Cathepsin D, but not cathepsin B, releases T cell stimulatory fragments from lysozyme that are functional in the context of multiple murine class II MHC molecules

Identification of cathepsin B from large yellow croaker (Pseudosciaena crocea) and its role in the processing of MHC class II-associated invariant chain

In teleost, cathepsin B has been identified from several species and shown to play roles in the host immune response during pathogen challenge. However, the mechanism of how cathepsin B modulates the immune response in teleosts remains poorly understood. In this study, we identified and characterized cathepsin B (LycCatB) and invariant chain (LycIi) from the large yellow croaker (Pseudosciaena crocea). Sequence comparison and phylogenetic analysis indicated that LycCatB and LycIi are highly conserved within teleosts. Quantitative RT-PCR analysis showed that LycCatB mRNA was widely expressed in all examined tissues. We then recombinantly expressed LycCatB and Lyc-TR-Ii (transmembrane domain removed Ii chain) in Pichia pastoris and Escherichiacoli, respectively. The recombinant LycCatB (rLycCatB) can hydrolyze the substrate Z-FR-AMC with a Km value of 40.68μM. Furthermore, co-incubation of rLycCatB with rLyc-TR-Ii led to an efficient cleavage of rLyc-TR-Ii in a time-dependant manner. These results indicated that cathepsin B may be involved in MHC class II-associated Ii processing in large yellow croaker, and provide new information helping to elucidate the immunological functions of teleost cathepsin B.

Relationship between protein digestibility and allergenicity: comparisons of pepsin and cathepsin

An association between protein allergenicity and resistance to pepsin digestion in the gastrointestinal tract has been proposed. However, although widely accepted, such an association is inconsistent with known labile allergens and resistant nonallergens. Given the central role of antigen presenting cells, and in particular dendritic cells (DC), in the development of allergic responses, the stability of allergens to intracellular processing may be more relevant than resistance to extracellular pepsin digestion. We have characterised the expression by DC of cathepsins (proteolytic enzymes), and compared the proteolytic activity of the most highly expressed cathepsin with pepsin for a range of 9 allergens and 4 putative nonallergens. Cathepsin expression in bone marrow-derived DC (BM-DC) derived from BALB/c strain mice was characterised by flow cytometry; cathepsins D, E and S were identified, with cathepsin D being the most highly expressed. Digestion studies revealed that the majority of allergens (5/9) were pepsin resistant, whereas non-allergens (3/4) were labile. If the generation of pepsin-resistant fragments was considered as a feature of allergenicity, this increased to 7/9 allergens and 4/4 nonallergens. In contrast, most of the proteins examined were resistant to cathepsin digestion, with significant digestion recorded for only 2/9 allergens and 2/4 non-allergens. Chemical reduction (to mimic intracellular reducing conditions) increased the susceptibility of proteins to digestion by cathepsins, but did not improve discrimination between allergens and nonallergens on this basis. These data confirm that there is a general relationship between resistance to digestion with pepsin and allergenicity. The relationship is not absolute, but the information gained from this characteristic does provide useful information in a weight of evidence approach for allergenicity assessment. The most abundant cathepsin detected in antigen processing BM-DC, cathepsin D, is not an appropriate substitute for pepsin. The hypothesis that pepsin stability may be a surrogate for stability to digestion within DC may still hold true, but consideration of a single enzyme in this context is possibly an oversimplification.

Amphisomal route of MHC class I cross-presentation in bacteria-infected dendritic cells

Dendritic cells (DCs) are among the first professional APCs encountered by the obligate intracellular bacterium Chlamydia during infection. Using an established mouse bone marrow-derived DC line, we show that DCs control chlamydial infection in multiple small inclusions characterized by restricted bacterial growth, impaired cytosolic export of the virulence factor chlamydial protease-like activity factor, and interaction with guanylate-binding protein 1, a host cell factor involved in the initiation of autophagy. During maturation of infected DCs, chlamydial inclusions disintegrate, likely because they lack chlamydial protease-like activity factor-mediated protection. Released cytosolic Chlamydia are taken up by autophagosomes and colocalize with cathepsin-positive amphisomal vacuoles, to which peptide transporter TAP and upregulated MHC class I (MHC I) are recruited. Chlamydial Ags are subsequently generated through routes involving preprocessing in amphisomes via cathepsins and entry into the cytosol for further processing by the proteasome. Finally, bacterial peptides are reimported into the endosomal pathway for loading onto recycling MHC I. Thus, we unravel a novel pathway of MHC I-mediated cross-presentation that is initiated with a host cellular attack physically disrupting the parasitophorous vacuole, involves autophagy to collect cytosolic organisms into autophagosomes, and concludes with complex multistep antigenic processing in separate cellular compartments.

Disulfide reduction in the endocytic pathway: immunological functions of gamma-interferon-inducible lysosomal thiol reductase

Gamma-interferon-inducible lysosomal thiol reductase (GILT) is constitutively expressed in most antigen presenting cells and is interferon γ inducible in other cell types via signal transducer and activator of transcription 1. Normally, N- and C-terminal propeptides are cleaved in the early endosome, and the mature protein resides in late endosomes and lysosomes. Correspondingly, GILT has maximal reductase activity at an acidic pH. Monocyte differentiation via Toll-like receptor 4 triggers secretion of a disulfide-linked dimer of the enzymatically active precursor, which may contribute to inflammation. GILT facilitates major histocompatibility complex (MHC) class II-restricted processing through reduction of protein disulfide bonds in the endocytic pathway and is hypothesized to expose buried epitopes for MHC class II binding. GILT can also facilitate the transfer of disulfide-containing antigens into the cytosol, enhancing their cross-presentation by MHC class I. A variety of antigens are strongly influenced by GILT-mediated reduction, including hen egg lysozyme, melanocyte differentiation antigens, and viral envelope glycoproteins. In addition, GILT is conserved among lower eukaryotes and likely has additional functions. For example, GILT expression increases the stability of superoxide dismutase 2 and decreases reactive oxygen species, which correlates with decreased cellular proliferation. It is also a critical host factor for infection with Listeria monocytogenes.

Emerging roles of cathepsin E in host defense mechanisms

Cathepsin E is an intracellular aspartic proteinase of the pepsin superfamily, which is predominantly expressed in certain cell types, including the immune system cells and rapidly regenerating gastric mucosal and epidermal keratinocytes. The intracellular localization of this protein varies with different cell types. The endosomal localization is primarily found in antigen-presenting cells and gastric cells. The membrane association is observed with certain cell types such as erythrocytes, osteoclasts, gastric parietal cells and renal proximal tubule cells. This enzyme is also found in the endoplasmic reticulum, Golgi complex and cytosolic compartments in various cell types. In addition to its intracellular localization, cathepsin E occurs in the culture medium of activated phagocytes and cancer cells as the catalytically active enzyme. Its strategic expression and localization thus suggests the association of this enzyme with specific biological functions of the individual cell types. Recent genetic and pharmacological studies have particularly suggested that cathepsin E plays an important role in host defense against cancer cells and invading microorganisms. This review focuses emerging roles of cathepsin E in immune system cells and skin keratinocytes, and in host defense against cancer cells. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Diminished intracellular invariant chain expression after vaccinia virus infection

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 APCs perturbs MHC class II-mediated Ag presentation. Exposure of human B cell lines to VV induced a substantial 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 APCs. 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 ongoing Ii proteolysis contributing to reduced steady-state Ii levels in these APC. Yet in contrast with CHX, VV infection of APCs 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.

Endosomal proteases influence the repertoire of MAGE-A3 epitopes recognized in vivo by CD4+ T cells

Little is known about the repertoire of MAGE-A3 CD4(+) T-cell epitopes recognized in vivo by neoplastic patients and how antigen processing influences epitope formation. Here, we first show that MAGE-A3-specific CD4(+) T cells are present in the blood of advanced melanoma patients. MAGE-A3(111-125), MAGE-A3(191-205), and MAGE-A3(281-300) were recognized by 7, 6, and 5 of the 11 patients tested, respectively. MAGE-A3(146-160) and MAGE-A3(171-185) were also recognized in two and one cases, whereas no recognition of MAGE-A3(161-175) and MAGE-A3(243-258) was observed. Cytokines produced were mainly interleukin 5 and/or granulocyte macrophage colony-stimulating factor, suggesting impairment of productive polarized Th1 responses. Secondly, proteases inhibitors were used to modulate in vitro the recognition by CD4(+) T-cells clones of dendritic cells loaded with MAGE-A3-expressing cell lysates. We found that formation of MAGE-A3(111-125) depended on both leupeptin-sensitive and pepstatin-sensitive proteases. In contrast, we found that MAGE-A3(161-175), which was never recognized ex vivo, was formed by leupeptin but destroyed by pepstatin-sensitive proteases. Collectively, our results show that (a) anti-MAGE-A3 CD4(+) T-cell immunity develops in vivo in neoplastic patients and is focused toward immunodominant epitopes, (b) the response in advanced disease is skewed toward a Th2 type, and (c) endosomal/lysosomal proteases in dendritic cells influence the repertoire of the epitopes recognized.

Functional Requirements for the Lysosomal Thiol Reductase GILT in MHC Class II-Restricted Antigen Processing

Ag processing and presentation via MHC class II is essential for activation of CD4(+) T lymphocytes. gamma-IFN-inducible lysosomal thiol reductase (GILT) is present in the MHC class II loading compartment and has been shown to facilitate class II Ag processing and recall responses to Ags containing disulfide bonds such as hen egg lysozyme (HEL). Reduction of proteins within the MHC class II loading compartment is hypothesized to expose residues for class II binding and protease trimming. In vitro analysis has shown that the active site of GILT involves Cys(46) and Cys(49), present in a CXXC motif that shares similarity with the thioredoxin family. To define the functional requirements for GILT in MHC class II Ag processing, a GILT-deficient murine B cell lymphoma line was generated and stably transduced with wild-type and cysteine mutants of GILT. Intracellular flow cytometric, immunoblotting, and immunofluorescence analyses demonstrated that wild-type and mutant GILT were expressed and maintained lysosomal localization. Transduction with wild-type GILT reconstituted MHC class II processing of a GILT-dependent HEL epitope. Mutation of either Cys(46) or Cys(49) abrogated MHC class II processing of a GILT-dependent HEL epitope. In addition, biochemical analysis of these mutants suggested that the active site facilitates processing of precursor GILT to the mature form. Precursor forms of GILT-bearing mutations in Cys(200) or Cys(211), previously found to display thiol reductase activity in vitro, could not mediate Ag processing. These studies demonstrate that the thiol reductase activity of GILT is its essential function in MHC class II-restricted Ag processing.

The Unveiling of Hidden T-Cell Determinants of a Native Antigen by Defined Mediators of Inflammation: Implications for the Pathogenesis of Autoimmunity

A major hypothesis for the induction of autoimmunity invokes the enhanced display of previously hidden (cryptic) epitopes under inflammatory conditions leading to the activation of self-reactive T cells. However, there is meager data that directly validate the influence of specific immune mediators on the upregulation of the presentation of cryptic determinants in vivo. We tested the effect on well-defined cryptic epitopes of hen eggwhite lysozyme (HEL) of the availability locally of a cytokine (IL-2, IL-4, IL-6, IL-10, TNF-alpha or granulocyte-macrophage colony-stimulating factor) at the antigen delivery site, or of the pretreatment of the immunogen with a cathepsin (Cat B, D, L or S) prior to use in vivo. Each of the three mouse strains (H-2(b/d/k)) tested revealed a unique profile of T-cell reactivity to different cryptic epitopes of HEL in response to a particular cytokine or cathepsin. These results provide proof of principle for the reversal of crypticity of self-epitopes by immune mediators in the local milieu. Moreover, co-immunization with an antigen and a cytokine offers a simple and reliable tool for studying the role of cryptic epitopes in autoimmunity. Our results also strengthen the rationale for the use of inhibitors of cytokine/cathepsin activity in the treatment of autoimmune diseases.

Biotinylated fluorescent peptide substrates for the sensitive and specific determination of cathepsin D activity

Cathepsin D (CatD) is a member of the mammalian aspartic protease family and is involved in cellular protein degradation and in several pathological processes. A sensitive and specific assay for the determination of CatD activity in biological samples was developed. The peptide amide substrates Amca-EDKPILF downward arrowFRLGK(biotin)-CONH2 (I), Amca-EEKPIC(Acm)F downward arrowFRLGK(biotin)-CONH2 (II) and Amca-EEKPISF downward arrowFRLGK(biotin)-CONH2 (III) contain a CatD cleavage site (F downward arrowF) flanked by a N-terminal Amca-fluorophore (7-amino-4-methylcoumarin-3-acetic acid) and a C-terminal biotin moiety. Substrates II and III proved to be specific substrates containing only one cleavage site for CatD. After cleavage of the Phe-Phe bond by CatD all biotin conjugated peptides were removed with streptavidin-coated magnetic beads. The remaining fluorescent peptides in solution represent the amount of digested substrate. The versatility of this CatD digest and pull down assay was demonstrated by measuring the activity of CatD in different subcellular fractions of human EBV-transformed B cells and human monocytes. The described method based on the designed CatD substrates represents a valuable tool for routine assays.

Identification of proteases employed by dendritic cells in the processing of protein purified derivative (PPD)

Dendritic cells (DC) are known to present exogenous protein Ag effectively to T cells. In this study we sought to identify the proteases that DC employ during antigen processing. The murine epidermal-derived DC line Xs52, when pulsed with PPD, optimally activated the PPD-reactive Th1 clone LNC.2F1 as well as the Th2 clone LNC.4k1, and this activation was completely blocked by chloroquine pretreatment. These results validate the capacity of XS52 DC to digest PPD into immunogenic peptides inducing antigen specific T cell immune responses. XS52 DC, as well as splenic DC and DCs derived from bone marrow degraded standard substrates for cathepsins B, C, D/E, H, J, and L, tryptase, and chymases, indicating that DC express a variety of protease activities. Treatment of XS52 DC with pepstatin A, an inhibitor of aspartic acid proteases, completely abrogated their capacity to present native PPD, but not trypsin-digested PPD fragments to Th1 and Th2 cell clones. Pepstatin A also inhibited cathepsin D/E activity selectively among the XS52 DC-associated protease activities. On the other hand, inhibitors of serine proteases (dichloroisocoumarin, DCI) or of cystein proteases (E-64) did not impair XS52 DC presentation of PPD, nor did they inhibit cathepsin D/E activity. Finally, all tested DC populations (XS52 DC, splenic DC, and bone marrow-derived DC) constitutively expressed cathepsin D mRNA. These results suggest that DC primarily employ cathepsin D (and perhaps E) to digest PPD into antigenic peptides.

Targeting plasmid-encoded proteins to the antigen presentation pathways

The antigen presentation pathways constitute a fulcrum on which adaptive immunity is balanced, and their manipulation should allow us to induce designer immune responses. The ease and rapidity with which DNA vaccines can be constructed and altered make them ideal candidates with which to test the various targeting strategies that have been conceived to date. These approaches and the mechanisms that may (or may not) underlie their success are reviewed in this article.

Lysosomal cysteine proteases regulate antigen presentation

Antigen presentation by both classical MHC class II molecules and the non-classical MHC class I-like molecule CD1D requires their entry into the endosomal/lysosomal compartment. Lysosomal cysteine proteases constitute an important subset of the enzymes that are present in this compartment and, here, we discuss the role of these proteases in regulating antigen presentation by both MHC class II and CD1D molecules.

Mass and composition matrix-assisted laser desorption ionization time of flight analysis enabling inference of the sequence of most peptides where the protein of origin is known

Mass spectrometers combining matrix-assisted laser-desorption ionization (MALDI) and time-of-flight analysis are among the most widely used in peptide analysis. They excel at accurate mass determinations on complex samples but, compared with tandem instruments, have very limited capacity to determine amino acid sequence through daughter ion analysis. Here we have investigated the sequence information that can be inferred from the masses of peptides in the special circumstance in which the peptides are known to be sub-sequences of known parent sequences. We show how sequence can be inferred from the measured m/z of a peptide (mass analysis) and examine the parameters that influence the level of confidence that can be placed in "inferred sequences". We further describe how specific amino acid modifications can be used with MALDI-TOF analysis to obtain partial composition information and demonstrate that combined mass and composition (MAC) analysis enables the sequences of most peptide ions to be inferred with very high confidence.

Microglial functions and proteases

There is accumulating evidence that intracellular and extracellular proteases of microglia contribute to various events in the central nervous system (CNS) through both nonspecific and limited proteolysis. Cathepsin E and cathepsin S, endosomal/lysosomal proteases, have been shown to play important roles in the major histocompatibility complex (MHC) class II-mediated antigen presentation of microglia by processing of exogenous antigens and degradation of the invariant chain associated with MHC class II molecules, respectively. Some members of cathepsins are also involved in neuronal death after secreted from microglia and clearance of phagocytosed amyloid- beta peptides. Tissue-type plasminogen activator, a serine protease, secreted from microglia participates in neuronal death, enhancement of N-methyl-D-aspartate receptor-mediated neuronal responses, and activation of microglia via either proteolytic or nonproteolytic activity. Calpain, a calcium-dependent cysteine protease, has been shown to play a pivotal role in the pathogenesis of multiple sclerosis by degrading myelin proteins extracellulary. Furthermore, matrix metalloproteases secreted from microglia also receive great attention as mediators of inflammation and tissue degradation through processing of pro-inflammatory cytokines and damage to the blood-brain barrier. The growing knowledge about proteolytic events mediated by microglial proteases will not only contribute to better understanding of microglial functions in the CNS but also may aid in the development of protease inhibitors as novel neuroprotective agents.

Analysis of MHC class II antigen processing by quantitation of peptides that constitute nested sets

Peptides associated with class II MHC molecules are of variable length because in contrast to peptides associated with class I MHC molecules, their amino and C termini are not constrained by the structure of the peptide interaction with the binding site. The proteolytic processing events that generate these peptides are still not well understood. To address this question, peptides extracted from HLA-DR*0401 were analyzed using two types of mass spectrometry instrumentation. This enabled identification of >700 candidate peptides in a single analysis and provided relative abundance information on 142 peptides contained in 11 nested sets of 3-36 members each. Peptides of 12 residues or less occurred only at low abundance, despite the fact that they were predicted to fully occupy the HLA-DR*0401 molecule in a single register. Conversely, the relative abundance of longer species suggested that proteolytic events occurring after MHC binding determine the final structure of most class II-associated peptides. Our data suggest that C-terminal residues of these peptides reflect the action of peptidases that cleave at preferred amino acids, while amino termini appear to be determined more by proximity to the class II MHC binding site. Thus, the analysis of abundance information for class II-associated peptides comprising nested sets has offered new insights into proteolytic processing of MHC class II-associated peptides.

Cathepsin L regulates CD4+ T cell selection independently of its effect on invariant chain: a role in the generation of positively selecting peptide ligands

CD4+ T cells are positively selected in the thymus on peptides presented in the context of major histocompatibility complex class II molecules expressed on cortical thymic epithelial cells. Molecules regulating this peptide presentation play a role in determining the outcome of positive selection. Cathepsin L mediates invariant chain processing in cortical thymic epithelial cells, and animals of the I-A(b) haplotype deficient in this enzyme exhibit impaired CD4+ T cell selection. To determine whether the selection defect is due solely to the block in invariant chain cleavage we analyzed cathepsin L-deficient mice expressing the I-A(q) haplotype which has little dependence upon invariant chain processing for peptide presentation. Our data indicate the cathepsin L defect in CD4+ T cell selection is haplotype independent, and thus imply it is independent of invariant chain degradation. This was confirmed by analysis of I-A(b) mice deficient in both cathepsin L and invariant chain. We show that the defect in positive selection in the cathepsin L-/- thymus is specific for CD4+ T cells that can be selected in a wild-type and provide evidence that the repertoire of T cells selected differs from that in wild-type mice, suggesting cortical thymic epithelial cells in cathepsin L knockout mice express an altered peptide repertoire. Thus, we propose a novel role for cathepsin L in regulating positive selection by generating the major histocompatibility complex class II bound peptide ligands presented by cortical thymic epithelial cells.

Adduction of soluble proteins with malondialdehyde-acetaldehyde (MAA) induces antibody production and enhances T-cell proliferation

BACKGROUND

The alcohol metabolites malondialdehyde and acetaldehyde can combine to form stable adducts (MAA) which are found in the livers of humans and rats after significant alcohol ingestion. While adducted proteins induce antibody responses in the absence of adjuvants, the mechanisms by which these responses occur are unknown. Thus, it was the purpose of these studies to investigate how MAA modification stimulates antibody and T-cell responses in the absence of adjuvants.

METHODS

Hen egg lysozyme (HEL) was modified with increasing levels of MAA and was used as an immunogen, and antibody and T-cell responses were determined. The role of scavenger receptors in the immunogenicity of MAA-adducted proteins was also investigated.

RESULTS

Maximum antibody response was induced after immunization with 1.8 nM MAA/nM HEL, and was primarily an IgG1 response to HEL as determined by inhibition ELISAs. T-cell proliferative responses after immunization with HEL-MAA were solely to HEL. Immunization with a scavenger receptor ligand in conjunction with HEL-MAA increased the predominant IgG1 response and sharply decreased the IgG2a response by approximately 50%. Binding of HEL-MAA by splenocytes was determined by flow cytometry to be approximately 15% greater than HEL alone, showing a doubling of the geometric mean fluorescence. Also, most of the cells that bound HEL-MAA were class II positive, indicating that antigen-presenting cells can bind the MAA-adducted HEL, and potentially initiate immune responses.

CONCLUSIONS

MAA modification of proteins induces antibody and T-cell proliferative responses in vivo. Initial studies suggest that these responses may be mediated by scavenger receptors that recognize MAA-adducted proteins. This suggests a mechanism by which proteins modified with oxidative products associated with chronic ethanol consumption may alter immune responses that may play an active role in the development and/or progression of alcoholic liver disease.

Involvement of cathepsin E in exogenous antigen processing in primary cultured murine microglia

We have attempted to elucidate an involvement of cathepsin E (CE) in major histocompatibility complex class II-mediated antigen presentation by microglia. In primary cultured murine microglia, CE was localized mainly in early endosomes and its expression level was markedly increased upon stimulation with interferon-gamma. Pepstatin A, a specific inhibitor of aspartic proteases, significantly inhibited interleukin-2 production from an OVA-(266-281)-specific T helper cell hybridomas upon stimulation with native OVA presented by interferon-gamma-treated microglia. However, pepstatin A failed to inhibit the presentation of OVA-(266-281) peptide. The possible involvement of CE in the processing of native OVA into antigenic peptide was further substantiated by that digested fragments of native OVA by CE could be recognized by OVA-specific Th cells. Cathepsin D also degraded native OVA into antigenic peptide, whereas microglia prepared from cathepsin D-deficient mice retained an ability for antigen presentation. On the other hand, the requirement for cysteine proteases such as cathepsins S and B in the processing of invariant chain (Ii) was confirmed by immunoblot analyses in the presence of their specific inhibitors. In conclusion, CE is required for the generation of an antigenic epitope from OVA but not for the processing of Ii in microglia.

A closer look at proteolysis and MHC-class-II-restricted antigen presentation

Antigen presentation by MHC class II molecules relies on the action of endocytic proteases, which are differentially expressed in antigen-presenting cells and are regulated by different components of the immune system. Endocytic enzymes process and convert exogenous antigens into peptidic determinants capable of interaction with MHC class II molecules. Chemical and genetic tools have recently been developed to study the role of lysosomal proteases in antigen presentation.

Proteolysis and antigen presentation by MHC class II molecules

Proteolysis is the primary mechanism used by all cells not only to dispose of unwanted proteins but also to regulate protein function and maintain cellular homeostasis. Proteases that reside in the endocytic pathway are the principal actors of terminal protein degradation. The proteases contained in the endocytic pathway are classified into four major groups based on the active-site amino acid used by the enzyme to hydrolyze amide bonds of proteins: cysteine, aspartyl, serine, and metalloproteases. The presentation of peptide antigens by major histocompatibility complex (MHC) class II molecules is strictly dependent on the action of proteases. Class II molecules scour the endocytic pathway for antigenic peptides to bind and present at the cell surface for recognition by CD4+ T cells. The specialized cell types that support antigen presentation by class II molecules are commonly referred to as professional antigen presenting cells (APCs), which include bone marrow-derived B lymphocytes, dendritic cells (DCs), and macrophages. In addition, the expression of certain endocytic proteases is regulated either at the level of gene transcription or enzyme maturation and their activity is controlled by the presence of endogenous protease inhibitors.

Cathepsin S and an asparagine-specific endoprotease dominate the proteolytic processing of human myelin basic protein in vitro

The biochemical characterization of antigen degradation is an important basis for a better understanding of both the immune response and autoimmune diseases mediated by MHC class II molecules. In this study we used high-performance liquid chromatography and mass spectrometry to analyze the processing of myelin basic protein (MBP), a potential autoantigen implicated in the pathogenesis of multiple sclerosis. We resolved the kinetics of MBP processing by lysosomal extracts or purified endocytic proteases, identified the major cleavage sites during this process and assigned them to the activity of proteolytic enzymes. Proteolytic processing of MBP is mostly guided along the hydrophobic regions of the protein. It is initiated by two proteolytic steps (after N(92) and S(110)) that are performed by an asparagine-specific endopeptidase (AEP) and by cathepsin (Cat) S, respectively. The resulting processing intermediates are converted into more than 60 different species of 20-40-mers due to the activity of endopeptidases including CatS, D and L. The fragments thus generated are subsequently degraded by C- or N-terminal trimming. Strikingly, the initial cleavages during MBP processing affect two immunodominant regions of the potential autoantigen [MBP(85-99) and MBP(111-129)] in an inverse manner. CatS directly generates the N terminus of the epitope MBP(111-129) in large quantities during the initial phase of processing, which might explain the immunogenicity of this region in spite of its relatively poor binding to HLA-DR4. In contrast, the dominant cleavage by AEP mediates the destruction of MBP(85-99) unless the epitope is protected, e.g. by binding to HLA-DR. Our results thus characterize the proteolytic events during processing of MBP on a molecular level and suggest a biochemical basis for the immunogenicity of the immunodominant epitopes, which could serve as a guideline for future therapeutic strategies.

CD4(+) T cells induced by a DNA vaccine: immunological consequences of epitope-specific lysosomal targeting

Our previous studies have shown that targeting DNA vaccine-encoded major histocompatibility complex class I epitopes to the proteasome enhanced CD8(+) T-cell induction and protection against lymphocytic choriomeningitis virus (LCMV) challenge. Here, we expand these studies to evaluate CD4(+) T-cell responses induced by DNA immunization and describe a system for targeting proteins and minigenes to lysosomes. Full-length proteins can be targeted to the lysosomal compartment by covalent attachment to the 20-amino-acid C-terminal tail of lysosomal integral membrane protein-II (LIMP-II). Using minigenes encoding defined T-helper epitopes from lymphocytic choriomeningitis virus, we show that the CD4(+) T-cell response induced by the NP(309-328) epitope of LCMV was greatly enhanced by addition of the LIMP-II tail. However, the immunological consequence of lysosomal targeting is not invariably positive; the CD4(+) T-cell response induced by the GP(61-80) epitope was almost abolished when attached to the LIMP-II tail. We identify the mechanism which underlies this marked difference in outcome. The GP(61-80) epitope is highly susceptible to cleavage by cathepsin D, an aspartic endopeptidase found almost exclusively in lysosomes. We show, using mass spectrometry, that the GP(61-80) peptide is cleaved between residues F(74) and K(75) and that this destroys its ability to stimulate virus-specific CD4(+) T cells. Thus, the immunological result of lysosomal targeting varies, depending upon the primary sequence of the encoded antigen. We analyze the effects of CD4(+) T-cell priming on the virus-specific antibody and CD8(+) T-cell responses which are mounted after virus infection and show that neither response appears to be accelerated or enhanced. Finally, we evaluate the protective benefits of CD4(+) T-cell vaccination in the LCMV model system; in contrast to DNA vaccine-induced CD8(+) T cells, which can confer solid protection against LCMV challenge, DNA vaccine-mediated priming of CD4(+) T cells does not appear to enhance the vaccinee's ability to combat viral challenge.

Differing processing requirements of four recombinant antigens containing a single defined T-cell epitope for presentation by major histocompatibility complex class II

A set of predictive rules governing the likelihood of generating a particular peptide-major histocompatibility complex (MHC) class II complex from an intact antigen has not been fully elucidated. We investigated the influence of positional and structural constraints in the region of the epitope by designing a set of recombinant antigens that each contained the well-characterized T-cell epitope moth cytochrome c (MCC) (88-103), which is specifically recognized by the monoclonal antibody (mAb) D4 when complexed with H-2Ek. Our model antigens contained MCC(88-103) either peripherally, at or towards the C-terminus, or internally. Their abilities to bind directly to soluble H-2Ek, and the extent of D4 epitope formation from them by antigen processing-competent and -incompetent cell lines, were determined. Here we report that three of these four antigens yielded MCC(88-103)/H-2Ek complexes independently of the conventional MHC class II antigen-processing and presentation pathway, and in each case the epitope was carried peripherally; two bound directly as intact proteins, probably as a result of spatial separation of the epitope from the major globular domain, and one was processed to peptide by a cell-surface protease. One protein, which carried the epitope inserted into an internal loop, acted as a conventional processing-dependent MCC(88-103) delivery vehicle. Thus, this epitope has different presentation requirements depending on its context. These antigens constitute a panel whose framework could be modified to further define predictive rules for antigen processing for presentation through the different MHC class II complex-generating pathways.

Substrate specificity of human cathepsin D using internally quenched fluorescent peptides derived from reactive site loop of kallistatin

Kallistatin, a serpin that specifically inhibits human tissue kallikrein, was demonstrated to be cleaved at the Phe-Phe bond in its reactive site loop (RSL) by cathepsin D. Internally quenched fluorescent peptides containing the amino acid sequence of kallistatin RSL were highly susceptible to hydrolysis by cathepsin D. Surprisingly, these peptides were efficiently hydrolyzed at Phe-Phe bond, despite having Lys and Ser at P2 and P2' positions, respectively, which was reported to be very unfavorable for substrates for cathepsin D. Due to the importance of cathepsin D in several physiological and pathological processes, we took the peptide containing kallistatin RSL sequence, Abz-Ala-Ile-Lys-Phe-Phe-Ser-Arg-Gln-EDDnp, as a reference substrate for a systematic specificity study of S3 to S3' protease subsites (EDDnp=N-[2,4-dinitrophenyl]-ethylenediamine and Abz=ortho-amino benzoic acid). We present in this paper some internally quenched fluorescent peptides that were efficient substrates for cathepsin D. They essentially differ from other previously described substrates by their higher kcat/Km values due, mainly, to low Km values, such as the substrate Abz-Ala-Ile-Ala-Phe-Phe-Ser-Arg-Gln-EDDnp (Km=0.27 microM, kcat=16.25 s(-1), kcat/Km=60185 microM(-1) x s(-1)).

Alpha1-antichymotrypsin and kallistatin hydrolysis by human cathepsin D

In the present paper, we demonstrate that alpha1-antichymotrypsin, a serpin with high inhibitory specificity toward cathepsin G, and kallistatin, a human serpin with high specificity toward tissue kallikrein, are digested by cathepsin D. Alpha1-Antichymotrypsin was hydrolyzed essentially in the reactive center loop at L-S, A-L, or L-V bonds; kallistatin was split into small fragments, but we detected the cleavage at F-F and F-S bonds in its reactive center loop in the first 15 min of digestion. In contrast to alpha1-antichymotrypsin, kallistatin is irreversibly inactivated at pH 4.0. Synthetic internally quenched fluorescent peptides containing sequences similar to the reactive center loops of these serpins were hydrolyzed by cathepsin D. The peptides derived from kallistatin were hydrolyzed more efficiently, and particularly relevant was the high susceptibility of the substrates Abz-AIKFFSAQTNRHILRFNRQ-EDDnp (Km = 0.08 microM, kcat = 2.4 s(-1)) and Abz-AIKFFSAQTNRQ-EDDnp (Km = 0.8 microM, kcat = 17.8 s(-1)), which were hydrolyzed at the F-F bond. Therefore, besides the description of a new class of very efficient internally quenched substrates for cathepsin D, we give evidence for the downregulation role of this proteinase on alpha1-antichymotrypsin and kallistatin. The acidification of extracellular milieu by tumor cells can result in activation of cathepsin D; as a consequence, kinins can be released, improving blood supply and leaving more cathepsin G available for the degradation of extracellular matrix.

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.

Lysosomal cathepsin B plays an important role in antigen processing, while cathepsin D is involved in degradation of the invariant chain inovalbumin-immunized mice

We previously reported that CA074, a specific inhibitor of cathepsin B, modulates specific immune responses from the T helper 2 (Th2) type to Th1 type in BALB/c mice infected with Leishmania major. In the present study, we found that a similar type of immune deviation was also induced in mice immunized with ovalbumin (OVA). However, treatment of mice with pepstatin A, a specific cathepsin D inhibitor, suppressed the OVA-specific proliferation of lymphocytes and blocked the development of both Th1 and Th2 cellular responses. These inhibitors did not appear to have any direct influence in vitro on functions of naive lymphocytes. OVA antigen (47 000 MW) was digested mainly into 40 000 MW protein in vitro by lysosomal proteases from naive BALB/c mice, and its digestion was markedly inhibited by the addition of CA074, but not by addition of pepstatin A, during incubation. However, pepstatin A strongly suppressed the degradation of the major histocompatibility complex class II-associated invariant chain (Ii) molecule in vivo and in vitro. Thus, cathepsin B appears to process antigens directed to preferential activation of Th2 cells, while cathepsin D may be responsible for the degradation of Ii, the processing of which is essential in initiating the antigen-specific activation of Th1 and Th2 CD4+ T cells. These lysosomal proteases may have different functions in regulating immune responses.

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.

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.

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

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

Proteases involved in MHC class II antigen presentation

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

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.

Characterization of Dictyostelium discoideum cathepsin D

Previous studies using magnetic purification of Dictyostelium discoideum endocytic vesicles led us to the identification of some major vesicle proteins. Using the same purification procedure, we have now focused our interest on a 44 kDa soluble vesicle protein. Microsequencing of internal peptides and subsequent cloning of the corresponding cDNA identified this protein as the Dictyostelium homolog of mammalian cathepsins D. The only glycosylation detected on Dictyostelium cathepsin D (CatD) is common antigen 1, a cluster of mannose 6-sulfate residues on N-linked oligosaccharide chains. CatD intracellular trafficking has been studied, showing the presence of the protein throughout the entire endocytic pathway. During the differentiation process, the catD gene presents a developmental regulation, which is also observed at the protein level. catD gene disruption does not alter significantly the cell behaviour, either in the vegetative form or the differentiation stage. However, modifications in the SDS-PAGE profiles of proteins bearing common antigen 1 were detected, when comparing parental and catD(-) cells. These modifications point to a possible role of CatD in the maturation of a few Dictyostelium lysosomal proteins.

Importance of thioredoxin in the proteolysis of an immunoglobulin G as antigen by lysosomal Cys-proteases

For disulphide-bonded antigens, reduction has been postulated to be a prerequisite for proteolytic antigen processing, with subsequent production of major histocompatibility complex (MHC) class II binding fragments. The murine monoclonal immunoglobulin G (IgG) CE25/B7 was used as a multimeric antigen in a human model. Native IgG is highly resistant to proteolysis and has been previously found to be partially reduced at early steps of cell processing to become a suitable substrate for endopeptidases. The role of the oxidoreductase thioredoxin (Trx) was assessed in the reduction of the IgG by cleavage of H-L and H-H disulphide bonds. Recombinant human Trx (rTrx) has been assayed in a proteolytic in vitro system on IgG using endosomal and lysosomal subcellular fractions from B lymphoblastoid cells. rTrx is required in a dose-dependent manner for development of efficient proteolysis, catalysed by thiol-dependent Cys-proteases, such as cathepsin B. We demonstrated that cathepsin B activity was stimulated by the addition of rTrx. Thus, we propose that Trx-dependent IgG proteolysis occurred, on the one hand by means of the unfolding of the IgG after disulphide reduction, becoming a substrate of lysosomal proteases, and on the other hand by Cys-proteases such as cathepsin B that are fully active upon the regeneration of their activity by hydrogen donors.

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

Cathepsins have been implicated in the degradation of proteins destined for the MHC class II processing pathway and in the proteolytic removal of invariant chain (Ii), a critical regulator of MHC class II function. Mice lacking the lysosomal cysteine proteinase cathepsin S (catS) demonstrated a profound inhibition of Ii degradation in professional APC in vivo. A marked variation in the generation of MHC class II-bound Ii fragments and presentation of exogenous proteins was observed between B cells, dendritic cells, and macrophages lacking catS. CatS-deficient mice showed diminished susceptibility to collagen-induced arthritis, suggesting a potential therapeutic target for regulation of immune responsiveness.

Tolerance and determinant hierarchy

The overall T cell response to a multideterminant antigen consists of the sum of responses to a limited number of different determinants on the protein. Antigen-presenting cells (APCs) are crucial in delimiting the determinants on the protein to which a response will be mounted. This influence is apparent at two levels. First, the determinants that are generated and displayed by APCs in the thymus are pivotal in shaping the T cell repertoire that will be available for responding to antigen determinants in the periphery. Second, antigen processing affects the selection of determinants that become displayed by the various peripheral APC populations that are involved in inducing and promoting a T cell response. We have studied the effect of the display hierarchy on tolerance induction to individual determinants in transgenic mice expressing different serum levels of hen egg lysozyme. We have also analysed aspects of the processing machinery that contribute to shaping the hierarchy of determinant display on MHC class II molecules: proteolysis and reduction of disulfide bonds.

Epitope dominance: evidence for reciprocal determinant spreading to glutamic acid decarboxylase in non-obese diabetic mice

Autoimmune T-cell responses to peptide determinants of several autoantigens have recently been characterized. These data suggest that, in some autoimmune models, such as experimental autoimmune encephalomyelitis, T-cell responses may diversify from a nested set of peptides to include many other peptide regions. A similar immune phenomenon pertaining to autoimmune diabetes (IDDM) is observed in NOD mice. We have explored a similar pattern of T-cell responses related to age and disease status in NOD mice termed epitope dominance, which describes immune responses toward a pronounced subset of determinants of the autoantigen glutamic acid decarboxylase (GAD). Our studies have identified a total of five GAD epitopes between the 65 and 67 kDa isoforms. The magnitude of T-cell responses to these various determinants was dependent on the stage of disease as well as on whether mice were protected from disease. The T-cell responses of these epitopes in NOD mice correlated with the predicted binding of these peptides to the NOD class II molecule I-Ag7. We therefore propose a model which implicates antigen presenting cells as critical entities in the propagation of dominant responses to the presentation of autoantigens to T cells, particularly in the Th 1 environment of the NOD mouse. This hypothesis presents a new framework for the discussion and interpretation of the kinetics of T-cell responses to different peptide epitopes in autoimmune diseases such as IDDM.

Cathepsins B and D are dispensable for major histocompatibility complex class II-mediated antigen presentation

Antigen presentation by major histocompatibility complex (MHC) class II molecules requires the participation of different proteases in the endocytic route to degrade endocytosed antigens as well as the MHC class II-associated invariant chain (Ii). Thus far, only the cysteine protease cathepsin (Cat) S appears essential for complete destruction of Ii. The enzymes involved in degradation of the antigens themselves remain to be identified. Degradation of antigens in vitro and experiments using protease inhibitors have suggested that Cat B and Cat D, two major aspartyl and cysteine proteases, respectively, are involved in antigen degradation. We have analyzed the antigen-presenting properties of cells derived from mice deficient in either Cat B or Cat D. Although the absence of these proteases provoked a modest shift in the efficiency of presentation of some antigenic determinants, the overall capacity of Cat B-/- or Cat D-/- antigen-presenting cells was unaffected. Degradation of Ii proceeded normally in Cat B-/- splenocytes, as it did in Cat D-/- cells. We conclude that neither Cat B nor Cat D are essential for MHC class II-mediated antigen presentation.

Substrate specificity of cathepsins D and E determined by N-terminal and C-terminal sequencing of peptide pools

Degradation of protein antigens by cellular proteases is a crucial step in the initiation of a T-cell-mediated immune response. But still little is known about the enzymes responsible for the processing of antigens, including their specificity. In this paper, we show that the combination of automated N-terminal sequencing with a newly developed method for C-terminal sequencing of peptide pools generated by the aspartic proteases cathepsins D and E is a fast and easy method to obtain detailed information of the substrate specificity of these endopeptidases. Using a 15-residue synthetic peptide library and a native protein as substrates, we confirm and extend the knowledge about the cleavage motif of cathepsin E where positions P1 and P1' of the substrate must be occupied exclusively by hydrophobic amino acids with aromatic or aliphatic side chains. However, Val and Ile residues are not allowed at position P1. Position P2' accepts a broad range of amino acids, including charged and polar ones. Additional requirements concerning the substrate positions P3' and P4' were also defined by pool sequencing. Furthermore, pool sequencing analysis of melittin digests with the aspartic proteases cathepsin D and E provided evidence that both enzymes share the same cleavage motif, identical to the one derived from the peptide library and the native protein. Therefore, pool sequencing analysis is a valuable and fast tool to determine the substrate specificity of any endopeptidase.

Endosomal proteases and antigen processing

T cells are activated by fragments of antigenic proteins bound to major histocompatibility complex (MHC) molecules and displayed on the cell surface. MHC class II proteins scavenge processed protein antigens from within endosomal compartments. The antigenic peptides are generated within these and other intracellular compartments using the array of proteolytic enzymes normally involved in terminal protein degradation. Antigen-presenting cells use different mechanisms to exploit and control the activity of these enzymes so as to ensure the generation of a wide variety of peptides, while preventing the destruction of antigenic epitopes by excessive proteolysis.

Degradation of mouse invariant chain: roles of cathepsins S and D and the influence of major histocompatibility complex polymorphism

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.

Antigen processing differences among APC

The hierarchy of display of determinants on a protein antigen is of critical importance with respect to which T cells will be selected during thymic development, as well as in the induction of mature responses. Activation of T cells will be dependent on unfolding, reduction and chain cleavage of the antigen, and the vagaries of competition with other determinants as well as hindrance in access to the major histocompatibility complex (MHC) or subsequently of the MHC/peptide complex to the T cell receptor. We here focus on a description of the parameters that determine the generation and display of determinants on MHC class II molecules by different types of antigen presenting cells in different locations.

Capture and processing of exogenous antigens for presentation on MHC molecules

Class I and class II MHC molecules bind peptides during their biosynthetic maturation and provide a continuously updated display of intracellular and environmental protein composition, respectively, for scrutiny by T cells. Receptor-mediated endocytosis, phagocytosis, and macropinocytosis all contribute to antigen uptake by class II MHC-positive antigen-presenting cells. Capture of antigenic peptides by class II MHC molecules is facilitated because antigen catabolism and class II MHC maturation take place in the same compartments or in communicating compartments of the endosome/lysosome system. These class II MHC-rich, multivesicular endosomes receive incoming antigen and can support not only antigen processing and class II MHC peptide loading but also the export of peptide/class II MHC complexes to the cell surface. A balance between production and destruction of antigenic peptides is achieved by the activity of local proteases and may be influenced by binding of antigen to other proteins both prior to the onset of processing (e.g. antibodies) and during antigen unfolding (e.g. MHC molecules). T cell determinants that can be released for MHC binding without a substantial processing requirement may be able to utilize a distinct minor population of cell surface class II MHC molecules that become available during peripheral recycling. Although peptides derived from exogenous protein sources are usually excluded from presentation on class I MHC molecules, recent evidence shows that this embargo may be lifted in certain professional antigen-presenting cells to increase the spectrum of antigens that may be displayed on class I MHC.

Amyloidogenic processing of human amyloid precursor protein in hippocampal neurons devoid of cathepsin D

betaA4-Amyloid peptide, the main component of the amyloid plaques in the brain of Alzheimer's disease patients is produced from amyloid precursor protein (APP) by proteolytical processing. Several lines of evidence suggest a direct role for cathepsin D, the major endosomal/lysosomal aspartic endopeptidase, in betaA4-amyloid peptide generation. Here we tested this hypothesis using primary cultures of hippocampal neurons derived from cathepsin D-deficient (knock out) mice and expressing wild-type human APP and two clinical APP variants via recombinant Semliki Forest virus. We demonstrate APP secretory processing, production of carboxyl-terminal amyloid fragments, and secretion of the betaA4-amyloid peptide in the complete absence of cathepsin D. The results rule out cathepsin D as a critical component of alpha-, beta-, or gamma-secretase and therefore as a primary target for drugs aimed at decreasing the betaA4-amyloid peptide burden in Alzheimer's disease.

Reduced immunogenicity of monomethoxypolyethylene glycol-modified lysozyme for activation of T cells

Chemical modification of proteins with monomethoxypolyethylene glycol (mPEG) will reduce the immunogenicity of proteins. In the present study, we evaluated the effect of mPEG modification on the capacity of hen egg-white lysozyme (HEL) to stimulate T cells. Lymph node cells (LNCs) from mice immunized with HEL or with mPEG-HEL conjugate were cultured with these antigens, then we measured the proliferation and IL-2 production. mPEG-modification lowered the T cell activating capacity of HEL, both in vitro and in vivo. Neither toxicity, nor antigen non-specific immunosuppressive capacity was observed with mPEG-HEL and unconjugated mPEG. Suppressor cells were unlikely to be generated in the mPEG-HEL-primed LNCs. We next examined the behavior of mPEG-HEL during antigen processing. The capacity of HEL and mPEG-HEL to be incorporated by live cells was much the same. However, the susceptibility to various proteases, including endosomal/lysosomal enzymes, was significantly decreased by mPEG modification. The increased resistance of mPEG-HEL to proteolytic degradation implied that the conjugate was poorly presented to T cells. This may be an important factor related to the low immunogenicity of mPEG modified proteins.