The double lysine motif of tapasin is a retrieval signal for retention of unstable MHC class I molecules in the endoplasmic reticulum

N-Glycosylation at Asn695 might suppress inducible nitric oxide synthase activity by disturbing electron transfer

Inducible nitric oxide synthase (iNOS) plays critical roles in the inflammatory response and host defense. Previous research on iNOS regulation mainly focused on its gene expression level, and much less is known about the regulation of iNOS function by N-glycosylation. In this study, we report for the first time that iNOS is N-glycosylated in vitro and in vivo. Mass spectrometry studies identified Asn695 as an N-glycosylation site of murine iNOS. Mutating Asn695 to Gln695 yields an iNOS that exhibits greater enzyme activity. The essence of nitric oxide synthase catalytic reaction is electron transfer process, which involves a series of conformational changes, and the linker between the flavin mononucleotide-binding domain and the flavin adenine dinucleotide-binding domain plays vital roles in the conformational changes. Asn695 is part of the linker, so we speculated that attachment of N-glycan to the Asn695 residue might inhibit activity by disturbing electron transfer. Indeed, our NADPH consumption results demonstrated that N-glycosylated iNOS consumes NADPH more slowly. Taken together, our results indicate that iNOS is N-glycosylated at its Asn695 residue and N-glycosylation of Asn695 might suppress iNOS activity by disturbing electron transfer.

SDS-PAGE for 35S Immunoprecipitation and Immunoprecipitation Western Blotting

This report discusses recent methods of sample preparation and gel electrophoresis for ³⁵S immunoprecipitation (IP) and IP western blotting. In both methods, IP is used to obtain purified proteins, and sodium dodecyl sulfate polyacrylamide gel electrophoresis is used to separate the proteins on a gel. In ³⁵S IP, the proteins are radiolabeled and visualized on film by fluorography; in IP blotting, proteins are transferred onto nitrocellulose paper, and antibodies are used to detect specific proteins. A similar IP and SDS-PAGE method can be used for both procedures, but IP blotting has the potential advantages of improvement in sensitivity for low-abundance proteins and enhanced specificity for identification of proteins from a mixture. Some of the technical adaptations discussed here to facilitate IP blotting and avoid loss of beads or purified proteins may also be useful for ³⁵S IP.

Whole genome duplications have provided teleosts with many roads to peptide loaded MHC class I molecules

Background

In sharks, chickens, rats, frogs, medaka and zebrafish there is haplotypic variation in MHC class I and closely linked genes involved in antigen processing, peptide translocation and peptide loading. At least in chicken, such MHCIa haplotypes of MHCIa, TAP2 and Tapasin are shown to influence the repertoire of pathogen epitopes being presented to CD8+ T-cells with subsequent effect on cell-mediated immune responses.

Results

Examining MHCI haplotype variation in Atlantic salmon using transcriptome and genome resources we found little evidence for polymorphism in antigen processing genes closely linked to the classical MHCIa genes. Looking at other genes involved in MHCI assembly and antigen processing we found retention of functional gene duplicates originating from the second vertebrate genome duplication event providing cyprinids, salmonids, and neoteleosts with the potential of several different peptide-loading complexes. One of these gene duplications has also been retained in the tetrapod lineage with orthologs in frogs, birds and opossum.

Conclusion

We postulate that the unique salmonid whole genome duplication (SGD) is responsible for eliminating haplotypic content in the paralog MHCIa regions possibly due to frequent recombination and reorganization events at early stages after the SGD. In return, multiple rounds of whole genome duplications has provided Atlantic salmon, other teleosts and even lower vertebrates with alternative peptide loading complexes. How this affects antigen presentation remains to be established.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1138-9) contains supplementary material, which is available to authorized users.

TAPBPR: a new player in the MHC class I presentation pathway

In order to provide specificity for T cell responses against pathogens and tumours, major histocompatibility complex (MHC) class I molecules present high-affinity peptides at the cell surface to T cells. A key player for peptide loading is the MHC class I-dedicated chaperone tapasin. Recently we discovered a second MHC class I-dedicated chaperone, the tapasin-related protein TAPBPR. Here, we review the major steps in the MHC class I pathway and the TAPBPR data. We discuss the potential function of TAPBPR in the MHC class I pathway and the involvement of this previously uncharacterised protein in human health and disease.

Intracellular Transport Routes for MHC I and Their Relevance for Antigen Cross-Presentation

Cross-presentation, in which exogenous antigens are presented via MHC I complexes, is involved both in the generation of anti-infectious and anti-tumoral cytotoxic CD8⁺ T cells and in the maintenance of immune tolerance. While cross-presentation was described almost four decades ago and while it is now established that some dendritic cell (DC) subsets are better than others in processing and cross-presenting internalized antigens, the involved molecular mechanisms remain only partially understood. Some of the least explored molecular mechanisms in cross-presentation concern the origin of cross-presenting MHC I molecules and the cellular compartments where antigenic peptide loading occurs. This review focuses on MHC I molecules and their intracellular trafficking. We discuss the source of cross-presenting MHC I in DCs as well as the role of the endocytic pathway in their recycling from the cell surface. Next, we describe the importance of the TAP peptide transporter for delivering peptides to MHC I during cross-presentation. Finally, we highlight the impact of innate immunity mechanisms on specific antigen cross-presentation mechanisms in which TLR activation modulates MHC I trafficking and TAP localization.

Topology of Endoplasmic Reticulum-Associated Cellular and Viral Proteins Determined with Split-GFP

The split green fluorescent protein (GFP) system was adapted for investigation of the topology of ER-associated proteins. A 215-amino acid fragment of GFP (S1-10) was expressed in the cytoplasm as a free protein or fused to the N-terminus of calnexin and in the ER as an intraluminal protein or fused to the C-terminus of calnexin. A 16-amino acid fragment of GFP (S11) was fused to the N- or C-terminus of the target protein. Fluorescence occurred when both GFP fragments were in the same intracellular compartment. After validation with the cellular proteins PDI and tapasin, we investigated two vaccinia virus proteins (L2 and A30.5) of unknown topology that localize to the ER and are required for assembly of the viral membrane. Our results indicated that the N- and C-termini of L2 faced the cytoplasmic and luminal sides of the ER, respectively. In contrast both the N- and C-termini of A30.5 faced the cytoplasm. The system offers advantages for quickly determining the topology of intracellular proteins: the S11 tag is similar in length to commonly used epitope tags; multiple options are available for detecting fluorescence in live or fixed cells; transfection protocols are adaptable to numerous expression systems and can enable high throughput applications.

Transport and quality control of MHC class I molecules in the early secretory pathway

Folding and peptide binding of major histocompatibility complex (MHC) class I molecules have been thoroughly researched, but the mechanistic connection between these biochemical events and the progress of class I through the early secretory pathway is much less well understood. This review focuses on the question how the partially assembled forms of class I (which lack high-affinity peptide and/or the light chain beta-2 microglobulin) are retained inside the cell. Such investigations offer researchers exciting chances to understand the connections between class I structure, conformational dynamics, peptide binding kinetics and thermodynamics, intracellular transport, and antigen presentation.

Peptide-independent stabilization of MHC class I molecules breaches cellular quality control

The intracellular trafficking of major histocompatibility complex class I (MHC-I) proteins is directed by three quality control mechanisms that test for their structural integrity, which is correlated to the binding of high-affinity antigenic peptide ligands. To investigate which molecular features of MHC-I these quality control mechanisms detect, we have followed the hypothesis that suboptimally loaded MHC-I molecules are characterized by their conformational mobility in the F-pocket region of the peptide-binding site. We have created a novel variant of an MHC-I protein, K(b)-Y84C, in which two α-helices in this region are linked by a disulfide bond that mimics the conformational and dynamic effects of bound high-affinity peptide. K(b)-Y84C shows a remarkable increase in the binding affinity to its light chain, beta-2 microglobulin (β2m), and bypasses all three cellular quality control steps. Our data demonstrate (1) that coupling between peptide and β2m binding to the MHC-I heavy chain is mediated by conformational dynamics; (2) that the folded conformation of MHC-I, supported by β2m, plays a decisive role in passing the ER-to-cell-surface transport quality controls; and (3) that β2m association is also tested by the cell surface quality control that leads to MHC-I endocytosis.

Tapasin facilitation of natural HLA-A and -B allomorphs is strongly influenced by peptide length, depends on stability, and separates closely related allomorphs

Despite an abundance of peptides inside a cell, only a small fraction is ultimately presented by HLA-I on the cell surface. The presented peptides have HLA-I allomorph-specific motifs and are restricted in length. So far, detailed length studies have been limited to few allomorphs. Peptide-HLA-I (pHLA-I) complexes of different allomorphs are qualitatively and quantitatively influenced by tapasin to different degrees, but again, its effect has only been investigated for a small number of HLA-I allomorphs. Although both peptide length and tapasin dependence are known to be important for HLA-I peptide presentation, the relationship between them has never been studied. In this study, we used random peptide libraries from 7- to 13-mers and studied binding in the presence and absence of a recombinant truncated form of tapasin. The data show that HLA-I allomorphs are differentially affected by tapasin, different lengths of peptides generated different amounts of pHLA-I complexes, and HLA-A allomorphs are generally less restricted than HLA-B allomorphs to peptides of the classical length of 8-10 aa. We also demonstrate that tapasin facilitation varies for different peptide lengths, and that the correlation between high degree of tapasin facilitation and low stability is valid for different random peptide mixes of specific lengths. In conclusion, these data show that tapasin has specificity for the combination of peptide length and HLA-I allomorph, and suggest that tapasin promotes formation of pHLA-I complexes with high on and off rates, an important intermediary step in the HLA-I maturation process.

The MHC I loading complex: a multitasking machinery in adaptive immunity

Recognition and elimination of virally or malignantly transformed cells are pivotal tasks of the adaptive immune system. For efficient immune detection, snapshots of the cellular proteome are presented as epitopes on major histocompatibility complex class I (MHC I) molecules for recognition by cytotoxic T cells. Knowledge about the track from the equivocal protein to the presentation of antigenic peptides has greatly expanded, leading to an astonishingly elaborate understanding of the MHC I peptide loading pathway. Here, we summarize the current view on this complex process, which involves ABC transporters, proteases, chaperones, and endoplasmic reticulum (ER) quality control. The contribution of individual proteins and subcomplexes is discussed, with a focus on the architecture and dynamics of the key player in the pathway, the peptide-loading complex (PLC).

Tapasin-related protein TAPBPR is an additional component of the MHC class I presentation pathway

Tapasin is an integral component of the peptide-loading complex (PLC) important for efficient peptide loading onto MHC class I molecules. We investigated the function of the tapasin-related protein, TAPBPR. Like tapasin, TAPBPR is widely expressed, IFN-γ-inducible, and binds to MHC class I coupled with β2-microglobulin in the endoplasmic reticulum. In contrast to tapasin, TAPBPR does not bind ERp57 or calreticulin and is not an integral component of the PLC. β2-microglobulin is essential for the association between TAPBPR and MHC class I. However, the association between TAPBPR and MHC class I occurs in the absence of a functional PLC, suggesting peptide is not required. Expression of TAPBPR decreases the rate of MHC class I maturation through the secretory pathway and prolongs the association of MHC class I on the PLC. The TAPBPR:MHC class I complex trafficks through the Golgi apparatus, demonstrating a function of TAPBPR beyond the endoplasmic reticulum/cis-Golgi. The identification of TAPBPR as an additional component of the MHC class I antigen-presentation pathway demonstrates that mechanisms controlling MHC class I expression remain incompletely understood.

SDS-PAGE for ³⁵S immunoprecipitation and immunoprecipitation western blotting

This report discusses recent methods of sample preparation and gel electrophoresis for (35)S immunoprecipitation (IP) and IP western blotting. In both methods, IP is used to obtain purified proteins, and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) is used to separate the proteins on a gel. In (35)S IP, the proteins are radiolabeled and visualized on film by fluorography; in IP blotting, proteins are transferred onto nitrocellulose paper, and antibodies are used to detect specific proteins. A similar IP and SDS-PAGE method can be used for both procedures, but IP blotting has the potential advantages of improvement in sensitivity for low-abundance proteins and enhanced specificity for identification of proteins from a mixture. Some of the technical adaptations discussed here to facilitate IP blotting and avoid loss of beads or purified proteins may also be useful for (35)S IP.

Molecular architecture of the MHC I peptide-loading complex: one tapasin molecule is essential and sufficient for antigen processing

The loading of antigen-derived peptides onto MHC class I molecules for presentation to cytotoxic T cells is a key process in adaptive immune defense. Loading of MHC I is achieved by a sophisticated machinery, the peptide-loading complex (PLC), which is organized around the transporter associated with antigen processing (TAP) with the help of several auxiliary proteins. As an essential adapter protein recruiting MHC I molecules to TAP, tapasin catalyzes peptide loading of MHC I. However, the exact stoichiometry and basic molecular architecture of TAP and tapasin within the PLC remains elusive. Here, we demonstrate that two tapasin molecules are assembled in the PLC, with one tapasin bound to each TAP subunit. However, one tapasin molecule bound either to TAP1 or TAP2 is sufficient for efficient MHC I antigen presentation. By specifically blocking the interaction between tapasin-MHC I complexes and the translocation complex TAP, the MHC I surface expression is impaired to the same extent as with soluble tapasin. Thus, the proximity of the peptide supplier TAP to the acceptor MHC I is crucial for antigen processing. In summary, the human PLC consists maximally of 2× tapasin-ERp57/MHC I per TAP complex, but one tapasin-ERp57/MHC I in the PLC is essential and sufficient for antigen processing.

Direct evidence that the N-terminal extensions of the TAP complex act as autonomous interaction scaffolds for the assembly of the MHC I peptide-loading complex

The loading of antigenic peptides onto major histocompatibility complex class I (MHC I) molecules is an essential step in the adaptive immune response against virally or malignantly transformed cells. The ER-resident peptide-loading complex (PLC) consists of the transporter associated with antigen processing (TAP1 and TAP2), assembled with the auxiliary factors tapasin and MHC I. Here, we demonstrated that the N-terminal extension of each TAP subunit represents an autonomous domain, named TMD₀, which is correctly targeted to and inserted into the ER membrane. In the absence of coreTAP, each TMD₀ recruits tapasin in a 1:1 stoichiometry. Although the TMD₀s lack known ER retention/retrieval signals, they are localized to the ER membrane even in tapasin-deficient cells. We conclude that the TMD₀s of TAP form autonomous interaction hubs linking antigen translocation into the ER with peptide loading onto MHC I, hence ensuring a major function in the integrity of the antigen-processing machinery.

Analysis of major histocompatibility complex class I folding: novel insights into intermediate forms

Folding around a peptide ligand is integral to the antigen presentation function of major histocompatibility complex (MHC) class I molecules. Several lines of evidence indicate that the broadly cross-reactive 34-1-2 antibody is sensitive to folding of the MHC class I peptide-binding groove. Here, we show that peptide-loading complex proteins associated with the murine MHC class I molecule K(d) are found primarily in association with the 34-1-2(+) form. This led us to hypothesize that the 34-1-2 antibody may recognize intermediately, as well as fully, folded MHC class I molecules. To further characterize the form(s) of MHC class I molecules recognized by 34-1-2, we took advantage of its cross-reactivity with L(d) . Recognition of the open and folded forms of L(d) by the 64-3-7 and 30-5-7 antibodies, respectively, has been extensively characterized, providing us with parameters against which to compare 34-1-2 reactivity. We found that the 34-1-2(+) L(d) molecules displayed characteristics indicative of incomplete folding, including increased tapasin association, endoplasmic reticulum retention, and instability at the cell surface. Moreover, we show that an L(d) -specific peptide induced folding of the 34-1-2(+) L(d) intermediate. Altogether, these results yield novel insights into the nature of MHC class I molecules recognized by the 34-1-2 antibody.

Molecular cloning and characterization of sea bass (Dicentrarchus labrax, L.) Tapasin

Mammalian tapasin (TPN) is a key member of the major histocompatibility complex (MHC) class I antigen presentation pathway, being part of the multi-protein complex called the peptide loading complex (PLC). Several studies describe its important roles in stabilizing empty MHC class I complexes, facilitating peptide loading and editing the repertoire of bound peptides, with impact on CD8(+) T cell immune responses. In this work, the gene and cDNA of the sea bass (Dicentrarchus labrax) glycoprotein TPN have been isolated and characterized. The coding sequence has a 1329 bp ORF encoding a 442-residue precursor protein with a predicted 24-amino acid leader peptide, generating a 418-amino acid mature form that retains a conserved N-glycosylation site, three conserved mammalian tapasin motifs, two Ig superfamily domains, a transmembrane domain and an ER-retention di-lysine motif at the C-terminus, suggestive of a function similar to mammalian tapasins. Similar to the human counterpart, the sea bass TPN gene comprises 8 exons, some of which correspond to separate functional domains of the protein. A three-dimensional homology model of sea bass tapasin was calculated and is consistent with the structural features described for the human molecule. Together, these results support the concept that the basic structure of TPN has been maintained through evolution. Moreover, the present data provides information that will allow further studies on cell-mediated immunity and class I antigen presentation pathway in particular, in this important fish species.

Productive association between MHC class I and tapasin requires the tapasin transmembrane/cytosolic region and the tapasin C-terminal Ig-like domain

The current model of antigen assembly with major histocompatibility complex (MHC) class I molecules posits that interactions between the tapasin N-terminal immunoglobulin (Ig)-like domain and the MHC class I peptide-binding groove permit tapasin to regulate antigen selection. Much less is known regarding interactions that might involve the tapasin C-terminal Ig-like domain. Additionally, the tapasin transmembrane/cytoplasmic region enables tapasin to bridge the MHC class I molecule to the transporter associated with antigen processing (TAP). In this investigation, we made use of two tapasin mutants to determine the relative contribution of the tapasin C-terminal Ig-like domain and the tapasin transmembrane/cytoplasmic region to the assembly of MHC class I molecules. Deletion of a loop within the tapasin C-terminal Ig-like domain (Δ334-342) prevented tapasin association with the MHC class I molecule K(d). Although tapasin Δ334-342 did not increase the efficiency of K(d) folding, K(d) surface expression was enhanced on cells expressing this mutant relative to tapasin-deficient cells. In contrast to tapasin Δ334-342, a soluble tapasin mutant lacking the transmembrane/cytoplasmic region retained the ability to bind to K(d) molecules, but did not facilitate K(d) surface expression. Furthermore, when soluble tapasin and tapasin Δ334-342 were co-expressed, soluble tapasin had a dominant negative effect on the folding and surface expression of not only K(d), but also D(b) and K(b). In addition, our molecular modeling of the MHC class I-tapasin interface revealed novel potential interactions involving tapasin residues 334-342. Together, these findings demonstrate that the tapasin C-terminal and transmembrane/cytoplasmic regions are critical to tapasin's capacity to associate effectively with the MHC class I molecule.

Tapasin discriminates peptide-human leukocyte antigen-A*02:01 complexes formed with natural ligands

A plethora of peptides are generated intracellularly, and most peptide-human leukocyte antigen (HLA)-I interactions are of a transient, unproductive nature. Without a quality control mechanism, the HLA-I system would be stressed by futile attempts to present peptides not sufficient for the stable peptide-HLA-I complex formation required for long term presentation. Tapasin is thought to be central to this essential quality control, but the underlying mechanisms remain unknown. Here, we report that the N-terminal region of tapasin, Tpn(1-87), assisted folding of peptide-HLA-A*02:01 complexes according to the identity of the peptide. The facilitation was also specific for the identity of the HLA-I heavy chain, where it correlated to established tapasin dependence hierarchies. Two large sets of HLA-A*02:01 binding peptides, one extracted from natural HLA-I ligands from the SYFPEITHI database and one consisting of medium to high affinity non-SYFPEITHI ligands, were studied in the context of HLA-A*02:01 binding and stability. We show that the SYFPEITHI peptides induced more stable HLA-A*02:01 molecules than the other ligands, although affinities were similar. Remarkably, Tpn(1-87) could functionally discriminate the selected SYFPEITHI peptides from the other peptide binders with high sensitivity and specificity. We suggest that this HLA-I- and peptide-specific function, together with the functions exerted by the more C-terminal parts of tapasin, are major features of tapasin-mediated HLA-I quality control. These findings are important for understanding the biogenesis of HLA-I molecules, the selection of presented T-cell epitopes, and the identification of immunogenic targets in both basic research and vaccine design.

The cell biology of major histocompatibility complex class I assembly: towards a molecular understanding

Major histocompatibility complex class I (MHC I) proteins protect the host from intracellular pathogens and cellular abnormalities through the binding of peptide fragments derived primarily from intracellular proteins. These peptide-MHC complexes are displayed at the cell surface for inspection by cytotoxic T lymphocytes. Here we reveal how MHC I molecules achieve this feat in the face of numerous levels of quality control. Among these is the chaperone tapasin, which governs peptide selection in the endoplasmic reticulum as part of the peptide-loading complex, and we propose key amino acid interactions central to the peptide selection mechanism. We discuss how the aminopeptidase ERAAP fine-tunes the peptide repertoire available to assembling MHC I molecules, before focusing on the journey of MHC I molecules through the secretory pathway, where calreticulin provides additional regulation of MHC I expression. Lastly we discuss how these processes culminate to influence immune responses.

The transporter associated with antigen processing (TAP) is active in a post-ER compartment

The translocation of cytosolic peptides into the lumen of the endoplasmic reticulum (ER) is a crucial step in the presentation of intracellular antigen to T cells by major histocompatibility complex (MHC) class I molecules. It is mediated by the transporter associated with antigen processing (TAP) protein, which binds to peptide-receptive MHC class I molecules to form the MHC class I peptide-loading complex (PLC). We investigated whether TAP is present and active in compartments downstream of the ER. By fluorescence microscopy, we found that TAP is localized to the ERGIC (ER-Golgi intermediate compartment) and the Golgi of both fibroblasts and lymphocytes. Using an in vitro vesicle formation assay, we show that COPII vesicles, which carry secretory cargo out of the ER, contain functional TAP that is associated with MHC class I molecules. Together with our previous work on post-ER localization of peptide-receptive class I molecules, our results suggest that loading of peptides onto class I molecules in the context of the peptide-loading complex can occur outside the ER.

Mutational analysis reveals a complex interplay of peptide binding and multiple biological features of HLA-B27

Molecular polymorphism influences the strong association of HLA-B27 with ankylosing spondylitis through an unknown mechanism. Natural subtypes and site-directed mutants were used to analyze the effect of altering the peptide-binding site of this molecule on its stability, interaction with tapasin, folding, and export. The disease-associated subtypes B*2705, B*2702, and B*2704 showed higher thermostability at 50 °C than all other subtypes and mutants, except some mimicking B*2702 polymorphism. The lowest values were found among pocket B mutants, most of which interacted strongly with tapasin, but otherwise there was no correlation between thermostability and tapasin interaction. Mutants resulting in increased hydrophobicity frequently acquired their maximal thermostability faster than those with increased polarity, suggesting that this process is largely driven by the thermodynamics of peptide binding. Folding, export, and tendency to misfold were influenced by polymorphism all along the peptide-binding site and were not specifically dependent on any particular region or structural feature. Frequent uncoupling of thermostability, folding/misfolding, and export can be explained by the distinct effect of mutations on the acquisition of a folded conformation, the optimization rate of B27-peptide complexes, and their quality control in the endoplasmic reticulum, all of which largely depend on the ways in which mutations alter peptide binding, without excluding additional effects on interactions with tapasin or other proteins involved in folding and export. The similarity of the generally disease-associated B*2707 to nondisease-associated subtypes in all the features analyzed suggests that molecular properties other than antigen presentation may not currently explain the relationship between HLA-B27 polymorphism and ankylosing spondylitis.

Tapasin edits peptides on MHC class I molecules by accelerating peptide exchange

The endoplasmic reticulum (ER) protein tapasin is essential for the loading of high-affinity peptides onto MHC class I molecules. It mediates peptide editing, i.e. the binding of peptides of successively higher affinity until class I molecules pass ER quality control and exit to the cell surface. The molecular mechanism of action of tapasin is unknown. We describe here the reconstitution of tapasin-mediated peptide editing on class I molecules in the lumen of microsomal membranes. We find that in a competitive situation between high- and low-affinity peptides, tapasin mediates the binding of the high-affinity peptide to class I by accelerating the dissociation of the peptide from an unstable intermediate of the binding reaction.

Calreticulin-dependent recycling in the early secretory pathway mediates optimal peptide loading of MHC class I molecules

Calreticulin is a lectin chaperone of the endoplasmic reticulum (ER). In calreticulin-deficient cells, major histocompatibility complex (MHC) class I molecules travel to the cell surface in association with a sub-optimal peptide load. Here, we show that calreticulin exits the ER to accumulate in the ER-Golgi intermediate compartment (ERGIC) and the cis-Golgi, together with sub-optimally loaded class I molecules. Calreticulin that lacks its C-terminal KDEL retrieval sequence assembles with the peptide-loading complex but neither retrieves sub-optimally loaded class I molecules from the cis-Golgi to the ER, nor supports optimal peptide loading. Our study, to the best of our knowledge, demonstrates for the first time a functional role of intracellular transport in the optimal loading of MHC class I molecules with antigenic peptide.

Intracellular assembly and trafficking of MHC class I molecules

The presentation of antigenic peptides by class I molecules of the major histocompatibility complex begins in the endoplasmic reticulum (ER) where the co-ordinated action of molecular chaperones, folding enzymes and class I-specific factors ensures that class I molecules are loaded with high-affinity peptide ligands that will survive prolonged display at the cell surface. Once assembled, class I molecules are released from the quality-control machinery of the ER for export to the plasma membrane where they undergo dynamic endocytic cycling and turnover. We review recent progress in our understanding of class I assembly, anterograde transport and endocytosis and highlight some of the events targeted by viruses as a means to evade detection by cytotoxic T cells and natural killer cells.

Presentation of telomerase reverse transcriptase, a self-tumor antigen, is down-regulated by histone deacetylase inhibition

Histone deacetylases (HDAC) modify the architecture of chromatin, leading to decreased gene expression, an effect that is reversed by HDAC inhibition. The balance between deacetylation and acetylation is central to many biological events including the regulation of cell proliferation and cancer but also the differentiation of immune T cells. The effects of HDAC inhibition on the interaction between antitumor effector T cells and tumor cells are not known. Here, we studied presentation of a universal self-tumor antigen, telomerase reverse transcriptase, in human tumor cells during HDAC inhibition. We found that HDAC inhibition with trichostatin A was associated with a decreased presentation and diminished killing of tumor cells by CTLs. Using gene array analysis, we found that HDAC inhibition resulted in a decrease of genes coding for proteasome catalytic proteins and for tapasin, an endoplasmic reticulum resident protein involved in the MHC class I pathway of endogenous antigen presentation. Our findings indicate that epigenetic changes in tumor cells decrease self-tumor antigen presentation and contribute to reduced recognition and killing of tumor cells by cytotoxic T lymphocytes. This mechanism could contribute to tumor escape from immune surveillance.

Amyloid precursor-like protein 2 increases the endocytosis, instability, and turnover of the H2-K(d) MHC class I molecule

The defense against the invasion of viruses and tumors relies on the presentation of viral and tumor-derived peptides to CTL by cell surface MHC class I molecules. Previously, we showed that the ubiquitously expressed protein amyloid precursor-like protein 2 (APLP2) associates with the folded form of the MHC class I molecule K(d). In the current study, APLP2 was found to associate with folded K(d) molecules following their endocytosis and to increase the amount of endocytosed K(d). In addition, increased expression of APLP2 was shown to decrease K(d) surface expression and thermostability. Correspondingly, K(d) thermostability and surface expression were increased by down-regulation of APLP2 expression. Overall, these data suggest that APLP2 modulates the stability and endocytosis of K(d) molecules.

Recent advances in antigen processing and presentation

Heterogeneous intracellular pathways and biochemical mechanisms are responsible for generating the glycoprotein complexes of peptide and major histocompatibility complex that are displayed on the surfaces of antigen-presenting cells for recognition by T lymphocytes. These pathways have a profound influence on the specificity of adaptive immunity and tolerance, as well as the context and consequences of antigen recognition by T cells in the thymus and periphery. The field of antigen processing and presentation has continued to advance since the publication of a focus issue on the topic in Nature Immunology in July 2004. Progress has been made on many fronts, including advances in understanding how proteases, accessory molecules and intracellular pathways influence peptide loading and antigen presentation in various cell types.

The Double Role of the Endoplasmic Reticulum Chaperone Tapasin in Peptide Optimization of HLA Class I Molecules

During the assembly of the HLA class I molecules with peptides in the peptide-loading complex, a series of transient interactions are made with ER-resident chaperones. These interactions culminate in the trafficking of the HLA class I molecules to the cell surface and presentation of peptides to CD8(+) T lymphocytes. Within the peptide-loading complex, the glycoprotein tapasin exhibits a relevant function. This immunoglobulin (Ig) superfamily member in the endoplasmic reticulum membrane tethers empty HLA class I molecules to the transporter associated with antigen-processing (TAP) proteins. This review will address the current concepts regarding the double role that tapasin plays in the peptide optimization and surface expression of the HLA class I molecules.

Multiple residues in the transmembrane helix and connecting peptide of mouse tapasin stabilize the transporter associated with the antigen-processing TAP2 subunit

The type I endoplasmic reticulum (ER) glycoprotein tapasin (Tpn) is essential for loading of major histocompatibility complex class I (MHC-I) molecules with an optimal spectrum of antigenic peptides and for stable expression of the heterodimeric, polytopic TAP peptide transporter. In a detailed mutational analysis, the transmembrane domain (TMD) and ER-luminal connecting peptide (CP) of mouse Tpn were analyzed for their capacity to stabilize the TAP2 subunit. Replacement of the TMD of Tpn by TMDs from calnexin or the Tpn-related protein, respectively, completely abolished TAP2 stabilization after transfection of Tpn-deficient cells, whereas TMDs derived from distantly related Tpn molecules (chicken and fish) were functional. A detailed mutational analysis of the TMD and adjacent residues in the ER-luminal CP of mouse Tpn was performed to elucidate amino acids that control the stabilization of TAP2. Single amino acid substitutions, including a conserved Lys residue in the center of the putative TMD, did not affect TAP2 expression levels. Mutation of this Lys plus four additional residues, predicted to be neighbors in an assumed alpha-helical TMD arrangement, abrogated the TAP2-stabilizing capacity of Tpn. In the presence of a wild-type TMD, also the substitution of a highly conserved Glu residue in the CP of Tpn strongly affected TAP2 stabilization. Defective TAP2 stabilization resulted in impaired cell surface expression of MHC-I molecules. This study thus defines a novel, spatially arranged motif in the TMD of Tpn essential for stable expression of the TAP2 protein and a novel protein interaction mode involving an ER-luminal Glu residue close to the membrane.