Self-defense: the fruit fly style

Dendritic cells in hepatitis and liver transplantation

Dendritic cells (DCs) play a key role in innate immune responses and are also the most effective cells for the activation of T cell immunity. They acquire antigen and process it; then they display it on the cell surface bound in a noncovalent complex with human leukocyte antigen molecules of class I (human leukocyte antigens A, B, and C) and class II (human leukocyte antigen DR). These cells are subdivided into 3 main subsets: 2 called myeloid dendritic cells (mDC) or classical DCs of types 1 and 2, and 1 called plasmacytoid dendritic cells (pDCs). The mDCs process and present antigen while the pDCs also strongly respond to viral signals by secreting type 1 interferon. In the liver, all of these subsets are present. However, their relative abundance is different from the peripheral blood, and it is further modified by liver disease. It appears that in relation to T cell tolerance, both mDCs and pDCs are influenced by the liver milieu and promote T cell inactivation. However, in antiviral responses and in ischemia/reperfusion injury, where innate immune functions are more important, mDCs and pDCs have distinct roles. Liver Transplantation 23 1433-1439 2017 AASLD.

Co-operative induction of pro-inflammatory signaling by Toll-like receptors

Toll-like receptors (TLRs) mediate detection of a broad range of pathogens and pathogen-derived products including LPS, peptidoglycan, bacterial lipopeptides, and lipoteichoic acid. Recent evidence indicates that the broad specificity of TLRs may be a consequence of the interactions between different TLRs. In this report, we demonstrate that while a constitutively active TLR4 homodimer can induce the production of pro-inflammatory cytokines, homodimers of TLR2 and TLR6 cannot. However, when co-expressed in the same cell, constitutively active TLR2 and TLR6 strongly induce cytokine production, indicating that these TLRs require partners to productively signal. Since TLR4 signals as a homodimer, while TLR2 and TLR6 do not, it is clear that, despite the conservation of their cytoplasmic signaling domains, the mechanisms by which they initiate signaling are different. We have localized the region of TLR4 that mediates its ability to signal as a homodimer to the membrane-proximal half of the cytoplasmic tail of the receptor.

U1-RNP and Toll-like receptors in the pathogenesis of mixed connective tissue diseasePart II. Endosomal TLRs and their biological significance in the pathogenesis of mixed connective tissue disease

Mixed connective tissue disease (MCTD) is a chronic autoimmune immunopathological disease of unknown etiology, which is characterized by the presence of various clinical symptoms and the presence of autoantibodies against U1-RNP particles. The U1-RNP component engages immune cells and their receptors in a complex network of interactions that ultimately lead to autoimmunity, inflammation, and tissue injury. The anti-U1-RNP autoantibodies form an immune complex with self-RNA, present in MCTD serum, which can act as endosomal Toll-like receptor (TLR) ligands. Inhibition of TLRs by nucleic acids is a promising area of research for the development of novel therapeutic strategies against pathogenic infection, tumorigenesis and autoimmunity. In this review we summarize current knowledge of endogenous TLRs and discuss their biological significance in the pathogenesis of MCTD. In part I we described the structure, biological function and significance of the U1-RNP complex in MCTD.

Prevalence and clinical significance of sterile intra-amniotic inflammation in patients with preterm labor and intact membranes

PROBLEM

Inflammation and infection play a major role in preterm birth. The purpose of this study was to (i) determine the prevalence and clinical significance of sterile intra-amniotic inflammation and (ii) examine the relationship between amniotic fluid (AF) concentrations of high mobility group box-1 (HMGB1) and the interval from amniocentesis to delivery in patients with sterile intra-amniotic inflammation.

METHOD OF STUDY

AF samples obtained from 135 women with preterm labor and intact membranes were analyzed using cultivation techniques as well as broad-range PCR and mass spectrometry (PCR/ESI-MS). Sterile intra-amniotic inflammation was defined when patients with negative AF cultures and without evidence of microbial footprints had intra-amniotic inflammation (AF interleukin-6 ≥ 2.6 ng/mL).

RESULTS

(i) The frequency of sterile intra-amniotic inflammation was significantly greater than that of microbial-associated intra-amniotic inflammation [26% (35/135) versus 11% (15/135); (P = 0.005)], (ii) patients with sterile intra-amniotic inflammation delivered at comparable gestational ages had similar rates of acute placental inflammation and adverse neonatal outcomes as patients with microbial-associated intra-amniotic inflammation, and (iii) patients with sterile intra-amniotic inflammation and high AF concentrations of HMGB1 (≥8.55 ng/mL) delivered earlier than those with low AF concentrations of HMGB1 (P = 0.02).

CONCLUSION

(i) Sterile intra-amniotic inflammation is more frequent than microbial-associated intra-amniotic inflammation, and (ii) we propose that danger signals participate in sterile intra-amniotic inflammation in the setting of preterm labor.

Gene expression specificity of the mussel antifungal mytimycin (MytM)

We previously reported the nucleotide sequences and diversity of mytimycin (MytM) from the Mediterranean mussel, Mytilus galloprovincialis. Using real-time PCR (q-PCR), we observed that the MytM gene was mainly expressed in circulating hemocytes and to a less extent in the mantle. In vivo challenge with bacteria or with the yeast, Candida albicans, did not increase the expression as measured by q-PCR in hemocytes. By contrast, injection of the filamentous fungus, Fusarium oxysporum, induced a sudden and strong increase of expression at 9h p.i. (stimulation index of 25.7 ± 2.1). Optimum stimulating dose was 10(4) spores of F. oxysporum per mussel. In the same samples, AMP mytilin and myticin showed no stimulation. Consequently, we hypothesized the existence of 2 different signal transduction pathways, one activated by bacteria and yeast, the other triggered by filamentous fungi. A second challenge performed with F. oxysporum 24 h after the first challenge induced an increase of MytM gene expression (stimulation index of 3.5 ± 1.7). However, this second increase was significantly lower than the first, suggesting less efficient response rather than significant protection.

The role of innate immune-stimulated epithelial apoptosis during gastrointestinal inflammatory diseases

The maintenance of mucosal barrier equilibrium in the intestine requires a delicate and dynamic balance between enterocyte loss by apoptosis and the generation of new cells by proliferation from stem cell precursors at the base of the intestinal crypts. When the balance shifts towards either excessive or insufficient apoptosis, a broad range of gastrointestinal diseases can manifest. Recent work from a variety of laboratories has provided evidence in support of a role for receptors of the innate immune system, including Toll-like receptors 2, 4, and 9 as well as the intracellular pathogen recognition receptor NOD2/CARD15, in the initiation of enterocyte apoptosis. The subsequent induction of enterocyte apoptosis in response to the activation of these innate immune receptors plays a key role in the development of various intestinal diseases, including necrotizing enterocolitis, Crohn's disease, ulcerative colitis, and intestinal cancer. This review will detail the regulatory pathways that govern enterocyte apoptosis, and will explore the role of the innate immune system in the induction of enterocyte apoptosis in gastrointestinal disease.

Insights into the role of Toll-like receptors in modulation of T cell responses

The innate immune receptors, such as Toll-like receptors (TLRs), are intimately involved in the early sensing of invading microorganisms or their structural components. Engagement of TLRs with their ligands results in activation of several downstream intracellular pathways leading to activation of innate and adaptive immune system cells. It was initially thought that TLRs are primarily expressed by antigen-presenting cells (APCs), such as macrophages and dendritic cells, and that interactions between microbial ligands and TLRs in these cells will indirectly result in activation of cells of the adaptive immune system, especially T cells. However, it has now become evident that TLRs are also expressed by various T cell subsets, such as conventional αβT cells, regulatory T cells, and γδT cells as well as natural killer T cells. Importantly, it appears that at least in some of these T cell subsets, TLRs are functionally active, because stimulation of these cells with TLR agonists in the absence of APCs results in exertion of effector or regulatory functions of T cells. The present review attempts to summarize the recent findings related to TLR expression in different T cell subsets and the direct role of TLRs in the induction and regulation of T cell responses, including those responses that occur at mucosal surfaces. In addition, the potential use of TLR agonists for steering T cell responses as a prophylactic or therapeutic strategy in the context of infectious, allergic or autoimmune diseases is explored.

NRAMP genes function in Arabidopsis thaliana resistance to Erwinia chrysanthemi infection

AtNRAMP3 and AtNRAMP4 are two Arabidopsis metal transporters sharing about 50% sequence identity with mouse NRAMP1. The NRAMP1/Slc11A1 metal ion transporter plays a crucial role in the innate immunity of animal macrophages targeted by intracellular bacterial pathogens. AtNRAMP3 and AtNRAMP4 localize to the vacuolar membrane. We found that AtNRAMP3 is upregulated in leaves challenged with the bacterial pathogens Pseudomonas syringae and Erwinia chrysanthemi, whereas AtNRAMP4 expression is not modified. Using single and double nramp3 and nramp4 mutants, as well as lines ectopically expressing either of these genes, we show that AtNRAMP3 and, to a lesser extent, AtNRAMP4 are involved in Arabidopsis thaliana resistance against the bacterial pathogen E. chrysanthemi. The susceptibility of the double nramp3 nramp4 mutant is associated with the reduced accumulation of reactive oxygen species and ferritin (AtFER1), an iron storage protein known to participate in A. thaliana defense. Interestingly, roots from infected plants accumulated transcripts of AtNRAMP3 as well as the iron-deficiency markers IRT1 and FRO2. This finding suggests the existence of a shoot-to-root signal reminiscent of an iron-deficiency signal activated by pathogen infection. Our data indicate that the functions of NRAMP proteins in innate immunity have been conserved between animals and plants.

Systems biology approaches for the study of multiple sclerosis

Multiple sclerosis (MS) is a progressive neurological disease caused by an autoimmune attack to the central nervous system (CNS). MS is thought to result from a complex interaction between genetic and environmental factors. In this review we analyze the contribution of genomics, trancriptomics and proteomics in delineating these factors, as well as their utility for the monitoring of disease progression, the identification of new targets for therapeutic intervention and the early detection of individuals at risk.

Improving defences at the portal of HIV entry: mucosal and innate immunity

The authors discuss the key scientific issues in mucosal and innate immunity related to immune protection against HIV infection.

The science of adjuvants

Adjuvants are substances that boost the immunogenicity of vaccines. However, most successful vaccines have been derived empirically and are capable of inducing robust T- and B-cell immunity without any adjuvant additives. Emerging evidence suggests that such live vaccines induce innate immune activation via a range of stimuli, including ligands specific for Toll-like receptors, which, in effect, serve as their own adjuvants. In contrast to these live vaccines, subunit vaccines need to be supplemented with adjuvants to boost their immunogenicity. However, there is a paucity of licensed adjuvants for clinical use and, thus, there is a critical need to develop safe and effective adjuvants. In this context, recent advances in innate immunity are beginning to offer new insights into how empiric vaccines and adjuvants mediate their efficacy. In this article, we review the latest progress and emerging concepts in adjuvant development, which includes novel findings in innate immune biology and their impact on vaccinology.

The effect of 17beta-estradiol on IL-6 secretion and NF-kappaB DNA-binding activity in human retinal pigment epithelial cells

Toll-like receptors (TLRs) and inflammatory cascades participate in the pathology of age-related macular degeneration (AMD). The effect of estrogens on the development of AMD is poorly understood, although many studies indicate that these compounds can modulate inflammatory responses. In this study, we investigated the regulatory role of TLR agonists and 17beta-estradiol (E(2)) on IL-6 expression and NF-kappaB DNA-binding activity in human retinal pigment epithelial cells (ARPE-19). The inflammatory response of ARPE-19 cells to various TLR agonists, e.g. Pam, zymosan, flagellin, SLTA and lipopolysaccharide (LPS) exposures were examined via the secretion of IL-6 cytokine as analyzed by ELISA. In addition, the IL-6 responses to the estrogen-receptor agonist, E(2), and to the estrogen-receptor antagonist ICI 182.780 as well as to the NF-kappaB inhibitor helenalin were compared. The DNA-binding activity of NF-kappaB transcription factor of nuclear cell extracts was analyzed by the gel mobility shift assay (EMSA). TLR4 gene expression was studied by quantitave PCR. The TLR4 agonist, LPS, caused a clear IL-6 response that was attenuated by E(2) in ARPE-19-cells. The anti-inflammatory properties of E(2) were mediated through estrogen receptors and were associated with decreased NF-kappaB DNA-binding activity. The level of TLR4 gene expression was not affected by LPS exposure. Our results indicate that IL-6 expression is regulated through NF-kappaB transcription factor and stereoid-receptor signalling pathways in ARPE-19 cells.

THE IN VIVO ANTITUMORAL EFFECTS OF LIPOPOLYSACCHARIDE AGAINST GLIOBLASTOMA MULTIFORME ARE MEDIATED IN PART BY TOLL-LIKE RECEPTOR 4

OBJECTIVE

Toll-like receptor 4 (Tlr-4) mediates many biological effects of lipopolysaccharide (LPS), which has antitumoral effects on glioblastoma both in vivo and in vitro. However, the precise role of Tlr-4 in these antitumoral effects remains unknown.

METHODS

The role of Tlr-4 in the antitumoral effect of LPS on glioblastomas was assessed in wild-type BALB/c mice and in Tlr-4 knockout (KO) BALB/c mice. Mice were implanted with DBT glioblastoma cells intracranially or subcutaneously, were treated with intratumoral LPS, and were assessed by histopathological examination for degrees of tumor progression and inflammation. Flow cytometry and Western blotting with antibodies to the Tlr-4 receptor and flow cytometry to the related CD14 moiety were performed to quantitate the expression levels of these two receptors by glioblastoma cells.

RESULTS

For subcutaneous tumors, LPS caused near complete tumor elimination in wild-type mice, but only a 50% reduction in Tlr-4 KO mice. For mice implanted with intracranial glioblastomas, LPS increased survival times modestly in wild-type mice, but showed no benefit in the Tlr-4 KO mice. There were no histological differences among wild-type and Tlr-4 KO mice, except for tumor size. In both models, an early neutrophilic and later macrophage-rich inflammatory infiltrate were seen after LPS administration. Quantitative flow cytometry and Western blotting showed no Tlr-4 receptor or CD14 expression in murine and human glioblastoma cells in vitro, and Western blotting suggested that Tlr-4 effects are mediated by nontumoral elements such as microglia and inflammatory cells.

CONCLUSION

LPS-induced antitumoral effects on glioblastoma multiforme are mediated, in part, by the Tlr-4 receptor. Further understanding of this process may lead to novel treatment strategies for this uniformly fatal disease.

The endocytic pathway mediates cell entry of dsRNA to induce RNAi silencing

Many metazoan cells can take up exogenous double-stranded (ds) RNA and use it to initiate an RNA silencing response, however, the mechanism for this uptake is ill-defined. Here, we identify the pathway for dsRNA uptake in Drosophila melanogaster S2 cells. Biochemical and cell biological analyses, and a genome-wide screen for components of the dsRNA-uptake machinery, indicated that dsRNA is taken up by an active process involving receptor-mediated endocytosis. Pharmacological inhibition of endocytic pathways disrupted exogenous dsRNA entry and the induction of gene silencing. This dsRNA uptake mechanism seems to be evolutionarily conserved, as knockdown of orthologues in Caenorhabditis elegans inactivated the RNA interference response in worms. Thus, this entry pathway is required for systemic RNA silencing in whole organisms. In Drosophila cells, pharmacological evidence suggests that dsRNA entry is mediated by pattern-recognition receptors. The possible role of these receptors in dsRNA entry may link RNA interference (RNAi) silencing to other innate immune responses.

The endocytic pathway mediates cell entry of dsRNA to induce RNAi silencing

Many metazoan cells can take up exogenous double-stranded (ds) RNA and use it to initiate an RNA silencing response, however, the mechanism for this uptake is ill-defined. Here, we identify the pathway for dsRNA uptake in Drosophila melanogaster S2 cells. Biochemical and cell biological analyses, and a genome-wide screen for components of the dsRNA-uptake machinery, indicated that dsRNA is taken up by an active process involving receptor-mediated endocytosis. Pharmacological inhibition of endocytic pathways disrupted exogenous dsRNA entry and the induction of gene silencing. This dsRNA uptake mechanism seems to be evolutionarily conserved, as knockdown of orthologues in Caenorhabditis elegans inactivated the RNA interference response in worms. Thus, this entry pathway is required for systemic RNA silencing in whole organisms. In Drosophila cells, pharmacological evidence suggests that dsRNA entry is mediated by pattern-recognition receptors. The possible role of these receptors in dsRNA entry may link RNA interference (RNAi) silencing to other innate immune responses.

Transferrin and the innate immune response of fish: identification of a novel mechanism of macrophage activation

We have previously demonstrated that a non-cytokine serum protein called transferrin was a primary activating molecule of the goldfish (Carassius auratus) macrophage antimicrobial response. The ability of the enzymatically cleaved forms of this protein to modulate fish macrophage function is novel and may represent a primitive and evolutionary conserved mechanism for the induction of NO response of macrophages. In the present study we confirm our earlier findings using immunoaffinity purified goldfish transferrin from mitogen-stimulated leukocyte supernatants. In addition we demonstrate that: (1). products released by necrotic/damaged cells contain transferrin-cleaving activity; (2). the cleavage site is located within the bridge peptide connecting the two lobes of the transferrin molecule; (3). transferrin is expressed by activated goldfish macrophages but not mitogen-stimulated kidney leukocytes; and (4). addition of transferrin significantly enhanced the killing response of goldfish macrophages exposed to different pathogens or pathogen products (e.g. lipopolysaccharide, Mycobacterium chelonei, Trypanosoma danilewskyi, Aeromonas salmonicida, and Leishmania major). We propose a model of fish macrophage activation that is mediated by a non-cytokine host protein (i.e. transferrin) in combination with highly conserved innate immunity recognition receptors that are almost certain to exist in teleost.

Modulation of tissue Toll-like receptor 2 and 4 during the early phases of polymicrobial sepsis correlates with mortality

OBJECTIVE

To determine whether there was a correlation between induction of polymicrobial sepsis, modulation of tissue Toll-like receptor (TLR) gene, and protein expression and survival outcome.

DESIGN

Prospective, randomized animal study.

SETTING

University medical school research laboratory.

SUBJECTS

Age- and weight-matched ICR/HSD mice.

INTERVENTIONS

Sepsis was induced by cecal ligation and puncture (CLP). No-surgery and sham (laparotomy)-operated mice were controls. We also examined tissue TLR2 and TLR4 messenger RNA and TLR4 protein levels in mice treated with an immunomodulator that increases survival in polymicrobial sepsis. In the immunomodulator study, mice were treated with glucan phosphate (50 mg/kg, intraperitoneally) 1 hr before CLP. No-surgery, sham surgery, glucan + no-surgery, sham surgery + glucan, and CLP groups were employed as controls.

MEASUREMENTS AND MAIN RESULTS

Total RNA was isolated from liver, lung, and spleen at 0, 1, 3, 6, 8, and 24 hrs after CLP. TLR gene expression was assessed by reverse transcription-polymerase chain reaction. Tissue TLR4 protein levels were evaluated at 24 hrs by Western blot and immunohistochemistry. CLP sepsis increased (p <.05) liver and lung TLR2 and TLR4 gene expression compared with controls. TLR4 protein concentrations also were increased. Increased TLR2/4 gene and TLR4 protein expression correlated with mortality. Immunoprophylaxis with glucan phosphate increased (p <.001) long-term survival (20% vs. 70%) but inhibited (p <.05) CLP-induced increases in tissue TLR2 and TLR4 messenger RNA expression as well as TLR4 protein expression.

CONCLUSIONS

Early increases in TLR2/4 gene and TLR4 protein expression correlated with mortality, whereas blunting TLR gene and protein expression correlated with improved long-term survival. This suggests that early up-regulation of tissue TLR2/4 may play a role in the proinflammatory response and pathophysiology of polymicrobial sepsis.

Cutting edge: mouse pellino-2 modulates IL-1 and lipopolysaccharide signaling

Pellino is a Drosophila protein originally isolated in a two-hybrid screen for proteins interacting with the serine/threonine kinase, pelle. Although mammalian homologs have been identified in mouse and man, the function of pellino is as yet unknown. In this study, the cloning, expression pattern, and a preliminary characterization of mouse pellino-2 is described. These studies reveal that mouse pellino-2 is expressed during embryogenesis and in a tissue-restricted manner in the adult. IL-1 induces the association of mouse pellino-2 with the mouse pelle-like kinase/IL-1R-associated kinase protein, a mammalian homolog of pelle. Ectopic pellino-2 expression did not result in NF-kappaB activation. However, ectopic expression of a mouse pellino-2 antisense construct inhibited IL-1 or LPS-induced activation of NF-kappaB-dependent IL-8 promoter activity. Our data reveal that mouse pellino-2 is a tissue-restricted component of a signaling pathway that couples the mouse pelle-like kinase/IL-1R-associated kinase protein to IL-1- or LPS-dependent signaling.

Human Vascular Endothelial Cells Express Pattern Recognition Receptors for Fungal Glucans Which Stimulates Nuclear Factor κB Activation and Interleukin 8 Production

Fungal cell wall glucans nonspecifically stimulate various aspects of innate immunity via interaction with membrane receptors on macrophages, neutrophils, and natural killer cells. We investigated the binding of water-soluble glucans in primary cultures of normal human coronary or dermal vascular endothelial cells (VECs). Membranes from VECs exhibited saturable binding. Competition studies demonstrated the presence of at least two glucan binding sites on VECs. Glucan phosphate competed for all binding sites with a KD of 3.7 μm for coronary VECs and 11 μm for dermal VECs, respectively. Laminarin, a low molecular weight glucan, competed for 47 to 51 per cent of binding ( KD = 2.8–2.9 μm), indicating the presence of at least two binding sites. Glucan (1 μg/mL) stimulated VEC nuclear factor κB nuclear binding activity and Interleukin 8 expression—but not that of vascular endothelial growth factor—in a time-dependent manner. This is the first report of pattern recognition receptors for glucan on human VECs. It also provides the first evidence that glucans can directly modulate the functional activity of VECs by stimulating cytokine gene. These results provide new insights into the mechanisms by which the host recognizes and responds to fungal cell wall products and suggests that the response to glucans may not be confined to leukocytes.

Bacterial lipopolysaccharides and innate immunity

Bacterial lipopolysaccharides (LPS) are the major outer surface membrane components present in almost all Gram-negative bacteria and act as extremely strong stimulators of innate or natural immunity in diverse eukaryotic species ranging from insects to humans. LPS consist of a poly- or oligosaccharide region that is anchored in the outer bacterial membrane by a specific carbohydrate lipid moiety termed lipid A. The lipid A component is the primary immunostimulatory centre of LPS. With respect to immunoactivation in mammalian systems, the classical group of strongly agonistic (highly endotoxic) forms of LPS has been shown to be comprised of a rather similar set of lipid A types. In addition, several natural or derivatised lipid A structures have been identified that display comparatively low or even no immunostimulation for a given mammalian species. Some members of the latter more heterogeneous group are capable of antagonizing the effects of strongly stimulatory LPS/lipid A forms. Agonistic forms of LPS or lipid A trigger numerous physiological immunostimulatory effects in mammalian organisms, but--in higher doses--can also lead to pathological reactions such as the induction of septic shock. Cells of the myeloid lineage have been shown to be the primary cellular sensors for LPS in the mammalian immune system. During the past decade, enormous progress has been obtained in the elucidation of the central LPS/lipid A recognition and signaling system in mammalian phagocytes. According to the current model, the specific cellular recognition of agonistic LPS/lipid A is initialized by the combined extracellular actions of LPS binding protein (LBP), the membrane-bound or soluble forms of CD14 and the newly identified Toll-like receptor 4 (TLR4)*MD-2 complex, leading to the rapid activation of an intracellular signaling network that is highly homologous to the signaling systems of IL-1 and IL-18. The elucidation of structure-activity correlations in LPS and lipid A has not only contributed to a molecular understanding of both immunostimulatory and toxic septic processes, but has also re-animated the development of new pharmacological and immunostimulatory strategies for the prevention and therapy of infectious and malignant diseases.

Recent developments in adjuvants for vaccines against infectious diseases

New generation vaccines, particularly those based on recombinant proteins and DNA, are likely to be less reactogenic than traditional vaccines, but are also less immunogenic. Therefore, there is an urgent need for the development of new and improved vaccine adjuvants. Adjuvants can be broadly separated into two classes, based on their principal mechanisms of action; vaccine delivery systems and 'immunostimulatory adjuvants'. Vaccine delivery systems are generally particulate e.g. emulsions, microparticles, iscoms and liposomes, and mainly function to target associated antigens into antigen presenting cells (APC). In contrast, immunostimulatory adjuvants are predominantly derived from pathogens and often represent pathogen associated molecular patterns (PAMP) e.g. LPS, MPL, CpG DNA, which activate cells of the innate immune system. Once activated, cells of innate immunity drive and focus the acquired immune response. In some studies, delivery systems and immunostimulatory agents have been combined to prepare adjuvant delivery systems, which are designed for more effective delivery of the immunostimulatory adjuvant into APC. Recent progress in innate immunity is beginning to yield insight into the initiation of immune responses and the ways in which immunostimulatory adjuvants may enhance this process. However, a rational approach to the development of new and more effective vaccine adjuvants will require much further work to better define the mechanisms of action of existing adjuvants. The discovery of more potent adjuvants may allow the development of vaccines against infectious agents such as HIV which do not naturally elicit protective immunity. New adjuvants may also allow vaccines to be delivered mucosally.

Fibrinogen stimulates macrophage chemokine secretion through toll-like receptor 4

Extravascular fibrin deposition is an early and persistent hallmark of inflammatory responses. Fibrin is generated from plasma-derived fibrinogen, which escapes the vasculature in response to endothelial cell retraction at sites of inflammation. Our ongoing efforts to define the physiologic functions of extravasated fibrin(ogen) have led to the discovery, reported here, that fibrinogen stimulates macrophage chemokine secretion. Differential mRNA expression analysis and RNase protection assays revealed that macrophage inflammatory protein-1alpha (MIP-1alpha), MIP-1beta, MIP-2, and monocyte chemoattractant protein-1 are fibrinogen inducible in the RAW264.7 mouse macrophage-like cell line, and ELISA confirmed that both RAW264.7 cells and primary murine thioglycolate-elicited peritoneal macrophages up-regulate the secretion of monocyte chemoattractant protein-1 >100-fold upon exposure to fibrinogen. Human U937 and THP-1 precursor-1 (THP-1) monocytic cell lines also secreted chemokines in response to fibrinogen, upon activation with IFN-gamma and differentiation with vitamin D(3), respectively. LPS contamination could not account for our observations, as fibrinogen-induced chemokine secretion was sensitive to heat denaturation and was unaffected by the pharmacologic LPS antagonist polymyxin B. Nevertheless, fibrinogen- and LPS-induced chemokine secretion both apparently required expression of functional Toll-like receptor 4, as each was diminished in macrophages derived from C3H/HeJ mice. Thus, innate responses to fibrinogen and bacterial endotoxin may converge at the evolutionarily conserved Toll-like recognition molecules. Our data suggest that extravascular fibrin(ogen) induces macrophage chemokine expression, thereby promoting immune surveillance at sites of inflammation.

The role of MyD88 and TLR4 in the LPS-mimetic activity of Taxol

Taxol can mimic bacterial lipopolysaccharide (LPS) by activating mouse macrophages in a cell cycle-independent, LPS antagonist-inhibitable manner. Macrophages from C3H/HeJ mice, which have a spontaneous mutation in Toll-like receptor 4 (TLR4), are hyporesponsive to both LPS and Taxol, suggesting that LPS and Taxol may share a signaling pathway involving TLR4. To determine whether TLR4 and its interacting adaptor molecule MyD88 are necessary for Taxol's LPS mimetic actions, we examined Taxol responses of primary macrophages from genetically defective mice lacking either TLR4 (C57BL/10ScNCr) or MyD88 (MyD88 knockout). When stimulated with Taxol, macrophages from wild-type mice responded robustly by secreting both TNF and NO, while macrophages from either TLR4-deficient C57BL/10ScNCr mice or MyD88 knockout mice produced only minimal amounts of TNF and NO. Taxol-induced NF-kappa B-driven luciferase activity was reduced after transfection of RAW 264.7 macrophages with a dominant negative version of mouse MyD88. Taxol-induced microtubule-associated protein kinase (MAPK) activation and NF-kappa B nuclear translocation were absent from TLR4-null macrophages, but were preserved in MyD88 knockout macrophages with a slight delay in kinetics. Neither Taxol-induced NF-kappa B activation, nor I kappa B degradation was affected by the presence of phosphatidylinositol 3-kinase inhibitors. These results suggest that Taxol and LPS not only share a TLR4/MyD88-dependent pathway in generating inflammatory mediators, but also share a TLR4-dependent/MyD88-independent pathway leading to activation of MAPK and NF-kappa B.

Normal human fibroblasts express pattern recognition receptors for fungal (1-->3)-beta-D-glucans

Fungal cell wall glucans nonspecifically stimulate various aspects of innate immunity. Glucans are thought to mediate their effects via interaction with membrane receptors on macrophages, neutrophils, and NK cells. There have been no reports of glucan receptors on nonimmune cells. We investigated the binding of a water-soluble glucan in primary cultures of normal human dermal fibroblasts (NHDF). Membranes from NHDF exhibited saturable binding with an apparent dissociation constant (K(D)) of 8.9 +/- 1.9 microg of protein per ml and a maximum binding of 100 +/- 8 resonance units. Competition studies demonstrated the presence of at least two glucan binding sites on NHDF. Glucan phosphate competed for all binding sites, with a K(D) of 5.6 microM (95% confidence interval [CI], 3.0 to 11 microM), while laminarin competed for 69% +/- 6% of binding sites, with a K(D) of 3.7 microM (95% CI, 1.9 to 7.3 microM). Glucan (1 microg/ml) stimulated fibroblast NF-kappaB nuclear binding activity and interleukin 6 (IL-6) gene expression in a time-dependent manner. NF-kappaB was activated at 4, 8, and 12 h, while IL-6 mRNA levels were increased by 48% at 8 h. This is the first report of pattern recognition receptors for glucan on human fibroblasts and the first demonstration of glucan binding sites on cells other than leukocytes. It also provides the first evidence that glucans can directly modulate the functional activity of NHDF. These results provide new insights into the mechanisms by which the host recognizes and responds to fungal (1-->3)-beta-D-glucans and suggests that the response to glucans may not be confined to cells of the immune system.

Phylogenetic and functional relationships between coagulation and the innate immune response

Activation of the coagulation system frequently accompanies systemic inflammatory states and is an almost invariable consequence of septic shock. The simultaneous activation of the innate immune response and the coagulation system after injury is a phylogenetically ancient, adaptive response that can be traced back to the early stages of eukaryotic evolution. Most invertebrate species lack differentiated phagocytic cells and platelets. They possess a common cellular and humoral pathway of inflammation and clotting after a breach in their internal milieu by either trauma or infection. The close linkage between clotting and inflammation has been preserved throughout vertebrate evolution and is readily demonstrable in human physiologic responses to a variety of potentially injurious stimuli. The same proinflammatory stimuli that activate the human clotting cascade also activate the phagocytic effector cells (neutrophils, monocytes, and macrophages). The complex and highly integrated linkage between systemic inflammation and coagulation are reviewed in this article.

Toll-like receptor 4 imparts ligand-specific recognition of bacterial lipopolysaccharide

Lipopolysaccharide (LPS) is the main inducer of shock and death in Gram-negative sepsis. Recent evidence suggests that LPS-induced signal transduction begins with CD14-mediated activation of 1 or more Toll-like receptors (TLRs). The lipid A analogues lipid IVa and Rhodobacter sphaeroides lipid A (RSLA) exhibit an uncommon species-specific pharmacology. Both compounds inhibit the effects of LPS in human cells but display LPS-mimetic activity in hamster cells. We transfected human TLR4 or human TLR2 into hamster fibroblasts to determine if either of these LPS signal transducers is responsible for the species-specific pharmacology. RSLA and lipid IVa strongly induced NF-kappaB activity and IL-6 release in Chinese hamster ovary fibroblasts expressing CD14 (CHO/CD14), but these compounds antagonized LPS antagonists in CHO/CD14 fibroblasts that overexpressed human TLR4. No such antagonism occurred in cells overexpressing human TLR2. We cloned TLR4 from hamster macrophages and found that human THP-1 cells expressing the hamster TLR4 responded to lipid IVa as an LPS mimetic, as if they were hamster in origin. Hence, cells heterologously overexpressing TLR4 from different species acquired a pharmacological phenotype with respect to recognition of lipid A substructures that corresponded to the species from which the TLR4 transgene originated. These data suggest that TLR4 is the central lipid A-recognition protein in the LPS receptor complex.

Regulatory roles for CD14 and phosphatidylinositol in the signaling via toll-like receptor 4-MD-2

The complex consisting of Toll-like receptor 4 (TLR4) and associated MD-2 signals the presence of lipopolysaccharide (LPS) when it is expressed in cell lines. We here show that normal human mononuclear cells express TLR4 and signal LPS via TLR4. CD14 is a molecule that binds to LPS and facilitates its signaling. Little is known, however, about the relationship of CD14 with TLR4-MD-2. We show that CD14 helps TLR4-MD-2 to sense and signal the presence of LPS. CD14 has also been implicated in recognition of apoptotic cells, which leads to phagocytosis without activation. Membrane phospholipids such as phosphatidylserine (PS) or phosphatidylinositol (PtdIns) are thought to serve as the ligands for CD14 in apoptotic cells. We find that PtdIns acts as an LPS antagonist in the signaling via TLR4-MD-2. TLR4-MD-2 seems to discriminate LPS from phospholipids. The signaling via TLR4-MD-2 is thus regulated by CD14 and phospholipid such as PtdIns.

Invertebrates versus vertebrates innate immunity: In the light of evolution

Invertebrates use a nonadaptive, innate immunity, the expression of germline encoded receptors, to identify the allogeneic and xenogeneic attributes. Vertebrates also have the capacity to express ontogenically related adaptive immunity which is a somatically selected gene rearrangement process. Several commonly accepted generalizations are utilized to explain the enigmatic lack of the adaptive immunity in invertebrates. All point to the primitive nature of the innate immunity and the primitive organization of the body plan and the life history patterns of invertebrates. Seven of the most common generalizations are reviewed and confuted by virtue of a biased literature presentation. Subsequently, three evolutionary puzzles are raised and the accepted paradigm that the vertebrate immunity is pathogenically directed is further challenged. This leads to an alternative idea suggesting that preserving the individuality against the threat of invading conspecific cells might have been the original function of the immune system. This ancient system has been co-opted later on to serve as a defence mechanism against pathogens. The secondary role arose in the form of a multiplicity of newly developed phenomena, one of them being the vertebrate adaptive immunity. This proposal is supported by the fact that vertebrates still exhibit two distinct but common types of naturally occurring transplantation events (natural chimerism) and by a variety of recent studies, providing evidence for the crucial role of the vertebrate's innate immunity in signalling and triggering the acquired effector mechanisms.

The Toll-like receptor 2 is recruited to macrophage phagosomes and discriminates between pathogens

Macrophages orchestrate innate immunity by phagocytosing pathogens and coordinating inflammatory responses. Effective defence requires the host to discriminate between different pathogens. The specificity of innate immune recognition in Drosophila is mediated by the Toll family of receptors; Toll mediates anti-fungal responses, whereas 18-wheeler mediates anti-bacterial defence. A large number of Toll homologues have been identified in mammals, and Toll-like receptor 4 is critical in responses to Gram-negative bacteria. Here we show that Toll-like receptor 2 is recruited specifically to macrophage phagosomes containing yeast, and that a point mutation in the receptor abrogates inflammatory responses to yeast and Gram-positive bacteria, but not to Gram-negative bacteria. Thus, during the phagocytosis of pathogens, two classes of innate immune receptors cooperate to mediate host defence: phagocytic receptors, such as the mannose receptor, signal particle internalization, and the Toll-like receptors sample the contents of the vacuole and trigger an inflammatory response appropriate to defence against the specific organism.

Toll4 (TLR4) expression in cardiac myocytes in normal and failing myocardium

Expression of innate immune response proteins, including IL-1beta, TNF, and the cytokine-inducible isoform of nitric oxide synthase (iNOS), have been documented in the hearts of humans and experimental animals with heart failure regardless of etiology, although the proximal events leading to their expression are unknown. Noting that expression of a human homologue of Drosophila Toll, a proximal innate immunity transmembrane signaling protein in the fly, now termed human Toll-like receptor 4 (hTLR4), appeared to be relatively high in the heart, we examined TLR4 mRNA and protein abundance in isolated cellular constituents of cardiac muscle and in normal and abnormal murine, rat, and human myocardium. TLR4 expression levels in cardiac myocytes and in coronary microvascular endothelial cells could be enhanced by either LPS or IL-1beta, an effect inhibited by the oxygen radical scavenger PDTC. Transfection of a constitutively active TLR4 construct, CD4/hTLR4, resulted in activation of a nuclear factor-kappaB reporter construct, but not of an AP-1 or an iNOS reporter construct, in cardiac myocytes. In normal murine, rat, and human myocardium, TLR4 expression was diffuse, and presumably cytoplasmic, in cardiac myocytes. However, in remodeling murine myocardium remote from sites of ischemic injury and in heart tissue from patients with idiopathic dilated cardiomyopathy, focal areas of intense TLR4 staining were observed in juxtaposed regions of 2 or more adjacent myocytes; this staining was not observed in control myocardium. Increased expression and signaling by TLR4, and perhaps other Toll homologues, may contribute to the activation of innate immunity in injured myocardium.

Signaling Events Induced by Lipopolysaccharide-Activated Toll-Like Receptor 2

Human Toll-like receptor 2 (TLR2) is a signaling receptor that responds to LPS and activates NF-κB. Here, we investigate further the events triggered by TLR2 in response to LPS. We show that TLR2 associates with the high-affinity LPS binding protein membrane CD14 to serve as an LPS receptor complex, and that LPS treatment enhances the oligomerization of TLR2. Concomitant with receptor oligomerization, the IL-1R-associated kinase (IRAK) is recruited to the TLR2 complex. Intracellular deletion variants of TLR2 lacking C-terminal 13 or 141 aa fail to recruit IRAK, which is consistent with the inability of these mutants to transmit LPS cellular signaling. Moreover, both deletion mutants could still form complexes with wild-type TLR2 and act in a dominant-negative (DN) fashion to block TLR2-mediated signal transduction. DN constructs of myeloid differentiation protein, IRAK, TNF receptor-associated factor 6, and NF-κB-inducing kinase, when coexpressed with TLR2, abrogate TLR2-mediated NF-κB activation. These results reveal a conserved signaling pathway for TLR2 and IL-1Rs and suggest a molecular mechanism for the inhibition of TLR2 by DN variants.

Toll-like receptor-2 mediates lipopolysaccharide-induced cellular signalling

Vertebrates and invertebrates initiate a series of defence mechanisms following infection by Gram-negative bacteria by sensing the presence of lipopolysaccharide (LPS), a major component of the cell wall of the invading pathogen. In humans, monocytes and macrophages respond to LPS by inducing the expression of cytokines, cell-adhesion proteins, and enzymes involved in the production of small proinflammatory mediators. Under pathophysiological conditions, LPS exposure can lead to an often fatal syndrome known as septic shock. Sensitive responses of myeloid cells to LPS require a plasma protein called LPS-binding protein and the glycosylphosphatidylinositol-anchored membrane protein CD14. However, the mechanism by which the LPS signal is transduced across the plasma membrane remains unknown. Here we show that Toll-like receptor 2 (TLR2) is a signalling receptor that is activated by LPS in a response that depends on LPS-binding protein and is enhanced by CD14. A region in the intracellular domain of TLR2 with homology to a portion of the interleukin (IL)-1 receptor that is implicated in the activation of the IL-1-receptor-associated kinase is required for this response. Our results indicate that TLR2 is a direct mediator of signalling by LPS.

MyD88 is an adaptor protein in the hToll/IL-1 receptor family signaling pathways

The Toll-mediated signaling cascade using the NF-kappaB pathway has been shown to be essential for immune responses in adult Drosophila, and we recently reported that a human homolog of the Drosophila Toll protein induces various immune response genes via this pathway. We now demonstrate that signaling by the human Toll receptor employs an adaptor protein, MyD88, and induces activation of NF-kappaB via the Pelle-like kinase IRAK and the TRAF6 protein, similar to IL-1R-mediated NF-kappaB activation. However, we find that Toll and IL-1R signaling pathways are not identical with respect to AP-1 activation. Finally, our findings implicate MyD88 as a general adaptor/regulator molecule for the Toll/IL-1R family of receptors for innate immunity.

Genetics of plant-pathogen interactions

Progress has occurred in understanding the function of disease-resistance genes that govern the resistance of plants to pathogens, and pathogen-produced molecules, called elicitors, that resistance genes key on. Data support the elicitor-receptor model wherein resistant plants contain receptors for pathogen elicitors. This recognition may be complex, however, involving delivery of elicitors to plant cells by specialized pathogen secretion systems and their processing prior to perception. Furthermore, elicitor receptors may not be the resistance gene proteins that govern specificity of the system. It is now also recognized that many elicitors function as virulence factors for the pathogen but have been co-opted by plants as triggers for active resistance. Major recent advances in the cloning and sequencing of clustered plant disease-resistance genes are providing information on the basis of their recognitional specificities and offer the opportunity to engineer new genes that recognize refractory pathogens or exhibit increased efficacy and durability. In combination with the transformation of cloned disease-resistance genes into new plant species, these approaches should facilitate disease control strategies in practical agriculture.