Processing pathways for presentation of cytosolic antigen to MHC class II-restricted T cells

Introducing Molecular Chaperones into the Causality and Prospective Management of Autoimmune Hepatitis

Molecular chaperones influence the immunogenicity of peptides and the activation of effector T cells, and their pathogenic roles in autoimmune hepatitis are unclear. Heat shock proteins are pivotal in the processing and presentation of peptides that activate CD8+ T cells. They can also induce regulatory B and T cells and promote immune tolerance. Tapasin and the transporter associated with antigen processing-binding protein influence the editing and loading of high-affinity peptides for presentation by class I molecules of the major histocompatibility complex. Their over-expression could enhance the autoimmune response, and their deficiency could weaken it. The lysosome-associated membrane protein-2a isoform in conjunction with heat shock cognate 70 supports the importation of cytosolic proteins into lysosomes. Chaperone-mediated autophagy can then process the peptides for activation of CD4+ T cells. Over-expression of autophagy in T cells may also eliminate negative regulators of their activity. The human leukocyte antigen B-associated transcript three facilitates the expression of class II peptide receptors, inhibits T cell apoptosis, prevents T cell exhaustion, and sustains the immune response. Immunization with heat shock proteins has induced immune tolerance in experimental models and humans with autoimmune disease by inducing regulatory T cells. Therapeutic manipulation of other molecular chaperones may promote T cell exhaustion and induce tolerogenic dendritic cells. In conclusion, molecular chaperones constitute an under-evaluated family of ancillary proteins that could affect the occurrence, severity, and outcome of autoimmune hepatitis. Clarification of their contributions to the immune mechanisms and clinical activity of autoimmune hepatitis could have therapeutic implications.

The Autophagy Receptor TAX1BP1 (T6BP) improves antigen presentation by MHC-II molecules

CD4⁺ T lymphocytes play a major role in the establishment and maintenance of immunity. They are activated by antigenic peptides derived from extracellular or newly synthesized (endogenous) proteins presented by the MHC-II molecules. The pathways leading to endogenous MHC-II presentation remain poorly characterized. We demonstrate here that the autophagy receptor, T6BP, influences both autophagy-dependent and -independent endogenous presentation of HIV- and HCMV-derived peptides. By studying the immunopeptidome of MHC-II molecules, we show that T6BP affects both the quantity and quality of peptides presented. T6BP silencing induces the mislocalization of the MHC-II-loading compartments and rapid degradation of the invariant chain (CD74) without altering the expression and internalization kinetics of MHC-II molecules. Defining the interactome of T6BP, we identify calnexin as a T6BP partner. We show that the calnexin cytosolic tail is required for this interaction. Remarkably, calnexin silencing replicates the functional consequences of T6BP silencing: decreased CD4⁺ T cell activation and exacerbated CD74 degradation. Altogether, we unravel T6BP as a key player of the MHC-II-restricted endogenous presentation pathway, and we propose one potential mechanism of action.

Targeting Cross-Presentation as a Route to Improve the Efficiency of Peptide-Based Cancer Vaccines

Cross-presenting dendritic cells (DC) offer an attractive target for vaccination due to their unique ability to process exogenous antigens for presentation on MHC class I molecules. Recent reports have established that these DC express unique surface receptors and play a critical role in the initiation of anti-tumor immunity, opening the way for the development of vaccination strategies specifically targeting these cells. This study investigated whether targeting cross-presenting DC by two complementary mechanisms could improve vaccine effectiveness, in both a viral setting and in a murine melanoma model. Our novel vaccine construct contained the XCL1 ligand, to target uptake to XCR1+ cross-presenting DC, and a cell penetrating peptide (CPP) with endosomal escape properties, to enhance antigen delivery into the cross-presentation pathway. Using a prime-boost regimen, we demonstrated robust expansion of antigen-specific T cells following vaccination with our CPP-linked peptide vaccine and protective immunity against HSV-1 skin infection, where vaccine epitopes were natively expressed by the virus. Additionally, our novel vaccination strategy slowed tumor outgrowth in a B16 murine melanoma model, compared to adjuvant only controls, suggesting antigen-specific anti-tumor immunity was generated following vaccination. These findings suggest that novel strategies to target the antigen cross-presentation pathway in DC may be beneficial for the generation of anti-tumor immunity.

Antigen processing and presentation

Dendritic cells are at the center of immune responses. They are defined by their ability to sense the environment, take up and process antigen, migrate to secondary lymphoid organs, where they present antigens to the adaptive immune system. In particular, they present lipids and proteins from pathogens, which they encountered in peripheral tissues, to T cells in order to induce a specific effector immune response. These complex antigens need to be broken down into peptides of a certain length in association with Major Histocompatibility Complex (MHC) molecules. Presentation of MHC/antigen complexes alongside costimulatory molecules and secretion of proinflammatory cytokines will induce an appropriate immune response. This interaction between dendritic cells and T cells takes place at defined locations within secondary lymphoid organs. In this review, we discuss the current knowledge and recent advances on the cellular and molecular mechanisms that underlie antigen processing and the subsequent presentation to T lymphocytes.

Anti-malarial drugs: possible mechanisms of action in autoimmune disease and prospects for drug development

A wide variety of mechanisms of anti-rheumatic action have been proposed for antimalarial agents. The molecular actions of chloroquine have been most thoroughly studied in vitro and in vivo, but it is likely that hydroxychloroquine works by a similar mechanism. Both agents are weak diprotic bases that can pass through the lipid cell membrane and preferentially concentrate in acidic cyto-plasmic vesicles. The resulting slight elevation of pH within these vesicles in macrophages or other antigen-presenting cells may influence the immune response to autoantigens. We hypothesize that anti-malarial agents influence the association of autoantigenic peptides with class II MHC molecules in the compartment for peptide loading and/or the subsequent processing and transport of the peptide-MHC complex to the cell membrane. This model of anti-malarial action provides a method to test additional drugs for their ability to modulate the immune response.

The CCR4-NOT complex contributes to repression of Major Histocompatibility Complex class II transcription

The multi-subunit CCR4 (carbon catabolite repressor 4)-NOT (Negative on TATA) complex serves as a central coordinator of all different steps of eukaryotic gene expression. Here we performed a systematic and comparative analysis of cells where the CCR4-NOT subunits CNOT1, CNOT2 or CNOT3 were individually downregulated using doxycycline-inducible shRNAs. Microarray experiments showed that downregulation of either CNOT subunit resulted in elevated expression of major histocompatibility complex class II (MHC II) genes which are found in a gene cluster on chromosome 6. Increased expression of MHC II genes after knock-down or knock-out of either CNOT subunit was seen in a variety of cell systems and also in naïve macrophages from CNOT3 conditional knock-out mice. CNOT2-mediated repression of MHC II genes occurred also in the absence of the master regulator class II transactivator (CIITA) and did not cause detectable changes of the chromatin structure at the chromosomal MHC II locus. CNOT2 downregulation resulted in an increased de novo transcription of mRNAs whereas tethering of CNOT2 to a regulatory region governing MHC II expression resulted in diminished transcription. These results expand the known repertoire of CCR4-NOT members for immune regulation and identify CNOT proteins as a novel group of corepressors restricting class II expression.

Fighting Viral Infections and Virus-Driven Tumors with Cytotoxic CD4+ T Cells

CD4+ T cells have been and are still largely regarded as the orchestrators of immune responses, being able to differentiate into distinct T helper cell populations based on differentiation signals, transcription factor expression, cytokine secretion, and specific functions. Nonetheless, a growing body of evidence indicates that CD4+ T cells can also exert a direct effector activity, which depends on intrinsic cytotoxic properties acquired and carried out along with the evolution of several pathogenic infections. The relevant role of CD4+ T cell lytic features in the control of such infectious conditions also leads to their exploitation as a new immunotherapeutic approach. This review aims at summarizing currently available data about functional and therapeutic relevance of cytotoxic CD4+ T cells in the context of viral infections and virus-driven tumors.

The elucidation of non-classical MHC class II antigen processing through the study of viral antigens

By convention, CD4+ T cells are activated predominantly by Major Histocompatibility Complex class II-bound peptides derived from extracellular (exogenous) antigens. It has been known for decades that alternative sources of antigen, particularly those synthesized within the antigen-presenting cell, can also supply peptides but the impact on TCD4+ responses, sometimes considerable, has only recently become appreciated. This review focuses on the contributions that studies of viral antigen have made to this shift in perspective, concluding with discussions of relevance to rational vaccine design, autoimmunity and cancer immunotherapy.

Variations in MHC Class II Antigen Processing and Presentation in Health and Disease

MHC class II (MHC-II) molecules are critical in the control of many immune responses. They are also involved in most autoimmune diseases and other pathologies. Here, we describe the biology of MHC-II and MHC-II variations that affect immune responses. We discuss the classic cell biology of MHC-II and various perturbations. Proteolysis is a major process in the biology of MHC-II, and we describe the various components forming and controlling this endosomal proteolytic machinery. This process ultimately determines the MHC-II-presented peptidome, including cryptic peptides, modified peptides, and other peptides that are relevant in autoimmune responses. MHC-II also variable in expression, glycosylation, and turnover. We illustrate that MHC-II is variable not only in amino acids (polymorphic) but also in its biology, with consequences for both health and disease.

Endogenous Antigen Presentation of MHC Class II Epitopes through Non-Autophagic Pathways

Antigenic peptides presented by major histocompatibility complex (MHC) class II molecules are generally derived from exogenous proteins acquired by antigen presenting cells. However, in some circumstances, MHC class II molecules can present intracellular proteins expressed within the antigen-presenting cells. There are several described pathways by which endogenous antigens are degraded and gain access to MHC class II molecules. These include autophagy and other non-autophagic pathways; the latter category includes the MHC class I-like pathways, heat shock protein 90-mediated pathways, and internalization from the plasma membrane. This review will summarize and discuss the non-autophagic pathways.

Antigen Presentation, Autoantigens, and Immune Regulation in Multiple Sclerosis and Other Autoimmune Diseases

Antigen presentation is in the center of the immune system, both in host defense against pathogens, but also when the system is unbalanced and autoimmune diseases like multiple sclerosis (MS) develop. It is not just by chance that a major histocompatibility complex gene is the major genetic susceptibility locus in MS; a feature that MS shares with other autoimmune diseases. The exact etiology of the disease, however, has not been fully understood yet. T cells are regarded as the major players in the disease, but most probably a complex interplay of altered central and peripheral tolerance mechanisms, T-cell and B-cell functions, characteristics of putative autoantigens, and a possible interference of environmental factors like microorganisms are at work. In this review, new data on all these different aspects of antigen presentation and their role in MS will be discussed, probable autoantigens will be summarized, and comparisons to other autoimmune diseases will be drawn.

Modified vaccinia virus Ankara-infected dendritic cells present CD4+ T-cell epitopes by endogenous major histocompatibility complex class II presentation pathways

CD4(+) T lymphocytes play a central role in the immune system and mediate their function after recognition of their respective antigens presented on major histocompatibility complex II (MHCII) molecules on antigen-presenting cells (APCs). Conventionally, phagocytosed antigens are loaded on MHCII for stimulation of CD4(+) T cells. Certain epitopes, however, can be processed directly from intracellular antigens and are presented on MHCII (endogenous MHCII presentation). Here we characterized the MHCII antigen presentation pathways that are possibly involved in the immune response upon vaccination with modified vaccinia virus Ankara (MVA), a promising live viral vaccine vector. We established CD4(+) T-cell lines specific for MVA-derived epitopes as tools for in vitro analysis of MHCII antigen processing and presentation in MVA-infected APCs. We provide evidence that infected APCs are able to directly transfer endogenous viral proteins into the MHCII pathway to efficiently activate CD4(+) T cells. By using knockout mice and chemical inhibitory compounds, we further elucidated the molecular basis, showing that among the various subcellular pathways investigated, proteasomes and autophagy are key players in the endogenous MHCII presentation during MVA infection. Interestingly, although proteasomal processing plays an important role, neither TAP nor LAMP-2 was found to be involved in the peptide transport. Defining the molecular mechanism of MHCII presentation during MVA infection provides a basis for improving MVA-based vaccination strategies by aiming for enhanced CD4(+) T-cell activation by directing antigens into the responsible pathways.

IMPORTANCE

This work contributes significantly to our understanding of the immunogenic properties of pathogens by deciphering antigen processing pathways contributing to efficient activation of antigen-specific CD4(+) T cells. We identified autophagosome formation, proteasomal activity, and lysosomal integrity as being crucial for endogenous CD4(+) T-cell activation. Since poxvirus vectors such as MVA are already used in clinical trials as recombinant vaccines, the data provide important information for the future design of optimized poxviral vaccines for the study of advanced immunotherapy options.

Parasite fate and involvement of infected cells in the induction of CD4+ and CD8+ T cell responses to Toxoplasma gondii

During infection with the intracellular parasite Toxoplasma gondii, the presentation of parasite-derived antigens to CD4⁺ and CD8⁺ T cells is essential for long-term resistance to this pathogen. Fundamental questions remain regarding the roles of phagocytosis and active invasion in the events that lead to the processing and presentation of parasite antigens. To understand the most proximal events in this process, an attenuated non-replicating strain of T. gondii (the cpsII strain) was combined with a cytometry-based approach to distinguish active invasion from phagocytic uptake. In vivo studies revealed that T. gondii disproportionately infected dendritic cells and macrophages, and that infected dendritic cells and macrophages displayed an activated phenotype characterized by enhanced levels of CD86 compared to cells that had phagocytosed the parasite, thus suggesting a role for these cells in priming naïve T cells. Indeed, dendritic cells were required for optimal CD4⁺ and CD8⁺ T cell responses, and the phagocytosis of heat-killed or invasion-blocked parasites was not sufficient to induce T cell responses. Rather, the selective transfer of cpsII-infected dendritic cells or macrophages (but not those that had phagocytosed the parasite) to naïve mice potently induced CD4⁺ and CD8⁺ T cell responses, and conferred protection against challenge with virulent T. gondii. Collectively, these results point toward a critical role for actively infected host cells in initiating T. gondii-specific CD4⁺ and CD8⁺ T cell responses.

CD4⁺ and CD8⁺ T cells are critical for controlling many infections. To generate a T cell response during infection, T cells must encounter the microbial peptides that they recognize bound to MHC molecules on the surfaces of other cells, such as dendritic cells. It is currently unclear how dendritic cells acquire the antigens they present to T cells during infection with many intracellular pathogens. It is possible that these antigens are phagocytosed and processed by dendritic cells, or antigens may be presented by cells that are infected by pathogens such as Toxoplasma gondii, which invades host cells independently of phagocytosis. To differentiate these pathways, we developed a novel technique to track the fate of T. gondii in vivo that distinguishes actively infected cells from those that phagocytosed parasites. This technique was used to examine each of these cell populations. We also used pharmacological inhibitors of parasite invasion, and the transfer of sort-purified infected or uninfected dendritic cells and macrophages to determine what roles phagocytosis and active invasion have in the initiation of T cell responses. Our results demonstrate that phagocytosis of parasites is not sufficient to induce CD4⁺ or CD8⁺ T cell responses, whereas infected cells are critical for this process.

Toward a Network Model of MHC Class II-Restricted Antigen Processing

The standard model of Major Histocompatibility Complex class II (MHCII)-restricted antigen processing depicts a straightforward, linear pathway: internalized antigens are converted into peptides that load in a chaperone dependent manner onto nascent MHCII in the late endosome, the complexes subsequently trafficking to the cell surface for recognition by CD4(+) T cells (TCD4+). Several variations on this theme, both moderate and radical, have come to light but these alternatives have remained peripheral, the conventional pathway generally presumed to be the primary driver of TCD4+ responses. Here we continue to press for the conceptual repositioning of these alternatives toward the center while proposing that MHCII processing be thought of less in terms of discrete pathways and more in terms of a network whose major and minor conduits are variable depending upon many factors, including the epitope, the nature of the antigen, the source of the antigen, and the identity of the antigen-presenting cell.

Genetic analysis of interferon induced thyroiditis (IIT): evidence for a key role for MHC and apoptosis related genes and pathways

Autoimmune thyroid diseases (AITD) have become increasingly recognized as a complication of interferon-alpha (IFNα) therapy in patients with chronic Hepatitis C virus (HCV) infection. Interferon-induced thyroiditis (IIT) can manifest as clinical thyroiditis in approximately 15% of HCV patients receiving IFNα and subclinical thyroiditis in up to 40% of patients, possibly resulting in either dose reduction or discontinuation of IFNα treatment. However, the exact mechanisms that lead to the development of IIT are unknown and may include IFNα-mediated immune-recruitment as well as direct toxic effects on thyroid follicular cells. We hypothesized that IIT develops in genetically predisposed individuals whose threshold for developing thyroiditis is lowered by IFNα. Therefore, our aim was to identify the susceptibility genes for IIT. We used a genomic convergence approach combining genetic association data with transcriptome analysis of genes upregulated by IFNα. Integrating results of genetic association, transcriptome data, pathway, and haplotype analyses enabled the identification of 3 putative loci, SP100/110/140 (2q37.1), HLA (6p21.3), and TAP1 (6p21.3) that may be involved in the pathogenesis of IIT. Immune-regulation and apoptosis emerged as the predominant mechanisms underlying the etiology of IIT.

Sculpting MHC class II-restricted self and non-self peptidome by the class I Ag-processing machinery and its impact on Th-cell responses

It is generally assumed that the MHC class I antigen (Ag)-processing (CAP) machinery - which supplies peptides for presentation by class I molecules - plays no role in class II-restricted presentation of cytoplasmic Ags. In striking contrast to this assumption, we previously reported that proteasome inhibition, TAP deficiency or ERAAP deficiency led to dramatically altered T helper (Th)-cell responses to allograft (HY) and microbial (Listeria monocytogenes) Ags. Herein, we tested whether altered Ag processing and presentation, altered CD4(+) T-cell repertoire, or both underlay the above finding. We found that TAP deficiency and ERAAP deficiency dramatically altered the quality of class II-associated self peptides suggesting that the CAP machinery impacts class II-restricted Ag processing and presentation. Consistent with altered self peptidomes, the CD4(+) T-cell receptor repertoire of mice deficient in the CAP machinery substantially differed from that of WT animals resulting in altered CD4(+) T-cell Ag recognition patterns. These data suggest that TAP and ERAAP sculpt the class II-restricted peptidome, impacting the CD4(+) T-cell repertoire, and ultimately altering Th-cell responses. Together with our previous findings, these data suggest multiple CAP machinery components sequester or degrade MHC class II-restricted epitopes that would otherwise be capable of eliciting functional Th-cell responses.

Beyond the classical: influenza virus and the elucidation of alternative MHC class II-restricted antigen processing pathways

CD4+ T cells (T(CD4+)) are activated by peptides, generally 13-17 amino acids in length, presented at the cell surface in combination with highly polymorphic MHC class II molecules. According to the classical model, these peptides are generated by endosomal digestion of internalized antigen and loaded onto MHC class II molecules in the late endosome. Historically, this "exogenous" pathway has been defined through the extensive use of purified proteins. However, the relatively recent use of clinically relevant antigens, those of influenza virus in our case, has revealed several additional pathways of peptide production, including some that are truly "endogenous", entailing synthesis of the protein within the infected cell. Indeed, some peptides appear to be created only via endogenous processing. The cell biology that underlies these alternative pathways remains poorly understood as do their relative contributions to defence against infectious agents and cancer, and the triggering of autoimmune diseases.

Proteasomes, TAP, and endoplasmic reticulum-associated aminopeptidase associated with antigen processing control CD4+ Th cell responses by regulating indirect presentation of MHC class II-restricted cytoplasmic antigens

Cytoplasmic Ags derived from viruses, cytosolic bacteria, tumors, and allografts are presented to T cells by MHC class I or class II molecules. In the case of class II-restricted Ags, professional APCs acquire them during uptake of dead class II-negative cells and present them via a process called indirect presentation. It is generally assumed that the cytosolic Ag-processing machinery, which supplies peptides for presentation by class I molecules, plays very little role in indirect presentation of class II-restricted cytoplasmic Ags. Remarkably, upon testing this assumption, we found that proteasomes, TAP, and endoplasmic reticulum-associated aminopeptidase associated with Ag processing, but not tapasin, partially destroyed or removed cytoplasmic class II-restricted Ags, such that their inhibition or deficiency led to dramatically increased Th cell responses to allograft (HY) and microbial (Listeria monocytogenes) Ags, both of which are indirectly presented. This effect was neither due to enhanced endoplasmic reticulum-associated degradation nor competition for Ag between class I and class II molecules. From these findings, a novel model emerged in which the cytosolic Ag-processing machinery regulates the quantity of cytoplasmic peptides available for presentation by class II molecules and, hence, modulates Th cell responses.

Crohn disease: a current perspective on genetics, autophagy and immunity

Crohn disease (CD) is a chronic and debilitating inflammatory condition of the gastrointestinal tract. Prevalence in Western populations is 100-150/100,000 and somewhat higher in Ashkenazi Jews. Peak incidence is in early adult life, although any age can be affected and a majority of affected individuals progress to relapsing and chronic disease. Medical treatments rely significantly on empirical corticosteroid therapy and immunosuppression, and intestinal resectional surgery is frequently required. Thus, 80% of patients with CD come to surgery for refractory disease or complications. It is hoped that an improved understanding of pathogenic mechanisms, for example by studying the genetic basis of CD and other forms of inflammatory bowel diseases (IBD), will lead to improved therapies and possibly preventative strategies in individuals identified as being at risk.

Crohn disease: a current perspective on genetics, autophagy and immunity

Crohn disease (CD) is a chronic and debilitating inflammatory condition of the gastrointestinal tract. Prevalence in Western populations is 100-150/100,000 and somewhat higher in Ashkenazi Jews. Peak incidence is in early adult life, although any age can be affected and a majority of affected individuals progress to relapsing and chronic disease. Medical treatments rely significantly on empirical corticosteroid therapy and immunosuppression, and intestinal resectional surgery is frequently required. Thus, 80% of patients with CD come to surgery for refractory disease or complications. It is hoped that an improved understanding of pathogenic mechanisms, for example by studying the genetic basis of CD and other forms of inflammatory bowel diseases (IBD), will lead to improved therapies and possibly preventative strategies in individuals identified as being at risk.

Alternative endogenous protein processing via an autophagy-dependent pathway compensates for Yersinia-mediated inhibition of endosomal major histocompatibility complex class II antigen presentation

Extracellular Yersinia pseudotuberculosis employs a type III secretion system (T3SS) for translocating virulence factors (Yersinia outer proteins [Yops]) directly into the cytosol of eukaryotic cells. Recently, we used YopE as a carrier molecule for T3SS-dependent secretion and translocation of listeriolysin O (LLO) from Listeria monocytogenes. We demonstrated that translocation of chimeric YopE/LLO into the cytosol of macrophages by Yersinia results in the induction of a codominant antigen-specific CD4 and CD8 T-cell response in orally immunized mice. In this study, we addressed the requirements for processing and major histocompatibility complex (MHC) class II presentation of chimeric YopE proteins translocated into the cytosol of macrophages by the Yersinia T3SS. Our data demonstrate the ability of Yersinia to counteract exogenous MHC class II antigen presentation of secreted hybrid YopE by the action of wild-type YopE and YopH. In the absence of exogenous MHC class II antigen presentation, an alternative pathway was identified for YopE fusion proteins originating in the cytosol. This endogenous antigen-processing pathway was sensitive to inhibitors of phagolysosomal acidification and macroautophagy, but it did not require the function either of the proteasome or of transporters associated with antigen processing. Thus, by an autophagy-dependent mechanism, macrophages are able to compensate for the YopE/YopH-mediated inhibition of the endosomal MHC class II antigen presentation pathway for exogenous antigens. This is the first report demonstrating that autophagy might enable the host to mount an MHC class II-restricted CD4 T-cell response against translocated bacterial virulence factors. We provide critical new insights into the interaction between the mammalian immune system and a human pathogen.

Antigen processing via autophagy--not only for MHC class II presentation anymore?

T cells monitor intracellular and extracellular protein composition via proteolytic products that are displayed to them on major histocompatibility complex (MHC) molecules. For this purpose it has been documented that MHC class II molecules, which were originally thought to just display lysosomal products of endocytosed proteins to CD4(+) helper T cells, can also present intracellular substrates of autophagic pathways. This has triggered the interest of immunologists into the role of autophagy in antigen processing in general, and recently additional autophagic mechanisms for intracellular and extracellular antigen processing onto MHC class I molecules for presentation to CD8(+) cytolytic T cells have been revealed. Here, I will review the contribution of autophagy for MHC class I and class II antigen processing and presentation to T cells.

Expression of antigen processing and presenting molecules by Schwann cells in inflammatory neuropathies

Schwann cells are the myelinating glia cells of the peripheral nervous system (PNS) and can become targets of an autoimmune response in inflammatory neuropathies like the Guillain-Barré syndrome (GBS). Professional antigen presenting cells (APCs) are known to promote autoimmune responses in target tissues by presenting self-antigens. Other cell types could participate in local autoimmune responses by acting as nonprofessional APCs. Using a combined approach of immunocytochemistry, immunohistochemistry, and flow cytometry analysis we demonstrate that human Schwann cells express the antigen processing and presenting machinery (APM) in vitro and in vivo. Moreover, cultured human Schwann cells increase the expression of proteasome subunit delta (Y), antigen peptide transporter TAP2, and HLA Class I and HLA Class II complexes in an inflammatory environment. In correlation with this observation, Schwann cells in sural nerve biopsies from GBS patients show increased expression of antigen processing and presenting molecules. Furthermore, cultured human Schwann cells can proteolytically digest fluorescently-labeled nonmammalian antigen ovalbumin. Taken together, our data suggest antigen processing and presentation as a possible function of Schwann cells that may contribute to (auto)immune responses within peripheral nerves.

HLA-DR: molecular insights and vaccine design

Vaccines are one of the most cost effective methods to control infectious diseases and at the same time one of the most complex products of the pharmaceutical industry. In contrast to other drugs, vaccines are used mainly in healthy individuals, often in children. For this reason, very high standards are set for their production. Subunit vaccines, especially peptide vaccines, can provide a safe and cost-effective alternative to vaccines produced from attenuated or inactivated pathogen preparations. Biochemical and structural studies of class II MHC-peptide complexes are beginning to provide a conceptual foundation for the rational design of subunit and peptide vaccines. In this review, we show how analysis of peptide-class II MHC complexes together with developing understanding of antigen processing pathways has opened the door to understanding the major rules that govern selection of T cell epitopes. We review progress towards computational prediction of such epitopes, and efforts to evaluate algorithms that incorporate various structural and/or biochemical aspects of the MHC-peptide interaction. Finally, using malaria as a model, we describe the development of a minimal subunit vaccine for the human malaria parasite Plasmodium falciparum.

How can a chemical element elicit complex immunopathology? Lessons from mercury-induced autoimmunity

Although most autoimmune diseases develop without a manifest cause, epidemiological studies indicate that external factors play an important role in triggering or aggravating autoimmune processes in genetically predisposed individuals. Nevertheless, most autoimmune disease-promoting environmental agents are unknown because their relationships to immune function are not understood. Thus, the study of animal models of chemically-induced autoimmunity should shed light on the pathways involved and allow us to identify these agents. The rodent model of heavy metal-induced autoimmunity is one of the most intriguing experimental systems available to address such questions. Although the ultimate pathophysiology of this model remains mysterious, recent studies have started to elucidate the mechanisms by which heavy metal exposure leads to immune activation and loss of self-tolerance.

Autophagy and its role in MHC-mediated antigen presentation

Intracellular degradation by autophagy plays a role in the maintenance of cellular homeostasis under normal conditions and during periods of cellular stress. Autophagy has also been implicated in several other cellular processes including immune recognition and responsiveness. More specifically, autophagy has been identified as a route by which cytoplasmic and nuclear Ag are delivered to MHC class II molecules for presentation to CD4(+) T cells. Autophagy has also recently been implicated in MHC class I cross-presentation of tumor Ag and the activation of CD8(+) T cells. This review discusses the role of autophagy in modulating MHC class I and class II Ag presentation as well as its implication in regulating autoimmunity and tolerance, tumor immunity, and host defense against intracellular pathogens.

Variation in the ATP-binding cassette transporter 2 gene is a separate risk factor for systemic lupus erythematosus within the MHC

The ATP-binding cassette transporter (TAP) proteins are functionally relevant candidates for predisposition to systemic lupus erythematosus (SLE) by virtue of their role in autoantigen presentation and location in the major histocompatibility complex (MHC). We tested if variation in the TAP genes (TAP1 and TAP2) is associated with SLE. We genotyped tag single nucleotide polymorphisms (SNPs) and performed family-based association analysis on 390 Caucasian pedigrees. We found significant evidence of association between TAP2 and SLE (rs241453, P=1.33 x 10(-6)). Conditional logistic regression analysis suggests that this TAP2 effect is separate from the HLA-DRB1 alleles. Our analyses show that both rs241453 (P=1.6 x 10(-4)) and HLA-DRB1*03xx (P=2.3 x 10(-4)) have significant autonomous effects not due to linkage disequilibrium. Moreover, these loci exhibit a significant statistical interaction (P<1.0 x 10(-6)), demonstrated by an increase in the odds ratio for the TAP2 association from OR=2.00 (95% confidence interval (CI)=1.17-3.42) in HLA-DRB1*03xx-negative subjects to OR=4.29 (CI=1.88-9.76) in the subjects with at least one HLA-DRB1*03xx allele group. We report the largest association study of the TAP genes with SLE to date, and the first to test for its separate effect and interaction with the HLA alleles consistently associated with SLE.

Down-regulation of MHC class II expression through inhibition of CIITA transcription by lytic transactivator Zta during Epstein-Barr virus reactivation

The presentation of peptides to T cells by MHC class II molecules is of critical importance in specific recognition to a pathogen by the immune system. The level of MHC class II directly influences T lymphocyte activation. The aim of this study was to identify the possible mechanisms of the down-regulation of MHC class II expression by Zta during EBV lytic cycle. The data in the present study demonstrated that ectopic expression of Zta can strongly inhibit the constitutive expression of MHC class II and CIITA in Raji cells. The negative effect of Zta on the CIITA promoter activity was also observed. Scrutiny of the DNA sequence of CIITA promoter III revealed the presence of two Zta-response element (ZRE) motifs that have complete homology to ZREs in the DR and left-hand side duplicated sequence promoters of EBV. By chromatin immunoprecipitation assays, the binding of Zta to the ZRE(221) in the CIITA promoter was verified. Site-directed mutagenesis of three conserved nucleotides of the ZRE(221) substantially disrupted Zta-mediated inhibition of the CIITA promoter activity. Oligonucleotide pull-down assay showed that mutation of the ZRE(221) dramatically abolished Zta binding. Analysis of the Zta mutant lacking DNA binding domain revealed that the DNA-binding activity of Zta is required for the trans repression of CIITA. The expression of HLA-DRalpha and CIITA was restored by Zta gene silencing. The data indicate that Zta may act as an inhibitor of the MHC class II pathway, suppressing CIITA transcription and thus interfering with the expression of MHC class II molecules.

Autophagy, antiviral immunity, and viral countermeasures

The autophagy pathway likely evolved not only to maintain cellular and tissue homeostasis but also to protect cells against microbial attack. This conserved mechanism by which cytoplasmic cargo is delivered to the endolysosomal system is now recognized as a central player in coordinating the host response to diverse intracellular pathogens, including viruses. As an endolysosomal delivery system, autophagy functions in the transfer of viruses from the cytoplasm to the lysosome where they are degraded, in the transfer of viral nucleic acids to endosomal sensors for the activation of innate immunity, and in the transfer of endogenous viral antigens to MHC class II compartments for the activation of adaptive immunity. Viruses have, in turn, evolved different strategies to antagonize, and potentially, to exploit the host autophagic machinery. Moreover, through mechanisms not yet well understood, autophagy may dampen host innate immune and inflammatory responses to viral infection. This review highlights the roles of autophagy in antiviral immunity, viral strategies to evade autophagy, and potential negative feedback functions of autophagy in the host antiviral response.

Macroautophagy, endogenous MHC II loading and T cell selection: the benefits of breaking the rules

Functional and biochemical assays indicate a substantial contribution of intracellularly derived peptides to the MHC class II 'ligandome'. Macroautophagy, a process traditionally known for its role in cellular housekeeping and adaptation to nutrient withdrawal, is an attractive candidate pathway for endogenous MHC class II loading. Work in cell culture systems, including antigen presentation assays, co-localization studies and sequencing of MHC class II bound peptides, demonstrates that substrates of autophagy can be loaded onto MHC class II. Advances in the development of mouse models to monitor or genetically disrupt macroautophagy now provide the basis for elucidating the immunological relevance of autophagy in vivo. Here, we will discuss recent findings suggesting a crucial role of macroautophagy in thymic epithelial cells for the generation of peptide/MHC class II ligands for positive selection and induction of T cell tolerance.

Immunology of mercury

The heavy metal mercury is ubiquitously distributed in the environment resulting in permanent low-level exposure in human populations. Mercury can be encountered in three main chemical forms (elemental, inorganic, and organic) which can affect the immune system in different ways. In this review, we describe the effects of these various forms of mercury exposure on immune cells in humans and animals. In genetically susceptible mice or rats, subtoxic doses of mercury induce the production of highly specific autoantibodies as well as a generalized activation of the immune system. We review studies performed in this model and discuss their implications for the role of environmental chemicals in human autoimmunity.

Autophagy in MHC class II presentation of endogenous antigens

Macroautophagy is a catabolic process for the lysosomal turnover of cell organelles and protein aggregates. Lysosomal degradation products are displayed by major histocompatibility class II molecules to CD4(+) T cells in the steady state for tolerance induction and during infections to mount adaptive immune responses. It has recently been shown that macroautophagy substrates can also give rise to MHC class II ligands. We review here the breadth of antigens that may utilize this pathway and the possible implications of this alternate route to MHC class II antigen presentation for immunity and tolerance. Based on this discussion, it is apparent that the regulation of macroautophagy may be beneficial in various disease settings in order to enhance adaptive immune responses or to reduce autoimmunity.

Host responses in the bursa of Fabricius of chickens infected with virulent Marek's disease virus

The bursa of Fabricius serves as an important tissue in the process of Marek's disease virus (MDV) pathogenesis, since B cells of the bursa harbor the cytolytic phase of MDV replication cycle. In the present study, host responses associated with MDV infection in the bursa of Fabricius of chickens were investigated. The expression of MDV phosphoprotein (pp)38 antigen, MDV glycoprotein (gB) and MDV viral interleukin (vIL)-8 transcripts was at the highest at 4 days post-infection (d.p.i.) and then showed a declining trend. On the contrary, the expression of meq (MDV EcoRI Q) gene as well as the viral genome load increased gradually until day 14 post-infection. The changes in viral parameters were associated with significantly higher infiltration of macrophages and T cell subsets, particularly CD4+ T cells into the bursa of Fabricius. Of the genes examined, the expression of interferon (IFN)-alpha, IFN-gamma genes and inducible nitric oxide synthase (iNOS) was significantly up-regulated in response to MDV infection in the bursa of Fabricius. The results suggest a role for these cells and cytokines in MDV-induced responses in the bursa of Fabricius.

Autophagy in CD4+ T-cell immunity and tolerance

Autophagy is a homeostatic process that enables eukaryotic cells to deliver cytoplasmic constituents for lysosomal degradation, to recycle nutrients and to survive during starvation. In addition to these primordial functions, autophagy has emerged as a key mechanism in orchestrating innate and adaptive immune responses to intracellular pathogens. Autophagy restricts viral infections as well as replication of intracellular bacteria and parasites and delivers pathogenic determinants for TLR stimulation and for MHC class II presentation to the adaptive immune system. Apart from its role in defense against pathogens, autophagy-mediated presentation of self-antigens in the steady state could have a crucial role in the induction and maintenance of CD4(+) T-cell tolerance. This review describes the mechanisms by which the immune system utilizes autophagic degradation of cytoplasmic material to regulate adaptive immune responses.

Enhancement of experimental Graves' disease by intranasal administration of a T cell epitope of the thyrotropin receptor

We previously showed that immunization of mice with murine fibroblasts transfected with the thyrotropin receptor (TSHR) and a murine major histocompatibility complex (MHC) class II molecule induces immune thyroid disease with the humoral and histological features of human Graves' disease in about 20% of mice. In this model, based on the proliferative response of T cells from hyperthyroid mice to a panel of overlapping TSHR peptides, we now demonstrate that TSHR 121-140 peptide contains an immunodominant T cell epitope. Supporting this conclusion, spleen cells from mice immunized with TSHR 121-140 peptide showed a strong proliferative response to fibroblasts transfected with the TSHR and a murine I-A(k) molecule, but not either alone. Also, intranasal administration of 100 mug of TSHR 121-140 peptide led to suppressed proliferative response of lymph node cells to the peptide. Interestingly, however, administration of this peptide enhanced, rather than suppressed, the frequency and severity of Graves' disease induced by the immunization of the fibroblasts transfected with the TSHR and a murine I-A(k) molecule. Spleen cells from hyperthyroid mice that were pretreated with intranasal peptide tended to produce lesser amounts of IL-4, IL-10 and IFN-gamma than those from normothyroid control mice. Although precise mechanisms of this enhancement remain to be determined, the results suggest that attempts to treat Graves' disease by intranasal administration of an immunodominant TSHR T cell epitope may aggravate, not prevent, the disease.

Cytosol to lysosome transport of intracellular antigens during immune surveillance

The delivery of intracellular substrates such as misfolded proteins and damaged organelles from the cytosol to the lysosome for degradation is crucial for cell survival. Multiple transport pathways including bulk autophagy (microautophagy and macroautophagy) and chaperone‐mediated autophagy (CMA) have been identified to efficiently facilitate this transit of macromolecules from the cytoplasm to acidic vacuolar organelles. While autophagy plays a role in the general housekeeping of cells, it also functions in more specialized processes such as development and differentiation, responses to physiological stress and immunity. The presentation of both exogenous and endogenous antigens (Ag) by major histocompatibility complex (MHC) class II molecules to CD4⁺ T lymphocytes is critical for the induction of tolerance to self Ag as well as the development of immunity against intracellular pathogens and tumors. Here, we discuss the class II‐mediated presentation of several endogenous Ag, dependent on either macroautophagy or CMA for their transport from the cytosol to endosomal/lysosomal compartments. Thus, the various pathways of autophagy as routes of cytoplasmic Ag delivery to lysosomes have significant implications for the MHC class II‐mediated immune response to intracellular pathogens and cancer.

Initiation of primary anti-vaccinia virus immunity in vivo

The primary focus of our work is the initiation of an antiviral immune response. While we employ many experimental systems to address this fundamental issue, much of our work revolves around the use of vaccinia virus. Concerns over the negative effects of vaccination have prevented the return of the smallpox immunization program to the general population and underscored the importance of understanding the primary immune response to vaccinia virus. This response is comprised of a complex symphony of immune system components employing a variety of different mechanisms. In this review, we will both highlight the roles of many of these components and touch on the applications of vaccinia virus in the laboratory and the clinic.

The impact of T cell intrinsic antigen adaptation on peripheral immune tolerance

Overlapping roles have been ascribed for T cell anergy, clonal deletion, and regulation in the maintenance of peripheral immunological tolerance. A measurement of the individual and additive impacts of each of these processes on systemic tolerance is often lacking. In this report we have used adoptive transfer strategies to tease out the unique contribution of T cell intrinsic receptor calibration (adaptation) in the maintenance of tolerance to a systemic self-antigen. Adoptively transferred naïve T cells stably calibrated their responsiveness to a persistent self-antigen in both lymphopenic and T cell-replete hosts. In the former, this state was not accompanied by deletion or suppression, allowing us to examine the unique contribution of adaptation to systemic tolerance. Surprisingly, adapting T cells could chronically help antigen-expressing B cells, leading to polyclonal hypergammaglobulinemia and pathology, in the form of mild arthritis. The helper activity mediated by CD40L and cytokines was evident even if the B cells were introduced after extended adaptation of the T cells. In contrast, in the T cell-replete host, neither arthritis nor autoantibodies were induced. The containment of systemic pathology required host T cell-mediated extrinsic regulatory mechanisms to synergize with the cell intrinsic adaptation process. These extrinsic mechanisms prevented the effector differentiation of the autoreactive T cells and reduced their precursor frequency, in vivo.

A Role for Intercellular Antigen Transfer in the Recognition of EBV-Transformed B Cell Lines by EBV Nuclear Antigen-Specific CD4+T Cells

The CD4+ T cell response to EBV may have an important role in controlling virus-driven B lymphoproliferation because CD4+ T cell clones to a subset of EBV nuclear Ag (EBNA) epitopes can directly recognize virus-transformed lymphoblastoid cell lines (LCLs) in vitro and inhibit their growth. In this study, we used a panel of EBNA1, 2, 3A, and 3C-specific CD4+ T cell clones to study the route whereby endogenously expressed EBNAs access the HLA class II-presentation pathway. Two sets of results spoke against a direct route of intracellular access. First, none of the clones recognized cognate Ag overexpressed in cells from vaccinia vectors but did recognize Ag fused to an endo/lysosomal targeting sequence. Second, focusing on clones with the strongest LCL recognition that were specific for EBNA2- and EBNA3C-derived epitopes LCL recognition was unaffected by inhibiting autophagy, a postulated route for intracellular Ag delivery into the HLA class II pathway in LCL cells. Subsequently, using these same epitope-specific clones, we found that Ag-negative cells with the appropriate HLA-restricting allele could be efficiently sensitized to CD4+ T cell recognition by cocultivation with Ag-positive donor lines or by exposure to donor line-conditioned culture medium. Sensitization was mediated by a high m.w. antigenic species and required active Ag processing by recipient cells. We infer that intercellular Ag transfer plays a major role in the presentation of EBNA-derived CD4 epitopes by latently infected target cells.

Chaperone-mediated autophagy in aging and disease

Different mechanisms target intracellular components for their degradation into lysosomes through what is known as autophagy. In mammals, three main forms of autophagy have been described: macroautophagy, microautophagy, and chaperone-mediated autophagy (CMA). CMA is the only autophagic pathway that allows selective degradation of soluble proteins in lysosomes. In contrast to the other mammalian forms of autophagy, CMA does not require vesicle formation or major changes in the lysosomal membrane. Instead, substrate proteins directly cross the lysosomal membrane to reach the lumen, where they are rapidly degraded. The substrate proteins are targeted to the lysosomal membrane by recognition of a targeting motif (a KFERQ-like motif), by a chaperone complex, consisting of hsc70 and its cochaperones, in the cytoplasm. Once at the lysosomal membrane, the protein interacts with a lysosomal receptor for this pathway, lysosomal associated membrane protein type 2A (LAMP-2A), and it is translocated across the membrane into the lysosomal lumen assisted by a lysosome resident chaperone. These two characteristics--selectivity and direct substrate translocation--determine the particular role of CMA in different physiological and pathological conditions. In this chapter, we cover current findings on the molecular mechanisms for CMA and the possible pathophysiological relevance of this selective lysosomal degradation.

A role for the Hsp90 molecular chaperone family in antigen presentation to T lymphocytes via major histocompatibility complex class II molecules

The heat shock protein (HSP) Hsp90 is known to chaperone cytosolic peptides for MHC class I (MHCI)-restricted antigen presentation to T lymphocytes. We now demonstrate a role for Hsp90 activity in presentation of antigens on MHCII. Treatment of mouse antigen-presenting cells (APC) with the pharmacological Hsp90 inhibitor, geldanamycin, inhibited MHCII-mediated presentation of endocytosed and cytosolic proteins as well as synthetic peptides to specific T cells. Ectopic expression of human Hsp90 in APC enhanced MHCII-mediated antigen presentation. Further, pharmacological Hsp90 inhibition reduced, while retroviral Hsp90 overexpression enhanced, the levels of stable compact MHCII heterodimers correlating with the antigen presentation phenotype. Pharmacological inhibition of Hsp90 activity in IFN-gamma-treated APC resulted in severe abrogation of MHCII-restricted presentation of cytosolic antigen, but only partially inhibited exogenous antigen presentation. Our data suggest a major role for Hsp90 activity in MHCII-mediated antigen presentation pathways, and implicate IFN-gamma-inducible Hsp90-independent mechanisms.

Differential MHC class II presentation of a pathogenic autoantigen during health and disease

Glucose-6-phosphate isomerase (GPI) is the target autoantigen recognized by KRN T cells in the K/BxN model of rheumatoid arthritis. T cell reactivity to this ubiquitous Ag results in the recruitment of anti-GPI B cells and subsequent immune complex-mediated arthritis. Because all APCs have the capacity to process and present this autoantigen, it is unclear why systemic autoimmunity with polyclonal B cell activation does not ensue. To this end, we examined how GPI is presented by B cells relative to other immunologically relevant APCs such as dendritic cells (DCs) and macrophages in the steady state, during different phases of arthritis development, and after TLR stimulation. Although all APCs can process and present the GPI:I-A(g7) complex, they do so with different efficiencies. DCs are the most potent at baseline and become progressively more potent with disease development correlating with immune complex uptake. Interestingly, in vivo and in vitro maturation of DCs did not enhance GPI presentation, suggesting that DCs use mechanisms to regulate the presentation of self-peptides. Non-GPI-specific B cells are the weakest APCs (100-fold less potent than DCs) and fail to productively engage KRN T cells at steady state and during arthritis. However, the ability to stimulate KRN T cells is strongly enhanced in B cells after TLR ligation and provides a mechanism whereby polyclonal B cells may be activated in the wake of an acute infection.

Immune surveillance of intracellular pathogens via autophagy

MHC class II molecules are thought to present peptides derived from extracellular proteins to CD4+ T cells, which are important mediators of adaptive immunity to infections. In contrast, autophagy delivers constitutively cytosolic material for lysosomal degradation and has so far been recognized as an efficient mechanism of innate immunity against bacteria and viruses. Recent studies, however, link these two pathways and suggest that intracellular cytosolic and nuclear antigens are processed for MHC class II presentation after autophagy.

TAP and TAP-like--brothers in arms?

The transporter associated with antigen processing like (TAPL, ABCB9) is a member of the ATP-binding cassette (ABC) transporter family. Moreover, TAPL belongs to the TAP family due to its high sequence homology to TAP1 and TAP2. TAPL forms a homodimer which is localized in lysosomes with a minor fraction in the ER. It functions as an ATP-dependent peptide transporter which shows a broad peptide specificity ranging from 6-mer up to 59-mer peptides. In contrast to TAP, TAPL transports peptides with low affinity but high efficiency. This review will briefly summarize current knowledge about the structural organization and possible physiological function of TAPL in antigen processing and presentation.

Association between the TAP2 gene codon 665 polymorphism and Graves' disease

A total of 95 patients with active Graves' disease (GD) and 105 normal healthy subjects were enrolled in this study, which attempted to determine whether single-site polymorphisms of the transporter associated with antigen processing 2 (TAP2) gene contribute to an individual's susceptibility to GD. Such polymorphisms were detected using polymerase chain reaction (PCR)-based restriction analysis. Associations between GD and the three site polymorphisms of the TAP2 gene at codons 379, 565, and 665 were investigated. The results of the genotype analysis revealed that the frequency of the GG homozygote's presence at codon 665 was lower, and that of the AA homozygote's presence was greater in GD patients (15.8% and 36.8%, respectively) compared to normal controls (34.3% and 16.2%, respectively; P<0.001). The OR (OD) for the risk of occurrence for the AA homozygote and AG heterozygote compared to the GG homozygote (as was the case for the GD patients) was respectively 4.941 and 2.117, with respective 95% confidence intervals (CI) of 2.303-10.598 and 1.020-4.369. The allelic analysis also demonstrated reduced G and enhanced A allele frequencies for GD patients compared to controls (respectively 39.5% vs. 59.0% [G allele], and 60.5% vs. 41.0% [A allele]; P=0.0001; OR=2.219, 95% CI: 1.449-3.395). By contrast, the differences between patient and control groups for the frequency of appearance of genotypes and allelic variants at codon 379 (P=0.522 and P=0.306, respectively) and codon 565 (P=0.199 and P=0.157, respectively) did not appear to be significant. These data reveal that the single-site polymorphism of the TAP2 gene at codon 665 may be an indicator for predicting GD development.