The requirement for DM in class II-restricted antigen presentation and SDS-stable dimer formation is allele and species dependent

The CD4+ T cell repertoire specific for citrullinated peptides shows evidence of immune tolerance

Rheumatoid arthritis occurs most often in people who express HLA-DR molecules containing a five aa "shared epitope" in the β chain. These MHCII molecules preferentially bind citrullinated peptides formed by posttranslational modification of arginine. Citrullinated peptide:HLA-DR complexes may act as arthritis-initiating neo-antigens for CD4+ T cells. Here, we used fluorophore-conjugated HLA-DR tetramers containing citrullinated peptides from human cartilage intermediate layer protein, fibrinogen, vimentin, or enolase 1 to track cognate CD4+ T cells. Immunization of HLA-DR transgenic mice with citrullinated peptides from vimentin or enolase 1 failed to cause any expansion of tetramer-binding cells, whereas immunization with citrullinated peptides from cartilage intermediate layer protein or fibrinogen elicited some expansion. The expanded tetramer-binding populations, however, had lower T helper 1 and higher regulatory T cell frequencies than populations elicited by viral peptides. These results indicate that HLA-DR-bound citrullinated peptides are not neo-antigens and induce varying degrees of immune tolerance that could pose a barrier to rheumatoid arthritis.

Protein Vaccination Directs the CD4+ T Cell Response toward Shared Protective Epitopes That Can Be Recalled after Influenza Virus Infection

Vaccination is widely used to generate protective immunity against influenza virus. CD4⁺ T cells contribute in diverse ways to protective immunity, most notably, in the provision of help for the production of neutralizing antibodies. Several recent reports have suggested that influenza virus infection elicits CD4⁺ T cells whose specificity only partially overlaps that of T cells elicited by vaccination. This finding has raised serious concerns regarding the utility of currently licensed inactivated influenza virus vaccines and novel protein-based vaccines. Here, using controlled animal models that allowed a broad sampling of the CD4⁺ T cell repertoire, we evaluated protein vaccine- versus infection-generated CD4⁺ T cell epitopes. Our studies revealed that all the infection-elicited CD4⁺ T cell epitope specificities are also elicited by protein vaccination, although the immunodominance hierarchies can differ. Finally, using a reverse-engineered influenza virus and a heterologous protein vaccination and infection challenge strategy, we show that protein vaccine-elicited CD4⁺ memory T cells are recalled and boosted after infection and provide early help to accelerate hemagglutinin (HA)-specific antibody responses. The early CD4⁺ T cell response and HA-specific antibody production are associated with lowered viral titers during the infection challenge. Our data lend confidence to the ability of current protein-based vaccines to elicit influenza virus-specific CD4⁺ T cells that can potentiate protective immunity upon influenza virus infection.IMPORTANCE Most current and new influenza vaccine candidates consist of a single influenza virus protein or combinations of influenza virus proteins. For these vaccines to elicit CD4⁺ T cells that can be recalled after infection, the peptide epitopes should be shared between the two modes of confrontation. Recently, questions regarding the relatedness of epitope selection by influenza virus infection and protein vaccination have been raised. However, the studies reported here show that the specificity of CD4⁺ T cells elicited by protein-based vaccines overlaps that of T cells elicited by infection and that CD4⁺ T cells primed by protein vaccines are recalled and contribute to protection of the host from a future infection.

Diverse Epitope Specificity, Immunodominance Hierarchy, and Functional Avidity of Effector CD4 T Cells Established During Priming Is Maintained in Lung After Influenza A Virus Infection

One of the major contributions to protective immunity to influenza viruses that is provided by virus-specific CD4 T cells is delivery of effector function to the infected lung. However, there is little known about the selection and breadth of viral epitope-specific CD4 T cells that home to the lung after their initial priming. In this study, using a mouse model of influenza A infection and an unbiased method of epitope identification, the viral epitope-specific CD4 T cells elicited after infection were identified and quantified. We found that a very diverse specificity of CD4 T cells is primed by infection, including epitopes from hemagglutinin, neuraminidase, matrix protein, nucleoprotein, and non-structural protein-1. Using peptide-specific cytokine EliSpots, the diversity and immunodominance hierarchies established in the lung-draining lymph node were compared with specificities of CD4 T cells that home to the lung. Our studies revealed that CD4 T cells of all epitope specificities identified in peripheral lymphoid tissue home back to the lung and that most of these lung-homing cells are localized within the tissue rather than the pulmonary vasculature. There is a striking shift of CD4 T cell functionality that enriches for IFN-γ production as cells are primed in the lymph node, enter the lung vasculature, and finally establish residency in the tissue, but with no apparent shifts in their functional avidity. We conclude that CD4 T cells of broad viral epitope specificity are recruited into the lung after influenza infection, where they then have the opportunity to encounter infected or antigen-bearing antigen-presenting cells.

Immunodominance in CD4 T-cell responses: implications for immune responses to influenza virus and for vaccine design

CD4 T cells play a primary role in regulating immune responses to pathogenic organisms and to vaccines. Antigen-specific CD4 T cells provide cognate help to B cells, a requisite event for immunoglobulin switch and affinity maturation of B cells that produce neutralizing antibodies and also provide help to cytotoxic CD8 T cells, critical for their expansion and persistence as memory cells. Finally, CD4 T cells may participate directly in pathogen clearance via cell-mediated cytotoxicity or through production of cytokines. Understanding the role of CD4 T-cell immunity to viruses and other pathogens, as well as evaluation of the efficacy of vaccines, requires insight into the specificity of CD4 T cells. This review focuses on the events within antigen-presenting cells that focus CD4 T cells toward a limited number of peptide antigens within the pathogen or vaccine. The molecular events are discussed in light of the special challenges that the influenza virus poses, owing to the high degree of genetic variability, unpredictable pathogenicity and the repeated encounters that human populations face with this highly infectious pathogenic organism.

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.

HLA-DM: arbiter conformationis

The recognition by CD4(+) T cells of peptides bound to class II MHC (MHCII) molecules expressed on the surface of antigen-presenting cells is a key step in the initiation of an adaptive immune response. Presentation of peptides is the outcome of an intracellular selection process occurring in dedicated endosomal compartments involving, among others, an MHCII-like molecule named HLA-DM (DM). The impact of DM on the epitope selection machinery has been known for more than 15 years. However, the mechanism by which DM skews the presented repertoire in favour of kinetically stable complexes has remained elusive. Here, a review of the most recent observations in the field is presented, pointing to the possibility that DM decides the survival of a peptide-MHCII complex (pMHCII) on the basis of its conformational flexibility, which is a function of the 'tightness' of interaction between the peptide and the MHCII at a specific region of the binding site.

The mechanism of HLA-DM induced peptide exchange in the MHC class II antigen presentation pathway

HLA-DM serves a critical function in the loading and editing of peptides on MHC class II (MHCII) molecules. Recent data showed that the interaction cycle between MHCII molecules and HLA-DM is dependent on the occupancy state of the peptide binding groove. Empty MHCII molecules form stable complexes with HLA-DM, which are disrupted by binding of high-affinity peptide. Interestingly, MHCII molecules with fully engaged peptides cannot interact with HLA-DM, and prior dissociation of the peptide N-terminus from the groove is required for HLA-DM binding. There are significant similarities to the peptide loading process for MHC class I molecules, even though it is executed by a distinct set of proteins in a different cellular compartment.

HLA-DM captures partially empty HLA-DR molecules for catalyzed removal of peptide

The mechanisms of HLA-DM catalyzed peptide exchange remain uncertain. We found that all stages of the interaction of DM with HLA-DR were dependent on the occupancy state of the peptide binding groove. High-affinity peptides were protected from removal by DM through two mechanisms: peptide binding induced dissociation of a long-lived complex of empty DR and DM, and high-affinity DR-peptide complexes bound DM only very slowly. Non-binding covalent DR-peptide complexes were converted to efficient DM binders upon truncation of an N-terminal peptide segment that emptied the P1 pocket and disrupted conserved hydrogen bonds to MHC. DM thus only binds to DR conformers in which a critical part of the binding site is vacant, due to spontaneous peptide motion.

Generation of MHC class II-peptide ligands for CD4 T-cell allorecognition of MHC class II molecules

PURPOSE OF REVIEW

The molecular and cellular mechanisms that underlie allorecognition of MHC class II molecules have been the subject of much debate and experimentation in recent decades. In this review, we discuss several aspects of MHC class II structure, peptide acquisition and TcR-MHC-peptide interactions that have particular relevance to recognition of cells bearing allogeneic class II molecules.

RECENT FINDINGS

First, MHC polymorphism is heavily biased toward those amino acids that influence stable peptide binding by MHC class II. Second, the peptide repertoire presented by class II molecules is highly diverse and can be edited substantially by the molecular catalyst HLA-DM and by tissue-specific expression of HLA-DO, stress and cytokines. Third, T-cell receptor docking onto MHC peptide consistently involves substantial contacts with the bound peptide in the MHC class II molecule. Finally, there is increasing evidence that T-cell recognition of MHC is, in part, germline encoded through T-cell-receptor V region contacts with MHC class II alpha helices.

SUMMARY

Together, these conclusions support the view that allorecognition of MHC class II molecules is likely to parallel key aspects of conventional CD4 T-cell recognition, with allele-dependent variation in peptide representation accounting in large part for the high precursor frequency of alloreactive CD4 T cells.

Presenting exogenous antigen to T cells

Antigen processing and presentation experiments can be done with a wide variety of antigen-presenting cells (APCs). Most experiments will use one of the "professional" APC types: dendritic cells (DCs), macrophages, and B lymphocytes. Other types of cells may be used for antigen presentation in some circumstances. Each type of professional APC has an important antigen-presentation function, but the different APC types contribute to different aspects of the immune response. Therefore, selection of an APC type for study must include consideration of the stage or aspect of immune response that is to be modeled in the experiment. An important technical distinction for some types of experiments is whether the APCs are adherent or nonadherent, since this dictates the procedures that must be used to wash the cells as the medium is changed.

The relationship between immunodominance, DM editing, and the kinetic stability of MHC class II:peptide complexes

Immunodominance refers to the restricted antigen specificity of T cells detected in the immune response after immunization with complex antigens. Despite the presence of many potential peptide epitopes within these immunogens, the elicited T-cell response apparently focuses on a very limited number of peptides. Over the last two decades, a number of distinct explanations have been put forth to explain this very restricted specificity of T cells, many of which suggest that endosomal antigen processing restricts the array of peptides available to recruit CD4 T cells. In this review, we present evidence from our laboratory that suggest that immunodominance in CD4 T-cell responses is primarily due to an intrinsic property of the peptide:class II complexes. The intrinsic kinetic stability of peptide:class II complexes controls DM editing within the antigen-presenting cells and thus the initial epitope density on priming dendritic cells. Additionally, we hypothesize that peptides that possess high kinetic stability interactions with class II molecules display persistence at the cell surface over time and will more efficiently promote T-cell signaling and differentiation than competing, lower-stability peptides contained within the antigen. We discuss this model in the context of the existing data in the field of immunodominance.

A defective viral superantigen-presenting phenotype in HLA-DR transfectants is corrected by CIITA

Activation of T lymphocytes by mouse mammary tumor virus superantigen (vSAg) requires binding to MHC class II molecules. The subcellular location where functional interactions occur between MHC class II molecules and vSAgs is still a matter of debate. To gain further insight into this issue, we have used human epithelial HeLa cells expressing HLA-DR1. Surprisingly, the human cells were unable to present transfected vSAg7 or vSAg9 to a series of murine T cell hybridomas. The defect is not related to a lack of vSAg processing, because these cells can indirectly activate T cells after coculture in the presence of B lymphocytes. However, after IFN-gamma treatment, the HeLa DR1(+) cells became apt at directly presenting the vSAg. Furthermore, transfection of CIITA was sufficient to restore presentation. Reconstitution experiments demonstrated the necessity of coexpressing HLA-DM and invariant chain (Ii) for efficient vSAg presentation. Interestingly, inclusion of a dileucine motif in the DRbeta cytoplasmic tail bypassed the need for HLA-DM expression and allowed the efficient presentation of vSAg7 in the presence of Ii. A similar trafficking signal was included in vSAg7 by replacing its cytoplasmic tail with the one of Ii. However, sorting of this chimeric Ii/vSAg molecule to the endocytic pathway completely abolished both its indirect and direct presentation. Together, our results suggest that functional vSAgs-DR complexes form after the very late stages of class II maturation, most probably at the cell surface.

Replacement of the membrane proximal region of I-Ad MHC class II molecule with I-E-derived sequences promotes production of an active and stable soluble heterodimer without altering peptide-binding specificity

The MHC class II molecule I-A is the murine homologue of HLA-DQ in humans. The I-A and DQ heterodimers display considerable heterodimer instability compared with their I-E and HLA-DR counterparts. This isotype-specific behavior makes the production of soluble I-A and DQ molecules very difficult. We have developed a strategy for production of soluble I-A(d) molecules involving expression of I-A(d) as a glycosil phosphatidyl inositol (PI) anchored chimera in Chinese Hamster Ovary (CHO) cells. The regions comprising the membrane proximal segments of I-A(d) alpha and beta chains were substituted for the corresponding regions of I-E, and the derived constructs were expressed in CHO cells. Procedures for purification of the soluble class II molecules were optimized and the WT and chimeric molecule were compared for structure, biochemical stability and functionality. Our analysis revealed that the substitutions in the membrane proximal domains improved cell surface expression and thermal stability of I-A(d) without altering the peptide binding specificity of the class II molecule. The results suggest that similar strategies could be used to increase the stability of other unstable class II molecules for in vitro studies.

Presentation of Endogenously Synthesized MHC Class II-Restricted Epitopes by MHC Class II Cancer Vaccines Is Independent of Transporter Associated with Ag Processing and the Proteasome

Cell-based vaccines consisting of invariant chain-negative tumor cells transfected with syngeneic MHC class II (MHC II) and costimulatory molecule genes are prophylactic and therapeutic agents for the treatment of murine primary and metastatic cancers. Vaccine efficacy is due to direct presentation of endogenously synthesized, MHC II-restricted tumor peptides to CD4+ T cells. Because the vaccine cells lack invariant chain, we have hypothesized that, unlike professional APC, the peptide-binding groove of newly synthesized MHC II molecules may be accessible to peptides, allowing newly synthesized MHC II molecules to bind peptides that have been generated in the proteasome and transported into the endoplasmic reticulum via the TAP complex. To test this hypothesis, we have compared the Ag presentation activity of multiple clones of TAP-negative and TAP-positive tumor cells transfected with I-Ak genes and the model Ag hen egg white lysozyme targeted to the endoplasmic reticulum or cytoplasm. Absence of TAP does not diminish Ag presentation of three hen egg white lysozyme epitopes. Likewise, cells treated with proteasomal and autophagy inhibitors are as effective APC as untreated cells. In contrast, drugs that block endosome function significantly inhibit Ag presentation. Coculture experiments demonstrate that the vaccine cells do not release endogenously synthesized molecules that are subsequently endocytosed and processed in endosomal compartments. Collectively, these data indicate that vaccine cell presentation of MHC II-restricted endogenously synthesized epitopes occurs via a mechanism independent of the proteasome and TAP complex, and uses a pathway that overlaps with the classical endosomal pathway for presentation of exogenously synthesized molecules.

DM and DO shape the repertoire of peptide–MHC-class-II complexes

The presentation of antigenic peptides by MHC class II molecules is essential for activation of CD4+ T cells. The formation of most peptide-MHC-class-II complexes is influenced by the actions of two specialized accessory proteins--DM and DO--located in the endosomal/lysosomal system where peptide loading occurs. DM removes class-II-associated invariant-chain peptide (CLIP) from newly synthesized class II molecules, but by now it is clearly established that this is only a special case of the general peptide-editing function of DM. Recent data have begun to explain the molecular basis for the editing activity. The other accessory protein, DO, modulates DM activity in vitro, but the physiological importance of DO is unclear. New evidence from several laboratories has provided clues that may soon change this.

Regulation of Antigen Presentation and Cross-Presentation in the Dendritic Cell Network: Facts, Hypothesis, and Immunological Implications

Dendritic cells (DCs) are central to the maintenance of immunological tolerance and the initiation and control of immunity. The antigen-presenting properties of DCs enable them to present a sample of self and foreign proteins, contained within an organism at any given time, to the T-cell repertoire. DCs achieve this communication with T cells by displaying antigenic peptides bound to MHC I and MHC II molecules. Here we review the studies carried out over the past 15 years to characterize these antigen presentation mechanisms, emphasizing their significance in relation to DC function in vivo. The life cycles of different DC populations found in vivo are described. Furthermore, we provide a critical assessment of the studies that examine the mechanisms controlling DC MHC class II antigen presentation, which have often reached contradictory conclusions. Finally, we review findings pertaining to the biological mechanisms that enable DCs to present exogenous antigens on their MHC class I molecules, a process known as cross-presentation. Throughout, we highlight what we consider to be major knowledge gaps in the field and speculate on possible directions for future research.

The exogenous pathway for antigen presentation on major histocompatibility complex class II and CD1 molecules

The endosomes and lysosomes of antigen-presenting cells host the processing and assembly reactions that result in the display of peptides on major histocompatibility complex (MHC) class II molecules and lipid-linked products on CD1 molecules. This environment is potentially hostile for T cell epitope and MHC class II survival, and the influence of regulators of protease activity and specialized chaperones that assist MHC class II assembly is crucial. At present, evidence indicates that individual proteases make both constructive and destructive contributions to antigen processing for MHC class II presentation to CD4 T cells. Some features of CD1 antigen capture within the endocytic pathway are also discussed.

Cutting edge: H-2DM is responsible for the large differences in presentation among peptides selected by I-Ak during antigen processing

We quantitated the amounts of peptides from hen egg-white lysozyme presented by I-A(k) molecules in APC lines. The large chemical gradient of presentation of the four hen egg-white lysozyme epitopes observed in cell lines expressing HLA-DM or H-2DM (referred to in this study as DM) was significantly diminished in the T2.A(k) line lacking DM. Differences in levels of presentation between wild-type and DM-deficient APC were observed for all four epitopes, but differences were most evident for the highest affinity epitope. As a result of these quantitative differences in display, presentation of all four epitopes to T cells was impaired in the line lacking DM. The binding affinity of the pool of naturally processed peptides from DM-expressing lines was higher than that from the DM-deficient line. Thus, using a direct biochemical approach in APC, we demonstrate that DM influences the selection of peptides bound to MHC class II by favoring high affinity peptides.

Regulation of the class II MHC pathway in primary human monocytes by granulocyte-macrophage colony-stimulating factor

GM-CSF stimulates the growth and differentiation of hematopoietic progenitors and also affects mature cell function. These effects have led to the use of GM-CSF as a vaccine adjuvant with promising results; however, the mechanisms underlying GM-CSF-mediated immune potentiation are incompletely understood. In this study, we investigated the hypothesis that the immune stimulatory role of GM-CSF is in part due to effects on class II MHC Ag presentation. We find that, in primary human monocytes treated for 24-48 h, GM-CSF increases surface class II MHC expression and decreases the relative level of the invariant chain-derived peptide, CLIP, bound to surface class II molecules. GM-CSF also increases expression of the costimulatory molecules CD86 and CD40, but not the differentiation marker CD1a or CD16. Furthermore, GM-CSF-treated monocytes are better stimulators in a mixed leukocyte reaction. Additional analyses of the class II pathway revealed that GM-CSF increases total protein and RNA levels of HLA-DR, DM, and DOalpha. Expression of class II transactivator (CIITA) types I and III, but not IV, transcripts increases in response to GM-CSF. Furthermore, GM-CSF increases the amount of CIITA associated with the DR promoter. Thus, our data argue that the proinflammatory role of GM-CSF is mediated in part through increased expression of key molecules involved in the class II MHC pathway via induction of CIITA.

The E5 protein of human papillomavirus type 16 perturbs MHC class II antigen maturation in human foreskin keratinocytes treated with interferon-gamma

Major histocompatibility complex (MHC) class II antigens are expressed on human foreskin keratinocytes (HFKs) following exposure to interferon gamma. The expression of MHC class II proteins on the cell surface may allow keratinocytes to function as antigen-presenting cells and induce a subsequent immune response to virus infection. Invariant chain (Ii) is a chaperone protein which plays an important role in the maturation of MHC class II molecules. The sequential degradation of Ii within acidic endocytic compartments is a key process required for the successful loading of antigenic peptide onto MHC class II molecules. Since human papillomavirus (HPV) 16 E5 can inhibit the acidification of late endosomes in HFKs, the E5 protein may be able to affect proper peptide loading onto the MHC class II molecule. To test this hypothesis, HFKs were infected with either control virus or a recombinant virus expressing HPV16 E5 and the infected cells were subsequently treated with interferon-gamma. ELISAs revealed a decrease of MHC class II expression on the surface of E5-expressing cells compared with control virus-infected cells after interferon treatment. Western blot analysis showed that, in cells treated with interferon gamma, E5 could prevent the breakdown of Ii and block the formation of peptide-loaded, SDS-stable mature MHC class II dimers, correlating with diminished surface MHC class II expression. These data suggest that HPV16 E5 may be able to decrease immune recognition of infected keratinocytes via disruption of MHC class II protein function.

DM loss in k haplotype mice reveals isotype-specific chaperone requirements

DM actions as a class II chaperone promote capture of diverse peptides inside the endocytic compartment(s). DM mutant cells studied to date express class II bound by class II-associated invariant chain-derived peptide (CLIP), a short proteolytic fragment of the invariant chain, and exhibit defective peptide-loading abilities. To evaluate DM functional contributions in k haplotype mice, we engineered a novel mutation at the DMa locus via embryonic stem cell technology. The present experiments demonstrate short-lived A(k)/CLIP complexes, decreased A(k) surface expression, and enhanced A(k) peptide binding activities. Thus, we conclude that DM loss in k haplotype mice creates a substantial pool of empty or loosely occupied A(k) conformers. On the other hand, the mutation hardly affects E(k) activities. The appearance of mature compact E(k) dimers, near normal surface expression, and efficient Ag presentation capabilities strengthen the evidence for isotype-specific DM requirements. In contrast to DM mutants described previously, partial occupancy by wild-type ligands is sufficient to eliminate antiself reactivity. Mass spectrometry profiles reveal A(k)/CLIP and a heterogeneous collection of relatively short peptides bound to E(k) molecules. These experiments demonstrate that DM has distinct roles depending on its specific class II partners.

Hierarchical contributions of allorecognition pathways in chronic lung rejection

The role of allorecognition in initiating lung graft rejection is not clearly defined. Using the heterotopic tracheal transplantation model, we examined the contributions of the indirect and direct allorecognition pathways in chronic airway rejection. Fully mismatched, wild-type grafts were transplanted into major histocompatibility complex (MHC) II-/-, class II-like accessory molecule (H2-DMalpha)-/- using MHC I-/- and wild-type allorecipients as control subjects. Similarly, MHC I-/-, MHC II-/-, or MHC I/II-/- allografts were transplanted into wild-type mice with appropriate control subjects. Grafts from nonimmunosuppressed recipients were evaluated at Weeks 2, 4, and 6. Grafts transplanted into MHC II-/- and H2-DMalpha-/- allorecipients showed a more intact epithelium and reduced lumen obliteration compared with grafts transplanted into wild-type or MHC I-/- allorecipients (p < 0.05 for each). These grafts exhibited abundant CD4+ and CD8+ cell infiltrates similar to control allografts. MHC I-/- and MHC I/II-/- but not MHC II-/- allografts placed in wild-type animals demonstrated less severe rejection compared with allograft control subjects (p < 0.05 for each). Although the indirect allorecognition pathway has the strongest influence on rejection, the direct pathway is sufficient to ultimately cause chronic airway rejection. In addition, these results suggest that MHC class I molecules are the principal alloantigens in the mouse heterotopic tracheal model of obliterative bronchiolitis.

Transmembrane domain-mediated colocalization of HLA-DM and HLA-DR is required for optimal HLA-DM catalytic activity

HLA-DM catalyzes peptide loading and exchange reactions by MHC class II molecules. Soluble recombinant DM, lacking transmembrane and cytoplasmic domains, was observed to have 200- to 400-fold less activity compared with the full-length protein in assays measuring DM-catalyzed peptide dissociation from purified HLA-DR1 in detergent solutions. Additional studies with truncated soluble DR1 demonstrated that transmembrane domains in DR1 molecules are also required for optimal activity. The potential requirement for specific interaction between the transmembrane domains of DM and DR was ruled out in experiments with chimeric DR1 molecules containing transmembrane domains from either DM or the unrelated protein CD80. These results suggested that the major role of the transmembrane domains is to facilitate colocalization of DM and DR in detergent micelles. The latter conclusion was further supported by the observation that HLA-DM-catalyzed peptide binding to certain murine class II proteins is increased by reducing the volume of detergent micelles. The importance of membrane colocalization was directly demonstrated in experiments in which DM and DR were reconstituted separately or together into membrane bilayers in unilamellar liposomes. Our findings demonstrate the importance of membrane anchoring in DM activity and underscore the potential importance of membrane localization in regulating peptide exchange by class II molecules.

Presentation of antigens by MHC class II molecules: getting the most out of them

The function of MHC class II molecules is to bind peptides derived from antigens that access the endocytic route of antigen presenting cells and display them on the plasma membrane for recognition by CD4(+) T cells. Formation of the MHC II-peptide complexes entails the confluence of the antigens and the MHC II molecules in the same compartments of the endocytic route. There, both the antigens and the MHC II molecules undergo a series of orchestrated changes that involve proteases, other hydrolases and chaperones, culminating in the generation of a wide repertoire of MHC II-peptide combinations. All the events that lead to formation of MHC II-peptide complexes show a considerable degree of flexibility; this lack of strict rules is advantageous in that it provides T cells with the maximum amount of information, ensuring that pathogens do not go undetected.

Relaxed DM requirements during class II peptide loading and CD4+ T cell maturation in BALB/c mice

Current ideas about DM actions have been strongly influenced by studies of mutant strains expressing the H-2(b) haplotype. To evaluate DM contributions to class II activities in BALB/c mice, we generated a novel mutation at the DMa locus via embryonic stem cell technology. Unlike long-lived A(b)/class II-associated invariant chain-derived peptide (CLIP) complexes, mature A(d) and E(d) molecules are loosely occupied by class II-associated invariant chain-derived peptide and are SDS unstable. BALB/c DM mutants weakly express BP107 conformational epitopes and toxic shock syndrome toxin-1 superantigen-binding capabilities, consistent with partial occupancy by wild-type ligands. Near normal numbers of mature CD4(+) T cells fail to undergo superantigen-mediated negative selection, as judged by TCR Vbeta usage. Ag presentation assays reveal consistent differences for A(d)- and E(d)-restricted T cells. Indeed, the mutation leads to decreased peptide capture by A(d) molecules, and in striking contrast causes enhanced peptide loading by E(d) molecules. Thus, DM requirements differ for class II structural variants coexpressed under physiological conditions in the intact animal.

HLA-DM recognizes the flexible conformation of major histocompatibility complex class II

DM facilitates formation of high affinity complexes of peptide-major histocompatibility complex (MHC) by release of class II MHC-associated invariant chain peptide (CLIP). This has been proposed to occur through discrimination of complex stability. By probing kinetic and conformational intermediates of the wild-type and mutant human histocompatibility leukocyte antigen (HLA)-DR1-peptide complexes, and examining their reactivities with DM, we propose that DM interacts with the flexible hydrophobic pocket 1 of DR1 and converts the molecule into a conformation that is highly peptide receptive. A more rigid conformation, generated upon filling of pocket 1, is less susceptible to DM effects. Thus, DM edits peptide-MHC by recognition of the flexibility rather than stability of the complex.

Tumor cells present MHC class II-restricted nuclear and mitochondrial antigens and are the predominant antigen presenting cells in vivo

MHC class II-restricted tumor Ags presented by class II(+) tumor cells identified to date are derived from proteins expressed in the cytoplasm or plasma membrane of tumor cells. It is unclear whether MHC class II(+) tumor cells present class II-restricted epitopes derived from other intracellular compartments, such as nuclei and/or mitochondria, and whether class II(+) tumor cells directly present Ag in vivo. To address these questions, a model Ag, hen egg lysozyme, was targeted to various subcellular compartments of mouse sarcoma cells, and the resulting cells were tested for presentation of three lysozyme epitopes in vitro and for presentation of nuclear Ag in vivo. In in vitro studies, Ags localized to all tested compartments (nuclei, cytoplasm, mitochondria, and endoplasmic reticulum) are presented in the absence invariant chain and H-2M. Coexpression of invariant chain and H-2M inhibit presentation of some, but not all, of the epitopes. In vivo studies demonstrate that class II(+) tumor cells, and not host-derived cells, are the predominant APC for class II-restricted nuclear Ags. Because class II(+) tumor cells are effective APC in vivo and probably present novel tumor Ag epitopes not presented by host-derived APC, their inclusion in cancer vaccines may enhance activation of tumor-reactive CD4(+) T cells.

Nonclassical MHC class II molecules

Major histocompatibility complex (MHC) class II molecules are cell surface proteins that present peptides to CD4(+) T cells. In addition to these wellcharacterized molecules, two other class II-like proteins are produced from the class II region of the MHC, HLA-DM (DM) and HLA-DO (DO) (called H2-M, or H2-DM and H2-O in the mouse). The function of DM is well established; it promotes peptide loading of class II molecules in the endosomal/lysosomal system by catalyzing the release of CLIP peptides (derived from the class II-associated invariant chain) in exchange for more stably binding peptides. While DM is present in all class II- expressing antigen presenting cells, DO is expressed mainly in B cells. In this cell type the majority of DM molecules are not present as free heterodimers but are instead associated with DO in tight heterotetrameric complexes. The association with DM is essential for the intracellular transport of DO, and the two molecules remain associated in the endosomal system. DO can clearly modify the peptide exchange activity of DM both in vitro and in vivo, but the physiological relevance of this interaction is still only partly understood.

H2-DMalpha(-/-) mice show the importance of major histocompatibility complex-bound peptide in cardiac allograft rejection

The role played by antigenic peptides bound to major histocompatibility complex (MHC) molecules is evaluated with H2-DMalpha(-/)- mice. These mice have predominantly class II-associated invariant chain peptide (CLIP)-, not antigenic peptide-bound, MHC class II. H2-DMalpha(-/)- donor heart grafts survived three times longer than wild-type grafts and slightly longer than I-A(beta)(b)-(/)- grafts. Proliferative T cell response was absent, and cytolytic response was reduced against the H2-DMalpha(-/)- grafts in vivo. Residual cytolytic T cell and antibody responses against intact MHC class I lead to eventual rejection. Removal of both H2-DMalpha and beta2-microglobulin (beta2m) in cardiac grafts lead to greater (8-10 times) graft survival, whereas removal of beta2m alone did not have any effect. These results demonstrate the significance of peptide rather than just allogeneic MHC, in eliciting graft rejection.

Phagosomes acquire nascent and recycling class II MHC molecules but primarily use nascent molecules in phagocytic antigen processing

Phagosomes contain class II MHC (MHC-II) and form peptide:MHC-II complexes, but the source of phagosomal MHC-II molecules is uncertain. Phagosomes may acquire nascent MHC-II or preexisting, recycling MHC-II that may be internalized from the plasma membrane. Brefeldin A (BFA) was used to deplete nascent MHC-II in murine macrophages to determine the relative contributions of nascent and recycling MHC-II molecules to phagocytic Ag processing. In addition, biotinylation of cell-surface proteins was used to assess the transport of MHC-II from the cell surface to phagosomes. BFA inhibited macrophage processing of latex bead-conjugated Ag for presentation to T cells, suggesting that nascent MHC-II molecules are important in phagocytic Ag processing. Furthermore, detection of specific peptide:MHC-II complexes in isolated phagosomes confirmed that BFA decreased formation of peptide:MHC-II complexes within phagosomes. Both flow organellometry and Western blot analysis of purified phagosomes showed that about two-thirds of phagosomal MHC-II was nascent (depleted by 3 h prior treatment with BFA) and primarily derived from intracellular sites. About one-third of phagosomal MHC-II was preexisting and primarily derived from the plasma membrane. BFA had little effect on phagosomal H2-DM or the degradation of bead-associated Ag. Thus, inhibition of phagocytic Ag processing by BFA correlated with depletion of nascent MHC-II in phagosomes and occurred despite the persistent delivery of plasma membrane-derived recycling MHC-II molecules and other Ag-processing components to phagosomes. These observations suggest that phagosomal Ag processing depends primarily on nascent MHC-II molecules delivered from intracellular sites, e.g., endocytic compartments.

Proteolysis in MHC class II antigen presentation: who's in charge?

Antigen-presenting cells (APC) degrade proteins intracellularly to generate peptides, which are then bound by products of the major histocompatibility complex (MHC) and exposed on the surface of the APC for recognition by T cells. The supply of antigenic peptides and their association with MHC molecules requires the concerted action of a cohort of accessory molecules that includes chaperones, transporters of peptides, and the proteases that degrade the antigens.

Early endosomal maturation of MHC class II molecules independently of cysteine proteases and H-2DM

Major histocompatibility complex (MHC) class II molecules bind and present to CD4(+) T cells peptides derived from endocytosed antigens. Class II molecules associate in the endoplasmic reticulum with invariant chain (Ii), which (i) mediates the delivery of the class II-Ii complexes into the endocytic compartments where the antigenic peptides are generated; and (ii) blocks the peptide-binding site of the class II molecules until they reach their destination. Once there, Ii must be removed to allow peptide binding. The bulk of Ii-class II complexes reach late endocytic compartments where Ii is eliminated in a reaction in which the cysteine protease cathepsin S and the accessory molecule H-2DM play an essential role. Here, we here show that Ii is also eliminated in early endosomal compartments without the intervention of cysteine proteases or H-2DM. The Ii-free class II molecules generated by this alternative mechanism first bind high molecular weight polypeptides and then mature into peptide-loaded complexes.

Intracellular transport and peptide loading of MHC class II molecules: regulation by chaperones and motors

MHC class II molecules are important in the onset and modulation of cellular immune responses. Studies on the intracellular transport of these molecules has provided insight into the way pathogens are processed and presented at the cell surface and may result in future immunological intervention strategies. Recent reviews have extensively described structural properties and early events in the biosynthesis of MHC class II (1-3). In this review, the focus will be on the function of the dedicated chaperone proteins Ii, DM and DO in the class II assembly, transport and peptide loading as well on proteins involved in transport steps late in the intracellular transport of MHC class II.

Proteases involved in MHC class II antigen presentation

Major histocompatibility complex class II antigen presentation requires the participation of lysosomal proteases in two convergent processes. First, the antigens endocytosed by the antigen-presenting cells must be broken down into antigenic peptides. Second, class II molecules are synthesized with their peptide-binding site blocked by invariant chain (Ii), and they acquire the capacity to bind antigens only after Ii has been degraded in the compartments where peptides reside. The study of genetically modified mice deficient in single lysosomal proteases has allowed us to determine their role in these processes. Cathepsins (Cat) B and D, previously considered major players in MHC class II antigen presentation, are dispensable for degradation of Ii and for generation of several antigenic determinants. By contrast, Cat S plays an essential role in removal of Ii in B cells and dendritic cells, whereas Cat L apparently does so in thymic epithelial cells. Accordingly, the absence of Cat S and L have major consequences for the onset of humoral immune responses and for T-cell selection, respectively. It is likely that other as yet uncharacterized lysosomal enzymes also play a role in Ii degradation and in generation of antigenic determinants. Experiments involving drugs that interfere with protein traffic suggest that more than one mechanism for Ii removal, probably involving different proteases, can co-exist in the same antigen-presenting cell. These findings may allow the development of protease inhibitors with possible therapeutic applications.

Quality control of MHC class II associated peptides by HLA-DM/H2-M

For many years the crucial components involved in MHC class II mediated antigen presentation have been thought to be known: polymorphic MHC class II molecules, the monomorphic invariant chain (li) and a set of conventional proteases that cleave antigenic proteins thereby generating ligands able to associate with MHC class II molecules. However, in 1994 it was found that without an additional molecule, HLA-DM (DM), efficient presentation of protein antigens cannot be achieved. Biochemical studies showed that DM acts as a molecular chaperone protecting empty MHC class II molecules from functional inactivation. In addition, it serves as a peptide editor: DM catalyzes not only the release of the invariant chain remnant CLIP, but of all sorts of low-stability peptides, and simultaneously favors binding of high-stability peptides. Through this quality control of peptide loading, DM enables APCs to optimize MHC restriction and to display their antigenic peptide cargo on the surface for prolonged periods of time to be scrutinized by T cells.

Mechanisms and consequences of peptide selection by the I-Ak class II molecule

Important quantitative parameters can be utilized to define the selection and the immunogenicity of protein antigens precisely at a biochemical and a cellular level. Here we describe a naturally processed family of peptides comprising the dominant hen egg white lysozyme epitope, its major contribution to surface I-Ak molecules, the primary and auxiliary peptide anchors involved in its selection, and its display of T-cell receptor contacts. In addition, we explore the importance of the processing events that lead to the generation of residues flanking the minimal core epitope, the quantification of T-cell responses directed toward the epitope, and the ability of the dominant epitope to form two unique conformations within the binding groove. Lastly, we address the relationship between this dominant and a minor lysozyme epitope.

Individual hydrogen bonds play a critical role in MHC class II: peptide interactions: implications for the dynamic aspects of class II trafficking and DM-mediated peptide exchange

Determination of the crystal structure of class II: peptide complexes has shown that in addition to pocket interactions involving the side chains of the peptide, peptide binding to MHC class II molecules is characterized by a series of hydrogen bonds which are contributed by genetically conserved amino acid residues in the class II molecule to the main chain of the peptide. Our experiments have revealed an unexpectedly large contribution of hydrogen bonds at the periphery of the MHC peptide binding pocket to MHC class II function. Kinetic studies have shown that peptide dissociation rates are profoundly accelerated by loss of a single hydrogen bonding residue. The magnitude of the effects seen with the loss in potential for a single hydrogen bond support a co-operative model in which individual bonds between class II and peptide are dependent on the integrity of neighboring interactions. Collectively our studies have revealed that MHC class II structure, peptide binding and intracellular trafficking events are critically dependent on the integrity of the hydrogen bonding network between class II molecules and its bound peptide.

Deviant trafficking of I-Ad mutant molecules is reflected in their peptide binding properties

I-Ad molecules harboring single amino acid changes in the conserved 80-82 region of the beta-chain show altered trafficking in invariant chain (Ii)-negative cell lines. Since residues beta81 and beta82 form hydrogen bonds with the backbone of bound peptide, alterations in this region may result in distinct MHC class II conformers that are targeted aberrantly. We examined the assembly and peptide binding properties of the mutant I-Ad molecules generated by in vitro translation. Indeed, loss of a single hydrogen bond at beta81, or of two hydrogen bonds at beta82, is sufficient to render I-Ad incapable of stable interaction with CLIP and other antigenic peptides, despite normal assembly with intact invariant chain. These results suggest that stable interaction of MHC class II molecules with peptide requires the integrity of the H-bond network between residues in the MHC class II alpha-helices and bound peptide, and that conformational features revealed by stable peptide binding are critical for MHC class II intracellular transport.

BALB/c Invariant Chain Mutant Mice Display Relatively Efficient Maturation of CD4+ T Cells in the Periphery and Secondary Proliferative Responses Elicited upon Peptide Challenge

Allelic differences are known to influence many important aspects of class II biosynthesis, including subunit assembly, Ii chain associations, and DM-mediated peptide loading. Mutant mouse strains lacking Ii chain expression have been previously studied on mixed genetic backgrounds. The present experiments describe cellular and functional characteristics of congenic BALB/c Ii chain mutants. As expected, class II surface expression was markedly decreased, but in contrast to I-Ad-transfected cell lines, serological analysis of BALB/c Ii chain-deficient spleen cells gave no evidence for discordant expression of class II conformational epitopes. Thus, we conclude that properly folded class II molecules are exported via the Ii chain-independent pathway. Functional assays demonstrate consistently superior peptide-loading capabilities, suggesting that these I-Ad molecules are empty or occupied by an easily displaced peptide(s). Defective B cell development was observed for three mutant strains established on diverse genetic backgrounds. Ii chain function is also essential for optimal class II surface expression by mature splenic dendritic cells. Surprisingly, we observe in BALB/c Ii chain mutants, relatively efficient maturation of CD4+ T cells in the periphery and secondary proliferative responses elicited upon peptide challenge. The milder phenotype displayed by BALB/c Ii chain mutants in comparison with class II functional defects previously described for mouse strains lacking Ii chain is likely to have an effect on disease susceptibility.

Class II antigen processing defects in two H2d mouse cell lines are caused by point mutations in the H2-DMa gene

The molecular nature of the defect in two mouse antigen processing-defective cell lines was examined. Both mutants were derived from the A20 (BALB/c, H2d) B cell line, and both were found to have defects in the H2-DMa gene. Mutant 3A5 exhibits severely reduced amounts of H2-DMa message, and no detectable DMalpha protein. cDNA sequence revealed a C-->T transition at nucleotide 118, introducing a premature stop codon in exon 2 of the H2-DMa gene. In contrast, mutant 2A2 exhibits reduced but detectable levels of H2-DMa message and DMalpha protein only after treatment with IL-4, which induces the expression of both the H2-DMa and the H2-DMb genes in B cells. In this mutant the cDNA sequence revealed a missense mutation in exon 3 resulting in the conversion of a conserved proline residue in the Ig-like domain to serine. Stable transfection with full-length H2-DMa cDNA reconstitutes the antigen processing capacity of both mutants, as demonstrated by the ability to present native antigen to T cell clones, and by restored class II SDS stability.

HLA-DM and invariant chain are expressed by thyroid follicular cells, enabling the expression of compact DR molecules

Thyroid follicular cells (TFC) in Graves' disease (GD) hyperexpress HLA class I and express ectopic HLA class II molecules, probably as a consequence of cytokines produced by infiltrating T cells. This finding led us to postulate that TFC could act as antigen-presenting cells, and in this way be responsible for the induction and/or maintenance of the in situ autoimmune T cell response. Invariant chain (li) and HLA-DM molecules are implicated in the antigen processing and presentation by HLA class II molecules. We have investigated the expression of these molecules by TFC from GD glands. The results demonstrate that class II+ TFC from GD patients also express li and HLA-DM, and this expression is increased after IFN-gamma stimulation. The level of HLA-DM expression by TFC was low but sufficient to catalyze peptide loading into the HLA class II molecules and form stable HLA class II-peptide complexes expressed at the surface of TFC. These results have implications for the understanding of the possible role of HLA class II+ TFC in thyroid autoimmune disease.

The Inability of the Nonobese Diabetic Class II Molecule to Form Stable Peptide Complexes Does Not Reflect a Failure to Interact Productively with DM

Sequence variability in MHC class II molecules plays a major role in genetically determined susceptibility to insulin-dependent diabetes mellitus (IDDM). It is not yet clear whether MHC class II polymorphism allows selective binding of diabetogenic peptides or regulates some key intracellular events associated with class II-restricted Ag presentation. In this study, we have employed gene transfer techniques to analyze the intracellular events that control peptide acquisition by the unique class II molecule expressed by nonobese diabetic mice (I-Ag7). This structurally unique class II molecule fails to demonstrate stable binding to antigenic peptides and fails to undergo the conformational change associated with stable peptide binding to class II molecules. The experiments reported here demonstrate that I-Ag7 can productively associate with two protein cofactors important in class II-restricted Ag presentation, invariant chain (Ii) and DM. DM participates in the removal of the Ii-derived class II-associated Ii chain peptide and the p12 degradation product from the I-Ag7 molecule. In addition, I-Ag7 undergoes a conformational change when DM is expressed within the APC. Finally, DM can mediate accumulation of peptide/class II complexes on the surface of APCs. Collectively, our experiments indicate that the failure of the I-Ag7 molecule to stably bind peptide cannot be attributed to a failure to interact with the DM or Ii glycoproteins.

The phenotype of H-2M-deficient mice is dependent on the MHC class II molecules expressed

For a broader view of the role of H-2M as an accessory molecule in antigen presentation, we investigated the degree to which different MHC class II isotypes and alleles depend on H-2M to function in vivo. We generated H-2M-deficient animals expressing Ek/b or Ak molecules in addition to the Ab molecules already present in the mutant strain, and compared the ability of the different MHC class II molecules to present antigen at the cell surface for recognition by T cells, and contribute to positive selection of CD4+ T cells in the thymus. Biochemical analyses were performed to assess MHC class II maturation, and to determine the peptide content of the molecules. In the absence of H-2M, Ek/b molecules contained a more heterogeneous set of class II-associated invariant chain peptides (CLIP) than Ab did, which, unlike Ab-CLIP complexes, were not SDS-stable. Unlike Ab molecules, both Ek/b and Ak efficiently presented exogenously added peptides to T cells in the absence of H-2M. In addition, epitopes from some proteins, especially those known to be invariant chain independent, were presented by Ak molecules in the mutant animals. To our surprise, expression of Ek/b overcame the positive selection defect observed in H-2M-deficient mice expressing Ab alone. In contrast, Ak expression did not augment positive selection of CD4+ T cells in the mutant animals. Some of these findings in vivo contrast significantly with findings from in vitro studies on murine MHC class II molecules in human DM-deficient cell lines.

Subtle conformational changes induced in major histocompatibility complex class II molecules by binding peptides

Intracellular trafficking of major histocompatibility complex (MHC) class II molecules is characterized by passage through specialized endocytic compartment(s) where antigenic peptides replace invariant chain fragments in the presence of the DM protein. These changes are accompanied by structural transitions of the MHC molecules that can be visualized by formation of compact SDS-resistant dimers, by changes in binding of mAbs, and by changes in T cell responses. We have observed that a mAb (25-9-17) that is capable of staining I-Ab on the surface of normal B cells failed to interact with I-Ab complexes with a peptide derived from the Ealpha chain of the I-E molecule but bound a similar covalent complex of I-Ab with the class II binding fragment (class II-associated invariant chain peptides) of the invariant chain. Moreover, 25-9-17 blocked activation of several I-Ab-reactive T cell hybridomas but failed to block others, suggesting that numerous I-Ab-peptide complexes acquire the 25-9-17(+) or 25-9-17(-) conformation. Alloreactive T cells were also able to discriminate peptide-dependent variants of MHC class II molecules. Thus, peptides impose subtle structural transitions upon MHC class II molecules that affect T cell recognition and may thus be critical for T cell selection and autiommunity.

Expression and Characterization of Recombinant Soluble Peptide: I-A Complexes Associated with Murine Experimental Autoimmune Diseases

Structural and functional studies of murine MHC class II I-A molecules have been limited by the low yield and instability of soluble, recombinant heterodimers. In the murine autoimmune diseases experimental autoimmune encephalomyelitis and collagen-induced arthritis, MHC class II molecules I-Au and I-Aq present peptides derived from myelin basic protein and type II collagen, respectively, to autoreactive T cells. To date, systems for the expression of these two I-A molecules in soluble form for use in structure-function relationship studies have not been reported. In the present study, we have expressed functional I-Au and I-Aq molecules using a baculovirus insect cell system. The chain pairing and stability of the molecules were increased by covalently linking the antigenic peptides to β-chains and adding carboxyl-terminal leucine zippers. Peptide:I-Aq complex quantitatively formed an SDS-stable dimer, whereas peptide:I-Au formed undetectable amounts. However, the two complexes did not show any significant difference in their response to thermal denaturation as assessed by circular dichroism analyses. The autoantigen peptide:I-A complexes were highly active in stimulating cognate T cells to secrete IL-2 and inducing Ag-specific apoptosis of the T cells. Interestingly, the T cells were stimulated by these soluble molecules in the apparent absence of experimentally induced cross-linking of TCRs, indicating that they may have therapeutic potential in autoimmune disease models.