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

Immune cells in dorsal root ganglia are associated with pruritus in a mouse model of allergic contact dermatitis and co-culture study

The interaction between the neuroimmune system plays a crucial role in itch sensation, yet most research has focused on immune cells within the skin. Our study seeks to explore the presence and functions of immune cells within the dorsal root ganglia (DRG) in the context of allergic contact dermatitis (ACD). Immunofluorescence and histological staining techniques were employed to identify immune cells, including T-cells, basophils, mast cells, and dendritic cells (DCs), within the DRG of BALB/c mice sensitized and challenged with toluene diisocyanate (TDI). Our findings revealed an increase in mast cells and DCs within the DRG under ACD condition. Additionally, when DRG neurons were cultured with mast cells, a higher proportion of neurons exhibited responses to non-histaminergic pruritogens compared to neurons cultured alone. This suggests that mast cells may contribute to heightened sensitivity to non-histaminergic pruritogens. Furthermore, we conducted transcriptomic analysis of DCs within the DRG using RNA sequencing, followed by pathway enrichment analysis. Our analysis revealed that sorted DCs are implicated in immune responses, inflammation, and itch, with notable upregulation of Cathepsin S (Ctss) and sphingosine-1-phosphate (S1P) phosphatase 2 (Sgpp2). Subsequent functional experiments targeting CTSS in co-culture studies validated suppressed response to pruritogen and agonists of TRPA1 and TRPV1, indicating a potential role in peripheral sensitization. Additionally, the co-culture study indicated that the neuroimmune interaction between DCs and DRG neurons might involve S1P metabolism and S1P receptor signaling. In conclusion, targeting DCs and exploring the non-histaminergic functions of mast cells within the DRG, holds promise as novel targets for treating pruritus.

Cathepsin S: A key drug target and signalling hub in immune system diseases

The lysosomal cysteine protease cathepsin S supports host defence by promoting the maturation of MHC class-II proteins. In contrast, increased cathepsin S activity mediates tissue destructive immune responses in autoimmune and inflammatory diseases. Therefore, cathepsin S is a key target in drug discovery programs. Here, we critically reviewed the specific mechanisms by which cathepsin S mediates autoimmune and hyperinflammatory responses to identify new targets for therapeutic immunomodulation. To this end, we performed literature review utilizing PubMed, drug database of US FDA, European Medicines Agency and the Drug-Gene Interaction Database. Cathepsin S destroys T cell epitopes and reduces endogenous antigen diversity, impairing negative selection of autoreactive T cells that could recognize these epitopes. Moreover, cathepsin S critically regulates inflammatory disease severity by generating proinflammatory molecules (PAR-1, PAR-2, IL-36γ, Fractalkine, Endostatin, Ephrin-B2), inactivating anti-inflammatory mediators (SLPI) and degrading molecules involved in antimicrobial and immunomodulatory responses (surfactant protein-A, LL-37, beta-defensins), inter-endothelial/-epithelial barrier function, gene repair and energy homeostasis. These pathways could be targeted by repositioning of existing drugs. These findings suggest that inhibiting cathepsin S or a specific downstream target of cathepsin S by repositioning of existing drugs could be a promising strategy for treating autoimmune and inflammatory diseases. Current cathepsin S inhibitors in clinical trials face challenges, highlighting the need for innovative inhibitors that function effectively in various cellular compartments with differing pH levels, without targeting the shared catalytic site of cysteine cathepsins.

The multifaceted roles of cathepsins in immune and inflammatory responses: implications for cancer therapy, autoimmune diseases, and infectious diseases

The cathepsin family comprises lysosomal proteases that play essential roles in various physiological processes, including protein degradation, antigen presentation, apoptosis, and tissue remodeling. Dysregulation of cathepsin activity has been linked to a variety of pathological conditions, such as cancer, autoimmune diseases, and neurodegenerative disorders. Understanding the functions of cathepsins is crucial for gaining insights into their roles in both health and disease, as well as for developing targeted therapeutic approaches. Emerging research underscores the significant involvement of cathepsins in immune cells, particularly T cells, macrophages, dendritic cells, and neutrophils, as well as their contribution to immune-related diseases. In this review, we systematically examine the impact of cathepsins on the immune system and their mechanistic roles in cancer, infectious diseases, autoimmune and neurodegenerative disorders, with the goal of identifying novel therapeutic strategies for these conditions.

Compensational role between cathepsins

Cathepsins, a family of lysosomal peptidases, play a crucial role in maintaining cellular homeostasis by regulating protein turnover and degradation as well as many specific regulatory actions that are important for proper cell function and human health. Alterations in the activity and expression of cathepsins have been observed in many diseases such as cancer, inflammation, neurodegenerative disorders, bone remodelling-related conditions and others. These changes are not exclusively harmful, but rather appear to be a compensatory response on the lack of one cathepsin in order to maintain tissue integrity. The upregulation of specific cathepsins in response to the inhibition or dysfunction of other cathepsins suggests a fine-tuned system of proteolytic balance and understanding the compensatory role of cathepsins may improve therapeutic potential of cathepsin's inhibitors. Selectively targeting one cathepsin or modulating their activity could offer new treatment strategies for a number of diseases. This review emphasises the need for comprehensive research into cathepsin biology in the context of disease. The identification of the specific cathepsins involved in compensatory responses, the elucidation of the underlying molecular mechanisms and the development of targeted interventions could lead to innovative therapeutic approaches.

Cysteine cathepsins: From diagnosis to targeted therapy of cancer

Cysteine cathepsins are a fascinating group of proteolytic enzymes that play diverse and crucial roles in numerous biological processes, both in health and disease. Understanding these proteases is essential for uncovering novel insights into the underlying mechanisms of a wide range of disorders, such as cancer. Cysteine cathepsins influence cancer biology by participating in processes such as extracellular matrix degradation, angiogenesis, immune evasion, and apoptosis. In this comprehensive review, we explore foundational research that illuminates the diverse and intricate roles of cysteine cathepsins as diagnostic markers and therapeutic targets for cancer. This review aims to provide valuable insights into the clinical relevance of cysteine cathepsins and explore their capacity to advance personalised and targeted medical interventions in oncology.

The Intricate Balance between Life and Death: ROS, Cathepsins, and Their Interplay in Cell Death and Autophagy

Cellular survival hinges on a delicate balance between accumulating damages and repair mechanisms. In this intricate equilibrium, oxidants, currently considered physiological molecules, can compromise vital cellular components, ultimately triggering cell death. On the other hand, cells possess countermeasures, such as autophagy, which degrades and recycles damaged molecules and organelles, restoring homeostasis. Lysosomes and their enzymatic arsenal, including cathepsins, play critical roles in this balance, influencing the cell's fate toward either apoptosis and other mechanisms of regulated cell death or autophagy. However, the interplay between reactive oxygen species (ROS) and cathepsins in these life-or-death pathways transcends a simple cause-and-effect relationship. These elements directly and indirectly influence each other's activities, creating a complex web of interactions. This review delves into the inner workings of regulated cell death and autophagy, highlighting the pivotal role of ROS and cathepsins in these pathways and their intricate interplay.

P2X4 signalling contributes to hyperactivity but not pain sensitization comorbidity in a mouse model of attention deficit/hyperactivity disorder

Introduction: Attention deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder characterized by hyperactivity, inattention, and impulsivity that often persist until adulthood. Frequent comorbid disorders accompany ADHD and two thirds of children diagnosed with ADHD also suffer from behavioural disorders and from alteration of sensory processing. We recently characterized the comorbidity between ADHD-like symptoms and pain sensitisation in a pharmacological mouse model of ADHD, and we demonstrated the implication of the anterior cingulate cortex and posterior insula. However, few studies have explored the causal mechanisms underlying the interactions between ADHD and pain. The implication of inflammatory mechanisms has been suggested but the signalling pathways involved have not been explored. Methods: We investigated the roles of purinergic signalling, at the crossroad of pain and neuroinflammatory pathways, by using a transgenic mouse line that carries a total deletion of the P2X4 receptor. Results: We demonstrated that P2X4 deletion prevents hyperactivity in the mouse model of ADHD. In contrast, the absence of P2X4 lowered thermal pain thresholds in sham conditions and did not affect pain sensitization in ADHD-like conditions. We further analysed microglia reactivity and the expression of inflammatory markers in wild type and P2X4KO mice. Our results revealed that P2X4 deletion limits microglia reactivity but at the same time exerts proinflammatory effects in the anterior cingulate cortex and posterior insula. Conclusion: This dual role of P2X4 could be responsible for the differential effects noted on ADHD-like symptoms and pain sensitization and calls for further studies to investigate the therapeutic benefit of targeting the P2X4 receptor in ADHD patients.

Evaluation of variable new antigen receptors (vNARs) as a novel cathepsin S (CTSS) targeting strategy

Aberrant activity of the cysteine protease Cathepsin S (CTSS) has been implicated across a wide range of pathologies. Notably in cancer, CTSS has been shown to promote tumour progression, primarily through facilitating invasion and migration of tumour cells and augmenting angiogenesis. Whilst an attractive therapeutic target, more efficacious CTSS inhibitors are required. Here, we investigated the potential application of Variable New Antigen Receptors (vNARs) as a novel inhibitory strategy. A panel of potential vNAR binders were identified following a phage display panning process against human recombinant proCTSS. These were subsequently expressed, purified and binding affinity confirmed by ELISA and SPR based approaches. Selected lead clones were taken forward and were shown to inhibit CTSS activity in recombinant enzyme activity assays. Further assessment demonstrated that our lead clones functioned by a novel inhibitory mechanism, by preventing the activation of proCTSS to the mature enzyme. Moreover, using an intrabody approach, we exhibited the ability to express these clones intracellularly and inhibit CTSS activity whilst lead clones were also noted to impede cell invasion in a tumour cell invasion assay. Collectively, these findings illustrate a novel mechanistic approach for inhibiting CTSS activity, with anti-CTSS vNAR clones possessing therapeutic potential in combating deleterious CTSS activity. Furthermore, this study exemplifies the potential of vNARs in targeting intracellular proteins, opening a range of previously "undruggable" targets for biologic-based therapy.

Lysosomal control of dendritic cell function

Lysosomal compartments undergo extensive remodeling during dendritic cell (DC) activation to meet the dynamic functional requirements of DCs. Instead of being regarded as stationary and digestive organelles, recent studies have increasingly appreciated the versatile roles of lysosomes in regulating key aspects of DC biology. Lysosomes actively control DC motility by linking calcium efflux to the actomyosin contraction, while enhanced DC lysosomal membrane permeability contributes to the inflammasome activation. Besides, lysosomes provide a platform for the transduction of innate immune signaling and the intricate host-pathogen interplay. Lysosomes and lysosome-associated structures are also critically engaged in antigen presentation and cross-presentation processes, which are pivotal for the induction of antigen-specific adaptive immune response. Through the current review, we emphasize that lysosome targeting strategies serve as vital DC-based immunotherapies in fighting against tumor, infectious diseases, and autoinflammatory disorders.

Type 2 Cystatins and Their Roles in the Regulation of Human Immune Response and Cancer Progression

Cystatins are a family of intracellular and extracellular protease inhibitors that inhibit cysteine cathepsins-a group of lysosomal cysteine proteases that participate in multiple biological processes, including protein degradation and post-translational cleavage. Cysteine cathepsins are associated with the development of autoimmune diseases, tumor progression, and metastasis. Cystatins are categorized into three subfamilies: type 1, type 2, and type 3. The type 2 cystatin subfamily is the largest, containing 10 members, and consists entirely of small secreted proteins. Although type 2 cystatins have many shared biological roles, each member differs in structure, post-translational modifications (e.g., glycosylation), and expression in different cell types. These distinctions allow the type 2 cystatins to have unique biological functions and properties. This review provides an overview of type 2 cystatins, including their biological similarities and differences, their regulatory effect on human immune responses, and their roles in tumor progression, immune evasion, and metastasis.

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.

Type I Cystatin Derived from Fasciola gigantica Suppresses Macrophage-Mediated Inflammatory Responses

There is an inverse relationship between the high incidence of helminth infection and the low incidence of inflammatory disease. Hence, it may be that helminth molecules have anti-inflammatory effects. Helminth cystatins are being extensively studied for anti-inflammatory potential. Therefore, in this study, the recombinant type I cystatin (stefin-1) of Fasciola gigantica (rFgCyst) was verified to have LPS-activated anti-inflammatory potential, including in human THP-1-derived macrophages and RAW 264.7 murine macrophages. The results from the MTT assay suggest that rFgCyst did not alter cell viability; moreover, it exerted anti-inflammatory activity by decreasing the production of proinflammatory cytokines and mediators, including IL-1β, IL-6, IL-8, TNF-α, iNOS, and COX-2 at the gene transcription and protein expression levels, as determined by qRT-PCR and Western blot analysis, respectively. Further, the secretion levels of IL-1β, IL-6, and TNF-α determined by ELISA and the NO production level determined by the Griess test were decreased. Furthermore, in Western blot analysis, the anti-inflammatory effects involved the downregulation of pIKKα/β, pIκBα, and pNF-κB in the NF-κB signaling pathway, hence reducing the translocation from the cytosol into the nucleus of pNF-κB, which subsequently turned on the gene of proinflammatory molecules. Therefore, cystatin type 1 of F. gigantica is a potential candidate for inflammatory disease treatment.

LAMP2 regulates autophagy in the thymic epithelium and thymic stroma-dependent CD4 T cell development

Within the thymus, thymic epithelial cells (TECs) provide dedicated thymic stroma microenvironments for T cell development. Because TEC functionality is sensitive to aging and cytoablative therapies, unraveling the molecular elements that coordinate their thymopoietic role has fundamental and clinical implications. Particularly, the selection of CD4 T cells depends on interactions between TCRs expressed on T cell precursors and self-peptides:MHC II complexes presented by cortical TECs (cTECs). Although the macroautophagy/autophagy-lysosomal protein degradation pathway is implicated in CD4 T cell selection, the molecular mechanism that controls the generation of selecting MHC II ligands remains elusive. LAMP2 (lysosomal-associated membrane protein 2) is a well-recognized mediator of autolysosome (AL) maturation. We showed that LAMP2 is highly expressed in cTECs. Notably, genetic inactivation of Lamp2 in thymic stromal cells specifically impaired the development of CD4 T cells that completed positive selection, without misdirecting MHC II-restricted cells into the CD8 lineage. Mechanistically, defects in autophagy in lamp2-deficient cTECs were linked to alterations in MHC II processing, which was associated with a marked reduction in CD4 TCR repertoire diversity selected within the lamp2-deficient thymic stroma. Together, our findings suggest that LAMP2 interconnects the autophagy-lysosomal axis and the processing of selecting self-peptides:MHC II complexes in cTECs, underling its implications for the generation of a broad CD4 TCR repertoire.Abbreviations: AIRE: autoimmune regulator (autoimmune polyendocrinopathy candidiasis ectodermal dystrophy); AL: autolysosome; AP: autophagosome; Baf-A1: bafilomycin A₁; B2M: beta-2 microglobulin; CTSL: cathepsin L; CD74/Ii: CD74 antigen (invariant polypeptide of major histocompatibility complex, class II antigen-associated); CFSE: carboxyfluorescein succinimidyl ester; CFU: colony-forming unit; CLIP: class II-associated invariant chain peptides; cTECs: cortical TECs dKO: double knockout; DN: double negative; DP: double positive; ENPEP/LY51: glutamyl aminopeptidase; FOXP3: forkhead box; P3 IFNG/IFNγ: interferon gamma; IKZF2/HELIOS: IKAROS family zinc finger 2; IL2RA/CD25: interleukin 2 receptor, alpha chain; KO: knockout; LAMP2: lysosomal-associated membrane protein 2; LIP: lymphopenia-induced proliferation; Lm: Listeria monocytogenes; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MHC: major histocompatibility complex; mTECs: medullary TECs; PRSS16/TSSP: protease, serine 16 (thymus); SELL/CD62L: selectin, lymphocyte; SP: single positive; TCR: T cell receptor; TCRB: T cell receptor beta chain; TECs: thymic epithelial cells; UEA-1: Ulex europaeus agglutinin-1; WT: wild-type.

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.

IFN-γR/STAT1 signaling in recipient hematopoietic antigen-presenting cells suppresses graft-versus-host disease

The absence of IFN-γ receptor (IFN-γR) or STAT1 signaling in donor cells has been shown to result in reduced induction of acute graft-versus-host disease (GVHD). In this study, we unexpectedly observed increased activation and expansion of donor lymphocytes in both lymphohematopoietic organs and GVHD target tissues of IFN-γR/STAT1-deficient recipient mice, leading to rapid mortality following the induction of GVHD. LPS-matured, BM-derived Ifngr1-/- Stat1-/- DCs (BMDCs) were more potent allogeneic stimulators and expressed increased levels of MHC II and costimulatory molecules. Similar effects were observed in human antigen-presenting cells (APCs) with knockdown of Stat1 by CRISPR/Cas9 and treatment with a JAK1/2 inhibitor. Furthermore, we demonstrated that the absence of IFN-γR/STAT1 signaling in hematopoietic APCs impaired the presentation of exogenous antigens, while promoting the presentation of endogenous antigens. Thus, the indirect presentation of host antigens to donor lymphocytes was defective in IFN-γR/STAT1-deficient, donor-derived APCs in fully donor chimeric mice. The differential effects of IFN-γR/STAT1 signaling on endogenous and exogenous antigen presentation could provide further insight into the roles of the IFN-γ/STAT1 signaling pathway in the pathogenesis of GVHD, organ rejection, and autoimmune diseases.

Essential Oil of Citrus sudachi Suppresses T Cell Activation Both In Vitro and In Vivo

The essential oil of Citrus sudachi (sudachi oil) is extracted from the peel of sudachi, a citrus plant. We investigated the effect of sudachi oil on immune function in both in vitro antigen (Ag) induced lymphocyte activation and in vivo Ag-specific immune response. In the in vitro study, the proliferative activity of splenocytes upon Ag-specific and non-specific stimulation was suppressed by treatment with sudachi oil in a dose-dependent manner. In addition, the expression level of Ag-presentation-related molecules and their Ag-presenting function on dendritic cells were suppressed by sudachi oil. To examine how sudachi oil regulates an Ag-specific immune response in vivo, mice were immunized with ovalbumin and the immune response of the mice was investigated. Ag-specific proliferation response of splenocytes from mice treated with sudachi essential oil was significantly suppressed. The results indicate that sudachi oil suppresses T cell and dendritic cell functions in vitro and Ag-specific T cell induction in vivo.

Cysteine cathepsins: A long and winding road towards clinics

Biomedical research often focuses on properties that differentiate between diseased and healthy tissue; one of the current focuses is elevated expression and altered localisation of proteases. Among these proteases, dysregulation of cysteine cathepsins can frequently be observed in inflammation-associated diseases, which tips the functional balance from normal physiological to pathological manifestations. Their overexpression and secretion regularly exhibit a strong correlation with the development and progression of such diseases, making them attractive pharmacological targets. But beyond their mostly detrimental role in inflammation-associated diseases, cysteine cathepsins are physiologically highly important enzymes involved in various biological processes crucial for maintaining homeostasis and responding to different stimuli. Consequently, several challenges have emerged during the efforts made to translate basic research data into clinical applications. In this review, we present both physiological and pathological roles of cysteine cathepsins and discuss the clinical potential of cysteine cathepsin-targeting strategies for disease management and diagnosis.

Cathepsin S (CTSS) activity in health and disease - A treasure trove of untapped clinical potential

Amongst the lysosomal cysteine cathepsin family of proteases, cathepsin S (CTSS) holds particular interest due to distinctive properties including a normal restricted expression profile, inducible upregulation and activity at a broad pH range. Consequently, while CTSS is well-established as a member of the proteolytic cocktail within the lysosome, degrading unwanted and damaged proteins, it has increasingly been shown to mediate a number of distinct, more selective roles including antigen processing and antigen presentation, and cleavage of substrates both intra and extracellularly. Increasingly, aberrant CTSS expression has been demonstrated in a variety of conditions and disease states, marking it out as both a biomarker and potential therapeutic target. This review seeks to contextualise CTSS within the cysteine cathepsin family before providing an overview of the broad range of pathologies in which roles for CTSS have been identified. Additionally, current clinical progress towards specific inhibitors is detailed, updating the position of the field in exploiting this most unique of proteases.

Antigen Presentation in the Lung

The lungs are constantly exposed to environmental and infectious agents such as dust, viruses, fungi, and bacteria that invade the lungs upon breathing. The lungs are equipped with an immune defense mechanism that involves a wide variety of immunological cells to eliminate these agents. Various types of dendritic cells (DCs) and macrophages (MACs) function as professional antigen-presenting cells (APCs) that engulf pathogens through endocytosis or phagocytosis and degrade proteins derived from them into peptide fragments. During this process, DCs and MACs present the peptides on their major histocompatibility complex class I (MHC-I) or MHC-II protein complex to naïve CD8⁺ or CD4⁺ T cells, respectively. In addition to these cells, recent evidence supports that antigen-specific effector and memory T cells are activated by other lung cells such as endothelial cells, epithelial cells, and monocytes through antigen presentation. In this review, we summarize the molecular mechanisms of antigen presentation by APCs in the lungs and their contribution to immune response.

Fractalkine/CX3CR1 Pathway in Neuropathic Pain: An Update

Injuries to the nervous system can result in a debilitating neuropathic pain state that is often resistant to treatment with available analgesics, which are commonly associated with several side-effects. Growing pre-clinical and clinical evidence over the last two decades indicates that immune cell-mediated mechanisms both in the periphery and in the Central Nervous System (CNS) play significant roles in the establishment and maintenance of neuropathic pain. Specifically, following peripheral nerve injury, microglia, which are CNS resident immune cells, respond to the activity of the first pain synapse in the dorsal horn of spinal cord and also to neuronal activity in higher centres in the brain. This microglial response leads to the production and release of several proinflammatory mediators which contribute to neuronal sensitisation under neuropathic pain states. In this review, we collect evidence demonstrating the critical role played by the Fractalkine/CX₃CR₁ signalling pathway in neuron-to-microglia communication in neuropathic pain states and explore how strategies that include components of this pathway offer opportunities for innovative targets for neuropathic pain.

Exploring the role of cathepsin in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease which is marked by leukocytes infiltration inside synovial tissue, joints and also inside synovial fluid which causes progressive destruction of joint cartilage. There are numerous genetical and lifestyle factors, responsible for rheumatoid arthritis. One such factor can be cysteine cathepsins, which act as proteolytic enzymes. These proteolytic enzyme gets activated at acidic pH and are found in lysosomes and are also termed as cysteine proteases. These proteases belong to papain family and have their elucidated role in musculoskeletal disorders. Numerous cathepsins have their targeted role in rheumatoid arthritis. These proteases are secreted through various cell types which includes matrix metalloproteases and papain like cysteine proteases. These proteases can potentially lead to bone and cartilage destruction which causes an immune response in case of inflammatory arthritis.

Cathepsin S is a novel target for age-related dry eye

Cathepsin S (Ctss) is a protease that is proinflammatory on epithelial cells. The purpose of this study was to investigate the role of Ctss in age-related dry eye disease. Ctss^-/- mice [in a C57BL/6 (B6) background] of different ages were compared to B6 mice. Ctss activity in tears and lacrimal gland (LG) lysates was measured. The corneal barrier function was investigated in naïve mice or after topical administration of Ctss eye drops 5X/day for two days. Eyes were collected, and conjunctival goblet cell density was measured in PAS-stained sections. Immunoreactivity of the tight junction proteins, ZO-1 and occludin, was investigated in primary human cultured corneal epithelial cells (HCEC) without or with Ctss, with or without a Ctss inhibitor. A significant increase in Ctss activity was observed in the tears and LG lysates in aged B6 compared to young mice. This was accompanied by higher Ctss transcripts and protein expression in LG and spleen. Compared to B6, 12 and 24-month-old Ctss^-/- mice did not display age-related corneal barrier disruption and goblet cell loss. Treatment of HCEC with Ctss for 48 h disrupted occludin and ZO-1 immunoreactivity compared to control cells. This was prevented by the Ctss inhibitor LY3000328 or Ctss-heat inactivation. Topical reconstitution of Ctss in Ctss^-/- mice for two days disrupted corneal barrier function. Aging on the ocular surface is accompanied by increased expression and activity of the protease Ctss. Our results suggest that cathepsin S modulation might be a novel target for age-related dry eye disease.

Alterations in Lysosome Homeostasis in Lipid-Related Disorders: Impact on Metabolic Tissues and Immune Cells

Lipid-related disorders, which primarily affect metabolic tissues, including adipose tissue and the liver are associated with alterations in lysosome homeostasis. Obesity is one of the more prevalent diseases, which results in energy imbalance within metabolic tissues and lysosome dysfunction. Less frequent diseases include Niemann-Pick type C (NPC) and Gaucher diseases, both of which are known as Lysosomal Storage Diseases (LSDs), where lysosomal dysfunction within metabolic tissues remains to be fully characterized. Adipocytes and hepatocytes share common pathways involved in the lysosome-autophagic axis, which are regulated by the function of cathepsins and CD36, an immuno-metabolic receptor and display alterations in lipid diseases, and thereby impacting metabolic functions. In addition to intrinsic defects observed in metabolic tissues, cells of the immune system, such as B cells can infiltrate adipose and liver tissues, during metabolic imbalance favoring inflammation. Moreover, B cells rely on lysosomes to promote the processing and presentation of extracellular antigens and thus could also present lysosome dysfunction, consequently affecting such functions. On the other hand, growing evidence suggests that cells accumulating lipids display defective inter-organelle membrane contact sites (MCSs) established by lysosomes and other compartments, which contribute to metabolic dysfunctions at the cellular level. Overall, in this review we will discuss recent findings addressing common mechanisms that are involved in lysosome dysregulation in adipocytes and hepatocytes during obesity, NPC, and Gaucher diseases. We will discuss whether these mechanisms may modulate the function of B cells and how inter-organelle contacts, emerging as relevant cellular mechanisms in the control of lipid homeostasis, have an impact on these diseases.

New Insights into the Role of Cysteine Cathepsins in Neuroinflammation

Neuroinflammation, which is mediated by microglia and astrocytes, is associated with the progression of neurodegenerative diseases. Increasing evidence shows that activated microglia induce the expression and secretion of various lysosomal cathepsins, particularly during the early stage of neuroinflammation. This trigger signaling cascade that aggravate neurodegeneration. To date, most research on neuroinflammation has focused on the role of cysteine cathepsins, the largest cathepsin family. Cysteine cathepsins are primarily responsible for protein degradation in lysosomes; however, they also play a role in regulating a number of other important physiological and pathological processes. This review focuses on the functional roles of cysteine cathepsins in the central nervous system during neuroinflammation, with an emphasis on their roles in the polarization of microglia and neuroinflammation signaling, which in turn causes neuronal death and thus neurodegeneration.

Peptides for T cell selection in the thymus

Major histocompatibility complex (MHC)-associated peptides generated and displayed by antigen-presenting cells in the thymus are essential for the generation of functional and self-tolerant T cells that protect our body from various pathogens. The peptides displayed by cortical thymic epithelial cells (cTECs) are generated by unique enzymatic machineries including the thymoproteasomes, and are involved in the positive selection of self-protective T cells. On the other hand, the peptides displayed by medullary thymic epithelial cells (mTECs) and thymic dendritic cells (DCs) are involved in further selection to establish self-tolerance in T cells. Although the biochemical nature of the peptide repertoire displayed in the thymus remains unclear, many studies have suggested a thymus-specific mechanism for the generation of MHC-associated peptides in the thymus. In this review, we summarize basic knowledge and recent advances in MHC-associated thymic peptides, focusing on the generation and function of thymoproteasome-dependent peptides specifically displayed by cTECs.

Partnering for the major histocompatibility complex class II and antigenic determinant requires flexibility and chaperons

Cytotoxic, or helper T cells recognize antigen via T cell receptors (TCRs) that can see their target antigen as short sequences of peptides bound to the groove of proteins of major histocompatibility complex (MHC) class I, and class II respectively. For MHC class II epitope selection from exogenous pathogens or self-antigens, participation of several accessory proteins, molecular chaperons, processing enzymes within multiple vesicular compartments is necessary. A major contributing factor is the MHC class II structure itself that uniquely offers a dynamic and flexible groove essential for epitope selection. In this review, I have taken a historical perspective focusing on the flexibility of the MHC II molecules as the driving force in determinant selection and interactions with the accessory molecules in antigen processing, HLA-DM and HLA-DO.

Distinct antigen uptake receptors route to the same storage compartments for cross-presentation in dendritic cells

An exclusive feature of dendritic cells (DCs) is their capacity to present exogenous antigens by MHC class I molecules, called cross‐presentation. Here, we show that protein antigen can be conserved in mature murine DCs for several days in a lysosome‐like storage compartment, distinct from MHC class II and early endosomal compartments, as an internal source for the supply of MHC class I ligands. Using two different uptake routes via Fcγ receptors and C‐type lectin receptors, we could show that antigens were routed towards the same endolysosomal compartments after 48 h. The antigen‐containing compartments lacked co‐expression of molecules involved in MHC class I processing and presentation including TAP and proteasome subunits as shown by single‐cell imaging flow cytometry. Moreover, we observed the absence of cathepsin S but selective co‐localization of active cathepsin X with protein antigen in the storage compartments. This indicates cathepsin S‐independent antigen degradation and a novel but yet undefined role for cathepsin X in antigen processing and cross‐presentation by DCs. In summary, our data suggest that these antigen‐containing compartments in DCs can conserve protein antigens from different uptake routes and contribute to long‐lasting antigen cross‐presentation.

Cartilage Protective and Immunomodulatory Features of Osteoarthritis Synovial Fluid-Treated Adipose-Derived Mesenchymal Stem Cells Secreted Factors and Extracellular Vesicles-Embedded miRNAs

Intra-articular administration of adipose-derived mesenchymal stem cells (ASCs), either in vitro expanded or within adipose tissue-based products obtained at point-of-care, has gained popularity as innovative regenerative medicine approach for osteoarthritis (OA) treatment. ASCs can stimulate tissue repair and immunomodulation through paracrine factors, both soluble and extracellular vesicles (EV) embedded, collectively defining the secretome. Interaction with the degenerative/inflamed environment is a crucial factor in understanding the finely tuned molecular message but, to date, the majority of reports have described ASC-secretome features in resting conditions or under chemical stimuli far from the in vivo environment of degenerated OA joints. In this report, the secretory profile of ASCs treated with native synovial fluid from OA patients was evaluated, sifting 200 soluble factors and 754 EV-embedded miRNAs. Fifty-eight factors and 223 EV-miRNAs were identified, and discussed in the frame of cartilage and immune cell homeostasis. Bioinformatics gave a molecular basis for M2 macrophage polarization, T cell proliferation inhibition and T reg expansion enhancement, as well as cartilage protection, further confirmed in an in vitro model of OA chondrocytes. Moreover, a strong influence on immune cell chemotaxis emerged. In conclusion, obtained molecular data support the regenerative and immunomodulatory properties of ASCs when interacting with osteoarthritic joint environment.

Covalent Small Molecule Immunomodulators Targeting the Protease Active Site

Cells of the immune system utilize multiple proteases to regulate cell functions and orchestrate innate and adaptive immune responses. Dysregulated protease activities are implicated in many immune-related disorders; thus, protease inhibitors have been actively investigated for pharmaceutical development. Although historically considered challenging with concerns about toxicity, compounds that covalently modify the protease active site represent an important class of agents, emerging not only as chemical probes but also as approved drugs. Here, we provide an overview of technologies useful for the study of proteases with the focus on recent advances in chemoproteomic methods and screening platforms. By highlighting covalent inhibitors that have been designed to target immunomodulatory proteases, we identify opportunities for the development of small molecule immunomodulators.

Serum proteins associated with periodontitis relapse post-surgery: A pilot study

BACKGROUND

The knowledge of which genes and proteins that are connected to the susceptibility to gingivitis with subsequent local tissue degradation seen in periodontitis is insufficient. Changes of serum proteins associated with recurrence of bleeding on probing (BOP) and increased periodontal pocket depths (PPD) after surgical treatment of periodontitis could reveal molecules that could be early signals of tissue destruction and/or of importance for systemic effects in other tissues or organs.

METHODS

We performed a longitudinal pilot study and followed 96 inflammation-related proteins over time in serum from patients who underwent surgical treatment of periodontitis (n= 21). The samples were taken before (time 0), and then at 3, 6, and 12 months after surgery. Changes in protein levels were analysed in relation to the clinical outcome measures, that is, proportion of surfaces affected by BOP and PPD.

RESULTS

Changes in treatment outcomes with early signs of relapse in periodontitis after surgical treatment, for example, increased BOP and PPDs, were during 12-months follow up associated with increased serum levels of high-sensitivity C-reactive protein (hs-CRP) and programmed death-ligand 1 (PD-L1), and reduced serum levels of cystatin-D protein.

CONCLUSION

This study shows that clinical signs of recurrence of periodontitis after surgery are reflected in serum, but larger studies are needed for verification. Our novel findings of an association between increased PD-L1- and decreased cystatin D-levels and recurrence in periodontitis are interesting because PD-L1 has been shown to facilitate bacterial infections and chronic inflammation and cystatin D to inhibit tissue destruction. Our results justify mechanistic studies regarding the role of these molecules in periodontitis.

Usefulness of Cathepsin S to Predict Risk for Obstructive Sleep Apnea among Patients with Type 2 Diabetes

Background

Obstructive sleep apnea (OSA) was highly prevalent in patients with type 2 diabetes (T2D). Cathepsin S (CTSS), a cysteine protease, is involved in the inflammatory activity in T2D and hypoxia conditions. The aim of the study was to evaluate whether CTSS could be involved in the inflammatory reaction of OSA in patients with T2D.

Methods

We included 158 participants in this study matched for age, gender, and body mass index in 4 groups (control, non-OSA&T2D, OSA&non-T2D, and OSA&T2D). After overnight polysomnography, we collected the clinical data including anthropometrical characteristics, blood pressure, and fasting blood samples in the morning. Plasma CTSS concentration was evaluated using the human Magnetic Luminex Assay.

Results

Compared with the control group, both the non-OSA&T2D group and the OSA&non-T2D group showed higher CTSS levels. Plasma CTSS expression was significantly increased in subjects with OSA&T2D compared to subjects with non-OSA&T2D. The OSA&T2D group had higher CTSS levels than the OSA&non-T2D group, but there were no statistically significant differences. Plasma CTSS levels showed significant correlation with the apnea-hypopnea index (AHI) (r = 0.559, P < 0.001) and plasma fasting blood glucose (r = 0.427, P < 0.001). After adjusting confounding factors, plasma CTSS levels were independently associated with the AHI (Beta: 0.386, 95% confidence intervals (CI): 21.988 to 57.781; P < 0.001). Furthermore, we confirmed the higher pinpoint accuracy of plasma CTSS in the diagnosis of OSA (area under the curve: 0.868).

Conclusions

Plasma CTSS expression was significantly elevated in the OSA&T2D group and was independently associated with the AHI; it could be a biomarker with a positive diagnostic value on diagnosing OSA among patients with T2D.

Cathepsin S as a potential therapeutic target for chronic pain

Chronic pain is a distressing yet poorly-treated condition that can arise as a result of diseases and injuries to the nervous system. The development of more efficacious therapies for chronic pain is essential and requires advances in our understanding of its underlying mechanisms. Clinical and preclinical evidence has demonstrated that immune responses play a crucial role in chronic pain. The lysosomal cysteine protease cathepsin S (CatS) plays a key role in such immune response. Here we discuss the preclinical evidence for the mechanistic importance of extracellular CatS in chronic pain focussing on studies utilising drugs and other pharmacological tools that target CatS activity. We also consider the use of CatS inhibitors as potential novel antihyperalgesics, highlighting that the route and timing of delivery would need to be tailored to the initial cause of pain in order to ensure the most effective use of such drugs.

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Cathepsin S plays a key extracellular role in the underlying mechanisms of chronic pain

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Pharmacological tools provide crucial evidence for this role and the therapeutic potential of targeting Cathepsin S

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The route of delivery and timing of cathepsin S inhibitor administration should be tailored to specific causes of chronic pain

Calcium-sensing receptor-mediated NLRP3 inflammasome response to calciprotein particles drives inflammation in rheumatoid arthritis

Increased extracellular Ca2+ concentrations ([Ca2+]ex) trigger activation of the NLRP3 inflammasome in monocytes through calcium-sensing receptor (CaSR). To prevent extraosseous calcification in vivo, the serum protein fetuin-A stabilizes calcium and phosphate into 70-100 nm-sized colloidal calciprotein particles (CPPs). Here we show that monocytes engulf CPPs via macropinocytosis, and this process is strictly dependent on CaSR signaling triggered by increases in [Ca2+]ex. Enhanced macropinocytosis of CPPs results in increased lysosomal activity, NLRP3 inflammasome activation, and IL-1β release. Monocytes in the context of rheumatoid arthritis (RA) exhibit increased CPP uptake and IL-1β release in response to CaSR signaling. CaSR expression in these monocytes and local [Ca2+] in afflicted joints are increased, probably contributing to this enhanced response. We propose that CaSR-mediated NLRP3 inflammasome activation contributes to inflammatory arthritis and systemic inflammation not only in RA, but possibly also in other inflammatory conditions. Inhibition of CaSR-mediated CPP uptake might be a therapeutic approach to treating RA.

Adenosine-to-inosine RNA editing in the immune system: friend or foe?

Our body expresses sensors to detect pathogens through the recognition of expressed molecules, including nucleic acids, lipids, and proteins, while immune tolerance prevents an overreaction with self and the development of autoimmune disease. Adenosine (A)-to-inosine (I) RNA editing, catalyzed by adenosine deaminases acting on RNA (ADARs), is a post-transcriptional modification that can potentially occur at over 100 million sites in the human genome, mainly in Alu repetitive elements that preferentially form a double-stranded RNA (dsRNA) structure. A-to-I conversion within dsRNA, which may induce a structural change, is required to escape from the host immune system, given that endogenous dsRNAs transcribed from Alu repetitive elements are potentially recognized by melanoma differentiation-associated protein 5 (MDA5) as non-self. Of note, loss-of-function mutations in the ADAR1 gene cause Aicardi-Goutières syndrome, a congenital autoimmune disease characterized by encephalopathy and a type I interferon (IFN) signature. However, the loss of ADAR1 in cancer cells with an IFN signature induces lethality via the activation of protein kinase R in addition to MDA5. This makes cells more sensitive to immunotherapy, highlighting the opposing immune status of autoimmune diseases (overreaction) and cancer (tolerance). In this review, we provide an overview of insights into two opposing aspects of RNA editing that functions as a modulator of the immune system in autoimmune diseases and cancer.

Leading the invasion: The role of Cathepsin S in the tumour microenvironment

Elevated expression of the cysteine protease Cathepsin S has been correlated with a number of different cancer types in recent years. As tools have been developed to enable more accurate examination of individual cathepsin species, our knowledge and appreciation of the role that this protease plays in facilitating cancer has increased exponentially. This review focuses on our current understanding of the role of Cathepsin S within tumours and the surrounding microenvironment. While various publications have shown that Cathepsin S can be derived from tumour cells themselves, a plethora of more recent studies have identified that Cathepsin S can also be derived from other cell types within the tumour microenvironment including endothelial cells, macrophages and T cells. Furthermore, specific proteolytic substrates cleaved by Cathepsin S have also been identified which have reinforced our hypothesis that this protease facilitates key steps within tumours leading to their invasion, angiogenesis and metastasis.

A novel cystatin derived from Trichinella spiralis suppresses macrophage-mediated inflammatory responses

Trichinella spiralis can modulate host immune responses to retain a suitable environment for its long-term survival. Incidentally, the parasite elicits regulatory effects through immunomodulatory molecule release, which can suppress host inflammation and may be used for the treatment of unrelated inflammatory diseases in someday. Here we identified and characterized a novel T. spiralis cystatin (TsCstN), which inhibits inflammation mediated by LPS-treated macrophages.Proteins contained in the excretory-secretory (ES) product of muscle-stage T. spiralis (ES-L1) were fractionated, and each was treated with mouse bone marrow-derived macrophages (mBMDMs) before LPS stimulation. The fractions that exhibited high immunomodulatory property by decreasing pro-inflammatory cytokines or increasing anti-inflammatory cytokines were identified by mass spectrometry. Incidentally, the conserved hypothetical protein (Tsp_04814) was selected for further characterization as it presented the most significant MS score. An annotation of Tsp_04814 using protein structural homology comparison suggested that it has high structural similarity to human cystatin E/M (TM score 0.690). The recombinant T. spiralis novel cystatin (rTsCstN) was expressed in Escherichia coli at a molecular weight of approximately 13 kDa. Mouse anti-rTsCstN polyclonal antibody (pAb) could detect native TsCstN in crude worm antigens (CWA) and ES-L1 and be predominantly localized in the stichosome and subcuticular cells. rTsCstN inhibited cysteine proteases in vitro, especially cathepsin L, at an optimal pH of 6. Besides, rTsCstN could be internalized into mBMDMs, which were mostly distributed in the cytoplasm and lysosome both before and after LPS stimulation. To evaluate the rTsCstN immunomodulatory properties on mBMDMs, rTsCstN was incubated with mBMDM before LPS stimulation; this demonstrated that rTsCstN suppressed pro-inflammatory cytokine production and MHC class II expression.T. spiralis L1-derived TsCstN was characterized as a novel cysteine protease inhibitor. The protein elicits an anti-inflammatory property by suppressing pro-inflammatory cytokines and interfering with the antigen presentation process through depletion of MHC class II expression.

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.

Increased adenosine-to-inosine RNA editing in rheumatoid arthritis

OBJECTIVE

Adenosine-to-inosine (A-to-I) RNA editing of Alu retroelements is a primate-specific mechanism mediated by adenosine deaminases acting on RNA (ADARs) that diversifies transcriptome by changing selected nucleotides in RNA molecules. We tested the hypothesis that A-to-I RNA editing is altered in rheumatoid arthritis (RA).

METHODS

Synovium expression analysis of ADAR1 was investigated in 152 RA patients and 50 controls. Peripheral blood mononuclear cells derived from 14 healthy subjects and 19 patients with active RA at baseline and after 12-week treatment were examined for ADAR1p150 and ADAR1p110 isoform expression by RT-qPCR. RNA editing activity was analysed by AluSx+ Sanger-sequencing of cathepsin S, an extracellular matrix degradation enzyme involved in antigen presentation.

RESULTS

ADAR1 was significantly over-expressed in RA synovium regardless of disease duration. Similarly, ADAR1p150 isoform expression was significantly increased in the blood of active RA patients. Individual nucleotide analysis revealed that A-to-I RNA editing rate was also significantly increased in RA patients. Both baseline ADAR1p150 expression and individual adenosine RNA editing rate of cathepsin S AluSx+ decreased after treatment only in those patients with good clinical response. Upregulation of the expression and/or activity of the RNA editing machinery were associated with a higher expression of edited Alu-enriched genes including cathepsin S and TNF receptor-associated factors 1,2,3 and 5.

CONCLUSION

A previously unrecognized regulation and role of ADAR1p150-mediated A-to-I RNA editing in post-transcriptional control in RA underpins therapeutic response and fuels inflammatory gene expression, thus representing an interesting therapeutic target.

Melatonin attenuates epidermal growth factor-induced cathepsin S expression in ARPE-19 cells: Implications for proliferative vitreoretinopathy

Abnormal proliferation and motility of retinal pigment epithelial cells leads to proliferative vitreoretinopathy (PVR). Melatonin is a known effective antitumour and anti-invasive agent, but whether it affects the formation and underlying mechanisms of PVR remains unclear. In this study, the results of the MTT assay, colony formation and propidium iodide (PI) staining with flow cytometry revealed that melatonin dose dependently inhibited epidermal growth factor (EGF)-induced proliferation of human ARPE-19 cells. Furthermore, melatonin reduced EGF-induced motility by suppressing cathepsin S (CTSS) expression. Pretreatment with ZFL (a CTSS inhibitor) or overexpression of CTSS (pCMV-CTSS) significantly inhibited EGF-induced cell motility when combined with melatonin. Epidermal growth factor induced the phosphorylation of AKT(S473)/mTOR (S2448) and transcription factor (c-Jun/Sp1) signaling pathways. Pretreatment of LY294002 (a PI3K inhibitor) or rapamycin (an mTOR inhibitor) markedly reduced EGF-induced motility and p-AKT/p-mTOR/c-Jun/Sp1 expression when combined with melatonin. Taken together, these data indicate that melatonin inhibited EGF-induced proliferation and motility of human ARPE-19 cells by activating the AKT/mTOR pathway, which is dependent on CTSS modulation of c-Jun/Sp1 signalling. Melatonin may be a promising therapeutic drug against PVR.

Novel targets for drug discovery in celiac disease

Celiac disease is a lifelong, immunological disorder induced by dietary protein-gluten, in a genetically susceptible populations, resulting in different clinical manifestations, the release of antibodies, and damage to the intestinal mucosa. The only recommended therapy for the disease is to strictly follow a gluten-free diet (GFD), which is difficult to comply with. A GFD is found to be ineffective in some active Celiac disease cases. Therefore, there is an unmet need for an alternative nondietary therapeutic approach. The review focuses on the novel drug targets for Celiac disease.

Lysosomes as a therapeutic target

Lysosomes are membrane-bound organelles with roles in processes involved in degrading and recycling cellular waste, cellular signalling and energy metabolism. Defects in genes encoding lysosomal proteins cause lysosomal storage disorders, in which enzyme replacement therapy has proved successful. Growing evidence also implicates roles for lysosomal dysfunction in more common diseases including inflammatory and autoimmune disorders, neurodegenerative diseases, cancer and metabolic disorders. With a focus on lysosomal dysfunction in autoimmune disorders and neurodegenerative diseases - including lupus, rheumatoid arthritis, multiple sclerosis, Alzheimer disease and Parkinson disease - this Review critically analyses progress and opportunities for therapeutically targeting lysosomal proteins and processes, particularly with small molecules and peptide drugs.

Differential effects of specific cathepsin S inhibition in biocompartments from patients with primary Sjögren syndrome

OBJECTIVE

Primary Sjögren syndrome (pSS) is characterized by T and B cell infiltration of exocrine glands. The cysteine protease cathepsin S (CatS) is crucially involved in MHCII processing and T cell stimulation, and elevated levels have been found in patients with RA, psoriasis and pSS. However, little is known about the functional characteristics and mechanisms of SS-A- and SS-B-specific T cells in pSS patients. We herein investigated the inhibition of CatS activity in different biocompartments of pSS patients including antigen-specific T cell responses.

METHODS

Ex vivo CatS activity was assessed in tears, plasma and saliva of 15 pSS patients and 13 healthy controls (HC) and in the presence or absence of the specific CatS inhibitor RO5459072. In addition, antigen (SS-A (60kD), SS-B, influenza H3N2, tetanus toxoid and SEB)-specific T cell responses were examined using ex vivo IFN-γ/IL-17 Dual ELISPOT and Bromdesoxyuridin (BrdU) proliferation assays in the presence or absence of RO5459072. Supernatants were analysed for IL-1β, IL-6, IL-10, TNF-α, IL-21, IL-22 and IL-23, using conventional ELISA.

RESULTS

CatS activity was significantly elevated in tear fluid, but not other biocompartments, was inversely associated with exocrinic function in pSS patients and could significantly be suppressed by RO5459072. Moreover, CatS inhibition by RO5459072 led to strong and dose-dependent suppression of SS-A/SS-B-specific T cell effector functions and cytokine secretion by CD14+ monocytes. However, RO5459072 was incapable of suppressing SS-A/SS-B-induced secretion of cytokines in CD14+ monocytes when T cells were absent, confirming a CatS/MHCII-mediated mechanism of suppression.

CONCLUSION

CatS activity in tear fluid seems to be a relevant biomarker for pSS disease activity. Conversely, CatS inhibition diminishes T cell and associated monokine responses towards relevant autoantigens in pSS. Thus, CatS inhibition may represent a promising novel treatment strategy in pSS.

In Vitro Digestion with Proteases Producing MHC Class II Ligands

Proteases generate peptides that bind to MHC class II molecules to interact with a wide diversity of CD4⁺ T cells. They are expressed in dedicated organelles: endosomes and lysosomes of professional antigen-presenting cells (pAPCs) such as B cells, macrophages, and dendritic cells. The identification of endosomal proteases which produce antigenic peptides is important for example for better vaccination and to prevent autoimmune diseases. Here, we describe a panel of techniques (in vitro digestion assays of protein with recombinant proteases or purified endosomes/lysosomes, T cell stimulation) to monitor the production of MHC class II ligands.

Ectromelia virus suppresses expression of cathepsins and cystatins in conventional dendritic cells to efficiently execute the replication process

Background

Cathepsins are a group of endosomal proteases present in many cells including dendritic cells (DCs). The activity of cathepsins is regulated by their endogenous inhibitors – cystatins. Cathepsins are crucial to antigen processing during viral and bacterial infections, and as such are a prerequisite to antigen presentation in the context of major histocompatibility complex class I and II molecules. Due to the involvement of DCs in both innate and adaptive immune responses, and the quest to understand the impact of poxvirus infection on host cells, we investigated the influence of ectromelia virus (ECTV) infection on cathepsin and cystatin levels in murine conventional DCs (cDCs). ECTV is a poxvirus that has evolved many mechanisms to avoid host immune response and is able to replicate productively in DCs.

Results

Our results showed that ECTV-infection of JAWS II DCs and primary murine GM-CSF-derived bone marrow cells down-regulated both mRNA and protein of cathepsin B, L and S, and cystatin B and C, particularly during the later stages of infection. Moreover, the activity of cathepsin B, L and S was confirmed to be diminished especially at later stages of infection in JAWS II cells. Consequently, ECTV-infected DCs had diminished ability to endocytose and process a soluble antigen. Close examination of cellular protein distribution showed that beginning from early stages of infection, the remnants of cathepsin L and cystatin B co-localized and partially co-localized with viral replication centers (viral factories), respectively. Moreover, viral yield increased in cDCs treated with siRNA against cathepsin B, L or S and subsequently infected with ECTV.

Conclusions

Taken together, our results indicate that infection of cDCs with ECTV suppresses cathepsins and cystatins, and alters their cellular distribution which impairs the cDC function. We propose this as an additional viral strategy to escape immune responses, enabling the virus to replicate effectively in infected cells.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1471-1) contains supplementary material, which is available to authorized users.

Cysteine Cathepsins and their Extracellular Roles: Shaping the Microenvironment

For a long time, cysteine cathepsins were considered primarily as proteases crucial for nonspecific bulk proteolysis in the endolysosomal system. However, this view has dramatically changed, and cathepsins are now considered key players in many important physiological processes, including in diseases like cancer, rheumatoid arthritis, and various inflammatory diseases. Cathepsins are emerging as important players in the extracellular space, and the paradigm is shifting from the degrading enzymes to the enzymes that can also specifically modify extracellular proteins. In pathological conditions, the activity of cathepsins is often dysregulated, resulting in their overexpression and secretion into the extracellular space. This is typically observed in cancer and inflammation, and cathepsins are therefore considered valuable diagnostic and therapeutic targets. In particular, the investigation of limited proteolysis by cathepsins in the extracellular space is opening numerous possibilities for future break-through discoveries. In this review, we highlight the most important findings that establish cysteine cathepsins as important players in the extracellular space and discuss their roles that reach beyond processing and degradation of extracellular matrix (ECM) components. In addition, we discuss the recent developments in cathepsin research and the new possibilities that are opening in translational medicine.

ECTV Abolishes the Ability of GM-BM Cells to Stimulate Allogeneic CD4 T Cells in a Mouse Strain-Independent Manner

Ectromelia virus (ECTV) is the etiological agent of mousepox, an acute and systemic disease with high mortality rates in susceptible strains of mice. Resistance and susceptibility to mousepox are triggered by the dichotomous T-helper (Th) immune response generated in infected animals, with strong protective Th1 or nonprotective Th2 profile, respectively. Th1/Th2 balance is influenced by dendritic cells (DCs), which were shown to differ in their ability to polarize naïve CD4⁺ T cells in different mouse strains. Therefore, we have studied the inner-strain differences in the ability of conventional DCs (cDCs), generated from resistant (C57BL/6) and susceptible (BALB/c) mice, to stimulate proliferation and activation of Th cells upon ECTV infection. We found that ECTV infection of GM-CSF-derived bone marrow (GM-BM) cells, composed of cDCs and macrophages, affected initiation of allogeneic CD4⁺ T cells proliferation in a mouse strain-independent manner. Moreover, infected GM-BM cells from both mouse strains failed to induce and even inhibited the production of Th1 (IFN-γ and IL-2), Th2 (IL-4 and IL-10) and Th17 (IL-17A) cytokines by allogeneic CD4⁺ T cells. These results indicate that in in vitro conditions ECTV compromises the ability of cDCs to initiate/polarize adaptive antiviral immune response independently of the host strain resistance/susceptibility to lethal infection.

The Phosphoinositide Kinase PIKfyve Promotes Cathepsin-S-Mediated Major Histocompatibility Complex Class II Antigen Presentation

Antigen presentation to T cells in major histocompatibility complex class II (MHC class II) requires the conversion of early endo/phagosomes into lysosomes by a process called maturation. Maturation is driven by the phosphoinositide kinase PIKfyve. Blocking PIKfyve activity by small molecule inhibitors caused a delay in the conversion of phagosomes into lysosomes and in phagosomal acidification, whereas production of reactive oxygen species (ROS) increased. Elevated ROS resulted in reduced activity of cathepsin S and B, but not X, causing a proteolytic defect of MHC class II chaperone invariant chain Ii processing. We developed a novel universal MHC class II presentation assay based on a bio-orthogonal "clickable" antigen and showed that MHC class II presentation was disrupted by the inhibition of PIKfyve, which in turn resulted in reduced activation of CD4⁺ T cells. Our results demonstrate a key role of PIKfyve in the processing and presentation of antigens, which should be taken into consideration when targeting PIKfyve in autoimmune disease and cancer.

Class II MHC antigen processing in immune tolerance and inflammation

Presentation of peptide antigens by MHC-II proteins is prerequisite to effective CD4 T cell tolerance to self and to recognition of foreign antigens. Antigen uptake and processing pathways as well as expression of the peptide exchange factors HLA-DM and HLA-DO differ among the various professional and non-professional antigen-presenting cells and are modulated by cell developmental state and activation. Recent studies have highlighted the importance of these cell-specific factors in controlling the source and breadth of peptides presented by MHC-II under different conditions. During inflammation, increased presentation of selected self-peptides has implications for maintenance of peripheral tolerance and autoimmunity.