Toll-like receptor 2 on inflammatory monocytes induces type I interferon in response to viral but not bacterial ligands

Virus recognition by Toll-7 activates antiviral autophagy in Drosophila

Innate immunity is highly conserved and relies on pattern recognition receptors (PRRs) such as Toll-like receptors (identified through their homology to Drosophila Toll) for pathogen recognition. Although Drosophila Toll is vital for immune recognition and defense, roles for the other eight Drosophila Tolls in immunity have remained elusive. Here we have shown that Toll-7 is a PRR both in vitro and in adult flies; loss of Toll-7 led to increased vesicular stomatitis virus (VSV) replication and mortality. Toll-7, along with additional uncharacterized Drosophila Tolls, was transcriptionally induced by VSV infection. Furthermore, Toll-7 interacted with VSV at the plasma membrane and induced antiviral autophagy independently of the canonical Toll signaling pathway. These data uncover an evolutionarily conserved role for a second Drosophila Toll receptor that links viral recognition to autophagy and defense and suggest that other Drosophila Tolls may restrict specific as yet untested pathogens, perhaps via noncanonical signaling pathways.

MDP up-regulates the gene expression of type I interferons in human aortic endothelial cells

Muramyldipeptide (MDP), the minimum essential structure responsible for the immuno-adjuvant activity of peptidoglycan, is recognized by intracellular nuclear-binding oligomerization domain 2 (NOD2). Here, we obtained evidence that the treatment of human aortic endothelial cells (HAECs) with MDP up-regulated the gene expression of type I interferons in a dose- and time-dependent manner. MDP also up-regulated the expression of the receptor NOD2, suggesting that MDP may induce a positive feedback response. The up-regulation of interferons was not dependent on the TNFa signaling, as HAECs did not express TNFa with the stimulation of MDP, and TNFa neutralizing antibody did not decrease the induction of IFNs induced by MDP. RT-PCR results showed that HAECs expressed the gene transcripts of interferon regulatory factor (IRF) 1, 2, 3, 9. The western blot results showed that MDP induced the phosphorylation of IRF3. These results suggested that MDP induced the up-regulation of gene transcript of interferons through the activation of IRF3 signaling pathway. Meanwhile, MDP induced the gene expression of pro-inflammatory cytokines, including IL-1ß, IL-8, and MCP-1. Taken together, these results suggested that HAECs may play roles in the anti-infection immune response and in the induction of innate immunity.

Type I interferons in regulation of mucosal immunity

The mucosal system is the first line of defense against many pathogens. It is continuously exposed to dietary and microbial antigens, and thus the host must maintain a homeostatic environment between commensal microbiota and pathogenic infections. Following infections and inflammatory events, a rapid innate immune response is evoked to dampen the inflammatory processes. Type I interferons, a family of pleiotropic cytokines and major products of the innate immune response, have a key role in these early immune events at the mucosa, as reviewed here. With the emergence of new discoveries of immune cell types in mucosal tissues and their reactions to commensal and pathogenic organisms, we also review the opportunities for exciting research in this field.

Both TLR2 and TRIF contribute to interferon-β production during Listeria infection

Synthesis of interferon-β (IFN-β) is an innate response to cytoplasmic infection with bacterial pathogens. Our recent studies showed that Listeria monocytogenes limits immune detection and IFN-β synthesis via deacetylation of its peptidoglycan, which renders the bacterium resistant to lysozyme degradation. Here, we examined signaling requirements for the massive IFN-β production resulting from the infection of murine macrophages with a mutant strain of L. monocytogenes, ΔpgdA, which is unable to modify its peptidoglycan. We report the identification of unconventional signaling pathways to the IFN-β gene, requiring TLR2 and bacterial internalization. Induction of IFN-β was independent of the Mal/TIRAP adaptor protein but required TRIF and the transcription factors IRF3 and IRF7. These pathways were stimulated to a lesser degree by wild-type L. monocytogenes. They operated in both resident and inflammatory macrophages derived from the peritoneal cavity, but not in bone marrow-derived macrophages. The novelty of our findings thus lies in the first description of TLR2 and TRIF as two critical components leading to the induction of the IFN-β gene and in uncovering that individual macrophage populations adopt different strategies to link pathogen recognition signals to IFN-β gene expression.

Respiratory and gastrointestinal epithelial modulation of the immune response during viral infection

Respiratory and enteric viral infections cause significant morbidity and mortality world-wide and represent a major socio-economic burden. Many of these viruses have received unprecedented public and media interest in recent years. A popular public misconception is that viruses are a threat to which the human body has only limited defences. However, the majority of primary and secondary exposures to virus are asymptomatic or induce only minor symptoms. The mucosal epithelial surfaces are the main portal of entry for viral pathogens and are centrally involved in the initiation, maintenance and polarisation of the innate and adaptive immune response to infection. This review describes the defences employed by the epithelium of the respiratory and gastrointestinal tracts during viral infections with focus on epithelial modulation of the immune response at the innate/adaptive interface.

Innate immune recognition of poxviral vaccine vectors

The study of poxviruses pioneered the field of vaccinology after Jenner's remarkable discovery that 'vaccination' with the phylogenetically related cowpox virus conferred immunity to the devastating disease of smallpox. The study of poxviruses continues to enrich the field of virology because the global eradication of smallpox provides a unique example of the potency of effective immunization. Other poxviruses have since been developed as vaccine vectors for clinical and veterinary applications and include modified vaccinia virus strains such as modified vaccinia Ankara and NYVAC as well as the avipox viruses, fowlpox virus and canarypox virus. Despite the empirical development of poxvirus-based vectored vaccines, it is only now becoming apparent that we need to better understand how the innate arm of the immune system drives adaptive immunity to poxviruses, and how this information is relevant to vaccine design strategies, which are the topics addressed in this article.

Sensing of RNA viruses: a review of innate immune receptors involved in recognizing RNA virus invasion

Our knowledge regarding the contribution of the innate immune system in recognizing and subsequently initiating a host response to an invasion of RNA virus has been rapidly growing over the last decade. Descriptions of the receptors involved and the molecular mechanisms they employ to sense viral pathogen-associated molecular patterns have emerged in great detail. This review presents an overview of our current knowledge regarding the receptors used to detect RNA virus invasion, the molecular structures these receptors sense, and the involved downstream signaling pathways.

CD14 controls the LPS-induced endocytosis of Toll-like receptor 4

The transport of Toll-like Receptors (TLRs) to various organelles has emerged as an essential means by which innate immunity is regulated. While most of our knowledge is restricted to regulators that promote the transport of newly synthesized receptors, the regulators that control TLR transport after microbial detection remain unknown. Here, we report that the plasma membrane localized Pattern Recognition Receptor (PRR) CD14 is required for the microbe-induced endocytosis of TLR4. In dendritic cells, this CD14-dependent endocytosis pathway is upregulated upon exposure to inflammatory mediators. We identify the tyrosine kinase Syk and its downstream effector PLCγ2 as important regulators of TLR4 endocytosis and signaling. These data establish that upon microbial detection, an upstream PRR (CD14) controls the trafficking and signaling functions of a downstream PRR (TLR4). This innate immune trafficking cascade illustrates how pathogen detection systems operate to induce both membrane transport and signal transduction.

TLR2 signaling depletes IRAK1 and inhibits induction of type I IFN by TLR7/9

Pathogens may signal through multiple TLRs with synergistic or antagonistic effects on the induction of cytokines, including type I IFN (IFN-I). IFN-I is typically induced by TLR9, but not TLR2. Moreover, we previously reported that TLR2 signaling by Mycobacterium tuberculosis or other TLR2 agonists inhibited TLR9 induction of IFN-I and IFN-I-dependent MHC-I Ag cross processing. The current studies revealed that lipopeptide-induced TLR2 signaling inhibited induction of first-wave IFN-α and IFN-β mRNA by TLR9, whereas induction of second-wave IFN-I mRNA was not inhibited. TLR2 also inhibited induction of IFN-I by TLR7, another MyD88-dependent IFN-I-inducing receptor, but did not inhibit IFN-I induction by TLR3 or TLR4 (both Toll/IL-1R domain-containing adapter-inducing IFN-β dependent, MyD88 independent). The inhibitory effect of TLR2 was not dependent on new protein synthesis or intercellular signaling. IL-1R-associated kinase 1 (IRAK1) was depleted rapidly (within 10 min) by TLR2 agonist, but not until later (e.g., 2 h) by TLR9 agonist. Because IRAK1 is required for TLR7/9-induced IFN-I production, we propose that TLR2 signaling induces rapid depletion of IRAK1, which impairs IFN-I induction by TLR7/9. This novel mechanism, whereby TLR2 inhibits IFN-I induction by TLR7/9, may shape immune responses to microbes that express ligands for both TLR2 and TLR7/TLR9, or responses to bacteria/virus coinfection.

Activation of natural killer cells during microbial infections

Natural killer (NK) cells are large granular lymphocytes that express a diverse array of germline encoded inhibitory and activating receptors for MHC Class I and Class I-like molecules, classical co-stimulatory ligands, and cytokines. The ability of NK cells to be very rapidly activated by inflammatory cytokines, to secrete effector cytokines, and to kill infected or stressed host cells, suggests that they may be among the very early responders during infection. Recent studies have also identified a small number of pathogen-derived ligands that can bind to NK cell surface receptors and directly induce their activation. Here we review recent studies that have begun to elucidate the various pathways by which viral, bacterial, and parasite pathogens activate NK cells. We also consider two emerging themes of NK cell–pathogen interactions, namely their contribution to adaptive immune responses and their potential to take on regulatory and immunomodulatory functions.

dsRNA sensors and plasmacytoid dendritic cells in host defense and autoimmunity

The innate immune system detects viruses through molecular sensors that trigger the production of type I interferons (IFN-I) and inflammatory cytokines. As viruses vary tremendously in size, structure, genomic composition, and tissue tropism, multiple sensors are required to detect their presence in various cell types and tissues. In this review, we summarize current knowledge of the diversity, specificity, and signaling pathways downstream of viral sensors and ask whether two distinct sensors that recognize the same viral component are complementary, compensatory, or simply redundant. We also discuss why viral sensors are differentially distributed in distinct cell types and whether a particular cell type dominates the IFN-I response during viral infection. Finally, we review evidence suggesting that inappropriate signaling through viral sensors may induce autoimmunity. The picture emerging from these studies is that disparate viral sensors in different cell types form a dynamic and integrated molecular network that can be exploited for improving vaccination and therapeutic strategies for infectious and autoimmune diseases.

Skin mast cells protect mice against vaccinia virus by triggering mast cell receptor S1PR2 and releasing antimicrobial peptides

Mast cells (MCs) are well-known effectors of allergic reactions and are considered sentinels in the skin and mucosa. In addition, through their production of cathelicidin, MCs have the capacity to oppose invading pathogens. We therefore hypothesized that MCs could act as sentinels in the skin against viral infections using antimicrobial peptides. In this study, we demonstrate that MCs react to vaccinia virus (VV) and degranulate using a membrane-activated pathway that leads to antimicrobial peptide discharge and virus inactivation. This finding was supported using a mouse model of viral infection. MC-deficient (Kit(wsh-/-)) mice were more susceptible to skin VV infection than the wild type animals, whereas Kit(wsh-/-) mice reconstituted with MCs in the skin showed a normal response to VV. Using MCs derived from mice deficient in cathelicidin antimicrobial peptide, we showed that antimicrobial peptides are one important antiviral granule component in in vivo skin infections. In conclusion, we demonstrate that MC presence protects mice from VV skin infection, MC degranulation is required for protecting mice from VV, neutralizing Ab to the L1 fusion entry protein of VV inhibits degranulation apparently by preventing S1PR2 activation by viral membrane lipids, and antimicrobial peptide release from MC granules is necessary to inactivate VV infectivity.

Toll-like receptors in bony fish: from genomics to function

Receptors that recognize conserved pathogen molecules are the first line of cellular innate immunity defense. Toll-like receptors (TLRs) are the best understood of the innate immune receptors that detect infections in mammals. Key features of the fish TLRs and the factors involved in their signaling cascade have high structural similarity to the mammalian TLR system. However, the fish TLRs also exhibit very distinct features and large diversity which is likely derived from their diverse evolutionary history and the distinct environments that they occupy. Six non-mammalian TLRs were identified in fish. TLR14 shares sequence and structural similarity with TLR1 and 2, and the other five (TLR19, 20, 21, 22 and 23) form a cluster of novel TLRs. TLR4 was lost from the genomes of most fishes, and the TLR4 genes found in zebrafish do not recognize the mammalian agonist LPS and are likely paralogous and not orthologous to mammalian TLR4 genes. TLR6 and 10 are also absent from all fish genomes sequenced to date. Of the at least 16 TLR types identified in fish, direct evidence of ligand specificity has only been shown for TLR2, TLR3, TLR5M, TLR5S and TLR22. The common carp TLR2 was shown to recognize the synthetic triacylated lipopeptide Pam(3)CSK(4) and lipopeptides from gram positive bacteria. The membrane-bound TLR5 (TLR5M) signaling in response to flagellin in rainbow trout is amplified through interaction with the soluble form (TLR5S) in a positive loop feedback. In Fugu, TLR3 is localized to the endoplasmic reticulum (ER) and recognizes relatively short dsRNA, while TLR22 has a surveillance function like the human cell-surface TLR3. Genome and gene duplications have been major contributors to the teleost's rich evolutionary history and genomic diversity. Duplicate or multi-copy TLR genes were identified for TLR3 and 7 in common carp, TLR4b, 5, 8 and 20 in zebrafish, TLR8a in rainbow trout and TLR22 in rainbow trout and Atlantic salmon. The main task for current and near-future fish TLRs research is to develop specificity assays to identify the ligands of all fish TLRs, which will advance comparative immunology research and will contribute to our understanding of disease resistance mechanisms in fish and the development of new adjuvants and/or more effective vaccines and therapeutics.

Type I interferons: diversity of sources, production pathways and effects on immune responses

Type I interferons (IFN-I) were first described over 50 years ago as factors produced by cells that interfere with virus replication and promote an antiviral state. Innate and adaptive immune responses to viruses are also greatly influenced by IFN-I. In this article we discuss the diversity of cellular sources of IFN-I and the pathways leading to IFN-I production during viral infections. Finally, we discuss the effects of IFN-I on cells of the immune system with emphasis on dendritic cells.

Transmembrane mutations in Toll-like receptor 9 bypass the requirement for ectodomain proteolysis and induce fatal inflammation

Recognition of nucleic acids as a signature of infection by Toll-like receptors (TLRs) 7 and 9 exposes the host to potential self-recognition and autoimmunity. It has been proposed that intracellular compartmentalization is largely responsible for reliable self versus nonself discrimination by these receptors. We have previously shown that TLR9 and TLR7 require processing prior to activation, which may further reinforce receptor compartmentalization and tolerance to self, yet this possibility remains untested. Here we report that residues within the TLR9 transmembrane (TM) region conferred the requirement for ectodomain proteolysis. TLR9 TM mutants responded to extracellular DNA, and mice expressing such receptors died from systemic inflammation and anemia. This inflammatory disease did not require lymphocytes and appeared to require recognition of self-DNA by dendritic cells. To our knowledge, these results provide the first demonstration that TLR-intrinsic mutations can lead to a break in tolerance.

CD11b⁺, Ly6G⁺ cells produce type I interferon and exhibit tissue protective properties following peripheral virus infection

The goal of the innate immune system is containment of a pathogen at the site of infection prior to the initiation of an effective adaptive immune response. However, effector mechanisms must be kept in check to combat the pathogen while simultaneously limiting undesirable destruction of tissue resulting from these actions. Here we demonstrate that innate immune effector cells contain a peripheral poxvirus infection, preventing systemic spread of the virus. These innate immune effector cells are comprised primarily of CD11b⁺Ly6C⁺Ly6G^- monocytes that accumulate initially at the site of infection, and are then supplemented and eventually replaced by CD11b⁺Ly6C⁺Ly6G⁺ cells. The phenotype of the CD11b⁺Ly6C⁺Ly6G⁺ cells resembles neutrophils, but the infiltration of neutrophils typically occurs prior to, rather than following, accumulation of monocytes. Indeed, it appears that the CD11b⁺Ly6C⁺Ly6G⁺ cells that infiltrated the site of VACV infection in the ear are phenotypically distinct from the classical description of both neutrophils and monocyte/macrophages. We found that CD11b⁺Ly6C⁺Ly6G⁺ cells produce Type I interferons and large quantities of reactive oxygen species. We also observed that depletion of Ly6G⁺ cells results in a dramatic increase in tissue damage at the site of infection. Tissue damage is also increased in the absence of reactive oxygen species, although reactive oxygen species are typically thought to be damaging to tissue rather than protective. These data indicate the existence of a specialized population of CD11b⁺Ly6C⁺Ly6G⁺ cells that infiltrates a site of virus infection late and protects the infected tissue from immune-mediated damage via production of reactive oxygen species. Regulation of the action of this population of cells may provide an intervention to prevent innate immune-mediated tissue destruction.

During a natural virus infection, small doses of infectious virus are deposited at a peripheral infection site, and then a “race” ensues, in which the replicating virus attempts to “outpace” the responding immune system of the host. In the early phases of infection, the innate immune system must contain the infection prior to the development of an effective adaptive response. Here we have characterized the cells of the innate immune system that move to a site of peripheral virus infection, and we find that a subset of these cells display atypical expression of cell surface molecules, timing of infiltration, and function. These cells protect the infected tissue from damage by producing reactive oxygen molecules, which are widely accepted to increase tissue damage. Therefore our findings indicate that during a peripheral virus infection, the typical rules governing the function of the innate immune system are altered to prevent tissue damage.

Natural killer cell activation by dendritic cells: balancing inhibitory and activating signals

Natural killer (NK) cells have originally been identified by their spontaneous cytolytic potential against tumor cells, which, however, might result from pre-activation due to prior pathogen exposure. Resting NK cells, on the contrary, require activation by bystander antigen-presenting cells to reach their full functional competence. In this review, we will summarize studies on how dendritic cells (DCs), the most potent type of antigen-presenting cell, communicate with human NK cells to activate them in secondary lymphoid organs and to integrate signals from activated NK cells at sites of inflammation for their own maturation. Furthermore, we will review aspects of the immunological synapse, which mediates this cross-talk. These studies provide the mechanistic understanding of how mature DCs can activate NK cells and survive to go on for the activation of adaptive immunity. This feature of DCs, to activate different waves of immune responses, could be harnessed for immunotherapies, including vaccinations.

Toll-like receptors: key players in antiviral immunity

TLRs are a family of innate receptors whose specificities are predetermined in the germline. Therefore, TLRs have evolved to recognize conserved features of microbes. Viruses typically lack the conserved features common to other pathogen classes, so the innate immune system has evolved to recognize viral nucleic acid as a hallmark of viral infection. In this review we discuss examples of TLR-mediated viral recognition and the functional consequences of this recognition for antiviral immunity.

Monocyte recruitment during infection and inflammation

Monocytes originate from progenitors in the bone marrow and traffic via the bloodstream to peripheral tissues. During both homeostasis and inflammation, circulating monocytes leave the bloodstream and migrate into tissues where, following conditioning by local growth factors, pro-inflammatory cytokines and microbial products, they differentiate into macrophage or dendritic cell populations. Recruitment of monocytes is essential for effective control and clearance of viral, bacterial, fungal and protozoal infections, but recruited monocytes also contribute to the pathogenesis of inflammatory and degenerative diseases. The mechanisms that control monocyte trafficking under homeostatic, infectious and inflammatory conditions are being unravelled and are the focus of this Review.

The type I interferon response during viral infections: a "SWOT" analysis

The type I interferon (IFN) response is a strong and crucial moderator for the control of viral infections. The strength of this system is illustrated by the fact that, despite some temporary discomfort like a common cold or diarrhea, most viral infections will not cause major harm to the healthy immunocompetent host. To achieve this, the immune system is equipped with a wide array of pattern recognition receptors and the subsequent coordinated type I IFN response orchestrated by plasmacytoid dendritic cells (pDCs) and conventional dendritic cells (cDCs). The production of type I IFN subtypes by dendritic cells (DCs), but also other cells is crucial for the execution of many antiviral processes. Despite this coordinated response, morbidity and mortality are still common in viral disease due to the ability of viruses to exploit the weaknesses of the immune system. Viruses successfully evade immunity and infection can result in aberrant immune responses. However, these weaknesses also open opportunities for improvement via clinical interventions as can be seen in current vaccination and antiviral treatment programs. The application of IFNs, Toll-like receptor ligands, DCs, and antiviral proteins is now being investigated to further limit viral infections. Unfortunately, a common threat during stimulation of immunity is the possible initiation or aggravation of autoimmunity. Also the translation from animal models to the human situation remains difficult. With a Strengths-Weaknesses-Opportunities-Threats ("SWOT") analysis, we discuss the interaction between host and virus as well as (future) therapeutic options, related to the type I IFN system.

A toll-like receptor 2 pathway regulates the Ppargc1a/b metabolic co-activators in mice with Staphylococcal aureus sepsis

Activation of the host antibacterial defenses by the toll-like receptors (TLR) also selectively activates energy-sensing and metabolic pathways, but the mechanisms are poorly understood. This includes the metabolic and mitochondrial biogenesis master co-activators, Ppargc1a (PGC-1α) and Ppargc1b (PGC-1β) in Staphylococcus aureus (S. aureus) sepsis. The expression of these genes in the liver is markedly attenuated inTLR2^−/− mice and markedly accentuated in TLR4^−/− mice compared with wild type (WT) mice. We sought to explain this difference by using specific TLR-pathway knockout mice to test the hypothesis that these co-activator genes are directly regulated through TLR2 signaling. By comparing their responses to S. aureus with WT mice, we found that MyD88-deficient and MAL-deficient mice expressed hepatic Ppargc1a and Ppargc1b normally, but that neither gene was activated in TRAM-deficient mice. Ppargc1a/b activation did not require NF-kβ, but did require an interferon response factor (IRF), because neither gene was activated in IRF-3/7 double-knockout mice in sepsis, but both were activated normally in Unc93b1-deficient (3d) mice. Nuclear IRF-7 levels in TLR2^−/− and TLR4^−/− mice decreased and increased respectively post-inoculation and IRF-7 DNA-binding at the Ppargc1a promoter was demonstrated by chromatin immunoprecipitation. Also, a TLR2-TLR4-TRAM native hepatic protein complex was detected by immunoprecipitation within 6 h of S. aureus inoculation that could support MyD88-independent signaling to Ppargc1a/b. Overall, these findings disclose a novel MyD88-independent pathway in S. aureus sepsis that links TLR2 and TLR4 signaling in innate immunity to Ppargc1a/b gene regulation in a critical metabolic organ, the liver, by means of TRAM, TRIF, and IRF-7.

Innate Immune Responses to AAV Vectors

Gene replacement therapy by in vivo delivery of adeno-associated virus (AAV) is attractive as a potential treatment for a variety of genetic disorders. However, while AAV has been used successfully in many models, other experiments in clinical trials and in animal models have been hampered by undesired responses from the immune system. Recent studies of AAV immunology have focused on the elimination of transgene-expressing cells by the adaptive immune system, yet the innate immune system also has a critical role, both in the initial response to the vector and in prompting a deleterious adaptive immune response. Responses to AAV vectors are primarily mediated by the TLR9–MyD88 pathway, which induces the production of pro-inflammatory cytokines by activating the NF-κB pathways and inducing type I IFN production; self-complementary AAV vectors enhance these inflammatory processes. Additionally, the alternative NF-κB pathway influences transgene expression in cells transduced by AAV. This review highlights these recent discoveries regarding innate immune responses to AAV and discusses strategies to ablate these potentially detrimental signaling pathways.

Herpes simplex virus type 2 virion host shutoff protein suppresses innate dsRNA antiviral pathways in human vaginal epithelial cells

Viruses that establish persistent infections have evolved numerous strategies to evade host innate antiviral responses. We functionally assessed the role of herpes simplex virus type 2 (HSV-2) virion host shutoff (vhs) protein on innate immune sensing pathways in human vaginal epithelial cells (VK2 ECs). Infection of cells with wild-type (WT) HSV-2 significantly decreased expression of innate immune sensors of viral infection, Toll-like receptor (TLR)2, TLR3, retinoic acid inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (Mda-5), relative to cells infected with a mutant that lacks vhs (vhsB) or mock-infected cells. Transfection with HSV-2 vhs similarly decreased expression of TLR2, TLR3, RIG-I and Mda-5, which was also confirmed in human embryonic kidney (HEK) 293 cells. vhsB infection of VK2 cells caused robust increases in the active form of interferon regulatory factor (IRF)3 and its translocation to the nucleus compared with the WT. Additionally, IRF3 activation by Sendai virus and polyinosinic : polycytidylic acid-induced stimulation of beta interferon (IFN-β) was significantly inhibited in vhs-transfected cells. Overall, our findings provide the first evidence that HSV-2 vhs plays roles in selectively inhibiting TLR3 and RIG-I/Mda-5, as well as TLR2-mediated antiviral pathways for sensing dsRNA and effectively suppresses IFN-β antiviral responses in human vaginal ECs.

Dissection of a type I interferon pathway in controlling bacterial intracellular infection in mice

Defence mechanisms against intracellular bacterial pathogens are incompletely understood. Our study characterizes a type I IFN-dependent cell-autonomous defence pathway directed against Legionella pneumophila, an intracellular model organism and frequent cause of pneumonia. We show that macrophages infected with L. pneumophila produced IFNβ in a STING- and IRF3- dependent manner. Paracrine type I IFNs stimulated upregulation of IFN-stimulated genes and a cell-autonomous defence pathway acting on replicating and non-replicating Legionella within their specialized vacuole. Our infection experiments in mice lacking receptors for type I and/or II IFNs show that type I IFNs contribute to expression of IFN-stimulated genes and to bacterial clearance as well as resistance in L. pneumophila pneumonia in addition to type II IFN. Overall, our study shows that paracrine type I IFNs mediate defence against L. pneumophila, and demonstrates a protective role of type I IFNs in in vivo infections with intracellular bacteria.

A modulatory interleukin-10 response to staphylococcal peptidoglycan prevents Th1/Th17 adaptive immunity to Staphylococcus aureus

Toll-like receptor (TLR) 2 on antigen-presenting cells (APCs) enables these cells to recognize peptidoglycan-embedded lipopeptides and glycopolymers in the Staphylococcus aureus cell wall and mount an inflammatory response to this microbe. TLR2 signalling can also modulate immunity to S. aureus by inducing an interleukin (IL)-10 response in APCs. What determines the balance between proinflammatory and modulatory responses to S. aureus is unknown. We show that the modulatory IL-10 response preferentially occurs upon CD14- and CD36-independent TLR2 signaling, triggering PI3K activation, and is restricted to monocytes and monocyte-derived macrophages (MΦs). In contrast, monocyte-derived dendritic cells (DCs) produce mostly IL-12 and IL-23. The differential APC polarization induced by staphylococcal peptidoglycan translates into differential T helper responses: MΦs primarily trigger IL-10 and weak IL-17 responses, whereas DCs trigger a robust Th1/Th17 response. Exploitation of TLR2 signalling plasticity by S. aureus may explain the wide range of outcomes of human encounters with this microbe.

Human cutaneous leishmaniasis: interferon-dependent expression of double-stranded RNA-dependent protein kinase (PKR) via TLR2

We investigated the type I interferon (IFN-1)/PKR axis in the outcome of the Leishmania (Leishmania) amazonensis infection, along with the underlying mechanisms that trigger and sustain this signaling pathway. Reporter assays of cell extracts from RAW-264.7 macrophages infected with L. (L.) amazonensis or HEK-293T cells cotransfected with TLR2 and PKR promoter constructions were employed. Primary macrophages of TLR2-knockout (KO) or IFNR-KO mice were infected, and the levels of PKR, IFN-1, and superoxide dismutase 1 (SOD1) transcript levels were investigated and compared. Immunohistochemical analysis of human biopsy lesions was evaluated for IFN-1 and PKR-positive cells. Leishmania infection increased the expression of PKR and IFN-β on induction of PKR-promoter activity. The observed effects required the engagement of TLR2. TLR2-KO macrophages expressed low IFN-β and PKR levels postinfection with a reduced parasite load. We also revealed the requirement of PKR signaling for Leishmania-induced IFN-1 expression, responsible for sustaining PKR expression and enhancing infection. Moreover, during infection, SOD1 transcripts increased and were also enhanced when IFN-1 was added to the cultures. Remarkably, SOD1 expression was abrogated in infected, dominant-negative PKR-expressing cells. Finally, lesions of patients with anergic diffuse cutaneous leishmaniasis exhibited higher levels of PKR/IFN-1-expressing cells compared to those with single cutaneous leishmaniasis. In summary, we demonstrated the mechanisms and relevance of the IFN-1/PKR axis in the Leishmania infection.

TLR7/9 versus TLR3/MDA5 signaling during virus infections and diabetes

IFN-I are pleiotropic cytokines that impact innate and adaptive immune responses. In this article, we discuss TLR7/9 versus TLR3/MDA5 signaling in antiviral responses and diabetes. pDCs are thought to have a critical role in antiviral defense because of their ability to rapidly secrete large amounts of IFN-I through TLR7/9 signaling. A recent study demonstrates that although pDCs are a source of IFN-I in vivo, their overall contribution to viral containment is limited and time-dependent, such that additional cellular sources of IFN-I are required to fully control viral infections. dsRNA sensors, such as TLR3 and MDA5, provide another important trigger for antiviral IFN-I responses, which can be exploited to enhance immune responses to vaccines. In the absence of infection, IFN-I production by pDCs or from signaling through dsRNA sensors has been implicated in the pathogenesis of autoimmune diseases such as diabetes. However, recent data demonstrate that IFN-I production via TLR3 and MDA5 is critical to counter diabetes caused by a virus with preferential tropism for pancreatic β-cells. This highlights the complexity of the host antiviral response and how multiple cellular and molecular components balance protective versus pathological responses.

Innate and adaptive immune responses to viral infection and vaccination

Recent accumulating evidence suggests that the human immune system possesses a variety of innate receptors that recognize, distinguish, and respond to viral infections and to vaccination. These include Toll-like receptors, C-type lectin receptors, RIG-I-like receptors, Nod-like receptors and possibly AIM2-like receptors. However, the precise mechanisms by which these receptors exert their critical roles in the induction of virus-specific adaptive immune responses have not been fully elucidated. In this review, we discuss recent advances in our understanding of the innate immune recognition of viruses and the differential connection to the adaptive immune responses induced by infection or vaccination, with a particular focus on the influenza virus.

Toll-like receptors and their crosstalk with other innate receptors in infection and immunity

Toll-like receptors (TLRs) are germline-encoded pattern recognition receptors (PRRs) that play a central role in host cell recognition and responses to microbial pathogens. TLR-mediated recognition of components derived from a wide range of pathogens and their role in the subsequent initiation of innate immune responses is widely accepted; however, the recent discovery of non-TLR PRRs, such as C-type lectin receptors, NOD-like receptors, and RIG-I-like receptors, suggests that many aspects of innate immunity are more sophisticated and complex. In this review, we will focus on the role played by TLRs in mounting protective immune responses against infection and their crosstalk with other PRRs with respect to pathogen recognition.

Presentation of type B peptide-MHC complexes from hen egg white lysozyme by TLR ligands and type I IFNs independent of H2-DM regulation

In APCs, presentation by MHC II molecules of the chemically dominant peptide from the protein hen egg white lysozyme (HEL) generates different conformational isomers of the peptide-MHC II complexes (pMHC). Type B pMHCs are formed in early endosomes from exogenous peptides in the absence of H2-DM, whereas in contrast, type A pMHC complexes are formed from HEL protein in late vesicles after editing by H2-DM. Thus, H2-DM edits off the more unstable pMHC complexes, which are not presented from HEL. In this study, we show that type B pMHC complexes were presented from HEL protein only after stimulation of dendritic cells (DC) with TLR ligands or type I IFN. Type I IFN contributed to most TLR ligand-induced type B pMHC generation, as presentation decreased in DC lacking the receptor for type I IFNs (IFNAR1(-/-)). In contrast, presentation of type A pMHC from HEL and from peptide was minimally affected by TLR ligands. The relative effectiveness of CD8α(+) DC or CD8α(-) DC in presenting type B pMHC complexes varied depending on the TLR ligand used. The mechanisms of generation of type B pMHC from HEL protein with TLR stimulation did not involve H2-DM or release of peptides. DC from H2-DM-deficient mice in the presence of TLR ligands presented type B pMHC. Such DC showed a slight enhancement of HEL catabolism, but peptide release was not evident. Thus, TLR ligands and type I IFN alter the pathways of presentation by MHC II molecules of DC such that type B pMHCs are generated from protein Ag.

IFN-β mediates suppression of IL-12p40 in human dendritic cells following infection with virulent Francisella tularensis

Active suppression of inflammation is a strategy used by many viral and bacterial pathogens, including virulent strains of the bacterium Francisella tularensis, to enable colonization and infection in susceptible hosts. In this study, we demonstrated that virulent F. tularensis strain SchuS4 selectively inhibits production of IL-12p40 in primary human cells via induction of IFN-β. In contrast to the attenuated live vaccine strain, infection of human dendritic cells with virulent SchuS4 failed to induce production of many cytokines associated with inflammation (e.g., TNF-α and IL-12p40). Furthermore, SchuS4 actively suppressed secretion of these cytokines. Assessment of changes in the expression of host genes associated with suppression of inflammatory responses revealed that SchuS4, but not live vaccine strain, induced IFN-β following infection of human dendritic cells. Phagocytosis of SchuS4 and endosomal acidification were required for induction of IFN-β. Further, using a defined mutant of SchuS4, we demonstrated that the presence of bacteria in the cytosol was required, but not sufficient, for induction of IFN-β. Surprisingly, unlike previous reports, induction of IFN-β by F. tularensis was not required for activation of the inflammasome, was not associated with exacerbation of inflammatory responses, and did not control SchuS4 replication when added exogenously. Rather, IFN-β selectively suppressed the ability of SchuS4-infected dendritic cells to produce IL-12p40. Together, these data demonstrated a novel mechanism by which virulent bacteria, in contrast to attenuated strains, modulate human cells to cause disease.

Innate immunity modulation in virus entry

Entry into a cell submits viruses to detection by pattern recognition receptors (PRRs) leading to an early innate anti-viral response. Several viruses evolved strategies to avoid or subvert PRR recognition at the step of virus entry to promote infection. Whereas viruses mostly escape from soluble PRR detection, endocytic/phagocytic PRRs, such as the mannose receptor or DC-SIGN, are commonly used for virus entry. Moreover, virion-incorporated proteins may also offer viruses a way to dampen anti-viral innate immunity upon virus entry, and entering viruses might usurp autophagy to improve their own infectivity.

Pattern recognition receptors and the innate immune response to viral infection

The innate immune response to viral pathogens is critical in order to mobilize protective immunity. Cells of the innate immune system detect viral infection largely through germline-encoded pattern recognition receptors (PRRs) present either on the cell surface or within distinct intracellular compartments. These include the Toll-like receptors (TLRs), the retinoic acid-inducble gene I-like receptors (RLRs), the nucleotide oligomerization domain-like receptors (NLRs, also called NACHT, LRR and PYD domain proteins) and cytosolic DNA sensors. While in certain cases viral proteins are the trigger of these receptors, the predominant viral activators are nucleic acids. The presence of viral sensing PRRs in multiple cellular compartments allows innate cells to recognize and quickly respond to a broad range of viruses, which replicate in different cellular compartments. Here, we review the role of PRRs and associated signaling pathways in detecting viral pathogens in order to evoke production of interferons and cytokines. By highlighting recent progress in these areas, we hope to convey a greater understanding of how viruses activate PRR signaling and how this interaction shapes the anti-viral immune response.

Suppression of TLR9 immunostimulatory motifs in the genome of a gammaherpesvirus

Multiple receptors within the innate immune system have evolved to recognize nucleic acids as signatures of viral infection. It is believed that this specificity is essential for viral detection, as viruses often lack other invariant features that can serve as suitable targets for innate receptors. One such innate receptor, TLR9, has been implicated in the detection of many dsDNA viruses. In this study, we investigate the detection of murine gammaherpesvirus 68 (MHV68) by TLR9. We find that the genomic DNA of the murine CMV, a very potent inducer of innate responses. Genome-wide analysis of the number of stimulatory versus nonstimulatory CpG motifs present in the genome of each virus reveals that the MHV68 genome contains only a fraction of the number of immunostimulatory motifs present in murine CMV. Notably, MHV68 appears to have selectively suppressed the number of stimulatory motifs through cytosine to thymine conversion. These data suggest that certain viruses may have evolved and modified their genomic content to avoid recognition by nucleic acid-sensing receptors of the innate immune system.

Platelet activation attracts a subpopulation of effector monocytes to sites of Leishmania major infection

Leishmania infection triggers the recruitment of Gr1⁺ monocytes to the site of infection via platelet-derived PDGF and subsequent CCL2 production.

Leishmania species trigger a brisk inflammatory response and efficiently induce cell-mediated immunity. We examined the mechanisms whereby leukocytes were recruited into lesions after Leishmania major infection of mice. We found that a subpopulation of effector monocytes expressing the granulocyte marker GR1 (Ly6C) is rapidly recruited into lesions, and these monocytes efficiently kill L. major parasites. The recruitment of this subpopulation of monocytes depends on the chemokine receptor CCR2 and the activation of platelets. Activated platelets secrete platelet-derived growth factor, which induces the rapid release of CCL2 from leukocytes and mesenchymal cells. This work points to a new role for platelets in host defense involving the selective recruitment of a subpopulation of effector monocytes from the blood to efficiently kill this intracellular parasite.

Type I interferon production induced by Streptococcus pyogenes-derived nucleic acids is required for host protection

Streptococcus pyogenes is a Gram-positive human pathogen that is recognized by yet unknown pattern recognition receptors (PRRs). Engagement of these receptor molecules during infection with S. pyogenes, a largely extracellular bacterium with limited capacity for intracellular survival, causes innate immune cells to produce inflammatory mediators such as TNF, but also type I interferon (IFN). Here we show that signaling elicited by type I IFNs is required for successful defense of mice against lethal subcutaneous cellulitis caused by S. pyogenes. Type I IFN signaling was accompanied with reduced neutrophil recruitment to the site of infection. Mechanistic analysis revealed that macrophages and conventional dendritic cells (cDCs) employ different signaling pathways leading to IFN-beta production. Macrophages required IRF3, STING, TBK1 and partially MyD88, whereas in cDCs the IFN-beta production was fully dependent on IRF5 and MyD88. Furthermore, IFN-beta production by macrophages was dependent on the endosomal delivery of streptococcal DNA, while in cDCs streptococcal RNA was identified as the IFN-beta inducer. Despite a role of MyD88 in both cell types, the known IFN-inducing TLRs were individually not required for generation of the IFN-beta response. These results demonstrate that the innate immune system employs several strategies to efficiently recognize S. pyogenes, a pathogenic bacterium that succeeded in avoiding recognition by the standard arsenal of TLRs.

IRF7: activation, regulation, modification and function

Interferon regulatory factor 7 (IRF7) was originally identified in the context of Epstein-Barr virus (EBV) infection, and has since emerged as the crucial regulator of type I interferons (IFNs) against pathogenic infections, which activate IRF7 by triggering signaling cascades from pathogen recognition receptors (PRRs) that recognize pathogenic nucleic acids. Moreover, IRF7 is a multifunctional transcription factor, underscored by the fact that it is associated with EBV latency, in which IRF7 is induced as well as activated by the EBV principal oncoprotein latent membrane protein-1 (LMP1). Aberrant production of type I IFNs is associated with many types of diseases such as cancers and autoimmune disorders. Thus, tight regulation of IRF7 expression and activity is imperative in dictating appropriate type I IFN production for normal IFN-mediated physiological functions. Posttranslational modifications have important roles in regulation of IRF7 activity, exemplified by phosphorylation, which is indicative of its activation. Furthermore, mounting evidence has shed light on the importance of regulatory ubiquitination in activation of IRF7. Albeit these exciting findings have been made in the past decade since its discovery, many questions related to IRF7 remain to be addressed.

TLR-based immune adjuvants

This work describes the nature and strength of the immune response induced by various Toll-like receptor ligands and their ability to act as vaccine adjuvants. It reviews the various ligands capable of triggering individual TLRs, and then focuses on the efficacy and safety of those agents for which clinical results are available.

Innate immune sensing of DNA viruses

DNA viruses are a significant contributor to human morbidity and mortality. The immune system protects against viral infections through coordinated innate and adaptive immune responses. While the antigen-specific adaptive mechanisms have been extensively studied, the critical contributions of innate immunity to anti-viral defenses have only been revealed in the very recent past. Central to these anti-viral defenses is the recognition of viral pathogens by a diverse set of germ-line encoded receptors that survey nearly all cellular compartments for the presence of pathogens. In this review, we discuss the recent advances in the innate immune sensing of DNA viruses and focus on the recognition mechanisms involved.

Phagosomal signaling by Borrelia burgdorferi in human monocytes involves Toll-like receptor (TLR) 2 and TLR8 cooperativity and TLR8-mediated induction of IFN-beta

Phagocytosed Borrelia burgdorferi (Bb) induces inflammatory signals that differ both quantitatively and qualitatively from those generated by spirochetal lipoproteins interacting with Toll-like receptor (TLR) 1/2 on the surface of human monocytes. Of particular significance, and in contrast to lipoproteins, internalized spirochetes induce transcription of IFN-β. Using inhibitory immunoregulatory DNA sequences (IRSs) specific to TLR7, TLR8, and TLR9, we show that the TLR8 inhibitor IRS957 significantly diminishes production of TNF-α, IL-6, and IL-10 and completely abrogates transcription of IFN-β in Bb-stimulated monocytes. We demonstrate that live Bb induces transcription of TLR2 and TLR8, whereas IRS957 interferes with their transcriptional regulation. Using confocal and epifluorescence microscopy, we show that baseline TLR expression in unstimulated monocytes is greater for TLR2 than for TLR8, whereas expression of both TLRs increases significantly upon stimulation with live spirochetes. By confocal microscopy, we show that TLR2 colocalization with Bb coincides with binding, uptake, and formation of the phagosomal vacuole, whereas recruitment of both TLR2 and TLR8 overlaps with degradation of the spirochete. We provide evidence that IFN regulatory factor (IRF) 7 is translocated into the nucleus of Bb-infected monocytes, suggesting its activation through phosphorylation. Taken together, these findings indicate that the phagosome is an efficient platform for the recognition of diverse ligands; in the case of Bb, phagosomal signaling involves a cooperative interaction between TLR2 and TLR8 in pro- and antiinflammatory cytokine responses, whereas TLR8 is solely responsible for IRF7-mediated induction of IFN-β.

Recent insights into the role of Toll-like receptors in viral infection

Toll-like receptors (TLRs) have a central role in innate immunity as they detect conserved pathogen-associated molecular patterns (PAMPs) on a range of microbes, including viruses, leading to innate immune activation and orchestration of the adaptive immune response. To date, a large number of viruses have been shown to trigger innate immunity via TLRs, suggesting that these receptors are likely to be important in the outcome to viral infection. This suggestion is supported by the observation that many viruses have evolved mechanisms not only to evade the innate immune system, but also to subvert it for the benefit of the virus. In this review we will discuss earlier evidence, mainly from knock-out mice studies, implicating TLRs in the innate immune response to viruses, in light of more recent clinical data demonstrating that TLRs are important for anti-viral immunity in humans.

Recognition of herpesviruses by the innate immune system

Advances in innate immunity over the past decade have revealed distinct classes of pattern recognition receptors (PRRs) that detect pathogens at the cell surface and in intracellular compartments. This has shed light on how herpesviruses, which are large disease-causing DNA viruses that replicate in the nucleus, are initially recognized during cellular infection. Surprisingly, this involves multiple PRRs both on the cell surface and within endosomes and the cytosol. In this article we describe recent advances in our understanding of innate detection of herpesviruses, how this innate detection translates into anti-herpesvirus host defence, and how the viruses seek to evade this innate detection to establish persistent infections.

Distinct inflammatory signals have physiologically divergent effects on epigenetic regulation of Foxp3 expression and Treg function

Foxp3 expression in regulatory T cells (Treg) is required for their development and suppressive function. How different inflammatory signals affect Foxp3 chromatin structure, expression and Tregs plasticity are not completely known. In the present study, the Toll-like receptor 2 (TLR2) ligand peptidoglycan inhibited Foxp3 expression in both natural Treg (nTreg) and TGFβ-driven adaptive Treg (aTreg). Inhibition was independent of paracrine Th1, Th2 and Th17 cytokines. PGN-induced T cell-intrinsic TLR2-Myd88-dependent IFR1 expression and induced IRF1 bound to IRF1 response elements (IRF-E) in the Foxp3 promoter and intronic enhancers, and negatively regulated Foxp3 expression. Inflammatory IL-6 and TLR2 signals induced divergent chromatin changes at the Foxp3 locus and regulated Treg suppressor function, and in an islet transplant model resulted in differences in their ability to prolong graft survival. These findings are important for understanding how different inflammatory signals can affect the transplantation tolerance and immunity.

TLR2 hypersensitivity of astrocytes as functional consequence of previous inflammatory episodes

Precedent inflammatory episodes may drastically modify the function and reactivity of cells. We investigated whether priming of astrocytes by microglia-derived cytokines alters their subsequent reaction to pathogen-associated danger signals not recognized in the quiescent state. Resting primary murine astrocytes expressed little TLR2, and neither the TLR2/6 ligand fibroblast-stimulating lipopeptide-1 (FSL1) nor the TLR1/2 ligand Pam(3)CysSK(4) (P3C) triggered NF-κB translocation or IL-6 release. We made use of single-cell detection of NF-κB translocation as easily detectable and sharply regulated upstream indicator of an inflammatory response or of c-Jun phosphorylation to measure restimulation events in astrocytes under varying conditions. Cells prestimulated with IL-1β, with a TLR3 ligand, with a complete cytokine mix consisting of TNF-α, IL-1β, and IFN-γ, or with media conditioned by activated microglia responded strongly to FSL1 or P3C stimulation, whereas the sensitivity of the NF-κB response to other pattern recognition receptors was unchanged. This sensitization to TLR2 ligands was associated with an initial upregulation of TLR2, displayed a "memory" window of several days, and was largely independent of the length of prestimulation. The altered signaling led to altered function, as FSL1 or P3C triggered the release of IL-6, CCL-20, and CXCL-2 in primed cells, but not in resting astrocytes. These data confirmed the hypothesis that astrocytes exposed to activated microglia assume a different functional phenotype involving longer term TLR2 responsiveness, even after the initial stimulation by inflammatory mediators has ended.

Lactobacillus acidophilus induces virus immune defence genes in murine dendritic cells by a Toll-like receptor-2-dependent mechanism

Lactobacilli are probiotics that, among other health-promoting effects, have been ascribed immunostimulating and virus-preventive properties. Certain Lactobacillus spp. have been shown to possess strong interleukin-12 (IL-12) -inducing properties. As IL-12 production depends on the up-regulation of type I interferons (IFNs), we hypothesized that the strong IL-12-inducing capacity of Lactobacillus acidophilus NCFM in murine bone-marrow-derived dendritic cells (DCs) is caused by an up-regulation of IFN-β, which subsequently induces IL-12 and the double-stranded RNA binding Toll-like receptor-3 (TLR-3). The expression of the genes encoding IFN-β, TLR-3, IL-12 and IL-10 in DCs upon stimulation with L. acidophilus NCFM was determined. Lactobacillus acidophilus NCFM induced a much stronger expression of Ifn-β, Il-12 and Il-10 compared with the synthetic double-stranded RNA ligand Poly I:C, whereas the levels of expressed Tlr-3 were similar. Whole genome microarray gene expression analysis revealed that other genes related to viral defence were significantly up-regulated and among the strongest induced genes in DCs stimulated with L. acidophilus NCFM. The ability to induce IFN-β was also detected in another L. acidophilus strain (X37), but was not a property of other probiotic strains tested, i.e. Bifidobacterium bifidum Z9 and Escherichia coli Nissle 1917. The IFN-β expression was markedly reduced in TLR-2(-/-) DCs, dependent on endocytosis, and the major cause of the induction of Il-12 and Tlr-3 in DCs stimulated with L. acidophilus NCFM. Collectively, our results reveal that certain lactobacilli trigger the expression of viral defence genes in DCs in a TLR-2 manner dependent on IFN-β.

Toll-like receptors as interferon-regulated genes and their role in disease

The Toll-like receptors (TLRs) are innate sensors that recognize both microbial and endogenous ligands, initiating the host defense response. TLRs initiate the potent proinflammatory response to infection, are the target for adjuvants, and are essential for the establishment and maturation of adaptive immunity. As such they have been the interest of widespread research and the target of therapeutic intervention on multiple diseases. It has become apparent that expression of a subset of TLRs (TLR1, TLR2, TLR3, TLR5, and TLR7) is induced by Type I interferons (IFN). The role and impact of IFN expression on TLR responses is therefore critical in understanding the role of TLRs in disease, particularly as IFN itself is a downstream gene induced by specific TLRs. In this review we discuss the function and role of IFN-regulated TLRs in disease and how the role of IFN may impact upon TLR induction of the immune response in diseases, particularly in mouse models.

The control of adaptive immune responses by the innate immune system

The mammalian immune system comprises an adaptive and an innate component. The innate immune system employs a limited number of germ-line-encoded pattern-recognition receptors (PRRs) that recognize invariant pathogen-associated molecular patterns (PAMPs). In contrast, the adaptive immune system depends on the generation of a diverse repertoire of antigen receptors on T and B lymphocytes and subsequent activation and clonal expansion of cells carrying the appropriate antigen-specific receptors. Induction of adaptive immunity not only depends on direct antigen recognition by the antigen receptors but also relies on essential signals that are delivered by the innate immune system. In recent years, we have witnessed the discovery of a still expanding array of different PRR systems that govern the generation of adaptive immunity. Here, we review our current understanding of innate control of adaptive immunity. In particular, we discuss how PRRs initiate adaptive immune responses in general, discuss specific mechanisms that shape the ensuing T and B cell responses, and highlight open questions that are still awaiting answers.