A point mutation in the IL-12R beta 2 gene underlies the IL-12 unresponsiveness of Lps-defective C57BL/10ScCr mice

Targeting TLR4 Signaling to Blunt Viral-Mediated Acute Lung Injury

Respiratory viral infections have been a long-standing global burden ranging from seasonal recurrences to the unexpected pandemics. The yearly hospitalizations from seasonal viruses such as influenza can fluctuate greatly depending on the circulating strain(s) and the congruency with the predicted strains used for the yearly vaccine formulation, which often are not predicted accurately. While antiviral agents are available against influenza, efficacy is limited due to a temporal disconnect between the time of infection and symptom development and viral resistance. Uncontrolled, influenza infections can lead to a severe inflammatory response initiated by pathogen-associated molecular patterns (PAMPs) or host-derived danger-associated molecular patterns (DAMPs) that ultimately signal through pattern recognition receptors (PRRs). Overall, these pathogen-host interactions result in a local cytokine storm leading to acute lung injury (ALI) or the more severe acute respiratory distress syndrome (ARDS) with concomitant systemic involvement and more severe, life threatening consequences. In addition to traditional antiviral treatments, blocking the host's innate immune response may provide a more viable approach to combat these infectious pathogens. The SARS-CoV-2 pandemic illustrates a critical need for novel treatments to counteract the ALI and ARDS that has caused the deaths of millions worldwide. This review will examine how antagonizing TLR4 signaling has been effective experimentally in ameliorating ALI and lethal infection in challenge models triggered not only by influenza, but also by other ALI-inducing viruses.

Tcf1+ cells are required to maintain the inflationary T cell pool upon MCMV infection

Cytomegalovirus-based vaccine vectors offer interesting opportunities for T cell-based vaccination purposes as CMV infection induces large numbers of functional effector-like cells that accumulate in peripheral tissues, a process termed memory inflation. Maintenance of high numbers of peripheral CD8 T cells requires continuous replenishment of the inflationary T cell pool. Here, we show that the inflationary T cell population contains a small subset of cells expressing the transcription factor Tcf1. These Tcf1⁺ cells resemble central memory T cells and are proliferation competent. Upon sensing viral reactivation events, Tcf1⁺ cells feed into the pool of peripheral Tcf1⁻ cells and depletion of Tcf1⁺ cells hampers memory inflation. TCR repertoires of Tcf1⁺ and Tcf1⁻ populations largely overlap, with the Tcf1⁺ population showing higher clonal diversity. These data show that Tcf1⁺ cells are necessary for sustaining the inflationary T cell response, and upholding this subset is likely critical for the success of CMV-based vaccination approaches.

Upon infection with cytomegalovirus, CD8⁺ T cells undergo prolific expansion in a process known as memory inflation. Here the authors define a population of Tcf1 expressing cells within the inflationary pool that is critical in fuelling this process.

Characterization of two novel mutations in IL-12R signaling in MSMD patients

Mendelian Susceptibility to Mycobacterial Disease (MSMD) is a rare syndrome with infections-among other complications-after Bacillus Calmette-Guerin (BCG) vaccination in children. We focused on the IL-12/IFN-γ pathway to identify new mutations in our patients. This study included 20 patients by vulnerability to mycobacteria and clinical manifestations of severe, recurrent infections. Blood samples were activated with BCG, BCG + IL-12 and BCG + IFN-γ. Cytokine levels were analyzed by ELISA. Measurements of IL-12Rβ1 and IL-12Rβ2 on the surface of peripheral blood mononuclear cells were performed by flow cytometry. To detect genetic defects, next-generation sequencing was performed by Thermo Fisher immunodeficiency panel. Flow cytometry analysis of 20 patients indicated reduction in IL-12R (β1/β2) expression in seven patients who showed incomplete production of IFN-γ by ELISA. In the patient with reduced IL-12 production, IFN-γR and IL-12R (β1/β2) expression levels were normal. Mutation analysis showed three previously reported mutations, two novel mutations in IL-12 R (β1/β2), and one previously reported mutation in IL-12.

Protective effects of total alkaloids from Dendrobium crepidatum against LPS-induced acute lung injury in mice and its chemical components

Dendrobium crepidatum was one of the sources of Herba Dendrobii, a famous and precious traditional Chinese medicine. Indolizine-type alkaloids are the main characteristic ingredients of D. crepidatum, which possesses a variety of changeable skeletons. In the present study, we found that the total alkaloids of D. crepidatum (TAD) can inhibit the production of nitric oxide (NO) in lipopolysaccharide (LPS)-activated macrophages and showed protective effects against LPS-induced acute lung injury (ALI) in mice through downregulating the TLR4-mediated MyD88/MAPK signaling pathway. Further phytochemical study showed that six previously undescribed indolizine-type compounds, including a racemic mixture (dendrocrepidine A-E) were isolated from TAD. Meanwhile, dendrocrepidine F was separated into a pair of enantiomers by a chiral chromatography, and their absolute configurations were assigned by single-crystal X-ray diffraction analysis. The isomer (-)-dendrocrepidine F showed higher anti-inflammatory effects by inhibiting NO production in LPS-treated macrophages with an IC₅₀ value of 13.3 μM. Taken together, indolizine-type alkaloids are the active components of D. crepidatum through downregulating the TLR4-mediated pathway, indicating some kind of therapy of TAD for ALI treatment.

Toll-like receptors elicit different recruitment kinetics of monocytes and neutrophils in mouse acute inflammation

Leukocyte recruitment is an important process in combating pathogens. The largest class of circulating leukocytes are neutrophils, which rapidly invade inflamed tissue, followed by inflammatory Ly6C⁺ monocytes. Ly6C^low monocytes patrol the endothelial wall routinely in the steady state. We recently reported early luminal recruitment of Ly6C^low monocytes, which preceded and orchestrated neutrophil arrival and extravasation in response to TLR7/8-mediated vascular inflammation. Here we dissected the kinetics of recruitment of monocytes and neutrophils and examined the dynamics of Ly6C^low monocytes in response to several other Toll-like receptor (TLR) agonists, using intravital confocal microscopy. We observed two types of kinetics in mesenteric veins. TLR2, TLR5 and TLR9 agonists caused early monocyte and neutrophil influx whereas TLR3 and TLR4 agonists rapidly recruited neutrophils and caused Ly6C^low monocytes to arrive at low levels later on. All TLR agonists, except TLR9, led Ly6C^low monocytes to meticulously patrol the vascular wall. Finally, these monocytes released pro-inflammatory cytokines and chemokines implicated in neutrophil recruitment in response to TLR2, TLR4, and TLR9 stimulation but not to TLR3 and TLR5 agonists. These results refine our understanding of the early events in the leukocyte recruitment cascade, including the patrolling behavior of Ly6C^low monocytes, in TLR-mediated acute vascular inflammation.

Doxorubicin-Induced Systemic Inflammation Is Driven by Upregulation of Toll-Like Receptor TLR4 and Endotoxin Leakage

Doxorubicin is one of the most effective chemotherapeutic agents used for cancer treatment, but it causes systemic inflammation and serious multiorgan side effects in many patients. In this study, we report that upregulation of the proinflammatory Toll-like receptor TLR4 in macrophages by doxorubicin is an important step in generating its toxic side effects. In patient serum, doxorubicin treatment resulted in leakage of endotoxin and inflammatory cytokines into circulation. In mice, doxorubicin damaged the intestinal epithelium, which also resulted in leakage of endotoxin from the gut flora into circulation. Concurrently, doxorubicin increased TLR4 expression in macrophages both in vitro and in vivo, which further enhanced the sensitivity of these cells to endotoxin. Either depletion of gut microorganisms or blockage of TLR4 signaling effectively decreased doxorubicin-induced toxicity. Taken together, our findings suggest that doxorubicin-triggered leakage of endotoxin into the circulation, in tandem with enhanced TLR4 signaling, is a candidate mechanism underlying doxorubicin-induced systemic inflammation. Our study provides new insights for devising relevant strategies to minimize the adverse effects of chemotherapeutic agents such as doxorubicin, which may extend its clinical uses to eradicate cancer cells. Cancer Res; 76(22); 6631-42. ©2016 AACR.

Role of interferons in LPS hypersensitivity

The innate immune response to Gram-negative bacteria depends mainly on the ability of the host to respond to the LPS component. Consequently, the state of LPS sensitivity at the time of infection and the numbers of invading bacteria ( i.e. the amounts of LPS) are primary factors determining the innate responses provoked by Gram-negative pathogens. LPS sensitivity increases following treatment of mice with live or killed micro-organisms. Two types of sensitization have been recognized, strong, IFN-γ-dependent and moderate IFN-γ-independent. IL-12 and IL-18 are intimately involved in the induction of IFN-γ by bacteria. We showed that Gram-negative bacteria induce IFN-γ in mice also by an IFN-β-dependent pathway that requires IL-18 and is independent of IL-12 signaling. This pathway is STAT4 dependent, the activation of which is directly linked to IFN-β. Further, IFN-β can be replaced by IFN-α. While different components of Gram-negative bacteria induce IL-12 and IL-18, LPS seems to be the only component in these bacteria capable of inducing IFN-β. Therefore, the IFN-β pathway of IFN-γ induction, unlike the IL-12 pathway, proceeds only in LPS responder mice. The IFN-α/β-dependent pathway is expected to play a role whenever IFN-α or IFN-β, and IL-18 are produced concomitantly during infection.

IL12RB2 Polymorphisms correlate with risk of lung adenocarcinoma

In a previous study, lack of IL-12 signaling in il12rb2 knock-out mice was found to predispose to lung adenocarcinoma (LAC). We asked whether specific polymorphisms of the human IL12RB2 gene may confer susceptibility to LAC. We studied IL12RB2 single nucleotide polymorphisms (SNPs) spanning from the promoter to the first untranslated exon of the gene. Genotypes of 49 individuals with LAC were compared with those of 93 healthy subjects. Two allele variants were found to be associated with increased susceptibility to LAC. One haplotype (hap), hap18, was more frequent in patients (18%) versus controls (6%) and significantly associated with increased probability of disease occurrence. Furthermore, IL-12 driven STAT4 phosphorylation in T cell blasts from healthy individuals was found to correlate with both single allele variants and haplotypes. In conclusion, genetically determined low signaling activity of IL-12R predisposes to the development of LAC.

Predominant role of interferon-γ in the host protective effect of CD8(+) T cells against Neospora caninum infection

It is well established that CD8⁺ T cells play an important role in protective immunity against protozoan infections. However, their role in the course of Neospora caninum infection has not been fully elucidated. Here we report that CD8-deficient mice infected with N. caninum presented higher parasitic loads in the brain and lungs and lower spleen and brain immunity-related GTPases than their wild-type counterparts. Moreover, adoptive transfer of splenic CD8⁺ T cells sorted from N. caninum-primed immunosufficient C57BL/10 ScSn mice prolonged the survival of infected IL-12-unresponsive C57BL/10 ScCr recipients. In both C57BL/6 and C57BL/10 ScSn mice CD8⁺ T cells are activated and produce interferon-γ (IFN-γ) upon challenged with N. caninum. The host protective role of IFN-γ produced by CD8⁺ T cells was confirmed in N. caninum-infected RAG2-deficient mice reconstituted with CD8⁺ T cells obtained from either IFN-γ-deficient or wild-type donors. Mice receiving IFN-γ-expressing CD8⁺ T cells presented lower parasitic burdens than counterparts having IFN-γ-deficient CD8⁺ T cells. Moreover, we observed that N. caninum-infected perforin-deficient mice presented parasitic burdens similar to those of infected wild-type controls. Altogether these results demonstrate that production of IFN-γ is a predominant protective mechanism conferred by CD8⁺ T cells in the course of neosporosis.

Whole-genome sequence analysis reveals differences in population management and selection of European low-input pig breeds

BACKGROUND

A major concern in conservation genetics is to maintain the genetic diversity of populations. Genetic variation in livestock species is threatened by the progressive marginalisation of local breeds in benefit of high-output pigs worldwide. We used high-density SNP and re-sequencing data to assess genetic diversity of local pig breeds from Europe. In addition, we re-sequenced pigs from commercial breeds to identify potential candidate mutations responsible for phenotypic divergence among these groups of breeds.

RESULTS

Our results point out some local breeds with low genetic diversity, whose genome shows a high proportion of regions of homozygosis (>50%) and that harbour a large number of potentially damaging mutations. We also observed a high correlation between genetic diversity estimates using high-density SNP data and Next Generation Sequencing data (r = 0.96 at individual level). The study of non-synonymous SNPs that were fixed in commercial breeds and also in any local breed, but with different allele, revealed 99 non-synonymous SNPs affecting 65 genes. Candidate mutations that may underlie differences in the adaptation to the environment were exemplified by the genes AZGP1 and TAS2R40. We also observed that highly productive breeds may have lost advantageous genotypes within genes involve in immune response--e.g. IL12RB2 and STAB1-, probably as a result of strong artificial in the intensive production systems in pig.

CONCLUSIONS

The high correlation between genetic diversity computed with the 60K SNP and whole genome re-sequence data indicates that the Porcine 60K SNP Beadchip provides reliable estimates of genomic diversity in European pig populations despite the expected bias. Moreover, this analysis gave insights for strategies to the genetic characterization of local breeds. The comparison between re-sequenced local pigs and re-sequenced commercial pigs made it possible to report candidate mutations to be responsible for phenotypic divergence among those groups of breeds. This study highlights the importance of low input breeds as a valuable genetic reservoir for the pig production industry. However, the high levels of ROHs, inbreeding and potentially damaging mutations emphasize the importance of the genetic characterization of local breeds to preserve their genomic variability.

Toll-like receptor 4 deficiency accelerates the development of insulin-deficient diabetes in non-obese diabetic mice

BACKGROUND/OBJECTIVE

Toll-like receptors (TLR) mediate the recognition of microbial constituents and stress-induced endogenous ligands by the immune system. They may also be involved in the maintenance or break down of tolerance against autologous antigens. The aim of our investigation was to study the consequence of TLR4 deficiency on the development of insulin-deficient diabetes in the NOD mouse.

METHODS

The TLR4 defect of the C57BL/10ScN mouse was backcrossed onto the NOD background and the effect of TLR4 deficiency on diabetes development was analysed by in vivo and in vitro studies.

RESULTS

Compared to animals with wildtype TLR4 expression (TLR4(+/+)), female NOD mice carrying a homozygous TLR4 defect (TLR4(-/-)), showed significant acceleration of diabetes development, with a younger age at diabetes onset (TLR4 (+/+) 177±22 d, TLR(-/-): 118±21 d; p<0.01). Pancreata of 120 d old TLR4(-/-) NOD mice revealed increased proportions of islets with advanced stages of immune cell infiltration compared to TLR4(+/+) mice (p<0.05). TLR4 deficiency did not affect the susceptibility of islet cells to the beta cell damaging mediators nitric oxide or the inflammatory cytokines tumor necrosis factor alpha, interleukin-1 beta and interferon gamma. The lack of TLR4 further had no effect on the frequency of regulatory T-cells but reduced their capacity to inhibit T-cell proliferation.

CONCLUSIONS

Our findings demonstrate that TLR4 deficiency results in an acceleration of diabetes development and immune cell infiltration of islets in NOD mice. We conclude that TLR4 is involved in the progression of the insulitis process thereby controlling the development of insulin-deficient diabetes in NOD mice.

Neuroexcitatory effects of morphine-3-glucuronide are dependent on Toll-like receptor 4 signaling

BACKGROUND

Multiple adverse events are associated with the use of morphine for the treatment of chronic non-cancer pain, including opioid-induced hyperalgesia (OIH). Mechanisms of OIH are independent of opioid tolerance and may involve the morphine metabolite morphine-3-glucuronide (M3G). M3G exhibits limited affinity for opioid receptors and no analgesic effect. Previous reports suggest that M3G can act via the Toll-like receptor 4 (TLR4)/myeloid differentiation protein-2 (MD-2) heterodimer in the central nervous system to elicit pain.

METHODS

Immunoblot and immunocytochemistry methods were used to characterize the protein expression of TLR4 present in lumbar dorsal root ganglion (DRG). Using in vitro intracellular calcium and current clamp techniques, we determined whether TLR4 activation as elicited by the prototypical agonists of TLR4, lipopolysaccharide (LPS) and M3G, contributed to changes in intracellular calcium and increased excitation. Rodents were also injected with M3G to determine the degree to which M3G-induced tactile hyperalgesia could be diminished using either a small molecule inhibitor of the MD-2/TLR4 complex in rats or TLR4 knockout mice. Whole cell voltage-clamp recordings were made from small- and medium-diameter DRG neurons (25 μm < DRG diameter <45 μm) for both control and M3G-treated neurons to determine the potential influence on voltage-gated sodium channels (NaVs).

RESULTS

We observed that TLR4 immunoreactivity was present in peptidergic and non-peptidergic sensory neurons in the DRG. Non-neuronal cells in the DRG lacked evidence of TLR4 expression. Approximately 15% of assayed small- and medium-diameter sensory neurons exhibited a change in intracellular calcium following LPS administration. Both nociceptive and non-nociceptive neurons were observed to respond, and approximately 40% of these cells were capsaicin-insensitive. Increased excitability observed in sensory neurons following LPS or M3G could be eliminated using Compound 15, a small molecule inhibitor of the TLR4/MD-2 complex. Likewise, systemic injection of M3G induced rapid tactile, but not thermal, nociceptive behavioral changes in the rat, which were prevented by pre-treating animals with Compound 15. Unlike TLR4 wild-type mice, TLR4 knockout mice did not exhibit M3G-induced hyperalgesia. As abnormal pain sensitivity is often associated with NaVs, we predicted that M3G acting via the MD-2/TLR4 complex may affect the density and gating of NaVs in sensory neurons. We show that M3G increases tetrodotoxin-sensitive and tetrodotoxin-resistant (NaV1.9) current densities.

CONCLUSIONS

These outcomes provide evidence that M3G may play a role in OIH via the TLR4/MD-2 heterodimer complex and biophysical properties of tetrodotoxin-sensitive and tetrodotoxin-resistant NaV currents.

Role of human TLR4 in respiratory syncytial virus-induced NF-κB activation, viral entry and replication

TLRs play a key role in innate immune defenses. It was previously reported that purified respiratory syncytial virus (RSV) fusion protein elicits an inflammatory response in hematopoietic cells, which required expression of TLR4 and its co-receptor CD14. However, a biological role of TLR4 in immunity to RSV, as initially proposed, has remained inconclusive and controversial. Here, we directly assess the role of human TLR4 and its co-receptors in NF-κB activation, viral entry and replication using intact virions rather than purified RSV components. We used HEK 293 reporter cells that are highly permissive for RSV and that either express or a lack a functional human TLR4/MD-2/CD14 complex. We demonstrate that RSV-mediated NF-κB activation, viral entry and replication are independent of the expression of a functional human TLR4/MD-2/CD14 complex and that, in turn, human TLR4 activation by LPS remains unaffected in RSV-infected cells. Thus, although isolated viral compounds such as purified RSV F protein may bind TLR4 and/or CD14, a direct interaction between intact RSV particles and the human TLR4 receptor complex does not seem to play a biological role in RSV pathogenesis.

Leukocyte- and platelet-derived microvesicle interactions following in vitro and in vivo activation of toll-like receptor 4 by lipopolysaccharide

Background

Pro-coagulant membrane microvesicles (MV) derived from platelets and leukocytes are shed into the circulation following receptor-mediated activation, cell-cell interaction, and apoptosis. Platelets are sentinel markers of toll-like receptor 4 (TLR4) activation. Experiments were designed to evaluate the time course and mechanism of direct interactions between platelets and leukocytes following acute activation of TLR4 by bacterial lipopolysaccharide (LPS).

Methodology/Principal Findings

Blood from age-matched male and female wild type (WT) and TLR4 gene deleted (dTLR4) mice was incubated with ultra-pure E. coli LPS (500 ng/ml) for up to one hour. At designated periods, leukocyte antigen positive platelets, platelet antigen positive leukocytes and cell-derived MV were quantified by flow cytometry. Numbers of platelet- or leukocyte-derived MV did not increase within one hour following in vitro exposure of blood to LPS. However, with LPS stimulation numbers of platelets staining positive for both platelet- and leukocyte-specific antigens increased in blood derived from WT but not dTLR4 mice. This effect was blocked by inhibition of TLR4 signaling mediated by My88 and TRIF. Seven days after a single intravenous injection of LPS (500 ng/mouse or 20 ng/gm body wt) to WT mice, none of the platelets stained for leukocyte antigen. However, granulocytes, monocytes and apoptotic bodies stained positive for platelet antigens.

Conclusions/Significance

Within one hour of exposure to LPS, leukocytes exchange surface antigens with platelets through TLR4 activation. In vivo, leukocyte expression of platelet antigen is retained after a single exposure to LPS following turn over of the platelet pool. Acute expression of leukocyte antigen on platelets within one hour of exposure to LPS and the sustained expression of platelet antigen on leukocytes following a single acute exposure to LPS in vivo explains, in part, associations of platelets and leukocytes in response to bacterial infection and changes in thrombotic propensity of the blood.

Early interleukin 6 production by leukocytes during ischemic acute kidney injury is regulated by TLR4

Although leukocytes infiltrate the kidney during ischemic acute kidney injury (AKI) and release interleukin 6 (IL6), their mechanism of activation is unknown. Here, we tested whether Toll-like receptor 4 (TLR4) on leukocytes mediated this activation by interacting with high-mobility group protein B1 (HMGB1) released by renal cells as a consequence of ischemic kidney injury. We constructed radiation-induced bone marrow chimeras using C3H/HeJ and C57BL/10ScNJ strains of TLR4 (-/-) mice and their respective TLR4 (+/+) wild-type counterparts and studied them at 4 h after an ischemic insult. Leukocytes adopted from TLR4 (+/+) mice infiltrated the kidneys of TLR4 (-/-) mice, and TLR4 (-/-) leukocytes infiltrated the kidneys of TLR4 (+/+) mice but caused little functional renal impairment in each case. Maximal ischemic AKI required both radiosensitive leukocytes and radioresistant renal parenchymal and endothelial cells from TLR4 (+/+) mice. Only TLR4 (+/+) leukocytes produced IL6 in vivo and in response to HMGB1 in vitro. Thus, following infiltration of the injured kidney, leukocytes produce IL6 when their TLR4 receptors interact with HMGB1 released by injured renal cells. This underscores the importance of TLR4 in the pathogenesis of ischemic AKI.

Th cells act via two synergistic pathways to promote antiviral CD8+ T cell responses

The mechanisms of how Th cells promote CD8(+) T cell responses during viral infections are largely unknown. In this study, we unraveled the mechanisms of T cell help for CD8(+) T cell responses during vaccinia virus infection. Our results demonstrate that Th cells promote vaccinia virus-specific CD8(+) T cell responses via two interconnected synergistic pathways: First, CD40L expressed by activated CD4(+) T cells instructs dendritic cells to produce bioactive IL-12p70, which is directly sensed by Ag-specific CD8(+) T cells, resulting in increased IL-2Rα expression. Second, Th cells provide CD8(+) T cells with IL-2, thereby enhancing their survival. Thus, Th cells are at the center of an important communication loop with a central role for IL-2/IL-2R and bioactive IL-12.

Involvement of the Toll-like receptor 4 pathway and use of TNF-alpha antagonists for treatment of the mucopolysaccharidoses

Enzyme replacement therapy is currently available for three of the mucopolysaccharidoses (MPSs) but has limited effects on the skeletal lesions. We investigated the involvement of the Toll-like receptor 4 (TLR4) signaling pathway in the pathogenesis of MPS bone and joint disease, and the use of the anti-TNF-alpha drug, Remicade (Centocor, Inc.), for treatment. TLR4 KO (TLR4(lps-/-)) mice were interbred with MPS VII mice to produce double-KO (DKO) animals. The DKO mice had longer and thinner faces and longer femora as revealed by micro-computed tomography analysis compared with MPS VII mice. Histological analyses also revealed more organized and thinner growth plates. The serum levels of TNF-alpha were normalized in the DKO animals, and the levels of phosphorylated STAT1 and STAT3 in articular chondrocytes were corrected. These findings led us to evaluate the effects of Remicade in MPS VI rats. When initiated at 1 month of age, i.v. treatment prevented the elevation of TNF-alpha, receptor activator of NF-kappaB, and other inflammatory molecules not only in the blood but in articular chondrocytes and fibroblast-like synoviocytes (FLSs). Treatment of 6-month-old animals also reduced the levels of these molecules to normal. The number of apoptotic articular chondrocytes in MPS VI rats was similarly reduced, with less infiltration of synovial tissue into the underlying bone. These studies revealed the important role of TLR4 signaling in MPS bone and joint disease and suggest that targeting TNF-alpha may have positive therapeutic effects.

CXCL10 impairs beta cell function and viability in diabetes through TLR4 signaling

In type 1 and type 2 diabetes (T1/T2DM), beta cell destruction by apoptosis results in decreased beta cell mass and progression of the disease. In this study, we found that the interferon gamma-inducible protein 10 plays an important role in triggering beta cell destruction. Islets isolated from patients with T2DM secreted CXCL10 and contained 33.5-fold more CXCL10 mRNA than islets from control patients. Pancreatic sections from obese nondiabetic individuals and patients with T2DM and T1DM expressed CXCL10 in beta cells. Treatment of human islets with CXCL10 decreased beta cell viability, impaired insulin secretion, and decreased insulin mRNA. CXCL10 induced sustained activation of Akt, JNK, and cleavage of p21-activated protein kinase 2 (PAK-2), switching Akt signals from proliferation to apoptosis. These effects were not mediated by the commonly known CXCL10 receptor CXCR3 but through TLR4. Our data suggest CXCL10 as a binding partner for TLR4 and as a signal toward beta cell failure in diabetes.

Toll-like receptor and IL-12 signaling control susceptibility to contact hypersensitivity

Allergic contact hypersensitivity (CHS) is a T cell–mediated inflammatory skin disease. Interleukin (IL)-12 is considered to be important in the generation of the allergen-specific T cell response. Loss of IL-12 function in IL-12Rβ2–deficient mice, however, did not ameliorate the allergic immune response, suggesting alternate IL-12–independent pathways in the induction of CHS. Because exposure to contact allergens always takes place in the presence of microbial skin flora, we investigated the potential role of Toll-like receptors (TLRs) in the induction of CHS. Using mice deficient in TLR4, the receptor for bacterial lipopolysaccharide (LPS), IL-12 receptor (R) β2, or both, we show that the concomitant absence of TLR4 and IL-12Rβ2, but not the absence of TLR4 or IL-12Rβ2 alone, prevented DC-mediated sensitization, generation of effector T cells, and the subsequent CHS response to 2,4,6-trinitro-1-chlorobenzene (TNCB), oxazolone, and fluorescein isothiocyanate. Introduction of the TLR4 transgene into the TLR4/IL-12Rβ2 mutant restored the CHS inducibility, showing a requirement for TLR4 in IL-12–independent CHS induction. Furthermore, the concomitant absence of TLR2 and TLR4 prevented the induction of CHS to TNCB in IL-12–competent mice. Finally, CHS was inducible in germ-free wild-type and IL-12Rβ2–deficient mice, but not in germ-free TLR4/IL-12Rβ2 double deficient mice, suggesting that the necessary TLR activation may proceed via endogenous ligands.

Toll-like receptors regulation of viral infection and disease

In recent years, it has become increasingly evident that mammalian Toll-like receptors (TLRs) play a critical role in determining the outcome of virus infection. TLRs have evolved to recognize viral nucleic acids, and promote the stimulation of innate and adaptive immune responses. Interestingly, the study of mice harboring deficiencies in various TLR proteins and their adaptors suggests that TLR activation promotes protective anti-viral immunity in some cases, while exacerbating virus-induced disease in others. In this report we describe the interactions of viruses with both the TLR system and the intracellular recognition system and highlight the role of TLRs in shaping the outcome of virus infection in both a positive and negative manner.

Pneumonia virus of mice: severe respiratory infection in a natural host

Pneumonia virus of mice (PVM; family Paramyxoviridae, genus Pneumovirus) is a natural mouse pathogen that is closely related to human and bovine respiratory syncytial viruses. Among the prominent features of this infection, robust replication of PVM takes place in bronchial epithelial cells in response to a minimal virus inoculum. Virus replication in situ results in local production of proinflammatory cytokines (MIP-1alpha, MIP-2, MCP-1 and IFNgamma) and granulocyte recruitment to the lung. If left unchecked, PVM infection and the ensuing inflammatory response ultimately lead to pulmonary edema, respiratory compromise and death. In this review, we consider the recent studies using the PVM model that have provided important insights into the role of the inflammatory response in the pathogenesis of severe respiratory virus infection. We also highlight several works that have elucidated acquired immune responses to this pathogen, including T cell responses and the development of humoral immunity. Finally, we consider several immunomodulatory strategies that have been used successfully to reduce morbidity and mortality when administered to PVM-infected, symptomatic mice, and thus hold promise as realistic therapeutic strategies for severe respiratory virus infections in human subjects.

The forward genetic dissection of afferent innate immunity

Recognition of the microbial world is mediated chiefly by a small group of immune receptors that activate a characteristic host inflammatory response, the innate immune response. Known as the Toll-like receptors (TLRs), these molecules are represented among most metazoans. In mammals, forward genetic analysis of the lipopolysaccharide (LPS) response led to the identification of TLR4 as the LPS receptor. Through a combination of forward and reverse genetic studies, a relatively detailed understanding of the functions of mammalian TLRs has now been achieved. As discussed here, mutagenesis has revealed proteins that participate in TLR signaling pathways, and informed our understanding of the subtleties of these molecules' structure and function.

Toll‐like receptor 4 suppression leads to islet allograft survival

Carbon monoxide (CO) exposure of an islet donor frequently leads to islet allograft long-term survival and tolerance in recipients. We show here that CO confers its protective effects at least in part by suppressing Toll-like receptor 4 (TLR4) up-regulation in pancreatic beta cells. TLR4 is normally up-regulated in islets during the isolation procedure; donor treatment with CO suppresses TLR4 expression in isolated islets as well as in transplanted grafts. TLR4 up-regulation allows initiation of inflammation, which leads to islet allograft rejection; islet grafts from TLR4-deficient mice survive indefinitely in BALB/c recipients and show significantly less inflammation at various days after transplantation compared with grafts from a control donor. Isolated islets preinfected with a TLR4 dominant negative virus before transplantation demonstrated prolonged survival in recipients. Despite the salutary effects of TLR4 suppression, HO-1 expression is still needed in the recipient for islet survival: TLR4-deficient islets were rejected promptly after being transplanted into recipients in which HO-1 activity was blocked. In addition, incubation of an insulinoma cell line, betaTC3, with an anti-TLR4 antibody protects those cells from cytokine-induced apoptosis. Our data suggest that TLR4 induction in beta cells is involved in beta cell death and graft rejection after transplantation. CO exposure protects islets from rejection by blocking TLR4 up-regulation.

Analysis of the immune response to Neospora caninum in a model of intragastric infection in mice

To study experimental Neospora caninum infection initiated at the gastrointestinal tract, Toll-like Receptor 4- and functional IL-12Rbeta2 chain-deficient C57BL/10 ScCr mice were challenged intragastrically with 5 x 10(6) N. caninum tachyzoites. All parasite-inoculated mice eventually died with disseminated infection. In contrast, immunocompetent BALB/c mice challenged with 1 x 10(7) N. caninum tachyzoites by the intragastric (i.g.) or the intraperitoneal (i.p.) route remained alive for at least 6 months. Expansion of splenic B- and T-cells, the latter displaying both activated and regulatory phenotypes, and increased levels of IFN-gamma and IL-10 mRNA were detected in both groups of infected BALB/c mice compared with non-infected controls, whereas in the Peyer's patches only IFN-gamma mRNA levels were found to be increased. Parasite-specific IgG1, IgG2a and IgA antibody levels were elevated in the sera of all infected mice, whereas increased N. caninum-specific IgA levels were detected in intestinal lavage fluids of i.g. challenged mice only. These results show that N. caninum infection can be successfully established in mice by i.g. administration of tachyzoites. They also show that the immune response elicited in i.g. or i.p. infected BALB/c mice, although conferring some degree of protection, was not sufficient for complete parasite clearance.

Cellular recognition of trimyristoylated peptide or enterobacterial lipopolysaccharide via both TLR2 and TLR4

Evidence for specific and direct bacterial product recognition through toll-like receptors (TLRs) has been emphasized recently. We analyzed lipopeptide analogues and enterobacterial lipopolysaccharide (eLPS) for their potential to activate cells through TLR2 and TLR4. Whereas bacterial protein palmitoylated at its N-terminal cysteine and N-terminal peptides derived thereof are known to induce TLR2-mediated cell activation, a synthetic acylhexapeptide mimicking a bacterial lipoprotein subpopulation for which N-terminal trimyristoylation is characteristic (Myr(3)CSK(4)) activated cells not only through TLR2 but also through TLR4. Conversely, highly purified eLPS triggered cell activation through overexpressed TLR2 in the absence of TLR4 expression if CD14 was coexpressed. Accordingly, TLR2(-/-) macrophages prepared upon gene targeting responded to Myr(3)CSK(4) challenge, whereas TLR2(-/-)/TLR4(d/d) cells were unresponsive. Through interferon-gamma (IFNgamma) priming, macrophages lacking expression of functional TLR4 and/or MD-2 acquired sensitivity to eLPS, whereas TLR2/TLR4 double deficient cells did not. Not only TLR2(-/-) mice but also TLR4(-/-) mice were resistant to Myr(3)CSK(4) challenge-induced fatal shock. d-Galactosamine-sensitized mice expressing defective TLR4 or lacking TLR4 expression acquired susceptibility to eLPS-driven toxemia upon IFNgamma priming, whereas double deficient mice did not. Immunization toward ovalbumin using Myr(3)CSK(4) as adjuvant was ineffective in TLR2(-/-)/TLR4(-/-) mice yet effective in wild-type, TLR2(-/-), or TLR4(-/-) mice as shown by analysis of ovalbumin-specific serum Ig concentration. A compound such as Myr(3)CSK(4) whose stimulatory activity is mediated by both TLR2 and TLR4 might constitute a preferable adjuvant. On the other hand, simultaneous blockage of both of the two TLRs might effectively inhibit infection-induced pathology.

TLR4 up-regulation at protein or gene level is pathogenic for lupus-like autoimmune disease

TLR4 is the receptor for the Gram-negative bacterial cell wall component LPS. TLR4 signaling is controlled by both positive and negative regulators to balance optimal immune response and potential sepsis. Unchecked TLR4 activation might result in autoimmune diseases, a hypothesis that has not been formally resolved. In this study, we found that TLR4 signaling to LPS can be positively enforced by expressing gp96 on cell surfaces through the chaperone function of, but not the direct signaling by, gp96; TLR4 as well as the commensal flora are essential for the production of anti-dsDNA Ab and the immune complex-mediated glomerulonephritis in transgenic mice that express surface gp96. Moreover, a similar constellation of autoimmunity was evident in mice that encode multiple copies of tlr4 gene. Our study has revealed that increased TLR4 signaling alone without exogenous insult can break immunological tolerance. It provides a strong experimental evidence for TLR4 dysregulation as an etiology of lupus-like renal disease.

In vivo effects of lipopolysaccharide and TLR4 on platelet production and activity: implications for thrombotic risk

Gram-negative bacteria release LPS, which activates Toll-like-receptor-4 (TLR4) in the host, initiating an inflammatory response to infection. Infection increases risk for thrombosis. Platelets contribute to defense from infection and to thrombosis. Experiments were designed to determine whether LPS, through TLR4 signaling, affects platelet phenotype. Platelet responses in wild-type (WT) mice and mice that lack the TLR4 gene (dTLR4) were compared following a single nonlethal injection of LPS (0.2 mg/kg iv). Compared with WT mice, mice without TLR4 had fewer circulating platelets with lower RNA content and were less responsive to thrombin-activated expression of P-selectin but were equally sensitive to aggregation or ATP secretion. One week following the LPS injection, the time it takes for the circulating platelet pool to turnover, the number of circulating platelets, thrombin-induced expression of P-selectin, and collagen-activated aggregation were increased comparably in both groups of mice. Therefore, the change of the platelet pool to an activated phenotype 1 wk after a single exposure to LPS appears to arise from a process that is independent of TLR4. The persistence of the effect 1 wk after the injection suggests that the changes reflect an action of LPS on megakaryocytes and their platelet progeny rather than on circulating platelets, which would have been cleared.

Neospora caninum: high susceptibility to the parasite in C57BL/10ScCr mice

C57BL/10ScCr mice, lack Toll-like receptor 4 and a functional Interleukin-12 receptor. Taking this into account, susceptibility of these mice to Neospora caninum infection was assessed comparatively to that of immunocompetent C57BL/10ScSn mice. C57BL/10ScCr mice inoculated intraperitoneally with 5x10(5)N. caninum tachyzoites showed a high susceptibility to this parasite. All infected C57BL/10ScCr mice were dead by day 8 post-infection whereas all control C57BL/10ScSn mice survived this parasitic challenge. Immunohistochemical analysis of infected C57BL/10ScCr mice showed N. caninum tachyzoites spread in the pancreas, liver, lung, intestine, heart and brain whereas no parasites were detected in similarly infected C57BL/10ScSn controls. The higher susceptibility of C57BL/10ScCr mice to neosporosis correlates with reduced interferon-gamma mRNA expression and increased IL-4 mRNA expression, comparatively to C57BL/10ScSn controls, detected in the spleen after the parasitic challenge. C57BL/10ScCr mice could thus be used as a new experimental model where to study immunobiological mechanisms associated with host susceptibility to neosporosis.

Suppression of pattern-recognition receptor TLR4 sensing does not alter lung responses to pneumovirus infection

Toll-like receptors (TLR) are an important component in the innate immune response to a wide variety of pathogens. Recently, a series of studies has addressed the hypothesis that TLR4 also participates in the host innate response against respiratory syncytial virus (RSV), the leading cause of lower respiratory tract infections in infants and young children. In most of the studies available, RSV, which is not a natural pathogen of mice, has been systematically used in mouse models of human bronchiolitis, with conflicting results. Pneumonia virus of mice (PVM), a member of the pneumovirus genus, shares many similarities with RSV. The serological and structural relationships that exist between them suggest that the immune response to these viruses may be similar in their respective natural hosts. To determine the role of TLR4 in host defense against PVM, TLR4-competent and TLR4-deficient mice were intranasally infected with PVM. Variation of body weight, pulmonary function values, histopathology, and pulmonary viral loads were analyzed. None of the investigated clinical, functional, histological and virological parameters was different between strains, which demonstrates that the sensitivity of the mouse to its natural pneumovirus infection is independent of the presence or absence of TLR4 sensing.

TLR-4 pathway mediates the inflammatory response but not bacterial elimination in E. coli pneumonia

We examined the role of Toll-like receptor (TLR)-4 in modifying the lung inflammatory response and its effects on the bacterial recovery from the lungs following inhaled Escherichia coli in two different strains of TLR-4 mutant mice that are hyporesponsive to LPS. The C57BL/10ScN(tlr4(lps-del)) mice containing a deletion mutation in the TLR-4 gene showed lower proinflammatory cytokine levels, lower lung MPO activity, and less parenchymal and peribronchial inflammation compared with the C57BL/10ScSn mice, a related TLR-4 wild-type substrain. However, the C57BL/10ScN(tlr4(lps-del)) mutant showed lower bacterial recovery in the lungs following inhaled E. coli associated with a rapid but transient increase in air space neutrophil counts at 6 h. In comparison, the C3H/HeJ(tlr4(Lps-d)) mutant mice containing a Pro712His substitution in TLR-4 demonstrated lower proinflammatory cytokine levels, lower lung MPO activity, and lower neutrophil accumulation in the air spaces but showed no differences in the bacterial burden of inhaled E. coli at 6 h, when compared with the TLR-4 wild-type C3H/HeSnJ mice. Thus two different TLR-4 mutants showed attenuated inflammatory responses in the lungs, but the reduced inflammatory responses were not consistently associated with either improved or impaired bacterial elimination from the lungs. Our findings indicate that the inflammatory response to inhaled E. coli is TLR-4 dependent, but bacterial elimination depends on other factors in addition to TLR-4.

A fetal sheep liver extract containing immunostimulatory substances including LPS acts as leukocyte activator in cells of LPS responder and non responder mice

Purified fractions from a fetal sheep liver extract (FSLE) were investigated, in a murine model, for induction of leukocyte stimulating activities. The fractions FSLE-1 and FSLE-2 induced splenocyte proliferation in vitro in C57Bl/10ScSn (LPS responder) mice comparable to LPS, and in C57Bl/10ScCr (LPS non responder) mice. They also stimulated the release of nitrogen radicals in bone marrow-derived macrophages (BMDM) from several mouse inbred strains including both C57Bl/10ScSn and C57Bl/10ScCr mice. Stimulation of NO production could be blocked by L-NMMA, an inhibitor of iNOS, and enhanced by the simultaneous addition of IFN-gamma. Moreover, stimulation of macrophages by FSLE-1 and FSLE-2 induced a cytostatic effect of the activated macrophages for Abelson 8-1 tumor cells. The stimulatory activity of the purified fractions is partially due to trace amounts of LPS derived from the fetal liver extract which was enriched during purification. Our results may help to explain the beneficial effect of the extract in patients which has been observed clinically.

Differential inflammatory activation of IL-6 (−/−) astrocytes

IL-6 is a major immunomodulatory cytokine with neuroprotective activity. The absence of interleukin-6 (IL-6) results in increased vulnerability of dopaminergic neurons to the neurotoxicant, MPTP, and a compromised reactive microgliosis. To determine how astrogliosis may contribute to nigrostriatal degeneration in IL-6 (-/-) mice, the inflammatory profiles of astrocytes of IL-6 genotype were compared. Fourteen cytokines and four chemokines were simultaneously assayed in the supernatants of LPS-stimulated primary astrocyte cultures. In a time course of 6, 18 and 48 h and LPS stimulations of 0, 0.1, 1, 10 and 100 ng/ml, IL-6 (-/-) astrocytes secreted significantly greater amounts of the pro-inflammatory cytokines IL-1alpha, IL-1beta and TNFalpha than did IL-6 (+/+) cells. Elevated levels of IL-10 and IL-12p40 were only detected at 48 h post-stimulation with greater IL-10 in IL-6 (-/-) supernatants and greater IL-12p40 in IL-6 (+/+) supernatants. IL-6 (+/+) astrocytes produced more G-CSF and GM-CSF when compared with IL-6 (-/-) astrocytes. Chemokine levels were greater in supernatants of IL-6 (+/+) astrocytes than IL-6 (-/-) cells prior to 48 h post-stimulation. At that time, higher levels of MIP-1alpha were maintained in IL-6 (+/+) supernatant, while similar levels of MCP-1 in supernatants of both IL-6 (+/+) and IL-6 (-/-) cells were measured. Additionally, LPS (100 ng/ml) resulted in greater levels of KC and Rantes in IL-6 (-/-) astrocyte supernatants compared with IL-6 (+/+) supernatants at that time. These results suggest that the autocrine modulatory activities of IL-6 affect multiple cytokine secretory pathways, which could participate in neurodegenerative processes.

The role of Toll-like receptors in the host response to viruses

The discovery of Toll-like receptors (TLR) has revolutionised our understanding of innate immunity. Numerous reviews have been written on the subject in the past few years. Here, we review the evidence that TLRs are involved in sensing and initiating anti-viral responses. There are now three strong lines of evidence that support such a role for TLRs. Firstly, TLRs 'recognise' virally derived molecules and are required for various virus-induced cellular effects. Secondly, TLRs trigger anti-viral signalling pathways leading to the induction of the interferon response. Thirdly, viral immune strategies employed against TLRs have been identified.

Expression of functional TLR4 confers proinflammatory responsiveness to Trypanosoma cruzi glycoinositolphospholipids and higher resistance to infection with T. cruzi

TLRs function as pattern recognition receptors in mammals and play an essential role in the recognition of microbial components. We found that the injection of glycoinositolphospholipids (GIPLs) from Trypanosoma cruzi into the peritoneal cavity of mice induced neutrophil recruitment in a TLR4-dependent manner: the injection of GIPL in the TLR4-deficient strain of mice (C57BL/10ScCr) caused no inflammatory response. In contrast, in TLR2 knockout mice, neutrophil chemoattraction did not differ significantly from that seen in wild-type controls. GIPL-induced neutrophil attraction and MIP-2 production were also severely affected in TLR4-mutant C3H/HeJ mice. The role of TLR4 was confirmed in vitro by testing genetically engineered mutants derived from TLR2-deficient Chinese hamster ovary (CHO)-K1 fibroblasts that were transfected with CD14 (CHO/CD14). Wild-type CHO/CD14 cells express the hamster TLR4 molecule and the mutant line, in addition, expresses a nonfunctional form of MD-2. In comparison to wild-type cells, mutant CHO/CD14 cells failed to respond to GIPLs, indicating a necessity for a functional TLR4/MD-2 complex in GIPL-induced NF-kappaB activation. Finally, we found that TLR4-mutant mice were hypersusceptible to T. cruzi infection, as evidenced by a higher parasitemia and earlier mortality. These results demonstrate that natural resistance to T. cruzi is TLR4 dependent, most likely due to TLR4 recognition of their GIPLs.

Chemokine gene expression in toll-like receptor-competent and -deficient mice infected with Leishmania major

We studied the expression of a subset of chemokines, including RANTES/CCL5, MIP-1alpha/CCL3, IP-10/CXCL10, and MCP-1/CCL2, in Toll-like receptor (TLR)-competent and -deficient mice after infection with Leishmania major. Chemokine expression at the site of infection (the footpad), in the draining lymph nodes and in the spleens of infected animals was determined by using two different methods of analysis. The results indicate that L. major infection causes overall upregulation of RANTES/CCL5, MIP-1alpha/CCL3, IP-10/CXCL10, and MCP-1/CCL2 in the footpads and lymph nodes, while expression of these chemokines is constitutive in the spleens of TLR4-competent mice (C57BL/10ScSn) and TLR4-deficient mice (C57BL10/ScN). Different patterns of expression were detected depending on the time postinfection, but there was little variation in the expression of these four chemokines in the presence or absence of TLR4.

Direct Toll-like receptor 2 mediated co-stimulation of T cells in the mouse system as a basis for chronic inflammatory joint disease

The pathogenesis of chronic inflammatory joint diseases such as adult and juvenile rheumatoid arthritis and Lyme arthritis is still poorly understood. Central to the various hypotheses in this respect is the notable involvement of T and B cells. Here we develop the premise that the nominal antigen-independent, polyclonal activation of preactivated T cells via Toll-like receptor (TLR)-2 has a pivotal role in the initiation and perpetuation of pathogen-induced chronic inflammatory joint disease. We support this with the following evidence. Both naive and effector T cells express TLR-2. A prototypic lipoprotein, Lip-OspA, from the etiological agent of Lyme disease, namely Borrelia burgdorferi, but not its delipidated form or lipopolysaccharide, was able to provide direct antigen-nonspecific co-stimulatory signals to both antigen-sensitized naive T cells and cytotoxic T lymphocyte (CTL) lines via TLR-2. Lip-OspA induced the proliferation and interferon (IFN)-γ secretion of purified, anti-CD3-sensitized, naive T cells from C57BL/6 mice but not from TLR-2-deficient mice. Induction of proliferation and IFN-γ secretion of CTL lines by Lip-OspA was independent of T cell receptor (TCR) engagement but was considerably enhanced after suboptimal TCR activation and was inhibitable by monoclonal antibodies against TLR-2.

Reduced myocardial ischemia-reperfusion injury in toll-like receptor 4-deficient mice

BACKGROUND

Myocardial ischemia and reperfusion-induced tissue injury involve a robust inflammatory response, but the proximal events in reperfusion injury remain incompletely defined. Toll-like receptor 4 (TLR4) is a proximal signaling receptor in innate immune responses to lipopolysaccharide of Gram-negative pathogens. TLR4 is also expressed in the heart and vasculature, but a role for TLR4 in the myocardial response to injury separate from microbial pathogens has not been examined. This study assessed the role of TLR4 in myocardial infarction and inflammation in a murine model of ischemia-reperfusion injury.

METHODS AND RESULTS

Myocardial ischemia-reperfusion (MIR) was performed on 2 strains of TLR4-deficient mice (C57/BL10 ScCr and C3H/HeJ) and controls (C57/BL10 ScSn and C3H/OuJ). Mice were subjected to 1 hour of coronary ligation, followed by 24 hours of reperfusion. TLR4-deficient mice sustained significantly smaller infarctions compared with control mice given similar areas at risk. Fewer neutrophils infiltrated the myocardium of TLR4-deficient Cr mice after MIR, indicated by less myeloperoxidase activity and fewer CD45/GR1-positive cells. The myocardium of TLR4-deficient Cr mice contained fewer lipid peroxides and less complement deposition compared with control mice after MIR. Serum levels of interleukin-12, interferon-gamma, and endotoxin were not increased after ischemia-reperfusion. Neutrophil trafficking in the peritoneum was similar in all strains after injection of thioglycollate.

CONCLUSIONS

TLR4-deficient mice sustain smaller infarctions and exhibit less inflammation after myocardial ischemia-reperfusion injury. The data suggest that in addition to its role in innate immune responses, TLR4 serves a proinflammatory role in murine myocardial ischemia-reperfusion injury.

Toll-like receptor 4 contributes to efficient control of infection with the protozoan parasite Leishmania major

The essential role of Toll-like receptors (TLR) in innate immune responses to bacterial pathogens is increasingly recognized, but very little is known about the role of TLRs in host defense against infections with eukaryotic pathogens. For the present study, we investigated whether TLRs contribute to the innate and acquired immune response to infection with the intracellular protozoan parasite Leishmania major. Our results show that TLR4 contributes to the control of parasite growth in both phases of the immune response. We also addressed the mechanism that results in killing or growth of the intracellular parasites. Control of parasite replication correlates with the early induction of inducible nitric oxide synthase in TLR4-competent mice, whereas increased parasite survival in host cells from TLR4-deficient mice correlates with a higher activity of arginase, an enzyme known to promote parasite growth. This is the first study showing that TLR4 contributes to the effective control of Leishmania infection in vivo.

Infection of C57BL/10ScCr and C57BL/10ScNCr mice with Leishmania major reveals a role for Toll-like receptor 4 in the control of parasite replication

The innate immune system is essential for host defense; it senses the presence of potentially pathogenic-invading microorganisms, and the contribution of Toll-like receptors (TLRs) to this response is increasingly recognized. In the present study, we investigated the contribution of TLR4 to the course of cutaneous leishmaniasis in vivo. We used C57BL/10ScNCr (TLR4(0/0)) and C57BL/10ScCr [TLR4/interleukin-12 (IL-12)Rbeta2(0/0)] mice and compared the course of Leishmania major infection, parasite load, cell recruitment, and cytokine profile with those of wild-type C57BL/10ScSn mice. Our results confirm the importance of IL-12 receptor-mediated signaling in resistance to L. major infections. Importantly, we show that the lack of TLR4 results in an increased permissiveness for parasite growth during the innate and adaptive phase of the immune response and in delayed healing of the cutaneous lesions. The use of the tlr4 transgenic mouse strain TCr5 demonstrated unequivocally that TLR4 contributes to the efficient control of Leishmania growth in vivo.

Development of TCR.ALPHA..BETA. CD8.ALPHA..ALPHA. Intestinal Intraepithelial Lymphocytes Is Promoted by Interleukin-15-Producing Epithelial Cells Constitutively Stimulated by Gram-Negative Bacteria via TLR4

The microbes present in the intestine have a strong influence on the development and maturation of lymphoid organs. The cross-talk mechanisms between intestinal intraepithelial lymphocytes (i-IEL) and noninvasive microbes are still poorly understood. The influence of microbes and lipopolysaccharides on the development of i-IEL, especially the TCR alpha beta(+) CD8 alpha alpha subset, was investigated using the different TLR4-mutant mouse strains C3H/HeJ, BALB/lps(d), and C57BL/10ScCr. Intestinal epithelial cells (i-EC) from TLR4-mutant strains did not express interleukin (IL)-15 mRNA, while IL-15 mRNA expression in i-EC from the corresponding wild-type, C3H/He, BALB/c, and C57BL/10ScSn mice was detected. The development of TCR alpha beta(+) CD8 alpha alpha cells in i-IEL significantly decreased in TLR4-mutant mice compared with the corresponding wild-type mice, while other T cell subsets in i-IEL showed similar percentages in the TLR4-mutant and wild-type mice. Adult thymectomized (ATx-) and lethally irradiated C3H/HeJ mice reconstituted with T cell-depleted bone marrow cells from C3H/He mice showed a significantly lower percentage of TCR alpha beta CD8 alpha alpha i-IEL than ATx-C3H/He mice after transfer of C3H/HeJ BM cells. The percentage of TCR alpha beta CD8 alpha alpha i-IEL and IL-15 mRNA expression in i-EC from BALB/lps(d) mice did not increase during Salmonella typhimurium infection but was significantly enhanced during Listeria monocytogenes infection. Our findings suggest that LPS induces IL-15 production by i-EC, resulting in the development of TCR alpha beta CD8 alpha alpha i-IEL.

Viruses and Toll-like receptors

Recently a number of viruses, including a poxvirus, herpesvirus, retrovirus and two paramyxoviruses, have been shown to activate cells via Toll-like receptor family members. Here we postulate that although activation via Toll-like receptor molecules can lead to anti-viral innate immune responses, in some cases viruses may use these responses to ameliorate infection.

Genetic control of the innate immune response

BACKGROUND

Susceptibility to infectious diseases is directed, in part, by the interaction between the invading pathogen and host macrophages. This study examines the influence of genetic background on host-pathogen interactions, by assessing the transcriptional responses of macrophages from five inbred mouse strains to lipopolysaccharide (LPS), a major determinant of responses to gram-negative microorganisms.

RESULTS

The mouse strains examined varied greatly in the number, amplitude and rate of induction of genes expressed in response to LPS. The response was attenuated in the C3H/HeJlpsd strain, which has a mutation in the LPS receptor Toll-like receptor 4 (TLR4). Variation between mouse strains allowed clustering into early (C57Bl/6J and DBA/2J) and delayed (BALB/c and C3H/ARC) transcriptional phenotypes. There was no clear correlation between gene induction patterns and variation at the Bcg locus (Slc11A1) or propensity to bias Th1 versus Th2 T cell activation responses.

CONCLUSION

Macrophages from each strain responded to LPS with unique gene expression profiles. The variation apparent between genetic backgrounds provides insights into the breadth of possible inflammatory responses, and paradoxically, this divergence was used to identify a common transcriptional program that responds to TLR4 signalling, irrespective of genetic background. Our data indicates that many additional genetic loci control the nature and the extent of transcriptional responses promoted by a single pathogen-associated molecular pattern (PAMP), such as LPS.

Overexpression of Toll-like receptor 4 amplifies the host response to lipopolysaccharide and provides a survival advantage in transgenic mice

Toll-like receptors are transmembrane proteins that are involved in the innate immune recognition of microbial constituents. Among them, Toll-like receptor 4 (Tlr4) is a crucial signal transducer for LPS, the major component of Gram-negative bacteria outer cell membrane. The contribution of Tlr4 to the host response to LPS and to infection with virulent Salmonella typhimurium was studied in four transgenic (Tg) strains including three overexpressing Tlr4. There was a good correlation between the level of Tlr4 mRNA expression and the sensitivity to LPS both in vitro and in vivo: Tg mice possessing the highest number of Tlr4 copies respond the most to LPS. Overexpression of Tlr4 by itself appears to have a survival advantage in Tg mice early during infection: animals possessing more than two copies of the gene survived longer and in a greater percentage to Salmonella infection. The beneficial effect of Tlr4 overexpression is greatly enhanced when the mice present a wild-type allele at natural resistance-associated macrophage protein 1, another critical innate immune gene involved in resistance to infection with Salmonella. Tlr4 and natural resistance-associated macrophage protein 1 exhibit functional epistatic interaction to improve the capacity of the host to control bacterial replication. However, this early improvement in disease resistance is not conducted later during infection, because mice overexpressing Tlr4 developed an excessive inflammatory response detrimental to the host.

TOLL-like receptors linking innate and adaptive immune response

Invading pathogens are controlled by the innate and adaptive arms of the immune system. Adaptive immunity, which is mediated by B and T lymphocytes, recognises pathogens by rearranged high affinity receptors. However, the establishment of adaptive immunity is often not rapid enough to eradicate microorganisms as it involves cell proliferation, gene activation and protein synthesis. More rapid defense mechanisms are provided by innate immunity, which recognises invading pathogens by germ-line-encoded pattern recognition receptors (PRR). Recent evidence shows that this recognition can mainly be attributed to the family of TOLL-like receptors (TLR). Binding of pathogen-associated molecular patterns (PAMP) to TLR induces the production of reactive oxygen and nitrogen intermediates (ROI and RNI), pro-inflammatory cytokines, and up-regulates expression of co-stimulatory molecules, subsequently initiating the adaptive immunity. In this review, we will summarize the discovery and the critical roles of the TLR family in host defense, briefly allude to signaling mechanisms mediating the response to TLR ligands, and will provide an update on current knowledge regarding the ligand specificity of these receptors and their role in immunity of domestic animals, particularly cattle.

Beneficial or deleterious effects of a preexisting hypersensitivity to bacterial components on the course and outcome of infection

Priming with heat-killed Propionibacterium acnes enhances the sensitivity of mice to lipopolysaccharide (LPS) and other biologically active bacterial components. We show that P. acnes priming has protective and deleterious effects on a subsequent serovar Typhimurium infection. It may result in a complete protection or prolonged survival, or it may accelerate mortality of the infected mice, depending on the number of serovar Typhimurium bacteria administered and on the degree of LPS hypersensitivity at the time of infection. Both effects of P. acnes-induced hypersensitivity are mediated by gamma interferon (IFN-gamma) and are based on a differential activation of the innate immune mechanisms which recognize and react against the LPS present in infecting bacteria. In P. acnes-primed mice null for LPS-binding protein (LBP(-/-) mice), the impaired LPS recognition, due to the absence of LBP, resulted in a higher resistance to serovar Typhimurium infection. A similar P. acnes priming of mice had a protective, but no deleterious effect on a subsequent L. monocytogenes infection. This effect was IFN-gamma dependent but independent of LBP.

TLR4 inactivation and rBPI(21) block burn-induced myocardial contractile dysfunction

Both large burns and severe gram-negative sepsis are associated with acute myocardial contractile dysfunction. Because others have reported that burn injury may be followed by transient endotoxemia, we hypothesized that bacterial endotoxin induces contractile impairment after burn trauma. We tested this hypothesis in two rodent models. In each model, postburn myocardial contractility was assessed using Langendorff preparations of excised hearts. In the first model, mice expressing either a mutant form of or no Toll-like receptor 4 (TLR4), a critical element of the mammalian endotoxin receptor, were resistant to postburn myocardial contractile dysfunction. In the second model, starting 30 min or 4 h after burn injury, rats were infused with recombinant bactericidal/permeability-increasing protein (rBPI(21)), a protein that binds and neutralizes endotoxin. Hearts from rBPI(21)-treated animals were completely protected from postburn contractile impairment. Because burn-induced contractile dysfunction can be prevented either by blocking signaling through the endotoxin receptor or by neutralizing circulating LPS, bacterial endotoxin may contribute to impaired myocardial contractility after burn injury.

Initial responses to endotoxins and Gram-negative bacteria

The innate immune system initiates host defence against invasive microbial pathogens using specific recognition mechanisms. Here we review the current concepts and the molecular basis of innate immune responses to bacterial infections, focusing our attention on the actors involved in the response to Gram-negative bacteria. Lipopolysaccharide (LPS) is the major virulence factor of Gram-negative bacteria. During the past decade, enormous progress has been obtained in the elucidation of LPS recognition and signalling in mammalian phagocytes. According to the current model, recognition of LPS is initialized by the cooperative interplay between the LPS-binding protein (LBP), the membrane-bound or soluble forms of CD14 and the recently identified Toll-like receptor 4 (TLR4)-MD-2 complex. Recognition of LPS leads to the rapid activation of an intracellular signalling pathway, highly homologous to the signalling pathway of interleukin-1, which results in the release of pro-inflammatory mediators. In vivo models in which animals are challenged with LPS or Gram-negative bacteria have highlighted opposite roles for LBP, CD14 and TLRs. Regarding LPS challenge, there is a large body of evidence in favour of a detrimental role played by LBP, CD14 and TLRs. These molecules sensitize the host to a LPS-induced uncontrolled acute inflammatory response that results in animal death. However, when the host is in the presence of virulent Gram-negative bacteria, the invading pathogens must be held in check by the innate immune system until a specific immune response is mounted. Under these conditions, LBP, CD14 and TLRs are required to trigger a pro-inflammatory response which is crucial for keeping infection under control. Therefore, caution should be the rule about the development of therapeutic approaches aimed at blocking the pro-inflammatory response during Gram-negative infections.

Toll-like receptor 4 (TLR4)-deficient murine macrophage cell line as an in vitro assay system to show TLR4-independent signaling of Bacteroides fragilis lipopolysaccharide

Bacterial lipopolysaccharides (LPS) activate cells of innate immunity, such as macrophages, by stimulating signaling through toll-like receptor 4 (TLR4). We and others have hypothesized that LPS derived from different bacterial species may function through TLR4-independent mechanisms. To test this hypothesis, we have generated using a nonviral transformation procedure a bone marrow-derived macrophage cell line called 10ScNCr/23 from mouse strain C57BL/10ScNCr. This mouse strain has a deletion of the TLR4 locus, causing the mouse strain to be nonresponsive to stimulation by LPS from Escherichia coli while responding normally to other bacterial substrates, such as lipoteichoic acid (LTA) from Staphylococcus aureus, which signal TLR4 independently. Stimulation with LTA induces five- and sixfold increases in 10ScNCr/23 cell line tumor necrosis factor alpha and macrophage inflammatory protein-2 (MIP-2) secretion, but no increases in either cytokine were found when cells were stimulated with E. coli LPS. Bacteroides fragilis-derived LPS, however, can effectively stimulate MIP-2 expression in the absence of functional TLR4 in the 10ScNCr/23 cell line. This gives rise to the notion that LPS from some bacterial species will utilize alternative receptors to stimulate the innate immune response.