A phospho-tyrosine-based signaling module using SPOP, CSK, and LYN controls TLR-induced IRF activity

Toll-like receptors (TLRs) recognize pathogen- and host-derived factors and control immune responses via the adaptor protein MyD88 and members of the interferon regulatory transcription factor (IRF) family. IRFs orchestrate key effector functions, including cytokine release, cell differentiation, and, under certain circumstances, inflammation pathology. Here, we show that IRF activity is generically controlled by the Src kinase family member LYN, which phosphorylates all TLR-induced IRFs at a conserved tyrosine residue, resulting in K48-linked polyubiquitination and proteasomal degradation of IRFs. We further show that LYN activity is controlled by the upstream kinase C-terminal Src kinase (CSK), whose activity, in turn, is controlled by the adaptor protein SPOP, which serves as molecular bridge to recruit CSK into the TLR signaling complex and to activate CSK catalytic activity. Consistently, deletion of SPOP or CSK results in increased LYN activity, LYN-directed IRF degradation, and inhibition of IRF transcriptional activity. Together, the data reveal a key regulatory mechanism for IRF family members controlling TLR biology.

A rapid and affordable point of care test for antibodies against SARS-CoV-2 based on hemagglutination and artificial intelligence interpretation

Diagnostic tests that detect antibodies (AB) against SARS-CoV-2 for evaluation of seroprevalence and guidance of health care measures are important tools for managing the COVID-19 pandemic. Current tests have certain limitations with regard to turnaround time, costs and availability, particularly in point-of-care (POC) settings. We established a hemagglutination-based AB test that is based on bi-specific proteins which contain a dromedary-derived antibody (nanobody) binding red blood cells (RBD) and a SARS-CoV-2-derived antigen, such as the receptor-binding domain of the Spike protein (Spike-RBD). While the nanobody mediates swift binding to RBC, the antigen moiety directs instantaneous, visually apparent hemagglutination in the presence of SARS-CoV-2-specific AB generated in COVID-19 patients or vaccinated individuals. Method comparison studies with assays cleared by emergency use authorization demonstrate high specificity and sensitivity. To further increase objectivity of test interpretation, we developed an image analysis tool based on digital image acquisition (via a cell phone) and a machine learning algorithm based on defined sample-training and -validation datasets. Preliminary data, including a small clinical study, provides proof of principle for test performance in a POC setting. Together, the data support the interpretation that this AB test format, which we refer to as 'NanoSpot.ai', is suitable for POC testing, can be manufactured at very low costs and, based on its generic mode of action, can likely be adapted to a variety of other pathogens.

A Rapid and Affordable Point-of-care Test for Detection of SARS-Cov-2-Specific Antibodies Based on Hemagglutination and Artificial Intelligence-Based Image Interpretation

Diagnostic tests that detect antibodies (AB) against SARS-CoV-2 for evaluation of seroprevalence and guidance of health care measures are important tools for managing the COVID-19 pandemic. Current tests have certain limitations with regard to turnaround time, costs and availability, particularly in point-of-care (POC) settings. We established a hemagglutination-based AB test (HAT) that is based on bi-specific proteins which contain a dromedary-derived antibody (nanobody) binding red blood cells (RBD) and a SARS-CoV-2-derived antigen, such as the receptor-binding domain of the Spike protein (Spike-RBD). While the nanobody mediates swift binding to RBC, the antigen moiety directs instantaneous, visually apparent hemagglutination in the presence of SARS-CoV-2-specific AB generated in COVID-19 patients or vaccinated individuals. Method comparison studies with assays cleared by emergency use authorization (EUA) demonstrate high specificity and sensitivity. To further increase objectivity of test interpretation, we developed an image analysis tool based on digital image acquisition (via a cell phone) and a machine learning algorithm based on defined sample-training and –validation datasets.

Preliminary data, including a small clinical study, provides proof of principle for test performance in a POC setting. Together, the data support the interpretation that this AB test format, which we refer to as ‘NanoSpot.ai’, is suitable for POC testing, can be manufactured at very low costs and, based on its generic mode of action, can likely be adapted to a variety of other pathogens.

Patrolling monocytes promote the pathogenesis of early lupus-like glomerulonephritis

Systemic lupus erythematosus (SLE) is a complex autoimmune disease with genetic and environmental contributions. Hallmarks of the disease are the appearance of immune complexes (IC) containing autoreactive Abs and TLR-activating nucleic acids, whose deposition in kidney glomeruli is suspected to promote tissue injury and glomerulonephritis (GN). Here, using a mouse model based on the human SLE susceptibility locus TNFAIP3-interacting protein 1 (TNIP1, also known as ABIN1), we investigated the pathogenesis of GN. We found that GN was driven by TLRs but, remarkably, proceeded independently of ICs. Rather, disease in 3 different mouse models and patients with SLE was characterized by glomerular accumulation of patrolling monocytes (PMos), a cell type with an emerging key function in vascular inflammation. Consistent with such function in GN, monocyte-specific deletion of ABIN1 promoted kidney disease, whereas selective elimination of PMos provided protection. In contrast to GN, PMo elimination did not protect from reduced survival or disease symptoms such as IC generation and splenomegaly, suggesting that GN and other inflammatory processes are governed by distinct pathogenic mechanisms. These data identify TLR-activated PMos as the principal component of an intravascular process that contributes to glomerular inflammation and kidney injury.

Triaryl Pyrazole Toll-Like Receptor Signaling Inhibitors: Structure-Activity Relationships Governing Pan- and Selective Signaling Inhibitors

The immune system uses members of the toll-like receptor (TLR) family to recognize a variety of pathogen- and host-derived molecules in order to initiate immune responses. Although TLR-mediated, pro-inflammatory immune responses are essential for host defense, prolonged and exaggerated activation can result in inflammation pathology that manifests in a variety of diseases. Therefore, small-molecule inhibitors of the TLR signaling pathway might have promise as anti-inflammatory drugs. We previously identified a class of triaryl pyrazole compounds that inhibit TLR signaling by modulation of the protein-protein interactions essential to the pathway. We have now systematically examined the structural features essential for inhibition of this pathway, revealing characteristics of compounds that inhibited all TLRs tested (pan-TLR signaling inhibitors) as well as compounds that selectively inhibited certain TLRs. These findings reveal interesting classes of compounds that could be optimized for particular inflammatory diseases governed by different TLRs.

Identification of Toll-like receptor signaling inhibitors based on selective activation of hierarchically acting signaling proteins

Toll-like receptors (TLRs) recognize various pathogen- and host tissue-derived molecules and initiate inflammatory immune responses. Exaggerated or prolonged TLR activation, however, can lead to etiologically diverse diseases, such as bacterial sepsis, metabolic and autoimmune diseases, or stroke. Despite the apparent medical need, no small-molecule drugs against TLR pathways are clinically available. This may be because of the complex signaling mechanisms of TLRs, which are governed by a series of protein-protein interactions initiated by Toll/interleukin-1 receptor homology domains (TIR) found in TLRs and the cytoplasmic adaptor proteins TIRAP and MyD88. Oligomerization of TLRs with MyD88 or TIRAP leads to the recruitment of members of the IRAK family of kinases and the E3 ubiquitin ligase TRAF6. We developed a phenotypic drug screening system based on the inducible homodimerization of either TIRAP, MyD88, or TRAF6, that ranked hits according to their hierarchy of action. From a bioactive compound library, we identified methyl-piperidino-pyrazole (MPP) as a TLR-specific inhibitor. Structure-activity relationship analysis, quantitative proteomics, protein-protein interaction assays, and cellular thermal shift assays suggested that MPP targets the TIR domain of MyD88. Chemical evolution of the original MPP scaffold generated compounds with selectivity for distinct TLRs that interfered with specific TIR interactions. Administration of an MPP analog to mice protected them from TLR4-dependent inflammation. These results validate this phenotypic screening approach and suggest that the MPP scaffold could serve as a starting point for the development of anti-inflammatory drugs.

EMMPRIN (CD 147) is a central activator of extracellular matrix degradation by Chlamydia pneumoniae-infected monocytes

Chlamydia (C.) pneumoniae are thought to infect monocytes and use them as vectors into the vessel wall, where they may accelerate atherosclerosis. We investigated the effects of C. pneumoniae on monocytic matrix metalloproteinase (MMP) activation with focus on the role of the extracellular matrix metalloproteinase inducer EMMPRIN. Human monocytes or monocytic MonoMac6 cells were infected with C. pneumoniae. Infection enhanced mRNA-and surface expression of EMMPRIN and Membrane-type-1 Matrix Metalloproteinase (MT1-MMP), plus the secretion of MMP-7, MMP-9 and the urokinase receptor (uPAR). Chlamydial heat shock protein 60 was identified to be partially responsible for EMMPRIN and MMP-9 induction, while C. trachomatis-infection had no stimulatory effect, indicatinga C. pneumoniae-specific activation pathway. Suppression of EMMPRIN by gene silencing almost completely hindered the induction of MT1-MMP and MMP-9 by C. pneumoniae, suggesting a predominant regulatory role for EMMPRIN. Moreover, C. pneumoniae-infected monocytes exhibited increased MMP-and plasmin-dependent migration through “matrigel”. Additionally, incubation of SMCs with supernatants of C. pneumoniae-infected monocytes induced MMP-2 activation, which was inhibited by IL1-Receptor antagonist or anti-IL-6-mAb, indicating paracrine intercellular activation pathways. In conclusion,C. pneumoniae induce MMP activity directly in monocytes through an EMMPRINdependent pathway and indirectly in SMCs via monocytederived cytokines.

SHARPIN controls the development of regulatory T cells

SHARPIN is an essential component of the linear ubiquitin chain assembly complex (LUBAC) complex that controls signalling pathways of various receptors, including the tumour necrosis factor receptor (TNFR), Toll-like receptor (TLR) and antigen receptor, in part by synthesis of linear, non-degrading ubiquitin chains. Consistent with SHARPIN's function in different receptor pathways, the phenotype of SHARPIN-deficient mice is complex, including the development of inflammatory systemic and skin diseases, the latter of which depend on TNFR signal transduction. Given the established function of SHARPIN in primary and malignant B cells, we hypothesized that SHARPIN might also regulate T-cell receptor (TCR) signalling and thereby control T-cell biology. Here, we focus primarily on the role of SHARPIN in T cells, specifically regulatory T (Treg) cells. We found that SHARPIN-deficient (Sharpin(cpdm/cpdm) ) mice have significantly reduced numbers of FOXP3(+) Treg cells in lymphoid organs and the peripheral blood. Competitive reconstitution of irradiated mice with mixed bone marrow from wild-type and SHARPIN-deficient mice revealed an overall reduced thymus population with SHARPIN-deficient cells with almost complete loss of thymic Treg development. Consistent with this cell-intrinsic function of SHARPIN in Treg development, TCR stimulation of SHARPIN-deficient thymocytes revealed reduced activation of nuclear factor-κB and c-Jun N-terminal kinase, establishing a function of SHARPIN in TCR signalling, which may explain the defective Treg development. In turn, in vitro generation and suppressive activity of mature SHARPIN-deficient Treg cells were comparable to wild-type cells, suggesting that maturation, but not function, of SHARPIN-deficient Treg cells is impaired. Taken together, these findings show that SHARPIN controls TCR signalling and is required for efficient generation of Treg cells in vivo, whereas the inhibitory function of mature Treg cells appears to be independent of SHARPIN.

Quantitative proteomic analysis of the influenza A virus nonstructural proteins NS1 and NS2 during natural cell infection identifies PACT as an NS1 target protein and antiviral host factor

Influenza A virus (IAV) replication depends on the interaction of virus proteins with host factors. The viral nonstructural protein 1 (NS1) is essential in this process by targeting diverse cellular functions, including mRNA splicing and translation, cell survival, and immune defense, in particular the type I interferon (IFN-I) response. In order to identify host proteins targeted by NS1, we established a replication-competent recombinant IAV that expresses epitope-tagged forms of NS1 and NS2, which are encoded by the same gene segment, allowing purification of NS proteins during natural cell infection and analysis of interacting proteins by quantitative mass spectrometry. We identified known NS1- and NS2-interacting proteins but also uncharacterized proteins, including PACT, an important cofactor for the IFN-I response triggered by the viral RNA-sensor RIG-I. We show here that NS1 binds PACT during virus replication and blocks PACT/RIG-I-mediated activation of IFN-I, which represents a critical event for the host defense. Protein interaction and interference with IFN-I activation depended on the functional integrity of the highly conserved RNA binding domain of NS1. A mutant virus with deletion of NS1 induced high levels of IFN-I in control cells, as expected; in contrast, shRNA-mediated knockdown of PACT compromised IFN-I activation by the mutant virus, but not wild-type virus, a finding consistent with the interpretation that PACT (i) is essential for IAV recognition and (ii) is functionally compromised by NS1. Together, our data describe a novel approach to identify virus-host protein interactions and demonstrate that NS1 interferes with PACT, whose function is critical for robust IFN-I production.

IMPORTANCE

Influenza A virus (IAV) is an important human pathogen that is responsible for annual epidemics and occasional devastating pandemics. Viral replication and pathogenicity depends on the interference of viral factors with components of the host defense system, particularly the type I interferon (IFN-I) response. The viral NS1 protein is known to counteract virus recognition and IFN-I production, but the molecular mechanism is only partially defined. We used a novel proteomic approach to identify host proteins that are bound by NS1 during virus replication and identified the protein PACT, which had previously been shown to be involved in virus-mediated IFN-I activation. We find that NS1 prevents PACT from interacting with an essential component of the virus recognition pathway, RIG-I, thereby disabling efficient IFN-I production. These observations provide an important piece of information on how IAV efficiently counteracts the host immune defense.

Hematopoietic progenitor cell lines with myeloid and lymphoid potential

Investigation of immune cell differentiation and function is limited by shortcomings of suitable and scalable experimental systems. Here we show that an estrogen–regulated form of HOXB8 that is retrovirally delivered into mouse bone marrow cells can be used along with FLT3 ligand to conditionally immortalize early hematopoietic progenitor cells (Hoxb8–FL). Hoxb8–FL cells have lost self–renewal capacity and megakaryocyte/ erythroid lineage potential, but sustain myeloid and lymphoid potential. Hoxb8–FL cells differentiate in vitro and in vivo into different myeloid and lymphoid cell types, including macrophages, granulocytes, dendritic cells and B– and T–lymphocytes, which are phenotypically and functionally indistinguishable from their primary counterparts. Quantitative in vitro cell lineage potential assays implicate that myeloid and B–cell potential of Hoxb8–FL cells is comparable to primary lymphoid–primed multipotent progenitors, while T–cell potential is comparatively reduced. Given the simplicity and unlimited proliferative capacity of Hoxb8–FL cells, this system provides unique opportunities to investigate cell differentiation and immune cell functions.

NIK prevents the development of hypereosinophilic syndrome-like disease in mice independent of IKKα activation

Immune cell-mediated tissue injury is a common feature of different inflammatory diseases, yet the pathogenetic mechanisms and cell types involved vary significantly. Hypereosinophilic syndrome (HES) represents a group of inflammatory diseases that is characterized by increased numbers of pathogenic eosinophilic granulocytes in the peripheral blood and diverse organs. On the basis of clinical and laboratory findings, various forms of HES have been defined, yet the molecular mechanism and potential signaling pathways that drive eosinophil expansion remain largely unknown. In this study, we show that mice deficient of the serine/threonine-specific protein kinase NF-κB-inducing kinase (NIK) develop a HES-like disease, reflected by progressive blood and tissue eosinophilia, tissue injury, and premature death at around 25-30 wk of age. Similar to the lymphocytic form of HES, CD4(+) T cells from NIK-deficient mice express increased levels of Th2-associated cytokines, and eosinophilia and survival of NIK-deficient mice could be prevented completely by genetic ablation of CD4(+) T cells. Experiments based on bone marrow chimeric mice, however, demonstrated that inflammation in NIK-deficient mice depended on radiation-resistant tissues, implicating that NIK-deficient immune cells mediate inflammation in a nonautonomous manner. Surprisingly, disease development was independent of NIK's known function as an IκB kinase α (IKKα) kinase, because mice carrying a mutation in the activation loop of IKKα, which is phosphorylated by NIK, did not develop inflammatory disease. Our data show that NIK activity in nonhematopoietic cells controls Th2 cell development and prevents eosinophil-driven inflammatory disease, most likely using a signaling pathway that operates independent of the known NIK substrate IKKα.

The E3 ubiquitin ligase mind bomb-2 (MIB2) protein controls B-cell CLL/lymphoma 10 (BCL10)-dependent NF-κB activation

B-cell CLL/lymphoma 10 (BCL10) is crucial for the activation of NF-κB in numerous immune receptor signaling pathways, including the T-cell receptor (TCR) and B-cell receptor signaling pathways. However, the molecular mechanisms that lead to signal transduction from BCL10 to downstream NF-κB effector kinases, such as TAK1 and components of the IKK complex, are not entirely understood. Here we used a proteomic approach and identified the E3 ligase MIB2 as a novel component of the activated BCL10 complex. In vitro translation and pulldown assays suggest direct interaction between BCL10 and MIB2. Overexpression experiments show that MIB2 controls BCL10-mediated activation of NF-κB by promoting autoubiquitination and ubiquitination of IKKγ/NEMO, as well as recruitment and activation of TAK1. Knockdown of MIB2 inhibited BCL10-dependent NF-κB activation. Together, our results identify MIB2 as a novel component of the activated BCL10 signaling complex and a missing link in the BCL10-dependent NF-κB signaling pathway.

Induction of a homeostatic circuit in lung tissue by microbial compounds

TGFbeta presented and activated by integrin alphavbeta6 expressed on alveolar epithelial cells (AEC) continuously inhibits the functionality of alveolar macrophages (AM). Despite this inhibition, AM retain their ability to respond to inhaled microorganisms. Herein we describe a homeostatic circuit through which the effects of microbial products on macrophages transiently circumvent this inhibition by repressing alphavbeta6 expression on AEC. Subsequent production of matrix metalloproteinases by activated AM activates latent TGFbeta, reinduces alphavbeta6 expression, and thereby reinstates tonic inhibition of AM function. Our results reveal how AM can be activated while minimizing their potential to inflict collateral damage to the adjacent lung tissue and indicate that tissue-specific microenvironmental factors shape organ-specific defense strategies against microbial invasion.

EMMPRIN (CD 147) is a central activator of extracellular matrix degradation by Chlamydia pneumoniae-infected monocytes. Implications for plaque rupture

Chlamydia (C.) pneumoniae are thought to infect monocytes and use them as vectors into the vessel wall, where they may accelerate atherosclerosis. We investigated the effects of C. pneumoniae on monocytic matrix metalloproteinase (MMP) activation with focus on the role of the extracellular matrix metalloproteinase inducer EMMPRIN. Human monocytes or monocytic MonoMac6 cells were infected with C. pneumoniae. Infection enhanced mRNA- and surface expression of EMMPRIN and Membrane-type-1 Matrix Metalloproteinase (MT1-MMP), plus the secretion of MMP-7, MMP-9 and the urokinase receptor (uPAR). Chlamydial heat shock protein 60 was identified to be partially responsible for EMMPRIN and MMP-9 induction, while C. trachomatis-infection had no stimulatory effect, indicating a C. pneumoniae-specific activation pathway. Suppression of EMMPRIN by gene silencing almost completely hindered the induction of MT1-MMP and MMP-9 by C. pneumoniae, suggesting a predominant regulatory role for EMMPRIN. Moreover, C. pneumoniae-infected monocytes exhibited increased MMP- and plasmin-dependent migration through "matrigel". Additionally, incubation of SMCs with supernatants of C. pneumoniae-infected monocytes induced MMP-2 activation, which was inhibited by IL1-Receptor antagonist or anti-IL-6-mAb, indicating paracrine intercellular activation pathways. In conclusion, C.pneumoniae induce MMP activity directly in monocytes through an EMMPRIN-dependent pathway and indirectly in SMCs via monocyte-derived cytokines.

Specificity in Toll-like receptor signalling through distinct effector functions of TRAF3 and TRAF6

Toll-like receptors (TLRs) are activated by pathogen-associated molecular patterns to induce innate immune responses and production of pro-inflammatory cytokines, interferons and anti-inflammatory cytokines. TLRs activate downstream effectors through adaptors that contain Toll/interleukin-1 receptor (TIR) domains, but the mechanisms accounting for diversification of TLR effector functions are unclear. To dissect biochemically TLR signalling, we established a system for isolating signalling complexes assembled by dimerized adaptors. Using MyD88 as a prototypical adaptor, we identified TNF receptor-associated factor 3 (TRAF3) as a new component of TIR signalling complexes that is recruited along with TRAF6. Using myeloid cells from TRAF3- and TRAF6-deficient mice, we show that TRAF3 is essential for the induction of type I interferons (IFN) and the anti-inflammatory cytokine interleukin-10 (IL-10), but is dispensable for expression of pro-inflammatory cytokines. In fact, TRAF3-deficient cells overproduce pro-inflammatory cytokines owing to defective IL-10 production. Despite their structural similarity, the functions of TRAF3 and TRAF6 are largely distinct. TRAF3 is also recruited to the adaptor TRIF (Toll/IL-1 receptor domain-containing adaptor-inducing IFN-beta) and is required for marshalling the protein kinase TBK1 (also called NAK) into TIR signalling complexes, thereby explaining its unique role in activation of the IFN response.

Induction and inhibition of the Th2 phenotype spread: implications for childhood asthma

The interactions between genetic and environmental factors play a major role in the development of childhood asthma. We hypothesized that a pre-existing Th2/asthmatic response can promote Th2 responses to newly encountered Ags (i.e., phenotype spread). To test this hypothesis, we developed a mouse model in which the requirements for the induction and inhibition of phenotype spread to a clinically relevant neo-allergen (i.e., ragweed) were investigated. Our results indicate that 1) phenotype spread to the neo-allergen can be induced only within the first 8 h after a bronchial challenge with the first Ag (OVA); 2) Th2 differentiation of naive CD4(+) T cells occurs in bronchial lymph nodes; 3) trafficking of naive CD4(+) T cells to local lymph nodes and IL-4 produced by OVA-activated Th2 cells play essential roles in the differentiation of naive CD4(+) T cells to Th2 cells; and 4) suppression of the production of chemokines involved in the homing of naive CD4(+) T and Th2 cells to bronchial lymph nodes by a TLR9 agonist inhibited phenotype spread and abrogated the consequent development of experimental asthma. These findings provide a mechanistic insight into Th2 phenotype spread and offer an animal model for testing relevant immunomodulatory interventions.

Toll-like receptor ligands: hygiene, atopy and therapeutic implications

PURPOSE OF REVIEW

Allergic and certain other inflammatory diseases have become more common in industrialized countries over the past few decades. One potential explanation for such trends is that with a decreased incidence of microbial exposures, as a result of modern public health practices, an important source of immune stimulation has been lost, with a consequent increase in inflammatory responses and their associated diseases. This review will focus on our current understanding of how microbial exposures impact on host immunity and the pathogenesis of allergic diseases.

RECENT FINDINGS

In the past decade, it has become clear that a number of molecular interactions between immunocytes and microbial compounds are mediated by Toll-like receptors on host cells. Moreover, recent investigations have suggested that ligands for different Toll-like receptors have the potential both to inhibit and promote the development of allergic hypersensitivities and diseases.

SUMMARY

On the basis of studies discussed herein, we speculate that physiological exposures to Toll-like receptor ligands have important yet complex effects on immune homeostasis and host susceptibility towards atopic diseases. Moreover, we anticipate that a fuller understanding of how physiological Toll-like receptor ligand exposures impact on immune development will lead to novel therapeutic interventions for the prevention and treatment of atopic diseases.

Immunoglobulin-like transcripts ILT2, ILT3 and ILT7 are expressed by human dendritic cells and down-regulated following activation

Immunoglobulin-like transcripts (ILT) represent novel immunoglobulin superfamily receptors that are expressed in myeloid, lymphoid and dendritic cells (DC). Here, we studied by gene expression profiling with DNA microarrays ILT expression in different DC subsets, including plasmacytoid DC (PDC), monocyte-derived DC (Mo-DC) and DC obtained by in vitro differentiation from CD34(+) progenitor cells, and DC activated in the presence of different activating agents. ILT2 and ILT3 were expressed in PDC, Mo-DC and DC obtained from CD34(+) cells. ILT7 mRNA was present in PDC, but absent in Mo-DC and DC obtained from CD34(+) cells, indicating that ILT7 mRNA expression seems to be a marker for PDC. CpG-DNA and inflammatory stimuli, such as TNF alpha, prostaglandin E2 (PGE2) and soluble CD40 ligand (sCD40L), and different combinations thereof are frequently employed for DC activation. Here, we demonstrate that ILT2 and ILT3 expression is down-regulated following DC activation by CpG-DNA and inflammatory stimuli at both mRNA and protein levels. Thus, activation of human DC with such stimuli involves down-regulation of inhibitory ILT2 and ILT3 receptors, and this could represent a novel mechanism contributing to DC activation.

Cutting edge: activation of Toll-like receptor 2 induces a Th2 immune response and promotes experimental asthma

Recognition of microbial components by APCs and their activation through Toll-like receptors (TLR) leads to the induction of adaptive immune responses. In this study, we show that activation of TLR2 by its synthetic ligand Pam3Cys, in contrast to activation of TLR9 by immunostimulatory DNA (ISS-ODN), induces a prominent Th2-biased immune response. Activation of APCs by Pam3Cys resulted in the induction of Th2-associated effector molecules like IL-13, and IL-1beta, GM-CSF and up-regulation of B7RP-1, but low levels of Th1-associated cytokines (IL-12, IFNalpha, IL-18, IL-27). Accordingly, TLR2 ligands aggravated experimental asthma. These data indicate that the type of TLR stimulation during the initial phase of immune activation determines the polarization of the adaptive immune response and may play a role in the initiation of Th2-mediated immune disorders, such as asthma.

Molecular basis for the immunostimulatory activity of guanine nucleoside analogs: activation of Toll-like receptor 7

Certain C8-substituted and N7, C8-disubstituted guanine ribonucleosides comprise a class of small molecules with immunostimulatory activity. In a variety of animal models, these agents stimulate both humoral and cellular immune responses. The antiviral actions of these guanosine analogs have been attributed to their ability to induce type I IFNs. However, the molecular mechanisms by which the guanosine analogs potentiate immune responses are not known. Here, we report that several guanosine analogs activate Toll-like receptor 7 (TLR7). 7-Thia-8-oxoguanosine, 7-deazaguanosine, and related guanosine analogs activated mouse immune cells in a manner analogous to known TLR ligands, inducing cytokine production in mouse splenocytes (IL-6 and IL-12, type I and II IFNs), bone marrow-derived macrophages (IL-6 and IL-12), and in human peripheral blood leukocytes (type I IFNs, tumor necrosis factor alpha and IL-12). The guanosine congeners also up-regulated costimulatory molecules and MHC I/II in dendritic cells. Genetic complementation studies in human embryonic kidney 293 cells confirmed that the guanosine analogs activate cells exclusively via TLR7. The stimulation of TLR7 by the guanosine analogs in human cells appears to require endosomal maturation because inhibition of this process with chloroquine significantly reduced the downstream activation of NF-kappaB. However, TLR8 activation by R-848 and TLR2 activation by [S-[2,3-bis(palmitoyloxy)-(2-RS)-propyl]-N-palmitoyl-R-Cys-S-Ser-Lys4-OH, trihydrochloride)] were not inhibited by chloroquine, whereas TLR9 activation by CpG oligodeoxynucleotides was abolished. In summary, we present evidence that guanosine analogs activate immune cells via TLR7 by a pathway that requires endosomal maturation. Thus, the B cell-stimulating and antiviral activities of the guanosine analogs may be explained by their TLR7-activating capacity.

Recruitment of Chlamydia pneumoniae-infected macrophages to the carotid artery wall in noninfected, nonatherosclerotic mice

OBJECTIVE

Monocyte recruitment into the subendothelium is a crucial step in atherogenesis. Chlamydia pneumoniae resides in circulating monocytes and in the atherosclerotic vascular wall. However, the role of C pneumoniae for monocyte recruitment is unknown. The aim of this study was to examine the impact of C pneumoniae on monocyte adhesion and migration.

METHODS AND RESULTS

C pneumoniae-infected, fluorescence-labeled mouse macrophages (ANA-1) were injected intravenously into noninfected, healthy mice. In vivo videomicroscopy showed increased rolling and firm adhesion to the carotid artery compared with noninfected macrophages. In vitro, C pneumoniae infection (yielding 25% to 35% infected monocytes) increased adhesion of human monocytes or MonoMac6 cells to human umbilical vein endothelial cells and improved cell migration through endothelial-like ECV604 cells. Cell adhesion was inhibited by antibody blockade of very late antigen-4, lymphocyte function-associated antigen-1, macrophage antigen-1, or urokinase receptor, which were found upregulated or activated on C pneumoniae infection (flow cytometry). In contrast, C trachomatis did not induce monocyte adhesion at comparable infection rates (25% to 35%), indicating a unique activation pathway for C pneumoniae. Polymyxin B did not affect C pneumoniae-induced adhesion, excluding a relevant role of lipopolysaccharide in this process.

CONCLUSIONS

These data indicate that C pneumoniae can direct monocytes to predilection sites of nonatherosclerotic vessel walls in vivo by activation of the integrin adhesion receptor system.

A subset of Toll-like receptor ligands induces cross-presentation by bone marrow-derived dendritic cells

Dendritic cells (DCs) are capable of cross-presenting exogenous Ag to CD8(+) CTLs. Detection of microbial products by Toll-like receptors (TLRs) leads to activation of DCs and subsequent orchestration of an adaptive immune response. We hypothesized that microbial TLR ligands could activate DCs to cross-present Ag to CTLs. Using DCs and CTLs in an in vitro cross-presentation system, we show that a subset of microbial TLR ligands, namely ligands of TLR3 (poly(inosinic-cytidylic) acid) and TLR9 (immunostimulatory CpG DNA), induces cross-presentation. In contrast to presentation of Ag to CD4(+) T cells by immature DCs, TLR-induced cross-presentation is mediated by mature DCs, is independent of endosomal acidification, and relies on cytosolic Ag processing machinery.

Activation of the immune system by bacterial CpG-DNA

The past decade has seen a remarkable process of refocusing in immunology. Cells of the innate immune system, especially macrophages and dendritic cells, have been at the centre of this process. These cells had been regarded by some scientists as non-specific, sometimes perhaps even confined to the menial job of serving T cells by scavenging antigen and presenting it to the sophisticated adaptive immune system. Only over the last few years has it become unequivocally clear that cells of the innate immunity hold, by variation of context and mode of antigen presentation, the power of shaping an adaptive immune response. The innate immune response, in turn, is to a significant degree the result of stimulation by so-called pathogen-associated molecular patterns (PAMPs). One compound with high stimulatory potential for the innate immune system is bacterial DNA. Here we will review recent evidence that bacterial DNA should be ranked with other PAMPs such as lipopolysaccharide (LPS) and lipoteichoic acid. We will further review our present knowledge of DNA recognition and DNA-dependent signal transduction in cells of the immune system.

Predominant role of toll-like receptor 2 versus 4 in Chlamydia pneumoniae-induced activation of dendritic cells

Chlamydia pneumoniae is an obligate intracellular human pathogen causing diseases such as pneumonia, bronchitis, and pharyngitis. Because of its intracellular replication, cell-mediated immune responses are needed to mediate successful defenses of the host. Because dendritic cells play a central role in linking innate immunity and Ag-specific cell-mediated immune responses we asked whether dendritic cells are activated upon contact with C. pneumoniae and whether known Toll like receptors (TLR) are involved in this process. Here we show that C. pneumoniae was taken up by bone marrow-derived murine dendritic cells. Ingested C. pneumoniae appeared to be unable to develop mature inclusion inside dendritic cells. Furthermore, upon contact with C. pneumoniae dendritic cells were potently stimulated because NF-kappaB was activated and translocated to the nucleus, cytokines like IL-12p40 and TNF-alpha were secreted, and expression of MHC class II molecules, CD40, CD80, and CD86 was up-regulated. Importantly, secretion of cytokines as well as translocation of NF-kappaB were dependent on the presence of TLR2 and independent from TLR4 with the exception of IL-12p40 secretion, which was attenuated in the absence of either a functional TLR2 or 4. In conclusion, we show here that recognition of the Gram-negative bacterium C. pneumoniae depends largely on TLR2 and only to a minor extent on TLR4.

Human TLR9 confers responsiveness to bacterial DNA via species-specific CpG motif recognition

The Toll-like receptor (TLR) family consists of phylogenetically conserved transmembrane proteins, which function as mediators of innate immunity for recognition of pathogen-derived ligands and subsequent cell activation via the Toll/IL-1R signal pathway. Here, we show that human TLR9 (hTLR9) expression in human immune cells correlates with responsiveness to bacterial deoxycytidylate-phosphate-deoxyguanylate (CpG)-DNA. Notably "gain of function" to immunostimulatory CpG-DNA is achieved by expressing TLR9 in human nonresponder cells. Transfection of either human or murine TLR9 conferred responsiveness in a CD14- and MD2-independent manner, yet required species-specific CpG-DNA motifs for initiation of the Toll/IL-1R signal pathway via MyD88. The optimal CpG motif for hTLR9 was GTCGTT, whereas the optimal murine sequence was GACGTT. Overall, these data suggest that hTLR9 conveys CpG-DNA responsiveness to human cells by directly engaging immunostimulating CpG-DNA.

Bacterial CpG-DNA triggers activation and maturation of human CD11c-, CD123+ dendritic cells

Human plasmacytoid precursor dendritic cells (ppDC) are a major source of type I IFN upon exposure to virus and bacteria, yet the stimulus causing their maturation into DCs is unknown. After PBMC activation with immunostimulatory bacterial DNA sequences (CpG-DNA) we found that ppDC are the primary source of IFN-alpha. In fact, either CpG-DNA or dsRNA (poly(I:C)) induced IFN-alpha from purified ppDC. Surprisingly, only CpG-DNA triggered purified ppDC survival, maturation, and production of TNF, GM-CSF, IL-6, and IL-8, but not IL-10 or IL-12. Known DC activators such as CD40 ligation triggered ppDC maturation, but only IL-8 production, while bacterial LPS was negative for all activation criteria. An additional finding was that only CpG-DNA could counteract IL-4-induced apoptosis in ppDC. Therefore, CpG-DNA represents a pathogen-associated molecular pattern for ppDC. In contrast to these finding, CpG-DNA, like LPS, caused TNF, IL-6, and IL-12 release from PBMC and purified monocytes; however, differentiation of monocytes into DCs with GM-CSF and IL-4 unexpectedly resulted in refractoriness to CpG-DNA, but not LPS. Taken together, these results suggest that within a DC subset a multiplicity of responses can be generated by distinct environmental stimuli and that responses to a given stimulus may be dissimilar between DC subsets.

Interaction of Chlamydia pneumoniae and human alveolar macrophages: infection and inflammatory response

The obligate intracellular pathogen Chlamydia pneumoniae is associated with chronic respiratory, atherosclerotic, and rheumatic disease. The alveolar macrophage (AM) is a potential target cell for the pathogen and may contribute to respiratory immunopathology. We therefore investigated in vitro the interaction between chlamydiae and macrophages with cocultures of C. pneumoniae and AM from 12 healthy volunteers. Inflammatory responses were evaluated through lucigenin-amplified chemiluminescence; secretion of tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), and interleukin 8 (IL-8); and expression of intercellular adhesion molecule-1 (ICAM-1) and human leukocyte antigen-DR (HLA-DR). C. pneumoniae readily induced productive infection in the AM. Inclusions containing replicating pathogens could be maintained for up to 120 h. Morphologically similar infection patterns were seen ex vivo in AM collected from six patients with known C. pneumoniae pneumonia. AM responded to the infection with a marked, dose-dependent release of reactive oxygen species, TNF-alpha, IL-1beta, and IL-8. ICAM-1 expression remained unchanged, but HLA-DR was significantly upregulated. Our data indicate that the release of antimicrobial mediators cannot prevent chlamydial infection and replication in AM, but may be involved in amplification of the local inflammatory response in C. pneumoniae pneumonia.