Toll-like receptors (TLRs) represent the main class of pattern-recognition receptors involved in sensing pathogenic microorganisms. The aim of the present study was to assess the role of TLR4 in the defense against Candida albicans infection. The outgrowth of C. albicans was 10-fold higher in TLR4-defective C3H/HeJ mice, compared with that in control C3H/HeN mice (P<.05). Production of tumor necrosis factor (TNF) and interleukin (IL)-1alpha and IL-1beta by mouse macrophages in response to C. albicans stimulation was not affected by TLR4, and the candidacidal capacities of the neutrophils and macrophages of C3H/HeJ mice were normal. In contrast, production of the CXC chemokines KC and macrophage inhibitory protein-2 was 40%-60% lower by the macrophages of C3H/HeJ mice (P<.05), which resulted in a 40% decrease in neutrophil recruitment to the site of infection. Candida-induced TNF and IL-1beta production by human peripheral blood mononuclear cells was significantly inhibited by blocking anti-TLR2 antibodies in vitro. In conclusion, TLR4-defective C3H/HeJ mice are more susceptible to C. albicans infection, and this is associated with impaired chemokine expression and neutrophil recruitment.

Here we report the presence of hyperphagia, obesity and insulin resistance in knockout mice deficient in IL-18 or IL-18 receptor, and in mice transgenic for expression of IL-18 binding protein. Obesity of Il18-/- mice resulted from accumulation of fat tissue based on increased food intake. Il18-/- mice also had hyperinsulinemia, consistent with insulin resistance and hyperglycemia. Insulin resistance was secondary to obesity induced by increased food intake and occurred at the liver level as well as at the muscle and fat-tissue level. The molecular mechanisms responsible for the hepatic insulin resistance in the Il18-/- mice involved an enhanced expression of genes associated with gluconeogenesis in the liver of Il18-/- mice, resulting from defective phosphorylation of STAT3. Recombinant IL-18 (rIL-18) administered intracerebrally inhibited food intake. In addition, rIL-18 reversed hyperglycemia in Il18-/- mice through activation of STAT3 phosphorylation. These findings indicate a new role of IL-18 in the homeostasis of energy intake and insulin sensitivity.

Toll-like receptor (TLR)10 is the only pattern-recognition receptor without known ligand specificity and biological function. We demonstrate that TLR10 is a modulatory receptor with mainly inhibitory effects. Blocking TLR10 by antagonistic antibodies enhanced proinflammatory cytokine production, including IL-1β, specifically after exposure to TLR2 ligands. Blocking TLR10 after stimulation of peripheral blood mononuclear cells with pam3CSK4 (Pam3Cys) led to production of 2,065 ± 106 pg/mL IL-1β (mean ± SEM) in comparison with 1,043 ± 51 pg/mL IL-1β after addition of nonspecific IgG antibodies. Several mechanisms mediate the modulatory effects of TLR10: on the one hand, cotransfection in human cell lines showed that TLR10 acts as an inhibitory receptor when forming heterodimers with TLR2; on the other hand, cross-linking experiments showed specific induction of the anti-inflammatory cytokine IL-1 receptor antagonist (IL-1Ra, 16 ± 1.7 ng/mL, mean ± SEM). After cross-linking anti-TLR10 antibody, no production of IL-1β and other proinflammatory cytokines could be found. Furthermore, individuals bearing TLR10 polymorphisms displayed an increased capacity to produce IL-1β, TNF-α, and IL-6 upon ligation of TLR2, in a gene-dose-dependent manner. The modulatory effects of TLR10 are complex, involving at least several mechanisms: there is competition for ligands or for the formation of heterodimer receptors with TLR2, as well as PI3K/Aktmediated induction of the anti-inflammatory cytokine IL-1Ra. Finally, transgenic mice expressing human TLR10 produced fewer cytokines when challenged with a TLR2 agonist. In conclusion, to our knowledge we demonstrate for the first time that TLR10 is a modulatory pattern-recognition receptor with mainly inhibitory properties.ighly conserved molecular structures of invading microorganisms are recognized by immune cells through patternrecognition receptors, of which Toll-like receptors (TLRs) are the most documented family. In humans, 10 members of the TLR family have been described (1). In general, specific ligation of TLRs leads to induction of proinflammatory mediators, such as cytokines and chemokines. One member of the TLR family however, TLR10, is considered an orphan receptor because of its still-unknown ligands and function.Human TLR10 is encoded on chromosome 4 within the TLR2 gene cluster, together with TLR1, TLR2, and TLR6, and shares all structural characteristics of the TLR family (2, 3). However, TLR10 differs from other TLRs by its lack of a classic downstream signaling pathway (4), despite its interaction with the myeloid differentiation primary response gene 88 adaptor protein (3). TLR10 is predominantly expressed in tissues rich in immune cells, such as spleen, lymph node, thymus, tonsil, and lung (2). Expression of TLR10 can be induced in B cells, dendritic cells, eosinophils, and neutrophils (3, 5, 6), as well as on nonimmune cells, such as trophoblasts (7). TLR1 and TLR6 are known to form heterodimers with TLR2, and this was shown for TLR10 as well (3,8). It is therefore ...

Lipoproteins can bind lipopolysaccharide (LPS) and decrease the LPS-stimulated production of pro-inflammatory cytokines. We investigated the effect of increased plasma concentrations of low-density-lipoproteins (LDL) on survival and cytokine production after a lethal challenge with either LPS or live Gram-negative bacteria in LDL receptor deficient mice (LDLR Ϫ / Ϫ ). The LDLR Ϫ / Ϫ mice challenged with LPS had an eightfold increased LD 50 when compared with the wild type controls (C57Bl/6J), while tumor necrosis factor ␣ (TNF ␣ ) and interleukin-1 ␣ (IL-1 ␣ ) plasma concentrations were decreased twofold. LDLR Ϫ / Ϫ mice had significantly lower and delayed mortality than control mice after infection with Klebsiella pneumoniae. No differences in the outgrowth of bacteria in the organs were present between the two groups, while circulating cytokine concentrations were decreased twofold in LDLR Ϫ / Ϫ mice. In contrast, the LPS-stimulated in vitro production of cytokines by peritoneal macrophages of LDLR Ϫ / Ϫ mice was significantly increased compared with controls. This increase was associated with enhanced specific binding of LPS to the macrophages of LDLR Ϫ / Ϫ mice. In conclusion, endogenous LDL can protect against the lethal effects of endotoxin and Gram-negative infection. At least part of this protection is achieved through decreased in vivo production of pro-inflammatory cytokines, in spite of increased cytokine production capacity. ( J. Clin. Invest. 1996. 97:1366-1372). Key words: lipopolysaccharide • gram-negative infection • tumor necrosis factor • interleukin-1 • low-density lipoproteins.

Toll-like receptors (TLR) are crucial for an efficient antifungal defense. We investigated the differential recognition of blastoconidia and hyphae of Candida albicans by TLRs. In contrast to Candida blastoconidia, which stimulated large amounts of gamma interferon (IFN-␥), the tissue-invasive Candida hyphae did not stimulate any IFN-␥ by human peripheral blood mononuclear cells (PBMC) or murine splenic lymphocytes. After stimulation with blastoconidia, the production of IFN-␥ was TLR4 dependent, as shown by the significantly decreased IFN-␥ production in anti-TLR4-treated PBMC and in splenic lymphocytes from TLR4-defective ScCr mice. In addition, peritoneal macrophages from ScCr mice produced less tumor necrosis factor ␣ (TNF-␣) than macrophages of control mice did when stimulated with Candida blastoconidia, but not with hyphae, indicating that TLR4-mediated signals are lost during hyphal germination. In contrast, macrophages from TLR2 knockout mice had a decreased production of TNF-␣ in response to both Candida blastoconidia and hyphae. Candida hyphae stimulated production of interleukin-10 through TLR2-dependent mechanisms. In conclusion, TLR4 mediates proinflammatory cytokine induction after Candida stimulation, whereas Candida recognition by TLR2 leads mainly to anti-inflammatory cytokine release. TLR4-mediated proinflammatory signals are lost during germination of Candida blastoconidia into hyphae. Phenotypic switching during germination may be an important escape mechanism of C. albicans, resulting in counteracting host defense.Candida albicans is a major fungal pathogen which can cause invasive infection, especially in immunocompromised hosts (11). During infection, the phenotypic switch between blastoconidia and hyphae is a virulence trait of C. albicans. Mutants that are locked in the yeast form are less virulent in experimental models of disseminated candidiasis (10, 25). Different mechanisms, such as inhibition of phagocytosis and killing or a differential induction of pro-versus anti-inflammatory cytokines, have been suggested to play a role in the increased invasiveness of hyphae. The molecular mechanism of the differential cytokine stimulation by blastoconidia and hyphae is not known.Previous research has shown that monocytes fail to phagocytose hyphae and produce low levels of interleukin-12 (IL-12) following hyphal stimulation (6, 14); the ingestion of hyphae by dendritic cells inhibits IL-12 production and induces IL-4 production, resulting in a T helper 2 (Th2) cytokine bias (7,20). Different signaling pathways may be induced by fungal blastoconidia and hyphae, as was recently demonstrated for Aspergillus fumigatus (19). A possible mechanism could be differential recognition by Toll-like receptors (TLR), which have been identified as the most important pattern recognition receptors for microbial ligands by host cells (1, 3). Both TLR2 and TLR4 are important for the stimulation of monocytes by A. fumigatus (19), but whereas conidia are recognized by both TLR2 and TLR4, leading to proinflammator...

Although Candida glabrata is an important pathogenic Candida species, relatively little is known about its innate immune recognition. Here, we explore the potential role of Dectin-2 for host defense against C. glabrata. Dectin-2-deficient (Dectin-2−/−) mice were found to be more susceptible to C. glabrata infections, showing a defective fungal clearance in kidneys but not in the liver. The increased susceptibility to infection was accompanied by lower production of T helper 1 (Th1) and Th17-derived cytokines by splenocytes of Dectin-2−/− mice, while macrophage-derived cytokines were less affected. These defects were associated with a moderate yet significant decrease in phagocytosis of the fungus by the Dectin-2−/− macrophages and neutrophils. Neutrophils of Dectin-2−/− mice also displayed lower production of reactive oxygen species (ROS) upon challenge with opsonized C. glabrata or C. albicans. This study suggests that Dectin-2 is important in host defense against C. glabrata and provides new insights into the host defense mechanisms against this important fungal pathogen.

Toll-like receptor-1 (TLR1) and TLR6 are receptors of the TLR family that form heterodimers with TLR2. The role of TLR1 and TLR6 for the recognition of the fungal pathogen Candida albicans was investigated. TLR1 is not involved in the recognition of C. albicans, and TLR1 knock-out (TLR1 À/À) mice showed a normal susceptibility to disseminated candidiasis. In contrast, recognition of C. albicans by TLR6 modulated the balance between Th1 and Th2 cytokines, and TLR6 knock-out mice displayed a defective production of IL-10 and an increased IFN-g release. Production of the monocyte-derived cytokines tumor necrosis factor, IL-1, and IL-6 was normal in TLR6 À/À mice, and this was accompanied by a normal susceptibility to disseminated candidiasis. In conclusion, TLR6 is involved in the recognition of C. albicans and modulates the Th1/Th2 cytokine balance, but this results in a mild phenotype with a normal susceptibility of TLR6 À/À mice to Candida infection.

Interleukin‐18 (IL‐18) is not only a co‐stimulus for the induction of interferon‐γ  but also has direct proinflammatory effects by inducing tumor necrosis factor‐α (TNF‐α), IL‐1β, IL‐8 and IL‐6. However, the cascade of events leading to induction of cytokines by IL‐18 is unclear. The aim of the present study was to investigate whether murine IL‐18 stimulates production of proinflammatory cytokines, and to assess whether induction of second‐wave cytokines such as IL‐6 by IL‐18 is driven by intermediary induction of endogenous cytokines of the TNF family or IL‐1β. When mouse peritoneal macrophages were stimulated in vitro with recombinant murine IL‐18, there was a dose‐dependent induction of TNF, IL‐1α, and IL‐1β. IL‐6 synthesis was also strongly induced by IL‐18 and, as revealed by studies in knockout mice, this production was not dependent on interactions between endogenous cytokines of the TNF/TNF receptor family: TNF‐α, lymphotoxin‐α, Fas/Fas ligand (L) or CD40/CD40L. Moreover, the induction of IL‐6 was also independent of endogenous IL‐1β, as macrophages isolated from IL‐1β deficient mice produced normal amounts of IL‐6 after stimulation with IL‐18. In conclusion, murine IL‐18 has pleiotropic proinflammatory activities by inducing production of TNF‐α, IL‐1α, IL‐1β and IL‐6, which could have important consequences for the pathophysiology of infectious and autoimmune diseases.