Enhanced dissociation of HLA-DR-bound peptides in the presence of HLA-DM

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.

CTCF controls expression and chromatin architecture of the human major histocompatibility complex class II locus

The major histocompatibility complex class II (MHC-II) locus includes a dense cluster of genes that function to initiate immune responses. Expression of insulator CCCTC binding factor (CTCF) was found to be required for expression of all MHC class II genes associated with antigen presentation. Ten CTCF sites that divide the MHC-II locus into apparent evolutionary domains were identified. To define the role of CTCF in mediating regulation of the MHC II genes, chromatin conformation capture assays, which provide an architectural assessment of a locus, were conducted across the MHC-II region. Depending on whether MHC-II genes and the class II transactivator (CIITA) were being expressed, two CTCF-dependent chromatin architectural states, each with multiple configurations and interactions, were observed. These states included the ability to express MHC-II gene promoter regions to interact with nearby CTCF sites and CTCF sites to interact with each other. Thus, CTCF organizes the MHC-II locus into a novel basal architecture of interacting foci and loop structures that rearranges in the presence of CIITA. Disruption of the rearranged states eradicated expression, suggesting that the formation of these structures is key to coregulation of MHC-II genes and the locus.

TLR-mediated B cell activation results in ectopic CLIP expression that promotes B cell-dependent inflammation

Infectious pathogens produce compounds called Toll ligands that activate TLRs on lymphocytes. Acute activation triggered by certain TLRs appears to "jump start" the innate immune response, characterized by the release of inflammatory cytokines and cellular expansion. In some individuals, there is a failure to control acute inflammation, resulting in postinfectious, chronic inflammation. Susceptibility to chronic inflammation is strongly associated with an individual's MHC genes. Recent clinical trials for several autoimmune diseases characterized by chronic inflammation suggest that B lymphocyte depletion therapies dampen chronic immune activation. However, currently, there is no known mechanism that accounts for the correlation among TLR activation, MHC genetics, and a pathological role for B-lymphocytes. Our hypothesis is that TLR-activated B cells (B cells that have been polyclonally activated in the absence of antigen-specific signals) are not controlled properly by T cell-dependent B cell death, thereby causing B cell-dependent chronic inflammation. Here, we show that treatment with Toll ligands results in polyclonal B cell activation accompanied by ectopic expression of CLIP. Furthermore, by adoptively transferring purified CLIP+ B cells in syngeneic animals, we find that CLIP+ B cells induce production of TNF-α by host T cells. Finally, we demonstrate that CLIP-targeted peptide competition results in the death of polyclonally activated CLIP+ B cells.

DM influences the abundance of major histocompatibility complex class II alleles with low affinity for class II-associated invariant chain peptides via multiple mechanisms

DM catalyses class II-associated invariant chain peptide (CLIP) release, edits the repertoire of peptides bound to major histocompatibility complex (MHC) class II molecules, affects class II structure, and thereby modulates binding of conformation-sensitive anti-class II antibodies. Here, we investigate the ability of DM to enhance the cell surface binding of monomorphic antibodies. We show that this enhancement reflects increases in cell surface class II expression and total cellular abundance, but notably these effects are selective for particular alleles. Evidence from analysis of cellular class II levels after cycloheximide treatment and from pulse-chase experiments indicates that DM increases the half-life of affected alleles. Unexpectedly, the pulse-chase experiments also revealed an early effect of DM on assembly of these alleles. The allelically variant feature that correlates with susceptibility to these DM effects is low affinity for CLIP; DM-dependent changes in abundance are reduced by invariant chain (CLIP) mutants that enhance CLIP binding to class II. We found evidence that DM mediates rescue of peptide-receptive DR0404 molecules from inactive forms in vitro and evidence suggesting that a similar process occurs in cells. Thus, multiple mechanisms, operating along the biosynthetic pathway of class II molecules, contribute to DM-mediated increases in the abundance of low-CLIP-affinity alleles.

Cutting edge: HLA-DM functions through a mechanism that does not require specific conserved hydrogen bonds in class II MHC-peptide complexes

HLA-DM catalyzes peptide dissociation and exchange in class II MHC molecules through a mechanism that has been proposed to involve the disruption of specific components of the conserved hydrogen bond network in MHC-peptide complexes. HLA-DR1 molecules with alanine substitutions at each of the six conserved H- bonding positions were expressed in cells, and susceptibility to DM catalytic activity was evaluated by measuring the release of CLIP. The mutants alphaN62A, alphaN69A, alphaR76A, and betaH81A DR1 were fully susceptible to DM-mediated CLIP release, and betaN82A resulted in spontaneous release of CLIP. Using recombinant soluble DR1 molecules, the amino acid betaN82 was observed to contribute disproportionately in stabilizing peptide complexes. Remarkably, the catalytic potency of DM with each beta-chain mutant was equal to or greater than that observed with wild-type DR1. Our results support the conclusion that no individual component of the conserved hydrogen bond network plays an essential role in the DM catalytic mechanism.

HLA-DM mediates peptide exchange by interacting transiently and repeatedly with HLA-DR1

The peptide editor HLA-DM (DM) catalyzes the exchange of peptides bound to MHC class II molecules within antigen presenting cells by generating a "peptide-receptive" MHC class II conformation (MHC(receptive)) to which peptides readily bind and rapidly unbind. While recent work has uncovered the determinants of DM recognition and effector functions, the nature of MHC(receptive) and its interaction with DM remains unclear. Here, we show that DM induces but does not stabilize MHC(receptive) in the absence of peptides. We demonstrate that DM is out-competed by certain superantigens, and increasing solvent viscosity inhibits DM-induced peptide association. We suggest that DM mediates peptide exchange by interacting transiently and repeatedly with MHC class II molecules, continually generating MHC(receptive). The simultaneous presence of peptide and DM in the milieu is thus crucial for the efficient generation of specific peptide-MHC class II complexes over time.

Ii-Key/HPV16 E7 hybrid peptide immunotherapy for HPV16+ cancers

Activation of antigen-specific CD4+ T cells is critical for vaccine design. We have advanced a novel technology for enhancing activation of antigen-specific CD4+ T helper cells whereby a fragment of the MHC class II-associated invariant chain (Ii-Key) is linked to an MHC class II epitope. An HLA-DR4-restricted HPV16 E7 epitope, HPV16 E7(8-22), was used to create a homologous series of Ii-Key/HPV16 E7 hybrids testing the influence of spacer length on in vivo enhancement of HPV16 E7(8-22)-specific CD4+ T lymphocyte responses. HLA-DR4-tg mice were immunized with Ii-Key/HPV16 E7(8-22) hybrids or the epitope-only peptide HPV16 E7(8-22). As measured by IFN-gamma ELISPOT assay of splenocytes from immunized mice, one of the Ii-Key/HPV16 E7(8-22) hybrids enhanced epitope-specific CD4+ T cell activation 5-fold compared to the HPV16 E7(8-22) epitope-only peptide. We further demonstrated that enhanced CD4+ T cell activation augments the CTL activity of a H-2D(b)-restricted HPV16 E7(49-57) epitope in HLA-DR4+ mice using an in vivo CTL assay. Binding assays indicated that the Ii-Key/HPV16 hybrid has increased affinity to HLA-DR4+ cells relative to the epitope-only peptide, which may explain its increased potency. In summary, Ii-Key hybrid modification of the HLA-DR4-restricted HPV16 E7(8-22) MHC class II epitope generates a potent immunotherapeutic peptide vaccine that may have potential for treating HPV16+ cancers in HLA-DR4+ patients.

Protective response to Leishmania major in BALB/c mice requires antigen processing in the absence of DM

Protection from the parasite Leishmania major is mediated by CD4 T cells. BALB/c mice are susceptible to L. major and show a nonprotective immunodominant CD4 T cell response to Leishmania homolog of activated receptor for c-kinase (LACK) 158-173. Host genes that underlie BALB/c susceptibility to L. major infections are poorly defined. DM, a nonclassical MHC class II molecule, due to its peptide editing properties has been shown to 1) edit the repertoire of peptides displayed by APC, and 2) focus the display of epitopes by APC to the immunodominant ones. We tested the hypothesis that deficiency of DM, by causing presentation of a different array of epitopes by infected APC than that presented by DM-sufficient APC, may change the course of L. major infection in the susceptible BALB/c mice. We show herein that unlike their susceptible wild-type counterparts, BALB/c mice deficient in DM are protected from infections with L. major. Furthermore, DM-deficient mice fail to display the immunodominant LACK 158-173 on infected APC. In its place, infected DM(-/-) hosts show elicitation of CD4 T cells specific for newer epitopes not presented by wild-type L. major-infected APC. Protection of BALB/c DM(-/-) mice is dependent on IFN-gamma. DM is thus a host susceptibility gene in BALB/c mice, and Ag processing in the absence of DM results in elicitation of a protective T cell response against L. major infections. This report suggests a novel mechanism to trigger host resistance against pathogens.

Understanding the focused CD4 T cell response to antigen and pathogenic organisms

Immunodominance is a term that reflects the final, very limited peptide specificity of T cells that are elicited during an immune response. Recent experiments in our laboratory compel us to propose a new paradigm for the control of immunodominance in CD4 T cell responses, stating that immunodominance is peptide-intrinsic and is dictated by the off-rate of peptides from MHC class II molecules. Our studies have revealed that persistence of peptide:class II complexes both predicts and controls CD4 T cell immunodominance and that this parameter can be rationally manipulated to either promote or eliminate immune responses. Mechanistically, we have determined that DM editing in APC is a key event that is influenced by the kinetic stability of class II:peptide complexes and that differential persistence of complexes also impacts the expansion phase of the immune response. These studies have important implications for rational vaccine design and for understanding the immunological mechanisms that limit the specificity of CD4 T cell responses.

Evidence for a human leucocyte antigen-DM-induced structural change in human leucocyte antigen-DObeta

Human leucocyte antigen (HLA)-DO is a non-classical major histocompatibility complex class II molecule which modulates the function of HLA-DM and the loading of antigenic peptides on molecules such as HLA-DR. The bulk of HLA-DO associates with HLA-DM and this interaction is critical for HLA-DO egress from the endoplasmic reticulum. HLA-DM assists the early steps of HLA-DO maturation presumably through the stabilization of the interactions between the N-terminal regions of the alpha and beta chains. To evaluate a possible role for HLA-DM in influencing the conformation of HLA-DO, we made use of a monoclonal antibody, Mags.DO5, that was raised against HLA-DO/DM complexes. Using transfected cells expressing mismatched heterodimers between HLA-DR and -DO chains, we found that the epitope for Mags.DO5 is located on the DObeta chain and that Mags.DO5 reactivity was increased upon cotransfection with HLA-DM. Our results suggest that HLA-DM influences the folding of HLA-DO in the endoplasmic reticulum. A mutant HLA-DO showing reduced capacity for endoplasmic reticulum egress was better recognized by Mags.DO5 in the presence of HLA-DM. On the other hand, an HLA-DO mutant capable of endoplasmic reticulum egress on its own was efficiently recognized by Mags.DO5, irrespective of the presence of HLA-DM. Taken together, our results suggest that HLA-DM acts as a private chaperone, directly assisting the folding of HLA-DO to promote egress from the endoplasmic reticulum.

HLA-DM mediates epitope selection by a "compare-exchange" mechanism when a potential peptide pool is available

Background

HLA-DM (DM) mediates exchange of peptides bound to MHC class II (MHCII) during the epitope selection process. Although DM has been shown to have two activities, peptide release and MHC class II refolding, a clear characterization of the mechanism by which DM facilitates peptide exchange has remained elusive.

Methodology/Principal Findings

We have previously demonstrated that peptide binding to and dissociation from MHCII in the absence of DM are cooperative processes, likely related to conformational changes in the peptide-MHCII complex. Here we show that DM promotes peptide release by a non-cooperative process, whereas it enhances cooperative folding of the exchange peptide. Through electron paramagnetic resonance (EPR) and fluorescence polarization (FP) we show that DM releases prebound peptide very poorly in the absence of a candidate peptide for the exchange process. The affinity and concentration of the candidate peptide are also important for the release of the prebound peptide. Increased fluorescence energy transfer between the prebound and exchange peptides in the presence of DM is evidence for a tetramolecular complex which resolves in favor of the peptide that has superior folding properties.

Conclusion/Significance

This study shows that both the peptide releasing activity on loaded MHCII and the facilitating of MHCII binding by a candidate exchange peptide are integral to DM mediated epitope selection. The exchange process is initiated only in the presence of candidate peptides, avoiding possible release of a prebound peptide and loss of a potential epitope. In a tetramolecular transitional complex, the candidate peptides are checked for their ability to replace the pre-bound peptide with a geometry that allows the rebinding of the original peptide. Thus, DM promotes a “compare-exchange” sorting algorithm on an available peptide pool. Such a “third party”-mediated mechanism may be generally applicable for diverse ligand recognition in other biological systems.

Complexes of two cohorts of CLIP peptides and HLA-DQ2 of the autoimmune DR3-DQ2 haplotype are poor substrates for HLA-DM

Atypical invariant chain (Ii) CLIP fragments (CLIP2) have been found in association with HLA-DQ2 (DQ2) purified from cell lysates. We mapped the binding register of CLIP2 (Ii 96-104) to DQ2 and found proline at the P1 position, in contrast to the canonical CLIP1 (Ii 83-101) register with methionine at P1. CLIP1/2 peptides are the predominant peptide species, even for DQ2 from HLA-DM (DM)-expressing cells. We hypothesized that DQ2-CLIP1/2 might be poor substrates for DM. We measured DM-mediated exchange of CLIP and other peptides for high-affinity indicator peptides and found it is inefficient for DQ2. DM-DQ-binding and DM chaperone effects on conformation and levels of DQ are also reduced for DQ2, compared with DQ1. We suggest that the unusual interaction of DQ2 with Ii and DM may provide a basis for the known disease associations of DQ2.

Immunodominance of CD4 T cells to foreign antigens is peptide intrinsic and independent of molecular context: implications for vaccine design

Immunodominance refers to the restricted peptide specificity of T cells that are detectable after an adaptive immune response. For CD4 T cells, many of the mechanisms used to explain this selectivity suggest that events related to Ag processing play a major role in determining a peptide's ability to recruit CD4 T cells. Implicit in these models is the prediction that the molecular context in which an antigenic peptide is contained will impact significantly on its immunodominance. In this study, we present evidence that the selectivity of CD4 T cell responses to peptides contained within protein Ags is not detectably influenced by the location of the peptide in a given protein or the primary sequence of the protein that bears the test peptide. We have used molecular approaches to change the location of peptides within complex protein Ags and to change the flanking sequences that border the peptide epitope to now include a protease site, and find that immunodominance or crypticity of a peptide observed in its native protein context is preserved. Collectively, these results suggest immunodominance of peptides contained in complex Ags is due to an intrinsic factor of the peptide, based upon the affinity of that peptide for MHC class II molecules. These findings are discussed with regard to implications for vaccine design.

Identification of the transcriptional response of human intestinal mucosa to Lactobacillus plantarum WCFS1 in vivo

Background

There is limited knowledge on the extent and dynamics of the mucosal response to commensal and probiotic species in the human intestinal lumen. This study aimed to identify the acute, time-dependent responses of intestinal mucosa to commensal Lactobacillus plantarum WCFS1 in vivo in two placebo-controlled human intervention studies in healthy volunteers. Transcriptional changes in duodenal mucosa upon continuous intraduodenal infusion of L. plantarum WCFS1 for one- and six h, respectively, were studied using oro- and nasogastric intubations with dedicated orogastric catheters and tissue sampling by standard flexible gastroduodenoscopy.

Results

One- and six-h exposure of small intestinal mucosa to L. plantarum WCFS1 induced differential expression of 669 and 424 gene reporters, respectively. While short-term exposure to L. plantarum WCFS1 inhibited fatty acid metabolism and cell cycle progression, cells switched to a more proliferative phase after prolonged exposure with an overall expression profile characterized by upregulation of genes involved in lipid metabolism, cellular growth and development. Cell death and immune responses were triggered, but cell death-executing genes or inflammatory signals were not expressed. Proteome analysis showed differential expression of several proteins. Only the microsomal protein 'microsomal triglyceride transfer protein' was regulated on both the transcriptional and the protein level in all subjects.

Conclusion

Overall, this study showed that intestinal exposure to L. plantarum WCFS1 induced consistent, time-dependent transcriptional responses in healthy intestinal mucosa. This extensive exploration of the human response to L. plantarum WCFS1 could eventually provide molecular support for specific or probiotic activity of this strain or species, and exemplifies the strength of the applied technology to identify the potential bio-activity of microbes in the human intestine.

Characterization of intracellular HLA-DR, DM and DO profile in K562 and HL-60 leukemic cells

Surface class-II antigen expression fires-up the antigen presentation process and development of immune response. The absence of surface HLA-DR is used in various systems to avoid immune recognition. Most leukemic cells use such mechanism to escape immune surveillance. Here, K562 and HL-60 leukemic cells were examined as to intracellular HLA-DR, -DM and -DO expression, if any. Immunofluorescence scored by UV-microscopy, flow cytometry or confocal microscope analysis detected intracellular pools of HLA-DR, -DO and to a lesser degree HLA-DM, whereas sub-cellular fractionation localized these molecules within endosomes. RT-PCR experiments revealed the presence of HLA-DRalphabeta, HLA-DMalphabeta and HLA-DObeta but not HLA-DOalpha transcripts. Despite the absence of the HLA-DOalpha chain, stable transfectants of K562 with a full length HLA-DObeta-EGFP construct showed that DObeta chain could be translocated to endosomes and form stable complexes with HLA-DR. Such complexes could be responsible for arresting HLA-DR molecules within endosomes, maintaining their surface class-II negative state.

Characteristics of carbohydrate antigen binding to the presentation protein HLA-DR

Zwitterionic polysaccharide antigens (ZPSs) were recently shown to activate T cells in a class II major histocompatibility complex (MHCII)-dependent fashion requiring antigen presenting cell (APC)-mediated oxidative processing although little is known about the mechanism or affinity of carbohydrate presentation (Cobb BA, Wang Q, Tzianabos AO, Kasper DL. 2004. Polysaccharide processing and presentation by the MHCII pathway. Cell. 117:677-687). A recent study showed that the helical conformation of ZPSs (Wang Y, Kalka-Moll WM, Roehrl MH, Kasper DL. 2000. Structural basis of the abscess-modulating polysaccharide A2 from Bacteroides fragilis. Proc Natl Acad Sci USA. 97:13478-13483; Choi YH, Roehrl MH, Kasper DL, Wang JY. 2002. A unique structural pattern shared by T-cell-activating and abscess-regulating zwitterionic polysaccharides. Biochemistry. 41:15144-15151) is closely linked with immunogenic activity (Tzianabos AO, Onderdonk AB, Rosner B, Cisneros RL, Kasper DL. 1993. Structural features of polysaccharides that induce intra-abdominal abscesses. Science. 262:416-419) and is stabilized by a zwitterionic charge motif (Kreisman LS, Friedman JH, Neaga A, Cobb BA. 2007. Structure and function relations with a T-cell-activating polysaccharide antigen using circular dichroism. Glycobiology. 17:46-55), suggesting a strong carbohydrate structure-function relationship. In this study, we show that PSA, the ZPS from Bacteroides fragilis, associates with MHCII at high affinity and 1:1 stoichiometry through a mechanism mirroring peptide presentation. Interestingly, PSA binding was mutually exclusive with common MHCII antigens and showed significant allelic differences in binding affinity. The antigen exchange factor HLA-DM that catalyzes peptide antigen association with MHCII also increased the rate of ZPS association and was required for APC presentation and ZPS-mediated T cell activation. Finally, the zwitterionic nature of these antigens was required only for MHCII binding, and not endocytosis, processing, or vesicular trafficking to MHCII-containing vesicles. This report is the first quantitative analysis of the binding mechanism of carbohydrate antigens with MHCII and leads to a novel model for nontraditional MHCII antigen presentation during bacterial infections.

MHC class II antigen presentation and immunological abnormalities due to deficiency of MHC class II and its associated genes

Antigen presentation by Major Histocompatibility Complex (MHC) class II molecules plays an important role in controlling immunity and autoimmunity. Multiple co-factors including the invariant chain (Ii), HLA-DM and HLA-DO are involved in this process. While the role for Ii and DM has been well defined, the biological function of DO remains obscure. Our data indicate that DO inhibits presentation of endogenous self-antigens and that developmentally-regulated DO expression enables antigen presenting cells to preferentially present different sources of peptide antigens at different stages of development. Disruption of this regulatory mechanism can result in not only immunodeficiency but also autoimmunity. Despite the fact that deletion of each of the three genes in experimental animals is associated with profound immunological abnormalities, no corresponding human diseases have been reported. This discrepancy suggests the possibility that primary immunodeficiencies due to a genetic defect of Ii, DM and DO in humans are under diagnosed or diagnosed as "common variable immunodeficiency", a category of immunodeficiency of heterogeneous or undefined etiology. Clinical tests for any of these potential genetic defects are not yet available. We propose the use of multi-color flow cytometry in conjunction with intracellular staining to detect expression of Ii, DM, DO in peripheral blood B cells as a convenient reliable screening test to identify individuals with defects in antigen presentation.

An MHC class II restriction bias in CD4 T cell responses toward I-A is altered to I-E in DM-deficient mice

The MHC-encoded cofactor DM catalyzes endosomal loading of peptides onto MHC class II molecules. Despite evidence from in vitro experiments that DM acts to selectively edit the repertoire of class II:peptide complexes, the consequence of DM expression in vivo, or a predictive pattern of DM activity in the specificity of CD4 T cell responses has remained unresolved. Therefore, to characterize DM function in vivo we used wild-type (WT) or DM-deficient (DM(-/-)) mice of the H-2(d) MHC haplotype and tested the hypothesis that DM promotes narrowing of the repertoire of class II:peptide complexes displayed by APC, leading to a correspondingly selective CD4 T cell response. Surprisingly, our results indicated that DM(-/-) mice do not exhibit a broadened CD4 T cell response relative to WT mice, but rather shift their immunodominance pattern to new peptides, a pattern associated with a change in class II isotype-restriction. Specifically, we found that CD4 T cell responses in WT mice were primarily restricted to the I-A class II molecule, whereas DM(-/-) mice recognize peptides in the context of I-E. The observed shift in isotype-restriction appeared to be due in part to a modification in the peripheral CD4 T cell repertoire available for peptide recognition.

The convergent roles of tapasin and HLA-DM in antigen presentation

Cytotoxic and helper T cells respond to peptides derived from endogenous and exogenous sources that bind to major histocompatibility complex (MHC) class I and class II molecules and are presented on antigen-presenting cells. MHC class I and class II structures and maturation pathways have evolved to optimize antigen presentation to their respective T cells. The accessory proteins tapasin and HLA-DM (DM) crucially influence the selection of peptides that bind to the MHC molecules. We discuss here the dynamic interactions of tapasin and DM with their corresponding MHC molecules that indicate striking parallels. Utilization of a common mode of peptide selection by two different, but related, biological systems argue for its mechanistic validity.

The convergent roles of tapasin and HLA-DM in antigen presentation

Cytotoxic and helper T cells respond to peptides derived from endogenous and exogenous sources that bind to major histocompatibility complex (MHC) class I and class II molecules and are presented on antigen-presenting cells. MHC class I and class II structures and maturation pathways have evolved to optimize antigen presentation to their respective T cells. The accessory proteins tapasin and HLA-DM (DM) crucially influence the selection of peptides that bind to the MHC molecules. We discuss here the dynamic interactions of tapasin and DM with their corresponding MHC molecules that indicate striking parallels. Utilization of a common mode of peptide selection by two different, but related, biological systems argue for its mechanistic validity.

Identification of the HLA-DM/HLA-DR interface

Human leukocyte antigen (HLA)-DM is a critical participant in antigen presentation that catalyzes the dissociation of the Class II-associated Invariant chain-derived Peptide (CLIP) from the major histocompatibility complex (MHC) Class II molecules. There is competition amongst peptides for access to an MHC Class II groove and it has been hypothesised that DM functions as a 'peptide editor' that catalyzes the replacement of one peptide for another within the groove. It is established that the DM catalyst interacts directly with the MHC Class II but the precise location of the interface is unknown. Here, we combine previously described mutational data with molecular docking and energy minimisation simulations to identify a putative interaction site of >4000A2 which agrees with known point mutational data for both the DR and DM molecule. The docked structure is validated by comparison with experimental data and previously determined properties of protein-protein interfaces. A possible dissociation mechanism is suggested by the presence of an acidic cluster near the N terminus of the bound peptide.

Recent advances in antigen processing and presentation

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

The relative energetic contributions of dominant P1 pocket versus hydrogen bonding interactions to peptide:class II stability: implications for the mechanism of DM function

Peptides are bound to MHC class II molecules by an array of hydrogen bonds between conserved MHC class II protein side-chains and the peptide backbone and through interactions between MHC protein pockets and peptide side-chain anchors. The crystal structure of murine I-A(k) protein with peptide shows a network of electrostatic interactions with the P1 aspartic acid anchor and an arginine in the P1 pocket that are thought to constitute the major stabilizing interaction between peptide and MHC. In this paper, have explored the relative energetic contribution of this dominant P1 pocket interaction with that made by a genetically conserved hydrogen bond which is formed by the beta 81 histidine residue and the main chain of the bound peptide. We have performed peptide dissociation experiments using antigenic peptides or variants that have altered side-chain interactions with the I-A(k) P1 pocket using either native I-A(k) or I-A(k) proteins mutated to disrupt the N-terminal hydrogen bond. The results demonstrate that the N-terminal hydrogen bonds in I-A(k) complexes make highly significant energetic contributions to the kinetic stabilities comparable to or greater than the energetic contribution of highly favorable P1 pocket interactions. Hence, we conclude that the kinetic stability of MHC class II:peptide complexes critically depends on two quite distinct molecular interactions between peptide and MHC located at the peptide's amino terminus. We discuss these results in light of the proposed mechanism for DM function.

Empty class II major histocompatibility complex created by peptide photolysis establishes the role of DM in peptide association

DM catalyzes the exchange of peptides bound to Class II major histocompatibility complex (MHC) molecules. Because the dissociation and association components of the overall reaction are difficult to separate, a detailed mechanism of DM catalysis has long resisted elucidation. UV irradiation of DR molecules loaded with a photocleavable peptide (caged Class II MHC molecules) enabled synchronous and verifiable evacuation of the peptide-binding groove and tracking of early binding events in real time by fluorescence polarization. Empty DR molecules generated by photocleavage rapidly bound peptide but quickly resolved into species with substantially slower binding kinetics. DM formed a complex with empty DR molecules that bound peptide with even faster kinetics than empty DR molecules just having lost their peptide cargo. Mathematical models demonstrate that the peptide association rate of DR molecules is substantially higher in the presence of DM. We therefore unequivocally establish that DM contributes directly to peptide association through formation of a peptide-loading complex between DM and empty Class II MHC. This complex rapidly acquires a peptide analogous to the MHC class I peptide-loading complex.

Expression Patterns of H2-O in Mouse B Cells and Dendritic Cells Correlate with Cell Function

In the endosomes of APCs, the MHC class II-like molecule H2-M catalyzes the exchange of class II-associated invariant chain peptides (CLIP) for antigenic peptides. H2-O is another class II-like molecule that modulates the peptide exchange activity of H2-M. Although the expression pattern of H2-O in mice has not been fully evaluated, H2-O is expressed by thymic epithelial cells, B cells, and dendritic cells (DCs). In this study, we investigated H2-O, H2-M, and I-A(b)-CLIP expression patterns in B cell subsets during B cell development and activation. H2-O was first detected in the transitional 1 B cell subset and high levels were maintained in marginal zone and follicular B cells. H2-O levels were down-regulated specifically in germinal center B cells. Unexpectedly, we found that mouse B cells may have a pool of H2-O that is not associated with H2-M. Additionally, we further evaluate H2-O and H2-M interactions in mouse DCs, as well as H2-O expression in bone marrow-derived DCs. We also evaluated H2-O, H2-M, I-A(b), and I-A(b)-CLIP expression in splenic DC subsets, in which H2-O expression levels varied among the splenic DC subsets. Although it has previously been shown that H2-O modifies the peptide repertoire, H2-O expression did not alter DC presentation of a number of endogenous and exogenous Ags. Our further characterization of H2-O expression in DCs, as well as the identification of a potential free pool of H2-O in mouse splenic B cells, suggest that H2-O may have a yet to be elucidated role in immune responses.

HLA-DM targets the hydrogen bond between the histidine at position beta81 and peptide to dissociate HLA-DR-peptide complexes

The peptide editor HLA-DM (DM) mediates exchange of peptides bound to major histocompatibility (MHC) class II molecules during antigen processing; however, the mechanism by which DM displaces peptides remains unclear. Here we generated a soluble mutant HLA-DR1 with a histidine-to-asparagine substitution at position 81 of the beta-chain (DR1betaH81N) to perturb an important hydrogen bond between MHC class II and peptide. Peptide-DR1betaH81N complexes dissociated at rates similar to the dissociation rates of DM-induced peptide-wild-type DR1, and DM did not enhance the dissociation of peptide-DR1betaH81N complexes. Reintroduction of an appropriate hydrogen bond (DR1betaH81N betaV85H) restored DM-mediated peptide dissociation. Thus, DR1betaH81N might represent a 'post-DM effect' conformation. We suggest that DM may mediate peptide dissociation by a 'hit-and-run' mechanism that results in conformational changes in MHC class II molecules and disruption of hydrogen bonds between betaHis81 and bound peptide.

Redox Regulation Facilitates Optimal Peptide Selection by MHC Class I during Antigen Processing

Activated CD8(+) T cells discriminate infected and tumor cells from normal self by recognizing MHC class I-bound peptides on the surface of antigen-presenting cells. The mechanism by which MHC class I molecules select optimal peptides against a background of prevailing suboptimal peptides and in a considerably proteolytic ER environment remained unknown. Here, we identify protein disulfide isomerase (PDI), an enzyme critical to the formation of correct disulfide bonds in proteins, as a component of the peptide-loading complex. We show that PDI stabilizes a peptide-receptive site by regulating the oxidation state of the disulfide bond in the MHC peptide-binding groove, a function that is essential for selecting optimal peptides. Furthermore, we demonstrate that human cytomegalovirus US3 protein inhibits CD8(+) T cell recognition by mediating PDI degradation, verifying the functional relevance of PDI-catalyzed peptide editing in controlling intracellular pathogens. These results establish a link between thiol-based redox regulation and antigen processing.

Small organic compounds enhance antigen loading of class II major histocompatibility complex proteins by targeting the polymorphic P1 pocket

Major histocompatibility complex (MHC) molecules are a key element of the cellular immune response. Encoded by the MHC they are a family of highly polymorphic peptide receptors presenting peptide antigens for the surveillance by T cells. We have shown that certain organic compounds can amplify immune responses by catalyzing the peptide loading of human class II MHC molecules HLA-DR. Here we show now that they achieve this by interacting with a defined binding site of the HLA-DR peptide receptor. Screening of a compound library revealed a set of adamantane derivatives that strongly accelerated the peptide loading rate. The effect was evident only for an allelic subset and strictly correlated with the presence of glycine at the dimorphic position beta86 of the HLA-DR molecule. The residue forms the floor of the conserved pocket P1, located in the peptide binding site of MHC molecule. Apparently, transient occupation of this pocket by the organic compound stabilizes the peptide-receptive conformation permitting rapid antigen loading. This interaction appeared restricted to the larger Gly(beta86) pocket and allowed striking enhancements of T cell responses for antigens presented by these "adamantyl-susceptible" MHC molecules. As catalysts of antigen loading, compounds targeting P1 may be useful molecular tools to amplify the immune response. The observation, however, that the ligand repertoire can be affected through polymorphic sites form the outside may also imply that environmental factors could induce allergic or autoimmune reactions in an allele-selective manner.

The impact of DM on MHC class II-restricted antigen presentation can be altered by manipulation of MHC-peptide kinetic stability

DM edits the peptide repertoire presented by major histocompatibility complex class II molecules by professional antigen-presenting cells (APCs), favoring presentation of some peptides over others. Despite considerable research by many laboratories, there is still significant uncertainty regarding the biochemical attributes of class II–peptide complexes that govern their susceptibility to DM editing. Here, using APCs that either do or do not express DM and a set of unrelated antigens, we found that the intrinsic kinetic stability of class II–peptide complexes is tightly correlated with the effects of DM editing within APCs. Furthermore, through the use of kinetic stability variants of three independent peptides, we demonstrate that increasing or decreasing the kinetic stability of class II–peptide complexes causes a corresponding alteration in DM editing. Finally, we show that the spontaneous kinetic stability of class II complexes correlates directly with the efficiency of presentation by DM⁺ APCs and the immunodominance of that class II–peptide complex during an immune response. Collectively, these results suggest that the pattern of DM editing in APCs can be intentionally changed by modifying class II–peptide interactions, leading to the desired hierarchy of presentation on APCs, thereby promoting recruitment of CD4 T cells specific for the preferred peptides during an immune response.

The human major histocompatibility complex class II HLA-DRB1 and HLA-DQA1 genes are separated by a CTCF-binding enhancer-blocking element

The human major histocompatibility complex class II (MHC-II) region encodes a cluster of polymorphic heterodimeric glycoproteins HLA-DR, -DQ, and -DP that functions in antigen presentation. Separated by approximately 44 kb of DNA, the HLA-DRB1 and HLA-DQA1 encode MHC-II proteins that function in separate MHC-II heterodimers and are diametrically transcribed. A region of high acetylation located in the intergenic sequences between HLA-DRB1 and HLA-DQA1 was discovered and termed XL9. The peak of acetylation coincided with sequences that bound the insulator protein CCCTC-binding factor as determined by chromatin immunoprecipitations and in vitro DNA binding studies. XL9 was also found to be associated with the nuclear matrix. The activity of the XL9 region was examined and found to be a potent enhancer-blocking element. These results suggest that the XL9 region may have evolved to separate the transcriptional units of the HLA-DR and HLA-DQ genes.

Small molecules that enhance the catalytic efficiency of HLA-DM

HLA-DM (DM) plays a critical role in Ag presentation to CD4 T cells by catalyzing the exchange of peptides bound to MHC class II molecules. Large lateral surfaces involved in the DM:HLA-DR (DR) interaction have been defined, but the mechanism of catalysis is not understood. In this study, we describe four small molecules that accelerate DM-catalyzed peptide exchange. Mechanistic studies demonstrate that these small molecules substantially enhance the catalytic efficiency of DM, indicating that they make the transition state of the DM:DR/peptide complex energetically more favorable. These compounds fall into two functional classes: two compounds are active only in the presence of DM, and binding data for one show a direct interaction with DM. The remaining two compounds have partial activity in the absence of DM, suggesting that they may act at the interface between DM and DR/peptide. A hydrophobic ridge in the DMbeta1 domain was implicated in the catalysis of peptide exchange because the activity of three of these enhancers was substantially reduced by point mutations in this area.

Amino-terminal flanking residues determine the conformation of a peptide-class II MHC complex

The peptide spanning residues 48-62 of hen egg white lysozyme presented by I-A(k) molecules gives rise to two T cell populations, types A and B, that recognize distinct conformers of the complex generated in late and recycling endosomes. The class II-like accessory molecule H2-DM functions as a conformational editor, eliminating the type B conformer in late endosomes. Here, we show that the conformation of the complex, and its susceptibility to editing by H2-DM, are determined by peptide amino-terminal flanking residues. Elimination of these residues abolished editing, permitting formation of the type B conformer in late endosomes. Substitutions at P(-2) affected the stability of the type B conformer, preventing its formation and/or editing, without hindering peptide binding or formation of the type A conformer of the complex. We conclude that interactions involving amino-terminal flanking residues stabilize peptide-MHC conformers and confer resistance to editing by H2-DM, influencing the nature of the T cell repertoire.

H2-O Expression in Primary Dendritic Cells

H2-O is a nonpolymorphic class II molecule whose biological role remains to be determined. H2-O modulates H2-M function, and it has been generally believed to be expressed only in B lymphocytes and thymic medullary epithelial cells, but not in dendritic cells (DCs). In this study, we report identification of H2-O expression in primary murine DCs. Similar to B cells, H2-O is associated with H2-M in DCs, and its expression is differentially regulated in DC subsets as well as during cell maturation and activation. Primary bone marrow DCs and plasmacytoid DCs in the spleen and lymph nodes express MHC class II and H2-M, but not the inhibitor H2-O. In contrast, myeloid DCs in secondary lymphoid organs express both H2-M and H2-O. In CD8alphaalpha(+) DCs, the ratio of H2-O to H2-M is higher than in CD8alphaalpha(-) DCs. In DCs generated from GM-CSF- and IL-4-conditioned bone marrow cultures, H2-O expression is not detected regardless of the maturation status of the cells. Administration of LPS induces in vivo activation of myeloid DCs, and this activation is associated with down-regulation of H2-O expression. Primary splenic DCs from H2-O(-/-) and H2-O(+/+) mice present exogenous protein Ags to T cell hybridomas similarly well, but H2-O(-/-) DCs induce stronger allogeneic CD4 T cell response than the H2-O(+/+) DCs in mixed leukocyte reactions. Our results suggest that H2-O has a broader role than previously appreciated in regulating Ag presentation.

Noble metals strip peptides from class II MHC proteins

Class II major histocompatibility complex (MHC) proteins are essential for normal immune system function but also drive many autoimmune responses. They bind peptide antigens in endosomes and present them on the cell surface for recognition by CD4(+) T cells. A small molecule could potentially block an autoimmune response by disrupting MHC-peptide interactions, but this has proven difficult because peptides bind tightly and dissociate slowly from MHC proteins. Using a high-throughput screening assay we discovered a class of noble metal complexes that strip peptides from human class II MHC proteins by an allosteric mechanism. Biochemical experiments indicate the metal-bound MHC protein adopts a 'peptide-empty' conformation that resembles the transition state of peptide loading. Furthermore, these metal inhibitors block the ability of antigen-presenting cells to activate T cells. This previously unknown allosteric mechanism may help resolve how gold(I) drugs affect the progress of rheumatoid arthritis and may provide a basis for developing a new class of anti-autoimmune drugs.

DM peptide-editing function leads to immunodominance in CD4 T cell responses in vivo

DM functions as a peptide editor for MHC class II-bound peptides. We examined the hypothesis that DM peptide editing plays a key role in focusing the in vivo CD4 T cell responses against complex pathogens and protein Ags to only one, or at most a few, immunodominant peptides. Most CD4 T cells elicited in the wild-type BALB/c (H-2d) mice infected with Leishmania major predominantly recognize a single epitope 158-173 within Leishmania homologue of activated receptor for c-kinase (LACK), as is the case when these mice are immunized with rLACK. Using DM-deficient (DM-/-) H-2d mice, we now show that in the absence of DM, the in vivo CD4 T cell responses to rLACK are skewed away from the immunodominant epitopes and are diversified to include two novel epitopes (LACK 33-48 and 261-276). DM-/- B10.BR (H-2k) mice showed similar results. These results constitute the first demonstration of the role of DM peptide editing in sculpting the specificity and immunodominance in in vivo CD4 T cell responses.

Conformational isomers of a peptide-class II major histocompatibility complex

The relative plasticity of peptide binding to class II major histocompatibility complex (MHC) molecules permits formation of multiple conformational isomers by the same peptide and MHC molecule; such conformers are specifically recognized by distinct subsets of T cells. Here, we review current knowledge and recent advances in our understanding of peptide-class II MHC conformational isomerism and the mechanisms that generate distinct MHC-peptide conformers. We focus on our studies of two T-cell subsets, type A and B, which recognize distinct conformers of the dominant epitope of hen egg white lysozyme presented by I-A(k). These conformers form via different pathways and in distinct intracellular vesicles: the type A conformer forms in late endosomes upon processing of native protein, while the more flexible type B conformer forms in early endosomes and at the cell surface. In this process, H2-DM acts as a conformational editor, eliminating the type B conformer in late endosomes. Type B T cells constitute a significant component of the naïve T-cell repertoire; furthermore, self-reactive type B T cells escape negative selection and are present in abundance in the periphery. Ongoing studies should elucidate the role of type B T cells in immunity to pathogens and in autoimmune pathology.

Achieving stability through editing and chaperoning: regulation of MHC class II peptide binding and expression

In antigen-presenting cells (APCs), loading of major histocompatibility complex class II (MHC II) molecules with peptides is regulated by invariant chain (Ii), which blocks MHC II antigen-binding sites in pre-endosomal compartments. Several molecules then act upon MHC II molecules in endosomes to facilitate peptide loading: Ii-degrading proteases, the peptide exchange factor, human leukocyte antigen-DM (HLA-DM), and its modulator, HLA-DO (DO). Here, we review our findings arguing that DM stabilizes a globally altered conformation of the antigen-binding groove by binding to a lateral surface of the MHC II molecule. Our data imply changes in the interactions between specificity pockets and peptide side chains, complementing data from others that suggest DM affects hydrogen bonds. Selective weakening of peptide/MHC interactions allows DM to alter the peptide repertoire. We also review our studies in cells that highlight the ability of several factors to modulate surface expression of MHC II molecules via post-Golgi mechanisms; these factors include MHC class II-associated Ii peptides (CLIP), DM, and microbial products that modulate MHC II traffic from endosomes to the plasma membrane. In this context, we discuss possible mechanisms by which the association of some MHC II alleles with autoimmune diseases may be linked to their low CLIP affinity.

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.

Right place, right time, right peptide: DO keeps DM focused

Peptide loading of major histocompatibility class II molecules is catalyzed in late endosomal and lysosomal compartments of cells by the catalytic action of human leukocyte antigen (HLA)-DM (H-2M in mice). In B cells, dendritic cells and thymic epithelial cells, the peptide loading of class II molecules is modified by the expression of the non-classical class II molecule, HLA-DO (H-2O in mice). Collectively, studies to date support that DO/H-2O expression inhibits the presentation of antigens acquired by cells via fluid phase endocytosis. However, in B cells, the expression of H-2O promotes the presentation of antigens internalized by the B-cell receptor. In this review, we summarize the literature pertaining to DO assembly, transport, and function, with an emphasis on the function of DO/H-2O.

The optimization of peptide cargo bound to MHC class I molecules by the peptide-loading complex

Major histocompatibility complex (MHC) class I complexes present peptides from both self and foreign intracellular proteins on the surface of most nucleated cells. The assembled heterotrimeric complexes consist of a polymorphic glycosylated heavy chain, non-polymorphic beta(2) microglobulin, and a peptide of typically nine amino acids in length. Assembly of the class I complexes occurs in the endoplasmic reticulum and is assisted by a number of chaperone molecules. A multimolecular unit termed the peptide-loading complex (PLC) is integral to this process. The PLC contains a peptide transporter (transporter associated with antigen processing), a thiooxido-reductase (ERp57), a glycoprotein chaperone (calreticulin), and tapasin, a class I-specific chaperone. We suggest that class I assembly involves a process of optimization where the peptide cargo of the complex is edited by the PLC. Furthermore, this selective peptide loading is biased toward peptides that have a longer off-rate from the assembled complex. We suggest that tapasin is the key chaperone that directs this action of the PLC with secondary contributions from calreticulin and possibly ERp57. We provide a framework model for how this may operate at the molecular level and draw parallels with the proposed mechanism of action of human leukocyte antigen-DM for MHC class II complex optimization.

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.

Chaperoning antigen presentation by MHC class II molecules and their role in oncogenesis

Tumor vaccine development aimed at stimulating the cellular immune response focuses mainly on MHC class I molecules. This is not surprising since most tumors do not express MHC class II or CD1 molecules. Nevertheless, the most successful targets for cancer immunotherapy, leukemia and melanoma, often do express MHC class II molecules, which leaves no obvious reason to ignore MHC class II molecules as a mediator in anticancer immune therapy. We review the current state of knowledge on the process of MHC class II-restricted antigen presentation and subsequently discuss the consequences of MHC class II expression on tumor surveillance and the induction of an efficient MHC class II mediated antitumor response in vivo and after vaccination.

CELL BIOLOGY OF ANTIGEN PROCESSING IN VITRO AND IN VIVO

The conversion of exogenous and endogenous proteins into immunogenic peptides recognized by T lymphocytes involves a series of proteolytic and other enzymatic events culminating in the formation of peptides bound to MHC class I or class II molecules. Although the biochemistry of these events has been studied in detail, only in the past few years has similar information begun to emerge describing the cellular context in which these events take place. This review thus concentrates on the properties of antigen-presenting cells, especially those aspects of their overall organization, regulation, and intracellular transport that both facilitate and modulate the processing of protein antigens. Emphasis is placed on dendritic cells and the specializations that help account for their marked efficiency at antigen processing and presentation both in vitro and, importantly, in vivo. How dendritic cells handle antigens is likely to be as important a determinant of immunogenicity and tolerance as is the nature of the antigens themselves.

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.

The expression of HLA-DO (H2-O) in B lymphocytes

HLA-DO (H2-O in mice) is a nonpolymorphic transmembrane alphabeta heterodimer encoded in the class II region of the major histocompatibility complex (MHC). It is expressed selectively in B lymphocytes and thymic medullary epithelial cells. DO forms a stable complex with the peptide-loading catalyst HLA-DM in the endoplasmic reticulum (ER); in the absence of DM, DO is unstable. During intracellular transport and distribution in the endosomal compartments, the ratio of DO to DM changes. In primary B cells, only approx 50% of DM molecules are associated with DO. DO appears to regulate the peptide-loading function of DM in the MHC class II antigen-presentation pathway. Although certain discrepancies are present, results from most studies indicate that DO (as well as H2-O) inhibits DM (H2-M) function; this inhibition is pH-dependent. As a consequence, DO restrains presentation of exogenous antigens delivered through nonreceptor-mediated mechanisms; in addition, DO alters the peptide repertoire that is associated with cell-surface class II molecules. The biological function of DO remains obscure, partially because of the lack of striking phenotypes in the H2-O knockout mice. Results from recent studies indicate that DO expression in B cells is dynamic, and highly regulated during B-cell development and B-cell activation, suggesting that the physiological role of DO is to tailor the antigen presentation function of the B-lineage cells to meet their primary function at each stage of B-cell development and maturation. Further investigations are needed in this direction.

“Chemical Analogues” of HLA-DM Can Induce a Peptide-receptive State in HLA-DR Molecules

We had recently identified small molecular compounds that are able to accelerate the ligand exchange reactions of HLA-DR molecules. Here we show that this acceleration is due to the induction of a "peptide-receptive" state. Dissociation experiments of soluble HLA-DR2.CLIP (class II-associated invariant chain peptide) complex and peptide-binding studies with "nonreceptive" empty HLA-DR1 and -DR2 molecules revealed that the presence of a small phenolic compound carrying an H-bond donor group (-OH) results in the drastic increase of both off- and on-rates. The rate-limiting step for ligand exchange, the transition of the major histocompatibility complex molecule from a nonreceptive into the receptive state, is normally mediated by interaction with the chaperone HLA-DM. In this respect, the effect of small molecules resembles that of the natural catalyst, except that they are still active at neutral pH. These "chemical analogues" of HLA-DM can therefore modulate the response of CD4+ T cells by editing the antigen composition of surface-bound class II major histocompatibility complex on living antigen-presenting cells.

Dissecting MHC class II export, B cell maturation, and DM stability defects in invariant chain mutant mice

Invariant (Ii) chain loss causes defective class II export, B cell maturation, and reduced DM stability. In this study, we compare Ii chain and class II mutant mouse phenotypes to dissect these disturbances. The present results demonstrate that ER retention of alphabeta complexes, and not beta-chain aggregates, disrupts B cell development. In contrast, we fail to detect class II aggregates in Ii chain mutant thymi. Ii chain loss in NOD mice leads to defective class II export and formation of alphabeta aggregates, but in this background, downstream signals are misregulated and mature B cells develop normally. Finally, Ii chain mutant strains all display reduced levels of DM, but mice expressing either p31 or p41 alone, and class II single chain mutants, are indistinguishable from wild type. We conclude that Ii chain contributions as a DM chaperone are independent of its role during class II export. This Ii chain/DM partnership favors class II peptide loading via conventional pathway(s).

Modification of peptide interaction with MHC creates TCR partial agonists

We report the creation of TCR partial agonists by the novel approach of manipulating the interaction between immunogenic peptide and MHC. Amino acids at MHC anchor positions of the I-E(k)-restricted hemoglobin (64-76) and moth cytochrome c (88-103) peptides were exchanged with MHC anchor residues from the low affinity class II invariant chain peptide (CLIP), resulting in antigenic peptides with altered affinity for MHC class II. Several low affinity peptides were identified as TCR partial agonists, as defined by the ability to stimulate cytolytic function but not proliferation. For example, a peptide containing methionine substitutions at positions one and nine of the I-E(k) binding motif acted as a partial agonist for two hemoglobin-reactive T cell clones (PL.17 and 3.L2). The identical MHC anchor substitutions in moth cytochrome c (88-103) also created a partial agonist for a mCC-reactive T cell (A.E7). Thus, peptides containing MHC anchor modifications mediated similar T cell responses regardless of TCR fine specificity or antigen reactivity. This data contrasts with the unique specificity among individual clones demonstrated using traditional altered peptide ligands containing substitutions at TCR contact residues. In conclusion, we demonstrate that altering the MHC anchor residues of the immunogenic peptide can be a powerful method to create TCR partial agonists.