Differential activation of the inflammasome by caspase-1 adaptors ASC and Ipaf

Specific adaptors regulate the activation of initiator caspases; for example, FADD and Apaf-1 engage caspases 8 and 9, respectively. The adaptors ASC, Ipaf and RIP2 have each been proposed to regulate caspase-1 (also called interleukin (IL)-1 converting enzyme), which is activated within the 'inflammasome', a complex comprising several adaptors. Here we show the impact of ASC-, Ipaf- or RIP2-deficiency on inflammasome function. ASC was essential for extracellular ATP-driven activation of caspase-1 in toll-like receptor (TLR)-stimulated macrophages. Accordingly, ASC-deficient macrophages exhibited defective maturation of IL-1beta and IL-18, and ASC-null mice were resistant to lipopolysaccharide-induced endotoxic shock. Furthermore, activation of caspase-1 in response to an intracellular pathogen (Salmonella typhimurium) was abrogated severely in ASC-null macrophages. Unexpectedly, Ipaf-deficient macrophages activated caspase-1 in response to TLR plus ATP stimulation but not S. typhimurium. Caspase-1 activation was not compromised by loss of RIP2. These data show that whereas ASC is key to caspase-1 activation within the inflammasome, Ipaf provides a special conduit to the inflammasome for signals triggered by intracellular pathogens. Notably, cell death triggered by stimuli that engage caspase-1 was ablated in macrophages lacking either ASC or Ipaf, suggesting a coupling between the inflammatory and cell death pathways.     

Cryopyrin activates the inflammasome in response to toxins and ATP

A crucial part of the innate immune response is the assembly of the inflammasome, a cytosolic complex of proteins that activates caspase-1 to process the proinflammatory cytokines interleukin (IL)-1beta and IL-18. The adaptor protein ASC is essential for inflammasome function, binding directly to caspase-1 (refs 3, 4), but the triggers of this interaction are less clear. ASC also interacts with the adaptor cryopyrin (also known as NALP3 or CIAS1). Activating mutations in cryopyrin are associated with familial cold autoinflammatory syndrome, Muckle-Wells syndrome and neonatal onset multisystem inflammatory disease, diseases that are characterized by excessive production of IL-1beta. Here we show that cryopyrin-deficient macrophages cannot activate caspase-1 in response to Toll-like receptor agonists plus ATP, the latter activating the P2X7 receptor to decrease intracellular K+ levels. The release of IL-1beta in response to nigericin, a potassium ionophore, and maitotoxin, a potent marine toxin, was also found to be dependent on cryopyrin. In contrast to Asc-/- macrophages, cells deficient in the gene encoding cryopyrin (Cias1-/-) activated caspase-1 and secreted normal levels of IL-1beta and IL-18 when infected with Gram-negative Salmonella typhimurium or Francisella tularensis. Macrophages exposed to Gram-positive Staphylococcus aureus or Listeria monocytogenes, however, required both ASC and cryopyrin to activate caspase-1 and secrete IL-1beta. Therefore, cryopyrin is essential for inflammasome activation in response to signalling pathways triggered specifically by ATP, nigericin, maitotoxin, S. aureus or L. monocytogenes.     

Semaphorin 7A initiates T-cell-mediated inflammatory responses through alpha1beta1 integrin

Semaphorins are axon guidance factors that assist growing axons in finding appropriate targets and forming synapses. Emerging evidence suggests that semaphorins are involved not only in embryonic development but also in immune responses. Semaphorin 7A (Sema7A; also known as CD108), which is a glycosylphosphatidylinositol-anchored semaphorin, promotes axon outgrowth through beta1-integrin receptors and contributes to the formation of the lateral olfactory tract. Although Sema7A has been shown to stimulate human monocytes, its function as a negative regulator of T-cell responses has also been reported. Thus, the precise function of Sema7A in the immune system remains unclear. Here we show that Sema7A, which is expressed on activated T cells, stimulates cytokine production in monocytes and macrophages through alpha1beta1 integrin (also known as very late antigen-1) as a component of the immunological synapse, and is critical for the effector phase of the inflammatory immune response. Sema7A-deficient (Sema7a-/-) mice are defective in cell-mediated immune responses such as contact hypersensitivity and experimental autoimmune encephalomyelitis. Although antigen-specific and cytokine-producing effector T cells can develop and migrate into antigen-challenged sites in Sema7a-/- mice, Sema7a-/- T cells fail to induce contact hypersensitivity even when directly injected into the antigen-challenged sites. Thus, the interaction between Sema7A and alpha1beta1 integrin is crucial at the site of inflammation. These findings not only identify a function of Sema7A as an effector molecule in T-cell-mediated inflammation, but also reveal a mechanism of integrin-mediated immune regulation.     

Bacterial RNA and small antiviral compounds activate caspase-1 through cryopyrin/Nalp3

Missense mutations in the CIAS1 gene cause three autoinflammatory disorders: familial cold autoinflammatory syndrome, Muckle-Wells syndrome and neonatal-onset multiple-system inflammatory disease. Cryopyrin (also called Nalp3), the product of CIAS1, is a member of the NOD-LRR protein family that has been linked to the activation of intracellular host defence signalling pathways. Cryopyrin forms a multi-protein complex termed 'the inflammasome', which contains the apoptosis-associated speck-like protein (ASC) and caspase-1, and promotes caspase-1 activation and processing of pro-interleukin (IL)-1beta (ref. 4). Here we show the effect of cryopyrin deficiency on inflammasome function and immune responses. Cryopyrin and ASC are essential for caspase-1 activation and IL-1beta and IL-18 production in response to bacterial RNA and the imidazoquinoline compounds R837 and R848. In contrast, secretion of tumour-necrosis factor-alpha and IL-6, as well as activation of NF-kappaB and mitogen-activated protein kinases (MAPKs) were unaffected by cryopyrin deficiency. Furthermore, we show that Toll-like receptors and cryopyrin control the secretion of IL-1beta and IL-18 through different intracellular pathways. These results reveal a critical role for cryopyrin in host defence through bacterial RNA-mediated activation of caspase-1, and provide insights regarding the pathogenesis of autoinflammatory syndromes.     

Involvement of receptor-interacting protein 2 in innate and adaptive immune responses

Host defences to microorganisms rely on a coordinated interplay between the innate and adaptive responses of immunity. Infection with intracellular bacteria triggers an immediate innate response requiring macrophages, neutrophils and natural killer cells, whereas subsequent activation of an adaptive response through development of T-helper subtype 1 cells (TH1) proceeds during persistent infection. To understand the physiological role of receptor-interacting protein 2 (Rip2), also known as RICK and CARDIAK, we generated mice with a targeted disruption of the gene coding for Rip2. Here we show that Rip2-deficient mice exhibit a profoundly decreased ability to defend against infection by the intracellular pathogen Listeria monocytogenes. Rip2-deficient macrophages infected with L. monocytogenes or treated with lipopolysaccharide (LPS) have decreased activation of NF-kappaB, whereas dominant negative Rip2 inhibited NF-kappaB activation mediated by Toll-like receptor 4 and Nod1. In vivo, Rip2-deficient mice were resistant to the lethal effects of LPS-induced endotoxic shock. Furthermore, Rip2 deficiency results in impaired interferon-gamma production in both TH1 and natural killer cells, attributed in part to defective interleukin-12-induced Stat4 activation. Our data reflect requirements for Rip2 in multiple pathways regulating immune and inflammatory responses.     

Interleukin-23 rather than interleukin-12 is the critical cytokine for autoimmune inflammation of the brain

Interleukin-12 (IL-12) is a heterodimeric molecule composed of p35 and p40 subunits. Analyses in vitro have defined IL-12 as an important factor for the differentiation of naive T cells into T-helper type 1 CD4+ lymphocytes secreting interferon-gamma (refs 1, 2). Similarly, numerous studies have concluded that IL-12 is essential for T-cell-dependent immune and inflammatory responses in vivo, primarily through the use of IL-12 p40 gene-targeted mice and neutralizing antibodies against p40. The cytokine IL-23, which comprises the p40 subunit of IL-12 but a different p19 subunit, is produced predominantly by macrophages and dendritic cells, and shows activity on memory T cells. Evidence from studies of IL-23 receptor expression and IL-23 overexpression in transgenic mice suggest, however, that IL-23 may also affect macrophage function directly. Here we show, by using gene-targeted mice lacking only IL-23 and cytokine replacement studies, that the perceived central role for IL-12 in autoimmune inflammation, specifically in the brain, has been misinterpreted and that IL-23, and not IL-12, is the critical factor in this response. In addition, we show that IL-23, unlike IL-12, acts more broadly as an end-stage effector cytokine through direct actions on macrophages.     

Interleukin-1 beta as a potent hyperalgesic agent antagonized by a tripeptide analogue

Interleukin-1 (IL-1) describes two inflammatory proteins, IL-1 alpha and IL-1 beta, produced by activated macrophages and other cell types and encoded by two genes. Their amino acid sequences have only 26% similarity, but their biological activities are comparable, with a few exceptions; indeed, both molecules appear to act at the same receptor. As IL-1 release prostaglandins which sensitize nociceptors in man and in experimental animals, we tested IL-1 alpha and IL-1 beta in rats for hyperalgesic (nociceptive) activity. Our results show that IL-1 beta given systemically is an extremely potent hyperalgesic agent with a probable peripheral site of action; IL-1 alpha is approximately 3,000 times less active than IL-1 beta. We have delineated the region of IL-1 beta mediating the hyperalgesic effect and developed an analgesic tripeptide analogue of IL-1 beta which antagonizes hyperalgesia evoked by IL-1 beta and by the inflammatory agent carrageenan.     

The role of autophagy during the early neonatal starvation period

At birth the trans-placental nutrient supply is suddenly interrupted, and neonates face severe starvation until supply can be restored through milk nutrients. Here, we show that neonates adapt to this adverse circumstance by inducing autophagy. Autophagy is the primary means for the degradation of cytoplasmic constituents within lysosomes. The level of autophagy in mice remains low during embryogenesis; however, autophagy is immediately upregulated in various tissues after birth and is maintained at high levels for 3-12 h before returning to basal levels within 1-2 days. Mice deficient for Atg5, which is essential for autophagosome formation, appear almost normal at birth but die within 1 day of delivery. The survival time of starved Atg5-deficient neonates (approximately 12 h) is much shorter than that of wild-type mice (approximately 21 h) but can be prolonged by forced milk feeding. Atg5-deficient neonates exhibit reduced amino acid concentrations in plasma and tissues, and display signs of energy depletion. These results suggest that the production of amino acids by autophagic degradation of 'self' proteins, which allows for the maintenance of energy homeostasis, is important for survival during neonatal starvation.     

Dendritic cell PAR1-S1P3 signalling couples coagulation and inflammation

Defining critical points of modulation across heterogeneous clinical syndromes may provide insight into new therapeutic approaches. Coagulation initiated by the cytokine-receptor family member known as tissue factor is a hallmark of systemic inflammatory response syndromes in bacterial sepsis and viral haemorrhagic fevers, and anticoagulants can be effective in severe sepsis with disseminated intravascular coagulation. The precise mechanism coupling coagulation and inflammation remains unresolved. Here we show that protease-activated receptor 1 (PAR1) signalling sustains a lethal inflammatory response that can be interrupted by inhibition of either thrombin or PAR1 signalling. The sphingosine 1-phosphate (S1P) axis is a downstream component of PAR1 signalling, and by combining chemical and genetic probes for S1P receptor 3 (S1P3) we show a critical role for dendritic cell PAR1-S1P3 cross-talk in regulating amplification of inflammation in sepsis syndrome. Conversely, dendritic cells sustain escalated systemic coagulation and are the primary hub at which coagulation and inflammation intersect within the lymphatic compartment. Loss of dendritic cell PAR1-S1P3 signalling sequesters dendritic cells and inflammation into draining lymph nodes, and attenuates dissemination of interleukin-1beta to the lungs. Thus, activation of dendritic cells by coagulation in the lymphatics emerges as a previously unknown mechanism that promotes systemic inflammation and lethality in decompensated innate immune responses.     

C5L2 is critical for the biological activities of the anaphylatoxins C5a and C3a

Complement-derived anaphylatoxins regulate immune and inflammatory responses through G-protein-coupled receptor (GPCR)-mediated signalling. C5L2 (also known as GPR77) is a relatively new GPCR thought to be a non-signalling receptor binding to C5a, on the basis of sequence information and experimental evidence. Here we show, using gene targeting, that C5L2 is required to facilitate C5a signalling in neutrophils, macrophages and fibroblasts in vitro. Deficiency of C5L2 results in reduced inflammatory cell infiltration, suggesting that C5L2 is critical for optimal C5a-mediated cell infiltration in certain in vivo settings. C5L2 is also involved in optimizing C3a-induced signals. Furthermore, like mice incapable of C3a/complement 3a receptor (C3aR) signalling, C5L2-deficient mice are hypersensitive to lipopolysaccharide (LPS)-induced septic shock, show reduced ovalbumin (OVA)-induced airway hyper-responsiveness and inflammation, and are mildly delayed in haematopoietic cell regeneration after gamma-irradiation. Our data indicate that C5L2 can function as a positive modulator for both C5a- and C3a-anaphylatoxin-induced responses.     

Specificity in Toll-like receptor signalling through distinct effector functions of TRAF3 and TRAF6

Toll-like receptors (TLRs) are activated by pathogen-associated molecular patterns to induce innate immune responses and production of pro-inflammatory cytokines, interferons and anti-inflammatory cytokines. TLRs activate downstream effectors through adaptors that contain Toll/interleukin-1 receptor (TIR) domains, but the mechanisms accounting for diversification of TLR effector functions are unclear. To dissect biochemically TLR signalling, we established a system for isolating signalling complexes assembled by dimerized adaptors. Using MyD88 as a prototypical adaptor, we identified TNF receptor-associated factor 3 (TRAF3) as a new component of TIR signalling complexes that is recruited along with TRAF6. Using myeloid cells from TRAF3- and TRAF6-deficient mice, we show that TRAF3 is essential for the induction of type I interferons (IFN) and the anti-inflammatory cytokine interleukin-10 (IL-10), but is dispensable for expression of pro-inflammatory cytokines. In fact, TRAF3-deficient cells overproduce pro-inflammatory cytokines owing to defective IL-10 production. Despite their structural similarity, the functions of TRAF3 and TRAF6 are largely distinct. TRAF3 is also recruited to the adaptor TRIF (Toll/IL-1 receptor domain-containing adaptor-inducing IFN-beta) and is required for marshalling the protein kinase TBK1 (also called NAK) into TIR signalling complexes, thereby explaining its unique role in activation of the IFN response.     

Vpx relieves inhibition of HIV-1 infection of macrophages mediated by the SAMHD1 protein

Macrophages and dendritic cells have key roles in viral infections, providing virus reservoirs that frequently resist antiviral therapies and linking innate virus detection to antiviral adaptive immune responses. Human immunodeficiency virus 1 (HIV-1) fails to transduce dendritic cells and has a reduced ability to transduce macrophages, due to an as yet uncharacterized mechanism that inhibits infection by interfering with efficient synthesis of viral complementary DNA. In contrast, HIV-2 and related simian immunodeficiency viruses (SIVsm/mac) transduce myeloid cells efficiently owing to their virion-associated Vpx accessory proteins, which counteract the restrictive mechanism. Here we show that the inhibition of HIV-1 infection in macrophages involves the cellular SAM domain HD domain-containing protein 1 (SAMHD1). Vpx relieves the inhibition of lentivirus infection in macrophages by loading SAMHD1 onto the CRL4(DCAF1) E3 ubiquitin ligase, leading to highly efficient proteasome-dependent degradation of the protein. Mutations in SAMHD1 cause Aicardi-Goutières syndrome, a disease that produces a phenotype that mimics the effects of a congenital viral infection. Failure to dispose of endogenous nucleic acid debris in Aicardi-Goutières syndrome results in inappropriate triggering of innate immune responses via cytosolic nucleic acids sensors. Thus, our findings show that macrophages are defended from HIV-1 infection by a mechanism that prevents an unwanted interferon response triggered by self nucleic acids, and uncover an intricate relationship between innate immune mechanisms that control response to self and to retroviral pathogens.     

Interplay of p53 and DNA-repair protein XRCC4 in tumorigenesis, genomic stability and development

XRCC4 is a non-homologous end-joining protein employed in DNA double strand break repair and in V(D)J recombination. In mice, XRCC4-deficiency causes a pleiotropic phenotype, which includes embryonic lethality and massive neuronal apoptosis. When DNA damage is not repaired, activation of the cell cycle checkpoint protein p53 can lead to apoptosis. Here we show that p53-deficiency rescues several aspects of the XRCC4-deficient phenotype, including embryonic lethality, neuronal apoptosis, and impaired cellular proliferation. However, there was no significant rescue of impaired V(D)J recombination or lymphocyte development. Although p53-deficiency allowed postnatal survival of XRCC4-deficient mice, they routinely succumbed to pro-B-cell lymphomas which had chromosomal translocations linking amplified c-myc oncogene and IgH locus sequences. Moreover, even XRCC4-deficient embryonic fibroblasts exhibited marked genomic instability including chromosomal translocations. Our findings support a crucial role for the non-homologous end-joining pathway as a caretaker of the mammalian genome, a role required both for normal development and for suppression of tumours.     

Enhanced bacterial clearance and sepsis resistance in caspase-12-deficient mice

Caspases function in both apoptosis and inflammatory cytokine processing and thereby have a role in resistance to sepsis. Here we describe a novel role for a caspase in dampening responses to bacterial infection. We show that in mice, gene-targeted deletion of caspase-12 renders animals resistant to peritonitis and septic shock. The resulting survival advantage was conferred by the ability of the caspase-12-deficient mice to clear bacterial infection more efficiently than wild-type littermates. Caspase-12 dampened the production of the pro-inflammatory cytokines interleukin (IL)-1beta, IL-18 (interferon (IFN)-gamma inducing factor) and IFN-gamma, but not tumour-necrosis factor-alpha and IL-6, in response to various bacterial components that stimulate Toll-like receptor and NOD pathways. The IFN-gamma pathway was crucial in mediating survival of septic caspase-12-deficient mice, because administration of neutralizing antibodies to IFN-gamma receptors ablated the survival advantage that otherwise occurred in these animals. Mechanistically, caspase-12 associated with caspase-1 and inhibited its activity. Notably, the protease function of caspase-12 was not necessary for this effect, as the catalytically inactive caspase-12 mutant Cys299Ala also inhibited caspase-1 and IL-1beta production to the same extent as wild-type caspase-12. In this regard, caspase-12 seems to be the cFLIP counterpart for regulating the inflammatory branch of the caspase cascade. In mice, caspase-12 deficiency confers resistance to sepsis and its presence exerts a dominant-negative suppressive effect on caspase-1, resulting in enhanced vulnerability to bacterial infection and septic mortality.     

A Toll-like receptor recognizes bacterial DNA

DNA from bacteria has stimulatory effects on mammalian immune cells, which depend on the presence of unmethylated CpG dinucleotides in the bacterial DNA. In contrast, mammalian DNA has a low frequency of CpG dinucleotides, and these are mostly methylated; therefore, mammalian DNA does not have immuno-stimulatory activity. CpG DNA induces a strong T-helper-1-like inflammatory response. Accumulating evidence has revealed the therapeutic potential of CpG DNA as adjuvants for vaccination strategies for cancer, allergy and infectious diseases. Despite its promising clinical use, the molecular mechanism by which CpG DNA activates immune cells remains unclear. Here we show that cellular response to CpG DNA is mediated by a Toll-like receptor, TLR9. TLR9-deficient (TLR9-/-) mice did not show any response to CpG DNA, including proliferation of splenocytes, inflammatory cytokine production from macrophages and maturation of dendritic cells. TLR9-/- mice showed resistance to the lethal effect of CpG DNA without any elevation of serum pro-inflammatory cytokine levels. The in vivo CpG-DNA-mediated T-helper type-1 response was also abolished in TLR9-/- mice. Thus, vertebrate immune systems appear to have evolved a specific Toll-like receptor that distinguishes bacterial DNA from self-DNA.