Immune complex-FcgammaR interaction modulates monocyte/macrophage molecules involved in inflammation and immune response.

The interaction between receptors for the Fc portion of IgG (FcgammaRs) 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-gamma-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-A(d)) 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.

Monocytes and neutrophils from tuberculosis patients are insensitive to anti-inflammatory effects triggered by the prototypic formyl peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP).

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)-gamma and interleukin (IL)-10 induced the overexpression of the receptor for the Fc portion of IgG I (FcgammaRI) in monocytes from tuberculosis (TB) patients, showing that these cells respond to IFN-gamma 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 FcgammaRI induced by IFN-gamma or IL-10. This effect was not observed in monocytes from TB patients. FMLP also induced the down-regulation of the expression of FcgammaRI in monocytes that had been activated already with IFN-gamma. 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 FcgammaRI. In contrast to normal donors, supernatants from FMLP-treated neutrophils from TB patients did not modify the basal level of expression of FcgammaRI 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 FcgammaRI in response to IFN-gamma and IL-10, and the unresponsiveness to the anti-inflammatory effects induced by formyl peptides.

The formyl peptide N-formyl-methionyl-leucyl-phenylalanine downregulates the expression of FcgammaRs in interferon-gamma-activated monocytes/macrophages in vitro and in vivo.

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. FcgammaRI is a receptor for the Fc portion of immunoglobulin G expressed in monocytes that mediates cytotoxicity and is upregulated by interferon-gamma (IFN-gamma) and interleukin-10 (IL-10). In this report, we demonstrate that N-formyl-methionyl-leucyl-phenylalanine (FMLP) downregulates the expression of FcgammaRI in IFN-gamma-treated monocytes, but not in IL-10-treated monocytes. We determine that supernatants obtained from monocytes treated with IFN-gamma and then exposed to FMLP induce the downregulation of FcgammaRI 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 FcgammaRI, 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 FcgammaRs in IFN-gamma-activated macrophages. Our results support the existence of a new mechanism through which FMLP could modulate the activity of monocytes/macrophages during bacterial infections.

Interleukin-1beta induces in vivo tolerance to lipopolysaccharide in mice.

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-1 (IL-1) and tumour necrosis factor alpha (TNF-), 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-1 were tolerant to LPS-induced shock. However, TNF- was unable to induce an LPS refractory state. Given the fact that 100 ng of IL-1 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-1 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-1 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-1 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.

Immune complexes (IC) down-regulate the basal and interferon-gamma-induced expression of MHC class II on human monocytes.

The interaction of Fc receptors for IgG (FcgammaRs) on monocytes/macrophages with immune complexes (IC) triggers regulatory and effector functions. Previous studies have shown that FcgammaR-IC interactions inhibit the IFN-gamma-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-gamma 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-gamma. We demonstrate that IC exert a drastic inhibition of basal and IFN-gamma-induced expression of MHC class II on human monocytes. This effect was mediated through the interaction of IC with both FcgammaRI and FcgammaRII. 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.

Activation of peripheral blood neutrophils from patients with active advanced tuberculosis.

Activation of peripheral blood neutrophils (PMN) was investigated in order to determine whether they might contribute to the inflammatory process during active advanced tuberculosis. Receptors for the Fc portion of IgG (FcgammaR) (FcgammaRI, FcgammaRII, and FcgammaRIIIB), CD66 (degranulation marker), and receptors for tumor necrosis factor-alpha (TNF-R55 and TNF-R75) were analyzed on PMN obtained from normal controls and tuberculosis patients (TB-PMN). Functional parameters such as cytotoxicity, superoxide anion generation triggered by N-formyl-methionyl-leucyl-phenyl-alanine (FMLP), and TNF-alpha and IL-1beta production were evaluated. A high expression of TNF-R55, CD66, and FcgammaRIIIB and the appearance of FcgammaRI were detected in TB-PMN. In addition, cytotoxicity, superoxide anion release, and TNF-alpha and IL-1beta production were enhanced in TB-PMN. Thus, in tuberculosis, the activation of PMN outside the focus of infection strongly suggests the possibility of a systemic inflammation that could modulate the inflammatory response.

Tolerance to lipopolysaccharide (LPS) regulates the endotoxin effects on Shiga toxin-2 lethality.

It has been suggested that Shiga toxin (Stx) is necessary but not sufficient for hemolytic uremic syndrome (HUS) development, and pro-inflammatory stimuli such as lipopolysaccharide (LPS) from Gram negative bacteria are needed. Taking into account that LPS is present in the natural infection during HUS development, detoxification or regulation of LPS activity could be crucial to define the course of the disease. The objective of the present study was to investigate whether tolerance to LPS and/or antibodies to LPS, are able to modify the LPS-induced modulation of Stx type-2 (Stx2) lethality in a mouse model. Our results demonstrate that the high levels of IgG anti-LPS antibodies in immunized mice did not modify the dual effects of LPS (enhancement or protection) on Stx2 action. This could be attributed to the fact that antibodies do not recognize the active portion of LPS molecule (lipid A). However, the enhancement of Stx2 toxicity exerted by LPS was inhibited in tolerant mice. This effect could be ascribed to the inhibition of LPS-induced TNF-alpha and IL-1beta secretion in tolerant animals, two cytokines known to be involved in the overexpression of Stx receptors. The phenomenon of LPS-induced protection on Stx2 toxicity was also inhibited in tolerant animals, although the mechanism involved in this effect is not clear. This is the first description which shows the influence of endotoxin tolerance on the evolution of experimental HUS. However, like in Gram negative infections, further knowledge on tolerance mechanism is necessary in order to achieve a comprehensive view of this phenomenon.

N-formyl-methionyl-leucyl-phenylalanine inhibits both gamma interferon- and interleukin-10-induced expression of FcgammaRI on human monocytes.

Three different classes of receptors for the Fc portion of immunoglobulin G (FcgammaRs), FcgammaRI, FcgammaRII, and FcgammaRIII, have been identified on human leukocytes. One of them, FcgammaRI, is a high-affinity receptor capable of induction of functions that include phagocytosis, respiratory burst, antibody-dependent cell-mediated cytotoxicity (ADCC), and secretion of cytokines. This receptor is expressed on mononuclear phagocytes, and this expression is regulated by cytokines and hormones such as gamma interferon (IFN-gamma), IFN-beta, interleukin-10 (IL-10), and glucocorticoids. We have recently demonstrated that the chemotactic peptide N-formyl-methionyl-leucyl-phenylalanine (FMLP) is capable of inducing a time-dependent downregulation of both FcgammaRIIIB and FcgammaRII in human neutrophils, altering FcgammaR-dependent functions. Considering the biological relevance of the regulation of FcgammaRI, we investigated the effect of FMLP on the overexpression of FcgammaRI induced by both IFN-gamma and IL-10 on human monocytes. We demonstrate that FMLP significantly abrogated IFN-gamma- and IL-10-induced FcgammaRI expression, although its basal level of expression was not altered. However, other IFN-gamma-mediated effects such as the overexpression of the major histocompatibility complex class II antigens and the enhancement of lipopolysaccharide-induced secretion of tumor necrosis factor alpha were not affected by FMLP treatment. The formyl peptide completely inhibited the IFN-gamma- and IL-10-induced enhancement of ADCC and phagocytosis carried out by adherent cells. The inhibitory effect of FMLP on FcgammaRI upregulation could exert an important regulatory effect during the evolution of bacterial infections.