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

Triggering antiviral response by RIG-I-related RNA helicases

TLRs detect several classes of virus-associated molecules, such as ssRNA, CpG-DNA and dsRNA, and transduce signals leading to the production of IFN. Recently discovered cytoplasmic RNA helicases, RIG-I and MDA5, selectively sense viral RNA species. Gene disruption studies revealed the critical but non-redundant function of RIG-I and MDA5 in host antiviral responses.

Protein kinase Calpha is involved in interferon regulatory factor 3 activation and type I interferon-beta synthesis

Protein kinase C (PKC) isoforms are critically involved in the regulation of innate immune responses. Herein, we investigated the role of conventional PKCalpha in the regulation of IFN-beta gene expression mediated by the Toll-like receptor 3 (TLR3) signaling pathway. Inhibition of conventional PKC (cPKC) activity in monocyte-derived dendritic cells or TLR3-expressing cells by an isoform-specific inhibitor, Gö6976, selectively inhibited IFN-beta synthesis induced by double-stranded RNA polyinosine-polycytidylic acid. Furthermore, reporter gene assays confirmed that PKCalpha regulates IFN-beta promoter activity, since overexpression of dominant negative PKCalpha but not PKCbeta(I) repressed interferon regulatory factor 3 (IRF-3)-dependent but not NF-kappaB-mediated promoter activity upon TLR3 engagement in HEK 293 cells. Dominant negative PKCalpha inhibited IRF-3 transcriptional activity mediated by overexpression of TIR domain-containing adapter inducing IFN-beta and Tank-binding kinase-1. Additional biochemical analysis demonstrated that Gö6976-treated dendritic cells exhibited IRF-3 phosphorylation, dimerization, nuclear translocation, and DNA binding activity analogous to their control counterparts in response to polyinosine-polycytidylic acid. In contrast, co-immunoprecipitation experiments revealed that TLR3-induced cPKC activity is essential for mediating the interaction of IRF-3 but not p65/RelA with the co-activator CREB-binding protein. Furthermore, PKCalpha knock-down with specific small interfering RNA inhibited IFN-beta expression and down-regulated IRF-3-dependent promoter activity, establishing PKCalpha as a component of TLR3 signaling that regulates IFN-beta gene expression by targeting IRF-3-CREB-binding protein interaction. Finally, we analyzed the involvement of cPKCs in other signaling pathways leading to IFN-beta synthesis. These experiments revealed that cPKCs play a role in the synthesis of IFN-beta induced via both TLR-dependent and -independent pathways.

Regulation of innate immunity by MAPK dual-specificity phosphatases: knockout models reveal new tricks of old genes

Throughout evolution, mammals have developed an elaborate network of positive and negative regulatory mechanisms, which provide balance between defensive measures against bacterial and viral pathogens and protective measures against unwarranted destruction of the host by the activated immune system. Kinases and phosphatases encompassing the MAPK pathway are key players in the orderly action of pro- and anti-inflammatory processes, forming numerous promiscuous interactions. Several lines of evidence demonstrate that the phosphorylation and activation status of kinases in the MAPK system has crucial impact on the outcome of downstream events that regulate cytokine production. At least 13 members of the family of dual-specificity phosphatases (DUSP) display unique substrate specificities for MAPKs. Despite the considerable amount of information obtained about the contribution of the different DUSP to MAPK-mediated signaling and innate immunity, the interpretation of available data remains problematic. The in vitro and ex vivo findings are often complicated by functional redundancy of signaling molecules and do not always accurately predict the situation in vivo. Until recently, DUSP research has been hampered by the lack of relevant mammalian knockout (KO) models, which is a powerful tool for delineating in vivo function and redundancy in gene families. This situation changed dramatically over the last year, and this review integrates recent insights into the precise biological role of the DUSP family in innate immunity gained from a comprehensive analysis of mammalian KO models.

TLR signaling

The TLR family senses the molecular signatures of microbial pathogens, and plays a fundamental role in innate immune responses. TLRs signal via a common pathway that leads to the expression of diverse inflammatory genes. In addition, each TLR elicits specific cellular responses to pathogens owing to differential usage of intracellular adapter proteins. Recent studies have revealed the importance of the subcellular localization of TLRs in pathogen recognition and signaling. TLR signaling pathways is negatively regulated by a number of cellular proteins to attenuate inflammation. Here, we describe recent advances in our understanding of the regulation of TLR-mediated signaling.

The murine liver is a potential target organ for IL-19, IL-20 and IL-24: Type I Interferons and LPS regulate the expression of IL-20R2

BACKGROUND/AIMS

The biological functions of the recently discovered IL-10-related cytokines IL-19, IL-20, IL-22, IL-24 and their receptors IL-20R1, IL-20R2 and IL-22R are not clear. Therefore, the expression of these cytokines and their receptors in the hepatic acute phase response to LPS was analysed. Type I interferons have important immunomodulatory functions in bacterial infections. We investigated if they influence release and organ-specific expression of TNF, IL-6 and IL-10 and the responsiveness of liver to IL-10 related cytokines during the reaction to LPS in vivo.

METHODS

B6 and congenic IFNAR-/- mice were intraperitoneally injected with 5mg/kg LPS. Systemic release of cytokines was quantified by ELISA. Organ-specific expression of cytokines and their receptors was evaluated by (semi quantitative or quantitative) RT-PCR.

RESULTS

The cytokines IL-19, IL-22 and the IL-20R2 receptor subunit are up-regulated by LPS in the liver of normal mice. IFNalpha/beta enhance the secretion and expression of IL-6 and IL-10 during the response to LPS, but also the up-regulation of IL-20R2 expression.

CONCLUSIONS

We show that the liver is a potential target for IL-19, IL-20 and IL-24. During an LPS response, IFNalpha/beta influence cytokine secretion and expression and possibly the response to IL-19 and IL-24.

Immunodeficiency: UNC-93B gets a toll call

Defects in innate immunity have moved to center stage in the past five years. This can be explained, at least in part, by the development of molecular assays and genetic tools that enable new approaches. Two recent studies using these elegant tools show that a transmembrane endoplasmic reticulum protein, UNC-93B, is essential for the normal response to signaling through the Toll-like receptors (TLRs) that respond to viral infection, TLR3, TLR7, TLR8 and TLR9.

D. Adjuvant-enhanced antibody responses in the absence of toll-like receptor signaling

Innate immune signals mediated by Toll-like receptors (TLRs) have been thought to contribute considerably to the antibody-enhancing effects of vaccine adjuvants. However, we report here that mice deficient in the critical signaling components for TLR mount robust antibody responses to T cell-dependent antigen given in four typical adjuvants: alum, Freund's complete adjuvant, Freund's incomplete adjuvant, and monophosphoryl-lipid A/trehalose dicorynomycolate adjuvant. We conclude that TLR signaling does not account for the action of classical adjuvants and does not fully explain the action of a strong adjuvant containing a TLR ligand. This may have important implications in the use and development of vaccine adjuvants.

Innate Immune Responses to EndosymbioticWolbachiaBacteria inBrugia malayiandOnchocerca volvulusAre Dependent on TLR2, TLR6, MyD88, and Mal, but Not TLR4, TRIF, or TRAM

The discovery that endosymbiotic Wolbachia bacteria play an important role in the pathophysiology of diseases caused by filarial nematodes, including lymphatic filariasis and onchocerciasis (river blindness) has transformed our approach to these disabling diseases. Because these parasites infect hundreds of millions of individuals worldwide, understanding host factors involved in the pathogenesis of filarial-induced diseases is paramount. However, the role of early innate responses to filarial and Wolbachia ligands in the development of filarial diseases has not been fully elucidated. To determine the role of TLRs, we used cell lines transfected with human TLRs and macrophages from TLR and adaptor molecule-deficient mice and evaluated macrophage recruitment in vivo. Extracts of Brugia malayi and Onchocerca volvulus, which contain Wolbachia, directly stimulated human embryonic kidney cells expressing TLR2, but not TLR3 or TLR4. Wolbachia containing filarial extracts stimulated cytokine production in macrophages from C57BL/6 and TLR4(-/-) mice, but not from TLR2(-/-) or TLR6(-/-) mice. Similarly, macrophages from mice deficient in adaptor molecules Toll/IL-1R domain-containing adaptor-inducing IFN-beta and Toll/IL-1R domain-containing adaptor-inducing IFN-beta-related adaptor molecule produced equivalent cytokines as wild-type cells, whereas responses were absent in macrophages from MyD88(-/-) and Toll/IL-1R domain-containing adaptor protein (TIRAP)/MyD88 adaptor-like (Mal) deficient mice. Isolated Wolbachia bacteria demonstrated similar TLR and adaptor molecule requirements. In vivo, macrophage migration to the cornea in response to filarial extracts containing Wolbachia was dependent on TLR2 but not TLR4. These results establish that the innate inflammatory pathways activated by endosymbiotic Wolbachia in B. malayi and O. volvulus filaria are dependent on TLR2-TLR6 interactions and are mediated by adaptor molecules MyD88 and TIRAP/Mal.

Increased macrophage activation mediated through toll-like receptors in rheumatoid arthritis

OBJECTIVE

Macrophages are the major source of inflammation mediators that are important in the pathogenesis of rheumatoid arthritis (RA). This study was undertaken to analyze macrophages obtained from the joints of RA patients in order to characterize the expression of Toll-like receptor 2 (TLR-2) and TLR-4 and the responses to TLR ligation.

METHODS

Cells were isolated from the synovial fluid (SF) of RA patients or patients with other forms of inflammatory arthritis. Cell surface TLR-2 and TLR-4 expression and intracellular tumor necrosis factor alpha (TNFalpha) and interleukin-8 (IL-8) expression by CD14+ macrophages were determined by flow cytometry. Peptidoglycan (PG) and lipopolysaccharide (LPS) were used as ligands for TLR-2 and TLR-4, respectively.

RESULTS

The expression of TLR-2 and TLR-4 was increased on CD14+ macrophages from the joints of RA patients compared with that on control in vitro-differentiated macrophages or control peripheral blood monocytes. Neither TLR-2 expression nor TLR-4 expression differed between RA and other forms of inflammatory arthritis. However, PG- and LPS-induced TNFalpha expression and IL-8 expression were greater with RA SF macrophages than with those obtained from the joints of patients with other forms of inflammatory arthritis or with control macrophages. PG-induced TNFalpha expression and IL-8 expression were highly correlated with TLR-2 expression in normal macrophages, but not with that in macrophages obtained from joints of RA patients or patients with other forms of inflammatory arthritis.

CONCLUSION

TLR-2 and TLR-4 ligation resulted in increased activation of RA synovial macrophages compared with those from patients with other forms of inflammatory arthritis or compared with control macrophages. Factors other than the level of TLR-2 and TLR-4 expression contributed to the increased activation of RA SF macrophages. These observations support the notion of a potential role for activation through TLR-2 and TLR-4 in the inflammation and joint destruction of RA.

Recognition and signaling by toll-like receptors

Toll-like receptors (TLRs) are transmembrane proteins that detect invading pathogens by binding conserved, microbially derived molecules and that induce signaling cascades for proinflammatory gene expression. A critical component of the innate immune system, TLRs utilize leucine-rich-repeat motifs for ligand binding and a shared cytoplasmic domain to recruit the adaptors MyD88, TRIF, TIRAP, and/or TRAM for downstream signaling. Despite significant domain conservation, TLRs induce gene programs that lead not only to the robust production of general proinflammatory mediators but also to the production of unique effectors, which provide pathogen-tailored immune responses. Here we review the mechanisms by which TLRs recognize pathogens and induce distinct signaling cascades.

Two tyrosine residues of Toll-like receptor 3 trigger different steps of NF-kappa B activation

Innate immune response to viral infection is often triggered by Toll-like receptor 3 (TLR3)-mediated signaling by double-stranded (ds) RNA, which culminates in the activation of the transcription factor NF-kappaB and induction of NF-kappaB-driven genes. We demonstrated that dsRNA-induced phosphorylation of two specific tyrosine residues, 759 and 858, of TLR3 was necessary and sufficient for complete activation of the NF-kappaB pathway. When Tyr-759 of TLR3 was mutated, gene induction was inhibited, although NF-kappaB was partially activated. It was released from IkappaB and translocated to the nucleus but failed to bind to the kappaB site of the target A20 gene promoter. This defect could be attributed to incomplete phosphorylation of the RelA (p65) subunit of NF-kappaB, as revealed by two-dimensional gel analyses of p65, isolated from dsRNA-treated cells expressing either wild type TLR3 or the Tyr-759 --> Phe mutant TLR3. Thus, two phosphotyrosine residues of TLR3 activate two distinct pathways, one leading to NF-kappaB release and the other leading to its phosphorylation.

TLR agonists induce regulatory dendritic cells to recruit Th1 cells via preferential IP-10 secretion and inhibit Th1 proliferation

Dendritic cells (DCs) and chemokines are important mediators linking innate and adaptive immunity on activation by Toll-like receptor (TLR) agonists. We previously identified a kind of regulatory DC subset (diffDCs) that differentiated from mature DCs under splenic stroma and that inhibited T-cell proliferation. The responsiveness of such regulatory DCs to TLR agonists and their pattern of chemokine production remain to be determined. Here, we report that the regulatory DCs secrete a higher level of CXCR3 chemokine IFN-gamma-induced protein-10 (IP-10) than immature DCs (imDCs), and more IP-10 is produced after stimulation with TLR-2, -4, -3, and -9 ligands. Blockade of IFN-alpha/beta inhibits IP-10 production by TLR agonist-activated regulatory DCs. We show that the increased IRF-3 and IFN-beta-induced STAT1 activation are responsible for the autocrine IFN-beta-dependent preferential production of IP-10 by regulatory DCs. In addition, stimulation with recombinant mouse IFN-alpha/beta induces more IP-10 production in regulatory DCs than that in imDCs. Moreover, the regulatory DCs selectively recruit more Th1 cells through IP-10 and inhibit Th1 proliferation. Our results demonstrate a new manner for regulatory DCs to down-regulate T-cell response by preferential IP-10 production and inhibition of recruited Th1 cell proliferation.

An IL-7 splicing-defect lymphopenia mouse model revealed by genome-wide mutagenesis

Homeostasis of the hematopoietic system is tightly regulated by an array of cytokines that control proliferation, differentiation and apoptosis of various cell lineages. To identify genes that are essential for hematopoietic homeostasis, we screened C57BL/6 mice that had been genome-wide mutagenized by N-ethyl-N-nitrosourea (ENU) to produce altered blood cell composition. We identified a mutant mouse line with a drastic reduction in the number of T and B cell lineages in lymphatic tissues and peripheral blood, as well as severe atrophy of the thymus and lymph nodes. Genotyping with a genome-wide single nucleotide polymorphism (SNP) marker set mapped the mutant phenotype to chromosome 3A and subsequent direct DNA sequencing revealed a G-to-A point mutation in the splicing donor site of the third exon of the candidate gene for IL-7, a lymphocyte survival cytokine. Such mutation resulted in skipping of exon 3 and production of an internally truncated IL-7 (DeltaE3-IL7). Furthermore, using recombinant proteins produced in a baculoviral system, we demonstrated that DeltaE3-IL7 had no detectable anti-apoptotic activity even at a dose that was 30 times more than that required for a wild-type protein to manifest a full activity in a naïve T cell survival assay. Our data suggest that this mutant mouse line provides an alternative animal model for the study of severe combined immunodeficiency (SCID) syndrome in humans.

SHP-2 phosphatase negatively regulates the TRIF adaptor protein-dependent type I interferon and proinflammatory cytokine production

The Toll-like receptor 3 (TLR3) and TLR4-signaling pathway that involves the adaptor protein TRIF activates type I interferon (IFN) and proinflammatory cytokine expression. Little is known about how TRIF pathway-dependent gene expression is regulated. SH2-containing protein tyrosine phosphatase 2 (SHP-2) is a widely expressed cytoplasmic tyrosine phosphatase. Here we demonstrate that SHP-2 negatively regulated TLR4- and TLR3-activated IFN-beta production. SHP-2 inhibited TLR3-activated but not TLR2-, TLR7-, and TLR9-activated proinflammatory cytokine IL-6 and TNF-alpha production. SHP-2 inhibited poly(I:C)-induced cytokine production by a phosphatase activity-independent mechanism. C-terminal domain of SHP-2 directly bound TANK binding kinase (TBK1) by interacting with the kinase domain of TBK1. SHP-2 deficiency increased TBK1-activated IFN-beta and TNF-alpha expression. TBK1 knockdown inhibited poly(I:C)-induced IL-6 production in SHP-2-deficient cells. SHP-2 also inhibited poly(I:C)-induced activation of MAP kinase pathways. These results demonstrate that SHP-2 specifically negatively regulate TRIF-mediated gene expression in TLR signaling, partially through inhibiting TBK1-activated signal transduction.

Toll like receptors and acute allograft rejection

Toll like receptors (TLR) are critical innate immune receptors expressed on a variety of cells including dendritic cells that are activated by the presence of invading pathogens. However, there is emerging evidence that TLR signaling participates in inflammation that may occur in the absence of overt infection. In solid organ transplantation there is increasing evidence, both in experimental and human studies, that TLR activation is involved in the innate immune recognition of allografts. Further investigation of how innate immunity impacts solid organ transplantation may lead to improved therapies for transplant recipients.

Macrophages and myeloid dendritic cells, but not plasmacytoid dendritic cells, produce IL-10 in response to MyD88- and TRIF-dependent TLR signals, and TLR-independent signals

We have previously reported that mouse plasmacytoid dendritic cells (DC) produce high levels of IL-12p70, whereas bone marrow-derived myeloid DC and splenic DC produce substantially lower levels of this cytokine when activated with the TLR-9 ligand CpG. We now show that in response to CpG stimulation, high levels of IL-10 are secreted by macrophages, intermediate levels by myeloid DC, but no detectable IL-10 is secreted by plasmacytoid DC. MyD88-dependent TLR signals (TLR4, 7, 9 ligation), Toll/IL-1 receptor domain-containing adaptor-dependent TLR signals (TLR3, 4 ligation) as well as non-TLR signals (CD40 ligation) induced macrophages and myeloid DC to produce IL-10 in addition to proinflammatory cytokines. IL-12p70 expression in response to CpG was suppressed by endogenous IL-10 in macrophages, in myeloid DC, and to an even greater extent in splenic CD8alpha(-) and CD8alpha(+) DC. Although plasmacytoid DC did not produce IL-10 upon stimulation, addition of this cytokine exogenously suppressed their production of IL-12, TNF, and IFN-alpha, showing trans but not autocrine regulation of these cytokines by IL-10 in plasmacytoid DC.

Point mutations in the melanocortin-4 receptor cause variable obesity in mice

Mutations in the melanocortin-4 receptor (MC4R) are associated with early-onset obesity in humans. Furthermore, a null Mc4r allele in mice leads to severe obesity due to hyperphagia and decreased energy expenditure. As part of independent N-ethyl- N-nitrosourea (ENU) mutagenesis screens, two obesity mutants, Fatboy and Southbeach, were isolated. Mapping revealed linkage to the melanocortin-4 receptor (Mc4r) and sequencing found single amino acid changes in Mc4r for each line. Expression of the mutant receptors in HEK 293 cells revealed defects in receptor signaling. The mutated Fatboy receptor (I194T) shows an increase in the effective concentration necessary for 50% of maximal signaling (EC(50)) when stimulated with alpha-MSH. Based on competitive binding, I194T is expressed on the cell surface at lower levels than the nonmutated receptor. In contrast, Southbeach (L300P) displays minimal receptor signaling when stimulated with the natural ligand alpha-MSH or the synthetic agonist NDP-alpha-MSH. Cell surface binding is absent, which usually indicates a lack of cell surface expression. However, antibody binding to Flag-tagged receptors by flow cytometry analysis and immunofluorescence demonstrates that L300P is translocated to the plasma membrane at a level comparable to the wild-type receptor. These results indicate a correlation with remaining receptor activity and the severity of the obesity in the mice homozygous for the mutations. Southbeach has less receptor activity and becomes more obese. These mutants will serve as good models for the variability in phenotype in humans carrying mutations in the MC4R gene.

IL-10 Inhibits Lipopolysaccharide-Induced CD40 Gene Expression through Induction of Suppressor of Cytokine Signaling-3

Costimulation between T cells and APCs is required for adaptive immune responses. CD40, an important costimulatory molecule, is expressed on a variety of cell types, including macrophages and microglia. The aberrant expression of CD40 is implicated in diseases including multiple sclerosis, rheumatoid arthritis, and Alzheimer's disease, and inhibition of CD40 signaling has beneficial effects in a number of animal models of autoimmune diseases. In this study, we discovered that IL-10, a cytokine with anti-inflammatory properties, inhibits LPS-induced CD40 gene expression. We previously demonstrated that LPS induction of CD40 in macrophages/microglia involves both NF-kappaB activation and LPS-induced production of IFN-beta, which subsequently activates STAT-1alpha. IL-10 inhibits LPS-induced IFN-beta gene expression and subsequent STAT-1alpha activation, but does not affect NF-kappaB activation. Our results also demonstrate that IL-10 inhibits LPS-induced recruitment of STAT-1alpha, RNA polymerase II, and the coactivators CREB binding protein and p300 to the CD40 promoter, as well as inhibiting permissive histone H3 acetylation (AcH3). IL-10 and LPS synergize to induce suppressor of cytokine signaling (SOCS)-3 gene expression in macrophages and microglia. Ectopic expression of SOCS-3 attenuates LPS-induced STAT activation, and inhibits LPS-induced CD40 gene expression, comparable to that seen by IL-10. These results indicate that SOCS-3 plays an important role in the negative regulation of LPS-induced CD40 gene expression by IL-10.

Uncovering the uncharacterized and unexpected: unbiased phenotype-driven screens in the mouse

Phenotype-based chemical mutagenesis screens for mouse mutations have undergone a transformation in the past five years from a potential approach to a practical tool. This change has been driven by the relative ease of identifying causative mutations now that the complete genome sequence is available. These unbiased screens make it possible to identify genes, gene functions and processes that are uniquely important to mammals. In addition, because chemical mutagenesis generally induces point mutations, these alleles often uncover previously unappreciated functions of known proteins. Here we provide examples of the success stories from forward genetic screens, emphasizing the examples that illustrate the discovery of mammalian-specific processes that could not be discovered in other model organisms. As the efficiency of sequencing and mutation detection continues to improve, it is likely that forward genetic screens will provide an even more important part of the repertoire of mouse genetics in the future.

Homeostasis of memory T cells

The pool of memory T cells is regulated by homeostatic mechanisms to persist for prolonged periods at a relatively steady overall size. Recent work has shown that two members of the common gamma chain (gammac) family of cytokines, interleukin-7 (IL-7) and IL-15, govern homeostasis of memory T cells. These two cytokines work in conjunction to support memory T-cell survival and intermittent background proliferation. Normal animals contain significant numbers of spontaneously arising memory-phenotype (MP) cells, though whether these cells are representative of true antigen-specific memory T cells is unclear. Nevertheless, it appears that the two types of memory cells do not display identical homeostatic requirements. For antigen-specific memory CD8+ T cells, IL-7 is primarily important for survival while IL-15 is crucial for their background proliferation. For memory CD4+ T cells, IL-7 has an important role, whereas the influence of IL-15 is still unclear.

Nonredundant roles of TIRAP and MyD88 in airway response to endotoxin, independent of TRIF, IL-1 and IL-18 pathways

Inhaled endotoxins induce an acute inflammatory response in the airways mediated through Toll-like receptor 4 (TLR4) and myeloid differentiation factor 88 (MyD88). However, the relative roles of the TLR4 adaptor proteins TIRAP and TRIF and of the MyD88-dependent IL-1 and IL-18 receptor pathways in this response are unclear. Here, we demonstrate that endotoxin-induced acute bronchoconstriction, vascular damage resulting in protein leak, Th1 cytokine and chemokine secretion and neutrophil recruitment in the airways are abrogated in mice deficient for either TIRAP or MyD88, but not in TRIF deficient mice. The contribution of other TLR-independent, MyD88-dependent signaling pathways was investigated in IL-1R1, IL-18R and caspase-1 (ICE)-deficient mice, which displayed normal airway responses to endotoxin. In conclusion, the TLR4-mediated, bronchoconstriction and acute inflammatory lung pathology to inhaled endotoxin critically depend on the expression of both adaptor proteins, TIRAP and MyD88, suggesting cooperative roles, while TRIF, IL-1R1, IL-18R signaling pathways are dispensable.

Viral targeting of interferon regulatory factor-3 and type I interferon gene transcription

Successful host defense against viruses depends on rapidly mounted defense mechanisms, which include the release of type I interferons (IFN)α/β and the transcription of IFN-stimulated genes. IFN limits viral replication and activates adaptive immunity. Much progress has now been made in delineating how the type I IFN response is triggered upon infection by different viruses. Progress in this regard relates to the identification of distinct families of pattern recognition receptors involved in the detection of viral nucleic acids, the discovery of adapter molecules, which couple signaling from these receptors to downstream effectors, and the characterization of key kinases responsible for the phosphorylation-induced activation of the IFN regulatory factors that control IFN gene transcription. In turn, we are learning that viruses encode a diversity of sophisticated mechanisms to block IFN induction at each of these levels and/or counteract IFN activity, thereby supporting viral replication and neutralizing the therapeutic action of IFNs.

HCV immunopathogenesis: virus-induced strategies against host immunity

Worldwide more than 170 million people are chronically infected with the hepatitis C virus (HCV), which is a frequent cause of chronic hepatitis, cirrhosis, and hepatocellular carcinoma. Unlike infection with other hepatotropic viruses, only a small percentage of acute HCV infections are cleared, and most infected individuals develop lifelong HCV infection in the absence of efficient treatment. It is believed that both viral and host factors contribute to the inability of the host immune system to clear the initial infection and lead to the high propensity of chronic HCV infection.

Adrenergic inhibition of innate anti-viral response: PKA blockade of Type I interferon gene transcription mediates catecholamine support for HIV-1 replication

Type I interferons (IFN-alpha and -beta) play a key role in anti-viral immunity, and we sought to define the molecular mechanisms by which the sympathetic nervous system (SNS) inhibits their effects. In peripheral blood leukocytes and plasmacytoid dendritic cells (pDC2), induction of interferon anti-viral activity by double-stranded RNA (poly-I:C) or CpG DNA was substantially inhibited by norepinephrine and by pharmacologic activation of the cAMP/PKA signaling pathway. This effect was specific to Type I interferons and driven by PKA-mediated repression of IFNA and IFNB gene transcription. Luciferase reporter analyses identified tandem interferon response factor-binding sites in positive regulatory domains I and III of the IFNB promoter as a key target of PKA inhibition. PKA suppression of Type I interferons was associated with impaired transcription of interferon response genes supporting the "anti-viral state", and was sufficient to account for norepinephrine-induced enhancement of HIV-1 replication. Given the ubiquitous role of Type I interferons in containing viral replication, PKA-mediated inhibition of IFN transcription could explain the stimulatory effects of catecholamines on a broad range of viral pathogens.

[Toll-dependent and toll-independent innate antiviral immunity]

Until recently, adaptive immunity and cytotoxic T cells were considered as the only essential components of the antiviral defence arsenal. Additional data that do not rule out the crucial role of these cells in the clearance of viral pathogens have, however, recently emerged. They indicate that innate immune cells such as macrophages, dendritic cells, gammadelta T cells as well as natural killer (NK) cells play a primordial role in this mechanism. It is now well established that innate immune cells can detect various pathogens (bacteria, viruses, fungi or parasites) very rapidly and respond to their presence through the activation of specific receptors. Once activated, these molecules trigger several signalling cascades that culminate in the establishment of very potent defence mechanisms. In addition, cytokines produced during this initial response are essential for the activation of the adaptive immune response which will add specificity and memory to the system. Among the innate immune receptors, attention has focused on the Toll-like receptors (TLR) and many reports indicate that some of the TLRs are clearly involved in defence against viral pathogens. However, new molecules, acting independently from any TLR, have recently been discovered. They define a second antiviral pathway which is presently the subject of intense research. In this article, we will review the role of the different molecules involved in each pathway within the framework of innate antiviral defence.

The role of hyaluronan degradation products as innate alloimmune agonists

Dendritic cells (DCs) play a key role in initiating alloimmunity yet the substances that activate them during the host response to transplantation remain elusive. In this study we examined the potential roles of endogenous innate immune agonists in activating dendritic cell-dependent alloimmunity. Using a murine in vitro culture system, we show that 135 KDa fragments of the extracellular matrix glycosaminoglycan hyaluronan induce dendritic cell maturation and initiate alloimmunity. Priming of alloimmunity by hyaluronan-activated DCs was dependent on signaling via TIR-associated protein, a Toll-like receptor (TLR) adaptor downstream of TLRs 2 and 4. However, this effect was independent of alternate TLR adaptors, MyD88 or Trif. Using an in vivo murine transplant model, we show that hyaluronan accumulated during skin transplant rejection. Examination of human lung transplant recipients demonstrated that increased levels of intragraft hyaluronan were associated with bronchiolitis obliterans syndrome. In conclusion, our study suggests that fragments of hyaluronan can act as innate immune agonists that activate alloimmunity.

Manipulation of the nuclear factor-kappaB pathway and the innate immune response by viruses

Viral and microbial constituents contain specific motifs or pathogen-associated molecular patterns (PAMPs) that are recognized by cell surface- and endosome-associated Toll-like receptors (TLRs). In addition, intracellular viral double-stranded RNA is detected by two recently characterized DExD/H box RNA helicases, RIG-I and Mda-5. Both TLR-dependent and -independent pathways engage the IkappaB kinase (IKK) complex and related kinases TBK-1 and IKKvarepsilon. Activation of the nuclear factor kappaB (NF-kappaB) and interferon regulatory factor (IRF) transcription factor pathways are essential immediate early steps of immune activation; as a result, both pathways represent prime candidates for viral interference. Many viruses have developed strategies to manipulate NF-kappaB signaling through the use of multifunctional viral proteins that target the host innate immune response pathways. This review discusses three rapidly evolving areas of research on viral pathogenesis: the recognition and signaling in response to virus infection through TLR-dependent and -independent mechanisms, the involvement of NF-kappaB in the host innate immune response and the multitude of strategies used by different viruses to short circuit the NF-kappaB pathway.

Renal collecting duct epithelial cells react to pyelonephritis-associated Escherichia coli by activating distinct TLR4-dependent and -independent inflammatory pathways

TLR4 plays a central role in resistance to pyelonephritis caused by uropathogenic Escherichia coli (UPEC). It has been suggested that renal tubule epithelial cells expressing TLRs may play a key role in inflammatory disorders and in initiating host defenses. In this study we used an experimental mouse model of ascending urinary tract infection to show that UPEC isolates preferentially adhered to the apical surface of medullary collecting duct (MCD) intercalated cells. UPEC-infected C3H/HeJ (Lps(d)) mice carrying an inactivating mutation of tlr4 failed to clear renal bacteria and exhibited a dramatic slump in proinflammatory mediators as compared with infected wild-type C3H/HeOuJ (Lps(n)) mice. However, the level of expression of the leukocyte chemoattractants MIP-2 and TNF-alpha still remained greater in UPEC-infected than in naive C3H/HeJ (Lps(d)) mice. Using primary cultures of microdissected Lps(n) MCDs that expressed TLR4 and its accessory molecules MD2, MyD88, and CD14, we also show that UPECs stimulated both a TLR4-mediated, MyD88-dependent, TIR domain-containing adaptor-inducing IFN-beta-independent pathway and a TLR4-independent pathway, leading to bipolarized secretion of MIP-2. Stimulation by UPECs of the TLR4-mediated pathway in Lps(n) MCDs leads to the activation of NF-kappaB, and MAPK p38, ERK1/2, and JNK. In addition, UPECs stimulated TLR4-independent signaling by activating a TNF receptor-associated factor 2-apoptosis signal-regulatory kinase 1-JNK pathway. These findings demonstrate that epithelial collecting duct cells are actively involved in the initiation of an immune response via several distinct signaling pathways and suggest that intercalated cells play an active role in the recognition of UPECs colonizing the kidneys.

Cell-Associated Double-Stranded RNA Enhances Antitumor Activity through the Production of Type I IFN

The efficacy of tumor cell vaccination largely depends on the maturation and activation status of the dendritic cell. Here we investigated the ability of soluble and tumor cell-associated dsRNA to serve as an adjuvant in the induction of protective adaptive antitumor responses. Our data showed that cell-associated dsRNA, but not soluble dsRNA, enhanced both tumor-specific CD8(+) and CD4(+) T cell responses. The cell-associated dsRNA increased the clonal burst of tumor-specific CD8(+) T cells and endowed them with an enhanced capacity for expansion upon a secondary encounter with tumor Ags, even when the CD8(+) T cells were primed in the absence of CD4(+) T cell help. The adjuvant effect of cell-associated dsRNA was fully dependent on the expression of TLR3 by the APCs and their subsequent production of type I IFNs, as the adjuvant effect of cell-associated dsRNA was completely abrogated in mice deficient in TLR3 or type I IFN signaling. Importantly, treatment with dsRNA-associated tumor cells increased the number of tumor-infiltrating lymphocytes and enhanced the survival of tumor-bearing mice. The data from our studies suggest that using cell-associated dsRNA as a tumor vaccine adjuvant may be a suitable strategy for enhancing vaccine efficacy for tumor cell therapy in cancer patients.

Toll-like receptors and their ligands control mesenchymal stem cell functions

Mesenchymal stem cells (MSCs) are widespread in adult organisms and may be involved in tissue maintenance and repair as well as in the regulation of hematopoiesis and immunologic responses. Thus, it is important to discover the factors controlling MSC renewal and differentiation. Here we report that adult MSCs express functional Toll-like receptors (TLRs), confirmed by the responses of MSCs to TLR ligands. Pam3Cys, a prototypic TLR-2 ligand, augmented interleukin-6 secretion by MSC, induced nuclear factor kappa B (NF-kappaB) translocation, reduced MSC basal motility, and increased MSC proliferation. The hallmark of MSC function is the capacity to differentiate into several mesodermal lineages. We show herein that Pam3Cys inhibited MSC differentiation into osteogenic, adipogenic, and chondrogenic cells while sparing their immunosuppressive effect. Our study therefore shows that a TLR ligand can antagonize MSC differentiation triggered by exogenous mediators and consequently maintains the cells in an undifferentiated and proliferating state in vitro. Moreover, MSCs derived from myeloid factor 88 (MyD88)-deficient mice lacked the capacity to differentiate effectively into osteogenic and chondrogenic cells. It appears that TLRs and their ligands can serve as regulators of MSC proliferation and differentiation and might affect the maintenance of MSC multipotency.

TLR signaling

Mammalian Toll-like receptors (TLRs) play a critical role in detection of invading pathogens as well as triggering of subsequent inflammatory and immune responses. Each TLR recognizes distinct microbial components and activates different signaling pathways by selective utilization of adaptor molecules. The signaling via TLRs is delivered from the cell surface and/or the endosome. Recently, the intracytoplasmic detection system of microbes has been identified in mammals as well. Peptidoglycan breakdown products and double-stranded RNA are sensed by NOD family and RNA helicase domain containing proteins, respectively. Thus, mammals make use of both receptor-type and intracellular proteins as detectors of invading pathogens.

Type I IFN Modulates Host Defense and Late Hyperinflammation in Septic Peritonitis

TLRs are considered important for the control of immune responses during endotoxic shock or polymicrobial sepsis. Signaling by TLRs may proceed through the adapter proteins MyD88 or TIR domain-containing adaptor inducinng IFN-beta. Both pathways can lead to the production of type I IFNs (IFN-alphabeta). In the present study, the role of the type I IFN pathway for host defense and immune pathology in sepsis was investigated using a model of mixed bacterial peritonitis. Systemic levels of IFN-alphabeta protein were markedly elevated during septic peritonitis. More detailed analyses revealed production of IFN-beta, but not IFN-alpha subtypes, and identified CD11b+ CD11c- macrophage-like cells as major producers of IFN-beta. The results further demonstrate that in IFN-alphabeta receptor I chain (IFNARI)-deficient mice, the early recruitment of neutrophils to the infected peritoneal cavity was augmented, most likely due to an increased local production of MCP-1 and leukotriene B4. In the absence of IFNARI, peritoneal neutrophils also exhibited enhanced production of reactive oxygen intermediates and elevated expression of Mac-1. Conversely, administration of recombinant IFN-beta resulted in reduced leukotriene B4 levels and decreased peritoneal neutrophil recruitment and activation. Analysis of the cytokine response to septic peritonitis revealed that IFNARI deficiency strongly attenuated late, but not early, hyperinflammation. In accordance with these findings, bacterial clearance and overall survival of IFNARI(-/-) mice were improved. Therefore, the present study reveals critical functions of the type I IFN pathway during severe mixed bacterial infections leading to sepsis. The results suggest that type I IFN exerts predominantly adverse effects under these conditions.

Toll-like Receptor Recognition Regulates Immunodominance in an Antimicrobial CD4+ T Cell Response

Although Toll-like Receptors (TLRs) play a major function in innate recognition of pathogens, their role in antigen processing and presentation in vivo is poorly understood. Here we establish that Toxoplasma gondii profilin, a TLR11 ligand present in the parasite, is an immunodominant antigen in the CD4(+) T cell response to the pathogen. The immunogenicity of profilin was entirely dependent on both TLR11 recognition and signaling through the adaptor myeloid differentiation factor 88 (MyD88). Selective responsiveness to this parasite protein was regulated at the level of antigen presentation by dendritic cells (DC) and required both TLR signaling and major histocompatibility complex (MHC) class II recognition acting in cis. These findings support a major influence of TLR recognition in antigen presentation by DC in vivo and establish a mechanism by which TLR ligand association regulates the immunogenicity of microbial antigens.

Contribution of Interferon-β to the Murine Macrophage Response to the Toll-like Receptor 4 Agonist, Lipopolysaccharide

Interferon-beta (IFN-beta) has been identified as the signature cytokine induced via the Toll-like receptor (TLR) 4, "MyD88-independent" signaling pathway in macrophages stimulated by Gram-negative bacterial lipopolysaccharide (LPS). In this study, we analyzed the responses of macrophages derived from wild-type (IFN-beta(+/+)) mice or mice with a targeted mutation in IFN-beta (IFN-beta(-/-)) to the prototype TLR4 agonist, Escherichia coli LPS. A comparison of basal and LPS-induced gene expression (by reverse transcription-PCR, real-time PCR, and Affymetrix microarray analyses) resulted in the identification of four distinct patterns of gene expression affected by IFN-beta deficiency. Analysis of a subset of each group of differentially regulated genes by computer-assisted promoter analysis revealed putative IFN-responsive elements in all genes examined. LPS-induced activation of intracellular signaling molecules, STAT1 Tyr-701, STAT1 Ser-727, and Akt, but not p38, JNK, and ERK MAPK proteins, was significantly diminished in IFN-beta(-/-) versus IFN-beta(+/+) macrophages. "Priming" of IFN-beta(-/-) macrophages with exogenous recombinant IFN-beta significantly increased levels of LPS-induced gene expression for induction of monocyte chemotactic protein 5, inducible nitric-oxide synthase, IP-10, and IL-12 p40 mRNA, whereas no increase or relatively small increases were observed for IL-1beta, IL-6, monocyte chemotactic protein 1, and MyD88 mRNA. Finally, IFN-beta(-/-) mice challenged in vivo with LPS exhibited increased survival when compared with wild-type IFN-beta(+/+) controls, indicating that IFN-beta contributes to LPS-induced lethality; however, not to the extent that one observes in mice with more complete pathway deficiencies (e.g. TLR4(-/-) or TRIF(-/-) mice). Collectively, these findings reveal unanticipated regulatory roles for IFN-beta in response to LPS in vitro and in vivo.

Innate Immune Activation and CD4+ T Cell Priming during Respiratory Fungal Infection

Aspergillus fumigatus is a mold that causes a spectrum of diseases, including lethal lung infections in immunocompromised humans and allergic asthma in atopic individuals. T helper 1 (Th1) CD4(+) T cells protect against invasive A. fumigatus infections whereas Th2 CD4(+) T cells exacerbate asthma upon inhalation of A. fumigatus spores. Herein, we demonstrate that A. fumigatus-specific T cells were rapidly primed in lymph nodes draining the lung and fully differentiated into interferon-gamma (IFN-gamma)-producing Th1 CD4(+) T cells upon arrival in the airways. T-bet induction in A. fumigatus-specific CD4(+) T cells was enhanced by MyD88-mediated signals in draining lymph nodes, but T cell proliferation, trafficking, and Th1 differentiation in the airways were Toll-like receptor (TLR) and MyD88 independent. Our studies demonstrate that CD4(+) T cell differentiation during respiratory fungal infection occurs incrementally, with TLR-mediated signals in the lymph node enhancing the potential for IFN-gamma production whereas MyD88-independent signals promote Th1 differentiation in the lung.

Protective and destructive immunity in the periodontium: Part 1--innate and humoral immunity and the periodontium

Based on the results of recent research in the field, the present paper will discuss the protective and destructive aspects of the innate vs. adaptive (humoral and cell-mediated) immunity associated with the bacterial virulent factors or antigenic determinants during periodontal pathogenesis. Attention will be focused on: (i) the Toll-like receptors (TLR), the innate immune repertoire for recognizing the unique molecular patterns of microbial components that trigger innate and adaptive immunity for effective host defenses, in some general non-oral vs. periodontal microbial infections; (ii) T-cell-mediated immunity, Th-cytokines, and osteoclastogenesis in periodontal disease progression; and (iii) some molecular techniques developed and used to identify critical microbial virulence factors or antigens associated with host immunity (using Actinobacillus actinomycetemcomitans and Porphyromonas gingivalis as the model species). Therefore, further understanding of the molecular interactions and mechanisms associated with the host's innate and adaptive immune responses will facilitate the development of new and innovative therapeutics for future periodontal treatments.

Type I Inteferon Gene Induction by the Interferon Regulatory Factor Family of Transcription Factors

Induction of type I interferons (IFNs) by viruses and other pathogens is crucial for innate immunity, and it is mediated by the activation of pattern-recognition receptors, such as Toll-like receptors and cytosolic receptors such as RIG-I and MDA5. The type I IFN induction is primarily controlled at the gene transcriptional level, wherein a family of transcription factors, interferon regulatory factors (IRFs), plays central roles. Here, we summarize the recent studies on IRFs, providing a paradigm of how genes are ingeniously regulated during immune responses. We also consider some evolutional aspects on the IFN-IRF system.

Natural killer T cells recognize diacylglycerol antigens from pathogenic bacteria

Natural killer T (NKT) cells recognize glycosphingolipids presented by CD1d molecules and have been linked to defense against microbial infections. Previously defined foreign glycosphingolipids recognized by NKT cells are uniquely found in nonpathogenic sphingomonas bacteria. Here we show that mouse and human NKT cells also recognized glycolipids, specifically a diacylglycerol, from Borrelia burgdorferi, which causes Lyme disease. The B. burgdorferi-derived, glycolipid-induced NKT cell proliferation and cytokine production and the antigenic potency of this glycolipid was dependent on acyl chain length and saturation. These data indicate that NKT cells recognize categories of glycolipids beyond those in sphingomonas and suggest that NKT cell responses driven by T cell receptor-mediated glycolipid recognition may provide protection against diverse pathogens.

TRIF-GEFH1-RhoB pathway is involved in MHCII expression on dendritic cells that is critical for CD4 T-cell activation

Dendritic cells (DC) play a central role in immune responses by presenting antigenic peptides to CD4+ T cells through MHCII molecules. Here, we demonstrate a TRIF-GEFH1-RhoB pathway is involved in MHCII surface expression on DC. We show the TRIF (TIR domain-containing adapter inducing IFNbeta)- but not the myeloid differentiation factor 88 (MyD88)-dependent pathway of lipopolysaccharide (LPS)-signaling in DC is crucial for the MHCII surface expression, followed by CD4+ T-cell activation. LPS increased the activity of RhoB, but not of RhoA, Cdc42, or Rac1/2 in a manor dependent on LPS-TRIF- but not LPS-Myd88-signaling. RhoB colocalized with MHCII+ lysosomes in DC. A dominant-negative (DN) form of RhoB (DN-RhoB) or RhoB's RNAi in DC inhibited the LPS-induced MHCII surface expression. Moreover, we found GEFH1 associated with RhoB, and DN-GEFH1 or GEFH1's RNAi suppressed the LPS-mediated RhoB activation and MHCII surface expression. DN-RhoB attenuated the DC's CD4+ T-cell stimulatory activity. Thus, our results provide a molecular mechanism relating how the MHCII surface expression is regulated during the maturation stage of DC. The activation of GEFH1-RhoB through the TRIF-dependent pathway of LPS in DC might be a critical target for controlling the activation of CD4+ T cells.

TLR signaling

The Toll-like receptor (TLR) family plays an instructive role in innate immune responses against microbial pathogens, as well as the subsequent induction of adaptive immune responses. TLRs recognize specific molecular patterns found in a broad range of microbial pathogens such as bacteria and viruses, triggering inflammatory and antiviral responses and dendritic cell maturation, which result in the eradication of invading pathogens. Individual TLRs interact with different combinations of adapter proteins and activate various transcription factors such as nuclear factor (NF)-kappaB, activating protein-1 and interferon regulatory factors, driving a specific immune response. This review outlines the recent advances in our understanding of TLR-signaling pathways and their roles in immune responses. Further, we also discuss a new concept of TLR-independent mechanisms for recognition of microbial pathogens.

N-(3-oxo-acyl)homoserine lactones signal cell activation through a mechanism distinct from the canonical pathogen-associated molecular pattern recognition receptor pathways

Innate immune system receptors function as sensors of infection and trigger the immune responses through ligand-specific signaling pathways. These ligands are pathogen-associated products, such as components of bacterial walls and viral nuclear acids. A common response to such ligands is the activation of mitogen-activated protein kinase p38, whereas double-stranded viral RNA additionally induces the phosphorylation of eukaryotic translation initiation factor 2alpha (eIF2alpha). Here we have shown that p38 and eIF2alpha phosphorylation represent two biochemical markers of the effects induced by N-(3-oxo-acyl)homoserine lactones, the secreted products of a number of Gram-negative bacteria, including the human opportunistic pathogen Pseudomonas aeruginosa. Furthermore, N-(3-oxo-dodecanoyl)homoserine lactone induced distension of mitochondria and the endoplasmic reticulum as well as c-jun gene transcription. These effects occurred in a wide variety of cell types including alveolar macrophages and bronchial epithelial cells, requiring the structural integrity of the lactone ring motif and its natural stereochemistry. These findings suggest that N-(3-oxo-acyl)homoserine lactones might be recognized by receptors of the innate immune system. However, we provide evidence that N-(3-oxo-dodecanoyl)homoserine lactone-mediated signaling does not require the presence of the canonical innate immune system receptors, Toll-like receptors, or two members of the NLR/Nod/Caterpillar family, Nod1 and Nod2. These data offer a new understanding of the effects of N-(3-oxo-dodecanoyl)homoserine lactone on host cells and its role in persistent airway infections caused by P. aeruginosa.

ENU-mutagenesis: insight into immune function and pathology

In random chemical mutagenesis, gene discovery is driven by phenotypes rather than by hypotheses. A standard dose of N-ethyl-N-nitrosourea results in approximately 30 coding mutations in male G1 mice, of which approximately 4 can be propagated to homozygosity in 3 generations. In recent years, large-scale screens of such G3 mice for phenotypes of interest to immunologists have revealed clues to the number of genes responsible for key immune responses, such as innate recognition of pathogens and autoantibody production. More than 20 of the phenotypes that exhibit a simple (Mendelian) pattern of inheritance have been mapped. Novel alleles have revealed new pathways of host defense, allergy and autoimmunity.

Role of toll-like receptors in spontaneous commensal-dependent colitis

Inflammatory bowel disease (IBD) is thought to result from a dysregulated interaction between the host immune system and its commensal microflora. Heterogeneity of disease susceptibility in humans and rodents suggest that multiple mechanisms are responsible for the etiology of IBD. In particular, deficiencies in anti-inflammatory and immune-suppressive mechanisms play an important role in the development of IBD. However, it is unknown how the indigenous microflora stimulates the immune system and how this response is regulated. To address these questions, we investigated the role of Toll-like receptor (TLR) signaling in the development of spontaneous, commensal-dependent colitis in interleukin (IL)-2- and IL-10-deficient mice. We report that colitis was dependent on TLR signaling in Il10(-/-) mice. In contrast, Il2(-/-) mice developed intestinal inflammation in the absence of TLR signaling pathways. These results demonstrate a differential role of innate immune recognition by TLRs in the development of commensal-dependent colitis.

Activation of common antiviral pathways can potentiate inflammatory responses to septic shock

Induction of the antiviral cytokine interferon alpha/beta (IFN-alpha/beta) is common in many viral infections. The impact of ongoing antiviral responses on subsequent bacterial infection is not well understood. In human disease, bacterial superinfection complicating a viral infection can result in significant morbidity and mortality. We injected mice with polyinosinic-polycytidylic (PIC) acid, a TLR3 ligand and known IFN-alpha/beta inducer as well as nuclear factor kappaB (NF-kappaB) activator to simulate very early antiviral pathways. We then challenged mice with an in vivo septic shock model characterized by slowly evolving bacterial infection to simulate bacterial superinfection early during a viral infection. Our data demonstrated robust induction of IFN-alpha in serum within 24 h of PIC injection with IFN-alpha/beta-dependent major histocompatibility antigen class II up-regulation on peritoneal macrophages. PIC pretreatment before septic shock resulted in augmented tumor necrosis factor alpha and interleukins 6 and 10 and heightened lethality compared with septic shock alone. Intact IFN-alpha/beta signaling was necessary for augmentation of the inflammatory response to in vivo septic shock and to both TLR2 and TLR4 agonists in vitro. To assess the NF-kappaB contribution to PIC-modulated inflammatory responses to septic shock, we treated with parthenolide, an NF-kappaB inhibitor before PIC and septic shock. Parthenolide did not inhibit IFN-alpha induction by PIC. Inhibition of NF-kappaB by parthenolide did reduce IFN-alpha-mediated potentiation of the cytokine response and lethality from septic shock. Our data demonstrate that pathways activated early during many viral infections can have a detrimental impact on the outcome of subsequent bacterial infection. These pathways may be critical to understanding the heightened morbidity and mortality from bacterial superinfection after viral infection in human disease.

Simultaneous deletion of MyD88 and Trif delays major histocompatibility and minor antigen mismatch allograft rejection

This study investigated whether ablation of all Toll-like receptor (TLR) signaling influenced skin allograft rejection across a complete donor/recipient mismatch of major histocompatibility and minor antigens. We predicted that defective TLR signaling would interfere with the activation of donor dendritic cells (DC) in vivo, by preventing DC activation in response to local environmental ("danger") signals. The ablation of TLR signals would therefore be associated with decreased activation of host T cells and decreased graft rejection. Using mice with deletions of the proximal TLR adapter proteins MyD88 or Trif, and those with simultaneous deletions of both MyD88 and Trif, we demonstrated that absence of both MyD88 and Trif adapter proteins prolonged skin graft survival, notably across a complete MHC and minor antigen barrier. Absence of MyD88 or Trif alone only had a modest effect on graft survival across even a minor MHC antigen difference. Prolonged survival of skin grafts from mice deficient in both MyD88 and Trif was associated with diminished migration of donor cells to draining lymph nodes and, subsequently, with delayed infiltration of host T cells into the grafted tissue.

Induction of dendritic cell maturation by pertussis toxin and its B subunit differentially initiate Toll-like receptor 4–dependent signal transduction pathways

OBJECTIVE

Pertussis toxin (PT) has the capacity to activate dendritic cells (DCs) for the augmentation of cell-mediated immune responses. To investigate the mechanism(s) by which PT activates DCs, we investigated the effects of PT and its B-oligomer (PTB) on the maturation of human and mouse DCs and determined whether PT could act as a pathogen-associated molecular pattern to activate one of the Toll-like receptors (TLRs).

METHODS

The effects of PT and PTB on the maturation of human and mouse DCs were analyzed in terms of surface marker expression, cytokine production, antigen-presenting capacity, and intracellular signaling. The participation of TLR4 in PT-induced signaling was determined by comparing the effect of PT on DCs derived from TLR4-deficient and wild-type mice, as well as by measuring PT-induced NF-kappaB activation in HEK293 cells transiently transfected to express various TLRs.

RESULTS

Although both promoted phenotypic and functional maturation DCs, however, unlike PT that induced DC production of interleukin (IL)-6, tumor necrosis factor-alpha, IL-12, and interferon-inducible protein, PTB was capable of stimulating the production of interferon-inducible protein. Bone marrow-derived DCs from C3H/HeJ mice with defective TLR-4 alleles were unresponsive to PT and PTB, whereas DCs from C3H/HeN mice responded. In addition, PT induced NF-kappaB activation and IL-8 production in HEK293 cells transfected with a combination of TLR4 and MD2 but not in nontransfected or TLR2-transfected HEK293 cells. Comparison of the patterns of cytokine induction and intracellular signaling events in DCs treated by PT and PTB revealed that although PT, like lipopolysaccharide, triggered both MyD88-dependent and -independent pathways, PTB preferentially triggered MyD88-independent pathways. Interestingly, mouse splenocyte proliferation in response to PT and PTB was only partially dependent on TLR4.

CONCLUSION

The data identify PT as another pathogen-associated molecular pattern that induces DC maturation in a TLR4-dependent manner. Unlike PT, which triggers both MyD88-dependent and -independent pathways, PTB only triggers the MyD88-independent pathway in DCs.