Modulation of the endosomal and lysosomal distribution of cathepsins B, L and S in human monocytes/macrophages

Endo-lysosomal assembly variations among human leukocyte antigen class I (HLA class I) allotypes

The extreme polymorphisms of human leukocyte antigen class I (HLA class I) proteins enable the presentation of diverse peptides to cytotoxic T lymphocytes. The canonical endoplasmic reticulum (ER) HLA class I assembly pathway enables presentation of cytosolic peptides, but effective intracellular surveillance requires multi-compartmental antigen sampling. Endo-lysosomes are generally sites of HLA class II assembly, but human monocytes and monocyte-derived dendritic cells (moDCs) also contain significant reserves of endo-lysosomal HLA class I molecules. We hypothesized variable influences of HLA class I polymorphisms upon outcomes of endo-lysosomal trafficking, as the stabilities and peptide occupancies of cell surface HLA class I molecules are variable. Consistent with this model, when the endo-lysosomal pH of moDCs is disrupted, HLA-B allotypes display varying propensities for reductions in surface expression, with HLA-B*08:01 or HLA-B*35:01 being among the most resistant or sensitive, respectively, among eight tested HLA-B allotypes. Perturbations of moDC endo-lysosomal pH result in accumulation of HLA-B*35:01 in LAMP1+ compartments and increase HLA-B*35:01 peptide receptivity. These findings reveal the intersection of the vacuolar cross-presentation pathway with a constitutive assembly pathway for some HLA-B allotypes. Notably, cross-presentation of epitopes derived from two soluble antigens was also more efficient for B*35:01 compared to B*08:01, even when matched for T cell response sensitivity, and more affected by cathepsin inhibition. Thus, HLA class I polymorphisms dictate the degree of endo-lysosomal assembly, which can supplement ER assembly for constitutive HLA class I expression and increase the efficiency of cross-presentation.

Molecular Determinants Regulating the Plasticity of the MHC Class II Immunopeptidome

In the last few years, advancement in the analysis of the MHC class II (MHC-II) ligandome in several mouse and human haplotypes has increased our understanding of the molecular components that regulate the range and selection of the MHC-II presented peptides, from MHC class II molecule polymorphisms to the recognition of different conformers, functional differences in endosomal processing along the endocytic tract, and the interplay between the MHC class II chaperones DM and DO. The sum of all these variables contributes, qualitatively and quantitatively, to the composition of the MHC II ligandome, altogether ensuring that the immunopeptidome landscape is highly sensitive to any changes in the composition of the intra- and extracellular proteome for a comprehensive survey of the microenvironment for MHC II presentation to CD4 T cells.

The bone marrow microenvironment of pre-B acute lymphoblastic leukemia at single-cell resolution

The bone marrow microenvironment (BMM) plays a key role in leukemia progression, but its molecular complexity in pre-B cell acute lymphoblastic leukemia (B-ALL), the most common cancer in children, remains poorly understood. To gain further insight, we used single-cell RNA sequencing to characterize the kinetics of the murine BMM during B-ALL progression. Normal pro- and pre-B cells were found to be the most affected at the earliest stages of disease and this was associated with changes in expression of genes regulated by the AP1-transcription factor complex and regulatory factors NELFE, MYC and BCL11A. Granulocyte–macrophage progenitors show reduced expression of the tumor suppressor long non-coding RNA Neat1 and disruptions in the rate of transcription. Intercellular communication networks revealed monocyte-dendritic precursors to be consistently active during B-ALL progression, with enriched processes including cytokine-mediated signaling pathway, neutrophil-mediated immunity and regulation of cell migration and proliferation. In addition, we confirmed that the hematopoietic stem and progenitor cell compartment was perturbed during leukemogenesis. These findings extend our understanding of the complexity of changes and molecular interactions among the normal cells of the BMM during B-ALL progression.

HIV Infection Induces Extracellular Cathepsin B Uptake and Damage to Neurons

HIV-associated neurocognitive disorders prevail in 20-50 percent of infected individuals. Macrophages transmigrate through the blood brain barrier during HIV-1 infection, triggering neuronal dysfunction. HIV-infected macrophages secrete cathepsin B (CATB), and serum amyloid p component (SAPC), inducing neuronal apoptosis by an unknown mechanism. We hypothesized that HIV infection facilitates CATB/SAPC secretion from macrophages followed by neuronal internalization, promoting dysfunction. SK-N-SH neuronal cells were exposed to active recombinant histidine-tagged cathepsin B (His-CATB). His-CATB entry was tracked by intracellular flow cytometry, and neuronal dysfunction was verified by western blot. Macrophage-derived extracellular vesicles (EVs) were tested for the presence of CATB and SAPC. Neurons internalized His-CATB, an effect that was partially decreased by pre-treatment with anti-CATB antibody. Pre-treatment with CATB and SAPC antibodies decreased cleavage of caspase-3 and restored synaptophysin in neurons. Neurons exposed to macrophage-conditioned media differentially internalized His-CATB, dependent on the HIV replication levels. Finally, CATB and SAPC were secreted in EVs. We report for the first time that CATB is secreted from macrophages both free and in EVs, and is internalized by neurons. Moreover, HIV-replication levels modulate the amount of CATB neuronal uptake, and neuronal dysfunction can be decreased with CATB antibodies. In conclusion, the CATB/SAPC complex represents a novel target against HIV-associated neurocognitive disorders.

Endosomal localization of TLR8 confers distinctive proteolytic processing on human myeloid cells

Nucleic acid-sensing TLRs are involved in both antimicrobial immune responses and autoimmune inflammation. TLR8 is phylogenetically and structurally related to TLR7 and TLR9, which undergo proteolytic processing in the endolysosomes to generate functional receptors. Recent structural analyses of human TLR8 ectodomain and its liganded form demonstrated that TLR8 is also cleaved, and both the N- and C-terminal halves contribute to ligand binding. However, the structures and ssRNA recognition mode of endogenous TLR8 in human primary cells are largely unknown. In this study, we show that proteolytic processing of TLR8 occurs in human monocytes and macrophages in a different manner compared with TLR7/9 cleavage. The insertion loop between leucine-rich repeats 14 and 15 in TLR8 is indispensable for the cleavage and stepwise processing that occurs in the N-terminal fragment. Both furin-like proprotein convertase and cathepsins contribute to TLR8 cleavage in the early/late endosomes. TLR8 recognizes viral ssRNA and endogenous RNA, such as microRNAs, resulting in the production of proinflammatory cytokines. Hence, localization sites of the receptors are crucial for the nucleic acid-sensing mode and downstream signaling.

Cathepsins limit macrophage necroptosis through cleavage of Rip1 kinase

It has recently been shown that programmed necrosis, necroptosis, may play a key role in the development of inflammation. Deciphering the regulation of this pathway within immune cells may therefore have implications in pathology associated with inflammatory diseases. We show that treatment of macrophages with the pan caspase inhibitor (zVAD-FMK) results in both increased phosphorylation and decreased cleavage of receptor interacting protein kinase-1 (Rip1), leading to necroptosis that is dependent on autocrine TNF signaling. Stimulation of cells with TLR agonists such as LPS in the presence of zVAD-FMK also induced Rip1-phosphorylation via a TNFR-independent mechanism. Further examination of Rip1 expression under these stimulatory conditions revealed a regulatory cleavage of Rip1 in macrophages that is not apparently attributable to caspase-8. Instead, we provide novel evidence that cysteine family cathepsins, which are highly abundant in myeloid cells, can also cleave Rip1 kinase. Using small interfering RNA knockdown, specific cathepsin inhibitors, and cell-free cleavage assays, we demonstrate that cysteine cathepsins B and S can directly cleave Rip1. Finally, we demonstrate that only through combined inhibition of cathepsins and caspase-8 could a potent induction of macrophage necroptosis be achieved. These data reveal a novel mechanism of regulation of necroptosis by cathepsins within macrophage cells.

Metabolism and antiretroviral activity of tenofovir alafenamide in CD4+ T-cells and macrophages from demographically diverse donors

BACKGROUND

Tenofovir alafenamide (TAF) is a novel investigational prodrug of tenofovir (TFV) that permits enhanced delivery of TFV into peripheral blood mononuclear cells (PBMCs) and lymphatic tissues. A critical step in the intracellular metabolic activation of TAF is mediated by the lysosomal protease cathepsin A (CatA). Here, we investigated CatA levels together with intracellular metabolism and antiretroviral activity of TAF in primary CD4+ T-lymphocytes (CD4s) and monocyte-derived macrophages (MDMs) isolated from a demographically diverse group of blood donors.

METHODS

CD4s and MDMs were prepared from fresh PBMCs. CatA levels were quantified in cell extracts by monitoring TAF hydrolysis using HPLC. Intracellular TAF metabolites were quantified by HPLC combined with mass spectrometry. Antiviral activities in activated CD4s and MDMs were determined using HIV-1 single-cycle reporter and p24 antigen production assays, respectively.

RESULTS

The levels of CatA and intracellular TAF metabolites differed minimally in CD4s and MDMs among 13 tested donors. TAF was >600-fold and 80-fold more potent than parent TFV in CD4s and MDMs, respectively, and its relative range of antiviral activity across all tested donors was comparable to that of other HIV-1 reverse transcriptase inhibitors, with mean ±sd (range) EC50 values of 11.0 ±3.4 (6.6-19.9) nM and 9.7 ±4.6 (2.5-15.7) nM in CD4s and MDMs, respectively.

CONCLUSIONS

These results indicate consistent intracellular metabolism and antiretroviral potency of TAF in relevant target cells of HIV-1 infection across multiple donors of variable gender, age and ethnicity, supporting further clinical investigation of TAF.

A novel Leishmania major amastigote assay in 96-well format for rapid drug screening and its use for discovery and evaluation of a new class of leishmanicidal quinolinium salts

In most laboratories, the screening for leishmanicidal compounds is carried out with Leishmania promastigotes or axenic amastigotes. However, the best approach to identify leishmanicidal compounds is the use of amastigotes residing in macrophages. Reporter gene-based assays are relatively new tools in the search for drugs against eucaryotic protozoa, permitting the development of faster, more automated assays. In this paper, we report on the establishment of a rapid screening assay in a 96-well format. A luciferase-transgenic (Luc-tg) Leishmania major strain was generated and used to infect bone marrow-derived macrophages (BMDM). Amastigote-infected BMDM were treated with different compound concentrations. Cells were lysed with a luciferin-containing buffer, and the resulting luminescence was measured to determine the half-maximal inhibitory concentration (IC50). To validate this new amastigote screening assay, a library of a new class of quinolinium salts was synthesized and tested for leishmanicidal activity. Some of the quinolinium salts showed very promising activities, with IC50s against intracellular amastigotes (IC50 < 1 μg/ml) and selectivity indices (SI > 20) that match the criteria of World Health Organization (WHO) for hits. Compound 21c (IC50 = 0.03 μg/ml; SI = 358) could become a new lead structure for the development of improved chemotherapeutic drugs against L. major. In summary, we describe the establishment of a new 96-well format assay with Luc-transgenic L. major for the rapid screening of compounds for leishmanicidal activity against intracellular amastigotes and its application to the identification of a new class of quinolinium salts with most promising leishmanicidal activity.

Increased nucleolar localization of SpiA3G in classically but not alternatively activated macrophages

Macrophages play a key role in innate immune response to pathogens and in tissue homeostasis, inflammation and repair. A serpin A3G (SpiA3G) is highly induced in classically activated macrophages. We show increased localization of SpiA3G in the nucleolus and co-localization with cathepsin L, upon classical, but not alternative activation of macrophages. Despite the increased expression of cathepsin L in the nuclei of classically activated macrophages, no cathepsin activity was detected. Since only pro-inflammatory, but not anti-inflammatory stimuli induce increased nucleolar localization of SpiA3G, we propose that SpiA3g translocation into the nucleolus is important in host defense against pathogens.

Antigen presentation by monocytes and monocyte-derived cells

Monocytes are circulating mononuclear phagocytes with a fundamental capacity to differentiate into macrophages. This differentiation can, in the presence of the right environmental cues, be re-directed instead to dendritic cells (DCs). Recent advances have been made in understanding the role of monocytes and their derivatives in presenting antigen to drive immune responses, and we review this topic herein. We briefly discuss the heterogeneity of monocytes in the blood and subsequently raise the possibility that one of the major monocyte phenotypes in the blood corresponds with a population of 'blood DCs' previously proposed to drive T-independent antibody reactions in the spleen. Then we evaluate the role of monocytes in T-dependent immunity, considering their role in acquiring antigens for presentation before exiting the bloodstream and their ability to differentiate into macrophages versus antigen-presenting DCs. Finally, we review recent literature on the role of monocyte-derived cells in cross-presentation and discuss the possibility that monocyte-derived cells participate critically in processing antigen for cross-priming, even if they do not present that antigen to T cells themselves.

Differential expression of cathepsin X in aging and pathological central nervous system of mice

Increasing evidence of a fundamental influence of cathepsins on inflammation has drawn interest in a thorough understanding of their role in physiological and pathological processes. Even though the number of identified cathepsins has more than doubled in the last years, information about their expression, regulation and function in the brain is still incomplete. In the present study we analyzed the regional, cellular and subcellular localization and the activity of the recently discovered cathepsin X in the normal, developing and pathological mouse brain. Our results show that CATX is: (i) is expressed in almost all cells in the mouse brain with a preference for glial cells; (ii) already widely expressed early in development and age-dependently upregulated in amount and activity; (iii) prominently localized in the lysosomal system but also scattered in the somal cytoplasm in the aged brain; (iv) upregulated in numerous glial cells of degenerating brain regions in a transgenic mouse model of amyotrophic lateral sclerosis; and (v) associated with plaques in a transgenic mouse model and in Alzheimer patients. These results strongly suggest that cathepsin X is an important player in degenerative processes during normal aging and in pathological conditions.

Interferon-gamma regulates cathepsin G activity in microglia-derived lysosomes and controls the proteolytic processing of myelin basic protein in vitro

The serine protease cathepsin (Cat) G dominates the proteolytic processing of the multiple sclerosis (MS)-associated autoantigen myelin basic protein (MBP) in lysosomes from primary human B cells and dendritic cells. This is in contrast to B-lymphoblastoid cell lines, where the asparagine endopeptidase (AEP) is responsible for this task. We have analysed microglia-derived lysosomal proteases for their ability to process MBP in vitro. In lysosomes derived from primary murine microglia, CatD, CatS, AEP and CatG were involved in the processing of MBP. Interestingly, when microglia were treated with interferon-gamma to mimic a T helper type 1-biased cytokine milieu in MS, CatG was drastically down-regulated, in contrast to CatS, CatB, CatL, CatD or AEP. This resulted in significantly increased stability of MBP and a selective lack of CatG-derived proteolytic fragments; however, it did not affect the gross pattern of MBP processing. Inhibition of serine proteases eliminated the processing differences between lysosomal extracts from resting microglia compared to interferon-stimulated microglia. Thus, the cytokine environment modulates lysosomal proteases in microglia by a selective down-regulation of CatG, leading to decreased MBP-processing by microglia-derived lysosomal proteases in vitro.

Immature monocytes acquire antigens from other cells in the bone marrow and present them to T cells after maturing in the periphery

Monocytes are circulating precursors for tissue macrophages and dendritic cells (DCs) but are not recognized to directly participate in antigen presentation. We developed techniques to label mouse monocyte subsets with particulate tracers in vivo. Gr-1^lo but not Gr-1^hi monocytes were stably labeled by intravenous injection of 0.5-μm microspheres. Gr-1^hi monocytes could be labeled when the microspheres were injected after systemic depletion of blood monocytes and spleen macrophages. In this condition, the phagocytic tracer was transferred to immature bone marrow monocytes by neutrophils and B cells that first carried the particles to the bone marrow. Moreover, antigens from B cells or proteins conjugated to the tracer particles were processed for presentation by monocytes and could induce T cell responses in the periphery. Cell-associated antigen taken up by bone marrow monocytes was retained intracellularly for presentation of the antigen days later when monocyte-derived DCs migrated to lymph nodes or in vitro after differentiation with granulocyte/macrophage colony-stimulating factor. These data reveal that immature monocytes unexpectedly sample antigen from the bone marrow environment and that they can present these antigens after they leave the bone marrow.

RANK ligand and interferon gamma differentially regulate cathepsin gene expression in pre-osteoclastic cells

Receptor activator of NF-kappaB ligand (RANKL) and interferon gamma (IFN-gamma) are critical and opposing mediators of osteoclastogenesis, exerting stimulatory and inhibitory effects, respectively. Cathepsin K (CTSK) is a secreted protease that plays an essential role in osteoclastic bone resorption. We have examined the role of IFN-gamma in the regulation of CTSK expression in the murine monocytic RAW 264.7 cell line, which can be readily differentiated to bone-resorbing osteoclasts upon RANKL treatment. Real-time RT-PCR reveals that RANKL stimulates CTSK mRNA expression in a dose- and time-dependent fashion, but that RANKL does not alter the expression of cathepsin L (CTSL) and cathepsin S (CTSS) mRNA. IFN-gamma stimulates both CTSL and CTSS expression after 3 days, but fails to significantly alter CTSK expression. IFN-gamma markedly inhibits the stimulation of CTSK mRNA and protein by RANKL, whereas RANKL suppresses the stimulation of CTSL and CTSS mRNA by IFN-gamma. IFN-gamma also ablates the RANKL induced osteoclastic differentiation of RAW cells. In RAW cells stably transfected with a CTSK promoter-luciferase plasmid containing the 1618 bp upstream of the transcription initiation site, IFN-gamma inhibits CTSK promoter activity and ablates its induction by RANKL. In conclusion, IFN-gamma and RANKL differentially regulate cathepsin K, S, and L gene expression in pre-osteoclastic cells, and there appears to be significant cross talk between the signal transduction pathways mediating the responses to RANKL and IFN-gamma.

Cathepsin B and L are involved in degradation of prions in GT1-1 neuronal cells

In scrapie-infected cells, the abnormal isoform of the prion protein, PrP(Sc), accumulates in endosomes/lysosomes. In this study, the involvement of two lysosomal proteases, cathepsin B and L, in cellular processing of PrP(Sc) was analyzed in immortalized neuronal gonadotropin-releasing hormone cells (GT1-1) infected with scrapie. Treatment with inhibitors of either cathepsin B or L resulted in accumulation of PrP(Sc). Such an increased accumulation also occurred when the activities of both cathepsins were inhibited using RNA interference. We conclude that cathepsin B and L are involved in the degradation of PrP(Sc) in scrapie-infected GT1-1 cells and that they can compensate for each other's functions. This study shows that specific proteases, abundantly present in neurons, have the capacity to degrade PrP(Sc).

Cysteine proteases as disease markers

This review comprises issues concerning cysteine cathepsins (CCs): human peptidases belonging to papain family (C1) of clan CA of cysteine proteases: cathepsins B, L, H, S, K, F, V, X, W, O and C. The involvement of these enzymes in physiological and pathological processes is described, especially with respect to their application as diagnostic and prognostic markers. They participate in precursor protein activation (including proenzymes and prohormones), MHC-II-mediated antigen presentation, bone remodeling, keratinocytes differentiation, hair follicle cycle, reproduction and apoptosis. Cysteine cathepsins upregulation has been demonstrated in many human tumors, including breast, lung, brain, gastrointestinal, head and neck cancer, and melanoma. Besides cancer diseases, they have been implied to participate in inflammatory diseases, such as inflammatory myopathies, rheumatoid arthritis, and periodontitis. Also, certain hereditary disorders are connected with mutations in CCs genes, what is observed in pycnodysostosis resulted from catK gene mutation and Papillon-Lefevre and Haim-Munk syndrome caused by catC gene defect. The potential application of cysteine cathepsins in diagnosis and/or prognosis is discussed in cancer diseases (breast, lung, head and neck, ovarian, gastrointestinal cancers, melanoma), as well as other disorders (periodontitis, rheumatoid arthritis, osteoarthritis).