Myeloid cell-derived proteases produce a proinflammatory form of IL-37 that signals via IL-36 receptor engagement

Interleukin-1 (IL-1) family cytokines are key barrier cytokines that are typically expressed as inactive, or partially active, precursors that require proteolysis within their amino termini for activation. IL-37 is an enigmatic member of the IL-1 family that has been proposed to be activated by caspase-1 and to exert anti-inflammatory activity through engagement of the IL-18R and SIGIRR. However, here we show that the longest IL-37 isoform, IL-37b, exhibits robust proinflammatory activity upon amino-terminal proteolysis by neutrophil elastase or cathepsin S. In sharp contrast, caspase-1 failed to process or activate IL-37 at concentrations that robustly activated its canonical substrate, IL-1β. IL-37 and IL-36 exhibit high structural homology, and, consistent with this, a K53-truncated form of IL-37, mimicking the cathepsin S-processed form of this cytokine, was found to exert its proinflammatory effects via IL-36 receptor engagement and produced an inflammatory signature practically identical to IL-36. Administration of K53-truncated IL-37b intraperitoneally into wild-type mice also elicited an inflammatory response that was attenuated in IL-36R^-/- animals. These data demonstrate that, in common with other IL-1 family members, mature IL-37 can also elicit proinflammatory effects upon processing by specific proteases.

IL-1 family cytokines serve as 'activity recognition receptors' for aberrant protease activity indicative of danger

Members of the extended IL-1 cytokine family play key roles as instigators of inflammation in numerous infectious and sterile injury contexts and are highly enriched at barrier surfaces such as the skin, lungs and intestinal mucosa. Because IL-1 family cytokines do not possess conventional ER-golgi trafficking and secretory signals, these cytokines are typically released into the extracellular space due to tissue damage resulting in necrosis, or pathogen detection resulting in pyroptosis. The latter feature, in combination with other factors, suggests that IL-1 family cytokines serve as canonical damage-associated molecular patterns (DAMPs), which instigate inflammation in response to tissue damage. However, IL-1 family cytokines also require a proteolytic activation step and diverse intracellular, extracellular and non-self proteases have been identified that are capable of processing and activating members of this family. This suggests that IL-1 family members function as sentinels for aberrant protease activity, which is frequently associated with infection or tissue damage. Here, we overview the diversity of proteases implicated in the activation of IL-1 family cytokines and suggest that this ancient cytokine family may have evolved to complement 'pattern recognition receptors', by serving as 'activity recognition receptors' enabling the detection of aberrant enzyme activity indicative of 'danger'.

TRAIL Receptors Serve as Stress-Associated Molecular Patterns to Promote ER-Stress-Induced Inflammation

Inflammation triggered by infection or cellular necrosis is initiated by a battery of pattern-recognition receptors, such as Toll-like receptors or IL-1 family receptors. Diverse forms of cell stress, such as ER stress or mitochondrial stress, can also promote inflammatory responses that contribute to the chronic inflammation observed in cancer, obesity, and other conditions. However, the molecular mechanisms of cell-stress-induced inflammation are poorly understood. Here, we show that ER stress initiated NF-κB activation and inflammation through transcriptional upregulation and ligand-independent activation of TRAIL receptors. ER-stress-induced TRAIL receptor activation resulted in caspase-8/FADD/RIPK1-dependent NF-κB activation and inflammatory cytokine production. Silencing or deletion of TRAIL receptors, or their downstream effectors caspase-8, FADD, or RIPK1, suppressed ER-stress-induced inflammation. Furthermore, chemotherapeutic stress-induced inflammatory responses were blunted in DR5/TRAIL-R null animals. We propose that, upon ER stress, TRAIL receptors serve as "stress-associated molecular patterns (SAMPs)" coupling ER stress to NF-κB-dependent inflammation.

Extracellular Neutrophil Proteases Are Efficient Regulators of IL-1, IL-33, and IL-36 Cytokine Activity but Poor Effectors of Microbial Killing

Neutrophil granule proteases are thought to function as anti-microbial effectors, cooperatively hydrolyzing microorganisms within phagosomes, or upon deployment into the extracellular space. However, evidence also suggests that neutrophil proteases play an important role in the coordination and escalation of inflammatory reactions, but how this is achieved has been obscure. IL-1 family cytokines are important initiators of inflammation and are typically released via necrosis but require proteolytic processing for activation. Here, we show that proteases liberated from activated neutrophils can positively or negatively regulate the activity of six IL-1 family cytokines (IL-1α, IL-1β, IL-33, IL-36α, IL-36β, and IL-36γ) with exquisite sensitivity. In contrast, extracellular neutrophil proteases displayed very poor bactericidal activity, exhibiting 100-fold greater potency toward cytokine processing than bacterial killing. Thus, in addition to their classical role as phagocytes, neutrophils play an important immunoregulatory role through deployment of their granule proteases into the extracellular space to process multiple IL-1 family cytokines.

To NET or not to NET:current opinions and state of the science regarding the formation of neutrophil extracellular traps

Since the discovery and definition of neutrophil extracellular traps (NETs) 14 years ago, numerous characteristics and physiological functions of NETs have been uncovered. Nowadays, the field continues to expand and novel mechanisms that orchestrate formation of NETs, their previously unknown properties, and novel implications in disease continue to emerge. The abundance of available data has also led to some confusion in the NET research community due to contradictory results and divergent scientific concepts, such as pro- and anti-inflammatory roles in pathologic conditions, demarcation from other forms of cell death, or the origin of the DNA that forms the NET scaffold. Here, we present prevailing concepts and state of the science in NET-related research and elaborate on open questions and areas of dispute.

Identification of small-molecule elastase inhibitors as antagonists of IL-36 cytokine activation

IL‐1 family cytokines act as apical initiators of inflammation in many settings and can promote the production of a battery of inflammatory cytokines, chemokines and other inflammatory mediators in diverse cell types. IL‐36α, IL‐36β and IL‐36γ, which belong to the extended IL‐1 family, have been implicated as key initiators of skin inflammation in psoriasis. IL‐36γ is highly upregulated in lesional skin from psoriatic individuals, and heritable mutations in the natural IL‐36 receptor antagonist result in a severe form of psoriasis. IL‐36 family cytokines are initially expressed as inactive precursors that require proteolytic processing for activation. The neutrophil granule‐derived protease elastase proteolytically processes and activates IL‐36α and IL‐36γ, increasing their biological activity ~ 500‐fold, and also robustly activates IL‐1α and IL‐33 through limited proteolytic processing. Consequently, inhibitors of elastase activity may have potential as anti‐inflammatory agents through antagonizing the activation of multiple IL‐1 family cytokines. Using in silico screening approaches, we have identified small‐molecule inhibitors of elastase that can antagonize activation of IL‐36γ by the latter protease. The compounds reported herein may have utility as lead compounds for the development of inhibitors of elastase‐mediated activation of IL‐36 and other IL‐1 family cytokines in inflammatory conditions, such as psoriasis.

Suppressing IL-36-driven inflammation using peptide pseudosubstrates for neutrophil proteases

Sterile inflammation is initiated by molecules released from necrotic cells, called damage-associated molecular patterns (DAMPs). Members of the extended IL-1 cytokine family are important DAMPs, are typically only released through necrosis, and require limited proteolytic processing for activation. The IL-1 family cytokines, IL-36α, IL-36β, and IL-36γ, are expressed as inactive precursors and have been implicated as key initiators of psoriatic-type skin inflammation. We have recently found that IL-36 family cytokines are proteolytically processed and activated by the neutrophil granule-derived proteases, elastase, and cathepsin G. Inhibitors of IL-36 processing may therefore have utility as anti-inflammatory agents through suppressing activation of the latter cytokines. We have identified peptide-based pseudosubstrates for cathepsin G and elastase, based on optimal substrate cleavage motifs, which can antagonize activation of all three IL-36 family cytokines by the latter proteases. Human psoriatic skin plaques displayed elevated IL-36β processing activity that could be antagonized by peptide pseudosubstrates specific for cathepsin G. Thus, antagonists of neutrophil-derived proteases may have therapeutic potential for blocking activation of IL-36 family cytokines in inflammatory conditions such as psoriasis.

Getting a gRIP on Flu by Casting the DAI

Influenza A initiates host cell death through unknown mechanisms. Thapa et al. (2016) in this issue of Cell Host & Microbe, along with recent work by Kuriakose et al. (2016), indicate that this virus provokes divergent modes of cell death, including apoptosis and necroptosis, via the nucleic acid sensor, DAI.

Neutrophil extracellular traps can serve as platforms for processing and activation of IL-1 family cytokines

Activated neutrophils can undergo a mode of regulated cell death, called NETosis, that results in the extrusion of chromatin into the extracellular space, thereby acting as extracellular traps for microorganisms. Neutrophil-derived extracellular traps (NETs) are comprised of DNA decorated with histones, antimicrobial proteins and neutrophil granule proteases, such as elastase and cathepsin G (Cat G). NET-associated factors are thought to enhance the antimicrobial properties of these structures and localisation of antimicrobial molecules on NETs may serve to increase their local concentration. Because neutrophil-derived proteases have been implicated in the processing and activation of several members of the extended interleukin (IL)-1 family, we wondered whether neutrophil NETs could also serve as platforms for the activation of proinflammatory cytokines. Here, we show that neutrophil NETs potently processed and activated IL-1α as well as IL-36 subfamily cytokines through NET-associated Cat G and elastase. Thus, in addition to their role as antimicrobial traps, NETs can also act as local sites of cytokine processing and activation.

Production of biologically active IL-36 family cytokines through insertion of N-terminal caspase cleavage motifs

Recent evidence has strongly implicated IL-36 cytokines as key initiators of inflammation in the skin barrier. IL-36 cytokines belong to the extended IL-1 family and, similar to most members of this family, are expressed as inactive precursors that require proteolytic processing for activation. Because the proteases responsible for activation of members of the IL-36 subfamily have not been reported, we have developed a method for the production of biologically active IL-36 through introduction of a caspase cleavage motif, DEVD, within the N-termini of these cytokines. Here, we show that DEVD-modified IL-36α, IL-36β and IL-36γ cytokines were highly soluble and were readily processed and activated by caspase-3. Caspase-3-processed IL-36 family cytokines exhibited robust biological activity on a range of responsive cell types, including primary keratinocytes. We also generated specific polyclonal antibodies against all three IL-36 family members through immunization with purified recombinant IL-36 cytokines. The modified forms of IL-36 described herein will be useful for production of large quantities of biologically active IL-36 for structure and function studies on these important proinflammatory cytokines.

Diverse Activators of the NLRP3 Inflammasome Promote IL-1β Secretion by Triggering Necrosis

The NLRP3 inflammasome is involved in caspase-1-dependent maturation of IL-1β in many contexts. A two-signal model has emerged for IL-1β maturation, with LPS providing "signal I" and diverse agents such as ATP, Nigericin, streptolysin O, uric acid crystals, and alum salts capable of acting as "signal II." In the absence of signal II, pro-IL-1β is upregulated but typically fails to be processed or released. What unites signal II stimuli has been debated, with the ability to promote K+ efflux suggested as a common factor, but the mechanism of IL-1β release remains unclear. Here, we show that all examined inflammasome signal II agents triggered necrosis, which was highly correlated with their ability to promote IL-1β release. IL-1β secretion occurred in tandem with the release of many additional proteins and was confined to necrotic cells. Thus, signal II agents initiate inflammation by promoting necrosis-driven IL-1β release, suggesting that IL-1β represents an inducible danger signal.