Glycan-regulated antigen processing of a protein in the endoplasmic reticulum can uncover cryptic cytotoxic T cell epitopes

Effects of Alkalinity Exposure on Antioxidant Status, Metabolic Function, and Immune Response in the Hepatopancreas of Macrobrachium nipponense

The oriental river prawn Macrobrachium nipponense is an important freshwater economic species in China, producing huge economic benefits. However, M. nipponense shows lower alkali tolerance than fish species, thus genetic selection is urgently needed in order to improve alkali tolerance in this species. In the present study, the effects of alkalinity exposure on the hepatopancreas of M. nipponense were measured under the alkali concentrations of 0 (control), 4, 8, and 12 mmol/L with the exposure time of 96 h through histological observations, measurement of antioxidant enzymes, metabolic profiling analysis, and transcriptome profiling analysis. The present study identified that the low concentration of alkali treatment (<4 mmol/L) did not result in morphological changes in the hepatopancreas and activity changes in antioxidant enzymes, while high-alkali treatment (>8 mmol/L) damaged the normal structures of the lumen and vacuoles and significantly stimulated the levels of superoxide dismutase, catalase, and total antioxidant capacity, indicating these antioxidant enzymes play essential roles in the protection of the body from the damage caused by the alkali treatment. Metabolic profiling analysis revealed that the main enriched metabolic pathways of differentially expressed metabolites in the present study were consistent with the metabolic pathways caused by environmental stress in plants and other aquatic animals. Transcriptome profiling analysis revealed that the alkali concentration of <8 mmol/L did not lead to significant changes in gene expression. The main enriched metabolic pathways were selected from the comparison between 0 mmol/L vs. 12 mmol/L, and some significantly up-regulated genes were selected from these metabolic pathways, predicting these selected metabolic pathways and genes are involved in the adaptation to alkali treatment in M. nipponense. The expressions of Ras-like GTP-binding protein, Doublesex and mab-3 related transcription factor 1a, and Hypothetical protein JAY84 are sensitive to changes in alkali concentrations, suggesting these three genes participated in the process of alkali adaptation in M. nipponense. The present study identified the effects of alkalinity exposure on the hepatopancreas of M. nipponense, including the changes in antioxidant status and the expressions of metabolites and genes, contributing to further studies of alkali tolerance in this species.

Differential polarization of C-terminal Src kinase between naive and antigen-experienced CD8+ T cells

In CD8(+) T cells, engagement of the TCR with agonist peptide:MHC molecules causes dynamic redistribution of surface molecules including the CD8 coreceptor to the immunological synapse. CD8 associates with the Src-family kinase (SFK) Lck, which, in turn, initiates the rapid tyrosine phosphorylation events that drive cellular activation. Compared with naive T cells, Ag-experienced CD8(+) T cells make shorter contacts with APC, are less dependent on costimulation, and are triggered by lower concentrations of Ag, yet the molecular basis of this more efficient response of memory T cells is not fully understood. In this article, we show differences between naive and Ag-experienced CD8(+) T cells in colocalization of the SFKs and their negative regulator, C-terminal Src kinase (Csk). In naive CD8(+) T cells, there was pronounced colocalization of SFKs and Csk at the site of TCR triggering, whereas in Ag-experienced cells, Csk displayed a bipolar distribution with a proportion of the molecules sequestered within a cytosolic area in the distal pole of the cell. The data show that there is differential redistribution of a key negative regulator away from the site of TCR engagement in Ag-experienced CD8(+) T cells, which might be associated with the more efficient responses of these cells on re-exposure to Ag.

Loss of CD4 T-cell-dependent tolerance to proteins with modified amino acids

The site-specific incorporation of the unnatural amino acid p-nitrophenylalanine (pNO(2)Phe) into autologous proteins overcomes self-tolerance and induces a long-lasting polyclonal IgG antibody response. To determine the molecular mechanism by which such simple modifications to amino acids are able to induce autoantibodies, we incorporated pNO(2)Phe, sulfotyrosine (SO(3)Tyr), and 3-nitrotyrosine (3NO(2)Tyr) at specific sites in murine TNF-α and EGF. A subset of TNF-α and EGF mutants with these nitrated or sulfated residues is highly immunogenic and induces antibodies against the unaltered native protein. Analysis of the immune response to the TNF-α mutants in different strains of mice that are congenic for the H-2 locus indicates that CD4 T-cell recognition is necessary for autoantibody production. IFN-γ ELISPOT analysis of CD4 T cells isolated from vaccinated mice demonstrates that peptides with mutated residues, but not the wild-type residues, are recognized. Immunization of these peptides revealed that a CD4 repertoire exists for the mutated peptides but is lacking for the wild-type peptides and that the mutated residues are processed, loaded, and presented on the I-A(b) molecule. Overall, our results illustrate that, although autoantibodies are generated against the endogenous protein, CD4 cells are activated through a neo-epitope recognition mechanism. Therefore, tolerance is maintained at a CD4 level but is broken at the level of antibody production. Finally, these results suggest that naturally occurring posttranslational modifications such as nitration may play a role in antibody-mediated autoimmune disorders.

Generation of MHC class I ligands in the secretory and vesicular pathways

CD8(+) T lymphocytes screen the surface of all cells in the body to detect pathogen infection or oncogenic transformation. They recognize peptides derived from cellular proteins displayed at the plasma membrane by major histocompatibility complex (MHC) class I molecules. Peptides are mostly by-products of cytosolic proteolytic enzymes. Peptidic ligands of MHC class I molecules are also generated in the secretory and vesicular pathways. Features of protein substrates, of proteases and of available MHC class I molecules for loading peptides in these compartments shape a singular collection of ligands that also contain different, longer, and lower affinity peptides than ligands produced in the cytosol. Especially in individuals who lack the transporters associated with antigen processing, TAP, and in infected and tumor cells where TAP is blocked, which thus have no supply of peptides derived from the cytosol, MHC class I ligands generated in the secretory and vesicular pathways contribute to shaping the CD8(+) T lymphocyte response.

Immunogenicity of biologically-derived therapeutics: assessment and interpretation of nonclinical safety studies

An evaluation of potential antibody formation to biologic therapeutics during the course of nonclinical safety studies and its impact on the toxicity profile is expected under current regulatory guidance and is accepted standard practice. However, approaches for incorporating this information in the interpretation of nonclinical safety studies are not clearly established. Described here are the immunological basis of anti-drug antibody formation to biopharmaceuticals (immunogenicity) in laboratory animals, and approaches for generating and interpreting immunogenicity data from nonclinical safety studies of biotechnology-derived therapeutics to support their progression to clinical evaluation. We subscribe that immunogenicity testing strategies should be adapted to the specific needs of each therapeutic development program, and data generated from such analyses should be integrated with available clinical and anatomic pathology, pharmacokinetic, and pharmacodynamic data to properly interpret nonclinical studies.

Immunization with an engineered mutant trans-sialidase highly protects mice from experimental Trypanosoma cruzi infection: a vaccine candidate

Chagas' disease is a major tropical disease for which a cure for chronic phase does not exist yet. Trypanosoma cruzi trans-sialidase (TS) seems to be involved in relevant processes such as infectivity, host survival and, very importantly, disease pathogenesis. In this study, we show that mice vaccinated with an engineered enzymatically deficient mutant TS containing the catalytic domain without the immunodominant SAPA (Shed Acute Phase Antigen) repeats, were highly protected against T. cruzi infection. Adult male BALB/c mice were immunized with mutant protein, purified from Pichia pastoris yeast, using three inoculations in Freund's adjuvant. All immunized mice were protected against challenge with a lethal dose of T. cruzi trypomastigotes. The protected immunized mice developed no clinical or tissue evidence of infection throughout the study. In contrast, 60-90% mortality and 100% occurrence of myocardial lesions were observed in the non-immunized counterparts. Titers of circulating antibody against TS did not correlate with protection, while anti-SAPA antibodies were coincident with disease severity. Further studies indicated that a single inoculation of mutant recombinant protein in Freund's complete adjuvant was not associated with blood or organic alterations, per se. Mutant TS vaccination seems to be a promising tool for immune intervention strategies in Chagas' disease, aimed at preventing T. cruzi-related heart tissue damage.

Traffic of proteins and peptides across membranes for immunosurveillance by CD8(+) T lymphocytes: a topological challenge

Cytotoxic CD8(+) T lymphocytes kill infected cells that display major histocompatibility complex (MHC) class I molecules presenting peptides processed from pathogen proteins. In general, the peptides are proteolytically processed from newly made endogenous antigens in the cytosol and require translocation to the endoplasmic reticulum (ER) for MHC class I loading. This last task is performed by the transporters associated with antigen processing (TAP). Sampling of suspicious pathogen-derived proteins reaches beyond the cytosol, and MHC class I loading can occur in other secretory or endosomal compartments besides the ER. Peptides processed from exogenous antigens can also be presented by MHC class I molecules to CD8(+) T lymphocytes, in this case requiring delivery from the extracellular medium to the processing and MHC class I loading compartments. The endogenous or exogenous antigen can be processed before or after its transport to the site of MHC class I loading. Therefore, mechanisms that allow the full-length protein or processed peptides to cross several subcellular membranes are essential. This review deals with the different intracellular pathways that allow the traffic of antigens to compartments proficient in processing and loading of MHC class I molecules for presentation to CD8(+) T lymphocytes and highlights the need to molecularly identify the transporters involved.

Cloning of idiotype immunoglobulin genes in B cell lymphomas by anchored PCR and production of individual recombinant idiotype vaccines in Escherichia coli

OBJECTIVES

Individual immunoglobulins expressed by B-cell lymphomas represent tumor-specific antigens ('idiotypes'). Immunization with idiotype in follicular lymphoma patients may induce specific immune responses, sustained progression-free survival, and disappearance of minimal residual disease. Manufacturing of idiotype vaccines has mostly relied on heterohybridomas established from viable lymphoma cells. This paper describes the feasibility of production of GMP-grade idiotype vaccines as recombinant Fab fragments in Escherichia coli.

METHODS

IgH and IgL transcripts were analyzed by anchored PCR from 106 lymphoma and nine control biopsies. Lymphoma-derived V segments were inserted into prokaryotic expression plasmids. Recombinant idiotype Fab fragments were expressed in E. coli in a fermentation system.

RESULTS

Idiotype IgH and IgL transcripts were identified in 95% of 106 lymphoma biopsies according to stringent clonality criteria. Large-scale idiotype expression was successful in 69 of 78 cases (89%) and yielded a median of 17 mg (range: 1.2-250 mg) recombinant Fab protein. After affinity chromatography, median vaccine purity was 99% heterodimeric Fab protein (range: 72-100%). Bacterial protein contamination was detectable in one vaccine only. Fab proteins with IgL lambda chains had a tendency for inferior yield and lesser purity than kappa-type Fabs. Among other structural idiotype features (isotype, V family usage, somatic hypermutation pattern, novel glycosylation sites, CDR III net charge), no consistent influences on Fab yield or purity were detected.

CONCLUSIONS

Anchored PCR cloning and subsequent protein expression in E. coli provides a reliable technological basis for clinical idiotype vaccination trials.

Processing of a Class I-Restricted Epitope from Tyrosinase Requires Peptide N-Glycanase and the Cooperative Action of Endoplasmic Reticulum Aminopeptidase 1 and Cytosolic Proteases

Although multiple components of the class I MHC processing pathway have been elucidated, the participation of nonproteasomal cytosolic enzymes has been largely unexplored. In this study, we provide evidence for multiple cytosolic mechanisms in the generation of an HLA-A*0201-associated epitope from tyrosinase. This epitope is presented in two isoforms containing either Asn or Asp, depending on the structure of the tyrosinase precursor. We show that deamidation of Asn to Asp is dependent on glycosylation in the endoplasmic reticulum (ER), and subsequent deglycosylation by peptide-N-glycanase in the cytosol. Epitope precursors with N-terminal extensions undergo a similar process. This is linked to an inability of ER aminopeptidase 1 to efficiently remove N-terminal residues, necessitating processing by nonproteasomal peptidases in the cytosol. Our work demonstrates that processing of this tyrosinase epitope involves recycling between the ER and cytosol, and an obligatory interplay between enzymes involved in proteolysis and glycosylation/deglycosylation located in both compartments.

Phase I trial of a novel intradermal idiotype vaccine in patients with advanced B-cell lymphoma: specific immune responses despite profound immunosuppression

The immunoglobulin receptor of B-cell lymphomas constitutes a specific tumor antigen (idiotype) and a target for active immunotherapy. Encouraging results have been reported in phase II trials after s.c. vaccination of follicular lymphoma patients during clinical remission with idiotype produced from eukaryotic cell lines and coupled to an immunogenic carrier macromolecule. We have developed a good manufacturing protocol for rapid expression of idiotype vaccines as recombinant Fab fragments in Escherichia coli. The objectives of this trial were to show safety and feasibility of intradermal immunization with this vaccine and to investigate whether immune responses were induced by this immunization route. Patients (n = 18) with advanced B-cell malignancies received repetitive intradermal vaccinations with 0.5 to 1.65 mg recombinant idiotype Fab fragment mixed with lipid-based adjuvant in combination with 150 mug granulocyte macrophage colony-stimulating factor s.c. at the same location. The patients' immune status was assessed by flow cytometry of peripheral blood lymphocytes and concomitant hepatitis B vaccination. Cellular and humoral immune responses to the vaccine were assessed by enzyme-linked immunospot and ELISA. Side effects of a total of 65 vaccinations were mild and did not affect the immunization schedule. No patient developed hepatitis B surface antibodies (anti-HBs) after two hepatitis B immunizations. Of 17 evaluable patients, five developed specific anti-vaccine antibodies, and eight developed anti-Fab T-cell responses. T-cell reactivity was independent of the cellular immune status and was idiotype specific as shown by statistical regression analysis (P = 0.0024) and epitope mapping studies. Intradermal administration of uncoupled recombinant idiotype with appropriate adjuvants may overcome profound clinical immunosuppression and induce specific immune responses.

A single chondroitin 6-sulfate oligosaccharide unit at Ser-2730 of human thyroglobulin enhances hormone formation and limits proteolytic accessibility at the carboxyl terminus. Potential insights into thyroid homeostasis and autoimmunity

We localized the site of type D (chondroitin 6-sulfate) oligosaccharide unit addition to human thyroglobulin (hTg). hTg was chromatographically separated into chondroitin 6-sulfate-containing (hTg-CS) and chondroitin 6-sulfate-devoid (hTg-CS0) molecules on the basis of their D-glucuronic acid content. In an ample number of hTg preparations, the fraction of hTg-CS in total hTg ranged from 32.0 to 71.6%. By exploiting the electrophoretic mobility shift and metachromasia conferred by chondroitin 6-sulfate upon the products of limited proteolysis of hTg, chondroitin 6-sulfate was first restricted to a carboxyl-terminal region, starting at residue 2514. A single chondroitin 6-sulfate-containing nonapeptide was isolated in pure form from the products of digestion of hTg with endoproteinase Glu-C, and its sequence was determined as LTAGXGLRE (residues 2726-2734, X being Ser2730 linked to the oligosaccharide chain). In an in vitro assay of enzymatic iodination, hTg-CS produced higher yields of 3,5,5 '-triiodothyronine (T3) (171%) and 3,5,3',5'-tetraiodothyronine (T4) (134%) than hTg-CS0. Unfractionated hTg behaved as hTg-CS. Thus, chondroitin 6-sulfate addition to a subset of hTg molecules enhanced the overall level of T4 and, in particular, T3 formation. Furthermore, the chondroitin 6-sulfate oligosaccharide unit of hTg-CS protected peptide bond Lys2714-Gly2715 from proteolysis, during the limited digestion of hTg-CS with trypsin. These findings provide insights into the molecular mechanism of regulation of the hormonogenic efficiency and of the T4/T3 ratio in hTg. The potential implications in the ability of hTg to function as an autoantigen and into the pathogenesis of thyroidal and extra-thyroidal manifestations of autoimmune thyroid disease are discussed.

Post-translational modifications of self antigens: implications for autoimmunity

Alterations in amino acid sequence can generate neo-epitopes from self proteins, causing autoaggressive immune attack. There is a range of possible post-translational modifications (PTMs) of mammalian proteins that can allow immune recognition of neo-self epitopes. These effects can vary from overt increase in affinity of MHC or T-cell receptor binding, to more subtle effects on the activity of proteolytic enzymes involved in antigen processing. Furthermore, intriguing insights into how the complex interactions between inflammation, enzyme activity and protein modification can direct self recognition are beginning to be unearthed.

Glycosylphosphatidylinositols in malaria pathogenesis and immunity: potential for therapeutic inhibition and vaccination

Glycosylphosphatidylinositols (GPIs) are found in the outer cell membranes of all eukaryotes. GPIs anchor a diverse range of proteins to the surface of Plasmodium falciparum, but may also exist free of protein attachment. In vitro and in vivo studies have established GPIs as likely candidate toxins in malaria, consistent with the prevailing paradigm that attributes induction of inflammatory cytokines, fever and other pathology to parasite toxins released when schizonts rupture. Although evolutionarily conserved, sufficient structural differences appear to exist that impart upon plasmodial GPIs the ability to activate second messengers in mammalian cells and elicit immune responses. In populations exposed to P. falciparum, the antibody response to purified GPIs is characterised by a predominance of immunoglobulin (Ig)G over IgM and an increase in the prevalence, level and persistence of responses with increasing age. It remains unclear, however, if these antibodies or other cellular responses to GPIs mediate anti-toxic immunity in humans; anti-toxic immunity may comprise either reduction in the severity of disease or maintenance of the malaria-tolerant state (i.e. persistent asymptomatic parasitaemia). P. falciparum GPIs are potentially amenable to specific therapeutic inhibition and vaccination; more needs to be known about their dual roles in malaria pathogenesis and protection for these strategies to succeed.

The processing of antigens delivered as DNA vaccines

The ability of DNA vaccines to provide effective immunological protection against infection and tumors depends on their ability to generate good CD4+ and CD8+ T-cell responses. Priming of these responses is a property of dendritic cells (DCs), and so the efficacy of DNA-encoded vaccines is likely to depend on the way in which the antigens they encode are processed by DCs. This processing could either be via the synthesis of the vaccine-encoded antigen by the DCs themselves or via its uptake by DCs following its synthesis in bystander cells that are unable to prime T cells. These different sources of antigen are likely to engage different antigen-processing pathways, which are the subject of this review. Understanding how to access different processing pathways in DCs may ultimately aid the rational development of plasmid-based vaccines to pathogens and to cancer.

Genetic immunization with LYT1 or a pool of trans-sialidase genes protects mice from lethal Trypanosoma cruzi infection

Genetic immunization with a limited set of genes has been demonstrated to be an effective means of protecting mice from a normally lethal challenge of Trypanosoma cruzi. The goal of this study was to expand the diversity of genes assessed as genetic vaccine candidates. Screening a T. cruzi amastigote cDNA expression library with anti-amastigote monoclonal antibodies resulted in the identification of two genes, the previously identified flagellar Ca(2+) binding protein, FCaBP, and a novel homologue of the adaptin AP-3 complex beta3 subunit, Tcbeta3. A third gene, LYT1, recently identified as a secreted T. cruzi protein involved in cell lysis and infectivity, and was selected. Although peptides from all three genes were found to be targets of cytotoxic T cell responses in chronically infected mice, only immunization with LYT1 protected mice from a normally lethal challenge of T. cruzi. As an alternative to testing individual T. cruzi genes as vaccines, pools of genes from the trans-sialidase (TS) and mucin families were assessed in vaccination studies. Immunization with pools of TS but not mucin genes provided protection against a normally lethal challenge of T. cruzi. This study demonstrates that the ability of T. cruzi proteins to elicit immune responses in infected hosts does not necessarily associate with the ability to induce protection and that both the products of single genes and multi-gene families may serve as effective vaccines.

Insights into antigen processing gained by direct analysis of the naturally processed class I MHC associated peptide repertoire

MHC class I molecules are responsible for the presentation of antigenic peptides to CD8+ T lymphocytes. Based on their relatively promiscuous binding of peptides, these molecules display information derived from a large fraction of proteins that are made inside the cell. This review describes our characterization of the peptides comprising this repertoire, with particular attention given to their complexity and quantities, their post-translational modification, and the pathways leading to their expression.

Exogenous peptides delivered by ricin require processing by signal peptidase for transporter associated with antigen processing-independent MHC class I-restricted presentation

In this study we demonstrate that a disarmed version of the cytotoxin ricin can deliver exogenous CD8(+) T cell epitopes into the MHC class I-restricted pathway by a TAP-independent, signal peptidase-dependent pathway. Defined viral peptide epitopes genetically fused to the N terminus of an attenuated ricin A subunit (RTA) that was reassociated with its partner B subunit were able to reach the early secretory pathway of sensitive cells, including TAP-deficient cells. Successful processing and presentation by MHC class I proteins was not dependent on proteasome activity or on recycling of MHC class I proteins, but rather on a functional secretory pathway. Our results demonstrated a role for signal peptidase in the generation of peptide epitopes associated at the amino terminus of RTA. We showed, first, that potential signal peptide cleavage sites located toward the N terminus of RTA can be posttranslationally cleaved by signal peptidase and, second, that mutation of one of these sites led to a loss of peptide presentation. These results identify a novel MHC class I presentation pathway that exploits the ability of toxins to reach the lumen of the endoplasmic reticulum by retrograde transport, and suggest a role for endoplasmic reticulum signal peptidase in the processing and presentation of MHC class I peptides. Because TAP-negative cells can be sensitized for CTL killing following retrograde transport of toxin-linked peptides, application of these results has direct implications for the development of novel vaccination strategies.

In vitro studies of core peptide-bearing immunopotentiating reconstituted influenza virosomes as a non-live prototype vaccine against hepatitis C virus

Evidence from both animal and human viral diseases indicate that cytotoxic T lymphocytes (CTL) are crucial in antiviral defense. However, a major problem to generate cytotoxic immunity is that in vivo exogenous antigens are usually presented via MHC class II pathway and normally fail to induce CTL. The aim of this study is to describe a novel non-live prototype vaccine based on immunopotentiating reconstituted influenza virosomes (IRIV) as vehicles to deliver HLA-A*0201-restricted hepatitis C virus (HCV) peptides (core 35-44 and 131-140) into the cytoplasm of at least three different target cell types [including T2, a transporter associated with antigen processing (TAP)-deficient cell line] resulting in MHC class I peptide presentation and lysis by peptide-specific CTL lines. Comparison of kinetics and analysis of the influence of peptide-stripping and Brefeldin A (BFA) reveal that there exists an endogenous, TAP-independent and BFA-sensitive pathway for virosomally delivered peptides. Moreover, virosomes containing influenza matrix peptide 58-66 can efficiently re-stimulate in vivo primed CTL and, importantly, IRIV containing HCV core peptides can even prime CTL from peripheral blood mononuclear cells of HCV(-) healthy blood donors in vitro. The fact that in vitro primed CTL are also able to specifically lyse target cells infected with recombinant vaccinia virus encoding the HCV core protein is of great importance for future studies based on in vivo mouse models. One of the most evident advantages of the virosomes in vivo will be their capability to protect the incorporated peptide from a large variety of degrading proteases.

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.

Post-translational protein modifications in antigen recognition and autoimmunity

It is estimated that 50-90% of the proteins in the human body are post-translationally modified. In the proper context, these modifications are necessary for the biological functions of a vast array of proteins and the effector functions of the cells in which they reside. However, it is now clear that some post-translational modifications can create new self antigens (Ags) or even mask Ags normally recognized by the immune system. In either case, they profoundly affect the recognition of Ag by bone marrow-derived cells, as well as their effector functions. How do post-translational protein modifications affect the processing of foreign and self Ags and what is their role in the origin of autoimmune responses?

An immunodominant MHC class II-restricted tumor antigen is conformation dependent and binds to the endoplasmic reticulum chaperone, calreticulin

There is accumulating evidence that CD4(+) T cell responses are important in antitumor immunity. Accordingly, we generated CD4(+) T cells against the murine CT26 colon cancer. Three of three independent CT26-specific CD4(+) hybridomas were found to recognize the high m.w. precursor of the env gene product gp90. The CD4(+) response was completely tumor specific in that the same glycoprotein expressed by other tumors was not recognized by the CT26-specific hybridomas. The recognition of gp90 by the hybridomas was strictly dependent on the conformation of gp90. Different procedures that disrupted the conformation of the glycoprotein, such as disulfide bond reduction and thermal denaturation, completely abrogated recognition of gp90 by all three hybridomas. In CT26 cells, but not in other tumor cells tested, a large proportion of gp90 was retained in the endoplasmic reticulum, mostly bound to the endoplasmic reticulum chaperone, calreticulin. Although calreticulin was not essential for the stimulation of the gp90-specific hybridomas, most of the antigenic form of gp90 was bound to it. The antigenicity of gp90 correlated well with calreticulin binding, reflecting the fact that specificity of binding of calreticulin to its substrate required posttranslational modifications that were also necessary for the generation of this tumor-specific CD4(+) epitope.

Multiple antigen-specific processing pathways for activating naive CD8+ T cells in vivo

Current knowledge of the processing of viral Ags into MHC class I-associated ligands is based almost completely on in vitro studies using nonprofessional APCs (pAPCs). This is two steps removed from real immune responses to pathogens and vaccines, in which pAPCs activate naive CD8(+) T cells in vivo. Rational vaccine design requires answers to numerous questions surrounding the function of pAPCs in vivo, including their abilities to process and present peptides derived from endogenous and exogenous viral Ags. In the present study, we characterize the in vivo dependence of Ag presentation on the expression of TAP by testing the immunogenicity of model Ags synthesized by recombinant vaccinia viruses in TAP1(-/-) mice. We show that the efficiency of TAP-independent presentation in vitro correlates with TAP-independent activation of naive T cells in vivo and provide the first in vivo evidence for proteolytic processing of antigenic peptides in the secretory pathway. There was, however, a clear exception to this correlation; although the presentation of the minimal SIINFEKL determinant from chicken egg OVA in vitro was strictly TAP dependent, it was presented in a TAP-independent manner in vivo. In vivo presentation of the same peptide from a fusion protein retained its TAP dependence. These results show that determinant-specific processing pathways exist in vivo for the generation of antiviral T cell responses. We present additional findings that point to cross-priming as the likely mechanism for these protein-specific differences.

Generation of MHC class I peptide antigens by protein processing in the secretory route by furin

Cytosolic degradation of endogenously synthesized proteins by the proteasome and translocation of processed peptides to the endoplasmic reticulum by the transporters associated with antigen presentation constitutes the classical route for antigen presentation by MHC class I proteins. We have previously defined an alternative pathway in the secretory route involving proteolytic maturation of precursor proproteins for chimeric hepatitis B virus secretory core protein HBe containing a class I epitope at its carboxy-terminus. We extend those results by demonstrating that intracellular delivery of the trans-Golgi network protease furin increases both proteolytic maturation and antigen presentation of the chimeric HBe proteins. An additional class I epitope from the HIV envelope gp160 protein was inserted into this COOH-terminal region of two different chimeric HBe proteins. This epitope was also presented to CTL in a transporter-independent manner involving furin, and protein maturation and antigen presentation were also enhanced by furin over-expression. Presentation of this second epitope was restricted by a different class I allele, thus suggesting that antigen presentation by this new pathway may apply to any antigenic epitope and class I molecule. These results define the furin proteolytic maturation pathway of HBe in the secretory route as a general antigen processing route for MHC class I presentation.

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.

Alternative pathways for processing exogenous and endogenous antigens that can generate peptides for MHC class I-restricted presentation

The concept of distinct endogenous and exogenous pathways for generating peptides for MHC-I and MHC-II-restricted presentation to CD4+ or CD8+ T cells fits well with the bulk of experimental data. Nevertheless, evidence is emerging for alternative processing pathways that generate peptides for MHC-I-restricted presentation. Using a well characterized, particulate viral antigen of prominent medical importance (the hepatitis B surface antigen), we summarize our evidence that the efficient, endolysosomal processing of exogenous antigens can lead to peptide-loaded MHC-I molecules. In addition, we describe evidence for endolysosomal processing of mutant, stress protein-bound, endogenous antigens that liberate peptides binding to (and presented by) MHC-I molecules. The putative biological role of alternative processing of antigens generating cytotoxic T-lymphocyte-stimulating epitopes is discussed.

Isoaspartyl post-translational modification triggers autoimmune responses to self-proteins

The normal functioning immune system is programmed to attack foreign pathogens and other foreign proteins while maintaining tolerance to self-proteins. The mechanisms by which tolerance is broken in the initiation of autoimmunity are not completely understood. In the present study, mice immunized with the murine cytochrome c peptide 90-104 showed no response by the B or T cell compartments. However, immunization with the isoaspartyl form of this peptide, where the linkage of Asp(93) to Leu(94) occurs through the beta-carboxyl group, resulted in strong B and T cell autoimmune responses. Antibodies elicited by immunization with the isoaspartyl form of self-peptide were cross-reactive in binding to both isoforms of cytochrome c peptide and to native cytochrome c self-protein. In a similar manner, immunization of mice with the isoaspartyl form of a peptide autoantigen of human systemic lupus erythematosus (SLE) resulted in strong B and T cell responses while mice maintained tolerance to the normal aspartyl form of self-antigen. Isoaspartyl linkages within proteins are enhanced in aging and stressed cells and arise under physiological conditions. These post-translationally modified peptides may serve as an early immunologic stimulus in autoimmune disease.

Effect of epitope flanking residues on the presentation of N-terminal cytotoxic T lymphocyte epitopes

We here demonstrate that placing two distinct influenza virus nucleoprotein epitopes at the N terminus of a cytosolic protein selectively blocks their presentation to specific cytotoxic T lymphocytes. The block is a cytosolic phenomenon, which can be overcome by distancing the epitope from the protein N terminus by two or more amino acids. Shortening the protein's C terminus fails to relieve the antigen presentation block. These results demonstrate that events at the N terminus of the target protein, rather than at its C terminus, are responsible for the lack of presentation of N-terminal epitopes. We also show that lack of presentation of N terminal epitopes is associated with a modification of the target protein which affects its electrophoretic mobility and isoelectric focusing point. This modification can be prevented by mutating the epitope's N-terminal flanking sequence, which results in an efficient presentation of the N-terminal epitope. Lack of presentation of the N-terminal epitopes results in a reduced ability of influenza-primed mice to clear acute infection with vaccinia virus encoding an N-terminal nucleoprotein epitope. Our results demonstrate that presentation of epitopes localized at the N terminus of cytosolic proteins can be modulated by events occurring at early stages of antigen processing.

Genes regulating MHC class I processing of antigen

The principal pathway of antigen processing that is associated with MHC class I involves three main steps: cytosolic peptide generation, peptide transport into the endoplasmic reticulum and peptide assembly with class I molecules. Recent advances suggest that additional cytosolic proteases complement the proteasome as a source of antigenic peptides. Peptide assembly involves several novel cofactors - including the proteins tapasin and ERp57, which may be important for stabilisation of empty class I molecules as well as quality control after peptide binding. Finally, genetic evidence suggests an important influence of an unidentified gene, in the MHC complex, on MHC class I processing.