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SUMMARY The interaction between receptors for the Fc portion of IgG (Fcγ Rs) from monocytes/macrophages and immune complexes (IC) triggers regulatory and effector functions. Recently, we have demonstrated that IC exert a drastic inhibition of basal and IFN-γ -induced expression of MHC class II on human monocytes. Taking into account that the regulation of MHC class II molecules is a crucial event in the immune response, in this report we extend our previous studies analysing the effect of STAT-1 phosphorylation in the down-regulatory process, the fate of the intracellular pool of MHC class II molecules and the effect of complement on MHC class II down-regulation induced by IC. We also studied the effect of IC on the expression of MHC class II (I-Ad ) in macrophages using a mouse model of chronic inflammation. We demonstrate that IC induce a depletion not only on surface expressed but also on intracellular MHC class II content and that IC-induced down-regulation of MHC class II is not mediated by the inhibition of STAT-1 phosphorylation. On the other hand, the effect of IC is not specific for the down-regulation of MHC class II, for it could be restricted to other molecules involved in inflammatory processes. Our experiments also show that the activation of the complement system could be a crucial step on the regulation of the effect of IC on MHC class II expression. In agreement with our in vitro experiments using human monocytes, IC treatment reduces the expression of MHC class II in a mouse model of chronic inflammation. [ABSTRACT FROM AUTHOR]

SUMMARY Tuberculosis is a chronic infectious disease caused by Mycobacterium tuberculosis where formyl peptides, which are cleavage products of bacterial and mitochondrial proteins, are present. In this study, we demonstrated that interferon gamma (IFN)-γ and interleukin (IL)-10 induced the overexpression of the receptor for the Fc portion of IgG I (Fcγ RI) in monocytes from tuberculosis (TB) patients, showing that these cells respond to IFN-γ and IL-10 signals. We also demonstrated that lower doses of IL-10 render monocytes from TB patients less responsive to higher doses of the cytokine. Although the prototypic formyl peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP) is a well-known proinflammatory agonist, we have demonstrated previously that preincubation of monocytes with FMLP inhibited the up-regulation of Fcγ RI induced by IFN-γ or IL-10. This effect was not observed in monocytes from TB patientes. FMLP also induced the down-regulation of the expression of Fcγ RI in monocytes that had been activated already with IFN-γ . However, this effect of FMLP was not observed in monocytes from TB patients and supernatants from monocytes obtained from these patients were incapable of inducing the down-regulation of Fcγ RI. In contrast to normal donors, supernatants from FMLP-treated neutrophils from TB patients did not modify the basal level of expression of Fcγ RI in monocytes from normal donors. In conclusion, in this study we demonstrated the existence of two novel mechanisms that may contribute to the pathological effects generated by M. tuberculosis: the enhancement of Fcγ RI in response to IFN-γ and IL-10, and the unresponsiveness to the anti-inflammatory effects induced by formyl peptides. [ABSTRACT FROM AUTHOR]

Abstract N -Formyl peptides are cleavage products of bacterial and mitochondrial proteins that have pro-inflammatory activities and play an important role in antibacterial host defence. FcγRI is a receptor for the Fc portion of immunoglobulin G expressed in monocytes that mediates cytotoxicity and is upregulated by interferon-γ (IFN-γ) and interleukin-10 (IL-10). In this report, we demonstrate that N -formyl-methionyl-leucyl-phenylalanine (FMLP) downregulates the expression of FcγRI in IFN-γ-treated monocytes, but not in IL-10-treated monocytes. We determine that supernatants obtained from monocytes treated with IFN-γ and then exposed to FMLP induce the downregulation of FcγRI in naïve monocytes. This effect is abrogated by the protease inhibitors phenylmethylsulphonyl fluoride and phosphoramidon, which inhibit serine and metalloproteases, respectively. Supernatants from FMLP-treated neutrophils also induce the downregulation of FcγRI, when added to naïve monocytes. Similar observations were obtained in vivo in a mouse model of chronic inflammation. In vivo , FMLP also downregulates the expression of FcγRs in IFN-γ-activated macrophages. Our results support the existence of a new mechanism through which FMLP could modulate the activity of monocytes/macrophages during bacterial infections. [ABSTRACT FROM AUTHOR]

The interaction of Fc receptors for IgG (FcγRs) on monocytes/macrophages with immune complexes (IC) triggers regulatory and effector functions. Previous studies have shown that FcγR–IC interactions inhibit the IFN-γ-induced expression of MHC class II in murine macrophages. However, the mechanism(s) responsible for these effects have not been elucidated. In addition, whether this IC-dependent effect also occurs in human cells is not known. Taking into account the fact that IC and IFN-γ are frequently found in infections and autoimmune disorders, together with the crucial role MHC class II molecules play in the regulation of immune response, we explored the effect and mechanism of IC-induced MHC class II down-regulation in human peripheral blood mononuclear cells (PBMC). This effect was studied either in the presence or absence of IFN-γ. We demonstrate that IC exert a drastic inhibition of basal and IFN-γ-induced expression of MHC class II on human monocytes. This effect was mediated through the interaction of IC with both FcγRI and FcγRII. Moreover, similar results were obtained using supernatants from IC-treated PBMC. The IC-induced down-regulation of MHC class II is abrogated by pepstatin and phosphoramidon, supporting the role of aspartic protease(s) and metalloprotease(s) in this process. In parallel with MHC class II expression, antigen presentation was markedly inhibited in the presence of IC. [ABSTRACT FROM AUTHOR]

The regulation of MHC-II molecules is a crucial event in the inmune response. We have demostrated that ovalbumin (OA)-rabbit IgG anti-OA IC down-regulate the basal and IFN-g-induced expression of MHC-II on human monocytes trough the secretion of proteases. Previous reports have shown that IC inhibit IFN-g-induced expression of FcgRI by preventing tyrosine phosphorylation of the STAT-1 and this mechanism was also proposed for the MHC-II inhibition. In this study we investigated whether MHC-II expression could be affected by the inhibition of STAT-1 phosphorylation, the effect of the complement on IC-induced down-regulation and the specificity of the IC effect. The results demonstrated that IFN-g was able to induce STAT-1 phosphorylation even in the presence of IC. Normal human serum (NHS) but not heat-inactivated serum (ØNHS), was capable of reducing the effect of IC on the basal expression of MHC-II on human monocytes. The expression of MHC-II was evaluated by flow cytometry and the results are expressed as the percentage of median of fluorescence intensity (%MFI) ± SEM of control cells: a)IC:41±9; b)IC+NHS:103±9; c)IC+ØNHS:46±11; n=5; a) and c) vs control p<0,01. IC were capable of down-regulating the basal expression of MHC-I, ICAM-1 and CD14 although CD11b expression was not modified. These results indicate that: 1) the effect of IC on MHC-II is not mediated through the inhibition of STAT-1 phosphorylation, 2) the activation of the complement system, probably by IC solubilization, could be a crucial step on the regulation of IC-dependent effects, 3) the effect of IC is not specific for MHC-II molecules. [ABSTRACT FROM AUTHOR]

SUMMARY Endotoxin or lipopolysaccharide (LPS) tolerance may be partially due to the secretion of potent anti-inflammatory cytokines following severe Gram-negative infections, or by low doses of LPS. In this work, we describe the effects of interleukin-1β (IL-1β) and tumour necrosis factor alpha (TNF-α), two early cytokines secreted after LPS exposure, in the induction of LPS tolerance. Our results demonstrate that mice treated with three daily doses of 100 ng of IL-1β were tolerant to LPS-induced shock. However, TNF-α was unable to induce an LPS refractory state. Given the fact that 100 ng of IL-1β increase the plasma levels of glucocorticoids, we evaluated whether a daily injection of dexamethasone (DEX) alone was able to reproduce the LPS-like tolerant state. However, no signs of LPS refractoriness were detected, except when DEX was administered concomitantly with a dose of IL-1β that does not induce corticosterone secretion (12 ng/mouse). This dose was found to induce in vitro up-regulation of the glucocorticoid receptors (GcR) of peritoneal macrophages following 24 h of treatment. In addition, we demonstrate that IL-1β is capable of inducing the down-regulation of Toll-like receptor 4 (TLR4), a crucial molecule in the signal transduction of LPS. Taken together, our results indicate that IL-1β can generate tolerance to LPS in vivo , and suggest that the regulation of mechanisms of the down-regulation of TLR4, as well as those involved in the expression of GcR and/or in the secretion of glucocorticoids, would be crucial for these effects. [ABSTRACT FROM AUTHOR]