Identification of Lps2 as a key transducer of MyD88-independent TIR signalling

Dendritic cells in viral infections

Antigen presenting cells (APCs) are recognized as key initiators of adaptive immunity, particularly to pathogens, by eliciting a rapid and potent immune attack on infected cells. Amongst APCs, dendritic cells (DCs) are specially equipped to initiate and regulate immune responses in a manner that depends on signals they receive from microbes and their cellular environment. To achieve this, they are equipped with highly efficient mechanisms that allow them to detect pathogens, to capture, process and present antigens, and to activate and guide the differentiation of T cells into effector and memory cells. DCs can no longer be considered as a homogeneous cell type performing a single function, but are heterogeneous both in phenotype, function and dependence on inflammatory stimuli for their formation and responsiveness. Recent studies of DC subtypes have highlighted the contrasting roles of different professional APCs in activating divergent arms of the immune response towards pathogens. In this review, we discuss the progress that has been made in dissecting the attributes of different DC subsets that migrate into, or reside permanently, within lymphoid tissues and their putative roles in the induction of the anti-viral immune response.

Chlamydia pneumoniae-induced foam cell formation requires MyD88-dependent and -independent signaling and is reciprocally modulated by liver X receptor activation

Chlamydia pneumoniae is detected by macrophages and other APCs via TLRs and can exacerbate developing atherosclerotic lesions, but how that occurs is not known. Liver X receptors (LXRs) centrally control reverse cholesterol transport, but also negatively modulate TLR-mediated inflammatory pathways. We isolated peritoneal macrophages from wild-type, TLR2, TLR3, TLR4, TLR2/4, MyD88, TRIF, MyD88/TRIF, and IFN regulatory factor 3 (IRF3) KO mice, treated them with live or UV-killed C. pneumoniae in the presence or absence of oxidized LDL, then measured foam cell formation. In some experiments, the synthetic LXR agonist GW3965 was added to macrophages infected with C. pneumoniae in the presence of oxidized LDL. Both live and UV-killed C. pneumoniae induced IRF3 activation and promoted foam cell formation in wild-type macrophages, whereas the genetic absence of TLR2, TLR4, MyD88, TRIF, or IRF3, but not TLR3, significantly reduced foam cell formation. C. pneumoniae-induced foam cell formation was significantly reduced by the LXR agonist GW3965, which in turn inhibited C. pneumoniae-induced IRF3 activation, suggesting a bidirectional cross-talk. We conclude that C. pneumoniae facilitates foam cell formation via activation of both MyD88-dependent and MyD88-independent (i.e., TRIF-dependent and IRF3-dependent) pathways downstream of TLR2 and TLR4 signaling and that TLR3 is not involved in this process. This mechanism could at least partly explain why infection with C. pneumoniae accelerates the development of atherosclerotic plaque and lends support to the proposal that LXR agonists might prove clinically useful in suppressing atherogenesis.

IFN-beta production by TLR4-stimulated innate immune cells is negatively regulated by GSK3-beta

TLR 4 stimulation of innate immune cells induces a MyD88-independent signaling pathway that leads to the production of IFN-beta. In this study, we demonstrate glycogen synthase kinase 3-beta (GSK3-beta) plays a fundamental role in this process. Suppression of GSK3-beta activity by either pharmacological inhibition, small interfering RNA-mediated gene silencing, or ectopic expression of a kinase-dead GSK3-beta mutant enhanced IFN-beta production by TLR4-stimulated macrophages. Conversely, ectopic expression of a constitutively active GSK3-beta mutant severely attenuated IFN-beta production. GSK3-beta was found to negatively control the cellular levels of the transcription factor c-Jun and its nuclear association with ATF-2. Small interfering RNA-mediated knockdown of c-Jun levels abrogated the ability of GSK3-beta inhibition to augment IFN-beta, demonstrating that the ability of GSK3 to control IFN-beta production was due to its ability to regulate c-Jun levels. The ability of GSK3 inhibition to control IFN-beta production was confirmed in vivo as mice treated with a GSK3 inhibitor exhibited enhanced systemic levels of IFN-beta upon LPS challenge. These findings identify a novel regulatory pathway controlling IFN-beta production by TLR4-stimulated innate immune cells.

Toll-like receptor signaling: a critical modulator of cell survival and ischemic injury in the heart

Toll-like receptors (TLRs) represent the first line of host defense against microbial infection and play a pivotal role in both innate and adaptive immunity. TLRs recognize invading pathogens through molecular pattern recognition, transduce signals via distinct intracellular pathways involving a unique set of adaptor proteins and kinases, and ultimately lead to the activation of transcription factors and inflammatory responses. Among 10 TLRs identified in humans, at least two exist in the heart, i.e., TLR2 and TLR4. In addition to the critical role of these in mediating cardiac dysfunction in septic conditions, emerging evidence suggests that the TLRs can also recognize endogenous ligands and may play an important role in modulating cardiomyocyte survival and in ischemic myocardial injury. In animal models of ischemia-reperfusion injury or in hypoxic cardiomyocytes in vitro, the administration of a sublethal dose of lipopolysaccharide, which signals through TLR4, reduces subsequent myocardial infarction, improves cardiac functions, and attenuates cardiomyocyte apoptosis. By contrast, a systemic deficiency of TLR2, TLR4, or myeloid differentiation primary-response gene 88, an adaptor critical for all TLR signaling, except TLR3, leads to an attenuated myocardial inflammation, a smaller infarction size, a better preserved ventricular function, and a reduced ventricular remodeling after ischemic injury. These loss-of-function studies suggest that both TLRs contribute to myocardial inflammation and ischemic injury in the heart although the exact contribution of cardiac (vs. circulatory cell) TLRs remains to be defined. These recent studies demonstrate an emerging role for TLRs as a critical modulator in both cell survival and tissue injury in the heart.

Signalling mechanisms for Toll-like receptor-activated neutrophil exocytosis: key roles for interleukin-1-receptor-associated kinase-4 and phosphatidylinositol 3-kinase but not Toll/IL-1 receptor (TIR) domain-containing adaptor inducing IFN-beta (TRIF)

Lipopolysaccharide (LPS) stimulates exocytosis in neutrophils. The signalling molecules involved in the regulation of this mechanism are currently unknown. Using neutrophils from interleukin-1-receptor-associated kinase (IRAK)-4- and Toll/IL-1 receptor (TIR) domain-containing adaptor inducing IFN-beta (TRIF)-deficient mice, we dissected the signalling pathways that control exocytosis. We analysed exocytosis of peroxidase-negative and azurophilic granules by following the mobilization of the beta2-integrin subunit CD11b and myeloperoxidase (MPO)-containing granules, respectively. IRAK-4-null neutrophils showed marked defects in both peroxidase-negative and azurophilic granule exocytosis in response to LPS. In contrast, the exocytic response to LPS of TRIF-deficient neutrophils was not different from that of wild-type cells. No differences were observed in the exocytosis of secretory organelles between IRAK-4-null and wild-type neutrophils when they were stimulated with the phorbol ester phorbol 12-myristate 13-acetate (PMA). Electron microscopy analysis showed that no morphological abnormalities were present in the granules of IRAK-4-deficient neutrophils, suggesting that the lack of exocytic response to LPS is not attributable to developmental abnormalities. Using pharmacological inhibitors, we found that p38 mitogen-activated protein kinase (p38MAPK) is essential for the exocytosis of all neutrophil secretory organelles in response to LPS. Interestingly, we found that phosphatidylinositol 3-kinase (PI3K) is essential for azurophilic granule exocytosis but not for the mobilization of other neutrophil granules in response to LPS. Azurophilic granule exocytosis in response to Listeria monocytogenes was dependent on PI3K but not IRAK-4 activity, suggesting that alternative signalling pathways are activated in IRAK-4-deficient neutrophils exposed to whole bacteria. Our results identified IRAK-4, p38MAPK and PI3K as important regulatory components with different roles in the signalling pathways that control Toll-like receptor ligand-triggered neutrophil exocytosis.

Key role of splenic myeloid DCs in the IFN-alphabeta response to adenoviruses in vivo

The early systemic production of interferon (IFN)-alphabeta is an essential component of the antiviral host defense mechanisms, but is also thought to contribute to the toxic side effects accompanying gene therapy with adenoviral vectors. Here we investigated the IFN-alphabeta response to human adenoviruses (Ads) in mice. By comparing the responses of normal, myeloid (m)DC- and plasmacytoid (p)DC-depleted mice and by measuring IFN-alphabeta mRNA expression in different organs and cells types, we show that in vivo, Ads elicit strong and rapid IFN-alphabeta production, almost exclusively in splenic mDCs. Using knockout mice, various strains of Ads (wild type, mutant and UV-inactivated) and MAP kinase inhibitors, we demonstrate that the Ad-induced IFN-alphabeta response does not require Toll-like receptors (TLR), known cytosolic sensors of RNA (RIG-I/MDA-5) and DNA (DAI) recognition and interferon regulatory factor (IRF)-3, but is dependent on viral endosomal escape, signaling via the MAP kinase SAPK/JNK and IRF-7. Furthermore, we show that Ads induce IFN-alphabeta and IL-6 in vivo by distinct pathways and confirm that IFN-alphabeta positively regulates the IL-6 response. Finally, by measuring TNF-alpha responses to LPS in Ad-infected wild type and IFN-alphabetaR(-/-) mice, we show that IFN-alphabeta is the key mediator of Ad-induced hypersensitivity to LPS. These findings indicate that, like endosomal TLR signaling in pDCs, TLR-independent virus recognition in splenic mDCs can also produce a robust early IFN-alphabeta response, which is responsible for the bulk of IFN-alphabeta production induced by adenovirus in vivo. The signaling requirements are different from known TLR-dependent or cytosolic IFN-alphabeta induction mechanisms and suggest a novel cytosolic viral induction pathway. The hypersensitivity to components of the microbial flora and invading pathogens may in part explain the toxic side effects of adenoviral gene therapy and contribute to the pathogenesis of adenoviral disease.

No longer an innocent bystander: epithelial toll-like receptor signaling in the development of mucosal inflammation

Diseases of mucosal inflammation represent important causes of morbidity and mortality, and have led to intense research efforts to understand the factors that lead to their development. It is well accepted that a breakdown of the normally impermeant epithelial barrier of the intestine, the lung, and the kidney is associated with the development of inflammatory disease in these organs, yet significant controversy exists as to how this breakdown actually occurs, and how such a breakdown may lead to inflammation. In this regard, much work has focused upon the role of the epithelium as an “innocent bystander,” a target of a leukocyte-mediated inflammatory cascade that leads to its destruction in the mucosal inflammatory process. However, recent evidence from a variety of laboratories indicates that the epithelium is not merely a passive component in the steps that lead to mucosal inflammation, but is a central participant in the process. In addressing this controversy, we and others have determined that epithelial cells express Toll-like receptors (TLRs) of the innate immune system, and that activation of TLRs by endogenous and exogenous ligands may play a central role in determining the balance between a state of “mucosal homeostasis,” as is required for optimal organ function, and “mucosal injury,” leading to mucosal inflammation and barrier breakdown. In particular, activation of TLRs within intestinal epithelial cells leads to the development of cellular injury and impairment in mucosal repair in the pathogenesis of intestinal inflammation, while activation of TLRs in the lung and kidney may participate in the development of pneumonitis and nephritis respectively. Recent work in support of these concepts is extensively reviewed, while essential areas of further study that are required to determine the significance of epithelial TLR signaling during states of health and disease are outlined.

Specific Lactobacillus species differentially activate Toll-like receptors and downstream signals in dendritic cells

BACKGROUND

Dendritic cells (DCs) regulate mucosal T-cell immunity and encounter several distinct bacteria of the gut flora, including lactobacilli. Gram-positive lactobacilli have been suggested to play an important role in exerting adjuvanticity effects on innate immune cells at mucosal sites.

AIMS & METHODS

In the present report, we studied the effects of specific Lactobacillus species on human monocyte derived DCs.

RESULTS

We show that lactobacilli activate DCs by differentially inducing the expression of Toll-like receptors and bioactive IL-12 in Lactobacillus-treated DCs. Further, these specific Lactobacillus spp. did not activate the phosphorylation of p38 MAPK, which might be a downstream effect of the remarkable capacity of lactobacilli to induce IL-12 in DCs that skew T cells significantly toward an IFN-gamma-secreting Th1 response.

CONCLUSION

These results highlight an important role of specific Lactobacillus spp. as adjuvants in triggering DC function, which in turn may determine the immunological outcome in an environment wherein innate cells reside.

Toll-like receptor-4 coordinates the innate immune response of the kidney to renal ischemia/reperfusion injury

Toll-like receptors (TLRs) can detect endogenous danger molecules released upon tissue injury resulting in the induction of a proinflammatory response. One of the TLR family members, TLR4, is constitutively expressed at RNA level on renal epithelium and this expression is enhanced upon renal ischemia/reperfusion (I/R) injury. The functional relevance of this organ-specific upregulation remains however unknown. We therefore investigated the specific role of TLR4 and the relative contribution of its two downstream signaling cascades, the MyD88-dependent and TRIF-dependent cascades in renal damage by using TLR4−/−, MyD88−/− and TRIF-mutant mice that were subjected to renal ischemia/reperfusion injury. Our results show that TLR4 initiates an exaggerated proinflammatory response upon I/R injury, as reflected by lower levels of chemokines and infiltrating granulocytes, less renal damage and a more preserved renal function in TLR4−/− mice as compared to wild type mice. In vitro studies demonstrate that renal tubular epithelial cells can coordinate an immune response to ischemic injury in a TLR4-dependent manner. In vivo we found that epithelial- and leukocyte-associated functional TLR4 contribute in a similar proportion to renal dysfunction and injury as assessed by bone marrow chimeric mice. Surprisingly, no significant differences were found in renal function and inflammation in MyD88−/− and TRIF-mutant mice compared with their wild types, suggesting that selective targeting of TLR4 directly may be more effective for the development of therapeutic tools to prevent I/R injury than targeting the intracellular pathways used by TLR4. In conclusion, we identified TLR4 as a cellular sentinel for acute renal damage that subsequently controls the induction of an innate immune response.

Evolution of MDA-5/RIG-I-dependent innate immunity: independent evolution by domain grafting

Type I Interferons (IFNs) are requisite components in antiviral innate immunity. Classically, a Toll-like receptor-dependent pathway induces type I interferons. However, recent recognition of melanoma differentiation associated gene-5 (MDA-5) and retinoic acid inducible gene-I (RIG-I) as primary sensors of RNA viruses for type I interferon induction highlights a potentially unique pathway for innate immunity. Our present investigation tracing the phylogenetic origin of MDA-5 and RIG-I domain arrangement (CARD1-CARD2-helicase-DEAD/DEAH) indicates that these proteins originated specifically in mammals, firmly linking this family of proteins with interferons in a highly derived evolutionary development of innate immunity. MDA-5, but not RIG-I, orthologs are found in fish, indicating that MDA-5 might have evolved before RIG-I. Our analyses also reveal that the MDA-5 and RIG-I domain arrangement evolved independently by domain grafting and not by a simple gene-duplication event of the entire four-domain arrangement, which may have been initiated by differential sensitivity of these proteins to viral infection.

Toll-like receptors: their roles in bacterial recognition and respiratory infections

Although respiratory infections cause significant morbidity and mortality throughout the world, the immunologic factors that mediate host susceptibility to these infections remain poorly understood. The lung contains a vast surface at the host-environment interface and acts as a crucial barrier to invading pathogens. The lung is equipped with specialized epithelial and hematopoietic cells, which express pattern recognition receptors that act as both sentinels and mediators of pulmonary innate immunity. Toll-like receptors (TLRs) mediate a particularly critical role in pathogen recognition and subsequent initiation of the host immune response. In this review, we will summarize current knowledge of TLRs and their bacterial ligands and explore their role in respiratory infections. Moreover, we will highlight recent advances in the role of TLRs in pulmonary infections from a human immunogenetics perspective.

MyD88 dependent signaling contributes to protective host defense against Burkholderia pseudomallei

Background

Toll-like receptors (TLRs) have a central role in the recognition of pathogens and the initiation of the innate immune response. Myeloid differentiation primary-response gene 88 (MyD88) and TIR-domain-containing adaptor protein inducing IFNβ (TRIF) are regarded as the key signaling adaptor proteins for TLRs. Melioidosis, which is endemic in SE-Asia, is a severe infection caused by the gram-negative bacterium Burkholderia pseudomallei. We here aimed to characterize the role of MyD88 and TRIF in host defense against melioidosis.

Methodology and Principal Findings

First, we found that MyD88, but not TRIF, deficient whole blood leukocytes released less TNFα upon stimulation with B. pseudomallei compared to wild-type (WT) cells. Thereafter we inoculated MyD88 knock-out (KO), TRIF mutant and WT mice intranasally with B. pseudomallei and found that MyD88 KO, but not TRIF mutant mice demonstrated a strongly accelerated lethality, which was accompanied by significantly increased bacterial loads in lungs, liver and blood, and grossly enhanced liver damage compared to WT mice. The decreased bacterial clearance capacity of MyD88 KO mice was accompanied by a markedly reduced early pulmonary neutrophil recruitment and a diminished activation of neutrophils after infection with B. pseudomallei. MyD88 KO leukocytes displayed an unaltered capacity to phagocytose and kill B. pseudomallei in vitro.

Conclusions

MyD88 dependent signaling, but not TRIF dependent signaling, contributes to a protective host response against B. pseudomallei at least in part by causing early neutrophil recruitment towards the primary site of infection.

Ross River virus envelope glycans contribute to type I interferon production in myeloid dendritic cells

Alphaviruses are mosquito-transmitted viruses that cause significant human disease, and understanding how these pathogens successfully transition from the mosquito vector to the vertebrate host is an important area of research. Previous studies demonstrated that mosquito and mammalian-cell-derived alphaviruses differentially induce type I interferons (alpha/beta interferon [IFN-alpha/beta]) in myeloid dendritic cells (mDCs), where the mosquito cell-derived virus is a poor inducer of IFN-alpha/beta compared to the mammalian-cell-derived virus. Furthermore, the reduced IFN-alpha/beta induction by the mosquito cell-derived virus is attributed to differential N-linked glycosylation. To further evaluate the role of viral envelope glycans in regulating the IFN-alpha/beta response, studies were performed to assess whether the mosquito cell-derived virus actively inhibits IFN-alpha/beta induction or is simply a poor inducer of IFN-alpha/beta. Coinfection studies using mammalian- and mosquito cell-derived Ross River virus (mam-RRV and mos-RRV, respectively) indicated that mos-RRV was unable to suppress IFN-alpha/beta induction by mam-RRV in mDC cultures. Additionally, a panel of mutant viruses lacking either individual or multiple N-linked glycosylation sites was used to demonstrate that N-linked glycans were essential for high-level IFN-alpha/beta induction by the mammalian-cell-derived virus. These results suggest that the failure of the mosquito cell-derived virus to induce IFN-alpha/beta is due to a lack of complex carbohydrates on the virion rather than the active suppression of the DC antiviral response.

Shiga toxin-mediated disease in MyD88-deficient mice infected with Escherichia coli O157:H7

Toll-like receptors (TLRs) are key factors of innate immunity that detect pathogen invasion and trigger a host response. TLR4 can mediate a response through adaptor molecules, MyD88 or TRIF. In the present study, streptomycin-treated MyD88(-/-), Tlr4(-/-), Trif (Lps2/Lps2), and C57BL/6 wild-type (WT) mice were infected with either Shiga toxin (Stx)-producing or non-producing Escherichia coli O157:H7. Moderate to severe clinical signs of disease developed in MyD88(-/-) (n = 21/21), Tlr4(-/-) (n = 12/16), Trif (Lps2/Lps2) (n = 7/15) and WT mice (n = 6/20) infected with Stx-producing E. coli O157:H7 but not in mice inoculated with the Stx non-producing strain (n = 0/54, P < 0.001). MyD88(-/-) mice infected with Stx-producing E. coli O157:H7 developed the most severe disease and had the highest bacterial burden. Hematological analysis of sick MyD88(-/-) mice showed reduced red blood cell counts and reticulocytosis, suggesting hemolysis. Thrombocytopenia developed in MyD88(-/-), Trif (Lps2/Lps2), and WT mice, and creatinine levels were elevated in both MyD88(-/-) and WT mice infected with the Stx-producing strain. Renal histopathology showed evidence of glomerular capillary congestion, tubular desquamation, and fibrinogen deposition, and intestinal histopathology showed mucosal injury, edema, and inflammation in sick mice. Administration of purified Stx2 to MyD88(-/-) and WT mice led to severe disease in both groups, suggesting that MyD88(-/-) mice are not more sensitive to Stx than WT mice. As MyD88(-/-) mice developed the most severe disease hematological and pathological changes, the results suggest that dysfunctional innate immune responses via MyD88 enhanced Stx-induced disease.

Putting endotoxin to work for us: monophosphoryl lipid A as a safe and effective vaccine adjuvant

The development of non-infectious subunit vaccines greatly increases the safety of prophylactic immunization, but also reinforces the need for a new generation of immunostimulatory adjuvants. Because adverse effects are a paramount concern in prophylactic immunization, few new adjuvants have received approval for use anywhere in the developed world. The vaccine adjuvant monophosphoryl lipid A is a detoxified form of the endotoxin lipopolysaccharide, and is among the first of a new generation of Toll-like receptor agonists likely to be used as vaccine adjuvants on a mass scale in human populations. Much remains to be learned about this compound's mechanism of action, but recent developments have made clear that it is unlikely to be simply a weak version of lipopolysaccharide. Instead, monophosphoryl lipid A's structure seems to have fortuitously retained several functions needed for stimulation of adaptive immune responses, while shedding those associated with pro-inflammatory side effects.

TLR- and CXCR1-dependent innate immunity: insights into the genetics of urinary tract infections

The susceptibility to urinary tract infection (UTI) is controlled by the innate immune response and Toll like receptors (TLRs) are the sentinels of this response. If productive, TLR4 signalling may initiate the symptomatic disease process. In the absence of TLR4 signalling the infected host instead develops an asymptomatic carrier state. The activation of mucosal TLR4 is also influenced by the properties of the infecting strain, and pathogens use their virulence factors to trigger 'pathogen-specific' TLR4 responses in the urinary tract but do not respond to the asymptomatic carrier strains in patients with asymptomatic bacteriuria (ABU). The TLR4 dependence has been demonstrated in mice and the relevance of low TLR4 function for protection for human disease was recently confirmed in children with asymptomatic bacteriuria, who expressed less TLR4 than age matched controls. Functional chemokines and functional chemokine receptors are crucial for neutrophil recruitment, and for the neutrophil dependent bacterial clearance. Interleukin (IL)-8 receptor deficient mice develop acute septic infections and chronic tissue damage, due to aberrant neutrophil function. This mechanism is relevant for human UTI as pyelonephritis prone children express low levels of the human CXCL8 (Il-8) receptor, CXC chemokine receptor 1 (CXCR1) and often have heterozygous CXCR1 polymorphisms. This review illustrates how intimately the innate response and the susceptibility to UTI are linked and sophisticated recognition mechanisms that rely on microbial virulence and on host TLR4 and CXCR1 signalling.

Inflammation and autoimmunity caused by a SHP1 mutation depend on IL-1, MyD88, and a microbial trigger

A recessive phenotype called spin (spontaneous inflammation) was induced by N-ethyl-N-nitrosourea (ENU) mutagenesis in C57BL/6J mice. Homozygotes display chronic inflammatory lesions affecting the feet, salivary glands and lungs, and antichromatin antibodies. They are immunocompetent and show enhanced resistance to infection by Listeria monocytogenes. TLR-induced TNF and IL-1 production are normal in macrophages derived from spin mice. The autoinflammatory phenotype of spin mice is fully suppressed by compound homozygosity for Myd88(poc), Irak4(otiose), and Il1r1-null mutations, but not Ticam1(Lps2), Stat1(m1Btlr), or Tnf-null mutations. Both autoimmune and autoinflammatory phenotypes are suppressed when spin homozygotes are derived into a germ-free environment. The spin phenotype was ascribed to a viable hypomorphic allele of Ptpn6, which encodes the tyrosine phosphatase SHP1, mutated in mice with the classical motheaten alleles me and me-v. Inflammation and autoimmunity caused by SHP1 deficiency are thus conditional. The SHP1-deficient phenotype is driven by microbes, which activate TLR signaling pathways to elicit IL-1 production. IL-1 signaling via MyD88 elicits inflammatory disease.

A catechol-O-methyltransferase that is essential for auditory function in mice and humans

We have identified a previously unannotated catechol-O-methyltranferase (COMT), here designated COMT2, through positional cloning of a chemically induced mutation responsible for a neurobehavioral phenotype. Mice homozygous for a missense mutation in Comt2 show vestibular impairment, profound sensorineuronal deafness, and progressive degeneration of the organ of Corti. Consistent with this phenotype, COMT2 is highly expressed in sensory hair cells of the inner ear. COMT2 enzymatic activity is significantly reduced by the missense mutation, suggesting that a defect in catecholamine catabolism underlies the auditory and vestibular phenotypes. Based on the studies in mice, we have screened DNA from human families and identified a nonsense mutation in the human ortholog of the murine Comt2 gene that causes nonsyndromic deafness. Defects in catecholamine modification by COMT have been previously implicated in the development of schizophrenia. Our studies identify a previously undescribed COMT gene and indicate an unexpected role for catecholamines in the function of auditory and vestibular sense organs.

The kinase activity of IL-1 receptor-associated kinase 4 is required for interleukin-1 receptor/toll-like receptor-induced TAK1-dependent NFkappaB activation

Two parallel interleukin-1 (IL-1)-mediated signaling pathways have been uncovered for IL-1R-TLR-mediated NFkappaB activation: TAK1-dependent and MEKK3-dependent pathways, respectively. The TAK1-dependent pathway leads to IKKalpha/beta phosphorylation and IKKbeta activation, resulting in classic NFkappaB activation through IkappaBalpha phosphorylation and degradation. The TAK1-independent MEKK3-dependent pathway involves IKKgamma phosphorylation and IKKalpha activation, resulting in NFkappaB activation through dissociation of phosphorylated IkappaBalpha from NFkappaB without IkappaBalpha degradation. IL-1 receptor-associated kinase 4 (IRAK4) belongs to the IRAK family of proteins and plays a critical role in IL-1R/TLR-mediated signaling. IRAK4 kinase-inactive mutant failed to mediate the IL-1R-TLR-induced TAK1-dependent NFkappaB activation pathway, but mediated IL-1-induced TAK1-independent NFkappaB activation and retained the ability to activate substantial gene expression, indicating a structural role of IRAK4 in mediating this alternative NFkappaB activation pathway. Deletion analysis of IRAK4 indicates the essential structural role of the IRAK4 death domain in receptor proximal signaling for mediating IL-1R-TLR-induced NFkappaB activation.

TRIF and IRF-3 binding to the TNF promoter results in macrophage TNF dysregulation and steatosis induced by chronic ethanol

Chronic ethanol (EtOH) abuse results in the development of steatosis, alcoholic hepatitis, and cirrhosis. Augmented TNF-alpha production by macrophages and Kupffer cells and signaling via the p55 TNF receptor have been shown to be critical for these effects of chronic EtOH; however, the molecular mechanisms leading to augmented TNF-alpha production remain unclear. Using cell culture models and in vivo studies we demonstrate that chronic EtOH results in increased TNF-alpha transcription, which is independent of NF-kappaB. Using reporter assays and chromatin immunoprecipitation we found that this increased transcription is due to increased IRF-3 binding to and transactivation of the TNF promoter. As IRF-3 is downstream from the TLR4 adaptor TIR-domain-containing adapter-inducing IFN-beta (Trif), we demonstrate that macrophages from Trif-/- mice are resistant to this dysregulation of TNF-alpha transcription by EtOH in vitro as well as EtOH-induced steatosis and TNF dysregulation in vivo. These data demonstrate that the Trif/IRF-3 pathway is a target to ameliorate liver dysfunction associated with chronic EtOH.

The antiviral adaptor proteins Cardif and Trif are processed and inactivated by caspases

The outcome of a viral infection depends on the interplay between the host's capacity to trigger potent antiviral responses and viral mechanisms that counteract them. Although Toll-like receptor (TLR)-3, which recognizes virally derived double-stranded (ds) RNA, transmits downstream antiviral signaling through the TIR adaptor Trif (TICAM-1), viral RNA-sensing RIG-like helicases (RLHs) use the mitochondrial-bound CARD protein Cardif (IPS-1/MAVS/VISA). The importance of these two antiviral signaling pathways is reflected by the fact that both adaptors are inhibited through specific cleavage triggered by the hepatitis C virus serine protease NS3-4A. Here, we show that inactivation can also occur through cellular caspases activated by various pro-apoptotic signals. Upon caspase-dependent cleavage both adaptors loose their capacity to activate the transcription factors interferon regulatory factors (IRF) and NF-kappaB. Importantly, poliovirus infection triggers a caspase-dependent cleavage of Cardif, suggesting that some viruses may activate caspases not only as a mean to facilitate shedding and replication, but also to impair antiviral responses.

TLRs and innate immunity

One of the most fundamental questions in immunology pertains to the recognition of non-self, which for the most part means microbes. How do we initially realize that we have been inoculated with microbes, and how is the immune response ignited? Genetic studies have made important inroads into this question during the past decade, and we now know that in mammals, a relatively small number of receptors operate to detect signature molecules that herald infection. One or more of these signature molecules are displayed by almost all microbes. These receptors and the signals they initiate have been studied in depth by random germline mutagenesis and positional cloning (forward genetics). Herein is a concise description of what has been learned about the Toll-like receptors, which play an essential part in the perception of microbes and shape the complex host responses that occur during infection.

TLR4-mediated activation of dendritic cells by the heat shock protein DnaK from Francisella tularensis

Francisella tularensis is the causative agent of tularemia, a severe, debilitating disease of humans and other mammals. As this microorganism is also classified as a "category-A pathogen" and a potential biowarfare agent, there is a need for an effective vaccine. Several antigens of F. tularensis, including the heat shock protein DnaK, have been proposed for use in a potential subunit vaccine. In this study, we characterized the innate immune response of murine bone marrow-derived dendritic cells (DC) to F. tularensis DnaK. Recombinant DnaK was produced using a bacterial expression system and purified using affinity, ion-exchange, and size-exclusion chromatography. DnaK induced the activation of MAPKs and NF-kappaB in DC and the production of the proinflammatory cytokines IL-6, TNF-alpha, and IL-12 p40, as well as low levels of IL-10. DnaK induced phenotypic maturation of DC, as demonstrated by an up-regulation of costimulatory molecules CD40, CD80, and CD86. DnaK stimulated DC through TLR4 and the adapters MyD88 and Toll/IL-1R domain-containing adaptor-inducing IFN-beta (TRIF) that mediated differential responses. DnaK induced activation of MAPKs and NF-kappaB in a MyD88- or TRIF-dependent manner. However, the presence of MyD88- and TRIF-dependent signaling pathways was essential for an optimal, DnaK-induced cytokine response in DC. In contrast, DnaK induced DC maturation in a TRIF-dependent, MyD88-independent manner. These results provide insight about the molecular interactions between an immunodominant antigen of F. tularensis and host immune cells, which is crucial for the rational design and development of a safe and efficacious vaccine against tularemia.

Regulation of maturation and activating potential in CD8+ versus CD8- dendritic cells following in vivo infection with vaccinia virus

DC maturation is known to be a necessary step in the generation of an effective immune response. We have used vaccinia virus (VACV) as a model to investigate the regulation of DC subsets in vivo following infection. While a number of in vitro studies have shown that DC infected with VACV fail to undergo maturation, the effect of VACV infection on the maturation of and cytokine production by DC subsets in vivo remains less defined. We have found that following systemic infection with vaccinia virus, both CD8+ and CD8- dendritic cells are infected. The number of infected DC peaked at 6 h and was highly decreased by 24 h post-infection. In both subsets, there was evidence of generalized upregulation of costimulatory molecules. Surprisingly, this included vaccinia infected DC, suggesting the regulation of DC maturation in vivo is much more complex and likely influenced by DC extrinsic signals. However, while we observed generalized upregulation of costimulatory molecules, IL-12 production was restricted to a subset of non-infected cells in both the CD8+ and CD8- DC populations. Importantly, the control of IL-12 production was differentially dependent on MyD88 signaling. IL-12 production was ablated in the absence of MyD88 in CD8- DC, while it was unchanged in CD8+ DC. These findings provide new insights into the control of DC maturation in vivo and demonstrate that the regulation of maturation in vivo following virus infection can be differentially controlled in distinct types of DC.

Dual signaling of MyD88 and TRIF is critical for maximal TLR4-induced dendritic cell maturation

TLR4 is a unique TLR because downstream signaling occurs via two separate pathways, as follows: MyD88 and Toll IL-1 receptor (TIR) domain-containing adaptor-inducing IFN-beta (TRIF). In this study, we compared and contrasted the interplay of these pathways between murine dendritic cells (DCs) and macrophages during LPS stimulation. During TLR4 activation, neither pathway on its own was critical for up-regulation of costimulatory molecules in DCs, whereas the up-regulation of costimulatory molecules was largely TRIF dependent in macrophages. LPS-induced secreted factors, of which type I IFNs were one of the active components, played a larger role in promoting the up-regulation of costimulatory molecules in macrophages than DCs. In both cell types, MyD88 and TRIF pathways together accounted for the inflammatory response to LPS activation. Furthermore, signaling of both adaptors allowed maximal T cell priming by LPS-matured DCs, with MyD88 playing a larger role than TRIF. In sum, in our experimental systems, TRIF signaling plays a more important role in LPS-induced macrophage activation than in DC activation.

Pyogenic bacterial infections in humans with MyD88 deficiency

MyD88 is a key downstream adapter for most Toll-like receptors (TLRs) and interleukin-1 receptors (IL-1Rs). MyD88 deficiency in mice leads to susceptibility to a broad range of pathogens in experimental settings of infection. We describe a distinct situation in a natural setting of human infection. Nine children with autosomal recessive MyD88 deficiency suffered from life-threatening, often recurrent pyogenic bacterial infections, including invasive pneumococcal disease. However, these patients were otherwise healthy, with normal resistance to other microbes. Their clinical status improved with age, but not due to any cellular leakiness in MyD88 deficiency. The MyD88-dependent TLRs and IL-1Rs are therefore essential for protective immunity to a small number of pyogenic bacteria, but redundant for host defense to most natural infections.

DiC14-amidine cationic liposomes stimulate myeloid dendritic cells through Toll-like receptor 4

DiC14-amidine cationic liposomes were recently shown to promote Th1 responses when mixed with allergen. To further define the mode of action of diC14-amidine as potential vaccine adjuvant, we characterized its effects on mouse and human myeloid dendritic cells (DC). First, we observed that, as compared with two other cationic liposomes, only diC14-amidine liposomes induced the production of IL-12p40 and TNF-alpha by mouse bone marrow-derived DC. DiC14-amidine liposomes also activated human DC, as shown by synthesis of IL-12p40 and TNF-alpha, accumulation of IL-6, IFN-beta and CXCL10 mRNA, and up-regulation of membrane expression of CD80 and CD86. DC stimulation by diC14-amidine liposomes was associated with activation of NF-kappaB, ERK1/2, JNK and p38 MAP kinases. Finally, we demonstrated in mouse and human cells that diC14-amidine liposomes use Toll-like receptor 4 to elicit both MyD88-dependent and Toll/IL-1R-containing adaptor inducing interferon IFN-beta (TRIF)-dependent responses.

High-throughput, single-cell analysis of macrophage interactions with fluorescently labeled Bacillus anthracis spores

The engulfment of Bacillus anthracis spores by macrophages is an important step in the pathogenesis of inhalational anthrax. However, from a quantitative standpoint, the magnitude to which macrophages interact with and engulf spores remains poorly understood, in part due to inherent limitations associated with commonly used assays. To analyze phagocytosis of spores by RAW264.7 macrophage-like cells in a high-throughput, nonsubjective manner, we labeled B. anthracis Sterne 7702 spores prior to infection with an Alexa Fluor 488 amine-reactive dye in a manner that did not alter their germination, growth kinetics, and heat resistance. Using flow cytometry, large numbers of cells exposed to labeled spores were screened to concurrently discriminate infected from uninfected cells and surface-associated from internalized spores. These experiments revealed that spore uptake was not uniform, but instead, highly heterogeneous and characterized by subpopulations of infected and uninfected cells, as well as considerable variation in the number of spores associated with individual cells. Flow cytometry analysis of infections demonstrated that spore uptake was independent of the presence or absence of fetal bovine serum, a germinant that, while routinely used in vitro, complicates the interpretation of the outcome of infections. Two commonly used macrophage cell lines, RAW264.7 and J774A.1 cells, were compared, revealing significant disparity between these two models in the rates of phagocytosis of labeled spores. These studies provide the experimental framework for investigating mechanisms of spore phagocytosis, as well as quantitatively evaluating strategies for interfering with macrophage binding and uptake of spores.

Brucella control of dendritic cell maturation is dependent on the TIR-containing protein Btp1

Brucella is an intracellular pathogen able to persist for long periods of time within the host and establish a chronic disease. We show that soon after Brucella inoculation in intestinal loops, dendritic cells from ileal Peyer's patches become infected and constitute a cell target for this pathogen. In vitro, we found that Brucella replicates within dendritic cells and hinders their functional activation. In addition, we identified a new Brucella protein Btp1, which down-modulates maturation of infected dendritic cells by interfering with the TLR2 signaling pathway. These results show that intracellular Brucella is able to control dendritic cell function, which may have important consequences in the development of chronic brucellosis.

A key determinant for intracellular pathogenic bacteria to induce infectious diseases is their ability to avoid recognition by the host immune system. Although most microorganisms internalized by host cells are efficiently cleared, Brucella behave as a Trojan horse causing a zoonosis called brucellosis that affects both humans and animals. Here we show that pathogenic Brucella are able to target host cell defense mechanisms by controlling the function of the sentinels of the immune system, the dendritic cells. In particular, the Brucella TIR-containing protein (Btp1) targets the Toll-like receptor 2 activation pathway, which is a major host response system involved in bacterial recognition. Btp1 is involved in the inhibition of dendritic cell maturation. The direct consequence is a control of inflammatory cytokine secretion and antigen presentation to T lymphocytes. These bacterial proteins are not specific for Brucella and have been identified in other pathogens and may be part of a general virulence mechanism used by several intracellular pathogens to induce disease.

The IRF family transcription factors in immunity and oncogenesis

The interferon regulatory factor (IRF) family, consisting of nine members in mammals, was identified in the late 1980s in the context of research into the type I interferon system. Subsequent studies over the past two decades have revealed the versatile and critical functions performed by this transcription factor family. Indeed, many IRF members play central roles in the cellular differentiation of hematopoietic cells and in the regulation of gene expression in response to pathogen-derived danger signals. In particular, the advances made in understanding the immunobiology of Toll-like and other pattern-recognition receptors have recently generated new momentum for the study of IRFs. Moreover, the role of several IRF family members in the regulation of the cell cycle and apoptosis has important implications for understanding susceptibility to and progression of several cancers.

The activation of macrophage and upregulation of CD40 costimulatory molecule in lipopolysaccharide-induced acute lung injury

To study the activation of macrophage and upregulation of costimulatory molecule of CD40 in lipopolysaccharide- (LPS-) induced acute lung injury (ALI) model, and to investigate the pathogenecy of ALI, mice were randomly divided into two groups. ALI model was created by injecting 0.2 mg/kg LPS in phosphate saline (PBS) in trachea. The pathologic changes of mice lungs were observed by HE staining at 24 and 48 hours after LPS treatment, then the alveolar septum damage, abnormal contraction, alveolar space hyperemia, and neutrophils or other inflammatory cells infiltration in the LPS group, but not in the control group, were observed. The expression of CD40 mRNA and CD40 protein molecules were higher in LPS group as compared to the control group by Northern blot and flow cytometry, respectively. Expression of Toll-like receptor-4 (TLR4) in activated macrophage (AMΦ) was higher in LPS group as compared to the control group by RT-PCR. The activation of NF-κB binding to NF-κB consensus oligos increased in LPS group by EMSA in macrophage. The concentrations of TNF-α, MIP-2, and IL-1β cytokines from bronchoalveolar lavage fluid (BALF) were increased significantly in LPS group as compared to the control group by ELISA. The activation of AM and upregulation of costimulatory molecule CD40 induced all kinds of inflammatory cytokines releasing, then led to ALI. Therefore, both of them played vital role in the process of development of ALI.

Upregulation of the TLR3 pathway by Kaposi's sarcoma-associated herpesvirus during primary infection

Kaposi's sarcoma-associated herpesvirus (KSHV) is associated with several different human malignancies, including Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. KSHV establishes lifelong latency in the host and modulates the host immune response. Innate immunity is critical for controlling de novo viral infection. Toll-like receptors (TLRs) are key components of the innate immune system, and they serve as pathogen recognition receptors that stimulate the host antiviral response. In particular, TLR3 has been implicated in RNA virus recognition. Currently, there is no information regarding how KSHV infection modulates any TLR pathway. We report the first evidence that KSHV upregulates TLR3 expression in human monocytes during primary infection. This is also the first demonstration of a human DNA tumor virus upregulating TLR3, a TLR that thus far has been associated with the recognition of RNA viruses. We found that KSHV upregulates the TLR3 pathway and induces TLR3-specific cytokines and chemokines, including beta 1 interferon (IFN-beta1) and CXCL10 (IP-10). Small interfering RNAs directed against TLR3 greatly reduced the ability of KSHV to upregulate IFN-beta1 and CXCL10 upon infection.

The role of Toll-like receptor-4 in pertussis vaccine-induced immunity

Background

The gram-negative bacterium Bordetella pertussis is an important causative agent of pertussis, an infectious disease of the respiratory tract. After introduction of whole-cell vaccines (wP) in the 1950's, pertussis incidence has decreased significantly. Because wP were found to be reactogenic, in most developed countries they have been replaced by acellular vaccines (aP). We have previously shown a role for Toll-like receptor 4 (Tlr4) in pertussis-infected mice and the pertussis toxin (Ptx)-IgG response in wP-vaccinated children, raising the issue of the relative importance of Tlr4 in wP vaccination of mice. Here we analyze the effects of wP and aP vaccination and B. pertussis challenge, in Tlr4-deficient C3H/HeJ and wild-type C3H/HeOuJ mice. aP consists of Ptx, filamentous hemagglutinin (FHA), and pertactin (Prn).

Results

We show an important role of Tlr4 in wP and (to a lesser extent) aP vaccination, induction of Th1 and Th17 cells by wP but not aP vaccination, and induction of Th17 cells by infection, confirming data by Higgins et al. (J Immunol 2006, 177:7980–9). Furthermore, in Tlr4-deficient mice, compared to wild-type controls (i) after vaccination only, Ptx-IgG (that was induced by aP but not wP vaccination), FHA-IgG, and Prn-IgG levels were similar, (ii) after infection (only), lung IL-1α and IL-1β expression were lower, (iii) after wP vaccination and challenge, Prn-IgG level and lung IL-5 expression were higher, while lung IL-1β, TNF-α, IFN-γ, IL-17, and IL-23 expression were lower, and lung pathology was absent, and (iv) after aP vaccination and challenge, Prn-IgG level and lung IL-5 expression were higher, while Ptx-IgG level was lower.

Conclusion

Tlr4 does not influence the humoral response to vaccination (without challenge), plays an important role in natural immunity, wP and aP efficacy, and induction of Th1 and Th17 responses, is critical for lung pathology and enhances pro-inflammatory cytokine production after wP vaccination and challenge, and diminishes Th2 responses after both wP and aP vaccination and challenge. wP vaccination does not induce Ptx-IgG. A role for LPS in the efficacy of wP underlines the usefulness of LPS analogs to improve bacterial subunit vaccines such as aP.

IRAK-4 kinase activity is required for IRAK-4-dependent innate and adaptive immune responses

Interleukin-1 receptor-associated kinase (IRAK)-4 is a serine-threonine kinase that plays an important role in innate and adaptive immune responses. While the requirement of IRAK-4 kinase activity has been studied in the context of IL-1R signaling, it is not clear whether IRAK-4 requires its kinase function for all of its roles in the immune system. IRAK-4 kinase-dead knock-in (IRAK-4KD/KD) mice were generated to further elucidate whether IRAK-4 kinase activity is required for IRAK-4 to induce cytokine production. IRAK-4KD/KD mice were impaired in their ability to produce cytokines in response to in vivo challenge with lipopolysaccharide (LPS), a potent TLR4 ligand. Cytokine production was also reduced in macrophages and dendritic cells from IRAK-4KD/KD mice in response to LPS and other TLR ligands. In addition, adaptive immune responses were impaired in IRAK-4KD/KD mice. Although in vitro T cell proliferation in response to TCR activation was unaffected in IRAK-4-deficient mice, in vivo T cell responses to lymphocytic choriomeningitits virus infection were significantly impaired in IRAK-4-knockout mice or mice expressing the kinase-dead mutant of IRAK-4. Collectively, these results indicate that IRAK-4 kinase activity is required for IRAK-4-dependent signaling in innate and adaptive immunity.

The ubiquitin-editing enzyme A20 restricts nucleotide-binding oligomerization domain containing 2-triggered signals

Muramyl dipeptide (MDP), a product of bacterial cell-wall peptidoglycan, activates innate immune cells by stimulating nucleotide-binding oligomerization domain containing 2 (NOD2) -dependent activation of the transcription factor NFkappaB and transcription of proinflammatory genes. A20 is a ubiquitin-modifying enzyme that restricts tumor necrosis factor (TNF) receptor and Toll-like receptor (TLR) -induced signals. We now show that MDP induces ubiquitylation of receptor- interacting protein 2 (RIP2) in primary macrophages. A20-deficient cells exhibit dramatically amplified responses to MDP, including increased RIP2 ubiquitylation, prolonged NFkappaB signaling, and increased production of proinflammatory cytokines. In addition, in vivo responses to MDP are exaggerated in A20-deficient mice and in chimeric mice bearing A20-deficient hematopoietic cells. These exaggerated responses occur independently of the TLR adaptors MyD88 and TRIF as well as TNF signals. These findings indicate that A20 directly restricts NOD2 induced signals in vitro and in vivo, and provide new insights into how these signals are physiologically restricted.

Signal regulatory protein alpha negatively regulates both TLR3 and cytoplasmic pathways in type I interferon induction

Recognition of double-stranded RNA (dsRNA) activates interferon-regulatory factor 3 (IRF3)-dependent expression of anti-viral factors. The innate immune system recognizes viral dsRNA through two distinct pathways. First, the Toll-like receptor 3 (TLR3) detects dsRNA phagocytosed in endosomes. In addition, the helicases retinoic acid induced protein I (RIG-I)/melanoma differentiation associated gene 5 (MDA5) binds cytoplasmic dsRNA generated during viral replication. Both RIG-I/MDA5 and TLR3 can bind polyriboinosinic:polyribocytidylic acid (poly(I:C)), the synthetic analog of viral dsRNA, and mediate type I IFN production. Here we show that signal regulatory protein (SIRP) alpha negatively regulates both TLR3- and RIG-1/MDA5-dependent anti-viral pathways. Suppression of SIRPalpha expression by RNA interference results in enhanced activation of IRF3 and MAPK pathways after poly(I:C) treatment, coupled with the up-regulation of IFN-beta and IFN-beta-inducible gene transcriptional activation. The requirement of phosphoinositide 3-kinase (PI3K) activity for the induction of IFN-beta and IFN-beta-inducible genes by dsRNA is supported by the observation that a PI3K inhibitor failed to activate IFN-beta and IFN-beta-inducible gene expression. PI3K, whose activity is essential for activation of IRF3, is recruited to the phosphorylated tyrosine residues of SIRPalpha upon poly(I:C) stimulation, which lead to a reduction in the activity of the downstream kinase AKT. Thus SIRPalpha may accomplish its inhibitory function in type I IFN induction, in part, through its association and sequestration of the signal transducer PI3K.

Chemical mutagenesis: a new strategy against the global threat of infectious diseases

The perpetual evolution of drug-resistant microbes, the overwhelming burden of acquired immune suppression due to HIV, the emergence or re-emergence of various pathogens (West Nile virus, pandemic influenza, Creutzfeld-Jacob disease), and increased fears of bioterrorism has drawn a great deal of new attention to infectious diseases. The pathogenesis of infection is characterized by complex interactions of potentially virulent microorganisms with host genetic and acquired factors. Chemical mutagenesis of the mouse genome provides a robust method to unravel this challenging problem. To deepen our understanding of the natural host response to pathogens, our team and others are interrogating the mouse genome to define genes that are crucial to the defense against infectious diseases (pathogen recognition, viral defense, bacterial defense, prion infection). In this review we highlight the current progress of these efforts and propose a toolbox for other groups that are interested in this endeavor.

TLR4/CD14-mediated PI3K activation is an essential component of interferon-dependent VSV resistance in macrophages

Phosphatidylinositol-3-phosphate kinase (PI3K) has been reported to exhibit anti-inflammatory roles as a negative modulator of the NF-kappaB pathway (MyD88- and Mal-dependent) triggered upon Toll-like receptor (TLR)4 activation by lipopolysaccharide (LPS). Here, we investigated the role of PI3K on the TLR4-dependent, MyD88-independent signaling cascade which is activated in macrophages infected by Vesicular Stomatitis Virus (VSV) and leads to interferon production, thus conferring antiviral protection. We show that VSV induces TLR4 (and CD14)-dependent Akt phosphorylation. We observed hypersusceptibility to viral infections after pharmacological inactivation of the PI3K pathway in macrophages, which indicates that normal PI3K functions are critical for type I interferon synthesis and viral resistance. Conversely, we noticed increased resistance in macrophages isolated from genetically modified mice in which the PI3K pathway is constitutively active. Our data, which demonstrate that PI3K-Akt axis is an important component of the TLR4-dependent antiviral mechanism, also indicate that pharmacological modulation of this pathway to regulate the inflammatory response could promote viral susceptibility.

Endoplasmic reticulum stress and the unfolded protein response are linked to synergistic IFN-beta induction via X-box binding protein 1

Type I IFN are strongly induced upon engagement of certain pattern recognition receptors by microbial products, and play key roles in regulating innate and adaptive immunity. It has become apparent that the endoplasmic reticulum (ER) stress-induced unfolded protein response (UPR), in addition to restoring ER homeostasis, also influences the expression of certain inflammatory cytokines. However, the extent to which UPR signaling regulates type I IFN remains unclear. Here we show that cells undergoing a UPR respond to TLR4 and TLR3 ligands, and intracellular dsRNA, with log-fold greater IFN-beta induction. This synergy is not dependent on autocrine type I IFN signaling, but unexpectedly requires the UPR transcription factor X-box binding protein 1 (XBP-1). Synergistic IFN-beta induction also occurs in HLA-B27/human beta(2)m-transgenic rat macrophages exhibiting a UPR as a consequence of HLA-B27 up-regulation, where it correlates with activation of XBP-1 splicing. Together these findings indicate that the cellular response to endogenous 'danger' that disrupts ER homeostasis is coupled to IFN-beta induction by XBP-1, which has implications for the immune response and the pathogenesis of diseases involving the UPR.

Cytomegalovirus M45 cell death suppression requires receptor-interacting protein (RIP) homotypic interaction motif (RHIM)-dependent interaction with RIP1

Herpesviruses such as cytomegaloviruses encode functions that modulate the innate response in diverse ways to counteract host sensing and delay host clearance during infection. The murine cytomegalovirus M45 protein interacts with receptor-interacting protein (RIP) 1 and RIP3 via a RIP homotypic interaction motif. Cell death suppression by M45 requires RIP homotypic interaction motif-dependent interaction with RIP1. This interaction also underlies the cell tropism role of M45 in preventing premature death of endothelial cells during murine cytomegalovirus infection. Thus, M45 is a viral inhibitor of RIP activation that provides a direct cell type-dependent replication benefit to the virus while modulating other biological processes signaling via the RIP1 adaptor such as activation of Toll-like receptor (TLR)3 as well as other mediators of cell death.

A TRIFfic perspective on acute lung injury

Acute lung injury (ALI) is a leading cause of death in people infected with H5N1 avian influenza virus or the SARS-coronavirus. Imai et al. (2008) now report that ALI is triggered by the signaling of oxidized phospholipids through Toll-like receptor 4 (TLR4) and the adaptor protein TRIF. These findings provide insight into the molecular pathogenesis of ALI, a condition for which treatment options are currently very limited.

Sequential control of Toll-like receptor-dependent responses by IRAK1 and IRAK2

Members of the IRAK family of kinases mediate Toll-like receptor (TLR) signaling. Here we show that IRAK2 was essential for sustaining TLR-induced expression of genes encoding cytokines and activation of the transcription factor NF-kappaB, despite the fact that IRAK2 was dispensable for activation of the initial signaling cascades. IRAK2 was activated 'downstream' of IRAK4, like IRAK1, and TLR-induced cytokine production was abrogated in the absence of both IRAK1 and IRAK2. Whereas the kinase activity of IRAK1 decreased within 1 h of TLR2 stimulation, coincident with IRAK1 degradation, the kinase activity of IRAK2 was sustained and peaked at 8 h after stimulation. Thus, IRAK2 is critical in late-phase TLR responses, and IRAK1 and IRAK2 are essential for the initial responses to TLR stimulation.

Phosphatase SHP-1 promotes TLR- and RIG-I-activated production of type I interferon by inhibiting the kinase IRAK1

Unbalanced production of proinflammatory cytokines and type I interferons in immune responses may lead to immunopathology; thus, the mechanisms that ensure the beneficial production of proinflammatory cytokines and type I interferons are of particular importance. Here we demonstrate that the phosphatase SHP-1 negatively regulated Toll-like receptor-mediated production of proinflammatory cytokines by inhibiting activation of the transcription factor NF-kappaB and mitogen-activated protein kinase. Simultaneously, SHP-1 increased the production of type I interferon mediated by Toll-like receptors and the helicase RIG-I by directly binding to and inhibiting activation of the kinase IRAK1. Our data demonstrate that SHP-1 contributes to immune homeostasis by balancing the production of proinflammatory cytokines and type I interferons in the innate immune response.