François Trottein

Identification of an IL-17–producing NK1.1neg iNKT cell population involved in airway neutrophilia

Invariant natural killer T (iNKT) cells are an important source of both T helper type 1 (Th1) and Th2 cytokines, through which they can exert beneficial, as well as deleterious, effects in a variety of inflammatory diseases. This functional heterogeneity raises the question of how far phenotypically distinct subpopulations are responsible for such contrasting activities. In this study, we identify a particular set of iNKT cells that lack the NK1.1 marker (NK1.1neg) and secrete high amounts of interleukin (IL)-17 and low levels of interferon (IFN)-γ and IL-4. NK1.1neg iNKT cells produce IL-17 upon synthetic (α-galactosylceramide [α-GalCer] or PBS-57), as well as natural (lipopolysaccharides or glycolipids derived from Sphingomonas wittichii and Borrelia burgdorferi), ligand stimulation. NK1.1neg iNKT cells are more frequent in the lung, which is consistent with a role in the natural immunity to inhaled antigens. Indeed, airway neutrophilia induced by α-GalCer or lipopolysaccharide instillation was significantly reduced in iNKT-cell–deficient Jα18−/− mice, which produced significantly less IL-17 in their bronchoalveolar lavage fluid than wild-type controls. Furthermore, airway neutrophilia was abolished by a single treatment with neutralizing monoclonal antibody against IL-17 before α-GalCer administration. Collectively, our findings reveal that NK1.1neg iNKT lymphocytes represent a new population of IL-17–producing cells that can contribute to neutrophil recruitment through preferential IL-17 secretion.

Peroxisome proliferator-activated receptor γ activators affect the maturation of human monocyte-derived dendritic cells

Peroxisome proliferator‐activated receptor γ  (PPARγ ), a member of the nuclear receptor superfamily, has recently been described as a modulator of macrophage functions and as an inhibitor of T cell proliferation. Here, we investigated the role of PPARγ  in dendritic cells (DC), the most potent antigen‐presenting cells. We showed that PPARγ  is highly expressed in immature human monocyte‐derived DC (MDDC) and that it may affect the immunostimulatory function of MDDC stimulated with lipopolysaccharide (LPS) or via CD40 ligand (CD40L). We found that the synthetic PPARγ  agonist rosiglitazone (as well as pioglitazone and troglitazone) significantly increases on LPS‐ and CD40L‐activated MDDC, the surface expression of CD36 (by 184% and 104%, respectively) and CD86 (by 54% and 48%), whereas it reduces the synthesis of CD80 (by 42% and 42%). Moreover, activation of PPARγ  resulted in a dramatic decreased secretion of the Th1‐promoting factor IL‐12 in LPS‐ and CD40L‐stimulated cells (by 47% and 62%), while the production of IL‐1β , TNF‐α , IL‐6 and IL‐10 was unaffected. Finally, PPARγ  ligands down‐modulate the synthesis of IFN‐γ ‐inducible protein‐10 (recently termed as CXCL10) and RANTES (CCL5), both chemokines involved in the recruitment of Th1 lymphocytes (by 49% and 30%), but not the levels of the Th2 cell‐attracting chemokines,macrophage‐derived chemokine (CCL22) and thymus and activation regulated chemokine (CCL17), in mature MDDC. Taken together, our data suggest that activation of PPARγ  in human DC may have an impact in the orientation of primary and secondary immune responses by favoring type 2 responses.

The role of PPARs in atherosclerosis

Peroxisome proliferator-activated receptors (PPARs) are lipid-activated transcription factors that regulate lipid and lipoprotein metabolism, glucose homeostasis and inflammation. The PPAR family consists of three proteins, alpha, beta/delta and gamma. Recent data suggest that PPAR alpha and gamma activation decreases atherosclerosis progression not only by correcting metabolic disorders, but also through direct effects on the vascular wall. PPARs modulate the recruitment of leukocytes to endothelial cells, control the inflammatory response and lipid homeostasis of monocytes/macrophages and regulate inflammatory cytokine production by smooth muscle cells. Experiments using animal models of atherosclerosis and clinical studies in humans strongly support an anti-atherosclerotic role for PPAR alpha and gamma in vivo. Thus, PPARs remain attractive therapeutic targets for the development of drugs used in the treatment of chronic inflammatory diseases such as atherosclerosis. Future research will aim for the development of more potent drugs with co-agonist activity on PPAR alpha, PPAR beta/delta and/or PPAR gamma as well as tissue and target gene-selective PPAR receptor modulators (SPPARMs).

Specific recognition of Candida albicans by macrophages requires galectin-3 to discriminate Saccharomyces cerevisiae and needs association with TLR2 for signaling.

Stimulation of cells of the macrophage lineage is a crucial step in the sensing of yeasts by the immune system. Glycans present in both Candida albicans and Saccharomyces cerevisiae cell walls have been shown to act as ligands for different receptors leading to different stimulating pathways, some of which need receptor co-involvement. However, among these ligand-receptor couples, none has been shown to discriminate the pathogenic yeast C. albicans. We explored the role of galectin-3, which binds C. albicans β-1,2 mannosides. These glycans are specifically and prominently expressed at the surface of C. albicans but not on S. cerevisiae. Using a mouse cell line and galectin-3-deleted cells from knockout mice, we demonstrated a specific enhancement of the cellular response to C. albicans compared with S. cerevisiae, which depended on galectin-3 expression. However, galectin-3 was not required for recognition and endocytosis of yeasts. In contrast, using PMA-induced differentiated THP-1, we observed that the presence of TLR2 was required for efficient uptake and endocytosis of both C. albicans and S. cerevisiae. TLR2 and galectin-3, which are expressed at the level of phagosomes containing C. albicans, were shown to be associated in differentiated macrophages after incubation with this sole species. These data suggest that macrophages differently sense C. albicans and S. cerevisiae through a mechanism involving TLR2 and galectin-3, which probably associate for binding of ligands expressing β-1,2 mannosides specific to the C. albicans cell wall surface.

Activation of the Prostaglandin D2 Receptor DP2/CRTH2 Increases Allergic Inflammation in Mouse

Allergic pathologies are often associated with IgE production, mast cell activation, and eosinophilia. PGD2 is the major eicosanoid, among several inflammatory mediators, released by mast cells. PGD2 binds to two membrane receptors, D prostanoid receptor (DP)1 and DP2, endowed with antagonistic properties. In humans, DP2 is preferentially expressed on type 2 lymphocytes, eosinophils, and basophils and mediates chemotaxis in vitro. Although not yet supported by in vivo studies, DP2 is thought to be important in the promotion of Th2-related inflammation. Herein, we demonstrate that mouse eosinophils express both DP1 and DP2 and that PGD2 exerts in vitro chemotactic effects on eosinophils through DP2 activation. Furthermore, 13,14-dihydro-15-keto-PGD2, a specific DP2 agonist not only increases eosinophil recruitment at inflammatory sites but also the pathology in two in vivo models of allergic inflammation: atopic dermatitis and allergic asthma. By contrast, DP1 activation tends to ameliorate the pathology in asthma. Taken together, these results support the hypothesis that DP2 might play a critical role in allergic diseases and underline the interest of DP2 antagonists in human therapy.

The orphan nuclear receptor RORα is a negative regulator of the inflammatory response

Retinoid‐related orphan receptor α (RORα) (NR1F1) is a member of the nuclear receptor superfamily whose biological functions are largely unknown. Since staggerer mice, which carry a deletion in the RORα gene, suffer from immune abnormalities, we generated an adenovirus encoding RORα1 to investigate its potential role in control of the inflammatory response. We demonstrated that RORα is expressed in human primary smooth‐muscle cells and that ectopic expression of RORα1 inhibits TNFα‐induced IL‐6, IL‐8 and COX‐2 expression in these cells. RORα1 negatively interferes with the NF‐κB signalling pathway by reducing p65 translocation as demonstrated by western blotting, immunostaining and electrophoretic mobility shift assays. This action of RORα1 on NF‐κB is associated with the induction of IκBα, the major inhibitory protein of the NF‐κB signalling pathway, whose expression was found to be transcriptionally upregulated by RORα1 via a ROR response element in the IκBα promoter. Taken together, these data identify RORα1 as a potential target in the treatment of chronic inflammatory diseases, including atherosclerosis and rheumatoid arthritis.

Prostaglandin D2 Inhibits Airway Dendritic Cell Migration and Function in Steady State Conditions by Selective Activation of the D Prostanoid Receptor 1

PGD2 is the major mediator released by mast cells during allergic responses, and it acts through two different receptors, the D prostanoid receptor 1 (DP1) and DP2, also known as CRTH2. Recently, it has been shown that PGD2 inhibits the migration of epidermal Langerhans cells to the skin draining lymph nodes (LNs) and affects the subsequent cutaneous inflammatory reaction. However, the role of PGD2 in the pulmonary immune response remains unclear. Here, we show that the intratracheal instillation of FITC-OVA together with PGD2 inhibits the migration of FITC+ lung DC to draining LNs. This process is mimicked by the DP1 agonist BW245C, but not by the DP2 agonist DK-PGD2. The ligation of DP1 inhibits the migration of FITC-OVA+ DCs only temporarily, but still inhibits the proliferation of adoptively transferred, OVA-specific, CFSE-labeled, naive T cells in draining LNs. These T cells produced lower amounts of the T cell cytokines IL-4, IL-10, and IFN-γ compared with T cells from mice that received FITC-OVA alone. Taken together, our data suggest that the activation of DP receptor by PGD2 may represent a pathway to control airway DC migration and to limit the activation of T cells in the LNs under steady state conditions, possibly contributing to homeostasis in the lung.

Peroxisome proliferator-activated receptor γ activators inhibit interleukin-12 production in murine dendritic cells

Peroxisome proliferator‐activated receptors (PPARs) are members of the nuclear receptor superfamily. They are divided into three subtypes (α, β or δ, and γ) and are involved in lipid and glucose homeostasis and in the control of inflammation. In this study, we analyzed the expression of PPARs in murine dendritic cells (DCs), the most potent antigen presenting cells. We find that immature as well as mature spleen‐derived DCs express PPARγ, but not PPARα, mRNA and protein. We also show that the PPARγ activator rosiglitazone does not interfere with the maturation of DCs in vitro nor modifies their ability to activate naive T lymphocytes in vivo. Finally, we present evidence that PPARγ activators down‐modulate the CD40‐induced secretion of interleukin‐12, a potent Th1‐driving factor. These data suggest a possible role for PPARγ in the regulation of immune responses.

Activation of the D prostanoid 1 receptor suppresses asthma by modulation of lung dendritic cell function and induction of regulatory T cells

Prostaglandins (PGs) can enhance or suppress inflammation by acting on different receptors expressed by hematopoietic and nonhematopoietic cells. Prostaglandin D2 binds to the D prostanoid (DP)1 and DP2 receptor and is seen as a critical mediator of asthma causing vasodilation, bronchoconstriction, and inflammatory cell influx. Here we show that inhalation of a selective DP1 agonist suppresses the cardinal features of asthma by targeting the function of lung dendritic cells (DCs). In mice treated with DP1 agonist or receiving DP1 agonist-treated DCs, there was an increase in Foxp3+ CD4+ regulatory T cells that suppressed inflammation in an interleukin 10–dependent way. These effects of DP1 agonist on DCs were mediated by cyclic AMP–dependent protein kinase A. We furthermore show that activation of DP1 by an endogenous ligand inhibits airway inflammation as chimeric mice with selective hematopoietic loss of DP1 had strongly enhanced airway inflammation and antigen-pulsed DCs lacking DP1 were better at inducing airway T helper 2 responses in the lung. Triggering DP1 on DCs is an important mechanism to induce regulatory T cells and to control the extent of airway inflammation. This pathway could be exploited to design novel treatments for asthma.

Activation of Peroxisome Proliferator-Activated Receptor-γ in Dendritic Cells Inhibits the Development of Eosinophilic Airway Inflammation in a Mouse Model of Asthma

Peroxisome proliferator-activated receptors (PPARs) are activated by an array of polyunsaturated fatty acid derivatives, oxidized fatty acids, and phospholipids and are proposed to be important modulators of immune and inflammatory responses. Recently, we showed that activation of PPAR-γ alters the maturation process of dendritic cells (DCs), the most potent antigen-presenting cells. In the present report, we investigated the possibility that, by targeting DCs, PPAR-γ activation may be involved in the regulation of the pulmonary immune response to allergens. Using a model of sensitization, based on the intratracheal transfer of ovalbumin (OVA)-pulsed DCs, we show that rosiglitazone, a selective PPAR-γ agonist, reduces the proliferation of Ag-specific T cells in the draining mediastinal lymph nodes but, surprisingly enough, dramatically increases the production of the immunoregulatory cytokine interleukin (IL)-10 by T cells, as compared to control mice sensitized with OVA-pulsed DCs. After aerosol challenge, the recruitment of eosinophils in the bronchoalveolar lavage fluids was strongly reduced compared to control mice. Finally, T cells from the mediastinal lymph nodes produced higher amounts of IL-10 and interferon-γ. Inhibition of IL-10 activity with anti-IL-10R antibodies partly restored the inflammation. The specificity of the phenomenon was confirmed by treating OVA-pulsed DCs with ciglitazone, another PPAR-γ agonist, and by using GW9662, a PPAR-γ antagonist. Our data suggest that PPAR-γ activation prevents induction of Th2-dependent eosinophilic airway inflammation and might contribute to immune homeostasis in the lung.