Impaired invariant chain degradation and antigen presentation and diminished collagen-induced arthritis in cathepsin S null mice

Cytokines regulate proteolysis in major histocompatibility complex class II-dependent antigen presentation by dendritic cells

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 alpha and interleukin (IL)-1beta 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 processing in the endocytic compartment

Proteolysis generates the peptides that bind to class II MHC molecules and, by destruction of the invariant chain, prepares the class II MHC molecule for capture of those peptides. A clearer picture is emerging of the proteases, protease inhibitors and other factors that together control the environment for class II MHC peptide loading. However, the details of invariant-chain processing and antigen processing may differ depending on the allele of class II and the antigen substrate under consideration.

Specific localization of lysosomal aminopeptidases in type II alveolar epithelial cells of the rat lung

We previously demonstrated that lysosomal cysteine proteinases, cathepsins B, H, and L were localized in lysosomes of alveolar macrophages and bronchial epithelial cells in the rat lung, while cathepsin H, a typical aminopeptidase, was additionally distributed in lamellar bodies containing surfactant in type II alveolar epithelial cells (ISHII et al., 1991). The present immunohistochemical study further examined the localization of lysosomal aminopeptidases, cathepsin C, and tripeptidyl peptidase I (TPP-I) in the rat lung. Western blotting confirmed the presence of cathepsin C and TPP-I as active forms in the pulmonary tissue, showing 25 kD and 47 kD, respectively. Immunohisto/cytochemical observations demonstrated that positive staining for cathepsin C and TPP-I was more intensely localized in alveolar epithelial regions than in bronchial or bronchiolar epithelial cells. By double immunostaining using confocal laser microscopy, immunoreactivity for cathepsin H was found to be co-localized with that for cathepsin C or TPP-I in both type II cells and macrophages. Moreover, when doubly stained with anti-cathepsin C and ED2, single-positive type II cells could be clearly distinguished from double-positive macrophages in the alveolar region. Immunoelectron microscopy revealed the gold labeling of cathepsin C or TPP-I in multivesicular and composite bodies, and lamellar bodies of Type II cells. These results showing that lysosomal aminopeptidases such as cathepsin H, cathepsin C and TPP-I are localized in lamellar bodies of type II alveolar epithelial cells strongly argue for the participation of lysosomal aminopeptidases in the formation process of surfactant containing specific proteins.

Osteoprotegerin, a crucial regulator of bone metabolism, also regulates B cell development and function

Osteoprotegerin (OPG) is a CD40-regulated gene in B cells and dendritic cells (DCs). We investigated the role of OPG in the immune system by generating opg(-/-) mice. Like its role as a regulator of bone metabolism, OPG also influences processes in the immune system, notably in B cell development. Ex vivo, opg(-/-) pro-B cells have enhanced proliferation to IL-7, and in opg(-/-) spleen, there is an accumulation of type 1 transitional B cells. Furthermore, opg(-/-) bone marrow-derived DCs are more effective in stimulating allogeneic T cells than control DCs. When challenged with a T-dependent Ag, opg(-/-) mice had a compromised ability to sustain an IgG3 Ag-specific response. Thus, in the immune system, OPG regulates B cell maturation and development of efficient Ab responses.

The cell biology of antigen presentation in dendritic cells

Dendritic cells are the most efficient antigen-presenting cells. They take up antigens and pathogens, generate MHC-peptide complexes, migrate from the sites of antigen acquisition to secondary lymphoid organs and, finally, they physically interact with and stimulate T lymphocytes. Indeed, dendritic cells are the only antigen-presenting cells that induce the activation of resting T cells, both in vitro and in vivo. Thus, dendritic cells initiate adaptive immune responses and determine tolerance. To do so, dendritic cells have developed unique membrane transport pathways. The molecular mechanisms responsible for the control of antigen uptake and processing, for the generation of MHC-peptide complexes and for their transport to the cell surface have been partially unraveled in the past two years.

Cathepsin L deficiency as molecular defect of furless: hyperproliferation of keratinocytes and pertubation of hair follicle cycling

Lysosomal cysteine proteinases of the papain family are involved in lysosomal bulk proteolysis, major histocompatibility complex class II mediated antigen presentation, prohormone processing, and extracellular matrix remodeling. Cathepsin L (CTSL) is a ubiquitously expressed major representative of the papain-like family of cysteine proteinases. To investigate CTSL in vivo functions, the gene was inactivated by gene targeting in embryonic stem cells. CTSL-deficient mice develop periodic hair loss and epidermal hyperplasia, acanthosis, and hyperkeratosis. The hair loss is due to alterations of hair follicle morphogenesis and cycling, dilatation of hair follicle canals, and disturbed club hair formation. Hyperproliferation of hair follicle epithelial cells and basal epidermal keratinocytes-both of ectodermal origin-are the primary characteristics underlying the mutant phenotype. Pathological inflammatory responses have been excluded as a putative cause of the skin and hair disorder. The phenotype of CTSL-deficient mice is reminiscent of the spontaneous mouse mutant furless (fs). Analyses of the ctsl gene of fs mice revealed a G149R mutation inactivating the proteinase activity. CTSL is the first lysosomal proteinase shown to be essential for epidermal homeostasis and regular hair follicle morphogenesis and cycling.

Pathologic gene expression in Gaucher disease: up-regulation of cysteine proteinases including osteoclastic cathepsin K

Deficiency of lysosomal acid β-glucosidase induces glycolipid storage in the macrophages of Gaucher disease but the pathways of multisystem tissue injury and destruction are unknown. To investigate the cognate molecular pathology of this inflammatory disorder, genes that were differentially expressed in spleen samples from a patient with Gaucher disease (Gaucher spleen) were isolated. Of 64 complementary DNA (cDNA) fragments sequenced from an enriched Gaucher cDNA library, 5 encode lysosomal proteins (cathepsins B, K, and S, α-fucosidase, and acid lipase), 10 encode other known proteins, and 2 represent novel sequences from human macrophage cell lines. Transcript abundance of the cathepsins, novel genes, pulmonary and activation-regulated chemokine (PARC), and NMB, a putative tumor suppressor gene, was greatly increased. Immunoblotting showed increased mature forms of all 3 cathepsins found in samples of Gaucher spleens. Immunofluorescence microscopy showed strong cathepsin B and K reactions in sinusoidal endothelium and Gaucher cells. The respective means, plus or minus SD, of cathepsin B, K, and S activities were 183 ± 35, 97 ± 39, and 91 ± 45 nmol/min/mg protein in 4 Gaucher spleens, and 26 ± 4, 10.5 ± 2, and 4.0 ± 2.1 nmol/min/mg protein in 3 control spleens. Plasma cathepsin B, K, and S activities were also elevated in Gaucher disease plasma (P < .001), but compared with control plasma samples, neither cathepsin B nor K activities were significantly elevated in 8 patients with nonglycosphingolipid lysosomal storage diseases or in 9 patients with other glycosphingolipidoses, which suggests disease specificity. All 3 cathepsin activities were increased 2-fold to 3-fold in Gaucher sera compared with control sera. In all 6 patients treated by enzyme replacement for 16-22 months, serum cathepsin activities decreased significantly (P < .01). Longitudinal studies confirmed the progressive reduction of proteinase activities during imiglucerase therapy but in 3 Gaucher patients with mild disease not so treated, serum cathepsin activities remained constant or increased during follow-up. Enhanced expression of cysteine proteinases may promote tissue destruction. Moreover, the first identification of aberrant cathepsin K expression in hematopoietic tissue other than osteoclasts implicates this protease in the breakdown of the matrix that characterizes lytic bone lesions in Gaucher disease.

Pathologic gene expression in Gaucher disease: up-regulation of cysteine proteinases including osteoclastic cathepsin K

Deficiency of lysosomal acid β-glucosidase induces glycolipid storage in the macrophages of Gaucher disease but the pathways of multisystem tissue injury and destruction are unknown. To investigate the cognate molecular pathology of this inflammatory disorder, genes that were differentially expressed in spleen samples from a patient with Gaucher disease (Gaucher spleen) were isolated. Of 64 complementary DNA (cDNA) fragments sequenced from an enriched Gaucher cDNA library, 5 encode lysosomal proteins (cathepsins B, K, and S, α-fucosidase, and acid lipase), 10 encode other known proteins, and 2 represent novel sequences from human macrophage cell lines. Transcript abundance of the cathepsins, novel genes, pulmonary and activation-regulated chemokine (PARC), and NMB, a putative tumor suppressor gene, was greatly increased. Immunoblotting showed increased mature forms of all 3 cathepsins found in samples of Gaucher spleens. Immunofluorescence microscopy showed strong cathepsin B and K reactions in sinusoidal endothelium and Gaucher cells. The respective means, plus or minus SD, of cathepsin B, K, and S activities were 183 ± 35, 97 ± 39, and 91 ± 45 nmol/min/mg protein in 4 Gaucher spleens, and 26 ± 4, 10.5 ± 2, and 4.0 ± 2.1 nmol/min/mg protein in 3 control spleens. Plasma cathepsin B, K, and S activities were also elevated in Gaucher disease plasma (P &lt; .001), but compared with control plasma samples, neither cathepsin B nor K activities were significantly elevated in 8 patients with nonglycosphingolipid lysosomal storage diseases or in 9 patients with other glycosphingolipidoses, which suggests disease specificity. All 3 cathepsin activities were increased 2-fold to 3-fold in Gaucher sera compared with control sera. In all 6 patients treated by enzyme replacement for 16-22 months, serum cathepsin activities decreased significantly (P &lt; .01). Longitudinal studies confirmed the progressive reduction of proteinase activities during imiglucerase therapy but in 3 Gaucher patients with mild disease not so treated, serum cathepsin activities remained constant or increased during follow-up. Enhanced expression of cysteine proteinases may promote tissue destruction. Moreover, the first identification of aberrant cathepsin K expression in hematopoietic tissue other than osteoclasts implicates this protease in the breakdown of the matrix that characterizes lytic bone lesions in Gaucher disease.

Evaluation of dipeptide alpha-keto-beta-aldehydes as new inhibitors of cathepsin S

A series of dipeptidyl alpha-keto-beta-aldehydes (glyoxals), prepared by solid-/solution-phase chemistries, were assessed for their inhibitory activity against cathepsin S, a lysosomal cysteine protease implicated in a number of important pathophysiological processes. The inhibitor Cbz-Phe-Leu-COCHO, which exhibits slow-binding kinetic characteristics, was found to be almost 400-fold more selective for cathepsin S (K(i) = 0.185 nM) than for cathepsin B (76 nM) and is, to our knowledge, the most potent, reversible, synthetic cathepsin S inhibitor reported to date.

Characterization of mouse cathepsin R, a new member of a family of placentally expressed cysteine proteases

A new mouse cysteine protease, termed cathepsin R, has been identified. The complete nucleotide sequence of this gene was derived from a set of cDNAs generated from 15.5-day mouse placenta. Sequence analysis revealed an open reading frame encoding a 334 amino acid long polypeptide closely related to placentally expressed cathepsins P, Q, and M. RT-PCR analysis indicated that cathepsin R is only expressed in placenta and thus is a new member of the emerging family of cathepsins whose expression is regulated during mouse embryonic development. Modeling and structural analysis suggests that cathepsin R will have a restricted substrate specificity when compared to that of cathepsin L.

Nonclassical MHC class II molecules

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

Protein degradation in MHC class II antigen presentation: opportunities for immunomodulation

Proteolysis is required for two steps of the MHC class II antigen-processing pathway, degradation of invariant chain and cleavage of protein antigens. Invariant chain dissociation from MHC is limited by a final proteolytic event which is tightly regulated in both temporal and tissue-specific ways. In contrast, enzymes involved in antigen proteolysis remain ill-defined. Gene 'knockout' experiments of housekeeping proteolytic enzymes suggest either that these enzymes do not play a major role, or that antigen proteolysis is too degenerate for this type of analysis. The possible role of two other proteinases, cathepsin E and aspariginyl endopeptidase is discussed. Finally, the data implicating antigen processing in repertoire generation is briefly considered. We conclude that selective regulation of endosomal proteolysis could have profound implications for control of immunity against infection, as well as in autoimmunity.

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

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

MHC class II-bound self peptides from autoimmune MRL/lpr mice reveal potential T cell epitopes for autoantibody production in murine systemic lupus erythematosus

The systemic lupus erythematosus-like syndrome in MRL/lpr mice involves high-titered IgG autoantibodies, particularly antinuclear Abs that target histones, DNA, and RNA particles. Although T cell help is required for the generation of antinuclear Abs, the epitopes recognized by such helper T cells are unknown. To address this question, we isolated and sequenced self peptides bound by MHC class II molecules from MRL/lpr mice. We identified a number of peptides that are not seen in similar preparations from nonautoimmune C3H animals. The "abnormal" peptide donors include histone, a protein component of a small nuclear ribonucleoprotein, ribosomal proteins, and RNA processing enzymes. We postulate that the peptides from these donors are T cell epitopes required for the generation of the most frequent antinuclear Abs specificities seen in MRL/lpr mice.

Cytoplasmic processing is a prerequisite for presentation of an endogenous antigen by major histocompatibility complex class II proteins

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.

Delta(9)-tetrahydrocannabinol selectively increases aspartyl cathepsin D proteolytic activity and impairs lysozyme processing by macrophages

Delta(9)-tetrahydrocannabinol (THC) causes an antigen-dependent defect in the ability of macrophages to activate helper T cells, and this drug-induced impairment is mediated through the peripheral CB2 receptor. Various requirements for the processing of the antigen, lysozyme, were examined to determine where along the pathway THC exerts its influence. A THC-exposed macrophage hybridoma inefficiently stimulated interleukin-2 secretion by a helper T cell hybridoma in response to native lysozyme and its reduced form, suggesting that disulfide bond reduction was unaffected. Cell surface expression of major histocompatibility complex class II molecules was normal on THC-exposed macrophages. The drug-exposed macrophages also competently presented a lysozyme peptide to the T cells, indicating that the class II molecules were functional. The proteolytic activity of two thiol cathepsins was unaltered, but aspartyl cathepsin D activity was significantly increased in THC-exposed macrophages. Thus, selective up-regulation of aspartyl cathepsin activity accompanied the deficiency in lysozyme processing and may contribute, at least in part, to the antigen-dependent processing defect in THC-exposed macrophages.

Role for cathepsin F in invariant chain processing and major histocompatibility complex class II peptide loading by macrophages

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 approximately 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.

Control of antigen presentation by a single protease cleavage site

Protein antigens require limited proteolytic processing to generate peptides for binding to class II MHC molecules, but the proteases and processing sites involved are largely unknown. Here we analyze the effect of eliminating the three major asparagine endopeptidase (AEP)-processing sites in the microbial antigen tetanus toxin C fragment. The mutant antigen is highly resistant to proteolysis by AEP and crude lysosomal extracts and is dramatically impaired in its ability to be processed and presented to T cells. Remarkably, processing at a single asparagine residue (1219) is obligatory for optimal presentation of many T cell epitopes in this antigen. These studies demonstrate that cleavage at a single processing site can be crucial for effective antigen presentation.

Antigen traffic pathways in dendritic cells

Dendritic cells (DC) are now believed to be the principal initiators of T cell-mediated immune responses. Their location in body tissues, migratory behaviour in response to inflammatory stimuli, endocytic properties, expression of MHC molecules and key T cell stimulatory molecules and many other attributes place these remarkable cells in a unique and influential position in the immune system. Progress in DC culture methods has recently allowed in-depth studies on the cell biological features that enable them to fulfill their crucial role in the immune response.

Widespread expression of tartrate-resistant acid phosphatase (Acp 5) in the mouse embryo

Tartrate-resistant acid phosphatase (TRAP, Acp 5) is considered to be a marker of the osteoclast and studies using 'knockout' mice have demonstrated that TRAP is critical for normal development of the skeleton. To investigate the distribution of TRAP in the mammalian embryo, cryostat sections of 18 d murine fetuses were examined by in situ hybridisation, immunohistochemistry and histochemical reactions in situ. Abundant expression of TRAP mRNA was observed in the skin and epithelial surfaces of the tongue, oropharynx and gastrointestinal tract including the colon, as well as the thymus, ossifying skeleton and dental papillae. TRAP protein was identified at the same sites, but the level of expression in the different tissues did not always correlate with apparent enzyme activity. The findings indicate that abundant TRAP expression is not confined to osteoclasts in bone, but occurs in diverse tissues harbouring cells of bone marrow origin, including dendritic cells and other cells belonging to the osteoclast/macrophage lineage.

Crystal structure of cathepsin X: a flip-flop of the ring of His23 allows carboxy-monopeptidase and carboxy-dipeptidase activity of the protease

BACKGROUND

Cathepsin X is a widespread, abundantly expressed papain-like mammalian lysosomal cysteine protease. It exhibits carboxy-monopeptidase as well as carboxy-dipeptidase activity and shares a similar activity profile with cathepsin B. The latter has been implicated in normal physiological events as well as in various pathological states such as rheumatoid arthritis, Alzheimer's disease and cancer progression. Thus the question is raised as to which of the two enzyme activities has actually been monitored.

RESULTS

The crystal structure of human cathepsin X has been determined at 2.67 A resolution. The structure shares the common features of a papain-like enzyme fold, but with a unique active site. The most pronounced feature of the cathepsin X structure is the mini-loop that includes a short three-residue insertion protruding into the active site of the protease. The residue Tyr27 on one side of the loop forms the surface of the S1 substrate-binding site, and His23 on the other side modulates both carboxy-monopeptidase as well as carboxy-dipeptidase activity of the enzyme by binding the C-terminal carboxyl group of a substrate in two different sidechain conformations.

CONCLUSIONS

The structure of cathepsin X exhibits a binding surface that will assist in the design of specific inhibitors of cathepsin X as well as of cathepsin B and thereby help to clarify the physiological roles of both proteases.

Lysosomal cysteine proteases: more than scavengers

Lysosomal cysteine proteases were believed to be mainly involved in intracellular protein degradation. Under special conditions they have been found outside lysosomes resulting in pathological conditions. With the discovery of a series of new cathepsins with restricted tissue distributions, it has become evident that these enzymes must be involved in a range of specific cellular tasks much broader than as simple housekeeping enzymes. It is therefore timely to review and discuss the various physiological roles of mammalian lysosomal papain-like cysteine proteases as well as their mechanisms of action and the regulation of their activity.

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

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

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

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

Cathepsins and compartmentalization in antigen presentation

Intracellular trafficking and cell surface expression of MHC class II molecules is a tightly regulated process and is to a large extent, determined by the fate of the class II chaperone, the invariant chain. Inhibition of endosomal proteases critical to invariant chain proteolysis reveals marked shunting of class II complexes to lysosomal compartments. Regulation of endosomal protease activity by expression of cystatin C directs class II cell surface expression during maturation of dendritic cells. These studies highlight the taut interactions between class-II-invariant-chain complexes and endosomal proteases during MHC class II maturation.

Cathepsin Q, a novel lysosomal cysteine protease highly expressed in placenta

The complete nucleotide sequence of a novel cathepsin cDNA derived from rat placenta was determined and is termed cathepsin Q. The predicted protein of 343 amino acid is a member of the family C1A protease related to cathepsin L. Rat cathepsin Q and its mouse counterpart were found highly expressed in placenta, whereas no detectable levels were found in lung, spleen, heart, brain, kidney, thymus, testicle, liver, or embryonic tissues. It is predicted that cathepsin Q will differ in catalytic specificity to another placental-specific protease, cathepsin P, indicating that these enzymes will have unique proteolytic functions in extra-embryonic tissues.

Bacterial proteins can be processed by macrophages in a transporter associated with antigen processing-independent, cysteine protease-dependent manner for presentation by MHC class I molecules

MHC class I molecules present peptides derived primarily from endogenously synthesized proteins on the cell surface as ligands for CD8+ T cells. However, CD8+ T cell responses to extracellular bacteria, virus-infected, or tumor cells can also be elicited because certain professional APC can generate peptide/MHC class I (MHC-I) complexes from exogenous sources. Whether the peptide/MHC-I complexes are generated because the exogenous proteins enter the classical cytosolic, TAP-dependent MHC-I processing pathway or an alternate pathway is controversial. Here we analyze the generation of peptide/MHC-I complexes from recombinant Escherichia coli as an exogenous Ag source that could be delivered to the phagosomes or directly into the cytosol. We show that peritoneal and bone marrow macrophages generate peptide/MHC-I complexes by the classical as well as an alternate, but relatively less efficient, TAP-independent pathway. Using a novel method to detect proteolytic intermediates we show that the generation of the optimal MHC-I binding peptide in the alternate pathway requires cysteine as well as other protease(s). This alternate TAP-independent pathway also operates in vivo and provides a potential mechanism for eliciting CD8+ T cell responses to exogenous Ags.

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

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

Animal models of rheumatoid arthritis and related inflammation

The major, extensively studied, experimentally-induced rat and mouse models of arthritis with features resembling rheumatoid arthritis are reviewed here. Etiopathogenetic studies that were recently published are emphasized. In summary, multiple triggering stimuli can induce disease in genetically-prone strains of inbred rats and mice. Multiple genetic loci, including both MHC and non-MHC, regulate disease expression in these animals. By comparison with other models of autoimmune disease, clustering of regulatory loci within and among species is increasingly becoming evident. At the cellular level, both innate and acquired immune systems are involved in the disease manifestations. At the molecular level, unbalanced chronic production of tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1, IL-6 and IL-12, as opposed to IL-4 and IL-10, is correlated with arthritis disease susceptibility and severity.

Proteases involved in MHC class II antigen presentation

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

The role of H2-O and HLA-DO in major histocompatibility complex class II-restricted antigen processing and presentation

The function of major histocompatibility complex (MHC) class II molecules is to sample exogenous antigens for presentation to CD4+ T helper cells. After synthesis in the endoplasmic reticulum, class II molecules are directed into the endosomal system by association with the invariant chain (Ii), which is sequentially cleaved, generating class II dimers loaded with Ii-derived peptides (CLIP). These class II-peptide complexes are physiological substrates for H2-M/HLA-DM, a resident of the endosomal/lysosomal system which facilitates the removal of CLIP from newly synthesised class II alpha beta dimers. Exchange of CLIP for antigenic class II-binding peptides is also promoted by the action of H2-M/HLA-DM, resulting in stable peptide-class II complexes that are transported to the cell surface for presentation to CD4+ T cells. Recent evidence suggests that this H2-M/HLA-DM-mediated 'peptide editing' is influenced by another MHC class II-encoded molecule, H2-O/HLA-DO. This non-polymorphic alpha beta heterodimer is associated with H2-M/HLA-DM during intracellular transport and within the endosomal system of B cells. H2-O/HLA-DO alters the peptide exchange function of H2-M/HLA-DM in a pH-dependent manner, so that H2-M/HLA-DM activity is limited to more acidic conditions, corresponding to lysosomal compartments. Indeed, H2-O/HLA-DO may serve to limit the presentation of antigens after fluid phase uptake by B cells, while augmenting presentation of antigens internalised via membrane Ig receptors. Such a mechanism may maintain the fidelity of the B-cell-CD4+ T-cell interaction, counteracting self reactivity arising from less stringent lymphocyte activation. Here, data evaluating the role of H2-O/HLA-DO shall be reviewed and its putative function discussed.

The role of lysosomal proteinases in MHC class II-mediated antigen processing and presentation

The recent analysis of cathepsin-deficient mice has shed light upon the role of lysosomal proteinases in the MHC class II processing and presentation pathway. Ubiquitous expression and involvement in the terminal degradation of proteins that intersect the endocytic pathway were previously perceived to be the hallmarks of these proteinases. However, recent evidence has demonstrated that several cathepsins are expressed in a tissue-specific fashion and that partial proteolysis of specific biological targets is a key function of cathepsins in antigen processing. Our work has focused on the differential expression of the cysteine proteinases cathepsins L (CL) and S (CS) and its pertinence to the generation of MHC class II: peptide complexes. Analysis of CL-deficient mice revealed a profound defect in invariant chain degradation in thymic cortical epithelial cells but not in bone marrow-derived antigen-presenting cells (APCs) (B cells, dendritic cells, and macrophages). The tissue-specific deficiency reflected the restricted pattern of expression of CL and CS in these cell types--CL is expressed in thymic cortical epithelial cells but not in DC or B cells, while CS exhibits the opposite expression pattern. The differential expression of proteinases by distinct APCs may affect the types of peptides that are presented to T cells and thereby the immune responses that are ultimately generated.

Cathepsin S controls the trafficking and maturation of MHC class II molecules in dendritic cells

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.

Phagocytic processing of antigens for presentation by class II major histocompatibility complex molecules

Microbes and other particulate antigens (Ags) are internalized by phagocytosis and then reside in plasma membrane-derived phagosomes. The contribution of phagosomes to the degradation of Ags has long been appreciated. It has been unclear, however, whether peptides derived from these degraded antigens bind class II major histocompatibility complex (MHC-II) molecules within phagosomes or within endocytic compartments that receive Ag fragments from phagosomes. Recent experiments have demonstrated that phagosomes containing Ag-conjugated latex beads express a full complement of Ag-processing molecules, e.g. MHC-II molecules, invariant chain, H2-DM and proteases sufficient to degrade bead- associated Ag. These phagosomes mediate the formation of peptide-MHC-II complexes, which are transported to the cell surface and presented to T cells. Phagosomes acquire both newly synthesized and plasma membrane-derived MHC-II molecules, but the formation of peptide-MHC-II complexes in phagosomes primarily involves newly synthesized MHC-II molecules. The content and traffic of phagosomal proteins vary considerably with the type of Ag ingested. Pathogenic microbes can alter phagosome composition and function to reduce Ag processing. For example, Mycobacterium tuberculosis blocks the maturation of phagosomes and reduces the ability of infected cells to present exogenous soluble protein Ags.

Cathepsin J, a novel murine cysteine protease of the papain family with a placenta-restricted expression

A novel mouse cysteine protease of the papain family was identified by searching the dbEST database. A 1.28 kb full-length cDNA was obtained which contains an open reading frame of 999 nucleotides and encodes a predicted polypeptide of 333 amino acids. The deduced polypeptide exhibits features characteristic of cysteine proteases of the papain type including the highly conserved residues of the catalytic triad, and was hence named cathepsin J. Cathepsin J represents the murine homologue of a previously described rat cathepsin L-related protein. Mature cathepsin J shows 59.3% identity to mouse cathepsin L and contains the characteristic ER(F/W)NIN motif within the propeptide indicating that this protease belongs to the subgroup of cathepsin L-like cysteine proteases. Northern blot analysis of various tissues revealed a placenta-restricted expression. This expression pattern may suggest a role of cathepsin J in embryo implantation and/or placental function. Ctsj was mapped to mouse chromosome 13 in the vicinity of cathepsin L suggesting that cathepsin J may have arisen by gene duplication from cathepsin L or a common ancestral gene.

Dipeptidyl peptidase I is required for the processing and activation of granzymes A and B in vivo

Dipeptidyl peptidase I (DPPI) is a lysosomal cysteine protease that has been implicated in the processing of granzymes, which are neutral serine proteases exclusively expressed in the granules of activated cytotoxic lymphocytes. In this report, we show that cytotoxic lymphocytes derived from DPPI-/- mice contain normal amounts of granzymes A and B, but these molecules retain their prodipeptide domains and are inactive. Cytotoxic assays with DPPI-/- effector cells reveal severe defects in the induction of target cell apoptosis (as measured by [(125)I]UdR release) at both early and late time points; this defect is comparable to that detected in perforin-/- or granzyme A-/- x B-/- cytotoxic lymphocytes. DPPI therefore plays an essential role in the in vivo processing and activation of granzymes A and B, which are required for cytotoxic lymphocyte granule-mediated apoptosis.