H

Lysosome/Endosome

MHC class II a/ß heterodimer

Inva

Invariant chain

Endoplasmic^ Reticulum

Antigen Presenting Cell

Fig. 1. The MHC class II presentation pathway. MHC class II a/p heterodimers assemble in the endoplasmic reticulum (ER) with the assistance of invariant chain (Ii). The cytoplasmic tail of Ii contains a motif that targets the MHC class II:Ii complex to the endosomal/lysosomal pathway. Maturation of the early endosomes leads to activation of lysosomal enzymes, including cysteinal proteases, which degrade endogenous and exogenous internalized proteins and Ii. Following Ii cleavage, the MHC class II peptide-binding groove remains occupied by the class II-associated invariant chain peptide (CLIP), which prevents peptide loading. Removal of CLIP and loading of peptides is mediated by the MHC-like molecule H-2M (DM). These newly assembled peptide:MHC class II complexes then traffic to the plasma membrane

Cysteine Proteases (leupeptin sensitive) Aspartyl or Cysteine Protease (leupeptin insensitive)

Internalized Antigen

Invariant Chain

Internalized Antigen

Invariant Chain

CD4+T cells

Heterodimer p31-► p22 -^p18-► p12 -^ Presentation to

CD4+T cells

Heterodimer

Endoplasmic Reticulum

Endosomes/Lysosomes

Fig. 2. Invariant chain degradation. Invariant chain (Ii) is degraded in a step-wise manner in the endosomes. The initial cleavage is likely mediated by a leupeptin-insensitive cysteine protease or by an aspartyl protease, whereas subsequent steps are mediated by leupeptin-sensitive cysteine proteases. Upon proteolytic removal of the bulk of Ii, MHC class II peptide-binding groove remains occupied by Ii-derived CLIP peptide (class II associated Ii peptide). In addition to Ii, lysosomal proteases degrade internalized exogenous and endogenous proteins in the endosomal/lysosomal compartment. MHC-like molecule H-2M (DM) facilitates exchange of CLIP for the peptides generated by these enzymes

MHC class II presentation pathway, i.e., degradation of invariant chain and generation of MHC class II binding peptides.

5.3 Lysosomal Cysteine Proteases

Lysosomal proteases belonging to a diverse protease family, known as the cathepsins, are involved in a number of important cellular processes. A number of these enzymes are cysteine proteases with an acidic pH

optimum characteristic of the lysosomal compartment (Turk et al. 2000, 2001). Development of irreversible inhibitors binding to the active site cysteine residue allowed for utilization of their radio-labeled or biotiny-lated derivatives as specific probes to assess the levels of active cysteine proteases in different cell types, and even in different intracellular compartments (Bogyo et al. 2000; Driessen et al. 1999; Shi et al. 2000). Some lysosomal cysteine proteases, such as cathepsin B, are ubiquitously expressed in different tissues, whereas expression of others is restricted to certain tissue types (Chapman et al. 1997; Nakagawa and Rudensky 1999). Active cathepsin S (Cat S) can be detected in the thyroid and the spleen, but not in the thymic tissue, while active cathepsin L (Cat L) is found in kidney, liver, and thymic tissue but not in the spleen (Chapman et al. 1997; Nakagawa and Rudensky 1999; Villadangos et al. 1999). The distinct expression patterns of Cat S and Cat L activity in lymphoid tissues led us and others to further investigate the specific cell type distribution and to analyze the nonredundant function of these enzymes in MHC class II presentation pathway. In the course of these studies, we found Cat S activity in B cells and DCs, while Cat L activity was lacking in these cells. Cortical thymic epithelial cells revealed the opposite pattern of expression of these two enzymes upon investigation of mRNA, protein, and enzymatic activity using active site labeling. In contrast to the above cell types, M^s express both Cat S and Cat L activity (Nakagawa et al. 1998,1999; C. Beers and A. Rudensky, unpublished observations) (Fig. 3).

It has been anticipated that function of any given cathepsin is redundant, since none of the cathepsin-deficient mice studied so far exhibit a generalized defect in lysosomal protein degradation (Deussing et al. 1998; Nakagawa et al. 1998, 1999; Pham and Ley 1999; Roth et al. 2000; Saftig et al. 1998; Shi et al. 1999). However, our analysis of Cat L and Cat S knockouts as well as studies by others have demonstrated that these lysosomal cysteine proteases play a specific nonredundant function in the MHC class II pathway.

Expression of Active Cathepsin S and Cathepsin L in Professional APCs

(detected by 125labelled inhibitor active site labeling)

Cathepsin S

Cathepsin L

Macrophage

B cells

Splenic/thymic DCs +

cTECs

Fig. 3. Expression of Cathepsin S and Cathepsin L in professional antigen-presenting cells. Cysteine protease activity is detected using a substrate-analog inhibitor that covalently binds to the active site cysteine residue of the cathep-sins. Macrophages, B cells, splenic and thymic DCs, and cTECs were purified from wild-type mice and incubated with the 125I-labeled substrate-analog inhibitor for 3 h. Active cathepsin S and cathepsin L were detected by separating cellular lysates using 12% SDS/PAGE. 125I-labeled cathepsins were visualized by autoradiography

5.4 MHC Class II Invariant Chain Degradation by Lysosomal Cysteine Proteases

Biochemical analyses of Cat S-deficient mice revealed a prominent accumulation of MHC class II-associated Ii degradation intermediates, p12 and p18, in Cat S-deficient B cells and DCs (Nakagawa et al. 1999; Shi et al. 1999). Proteolytic conversion of p12 Ii into CLIP is required for efficient loading of antigenic peptides and surface delivery of MHC class II peptide complexes. Therefore, antigen presentation studies utilizing a panel of MHC class II-restricted T cell hybridomas showed that the impaired Ii degradation in Cat S-/- bone marrow-derived APCs was associated with diminished presentation of the majority of exogenous antigens tested as compared to wild-type mice. Despite a defect in antigen presentation by B cells and DCs, the numbers of CD4+ T cells remain normal in Cat S-/- mice (Nakagawa et al. 1998,1999).

In contrast to Cat S-deficient mice, B cells and DCs from Cat L-deficient mice exhibit normal Ii degradation and MHC class II antigen processing and presentation consistent with the expression pattern of these two enzymes. However, a profound defect in Ii degradation became apparent upon analysis of Ii degradation intermediates in the thymic epithelium and purified cortical TECs, cells that mediate positive selection of CD4+ T cells (Benavides et al. 2001; Nakagawa et al. 1998). These cells show impaired late-stage Ii degradation and a marked decrease in CD4+ T cell numbers to about one-quarter of those present in littermate wild type animals. Overall expression of MHC class II on the cell surface of cortical TECs is normal. However, a substantial proportion of these MHC class II molecules is bound to p12 Ii fragments. Furthermore, the accumulation of Ii intermediates p12 and p18 within the endosomal/lysosomal compartment most likely prevents the proper loading of endogenous peptides, thereby limiting positive selection (Benavides et al. 2001; Nakagawa et al. 1998).

These data indicate that both Cat S and Cat L are required for late stages of Ii degradation in professional APCs, yet they function in distinct cell types. Cat L is critical for Ii processing in cortical TECs, cells that mediate positive selection of immature double-positive thymocytes, while Cat S plays a similarly critical role in bone marrow-derived cells that induce immune responses in the periphery and mediate central and peripheral tolerance. It is puzzling to speculate as to why there are two lysosomal cysteine proteases with an apparently similar role in the MHC class II presentation pathway, albeit functioning in separate cell types. One explanation is that these enzymes may also be involved in the generation of peptide fragments presented by MHC class II molecules. It can be further speculated that differences in the peptide repertoires displayed by positively selecting thymic cortical epithelial cells and tolerogenic dendritic cells in the thymus and in the periphery may be essential for development of a sizable peripheral CD4 T cell compartment and/or for diminishing potential risk of autoimmunity.

5.5 Role of Cathepsin S and L in Generation of MHC Class II-Bound Peptides

As discussed above, cathepsins have been implicated in the processing of exogenous and endogenous proteins for presentation by MHC class II molecules to CD4+ T cells in addition to their role in late stages of Ii degradation.

We have taken a direct approach to determine the role of Cat S and Cat L in the generation of T cell epitopes by engineering fibroblast cell lines that express either Cat S or Cat L, or neither enzyme. Peptides were eluted from purified MHC class II molecules and analyzed using tandem mass spectrometry (Hsieh et al. 2002). It was observed that the majority of abundant peptides expressed by MHC class II molecules were not dependent upon expression of either Cat S or Cat L, although their relative expression varied in Cat S, Cat L, and control cells. However, a subset of MHC class II eluted peptides was significantly affected by the presence or absence of these enzymes. Specifically, the generation of some peptides required Cat S, while others were dependent on Cat L expression. The majority of peptides were generated in the presence of either Cat S or Cat L. In addition, the level of distinct subsets of MHC class II binding peptides was greatly diminished in the presence of Cat S or Cat L or both of these enzymes. In aggregate, studies utilizing these model APC lines indicate that Cat S and Cat L are able to influence endogenous MHC class II peptides.

By generating mice deficient in both Cat S or Cat L and Ii, the effect of these enzymes on generation of the MHC class II-binding peptide in vivo can be assessed directly as the lack of Ii abolishes the need to discriminate between the effects of these enzymes on the peptide processing from a defect in Ii degradation. The results of such an analysis indicate that Cat L influences the generation of T cell peptide epitope repertoire expressed by cortical TECs in vivo. Therefore, Cat L is essential not only for late stages of Ii degradation in thymic cortical epithelial cells, but also for generation of class II-bound peptides involved in positive selection of CD4 T cells. Cat S does not seem to have such a nonredundant role because overall peptide generation in DCs from mice lacking both Cat S and Ii appeared normal. However, generation of some peptide epitopes was affected. For example, Cat S was shown to destroy a known endogenously generated IgM T cell epitope (Honey et al. 2002; Hsieh et al. 2002). A separate study implicated Cat S in generating some, but not all hen egg lysozyme T cell epitopes in B cells (Pluger et al. 2002). In summary, these studies indicate that Cat L is involved in the generation of the majority of peptides involved in positive selection of CD4+ T cells in cortical TECs, while Cat S influences some, but not the bulk, of MHC class II-bound peptides presented by DCs and B cells.

5.6 Regulation of Cathepsin S and L Activity and Their Role in Ii Degradation in Macrophages

Since macrophages display both Cat S and Cat L activity, we attempted to further investigate the relative involvement of these enzymes in MHC class II presentation in these cells, specifically, their contribution to the late stages of Ii degradation. In our earlier studies, we have observed a role for Cat S in regulating MHC II presentation of some exogenous antigens by M^s (Nakagawa et al. 1999). In contrast, we failed to observe a detectable role for Cat L in invariant chain cleavage in IFN-y-stimulated macrophages. In more recent experiments employing biosynthetic pulse-chase labeling of Cat S-, Cat L-deficient, or Cat SCat L double-deficient M^s in combination with immunoprecipitation of MHC class II molecules, we confirmed that Cat S is the predominant enzyme processing Ii in these cells (Beers et al. 2003). Furthermore, we discovered that lack of a detectable role of Cat L in this process is due to a substantial downregulation of its enzymatic activity upon IFN-y stimulation. Although this decline in Cat L activity coincided with a decrease in Cat L mRNA and an increase in secretion of mature Cat L protein, the level of intracellular mature Cat L protein did not change in IFN-y stimulated pM^s. This finding suggests that Cat L activity is diminished due to induced expression of a Cat L-specific inhibitor. Previously, the p41 isoform of Ii was shown to exhibit Cat L-specific inhibitory activity due to the presence of a characteristic thyroglobulin-like domain. Since IFN-y stimulation results in a drastic upregulation of MHC class II and associated accessory molecules including Ii, p41 was a logical candidate responsible for the observed inhibition of Cat L activity. However, we found that the p41 isoform of Ii did not contribute significantly to downregulation of Cat L activity in pM^s as comparable inhibition of Cat L activity was also observed in Ii-deficient pM^s upon IFN-y induction. More recently, our preliminary studies implicate cystatin F as a specific inhibitor of Cat L activity in IFN-y activated pM^s (C. Beers and A. Rudensky, unpublished observations). These results indicate that upon activation of pM^s in response to pro-inflammatory stimuli such as IFN-y, enzymatic activity of Cat L is specifically inhibited, such that Cat S mediates Ii degradation and regulates MHC class II maturation. Notably, a similar phenomenon was observed in DCs isolated from mice expressing Cat L transgene driven by DC-specific CD11c promoter. In the latter cells, like in IFN-y-stimulated pM^s, very little Cat L activity was observed, while high levels of the mature form of Cat L protein were present (Beers et al. 2003). Thus, during Th1 immune response dominated by IFN-y production, inhibition of Cat L activity in macrophages results in preferential usage of cathepsin S for MHC class II presentation by all types of bone marrow derived antigen-presenting cells. This suggests that as a result of differential regulation of Cat L and Cat S, the latter enzyme governs MHC class II presentation by all bone marrow-derived APCs in secondary lymphoid organs.

These findings open up the possibility of manipulating the efficiency of antigen presentation by MHC class II molecules.

5.7 Role of Cathepsin S and L in MHC Class II Presentation by Nonprofessional Antigen-Presenting Cells

As mentioned above, MHC class II expression can also be induced by bacterial cell products and inflammatory cytokines on different types of nonprofessional APCs, including intestinal epithelium, fibroblasts, and endothelium (Hershberg and Mayer 2000; Sartor 1994). Interestingly, expression of MHC class II molecules on these nonprofessional APCs has been implicated in immune-mediated inflammation and progression of, or resistance to, autoimmunity (Hershberg and Blumberg 1999). In particular, it has been suggested that antigen presentation by intestinal epithelial cells (IECs) may play an important role in MHC class II-mediated presentation to immunoregulatory T cells (Campbell et al. 1999; Hershberg et al. 1997).

Epithelial cells at environmental interfaces provide protection from potentially harmful agents including pathogens. In addition to serving as a physical barrier and to producing soluble mediators of immunity such as antimicrobial peptides, these cells frequently function as "nonprofessional" antigen-presenting cells. In this regard, intestinal epithelial cells (IECs) are particularly prominent because they express MHC class II molecules at a site of massive antigenic exposure, where antigens could be presented to a large number of intraepithelial and lamina propria T lymphocytes expressing either yS or a^ TCR. However, unlike bone marrow-derived professional APCs such as dendritic cells or B cells, little is known about the mechanisms of MHC class II presentation by IEC. Since thymic cortical epithelial cells, but not bone marrow-derived APCs, employ Cat L for Ii degradation and MHC class II peptide generation, we anticipated finding a similar role for Cat L in intestinal epithelial cells. Unexpectedly, we detected Cat S activity in epithelial cells of the small and large intestine. In contrast, Cat L activity was missing despite the presence of high levels of mature Cat L protein, indicating that Cat L activity is downmodulated. Further analysis revealed that Cat S is crucial for Ii degradation and antigen presentation by IEC (Beers et al. 2005).

In aggregate, our studies demonstrate that in vivo both professional and nonprofessional MHC class II-expressing APC utilize Cat S, but not Cat L, for MHC class II-mediated antigen presentation. In contrast, thymic cortical epithelial cells involved in positive selection of thymocytes are making use of Cat L for Ii degradation and generation of positively selecting ligands for CD4 T cells. The biological significance of differential expression of these two enzymes in distinct types of antigen-presenting cells in vivo remains unknown and needs to be addressed in future studies.

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