The absence of ER trimming alters peptides presented by non-classical MHC class I molecules (78.8)

The nature, function and pathways used to produce peptides for presentation by non-classical MHC class I molecules is poorly understood. Here we show that cells from mice lacking the ER aminopeptidase associated with antigen processing (ERAAP) present an immunologically distinct peptide-MHC class I repertoire. Wild-type mice immunized with ERAAP-deficient cells mount robust CD8 T cell responses specific for peptides presented by classical as well as non-classical MHC I molecules. These CD8 T cells produce pro-inflammatory cytokines and effectively kill ERAAP-deficient target cells in vitro as well as in vivo. Further, peptides presented by non-classical MHC class I molecules are expressed in a tissue-specific manner and elicit CD8 T cell responses both by direct and cross-priming. Thus, ERAAP trims the final peptides presented by classical as well as non-classical MHC class I molecules, and may be involved in trimming peptides for cross-presentation.

This research was supported by grants from the NIH (AI060040) to N.S.; N.A.N. is supported by the Irvington Institute Fellowship Program of the Cancer Research Institute.

Enhanced antigen presentation and CTL activity by transduction of mature rather than immature dendritic cells with octaarginine-modified liposomes

To improve uptake and cross-presentation of exogenous antigens (Ag) by dendritic cells (DCs), octaarginine-modified liposomes (R8-Lip) were used as a novel strategy for protein-Ag transduction. Immature DCs endocytose macromolecules efficiently. While mature DCs lose their ability to capture Ag, but have an increased capacity for T-cell activation. Thus Ag-transduction has been performed mostly in immature DCs. In the present study, R8-Lip were efficiently taken up by both immature and mature DCs. DCs transduced after maturation were highly efficient at cross-presentation of Ag and induced higher cytotoxic T-lymphocytes (CTL) activity than were DCs transduced before maturation. The mechanism of Ag presentation involved the escape of R8-Lip from endosomes to cytosol, which require the acidic environment. The Ag released was then processed by a proteasome-dependent pathway. This novel transduction approach is clinically applicable, easy to perform, and has more practical advantages than current protein transduction methods.

Endoplasmic reticulum aminopeptidase associated with antigen processing regulates quality of processed peptides presented by MHC class I molecules

Effective immune surveillance by CD8 T cells depends on the presentation of diverse peptides by MHC class I (pMHC I) molecules on the cell surface. The pMHC I repertoire is shaped in the endoplasmic reticulum (ER) by the ER aminopeptidase associated with Ag processing (ERAAP). The ERAAP activity is required for producing peptides of appropriate length for generating optimal pMHC I. Paradoxically, ERAAP also inhibits generation of certain peptides such as the SVL9 (SSVVGVWYL) peptide encoded by the H13(a) histocompatibility gene and presented by D(b) MHC by an unknown mechanism. In this study, we show that the presentation of the SVL9-D(b) complex is inhibited when other peptides compete for binding D(b). Conversely, improving the binding of SVL9 peptide to D(b) suppresses the inhibition. Interestingly, the inhibitory effect of competitor peptides is observed only when ERAAP is expressed in the same cells. Thus, ERAAP, in concert with MHC I molecules, regulates the quality of processed peptides presented on the cell surface.

A distinct translation initiation mechanism generates cryptic peptides for immune surveillance

MHC class I molecules present a comprehensive mixture of peptides on the cell surface for immune surveillance. The peptides represent the intracellular protein milieu produced by translation of endogenous mRNAs. Unexpectedly, the peptides are encoded not only in conventional AUG initiated translational reading frames but also in alternative cryptic reading frames. Here, we analyzed how ribosomes recognize and use cryptic initiation codons in the mRNA. We find that translation initiation complexes assemble at non-AUG codons but differ from canonical AUG initiation in response to specific inhibitors acting within the peptidyl transferase and decoding centers of the ribosome. Thus, cryptic translation at non-AUG start codons can utilize a distinct initiation mechanism which could be differentially regulated to provide peptides for immune surveillance.

Immunodominant, protective response to the parasite Toxoplasma gondii requires antigen processing in the endoplasmic reticulum

The parasite Toxoplasma gondii replicates in a specialized intracellular vacuole and causes disease in many species. Protection from toxoplasmosis is mediated by CD8(+) T cells, but the T. gondii antigens and host genes required for eliciting protective immunity are poorly defined. Here we identified GRA6, a polymorphic protein secreted in the parasitophorous vacuole, as the source of the immunodominant and protective decapeptide HF10 presented by the H-2L(d) major histocompatibility complex class I molecule. Presentation of the HF10-H-2L(d) ligand required proteolysis by ERAAP, the endoplasmic reticulum aminopeptidase associated with antigen processing. Consequently, expansion of protective CD8(+) T cell populations was impaired in T. gondii-infected ERAAP-deficient mice, which were more susceptible to toxoplasmosis. Thus, endoplasmic reticulum proteolysis is critical for eliciting protective immunity to a vacuolar parasite.

Coping with loss of perfection in the MHC class I peptide repertoire

The MHC class I molecules present thousands of peptides (pMHC I) on the cell surface for immune surveillance by CD8 T cells. The pMHC I repertoire normally contains peptides of perfect length and sequences suitable for binding each MHC I. The peptides are made by first fragmenting cytoplasmic proteins. The fragments are then transported into the endoplasmic reticulum (ER), where they are trimmed to appropriate length by the ER aminopeptidase associated with antigen processing (ERAAP) to generate the final pMHC I. Here, we review studies on the role of ERAAP in generating pMHC I from endogenous or viral proteins and their ability to elicit CD8 T cell responses. The absence of ERAAP profoundly disrupts the pMHC I repertoire which can have major consequences on the immune responses to endogenous and viral antigens.

The final touches make perfect the peptide-MHC class I repertoire

Major histocompatibility complex (MHC) class I molecules present short, perfectly cleaved peptides on the cell surface for immune surveillance by CD8(+) T cells. The pathway for generating these peptides begins in the cytoplasm, and the peptide-MHC I (pMHC I) repertoire is finalized in the endoplasmic reticulum. Recent studies show that the peptides for MHC I are customized by the ER aminopeptidase associated with antigen processing and by dynamic interactions within the MHC peptide-loading complex. Failure to customize the pMHC I repertoire has profound immunological consequences.

Tracking antigenic intermediates throughout the antigen processing pathway during Listeria monocytogenes infection (36.3)

Prior to their appearance on the cell surface where they are presented by MHC I molecules to T cells, antigenic peptides are created by a complex array of intracellular processing events. Using conventional T cell assays, only the final peptide can be efficiently detected on the cell surface. In order to detect the full range of antigenic intermediates that exist intracellularly throughout the antigen processing pathway, members of the Shastri lab have successfully developed the KOVAK system. KOVAK is a modified form of ovalbumin which contains enzymatically cleavable residues flanking the antigenic peptide, SHL8, thus allowing it to be liberated from the context of any antigenic intermediate and detected by SHL8-specific T cells following HPLC fractionation of cell extracts. Over the past year, we have utilized this system to track antigenic intermediates in the processing of KOVAK expressed by Listeria monocytogenes. Because Listeria enters the cell phagosomally and then injects itself into the cytoplasm, Listeria infection provides an excellent model to investigate the roles of various components in both the exogenous and endogenous pathways, and where these pathways merge intracellularly. So far we have successfully used this model in combination with knockout mice and inhibitors to delineate the roles of phagosomal proteases, cytoplasmic proteases and the proteasome, chaperones, TAP transport, and ERAAP in the processing of Listeria-derived antigens in infected macrophages. We will also investigate the roles of macrophage and dendritic cell maturation in the processing of Listeria-derived antigens.

Motif structure of minor histocompatibility antigen for MHC class II-restricted presentation (93.19)

Unlike major histocompatibility complex (MHC) class I, the processing pathway of endogenous MHC class II-restricted antigen has been poorly understood. We previously reported that endogenous MHC class II-restricted minor histocompatibility antigen that encoded murine H46 locus on chromosome 7 was IL-4-inducible gene 1 (IL4i1), which was continually expressed in bone marrow dendritic cells (BMDCs) (J.Exp.Med. 2003, 197: 375). In this study, we investigated the processing pathway of endogenous MHC class II-restricted IL4i1 antigen in BMDCs. IL4i1 antigen localized at RhoB-localized endosome, and MHC class II-restricted presentations were inhibited by endosomal vesicle trafficking inhibitor, such as bafilomycin. Whereas, lysosomal enzyme inhibitors, such as chloroquine, could not influence the presentations, suggesting that seem to be different from autophagy processing pathway. Next, we focused on the IL4i1 gene structure that does arrangement of tri-leucine-based motif in the downstream of tyrosine-based motif, designated YL motif. When YL motif was mutated, the antigen presentations were dramatically inhibited and this mutated-IL4i1 did not colocalize with RhoB-localized vesicle. These data suggested that YL motif would be a sorting signal for it where might occur antigen processing.

Nonsense Mediated Decay associated “Pioneer round of translation” peptides as source for MHC I mediated antigen presentation (93.3)

Nonsense mediated decay (NMD) plays a critical role in the mRNA quality control mechanism protecting cells from aberrant mRNAs containing premature termination codons (PTC). Recognition of these PTCs requires a “pioneer round of translation” prior to the bulk translation of mRNAs for cellular protein synthesis, therefore producing a pool of immediate early peptides.

Effectiveness of the immune surveillance system is insured by the timely and complete MHC I mediated presentation of a pool of precisely cleaved peptides representing all cellular proteins. So far sources of antigenic peptides are known to include regular degraded proteins, defective ribosomal products (Drips) and cryptic peptides starting with a non-AUG start codon. Here the contribution of peptides originating from the pioneer translation to the pool of peptides presented by MHC I molecules is described. In Hela cells RNAi mediated depletion of the essential NMD factor hUpf1 as well as the pioneer translation initiation complex factors CBP80 and PABP2 reveal impaired MHC I antigen presentation. In sharp contrast, targeted interference with conventional translation without effecting pioneer translation permits antigen presentation. Taken together these findings put products of the pioneer translation into an important position as novel resource for antigenic peptides and reveal a new relationship between NMD and immunosurveillance.

ERAAP synergizes with MHC class I molecules to make the final cut in the antigenic peptide precursors in the endoplasmic reticulum

The major histocompatibility complex class I molecules display peptides (pMHC I) on the cell surface for immune surveillance by CD8(+) T cells. These peptides are generated by proteolysis of intracellular polypeptides by the proteasome in the cytoplasm and then in the endoplasmic reticulum (ER) by the ER aminopeptidase associated with antigen processing (ERAAP). To define the unknown mechanism of ERAAP function in vivo, we analyzed naturally processed peptides in cells with or without appropriate MHC I and ERAAP. In the absence of MHC I, ERAAP degraded the antigenic precursors in the ER. However, MHC I molecules could bind proteolytic intermediates and were essential for generation of the final peptide by ERAAP. Thus, ERAAP synergizes with MHC I to generate the final pMHC I repertoire.

In the absence of aminopeptidase ERAAP, MHC class I molecules present many unstable and highly immunogenic peptides

Immunosurveillance by cytotoxic T cells requires that cells generate a diverse spectrum of peptides for presentation by major histocompatibility complex (MHC) class I molecules. Those peptides are generated by proteolysis, which begins in the cytoplasm and continues in the endoplasmic reticulum by the unique aminopeptidase ERAAP. The overall extent to which trimming by ERAAP modifies the peptide pool and the immunological consequences of ERAAP deficiency are unknown. Here we show that the peptide-MHC repertoire of ERAAP-deficient mice was missing many peptides. Furthermore, ERAAP-deficient cells presented many unstable and structurally unique peptide-MHC complexes, which elicited potent CD8+ T cell and B cell responses. Thus, ERAAP is a 'quintessential editor' of the peptide-MHC repertoire and, paradoxically, its absence enhances immunogenicity.

Hsp90alpha chaperones large C-terminally extended proteolytic intermediates in the MHC class I antigen processing pathway

Intracellular proteins are degraded in the antigen processing pathway to generate peptide-loaded MHC I complexes (pMHC I) for immune surveillance. The characteristics of the final pMHC I are clear but those of their precursors and their potential binding partners remain poorly defined. By using a unique method to biochemically detect preprocessed ovalbumin-derived antigenic peptides, we find that cells generate large, C-terminally extended proteolytic intermediates that are associated with the alpha isotype of hsp90 chaperone. Knockdown of hsp90alpha expression by siRNA resulted in the loss of these intermediates and decreased presentation of the final pMHC I on the cell surface. Generation of pMHC I was also inhibited by knockdown of the cochaperone CHIP that interacts with heat shock proteins, ubiquitinates their clients, and delivers them to the proteasome. Thus, hsp90alpha can serve as a chaperone for precursors of pMHC I at an early stage in the antigen processing pathway.

In vivo targeting of dendritic cells for activation of cellular immunity using vaccine carriers based on pH-responsive microparticles

Activating the immune system to trigger a specific response is a major challenge in vaccine development. In particular, activating sufficient cytotoxic T lymphocyte-mediated cellular immunity, which is crucial for the treatment of many diseases including cancer and AIDS, has proven to be especially challenging. In this study, antigens were encapsulated in acid-degradable polymeric particle carriers to cascade cytotoxic T lymphocyte activation. To target dendritic cells, the most potent antigen-presenting cells, the particle carriers, were further conjugated with monoclonal antibodies. A series of ex vivo and in vivo studies have shown increased receptor-mediated uptake of antibody-conjugated particles by dendritic cells as well as migration of particle-carrying dendritic cells to lymph nodes and stimulation of naïve T cells leading to enhanced cellular immune response as confirmed by specific cell lysis and IFN-gamma secretion.

The aminopeptidase ERAAP shapes the peptide repertoire displayed by major histocompatibility complex class I molecules

Major histocompatibility complex (MHC) class I molecules present thousands of peptides to allow CD8(+) T cells to detect abnormal intracellular proteins. The antigen-processing pathway for generating peptides begins in the cytoplasm, and the MHC molecules are loaded in the endoplasmic reticulum. However, the nature of peptide pool in the endoplasmic reticulum and the proteolytic events that occur in this compartment are unclear. We addressed these issues by generating mice lacking the endoplasmic reticulum aminopeptidase associated with antigen processing (ERAAP). We found that loss of ERAAP disrupted the generation of naturally processed peptides in the endoplasmic reticulum, decreased the stability of peptide-MHC class I complexes and diminished CD8(+) T cell responses. Thus, trimming of antigenic peptides by ERAAP in the endoplasmic reticulum is essential for the generation of the normal repertoire of processed peptides.

All the peptides that fit: the beginning, the middle, and the end of the MHC class I antigen-processing pathway

The end result of the antigen-processing pathway is the display of peptide-bound major histocompatibility complex I (pMHC I) molecules. The pMHC I molecules are expressed on the cell surface where they can be surveyed by CD8(+) T cells for abnormal proteins. MHC I molecules present a large repertoire of peptides that fit perfectly in their binding grooves and represent the otherwise hidden intracellular contents. Many peptides originate as defective ribosomal products in the cytoplasm. In a stepwise manner, the antigen-processing pathway generates and protects the proteolytic intermediates until they yield the final peptides that can fit the MHC I in the endoplasmic reticulum.

Acid-degradable particles for protein-based vaccines: enhanced survival rate for tumor-challenged mice using ovalbumin model

Acid-degradable protein-loaded polymer particles show promise for antigen-based vaccines due to their ability to activate cytotoxic T lymphocytes (CTLs) in vitro. Protein loadings and cytotoxic T lymphocyte activation efficiencies have now been enhanced through novel delivery vehicle designs. In particular, the use of a more hydrophilic acid-degradable cross-linker leads to increased water dispersibility and increased protein loading efficiency for the particles. A 2.5-fold increase in protein encapsulation allows the delivery of more protein antigen to antigen presenting cells (APCs) leading to a 20-fold rise in antigen presentation levels. The mechanism by which APCs internalize these particles was explored using the phagocytosis inhibitor, cytochalasin B. In addition, preliminary in vivo experiments were conducted to investigate the ability of the protein-loaded particles to provide immunity against tumors in mice, and an enhanced survival rate over the use of protein alone was observed, indicating that this vaccine delivery strategy has great practical potential.

Class I major histocompatibility complex presentation of antigens that escape from the parasitophorous vacuole of Toxoplasma gondii

The intracellular parasite Toxoplasma gondii, the causative agent of toxoplasmosis, induces a protective CD8 T-cell response in its host; however, the mechanisms by which T. gondii proteins are presented by the class I major histocompatibility complex remain largely unexplored. T. gondii resides within a specialized compartment, the parasitophorous vacuole, that sequesters the parasite and its secreted proteins from the host cell cytoplasm, suggesting that an alternative cross-priming pathway might be necessary for class I presentation of T. gondii antigens. Here we used a strain of T. gondii expressing yellow fluorescent protein and a secreted version of the model antigen ovalbumin to investigate this question. We found that presentation of ovalbumin secreted by the parasite requires the peptide transporter TAP (transporter associated with antigen processing) and occurs primarily in actively infected cells rather than bystander cells. We also found that dendritic cells are a major target of T. gondii infection in vivo and account for much of the antigen-presenting activity in the spleen. Finally, we obtained evidence that Cre protein secreted by T. gondii can mediate recombination in the nucleus of the host cell. Together, these results indicate that Toxoplasma proteins can escape from the parasitophorous vacuole into the host cytoplasm and be presented by the endogenous class I pathway, leading to direct recognition of infected cells by CD8 T cells.

Unanticipated antigens: translation initiation at CUG with leucine

Major histocompatibility class I molecules display tens of thousands of peptides on the cell surface for immune surveillance by T cells. The peptide repertoire represents virtually all cellular translation products, and can thus reveal a foreign presence inside the cell. These peptides are derived from not only conventional but also cryptic translational reading frames, including some without conventional AUG codons. To define the mechanism that generates these cryptic peptides, we used T cells as probes to analyze the peptides generated in transfected cells. We found that when CUG acts as an alternate initiation codon, it can be decoded as leucine rather than the expected methionine residue. The leucine start does not depend on an internal ribosome entry site–like mRNA structure, and its efficiency is enhanced by the Kozak nucleotide context. Furthermore, ribosomes scan 5′ to 3′ specifically for the CUG initiation codon in a eukaryotic translation initiation factor 2–independent manner. Because eukaryotic translation initiation factor 2 is frequently targeted to inhibit protein synthesis, this novel translation mechanism allows stressed cells to display antigenic peptides. This initiation mechanism could also be used at non-AUG initiation codons often found in viral transcripts as well as in a growing list of cellular genes.

Proteins have been identified for which a unique translational machinery makes use of unconventional start codons

The group II chaperonin TRiC protects proteolytic intermediates from degradation in the MHC class I antigen processing pathway

MHC class I molecules present precisely cleaved peptides of intracellular proteins on the cell surface. For most antigenic precursors, presentation requires transport of peptide fragments into the ER, but the nature of the cytoplasmic peptides and their chaperones is obscure. By tracking proteolytic intermediates in living cells, we show that intracellular proteolysis yields a mixture of antigenic peptides containing only N-terminal flanking residues for ER transport. Some of these peptides were bound to the group II chaperonin TRiC and were protected from degradation. Destabilization of TRiC by RNA interference inhibited the expression of peptide-loaded MHC I molecules on the cell surface. Thus, the TRiC chaperonin serves a function in protecting proteolytic intermediates in the MHC I antigen processing pathway.

Constitutive display of cryptic translation products by MHC class I molecules

Major histocompatibility complex (MHC) class I molecules display tens of thousands of peptides on the cell surface, derived from virtually all endogenous proteins, for inspection by cytotoxic T cells (CTLs). We show that, in normal mouse cells, MHC I molecules present a peptide encoded in the 3' "untranslated" region. Despite its rarity, the peptide elicits CTL responses and induces self-tolerance, establishing that immune surveillance extends well beyond conventional polypeptides. Furthermore, translation of this cryptic peptide occurs by a previously unknown mechanism that decodes the CUG initiation codon as leucine rather than the canonical methionine.