John R. Lukens

Mitochondria: diversity in the regulation of the NLRP3 inflammasome

Recent studies have identified new roles for mitochondria in the regulation of autoinflammatory processes. Emerging data suggests that the release of danger signals from mitochondria in response to stress and infection promotes the formation of the inflammatory signaling platform known as inflammasomes. Activation of inflammasomes by damaged mitochondria results in caspase-1-dependent secretion of the inflammatory cytokines interleukin-1β (IL-1β) and IL-18, and an inflammatory form of cell death referred to as pyroptosis. Here, we review recently described mechanisms that have been proposed to be involved in mitochondria-mediated regulation of inflammasome activation and inflammation. In addition, we highlight how aberrant regulation of mitochondria-induced inflammasome activation centrally contributes to the inflammatory process that is responsible for obesity and associated metabolic diseases.

Dietary modulation of the microbiome affects autoinflammatory disease

The incidences of chronic inflammatory disorders have increased considerably over the past three decades. Recent shifts in dietary consumption may have contributed importantly to this surge, but how dietary consumption modulates inflammatory disease is poorly defined. Pstpip2cmo mice, which express a homozygous Leu98Pro missense mutation in the Pombe Cdc15 homology family protein PSTPIP2 (proline-serine-threonine phosphatase interacting protein 2), spontaneously develop osteomyelitis that resembles chronic recurrent multifocal osteomyelitis in humans. Recent reports demonstrated a crucial role for interleukin-1β (IL-1β) in osteomyelitis, but deletion of the inflammasome components caspase-1 and NLRP3 failed to rescue Pstpip2cmo mice from inflammatory bone disease. Thus, the upstream mechanisms controlling IL-1β production in Pstpip2cmo mice remain to be identified. In addition, the environmental factors driving IL-1β-dependent inflammatory bone erosion are unknown. Here we show that the intestinal microbiota of diseased Pstpip2cmo mice was characterized by an outgrowth of Prevotella. Notably, Pstpip2cmo mice that were fed a diet rich in fat and cholesterol maintained a normal body weight, but were markedly protected against inflammatory bone disease and bone erosion. Diet-induced protection against osteomyelitis was accompanied by marked reductions in intestinal Prevotella levels and significantly reduced pro-IL-1β expression in distant neutrophils. Furthermore, pro-IL-1β expression was also decreased in Pstpip2cmo mice treated with antibiotics, and in wild-type mice that were kept under germ-free conditions. We further demonstrate that combined deletion of caspases 1 and 8 was required for protection against IL-1β-dependent inflammatory bone disease, whereas the deletion of either caspase alone or of elastase or neutrophil proteinase 3 failed to prevent inflammatory disease. Collectively, this work reveals diet-associated changes in the intestinal microbiome as a crucial factor regulating inflammasome- and caspase-8-mediated maturation of IL-1β and osteomyelitis in Pstpip2cmo mice.

Cutting Edge: Critical Role for PYCARD/ASC in the Development of Experimental Autoimmune Encephalomyelitis

Multiple sclerosis is an autoimmune disease in which self-reactive T cells attack oligodendrocytes that myelinate axons in the CNS. Experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, is dependent on caspase-1; however, the role of Nod-like receptors upstream of caspase-1 is unknown. Danger- and pathogen-associated molecular patterns activate Nod-like receptor 3, which activates caspase-1 through the adaptor protein, apoptosis-associated speck-like protein containing CARD (ASC). We report that the progression of EAE is dependent on ASC and caspase-1 but not Nod-like receptor 3. ASC(-/-) mice were even more protected from the progression of EAE than were caspase-1(-/-) mice, suggesting that an inflammasome-independent function of ASC contributes to the progression of EAE. We found that CD4(+) T cells deficient in ASC exhibited impaired survival; accordingly, ASC(-/-) mice had fewer myelin oligodendrocyte glycoprotein-specific T cells in the draining lymph nodes and CNS.

Protective Roles for Caspase-8 and cFLIP in Adult Homeostasis

Caspase-8 or cellular FLICE-like inhibitor protein (cFLIP) deficiency leads to embryonic lethality in mice due to defects in endothelial tissues. Caspase-8(-/-) and receptor-interacting protein kinase-3 (RIPK3)(-/-), but not cFLIP(-/-) and RIPK3(-/-), double-knockout animals develop normally, indicating that caspase-8 antagonizes the lethal effects of RIPK3 during development. Here, we show that the acute deletion of caspase-8 in the gut of adult mice induces enterocyte death, disruption of tissue homeostasis, and inflammation, resulting in sepsis and mortality. Likewise, acute deletion of caspase-8 in a focal region of the skin induces local keratinocyte death, tissue disruption, and inflammation. Strikingly, RIPK3 ablation rescues both phenotypes. However, acute loss of cFLIP in the skin produces a similar phenotype that is not rescued by RIPK3 ablation. TNF neutralization protects from either acute loss of caspase-8 or cFLIP. These results demonstrate that caspase-8-mediated suppression of RIPK3-induced death is required not only during development but also for adult homeostasis. Furthermore, RIPK3-dependent inflammation is dispensable for the skin phenotype.

RIP1-driven autoinflammation targets IL-1α independently of inflammasomes and RIP3

The protein-tyrosine phosphatase SHP-1 has critical roles in immune signalling, but how mutations in SHP-1 cause inflammatory disease in humans remains poorly defined. Mice homozygous for the Tyr208Asn amino acid substitution in the carboxy terminus of SHP-1 (referred to as Ptpn6spin mice) spontaneously develop a severe inflammatory syndrome that resembles neutrophilic dermatosis in humans and is characterized by persistent footpad swelling and suppurative inflammation. Here we report that receptor-interacting protein 1 (RIP1)-regulated interleukin (IL)-1α production by haematopoietic cells critically mediates chronic inflammatory disease in Ptpn6spin mice, whereas inflammasome signalling and IL-1β-mediated events are dispensable. IL-1α was also crucial for exacerbated inflammatory responses and unremitting tissue damage upon footpad microabrasion of Ptpn6spin mice. Notably, pharmacological and genetic blockade of the kinase RIP1 protected against wound-induced inflammation and tissue damage in Ptpn6spin mice, whereas RIP3 deletion failed to do so. Moreover, RIP1-mediated inflammatory cytokine production was attenuated by NF-κB and ERK inhibition. Together, our results indicate that wound-induced tissue damage and chronic inflammation in Ptpn6spin mice are critically dependent on RIP1-mediated IL-1α production, whereas inflammasome signalling and RIP3-mediated necroptosis are dispensable. Thus, we have unravelled a novel inflammatory circuit in which RIP1-mediated IL-1α secretion in response to deregulated SHP-1 activity triggers an inflammatory destructive disease that proceeds independently of inflammasomes and programmed necrosis.

Dealing with Danger in the CNS: The Response of the Immune System to Injury

Fighting pathogens and maintaining tissue homeostasis are prerequisites for survival. Both of these functions are upheld by the immune system, though the latter is often overlooked in the context of the CNS. The mere presence of immune cells in the CNS was long considered a hallmark of pathology, but this view has been recently challenged by studies demonstrating that immunological signaling can confer pivotal neuroprotective effects on the injured CNS. In this review, we describe the temporal sequence of immunological events that follow CNS injury. Beginning with immediate changes at the injury site, including death of neural cells and release of damage-associated molecular patterns (DAMPs), and progressing through innate and adaptive immune responses, we describe the cascade of inflammatory mediators and the implications of their post-injury effects. We conclude by proposing a revised interpretation of immune privilege in the brain, which takes beneficial neuro-immune communications into account.

IL-1 family cytokines trigger sterile inflammatory disease

Inflammation plays vital roles in protective responses against pathogens and tissue repair, however, improper resolution of inflammatory networks is centrally involved in the pathogenesis of many acute and chronic diseases. Extensive advances have been made in recent years to define the inflammatory processes that are required for pathogen clearance, however, in comparison, less is known about the regulation of inflammation in sterile settings. Over the past decade non-communicable chronic diseases that are potentiated by sterile inflammation have replaced infectious diseases as the major threat to global human health. Thus, improved understanding of the sterile inflammatory process has emerged as one of the most important areas of biomedical investigation during our time. In this review we highlight the central role that interleukin-1 family cytokines play in sterile inflammatory diseases.

Intrahepatic IL-10 Maintains NKG2A+Ly49− Liver NK Cells in a Functionally Hyporesponsive State

The tolerogenic nature of the liver allows daily exposure to gut-derived foreign Ags without causing inflammation, but it may facilitate persistent infection in the liver. NK cells play a central role in innate immunity, as well as in shaping the adaptive immune response. We hypothesized that the naive mouse liver maintains intrahepatic NK cells in a functionally hyporesponsive state. Compared with splenic NK cells, liver NK cells displayed a dampened IFN-γ response to IL-12/IL-18 stimulation. Importantly, the liver contains a significant population of functionally hyporesponsive NK cells that express high levels of the inhibitory receptor NKG2A and lack expression of MHC class I-binding Ly49 receptors. Adoptively transferred splenic NK cells that migrate to the liver displayed phenotypic and functional changes, suggesting that the liver environment modifies NK cell receptor expression and functional responsiveness. Notably, IL-10 is present at high levels within the liver, and in vivo blockade of IL-10R resulted in a decreased percentage of intrahepatic NKG2A+Ly49− NK cells. These data suggest that the liver environment regulates NK cell receptor expression and that IL-10 contributes to the regulation of liver NK cells, in part, by maintaining a greater percentage of the hyporesponsive NKG2A+Ly49− NK cells in the liver.

Signaling via the RIP2 Adaptor Protein in Central Nervous System-Infiltrating Dendritic Cells Promotes Inflammation and Autoimmunity

Peripheral peptidolgycan (PGN) is present within antigen-presenting cells in the central nervous system (CNS) of multiple sclerosis (MS) patients, possibly playing a role in neuroinflammation. Accordingly, PGN is linked with disease progression in the animal model of MS, experimental autoimmune encephalomyelitis (EAE), but the role of specific PGN-sensing proteins is unknown. Here we report that the progression of EAE was dependent on the intracellular PGN sensors NOD1 and NOD2 and their common downstream adaptor molecule, receptor interacting protein 2 (RIP2; also known as RIPK2 and RICK). We found that RIP2, but not toll-like receptor 2 (TLR2), played a critical role in the activation of CNS-infiltrating dendritic cells. Our results suggest that PGN in the CNS is involved in the pathogenesis of EAE through the activation of infiltrating dendritic cells via NOD1-, NOD2-, and RIP2-mediated pathways.

Inflammasome-Derived IL-1β Regulates the Production of GM-CSF by CD4+ T Cells and γδ T Cells

Recent findings have demonstrated an indispensable role for GM-CSF in the pathogenesis of experimental autoimmune encephalomyelitis. However, the signaling pathways and cell populations that regulate GM-CSF production in vivo remain to be elucidated. Our work demonstrates that IL-1R is required for GM-CSF production after both TCR- and cytokine-induced stimulation of immune cells in vitro. Conventional αβ and γδ T cells were both identified to be potent producers of GM-CSF. Moreover, secretion of GM-CSF was dependent on IL-1R under both IL-12– and IL-23–induced stimulatory conditions. Deficiency in IL-1R conferred significant protection from experimental autoimmune encephalomyelitis, and this correlated with reduced production of GM-CSF and attenuated infiltration of inflammatory cells into the CNS. We also find that GM-CSF production in vivo is not restricted to a defined CD4+ T cell lineage but is rather heterogeneously expressed in the effector CD4+ T cell population. In addition, inflammasome-derived IL-1β upstream of IL-1R is a critical regulator of GM-CSF production by T cells during priming, and the adapter protein, MyD88, promotes GM-CSF production in both αβ and γδ T cells. These findings highlight the importance of inflammasome-derived IL-1β and the IL-1R/MyD88 signaling axis in the regulation of GM-CSF production.