BackgroundHypomethylation 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.MethodsExperimental 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-1β 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.ResultsWe showed that mice deficient in cathepsin Z have reduced neuroinflammation and dramatically lowered circulating levels of IL-1β 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-1β, which in turn reduced the ability of mice to generate Th17 responses—critical steps in the pathogenesis of EAE and MS.ConclusionTogether, these data support a novel role for cathepsin Z in the propagation of IL-1β-driven neuroinflammation.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-017-0874-x) contains supplementary material, which is available to authorized users.

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.

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.

Although endosomes, lysosomes, and phagosomes require a reductive environment for the optimal activity of disulfide reductases and other thiol-dependent enzymes, how these reductive environments are established and maintained remain unknown. Our goal in this study was to begin to elucidate the redox control systems responsible for maintaining redox-sensitive enzymatic activities in the phagolysosome of murine macrophages. Through the use of specific inhibitors and genetic knockdown of known redox enzymes, we identified redox pathways that influence phagosomal disulfide reduction. In particular, known inhibitors of the NADPH-dependent selenoprotein, thioredoxin reductase, were shown to inhibit phagosomal disulfide reduction and phagosomal proteolysis. This was supported by the observation that conditional deletion of the selenocysteine tRNA in macrophages decreased phagosomal disulfide reduction capacity. In addition, pharmacologic inhibition of the pentose phosphate pathway decreased rates of disulfide reduction and proteolysis in the phagosome, implicating NADPH as a source of phagosomal reductive energy. Finally, by analyzing the effect of extracellular redox couples, such as cysteine:cystine on thiol-dependent phagosomal processes, we demonstrated that the extracellular space can additionally supply the phagosome with reductive energy. Collectively, these data demonstrate that defined cytosolic reductive pathways act in concert with the uptake of cysteine from the extracellular space to support thiol-dependent chemistries in the phagosome.

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