Pepstatin A-sensitive aspartic proteases in lysosome are involved in degradation of the invariant chain and antigen-processing in antigen presenting cells of mice infected with Leishmania major

The Role of Cysteine Peptidases in Hematopoietic Stem Cell Differentiation and Modulation of Immune System Function

Cysteine cathepsins are primarily involved in the degradation and recycling of proteins in endo-lysosomal compartments but are also gaining recognition as pivotal proteolytic contributors to various immune functions. Through their extracellular proteolytic activities within the hematopoietic stem cell niche, they are involved in progenitor cell mobilization and differentiation. Cysteine cathepsins, such as cathepsins L and S contribute to antigen-induced adaptive immunity through major histocompatibility complex class II antigen presentation whereas cathepsin X regulates T-cell migration. By regulating toll-like receptor signaling and cytokine secretion cysteine cathepsins activate innate immune cells and affect their functional differentiation. Cathepsins C and H are expressed in cytotoxic T lymphocytes and natural killer cells and are involved in processing of pro-granzymes into proteolytically active forms. Cytoplasmic activities of cathepsins B and L contribute to the maintenance of homeostasis of the adaptive immune response by regulating cell death of T and B lymphocytes. The expression pattern, localization, and activity of cysteine cathepsins is tightly connected to their function in immune cells. Furthermore, cysteine cathepsins together with their endogenous inhibitors, serve as mediators in the interplay between cancer and immune cells that results in immune cell anergy. The aim of the present article is to review the mechanisms of dysregulation of cysteine cathepsins and their inhibitors in relation to immune dysfunction to address new possibilities for regulation of their function.

HIV-1 Nef Targets HDAC6 to Assure Viral Production and Virus Infection

HIV Nef is a central auxiliary protein in HIV infection and pathogenesis. Our results indicate that HDAC6 promotes the aggresome/autophagic degradation of the viral polyprotein Pr55Gag to inhibit HIV-1 production. Nef counteracts this antiviral activity of HDAC6 by inducing its degradation and subsequently stabilizing Pr55Gag and Vif viral proteins. Nef appears to neutralize HDAC6 by an acidic/endosomal-lysosomal processing and does not need the downregulation function, since data obtained with the non-associated cell-surface Nef-G2A mutant - the cytoplasmic location of HDAC6 - together with studies with chemical inhibitors and other Nef mutants, point to this direction. Hence, the polyproline rich region P72xxP75 (69-77 aa) and the di-Leucin motif in the Nef-ExxxLL160-165 sequence of Nef, appear to be responsible for HDAC6 clearance and, therefore, required for this novel Nef proviral function. Nef and Nef-G2A co-immunoprecipitate with HDAC6, whereas the Nef-PPAA mutant showed a reduced interaction with the anti-HIV-1 enzyme. Thus, the P72xxP75 motif appears to be responsible, directly or indirectly, for the interaction of Nef with HDAC6. Remarkably, by neutralizing HDAC6, Nef assures Pr55Gag location and aggregation at plasma membrane, as observed by TIRFM, promotes viral egress, and enhances the infectivity of viral particles. Consequently, our results suggest that HDAC6 acts as an anti-HIV-1 restriction factor, limiting viral production and infection by targeting Pr55Gag and Vif. This function is counteracted by functional HIV-1 Nef, in order to assure viral production and infection capacities. The interplay between HIV-1 Nef and cellular HDAC6 may determine viral infection and pathogenesis, representing both molecules as key targets to battling HIV.

Deficiency of Selected Cathepsins Does Not Affect the Inhibitory Action of ECTV on Immune Properties of Dendritic Cells

Ectromelia virus (ECTV), an orthopoxvirus, undergoes productive replication in conventional dendritic cells (cDCs), resulting in the inhibition of their innate and adaptive immune functions. ECTV replication rate in cDCs is increased due to downregulation of the expression of cathepsins - cystein proteases that orchestrate several steps during DC maturation. Therefore, this study was aimed to determine if downregulation of cathepsins, such as B, L or S, disrupts cDC capacity to induce activating signals in T cells or whether infection of cDCs with ECTV further weakens their functions as antigen-presenting cells. Our results showed that cDCs treated with siRNA against cathepsin B, L and S synthesize similar amounts of pro-inflammatory cytokines and exhibit comparable ability to mature and stimulate alloreactive CD4⁺ T cells, as untreated wild type (WT) cells. Moreover, ECTV inhibitory effect on cDC innate and adaptive immune functions, observed especially after LPS treatment, was comparable in both cathepsin-silenced and WT cells. Taken together, the absence of cathepsins B, L and S has minimal, if any, impact on the inhibitory effect of ECTV on cDC immune functions. We assume that the virus-mediated inhibition of cathepsin expression in cDCs represents more a survival mechanism than an immune evasion strategy.

Lysosome-Dependent Activation of Human Dendritic Cells by the Vaccine Adjuvant QS-21

The adjuvant properties of the saponin QS-21 have been known for decades. It is a component of the Adjuvant System AS01 that is used in several vaccine candidates. QS-21 strongly potentiates both cellular and humoral immune responses to purified antigens, yet how it activates immune cells is largely unknown. Here, we report that QS-21 directly activated human monocyte-derived dendritic cells (moDCs) and promoted a pro-inflammatory transcriptional program. Cholesterol-dependent QS-21 endocytosis followed by lysosomal destabilization and Syk kinase activation were prerequisites for this response. Cathepsin B, a lysosomal cysteine protease, was essential for moDC activation in vitro and contributed to the adjuvant effects of QS-21 in vivo. Collectively, these findings provide new insights into the pathways involved in the direct activation of antigen-presenting cells by a clinically relevant QS-21 formulation.

Specific functions of lysosomal proteases in endocytic and autophagic pathways

Endolysosomal vesicles form a highly dynamic multifunctional cellular compartment that contains multiple highly potent proteolytic enzymes. Originally these proteases have been assigned to cooperate solely in executing the unselective ‘bulk proteolysis’ within the acidic milieu of the lysosome. Although to some degree this notion still holds true, evidence is accumulating for specific and regulatory functions of individual ‘acidic’ proteases in many cellular processes linked to the endosomal/lysosomal compartment. Here we summarize and discuss the functions of individual endolysosomal proteases in such diverse processes as the termination of growth factor signaling, lipoprotein particle degradation, infection, antigen presentation, and autophagy. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Protease expression by microorganisms and its relevance to crucial physiological/pathological events

The treatment of infections caused by fungi and trypanosomatids is difficult due to the eukaryotic nature of these microbial cells, which are similar in several biochemical and genetic aspects to host cells. Aggravating this scenario, very few antifungal and anti-trypanosomatidal agents are in clinical use and, therefore, therapy is limited by drug safety considerations and their narrow spectrum of activity, efficacy and resistance. The search for new bioactive agents against fungi and trypanosomatids has been expanded because progress in biochemistry and molecular biology has led to a better understanding of important and essential pathways in these microorganisms including nutrition, growth, proliferation, signaling, differentiation and death. In this context, proteolytic enzymes produced by these eukaryotic microorganisms are appointed and, in some cases, proven to be excellent targets for searching novel natural and/or synthetic pharmacological compounds, in order to cure or prevent invasive fungal/trypanosomatid diseases. With this task in mind, our research group and others have focused on aspartic-type proteases, since the activity of this class of hydrolytic enzymes is directly implicated in several facets of basic biological processes of both fungal and trypanosomatid cells as well as due to the participation in numerous events of interaction between these microorganisms and host structures. In the present paper, a concise revision of the beneficial effects of aspartic protease inhibitors, with emphasis on the aspartic protease inhibitors used in the anti-human immunodeficiency virus therapy, will be presented and discussed using our experience with the following microbial models: the yeast Candida albicans, the filamentous fungus Fonsecaea pedrosoi and the protozoan trypanosomatid Leishmania amazonensis.

Structure-based development of specific inhibitors for individual cathepsins and their medical applications

Specific inhibitors for individual cathepsins have been developed based on their tertiary structures of X-ray crystallography. Cathepsin B-specific inhibitors, CA-074 and CA-030, and cathepsin L specific inhibitors, CLIK-148 and CLIK-195, were designed as the epoxysuccinate derivatives. Cathepsin S inhibitor, CLIK-060, and cathepsin K inhibitor, CLIK-166, were synthesized. These inhibitors can use in vitro and also in vivo, and show no toxicity for experimental animals by the amounts used as the cathepsin inhibitor. Various cathepsins are used in the processing of antigenic proteins. The CLIK-060 treatment to the autoimmune disease, Sjögren model mice, led to strongly suppress the expression of the pathological symptoms. Cathepsins L or K participates to the degradation of bone collagen. The CLIK-148 protects osteoporosis in animals and also protects the bone metastasis of cancer cells. Cathepsin L also enhances insulin-induced glucose uptake into 3T3-L1 adipocytes, suggesting cathepsin L plays the roles in adipogenesis and glucose tolerance in type 2 diabetes.

Grassystatins A-C from marine cyanobacteria, potent cathepsin E inhibitors that reduce antigen presentation

In our efforts to explore marine cyanobacteria as a source of novel bioactive compounds, we discovered a statine unit-containing linear decadepsipeptide, grassystatin A (1), which we screened against a diverse set of 59 proteases. We describe the structure determination of 1 and two natural analogues, grassystatins B (2) and C (3), using NMR, MS, and chiral HPLC techniques. Compound 1 selectively inhibited cathepsins D and E with IC(50)s of 26.5 nM and 886 pM, respectively. Compound 2 showed similar potency and selectivity against cathepsins D and E (IC(50)s of 7.27 nM and 354 pM, respectively), whereas the truncated peptide analogue grassystatin C (3), which consists of two fewer residues than 1 and 2, was less potent against both but still selective for cathepsin E. The selectivity of compounds 1-3 for cathepsin E over D (20-38-fold) suggests that these natural products may be useful tools to probe cathepsin E function. We investigated the structural basis of this selectivity using molecular docking. We also show that 1 can reduce antigen presentation by dendritic cells, a process thought to rely on cathepsin E.

Molecular dynamics simulation of Leishmania major surface metalloprotease GP63 (leishmanolysin)

One of the molecular factors contributing to Leishmania sp. virulence and pathogenesis is the major surface metalloprotease GP63, alternatively called leishmanolysin, MSP, and PSP (EC 3.4.24.36). Here, the molecular dynamics simulation of Leishmania major GP63 in water at pH 7 is reported. Upon solvation, GP63 undergoes a sharp structural relaxation with respect to the crystal structure. The fluctuation pattern occurs essentially in solvent-exposed nonstructured regions. By contrast, the active site turns out to be rigid. Essential dynamics and dynamic-domain analyses, both carried out on the equilibrated portion of GP63, show that the fingerprint fluctuations of GP63 are practically characterized by the motion of a large part of the N-terminal domain. These results appear to be in line with substrate recognition and (pro)enzyme activation played by the N-terminal domain of GP63. A systematic analysis among a series of 10 homologs of GP63 also shows that the residues involved in the interdomain bending result highly conserved. This finding also suggests possible relationship between the maintainance of proteolytic activity and the similarity of the dynamical properties of the related enzymes.

Cathepsin S is not crucial to TSHR processing and presentation in a murine model of Graves' disease

By regulating invariant (Ii) chain processing and MHC class II peptide loading, the endosomal protease cathepsin S (Cat S) has a potential role in autoimmune susceptibility. Indeed, Cat S null mice are resistant to I-Ab-restricted experimental myasthenia gravis due to inadequate peptide presentation. To explore the role of Cat S in a Graves' disease model, I-Ad-restricted wild-type (WT) and Cat S(-/-) mice were immunized with adenovirus encoding the A subunit of thyroid stimulating hormone receptor (TSHR). TSHR adenovirus immunized mice develop Th1 T cells, TSHR antibodies, and a proportion become overtly hyperthyroid. Although TSHR presentation in vitro was initially impaired in Cat S(-/-) mice, subsequent TSHR presentation in vitro and disease development were similar in both groups but with higher antibody responses in Cat S null mice. WT and Cat S(-/-) mice recognized similar T cell epitopes from a panel of overlapping TSHR peptides. TSHR responses were found to be I-Ad-restricted and Cat S(-/-) I-Ad B cells had marked defects in Ii processing. These data imply that loading of TSHR peptides critical to TSHR antibody responses becomes Ii-independent. Contrasting findings among organ-specific murine autoimmune models imply that potential uses of Cat S inhibitors to ameliorate autoimmunity must be determined empirically.

Cathepsin S is required for murine autoimmune myasthenia gravis pathogenesis

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

Targeting plasmid-encoded proteins to the antigen presentation pathways

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

Microglial functions and proteases

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

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

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

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

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

Treatment with cathepsin L inhibitor potentiates Th2-type immune response in Leishmania major-infected BALB/c mice

Prior to the activation of CD4 (+) T cells, exogenous proteins must be digested by endo/lysosomal enzymes in antigen-presenting cells (APC) to produce antigenic peptides that are able to be presented on class II molecules of the MHC. Studies described here inspect the functional significance of cathepsin L inhibition for antigen processing and T (h) 1/T (h) 2 differentiation in experimental leishmaniasis. We first demonstrated using in vitro systems that cathepsin L is one of the candidate endo/lysosomal enzymes in processing of soluble Leishmania antigen (SLA) and that its specific inhibitor, CLIK148, modulated the processing of SLA. BALB/c mice are known to be susceptible to infection with Leishmania major. Interestingly, treatment of BALB/c mice with CLIK148 exacerbated the infection by enhancing the development of SLA-specific T (h) 2-type response such as production of IL-4 and generation of T (h) 2-dependent specific IgE/IgG1 antibodies. Moreover, addition of CLIK148 in incubation of a SLA-specific CD4 (+) T cell line with APC up-regulated the production of IL-4. However, CLIK148 did not exert any direct influence on the function of T cells themselves. Taken together, these findings suggest that treatment of host mice with CLIK148 affects the processing of SLA in APC, resulting in the potentiation of T (h) 2-type immune responses and thus leading to exacerbation of the infection. Furthermore, endo/lysosomal cathepsin L was found to be functionally distinct from previously described cathepsins B and D.

Splenic cathepsin L is maturated from the proform by interferon-gamma after immunization with exogenous antigens

The processing of foreign protein antigens into peptides requires the participation of various endo/lysosomal proteases in antigen-presenting cells (APCs). In this study, a proenzyme of cathepsin L, procathepsin L, was found to be present in the spleens of naive mice, as demonstrated by immunoblotting. Interestingly, the maturation of cathepsin L from procathepsin L was strongly induced when the host BALB/c mice were immunized with ovalbumin or soluble leishmanial antigen, despite the fact that mouse albumin, a kind of self-antigen, did not have such a potential. Furthermore, foreign antigens, but not self-antigens, could increase the activity of cathepsin L, probably being mediated by interferon-gamma, as demonstrated by in vivo and in vitro experiments. As cathepsin L matured, the efficiency of antigen processing was increased in APCs. These results suggest that endo/lysosomal cathepsin L plays an important role in the immune regulation via antigen processing even in peripheral lymphoid tissues as well as in the thymus.