Inherited disorders of human Toll-like receptor signaling: immunological implications

Ablation of SigH+ pDCs in B6.Nba2 mice prevents lupus-like disease development only if started before disease is fully established

Systemic lupus erythematosus is characterized by hyper-activation of the immune system, multi-organ inflammation, and end-organ damage. Type I interferons (IFN-I) have been strongly implicated a role in disease etiology as has the main IFN-I-producing cell subset, the plasmacytoid dendritic cell (pDC). The B6.Nba2 mouse model develops a lupus-like disease characterized by elevated IFN-I levels and pDC pathogenicity. We have previously shown that pDC ablation prior to disease development in B6.Nba2 mice effectively prevents disease; however, it remains unclear if a similar protection can be seen if pDC ablation is initiated during later disease stages. This is important as Systemic lupus erythematosus patients are rarely diagnosed until disease is well-established and thus preventative treatment is unlikely to take place. Here we show that ablation of pDCs in the B6.Nba2 mouse model must be initiated early in order to effectively block disease development and that sustained reduction in pDC numbers is necessary for sustained effects. Finally, targeting of pDCs have been hypothesized to affect immunity towards infectious agents, in particular virus and intracellular bacteria. We show here that pDC ablation in B6.Nba2 mice does not affect the anti-viral response to encephalomyocarditic virus or a model T-dependent antigen. In summary, pDC ablation does not affect general immunity, but needs to happen early and be sustained to prevent lupus-like disease development in B6.Nba2 mice.

Toll protein family structure, evolution and response of the whiteleg shrimp (Litopenaeus vannamei) to exogenous iridescent virus

Whiteleg shrimp is a widely cultured crustacean, but frequent disease outbreaks have decreased production and caused significant losses. Toll-like receptors (TLRs) comprise a large innate immune family that is involved in the innate immune response. However, understanding of their regulatory mechanism is limited. In this study, PacBio sequencing and Illumina sequencing were applied to the gill and hepatopancreas tissues of whiteleg shrimp and an integrated transcript gene set was established. The upregulation of Toll1, Toll2 and Toll3 transcripts in the hepatopancreas tissue of whiteleg shrimp after iridescent virus infection implies that these proteins are involved in the immune response to the virus; simultaneously, the TRAF6 and relish transcripts in the Toll pathway were also upregulated, implying that the Toll pathway was activated. We predicted the three-dimensional structure of the five Toll proteins in whiteleg shrimp and humans and constructed a phylogenetic tree of the Toll protein family. In addition, there was a large discrepancy of Toll1 between invertebrates and vertebrates, presumably because of the loss of Toll1 protein sequence during the evolution process from invertebrates to vertebrates. Our research will improve the cognition of Toll protein family in invertebrates in terms of evolution, structure and function and provide theoretical guidance for researchers in this field.

Mycobacterium tuberculosis RipA Dampens TLR4-Mediated Host Protective Response Using a Multi-Pronged Approach Involving Autophagy, Apoptosis, Metabolic Repurposing, and Immune Modulation

Reductive evolution has endowed Mycobacterium tuberculosis (M. tb) with moonlighting in protein functions. We demonstrate that RipA (Rv1477), a peptidoglycan hydrolase, activates the NFκB signaling pathway and elicits the production of pro-inflammatory cytokines, TNF-α, IL-6, and IL-12, through the activation of an innate immune-receptor, toll-like receptor (TLR)4. RipA also induces an enhanced expression of macrophage activation markers MHC-II, CD80, and CD86, suggestive of M1 polarization. RipA harbors LC3 (Microtubule-associated protein 1A/1B-light chain 3) motifs known to be involved in autophagy regulation and indeed alters the levels of autophagy markers LC3BII and P62/SQSTM1 (Sequestosome-1), along with an increase in the ratio of P62/Beclin1, a hallmark of autophagy inhibition. The use of pharmacological agents, rapamycin and bafilomycin A1, reveals that RipA activates PI3K-AKT-mTORC1 signaling cascade that ultimately culminates in the inhibition of autophagy initiating kinase ULK1 (Unc-51 like autophagy activating kinase). This inhibition of autophagy translates into efficient intracellular survival, within macrophages, of recombinant Mycobacterium smegmatis expressing M. tb RipA. RipA, which also localizes into mitochondria, inhibits the production of oxidative phosphorylation enzymes to promote a Warburg-like phenotype in macrophages that favors bacterial replication. Furthermore, RipA also inhibited caspase-dependent programed cell death in macrophages, thus hindering an efficient innate antibacterial response. Collectively, our results highlight the role of an endopeptidase to create a permissive replication niche in host cells by inducing the repression of autophagy and apoptosis, along with metabolic reprogramming, and pointing to the role of RipA in disease pathogenesis.

Invasive Bacterial Infections in Subjects with Genetic and Acquired Susceptibility and Impacts on Recommendations for Vaccination: A Narrative Review

The WHO recently endorsed an ambitious plan, “Defeating Meningitis by 2030”, that aims to control/eradicate invasive bacterial infection epidemics by 2030. Vaccination is one of the pillars of this road map, with the goal to reduce the number of cases and deaths due to Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae and Streptococcus agalactiae. The risk of developing invasive bacterial infections (IBI) due to these bacterial species includes genetic and acquired factors that favor repeated and/or severe invasive infections. We searched the PubMed database to identify host risk factors that increase the susceptibility to these bacterial species. Here, we describe a number of inherited and acquired risk factors associated with increased susceptibility to invasive bacterial infections. The burden of these factors is expected to increase due to the anticipated decrease in cases in the general population upon the implementation of vaccination strategies. Therefore, detection and exploration of these patients are important as vaccination may differ among subjects with these risk factors and specific strategies for vaccination are required. The aim of this narrative review is to provide information about these factors as well as their impact on vaccination against the four bacterial species. Awareness of risk factors for IBI may facilitate early recognition and treatment of the disease. Preventive measures including vaccination, when available, in individuals with increased risk for IBI may prevent and reduce the number of cases.

Viral antigens elicit augmented immune responses in primary Sjögren's syndrome

OBJECTIVES

Infections have been suggested in the pathogenesis of primary SS (pSS). Systematic studies of immune responses to microbial antigens in vivo may be performed during vaccination. In the present study, we therefore longitudinally followed patients with pSS and controls during split-virion influenza vaccination to identify pSS-specific cellular, transcriptomic and serological responses.

METHODS

Patients without treatment (pSSUntr, n = 17), on hydroxychloroquine-treatment (pSSHCQ, n = 8), and healthy controls (n = 16) were included. Antibody titres were determined by ELISA. Plasma proteins were measured by proximity extension assay. Monocyte gene expression was assessed by Nanostring. Routine laboratory tests were performed and clinical disease symptoms were registered by questionnaires.

RESULTS

pSSUntr developed higher vaccine-specific IgG titres compared with controls. Notably, anti-Ro52 autoantibody titres increased in pSSUntr but remained unchanged in pSSHCQ. No changes in disease symptoms including EULAR Sjögren's Syndrome Patient Reported Index score were registered. Twenty-four hours after vaccination, the leucocyte count in pSSUntr decreased, with a concomitant increase of CCL7 in plasma. Transcriptomic analysis in monocytes revealed differential vaccination-related expression of the NEMO/IKBKG gene, and its higher induced expression in pSSUntr associated with higher serological vaccine responses. Moreover, titres of vaccine-specific antibodies were associated with higher vaccination-induced NF-κB signalling and higher steady-state IFN signatures in monocytes, and with the levels of several plasma proteins with soluble PD-1 displaying the strongest association.

CONCLUSION

We observed augmented innate and adaptive immune responses in pSS following viral antigen exposure suggesting an underlying hyper-responsiveness to immune challenges, supporting a role for infections driving the immunopathology and acting as environmental risk factor for pSS.

Heritable defects of the human TLR signalling pathways

Recently, three human primary immunodeficiencies associated with impaired TLR signalling were described. Anhidrotic ectodermal dysplasia with immunodeficiency (EDA-ID), either X-linked recessive or autosomal dominant, is caused by hypomorphic mutations in NEMO or hypermorphic mutation in IKBA, respectively, both involved in nuclear factor-κB (NF-κB) activation. These patients present with abnormal development of ectoderm-derived structures and suffer from a broad spectrum of infectious diseases. In vitro studies of the patients' cells showed an impaired, but not abolished, NF-κB activation in response to a large set of stimuli, including TLR agonists. More recently, patients with autosomal recessive amorphic mutations in IRAK4 have been reported, presenting no developmental defect and a more restricted spectrum of infectious diseases, mostly caused by pyogenic encapsulated bacteria, principally, but not exclusively Gram-positive. In vitro studies carried out with these patients' cells showed a specific impairment of the Toll—interleukin-1 receptor (TIR)—interleukin-1 receptor associated kinase (IRAK) signalling pathway. NF-κB- and mitogen activated protein kinase (MAPK) pathways are impaired in response to all TIR agonists tested. These data, therefore, suggest that TLRs play a critical role in host defence against pyogenic bacteria, but may be dispensable or redundant for immunity to most other infectious agents in humans.

Pathogenesis of infection in surgical patients

Purpose of review

Despite the application of prophylactic antimicrobial therapy and advanced technologies, infection remains one of the most common causes of morbidity and mortality in surgical patients. Understanding the pathogenesis of surgical infection would offer new insights into the development of biomarkers to predict and stratify infection in patients, and to explore specific strategies to minimize this serious postoperative complication.

Recent findings

The acute nonspecific inflammatory response triggered by endogenous danger signals evoked by surgical insult is beneficial, while paradoxically associated with reduced resistance to infection. There is growing evidence indicating that primed inflammation by surgical insult exaggerates the dysregulation of the immune-inflammatory response to the invasion of pathogens postoperatively. Innate immune receptors, such as Toll-like receptors (TLRs), contribute to detecting both pathogen-associated molecular patterns and endogenous damage-associated molecular patterns, and to further amplifying inflammatory responses to infection. Current evidence shows the fascinating role of non-TLRs in the process of infection. Non-TLRs, such as membrane-associated triggering receptor expressed on myeloid cells family, cytosolic nucleotide-binding oligomerization domain-like receptors and nuclear receptor nuclear family 4 subgroup A receptors, are also crucial in triggering the immune responses and mounting an effective defense against surgical insults and the second hit of infection.

Summary

Understanding the pivotal role of non-TLRs in sensing exogenous and endogenous molecules, and the influence of primed systemic inflammation and depressed immune status on the defense against pathogen after surgical insult, would be helpful to fully explore the relevant sophisticated phenomena of surgical infection, and to elucidate the occurrence of heterogeneous constellations of clinical signs and symptoms among this special population.

EDA-ID and IP, two faces of the same coin: how the same IKBKG/NEMO mutation affecting the NF-κB pathway can cause immunodeficiency and/or inflammation

Anhidrotic Ectodermal Dysplasia with ImmunoDeficiency (EDA-ID, OMIM 300291) and Incontinentia Pigmenti (IP, OMIM 308300) are two rare diseases, caused by mutations of the IKBKG/NEMO gene. The protein NEMO/IKKγ is essential for the NF-κB activation pathway, involved in a variety of physiological and cellular processes, such as immunity, inflammation, cell proliferation, and survival. A wide spectrum of IKBKG/NEMO mutations have been identified so far, and, on the basis of their effect on NF-κB activation, they are considered hypomorphic or amorphic (loss of function) mutations. IKBKG/NEMO hypomorphic mutations, reducing but not abolishing NF-κB activation, have been identified in EDA-ID and IP patients. Instead, the amorphic mutations, abolishing NF-κB activation by complete IKBKG/NEMO gene silencing, cause only IP. Here, we present an overview of IKBKG/NEMO mutations in EDA-ID and IP patients and describe similarities and differences between the clinical/immunophenotypic and genetic aspects, highlighting any T and B lymphocyte defect, and paying particular attention to the cellular and molecular defects that underlie the pathogenesis of both diseases.

Recurrent invasive pneumococcal disease in children: underlying clinical conditions, and immunological and microbiological characteristics

Purpose

Clinical, immunological and microbiological characteristics of recurrent invasive pneumococcal disease (IPD) in children were evaluated, differentiating relapse from reinfection, in order to identify specific risk factors for both conditions.

Methods

All patients <18 years-old with recurrent IPD admitted to a tertiary-care pediatric center from January 2004 to December 2011 were evaluated. An episode of IPD was defined as the presence of clinical findings of infection together with isolation and/or pneumococcal DNA detection by Real-Time PCR in any sterile body fluid. Recurrent IPD was defined as 2 or more episodes in the same individual at least 1 month apart. Among recurrent IPD, we differentiated relapse (same pneumococcal isolate) from reinfection.

Results

593 patients were diagnosed with IPD and 10 patients died. Among survivors, 23 episodes of recurrent IPD were identified in 10 patients (1.7%). Meningitis was the most frequent form of recurrent IPD (10 episodes/4 children) followed by recurrent empyema (8 episodes/4 children). Three patients with recurrent empyema caused by the same pneumococcal clone ST306 were considered relapses and showed high bacterial load in their first episode. In contrast, all other episodes of recurrent IPD were considered reinfections. Overall, the rate of relapse of IPD was 0.5% and the rate of reinfection 1.2%. Five out of 7 patients with reinfection had an underlying risk factor: cerebrospinal fluid leak (n = 3), chemotherapy treatment (n = 1) and a homozygous mutation in MyD88 gene (n = 1). No predisposing risk factors were found in the remainder.

Conclusions

recurrent IPD in children is a rare condition associated with an identifiable risk factor in case of reinfection in almost 80% of cases. In contrast, recurrent IPD with pleuropneumonia is usually a relapse of infection.

Structural dynamic analysis of apo and ATP-bound IRAK4 kinase

Interleukin-1 receptor-associated kinases (IRAKs) are Ser/Thr protein kinases that play an important role as signaling mediators in the signal transduction facilitated by the Toll-like receptor (TLR) and interleukin-1 receptor families. Among IRAK family members, IRAK4 is one of the drug targets for diseases related to the TLR and IL-1R signaling pathways. Experimental evidence suggests that the IRAK4 kinase domain is phosphorylated in its activation loop at T342, T345, and S346 in the fully activated state. However, the molecular interactions of subdomains within the active and inactive IRAK4 kinase domain are poorly understood. Hence, we employed a long-range molecular dynamics (MD) simulation to compare apo IRAK4 kinase domains (phosphorylated and unphosphorylated) and ATP-bound phosphorylated IRAK4 kinase domains. The MD results strongly suggested that lobe uncoupling occurs in apo unphosphorylated IRAK4 kinase via the disruption of the R334/T345 and R310/T345 interaction. In addition, apo unphosphorylated trajectory result in high mobility, particularly in the N lobe, activation segment, helix αG, and its adjoining loops. The Asp-Phe-Gly (DFG) and His-Arg-Asp (HRD) conserved kinase motif analysis showed the importance of these motifs in IRAK4 kinase activation. This study provides important information on the structural dynamics of IRAK4 kinase, which will aid in inhibitor development.

rFliC prolongs allograft survival in association with the activation of recipient Tregs in a TLR5-dependent manner

Allorejection remains an obstacle for successful organ transplantation. Although different types of immunosuppressive agents are effective for controlling rejection and prolonging graft survival, drug treatment is limited because of side effects and toxicity. Therefore, it is necessary and urgent to identify new candidate drugs for inducing allotolerance. Recently, it has been reported that bacterial flagellin induces the immunosuppressive activity of regulatory T cells (Tregs) in humans in vitro. In the present study, we analyzed the effects of recombinant flagellin (rFliC) on allograft survival and explored the underlying mechanisms associated with the activation of recipient Tregs in a murine skin allotransplantation model. The results showed that rFliC administration (3 mg/kg, once per day for 3 days, i.p.) prolonged allograft survival (mean survival time: 18.4±1.1 days) compared to the control group (10±0.7 days, P<0.01). Additionally, higher positive expression of Toll-like receptor 5 (TLR5) was detected within the allograft administered with rFliC. The frequency of CD4(+)CD25(+)Foxp3(+) Tregs; the expression of Treg-related factors TLR5, Foxp3, TGF-β1 and IL-10; and the proliferation and suppression of Tregs were increased following rFliC administration compared to the control. Moreover, the increased expression of tolerance-related molecules and the proliferation of Tregs induced by rFliC were attenuated by an anti-TLR5 blocking antibody both in vivo and in vitro. In conclusion, rFliC administration prolongs the survival of allografts, which is associated with the activation of recipient Tregs in a TLR5-dependent manner. rFliC may be a new candidate for anti-allorejection therapy.

PepT1 expressed in immune cells has an important role in promoting the immune response during experimentally induced colitis

We and others have shown that the dipeptide cotransporter PepT1 is expressed in immune cells, including macrophages that are in close contact with the lamina propria of the small and large intestines. In the present study, we used PepT1-knockout (KO) mice to explore the role played by PepT1 in immune cells during dextran sodium sulfate (DSS)-induced colitis. DSS treatment caused less severe body weight loss, diminished rectal bleeding, and less diarrhea in PepT1-KO mice than in wild-type (WT) animals. A histological examination of colonic sections revealed that the colonic architecture was less disrupted and the extent of immune cell infiltration into the mucosa and submucosa following DSS treatment was reduced in PepT1-KO mice compared with WT animals. Consistent with these results, the DSS-induced colitis increase in colonic myeloperoxidase activity was significantly less in PepT1-KO mice than in WT littermates. The colonic levels of mRNAs encoding the inflammatory cytokines CXCL1, interleukin (IL)-6, monocyte chemotactic protein-1, IL-12, and interferon-γ were significantly lower in DSS-treated PepT1-KO mice than in DSS-treated WT animals. Colonic immune cells from WT had significantly higher level of proinflammatory cytokines then PepT1 KO. In addition, we observed that knocking down the PepT1 expression decreases chemotaxis of immune cells recruited during intestinal inflammation. Antibiotic treatment before DSS-induced colitis eliminated the differential expression of inflammatory cytokines between WT and PepT1-KO mice. In conclusion, PepT1 in immune cells regulates the secretion of proinflammatory cytokines triggered by bacteria and/or bacterial products, and thus has an important role in the induction of colitis. PepT1 may transport small bacterial products, such as muramyl dipeptide and the tripeptide L-Ala-gamma-D-Glu-meso-DAP, into macrophages. These materials may be sensed by members of the nucleotide-binding site-leucine-rich repeat family of intracellular receptors, ultimately resulting in altered homeostasis of the intestinal microbiota.

Experimental and natural infections in MyD88- and IRAK-4-deficient mice and humans

Most Toll-like-receptors (TLRs) and interleukin-1 receptors (IL-1Rs) signal via myeloid differentiation primary response 88 (MyD88) and interleukin-1 receptor-associated kinase 4 (IRAK-4). The combined roles of these two receptor families in the course of experimental infections have been assessed in MyD88- and IRAK-4-deficient mice for almost fifteen years. These animals have been shown to be susceptible to 46 pathogens: 27 bacteria, eight viruses, seven parasites, and four fungi. Humans with inborn MyD88 or IRAK-4 deficiency were first identified in 2003. They suffer from naturally occurring life-threatening infections caused by a small number of bacterial species, although the incidence and severity of these infections decrease with age. Mouse TLR- and IL-1R-dependent immunity mediated by MyD88 and IRAK-4 seems to be vital to combat a wide array of experimentally administered pathogens at most ages. By contrast, human TLR- and IL-1R-dependent immunity mediated by MyD88 and IRAK-4 seems to be effective in the natural setting against only a few bacteria and is most important in infancy and early childhood. The roles of TLRs and IL-1Rs in protective immunity deduced from studies in mutant mice subjected to experimental infections should therefore be reconsidered in the light of findings for natural infections in humans carrying mutations as discussed in this review.

TLR3 immunity to infection in mice and humans

TLR3 is a receptor for dsRNA, which is generated during most viral infections. However, other cellular processes may also produce dsRNA and there are other receptors for dsRNA. The role of TLR3 in protective immunity to viruses has been investigated in mice and humans with genetically impaired TLR3 responses. TLR3-deficient mice responded to experimental challenge with 16 different viruses in various ways. They were susceptible to eight viruses, normally resistant to three other viruses, and their survival rates were higher than those of wild-type mice following infection with four other viruses. Conflicting results were obtained for the other virus tested. These data are difficult to understand in terms of a simple pattern based on virus structure or tissue tropism. Surprisingly, the known human patients with inborn errors of the TLR3 pathway have remained healthy or developed encephalitis in the course of natural primary infection with HSV-1. These patients display no clear susceptibility to other infections, including viral infections, such as other forms of viral encephalitis and other HSV-1 diseases in particular. This restricted susceptibility to viruses seems to result from impaired TLR3-dependent IFN-α/β production by central nervous system (CNS)-resident non-hematopoietic cells infected with HSV-1. These studies neatly illustrate the value of combining genetic studies of experimental infections in mice and natural infections in humans, to elucidate the biological function of host molecules in protective immunity.

Inborn errors of anti-viral interferon immunity in humans

The three types of interferon (IFNs) are essential for immunity against at least some viruses in the mouse model of experimental infections, type I IFNs displaying the broadest and strongest anti-viral activity. Consistently, human genetic studies have shown that type II IFN is largely redundant for immunity against viruses in the course of natural infections. The precise contributions of human type I and III IFNs remain undefined. However, various inborn errors of anti-viral IFN immunity have been described, which can result in either broad or narrow immunological and viral phenotypes. The broad disorders impair the response to (STAT1, TYK2) or the production of at least type I and type III IFNs following multiple stimuli (NEMO), resulting in multiple viral infections at various sites, including herpes simplex encephalitis (HSE). The narrow disorders impair exclusively (TLR3) or mostly (UNC-93B, TRIF, TRAF3) the TLR3-dependent induction of type I and III IFNs, leading to HSE in apparently otherwise healthy individuals. These recent discoveries highlight the importance of human type I and III IFNs in protective immunity against viruses, including the TLR3-IFN pathway in protection against HSE.

Herpesviral infection and Toll-like receptor 2

In the last decade, substantial progress has been made in understanding the molecular mechanisms involved in the initial host responses to viral infections. Herpesviral infections can provoke an inflammatory cytokine response, however, the innate pathogen-sensing mechanisms that transduce the signal for this response are poorly understood. In recent years, it has become increasingly evident that the Toll-like receptors (TLRs), which are germline-encoded pattern recognition receptors (PRRs), function as potent sensors for infection. TLRs can induce the activation of the innate immunity by recruiting specific intracellular adaptor proteins to initiate signaling pathways, which then culminating in activation of the nuclear factor kappa B (NF-κB) and interferon-regulatory factors (IRFs) that control the transcription of genes encoding type I interferon (IFN I) and other inflammatory cytokines. Furthermore, activation of innate immunity is critical for mounting adaptive immune responses. In parallel, common mechanisms used by viruses to counteract TLR-mediated responses or to actively subvert these pathways that block recognition and signaling through TLRs for their own benefit are emerging. Recent findings have demonstrated that TLR2 plays a crucial role in initiating the inflammatory process, and surprisingly that the response TLR2 triggers might be overzealous in its attempt to counter the attack by the virus. In this review, we summarize and discuss the recent advances about the specific role of TLR2 in triggering inflammatory responses in herpesvirus infection and the consequences of the alarms raised in the host that they are assigned to protect.

Immunological variation between inbred laboratory mouse strains: points to consider in phenotyping genetically immunomodified mice

Inbred laboratory mouse strains are highly divergent in their immune response patterns as a result of genetic mutations and polymorphisms. The generation of genetically engineered mice (GEM) has, in the past, used embryonic stem (ES) cells for gene targeting from various 129 substrains followed by backcrossing into more fecund mouse strains. Although common inbred mice are considered "immune competent," many have variations in their immune system-some of which have been described-that may affect the phenotype. Recognition of these immune variations among commonly used inbred mouse strains is essential for the accurate interpretation of expected phenotypes or those that may arise unexpectedly. In GEM developed to study specific components of the immune system, accurate evaluation of immune responses must take into consideration not only the gene of interest but also how the background strain and microbial milieu contribute to the manifestation of findings in these mice. This article discusses points to consider regarding immunological differences between the common inbred laboratory mouse strains, particularly in their use as background strains in GEM.

Herpes simplex encephalitis in children with autosomal recessive and dominant TRIF deficiency

Herpes simplex encephalitis (HSE) is the most common sporadic viral encephalitis of childhood. Autosomal recessive (AR) UNC-93B and TLR3 deficiencies and autosomal dominant (AD) TLR3 and TRAF3 deficiencies underlie HSE in some children. We report here unrelated HSE children with AR or AD TRIF deficiency. The AR form of the disease was found to be due to a homozygous nonsense mutation that resulted in a complete absence of the TRIF protein. Both the TLR3- and the TRIF-dependent TLR4 signaling pathways were abolished. The AD form of disease was found to be due to a heterozygous missense mutation, resulting in a dysfunctional protein. In this form of the disease, the TLR3 signaling pathway was impaired, whereas the TRIF-dependent TLR4 pathway was unaffected. Both patients, however, showed reduced capacity to respond to stimulation of the DExD/H-box helicases pathway. To date, the TRIF-deficient patients with HSE described herein have suffered from no other infections. Moreover, as observed in patients with other genetic etiologies of HSE, clinical penetrance was found to be incomplete, as some HSV-1-infected TRIF-deficient relatives have not developed HSE. Our results provide what we believe to be the first description of human TRIF deficiency and a new genetic etiology for HSE. They suggest that the TRIF-dependent TLR4 and DExD/H-box helicase pathways are largely redundant in host defense. They further demonstrate the importance of TRIF for the TLR3-dependent production of antiviral IFNs in the CNS during primary infection with HSV-1 in childhood.

Old and new findings on lipopolysaccharide-binding protein: a soluble pattern-recognition molecule

LBP [LPS (lipopolysaccharide)-binding protein] was discovered approximately 25 years ago. Since then, substantial progress has been made towards our understanding of its function in health and disease. Furthermore, the discovery of a large protein family sharing functional and structural attributes has helped in our knowledge. Still, key questions are unresolved, and here an overview on the old and new findings on LBP is given. LBP is an acute-phase protein of the liver, but is also synthesized in other cells of the organism. While LBP is named after the ability to bind to LPS of Gram-negative bacteria, it also can recognize other bacterial compounds, such as lipopeptides. It has been shown that LBP is needed to combat infections; however, the main mechanism of action is still not clear. New findings on natural genetic variations of LBP leading to functional consequences may help in further elucidating the mechanism of LBP and its role in innate immunity and disease.

NEMO is a key component of NF-κB- and IRF-3-dependent TLR3-mediated immunity to herpes simplex virus

BACKGROUND

Children with germline mutations in Toll-like receptor 3 (TLR3), UNC93B1, TNF receptor-associated factor 3, and signal transducer and activator of transcription 1 are prone to herpes simplex virus-1 encephalitis, owing to impaired TLR3-triggered, UNC-93B-dependent, IFN-α/β, and/or IFN-λ-mediated signal transducer and activator of transcription 1-dependent immunity.

OBJECTIVE

We explore here the molecular basis of the pathogenesis of herpes simplex encephalitis in a child with a hypomorphic mutation in nuclear factor-κB (NF-κB) essential modulator, which encodes the regulatory subunit of the inhibitor of the Iκβ kinase complex.

METHODS

The TLR3 signaling pathway was investigated in the patient's fibroblasts by analyses of IFN-β, IFN-λ, and IL-6 mRNA and protein levels, by quantitative PCR and ELISA, respectively, upon TLR3 stimulation (TLR3 agonists or TLR3-dependent viruses). NF-κB activation was assessed by electrophoretic mobility shift assay and interferon regulatory factor 3 dimerization on native gels after stimulation with a TLR3 agonist.

RESULTS

The patient's fibroblasts displayed impaired responses to TLR3 stimulation in terms of IFN-β, IFN-λ, and IL-6 production, owing to impaired activation of both NF-κB and IRF-3. Moreover, vesicular stomatitis virus, a potent IFN-inducer in human fibroblasts, and herpes simplex virus-1, induced only low levels of IFN-β and IFN-λ in the patient's fibroblasts, resulting in enhanced viral replication and cell death, as reported for UNC-93B-deficient fibroblasts.

CONCLUSION

Herpes simplex encephalitis may occur in patients carrying NF-κB essential modulator mutations, due to the impairment of NF-κB- and interferon regulatory factor 3-dependent-TLR3-mediated antiviral IFN production.

TLR5 functions as an endocytic receptor to enhance flagellin-specific adaptive immunity

Innate immune activation via TLR induces dendritic cell maturation and secretion of inflammatory mediators, generating favorable conditions for naïve T-cell activation. Here, we demonstrate a previously unknown function for TLR5, namely that it enhances MHC class-II presentation of flagellin epitopes to CD4(+) T cells and is required for induction of robust flagellin-specific adaptive immune responses. Flagellin-specific CD4(+) T cells expanded poorly in TLR5-deficient mice immunized with flagellin, a deficiency that persisted even when additional TLR agonists were provided. Flagellin-specific IgG responses were similarly depressed in the absence of TLR5. In marked contrast, TLR5-deficient mice developed robust flagellin-specific T-cell responses when immunized with processed flagellin peptide. Surprisingly, the adaptor molecule Myd88 was not required for robust CD4(+) T-cell responses to flagellin, indicating that TLR5 enhances flagellin-specific CD4(+) T-cell responses in the absence of conventional TLR signaling. A requirement for TLR5 in generating flagellin-specific CD4(+) T-cell activation was also observed when using an in vitro dendritic cell culture system. Together, these data uncover an Myd88-independent function for dendritic cell TLR5 in enhancing the presentation of peptides to flagellin-specific CD4(+) T cells.

Mucosal immune system in neonatal period and early infancy

The major mechanisms of mucosal defense in the neonate consist of a variety of nonspecific barriers, and innate and specific adaptive immune responses. The functions of innate immunity in the mucosal surfaces are mediated by host-specific microbial–pathogen recognition receptors, designed to recognize unique microbial-associated molecular patterns integral to the structure of most microorganisms. Other mechanisms include many antimicrobial peptides, macrophages, dendritic cells, complement components and host-derived cellular and soluble products. The important elements of neonatal mucosal adaptive immunity include the inductive sites, such as the Peyer’s patches and other lymphoid structures in the respiratory and gastrointestinal tracts, the nasopharyngeal and sublingual tissues, the subepithelial and intraepithelial sites in most external mucosal surfaces, including the genital tract and the mammary glands. These sites contain lymphoid cells derived by the homing of antigen-activated cells from the inductive sites. Activated B cells, mostly IgA (up to 80%) are detected in the mucosal tissues shortly after birth. However, IgA-producing plasma cells are generally detected in the mucosa approximately 7–10 days of age. With progressive environmental antigenic stimulation, the number of circulating IgA cells increase significantly by 1 month of age. The postnatal development of mucosal immunity is critically influenced by the acquisition and nature of mucosal microflora, and the temporal nature and qualitative and quantitative aspects of dietary antigens and other environmental agents introduced in the neonatal period. Mucosal immune responses are generally protective against disease-producing organisms and environmental macromolecules. The mucosal immune responses may also be pathologic and foster the induction of immunologically mediated disease states and autoimmunity. The development of secretory IgA and other antimicrobial mucosal responses as well as the induction of tolerance in the neonatal period and early infancy are essential for the maintenance of mucosal homeostasis in early childhood and prevention of disease later in life.

Ligand-independent toll-like receptor signals generated by ectopic overexpression of MyD88 generate local and systemic antitumor immunity

Although critical for initiating and regulating immune responses, the therapeutic use of individual cytokines as anticancer immunotherapeutic agents has achieved only modest clinical success. Consequently, many current strategies have focused on the use of specific immunotherapeutic agonists that engage individual receptors of innate immune networks, such as the Toll-like receptor (TLR) system, each resulting in specific patterns of gene expression, cytokine production, and inflammatory outcome. However, these immunotherapeutics are constrained by variable cellular TLR expression and responsiveness to particular TLR agonists, as well as the specific cellular context of different tumors. We hypothesized that overexpression of MyD88, a pivotal regulator of multiple TLR signaling pathways, could circumvent these constraints and mimic coordinated TLR signaling across all cell types in a ligand-independent fashion. To explore this hypothesis, we generated an adenoviral vector expressing MyD88 and show that Ad-MyD88 infection elicits extensive Th1-specific transcriptional and secreted cytokine signatures in all murine and human cell types tested in vitro and in vivo. Importantly, in vivo intratumoral injection of Ad-MyD88 into established tumor masses enhanced adaptive immune responses and inhibited local tumor immunosuppression, resulting in significantly inhibited local and systemic growth of multiple tumor types. Finally, Ad-MyD88 infection of primary human dendritic cells, tumor-associated fibroblasts, and colorectal carcinoma cells elicited significant Th1-type cytokine responses, resulting in enhanced tumor cell lysis and expansion of human tumor antigen-specific T cells. Thus, Ad-MyD88 initiated robust antitumor activity in established murine tumor microenvironments and in human contexts, suggesting its potential effectiveness as a clinical immunotherapeutic strategy.

Immune deficiency caused by impaired expression of nuclear factor-kappaB essential modifier (NEMO) because of a mutation in the 5' untranslated region of the NEMO gene

BACKGROUND

Nuclear factor-kappaB (NF-kappaB) is a key transcription factor that regulates both innate and adaptive immunity as well as ectodermal development. Mutations in the coding region of the IkappaB kinase gamma/NF-kappaB essential modifier (NEMO) gene cause X-linked ectodermal dysplasia with immunodeficiency.

OBJECTIVE

To determine the genetic cause of recurrent sinopulmonary infections and dysgammaglobulinemia in a patient with a normal NEMO coding sequence and his affected brother.

METHODS

TNF-alpha and IFN-alpha production in response to Toll-like receptor (TLR) stimulation was analyzed by ELISA, NEMO mRNA levels were measured by quantitative PCR, and NEMO protein expression was measured by Western blotting. NF-kappaB activation was assessed by nuclear translocation of p65 and luciferase reporter gene assays.

RESULTS

TLR-induced TNF-alpha and IFN-alpha production by PBMCs was impaired in the patient and his brother. Sequencing of the patient's NEMO gene revealed a novel mutation in the 5' untranslated region, which was also present in the brother, resulting in abnormally spliced transcripts and a 4-fold reduction in mRNA levels. NEMO protein levels in EBV transformed B cells and fibroblasts from the index patient were 8-fold lower than normal controls. NF-kappaB p65 nuclear translocation in the patient's EBV B cells after TLR7 ligation was defective. NF-kappaB-dependent luciferase gene expression in IL-1-stimulated fibroblasts from the patient was impaired.

CONCLUSION

This is the first description of immune deficiency resulting from low expression of a normal NEMO protein.

Monogenic IL-1 mediated autoinflammatory and immunodeficiency syndromes: finding the right balance in response to danger signals

INTRODUCTION

Interleukin-1 was the first cytokine identified and is a powerful inducer of fever and inflammation. The biologically active receptor for IL-1, shares signaling pathways with some pathogen recognition receptors, the Toll-like receptors (TLRs) which early on suggested an important role in innate immune function.

DISCUSSION

The discovery that some intracellular "danger receptors", the NOD like receptors (NLRs) can assemble to form multimolecular platforms, the inflammasomes, that not only sense intracellular danger but also activate IL-1beta, has provided the molecular basis for the integration of IL-1 as an early response mediator in danger recognition. The critical role of balancing IL-1 production and signaling in human disease has recently been demonstrated in rare human monogenic diseases with mutations that affect the meticulous control of IL-1 production, release and signaling by leading to decreased or increased TLR/IL-1 signaling. In diseases of decreased TLR/IL-1 signaling (IRAK-4 and MyD88 deficiencies) patients are at risk for infections with gram positive organisms; and in diseases of increased signaling, patients develop systemic autoinflammatory diseases (cryopyrin-associated periodic syndromes (CAPS), and deficiency of the IL-1 receptor antagonist (DIRA)).

CONCLUSION

Monogenic defects in a number of rare diseases that affect the balance of TLR/IL-1 signaling have provided us with opportunities to study the systemic effects of IL-1 in human diseases. The molecular defects in CAPS and DIRA provided a therapeutic rationale for targeting IL-1 and the impressive clinical results from IL-1 blocking therapies have undoubtedly confirmed the pivotal role of IL-1 in human disease and spurred the exploration of modifying IL-1 signaling in a number of genetically complex common human diseases.

Toll-like receptors--sentries in the B-cell response

Toll-like receptors (TLR) play a central role in the initiation of the innate immune response to pathogens. Upon recognition of molecular motifs specific for microbial molecules TLR mediate pro-inflammatory cytokine secretion and enhance antigen presentation; in B cells they further promote expansion, class switch recombination and immunoglobulin secretion. As a result of their adjuvant properties, TLR ligands have become an integral component of antimicrobial vaccines. In spite of this, little is known of the direct effects of TLR engagement on B-lymphocyte function. The scope of this review is to outline the differences in TLR expression and reactivity in murine and human B-cell subsets and to provide an overview of the currently available literature. We will further discuss the possible roles of TLR in regulating B-cell effector functions and shaping antibody-mediated defence against microbial pathogens in vivo.

Robust TLR4-induced gene expression patterns are not an accurate indicator of human immunity

BACKGROUND

Activation of Toll-like receptors (TLRs) is widely accepted as an essential event for defence against infection. Many TLRs utilize a common signalling pathway that relies on activation of the kinase IRAK4 and the transcription factor NFkappaB for the rapid expression of immunity genes.

METHODS

21 K DNA microarray technology was used to evaluate LPS-induced (TLR4) gene responses in blood monocytes from a child with an IRAK4-deficiency. In vitro responsiveness to LPS was confirmed by real-time PCR and ELISA and compared to the clinical predisposition of the child and IRAK4-deficient mice to Gram negative infection.

RESULTS

We demonstrated that the vast majority of LPS-responsive genes in IRAK4-deficient monocytes were greatly suppressed, an observation that is consistent with the described role for IRAK4 as an essential component of TLR4 signalling. The severely impaired response to LPS, however, is inconsistent with a remarkably low incidence of Gram negative infections observed in this child and other children with IRAK4-deficiency. This unpredicted clinical phenotype was validated by demonstrating that IRAK4-deficient mice had a similar resistance to infection with Gram negative S. typhimurium as wildtype mice. A number of immunity genes, such as chemokines, were expressed at normal levels in human IRAK4-deficient monocytes, indicating that particular IRAK4-independent elements within the repertoire of TLR4-induced responses are expressed.

CONCLUSIONS

Sufficient defence to Gram negative immunity does not require IRAK4 or a robust, 'classic' inflammatory and immune response.

IRAK-4 inhibitors for inflammation

Interleukin-1 receptor-associated kinases (IRAKs) are key components in the signal transduction pathways utilized by interleukin-1 receptor (IL-1R), interleukin-18 receptor (IL-18R), and Toll-like receptors (TLRs). Out of four members in the mammalian IRAK family, IRAK-4 is considered to be the “master IRAK”, the only family member indispensable for IL-1R/TLR signaling. In humans, mutations resulting in IRAK-4 deficiency have been linked to susceptibility to bacterial infections, especially recurrent pyogenic bacterial infections. Furthermore, knock-in experiments by several groups have clearly demonstrated that IRAK-4 requires its kinase activity for its function. Given the critical role of IRAK-4 in inflammatory processes, modulation of IRAK-4 kinase activity presents an attractive therapeutic approach for the treatment of immune and inflammatory diseases. The recent success in the determination of the 3-dimensional structure of the IRAK-4 kinase domain in complex with inhibitors has facilitated the understanding of the mechanistic role of IRAK-4 in immunity and inflammation as well as the development of specific IRAK-4 kinase inhibitors. In this article, we review the biological function of IRAK-4, the structural characteristics of the kinase domain, and the development of small molecule inhibitors targeting the kinase activity. We also review the key pharmacophores required for several classes of inhibitors as well as important features for optimal protein/inhibitor interactions. Lastly, we summarize how these insights can be translated into strategies to develop potent IRAK-4 inhibitors with desired properties as new anti-inflammatory therapeutic agents.

Pathogen recognition and inflammatory signaling in innate immune defenses

The innate immune system constitutes the first line of defense against invading microbial pathogens and relies on a large family of pattern recognition receptors (PRRs), which detect distinct evolutionarily conserved structures on pathogens, termed pathogen-associated molecular patterns (PAMPs). Among the PRRs, the Toll-like receptors have been studied most extensively. Upon PAMP engagement, PRRs trigger intracellular signaling cascades ultimately culminating in the expression of a variety of proinflammatory molecules, which together orchestrate the early host response to infection, and also is a prerequisite for the subsequent activation and shaping of adaptive immunity. In order to avoid immunopathology, this system is tightly regulated by a number of endogenous molecules that limit the magnitude and duration of the inflammatory response. Moreover, pathogenic microbes have developed sophisticated molecular strategies to subvert host defenses by interfering with molecules involved in inflammatory signaling. This review presents current knowledge on pathogen recognition through different families of PRRs and the increasingly complex signaling pathways responsible for activation of an inflammatory and antimicrobial response. Moreover, medical implications are discussed, including the role of PRRs in primary immunodeficiencies and in the pathogenesis of infectious and autoimmune diseases, as well as the possibilities for translation into clinical and therapeutic applications.

Evolutionary dynamics of human Toll-like receptors and their different contributions to host defense

Infectious diseases have been paramount among the threats to health and survival throughout human evolutionary history. Natural selection is therefore expected to act strongly on host defense genes, particularly on innate immunity genes whose products mediate the direct interaction between the host and the microbial environment. In insects and mammals, the Toll-like receptors (TLRs) appear to play a major role in initiating innate immune responses against microbes. In humans, however, it has been speculated that the set of TLRs could be redundant for protective immunity. We investigated how natural selection has acted upon human TLRs, as an approach to assess their level of biological redundancy. We sequenced the ten human TLRs in a panel of 158 individuals from various populations worldwide and found that the intracellular TLRs—activated by nucleic acids and particularly specialized in viral recognition—have evolved under strong purifying selection, indicating their essential non-redundant role in host survival. Conversely, the selective constraints on the TLRs expressed on the cell surface—activated by compounds other than nucleic acids—have been much more relaxed, with higher rates of damaging nonsynonymous and stop mutations tolerated, suggesting their higher redundancy. Finally, we tested whether TLRs have experienced spatially-varying selection in human populations and found that the region encompassing TLR10-TLR1-TLR6 has been the target of recent positive selection among non-Africans. Our findings indicate that the different TLRs differ in their immunological redundancy, reflecting their distinct contributions to host defense. The insights gained in this study foster new hypotheses to be tested in clinical and epidemiological genetics of infectious disease.

The detrimental effects of microbial infections have led to the evolution of a variety of host defense mechanisms. A vast array of host innate immunity receptors, critical sensors of viruses, bacteria, and fungi, exist to achieve permanent surveillance of intruding pathogens. The best characterized class of microbial sensors is the Toll-like receptor (TLR) family, which elicits inflammatory and antimicrobial responses after activation by microbial products. Here we investigated how microbes have exerted selective pressure on the human TLR family to gain insights on the extent to which they are functionally important in the immune system. By resequencing the ten TLRs in different worldwide populations, we show that intracellular TLRs—principally specialized in viral recognition—evolve under strong purifying selection, indicating their essential role in host survival, while the remaining TLRs display higher levels of immunological redundancy. However, for this latter group of genes, we also show that mutations altering immune responses have been in some cases beneficial for host survival, as attested by the signature of positive selection favoring a reduced TLR1-mediated response in Europeans. Our findings taken together indicate that the different human TLRs differ in their biological relevance and provide clues to be experimentally tested in clinical, immunological, and epidemiological studies.

TLR4 polymorphisms and ageing: implications for the pathophysiology of age-related diseases

INTRODUCTION

Innate immunity provides a first line of host defense against infection by recognizing and killing microbes while simultaneously activating an instructive immune response. Toll-like receptors (TLRs) are principal mediators of rapid microbial recognition and function mainly by detection of pathogen-associated molecular patterns that do not exist in the host. Recognition of their ligands leads to a series of signaling events resulting in acute host responses, involved in killing pathogens.

DISCUSSION

We describe the involvement of TLR4 polymorphisms in ageing, and in particular in age-related diseases, suggesting the crucial role of molecules of innate immunity in pathophysiology of these diseases. Hence, we observed that pro-inflammatory alleles may be related to unsuccessful ageing, such as Alzheimer's disease, prostate cancer, and atherosclerosis; in contrast, the control of inflammation by anti-inflammatory alleles may result in increased longevity and successful ageing. Finally, a possible therapeutic approach to delay age-related diseases is outlined.

Recent advances in primary immunodeficiencies: identification of novel genetic defects and unanticipated phenotypes

Primary immunodeficiencies (PIDs) have traditionally been defined according to their immunologic phenotype. Far from being concluded, the search for human genes that, when mutated, cause PID is actively being pursued. During the last year, four novel genetic defects that cause severe combined immunodeficiency and severe congenital neutropenia have been identified. At the same time, the immunologic definition of primary immunodeficiencies has been expanded by the recognition that genetic defects affecting innate immunity may result in selective predisposition to certain infections, such as mycobacterial disease, herpes simplex encephalitis, and invasive pneumococcal infections. Studies of genetically determined susceptibility to infections have recently shown that immunologic defects may also account for novel infectious phenotypes, such as malaria or leprosy. Finally, a growing body of evidence indicates that primary immunodeficiencies may present with a noninfectious clinical phenotype that may be restricted to single organs, as in the case of atypical hemolytic uremic syndrome or pulmonary alveolar proteinosis. Overall, these achievements highlight the importance of human models, which often differ from the corresponding animal models.

Persistent systemic inflammation and atypical enterocolitis in patients with NEMO syndrome

The NEMO syndrome is a primary immunodeficiency with immune and non-immune manifestations. The immune deficiency is heterogeneous showing defects in humoral, innate, and cell-mediated immunity. While the clinical aspects of the immunodeficiency are increasingly well understood, little is known about autoimmune manifestations in NEMO patients. We therefore sought to examine serologic markers of systemic inflammation and intestinal pathology in a kindred of patients with the NEMO syndrome. We observed persistent elevation of erythrocyte sedimentation rates in five patients, and two were symptomatic, with a chronic but atypical enterocolitis. Though pathologic lesions in these two patients were consistent with acute inflammation, sustained clinical improvement was only achieved with systemic and/or topical glucocorticoid therapy. Our data suggest that some patients with the NEMO syndrome exhibit persistent elevation of inflammatory markers similar to systemic autoimmune diseases and may subsequently develop an atypical enterocolitis.

TLR Cross-Talk Mechanism of Hemorrhagic Shock-Primed Pulmonary Neutrophil Infiltration

Hemorrhage resulted from severe trauma renders patients susceptible to the development of acute lung injury (ALI). The accumulation of polymorphonuclear neutrophils (PMN) in the lung is a critical event in the development of ALI. PMN migration is a result of a cascade of cellular events, in which PMN, endothelial cells (EC), and macrophages (Mϕ) act in concert. Recent studies explored interrelated novel findings indicating that Toll-like receptors (TLRs) cross-talk mechanisms occurring in PMN, EC, and Mϕ are important determinants for hemorrhage-primed PMN migration. In Mϕ and EC, LPS acts through TLR4 signaling to up-regulate TLR2. Oxidant signaling derived from hemorrhage-activated PMN NAD(P)H oxidase enhances the TLR2 upregulation through PMN-Mϕ or PMN-EC interaction, resulting in an amplified release of cytokines and chemokines from the Mϕ and expression of adhesion molecules in the EC in response to TLR2 ligands, thereby promoting PMN migration. This review provides an insight of the mechanisms.

Polymorphisms of cytokine and innate immunity genes and GVHD

In the last 10 years, non-HLA genotypes have been investigated for their potential roles in the occurrence and severity of graft-versus-host disease (GVHD) as well as for their contribution to overall transplant-related mortality, infectious episodes, and overall survival. This chapter will review the latest results of cytokine gene polymorphisms between patient and donor which may cause the production of high or low levels of cytokines during the three-stage process of the GVHD 'cytokine storm'. More recent investigations into innate immunity and the interaction with subsequent downstream cytokine production and ultimate tissue damage are discussed. The potential of these non-HLA genetics to aid in predicting GVHD and post-transplant survival and the relevance of this information to the clinic are reviewed.

Revisiting human primary immunodeficiencies

Human primary immunodeficiencies (PIDs) are often thought to be confined to a few rare, familial, monogenic, recessive traits impairing the development or function of one or several leucocyte subsets and resulting in multiple, recurrent, opportunistic and fatal infections in infancy. We highlight here the rapidly growing number of exceptions to each of these conventional qualifications. Indeed, bona fide PIDs include common and sporadic illnesses and may present as dominant, or even polygenic traits; their pathogenesis may involve non haematopoietic cells, and they may result in single episode of illness, with a single or multiple morbid phenotypes, some of which may involve infection, in otherwise healthy adults. We need to increase awareness of the multitude of clinical presentations of human PIDs considerably and rapidly in the medical community. Human PIDs should be considered in a wide range of clinical situations.

Novel primary immunodeficiencies revealed by the investigation of paediatric infectious diseases

Human primary immunodeficiencies impairing myeloid and/or lymphoid cellular responses to activating receptors other than antigen receptors have recently been described in children with various infectious diseases. Germline mutations in NEMO and IKBA impair NF-kappaB-mediated signalling, at least in response to the stimulation of TLRs, IL-1Rs and TNFRs, and confer a broad predisposition to infections. Mutations in IRAK4 selectively impair TLRs other than TLR3 and most IL-1R responses, and confer a predisposition to pyogenic bacterial diseases, including invasive pneumococcal disease in particular. Mutations in TLR3 and UNC93B1 impair TLR3 responses and confer a predisposition to herpes simplex encephalitis. Mutations in STAT1 impair IFN-gamma and/or IFN-alpha/beta responses and predispose subjects to mycobacterial and viral diseases, respectively. Mutations in IFNGR1 and IFNGR2 impair IFN-gamma responses and confer a predisposition to mycobacterial diseases. Mutations in IL12B and IL12RB1 impair IL-12 and IL-23 responses and predispose subjects to infections caused by mycobacteria and Salmonella. Finally, mutations in TYK2 and STAT3 mostly impair IL-6R responses, conferring a predisposition to staphylococcal disease in particular. The infectious phenotypes associated with these novel leukocyte activation deficiencies are therefore collectively diverse, tightly dependent on the morbid gene and affected pathway, and individually narrow, often restricted to one or a few infectious diseases.

Review: variability of host-pathogen interaction

The course of every infection is different. The same pathogen can lead to subclinical, mild, severe or lethal infections in individuals. But is this just chance or determined by individual differences--on the side of the host as well as on the side of the pathogen? If so, we might need to consider these variations for treatment decisions. Indeed, we now understand that genetic polymorphisms and health status represent inborn and acquired risk factors. Similarly, pathogens impress with an increasing number of already identified virulence factors and host response modifiers. The emerging, more complex, view of the factors determining course and outcome of infections promises to enable more tailored and thus, hopefully, more effective treatment decisions.

Toll-like receptors--taking an evolutionary approach

The Toll receptor was initially identified in Drosophila melanogaster for its role in embryonic development. Subsequently, D. melanogaster Toll and mammalian Toll-like receptors (TLRs) have been recognized as key regulators of immune responses. After ten years of intense research on TLRs and the recent accumulation of genomic and functional data in diverse organisms, we review the distribution and functions of TLRs in the animal kingdom. We provide an evolutionary perspective on TLRs, which sheds light on their origin at the dawn of animal evolution and suggests that different TLRs might have been co-opted independently during animal evolution to mediate analogous immune functions.

CpG drives human transitional B cells to terminal differentiation and production of natural antibodies

The receptor TLR9, recognizing unmethylated bacterial DNA (CpG), is expressed by B cells and plays a role in the maintenance of serological memory. Little is known about the response of B cells stimulated with CpG alone, without additional cytokines. In this study, we show for the first time the phenotypic modification, changes in gene expression, and functional events downstream to TLR9 stimulation in human B cell subsets. In addition, we demonstrate that upon CpG stimulation, IgM memory B cells differentiate into plasma cells producing IgM Abs directed against the capsular polysaccharides of Streptococcus pneumoniae. This novel finding proves that IgM memory is the B cell compartment responsible for the defense against encapsulated bacteria. We also show that cord blood transitional B cells, corresponding to new bone marrow emigrants, respond to CpG. Upon TLR9 engagement, they de novo express AID and Blimp-1, genes necessary for hypersomatic mutation, class-switch recombination, and plasma cell differentiation and produce Abs with anti-pneumococcal specificity. Transitional B cells, isolated from cord blood, have not been exposed to pneumococcus in vivo. In addition, it is known that Ag binding through the BCR causes apoptotic cell death at this stage of development. Therefore, the ability of transitional B cells to sense bacterial DNA through TLR9 represents a tool to rapidly build up the repertoire of natural Abs necessary for our first-line defense at birth.

Recurrent staphylococcal cellulitis and subcutaneous abscesses in a child with autoantibodies against IL-6

We investigated an otherwise healthy patient presenting two episodes of staphylococcal cellulitis and abscesses, accompanied by high fever and biological signs of inflammation but, paradoxically, with no detectable increase in serum levels of C-reactive protein (CRP), an IL-6-responsive protein synthesized in the liver. Following in vitro activation of whole blood cells from the patient with multiple cytokines, TLR agonists, heat-killed bacteria, and mitogens, we observed a profound and specific impairment of IL-6 secretion. However, the patient's PBMCs, activated in the same conditions but in the absence of the patient's plasma, secreted IL-6 normally. The patient's serum contained high titers of IgG1 autoantibodies against IL-6, which specifically neutralized IL-6 production by control PBMCs as well as IL-6 responses in the human hepatocellular carcinoma cell line Hep3B. These anti-IL-6 autoantibodies were detected over a period of 4 years, in the absence of any other autoantibodies. Our results indicate that these Abs probably prevented an increase in CRP concentration during infection and that impaired IL-6-mediated immunity may have contributed to staphylococcal disease. Patients with severe bacterial infections and low serum CRP concentrations should be tested for anti-IL-6 autoantibodies, especially in the presence of other clinical and biological signs of inflammation.

Genetic susceptibility to herpes simplex virus 1 encephalitis in mice and humans

PURPOSE OF REVIEW

Herpes simplex encephalitis is a rare complication of herpes simplex virus 1 infection that strikes otherwise healthy individuals. Its pathogenesis has long remained elusive. We highlight the investigations dealing with the genetic basis of herpes simplex encephalitis in mice and humans.

RECENT FINDINGS

Mouse models have revealed the impact of various host genes on protective immunity to herpes simplex encephalitis through strain-dependent variability (forward genetics) and via targeted knockouts (reverse genetics). These studies established in particular the crucial role of IFNalpha/beta in immunity to herpes simplex virus 1, paving the way towards the elucidation of the genetic cause of human herpes simplex encephalitis. Two children with rare, specific STAT1 or NEMO mutations displayed a broad impairment of IFNalpha/beta and IFNlambda-mediated immunity and predisposition to several infectious diseases including herpes simplex encephalitis. In contrast, children with UNC93B1 and TLR3 mutations displayed a selective impairment of dsRNA-induced IFNalpha/beta and IFNlambda production and predisposition to isolated herpes simplex encephalitis.

SUMMARY

Herpes simplex encephalitis results from a series of monogenic primary immunodeficiencies that impair the TLR3 and UNC-93B-dependent production of IFNalpha/beta and IFNlambda in the central nervous system, at least in a fraction of children. This is not only crucial for the understanding of immunity to herpes simplex virus 1, but also for the diagnosis and treatment of herpes simplex encephalitis.

Inherited human IRAK-4 deficiency: an update

Interleukin-1 receptor-associated kinase-4 (IRAK-4), a member of the IRAK family, plays an essential role in Toll-like receptor- and IL-1 receptor-mediated signaling. We briefly review inherited human IRAK-4 deficiency, a recently described primary immunodeficiency leading to recurrent, invasive, pyogenic bacteria infection, and invasive pneumococcal disease in particular.

Tolerance and autoimmunity: lessons at the bedside of primary immunodeficiencies

The recent progress in the genetic characterization of many primary immunodeficiencies (PIDs) allows for a better understanding of immune molecular and cellular mechanisms. The present chapter discusses associations between PIDs and autoimmune diseases (AIDs) in this new light. PIDs are classified according to the frequency of association with AIDs, defining four groups of conditions: systematic (more than 80% of all patients), strong (10-80%), mild (less than 10%), and absent (no available descriptions). Several general conclusions could be drawn: (1) pathological autoimmune (AI) manifestations are very frequently associated with PIDs, indicating that, contrary to conventional notions, antimicrobial protection and natural tolerance to body tissues share many basic mechanisms; (2) in some gene defects, association is so strong that one could speak of "monogenic" AIDs; (3) basic types of PIDs are selectively associated with AID of a particular set of target tissues; (4) while for some gene defects, current theory satisfactorily explains pathogenesis of the corresponding AID, other situations suggest extensive gaps in the present understanding of natural tolerance; and (5) not exceptionally, observations on the AI phenotype for the same gene defect in mouse and man are not concordant, perhaps owing to the limited genetic diversity of mouse models, often limited to a single mouse strain. Overall, clinical observations on PID support the new paradigm of "dominant" tolerance to self-components, in which AID owes to deficits in immune responses (i.e., in regulatory mechanisms), rather than from excessive reactivity.