As the avidity of both receptor-MHC interactions is low, it is believed that TCRalphabeta and CD8alphabeta heterodimers collaborate in T-cell recognition. We have established a TCR/CD3-CD8 capture ELISA, which can measure the interaction of pMHC with CD8alphabeta molecules and with TCR/CD3 complexes. The major findings are: (1) TCR/CD3 complexes derived from in vitro activated T cells and captured by anti-CD3 MoAb, do bind specific pMHC and (2) CD8+ T cells express at least three forms of CD8alphabeta molecules: single CD8alphabeta, CD3-CD8 and TCR/CD3-CD8 complexes. Only the latter complexes are associated with CD3zeta homodimers, and the quantity of TCR/CD3-CD8 complexes relative to total CD8alphabeta molecules appears to increase and to be selected into sucrose-gradient microdomains as a function of TCRalphabeta-mediated T-cell activation.

The Use of Mass Spectrometry to Identify Antigens from Proteasome Processing

Mass spectrometry (MS) is a powerful tool for the characterization of antigenic peptides that play a major role in the immune system. Most of the major histocompatibility complex (MHC) class I peptides are generated during the degradation of intracellular proteins by the proteasome, a catalytic complex present in all eukaryotic cells. This chapter focuses on the contribution of MS to the understanding of the mechanisms of antigen processing by the proteasome. This knowledge may be valuable for the design of specific inhibitors of proteasome, which has recently been recognized as a therapeutic target in cancer therapies and for the development of efficient peptidic vaccines in immunotherapies. Examples from the literature have been chosen to illustrate how MS data can contribute first to the understanding of the mechanisms of proteasomal processing and, second, to the understanding of the crucial role of proteasome in cytotoxic T lymphocytes (CTL) activation. The general strategy based on MS analyses used in these studies is also described.

Effect of ajoene, a natural antitumor small molecule, on human 20S proteasome activity in vitro and in human leukemic HL60 cells

The pharmacologic properties of ajoene, the major sulfur-containing compound purified from garlic, and its possible role in the prevention and treatment of cancer has received increasing attention. Several studies demonstrated that induction of apoptosis and cell cycle blockade are typical biologic effects observed in tumor cells after proteasome inhibition. The proteasome is responsible for the degradation of a variety of intracellular proteins and plays a key role in the regulation of many cellular processes. The aim of the present work was therefore to explore the effects of ajoene on the proteasome activities. In vitro activities of 20S proteasome purified from human erythrocytes on fluorogenic peptide substrates specific for trypsin-like, chymotrypsin-like and peptidylglutamyl peptide hydrolyzing activities revealed that ajoene inhibited the trypsin-like activity in a dose- and time-dependent manner. Further, the ability of 20S proteasome to degrade the OVA(51-71) peptide, a model proteasomal substrate, was partially but significantly inhibited by ajoene. In addition, when human leukemia cell line HL60 was treated with ajoene, both trypsin- and chymotrypsin-like activities were affected, cells arrested in G2/M phase and total amount of cytosolic proteasome increased. All these data clearly indicate that ajoene may affect proteasome function and activity both in vitro and in the living cell. This is a novel aspect in the biologic profile of this garlic compound giving new insights into the understanding of the molecular mechanisms of its potential antitumor action.

Characterization of protein variants and post-translational modifications: ESI-MSn analyses of intact proteins eluted from polyacrylamide gels

We have developed a strategy to characterize protein isoforms, resulting from single-point mutations and post-translational modifications. This strategy is based on polyacrylamide gel electrophoresis separation of protein isoforms, mass spectrometry (MS) and MSn analyses of intact proteins, and tandem MS analyses of proteolytic peptides. We extracted protein isoforms from polyacrylamide gels by passive elution using SDS, followed by nanoscale hydrophilic phase chromatography for SDS removal. We performed electrospray ionization MS analyses of the intact proteins to determine their molecular mass, allowing us to draw hypotheses on the nature of the modification. In the case of labile post-translational modifications, like phosphorylations and glycosylations, we conducted electrospray ionization MSn analyses of the intact proteins to confirm their presence. Finally, after digestion of the proteins in solution, we performed tandem MS analyses of the modified peptides to locate the modifications. Using this strategy, we have determined the molecular mass of 5-10 pmol of a protein up to circa 50 kDa loaded on a gel with a 0.01% mass accuracy. The efficiency of this approach for the characterization of protein variants and post-translational modifications is illustrated with the study of a mixture of kappa-casein isoforms, for which we were able to identify the two major variants and their phosphorylation site and glycosylation motif. We believe that this strategy, which combines two-dimensional gel electrophoresis and mass spectrometric analyses of gel-eluted intact proteins using a benchtop ion trap mass spectrometer, represents a promising approach in proteomics.

T cells infiltrate the brain in murine and human transmissible spongiform encephalopathies

CD4 and CD8 T lymphocytes infiltrate the parenchyma of mouse brains several weeks after intracerebral, intraperitoneal, or oral inoculation with the Chandler strain of mouse scrapie, a pattern not seen with inoculation of prion protein knockout (PrP(-/-)) mice. Associated with this cellular infiltration are expression of MHC class I and II molecules and elevation in levels of the T-cell chemokines, especially macrophage inflammatory protein 1beta, IFN-gamma-inducible protein 10, and RANTES. T cells were also found in the central nervous system (CNS) in five of six patients with Creutzfeldt-Jakob disease. T cells harvested from brains and spleens of scrapie-infected mice were analyzed using a newly identified mouse PrP (mPrP) peptide bearing the canonical binding motifs to major histocompatibility complex (MHC) class I H-2(b) or H-2(d) molecules, appropriate MHC class I tetramers made to include these peptides, and CD4 and CD8 T cells stimulated with 15-mer overlapping peptides covering the whole mPrP. Minimal to modest K(b) tetramer binding of mPrP amino acids (aa) 2 to 9, aa 152 to 160, and aa 232 to 241 was observed, but such tetramer-binding lymphocytes as well as CD4 and CD8 lymphocytes incubated with the full repertoire of mPrP peptides failed to synthesize intracellular gamma interferon (IFN-gamma) or tumor necrosis factor alpha (TNF-alpha) cytokines and were unable to lyse PrP(-/-) embryo fibroblasts or macrophages coated with (51)Cr-labeled mPrP peptide. These results suggest that the expression of PrP(sc) in the CNS is associated with release of chemokines and, as shown previously, cytokines that attract and retain PrP-activated T cells and, quite likely, bystander activated T cells that have migrated from the periphery into the CNS. However, these CD4 and CD8 T cells are defective in such an effector function(s) as IFN-gamma and TNF-alpha expression or release or lytic activity.

Mapping and structural dissection of human 20 S proteasome using proteomic approaches

The proteasome, a proteolytic complex present in all eukaryotic cells, is part of the ATP-dependent ubiquitin/proteasome pathway. It plays a critical role in the regulation of many physiological processes. The 20 S proteasome, the catalytic core of the 26 S proteasome, is made of four stacked rings of seven subunits each (alpha7beta7beta7alpha7). Here we studied the human 20 S proteasome using proteomics. This led to the establishment of a fine subunit reference map and to the identification of post-translational modifications. We found that the human 20 S proteasome, purified from erythrocytes, exhibited a high degree of structural heterogeneity, characterized by the presence of multiple isoforms for most of the alpha and beta subunits, including the catalytic ones, resulting in a total of at least 32 visible spots after Coomassie Blue staining. The different isoforms of a given subunit displayed shifted pI values, suggesting that they likely resulted from post-translational modifications. We then took advantage of the efficiency of complementary mass spectrometric approaches to investigate further these protein modifications at the structural level. In particular, we focused our efforts on the alpha7 subunit and characterized its N-acetylation and its phosphorylation site localized on Ser(250).

A new generation of Melan-A/MART-1 peptides that fulfill both increased immunogenicity and high resistance to biodegradation: implication for molecular anti-melanoma immunotherapy

Intense efforts of research are made for developing antitumor vaccines that stimulate T cell-mediated immunity. Tumor cells specifically express at their surfaces antigenic peptides presented by MHC class I and recognized by CTL. Tumor antigenic peptides hold promise for the development of novel cancer immunotherapies. However, peptide-based vaccines face two major limitations: the weak immunogenicity of tumor Ags and their low metabolic stability in biological fluids. These two hurdles, for which separate solutions exist, must, however, be solved simultaneously for developing improved vaccines. Unfortunately, attempts made to combine increased immunogenicity and stability of tumor Ags have failed until now. Here we report the successful design of synthetic derivatives of the human tumor Ag Melan-A/MART-1 that combine for the first time both higher immunogenicity and high peptidase resistance. A series of 36 nonnatural peptide derivatives was rationally designed on the basis of knowledge of the mechanism of degradation of Melan-A peptides in human serum and synthesized. Eight of them were efficiently protected against proteolysis and retained the antigenic properties of the parental peptide. Three of the eight analogs were twice as potent as the parental peptide in stimulating in vitro Melan-specific CTL responses in PBMC from normal donors. We isolated these CTL by tetramer-guided cell sorting and expanded them in vitro. The resulting CTL efficiently lysed tumor cells expressing Melan-A Ag. These Melan-A/MART-1 Ag derivatives should be considered as a new generation of potential immunogens in the development of molecular anti-melanoma vaccines.

Pleiotropic effects of post-translational modifications on the fate of viral glycopeptides as cytotoxic T cell epitopes

The fate of viral glycopeptides as cytotoxic T lymphocyte (CTL) epitopes is unclear. We have dissected the mechanisms of antigen presentation and CTL recognition of the peptide GP392-400 (WLVTNGSYL) from the lymphocytic choriomeningitis virus (LCMV) and compared them with those of the previously reported GP92-101 antigen (CSANNSHHYI). Both GP392-400 and GP92-101 bear a glycosylation motif, are naturally N-glycosylated in the mature viral glycoproteins, bind to major histocompatibility complex H-2D(b) molecules, and are immunogenic. However, post-translational modifications differentially affected GP92-101 and GP392-400. Upon N-glycosylation or de-N-glycosylation, a marked decrease in major histocompatibility complex binding was observed for GP392-400 but not for GP92-101. Further, under its N-glycosylated or de-N-glycosylated form, GP392-400 then lost its initial ability to generate a CTL response in mice, whereas GP92-101 was still immunogenic under the same conditions. The genetically encoded form of GP392-400, which on the basis of its immunogenicity could still be presented with H-2D(b) during the course of LCMV infection, does not in fact appear at the surface of LCMV-infected cells. Our results show that post-translational modifications of viral glycopeptides can have pleiotropic effects on their presentation to and recognition by CTL that contribute to either creation of neo-epitopes or destruction of potential epitopes.

Common antiviral cytotoxic t-lymphocyte epitope for diverse arenaviruses

Members of the Arenaviridae family have been isolated from mammalian hosts in disparate geographic locations, leading to their grouping as Old World types (i.e., lymphocytic choriomeningitis virus [LCMV], Lassa fever virus [LFV], Mopeia virus, and Mobala virus) and New World types (i.e., Junin, Machupo, Tacaribe, and Sabia viruses) (C. J. Peters, M. J. Buchmeier, P. E. Rollin, and T. G. Ksiazek, p. 1521-1551, in B. N. Fields, D. M. Knipe, and P. M. Howley [ed.], Fields virology, 3rd ed., 1996; P. J. Southern, p. 1505-1519, in B. N. Fields, D. M. Knipe, and P. M. Howley [ed.], Fields virology, 3rd ed., 1996). Several types in both groups-LFV, Junin, Machupo, and Sabia viruses-cause severe and often lethal human diseases. By sequence comparison, we noted that eight Old World and New World arenaviruses share several amino acids with the nucleoprotein (NP) that consists of amino acids (aa) 118 to 126 (NP 118-126) (RPQASGVYM) of LCMV that comprise the immunodominant cytotoxic T-lymphocyte (CTL) epitope for H-2(d) mice (32). This L(d)-restricted epitope constituted >97% of the total bulk CTLs produced in the specific antiviral or clonal responses of H-2(d) BALB mice. NP 118-126 of the Old World arenaviruses LFV, Mopeia virus, and LCMV and the New World arenavirus Sabia virus bound at high affinity to L(d). The primary H-2(d) CTL anti-LCMV response as well as that of a CTL clone responsive to LCMV NP 118-126 recognized target cells coated with NP 118-126 peptides derived from LCMV, LFV, and Mopeia virus but not Sabia virus, indicating that a common functional NP epitope exists among Old World arenaviruses. Use of site-specific amino acid exchanges in the NP CTL epitope among these arenaviruses identified amino acids involved in major histocompatibility complex binding and CTL recognition.

Structural and functional identification of major histocompatibility complex class I-restricted self-peptides as naturally occurring molecular mimics of viral antigens. Possible role in CD8+ T cell-mediated, virus-induced autoimmune disease

Structural similarity (molecular mimicry) between viral epitopes and self-peptides can lead to the induction of autoaggressive CD4(+) as well as CD8(+) T cell responses. Based on the flexibility of T cell receptor/antigen/major histocompatibility complex recognition, it has been proposed that a self-peptide could replace a viral epitope for T cell recognition and therefore participate in pathophysiological processes in which T cells are involved. To address this issue, we used, as a molecular model of viral antigen, the H-2D(b)-restricted immunodominant epitope nucleoprotein (NP)-(396-404) (FQPQNGQFI) of lymphocytic choriomeningitis virus (LCMV). We identified peptide sequences from murine self-proteins that share structural and functional homology with LCMV NP-(396-404) and that bound to H-2D(b) with high affinity. One of these self-peptides, derived from tumor necrosis factor receptor I (FGPSNWHFM, amino acids 302-310), maintained LCMV-specific CD8(+) T cells in an active state as observed both in vitro in cytotoxic assays and in vivo in a model of virus-induced autoimmune diabetes, the rat insulin promoter-LCMV NP transgenic mouse. The natural occurrence and molecular concentration at the surface of H-2(b) spleen cells of tumor necrosis factor receptor I-(302-310) were determined by on-line micro-high pressure liquid chromatography/mass spectrometry and supported its biological relevance.

Cutting edge: CTLs rapidly capture membrane fragments from target cells in a TCR signaling-dependent manner

Upon encounter of a CTL with a target cell carrying foreign Ags, the TCR internalizes with its ligand, the peptide-MHC class I complex. However, it is unclear how this can happen mechanistically because MHC molecules are anchored to the target cell's surface via a transmembrane domain. By using antigenic peptides and lipids that were fluorescently labeled, we found that CTLs promptly capture target cell membranes together with the antigenic peptide as well as various other surface proteins. This efficient and specific capture process requires sustained TCR signaling. Our observations indicate that this process allows efficient acquisition of the Ag by CTL, which may in turn regulate lymphocyte activation or elimination.

Molecular and functional dissection of the H-2Db-restricted subdominant cytotoxic T-cell response to lymphocytic choriomeningitis virus

Infection of H-2b mice with lymphocytic choriomeningitis virus (LCMV) generates an H-2Db-restricted cytotoxic T-lymphocyte (CTL) response whose subdominant component is directed against the GP92-101 (CSANNSHHYI) epitope. The aim of this study was to identify the functional parameters accounting for this subdominance. We found that the two naturally occurring (genetically encoded and posttranslationally modified) forms of LCMV GP92-101 were immunogenic, did not act as T-cell antagonists, and bound efficiently to but were unable to form stable complexes with H-2Db, a crucial factor for immunodominance. Thus, the H-2Db-restricted subdominant CTL response to LCMV resulted not from altered T-cell activation but from impaired major histocompatibility complex presentation properties.

Processing of some antigens by the standard proteasome but not by the immunoproteasome results in poor presentation by dendritic cells

By stimulating human lymphocytes with an autologous renal carcinoma, we obtained CTL recognizing an antigen derived from a novel, ubiquitous protein. The CTL failed to lyse autologous EBV-transformed B cells, even though the latter express the protein. This is due to the presence in these cells of immunoproteasomes, which, unlike standard proteasomes, cannot produce the antigenic peptide. We show that dendritic cells also carry immunoproteasomes and fail to present this antigen. This may explain why the relevant CTL escape thymic deletion and are not regularly activated in the periphery. Lack of cleavage by the immunoproteasome was also observed for melanoma differentiation antigen Melan-A26-35/HLA-A2, currently used for antitumoral vaccination. For immunization with such antigens, proteins should be less suitable than peptides, which do not require proteasome digestion in dendritic cells.

Genetically encoded and post-translationally modified forms of a major histocompatibility complex class I-restricted antigen bearing a glycosylation motif are independently processed and co-presented to cytotoxic T lymphocytes

The mechanisms by which antigenic peptides bearing a glycosylation site may be processed from viral glycoproteins, post-translationally modified, and presented by major histocompatibility complex class I molecules remain poorly understood. With the aim of exploring these processes, we have dissected the structural and functional properties of the MHC-restricted peptide GP92-101 (CSANNSHHYI) generated from the lymphocytic choriomeningitis virus (LCMV) GP1 glycoprotein. LCMV GP92-101 bears a glycosylation motif -NXS- that is naturally N-glycosylated in the mature viral glycoprotein, displays high affinity for H-2D(b) molecules, and elicits a CD8(+) cytotoxic T lymphocyte response. By analyzing the functional properties of natural and synthetic peptides and by identifying the viral sequence(s) from the pool of naturally occurring peptides, we demonstrated that multiple forms of LCMV GP92-101 were generated from the viral glycoprotein and co-presented at the surface of LCMV-infected cells. They corresponded to non-glycosylated and post-translationally modified sequences (conversion of Asn-95 to Asp or alteration of Cys-92). The glycosylated form, despite its potential immunogenicity, was not detected. These data illustrate that distinct, non-mutually exclusive antigen presentation pathways may occur simultaneously within a cell to generate structurally and functionally different peptides from a single genetically encoded sequence, thus contributing to increasing the diversity of the T cell repertoire.

Single H2Kb, H2Db and double H2KbDb knockout mice: peripheral CD8+ T cell repertoire and anti-lymphocytic choriomeningitis virus cytolytic responses

Single H2Kb, H2Db and double H2KbDb homozygous knockout (KO) mice were generated and their peripheral CD8+ T cell repertoires compared to that of C57BL/6 (B6) mice. Limited (10-20%, H2Db), substantial (30-50%, H2Kb) and profound (90%, H2KbDb) reduction of peripheral CD8+ T cells was observed in KO mice, without Vbeta diversity alteration. Classical class Ia molecules therefore ensure most but not all of the peripheral CD8+ T cell repertoire education. As expected, H2Kb but also H2Db KO mice developed choriomeningitis following intracranial infection by lymphocytic choriomeningitis virus with the same kinetics, lethality and CD8+ cell implication as wild-type B6 mice. By contrast, H2KbDb (class Ia-Ib+) KO mice survived. Choriomeningitis of H2Db KO mice was linked to the development of a subdominant (in normal B6 mice) H2Kb-restricted cytotoxic T lymphocyte response. Mice expressing a restricted set of histocompatibility class I molecules should represent useful tools to evaluate the immunological potentials of individual MHC class I molecules.

Use of a high-affinity peptide that aborts MHC-restricted cytotoxic T lymphocyte activity against multiple viruses in vitro and virus-induced immunopathologic disease in vivo

Binding of a specific peptide(s) from a viral protein to major histocompatibility complex (MHC) class I molecules is a critical step in the activation of CD8(+) cytotoxic T lymphocytes (CTLs). Once activated, CTLs can cause lethal disease in an infected host, for example, by killing virus-containing ependymal and ventricular cells in the central nervous system or viral protein-expressing beta cells in the pancreatic islets of Langerhans. Here we describe the usage of a designed (not natural) high-affinity peptide to compete with viral peptide(s)-MHC binding. This peptide blocks virus-induced CTL-mediated disease both in the CNS and in the pancreatic islets in vivo. Further, the blocking peptide aborts MHC-restricted killing of target cells by CTLs generated to three separate viruses: lymphocytic choriomeningitis virus, influenza virus, and simian virus 40.

A structure-based approach to designing non-natural peptides that can activate anti-melanoma cytotoxic T cells

Tumor antigens presented by major histocompatibility complex (MHC) class I molecules and recognized by CD8(+) cytotoxic T lymphocytes (CTLs) may generate an efficient antitumor immune response after appropriate immunization. Antigenic peptides can be used in vivo to induce antitumor or antiviral immunity. The efficiency of naked peptides may be greatly limited by their degradation in the biological fluids. We present a rational, structure-based approach to design structurally modified, peptidase-resistant and biologically active analogues of human tumor antigen MAGE-1.A1. This approach is based on our understanding of the peptide interaction with the MHC and the T cell receptor and its precise degradation pathway. Knowledge of these mechanisms led to the design of a non-natural, minimally modified analogue of MAGE-1.A1, [Aib2, NMe-Ser8]MAGE-1.A1, which was highly peptidase-resistant and bound to MHC and activated MAGE-1.A1-specific anti-melanoma CTLs. Thus, we showed that it is possible to structurally modify peptide epitopes to obtain analogues that are still specifically recognized by CTLs. Such analogues may represent interesting leads for antitumor synthetic vaccines.

Single H2Kb, H2Db and double H2KbDb knockout mice: peripheral CD8+ T cell repertoire and anti-lymphocytic choriomeningitis virus cytolytic responses

Single H2Kb, H2Db and double H2KbDb homozygous knockout (KO) mice were generated and their peripheral CD8+ T cell repertoires compared to that of C57BL/6 (B6) mice. Limited (10-20%, H2Db), substantial (30-50%, H2Kb) and profound (90%, H2KbDb) reduction of peripheral CD8+ T cells was observed in KO mice, without Vbeta diversity alteration. Classical class Ia molecules therefore ensure most but not all of the peripheral CD8+ T cell repertoire education. As expected, H2Kb but also H2Db KO mice developed choriomeningitis following intracranial infection by lymphocytic choriomeningitis virus with the same kinetics, lethality and CD8+ cell implication as wild-type B6 mice. By contrast, H2KbDb (class Ia-Ib+) KO mice survived. Choriomeningitis of H2Db KO mice was linked to the development of a subdominant (in normal B6 mice) H2Kb-restricted cytotoxic T lymphocyte response. Mice expressing a restricted set of histocompatibility class I molecules should represent useful tools to evaluate the immunological potentials of individual MHC class I molecules.

In vivo treatment with a MHC class I-restricted blocking peptide can prevent virus-induced autoimmune diabetes

We tested the in vivo potential of a MHC class I-restricted blocking peptide to sufficiently lower an anti-viral CTL response for preventing virus-induced CTL-mediated autoimmune diabetes (insulin-dependent diabetes mellitus (IDDM)) in vivo without affecting systemic viral clearance. By designing and screening several peptides with high binding affinities to MHC class I H-2Db for best efficiency in blocking killing of target cells by lymphocytic choriomeningitis virus (LCMV) and other viral CTL, we identified the peptide for this study. In vitro, it selectively lowered CTL killing restricted to the Db allele, which correlated directly with the affinity of the respective epitopes. Expression of the blocking peptide in the target cell lowered recognition of all Db-restricted LCMV epitopes. In addition, in vitro expansion of LCMV memory CTL was prevented, resulting in decreased IFN-gamma secretion. In vivo, a 2-wk treatment with this peptide lowered the LCMV Db-restricted CTL response by over threefold without affecting viral clearance. However, the CTL reduction by the peptide treatment was sufficient to prevent LCMV-induced IDDM in rat insulin promoter-LCMV-glycoprotein transgenic mice. Following LCMV infection, these mice develop IDDM, which depends on Db-restricted anti-self (viral) CTL. Precursor numbers of splenic LCMV-CTL in peptide-treated mice were reduced, but their cytokine profile was not altered, indicating that the peptide did not induce regulatory cells. Further, non-LCMV-CTL recognizing the blocking peptide secreted IFN-gamma and did not protect from IDDM. This study demonstrates that in vivo treatment with a MHC class I blocking peptide can prevent autoimmune disease by directly affecting expansion of autoreactive CTL.

In Vivo Treatment with a MHC Class I-Restricted Blocking Peptide Can Prevent Virus-Induced Autoimmune Diabetes

We tested the in vivo potential of a MHC class I-restricted blocking peptide to sufficiently lower an anti-viral CTL response for preventing virus-induced CTL-mediated autoimmune diabetes (insulin-dependent diabetes mellitus (IDDM)) in vivo without affecting systemic viral clearance. By designing and screening several peptides with high binding affinities to MHC class I H-2Db for best efficiency in blocking killing of target cells by lymphocytic choriomeningitis virus (LCMV) and other viral CTL, we identified the peptide for this study. In vitro, it selectively lowered CTL killing restricted to the Db allele, which correlated directly with the affinity of the respective epitopes. Expression of the blocking peptide in the target cell lowered recognition of all Db-restricted LCMV epitopes. In addition, in vitro expansion of LCMV memory CTL was prevented, resulting in decreased IFN-γ secretion. In vivo, a 2-wk treatment with this peptide lowered the LCMV Db-restricted CTL response by over threefold without affecting viral clearance. However, the CTL reduction by the peptide treatment was sufficient to prevent LCMV-induced IDDM in rat insulin promoter-LCMV-glycoprotein transgenic mice. Following LCMV infection, these mice develop IDDM, which depends on Db-restricted anti-self (viral) CTL. Precursor numbers of splenic LCMV-CTL in peptide-treated mice were reduced, but their cytokine profile was not altered, indicating that the peptide did not induce regulatory cells. Further, non-LCMV-CTL recognizing the blocking peptide secreted IFN-γ and did not protect from IDDM. This study demonstrates that in vivo treatment with a MHC class I blocking peptide can prevent autoimmune disease by directly affecting expansion of autoreactive CTL.

Analysis of the degradation mechanisms of MHC class I-presented tumor antigenic peptides by high performance liquid chromatography/electrospray ionization mass spectrometry: application to the design of peptidase-resistant analogs

Peptide vaccines based on the use of MHC class I restricted epitopes are currently assayed for anti-tumor and anti-viral immunotherapy. With the aim of designing minimally modified, peptidase-resistant analogs, we developed a rational approach based on a detailed understanding of the degradation mechanism of peptides in serum. Degradation of murine tumor antigen P198 and human tumor antigen MAGE-3.A1 was followed by on line high performance liquid chromatography/electrospray ionization mass spectrometry (HPLC/ESI-MS). This method provided high precision and sensitivity for rapid and direct analysis of degradation fragments in a complex mixture and, very importantly, precise identification of transient degradation fragments present at low concentrations. The design of structurally modified analogs, and the analysis of their degradation by on-line HPLC/ESI-MS, allowed us to to demonstrate the efficiency of local modifications in the protection of a given peptide bond towards a specific peptidase activity.

The signal sequence of lymphocytic choriomeningitis virus contains an immunodominant cytotoxic T cell epitope that is restricted by both H-2D(b) and H-2K(b) molecules

Infection of H-2b mice with lymphocytic choriomeningitis virus (LCMV) generates three well-characterized H-2D(b)-restricted immunodominant epitopes delineated in the NP, GP1, and GP2 proteins. Here we report that the H-2D(b)-restricted GP1 epitope GP33-41/43 (KAVYNFATC/GI) located in the signal sequence of LCMV is also the immunodominant epitope recognized by CTL at the surface of the same infected cells in the context of H-2K(b) restriction. The GP1 epitope bound to H-2D(b) and H-2K(b) molecules with comparable affinities. The respective binding processes involved different sets of peptide anchoring residues and required dramatically different conformations of the peptide backbone as well as rearrangement of residue side chains. The 10-mer peptide GP34-43 (AVYNFATCGI) was the optimal H-2K(b)-binding sequence and the 8-mer peptide GP34-41 (AVYNFATC) the minimal sequence for optimal H-2K(b)-restricted CTL recognition. Comparison of lytic activities of primary splenic anti-LCMV CTL from C57BL/6 (D(b+)/K(b+)), B10A.[5R] (D(b-)/K(b+)), and B10A.[2R] (D(b+)/K(b-)) mice against LCMV-infected or peptide-coated target cells expressing either one or the two MHC alleles revealed that the H-2K(b)-restricted component of the anti-GP1 CTL response was mounted independently of but as efficiently as its H-2D(b) counterpart. Analysis of the immune response against a GP1 variant that escapes CTL recognition showed that the GP1 epitope: (i) was likely the only immunodominant LCMV epitope in the context of H-2K(b), and (ii) could efficiently evade H-2D(b) and H-2K(b)-restricted CTL mediated lysis.

Measles virus recognizes its receptor, CD46, via two distinct binding domains within SCR1-2

Measles virus (MV) enters cells by attachment of the viral hemagglutinin to the major cell surface receptor CD46 (membrane cofactor protein). CD46 is a transmembrane glycoprotein whose ectodomain is largely composed of four conserved modules called short consensus repeats (SCRs). We have previously shown that MV interacts with SCR1 and SCR2 of CD46. (M. Manchester et al. (1995) Proc. Natl. Acad. Sci. USA 92, 2303-2307) Here we report mapping the MV interaction with SCR1 and SCR2 of CD46 using a combination of peptide inhibition and mutagenesis studies. By testing a series of overlapping peptides corresponding to the 126 amino acid SCR1-2 region for inhibition of MV infection, two domains were identified that interacted with MV. One domain was found within SCR1 (amino acids 37-56) and another within SCR2 (amino acids 85-104). These results were confirmed by constructing chimeras with complementary regions from structurally similar, but non-MV-binding, SCRs of decay accelerating factor (DAF; CD55). These results indicate that MV contacts at least two distinct sites within SCR1-2.

Antiviral pressure exerted by HIV-1-specific cytotoxic T lymphocytes (CTLs) during primary infection demonstrated by rapid selection of CTL escape virus

The HIV-1-specific cytotoxic T lymphocyte (CTL) response is temporally associated with the decline in viremia during primary HIV-1 infection, but definitive evidence that it is of importance in virus containment has been lacking. Here we show that in a patient whose early CTL response was focused on a highly immunodominant epitope in gp 160, there was rapid elimination of the transmitted virus strain and selection for a virus population bearing amino acid changes at a single residue within this epitope, which conferred escape from recognition by epitope-specific CTL. The magnitude (> 100-fold), kinetics (30-72 days from onset of symptoms) and genetic pathways of virus escape from CTL pressure were comparable to virus escape from antiretroviral therapy, indicating the biological significance of the CTL response in vivo. One aim of HIV-1 vaccines should thus be to elicit strong CTL responses against multiple codominant viral epitopes.

Binding of viral antigens to major histocompatibility complex class I H-2Db molecules is controlled by dominant negative elements at peptide non-anchor residues. Implications for peptide selection and presentation

Binding of viral antigens to major histocompatibility complex (MHC) class I molecules is a critical step in the activation process of CD8(+) cytotoxic T lymphocytes. In this study, we investigated the impact of structural factors at non-anchor residues in peptide-MHC interaction using the model of lymphocytic choriomeningitis virus (LCMV) infection of its natural host, the mouse. Altering viral genes by making reassortants, recombinants, and using synthetic peptides, CD8(+) cytotoxic T lymphocytes were shown to recognize only three H-2Db-restricted epitopes, GP amino acids 33-41/43, GP 276-286, and NP 396-404. However, LCMV NP and GP proteins contain 31 other peptides bearing the H-2Db motif. These 34 LCMV peptides and 11 other known H2-Db-restricted peptides were synthesized and examined for MHC binding properties. Despite the presence of the H-2Db binding motif, the majority of LCMV peptides showed weak or no affinity for H-2Db. We observed that dominant negative structural elements located at non-anchor positions played a crucial role in peptide-MHC interaction. By comparative sequence analysis of strong versus non-binders and using molecular modeling, we delineated these negative elements and evaluated their impact on peptide-MHC interaction. Our findings were validated by showing that a single mutation of a favorable non-anchor residue in the sequence of known viral epitopes for a negative element resulted in dramatic reduction of antigen presentation properties, while conversely, substitution of one negative for a positive element in the sequence of a non-binder conferred to the peptide an ability to now bind to MHC molecules.

Characterization of an epitope of the human cytomegalovirus protein IE1 recognized by a CD4+ T cell clone

CD4+ T cells specific for human cytomegalovirus (HCMV) IE1 protein are potential effectors of the control of HCMV infection through cytokine production. Better knowledge of major histocompatibility complex (MHC)-peptide-T cell receptor (TcR) interactions in the CD4+ T cell response should result in a better design of immunizing peptides and is a prerequisite for the development of vaccines or anti-cytomegalovirus therapy. In this study, the recombinant protein comprising residues 86-491 encoded by exon 4 of IE1 (GST-e4) was cleaved by enzymatic digestion and analyzed by high pressure liquid chromatography-mass spectroscopy (HPLC-MS). We identified the 14-residue epitope 162-DKREMWMACIKELH-175 recognized by an HLA-DR8-restricted clone, BeA3. Synthetic elongated, truncated and di-Ala-substituted peptides of the 18-mer IE1 158-IVPEDKREMWMACIKELH-175 sequence were used to analyze the amino acid motifs involved in binding to HLA-DR8 and recognition by the BeA3 clone. Substitutions which abolished (MW --> AA), or decreased (RE --> AA and MA --> AA) T cell clone proliferation, cytokine production and cytotoxicity were identified. Loss of T cell function induced by the MW --> AA substitution was associated with poor HLA-DR8 binding. Decreased T cell function (RE --> AA and MA --> AA) was associated with good HLA-DR8 binding, which suggested that these motifs were involved in TcR binding. Other substitutions induced potentiation of the T cell clone response: the IV --> AA substitution induced stronger proliferation, but equivalent cytokine production, when compared with the reference peptide IE1 (158-175). CI --> AA substitution induced strong potentiation of HLA-DR8 binding, proliferation and interferon-gamma and interleukin-4 production, possibly due to the removal of negative effects of Cys, Ile, or both side chains. Cytotoxicity was not improved by any substitution. Our results show modulation of the CD4+ T cell response according to the peptide residues involved in the HLA-DR8-peptide-TcR interaction.

Discriminated selection among viral peptides with the appropriate anchor residues: implications for the size of the cytotoxic T-lymphocyte repertoire and control of viral infection

Structural characterization of peptides restricted by major histocompatibility complex (MHC) class I molecules has identified residues critical for MHC class I binding and for T-cell receptor recognition. For example, optimal peptides fitting into the murine MHC class I Db groove are 9 to 11 amino acids long and require as MHC anchor residues an Asn (N) at position 5 and also either a hydrophobic residue, a Met (M) or a Cys (C), at the carboxy terminus. The three known Db-restricted peptides of lymphocytic choriomeningitis virus (LCMV) are glycoproteins GP1 (amino acids [aa] 33 KAVYNFATC), GP2 (aa 276 SGVENPGGYCL), and nucleoprotein NP (aa 396 FQPQNGQFI). In addition to these two GP and one NP peptides, computer search revealed 11 other GP peptide sequences and 20 additional NP sequences that contained the Db binding motif. By Db competitive binding analysis, only two of these 11 GP peptides and 1 of these 20 NP peptides bound to the MHC Db molecule with an affinity equivalent to the measured affinities for the three known GP1, GP2, and NP cytotoxic T-lymphocyte (CTL) epitopes. No CTL specific for these three peptides were generated when H-2b mice were inoculated with viral variants in which either the two known GP epitopes (GP1 and GP2; termed GPV) or the GPV and NP epitopes (termed GPV + NPV) were mutated. However, a novel CD8+ anti-LCMV CTL response ordinarily not seen in H-2b mice inoculated with wild-type virus was noted when such mice were inoculated with the GPV + NPV-mutated variant. This result indicates that (i) despite large numbers of peptides containing the appropriate anchor residues within a viral protein, only a restricted number induce CTL, thereby maintaining a limited CTL repertoire, (ii) despite the limited repertoire, the immune system retains the flexibility to generate an immune response(s) to a previously silent protein(s), suggesting a hierarchial control mechanism, and (iii) identification of a primary amino acid sequence is not sufficient, per se, to predict CTL epitopes, and peptide conformations are likely more complex than indicated by simple linear sequence comparisons.

Relative implication of peptide residues in binding to major histocompatibility complex class I H-2Db: application to the design of high-affinity, allele-specific peptides

The H-2Db peptide sequence SMIENLEYM was manipulated (N- and C-terminus truncation and alanine substitution) to determine the role of structural elements (peptide ends and residue side chains) in binding to H-2Db. We found that good binding affinity could be obtained by compensating the minimal binding condition for one element by the optimal condition of the other element. In particular, we showed, that although the minimal binding sequence could be as short as a heptamer (deletion of positions 1 and 2), it needed the presence of optimal amino acids at other positions (IENLEYM). Conversely, the structurally minimal peptide would accept multiple alanine residues, but required the optimal nonameric length (AAAENAEAA). Positions 1, 2, 3, 4, 5, 7 and 9, but not 6 and 8, were involved in the H-2Db-peptide interaction. Most residues interacted directly with the MHC molecule via their main chain (amino and carboxyl) atoms (positions 1 and 2), their side chains (positions 3 and 5), or both (position 9). Positions 4 and 7 were found to play an indirect role, probably by influencing the secondary structure. At the C-terminus, the presence of a residue at position 9, but not the hydrophobic nature of its side chain, was mandatory for binding. At the N-terminus, the role of the residue at position 1 was of either minor or critical importance depending on the presence or not of a strong auxiliary anchor at position 3. The indirect contribution of residue side chains at positions 4 and 7 reflected the importance of dynamic components in the binding process. Based on these results, we designed a series of high-affinity, H-2Db selective peptides derived from the sequence X1 AIX4NAEAL, where X1 = Y or K and X4 = E or K. After radioiodination or fluorescent (FITC) labelling, these peptides bound strongly and specifically to the surface of viable H-2Db-expressing cells. Rationally designed synthetic peptides, either alone or in a stable complex with MHC, might be of value for controlling CTL activity.

CTL escape viral variants. I. Generation and molecular characterization

Cytotoxic T lymphocytes (CTL) play a pivotal role in preventing persistent viral infections and aborting acute infections. H-2Db-restricted CTL optimally recognize a specific peptide of 9 to 11 amino acids (aa) derived from a viral protein and held in place (restricted) by a MHC class I glycoprotein on the surfaces of infected cells. Only three peptide sequences with the appropriate Db motif from lymphocytic choriomeningitis virus Armstrong strain (LCMV) are known to be presented to CTL by H-2Db molecules; they are from the glycoproteins (GP), residues 33-41 KAVYNFATC (GP1) and 276-286 SGVENPGGYCL (GP2), and the nucleoprotein (NP), 396-404 FQPQNGQFI. Incubation of virally infected H-2b cells with CTL clones that recognize only GP1, GP2, or NP leads to the selection of viral variants which upon infecting cells bearing H-2b molecules, escape recognition by CTL of the appropriate specificity. Nucleic acid sequencing showed a single mutation in GP1 (aa 38 F-->L), GP2 (aa 282 G-->D), or NP (aa 403 F-->L) in the variant viruses. When wild-type (wt) LCMV peptides and the three variant peptides (GP1, GP2, NP) were synthesized and subjected to a competitive inhibition binding assay, no differences in binding affinity for H-2Db were found between the wt and variant peptides. Uninfected cells coated with the wt peptide were recognized and lysed by the appropriate CTL clone or by in vivo-primed bulk CTL, but similar targets coated with the GP1, GP2, or NP variant peptides were not. This result, coupled with computer graphic analysis of these variant peptides with the recently solved three-dimensional structure for the Db MHC class I molecule, placed the side chain of the mutated residues on the outer surface of the MHC-peptide complex and accessible to the T cell receptor. Ala substitution at GP residue 38 or 282 or at NP 403 also abrogated CTL recognition and lysis. Inoculation of any one of the mutated viral variants into mice produced an effective CTL response to the other two nonmutated GP or NP peptides, suggesting that production of biologically relevant CTL escape virus variants in vivo requires selection of mutations in more than one and likely all the CTL epitopes, a low probability event.

Optimal lymphocytic choriomeningitis virus sequences restricted by H-2Db major histocompatibility complex class I molecules and presented to cytotoxic T lymphocytes

Infection with lymphocytic choriomeningitis virus induces the generation of CD8+ cytotoxic T lymphocytes (CTL). In the H-2b mouse, this cellular immune response is directed against three viral structural epitopes (GP1, GP2, and NP) presented by the major histocompatibility complex (MHC) class I H-2Db molecules. This study was undertaken to delineate which sequence of each of these three epitopes is optimal for MHC binding and CTL recognition. The first step was to synthesize the relevant peptides truncated at the N or C terminus and flanking the crucial H-2Db-anchoring Asn residue in position 5. These peptides were then tested (i) for their binding properties in two H-2Db-specific assays with viable cells (upregulation of H-2Db expression on the surface of RMA-S cells and competition against the Db-restricted peptide 125I-gp276-286 on T2-Db cells) and (ii) for their abilities to sensitize H-2b target cells for CTL lysis in vitro. For optimal antigenic presentation, all three epitopes required the MHC-anchoring Asn residue at position 5 of their sequences. The results clearly and unambiguously delineated optimal lengths for two of the epitopes and two options for the third. NP appeared as a conventional 9-amino-acid (aa)-long peptide, np396-404 (FQPQNGQFI). GP2 was defined as a longer peptide (11 aa), gp276-286 (SGVENPGGYCL). Characterization of the GP1 epitope was more complex: the 9-aa-long peptide gp33-41 (KAVYNFATC) and the carboxyl-extended 11-aa-long peptide gp33-43 (KAVYN FATCGI) were both established as possible optimal sequences depending on the cell line used to test binding and lysis.

Localization of two cytotoxic T lymphocyte epitopes and three anchoring residues on a single nonameric peptide that binds to H-2Ld and is recognized by cytotoxic T lymphocytes against mouse tumor P815

Mouse mastocytoma P815 expresses tumor antigens P815A and P815B encoded by a single gene called P1A and carried by a single peptide named P1A 35-43 (NH2-Leu-Pro-Tyr-Leu-Gly-Trp-Leu-Val-Phe-COOH). P1A 35-43 is presented to anti-P815A and anti-P815B cytotoxic T lymphocytes (CTL) by major histocompatibility complex (MHC) H-2Ld molecules. In order to determine the individual role played by each amino acid residue of P1A 35-43 in binding to H-2Ld and in recognition by anti-A and anti-B T cell receptors (TcR), a series of P1A35-43 peptides substituted by alanine at single positions was synthesized and tested for binding to H-2Ld and for CTL recognition. Binding to H-2Ld was estimated by measuring the ability of the peptide to up-regulate cell surface expression of H-2Ld. We found that three residues were important for interaction of P1A 35-43 with H-2Ld. Two of them, Pro at position 2 and Phe at position 9 were consistent with the described H-2Ld binding motif. A third residue, Trp at position 6, was also required for effective MHC binding of the tumor antigen. CTL sensitization assays showed that alanine substitution at position 7 (Leu) or at position 8 (Val) dramatically affected peptide recognition by anti-A CTL while positions 3 (Tyr) and 4 (Leu) were critical for recognition by anti-B CTL. We conclude that Pro2, Trp6 and Phe9 constitute the anchor residues of P1A 35-43 to H-2Ld, whereas the dipeptidyl sequences Tyr3-Leu4 and Leu7-Val8 form the core epitopes recognized by the TcR of anti-P815B and anti-P815A CTL, respectively.

Thymic selection and adaptability of cytotoxic T lymphocyte responses in transgenic mice expressing a viral protein in the thymus

Upon primary challenge with lymphocytic choriomeningitis virus (LCMV), H-2d (BALB/cByJ) mice mount a cytotoxic T lymphocyte (CTL) response to a single immunodominant domain of the viral nucleoprotein (NP) but no detectable response to the viral glycoprotein (GP). To manipulate this CTL response, the viral NP gene was expressed in the thymus and peripheral T lymphocytes using the murine Thy1.2 promoter. As a result, such Thy1.2-NP (H-2d) transgenic (tg) mice deleted their high-affinity anti-LCMV-NP CTL, but generated equal numbers of lower-affinity NP CTL. Further, they made an alternative anti-LCMV-GP CTL response that is not normally found in non-tg mice indicating a hierarchial control of the CTL response. Unlike the H-2d mice, H-2b (C57Bl/6J) mice normally mount a CTL response to both LCMV-GP and -NP. When the LCMV-NP was expressed using the Thy1.2 promoter in these H-2b mice, the LCMV-NP-specific CTL response was completely aborted and no CTL to new, alternative viral epitopes were generated. Dilutions of H-2b or H-2d NP peptides indicated that 3-4 logs less H-2b NP peptide was required to sensitize syngeneic target cells for CTL-specific lysis, suggesting that the differing affinities of H-2b and H-2d major histocompatibility complex molecules for their peptides likely account for the total removal of NP CTL in the H-2b mice but only partial removal in H-2d mice made to express thymic NP. Thymic grafting experiments done with thymi from newborn Thy1.2-NP tg mice show that selection processes studied in this model are of central (thymic) origin and are not caused by Thy1.2-positive LCMV-NP-expressing T lymphocytes in the periphery.

Antigen presentation and cytotoxic T lymphocyte killing studied in individual, living cells

Interactions between individual, living fibroblasts and cytotoxic T lymphocyte (CTL) clones were analyzed by using video-enhanced differential interference contrast and fluorescence microscopy in a multimode configuration. Fibroblasts expressing known major histocompatibility complex I alleles (MC57: H-2b; Balb: H-2d) were sensitized for killing by incubating or microinjecting them with peptide fragments of lymphocytic choriomeningitis virus. Previous determination of the CTL clones' specificity for these peptides and MHC-I alleles enabled us to study CTL killing of fibroblasts, and nonlethal CTL interaction with targets due to "mismatches" of the CTL, target, and/or peptide. During viral peptide-specific MHC-restricted CTL killing, distinct morphological alterations were observed (CTL shape changes, movements of granules in CTL cytoplasm, and target cell contraction and blebbing). When no killing occurred, CTL engaged in prolonged, nonrandom movement on the target cells. Alloreactive and virus-specific CTL displayed the same morphology during killing. To study antigen presentation further within individual, living cells, a LCMV glycoprotein peptide (aa 272-286, LSDSSGVENPGGYCL) was covalently labeled with tetramethylrhodamine. In 51Cr release assays, the labeled peptide specifically induced potent CTL killing, but neither labeled nor unlabeled peptide proved toxic for unsensitized targets. Microinjection of the labeled peptide into the cytoplasm of fibroblast cells led to CTL killing of those cells, yet nearby uninjected cells contacted by CTL were not killed, indicating that killing was due to presentation of microinjected peptide rather than binding of extracellular peptide to cell surface MHC. Peptide-injected target cells were killed only when combined with CTL specific for the peptide and for the MHC allele of the injected cell.