The pore-forming apolipoprotein APOL7C drives phagosomal rupture and antigen cross-presentation by dendritic cells.

Conventional dendritic cells (cDCs) generate protective cytotoxic T lymphocyte (CTL) responses against extracellular pathogens and tumors. This is achieved through a process known as cross-presentation (XP), and, despite its biological importance, the mechanism(s) driving XP remains unclear. Here, we show that a cDC-specific pore-forming protein called apolipoprotein L 7C (APOL7C) is up-regulated in response to innate immune stimuli and is recruited to phagosomes. Association of APOL7C with phagosomes led to phagosomal rupture and escape of engulfed antigens to the cytosol, where they could be processed via the endogenous MHC class I antigen processing pathway. Accordingly, mice deficient in APOL7C did not efficiently prime CD8+ T cells in response to immunization with bead-bound and cell-associated antigens. Together, our data indicate the presence of dedicated apolipoproteins that mediate the delivery of phagocytosed proteins to the cytosol of activated cDCs to facilitate XP.

Blockade of Proteinase-Activated Receptor 2 (PAR2) Attenuates Neuroinflammation in Experimental Autoimmune Encephalomyelitis.

Proteinase-activated receptor-2 (PAR2), which modulates inflammatory responses, is elevated in the central nervous system in multiple sclerosis (MS) and in its murine model, experimental autoimmune encephalomyelitis (EAE). In PAR2-null mice, disease severity of EAE is markedly diminished. We therefore tested whether inhibiting PAR2 activation in vivo might be a viable strategy for the treatment of MS. Using the EAE model, we show that a PAR2 antagonist, the pepducin palmitoyl-RSSAMDENSEKKRKSAIK-amide (P2pal-18S), attenuates EAE progression by affecting immune cell function. P2pal-18S treatment markedly diminishes disease severity and reduces demyelination, as well as the infiltration of T-cells and macrophages into the central nervous system. Moreover, P2pal-18S decreases granulocyte-macrophage colony-stimulating factor (GM-CSF) production and T-cell activation in cultured splenocytes and prevents macrophage polarization in vitro. We conclude that PAR2 plays a key role in regulating neuroinflammation in EAE and that PAR2 antagonists represent promising therapeutic agents for treating MS and other neuroinflammatory diseases. SIGNIFICANCE STATEMENT: Proteinase-activated receptor-2 modulates inflammatory responses and is increased in multiple sclerosis lesions. We show that the proteinase-activated receptor-2 antagonist palmitoyl-RSSAMDENSEKKRKSAIK-amide reduces disease in the murine experimental autoimmune encephalomyelitis model of multiple sclerosis by inhibiting T-cell and macrophage activation and infiltration into the central nervous system, making it a potential treatment for multiple sclerosis.

The Derivation and Use of HoxB8-Driven Conditionally Immortalized Macrophages.

The use of Hox-driven conditionally immortalized immune cells has significantly increased in biomedical research over the past 15 years. HoxB8-driven conditionally immortalized myeloid progenitor cells maintain their ability to differentiate into functional macrophages. There are multiple benefits to this conditional immortalization strategy including the ability for unlimited propagation, genetic mutability, primary-like immune cells (macrophages, dendritic cells, and granulocytes) on demand, derivation from variety of mouse strains, and simple cryopreservation and reconstitution. In this chapter, we will discuss how to derive and use these HoxB8-conditionally immortalized myeloid progenitor cells.

Approaches to Measuring Reductive and Oxidative Events in Phagosomes.

The phagosome is a redox-active organelle. Numerous reductive and oxidative systems play both direct and indirect roles in phagosomal function. With the advent of newer methodologies to study these redox events in live cells, the details of how redox conditions change within the maturing phagosome, how they are regulated, and how they influence other phagosomal functions can be investigated. In this chapter, we detail phagosome-specific, fluorescence-based assays that measure disulfide reduction and the production of reactive oxygen species in live phagocytes such as macrophages and dendritic cells, in real time.

Multiplexed Phagosomal Assays for the Detection and Quantification of Bidirectional Exchange Between the Phagolysosomal Lumen and Extracellular Space.

The phagolysosome is an antimicrobial and degradative organelle that plays a key role in macrophage-mediated inflammation and homeostasis. Before being presented to the adaptive immune system, phagocytosed proteins must first be processed into immunostimulatory antigens. Until recently, little attention has been given to how other processed PAMPs and DAMPs can stimulate an immune response if they are sequestered in the phagolysosome. Eructophagy is a newly described process in macrophages that releases partially digested immunostimulatory PAMPs and DAMPs extracellularly from the mature phagolysosome to activate vicinal leukocytes. This chapter outlines approaches to observe and quantify eructophagy by simultaneously measuring several phagosomal parameters of individual phagosomes. These methods use specifically designed experimental particles capable of conjugating to multiple reporter/reference fluors in combination with real-time automated fluorescent microscopy. Through the use of high-content image analysis software, each phagosomal parameter can be evaluated quantitatively or semiquantitatively during post-analysis.

Eructophagy: macrophages use autophagic machinery to burp out parts of their meal.

The phagolysosome is an antimicrobial and degradative organelle that plays key roles in macrophage-mediated inflammatory and homeostatic functions. Whereas mature phagolysosomes are known to sequester and degrade their contents into basic nutrients, they were not previously assigned an active role in amplifying inflammation. We have described a novel macrophage process in which partially digested immunostimulatory PAMPs are released extracellularly from the mature phagolysosome via discrete events we term eructophagy. Eructophagy is induced by proinflammatory stimuli, negatively regulated by IL4 and MTOR, and is dependent on key autophagy proteins, including fusion machinery of degradative and secretory autophagy. We propose that macrophages use eructophagy to release processed PAMPs/DAMPs to amplify local inflammation.

Macrophages disseminate pathogen associated molecular patterns through the direct extracellular release of the soluble content of their phagolysosomes.

Recognition of pathogen-or-damage-associated molecular patterns is critical to inflammation. However, most pathogen-or-damage-associated molecular patterns exist within intact microbes/cells and are typically part of non-diffusible, stable macromolecules that are not optimally immunostimulatory or available for immune detection. Partial digestion of microbes/cells following phagocytosis potentially generates new diffusible pathogen-or-damage-associated molecular patterns, however, our current understanding of phagosomal biology would have these molecules sequestered and destroyed within phagolysosomes. Here, we show the controlled release of partially-digested, soluble material from phagolysosomes of macrophages through transient, iterative fusion-fission events between mature phagolysosomes and the plasma membrane, a process we term eructophagy. Eructophagy is most active in proinflammatory macrophages and further induced by toll like receptor engagement. Eructophagy is mediated by genes encoding proteins required for autophagy and can activate vicinal cells by release of phagolysosomally-processed, partially-digested pathogen associated molecular patterns. We propose that eructophagy allows macrophages to amplify local inflammation through the processing and dissemination of pathogen-or-damage-associated molecular patterns.

Extracellular cathepsin Z signals through the α5 integrin and augments NLRP3 inflammasome activation.

Respiratory silicosis is a preventable occupational disease that develops secondary to the aspiration of crystalline silicon dioxide (silica) into the lungs, activation of the NLRP3 inflammasome, and IL-1ß production. Cathepsin Z has been associated with the development of inflammation and IL-1ß production; however, the mechanism of how cathepsin Z leads to IL-1ß production is unknown. Here, the requirement for cathepsin Z in silicosis was determined using WT mice and mice deficient in cathepsin Z. The activation of the NLRP3 inflammasome in macrophages was studied using WT and cathepsin Z-deficient bone marrow-derived murine dendritic cells and the human monocytic cell line THP-1. The cells were activated with silica, and IL-1ß release was determined using enzyme-linked immunosorbent assay or IL-1ß bioassays. The relative contribution of the active domain or integrin-binding domain of cathepsin Z was studied using recombinant cathepsin Z constructs and the α5 integrin neutralizing antibody. We report that the lysosomal cysteine protease cathepsin Z potentiates the development of inflammation associated with respiratory silicosis by augmenting NLRP3 inflammasome-derived IL-1ß expression in response to silica. The secreted cathepsin Z functions nonproteolytically via the internal integrin-binding domain to impact caspase-1 activation and the production of active IL-1ß through integrin α5 without affecting the transcription levels of NLRP3 inflammasome components. This work reveals a regulatory pathway for the NLRP3 inflammasome that occurs in an outside-in fashion and provides a link between extracellular cathepsin Z and inflammation. Furthermore, it reveals a level of NLRP3 inflammasome regulation that has previously only been found downstream of extracellular pathogens.

Listeria exploits IFITM3 to suppress antibacterial activity in phagocytes.

The type I interferon (IFN) signaling pathway has important functions in resistance to viral infection, with the downstream induction of interferon stimulated genes (ISG) protecting the host from virus entry, replication and spread. Listeria monocytogenes (Lm), a facultative intracellular foodborne pathogen, can exploit the type I IFN response as part of their pathogenic strategy, but the molecular mechanisms involved remain unclear. Here we show that type I IFN suppresses the antibacterial activity of phagocytes to promote systemic Lm infection. Mechanistically, type I IFN suppresses phagosome maturation and proteolysis of Lm virulence factors ActA and LLO, thereby promoting phagosome escape and cell-to-cell spread; the antiviral protein, IFN-induced transmembrane protein 3 (IFITM3), is required for this type I IFN-mediated alteration. Ifitm3-/- mice are resistant to systemic infection by Lm, displaying decreased bacterial spread in tissues, and increased immune cell recruitment and pro-inflammatory cytokine signaling. Together, our findings show how an antiviral mechanism in phagocytes can be exploited by bacterial pathogens, and implicate IFITM3 as a potential antimicrobial therapeutic target.

A non-immunological role for γ-interferon-inducible lysosomal thiol reductase (GILT) in osteoclastic bone resorption.

The extracellular bone resorbing lacuna of the osteoclast shares many characteristics with the degradative lysosome of antigen-presenting cells. γ-Interferon-inducible lysosomal thiol reductase (GILT) enhances antigen processing within lysosomes through direct reduction of antigen disulfides and maintenance of cysteine protease activity. In this study, we found the osteoclastogenic cytokine RANKL drove expression of GILT in osteoclast precursors in a STAT1-dependent manner, resulting in high levels of GILT in mature osteoclasts, which could be further augmented by γ-interferon. GILT colocalized with the collagen-degrading cysteine protease, cathepsin K, suggesting a role for GILT inside the osteoclastic resorption lacuna. GILT-deficient osteoclasts had reduced bone-resorbing capacity, resulting in impaired bone turnover and an osteopetrotic phenotype in GILT-deficient mice. We demonstrated that GILT could directly reduce the noncollagenous bone matrix protein SPARC, and additionally, enhance collagen degradation by cathepsin K. Together, this work describes a previously unidentified, non-immunological role for GILT in osteoclast-mediated bone resorption.

Better Together: Current Insights Into Phagosome-Lysosome Fusion.

Following phagocytosis, the nascent phagosome undergoes maturation to become a phagolysosome with an acidic, hydrolytic, and often oxidative lumen that can efficiently kill and digest engulfed microbes, cells, and debris. The fusion of phagosomes with lysosomes is a principal driver of phagosomal maturation and is targeted by several adapted intracellular pathogens. Impairment of this process has significant consequences for microbial infection, tissue inflammation, the onset of adaptive immunity, and disease. Given the importance of phagosome-lysosome fusion to phagocyte function and the many virulence factors that target it, it is unsurprising that multiple molecular pathways have evolved to mediate this essential process. While the full range of these pathways has yet to be fully characterized, several pathways involving proteins such as members of the Rab GTPases, tethering factors and SNAREs have been identified. Here, we summarize the current state of knowledge to clarify the ambiguities in the field and construct a more comprehensive phagolysosome formation model. Lastly, we discuss how other cellular pathways help support phagolysosome biogenesis and, consequently, phagocyte function.

Application of a Sulfoxonium Ylide Electrophile to Generate Cathepsin X-Selective Activity-Based Probes.

Cathepsin X/Z/P is cysteine cathepsin with unique carboxypeptidase activity. Its expression is associated with cancer and neurodegenerative diseases, although its roles during normal physiology are still poorly understood. Advances in our understanding of its function have been hindered by a lack of available tools that can specifically measure the proteolytic activity of cathepsin X. We present a series of activity-based probes that incorporate a sulfoxonium ylide warhead, which exhibit improved specificity for cathepsin X compared to previously reported probes. We apply these probes to detect cathepsin X activity in cell and tissue lysates, in live cells and in vivo, and to localize active cathepsin X in mouse tissues by microscopy. Finally, we utilize an improved method to generate chloromethylketones, necessary intermediates for synthesis of acyloxymethylketones probes, by way of sulfoxonium ylide intermediates. In conclusion, the probes presented in this study will be valuable for investigating cathepsin X pathophysiology.

Growth hormone-mediated reprogramming of macrophage transcriptome and effector functions.

Macrophages are an important component of the innate immune response. Priming and activation of macrophages is stimulated by cytokines (i.e IFNγ). However, growth hormone (GH) can also stimulate macrophage activation. Based on these observations, the goal of this work was to 1) to compare the transcriptome profile of macrophages activated in vitro with GH and IFNγ, and 2) to assess the impact of GH on key macrophage functional properties like reactive oxygen species (ROS) production and phagosomal proteolysis. To assess the global transcriptional and functional impact of GH on macrophage programming, bone marrow derived macrophages were treated with GH or IFNγ. Our data strongly support a potential link between GH, which wanes with age, and impaired macrophage function. The notable overlap of GH with IFNγ-induced pathways involved in innate immune sensing of pathogens and antimicrobial responses argue for an important role for GH in macrophage priming and maturation. By using functional assays that report on biochemical activities within the lumen of phagosomes, we have also shown that GH alters physiologically relevant processes such as ROS production and proteolysis. These changes could have far reaching impacts on antimicrobial capacity, signaling, and antigen presentation.

17-DMAG regulates p21 expression to induce chondrogenesis in vitro and in vivo.

Cartilage degeneration after injury affects a significant percentage of the population, including those that will go on to develop osteoarthritis (OA). Like humans, most mammals, including mice, are incapable of regenerating injured cartilage. Interestingly, it has previously been shown that p21 (Cdkn1a) knockout (p21-/-) mice demonstrate auricular (ear) cartilage regeneration. However, the loss of p21 expression is highly correlated with the development of numerous types of cancer and autoimmune diseases, limiting the therapeutic translation of these findings. Therefore, in this study, we employed a screening approach to identify an inhibitor (17-DMAG) that negatively regulates the expression of p21. We also validated that this compound can induce chondrogenesis in vitro (in adult mesenchymal stem cells) and in vivo (auricular cartilage injury model). Furthermore, our results suggest that 17-DMAG can induce the proliferation of terminally differentiated chondrocytes (in vitro and in vivo), while maintaining their chondrogenic phenotype. This study provides new insights into the regulation of chondrogenesis that might ultimately lead to new therapies for cartilage injury and/or OA.

Author Correction: Smac mimetics and oncolytic viruses synergize in driving anticancer T-cell responses through complementary mechanisms.

The originally published version of this article contained an error in the spelling of the author Pankaj Tailor, which was incorrectly given as Pankaj Taylor. This has now been corrected in both the PDF and HTML versions of the article.

The phagosome and redox control of antigen processing.

In addition to debris clearance and antimicrobial function, versatile organelles known as phagosomes play an essential role in the processing of exogenous antigen in antigen presenting cells. While there has been much attention on human leukocyte antigen haplotypes in the determination of antigenic peptide repertoires, the lumenal biochemistries within phagosomes and endosomes are emerging as equally-important determinants of peptide epitope composition and immunodominance. Recently, the lumenal redox microenvironment within these degradative compartments has been shown to impact two key antigenic processing chemistries: proteolysis by lysosomal cysteine proteases and disulfide reduction of protein antigens. Through manipulation of the balance between oxidative and reductive capacities in the phagosome-principally by modulating NADPH oxidase (NOX2) and γ-interferon-inducible lysosomal thiol reductase (GILT) activities-studies have demonstrated changes to antigen processing patterns leading to modified repertoires of antigenic peptides available for presentation, and subsequently, altered disease progression in T cell-driven autoimmunity. This review focuses on the mechanisms and consequences of redox-mediated phagosomal antigen processing, and the potential downstream implications to tolerance and autoimmunity.

The cooling compound icilin attenuates autoimmune neuroinflammation through modulation of the T-cell response.

The synthetic supercooling drug, icilin, and its primary receptor target, the cation channel transient receptor potential (TRP) melastatin-8 (TRPM8), have been described as potent negative regulators of inflammation in the colon. The aim of this study was to determine whether the anti-inflammatory action of icilin could potentially be used to treat autoimmune neuroinflammatory disorders, such as multiple sclerosis (MS). During experimental autoimmune encephalomyelitis (EAE)-a CD4+ T cell-driven murine model of MS-we found that both wild-type (WT) and TRPM8-deficient EAE mice were protected from disease progression during icilin treatment, as evidenced by delays in clinical onset and reductions in neuroinflammation. In vitro, icilin potently inhibited the proliferation of murine and human CD4+ T cells, with the peripheral expansion of autoantigen-restricted T cells similarly diminished by the administration of icilin in mice. Attenuation of both TRPM8-/- and TRP ankyrin-1-/- T-cell proliferation by icilin was consistent with the WT phenotype, which suggests a mechanism that is independent of these channels. In addition, icilin treatment altered the expressional profile of activated CD4+ T cells to one that was indicative of restricted effector function and limited neuroinflammatory potential. These findings identify a potent anti-inflammatory role for icilin in lymphocyte-mediated neuroinflammation and highlight clear pleiotropic effects of the compound beyond classic TRP channel activation.-Ewanchuk, B. W., Allan, E. R. O., Warren, A. L., Ramachandran, R., Yates, R. M. The cooling compound icilin attenuates autoimmune neuroinflammation through modulation of the T-cell response.

Smac mimetics and oncolytic viruses synergize in driving anticancer T-cell responses through complementary mechanisms.

Second mitochondrial activator of caspase (Smac)-mimetic compounds and oncolytic viruses were developed to kill cancer cells directly. However, Smac-mimetic compound and oncolytic virus therapies also modulate host immune responses in ways we hypothesized would complement one another in promoting anticancer T-cell immunity. We show that Smac-mimetic compound and oncolytic virus therapies synergize in driving CD8+ T-cell responses toward tumors through distinct activities. Smac-mimetic compound treatment with LCL161 reinvigorates exhausted CD8+ T cells within immunosuppressed tumors by targeting tumor-associated macrophages for M1-like polarization. Oncolytic virus treatment with vesicular stomatitis virus (VSVΔM51) promotes CD8+ T-cell accumulation within tumors and CD8+ T-cell activation within the tumor-draining lymph node. When combined, LCL161 and VSVΔM51 therapy engenders CD8+ T-cell-mediated tumor control in several aggressive mouse models of cancer. Smac-mimetic compound and oncolytic virus therapies are both in clinical development and their combination therapy represents a promising approach for promoting anticancer T-cell immunity.Oncolytic viruses (OV) and second mitochondrial activator of caspase (Smac)-mimetic compounds (SMC) synergistically kill cancer cells directly. Here, the authors show that SMC and OV therapies combination also synergize in vivo by promoting anticancer immunity through an increase in CD8+ T-cell response.

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

BACKGROUND: Hypomethylation of the cathepsin Z locus has been proposed as an epigenetic risk factor for multiple sclerosis (MS). Cathepsin Z is a unique lysosomal cysteine cathepsin expressed primarily by antigen presenting cells. While cathepsin Z expression has been associated with neuroinflammatory disorders, a role for cathepsin Z in mediating neuroinflammation has not been previously established. METHODS: Experimental autoimmune encephalomyelitis (EAE) was induced in both wildtype mice and mice deficient in cathepsin Z. The effects of cathepsin Z-deficiency on the processing and presentation of the autoantigen myelin oligodendrocyte glycoprotein, and on the production of IL-1ß and IL-18 were determined in vitro from cells derived from wildtype and cathepsin Z-deficient mice. The effects of cathepsin Z-deficiency on CD4+ T cell activation, migration, and infiltration to the CNS were determined in vivo. Statistical analyses of parametric data were performed by one-way ANOVA followed by Tukey post-hoc tests, or by an unpaired Student's t test. EAE clinical scoring was analyzed using the Mann-Whitney U test. RESULTS: We showed that mice deficient in cathepsin Z have reduced neuroinflammation and dramatically lowered circulating levels of IL-1ß during EAE. Deficiency in cathepsin Z did not impact either the processing or the presentation of MOG, or MOG- specific CD4+ T cell activation and trafficking. Consistently, we found that cathepsin Z-deficiency reduced the efficiency of antigen presenting cells to secrete IL-1ß, which in turn reduced the ability of mice to generate Th17 responses-critical steps in the pathogenesis of EAE and MS. CONCLUSION: Together, these data support a novel role for cathepsin Z in the propagation of IL-1ß-driven neuroinflammation.

Gestational bisphenol-A exposure lowers the threshold for autoimmunity in a model of multiple sclerosis.

Environmental and hormonal factors are implicated in dysimmunity in multiple sclerosis. We investigated whether bisphenol-A, a prominent contaminant with endocrine-disrupting capabilities, altered susceptibility in an inflammatory model of multiple sclerosis. We found that gestational, but not adult, exposure to bisphenol-A increased the development of experimental autoimmune encephalomyelitis in adulthood in male, but not female, mice when a suboptimal disease-inducing immunization was used. Gestational bisphenol-A in male mice primed macrophages in adulthood and raised granulocyte-colony stimulating factor and neutrophil counts/activity postsuboptimal immunization. Neutralizing granulocyte-colony stimulating factor blocked susceptibility to disease in bisphenol-A mice. Early life exposure to bisphenol-A may represent an environmental consideration in multiple sclerosis.