Suppression of osteoprotegerin expression by prostaglandin E2 is crucially involved in lipopolysaccharide-induced osteoclast formation

LPS is a potent stimulator of bone resorption in inflammatory diseases. The mechanism by which LPS induces osteoclastogenesis was studied in cocultures of mouse osteoblasts and bone marrow cells. LPS stimulated osteoclast formation and PGE(2) production in cocultures of mouse osteoblasts and bone marrow cells, and the stimulation was completely inhibited by NS398, a cyclooxygenase-2 inhibitor. Osteoblasts, but not bone marrow cells, produced PGE(2) in response to LPS. LPS-induced osteoclast formation was also inhibited by osteoprotegerin (OPG), a decoy receptor of receptor activator of NF-kappaB ligand (RANKL), but not by anti-mouse TNFR1 Ab or IL-1 receptor antagonist. LPS induced both stimulation of RANKL mRNA expression and inhibition of OPG mRNA expression in osteoblasts. NS398 blocked LPS-induced down-regulation of OPG mRNA expression, but not LPS-induced up-regulation of RANKL mRNA expression, suggesting that down-regulation of OPG expression by PGE(2) is involved in LPS-induced osteoclast formation in the cocultures. NS398 failed to inhibit LPS-induced osteoclastogenesis in cocultures containing OPG knockout mouse-derived osteoblasts. IL-1 also stimulated PGE(2) production in osteoblasts and osteoclast formation in the cocultures, and the stimulation was inhibited by NS398. As seen with LPS, NS398 failed to inhibit IL-1-induced osteoclast formation in cocultures with OPG-deficient osteoblasts. These results suggest that IL-1 as well as LPS stimulates osteoclastogenesis through two parallel events: direct enhancement of RANKL expression and suppression of OPG expression, which is mediated by PGE(2) production.     

IL-6 inhibits lipopolysaccharide-induced tumor necrosis factor production in cultured human monocytes, U937 cells, and in mice

Incubation of the human U937 histiocytic lymphoma cell line with granulocyte-macrophage colony stimulating factor (GM-CSF) rendered the cells responsive to induction of TNF by LPS. Treatment with IL-6 reduced TNF production in GM-CSF-primed U937 cells. The inhibitory effect was most pronounced (approximately equal to 80%) when IL-6 was added either along with GM-CSF or within the first 3 h of GM-CSF treatment. Both GM-CSF or IL-6 inhibited [3H]TdR uptake in U937 cells, and simultaneous treatment with GM-CSF and IL-6 resulted in an additive inhibitory effect on cell proliferation. However, the inhibition of TNF production could not be explained by the inhibitory effect of IL-6 on cell growth, nor was it due to a reduction in cell viability. An inhibition of TNF production by IL-6 was also demonstrated in cultured human peripheral blood monocytes. Treatment with IL-6 also resulted in a dose-dependent reduction of the 17-kDa TNF band revealed by SDS-PAGE after labeling monocytes with [35S]cysteine and immunoprecipitation with anti-TNF mAb. In addition, treatment with IL-6 resulted in a reduction of monocyte in vitro cytotoxicity for tumor target cells. Finally, in mice sensitized by the administration of Bacillus Calmette-Guérin, the injection of IL-6 significantly reduced the levels of TNF found in the serum upon challenge with LPS. Inasmuch as TNF is known to be an inducer of IL-6, the inhibitory action of IL-6 on TNF production may represent the negative arm of a regulatory circuit. The inhibitory action of IL-6 on TNF production is consistent with a predominantly antiinflammatory role of IL-6 in the intact organism.     

Granulocyte colony-stimulating factor treatment protects rodents against lipopolysaccharide-induced toxicity via suppression of systemic tumor necrosis factor-alpha.

Pretreatment with recombinant human granulocyte CSF (G-CSF) protected mice in two different models of septic shock. Intravenous injection of 250 micrograms/kg G-CSF to mice prevented lethality induced by 5 mg/kg LPS. Injection of 50 micrograms/kg G-CSF protected galactosamine-sensitized mice against LPS-induced hepatitis. In either case, this protection was accompanied by a suppression of LPS-induced serum TNF activity. In contrast, when galactosamine-sensitized mice were pretreated with 50 micrograms/kg murine recombinant granulocyte/macrophage CSF instead of G-CSF and subsequently challenged with LPS, serum TNF activity was significantly enhanced and mortality was increased. The suppressive effect of G-CSF on LPS-induced TNF production was also demonstrated in rats. In vivo, no TNF was detectable in the blood of LPS-treated rats, which had been pretreated with G-CSF. Ex vivo, alveolar macrophages, bone marrow macrophages, Kupffer cells, or peritoneal macrophages prepared from G-CSF-treated rats produced significantly less TNF upon stimulation with LPS than corresponding populations from control rats. However, when these macrophage populations were incubated with G-CSF in vitro, LPS-induced TNF production was unaffected. These data suggest that the G-CSF-mediated suppression of TNF production is not a direct effect of G-CSF on macrophages. To examine whether, independent of the protection against LPS, G-CSF treatment still activated neutrophils, it was demonstrated that granulocytes from G-CSF-treated rats were primed for PMA-induced oxidative burst and for ionophore/arachidonic acid-stimulated lipoxygenase product formation. The experiments of this study support the notion that G-CSF is a negative feedback signal for macrophage-derived TNF-alpha production during Gram-negative sepsis.     

Ciliary neurotrophic factor inhibits brain and peripheral tumor necrosis factor production and, when coadministered with its soluble receptor, protects mice from lipopolysaccharide toxicity.

BACKGROUND: The receptor of ciliary neurotrophic factor (CNTF) contains the signal transduction protein gp130, which is also a component of the receptors of cytokines such as interleukin (IL)-6, leukemia-inhibitory factor (LIF), IL-11, and oncostatin M. This suggests that these cytokines might share common signaling pathways. We previously reported that CNTF augments the levels of corticosterone (CS) and of IL-6 induced by IL-1 and induces the production of the acute-phase protein serum amyloid A (SAA). Since the elevation of serum CS is an important feedback mechanism to limit the synthesis of proinflammatory cytokines, particularly tumor necrosis factor (TNF), we have investigated the effect of CNTF on both TNF production and lipopolysaccharide (LPS) toxicity. MATERIALS AND METHODS: To induce serum TNF levels, LPS was administered to mice at 30 mg/kg i.p. and CNTF was administered as a single dose of 10 micrograms/mouse i.v., either alone or in combination with its soluble receptor sCNTFR alpha at 20 micrograms/mouse. Serum TNF levels were the measured by cytotoxicity on L929 cells. In order to measure the effects of CNTF on LPS-induced TNF production in the brain, mice were injected intracerebroventricularly (i.c.v.) with 2.5 micrograms/kg LPS. Mouse spleen cells cultured for 4 hr with 1 microgram LPS/ml, with or without 10 micrograms CNTF/ml, were also analyzed for TNF production. RESULTS: CNTF, administered either alone or in combination with its soluble receptor, inhibited the induction of serum TNF levels by LPS. This inhibition was also observed in the brain when CNTF and LPS were administered centrally. In vitro, CNTF only marginally affected TNF production by LPS-stimulated mouse splenocytes, but it acted synergistically with dexamethasone (DEX) in inhibiting TNF production. Most importantly, CNTF administered together with sCNTFR alpha protected mice against LPS-induced mortality. CONCLUSIONS: These data suggest that CNTF might act as a protective cytokine against TNF-mediated pathologies both in the brain and in the periphery.     

Selection of novel analogs of thalidomide with enhanced tumor necrosis factor alpha inhibitory activity.

BACKGROUND: Tumor necrosis factor alpha (TNF alpha) is thought to mediate both protective and detrimental manifestations of the inflammatory response. Recently, thalidomide (alpha-N-phthalimidoglutarimide) was shown to partially inhibit monocyte TNF alpha production (by 50-70%) both in vivo and in vitro. More efficient inhibition of TNF alpha may, however, be necessary to rescue the host from more acute and extensive toxicities of TNF alpha-mediated inflammation. MATERIALS AND METHODS: Three structural analogues of thalidomide were selected for study based on increased activity against TNF alpha production. The parent drug and the analogs were tested in vitro in human peripheral blood mononuclear cell cultures for their effects on lipopolysaccharide (LPS) induced cytokine protein and mRNA production using ELISAs and Northern blot hybridization. The in vitro effects of the drugs were then confirmed in vivo in a mouse model of LPS induced lethality. RESULTS: The new compounds (two esters and one amide) showed increased inhibition of TNF alpha production by LPS-stimulated human monocytes, relative to the parent drug thalidomide. The analogs and the parent drug enhanced the production of interleukin 10 (IL-10), but had little effect on IL-6 and IL-1 beta protein and mRNA production. When tested in vivo, the amide analog protected 80% of LPS-treated mice against death from endotoxin induced shock. CONCLUSIONS: Analogs of thalidomide designed to better inhibit TNF alpha production in vitro have correspondingly greater efficacy in vivo. These finding may have therapeutic implication for the treatment of human diseases characterized by acute and extensive TNF alpha production such as tuberculous meningitis or toxic shock.     

N-acetylcysteine and glutathione as inhibitors of tumor necrosis factor production.

TNF is a major mediator in the pathogenesis of endotoxic shock, and its inhibition has a protective effect in various animal models of sepsis or endotoxin (lipopolysaccharide, LPS) toxicity. LPS treatment also induces an oxidative damage mediated by increased production of reactive oxygen intermediates. N-Acetylcysteine (NAC) is an antioxidant and a precursor of the synthesis of glutathione (GSH) and was reported to protect against LPS toxicity and LPS-induced pulmonary edema. In this study we investigated the effect of NAC on TNF production and LPS lethality in mice. The results indicated that oral administration of NAC protects against LPS toxicity and inhibits the increase in serum TNF levels in LPS-treated mice. The inhibition was not confined to the released form of TNF, since NAC also inhibited LPS-induced spleen-associated TNF. On the other hand, the inhibitor of GSH synthesis, DL-buthionine-(SR)-sulfoximine (BSO), had the opposite effect of potentiating LPS-induced TNF production, and this was associated with a decrease in liver GSH levels. Repletion of liver GSH with NAC reversed this effect. NAC was also active in inhibiting TNF production and hepatotoxicity in mice treated with LPS in association with a sensitizing dose of Actinomycin D. These data indicate that GSH can be an endogenous modulator of TNF production in vivo. On the other hand, NAC pretreatment did not inhibit other effects of LPS, particularly induction of serum IL-6, spleen IL-1 alpha, and corticosterone, in the same experimental model, suggesting that the observed effect could be specific for TNF.     

Regulation of lipopolysaccharide-induced tumor necrosis factor production by cyclosporin A in mice primed with muramyl dipeptide

Abstract The effect of cyclosporin A (CsA) on tumor necrosis factor (TNF) or interleukin-6 (IL-6) production was evaluated in vivo in primed or unprimed mice challenged with lipopolysaccharide (LPS). Both pretreatment with BCG infection or with muramyl dipeptide (MDP) prior to LPS challenge resulted in an increase in the cytokine bioactivity level in the blood. CsA administration inhibited the TNF production. In unprimed mice, either normal or sensitized to LPS lethality by galactosamine treatment, a marked decrease in the cytokine level was observed after injection of CsA. After adrenalectomy, the yield of both TNF and IL-6 following LPS injection was markedly elevated but decreased by CsA administration. Ex vivo experiments have shown that the inhibitory effect of CsA could be demonstrated at the level of macrophages from mice previously given the drug. If mice had received MDP, in vitro responses of cells to LPS were enhanced but again CsA decreased the mRNA expression and protein secretion.     

Differential regulation of IL 6, IL 1 A, IL 1 beta and TNF alpha production in LPS-stimulated human monocytes: role of cyclic AMP.

Interleukin 6 (IL 6), IL 1 alpha, IL beta and tumor necrosis factor (TNF) alpha are four cytokines induced in monocytes by lipopolysaccharide (LPS); however, it is unclear whether the mechanisms which control their production are similar. In this study, we report the effects of prostaglandin E2 (PGE2), and two other cAMP-elevating agents, dibutyryl cAMP and 3-isobutyl-1-methyl-xanthine, on the in vitro LPS-induced production of IL 6, IL 1 alpha, IL 1 beta and TNF alpha by human monocytes. The production of these four cytokines was found to be selectively regulated in monocytes, by increases in intracellular cAMP levels. In effect, such agents enhanced, in a dose-dependent manner, both extracellular and cell-associated IL 6 production by LPS-stimulated monocytes. In contrast, it was confirmed, using the same samples, that these cAMP-elevating agents inhibit both extracellular and cell-associated TNF alpha production in a dose-dependent manner. IL 1 alpha and IL 1 beta production, measured by means of specific immunoreactive assays, were not significantly modified. Kinetic analysis showed that the potentiating effect of cAMP on IL 6 production, along with its inhibiting effect on TNF alpha production, could be seen as early as 1 hr after LPS stimulation. These results demonstrate that IL 6, TNF alpha, IL 1 alpha and IL 1 beta production can be differently modulated by an agent, PGE2, which is produced simultaneously by LPS-stimulated monocytes. Such differential autocrine modulation may play an important role in the regulation of the production of cytokines participating in immune and inflammatory responses.     

Biphasic production of IL-8 in lipopolysaccharide (LPS)-stimulated human whole blood. Separation of LPS- and cytokine-stimulated components using anti-tumor necrosis factor and anti-IL-1 antibodies.

TNF, IL-1, and IL-6 are integral components of the cytokine cascade released in the response to inflammatory stimuli such as LPS. IL-8 is produced both in response to LPS as well as TNF and IL-1. The early, local production of TNF and IL-1 may therefore contribute to the subsequent expression of IL-8. This hypothesis was tested using LPS-stimulated human whole blood as an ex vivo model of local cytokine production. The production of TNF, IL-1 alpha, IL-1 beta, IL-6, and IL-8 was found to be responsive to a wide range of LPS concentrations (0.1 ng/ml-10 micrograms/ml). These cytokines were first detected between 1 to 4 h post-LPS stimulation, and reached plateau levels after 6 to 12 h. IL-8, however, also displayed a secondary wave of production, with the levels again increasing between 12 to 24 h. The IL-8 present in the plasma after LPS stimulation was biologically active, as assessed by neutrophil chemotaxis. In further studies, addition of anti-TNF and anti-IL-1 neutralizing antibodies, alone and in combination, to LPS-stimulated blood resulted in nearly complete ablation of the secondary phase of IL-8 synthesis at both the levels of protein and mRNA, while leaving the first, LPS-mediated phase of IL-8 synthesis unaffected. This model of cytokine production in human whole blood may reflect the sequence of events in a localized environment of inflammation where both a primary stimulus and the induced early cytokine mediators may serve to elicit multiple, temporally distinct phases of IL-8 production.     

Lipopolysaccharide-dependent prostaglandin E(2) production is regulated by the glutathione-dependent prostaglandin E(2) synthase gene induced by the Toll-like receptor 4/MyD88/NF-IL6 pathway

Macrophages produce a large amount of PGE(2) during inflammation. This lipid mediator modulates various immune responses. PGE(2) acts on macrophages and inhibits production of cytokines such as TNF-alpha and IL-12. Membrane-bound glutathione-dependent PGE(2) synthase (mPGES) has been shown to be a terminal enzyme of the cyclooxygenase-2-mediated PGE(2) biosynthesis. Here we identified mPGES as a molecule that is induced by LPS in macrophages. The expression of mPGES was not induced by LPS in mice lacking Toll-like receptor 4 or MyD88. Furthermore, mice deficient in NF-IL6 showed neither induction of mPGES nor biosynthesis of PGE(2) in response to LPS, indicating that mPGES expression in response to LPS is regulated by a Toll-like receptor 4/MyD88/NF-IL6-dependent signaling pathway. We generated mPGES-deficient mice and investigated the role of mPGES in vivo. The mice showed no augmentation of the PGE(2) production in response to LPS. However, they were not impaired in the LPS-induced production of inflammatory cytokines and showed normal response to the LPS-induced shock. Thus, mPGES is critically involved in the biosynthesis of PGE(2) induced by LPS, but is dispensable for the modulation of inflammatory responses.     

ANG II is involved in the LPS-induced production of proinflammatory cytokines in dehydrated rats

We have previously reported results that led us to speculate that ANG II is involved in the LPS-induced production of proinflammatory cytokines, especially under dehydrated conditions. To test this possibility, in this study we examined the effects of an angiotensin-converting enzyme (ACE) inhibitor and an antagonist of the type-1 ANG II receptor (AT(1) receptor) on the LPS-induced production of the proinflammatory cytokines IL-1 and IL-6 in dehydrated rats. A single intravenous injection of LPS induced a marked increase in the expression of IL-1beta mRNA in the liver, an effect that was significantly attenuated by pretreatment with the ACE inhibitor. Furthermore, the ACE inhibitor reduced the LPS-induced increase in the hepatic concentration of IL-1beta protein. When the AT(1)-receptor antagonist was given intravenously before the LPS, the increase in the hepatic concentration of IL-1beta was significantly reduced. Finally, the ACE inhibitor reduced the LPS-induced increase in the plasma concentration of IL-6. These results represent the first in vivo evidence that ANG II and its AT(1) receptor play important roles in the production of proinflammatory cytokines that is induced by LPS under dehydrated conditions.     

Immunostimulating effects of muramyl dipeptide, glucosaminyl muramyl dipeptide and their synthetic derivatives in vitro

The effect of muramyldipeptide (MDP), glucosaminylmuramyldipeptide (GMDP) and their six synthetic derivatives on production of tumor necrosis factor (TNF), interleukin-1 (IL-1) and interleukin-2 (IL-2) by murine spleen cells in vitro was studied. MDP induced insignificant TNF production and did not stimulate production of IL-1 by the murine splenocytes within a 24-hour cultivation period whereas in combination with lipopolysaccharide (LPS) it induced significant production of both the cytokins. GMDP induced marked production of TNF (54 per cent cytotoxic index) and IL-1 (stimulation index 8). Addition of LPS in an amount of 10 ng/ml increased production of TNF by the murine splenocytes under the effect of GMDP but had no effect on production of IL-1. Neither MDP nor GMDP even in combination with LPS induced production of IL-2 by splenocytes of mice DVA/2 and C57B1/6 at activation for 24 hours. All the synthetic derivatives of MDP and GMDP except the MDP polymer activated TNF production by the murine spleen cells. GMDP lysine had the highest effect: 67 per cent cytotoxic index. In combination with LPS its cytotoxic index amounted to 87 per cent. The TNF activity was always higher when LPS in an amount of 10 ng/ml was added to the glycopeptides.     

Anti-tumor necrosis factor antibody therapy fails to prevent lethality after cecal ligation and puncture or endotoxemia.

Cytokines have been studied intensively to delineate their role in the altered pathophysiology observed in septic shock. We studied the role of TNF in the lethality of two well characterized models of septic shock by inhibiting TNF's activity with a specific antibody. In the first model, sepsis was induced by cecal ligation and puncture (CLP), and in the second model sepsis was induced by either an i.p. or i.v. injection of LPS. After CLP, plasma endotoxin was detectable within 4 h and reached a peak at 8 h (136 +/- 109 ng/ml). TNF bioactivity peaked at 12 h (528 +/- 267 pg/ml) at a significantly higher level than sham-operated control mice (64 +/- 31 pg/ml). After i.p. LPS, TNF peaked much more quickly (90 min) compared with CLP and at a significantly higher level (107,900 +/- 25,000 pg/ml). Another cytokine studied in septic shock, IL-6, peaked at 12 h after CLP at 1011 +/- 431 pg/ml, and at 90 min after lethal LPS at 16,300 +/- 3,700 pg/ml. Mice were treated with an anti-TNF antibody that has been shown previously to inhibit in vivo TNF activity. Antibody treatment of mice subjected to CLP significantly reduced TNF bioactivity but did not reduce mortality or pulmonary neutrophilic infiltration. In the i.v. LPS model, anti-TNF antibody treatment concomitant with LPS injection reduced plasma TNF activity from 80,000 +/- 20,000 pg/ml to undetectable levels. However, anti-TNF treatment immediately before either i.v. or i.p. LPS did not reduce mortality. Additionally, when the antibody was administered 4 h before the lethal i.v. LPS, there was no reduction in lethality. These data show that in two separate models of septic shock blockade of TNF biologic activity will not prevent lethality.     

Saccharated colloidal iron enhances lipopolysaccharide-induced nitric oxide production in vivo

We investigated the effects of iron on the production of nitric oxide (NO), inducible NO synthase (iNOS), and plasma cytokines induced by lipopolysaccharide (LPS) in vivo. Male Wistar rats were preloaded with a single intravenous injection of saccharated colloidal iron (Fesin, 70 mg iron/kg body weight) or normal saline as a control, and then given an intraperitoneal injection of LPS (5.0 mg/kg body weight). Rats, preloaded with iron, had evidence of both iron deposition and strong iNOS induction in liver Kupffer cells upon injection of LPS; phagocytic cells in the spleen and lung had similar findings. LPS-induced NO production in iron-preloaded rats was significantly higher than control rats as accessed by NO-hemoglobin levels measured by ESR (electron spin resonance) and NOx (nitrate plus nitrite) levels. Western blot analysis showed that iron preloading significantly enhanced LPS-induced iNOS induction in the liver, but not in the spleen or lung. LPS-induced plasma levels of IL-6, IL-1beta, and TNF-alpha were also significantly higher in iron-preloaded rats as shown by ELISA, but IFN-gamma levels were unchanged. We conclude that colloidal-iron phagocytosed by liver Kupffer cells enhanced LPS-induced NO production in vivo, iNOS induction in the liver, and release of IL-6, IL-1beta, and TNF-alpha.     

Granulocyte colony-stimulating factor decreases tumor necrosis factor production in whole blood: role of interleukin-10 and prostaglandin E(2)

Previous reports have indicated that the administration of granulocyte colony-stimulating factor (G-CSF) decreases ex vivo tumor necrosis factor (TNF) production in humans. In this study, we report that daily pretreatment of mice with G-CSF for three days decreases ex vivo lipopolysaccharide (LPS)-induced TNF production in whole blood. Conversely, production of interleukin-10 (IL-10) and prostaglandin E(2) (PGE(2)) is increased. The inhibitory effect of G-CSF pretreatment on TNF production is partially reversed by addition of an anti-IL-10 antibody, and completely reversed by combined addition of anti-IL-10 antibody and the cyclooxygenase (COX) inhibitor, ketoprofen. These results suggest that G-CSF decreases TNF production in this experimental model by increasing production of IL-10 and PGE(2), which are both known inhibitors of TNF production.     

Dendritic cells issued in vitro from bone marrow produce PGE(2) that contributes to the immunomodulation induced by antigen-presenting cells.

Given that preliminary work has indicated that prostaglandins can play a role in modulating dendritic cell (DC) functions, we addressed the prostaglandin E(2) (PGE(2)) biosynthetic capacity of mouse DC produced in vitro from bone marrow cells. We observed production of significant amounts of PGE(2), which was reduced by at least 80% when cells were incubated in the presence of indomethacin, a COX-1 preferential inhibitor. Indeed, when tested by Western blot analysis with specific COX-1 and COX-2 antibodies, only COX-1 expression could be detected in the bone marrow (BM)-DC. For lipopolysaccharide (LPS)-treated BM-DC, inhibition of PGE(2) production by indomethacin or by NS-398 (a COX-2-selective inhibitor) used alone was less potent. After LPS treatment of BM-DC, COX-1 and COX-2 expression was potent, and inhibition of PGE(2) synthesis needed the presence of both indomethacin and NS-398. We also observed that exogenous PGE(2) diminished the expression of MHC class II molecules by BM-DC and that prostaglandin and indomethacin had antagonistic effects on cell proliferation during the mixed lymphocyte reaction using BM-DC as stimulatory cells. This assessment of PGE(2) suggests that endogenous PGE(2) produced by DC might play a role as an immunomodulating factor during the immune response. This hypothesis is sustained by the fact that IL-12 production by BM-DC is modulated by exogenous PGE(2) as well as endogenous prostaglandin, since either the addition of exogenous PGE(2) or the presence of LPS (which increases endogenous PGE(2) synthesis) decreases IL-12 production, while NS-398 (which decreases LPS-induced PGE(2) synthesis) increases IL-12 synthesis.     

Functional consequences of CD36 downregulation by TLR signals

TLR recognition activates the secretion of pro- and anti-inflammatory cytokines and it also modulates the expression of crucial molecules involved in phagocytosis and antimicrobial activity. Scavenger receptors can act as TLR co-receptors or facilitate antigen loading. However, it remains unknown whether TLR can modulate the expression of these scavenger receptors. We stimulated human peripheral blood mononuclear cells (PBMC) with TLR2 (Pam3CSK4 and FSL1) and TLR4 ligand lipopolysaccharide (LPS) and then analyzed CD36 expression on different monocyte subpopulations by flow cytometry. TLR2 and TLR4 ligands can downregulate CD36 on the surface of monocytes, guiding the protein to intracellular compartments. Even though TLR-activation induced TNFα, IL-10 and IL-6 production, only recombinant TNFα was able to downregulate CD36. Neutralizing anti-TNFα antibodies showed that the Pam3CSK4 and FSL1-induced downregulation was partially mediated by TNFα but not by IL-6 or IL-10. However, LPS-induced downregulation could have also been caused by direct TLR4 targeting and signaling, and/or mediated by other unknown factors. CD36 downregulation reduced the capability of monocytes to phagocyte apoptotic neutrophils. In conclusion, modulation of scavenger receptor expression by TLR targeting on monocytes has functional consequences. Characterization this complex regulation may help us to understand this innate response and develop specific therapeutic drugs for each mechanism.     

Role of transforming growth factor-beta(1) in down-regulating TNF production by alveolar macrophages during asbestos-induced pulmonary fibrosis

Activation of alveolar macrophages (AM) for tumour necrosis factor production is suppressed initially during the inflammatory response to fibrogenic dusts. We investigated the mechanisms involved in TNF suppression, notably the role of other AM-derived mediators including prostaglandin E(2) (PGE(2)), transforming growth factor-beta(1) (TGF-beta(1)), and interleukin 6 (IL-6). The action of PGE(2) and TGF-beta(1), on AM was different. At physiologically relevant doses (25-300 pg/ml), PGE(2) did not cause significant inhibition of Hpopolysaccharide (Lps)-induced TNF release by AM in vitro but stimulated IL-6 (up to six fold), an inhibitor of AM-derived TNT. In contrast, TGF-beta(1) (0.5-50 ng/ml) inhibited both LPS-induced TNT and IL-6 release by 50% but had no effect on PGE(2) production by AM. To determine the respective contribution of these different inhibitors in TNF suppression, AM from rats exposed to fibrogenic asbestos for weeks were treated with neutralizing antibody against TGF-beta(1) or indomethacin, an inhibitor of PGE(2) synthesis. Treatment of rat AM with anti-TGF-beta(1) but not indomethacin, abrogated the observed TNT suppression. These results suggest that an autocrine, TGF-beta(1)-dependent mechanism is involved in the down-regulation of TNF production by rat AM from animals with lung fibrosis.