Primitive Toll signalling: bugs, flies, worms and man

Proteobacteria and Firmicutes Secreted Factors Exert Distinct Effects on Pseudomonas aeruginosa Infection under Normoxia or Mild Hypoxia

Microbiota may alter a pathogen's virulence potential at polymicrobial infection sites. Here, we developed a multi-modal Drosophila assay, amenable to the assessment of human bacterial interactions using fly survival or midgut regeneration as a readout, under normoxia or mild hypoxia. Deploying a matrix of 12 by 33 one-to-one Drosophila co-infections via feeding, we classified bacterial interactions as neutral, synergistic, or antagonistic, based on fly survival. Twenty six percent of these interactions were antagonistic, mainly occurring between Proteobacteria. Specifically, Pseudomonas aeruginosa infection was antagonized by various Klebsiella strains, Acinetobacter baumannii, and Escherichia coli. We validated these interactions in a second screen of 7 by 34 one-to-one Drosophila co-infections based on assessments of midgut regeneration, and in bacterial co-culture test tube assays, where antagonistic interactions depended on secreted factors produced upon high sugar availability. Moreover, Enterococci interacted synergistically with P. aeruginosa in flies and in test tubes, enhancing the virulence and pyocyanin production by P. aeruginosa. However, neither lactic acid bacteria nor their severely hypoxic culture supernatants provided a survival benefit upon P. aeruginosa infection of flies or mice, respectively. We propose that at normoxic or mildly hypoxic sites, Firmicutes may exacerbate, whereas Proteobacteria secreted factors may ameliorate, P. aeruginosa infections.

Aeromonas salmonicida activates rainbow trout IgM+ B cells signalling through Toll like receptors

As B cells are singularly equipped with a B cell receptor (BCR) and a range of innate receptors, they are able to integrate both antigen-specific and innate signals, with the latter being essential to reach an adequate level of activation. Whether teleost B cells sense pathogens through innate mechanisms has not yet been explored, despite the fact that fish B cells display a wider array of innate receptors than many mammalian B cell subsets. Hence, in the current study, we have investigated the effects of inactivated Aeromonas salmonicida, a Gram negative rainbow trout pathogen, on trout splenic IgM⁺ B cells in vitro in the presence or absence of different inhibitors of Toll-like receptor (TLR) signalling, to establish to what degree innate signals are contributing to the activation of B cells in teleosts. Our results demonstrate that most of the effects that A. salmonicida exerts on trout IgM⁺ B cells are significantly blocked in the presence of inhibitors of MyD88 and TRIF, important nodes in TLR signal pathways. Thus, the data presented demonstrates that, also in teleost, TLR signalling is essential for the activation of IgM⁺ B cells. These results will be useful for the future optimization of novel vaccines and adjuvants.

Cytokine release syndrome: inhibition of pro-inflammatory cytokines as a solution for reducing COVID-19 mortality

Coronavirus disease (COVID-19) reached pandemic proportions at the beginning of 2020 and continues to be a worldwide concern. End organ damage and acute respiratory distress syndrome are the leading causes of death in severely or critically ill patients. The elevated cytokine levels in severe patients in comparison with mildly affected patients suggest that cytokine release syndrome (CRS) occurs in the severe form of the disease. In this paper, the significant role of pro-inflammatory cytokines, including IL-1, IL-6, and TNF-alpha, and their mechanism of action in the CRS cascade is explained. Potential therapeutic approaches involving anti-IL-6 and anti-TNF-alpha antibodies to fight COVID-19 and reduce mortality rate in severe cases are also discussed.

The family of the interleukin-1 receptors

The extracellular forms of the IL-1 cytokines are active through binding to specific receptors on the surface of target cells. IL-1 ligands bind to the extracellular portion of their ligand-binding receptor chain. For signaling to take place, a non-binding accessory chain is recruited into a heterotrimeric complex. The intracellular approximation of the Toll-IL-1-receptor (TIR) domains of the 2 receptor chains is the event that initiates signaling. The family of IL-1 receptors (IL-1R) includes 10 structurally related members, and the distantly related soluble protein IL-18BP that acts as inhibitor of the cytokine IL-18. Over the years the receptors of the IL-1 family have been known with many different names, with significant confusion. Thus, we will use here a recently proposed unifying nomenclature. The family includes several ligand-binding chains (IL-1R1, IL-1R2, IL-1R4, IL-1R5, and IL-1R6), 2 types of accessory chains (IL-1R3, IL-1R7), molecules that act as inhibitors of signaling (IL-1R2, IL-1R8, IL-18BP), and 2 orphan receptors (IL-1R9, IL-1R10). In this review, we will examine how the receptors of the IL-1 family regulate the inflammatory and anti-inflammatory functions of the IL-1 cytokines and are, more at large, involved in modulating defensive and pathological innate immunity and inflammation. Regulation of the IL-1/IL-1R system in the brain will be also described, as an example of the peculiarities of organ-specific modulation of inflammation.

Toll-like receptors in immunity and inflammatory diseases: Past, present, and future

The immune system is a very diverse system of the host that evolved during evolution to cope with various pathogens present in the vicinity of environmental surroundings inhabited by multicellular organisms ranging from achordates to chordates (including humans). For example, cells of immune system express various pattern recognition receptors (PRRs) that detect danger via recognizing specific pathogen-associated molecular patterns (PAMPs) and mount a specific immune response. Toll-like receptors (TLRs) are one of these PRRs expressed by various immune cells. However, they were first discovered in the Drosophila melanogaster (common fruit fly) as genes/proteins important in embryonic development and dorso-ventral body patterning/polarity. Till date, 13 different types of TLRs (TLR1-TLR13) have been discovered and described in mammals since the first discovery of TLR4 in humans in late 1997. This discovery of TLR4 in humans revolutionized the field of innate immunity and thus the immunology and host-pathogen interaction. Since then TLRs are found to be expressed on various immune cells and have been targeted for therapeutic drug development for various infectious and inflammatory diseases including cancer. Even, Single nucleotide polymorphisms (SNPs) among various TLR genes have been identified among the different human population and their association with susceptibility/resistance to certain infections and other inflammatory diseases. Thus, in the present review the current and future importance of TLRs in immunity, their pattern of expression among various immune cells along with TLR based therapeutic approach is reviewed.

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TLRs are first described PRRs that revolutionized the biology of host-pathogen interaction and immune response

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The discovery of different TLRs in humans proved milestone in the field of innate immunity and inflammation

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The pattern of expression of all the TLRs expressed by human immune cells

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An association of various TLR SNPs with different inflammatory diseases

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Currently available drugs or vaccines based on TLRs and their future in drug targeting along with the role in reproduction, and regeneration

Systemic Inflammatory Response Syndrome After Major Abdominal Surgery Predicted by Early Upregulation of TLR4 and TLR5

OBJECTIVES

To study innate immune pathways in patients undergoing hepatopancreaticobiliary surgery to understand mechanisms leading to enhanced inflammatory responses and identifying biomarkers of adverse clinical consequences.

BACKGROUND

Patients undergoing major abdominal surgery are at risk of life-threatening systemic inflammatory response syndrome (SIRS) and sepsis. Early identification of at-risk patients would allow tailored postoperative care and improve survival.

METHODS

Two separate cohorts of patients undergoing major hepatopancreaticobiliary surgery were studied (combined n = 69). Bloods were taken preoperatively, on day 1 and day 2 postoperatively. Peripheral blood mononuclear cells and serum were separated and immune phenotype and function assessed ex vivo.

RESULTS

Early innate immune dysfunction was evident in 12 patients who subsequently developed SIRS (postoperative day 6) compared with 27 who did not, when no clinical evidence of SIRS was apparent (preoperatively or days 1 and 2). Serum interleukin (IL)-6 concentration and monocyte Toll-like receptor (TLR)/NF-κB/IL-6 functional pathways were significantly upregulated and overactive in patients who developed SIRS (P < 0.0001). Interferon α-mediated STAT1 phosphorylation was higher preoperatively in patients who developed SIRS. Increased TLR4 and TLR5 gene expression in whole blood was demonstrated in a separate validation cohort of 30 patients undergoing similar surgery. Expression of TLR4/5 on monocytes, particularly intermediate CD14CD16 monocytes, on day 1 or 2 predicted SIRS with accuracy 0.89 to 1.0 (areas under receiver operator curves).

CONCLUSIONS

These data demonstrate the mechanism for IL-6 overproduction in patients who develop postoperative SIRS and identify markers that predict patients at risk of SIRS 5 days before the onset of clinical signs.

Functional conservation of an ancestral Pellino protein in helminth species

The immune system of H. sapiens has innate signaling pathways that arose in ancestral species. This is exemplified by the discovery of the Toll-like receptor (TLR) pathway using free-living model organisms such as Drosophila melanogaster. The TLR pathway is ubiquitous and controls sensitivity to pathogen-associated molecular patterns (PAMPs) in eukaryotes. There is, however, a marked absence of this pathway from the plathyhelminthes, with the exception of the Pellino protein family, which is present in a number of species from this phylum. Helminth Pellino proteins are conserved having high similarity, both at the sequence and predicted structural protein level, with that of human Pellino proteins. Pellino from a model helminth, Schistosoma mansoni Pellino (SmPellino), was shown to bind and poly-ubiquitinate human IRAK-1, displaying E3 ligase activity consistent with its human counterparts. When transfected into human cells SmPellino is functional, interacting with signaling proteins and modulating mammalian signaling pathways. Strict conservation of a protein family in species lacking its niche signalling pathway is rare and provides a platform to examine the ancestral functions of Pellino proteins that may translate into novel mechanisms of immune regulation in humans.

Structural biology of the IL-1 superfamily: key cytokines in the regulation of immune and inflammatory responses

Interleukin-1 superfamily of cytokines (IL-1, IL-18, IL-33) play key roles in inflammation and regulating immunity. The mechanisms of agonism and antagonism in the IL-1 superfamily have been pursued by structural biologists for nearly 20 years. New insights into these mechanisms were recently provided by the crystal structures of the ternary complexes of IL-1β and its receptors. We will review here the structural biology related to receptor recognition by IL-1 superfamily cytokines and the regulation of its cytokine activities by antagonists.

Cytokine components and mucosal immunity in the oviduct of Xenopus laevis (amphibia, pipidae)

Most studies on the mucosal immunity in female reproductive tissues have been performed in mammals. In all species, apart from their reproductive strategies, immunity in the genital mucosa is required to defend the host against luminal pathogens. In this study we investigated the role of the innate immunity of the oviductal mucosa of Xenopus laevis, an amphibian characterized by external fertilization. In particular we examined the expression and localization of Interleukin-1β (IL1B), Macrophage migration inhibitory factor (MIF) and Interleukin-1 receptor type 1 (IL1R1) in different oviductal portions including an upper glandular region, an intermediate and a lower aglandular region (the ovisac). Tissues were examined by immunohistochemistry and western blot using polyclonal antibodies against human molecules. IL1B, MIF and IL1R1 were all shown in the three oviductal regions examined, albeit with a general increase towards the external environment. A substantial difference among the cytokine components was also observed mainly in the epithelium of the glandular and intermediate regions. Specifically, all three molecules were expressed by the luminal ciliated cells while only IL1R1 was present in the unciliated cells at the bottom of the epithelial ingrowths. The expression of IL1R1 in these cells appeared as a continuous layer separating the epithelium from the underlying tissues. While supporting the role of the innate immune system for host's defense against pathogens, the peculiar distribution of the cytokine components in the oviduct of X. laevis suggests novel immunologic strategies useful to assure gland secretion essential for egg formation and fertilization.

Differential gene expression during the moult cycle of Antarctic krill (Euphausia superba)

Background

All crustaceans periodically moult to renew their exoskeleton. In krill this involves partial digestion and resorption of the old exoskeleton and synthesis of new cuticle. Molecular events that underlie the moult cycle are poorly understood in calcifying crustaceans and even less so in non-calcifying organisms such as krill. To address this we constructed an Antarctic krill cDNA microarray in order to generate gene expression profiles across the moult cycle and identify possible activation pathways.

Results

A total of 26 different cuticle genes were identified that showed differential gene expression across the moult cycle. Almost all cuticle genes were up regulated during premoult and down regulated during late intermoult. There were a number of transcripts with significant sequence homology to genes potentially involved in the synthesis, breakdown and resorption of chitin. During early premoult glutamine synthetase, a gene involved in generating an amino acid used in the synthesis of glucosamine, a constituent of chitin, was up regulated more than twofold. Mannosyltransferase 1, a member of the glycosyltransferase family of enzymes that includes chitin synthase was also up regulated during early premoult. Transcripts homologous to a β-N-acetylglucosaminidase (β-NAGase) precursor were expressed at a higher level during late intermoult (prior to apolysis) than during premoult. This observation coincided with the up regulation during late intermoult, of a coatomer subunit epsilon involved in the production of vesicles that maybe used to transport the β-NAGase precursors into the exuvial cleft. Trypsin, known to activate the β-NAGase precursor, was up regulated more than fourfold during premoult. The up regulation of a predicted oligopeptide transporter during premoult may allow the transport of chitin breakdown products across the newly synthesised epi- and exocuticle layers.

Conclusion

We have identified many genes differentially expressed across the moult cycle of krill that correspond with known phenotypic structural changes. This study has provided a better understanding of the processes involved in krill moulting and how they may be controlled at the gene expression level.

Drosophila melanogaster as a model host for the Burkholderia cepacia complex

Background

Colonization with bacterial species from the Burkholderia cepacia complex (Bcc) is associated with fast health decline among individuals with cystic fibrosis. In order to investigate the virulence of the Bcc, several alternative infection models have been developed. To this end, the fruit fly is increasingly used as surrogate host, and its validity to enhance our understanding of host-pathogen relationships has been demonstrated with a variety of microorganisms. Moreover, its relevance as a suitable alternative to mammalian hosts has been confirmed with vertebrate organisms.

Methodology/Principal Findings

The aim of this study was to establish Drosophila melanogaster as a surrogate host for species from the Bcc. While the feeding method proved unsuccessful at killing the flies, the pricking technique did generate mortality within the populations. Results obtained with the fruit fly model are comparable with results obtained using mammalian infection models. Furthermore, validity of the Drosophila infection model was confirmed with B. cenocepacia K56-2 mutants known to be less virulent in murine hosts or in other alternative models. Competitive index (CI) analyses were also performed using the fruit fly as host. Results of CI experiments agree with those obtained with mammalian models.

Conclusions/Significance

We conclude that Drosophila is a useful alternative infection model for Bcc and that fly pricking assays and competition indices are two complementary methods for virulence testing. Moreover, CI results indicate that this method is more sensitive than mortality tests.

The role of epithelial integrin receptors in recognition of pulmonary pathogens

Integrins are a large family of heterodimeric transmembrane cell adhesion receptors. During the last decade, it has become clear that integrins significantly participate in various host-pathogen interactions involving pathogenic bacteria, fungi, and viruses. Many bacteria possess adhesins that can bind either directly or indirectly to integrins. However, there appears to be an emerging role for integrins beyond simply adhesion molecules. Given the conserved nature of integrin structure and function, and the diversity of the pathogens which use integrins, it appears that they may act as pattern recognition receptors important for the innate immune response. Several clinically significant bacterial pathogens target lung epithelial integrins, and this review will focus on exploring various structures and mechanisms involved in these interactions.

Beta-secretase: structure, function, and evolution

The most popular current hypothesis is that Alzheimer's disease (AD) is caused by aggregates of the amyloid peptide (Abeta), which is generated by cleavage of the Abeta protein precursor (APP) by beta-secretase (BACE-1) followed by gamma-secretase. BACE-1 cleavage is limiting for the production of Abeta, making it a particularly good drug target for the generation of inhibitors that lower Abeta. A landmark discovery in AD was the identification of BACE-1 (a.k.a. Memapsin-2) as a novel class of type I transmembrane aspartic protease. Although BACE-2, a homologue of BACE-1, was quickly identified, follow up studies using knockout mice demonstrated that BACE-1 was necessary and sufficient for most neuronal Abeta generation. Despite the importance of BACE-1 as a drug target, development has been slow due to the incomplete understanding of its function and regulation and the difficulties in developing a brain penetrant drug that can specifically block its large catalytic pocket. This review summarizes the biological properties of BACE-1 and attempts to use phylogenetic perspectives to understand its function. The article also addresses the challenges in discovering a selective drug-like molecule targeting novel mechanisms of BACE-1 regulation.

The interleukin-1 receptor family

The cytokines IL-1 and IL-18 are key molecules both in the innate and in the adaptive immune response. Their activity is mediated by specific receptors present on the membrane of target cells. It has become apparent that these receptors are members of a larger family of related receptors, most of which are apparently involved in the mechanisms of host defense. Thus, the large Toll/IL-1R (TIR) superfamily encompasses the Ig domain family (IL-1 receptors, IL-18 receptors, and IL-1R-like receptors), the leucine-rich domain family [the Toll-like receptors (TLR) and similar receptors], and a series of TIR domain-containing intracellular adapter molecules. The TIR superfamily is defined by a common intracellular TIR domain, involved in the initiation of signaling. A group of TIR domain-containing adapters (MyD88, TIRAP, TRIF, and TRAM) are differentially recruited to the Toll/IL-1 receptors, contributing to the specificity of signaling. Recent studies have also begun to unravel the mechanisms of negative regulation of the Toll/IL-1 receptors. The orphan receptor TIR8/SIGIRR, a member of TIR superfamily, while unable to initiate signaling, can negatively modulate the TIR-mediated responses. Other negative regulators of the Toll/IL-1R family include T1/ST2, some soluble forms of TLR, and MyD88s. The coordinated positive and negative regulation of the TIR activation ensures the appropriate modulation of the innate and inflammatory responses and avoids the risk of pathological derangement. This chapter will consider in detail the characteristics and functional role of the Ig domain receptor subfamily in the regulation of host defense and their possible role in pathology.

Identifying components of the NF-kappaB pathway in the beneficial Euprymna scolopes-Vibrio fischeri light organ symbiosis

The Toll/NF-kappaB pathway is a common, evolutionarily conserved innate immune pathway that modulates the responses of animal cells to microbe-associated molecular patterns (MAMPs). Because MAMPs have been implicated as critical elements in the signaling of symbiont-induced development, an expressed sequence tag library from the juvenile light organ of Euprymna scolopes was used to identify members of the Toll/NF-kappaB pathway. Full-length transcripts were identified by using 5' and 3' RACE PCR. Seven transcripts critical for MAMP-induced triggering of the Toll/NF-kappaB phosphorylation cascade have been identified, including receptors, signal transducers, and a transcription factor. Further investigations should elucidate the role of the Toll/NF-kappaB pathway in the initiation of the beneficial symbiosis between E. scolopes and Vibrio fischeri.

Toll-like receptor mRNA expression patterns in human dendritic cells and monocytes

The innate immune system recognises a wide spectrum of pathogens without a need for prior exposure. The main cells responsible are monocytes, macrophages, dendritic cells (DC) and neutrophils phagocytose microbial pathogens triggering a cytokine network resulting in the development of inflammatory and specific immune responses. Findings in the Toll-like receptor (TLR) family, initially discovered in Drosophila, further elucidated these processes. Toll-like receptors induce activation of an innate immune response and at present ten TLRs have been identified, named TLRs 1-10. In addition to the ignition of the innate immune response, evidence implicates the TLR family in a spectrum of systemic disorders following bacterial infections including sepsis and multiple organ failure, and can be detrimental, leading to tissue injury. In this project, our main goal was to investigate the effects of a TLR4 ligand, lipolysaccharide (LPS) in human DC and monocytes. Our hypothesis is that different professional APCs, express different mRNA TLR transcripts. Our findings indicate that TLR expression patterns change in relation to the pathogen involved and in the case of DC, and the maturation stage the latter are upon challenging. Our results and interpretation showed significant alteration of transcript expression patterns upon LPS challenge in all cell subsets, with DC subsets expressing different TLR mRNA patterns as they go through different maturation stages.

Type I interferons and the innate immune response--more than just antiviral cytokines

The role of type I interferon (referred to as IFN in this review) in early antiviral immunity is well known. More recently IFN has been shown to be a potent regulator of adaptive immunity. It is now becoming clear that a broad range of viruses, bacteria and even parasites express ligands capable of stimulating a growing number of signalling pathways that results in, often subtype specific, induction of IFN. Of particular interest are the signalling pathways associated with the Toll-like receptors. This family of receptors, each able to induce signals in response to a variety of ligands, initiates the pro-inflammatory response. They also contain members that have the capacity to induce IFN, making use of, and perhaps promoting the evolution of its pleiotropic responses. Greater knowledge of the events that result in induction of IFN is necessary in understanding the specificity of expression of an increasingly complex and important aspect of our immune system. This may reveal to us further therapeutic opportunities, either in the use of IFN or in the manipulation of their expression. This review details the established knowledge and recent advances made in understanding how and under what circumstances the IFNs are expressed, starting with brief overviews of IFN and Toll-like receptors before following the molecular processes from induction of IFN, activation of the JAK-STAT pathway and finally the expression of interferon stimulated genes and their functions.

Cg-Rel, the first Rel/NF-kappaB homolog characterized in a mollusk, the Pacific oyster Crassostrea gigas

We report here the identification and functional characterization of Cg-Rel, a gene encoding the Crassostrea gigas homolog of Rel/NF-kappaB transcription factors found in insects and mammals. Sequence and phylogenetic analysis showed that Cg-Rel shares the structural organization of Rel/NF-kappaB transcription factors of class II. It includes a Rel homology domain as well as a C-terminal transactivation domain (TD). Overexpression of Cg-Rel in the Drosophila S2 cell line activated the expression of a NF-kappaB-dependent reporter gene, whereas transfection with a Cg-Rel construct containing a C-terminal deletion of the TD or using a reporter gene with mutated kappaB binding sites failed to activate expression. These results suggest that Cg-Rel is a functional member of the Rel family of transcription factors, making this the sixth structurally homologous component of the Rel/NF-kappaB pathway characterized in C. gigas. Based on homology to other invertebrates' Rel/NF-kappaB cascade, the function of the oyster pathway may serve to regulate genes involved in innate defense and/or development. These findings serve to highlight a potentially important regulatory pathway to the study of oyster immunology, hence allowing comparison of the immune system in vertebrates and invertebrates, an important key issue to understand its evolution.

Characterization of Pellino2, a substrate of IRAK1 and IRAK4

Interleukin-1 (IL-1) receptor-associated kinases (IRAKs) are central components of Toll/IL-1 receptor (TIR) signaling pathways. In an attempt to discover novel signal transducers in TIR signaling, we identified human Pellino2 as an interaction partner of IRAK4. Pellino2 interacts with kinase-active as well as kinase-inactive IRAK1 and IRAK4. Furthermore, Pellino2 is one of the first substrates identified for IRAK1 and IRAK4. Functional studies using overexpression or RNAi knock-down of Pellino2 suggest a role of Pellino2 as a scaffolding protein similar to Pellino1. However, unlike Pellino1, Pellino2 does not seem to activate a specific transcription factor, but links TIR signaling to basic cellular processes.

In silico identification of components of the Toll-like receptor (TLR) signaling pathway in clustered chicken expressed sequence tags (ESTs)

We have described a bioinformatic approach that involves the clustering of expressed sequence tags (ESTs) to reveal homologs of the Toll-like receptor (TLR) pathway in the chicken. Homology searching of proteins, predicted to be encoded by these EST clusters, resulted in the in silico identification of full-length sequences for Toll-interacting protein (Tollip), IL-1 receptor-associated kinase 4 (IRAK-4), myeloid differentiation factor 88 adapter-like (Mal), TGF beta-activated kinase 1 binding protein 1 (TAB1). We also determined partial sequence information for myeloid differentiation factor 88 (MyD88), two novel TLRs, TNF receptor-associated factor 6 (TRAF6), TGF beta-activated kinase 1 (TAK1), TAB2, inhibitor of nuclear factor kappa B kinase alpha (IKK alpha) and IKK beta. This bioinformatics study has confirmed the evolutionary conservation of the TLR pathway in chicken and demonstrated its essential homology to the TLR pathway in mammals. We have identified in silico the full-length sequence for liver-expressed antimicrobial peptide 2 (LEAP-2). This is the first time a non-mammalian LEAP-2 has been described.

Prediction of the prototype of the human Toll-like receptor gene family from the pufferfish, Fugu rubripes, genome

The insect Toll family of proteins and their mammalian counterparts seemingly shared one common ancestor and evolved independently. Here we demonstrated that the prototype of the mammalian-type (M-type) Toll family is shared by the fish and humans. According to the draft of the pufferfish Fugu genome project, the signature Toll-IL-1 receptor homology domain (TIR domain) has been conserved during evolution. FuguTLR2, 3, 5, 7, 8 and 9 members correspond structurally to respective mammalian TLRs. One Fugu TLR showed equally high amino acid identity to human TLR1, 6 and 10, and we named it FuguTLR1. Fugu rubripes has genes for TLR21 and 22, which are unique to fish. One possible interpretation of these findings is that TLR1, 2, 3, 4, 5, 7, 8, 9, 21 and 22 existed in the ancestral genome common to fish and mammals, and that TLR4 was lost in the fish lineage, while TLR21 and 22 were lost in the mammalian lineage. Strikingly, a solitary ascidian, Halocynthia roretzi, has only a few Toll-like proteins, which, like Caenorhabditis elegans Toll, represent primitive ones before the expansion of the Toll family. Therefore, the expansion of TLR genes should have occurred earlier than fish, but not C. intestinalis, separated evolutionarily from mammals. These results infer that the appearance of the M-type innate system was completed before or concomitant with the appearance of acquired immunity. We interpret the present data to mean that the differences of TLRs identified in this study between fishes and humans may be rather peripheral, partially due to selection pressure exerted by pathogens in distinct environments.

Cross-species infections and their analysis

The ability of certain pathogens to infect multiple hosts has led to the development of genetically tractable nonvertebrate hosts to elucidate the molecular mechanisms of interactions between these pathogens and their hosts. The use of plant, insect, nematode, and protozoan hosts to study human pathogens has facilitated the elucidation of molecular nature of pathogenesis and host responses. Analyses of virulence of multihost pathogens on their respective hosts revealed that pathogens utilize many universal offensive strategies to overcome host defenses, irrespective of the evolutionary lineage of the host. Likewise, genetic dissections of the defense response of the nonvertebrate hosts have also shown that key features underlying host defense responses are highly conserved. This review summarizes how the information gained from the analysis of cross-species infections contributes to our understanding of host-pathogen interactions.

Making and breaking tolerance

The lymphocyte's decision between tolerance and immunity/autoimmunity is regulated at many levels. Two important parameters in this decision are the maturation state of the antigen presenting cells (APCs) and the amount of self antigen that is detected by the immune system. Maturation of APCs occurs as a consequence of signals received by the innate immune system and may lead to the breakdown of tolerance. Particularly relevant to this process are the Toll-like receptors and mechanisms of cross presentation of self antigens. In addition, genetic alterations in a variety of cell surface receptors, signalling components and regulators of apoptosis/survival can break tolerance and lead to autoimmunity in vivo.

Summary and comparison of the signaling mechanisms of the Toll/interleukin-1 receptor family

The Toll/interleukin-1 (IL-1) receptor (TIR) family comprises two groups of transmembrane proteins, which share functional and structural properties. The members of the IL-1 receptor (IL-1R) subfamily are characterized by three extracellular immunoglobulin (Ig)-like domains. They form heterodimeric signaling receptor complexes consisting of receptor and accessory proteins. The members of the Toll-like receptor (TLR) subfamily recognize alarm signals that can be derived either from pathogens or the host itself. TLRs possess leucine-rich repeats in their extracellular part. TLRs can form dimeric receptor complexes consisting of two different TLRs or homodimers in the case of TLR4. The TLR4 receptor complex requires supportive molecules for optimal response to its ligand lipopolysaccharide (LPS). A hallmark of the TIR family is the cytoplasmic TIR domain that is indispensable for signal transduction. The TIR domain serves as a scaffold for a series of protein-protein interactions which result in the activation of a unique signaling module consisting of MyD88, interleukin-1 receptor associated kinase (IRAK) family members and Tollip, which is used exclusively by TIR family members. Subsequently, several central signaling pathways are activated in parallel, the activation of NFkappaB being the most prominent event of the inflammatory response. Recent developments indicate that in addition to the common signaling module MyD88/IRAK/Tollip, other molecules can modulate signaling by TLRs, especially of TLR4, resulting in differential biological answers to distinct pathogenic structures. Subtle differences in TLR signaling pathways are now becoming apparent, which reveal how the innate immune system decides at a very early stage the direction in which the adaptive immune response will develop. The creation of pathogen-specific mediator environments by dendritic cells defines whether a cellular or humoral response will be activated in response to the pathogen.

Involvement of TIRAP/MAL in signaling for the activation of interferon regulatory factor 3 by lipopolysaccharide

Infections of bacteria and viruses induce host defense reactions known as innate responses that include the production of cytokines and chemokines. The production of type I interferon (IFN) is known to be induced by viral double-stranded (ds) RNA or bacterial lipopolysaccharide (LPS). Although important functions for the transcription factors NF-kappaB and interferon regulatory factor-3 (IRF-3) are indicated, the molecular signals leading to the activation of IFN genes have yet to be elucidated. We provide several lines of evidence that LPS and dsRNA trigger distinct intracellular signals upstream. Notably, our investigation revealed a critical function for TIRAP/MAL, a signaling adapter for Toll-like receptor (TLR) 4, in LPS-induced but not dsRNA-induced activation of IRF-3. These results highlight cross-talk between TLR-mediated and virus/dsRNA-induced signals resulting in activation of the IFN system.

IRAK-4: a novel member of the IRAK family with the properties of an IRAK-kinase

Toll/IL-1 receptor family members are central components of host defense mechanisms in a variety of species. One well conserved element in their signal transduction is Ser/Thr kinases, which couple early signaling events in a receptor complex at the plasma membrane to larger signalosomes in the cytosol. The fruit fly Drosophila melanogaster has one member of this family of kinases, termed Pelle. The complexity of this pathway is vastly increased in vertebrates, and several Pelle homologs have been described and termed IL-1 receptor-associated kinase (IRAK). Here we report the identification of a novel and distinct member of the IRAK family, IRAK-4. IRAK-4 is the closest human homolog to Pelle. Endogenous IRAK-4 interacts with IRAK-1 and TRAF6 in an IL-1-dependent manner, and overexpression of IRAK-4 can activate NF-kappa B as well as mitogen-activated protein (MAP) kinase pathways. Most strikingly, and in contrast to the other IRAKs, IRAK-4 depends on its kinase activity to activate NF-kappa B. In addition, IRAK-4 is able to phosphorylate IRAK-1, and overexpression of dominant-negative IRAK-4 is blocking the IL-1-induced activation and modification of IRAK-1, suggesting a role of IRAK-4 as a central element in the early signal transduction of Toll/IL-1 receptors, upstream of IRAK-1.

Tackling both sides of the host-pathogen equation with Caenorhabditis elegans

If one is interested in dissecting the complex interactions that exist between host and pathogen, the nematode worm Caenorhabditis elegans is perhaps not the first model host that comes to mind. In this review I will introduce 'the worm' and try to show how it is, in fact, well suited to the identification of universal virulence factors and holds great promise for the study of conserved mechanisms of innate immunity.

Evolution of effectors and receptors of innate immunity

The bony fishes are derived from one of the earliest divergent vertebrate lineages to have both innate and acquired immune systems. They are considered by some to be an ideal model to study the underpinnings of immune systems precisely because of their phylogenetic position and the fact that their adaptive immune systems have not been elaborated to the extent seen in mammals. By the same token, examination of innate immune systems in invertebrates and early chordates can provide insight into how homologous systems operate in fish and higher vertebrates. Herein, we provide an overview of the molecular evidence that we hope helps clarify the evolutionary relationships of innate immune molecules identified in bony fishes. The innate immune systems being considered include select chemokines (CC and CXC chemokines and their receptors), cytokines (IL-1, IL-8, interferons, TGF-beta, TNF-alpha), acute phase proteins (SAA, SAP, CRP, alpha2M, and the complement components--C3-C9, MASP, MBL, Bf), NK cell receptors, and molecules upstream and downstream of the Toll signaling pathways.