Interaction between HLA-DM and HLA-DR involves regions that undergo conformational changes at lysosomal pH

Antigenic peptide loading of major histocompatibility complex class II molecules is enhanced by lysosomal pH and catalyzed by the HLA-DM molecule. The physical mechanism behind the catalytic activity of DM was investigated by using time-resolved fluorescence anisotropy (TRFA) and fluorescence binding studies with the dye 8-anilino-1-naphthalenesulfonic acid (ANS). We demonstrate that the conformations of both HLA-DM and HLA-DR3, irrespective of the composition of bound peptide, are pH sensitive. Both complexes reversibly expose more nonpolar regions upon protonation. Interaction of DM with DR shields these hydrophobic domains from the aqueous environment, leading to stabilization of the DM and DR conformations. At lysosomal pH, the uncovering of additional hydrophobic patches leads to a more extensive DM-DR association. We propose that DM catalyzes class II peptide loading by stabilizing the low-pH conformation of DR, favoring peptide exchange. The DM-DR association involves a larger hydrophobic surface area with DR/class II-associated invariant chain peptides (CLIP) than with stable DR/peptide complexes, explaining the preferred association of DM with the former. The data support a release mechanism of DM from the DM-DR complex through reduction of the interactive surface, upon binding of class II molecules with antigenic peptide or upon neutralization of the DM-DR complex at the cell surface.

Inefficient peptide binding by cell-surface class II MHC molecules

The efficiency of peptide loading onto surface class II MHC molecules in intact APC was investigated, using a previously defined europium immunoassay as well as a simplified Western blot procedure. Conditions normally employed for peptide loading in T cell stimulation assays were suboptimal for peptide binding, which is enhanced at low pH, in the presence of protease inhibitors, and the absence of competing serum proteins. In contrast to some earlier reports, our results indicate that the rate of peptide loading by class II molecules is not enhanced in the environment of the plasma membrane. Peptide association rates were similar for purified and cell-surface class II molecules. As previously reported, rapid peptide binding can be achieved by reconstituting class II molecules into total cellular membranes. We report that this activity is due solely to HLA-DM (which is not present at the cell surface), since it can be specifically removed by immunodepletion with an anti-DM mAb. Thus, we find no evidence for additional cellular cofactors capable of catalyzing peptide binding to class II molecules.

Novel mutants define genes required for the expression of human histocompatibility leukocyte antigen DM: evidence for loci on human chromosome 6p

We and others have shown that the products of the HLA-DM locus are required for the intracellular assembly of major histocompatibility complex class II molecules with cognate peptides for antigen presentation. HLA-DM heterodimers mediate the dissociation of invariant chain (Ii)-derived class II-associated Ii peptides (CLIP) from class II molecules and facilitate the loading of class II molecules with antigenic peptides. Here we describe novel APC mutants with defects in the formation of class II-peptide complexes. These mutants express class II molecules which are conformationally altered, and an aberrantly high percentage of these class II molecules are associated with Ii-derived CLIP. This phenotype resembles that of DM null mutants. However, we show that the defects in two of these new mutants do not map to the DM locus. Nevertheless, our evidence suggests that the antigen processing defective phenotype in these mutants results from deficient DM expression. These mutants thus appear to define genes in which mutations have differential effects on the expression of conventional class II molecules and DM molecules. Our data are most consistent with these factors mapping to human chromosome 6p. Previous data have suggested that the expression of DM and class II genes are coordinately regulated. The results reported here suggest that DM and class II can also be differentially regulated, and that this differential regulation has significant effects on class II-restricted antigen processing.

Major histocompatibility complex class II compartments in human and mouse B lymphoblasts represent conventional endocytic compartments

In most human and mouse antigen-presenting cells, the majority of intracellular major histocompatibility complex (MHC) class II molecules resides in late endocytic MHC class II compartments (MIICs), thought to function in antigen processing and peptide loading. However, in mouse A20 B cells, early endocytic class II-containing vesicles (CIIVs) have been reported to contain most of the intracellular MHC class II molecules and have also been implicated in formation of MHC class II-peptide complexes. To address this discrepancy, we have studied in great detail the endocytic pathways of both a human (6H5.DM) and a mouse (A20.Ab) B cell line. Using quantitative immunoelectron microscopy on cryosections of cells that had been pulse-chased with transferrin-HRP or BSA-gold as endocytic tracers, we have identified up to six endocytic subcompartments including an early MIIC type enriched in invariant chain, suggesting that it serves as an important entrance to the endocytic pathway for newly synthesized MHC class II/invariant chain complexes. In addition, early MIICs represented the earliest endocytic compartment containing MHC class II- peptide complexes, as shown by using an antibody against an abundant endogenous class II-peptide complex. The early MIIC exhibited several though not all of the characteristics reported for the CIIV and was situated just downstream of early endosomes. We have not encountered any special class II-containing endocytic structures besides those normally present in nonantigen-presenting cells. Our results therefore suggest that B cells use conventional endocytic compartments rather than having developed a unique compartment to accomplish MHC class II presentation.

Negative regulation by HLA-DO of MHC class II-restricted antigen processing

HLA-DM is a major histocompatibility complex (MHC) class II-like molecule that facilitates antigen processing by catalyzing the exchange of invariant chain-derived peptides (CLIP) from class II molecules for antigenic peptides. HLA-DO is a second class II-like molecule that physically associates with HLA-DM in B cells. HLA-DO was shown to block HLA-DM function. Purified HLA-DM-DO complexes could not promote peptide exchange in vitro. Expression of HLA-DO in a class II+ and DM+, DO- human T cell line caused the accumulation of class II-CLIP complexes, indicating that HLA-DO blocked DM function in vivo and suggesting that HLA-DO is an important modulator of class II-restricted antigen processing.

Targeting Epstein-Barr virus nuclear antigen 1 (EBNA1) through the class II pathway restores immune recognition by EBNA1-specific cytotoxic T lymphocytes: evidence for HLA-DM-independent processing

Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA1) is the only viral protein consistently expressed in all malignancies associated with EBV and there is now convincing evidence to suggest that EBNA1 is not recognized by MHC class I-restricted cytotoxic T lymphocytes (CTL). The lack of recognition of EBNA1 has been attributed to a cis-acting inhibitory effect of glycine-alanine repetitive (G-Ar) sequences on the endogenous processing of this antigen through the class I pathway. In the present study we have explored the possibility of targeting EBNA1 through an alternative mechanism using the MHC class II pathway. Using purified EBNA1 protein, we demonstrate here that CD4+ CTL can efficiently recognize EBV-transformed B cells and Burkitt's lymphoma cells following exogenous sensitization with this antigen, and this immune recognition is not affected by the G-Ar domain within EBNA1. Analysis of the processing mechanism revealed that intracellular loading of class II molecules with an EBNA1 epitope occurs through an HLA-DM-independent pathway. These results highlight a novel mechanism for immune recognition of EBNA1 and also demonstrate that the G-Ar-mediated protection from processing can be overridden if this antigen is presented through the class II pathway.

Inhibition of IL-10 protein synthesis induces major histocompatibility complex class II gene expression in class II-deficient patients

Major histocompatibility complex (MHC) class II deficiency is an inherited autosomal recessive combined immunodeficiency, characterized by a lack of constitutive expression of the human leukocyte antigen (HLA) class II genes. The patients investigated in this study are histoidentical twin brothers with a new phenotype in MHC class II deficiency. Examination of HLA-D locus genes in their fractionated peripheral mononuclear cells (MNCs) revealed an unusual and uncoordinated mRNA pattern. Here we analyzed the distribution of pro- and anti-inflammatory cytokines expressed in these patients' adherent and nonadherent MNCs. We show that gene expression of IL-1 alpha, IL-1 beta, IL-6, granulocyte-colony-stimulating factor, and IL-10 was induced in both cell fractions, whereas increased mRNA levels of interferon-gamma and the inducible nitric oxide synthase were exclusively detected in the patients' nonadherent MNCs. As IL-10 is known to be able to downregulate transcription of MHC class II and expression of IL-10 in the patients' MNCs was increased, we investigated the regulatory function of this cytokine. Interestingly, inhibition of IL-10 protein synthesis with IL-10-specific antisense oligonucleotide DNA (IL-10-AS-ODN) induced HLA-D locus genes in these MHC class II-deficient patients. Exposure of the nonadherent cell fraction to IL-10-AS-ODN resulted in a profound induction of a previously absent DR beta 1 and DP alpha gene expression. HLA-DQ beta mRNA levels, however, were increased in both the adherent and the nonadherent MNC population. Albeit expression of HLA-D locus genes was inducible via inhibition of IL-10 translation, surface expression of HLA class II antigens on the patients' MNCs was essentially negative. The data presented support the concept of a coordinated network of pro- and anti-inflammatory cytokine regulation and this network obviously has a significant role in the cell-type-specific regulation of MHC class II expression.

The mannose receptor functions as a high capacity and broad specificity antigen receptor in human dendritic cells

Dendritic cells, in contrast to B lymphocytes, must be able to efficiently internalize a diverse array of antigens for processing and loading onto major histocompatibility complex (MHC) class II molecules. Here we characterize the mannose receptor pathway in dendritic cells and show that mannose receptor-mediated uptake of antigens results in a approximately 100-fold more efficient presentation to T cells, as compared to antigens internalized via fluid phase. Immunocytochemistry as well as subcellular fractionation revealed the localization of the mannose receptor and MHC class II molecules in distinct subcellular compartments. The mannose receptor thus functions in rapid internalization and concentration of a variety of glycosylated antigens that become available for processing and presentation. This may contribute to the unique capacity of dendritic cells to generate primary T cell responses against infectious agents.

Regulation of HLA class I and II expression by interferons and influenza A virus in human peripheral blood mononuclear cells

HLA class I and II molecules play a central role in regulating host immune responses against microbial infections because they present foreign antigens to CD8+ and CD4+ T lymphocytes, respectively. Many cytokines, especially interferons (IFN), are known to upregulate human leucocyte antigen (HLA) class I and II gene expression, but the kinetics, expression levels and viral regulation of HLA genes in primary human cells have not been well documented. Stimulation of peripheral blood mononuclear cells (PBMC) with IFN-alpha and IFN-gamma resulted in a 1.5- to twofold increase in HLA class I and beta 2-microglobulin expression in lymphocytes and monocytes. Lymphocytes did not express any detectable HLA class II either basally or after IFN induction. In monocytes, instead, a high basal class II expression was found and it was further induced by IFN-alpha (up to twofold) and especially by IFN-gamma (up to fivefold). In granulocyte-macrophage colony-stimulating factor (GM-CSF) differentiated human macrophages, basal HLA class I and II protein expression levels were high but IFN-gamma stimulation was able to further enhance their expression. Accordingly, class I and II mRNA expression was elevated by IFN-gamma, whereas IFN-alpha practically had no effect on HLA class I mRNA levels. Influenza A virus infection of macrophages resulted in temporary increases in HLA class I, beta 2-microglobulin and class II antigen expression. Neutralization of virus-induced IFN production by antibodies against type I and II IFNs prevented the virus-induced upregulation of HLA antigens. At late times of infection, as analysed by steady-state mRNA expression, both HLA class I and II mRNA were strongly reduced. These results suggest that IFNs are important regulators of HLA genes and responsible for a temporary increase in HLA antigen expression during influenza A virus infection.

Binding of copolymer 1 and myelin basic protein leads to clustering of class II MHC molecules on antigen-presenting cells

Copolymer 1 (Cop 1), a synthetic copolymer of amino acids, effective in suppression of experimental allergic encephalomyelitis (EAE) and myelin basic protein (MBP), was shown to bind extensively and promiscuously to the class II MHC molecules on antigen-presenting cells (APC) without prior processing. In the case of human APC, binding has earlier been demonstrated to DR but not DQ or class I molecules. In the present study, we examined whether binding of Cop 1 and MBP affects MHC class II expression on the cell membrane. Biotinylated derivatives of these antigens were used to monitor their direct binding to MHC molecules on living APC by flow cytometry using phycoerythrin-streptavidin, while the levels of MHC surface expression were monitored by staining with FITC-conjugated anti-class I- and class II-specific antibodies. When Cop 1 or MBP were incubated with the APC, intensity of cell staining with anti-DR, but not with anti-DQ or anti-class I antibodies, was significantly increased, compared to the staining of control APC not reacted with these antigens. In contrast, staining intensity was unaffected when p84-102, a human immunodominant epitope of MBP, or ovalbumin (OVA), a protein which undergoes proteolytic degradation prior to binding, were incubated with the APC. Cycloheximide, a protein synthesis inhibitor, had no effect on the enhanced staining intensity with anti-DR antibody of cells treated with Cop 1 or MBP, whereas it inhibited the enhanced staining of both DR and DQ molecules caused by the respective antibodies, in the absence of these antigens. Brefeldin A, a protein transport inhibitor, lowered the levels of staining intensity with anti-DR and anti-DQ antibodies in both cases, with and without antigen added to the APC. Fluorescence microscopic analysis revealed that cells incubated with Cop 1 or MBP, but not with p84-102 or OVA, exhibit both bright staining of the cell membrane and clusters produced by the aggregation of DR molecules with these antigens. Taken together, these observations indicate that Cop 1 and MBP, due to their polyvalent character, lead to increased fluorescence intensity of their complexes with HLA-DR, possibly due to recruitment and clustering of previously synthesized DR molecules. This can explain the efficient binding of these antigens to the MHC class II molecules.

Epstein-Barr virus uses HLA class II as a cofactor for infection of B lymphocytes

Infection of B lymphocytes by Epstein-Barr virus (EBV) requires attachment of virus via binding of viral glycoprotein gp350 to CD21 on the cell surface. Penetration of the cell membrane additionally involves a complex of three glycoproteins, gH, gL, and gp42. Glycoprotein gp42 binds to HLA-DR. Interference with this interaction with a soluble form of gp42, with a monoclonal antibody (MAb) to gp42, or with a MAb to HLA-DR inhibited virus infection. It was not possible to superinfect cells that failed to express HLA-DR unless expression was restored by transfection or creation of hybrid cell lines with complementing deficiencies in expression of HLA class II. HLA class II molecules thus serve as cofactors for infection of human B cells.

An iterative algorithm for converting a class II MHC binding motif into a quantitative predictive model

Biochemists and molecular biologists have suggested motifs for characterizing the binding of peptide fragments and class II major histocompatibility complex (MHC) molecules based on laboratory results and crystal structures. The iterative algorithm presented here converts a suggested motif into a quantitative data-based model. The database accessed consists of peptide fragments known to bind or not bind to class II MHC molecules of particular haplotypes. Stepwise discriminant analysis is utilized to increase or decrease motif coefficients until the resulting motif classifies all binders and non-binders correctly. Stepwise discriminant analysis is a standard multivariate statistical procedure and is available in comprehensive commercial statistical packages. Program 7M of BMDP Statistical Software was used in this study.

Characterization of the major histocompatibility complex class II binding site on LAG-3 protein

The lymphocyte activation gene-3 (LAG-3), selectively transcribed in human activated T and NK cells, encodes a ligand for major histocompatibility complex (MHC) class II molecules. Like CD4, LAG-3 ectodomain is composed of four Ig-like domains (D1-D4). Nothing is known about the LAG-3 regions or residues required to form a stable MHC class II binding site. In contrast to CD4, soluble LAG-3 molecules stably interact with MHC class II molecules expressed on the cell surface. In addition, the first two N-terminal domains of soluble LAG-3 (D1 and D2) molecules, alone, are capable of binding MHC class II. From a LAG-3 model structure, we designed mutants and tested their ability to bind MHC class II molecules in an intercellular adhesion assay. We found residues on the membrane-distal, CDR1-2-containing top face of D1 that are essential for either binding or repulsing MHC class II proteins. Most of these residues are clustered at the base of a large extra-loop structure that is a hallmark of the LAG-3 D1 Ig-like domain. In addition, as for CD4, oligomerization of LAG-3 on the cell surface may be required to form a stable MHC binding site because mutation of three residues in the ABED beta-strands containing side of D1 results in a dominant negative effect (i.e., binding inhibition of coexpressed wild-type LAG-3).

Intact proteins can bind to class II histocompatibility molecules with high affinity

The ability of intact protein antigens to bind to purified class II histocompatibility molecules was investigated. Intact bovine ribonuclease (RNase) inhibited peptide binding to DR1 with a potency similar to that of a high affinity peptide or irreversibly denatured RNase. Similarly, horse myoglobin (Mb) was a potent inhibitor of peptide binding to I-E(k). I-E(k)-Mb complexes were directly visualized as a distinct band with reduced mobility on SDS PAGE. Direct binding experiments with biotin-labeled proteins demonstrated that Mb and RNase bind to class II molecules through the peptide-binding groove with high affinity, and that binding occurs in the absence of detergent. The possibility that HLA-DM can catalyse the binding of intact protein antigens was supported by the observation that DM enhances the binding of biotin-RNase to DR1. Our results provide further support for the hypothesis that intact, partially unfolded protein antigens can act as ligands for initial interaction with class II molecules.

Endothelial expression of CD40 in renal cell carcinoma

Recently, the immunoregulative molecule CD40 has also been introduced as a potential surface determinant of endothelial cells that can be induced by various cytokines and thus might be involved in inflammatory vascular reactions. In this study, the ubiquitous endothelial expression of CD40 within the neovascularized areas of renal cell carcinoma is demonstrated. The strong capillary expression of CD40 in 12 tumor samples is contrasted by the absence of endothelial CD40 in the corresponding tumor-free kidney specimens in which only certain tubular segments and few interstitial cells carry CD40. Northern hybridization studies confirmed the presence of CD40 RNA in cytokine-treated endothelial cells and in renal cell carcinoma, whereas no hybridization signal was obtained with normal kidney tissue. That the presence of tumor cells is pertinent to the endothelial expression of CD40 could be substantiated by in vitro experiments, when a renal carcinoma cell line and its supernatant, but not normal kidney cells, could induce CD40 on endothelial cells in culture. According to further experimental results, the carcinoma-derived, CD40-inducing factor(s) is not represented within a variety of pleiotropic cytokines including IFN-gamma, interleukin 1, interleukin 6, and tumor necrosis factor alpha, or common angiogenic factors such as basic fibroblast growth factor, vascular endothelial cell growth factor, angiogenin, and erythropoietin. The immunohistological results showing a widespread, even distribution of CD40 in tumor capillaries suggest that within renal cell carcinoma, the appearance of endothelial CD40 may also be related to angiogenesis in addition to inflammation.

Assembly of an abundant endogenous major histocompatibility complex class II/peptide complex in class II compartments

To identify the intracellular site(s) of formation of an endogenous class II/peptide complex in a human B cell line, we employed kinetic pulse-chase labeling experiments followed by subcellular fractionation by Percoll density gradient centrifugation and immunogold labeling on ultrathin cryosections. For direct demonstration of assembly of such complexes, we used the monoclonal antibody YAe, which detects an endogenous complex of the mouse class II molecule I-Ab with a 17-amino acid peptide derived from the alpha chain of HLA-DR (DR alpha52-68). We show that in human B lymphocytes, these class II/peptide complexes assemble and transiently accumulate in major histocompatibility complex class II-enriched compartments before reaching the cell surface.

HLA-DM acts as a molecular chaperone and rescues empty HLA-DR molecules at lysosomal pH

HLA-DM (DM) is a nonclassical MHC class II molecule that interacts with classical MHC II molecules in acidic compartments. During this association DM is supposed to catalyze the release of invariant chain (Ii)-derived CLIP peptides, as well as other peptides bound with low kinetic stability. Here we provide evidence that in lysosomal compartments of B cells a considerable fraction of DM is stably associated with empty DR alphabeta dimers, thereby preventing their functional inactivation and aggregation. Upon encounter with cognate peptide, the DM-associated DR molecules can be rapidly loaded and no longer bind to DM. Thus, DM seems to act as a dedicated class II-specific chaperone. In view of the suggested shortage of DM-resistant self-peptides in the loading compartment, empty class II molecules that are chaperoned by DM may enable the antigen-processing system to respond promptly to the challenge by newly entering antigens.

Early endosomes and a late endocytic compartment generate different peptide-class II MHC complexes via distinct processing mechanisms

Class II MHC Ag-processing compartments and mechanisms were compared for four antigenic epitopes from hen egg lysozyme (HEL) and RNase. T cell assays on subcellular fractions of Ag-pulsed macrophages detected the initial appearance of HEL-(48-61):I-Ak, HEL-(34-45):I-Ak, and RNase-(90-105):I-Ek complexes in a high density late endocytic compartment. In contrast, RNase-(42-56):I-Ak complexes never appeared in high density compartments, but were rapidly generated in low density endosomes. This early endosomal processing mechanism was 1) chloroquine inhibitable; 2) less sensitive than the late endocytic mechanism to 20 degrees C inhibition; 3) partially resistant to depletion of nascent class II MHC molecules with brefeldin A, suggesting some use of recycled class II MHC molecules, whereas the late endocytic processing mechanism was blocked by brefeldin A; and 4) involved in the processing of a DM-independent complex (RNase-(42-56):I-Ak). Thus, distinct processing compartments and mechanisms are identified for different epitopes even within a single Ag.

In vitro targeting and specific transfection of human neuroblastoma cells by chCE7 antibody-mediated gene transfer

We developed a new vector for gene targeting of neuroblastoma (NB) cells, based on the utilization o a monoclonal antibody (chCE7) covalently linked to polylysine (PL). In the presence of chloroquine, chCE7-PL-DNA complexes transfected NB cells as efficiently as DOTAP, transfectam, TF-X50, or lipofectamine. This was demonstrated by transfection of the luciferase or beta-galactosidase reporter genes in three different NB cell lines. This transfection was specific, since it was inhibited in the presence of competing unconjugated chCE7 antibody (Ab), and was not observed in cell lines negative for the CE7 antigen. We tested the potential biological activity of a plasmid coding for gamma-interferon (gamma IFN) transfected with chCE7-PL. HLA ABC expression on NB cells was induced after transfection with pCMV-gamma IFN at a higher level than after incubation with 1000 IU/ml of purified gamma IFN. Moreover, these HLA ABC-positive NB cells were able to activate autologous cytotoxic T lymphocytes in vitro. Thus chCE7-PL is able to target a plasmid to NB cells and to allow the expression of the transfected gene in a biologically active form.

Induction of cell cycle arrest and B cell terminal differentiation by CDK inhibitor p18(INK4c) and IL-6

Cell cycle arrest and cell death are tightly coupled to terminal differentiation of B cells to plasma cells in vivo. This process was recapitulated in vitro by stimulation of IgG-bearing human B lymphoblastoid cells with interleukin-6 (IL-6), which led to orderly cell cycle arrest, differentiation, and apoptosis. In terminally differentiated plasmacytoid cells, phosphorylation of pRb was suppressed, correlating with the activation of the D-type cyclin-dependent kinase (CDK) inhibitors p18(INK4c) and p21(WAF1/CIP1). The expression of CDK6, however, remained unchanged. Activation of p18 by IL-6 was rapid, concomitant with marked enhancement of its association with CDK6 and cell cycle arrest. Overexpression of p18 in IgM-bearing lymphoblastoid cells, which differentiated in response to IL-6 but did not exit the cell cycle, reconstituted coupled differentiation and cell cycle arrest. Thus, CDK inhibitors, in particular p18, are likely to play a pivotal role in controlling cell cycle arrest and cell death in terminal differentiation of late-stage B cells to plasma cells via inhibition of pRb phosphorylation by CDK6.

HLA-DM is present in one-fifth the amount of HLA-DR in the class II peptide-loading compartment where it associates with leupeptin-induced peptide (LIP)-HLA-DR complexes

HLA-DM has been shown in vitro to catalyze the release of invariant chain (Ii) derived peptides from the peptide-binding groove of class II molecules, thereby facilitating the binding of antigenic peptides. Previous studies showed that at steady state, the majority of DM resides in the class II peptide-loading compartment (IIPLC) where Ii dissociates from class II molecules and antigenic peptides are bound. Here we characterize the expression of DM in vivo in subcellular fractions containing the IIPLC. Using quantitative immunoblotting, we show that in the cell as a whole, class II molecules are expressed in 23-fold molar excess of DM. However, DM is concentrated in the IIPLC, where it is present in a considerably higher concentration relative to the class II molecules, in a molar ratio of 5DR:1 DM. This molar ratio of DM to DR in the IIPLC in vivo is consistent with the catalytic function proposed for DM from studies in vitro. We also provide both biochemical and genetic evidence that DM associates with complexes which contain Ii fragments and class II molecules in the IIPLC. Such complexes are only observed in leupeptin-treated cells in which Ii fails to be completely degraded and complexes containing the leupeptin-induced fragment of Ii (LIP) and class II molecules accumulate in the IIPLC. This observation is consistent with LIP-class II complexes being a substrate for DM in vivo and suggests that interactions of DM and LIP-class II are extremely transient under normal conditions.

Expression of major histocompatibility complex class II molecules in HeLa cells promotes the recruitment of AP-1 Golgi-specific assembly proteins on Golgi membranes

The newly synthesized major histocompatibility complex (MHC) class II molecules, an alphabeta dimer associated with the Ii invariant chain, must be targeted to endosomal, lysosomal enzyme-rich compartments in order to bind and present immunogenic peptides. The precise route followed by this complex at the exit of the trans-Golgi network, the last sorting station of the biosynthetic pathway, is poorly understood. We show here that overexpression of alphabetaIi complexes in HeLa cells promotes the first step of clathrin-coat assembly in vitro, that is the ARF-dependent translocation of AP-1 Golgi-specific assembly proteins on membranes. In contrast, alphabeta dimers alone or associated with Ii lacking most of its cytoplasmic domain fail to recruit AP-1. This study strongly suggests that the invariant chain (Ii) is responsible for the AP-1-dependent sorting of the alphabeta dimers in the trans-Golgi network of HeLa cells and that the MHC class II molecules are, like the mannose 6-phosphate receptors, transported directly from this compartment to endosomes via clathrin-coated vesicles.

Editing of the HLA-DR-peptide repertoire by HLA-DM

Antigenic peptide loading of classical major histocompatibility complex (MHC) class II molecules requires the exchange of the endogenous invariant chain fragment CLIP (class II associated Ii peptide) for peptides derived from antigenic proteins. This process is facilitated by the non-classical MHC class II molecule HLA-DM (DM) which catalyzes the removal of CLIP. Up to now it has been unclear whether DM releases self-peptides other than CLIP and thereby modifies the peptide repertoire presented to T cells. Here we report that DM can release a variety of peptides from HLA-DR molecules. DR molecules isolated from lymphoblastoid cell lines were found to carry a sizeable fraction of self-peptides that are sensitive to the action of DM. The structural basis for this DM sensitivity was elucidated by high-performance size exclusion chromatography and a novel mass spectrometry binding assay. The results demonstrate that the overall kinetic stability of a peptide bound to DR determines its sensitivity to removal by DM. We show that DM removes preferentially those peptides that contain at least one suboptimal side chain at one of their anchor positions or those that are shorter than 11 residues. These findings provide a rationale for the previously described ligand motifs and the minimal length requirements of naturally processed DR-associated self-peptides. Thus, in endosomal compartments, where peptide loading takes place, DM can function as a versatile peptide editor that selects for high-stability MHC class II-peptide complexes by kinetic proofreading before these complexes are presented to T cells.

F(c)gammaRI-targeted fusion proteins result in efficient presentation by human monocytes of antigenic and antagonist T cell epitopes

A major challenge for using native or modified T cell epitopes to induce or suppress immunity relates to poor localization of peptides to antigen presenting cells (APCs) in vivo. In this study, we demonstrate enhanced presentation of antigenic and antagonistic peptides by targeting them to the type I Fc receptor for IgG (F(c)gammaRI, CD64) on human monocytes. A Th epitope of tetanus toxoid, TT830, and the antagonistic peptide for TT830, TT833S, were genetically grafted into the constant region of the heavy chain of the humanized anti-CD64 mAb 22 and expressed as monovalent fusion proteins, Fab22-TT830 and Fab22-TT833S. These CD64-targeted peptides were up to 1,000- and 100-fold more efficient than the parent peptides for T cell stimulation and antagonism, respectively, suggesting that such fusion proteins could effectively increase the delivery of peptides to APCs in vivo. Moreover, the F(c)gammaRI-targeted antagonistic peptide inhibited proliferation of TT830-specific T cells even when APCs were first pulsed with native peptide, a situation comparable with that which would be encountered in vivo when attempting to ameliorate an autoimmune response. These data suggest that targeted presentation of antagonistic peptides could lead to promising Ag-specific therapies for T cell-mediated autoimmune diseases.