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Leptin is associated with cardiometabolic complications of obesity, such as metabolic syndrome and atherosclerosis. In obese men, the presence of metabolic syndrome is associated with higher circulating leptin and interleukin (IL)-6 concentrations and increased monocyte cytokine production capacity. Here, we investigated the effects of leptin on monocyte function and systemic inflammatory markers in obese individuals. We specifically explored whether leptin can induce long-term changes in innate immune function by inducing innate immune memory (also called trained immunity). We exposed human primary monocytes for 24 h to relevant leptin concentrations in vitro and measured cytokine production. In addition, after removing leptin, we incubated monocytes for 5 d in culture medium, and we restimulated them on day 6 to assess cytokine production capacity, phagocytosis, and foam cell formation. Direct stimulation with leptin did not induce cytokine production, but exposure to 50 ng/mL leptin augmented lipopolysaccharide- and R848-induced tumor necrosis factor α (TNF-α) production after 1 wk. In a separate in vivo study in a cohort of 302 obese subjects (body mass index [BMI] >27 kg/m2, 55 to 81 yr), we measured circulating leptin, inflammatory markers, and cytokine production upon ex vivo stimulation of isolated peripheral blood mononuclear cells. Circulating leptin concentrations positively correlated with circulating IL-1β and IL-6, which was more pronounced in men than in women. Four single nucleotide polymorphisms in the leptin gene influenced circulating IL-6 concentrations in men, suggesting a direct effect of leptin on IL-6. In conclusion, in vitro, leptin does not directly stimulate monocytes to produce cytokines, yet induces long-term monocyte hyperresponsiveness, i.e. trained immunity. In obese subjects, leptin is associated with circulating IL-6 in a sex-dependent manner. The underlying mechanisms of the sex-specific effect of leptin on innate immune cells remain to be further investigated., Competing Interests: Conflict of interest statement. None declared., (© The Author(s) 2023. Published by Oxford University Press on behalf of Society for Leukocyte Biology.)

Bacillus Calmette-Guérin vaccine is well known for inducing trained immunity in myeloid and natural killer cells, which can explain its cross-protective effect against heterologous infections. Although displaying functional characteristics of both adaptive and innate immunity, γδ T-cell memory has been only addressed in a pathogen-specific context. In this study, we aimed to determine whether human γδ T cells can mount trained immunity and therefore contribute to the cross-protective effect of the Bacillus Calmette-Guérin vaccine. We investigated in vivo induction of innate memory in γδ T cells by Bacillus Calmette-Guérin vaccination in healthy human volunteers by combining single-cell RNA sequencing technology with immune functional assays. The total number of γδ T cells and membrane markers of activation was not influenced by Bacillus Calmette-Guérin vaccination. In contrast, Bacillus Calmette-Guérin changed γδ T cells' transcriptional programs and increased their responsiveness to heterologous bacterial and fungal stimuli, including lipopolysaccharide and Candida albicans, as simultaneously characterized by higher tumor necrosis factor and interferon γ production, weeks after vaccination. Human γδ T cells in adults display the potential to develop a trained immunity phenotype after Bacillus Calmette-Guérin vaccination., Competing Interests: Conflict of interest statement. L.A.B.J. and M.G.N. are scientific founders of TTxD and Lemba. All authors claim no conflict of interest., (© The Author(s) 2023. Published by Oxford University Press on behalf of Society for Leukocyte Biology.)

Rationale: To evaluate whether common nonsynonymous variants [single-nucleotide polymorphisms (SNPs) or SNP haplotypes] in the β2-adrenergic receptor render subjects more susceptible to norepinephrine-induced immunosuppression and whether they are associated with dysregulated ex vivo and in vivo inflammatory responses., Methods: Peripheral blood mononuclear cells from healthy volunteers (main cohort: n = 106, secondary cohort: n = 408) were ex vivo stimulated with various stimuli and production of cytokines was assessed. Additionally, ex vivo modulation of cytokine production by norepinephrine was evaluated in the main cohort. Volunteers from the main cohort also underwent experimental endotoxemia (administration of 1 ng/kg lipopolysaccharide), during which in vivo plasma cytokine concentrations and clinical inflammatory parameters were measured. Subjects were genotyped, common SNPs in the ADRB2 gene were extracted (rs1042711, rs1042713, and rs1042714), and the presence of haplotypes was identified (CysGlyGln, CysArgGln, and ArgGlyGlu)., Results: In both cohorts, presence of ADRB2 SNPs or haplotypes was not associated with altered ex vivo cytokine responses. Norepinephrine attenuated production of the proinflammatory cytokines TNF and IL-6 [-26% (-22% to -30%) and -14% (-9% to -18%), respectively, both P < 0.0001] and enhanced release of the anti-inflammatory IL-10 [+9% (+3% to +15%), P = 0.003]. These effects were not modulated by the presence of ADRB2 SNPs or haplotypes (all P values >0.37). In addition, no influence of SNPs or haplotypes on in vivo cytokine concentrations or clinical inflammatory parameters was observed (P values >0.14)., Conclusions: Common nonsynonymous variants in the ADRB2 gene influence neither ex vivo cytokine production or norepinephrine-mediated immunosuppression nor the systemic in vivo inflammatory response induced by lipopolysaccharide administration in healthy volunteers., (© The Author(s) 2023. Published by Oxford University Press on behalf of Society for Leukocyte Biology.)

Atherosclerotic cardiovascular diseases (CVD) are among the leading causes of death in the world. Monocyte-derived macrophages are key players in the pathophysiology of atherosclerosis. Innate immune memory following exposure of monocytes to atherogenic compounds, such as oxidized low-density lipoproteins (oxLDL), termed trained immunity, can contribute to atherogenesis. The current study aimed to elucidate intracellular mechanisms of oxLDL-induced trained immunity. Using untargeted intracellular metabolomics in isolated human primary monocytes, we show that oxLDL-induced trained immunity results in alterations in the balance of intracellular steroid hormones in monocytes. This was reflected by a decrease in extracellular progesterone concentrations following LPS stimulation. To understand the potential effects of steroid hormones on trained immunity, monocytes were costimulated with oxLDL and the steroid hormones progesterone, hydrocortisone, dexamethasone, β-estradiol, and dihydrotestosterone. Progesterone showed a unique ability to attenuate the enhanced TNFα and IL-6 production following oxLDL-induced trained immunity. Single nucleotide polymorphisms in the nuclear glucocorticoid, progesterone, and mineralocorticoid receptor were shown to correlate with ex vivo oxLDL-induced trained immunity in 243 healthy volunteers. Pharmacologic inhibition experiments revealed that progesterone exerts the suppression of TNFα in trained immunity via the nuclear glucocorticoid and mineralocorticoid receptors. Our data show that progesterone has a unique ability to suppress oxLDL-induced trained immunity. We hypothesize that this effect might contribute to the lower incidence of CVD in premenopausal women., (© 2022 The Authors. Journal of Leukocyte Biology published by Wiley Periodicals LLC on behalf of Society for Leukocyte Biology.)

Trained immunity is a de facto memory of innate immune cells, resulting in a long-term increase in innate host defense mechanisms after infection. The long-term heterologous protection conferred by trained immunity is mediated through epigenetic and functional reprogramming of hematopoietic stem and progenitor cells. Because the spleen is a reservoir of undifferentiated monocytes and is considered the prime organ for extramedullary hematopoiesis, we investigated the role of the spleen in the establishment of trained immunity. A β-glucan-induced trained immunity mouse model was performed in previously sham-operated or splenectomized animals. Removal of the spleen did not modulate the proinflammatory cytokine production of in vivo trained peritoneal cells, nor did it ablate the increased percentage of proinflammatory circulatory monocytes and natural killer cells seen in trained animals. However, spleen removal prevented neutrophilia, an important characteristic of trained immunity. These data point to a limited role of the spleen in trained immunity. The pathophysiologic relevance of the spleen in the induction of neutrophilia during trained immunity remains to be fully explored., (© 2021 The Authors. Journal of Leukocyte Biology published by Wiley Periodicals LLC on behalf of Society for Leukocyte Biology.)

Trained immunity is the acquisition of a hyperresponsive phenotype by innate immune cells (such as monocytes and macrophages) after an infection or vaccination, a de facto nonspecific memory dependent on epigenetic and metabolic reprogramming of these cells. We have recently shown that induction of trained immunity is dependent on IL-1β. Here, we show that recombinant IL-38, an anti-inflammatory cytokine of the IL-1-family, was able to induce long-term inhibitory changes and reduce the induction of trained immunity by β-glucan in vivo in C57BL/6 mice and ex vivo in their bone marrow cells. IL-38 blocked mTOR signaling and prevented the epigenetic and metabolic changes induced by β-glucan. In healthy subjects, the IL1F10 associated single nucleotide polymorphism rs58965312 correlated with higher plasma IL-38 concentrations and reduced induction of trained immunity by β-glucan ex vivo. These results indicate that IL-38 induces long-term anti-inflammatory changes and also inhibits the induction of trained immunity. Recombinant IL-38 could therefore potentially be used as a therapeutic intervention for diseases characterized by exacerbated trained immunity., (© 2021 The Authors. Journal of Leukocyte Biology published by Wiley Periodicals LLC on behalf of Society for Leukocyte Biology.)

Secondary infections are a major complication of sepsis and associated with a compromised immune state, called sepsis-induced immunoparalysis. Molecular mechanisms causing immunoparalysis remain unclear; however, changes in cellular metabolism of leukocytes have been linked to immunoparalysis. We investigated the relation of metabolic changes to antimicrobial monocyte functions in endotoxin-induced immunotolerance, as a model for sepsis-induced immunoparalysis. In this study, immunotolerance was induced in healthy males by intravenous endotoxin (2 ng/kg, derived from Escherichia coli O:113) administration. Before and after induction of immunotolerance, circulating CD14 + monocytes were isolated and assessed for antimicrobial functions, including cytokine production, oxidative burst, and microbial (Candida albicans) killing capacity, as well metabolic responses to ex vivo stimulation. Next, the effects of altered cellular metabolism on monocyte functions were validated in vitro. Ex vivo lipopolysaccharide stimulation induced an extensive rewiring of metabolism in naive monocytes. In contrast, endotoxin-induced immunotolerant monocytes showed no metabolic plasticity, as they were unable to adapt their metabolism or mount cytokine and oxidative responses. Validation experiments showed that modulation of metabolic pathways, affected by immunotolerance, influenced monocyte cytokine production, oxidative burst, and microbial (C. albicans) killing in naive monocytes. Collectively, these data demonstrate that immunotolerant monocytes are characterized by a loss of metabolic plasticity and these metabolic defects impact antimicrobial monocyte immune functions. Further, these findings support that the changed cellular metabolism of immunotolerant monocytes might reveal novel therapeutic targets to reverse sepsis-induced immunoparalysis., (©2019 The Authors. Society for Leukocyte Biology Published by Wiley Periodicals, Inc.)

During the last few years, a growing body of evidence has shown that immunological memory is not an exclusive trait of lymphocytes, as many inflammatory insults can alter the functionality and the responsiveness of the innate immune system in the long term. Innate immune cells, such as monocytes, macrophages, dendritic cells, and NK cells can be influenced by the encounters with inflammatory stimuli, undergoing functional reprogramming and developing changed responses to subsequent chellenges. The long-term reprogramming depends on the rewiring of cell metabolism and epigenetic processes, and they stay at the basis of induction of both innate immune memory (also termed trained immunity) and innate immune tolerance. Here, we review the central role that the effects of this long-term reprogramming of innate immune cells plays in a number of clinically relevant conditions such as vaccination, atherosclerosis, sepsis, and cancer., (©2018 Society for Leukocyte Biology.)

The anti-tuberculosis vaccine Bacillus Calmette-Guérin (BCG) is a well-known immune modulator that induces nonspecific protective effects against heterologous infections through induction of innate immune memory, also termed "trained immunity." In randomized trials in low weight newborns, BCG vaccination reduced neonatal mortality due to decreased incidence of sepsis and respiratory infections. In many studies, sex-differential nonspecific effects of vaccines have been observed, but the mechanisms behind these differential effects are unknown. We investigated whether the important sex hormones estrogen and dihydrotestosterone (DHT) influence BCG-induced trained immunity in human primary monocytes. Although addition of estradiol and DHT to BCG inhibited the production of proinflammatory cytokines after direct stimulation of human monocytes, they did not influence the induction of trained immunity by BCG. In addition, estradiol or DHT did not induce training or tolerance in monocytes themselves. We conclude that these important sex hormones are unlikely to explain the sex-differential effects after BCG vaccination. Future studies should focus on the investigation of alternative mechanisms as an explanation for sex-differential nonspecific effects of BCG vaccination., (©2018 Society for Leukocyte Biology.)

Cancer immunotherapy has steadily progressed during the past decades, with checkpoint inhibitor therapy becoming the latest and one of the most promising treatments. Despite the progress, most of the patients do not respond or develop resistance, and novel additional approaches are needed to improve the clinical effectiveness of immunotherapy. Trained immunity (TI) has been described recently as a process of epigenetic and metabolic reprogramming that induces a long-term enhanced function of innate immune cells. TI is considered to have beneficial effects in improving host response to infections and vaccination, and increasing evidence suggests that TI-mediated mechanisms also have useful and potent antitumor effects. We hypothesized that novel and more effective approaches for immunotherapy in cancer may involve induction of TI, alone or in combination with current immunotherapies., (© Society for Leukocyte Biology.)

In recent years, it has become appreciated that immune cells have different metabolic profiles depending on their activation status. During sepsis, circulating leukocytes go through a hyperinflammatory state, which can be accompanied or followed by defective antimicrobial defenses (also described as immune tolerance or paralysis). In this review, the modulation of different cellular metabolic pathways during sepsis in monocytes and macrophages will be discussed. Glycolysis is studied extensively in sepsis and is up-regulated in hyperinflammatory cells, whereas in immune tolerance, it is often down-regulated. Few data are available on other metabolic pathways in immune cells from patients with sepsis. The pentose phosphate pathway is up-regulated during acute hyperinflammatory responses, whereas fatty acid β-oxidation is increased later during sepsis and is associated with an anti-inflammatory (M2) phenotype of macrophages. Within the amino acid metabolism we will discuss the most studied metabolites. Collectively, these data argue that exploration of the immunometabolic pathways in sepsis is an important area of research, and the targeting of metabolic pathways may represent a promising novel strategy as a therapy of sepsis., (© Society for Leukocyte Biology.)

We have previously demonstrated that inhibition of autophagy increased the Borrelia burgdorferi induced innate cytokine production in vitro, but little is known regarding the effect of autophagy on in vivo models of Borrelia infection. Here, we showed that ATG7-deficient mice that were intra-articular injected with Borrelia spirochetes displayed increased joint swelling, cell influx, and enhanced interleukin-1β and interleukin-6 production by inflamed synovial tissue. Because both interleukin-1β and interleukin-6 are linked to the development of adaptive immune responses, we examine the function of autophagy on Borrelia induced adaptive immunity. Human peripheral blood mononuclear cells treated with autophagy inhibitors showed an increase in interleukin-17, interleukin-22, and interferon-γ production in response to exposure to Borrelia burgdorferi. Increased IL-17 production was dependent on IL-1β release but, interestingly, not on interleukin-23 production. In addition, cytokine quantitative trait loci in ATG9B modulate the Borrelia induced interleukin-17 production. Because high levels of IL-17 have been found in patients with confirmed, severe, chronic borreliosis, we propose that the modulation of autophagy may be a potential target for anti-inflammatory therapy in patients with persistent Lyme disease., (© Society for Leukocyte Biology.)

BCG vaccination is associated with a reduced mortality from nonmycobacterial infections. This is likely to be mediated by a combination of innate-immune memory ("trained immunity") and heterologous effects on adaptive immunity. As such, BCG could be used to boost host immunity but not in immunocompromised hosts, as it is a live, attenuated vaccine. Therefore, we assessed whether killed γBCG has similar potentiating effects. In an in vitro model of trained immunity, human monocytes were incubated with γBCG for 24 h and restimulated after 6 d. Cytokine production and the role of pattern recognition receptors and histone methylation markers were assessed. The in vivo effects of γBCG vaccination were studied in a proof-of-principle trial in 15 healthy volunteers. γBCG induced trained immunity in vitro via the NOD2 receptor pathway and up-regulation of H3K4me3 histone methylation. However, these effects were less strong than those induced by live BCG. γBCG vaccination in volunteers had only minimal effects on innate immunity, whereas a significant increase in heterologous Th1/Th17 immunity was observed. Our results indicate that γBCG induces long-term training of innate immunity in vitro. In vivo, γBCG induces mainly heterologous effects on the adaptive-immune system, whereas effects on innate cytokine production are limited., (© Society for Leukocyte Biology.)

An increasing body of evidence shows that the innate immune system has adaptive characteristics that involve a heterologous memory of past insults. Both experimental models and proof-of-principle clinical trials show that innate immune cells, such as monocytes, macrophages, and NK cells, can provide protection against certain infections in vaccination models independently of lymphocytes. This process is regulated through epigenetic reprogramming of innate immune cells and has been termed "trained immunity." It has been hypothesized that induction of trained immunity is responsible for the protective, nonspecific effects induced by vaccines, such as BCG, measles vaccination, and other whole-microorganism vaccines. In this review, we will present the mechanisms of trained immunity responsible for the long-lasting effects of vaccines on the innate immune system., (© Society for Leukocyte Biology.)

Epidemiologic studies suggest that VAS has long-lasting immunomodulatory effects. We hypothesized that ATRA inhibits inflammatory cytokines in a model of trained immunity in monocytes by inducing epigenetic reprogramming through histone modifications. We used an previously described in vitro model of trained immunity, in which adherent monocytes of healthy volunteers were incubated for 24 h with BCG in the presence or absence of ATRA. After washing the cells, they were incubated for an additional 6 d in culture medium and restimulated with microbial ligands, and cytokine production was assessed. ATRA inhibited cytokine responses upon restimulation of monocytes, and this effect was exerted through increased expression of SUV39H2, a histone methyltransferase that induces the inhibitory mark H3K9me3. H3K9me3 at promoter sites of several cytokines was up-regulated by ATRA, and inhibition of SUV39H2 restored cytokine production. In addition to H3K9me3, the stimulatory histone mark H3K4me3 was down-regulated by ATRA at several promoter locations of cytokine genes. Therefore, we can conclude that ATRA inhibits cytokine production in models of direct stimulation or BCG-induced trained immunity and that these effects are mediated by histone modifications., (© Society for Leukocyte Biology.)

TLRs play a major role in microbe-host interactions and innate immunity. Of the 10 functional TLRs described in humans, TLR2 is unique in its requirement to form heterodimers with TLR1 or TLR6 for the initiation of signaling and cellular activation. The ligand specificity of TLR2 heterodimers has been studied extensively, using specific bacterial and synthetic lipoproteins to gain insight into the structure-function relationship, the minimal active motifs, and the critical dependence on TLR1 or TLR6 for activation. Different from that for specific well-defined TLR2 agonists, recognition of more complex ligands like intact microbes or molecules from endogenous origin requires TLR2 to interact with additional coreceptors. A breadth of data has been published on ligand-induced interactions of TLR2 with additional pattern recognition receptors such as CD14, scavenger receptors, integrins, and a range of other receptors, all of them important factors in TLR2 function. This review summarizes the roles of TLR2 in vivo and in specific immune cell types and integrates this information with a detailed review of our current understanding of the roles of specific coreceptors and ligands in regulating TLR2 functions. Understanding how these processes affect intracellular signaling and drive functional immune responses will lead to a better understanding of host-microbe interactions and will aid in the design of new agents to target TLR2 function in health and disease.

Inflammation in response to infection or trauma can lead to CARS, which is characterized by leukocyte dysfunction. In this study, we used a human model system for CARS to study the effect of GM-CSF and IFN-γ treatment on this immunoparalyzed state. Healthy human volunteers were treated with GM-CSF (4 μg/kg), IFN-γ (100 μg), or placebo in between two challenges with Escherichia coli LPS/endotoxin (2 ng/kg). Serial leukocyte blood counts were measured. Neutrophil subsets were discriminated using CD16 and CD62L expression. LPS rechallenge resulted in increased mobilization of mature neutrophils, whereas banded neutrophils decreased. GM-CSF and IFN-γ treatment did not restore these changes. GM-CSF treatment did, however, increase the number of CD16(bright)/CD62L(dim) neutrophils that were previously shown be able to suppress T cell proliferation. IFN-γ treatment decreased neutrophilia seen after LPS rechallenge. Our study shows that LPS rechallenge was associated with changes in the distribution of neutrophil subsets, whereas no additional changes in kinetics of other granulocyte populations were observed. GM-CSF and IFN-γ treatment induced a shift in granulocyte composition toward an anti-inflammatory direction by increasing CD16(bright)/CD62L(dim) cells or decreasing neutrophil counts, respectively.

TREM-1 is an important signaling receptor expressed on neutrophils and monocytes that plays an important role in systemic infections. Here, we review the intracellular signaling pathways that mediate the immunological effects of TREM-1. Because of the absence of signaling motifs, TREM-1 constitutively associates with DAP12 for induction of intracellular signals. After phosphorylation of DAP12, production of chemokines and cytokines is induced. Moreover, TREM-1 also modulates signaling pathways induced by known classes of PRRs, such as TLRs and NLRs. The exact mechanisms through which TREM-1 influences TLR and NLR pathways are still largely elusive.

In the mucosa, the immune pathways discriminating between colonizing and invasive Candida, thus inducing tolerance or inflammation, are poorly understood. Th17 responses induced by Candida albicans hyphae are central for the activation of mucosal antifungal immunity. An essential step for the discrimination between yeasts and hyphae and induction of Th17 responses is the activation of the inflammasome by C. albicans hyphae and the subsequent release of active IL-1β in macrophages. Inflammasome activation in macrophages results from differences in cell-wall architecture between yeasts and hyphae and is partly mediated by the dectin-1/Syk pathway. These results define the dectin-1/inflammasome pathway as the mechanism that enables the host immune system to mount a protective Th17 response and distinguish between colonization and tissue invasion by C. albicans.

In the present study, we dissected the pathways that trigger the IL-17A responses by MTB. Dectin-1 and TLR4 were shown to be involved in MTB-induced IL-17A production, and blockade of the NOD2, TLR2, or MR had no effect on IL-17A. The MAPK Erk, known to mediate transcription of IL-1beta mRNA, was strongly involved in the IL-17A production induced by MTB. The intracellular enzymes caspase-1 and serine proteases, which process pro-IL-1beta into the active IL-1beta, were also crucial for the induction of IL-17A. Lastly, the MTB-induced IL-17A response was strongly dependent on signaling through the IL-1R but not the IL-6R pathway. In conclusion, the MTB-induced IL-17A response relies strongly on the endogenous IL-1 pathway and IL-1R signaling. TLR4 and dectin-1 are the main receptors responsible for mediating the signals responsible for IL-17A production by MTB. These findings contribute to a better understanding of the host response to mycobacteria and provide the opportunity to explore potential, novel, therapeutic strategies against TB.

Mycobacterium paratuberculosis has been suggested to be involved in the pathogenesis of Crohn's disease (CD). The importance of microorganisms in CD is supported by the association of CD with mutations in the intracellular pathogen recognition receptor (PRR) nucleotide-binding oligomerization domain 2 (NOD2). The aim of this study is to investigate the PRR involved in the recognition of M. paratuberculosis. Methods used include in vitro stimulation of transfected cell lines, murine macrophages, and human PBMC. M. paratuberculosis stimulated human TLR2 (hTLR2)-Chinese hamster ovary (CHO) cells predominantly and hTLR4-CHO cells modestly. Macrophages from TLR2 and TLR4 knockout mice produced less cytokines compared with controls after stimulation with M. paratuberculosis. TLR4 inhibition in human PBMC reduced cytokine production only after stimulation with live M. paratuberculosis. TLR-induced TNF-alpha, IL-1beta, and IL-10 production is mediated through MyD88, whereas Toll-IL-1R domain-containing adaptor inducing IFN-beta (TRIF) promoted the release of IL-1beta. hNOD2-human embryo kidney (HEK) cells, but not hNOD1-HEK cells, responded to stimulation with M. paratuberculosis. PBMC of individuals homozygous for the 3020insC NOD2 mutation showed a 70% defective cytokine response after stimulation with M. paratuberculosis. These results demonstrate that TLR2, TLR4, and NOD2 are involved in the recognition of M. paratuberculosis by the innate immune system.

Triggering receptor expressed on myeloid cells-1 (TREM-1) is a member of a new family of myeloid receptors, encoded by a gene cluster linked to the MHC. Engagement of TREM-1 stimulates intracellular signals, resulting in activation of phagocytosis, neutrophil degranulation, and amplification of cytokine production induced by TLRs. In the present study, a novel property following engagement of TREM-1 is described, namely the amplification of cytokine production induced by the second major class of pattern recognition receptors, the NAIP, CIITA, HET-E, TP-1-leucine-rich repeat (NACHT-LRR; NLR) receptors, which recognize intracellular microorganisms through sensing their muropeptide components of peptidoglycan. The TREM-1/NLR synergism was observed for the production of TNF-alpha, IL-1beta, and IL-6, leading to an increase in cytokine production up to tenfold greater than the additive value of TREM-1 or muropeptide stimulation alone. Several putative mechanisms are proposed to be involved in the synergism between NLRs and TREM-1, including the increase in TREM-1 expression by NLR ligands, and of the expression of nucleotide oligomerization domain-2 receptor by TREM-1 engagement. In contrast, although caspase-1 modulates IL-1beta and IL-6 production after stimulation with anti-TREM-1 antibodies or NLR ligands, it does not appear to be responsible for the synergism between these two pathways. These findings demonstrate that TREM-1 acts on both major recognition pathways of bacterial structures: the extracellular TLR receptors, and the intracellular NLR molecules. This latter finding supports the concept that TREM-1 provides optimal amplification of cytokine-induced inflammation during the initiation of host defense.

The nucleotide oligomerization domain 2 (NOD2) 3020insC (NOD2fs) mutation increases susceptibility to Crohn's disease (CD), but the mechanism remains controversial. Loss-of-function and gain-of-function phenotypes have been described as a result of NOD2fs. Here, we show that dendritic cells (DC) derived from CD patients homozygous for this mutation respond normally to purified Toll-like receptor (TLR) ligands but fail to up-regulate the costimulatory molecules CD80 and CD86 in response to the NOD2 ligand muramyl dipeptide (MDP). Moreover, they lack MDP-induced enhancement of TLR-mediated tumor necrosis factor alpha, interleukin (IL)-12, and IL-10 production, which is observed in control DC with intact NOD2. These data indicate that the NOD2fs mutation results in a loss-of-function phenotype in human myeloid DC and imply decreased immune regulation by IL-10 as a possible mechanism for this mutation in CD.

Toll-like receptors (TLRs) have been identified as a major class of pattern-recognition receptors. Recognition of pathogen-associated molecular patterns (PAMPs) by TLRs, alone or in heterodimerization with other TLR or non-TLR receptors, induces signals responsible for the activation of genes important for an effective host defense, especially proinflammatory cytokines. Although a certain degree of redundancy exists between signals induced by the various TLRs, recent studies have identified intracellular pathways specific for individual TLRs. This leads to the release of cytokine profiles specific for particular PAMPs, and thus, TLRs confer a certain degree of specificity to the innate-immune response. In addition to the activation of the innate-immune response, TLR-mediated recognition represents a link between the innate- and acquired-immune systems, by inducing the maturation of dendritic cells and directing the T helper responses. Alternatively, recent data have also suggested TLR-mediated escape mechanisms used by certain pathogenic microorganisms, especially through TLR2 induction of anti-inflammatory cytokines. Finally, the crucial role of TLRs for the host defense against infections has been strengthened recently by the description of patients partially defective in the TLR-activation pathways.

To determine the relative contribution of lipopolysaccharide (LPS) and non-LPS components of Neisseria meningitidis to the pathogenesis of meningococcal sepsis, this study quantitatively compared cytokine induction by isolated LPS, wild-type serogroup B meningococci (strain H44/76), and LPS-deficient mutant meningococci (strain H44/76[pLAK33]). Stimulation of human peripheral-blood mononuclear cells with wild-type and LPS-deficient meningococci showed that non-LPS components of meningococci are responsible for a substantial part of tumor necrosis factor (TNF)-alpha and interleukin (IL)-1beta production and virtually all interferon (IFN)-gamma production. Based on tricine sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of LPS in proteinase K-treated lysates of N. meningitidis H44/76, a quantitative comparison was made between the cytokine-inducing capacity of isolated and purified LPS and LPS-containing meningococci. At concentrations of >10(7) bacteria/mL, intact bacteria were more potent cytokine inductors than equivalent amounts of isolated LPS, and cytokine induction by non-LPS components was additive to that by LPS. Experiments with mice showed that non-LPS components of meningococci were able to induce cytokine production and mortality. The principal conclusion is that non-LPS parts of N. meningitidis may play a role in the pathogenesis of meningococcal sepsis by inducing substantial TNF-alpha, IL-1beta, and IFN-gamma production.