Lysosomal cysteine proteases and antigen presentation

Aprotinin (I): Understanding the Role of Host Proteases in COVID-19 and the Importance of Pharmacologically Regulating Their Function

Proteases are produced and released in the mucosal cells of the respiratory tract and have important physiological functions, for example, maintaining airway humidification to allow proper gas exchange. The infectious mechanism of severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2), which causes coronavirus disease 2019 (COVID-19), takes advantage of host proteases in two ways: to change the spatial conformation of the spike (S) protein via endoproteolysis (e.g., transmembrane serine protease type 2 (TMPRSS2)) and as a target to anchor to epithelial cells (e.g., angiotensin-converting enzyme 2 (ACE2)). This infectious process leads to an imbalance in the mucosa between the release and action of proteases versus regulation by anti-proteases, which contributes to the exacerbation of the inflammatory and prothrombotic response in COVID-19. In this article, we describe the most important proteases that are affected in COVID-19, and how their overactivation affects the three main physiological systems in which they participate: the complement system and the kinin-kallikrein system (KKS), which both form part of the contact system of innate immunity, and the renin-angiotensin-aldosterone system (RAAS). We aim to elucidate the pathophysiological bases of COVID-19 in the context of the imbalance between the action of proteases and anti-proteases to understand the mechanism of aprotinin action (a panprotease inhibitor). In a second-part review, titled "Aprotinin (II): Inhalational Administration for the Treatment of COVID-19 and Other Viral Conditions", we explain in depth the pharmacodynamics, pharmacokinetics, toxicity, and use of aprotinin as an antiviral drug.

Whole genome identification of olive flounder (Paralichthys olivaceus) cathepsin genes: Provides insights into its regulation on biotic and abiotic stresses response

Cathepsins are major lysosomal enzymes involved in essential physiological processes, including protein degradation, tissue differentiation, and innate or adaptive responses. Several kinds of cathepsins have been reported in teleost fishes, but no characterization have been performed for the inflammatory response of cathepsin family in olive flounder until now. In our current study, a total of 17 cathepsins in olive flounder were systematically identified and characterized. Phylogenetic analysis clearly indicated that the cathepsin genes was highly conserved. Analysis of structure and motifs exhibited high sequence similarity of cathepsin genes in olive flounder. Expression profiles of cathepsin genes in different tissues and developmental stages showed that cathepsins were temporally and spatially specific. RNA-seq analysis of bacteria and temperature stresses revealed that members of cathepsin were involved in inflammatory responses. Collectively, our findings would provide a further reference for understanding the molecular mechanisms of cathepsins in olive flounder.

Cathepsin S inhibition in dendritic cells prevents Th17 cell differentiation in perivascular adipose tissues following vascular injury in diabetic rats

In the context of diabetes mellitus (DM), the circulating cathepsin S (CTSS) level is significantly higher in the cardiovascular disease group. Therefore, this study was designed to investigate the role of CTSS in restenosis following carotid injury in diabetic rats. To induce DM, 60 mg/kg of streptozotocin (STZ) in citrate buffer was injected intraperitoneally into Sprague-Dawley rats. After successful modeling of DM, wire injury of the rat carotid artery was performed, followed by adenovirus transduction. Levels of blood glucose and Th17 cell surface antigens including ROR-γt, IL-17A, IL-17F, IL-22, and IL-23 in perivascular adipose tissues (PVAT) were evaluated. For in vitro analysis, human dendritic cells (DCs) were treated with 5.6-25 mM glucose for 24 h. The morphology of DCs was observed using an optical microscope. CD4+ T cells derived from human peripheral blood mononuclear cells were cocultured with DCs for 5 days. Levels of IL-6, CTSS, ROR-γt, IL-17A, IL-17F, IL-22 and IL-23 were measured. Flow cytometry was conducted to detect DC surface biomarkers (CD1a, CD83, and CD86) and Th17 cell differentiation. The collected DCs presented a treelike shape and were positive for CD1a, CD83, and CD86. Glucose impaired DC viability at the dose of 35 mM. Glucose treatment led to an increase in CTSS and IL-6 expression in DCs. Glucose-treated DCs promoted the differentiation of Th17 cells. CTSS depletion downregulated IL-6 expression and inhibited Th17 cell differentiation in vitro and in vivo. CTSS inhibition in DCs inhibits Th17 cell differentiation in PVAT tissues from diabetic rats following vascular injury.

Cathepsins in oral diseases: mechanisms and therapeutic implications

Cathepsins are a type of lysosomal globulin hydrolase and are crucial for many physiological processes, including the resorption of bone matrix, innate immunity, apoptosis, proliferation, metastasis, autophagy, and angiogenesis. Findings regarding their functions in human physiological processes and disorders have drawn extensive attention. In this review, we will focus on the relationship between cathepsins and oral diseases. We highlight the structural and functional properties of cathepsins related to oral diseases, as well as the regulatory mechanisms in tissue and cells and their therapeutic uses. Elucidating the associated mechanism between cathepsins and oral diseases is thought to be a promising strategy for the treatment of oral diseases and may be a starting point for further studies at the molecular level.

Cysteine Cathepsins in Breast Cancer: Promising Targets for Fluorescence-Guided Surgery

The majority of breast cancer patients is treated with breast-conserving surgery (BCS) combined with adjuvant radiation therapy. Up to 40% of patients has a tumor-positive resection margin after BCS, which necessitates re-resection or additional boost radiation. Cathepsin-targeted near-infrared fluorescence imaging during BCS could be used to detect residual cancer in the surgical cavity and guide additional resection, thereby preventing tumor-positive resection margins and associated mutilating treatments. The cysteine cathepsins are a family of proteases that play a major role in normal cellular physiology and neoplastic transformation. In breast cancer, the increased enzymatic activity and aberrant localization of many of the cysteine cathepsins drive tumor progression, proliferation, invasion, and metastasis. The upregulation of cysteine cathepsins in breast cancer cells indicates their potential as a target for intraoperative fluorescence imaging. This review provides a summary of the current knowledge on the role and expression of the most important cysteine cathepsins in breast cancer to better understand their potential as a target for fluorescence-guided surgery (FGS). In addition, it gives an overview of the cathepsin-targeted fluorescent probes that have been investigated preclinically and in breast cancer patients. The current review underscores that cysteine cathepsins are highly suitable molecular targets for FGS because of favorable expression and activity patterns in virtually all breast cancer subtypes. This is confirmed by cathepsin-targeted fluorescent probes that have been shown to facilitate in vivo breast cancer visualization and tumor resection in mouse models and breast cancer patients. These findings indicate that cathepsin-targeted FGS has potential to improve treatment outcomes in breast cancer patients.

The Key Role of Lysosomal Protease Cathepsins in Viral Infections

Cathepsins encompass a family of lysosomal proteases that mediate protein degradation and turnover. Although mainly localized in the endolysosomal compartment, cathepsins are also found in the cytoplasm, nucleus, and extracellular space, where they are involved in cell signaling, extracellular matrix assembly/disassembly, and protein processing and trafficking through the plasma and nuclear membrane and between intracellular organelles. Ubiquitously expressed in the body, cathepsins play regulatory roles in a wide range of physiological processes including coagulation, hormone secretion, immune responses, and others. A dysregulation of cathepsin expression and/or activity has been associated with many human diseases, including cancer, diabetes, obesity, cardiovascular and inflammatory diseases, kidney dysfunctions, and neurodegenerative disorders, as well as infectious diseases. In viral infections, cathepsins may promote (1) activation of the viral attachment glycoproteins and entry of the virus into target cells; (2) antigen processing and presentation, enabling the virus to replicate in infected cells; (3) up-regulation and processing of heparanase that facilitates the release of viral progeny and the spread of infection; and (4) activation of cell death that may either favor viral clearance or assist viral propagation. In this review, we report the most relevant findings on the molecular mechanisms underlying cathepsin involvement in viral infection physiopathology, and we discuss the potential of cathepsin inhibitors for therapeutical applications in viral infectious diseases.

Genome-Wide Identification, Evolutionary Analysis, and Expression Patterns of Cathepsin Superfamily in Black Rockfish (Sebastes schlegelii) following Aeromonas salmonicida Infection

Cathepsins are lysosomal cysteine proteases belonging to the papain family and play crucial roles in intracellular protein degradation/turnover, hormone maturation, antigen processing, and immune responses. In the present study, 18 cathepsins were systematically identified from the fish S. schlegelii genome. Phylogenetic analysis indicated that cathepsin superfamilies are categorized into eleven major clusters. Synteny and genome organization analysis revealed that whole-genome duplication led to the expansion of S. schlegelii cathepsins. Evolutionary rate analyses indicated that the lowest Ka/Ks ratios were observed in CTSBa (0.13) and CTSBb (0.14), and the highest Ka/Ks ratios were observed in CTSZa (1.97) and CTSZb (1.75). In addition, cathepsins were ubiquitously expressed in all examined tissues, with high expression levels observed in the gill, intestine, head kidney, and spleen. Additionally, most cathepsins were differentially expressed in the head kidney, gill, spleen, and liver following Aeromonas salmonicida infection, and their expression signatures showed tissue-specific and time-dependent patterns. Finally, protein–protein interaction network (PPI) analyses revealed that cathepsins are closely related to a few immune-related genes, such as interleukins, chemokines, and TLR genes. These results are expected to be valuable for comparative immunological studies and provide insights for further functional characterization of cathepsins in fish species.

GILT Expression in Human Melanoma Cells Enhances Generation of Antigenic Peptides for HLA Class II-Mediated Immune Recognition

Melanoma is an aggressive skin cancer that has become increasingly prevalent in western populations. Current treatments such as surgery, chemotherapy, and high-dose radiation have had limited success, often failing to treat late stage, metastatic melanoma. Alternative strategies such as immunotherapies have been successful in treating a small percentage of patients with metastatic disease, although these treatments to date have not been proven to enhance overall survival. Several melanoma antigens (Ags) proposed as targets for immunotherapeutics include tyrosinase, NY-ESO-1, gp-100, and Mart-1, all of which contain both human leukocyte antigen (HLA) class I and class II-restricted epitopes necessary for immune recognition. We have previously shown that an enzyme, gamma-IFN-inducible lysosomal thiol-reductase (GILT), is abundantly expressed in professional Ag presenting cells (APCs), but absent or expressed at greatly reduced levels in many human melanomas. In the current study, we report that increased GILT expression generates a greater pool of antigenic peptides in melanoma cells for enhanced CD4+ T cell recognition. Our results suggest that the induction of GILT in human melanoma cells could aid in the development of a novel whole-cell vaccine for the enhancement of immune recognition of metastatic melanoma.

Revisiting the Principles of Designing a Vaccine

Immune principles formulated by Jenner, Pasteur, and early immunologists served as fundamental propositions for vaccine discovery against many dreadful pathogens. However, decisive success in the form of an efficacious vaccine still eludes for diseases such as tuberculosis, leishmaniasis, and trypanosomiasis. Several antileishmanial vaccine trials have been undertaken in past decades incorporating live, attenuated, killed, or subunit vaccination, but the goal remains unmet. In light of the above facts, we have to reassess the principles of vaccination by dissecting factors associated with the hosts' immune response. This chapter discusses the pathogen-associated perturbations at various junctures during the generation of the immune response which inhibits antigenic processing, presentation, or remodels memory T cell repertoire. This can lead to ineffective priming or inappropriate activation of memory T cells during challenge infection. Thus, despite a protective primary response, vaccine failure can occur due to altered immune environments in the presence of pathogens.

Genome-wide identification, expression signature and immune functional analysis of two cathepsin S (CTSS) genes in turbot (Scophthalmus maximus L.)

Cathepsins, a superfamily of hydrolytic enzymes produced and enclosed within lysosomes, play multiple roles at physiological and pathological states. Cathepsin S is a lysosomal cysteine endopeptidase of the papain family, and exerts critical roles in the regulation of MHC class II immune responses. In the present study, we captured two Cathepsin S genes in turbot (SmCTSS1 and SmCTSS2.1), characterized their expression patterns following V. anguillarum and S. iniae infections, and explored their binding ability and agglutination capability. Firstly, the SmCTSS1 contained a 990 bp ORF encoding 329 amino acids, while SmCTSS2.1 contained a 1,014 bp ORF encoding 337 amino acids. The phylogenetic analysis revealed that both genes showed the closest relationship to their counterparts of Japanese flounder (Paralichthys olivaceus). In addition, both genes were ubiquitously expressed in all examined healthy tissues, with the highest expression level observed in spleen and intestine, respectively, while the lowest expression level both observed in liver. Both SmCTSS1 and SmCTSS2.1 were significantly differentially expressed, and exhibited general down-regulations at most time points in skin and intestine after two bacterial infections. Finally, both rSmCTSS1 and rSmCTSS2.1 showed significant binding ability to three examined microbial ligands (LPS, PGN and LTA), and strong agglutination effect to different bacteria (E. tarda, S. agalactiae, S. aureus and V. anguillarum). Collectively, this study provided valuable data for understanding the roles of CTSS in the host defense against bacterial infections in turbot, and indicated the potential vital roles of CTSS in innate immune responses of teleost species.

Characterization, expression, enzymatic activity, and functional identification of cathepsin S from black rockfish Sebastes schlegelii

Cathepsin S belong to the cathepsin L-like family of cysteine cathepsins. It is well known that Cathepsin S participate in various physiological processes and host immune defense in mammals. However, in teleost fish, the function of cathepsin S is less investigated. In the present study, a cathepsin S homologue (SsCTSS) from the teleost fish black rockfish (Sebastes schlegelii) were identified and examined at expression and functional levels. In silico analysis showed that three domains, including signal peptide, cathepsin propeptide inhibitor I29 domain, and functional domain Pept_C1, were existed in the cathepsin. SsCTSS possesses a peptidase domain with three catalytically essential residues (Cys25, His162, and Asn183). Phylogenetic profiling indicated that SsCTSS are evolutionally close to the cathepsin S of other teleost fish. The expression of SsCTSS in immune-related tissues was upregulated in a time-dependent manner upon bacterial pathogen infection. Purified recombinant SsCTSS (rSsCTSS) exhibited apparent peptidase activity, which was remarkably declined in the presence of the cathepsin inhibitor E-64. rSsCTSS showed strong binding ability to LPS and PGN, the major constituents of the outer membranes of Gram-negative and Gram-positive bacteria, respectively. rSsCTSS also exhibited the capability of agglutination to different bacteria. The knockdown of SsCTSS attenuated the ability of host to eliminate pathogenic bacteria. Taken together, our results suggested that SsCTSS functions as cysteine protease which might be involved in the antibacterial immunity of black rockfish.

Antibiotic Perturbation of Gut Microbiota Dysregulates Osteoimmune Cross Talk in Postpubertal Skeletal Development

Commensal gut microbiota-host immune responses are experimentally delineated via gnotobiotic animal models or alternatively by antibiotic perturbation of gut microbiota. Osteoimmunology investigations in germ-free mice, revealing that gut microbiota immunomodulatory actions critically regulate physiologic skeletal development, highlight that antibiotic perturbation of gut microbiota may dysregulate normal osteoimmunological processes. We investigated the impact of antibiotic disruption of gut microbiota on osteoimmune response effects in postpubertal skeletal development. Sex-matched C57BL/6T mice were administered broad-spectrum antibiotics or vehicle-control from the age of 6 to 12 weeks. Antibiotic alterations in gut bacterial composition and skeletal morphology were sex dependent. Antibiotics did not influence osteoblastogenesis or endochondral bone formation, but notably enhanced osteoclastogenesis. Unchanged Tnf or Ccl3 expression in marrow and elevated tumor necrosis factor-α and chemokine (C-C motif) ligand 3 in serum indicated that the pro-osteoclastic effects of the antibiotics are driven by increased systemic inflammation. Antibiotic-induced broad changes in adaptive and innate immune cells in mesenteric lymph nodes and spleen demonstrated that the perturbation of gut microbiota drives a state of dysbiotic hyperimmune response at secondary lymphoid tissues draining local gut and systemic circulation. Antibiotics up-regulated the myeloid-derived suppressor cells, immature myeloid progenitor cells known for immunosuppressive properties in pathophysiologic inflammatory conditions. Myeloid-derived suppressor cell-mediated immunosuppression can be antigen specific. Therefore, antibiotic-induced broad suppression of major histocompatibility complex class II antigen presentation genes in bone marrow discerns that antibiotic perturbation of gut microbiota dysregulates critical osteoimmune cross talk.

Endolysosomal Degradation of Allergenic Ole e 1-Like Proteins: Analysis of Proteolytic Cleavage Sites Revealing T Cell Epitope-Containing Peptides

Knowledge of the susceptibility of proteins to endolysosomal proteases provides valuable information on immunogenicity. Though Ole e 1-like proteins are considered relevant allergens, little is known about their immunogenic properties and T cell epitopes. Thus, six representative molecules, i.e., Ole e 1, Fra e 1, Sal k 5, Che a 1, Phl p 11 and Pla l 1, were investigated. Endolysosomal degradation and peptide generation were simulated using microsomal fractions of JAWS II dendritic cells. Kinetics and peptide patterns were evaluated by gel electrophoresis and mass spectrometry. In silico MHC (major histocompatibility complex) class II binding prediction was performed with ProPred. Cleavage sites were assigned to the primary and secondary structure, and in silico docking experiments between the protease cathepsin S and Ole e 1 were performed. Different kinetics during endolysosomal degradation were observed while similar peptide profiles especially at the C-termini were detected. Typically, the identified peptide clusters comprised the previously-reported T cell epitopes of Ole e 1, consistent with an in silico analysis of the T cell epitopes. The results emphasize the importance of the fold on allergen processing, as also reflected by conserved cleavage sites located within the large flexible loop. In silico docking and mass spectrometry results suggest that one of the first Ole e 1 cleavages might occur at positions 107–108. Our results provided kinetic and structural information on endolysosomal processing of Ole e 1-like proteins.

A role for cathepsin Z in neuroinflammation provides mechanistic support for an epigenetic risk factor in multiple sclerosis

BACKGROUND

Hypomethylation of the cathepsin Z locus has been proposed as an epigenetic risk factor for multiple sclerosis (MS). Cathepsin Z is a unique lysosomal cysteine cathepsin expressed primarily by antigen presenting cells. While cathepsin Z expression has been associated with neuroinflammatory disorders, a role for cathepsin Z in mediating neuroinflammation has not been previously established.

METHODS

Experimental autoimmune encephalomyelitis (EAE) was induced in both wildtype mice and mice deficient in cathepsin Z. The effects of cathepsin Z-deficiency on the processing and presentation of the autoantigen myelin oligodendrocyte glycoprotein, and on the production of IL-1β and IL-18 were determined in vitro from cells derived from wildtype and cathepsin Z-deficient mice. The effects of cathepsin Z-deficiency on CD4+ T cell activation, migration, and infiltration to the CNS were determined in vivo. Statistical analyses of parametric data were performed by one-way ANOVA followed by Tukey post-hoc tests, or by an unpaired Student's t test. EAE clinical scoring was analyzed using the Mann-Whitney U test.

RESULTS

We showed that mice deficient in cathepsin Z have reduced neuroinflammation and dramatically lowered circulating levels of IL-1β during EAE. Deficiency in cathepsin Z did not impact either the processing or the presentation of MOG, or MOG- specific CD4+ T cell activation and trafficking. Consistently, we found that cathepsin Z-deficiency reduced the efficiency of antigen presenting cells to secrete IL-1β, which in turn reduced the ability of mice to generate Th17 responses-critical steps in the pathogenesis of EAE and MS.

CONCLUSION

Together, these data support a novel role for cathepsin Z in the propagation of IL-1β-driven neuroinflammation.

Role of Cathepsins in Mycobacterium tuberculosis Survival in Human Macrophages

Cathepsins are proteolytic enzymes that function in the endocytic pathway, especially in lysosomes, where they contribute directly to pathogen killing or indirectly, by their involvement in the antigen presentation pathways. Mycobacterium tuberculosis (MTB) is a facultative intracellular pathogen that survives inside the macrophage phagosomes by inhibiting their maturation to phagolysosomes and thus avoiding a low pH and protease-rich environment. We previously showed that mycobacterial inhibition of the proinflammatory transcription factor NF-κB results in impaired delivery of lysosomal enzymes to phagosomes and reduced pathogen killing. Here, we elucidate how MTB also controls cathepsins and their inhibitors, cystatins, at the level of gene expression and proteolytic activity. MTB induced a general down-regulation of cathepsin expression in infected cells, and inhibited IFNγ-mediated increase of cathepsin mRNA. We further show that a decrease in cathepsins B, S and L favours bacterial survival within human primary macrophages. A siRNA knockdown screen of a large set of cathepsins revealed that almost half of these enzymes have a role in pathogen killing, while only cathepsin F coincided with MTB resilience. Overall, we show that cathepsins are important for the control of MTB infection, and as a response, it manipulates their expression and activity to favour its intracellular survival.

Proteolytic clipping of histone tails: the emerging role of histone proteases in regulation of various biological processes

Chromatin is a dynamic DNA scaffold structure that responds to a variety of external and internal stimuli to regulate the fundamental biological processes. Majority of the cases chromatin dynamicity is exhibited through chemical modifications and physical changes between DNA and histones. These modifications are reversible and complex signaling pathways involving chromatin-modifying enzymes regulate the fluidity of chromatin. Fluidity of chromatin can also be impacted through irreversible change, proteolytic processing of histones which is a poorly understood phenomenon. In recent studies, histone proteolysis has been implicated as a regulatory process involved in the permanent removal of epigenetic marks from histones. Activities responsible for clipping of histone tails and their significance in various biological processes have been observed in several organisms. Here, we have reviewed the properties of some of the known histone proteases, analyzed their significance in biological processes and have provided future directions.

Cysteine protease cathepsins and matrix metalloproteinases in the development of abdominal aortic aneurysms

Both cysteine protease cathepsins and matrix metalloproteinases are implicated in the pathogenesis of abdominal aortic aneurysms (AAAs) in humans and animals. Blood and aortic tissues from humans or animals with AAAs contain much higher levels of these proteases, and often lower levels of their endogenous inhibitors, than do blood and aortic tissues from healthy subjects. Protease- and protease inhibitor-deficient mice and synthetic protease inhibitors have affirmed that cysteinyl cathepsins and matrix metalloproteinases both participate directly in AAA development in several experimental model systems. Here, we summarize our current understanding of how proteases contribute to the pathogenesis of AAA, and discuss whether proteases or their inhibitors may serve as diagnostic biomarkers or potential therapeutic targets for this common human arterial disease.

Proteomics reveals acute pro-inflammatory and protective responses in rat Kupffer cells and hepatocytes after chemical initiation of liver cancer and after LPS and IL-6

Inflammation is a key event in the development of liver cancer. We studied early inflammatory responses of Kupffer cells (KCs) and hepatocyte (HC) after cancer initiation. The chemical carcinogen N-nitrosomorpholine (NNM) was used in a rat model. We applied a comprehensive analytical strategy including metabolic labeling, 2-D PAGE, LC-MS/MS-based spot identification and shotgun proteomics and thus determined the rates of synthesis of individual proteins, compared whole tissue with isolated constituent cells and performed in vivo to in vitro comparisons of NNM effects. NNM increased synthesis of overall and 138 individual proteins identified in HC and/or KC, indicating reprogramming of metabolism favoring protection, repair and replacement of cell constituents in HC and KC. Secretome analysis by 2-D PAGE and shotgun proteomics of HC revealed the induction of acute phase proteins, in case of KC of proteases, cytokines and chemokines, indicating inflammatory effects. All responses were induced rapidly, independently of signals from other cells, and closely mimicked the pro-inflammatory and protective effects of inflammation modulators LPS in KC and IL-6 in HC. In conclusion, the carcinogen NNM exerts pro-inflammatory effects in the liver, partially by direct activation of KC. The acute inflammation and its protective component will enhance formation, survival and proliferation of initiated cells and may therefore act synergistically with the genotoxic action of the carcinogen.

Overexpression of cathepsin S induces chronic atopic dermatitis in mice

Atopic dermatitis (AD) is a chronically relapsing, non contagious pruritic skin disease with two phases: acute and chronic. Cysteine protease cathepsin S (CTSS) is involved in inflammatory processes, possibly leading to atherosclerosis and asthma. Recently, it has been reported that CTSS can arouse a predominant sensation of itch accompanied by classical ligand–receptor signaling [corrected]. Recently, CTSS was shown to be a ligand for proteinase-activated receptor-2 (PAR-2), which is associated with itching. In this study, we show that CTSS-overexpressing transgenic (TG) mice spontaneously develop a skin disorder similar to chronic AD. The results of this study suggest that CTSS overexpression triggers PAR-2 expression in dendritic cells (DCs), resulting in the promotion of CD4(+) differentiation, which is involved in major histocompatibility complex (MHC) class II expression. In addition, we investigated mast cells and macrophages and found significantly higher mean levels of T helper type 1 (Th1) cell-associated cytokines than T helper type 2 (Th2) cell-associated cytokines in CTSS-overexpressing TG mice. These results suggest that increased PAR-2 expression in DCs as a result of CTSS overexpression induces scratching behavior and Th1 cell-associated cytokine expression, and can trigger chronic AD symptoms.

Regulation of cathepsin G reduces the activation of proinsulin-reactive T cells from type 1 diabetes patients

Autoantigenic peptides resulting from self-proteins such as proinsulin are important players in the development of type 1 diabetes mellitus (T1D). Self-proteins can be processed by cathepsins (Cats) within endocytic compartments and loaded to major histocompatibility complex (MHC) class II molecules for CD4⁺ T cell inspection. However, the processing and presentation of proinsulin by antigen-presenting cells (APC) in humans is only partially understood. Here we demonstrate that the processing of proinsulin by B cell or myeloid dendritic cell (mDC1)-derived lysosomal cathepsins resulted in several proinsulin-derived intermediates. These intermediates were similar to those obtained using purified CatG and, to a lesser extent, CatD, S, and V in vitro. Some of these intermediates polarized T cell activation in peripheral blood mononuclear cells (PBMC) from T1D patients indicative for naturally processed T cell epitopes. Furthermore, CatG activity was found to be elevated in PBMC from T1D patients and abrogation of CatG activity resulted in functional inhibition of proinsulin-reactive T cells. Our data suggested the notion that CatG plays a critical role in proinsulin processing and is important in the activation process of diabetogenic T cells.

Nucleic acid recognition by Toll-like receptors is coupled to stepwise processing by cathepsins and asparagine endopeptidase

TLR3, TLR7, and TLR9 are cleaved in the same step-wise manner in all immune cell types examined.

Toll-like receptor (TLR) 9 requires proteolytic processing in the endolysosome to initiate signaling in response to DNA. However, recent studies conflict as to which proteases are required for receptor cleavage. We show that TLR9 proteolysis is a multistep process. The first step removes the majority of the ectodomain and can be performed by asparagine endopeptidase (AEP) or cathepsin family members. This initial cleavage event is followed by a trimming event that is solely cathepsin mediated and required for optimal receptor signaling. This dual requirement for AEP and cathepsins is observed in all cell types that we have analyzed, including mouse macrophages and dendritic cells. In addition, we show that TLR7 and TLR3 are processed in an analogous manner. These results define the core proteolytic steps required for TLR9 function and suggest that receptor proteolysis may represent a general regulatory strategy for all TLRs involved in nucleic acid recognition.

Antigen processing and presentation in multiple sclerosis

CD4(+) T cells play a central role in the pathogenesis of multiple sclerosis (MS). Generation, activation and effector function of these cells crucially depends on their interaction with MHC II-peptide complexes displayed by antigen presenting cells (APC). Processing and presentation of self antigens by different APC therefore influences the disease course at all stages. Selection by thymic APC leads to the generation of autoreactive T cells, which can be activated by peripheral APC. Reactivation by central nervous system APC leads to the initiation of the inflammatory response resulting in demyelination. In this review we will focus on how MHC class II antigenic epitopes are created by different APC from the thymus, the periphery and from the brain, and will discuss the relevance of the balance between creation and destruction of such epitopes in the context of MS. A solid understanding of these processes offers the possibility for designing future therapeutic strategies.

Burkitt lymphoma: pathogenesis and immune evasion

B-cell lymphomas arise at distinct stages of cellular development and maturation, potentially influencing antigen (Ag) presentation and T-cell recognition. Burkitt lymphoma (BL) is a highly malignant B-cell tumor associated with Epstein-Barr Virus (EBV) infection. Although BL can be effectively treated in adults and children, leading to high survival rates, its ability to mask itself from the immune system makes BL an intriguing disease to study. In this paper, we will provide an overview of BL and its association with EBV and the c-myc oncogene. The contributions of EBV and c-myc to B-cell transformation, proliferation, or attenuation of cellular network and immune recognition or evasion will be summarized. We will also discuss the various pathways by which BL escapes immune detection by inhibiting both HLA class I- and II-mediated Ag presentation to T cells. Finally, we will provide an overview of recent developments suggesting the existence of BL-associated inhibitory molecules that may block HLA class II-mediated Ag presentation to CD4+ T cells, facilitating immune escape of BL.

Cathepsin A is expressed in primary human antigen-presenting cells

Cathepsins are expressed in antigen-presenting cells (APC). These cathepsins are known to regulate antigen processing and degradation of the invariant chain (Ii) into the class II-associated Ii peptide (CLIP), which occupies the peptide-binding groove of the major histocompatibility complex (MHC) class II molecule. Previous studies have identified the serine carboxypeptidase cathepsin A (CatA) in various tissues and cells; however, it is not clear whether CatA is also expressed in primary human APC. We demonstrate the expression of CatA in B lymphoblastoid cells (BLC), primary human B cells, both subsets of myeloid dendritic cells (mDC1 and mDC2), as well as in plasmacytoid DC. PMSF or lactacystin-mediated inhibition of serine proteases in BLC-derived lysosomal proteases resulted in the inhibition of amino acid release from the C-terminal end of two model peptides. This inhibition did not occur by using a proline rich peptide. Our data suggest that CatA is involved in the C-terminal fine-tuning of antigenic T cell epitopes in human APC.

Insights into the Role of GILT in HLA Class II Antigen Processing and Presentation by Melanoma

Metastatic melanoma is one of the deadliest of skin cancers and is increasing in incidence. Since current treatment regimens are ineffective at controlling and/or curing the disease, novel approaches, such as immunotherapy, for treating this malignant disease are being explored. In this review, we discuss potential melanoma antigens (Ags) and their role in utilizing the HLA class II pathway to elicit tumor Ag-specific CD4+ T cell responses in order to effectively induce long-lasting CD8+ antitumor memory. We also discuss the role of endolysosomal cathepsins and Gamma-Interferon-inducible Lysosomal Thiol reductase (GILT) in Ag processing and presentation, and at enhancing CD4+ T cell recognition of melanoma cells. This review also summarizes our current knowledge on GILT and highlights a novel mechanism of GILT-mediated immune responses against melanoma cells. At the end, we propose a strategy employing GILT in the development of a potential whole cell vaccine for combating metastatic melanoma.

Cathepsin G: roles in antigen presentation and beyond

Contributions from multiple cathepsins within endosomal antigen processing compartments are necessary to process antigenic proteins into antigenic peptides. Cysteine and aspartyl cathepsins have been known to digest antigenic proteins. A role for the serine protease, cathepsin G (CatG), in this process has been described only recently, although CatG has long been known to be a granule-associated proteolytic enzyme of neutrophils. In line with a role for this enzyme in antigen presentation, CatG is found in endocytic compartments of a variety of antigen presenting cells. CatG is found in primary human monocytes, B cells, myeloid dendritic cells 1 (mDC1), mDC2, plasmacytoid DC (pDC), and murine microglia, but is not expressed in B cell lines or monocyte-derived DC. Purified CatG can be internalized into endocytic compartments in CatG non-expressing cells, widening the range of cells where this enzyme may play a role in antigen processing. Functional assays have implicated CatG as a critical enzyme in processing of several antigens and autoantigens. In this review, historical and recent data on CatG expression, distribution, function and involvement in disease will be summarized and discussed, with a focus on its role in antigen presentation and immune-related events.

Application of specific cell permeable cathepsin G inhibitors resulted in reduced antigen processing in primary dendritic cells

The serine protease cathepsin G (CatG) is expressed in primary antigen-presenting cells and regulates autoantigen processing in CatG pre-loaded fibroblasts. To further investigate the function of CatG in the major histocompatibility complex (MHC) class II loading compartments, a specific, cell permeable CatG-inhibitor is needed. In this study, several CatG-inhibitors were tested for their ability to penetrate the cell membrane of peripheral blood mononuclear cells (PBMC). We find that the commercially available reversible CatG-specific inhibitor I (CatG inhibitor) and the irreversible Suc-Val-Pro-Phe(P) (OPh)(2) (Suc-VPF) are both cell permeable and specifically inhibit intracellular CatG in the PBMC. Furthermore, selective inhibition of CatG resulted in reduced tetanus toxin C-fragment (TTC) and hemagglutinin (HA) processing and presentation to CD4(+) T cells. We conclude that these CatG inhibitors can be used for both antigen-processing studies and for modulation of T cell response in situ and in vivo.

Gamma-IFN-inducible-lysosomal thiol reductase modulates acidic proteases and HLA class II antigen processing in melanoma

HLA class II-restricted antigen (Ag) processing and presentation are important for the activation of CD4+ T cells, which are the central orchestrating cells of immune responses. The majority of melanoma cells either expresses, or can be induced to express, HLA class II proteins. Thus, they are prime targets for immune mediated elimination by class II-restricted CD4+ T cells. We have previously shown that human melanoma cells lack an important enzyme, gamma interferon-inducible lysosomal thiol-reductase (GILT), capable of perturbing immune recognition of these tumors. Here, we show that GILT expression in human melanoma cells enhances Ag processing and presentation via HLA class II molecules. We also show that GILT expression influences the generation of active forms of cysteinyl proteases, cathepsins B, L and S, as well as an aspartyl protease cathepsin D in melanoma cells. Mechanistic studies revealed that GILT does not regulate acidic cathepsins at the transcriptional level; rather it colocalizes with the cathepsins and influences HLA class II Ag processing. GILT expression in melanoma cells also elevated HLA-DM molecules, which favor epitope loading onto class II in the endolysosomal compartments, enhancing CD4+ T cell recognition. These data suggest that GILT-expressing melanoma cells could prove to be very promising for direct antigen presentation and CD4+ T cell recognition, and may have direct implications for the design of cancer vaccines.

Emerging drugs for psoriasis

Psoriasis is a chronic immune-mediated inflammatory skin disease characterised by abnormal keratinocyte differentiation and proliferation. The immunopathogenesis is complex and novel evidence shows the involvement of both innate and adaptive immune response. Type 1 T cells and their effector cytokines play a pivotal role. Several drugs under preclinical and clinical development for psoriasis are directed against the immune response, targeting activation or proliferation of T cells, their trafficking and skin-homing, or effector cytokines. Among these, great attention has been given to TNF-alpha, following the demonstration of effectiveness of anti-TNF-alpha biologicals, and to IFN-gamma inducers. Another appealing approach concerns drugs capable of inducing immunological tolerance. Progress made in the recognition of intracellular events has prompted the development of small molecules and oligonucleotides that can inhibit specific molecular targets. There is, however, a plethora of other emerging drugs, clearly suggestive of the current interest for psoriasis, which are briefly described in this paper.