Crystalline Silica Impairs Efferocytosis Abilities of Human and Mouse Macrophages: Implication for Silica-Associated Systemic Sclerosis

Inhalation of crystalline silica (SiO2) is a risk factor of systemic autoimmune diseases such as systemic sclerosis (SSc) and fibrotic pulmonary disorders such as silicosis. A defect of apoptotic cell clearance (i.e., efferocytosis, a key process in the resolution of inflammation) is reported in macrophages from patients with fibrotic or autoimmune diseases. However, the precise links between SiO2 exposure and efferocytosis impairment remain to be determined. Answering to this question may help to better link innate immunity and fibrosis. In this study, we first aim to determine whether SiO2 might alter efferocytosis capacities of human and mouse macrophages. We secondly explore possible mechanisms explaining efferocytosis impairment, with a specific focus on macrophage polarization and on the RhoA/ROCK pathway, a key regulator of cytoskeleton remodeling and phagocytosis. Human monocyte-derived macrophages (MDM) and C57BL/6J mice exposed to SiO2 and to CFSE-positive apoptotic Jurkat cells were analyzed by flow cytometry to determine their efferocytosis index (EI). The effects of ROCK inhibitors (Y27632 and Fasudil) on EI of SiO2-exposed MDM and MDM from SSc patients were evaluated in vitro. Our results demonstrated that SiO2 significantly decreased EI of human MDM in vitro and mouse alveolar macrophages in vivo. In human MDM, this SiO2-associated impairment of efferocytosis, required the expression of the membrane receptor SR-B1 and was associated with a decreased expression of M2 polarization markers (CD206, CD204, and CD163). F-actin staining, RhoA activation and impairment of efferocytosis, all induced by SiO2, were reversed by ROCK inhibitors. Moreover, the EI of MDM from SSc patients was similar to the EI of in vitro- SiO2-exposed MDM and Y27632 significantly increased SSc MDM efferocytosis capacities, suggesting a likewise activation of the RhoA/ROCK pathway in SSc. Altogether, our results demonstrate that SiO2 exposure may contribute to the impairment of efferocytosis capacities of mouse and human macrophages but also of MDM in SiO2-associated autoimmune diseases and fibrotic disorders such as SSc; in this context, the silica/RhoA/ROCK pathway may constitute a relevant therapeutic target.

Efferocytosis capacities of blood monocyte-derived macrophages in systemic sclerosis

A defect in the apoptotic cell clearance (efferocytosis) by phagocytic cells may participate in autoimmunity and chronic inflammation. The mechanisms leading to the emergence of autoimmunity in systemic sclerosis (SSc) are still to be determined. In this study, the efferocytosis capacities of blood monocyte-derived macrophages (MDM) from patients with SSc were evaluated. Blood monocytes obtained from patients with SSc and healthy donors (HD) were differentiated in vitro into macrophages. The capacities of MDM to engulf CFSE+ apoptotic Jurkat human T lymphocytes were compared between SSc MDM and HD using flow cytometry. The expression of classical engulfing receptors in SSc MDM and HD MDM was also evaluated and their involvement in the modulation of efferocytosis was confirmed using a siRNA approach. The mean phagocytic index (PI) reflecting efferocytosis capacities of SSc MDM (PI = 19.3 ± 3.0; n = 21) was significantly decreased in comparison with the PI of HD MDM (PI = 35.9 ± 3.0; n = 31; P < 0.001). In comparison with HD, SSc MDM exhibited a downregulated expression of scavenger receptor (SR)-B1, SR-A1 and integrin β5 (ITGβ5). In HD MDM, the extinction of these receptors was followed by a reduction of efferocytosis only for the repression of ITGβ5, suggesting a possible selective role of this integrin in the impaired efferocytosis observed in SSc. As efferocytosis may be at the crossroads of inflammation, autoimmunity and fibrosis, in showing impaired efferocytosis capacities of blood MDM in SSc, our study offers new pathogenesis considerations for the involvement of macrophages in the autoimmune processes driving this disorder.

Distinct Properties of Human M-CSF and GM-CSF Monocyte-Derived Macrophages to Simulate Pathological Lung Conditions In Vitro: Application to Systemic and Inflammatory Disorders with Pulmonary Involvement

Macrophages play a central role in the pathogenesis of inflammatory and fibrotic lung diseases. However, alveolar macrophages (AM) are poorly available in humans to perform in vitro studies due to a limited access to broncho-alveolar lavage (BAL). In this study, to identify the best alternative in vitro model for human AM, we compared the phenotype of AM obtained from BAL of patients suffering from three lung diseases (lung cancers, sarcoidosis and Systemic Sclerosis (SSc)-associated interstitial lung disease) to human blood monocyte-derived macrophages (MDMs) differentiated with M-CSF or GM-CSF. The expression of eight membrane markers was evaluated by flow cytometry. Globally, AM phenotype was closer to GM-CSF MDMs. However, the expression levels of CD163, CD169, CD204, CD64 and CD36 were significantly higher in SSc-ILD than in lung cancers. Considering the expression of CD204 and CD36, the phenotype of SSc-AM was closer to MDMs, from healthy donors or SSc patients, differentiated by M-CSF rather than GM-CSF. The comparative secretion of IL-6 by SSc-MDMs and SSc-AM is concordant with these phenotypic considerations. Altogether, these results support the M-CSF MDM model as a relevant in vitro alternative to simulate AM in fibrotic disorders such as SSc.

Repression of interferon β-regulated cytokines by the JAK1/2 inhibitor ruxolitinib in inflammatory human macrophages

Ruxolitinib is a Janus kinase (JAK) 1/2 inhibitor, currently used in the treatment of myeloproliferative neoplasms. It exerts potent anti-inflammatory activity, but the involved molecular and cellular mechanisms remain poorly understood. In order to gain insights about this point, ruxolitinib effects towards expression of main inflammatory cytokines were studied in human macrophages, which constitute a key-cell type implicated in inflammation. Analysis of mRNA expression of cytokines (n=84) by PCR array indicated that, among those induced by the pro-inflammatory stimulus lipopolysaccharide (LPS) (n=44), 61.4% (n=27) were repressed by 5μM ruxolitinib. The major inflammatory cytokines, interleukin (IL) 6 and tumor necrosis factor α, were notably down-regulated by ruxolitinib at both the mRNA and protein level. Other repressed cytokines included IL27 and the chemokines CCL2, CXCL9, CXCL10 and CXCL11, but not IL1β. The interferon (IFN) β/JAK/signal transducer and activator of transcription (STAT) pathway, well-activated by LPS in human macrophages as demonstrated by increased secretion of IFNβ, STAT1 phosphorylation, and up-regulation of reference IFNβ-responsive genes, was concomitantly blocked by the JAK inhibitor. Most of cytokines targeted by ruxolitinib were shown to be regulated by IFNβ in a JAK-sensitive manner. In addition, counteracting the IFNβ/JAK/STAT cascade using a blocking monoclonal antibody directed against IFNβ receptor resulted in a similar profile of cytokine repression to that observed in response to the JAK inhibitor. Overall, these data provide evidence for ruxolitinib-mediated repression of inflammatory cytokines in human macrophages through inhibition of the LPS/IFNβ/JAK/STAT signalling pathway, which probably contributes to the anti-inflammatory effects of the JAK inhibitor.

Mutation of the oxaloacetate decarboxylase gene of Lactococcus lactis subsp. lactis impairs the growth during citrate metabolism

AIMS

Citrate metabolism generates metabolic energy through the generation of a membrane potential and a pH gradient. The purpose of this work was to study the influence of oxaloacetate decarboxylase in citrate metabolism and intracellular pH maintenance in relation to acidic conditions.

METHODS AND RESULTS

A Lactococcus lactis oxaloacetate decarboxylase mutant [ILCitM (pFL3)] was constructed by double homologous recombination. During culture with citrate, and whatever the initial pH, the growth rate of the mutant was lower. In addition, the production of diacetyl and acetoin was altered in the mutant strain. However, our results indicated no relationship with a change in the maintenance of intracellular pH. Experiments performed on resting cells clearly showed that oxaloacetate accumulated temporarily in the supernatant of the mutant. This accumulation could be involved in the perturbations observed during citrate metabolism, as the addition of oxaloacetate in M17 medium inhibited the growth of L. lactis.

CONCLUSIONS

The mutation of oxaloacetate decarboxylase perturbed citrate metabolism and reduced the benefits of its utilization during growth under acidic conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study allows a better understanding of citrate metabolism and the role of oxaloacetate decarboxylase in the tolerance of lactic acid bacteria to acidic conditions.

Effect of nitrogen limitation and nature of the feed upon Oenococcus oeni metabolism and extracellular protein production

AIMS

The aim of the study was to characterize the effect of various nitrogen sources on Oenococcus oeni growth, carbon source utilization, extracellular protease activity and extracellular proteins. More generally, the goal is to understand how nitrogen-based additives might act to enhance malolactic fermentation in wine.

METHODS AND RESULTS

Five yeast extracts were used. As the amino acid and nitrogen analyses revealed, they were similar in global amino acid composition, except for arginine level. Nevertheless the ratio of amino acids between free/bound, and low/high molecular weight fractions were highly different. One of the yeast extracts led to a significant protease activity in the supernatant and to a poor final biomass of the IOB84.13 strain compared to the other ones. For the IOB84.13 strain specifically, arginine addition to the arginine poor yeast extract did not restore growth. 35S-methionine-labelled extracellular proteins were separated by SDS-PAGE. Signals were detected in all media early in the growth phase and were maintained during 48 h of culture.

CONCLUSIONS

A significant protease activity was detected for O. oeni supernatants during growth under nitrogen limitation but only for certain nitrogen sources. Moreover, the activity was strain dependent. Peptides (0.5-10 kDa) seemed to be more favourable for growth of wine bacteria than <0.5 kDa nitrogen sources. The extracellular protein signal patterns differed more greatly between the bacterial strains tested than between the nitrogen molecules in the medium.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study extensively considering the role of the nitrogen source composition and level upon O. oeni growth and metabolism.