Regulation of CD1 function and NK1.1(+) T cell selection and maturation by cathepsin S

Novel lipid antigens for NKT cells in cancer

Cancer immunotherapy aims to unleash the power of the immune system against tumors without the side effects of traditional chemotherapy. Immunotherapeutic methods vary widely, but all follow the same basic principle: overcome the barriers utilized by cancers to avoid immune destruction. These approaches often revolve around classical T cells, such as with CAR T cells and neoantigen vaccines; however, the utility of the innate-like iNKT cell in cancer immunotherapy has gained significant recognition. iNKT cells parallel classic T cell recognition of peptide antigens presented on MHC through their recognition of lipid antigens presented on the MHC I-like molecule CD1d. Altered metabolism and a lipogenic phenotype are essential properties of tumor cells, representing a unique feature that may be exploited by iNKT cells. In this review, we will cover properties of iNKT cells, CD1d, and lipid antigen presentation. Next, we will discuss the cancer lipidome and how it may be exploited by iNKT cells through a window of opportunity. Finally, we will review, in detail, novel lipid antigens for iNKT cells in cancer.

CD1-mediated immune responses in mucosal tissues: molecular mechanisms underlying lipid antigen presentation system

The cluster of differentiation 1 (CD1) molecule differs from major histocompatibility complex class I and II because it presents glycolipid/lipid antigens. Moreover, the CD1-restricted T cells that recognize these self and foreign antigens participate in both innate and adaptive immune responses. CD1s are constitutively expressed by professional and nonprofessional antigen-presenting cells in mucosal tissues, namely, the skin, lung, and intestine. This suggests that CD1-reactive T cells are involved in the immune responses of these tissues. Indeed, evidence suggests that these cells play important roles in diverse diseases, such as inflammation, autoimmune disease, and infection. Recent studies elucidating the molecular mechanisms by which CD1 presents lipid antigens suggest that defects in these mechanisms could contribute to the activities of CD1-reactive T cells. Thus, improving our understanding of these mechanisms could lead to new and effective therapeutic approaches to CD1-associated diseases. In this review, we discuss the CD1-mediated antigen presentation system and its roles in mucosal tissue immunity.

Involvement of Cathepsins in Innate and Adaptive Immune Responses in Periodontitis

Periodontitis is an infectious disease whereby the chronic inflammatory process of the periodontium stimulated by bacterial products induces specific host cell responses. The activation of the host cell immune system upregulates the production of inflammatory mediators, comprising cytokines and proteolytic enzymes, which contribute to inflammation and bone destruction. It has been well known that periodontitis is related to systemic inflammation which links to numerous systemic diseases, including diabetes and arteriosclerosis. Furthermore, periodontitis has been reported in association with neurodegenerative diseases such as Alzheimer's disease (AD) in the brain. Regarding immune responses and inflammation, cathepsin B (CatB) plays pivotal role for the induction of IL-1β, cathepsin K- (CatK-) dependent active toll-like receptor 9 (TLR9) signaling, and cathepsin S (CatS) which involves in regulating both TLR signaling and maturation of the MHC class II complex. Notably, both the production and proteolytic activities of cathepsins are upregulated in chronic inflammatory diseases, including periodontitis. In the present review, we focus on the roles of cathepsins in the innate and adaptive immune responses within periodontitis. We believe that understanding the roles of cathepsins in the immune responses in periodontitis would help to elucidate the therapeutic strategies of periodontitis, thus benefit for reduction of systemic diseases as well as neurodegenerative diseases in the global aging society.

Differential Role of Cathepsins S and B In Hepatic APC-Mediated NKT Cell Activation and Cytokine Secretion

Natural killer T (NKT) cells exhibit a specific tissue distribution, displaying the liver the highest NKT/conventional T cell ratio. Upon antigen stimulation, NKT cells secrete Th1 cytokines, including interferon γ (IFNγ), and Th2 cytokines, including IL-4 that recruit and activate other innate immune cells to exacerbate inflammatory responses in the liver. Cysteine cathepsins control hepatic inflammation by regulating κB-dependent gene expression. However, the contribution of cysteine cathepsins other than Cathepsin S to NKT cell activation has remained largely unexplored. Here we report that cysteine cathepsins, cathepsin B (CTSB) and cathepsin S (CTSS), regulate different aspects of NKT cell activation. Inhibition of CTSB or CTSS reduced hepatic NKT cell expansion in a mouse model after LPS challenge. By contrast, only CTSS inhibition reduced IFNγ and IL-4 secretion after in vivo α-GalCer administration. Accordingly, in vitro studies reveal that only CTSS was able to control α-GalCer-dependent loading in antigen-presenting cells (APCs), probably due to altered endolysosomal protein degradation. In summary, our study discloses the participation of cysteine cathepsins, CTSB and CTSS, in the activation of NKT cells in vivo and in vitro.

Interferon-γ treatment in vitro elicits some of the changes in cathepsin S and antigen presentation characteristic of lacrimal glands and corneas from the NOD mouse model of Sjögren's Syndrome

Inflammation and impaired secretion by lacrimal and salivary glands are hallmarks of the autoimmune disease, Sjögren’s Syndrome. These changes in the lacrimal gland promote dryness and inflammation of the ocular surface, causing pain, irritation and corneal damage. The changes that initiate and sustain autoimmune inflammation in the lacrimal gland are not well-established. Here we demonstrate that interferon-γ (IFN-γ) is significantly elevated in lacrimal gland and tears of the male NOD mouse, a model of autoimmune dacryoadenitis which exhibits many ocular characteristics of Sjögren’s Syndrome, by 12 weeks of age early in lacrimal gland inflammation. Working either with primary cultured lacrimal gland acinar cells from BALB/c mice and/or rabbits, in vitro IFN-γ treatment for 48 hr decreased expression of Rab3D concurrent with increased expression of cathepsin S. Although total cellular cathepsin S activity was not commensurately increased, IFN-γ treated lacrimal gland acinar cells showed a significant increase in carbachol-stimulated secretion of cathepsin S similar to the lacrimal gland in disease. In vitro IFN-γ treatment did not increase the expression of most components of major histocompatibility complex (MHC) class II-mediated antigen presentation although antigen presentation was slightly but significantly stimulated in primary cultured lacrimal gland acinar cells. However, exposure of cultured human corneal epithelial cells to IFN-γ more robustly increased expression and activity of cathepsin S in parallel with increased expression and function of MHC class II-mediated antigen presentation. We propose that early elevations in IFN-γ contribute to specific features of ocular disease pathology in Sjögren’s Syndrome.

Cathepsin S Is Involved in Th17 Differentiation Through the Upregulation of IL-6 by Activating PAR-2 after Systemic Exposure to Lipopolysaccharide from Porphyromonas gingivalis

Positive links have been found between periodontitis and numerous diseases in humans via persistent inflammation throughout the body. However, the main factors responsible for maintaining this pro-inflammatory condition are poorly understood. The spleen, the largest secondary immune organ, is a central hub regulating the immune response/inflammation due to the dendritic cell (DC) response to CD4⁺ T cell subtype differentiation, and lysosomal proteinase cathepsin S (CatS) is known to be involved in DC functions. In the present study, we found that CatS-induced IL-6 production by splenic DCs subsequently promotes Th17 differentiation, in response to systemic exposure to lipopolysaccharide derived from Porphyromonas gingivalis (PgLPS). The population of CD11c⁺ DCs was significantly increased in the splenic marginal zone (MZ) locally of wild-type (DBA/2) mice with splenomegaly but not in that of CatS deficient (CatS^-/-) mice after systemic exposure to PgLPS for 7 consecutive days (5 mg/kg/day, intraperitoneal). Similarly, the population of Th17⁺CD4⁺ T cells was also significantly increased in the splenic MZ of wild-type mice but not in that of CatS^-/- mice after PgLPS exposure. Furthermore, the increase in the Th17⁺ CD4⁺ T cell population paralleled increases in the levels of CatS and IL-6 in CD11c⁺ cells in the splenic MZ. In isolated primary splenic CD11c⁺ cells, the mRNA expression and the production of IL-6 was dramatically increased in wild-type mice but not in CatS^-/- mice after direct stimulation with PgLPS (1 μg/ml), and this PgLPS-induced increase in the IL-6 expression was completely abolished by pre-treatment with Z-Phe-Leu-COCHO (Z-FL), the specific inhibitor of CatS. The PgLPS activated protease-activated receptor (PAR) 2 in the isolated splenic CD11c⁺ cells was also significantly inhibited by CatS deficiently. In addition, the PgLPS-induced increase in the IL-6 production by splenic CD11c⁺ cells was completely abolished by pre-treatment with FSLLRY-NH₂, a PAR2 antagonist, as well as Akti, a specific inhibitor of Akt. These findings indicate that CatS plays a critical role in driving splenic DC-dependent Th17 differentiation through the upregulation of IL-6 by activating PAR2 after exposure to components of periodontal bacteria. Therefore, CatS-specific inhibitors may be effective in alleviating periodontitis-related immune/inflammation.

Zerumbone modulates CD1d expression and lipid antigen presentation pathway in breast cancer cells

Natural Killer T (NKT) cells based cancer immunotherapy is an evolving area of cancer therapy, but tumors escape from this treatment modality by altering CD1d expression and its antigen presentation pathway. Here, we have studied the relation of CD1d expression in various breast cancer cell lines to their viability and progression. We observed a novel phenomenon that CD1d expression level increases with the progressive stage of the cancer. A small molecule, zerumbone (ZER) caused down-regulation of CD1d that was accompanied by breast cancer cell growth in vitro. The growth inhibitory effect of ZER against breast cancer cells was augmented by treatment with anti-CD1d mAb. This effect was mediated by G1-phase cell cycle arrest and apoptosis induction coupled with an increase in mitochondrial membrane depolarization. CD1d expression and cell proliferation were inhibited by both CD1d siRNA and ZER. The α-galactosylceramide, a ligand for CD1d, showed increased CD1d expression as well as cell proliferation which was opposite to the effects of ZER. This study shows that, CD1d overexpression is associated with the progressive stages of breast cancer and ZER could be an adjuvant to potentiate cancer immunotherapy.

CD74 Deficiency Mitigates Systemic Lupus Erythematosus-like Autoimmunity and Pathological Findings in Mice

CD74 mediates MHC class-II antigenic peptide loading and presentation and plays an important role in the pathogenesis of autoimmune diseases, including systemic lupus erythematosus. C57BL/6 Fas^lpr mice that develop spontaneous lupus-like autoimmunity and pathology showed elevated CD74 expression in the inflammatory cell infiltrates and the adjacent tubular epithelial cells (TECs) in kidneys affected by lupus nephritis but negligible levels in kidneys from age-matched wild-type mice. The inflammatory cytokine IFN-γ or IL-6 induced CD74 expression in kidney TECs in vitro. The presence of kidney TECs from Fas^lpr mice, rather than from wild-type mice, produced significantly stronger histones, dsDNA, and ribonucleoprotein-Smith Ag complex-induced CD4⁺ T cell activation. Splenocytes from CD74-deficient Fas^lprCd74^-/- mice had muted responses in a MLR and to the autoantigen histones. Compared with Fas^lprCd74^+/+ mice, Fas^lprCd74^-/- mice had reduced kidney and spleen sizes, splenic activated T cells and B cells, serum IgG and autoantibodies, urine albumin/creatinine ratio, kidney Periodic acid-Schiff score, IgG and C3 deposition, and serum IL-6 and IL-17A levels, but serum IL-2 and TGF-β levels were increased. Study of chronic graft-versus-host C57BL/6 mice that received donor splenocytes from B6.C-H2^bm12 /KhEg mice and those that received syngeneic donor splenocytes yielded similar observations. CD74 deficiency reduced lupus-like autoimmunity and kidney pathology in chronic graft-versus-host mice. This investigation establishes the direct participation of CD74 in autoimmunity and highlights a potential role for CD74 in kidney TECs, together with professional APCs in systemic lupus erythematosus.

Peripheral role of cathepsin S in Th1 cell-dependent transition of nerve injury-induced acute pain to a chronic pain state

There is increasing evidence that CD4(+) T-cell-dependent responses are associated with the maintenance of neuropathic pain. However, little is known about the precise mechanism(s) underlying the activation of CD4(+) T-cells. We herein show that inhibition of cathepsin S (CatS) activity, either through genetic deletion or via a pharmacological inhibitor, Z-Phe-Leu-COCHO (Z-FL), significantly attenuated the maintenance of tactile allodynia, splenic hypertrophy, increased number of splenic CD4(+) T-cells and the final cleavage step of the MHC class II-associated invariant chain following peripheral nerve injury. It was also noted that splenectomy significantly attenuated the peripheral nerve injury-induced tactile allodynia, whereas the adoptive transfer of splenic CD4(+) T-cells from neuropathic wild-type mice significantly increased the pain level of splenectomized wild-type or CatS(-/-) mice. Furthermore, CatS deficiency or Z-FL treatment also significantly inhibited the infiltration of CD4(+) T-cells that expressed interferon-γ (IFN-γ) in the dorsal spinal cord. Signal transducer and activator of transcription 1, a molecule downstream of IFN-γ receptor activation, was activated exclusively in microglia 7 d after peripheral nerve injury. Moreover, CatS deficiency, Z-FL treatment, or splenectomy significantly attenuated the proliferation of microglia 14 d after peripheral nerve injury. These results show a peripheral pivotal role of CatS in the development of neuropathic pain through the antigen-specific activation of CD4(+) T-cells. After activation, CD4(+) T-cells infiltrate into the dorsal spinal cord and secrete IFN-γ to reactivate microglia, which contribute to the transition of acute pain to a chronic pain state.

Novel insights into lipid antigen presentation

T cells recognizing lipid antigens are present in large numbers in circulating blood. They exert multiple functions including immunoregulation, tumour surveillance and protection during infection. Here, we review the latest information on the mechanisms of lipid antigen presentation by CD1 molecules. Recent studies have provided insight into CD1 trafficking within the cell, lipid distribution and handling, CD1 maturation, lipid antigen processing and loading. The structural resolution of all human CD1 molecules has revealed unique features that correlate with function. Molecular mechanisms regulating CD1 expression and multiple evasion mechanisms evolved by viral and bacterial pathogens have been disclosed. With rapid progression, these studies have decoded lipid-specific immunity and have revealed the important immunological role of this type of antigen recognition.

Critical role for invariant chain in CD1d-mediated selection and maturation of Vα14-invariant NKT cells

The development and maturation of Vα14 invariant (i)NKT cells in mice requires CD1d-mediated lipid antigen presentation in the thymus and the periphery. Cortical thymocytes mediate positive selection, while professional APCs are involved in thymic negative selection and in terminal maturation of iNKT cells in the periphery. CD1d requires entry in the endosomal pathway to allow antigen acquisition for assembly as lipid/CD1d complexes for display to iNKT cells. This process involves tyrosine-based sorting motifs in the CD1d cytoplasmic tail and invariant chain (Ii) that CD1d associates with in the endoplasmic reticulum. The function of Ii in iNKT cell thymic development and peripheral maturation had not been fully understood. Using mice deficient in Ii and the Ii-processing enzyme cathepsin S (catS), we addressed this question. Ii(-/-) mice but not catS(-/-) mice developed significantly fewer iNKT cells in thymus, that were less mature as measured by CD44 and NK1.1 expression. Ii(-/-) mice but not catS(-/-) mice developed fewer Vβ7(+) cells in their iNKT TCR repertoire than WT counterparts, indicative of a change in endogenous glycolipid antigen/CD1d-mediated iNKT cell selection. Finally, using a Mycobacterium tuberculosis infection model in macrophages, we show that iNKT developed in Ii(-/-) but not catS(-/-) mice have defective effector function. Our data support a role for professional APCs expressing Ii, but no role for catS in the thymic development and peripheral terminal maturation of iNKT cells.

Deficiency of antigen-presenting cell invariant chain reduces atherosclerosis in mice

BACKGROUND

Adaptive immunity and innate immunity play important roles in atherogenesis. Invariant chain (CD74) mediates antigen-presenting cell antigen presentation and T-cell activation. This study tested the hypothesis that CD74-deficient mice have reduced numbers of active T cells and resist atherogenesis.

METHODS AND RESULTS

In low-density lipoprotein receptor-deficient (Ldlr(-/-)) mice, CD74 deficiency (Ldlr(-/-)Cd74(-/-)) significantly reduced atherosclerosis and CD25(+)-activated T cells in the atheromata. Although Ldlr(-/-)Cd74(-/-) mice had decreased levels of plasma immunoglobulin (Ig) G1, IgG2b, and IgG2c against malondialdehyde-modified LDL (MDA-LDL), presumably as a result of impaired antigen-presenting cell function, Ldlr(-/-)Cd74(-/-) mice showed higher levels of anti-MDA-LDL IgM and IgG3. After immunization with MDA-LDL, Ldlr(-/-)Cd74(-/-) mice had lower levels of all anti-MDA-LDL Ig isotypes compared with Ldlr(-/-) mice. As anticipated, only Ldlr(-/-) splenocytes responded to in vitro stimulation with MDA-LDL, producing Th1/Th2 cytokines. Heat shock protein-65 immunization enhanced atherogenesis in Ldlr(-/-) mice, but Ldlr(-/-) Cd74(-/-) mice remained protected. Compared with Ldlr(-/-) mice, Ldlr(-/-)Cd74(-/-) mice had higher anti-MDA-LDL autoantibody titers, fewer lesion CD25(+)-activated T cells, impaired release of Th1/Th2 cytokines from antigen-presenting cells after heat shock protein-65 stimulation, and reduced levels of all plasma anti-heat shock protein-65 Ig isotypes. Cytofluorimetry of splenocytes and peritoneal cavity cells of MDA-LDL- or heat shock protein-65-immunized mice showed increased percentages of autoantibody-producing marginal zone B and B-1 cells in Ldlr(-/-)Cd74(-/-) mice compared with Ldlr(-/-) mice.

CONCLUSIONS

Invariant chain deficiency in Ldlr(-/-) mice reduced atherosclerosis. This finding was associated with an impaired adaptive immune response to disease-specific antigens. Concomitantly, an unexpected increase in the number of innate-like peripheral B-1 cell populations occurred, resulting in increased IgM/IgG3 titers to the oxidation-specific epitopes.

Diverse regulatory roles for lysosomal proteases in the immune response

The innate and adaptive immune system utilise endocytic protease activity to promote functional immune responses. Cysteine and aspartic proteases (cathepsins) constitute a subset of endocytic proteases, the immune function of which has been described extensively. Although historically these studies have focused on their role in processes such as antigen presentation and zymogen processing within the endocytic compartment, recent discoveries have demonstrated a critical role for these proteases in other intracellular compartments, and within the extracellular milieu. It has also become clear that their pattern of expression and substrate specificities are more diverse than was first envisaged. Here, we discuss recent advances addressing the role of lysosomal proteases in various aspects of the immune response. We pay attention to reports demonstrating cathepsin activity outside of its canonical endosome/lysosome microenvironment.

Genetic and pharmacologic alteration of cathepsin expression influences reovirus pathogenesis

The cathepsin family of endosomal proteases is required for proteolytic processing of several viruses during entry into host cells. Mammalian reoviruses utilize cathepsins B (Ctsb), L (Ctsl), and S (Ctss) for disassembly of the virus outer capsid and activation of the membrane penetration machinery. To determine whether cathepsins contribute to reovirus tropism, spread, and disease outcome, we infected 3-day-old wild-type (wt), Ctsb(-/-), Ctsl(-/-), and Ctss(-/-) mice with the virulent reovirus strain T3SA+. The survival rate of Ctsb(-/-) mice was enhanced in comparison to that of wt mice, whereas the survival rates of Ctsl(-/-) and Ctss(-/-) mice were diminished. Peak titers at sites of secondary replication in all strains of cathepsin-deficient mice were lower than those in wt mice. Clearance of the virus was delayed in Ctsl(-/-) and Ctss(-/-) mice in comparison to the levels for wt and Ctsb(-/-) mice, consistent with a defect in cell-mediated immunity in mice lacking cathepsin L or S. Cathepsin expression was dispensable for establishment of viremia, but cathepsin L was required for maximal reovirus growth in the brain. Treatment of wt mice with an inhibitor of cathepsin L led to amelioration of reovirus infection. Collectively, these data indicate that cathepsins B, L, and S influence reovirus pathogenesis and suggest that pharmacologic modulation of cathepsin activity diminishes reovirus disease severity.

CD1c bypasses lysosomes to present a lipopeptide antigen with 12 amino acids

The recent discovery of dideoxymycobactin (DDM) as a ligand for CD1a demonstrates how a nonribosomal lipopeptide antigen is presented to T cells. DDM contains an unusual acylation motif and a peptide sequence present only in mycobacteria, but its discovery raises the possibility that ribosomally produced viral or mammalian proteins that commonly undergo lipidation might also function as antigens. To test this, we measured T cell responses to synthetic acylpeptides that mimic lipoproteins produced by cells and viruses. CD1c presented an N-acyl glycine dodecamer peptide (lipo-12) to human T cells, and the response was specific for the acyl linkage as well as the peptide length and sequence. Thus, CD1c represents the second member of the CD1 family to present lipopeptides. lipo-12 was efficiently recognized when presented by intact cells, and unlike DDM, it was inactivated by proteases and augmented by protease inhibitors. Although lysosomes often promote antigen presentation by CD1, rerouting CD1c to lysosomes by mutating CD1 tail sequences caused reduction in lipo-12 presentation. Thus, although certain antigens require antigen processing in lysosomes, others are destroyed there, providing a hypothesis for the evolutionary conservation of large CD1 families containing isoforms that survey early endosomal pathways.

Leukocyte cathepsin S is a potent regulator of both cell and matrix turnover in advanced atherosclerosis

OBJECTIVE

A dysbalance of proteases and their inhibitors is instrumental in remodeling of atherosclerotic plaques. One of the proteases implicated in matrix degradation is cathepsin-S (CatS). To address its role in advanced lesion composition, we generated chimeric LDLr(-/-) mice deficient in leukocyte CatS by transplantation with CatS(-/-)xLDLr(-/-) or with LDLr(-/-) bone marrow and administered a high-fat diet.

METHODS AND RESULTS

No difference in aortic root lesion size could be detected between CatS(+/+) and CatS(-/-) chimeras. However, leukocyte CatS deficiency markedly changed plaque morphology and led to a dramatic reduction in necrotic core area by 77% and an abundance of large foam cells. Plaques of CatS(-/-) chimeras contained 17% more macrophages, 62% less SMCs, and 33% less intimal collagen. The latter two could be explained by a reduced number of elastic lamina fractures. Moreover, macrophage apoptosis was reduced by 60% with CatS deficiency. In vitro, CatS was found to be involved in cholesterol metabolism and in macrophage apoptosis in a collagen and fibronectin matrix.

CONCLUSIONS

Leukocyte CatS deficiency results in considerably altered plaque morphology, with smaller necrotic cores, reduced apoptosis, and decreased SMC content and collagen deposition and may thus be critical in plaque stability.

CD1d-restricted glycolipid antigens: presentation principles, recognition logic and functional consequences

Invariant natural killer T (iNKT) cells are innate lymphocytes whose functions are regulated by self and foreign glycolipid antigens presented by the antigen-presenting molecule CD1d. Activation of iNKT cells in vivo results in rapid release of copious amounts of effector cytokines and chemokines with which they regulate innate and adaptive immune responses to pathogens, certain types of cancers and self-antigens. The nature of CD1d-restricted antigens, the manner in which they are recognised and the unique effector functions of iNKT cells suggest an innate immunoregulatory role for this subset of T cells. Their ability to respond fast and our ability to steer iNKT cell cytokine response to altered lipid antigens make them an important target for vaccine design and immunotherapies against autoimmune diseases. This review summarises our current understanding of CD1d-restricted antigen presentation, the recognition of such antigens by an invariant T-cell receptor on iNKT cells, and the functional consequences of these interactions.

Phage display-derived recombinant antibodies with TCR-like specificity against alpha-galactosylceramide and its analogues in complex with human CD1d molecules

The glycolipid alpha-galactosylceramide (alpha-GalCer) is a potent activator of invariant natural killer T (iNKT) cells and has been shown to be an effective agent against cancer, infections and autoimmune diseases. The effectiveness of alpha-GalCer and its alkyl chain analogues depends on efficient loading and presentation by the antigen-presenting molecule CD1d. To monitor the ability of CD1d to present the glycolipids, we have used a phage display strategy to generate recombinant antibodies with T cell receptor-like (TCRL) specificity against the human CD1d (hCD1d)-alpha-GalCer complex. These Fab fragments were able to detect specifically hCD1d-alpha-GalCer complexes in cell-free systems such as surface plasmon resonance and ELISA, as well as on the surface of hCD1d(+) antigen-presenting cells (APC) by flow cytometry and immunofluorescence microscopy, the latter of which could also detect intracellular complexes. We show that our TCRL antibodies can stain dendritic cells from CD11c-hCD1d-transgenic mice administered in vivo with alpha-GalCer and its analogues. Furthermore, the antibody was also able to detect the presentation by hCD1d molecules of analogues of alpha-GalCer with the same polar head structure. Using this reagent, we were able to confirm directly that the alpha-GalCer analogue C20:2 preferentially loads onto cell surface CD1d rapidly without the need for internalization, while the loading of alpha-GalCer is improved with longer incubation times on professional APC. This reagent will be essential for assessing the loading and presenting capabilities of hCD1d of alpha-GalCer and its analogues.

Cysteine cathepsins: cellular roadmap to different functions

Cysteine cathepsins belong to the papain-like family C1 of clan CA cysteine peptidases. These enzymes are ubiquitously expressed and exert their proteolytic activity mainly, but not exclusively within the compartments along the endocytic pathway. Moreover, cysteine cathepsins are active in pericellular environments as soluble enzymes or bound to cell surface receptors at the plasma membrane, and possibly even within secretory vesicles, the cytosol, mitochondria, and within the nuclei of eukaryotic cells. Proteolytic actions performed by cysteine cathepsins are essential in the maintenance of homeostasis and depend heavily upon their correct sorting and trafficking within cells. As a consequence, the numerous and diverse approaches to identification, qualitative and quantitative determination, and visualization of cysteine cathepsin functions in vitro, in situ, and in vivo cover the entire spectrum of biochemistry, molecular and cell biology. This review focuses upon the transport pathways directing cysteine cathepsins to their points of action and thus emphasizes the broader role and functionality of cysteine cathepsins in a number of specific cellular locales. Such understanding will provide a foundation for future research investigating the involvement of these peptidases with their substrates, inhibitors, and the intertwined proteolytic networks at the hubs of complex biological systems.

Pathways of CD1 and lipid antigen delivery, trafficking, processing, loading, and presentation

Specific T cell responses to a variety of self and microbial lipids depend on proper assembly and intracellular trafficking of CD 1 molecules that intersect with and load processed lipid antigens. These pathways involve unique membrane trafficking and chaperones that are distinct from those utilized for major histocompatibility complex (MHC)-mediated presentation of peptide antigens, and thus define unique lipid antigen presentation pathways. Furthermore, recent studies have identified components of lipid metabolism that participate in lipid delivery, uptake, processing and loading onto CD1 molecules. Defects in these pathways result in impaired T cell development and function, underscoring their critical role in the lipid-specific T cell immune responses.

Efficient activation of Valpha14 invariant NKT cells by foreign lipid antigen is associated with concurrent dendritic cell-specific self recognition

A burst release of cytokines by Valpha14 invariant NKT (iNKT) cells upon their TCR engagement critically regulates innate and adaptive immune responses. However, it remains unclear in vivo why iNKT cells respond efficiently to microbial or intracellular lipid Ags that are at low levels or that possess suboptimal antigenicity. We found that dendritic cells (DCs) potentiated iNKT cells to respond to a minimal amount of ligand alpha-galactosylceramide (alphaGalCer) through CD1d-dependent autoreactive responses that require endosomal processing and CD1d trafficking. The ability of potentiation of NKT cells was DC specific and did not depend on costimulatory signals and IL-12 production by DCs. However, DCs that failed to synthesize a major endogenous lipid Ag isoglobotrihexosylceramide were unable to potentiate NKT cells for efficient activation. Further analysis showed that differences in the level and pattern of endogenous lipid Ag presentation differentiate DCs and B cells for effective potentiation and subsequent activation of iNKT cells in the presence of an exogenous Ag. Thus, CD1d-dependent potentiation by DCs may be crucial for iNKT cell-mediated immunity against infectious agents.

MTP regulated by an alternate promoter is essential for NKT cell development

Microsomal triglyceride transfer protein (MTP), an endoplasmic reticulum lipid transfer protein critical for apolipoprotein B (apoB) secretion, regulates CD1d antigen presentation. We identified MTP variant 1 (MTPv1), a novel splice variant of mouse MTP, by polymerase chain reaction and Northern analysis in non-apoB-secreting tissues, including thymocytes and antigen-presenting cells (APCs). Edman degradation of MTPv1 isolated from transfected cells revealed three unique residues; however, recombinant MTP and MTPv1 had an equivalent protein disulfide isomerase association, subcellular localization, triglyceride transfer, phospholipid transfer, response to inhibitors, and ability to support apoB secretion. MTP and MTPv1 efficiently transferred phosphatidylethanolamine to CD1d in vitro. NKT cells fail to develop in fetal thymic organ culture (FTOC) treated with MTP antagonists. MTP-inhibited FTOCs produced negligible numbers of CD1d tetramer-positive cells and exhibited marked defects in IL-4 production upon stimulation with anti-CD3 or alpha-galactosylceramide-pulsed APCs. CD1d expression on CD4(+)CD8(+) FTOC cells was unaffected by MTP inhibition. Thus, our results demonstrate that MTPv1 in thymocytes is critical to NKT cell development. We hypothesize that, when MTP is inactive, CD1d traffics to the cell surface and presents no lipid or a lipid that is incapable of mediating NKT cell selection and/or is refractory to lysosomal editing.

Endosomal proteases in antigen presentation

Processing of cytoplasmic and endocytized foreign antigens were once thought to be completely separate pathways of peptide generation for MHC class I and class II peptide display. However, recent studies show actually that there is extensive overlap owing, in part, to the positioning of endosomal proteases at sites in which MHC class I and class II paths cross. The questions of how antigens access and persist at sites of endosomal processing are proving to be as important as those regarding the proteases themselves. However, individual proteases within the set of endosomal enzymes have emerged both as targets for microbial evasion from MHC-class-II-dependent immunity and as required elements in autoimmunity. Repeated implication of cathepsins in processing medically important antigens holds promise for therapeutic manipulation of proteases in diseases in which there is disordered adaptive immunity.

The lysosomal cysteine proteases in MHC class II antigen presentation

The endosomal pathway of antigen presentation leads to the display of peptides on major histocompatibility complex (MHC) class II molecules at the cell surface of antigen-presenting cells (APCs). The pathway involves two major steps, invariant chain degradation and antigen processing, which take place in the late endosomes/lysosomes. So far, of the known lysosomal proteases, only cathepsin L and cathepsin S have been shown to have a non-redundant role in endosomal presentation in vivo. Besides being engaged in the degradation of invariant chain, these enzymes also mediate the processing of antigens in distinct cell types. Surprisingly, these enzymes are active in different types of APCs, and this defined expression pattern seems to be enforced by regulatory mechanisms acting on multiple levels.

The crystal structure of human CD1d with and without alpha-galactosylceramide

The glycolipid alpha-galactosylceramide binds with high affinity to CD1d and stimulates natural killer T cells. Here we report the crystal structure of human CD1d in complex with synthetic alpha-galactosylceramide at a resolution of 3.0 A. The structure shows a tightly fit lipid in the CD1d binding groove, with the sphingosine chain bound in the C' pocket and the longer acyl chain anchored in the A' pocket. We also present the CD1d structure without lipid, which has a more open conformation of the binding groove, suggesting a dual conformation of CD1d in which the 'open' conformation is more able to load lipids. These structures provide clues as to how CD1 molecules load glycolipids as well as data to guide the design of new therapeutic agents.

The human CD1-restricted T cell repertoire is limited to cross-reactive antigens: implications for host responses against immunologically related pathogens

The repertoires of CD1- and MHC-restricted T cells are complementary, permitting the immune recognition of both lipid and peptide Ags, respectively. To compare the breadth of the CD1-restricted and MHC-restricted T cell repertoires, we evaluated T cell responses against lipid and peptide Ags of mycobacteria in leprosy, comparing tuberculoid patients, who are able to restrict the pathogen, and lepromatous patients, who have disseminated infection. The striking finding was that in lepromatous leprosy, T cells did not efficiently recognize lipid Ags from the leprosy pathogen, Mycobacterium leprae, or the related species, Mycobacterium tuberculosis, yet were able to efficiently recognize peptide Ags from M. tuberculosis, but not M. leprae. To identify a mechanism for T cell unresponsiveness against mycobacterial lipid Ags in lepromatous patients, we used T cell clones to probe the species specificity of the Ags recognized. We found that the majority of M. leprae-reactive CD1-restricted T cell clones (92%) were cross-reactive for multiple mycobacterial species, whereas the majority of M. leprae-reactive MHC-restricted T cells were species specific (66%), with a limited number of T cell clones cross-reactive (34%) with M. tuberculosis. In comparison with the MHC class II-restricted T cell repertoire, the CD1-restricted T cell repertoire is limited to recognition of cross-reactive Ags, imparting a distinct role in the host response to immunologically related pathogens.

Cathepsin S is required for murine autoimmune myasthenia gravis pathogenesis

Because presentation of acetylcholine receptor (AChR) peptides to T cells is critical to the development of myasthenia gravis, we examined the role of cathepsin S (Cat S) in experimental autoimmune myasthenia gravis (EAMG) induced by AChR immunization. Compared with wild type, Cat S null mice were markedly resistant to the development of EAMG, and showed reduced T and B cell responses to AChR. Cat S null mice immunized with immunodominant AChR peptides showed weak responses, indicating failed peptide presentation accounted for autoimmune resistance. A Cat S inhibitor suppressed in vitro IFN-gamma production by lymph node cells from AChR-immunized, DR3-bearing transgenic mice. Because Cat S null mice are not severely immunocompromised, Cat S inhibitors could be tested for their therapeutic potential in EAMG.

Regulation of Antigen Presentation and Cross-Presentation in the Dendritic Cell Network: Facts, Hypothesis, and Immunological Implications

Dendritic cells (DCs) are central to the maintenance of immunological tolerance and the initiation and control of immunity. The antigen-presenting properties of DCs enable them to present a sample of self and foreign proteins, contained within an organism at any given time, to the T-cell repertoire. DCs achieve this communication with T cells by displaying antigenic peptides bound to MHC I and MHC II molecules. Here we review the studies carried out over the past 15 years to characterize these antigen presentation mechanisms, emphasizing their significance in relation to DC function in vivo. The life cycles of different DC populations found in vivo are described. Furthermore, we provide a critical assessment of the studies that examine the mechanisms controlling DC MHC class II antigen presentation, which have often reached contradictory conclusions. Finally, we review findings pertaining to the biological mechanisms that enable DCs to present exogenous antigens on their MHC class I molecules, a process known as cross-presentation. Throughout, we highlight what we consider to be major knowledge gaps in the field and speculate on possible directions for future research.

Natural Killer T Cells in Mucosal Homeostasis

The mucosal-associated lymphoid tissues (MALT), including the gut-associated lymphoid tissues, are a tightly regulated environment. In fact, it might be stated that on the basis of studies from animal models of inflammatory bowel disease (IBD), the major means of peripheral regulation of immune responses in the intestine is not necessarily from processes such as deletion or anergy, but more likely from the controls imposed upon responses due to the activities of a variety of regulatory subsets of cells. One type of regulatory cellular subset that has recently gained attention is the subset of T cells that are associated with CD1d-restricted responses. Recently, CD1d-restricted T cells have been increasingly appreciated to play a significant role in mucosal tissues of the intestine and lung, for example. Insights from these studies have clearly elevated these cells to particular importance in the regulation of a variety of infectious and inflammatory conditions, such as those associated with idiopathic IBD. In this review, we focus on recent observations on the characteristics of CD1d-restricted pathways in mucosal compartments, after a brief introduction into the biology of CD1d and CD1d-restricted T cells.

New insights into pathways for CD1-mediated antigen presentation

Recent studies of CD1 structure and intracellular trafficking have demonstrated significant differences among the CD1 isoforms (CD1a, CD1b, CD1c and CD1d). The molecular and structural basis for the differential trafficking of CD1 molecules has also been delineated. These observations broaden our understanding of why the immune system has evolved multiple CD1 isoforms to survey different cellular compartments for lipid antigen presentation, to provide host defense against the microbial world and to offer immunoregulation with relevance to tumor immunity and autoimmunity.

The exogenous pathway for antigen presentation on major histocompatibility complex class II and CD1 molecules

The endosomes and lysosomes of antigen-presenting cells host the processing and assembly reactions that result in the display of peptides on major histocompatibility complex (MHC) class II molecules and lipid-linked products on CD1 molecules. This environment is potentially hostile for T cell epitope and MHC class II survival, and the influence of regulators of protease activity and specialized chaperones that assist MHC class II assembly is crucial. At present, evidence indicates that individual proteases make both constructive and destructive contributions to antigen processing for MHC class II presentation to CD4 T cells. Some features of CD1 antigen capture within the endocytic pathway are also discussed.

CD1: antigen presentation and T cell function

This review summarizes the major features of CD1 genes and proteins, the patterns of intracellular trafficking of CD1 molecules, and how they sample different intracellular compartments for self- and foreign lipids. We describe how lipid antigens bind to CD1 molecules with their alkyl chains buried in hydrophobic pockets and expose their polar lipid headgroup whose fine structure is recognized by the TCR of CD1-restricted T cells. CD1-restricted T cells carry out effector, helper, and adjuvant-like functions and interact with other cell types including macrophages, dendritic cells, NK cells, T cells, and B cells, thereby contributing to both innate and adaptive immune responses. Insights gained from mice and humans now delineate the extensive range of diseases in which CD1-restricted T cells play important roles and reveal differences in the role of CD1a, CD1b, and CD1c in contrast to CD1d. Invariant TCR alpha chains, self-lipid reactivity, and rapid effector responses empower a subset of CD1d-restricted T cells (NKT cells) to have unique effector functions without counterpart among MHC-restricted T cells. This review describes the function of CD1-restricted T cells in antimicrobial responses, antitumor immunity, and in regulating the balance between tolerance and autoimmunity.

Lysosomal cysteine proteases in atherosclerosis

Atherosclerosis is an inflammatory disease characterized by extensive remodeling of the extracellular matrix architecture of the arterial wall. Although matrix metalloproteinases and serine proteases participate in these pathologic events, recent data from atherosclerotic patients and animals suggest the participation of lysosomal cysteine proteases in atherogenesis. Atherosclerotic lesions in humans overexpress the elastolytic and collagenolytic cathepsins S, K, and L but show relatively reduced expression of cystatin C, their endogenous inhibitor, suggesting a shift in the balance between cysteine proteases and their inhibitor that favors remodeling of the vascular wall. Extracts of human atheromatous tissue show greater elastolytic activity in vitro than do those from healthy donors. The cysteinyl protease inhibitor E64d limits this increased elastolysis, indicating involvement of cysteine proteases in elastin degradation during atherogenesis. Furthermore, inflammatory cytokines augment expression and secretion of active cysteine proteases from cultured monocyte-derived macrophages, vascular smooth muscle cells, and endothelial cells and increase degradation of extracellular elastin and collagen. Cathepsin S-deficient cells or those treated with E64d show significantly impaired elastolytic or collagenolytic activity. Additionally, recent in vivo studies of atherosclerosis-prone, LDL receptor-null mice lacking cathepsin S show participation of this enzyme in the initial infiltration of leukocytes, medial elastic lamina degradation, endothelial cell invasion, and neovascularization, illustrating an important role for cysteine proteases in arterial remodeling and atherogenesis.

Parallels and distinctions between T and NKT cell development in the thymus

NKT cells are emerging as an extremely influential regulatory subset of T lymphocytes that are functionally and developmentally distinct from their mainstream counterparts. Like other T cells, NKT cells are thymus-dependent but their apparently unique pathway of differentiation is poorly characterized. Given the strong association between NKT cell deficiency and increased incidences of autoimmunity and cancer it is imperative that the mechanisms by which NKT cells are generated becomes better understood. This review examines what is known about NKT cell development in the thymus and highlights elements of the pathway that differ significantly from mainstream T-cell development. It is here that NKT cell-specific disorders may originate and may best be addressed.

The Third Way: Progress on pathways of antigen processing and presentation by CD1

CD1 proteins are a third family of antigen presenting molecules that bind bacterial and autologous lipid antigens for presentation to T cells. With the solution of the crystal structures of several complexes of CD1 molecules with lipids, a greater appreciation has been gained of the adaptability of CD1 in binding lipid antigens with diverse structural features. Biochemical studies of the interactions between the TCR and CD1-lipid complexes have revealed striking contrasts with TCR that bind to peptides presented by MHC-encoded class I and class II molecules. The sphingolipid activating proteins (SAP) have recently been found to facilitate the transfer of lipid antigens onto CD1 molecules. This helps to provide an explanation as to how the thermodynamic barrier, caused by loading hydrophobic lipid antigens in a hydrophilic environment, can be overcome. Mechanisms of CD1 endosomal trafficking are being delineated, including the means by which adaptor proteins induce the localization of some types of CD1 molecules to lysosomes, where they bind antigens. Unlike MHC class I and class II proteins, specialized molecules that function solely in chaperoning CD1 molecules, or in facilitating their antigen loading, have not been found. This suggests that the CD1 antigen presenting system, which diverged early in vertebrate evolution from MHC antigen presenting molecules, is a simpler system with a character closer to the primordial antigen presenting function.

Genetic control of NKT cell numbers

NKT cells play a critical role in shaping the character and strength of a wide range of immune responses, including those against pathogens, tumours, allografts and autologous tissues. Because numbers of NKT cells affect clinical outcomes in a wide range of disease models, and this characteristic demonstrates allelic variation, the mapping of the locations and identification of the coding sequences of these genes has become a matter of significant importance. Here, we review the results to date that examine the effects of targeted deletion of a number of candidate genes, as well as the congenic and genetic linkage analyses that have attempted to localize allelic loci that affect NKT cell numbers. Although a number of candidate genes have been examined, there is no evidence that any of these contribute to variation in NKT cell numbers in natural populations. Two of the most important genetic regions controlling NKT cell numbers are Nkt1 on chromosome 1, which may contribute to lupus susceptibility, and Nkt2 on chromosome 2, which appears to contribute to diabetes susceptibility. Of great interest is a third locus on chromosome 18, identified in a novel congenic line, which can confer an absolute deficiency in this important immunoregulatory lymphocyte population.

Effect of Decreasing the Affinity of the Class II-Associated Invariant Chain Peptide on the MHC Class II Peptide Repertoire in the Presence or Absence of H-2M1

The class II-associated invariant chain peptide (CLIP) region of the invariant chain (Ii) directly influences MHC class II presentation by occupying the MHC class II peptide-binding groove, thereby preventing premature loading of peptides. Different MHC class II alleles exhibit distinct affinities for CLIP, and a low affinity interaction has been associated with decreased dependence upon H-2M and increased susceptibility to rheumatoid arthritis, suggesting that decreased CLIP affinity alters the MHC class II-bound peptide repertoire, thereby promoting autoimmunity. To examine the role of CLIP affinity in determining the MHC class II peptide repertoire, we generated transgenic mice expressing either wild-type human Ii or human Ii containing a CLIP region of low affinity for MHC class II. Our data indicate that although degradation intermediates of Ii containing a CLIP region with decreased affinity for MHC class II do not remain associated with I-A(b), this does not substantially alter the peptide repertoire bound by MHC class II or increase autoimmune susceptibility in the mice. This implies that the affinity of the CLIP:MHC class II interaction is not a strong contributory factor in determining the probability of developing autoimmunity. In contrast, in the absence of H-2M, MHC class II peptide repertoire diversity is enhanced by decreasing the affinity of CLIP for MHC class II, although MHC class II cell surface expression is reduced. Thus, we show clearly, in vivo, the critical chaperone function of H-2M, which preserves MHC class II molecules for high affinity peptide binding upon dissociation of Ii degradation intermediates.

Lysosomal localization of murine CD1d mediated by AP-3 is necessary for NK T cell development

The presentation of lipid and glycolipid Ags to T cells is mediated through CD1 molecules. In the mouse and rat only a single isoform, CD1d, performs these functions, while humans and all other mammals studied have members of both group I (CD1a, -b, and -c) and group II (CD1d) isoforms. Murine CD1d contains a cytoplasmic tyrosine-based sorting motif that is similar to motifs recognized by adaptor protein complexes that sort transmembrane proteins. Here we show that the adaptor protein complex, AP-3, directly interacts with murine CD1d and controls its targeting to lysosomes. AP-3 deficiency results in a redistribution of CD1d from lysosomes to the cell surface of thymocytes, B cell-depleted splenocytes, and dendritic cells. The altered trafficking of CD1d in AP-3-deficient mice results in a significant reduction of NK1.1(+)TCR-beta(+) and CD1d tetramer-positive cells, consistent with a defect in CD1d self-Ag presentation and thymocyte-positive selection. The AP-3 complex has recently been shown to associate with the human CD1b isoform, which has an intracellular distribution pattern similar to that of murine CD1d. We propose that lysosomal sampling may be so critical for efficient host defense that mice have evolved mechanisms to target their single CD1 isoform to lysosomes for sampling lipid Ags. Here we show the dominant mechanism for this trafficking is mediated by AP-3.

The adaptor protein AP-3 is required for CD1d-mediated antigen presentation of glycosphingolipids and development of Valpha14i NKT cells

Relatively little is known about the pathway leading to the presentation of glycolipids by CD1 molecules. Here we show that the adaptor protein complex 3 (AP-3) is required for the efficient presentation of glycolipid antigens that require internalization and processing. AP-3 interacts with mouse CD1d, and cells from mice deficient for AP-3 have increased cell surface levels of CD1d and decreased expression in late endosomes. Spleen cells from AP-3-deficient mice have a reduced ability to present glycolipids to natural killer T (NKT) cells. Furthermore, AP-3-deficient mice have a significantly reduced NKT cell population, although this is not caused by self-tolerance that might result from increased CD1d surface levels. These data suggest that the generation of the endogenous ligand that selects NKT cells may also be AP-3 dependent. However, the function of MHC class II-reactive CD4+ T lymphocytes is not altered by AP-3 deficiency. Consistent with this divergence from the class II pathway, NKT cell development and antigen presentation by CD1d are not reduced by invariant chain deficiency. These data demonstrate that the AP-3 requirement is a particular attribute of the CD1d pathway in mice and that, although MHC class II molecules and CD1d are both found in late endosomes or lysosomes, different pathways mediate their intracellular trafficking.

Antigen presentation in vaccine development

A variety of microorganisms, nutrients or toxins are generally intrude our body through mucosal tissues or skin, where equipment for both preventing their invasions and catching their information to activate internal immune systems for adapting surroundings is arranged. Among the equipment, cells in charge of innate immunity, particularly dendritic cells (DCs), having an excellent capacity for prompt recognition of invaded pathogens via toll-like receptors (TLRs) to alert B and T cells for establishing aquired/adaptive immunity by presenting their processed antigenic fragments, have been paid great attention. These TLR-activated, antigen captured DCs are divided into two groups; one is pathogen-retaining unit and the other is pathogen-controlling unit. The latter DCs present processed antigenic molecules from the pathogens to competent alphabeta T cells together with special containers, such as class I, class II MHC and CD1 to generate specific cellular immunity. The former two MHC molecules can present processed peptide antigens, whereas the last CD1 molecule can present glycolipid/lipid antigens. In contrast, B lymphocytes that captured antigens via their specific immunoglobulin (Ig) receptors present digested peptide fragments with their class II MHC to stimulate suitable CD4(+) helper T cells which in turn secrete various cytokines to efficiently expand and maintain antibody production from that partner B cells to establish humoral immunity. These alphabeta T cells and antibodies, recognize either processed antigenic peptide or glycolipid fragments, and thus, identification of these epitopes enables us to generate artificial pathogen-specific vaccines. Based on the recent findings about precise mechanisms of antigen processing and presentation orchestrated at the surface compartment, future development of vaccines against various pathogens are discussed.

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.

Innate self recognition by an invariant, rearranged T-cell receptor and its immune consequences

This review attempts to illuminate the glycolipid antigen presentation properties of CD1d, how CD1d controls the function of natural T (iNKT) cells and how CD1d and iNKT cells interact to jump-start the immune system. It is postulated that the CD1d-iNKT cell system functions as a sensor, sensing alterations in cellular lipid content by virtue of its affinity for such ligands. The presentation of a neo-self glycolipid, presumably by infectious assault of antigen-presenting cells, activates iNKT cells, which promptly release pro-inflammatory and anti-inflammatory cytokines and jump-start the immune system.

Human cathepsin S, but not cathepsin L, degrades efficiently MHC class II-associated invariant chain in nonprofessional APCs

MHC class II-restricted antigen presentation plays a central role in the immune response against exogenous antigens. The association of invariant (Ii) chain with MHC class II dimers is required for proper antigen presentation to CD4+ T cells by antigen-presenting cells. MHC class II complexes first traffic through the endocytic pathway to allow Ii chain degradation and antigenic peptide loading before their arrival at the cell surface. In recent years, a considerable effort has been directed toward the identification of proteases responsible for Ii chain degradation. Targeted gene deletion in mice has allowed a precise description of the cysteine proteases involved in the last step of Ii chain degradation. By using nonspecialized cellular models expressing MHC II molecules, we are now exploring the contribution of known cysteine proteases to human Ii chain processing. Surprisingly and contrary to the situation in mouse, cathepsin S was found to be the only human cysteine protease able to efficiently degrade the Ii-p10 fragment in epithelial cells. This selectivity has implications for thymic selection and indicates that differences between man and mice are probably more profound at this level than expected.

Antigen processing and presentation in human muscle: cathepsin S is critical for MHC class II expression and upregulated in inflammatory myopathies

The immunological properties of muscle cells are of critical importance for both the pathogenesis of inflammatory muscle disorders as well as for understanding and controlling novel therapeutic strategies. Muscle cells can present antigens to both CD4 and CD8 cells. However, the cellular biochemistry of antigen processing and presentation by muscle cells is not clear. Cathepsins play a central role in the generation of antigenic peptide and control transport and maturation of MHC class II molecules. To further elucidate the molecular basis for the MHC class II-mediated antigen presentation by muscle cells, we here analyzed cultured human myoblasts and biopsies from inflammatory myopathies with respect to the expression and function of the constituents of the MHC class II antigen presentation machinery. We identified cathepsin S (CatS) as the dominant endocytic protease that is specifically upregulated under inflammatory conditions to significant mRNA levels, synchronously with HLA-DR, -DM and the class II invariant chain (Ii), both in muscle biopsies from affected individuals with inflammatory myopathies and in human myoblasts cultured in the presence of IFN-gamma. This led to translation of the mature CatS polypeptide that was enzymatically active in human myoblasts under inflammatory conditions. By contrast, expression of CatL and CatB was unaffected by IFN-gamma at both the expression and activity levels. CatS activity is required for efficient surface display of MHC class II in this cell type: functional inhibition of CatS using a CatS-selective inhibitor reduced the levels of surface class II alphabeta:peptide complexes on stimulated myoblasts by almost 50%. Surprisingly, and in contrast to B cells and dendritic cells, this was not due to inefficient processing of Ii in the absence of CatS, which was unaffected by the elimination of CatS activity. We therefore conclude that CatS is involved in the regulation of class II expression in human myoblasts independently from Ii processing.