Inhibition of invariant chain processing, antigen-induced proliferative responses, and the development of collagen-induced arthritis and experimental autoimmune encephalomyelitis by a small molecule cysteine protease inhibitor

Disease status in human and experimental arthritis, and response to TNF blockade, is associated with MHC class II invariant chain (CD74) isoform expression

Splice variants of CD74 differentially modulate the activity of cathepsin L (CTSL). As CD74 and CTSL participate in the pathogenesis of inflammatory diseases such as rheumatoid arthritis (RA), we determined whether splice variants of CD74 could be biomarkers of disease activity. Gene expression was measured in mice with collagen-induced arthritis using quantitative PCR (qPCR). In vitro studies using murine macrophage/DC-lineage cells determined the relative influence of macrophage phenotype on isoform expression and the potential to produce CTSL in response to TNF. CD74 splice variants were measured in human RA synovium and RA patients' monocytes. In arthritic mice, the expression of the p41 CD74 isoform was significantly higher in severely affected paws compared with unaffected paws or the paws of naïve mice; the p41 isoform significantly correlated with the expression of TNF in arthritic paws. Compared with M2-like macrophages, M1-like macrophages expressed increased levels of CD74 and had higher expression, secretion and activity of CTSL. RA patients that responded to TNF blockade had significantly higher expression levels of CD74 in circulating monocytes after treatment, compared with non-responders. The expression of the human CD74 isoform a was significantly higher in RA synovia, compared with osteoarthritis synovia, and was associated with CSTL enzymatic activity. This study is the first to demonstrate differential expression of the CD74 p41 isoform in an auto-immune disorder and in response to therapy. The differential expression of CD74 splice variants indicates an association, and potentially a mechanistic role, in the pathogenesis of RA.

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

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

Redundancy between Cysteine Cathepsins in Murine Experimental Autoimmune Encephalomyelitis

The cysteine cathepsins B, S, and L are functionally linked to antigen processing, and hence to autoimmune disorders such as multiple sclerosis. Stemming from several studies that demonstrate that mice can be protected from experimental autoimmune encephalomyelitis (EAE) through the pharmacologic inhibition of cysteine cathepsins, it has been suggested that targeting these enzymes in multiple sclerosis may be of therapeutic benefit. Utilizing mice deficient in cysteine cathepsins both individually and in combination, we found that the myelin-associated antigen myelin oligodendrocyte glycoprotein (MOG) was efficiently processed and presented by macrophages to CD4+ T cells in the individual absence of cathepsin B, S or L. Similarly, mice deficient in cathepsin B or S were susceptible to MOG-induced EAE and displayed clinical progression and immune infiltration into the CNS, similar to their wild-type counterparts. Owing to a previously described CD4+ T cell deficiency in mice deficient in cathepsin L, such mice were protected from EAE. When multiple cysteine cathepsins were simultaneously inhibited via genetic deletion of both cathepsins B and S, or by a cathepsin inhibitor (LHVS), MHC-II surface expression, MOG antigen presentation and EAE were attenuated or prevented. This study demonstrates the functional redundancy between cathepsin B, S and L in EAE, and suggests that the inhibition of multiple cysteine cathepsins may be needed to modulate autoimmune disorders such as multiple sclerosis.

Recent advances of cluster of differentiation 74 in cancer

Cluster of differentiation 74 (CD74) performs multiple roles in B cells, T cells, and antigen-presenting cells within the immune system; it also participates in major histocompatibility complex class II-restricted antigen presentation and inflammation. Recently, a role for CD74 in carcinogenesis has been described. CD74 promotes cell proliferation and motility and prevents cell death in a macrophage migration inhibitory factor-dependent manner. Its roles as an accessory signal receptor on the cell surface and the ability to interact with other signaling molecules make CD74 an attractive therapeutic target for the treatment of cancer. This review focuses on the original role of CD74 in the immune system and its emerging tumor-related functions. First, the structure of CD74 will be summarized. Second, the current understandings about the expression, cellular localization, molecular mechanisms and signaling pathways of CD74 in immunity and cancer will be reviewed. Third, the examples that suggest CD74 is a promising molecular therapeutic target are reviewed and discussed. Although the safety and efficacy of CD74-targeted strategies are under development, deeply understanding of the regulation of CD74 will hold promise for the use of CD74 as a therapeutic target and may develop the CD74-targeted therapeutic agents such as neutralized antibody and compounds.

Spinal cathepsin S and fractalkine contribute to chronic pain in the collagen-induced arthritis model

OBJECTIVE

The induction of rheumatoid arthritis (RA) by active and passive immunization of mice results in the development of pain at the same time as the swelling and inflammation, with both peripheral and central sensitization contributing to joint pain. The purpose of this study was to examine the development of pain in the rat model of collagen-induced arthritis (CIA) and to evaluate the contribution of neuroimmune interactions to established arthritis pain.

METHODS

Mechanical hypersensitivity was assessed in female Lewis rats before and up to 18 days after induction of CIA by immunization with type II collagen. The effect of selective inhibitors of microglia were then evaluated by prolonged intrathecal delivery of a cathepsin S (CatS) inhibitor and a fractalkine (FKN) neutralizing antibody, from day 11 to day 18 following immunization.

RESULTS

Rats with CIA developed significant mechanical hypersensitivity, which started on day 9, before the onset of clinical signs of arthritis. Mechanical hypersensitivity peaked with the severity of the disease, when significant microglial and astrocytic responses, alongside T cell infiltration, were observed in the spinal cord. Intrathecal delivery of microglial inhibitors, a CatS inhibitor, or an FKN neutralizing antibody attenuated mechanical hypersensitivity and spinal microglial response in rats with CIA.

CONCLUSION

The inhibition of microglial targets by centrally penetrant CatS inhibitors and CX(3) CR1 receptor antagonists represents a potential therapeutic avenue for the treatment of pain in RA.

Microglial signalling mechanisms: Cathepsin S and Fractalkine

A recent major conceptual advance has been the recognition of the importance of immune system-neuron interactions in the modulation of spinal pain processing. In particular, pro-inflammatory mediators secreted by immune competent cells such as microglia modulate nociceptive function in the injured CNS and following peripheral nerve damage. Chemokines play a pivotal role in mediating neuronal-microglial communication which leads to increased nociception. Here we examine the evidence that one such microglial mediator, the lysosomal cysteine protease Cathepsin S (CatS), is critical for the maintenance of neuropathic pain via cleavage of the transmembrane chemokine Fractalkine (FKN). Both CatS and FKN mediate critical physiological functions necessary for immune regulation. As key mediators of homeostatic functions it is not surprising that imbalance in these immune processes has been implicated in autoimmune disorders including Multiple Sclerosis and Rheumatoid Arthritis, both of which are associated with chronic pain. Thus, impairment of the CatS/FKN signalling pair constitutes a novel therapeutic approach for the treatment of chronic pain.

Azepanone-based inhibitors of human cathepsin S: optimization of selectivity via the P2 substituent

A series of azepanone inhibitors of cathepsin S is described. Selectivity over both cathepsin K and cathepsin L was achieved by varying the P2 substituent. Ultimately, a balanced potency and selectivity profile was achieved in compound 39 possessing a 1-methylcyclohexyl alanine at P2 and nicotinamide as the P' substituent. The cellular potency of selected analogs is also described.

Invariant chain processing is independent of cathepsin variation between primary human B cells/dendritic cells and B-lymphoblastoid cells

As part of the endocytic antigen processing pathway, proteolytic cleavage of the invariant chain (Ii) is important for the generation of class II-associated invariant chain peptide (CLIP). CLIP remains associated with the major histocompatibility complex (MHC) class II molecule to prevent premature loading of antigenic peptides. Cysteine proteases, such as Cathepsin S (CatS), CatL, or CatV, play a pivotal role in the final stage of Ii degradation depending on the cell type studied. Less is known regarding the early stages of Ii processing. We therefore explored whether the serine protease CatG is involved in the initial step of Ii degradation in primary antigen presenting cells (APC), since the cathepsin distribution differs between primary APC and cell lines. While primary human B cells and dendritic cells (DC) do harbor CatG, this protease is absent in B-lymphoblastoid cells (BLC) or monocyte-derived DC generated in vitro. In addition, other proteases, such as CatC, CatL, and the asparagine endoprotease (AEP), are active in BLC and monocyte-derived DC. Here we demonstrate that CatG progressively degraded Ii in vitro resulting in several intermediates. However, pharmacological inhibition of CatG in primary B cells and DC did not alter Ii processing, indicating that CatG is dispensable in Ii degradation. Interestingly, stalling of cysteine proteases by inhibition in BLC vs. primary B cells and DC did not result in any differences in the generation of distinct Ii intermediates between the cells tested, suggesting that Ii processing is independent of the cathepsin variation within professional human APC.

CD74: an emerging opportunity as a therapeutic target in cancer and autoimmune disease

INTRODUCTION

CD74, also known as the invariant chain, participates in several key processes of the immune system, including antigen presentation, B-cell differentiation and inflammatory signaling. Despite being described more than 3 decades ago, new functions and novel interactions for this evolutionarily conserved molecule are still being unraveled. As a participant in several immunological processes and an indicator of disease in some conditions, it has potential as a therapeutic target.

AREAS COVERED

The relationship between the structure of CD74 variants and their physiological functions is detailed in this review. The function of CD74 in several cell lineages is examined with a focus on the interactions with cathepsins and, in an inflammatory milieu, the pro-inflammatory cytokine macrophage migratory inhibitory factor. The role of CD74 signaling in inflammatory and carcinogenic processes is outlined as is the use of CD74 as a therapeutic target (in cancer) and tool (as a vaccine).

EXPERT OPINION

CD74 has several roles within the cell and throughout the immune system. Most prominent amongst these are the complex relationships with MIF and cathepsins. Modulation of CD74 function shows promise for the effective amelioration of disease.

The resorptive apparatus of osteoclasts supports lysosomotropism and increases potency of basic versus non-basic inhibitors of cathepsin K

In mice and humans, the effect of genetic deficiency of cathepsin K (catK) is impaired bone resorption, or osteopetrosis. Inhibition of catK is therefore a promising strategy for the treatment of osteoporosis. The enzyme acts in an acid environment. This provides a further potential opportunity: if the inhibitor is basic it is more likely to accumulate in membrane-bound acidic compartments (lysosomotropism), so minimizing off-target effects. However, the resorptive hemivacuole is not membrane-bound, and so might not retain lysosomotropic compounds. We therefore elected to determine whether the osteoclastic resorptive apparatus supports such accumulation. First, we attempted to compare the persistence of a lysosomotropic dye in the hemivacuole versus intracellular vesicles. To our surprise the dye could not be detected in the ruffled border region by confocal microscopy. We found that this could be explained by the tight packing of the folds of the ruffled border, and their close apposition to the bone surface. We also found that the dye persisted similarly in resorbing osteoclasts and macrophages, consistent with the notion that resorbing osteoclasts support lysosomotropism. Next, we compared the ability of basic and non-basic inhibitors of catK to suppress bone resorption by human osteoclasts. We found that basic compounds were considerably more potent than non-basic compounds at suppression of osteoclastic resorption than would be anticipated from their potency as enzyme inhibitors. Also consistent with osteoclastic lysosomotropism, basic inhibitors suppressed resorption for substantially longer than a non-basic inhibitor after washout from cell cultures. Furthermore, selectivity of basic inhibitors for inhibition of catK versus other cathepsins persisted: concentrations that inhibited catK in osteoclasts had no detectable effect on cathepsin S (catS) in a cell-based assay. This data is consistent with accumulation and enrichment of such basic inhibitors in the resorptive apparatus of the osteoclast, allowing for prolonged efficacy at the intended site of action. Our results suggest a major advantage for lysosomotropic compounds as inhibitors of bone resorption by osteoclasts in osteoporosis and other diseases caused by excessive osteoclastic activity.

The immunomodulatory action of sialostatin L on dendritic cells reveals its potential to interfere with autoimmunity

Sialostatin L (SialoL) is a secreted cysteine protease inhibitor identified in the salivary glands of the Lyme disease vector Ixodes scapularis. In this study, we reveal the mechanisms of SialoL immunomodulatory actions on the vertebrate host. LPS-induced maturation of dendritic cells from C57BL/6 mice was significantly reduced in the presence of SialoL. Although OVA degradation was not affected by the presence of SialoL in dendritic cell cultures, cathepsin S activity was partially inhibited, leading to an accumulation of a 10-kDa invariant chain intermediate in these cells. As a consequence, in vitro Ag-specific CD4(+) T cell proliferation was inhibited in a time-dependent manner by SialoL, and further studies engaging cathepsin S(-/-) or cathepsin L(-/-) dendritic cells confirmed that the immunomodulatory actions of SialoL are mediated by inhibition of cathepsin S. Moreover, mice treated with SialoL displayed decreased early T cell expansion and recall response upon antigenic stimulation. Finally, SialoL administration during the immunization phase of experimental autoimmune encephalomyelitis in mice significantly prevented disease symptoms, which was associated with impaired IFN-gamma and IL-17 production and specific T cell proliferation. These results illuminate the dual mechanism by which a human disease vector protein modulates vertebrate host immunity and reveals its potential in prevention of an autoimmune disease.

Inhibition of cathepsin K reduces bone erosion, cartilage degradation and inflammation evoked by collagen-induced arthritis in mice

Cathepsin K (EC 3.4.22.38) is expressed by osteoclasts and synovial fibroblasts and its proteolytic activity is hypothesized to play a role in the pathology of rheumatoid arthritis. This study explored the effects of the cathepsin K inhibitor N-(1-{[(Cyanomethyl)amino]carbonyl}cyclohexyl)-4-[2-(4-methylpiperazin-1-yl)-1,3-thiazol-4-yl]benzamide (L-006235) in murine collagen-induced arthritis. L-006235 is a potent inhibitor of recombinant human and murine cathepsin K, enzymes (K(i):0.073 nM and IC(50): 2.4 nM, respectively) and at the cellular level in human osteoclasts (IC(50): 28 nM) with ~1000-fold selectivity against cathepsin S. L-006235 did not result in splenic invariant chain p10 accumulation, a specific marker of cathepsin S inhibition. L-006235 was dosed daily (25 mg/kg, p.o.), either prophylactically (days 0-42) or therapeutically (14 days post onset of disease) to DBA/1J mice subjected to collagen-induced arthritis. Disease severity was scored during the course of the study. Histological evaluation of cartilage and bone degradation together with related biomarkers namely, deoxypyridinoline, cartilage oligomeric matrix protein and C-terminal telopeptide degradation product of type I collagen (CTX-I) were analyzed after the study. After prophylactic or therapeutic administration, L-006235 significantly reduced biomarkers reflecting bone and cartilage degradation. Pathological changes at the histological level were significantly reduced after prophylactic treatment (P<0.01), but not after therapeutic treatment. Prophylactic treatment with L-006235 delayed disease onset (P<0.01) and reduced the disease severity score (P<0.05). Inhibition of cathepsin K activity exerts beneficial effects on collagen-induced arthritis in mice and thus warrants further investigation as a therapeutic intervention in human rheumatoid arthritis.