The major histocompatibility complex (MHC) class II–associated invariant chain (Ii) regulates intracellular trafficking and peptide loading of MHC class II molecules. Such loading occurs after endosomal degradation of the invariant chain to a ∼3-kD peptide termed CLIP (class II–associated invariant chain peptide). Cathepsins L and S have both been implicated in degradation of Ii to CLIP in thymus and peripheral lymphoid organs, respectively. However, macrophages from mice deficient in both cathepsins S and L can process Ii and load peptides onto MHC class II dimers normally. Both processes are blocked by a cysteine protease inhibitor, indicating the involvement of an additional Ii-processing enzyme(s). Comparison of cysteine proteases expressed by macrophages with those found in splenocytes and dendritic cells revealed two enzymes expressed exclusively in macrophages, cathepsins Z and F. Recombinant cathepsin Z did not generate CLIP from Ii–MHC class II complexes, whereas cathepsin F was as efficient as cathepsin S in CLIP generation. Inhibition of cathepsin F activity and MHC class II peptide loading by macrophages exhibited similar specificity and activity profiles. These experiments show that cathepsin F, in a subset of antigen presenting cells (APCs), can efficiently degrade Ii. Different APCs can thus use distinct proteases to mediate MHC class II maturation and peptide loading.
cysteine protease; antigen presentation; protease inhibitor; proteolysis; antigen presenting cell
MHC class II molecules display antigenic peptides on cell surfaces for recognition by CD4(+) T cells. Proteolysis is required in this process both for degradation of invariant chain (Ii) from class II-Ii complexes to allow subsequent binding of peptides, and for generation of the antigenic peptides. The cysteine endoprotease, cathepsin S, mediates Ii degradation in human and mouse antigen-presenting cells. Studies described here examine the functional significance of cathepsin S inhibition on antigen presentation and immunity. Specific inhibition of cathepsin S in A20 cells markedly impaired presentation of an ovalbumin epitope by interfering with class II-peptide binding, not by obstructing generation of the antigen. Administration of a cathepsin S inhibitor to mice in vivo selectively inhibited activity of cathepsin S in splenocytes, resulting in accumulation of a class II-associated Ii breakdown product, attenuation of class II-peptide complex formation, and inhibition of antigen presentation. Mice treated with inhibitor had an attenuated antibody response when immunized with ovalbumin but not the T cell-independent antigen TNP-Ficoll. In a mouse model of pulmonary hypersensitivity, treatment with the inhibitor also abrogated a rise in IgE titers and profoundly blocked eosinophilic infiltration in the lung. Thus, inhibition of cathepsin S in vivo alters Ii processing, antigen presentation, and immunity. These data identify selective inhibition of cysteine proteases as a potential therapeutic strategy for asthma and autoimmune disease processes.
Major histocompatibility complex class II molecules are synthesized as a nonameric complex consisting of three αβ dimers associated with a trimer of invariant (Ii) chains. After exiting the TGN, a targeting signal in the Ii chain cytoplasmic domain directs the complex to endosomes where Ii chain is proteolytically processed and removed, allowing class II molecules to bind antigenic peptides before reaching the cell surface. Ii chain dissociation and peptide binding are thought to occur in one or more postendosomal sites related either to endosomes (designated CIIV) or to lysosomes (designated MIIC). We now find that in addition to initially targeting αβ dimers to endosomes, Ii chain regulates the subsequent transport of class II molecules. Under normal conditions, murine A20 B cells transport all of their newly synthesized class II I-Ab αβ dimers to the plasma membrane with little if any reaching lysosomal compartments. Inhibition of Ii processing by the cysteine/serine protease inhibitor leupeptin, however, blocked transport to the cell surface and caused a dramatic but selective accumulation of I-Ab class II molecules in lysosomes. In leupeptin, I-Ab dimers formed stable complexes with a 10-kD NH2-terminal Ii chain fragment (Ii-p10), normally a transient intermediate in Ii chain processing. Upon removal of leupeptin, Ii-p10 was degraded and released, I-Ab dimers bound antigenic peptides, and the peptide-loaded dimers were transported slowly from lysosomes to the plasma membrane. Our results suggest that alterations in the rate or efficiency of Ii chain processing can alter the postendosomal sorting of class II molecules, resulting in the increased accumulation of αβ dimers in lysosome-like MIIC. Thus, simple differences in Ii chain processing may account for the highly variable amounts of class II found in lysosomal compartments of different cell types or at different developmental stages.
Antigen-presenting cells (APC) degrade endocytosed antigens into peptides that are bound and presented to T cells by major histocompatibility complex (MHC) class II molecules. Class II molecules are delivered to endocytic compartments by the class II accessory molecule invariant chain (Ii), which itself must be eliminated to allow peptide binding. The cellular location of Ii degradation, as well as the enzymology of this event, are important in determining the sets of antigenic peptides that will bind to class II molecules. Here, we show that the cysteine protease cathepsin S acts in a concerted fashion with other cysteine and noncysteine proteases to degrade mouse Ii in a stepwise fashion. Inactivation of cysteine proteases results in incomplete degradation of Ii, but the extent to which peptide loading is blocked by such treatment varies widely among MHC class II allelic products. These observations suggest that, first, class II molecules associated with larger Ii remnants can be converted efficiently to class II–peptide complexes and, second, that most class II–associated peptides can still be generated in cells treated with inhibitors of cysteine proteases. Surprisingly, maturation of MHC class II in mice deficient in cathepsin D is unaffected, showing that this major aspartyl protease is not involved in degradation of Ii or in generation of the bulk of antigenic peptides.
Hepatocytes are the main source of hepatitis C virus (HCV) replication and contain the maximum viral load in an infected person. Chronic HCV infection is characterized by weak cellular immune responses to viral proteins. Cathepsin S is a lysosomal cysteine protease and controls HLA-DR–antigen complex presentation through the degradation of the invariant chain. In this study, we examined the effect of HCV proteins on cathepsin S expression and found it to be markedly decreased in dendritic cells (DCs) exposed to HCV or in hepatocytes expressing HCV proteins. The downregulation of cathepsin S was mediated by HCV core and NS5A proteins involving inhibition of the transcription factors interferon regulatory factor 1 (IRF-1) and upstream stimulatory factor 1 (USF-1) in gamma interferon (IFN-γ)-treated hepatocytes. Inhibition of cathepsin S by HCV proteins increased cell surface expression of the invariant chain. In addition, hepatocytes stably transfected with HCV core or NS5A inhibited HLA-DR expression. Together, these results suggested that HCV has an inhibitory role on cathepsin S-mediated major histocompatibility complex (MHC) class II maturation, which may contribute to weak immunogenicity of viral antigens in chronically infected humans.
Within antigen presenting cells (APC), ubiquitination regulates the trafficking of immune modulators such as MHC class II and CD86 (B7.2) molecules. MARCH1 (membrane-associated RING-CH), a newly identified ubiquitin E3 ligase expressed in APC, ubiquitinates MHC class II thereby reducing its surface expression. Following LPS-induced maturation of dendritic cells (DC), MARCH1 mRNA is downregulated and MHC class II is redistributed to the cell surface from endosomal compartments. Here, we show that MARCH1 expression is also regulated at the post-transcriptional level. In primary DC and APC cell lines of murine origin, MARCH1 had a half-life of less than 30 min. MARCH1 degradation appears to occur partly in lysosomes, since inhibiting lysosomal activity stabilized MARCH1. Similar stabilization was observed when MARCH1-expressing cells were treated with cysteine protease inhibitors. Mutational analyses of MARCH1 defined discrete domains required for destabilization, proper localization, and functional interaction with substrates. Together, these data suggest that MARCH1 expression is regulated at a post-transcriptional level by trafficking within the endo-lysosomal pathway where MARCH1 is proteolysed. The short half-life of MARCH1 permits very rapid changes in the levels of the protein in response to changes in the mRNA, resulting in efficient induction of antigen presentation once APC receive maturational signals.
B lymphocytes contain a novel population of endocytic vesicles involved in the transport of newly synthesized major histocompatibility complex (MHC) class II alpha beta chains and alpha beta peptide complexes to the cell surface. We now present evidence that these class II-enriched vesicles (CIIV) are also likely to be a site for the loading of immunogenic peptides onto MHC molecules. We used the serine protease inhibitor leupeptin to accumulate naturally occurring intermediates in the degradation of alpha beta-invariant chain complexes and to slow the intracellular transport of class II molecules. As expected, leupeptin caused an accumulation of Ii chain and class II molecules (I-A(d)) in endosomes and lysosomes. More importantly, however, it enhanced the selective accumulation of a 10-kD invariant chain fragment associated with sodium dodecyl sulfate (SDS)-labile (empty) alpha beta dimers in CIIV. This was followed by the dissociation of the 10-kD fragment, formation of SDS-stable (peptide-loaded) alpha beta dimers, and their subsequent appearance at the cell surface. Thus, CIIV are likely to serve as a specialized site, distinct from endosomes and lysosomes, that hosts the final steps in the dissociation of invariant chain from class II molecules and the loading of antigen-derived peptides onto newly synthesized alpha beta dimers.
Before a class II molecule can be loaded with antigenic material and reach the surface to engage CD4+ T cells, its chaperone, the class II-associated invariant chain (Ii), is degraded in a stepwise fashion by proteases in endocytic compartments. We have dissected the role of cathepsin S (CatS) in the trafficking and maturation of class II molecules by combining the use of dendritic cells (DC) from CatS−/− mice with a new active site–directed probe for direct visualization of active CatS. Our data demonstrate that CatS is active along the entire endocytic route, and that cleavage of the lysosomal sorting signal of Ii by CatS can occur there in mature DC. Genetic disruption of CatS dramatically reduces the flow of class II molecules to the cell surface. In CatS−/− DC, the bulk of major histocompatibility complex (MHC) class II molecules is retained in late endocytic compartments, although paradoxically, surface expression of class II is largely unaffected. The greatly diminished but continuous flow of class II molecules to the cell surface, in conjunction with their long half-life, can account for the latter observation. We conclude that in DC, CatS is a major determinant in the regulation of intracellular trafficking of MHC class II molecules.
major histocompatibility complex class II; cathepsins; dendritic cells; antigen presentation; biological transport
MHC class II-restricted Ag processing requires protein degradation in the endocytic pathway for the activation of CD4+ T cells. Gamma-interferon-inducible lysosomal thiol reductase (GILT) facilitates Ag processing by reducing protein disulfide bonds in this compartment. Lysosomal cysteine protease cathepsin S (CatS) contains disulfide bonds and mediates essential steps in MHC class II-restricted processing, including proteolysis of large polypeptides and cleavage of the invariant chain. We sought to determine whether GILT’s reductase activity regulates CatS expression and function. Confocal microscopy confirmed that GILT and CatS colocalized within lysosomes of B cells. GILT expression posttranscriptionally decreased the steady-state protein expression of CatS in primary B cells and B-cell lines. GILT did not substantially alter the expression of other lysosomal proteins, including H2-M, H2-O, or CatL. GILT’s reductase active site was necessary for diminished CatS protein levels, and GILT expression decreased the half-life of CatS, suggesting that GILT-mediated reduction of protein disulfide bonds enhances CatS degradation. GILT expression decreased the proteolysis of a CatS selective substrate. This study illustrates a physiologic mechanism that regulates CatS and has implications for fine tuning MHC class II-restricted Ag processing and for the development of CatS inhibitors, which are under investigation for the treatment of autoimmune disease.
Antigen presentation/processing; Antigen presenting cells; B cells; Cathepsin S; Gamma-interferon-inducible lysosomal thiol reductase (GILT)
During infection, parasites evade the host immune system by modulating or exploiting the immune system; e.g., they suppress expression of major histocompatibility complex class II molecules or secrete cytokine-like molecules. However, it is not clear whether helminths disturb the immune responses of their hosts by controlling the antigen-processing pathways of the hosts. In this study, we identified a new cysteine protease inhibitor, nippocystatin, derived from excretory-secretory (ES) products of an intestinal nematode, Nippostrongylus brasiliensis. Nippocystatin, which belongs to cystatin family 2, consists of 144 amino acids and is secreted as a 14-kDa mature form. In vivo treatment of ovalbumin (OVA)-immunized mice with recombinant nippocystatin (rNbCys) profoundly suppressed OVA-specific proliferation of splenocytes but not non-antigen-specific proliferation of splenocytes. OVA-specific cytokine production was also greatly suppressed in rNbCys-treated mice. Although the serum levels of both OVA-specific immunoglobulin G1 (IgG1) and IgG2a were not affected by rNbCys treatment, OVA-specific IgE was preferentially downregulated in rNbCys-treated mice. In vitro rNbCys inhibited processing of OVA by lysosomal cysteine proteases from the spleens of mice. Mice with anti-nippocystatin antibodies became partially resistant to infection with N. brasiliensis. Based on these findings, N. brasiliensis appears to skillfully evade host immune systems by secreting nippocystatin, which modulates antigen processing in antigen-presenting cells of hosts.
Stepwise degradation of the invariant chain (Ii) is required for the binding of antigenic peptides to MHC class II molecules. Cathepsin (Cat) L in the murine thymus and Cat S in peripheral APCs have both been implicated in the last step of Ii degradation that gives rise to the class II–associated invariant chain peptides (CLIP). Cat V has been recently described as highly homologous to Cat L and exclusively expressed in human thymus and testis, but with no mouse orthologue. We report that Cat V is the dominant cysteine protease in cortical human thymic epithelial cells, while Cat L and Cat S seem to be restricted to dendritic and macrophage-like cells. Active Cat V in thymic lysosomal preparations was demonstrated by active-site labeling. Recombinant Cat V was capable of converting Ii into CLIP efficiently, suggesting that Cat V is the protease that controls the generation of αβ-CLIP complexes in the human thymus, in analogy to Cat L in mouse. Comparison of Cat V expression between thymi from patients with myasthenia gravis and healthy controls revealed a significantly higher expression level in the pathological samples, suggesting a potential involvement of this protease in the immunopathogenesis of myasthenia gravis, an autoimmune disease almost invariably associated with thymic pathology.
Human herpesvirus 8 (HHV8) downregulates major histocompatibility complex (MHC) class I complexes from the plasma membrane via two of its genes, K3 and K5. The N termini of K3 and K5 contain a plant homeodomain (PHD) predicted to be structurally similar to RING domains found in E3 ubiquitin ligases. In view of the importance of the ubiquitin-proteasome system in sorting within the endocytic pathway, we analyzed its role in downregulation of MHC class I complexes in cells expressing K3. Proteasome inhibitors as well as cysteine and aspartyl protease inhibitors stabilize MHC class I complexes in cells expressing K3. However, proteasome inhibitors differentially affect sorting of MHC class I complexes within the endocytic pathway and prevent their delivery to a dense endosomal compartment. In this compartment, the cytoplasmic tail of MHC class I complexes is cleaved by cysteine proteases. The complex is then cleaved within the plane of the membrane by an aspartyl protease, resulting in a soluble MHC class I fragment composed of the lumenal domain of the heavy chain, β2-microglobulin (β2m), and peptide. We conclude that K3 not only directs internalization, but also targets MHC class I complexes to a dense endocytic compartment on the way to lysosomes in a ubiquitin-proteasome-dependent manner.
During their final differentiation or maturation, dendritic cells (DCs) redistribute their major histocompatibility complex (MHC) class II products from intracellular compartments to the plasma membrane. Using cells arrested in the immature state, we now find that DCs also regulate the initial intracellular formation of immunogenic MHC class II–peptide complexes. Immature DCs internalize the protein antigen, hen egg lysozyme (HEL), into late endosomes and lysosomes rich in MHC class II molecules. There, despite extensive colocalization of HEL protein and MHC class II products, MHC class II–peptide complexes do not form unless the DCs are exposed to inflammatory mediators such as tumor necrosis factor α, CD40 ligand, or lipoplolysaccharide. The control of T cell receptor (TCR) ligand formation was observed using the C4H3 monoclonal antibody to detect MHC class II–HEL peptide complexes by flow cytometry and confocal microscopy, and with HEL-specific 3A9 transgenic T cells to detect downregulation of the TCR upon MHC–peptide encounter. Even the binding of preprocessed HEL peptide to MHC class II is blocked in immature DCs, including the formation of C4H3 epitope in MHC class II compartments, suggesting an arrest to antigen presentation at the peptide-loading step, rather than an enhanced degradation of MHC class II–peptide complexes at the cell surface, as described in previous work. Therefore, the capacity of late endosomes and lysosomes to produce MHC class II–peptide complexes can be strictly controlled during DC differentiation, helping to coordinate antigen acquisition and inflammatory stimuli with formation of TCR ligands. The increased ability of maturing DCs to load MHC class II molecules with antigenic cargo contributes to the >100-fold enhancement of the subsequent primary immune response observed when immature and mature DCs are compared as immune adjuvants in culture and in mice.
dendritic cell; maturation; MHC class II–peptide complex; lysosome; inflammation
In human B cells, class II molecules of the major histocompatibility complex (MHC-II) accumulate in an endosomal/lysosomal compartment, the MIIC, in which they may encounter and bind peptides. An additional molecule required for MHC-II peptide binding, HLA-DM (DM), has also been localized to the MIIC. Neither the relationship of the MIIC to the endosomal system nor the mechanisms by which DM localizes to the MIIC are understood. To address these issues, DM localization was analyzed in cells that do or do not express MHC-II. DM alpha beta heterodimers were localized in transfected MHC-II-negative HeLa and NRK cells, in the absence of the MHC-II-associated invariant chain, to a prelysosomal/lysosomal compartment by immunofluorescence microscopy. To identify a potential targeting determinant, we analyzed the localization of a chimeric protein, T-T-Mb, in which the cytoplasmic tail of murine DM beta (Mb) was appended to the lumenal and transmembrane domains of a cell surface protein, Tac. Like intact DM, T- T-Mb was localized to a lysosomal compartment in HeLa and NRK cells, as judged by immunofluorescence and immunoelectron microscopy. T-T-Mb was rapidly degraded in this compartment by a process that was blocked by inhibitors of lysosomal proteolysis. The DM beta cytoplasmic tail also mediated internalization of anti-Tac antibody from the cell surface and delivery to lysosomes. Deletion from the DM beta cytoplasmic tail of the tyrosine-based motif, YTPL, resulted in cell surface expression of T-T-Mb and a loss of both degradation and internalization; alanine scanning mutagenesis showed that the Y and L residues were critical for these functions. Similarly, mutation of the same Y residue within full- length DM beta resulted in cell surface expression of DM alpha beta heterodimers. Lastly, T-T-Mb was localized by immunoelectron microscopy to the MIIC in a human B lymphoblastoid cell line. Our results suggest that a motif, YTPL, in the cytoplasmic tail of the beta chain of DM is sufficient for targeting either to lysosomes or to the MIIC.
Fine control of lysosomal degradation for limited processing of internalized antigens is a hallmark of professional antigen presenting cells. Previous work in mice has shown that dendritic cells (DCs) contain lysosomes with remarkably low protease content. Combined with the ability to modulate lysosomal pH during phagocytosis and maturation, murine DCs enhance their production of class II MHC-peptide complexes for presentation to T cells.
In this study we extend these findings to human DCs and distinguish between different subsets of DCs based on their ability to preserve internalized antigen. Whereas DCs derived in vitro from CD34+ hematopoietic progenitor cells or isolated from peripheral blood of healthy donors are protease poor, DCs derived in vitro from monocytes (MDDCs) are more similar to macrophages (MΦs) in protease content. Unlike other DCs, MDDCs also fail to reduce their intralysosomal pH in response to maturation stimuli. Indeed, functional characterization of lysosomal proteolysis indicates that MDDCs are comparable to MΦs in the rapid degradation of antigen while other human DC subtypes are attenuated in this capacity.
Human DCs are comparable to murine DCs in exhibiting a markedly reduced level of lysosomal proteolysis. However, as an important exception to this, human MDDCs stand apart from all other DCs by a heightened capacity for proteolysis that resembles that of MΦs. Thus, caution should be exercised when using human MDDCs as a model for DC function and cell biology.
The US2-11 region of human and rhesus cytomegalovirus encodes a conserved family of glycoproteins that inhibit MHC-I assembly with viral peptides, thus preventing cytotoxic T cell recognition. Since HCMV lacking US2-11 is no longer able to block assembly and transport of MHC-I, we examined whether this is also observed for RhCMV lacking the corresponding region. Unexpectedly, recombinant RhCMV lacking US2-11 was still able to inhibit MHC-I expression in infected fibroblasts, suggesting the presence of an additional MHC-I evasion mechanism. Progressive deletion analysis of RhCMV-specific genomic regions revealed that MHC-I expression is fully restored upon additional deletion of rh178. The protein encoded by this RhCMV-specific open reading frame is anchored in the endoplasmic reticulum membrane. In the presence of rh178, RhCMV prevented MHC-I heavy chain (HC) expression, but did not inhibit mRNA transcription or association of HC mRNA with translating ribosomes. Proteasome inhibitors stabilized a HC degradation intermediate in the absence of rh178, but not in its presence, suggesting that rh178 prevents completion of HC translation. This interference was signal sequence-dependent since replacing the signal peptide with that of CD4 or murine HC rendered human HCs resistant to rh178. We have identified an inhibitor of antigen presentation encoded by rhesus cytomegalovirus unique in both its lack of homology to any other known protein and in its mechanism of action. By preventing signal sequence-dependent HC translocation, rh178 acts prior to US2, US3 and US11 which attack MHC-I proteins after protein synthesis is completed. Rh178 is the first viral protein known to interfere at this step of the MHC-I pathway, thus taking advantage of the conserved nature of HC leader peptides, and represents a new mechanism of translational interference.
To avoid immune detection by cytotoxic T lymphocytes, viruses interfere with antigen presentation by major histocompatibility complex class I (MHC-I) molecules. We have discovered a unique cytomegaloviral protein that interferes with the biosynthesis of MHC-I heavy chains and was thus termed viral inhibitor of heavy chain expression (VIHCE). We show that VIHCE does not affect transcription of MHC-I mRNA or the formation of poly-ribosomes. Surprisingly, however, very little MHC-I protein is detected, even when proteasomal protein degradation is inhibited, suggesting incomplete protein translation. Interestingly, VIHCE requires the proper MHC-I signal peptide, suggesting that CMV takes advantage of the high conservation of MHC-I signal peptides and interferes with protein translation by inhibiting signal sequence-dependent protein translocation. This is the first description of a viral protein that specifically targets the translation of a cellular immuno-stimulatory protein.
For a long time the lysosomal pathway was thought to be exclusively one for catabolism and recycling of material taken up by endocytosis from the external milieu or from the cytosol by autophagy. At least in the immune system it is clear now that endo/lysosomal proteolysis generates crucially important information, in particular peptides that bind class II MHC molecules to create ligands for survey by the diverse antigen receptors of the T lymphocyte system. This process of antigen processing and presentation is used to display not only foreign but also self peptides and therefore is important for ‘self’ tolerance as well as immunity to pathogens. Some cells, macrophages and particularly dendritic cells can load peptides on class I MHC molecules in the endosome system through the important, though still not fully characterised, pathway of cross-presentation. Here I try to provide a brief review of how this area developed focussing to some extent our own contributions to understanding the class II MHC pathway. I also mention briefly recent work of others showing that proteolysis along this pathway turns out to regulate immune signalling events in the innate immune system such as the activation of some members of the Toll-like receptor family. Finally, our recent work on the endo/lysosome targeted protease inhibitor cystatin F, suggests that auto-regulation of protease activity in some immune cells occurs. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.
► The original view of lysosomes as exclusively destructive organelles has changed. ► Controlled proteolysis in endo-lysosomes is needed for immunity. ► Antigen presentation and sensing of some microbial products requires protease action. ► Cystatin F may attenuate potentially toxic protease levels in some immune cells.
Vaccinia virus (VV) has been used as a vaccine to eradicate smallpox and as a vaccine for HIV and tumors. However, the immunoevasive properties of VV, have raised safety concerns. VV infection of APC perturbs MHC class II-mediated Ag presentation. Exposure of human B cell lines to VV induced a dramatic reduction in cellular expression of the class II chaperone, invariant chain (Ii) during the late stages (i.e. 8–10 h) of infection. Yet, cell viability and surface expression of MHC class II molecules were maintained up to 24 h after exposure to virus. Reductions in Ii and class II mRNA levels were detected as early as 6 h after VV infection of APC. To examine whether VV was acting solely to disrupt host protein synthesis, B cells were treated with an inhibitor of translation, cycloheximide (CHX). Within 1 h of B cell CHX treatment, Ii protein expression decreased coupled with a loss of class II presentation. Analysis of Ii degradation in VV or CHX treated cells, revealed on-going Ii proteolysis contributing to reduced steady state Ii levels in these APC. Yet in contrast with CHX, VV infection of APC altered lysosomal protease expression and Ii degradation. Virus infection induced cellular cathepsin L expression while reducing the levels of other lysosomal proteases. These results demonstrate that at late stages of VV infection, reductions in cellular Ii levels coupled with changes in lysosomal protease activity, contribute in part to defects in class II presentation.
antigen presentation; viral; B cell
Cytomegalovirus (CMV) efficiently evades many host immune defenses and encodes a number of proteins that prevent antigen presentation by major histocompatibility complex class I (MHC-I) molecules in order to evade recognition and killing of infected cells by cytotoxic CD8+ T cells. We recently showed that rhesus CMV-specific Rh178 intercepts MHC-I protein translation before interference of MHC-I maturation by homologues of the human CMV US6 family. Here, we demonstrate that Rh178 localizes to the membrane of the endoplasmic reticulum, displaying a short luminal and large cytosolic domain, and that the membrane-proximal cytosolic portion is essential for inhibition of MHC-I expression. We further observed that Rh178 does not require synthesis of full-length MHC-I heavy chains but is capable of inhibiting the translation of short, unstable amino-terminal fragments of MHC-I. Moreover, the transfer of amino-terminal fragments containing the MHC-I signal peptide renders recipient proteins susceptible to targeting by Rh178. The cytosolic orientation of Rh178 and its ability to target protein fragments carrying the MHC-I signal peptide are consistent with Rh178 intercepting partially translated MHC-I heavy chains after signal recognition particle-dependent transfer to the endoplasmic reticulum membrane. However, interference with MHC-I translation by Rh178 seems to occur prior to SEC61-dependent protein translocation, since inhibition of MHC-I translocation by eeyarestatin 1 resulted in a full-length degradation intermediate that can be stabilized by proteasome inhibitors. These data are consistent with Rh178 blocking protein translation of MHC-I heavy chains at a step prior to the start of translocation, thereby downregulating MHC-I at a very early stage of translation.
The major histocompatibility complex (MHC) class II-associated invariant chain (Ii) is thought to act as a chaperone that assists class II during folding, assembly, and transport. To define more precisely the role of Ii chain in regulating class II function, we have investigated in detail the biosynthesis, transport, and intracellular distribution of class II molecules in splenocytes from mice bearing a deletion of the Ii gene. As observed previously, the absence of Ii chain caused significant reduction in both class II-restricted antigen presentation and expression of class II molecules at the cell surface because of the intracellular accumulation of alpha and beta chains. Whereas much of the newly synthesized MHC molecules enter a high molecular weight aggregate characteristic of misfolded proteins, most of the alpha and beta chains form dimers and acquire epitopes characteristic of properly folded complexes. Although the complexes do not bind endogenously processed peptides, class II molecules that reach the surface are competent to bind peptides added to the medium, further demonstrating that at least some of the complexes fold properly. Similar to misfolded proteins, however, the alpha and beta chains are poorly terminally glycosylated, suggesting that they fail to reach the Golgi complex. As demonstrated by double label confocal and electron microscope immunocytochemistry, class II molecules were found in a subcompartment of the endoplasmic reticulum and in a population of small nonlysosomal vesicles possibly corresponding to the intermediate compartment or cis-Golgi network. Thus, although alpha and beta chains can fold and form dimers on their own, the absence of Ii chain causes them to be recognized as "misfolded" and retained in the same compartments as bona fide misfolded proteins.
Mycobacterium tuberculosis (Mtb) resides in a long-lived phagosomal compartment that resists maturation. The manner by which Mtb antigens are processed and presented on MHC Class I molecules is poorly understood. Using human dendritic cells and IFN-γ release by CD8+ T cell clones, we examined the processing and presentation pathway for two Mtb–derived antigens, each presented by a distinct HLA-I allele (HLA-Ia versus HLA-Ib). Presentation of both antigens is blocked by the retrotranslocation inhibitor exotoxin A. Inhibitor studies demonstrate that, after reaching the cytosol, both antigens require proteasomal degradation and TAP transport, but differ in the requirement for ER–golgi egress and new protein synthesis. Specifically, presentation by HLA-B8 but not HLA-E requires newly synthesized HLA-I and transport through the ER–golgi. Phenotypic analysis of the Mtb phagosome by flow organellometry revealed the presence of Class I and loading accessory molecules, including TAP and PDI. Furthermore, loaded HLA-I:peptide complexes are present within the Mtb phagosome, with a pronounced bias towards HLA-E:peptide complexes. In addition, protein analysis also reveals that HLA-E is enriched within the Mtb phagosome compared to HLA-A2. Together, these data suggest that the phagosome, through acquisition of ER–localized machinery and as a site of HLA-I loading, plays a vital role in the presentation of Mtb–derived antigens, similar to that described for presentation of latex bead-associated antigens. This is, to our knowledge, the first description of this presentation pathway for an intracellular pathogen. Moreover, these data suggest that HLA-E may play a unique role in the presentation of phagosomal antigens.
Major Histocompatibility Complex Class I (MHC-I) generally serves to present peptides derived from cytosolic proteins to CD8+ T lymphocytes, thereby alerting the immune system that the cell is infected. The machinery required for MHC-I antigen processing and presentation is localized to the cytosol and endoplasmic reticulum (ER). After phagocytosis of bacteria, it is unclear how bacterial antigens are presented by MHC-I, as they are segregated from this machinery. In this report, we examine processing and presentation of two proteins derived from the intracellular pathogen, Mycobacterium tuberculosis (Mtb). We find that Mtb proteins are able to access the cytosol where they are degraded by the proteasome. Mtb proteins reach the cytosol by retrotranslocation, a process which normally functions to transport misfolded proteins from the ER to the cytosol. Furthermore, the Mtb phagosome contains ER–derived members of the MHC-I peptide loading complex, which aid in loading peptides onto MHC-I molecules. Finally, we detect loaded HLA-I:peptide complexes in the phagosome, demonstrating that loading can occur in the Mtb phagosome. Together, these findings suggest that the Mtb phagosome, through acquisition of ER–derived MHC-I machinery and as a site of MHC-I loading, plays a vital role in presentation of Mtb–derived peptides on MHC-I.
Endo/lysosomal proteases control two key events in antigen (Ag) presentation: the degradation of protein Ag and the generation of peptide-receptive major histocompatibility complex (MHC) class II molecules. Here we show that the proinflammatory cytokines tumor necrosis factor α and interleukin (IL)-1β rapidly increase the activity of cathepsin (cat) S and catB in human dendritic cells (DCs). As a consequence, a wave of MHC class II sodium dodecyl sulfate stable dimer formation ensues in a catS-dependent fashion. In contrast, the antiinflammatory cytokine IL-10 renders DCs incapable of upregulating catS and catB activity and in fact, attenuates the level of both enzymes. Suppressed catS and catB activity delays MHC class II sodium dodecyl sulfate stable dimer formation and impairs Ag degradation. In DCs exposed to tetanus toxoid, IL-10 accordingly reduces the number of MHC class II–peptide complexes accessible to tetanus toxoid–specific T cell receptors, as analyzed by measuring T cell receptor downregulation in Ag-specific T cell clones. Thus, the control of protease activity by pro- and antiinflammatory cytokines is an essential feature of the Ag presentation properties of DCs.
antigen-presenting cell; cathepsin; class II maturation; antigen degradation; TCR
While it has long been known that human CD4+ T cells can express functional class II MHC molecules, the role of lysosomal proteases in T cell class II MHC processing and presentation pathway is unknown. Using CD4+ T cell clones that constitutively express class II MHC, we determined that cathepsin S is necessary for invariant chain proteolysis in T cells. CD4+HLA-DR+ T cells downregulated cathepsin S expression and activity 18 hours after activation, thereby ceasing nascent class II MHC product formation. This blockade resulted in the loss of the invariant chain fragment CLIP from the cell surface, suggesting that—like professional APC—CD4+ HLA-DR+ cells modulate self-antigen presentation as a consequence of activation. Furthermore, cathepsin S expression and activity, and concordantly cell surface CLIP expression, was reduced in HLA-DR+ CD4+ T cells as compared to B cells both in vitro and ex vivo.
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The invariant chain (Ii) is associated with major histocompatibility complex class II molecules during early stages of their intracellular transport. In an acidic endosomal/lysosomal compartment, it is proteolytically cleaved and removed from class II heterodimers. Participation of aspartic and cysteine proteases has been observed in in vitro degradation of Ii, but the specific enzymes responsible for its in vivo processing are as yet undefined. We have previously isolated a noncovalent complex of the lysosomal cysteine protease cathepsin L with a peptide fragment derived from the p41 form of Ii from human kidney. Here we show that this Ii fragment, which is identical to the alternatively spliced segment of p41, is a very potent competitive inhibitor of cathepsin L (equilibrium inhibition constant Ki = 1.7 X 10(-12) M). It inhibits two other cysteine proteases, cathepsin H and papain, but to much lesser extent. Cysteine proteases cathepsins B, C, and S, as well as representatives of serine, aspartic, and metalloproteases, are not inhibited at all. These findings suggest a novel role for p41 in the regulation of various proteolytic activities during antigen processing and presentation. The Ii inhibitory fragment shows no sequence homology with the known cysteine protease inhibitors, and may, therefore, represent a new class.
Biochemical and functional studies have demonstrated major histocompatibility complex (MHC) class II–restricted presentation of select epitopes derived from cytoplasmic antigens, with few insights into the processing reactions necessary for this alternate pathway. Efficient presentation of an immunodominant epitope derived from glutamate decarboxylase (GAD) was observed regardless of whether this antigen was delivered exogenously or via a cytoplasmic route into human histocompatibility leukocyte antigen class II–DR4+ antigen-presenting cells. Presentation of exogenous as well as cytoplasmic GAD required the intersection of GAD peptides and newly synthesized class II proteins. By contrast, proteolytic processing of this antigen was highly dependent upon the route of antigen delivery. Exogenous GAD followed the classical pathway for antigen processing, with an absolute requirement for endosomal/lysosomal acidification as well as cysteine and aspartyl proteases resident within these organelles. Presentation of endogenous GAD was dependent upon the action of cytoplasmic proteases, including the proteasome and calpain. Thus, translocation of processed antigen from the cytoplasm into membrane organelles is necessary for class II–restricted presentation via this alternate pathway. Further trimming of these peptides after translocation was mediated by acidic proteases within endosomes/lysosomes, possibly after or before class II antigen binding. These studies suggest that processing of exogenous and cytoplasmic proteins occurs through divergent but overlapping pathways. Furthermore, two cytoplasmic proteases, the proteasome and calpain, appear to play important roles in MHC class II–restricted antigen presentation.
proteasome; calpain; cathepsin; cytoplasm; protease