Hedi Harizi

Cyclooxygenase-2-Issued Prostaglandin E2 Enhances the Production of Endogenous IL-10, Which Down-Regulates Dendritic Cell Functions

PGE2 is a well-known immunomodulator produced in the immune response by APCs, such as dendritic cells (DCs), the most potent APC of the immune system. We investigated the PGE2 biosynthetic capacity of bone marrow-derived DC (BM-DC) and the effects of PG on the APC. We observed that BM-DC produce PGE2 and other proinflammatory mediators, such as leukotriene B4 and NO, after LPS exposure. Constitutively present in BM-DC, cyclooxygenase (COX)-1 did not contribute significantly to the total pool of PGE2 compared with the LPS-induced COX-2-produced PGE2. Treatment of BM-DC with exogenous PGE2 induced the production of large amounts of IL-10 and less IL-12p70. In addition, selective inhibition of COX-2, but not COX-1, was followed by significant decrements in PGE2 and IL-10, a concomitant restoration of IL-12 production, and an enhancement of DC stimulatory potential. In contrast, we found no demonstrable role for leukotriene B4 or NO. In view of the potential of PGE2 to stimulate IL-10, we examined the possibility that the suppressive effect of PGE2 is mediated via IL-10. We found that exogenous IL-10 inhibits IL-12p70 production in the presence of NS-398, a COX-2 selective inhibitor, while the inhibitory effects of PGE2 were totally reversed by anti-IL-10. We conclude that COX-2-mediated PGE2 up-regulates IL-10, which down-regulates IL-12 production and the APC function of BM-DC.

Arachidonic-acid-derived eicosanoids: roles in biology and immunopathology

Arachidonic acid (AA)-derived eicosanoids belong to a complex family of lipid mediators that regulate a wide variety of physiological responses and pathological processes. They are produced by various cell types through distinct enzymatic pathways and act on target cells via specific G-protein-coupled receptors. Although originally recognized for their capacity to elicit biological responses such as vascular homeostasis, protection of the gastric mucosa and platelet aggregation, eicosanoids are now understood to regulate immunopathological processes ranging from inflammatory responses to chronic tissue remodelling, cancer, asthma, rheumatoid arthritis and autoimmune disorders. Here, we review the major properties of eicosanoids and their expanding roles in biology and medicine.

Prostaglandin E2 modulates dendritic cell function via EP2 and EP4 receptor subtypes

We have reported previously that PGE2 inhibits dendritic cells (DC) functions. Because E prostanoid receptor (EPR) subtypes involved in this action are unknown, expression and functions of these receptors were examined in DC. Western blot and flow cytometry analyses showed that all EPRs were coexpressed in DC. In a dose‐dependent manner, lipopolysaccharide (LPS) enhanced EP2R/EP4R but not EP1R/EP3R expressions. NS‐398, a cyclooxygenase (COX)‐2‐selective inhibitor, suppressed LPS‐enhanced EP2R/EP4R expression, suggesting that COX‐2‐issued prostaglandin E2 (PGE2) modulates DC function through stimulation of specific EPR subtypes. Using selective agonists, we found that butaprost, an EP2R agonist, and PGE1 alcohol, an EP2R and EP2R/EP4R agonist, inhibited major histocompatibility complex class II expression and enhanced interleukin‐10 production from DC. However, no effect was observed with sulprostone and 17‐phenyl‐ω‐trinor‐PGE2, selective agonists for EP1R and EP1R/EP3R, respectively. Treatment of DC with dibutyryl cyclic adenosine monophosphate (cAMP), an analog of cAMP, mimics PGE2‐induced, inhibitory effects. Taken together, our data demonstrate that EP2R/EP4R are efficient for mediating PGE2‐induced modulation of DC functions.

Prostanoids and their receptors that modulate dendritic cell-mediated immunity.

Dendritic cells (DC) are essential for the initiation of immune responses by capturing, processing and presenting antigens to T cells. In addition to their important role as professional APC, they are able to produce immunosuppressive and pro‐inflammatory prostanoids from arachidonic acid (AA) by the action of cyclooxygenase (COX) enzymes. In an autocrine and paracrine fashion, the secreted lipid mediators subsequently modulate the maturation, cytokine production, Th‐cell polarizing ability, chemokine receptor expression, migration, and apoptosis of these extremely versatile APC. The biological actions of prostanoids, including their effects on APC‐mediated immunity and acute inflammatory responses, are exerted by G protein‐coupled receptors on plasma membrane. Some COX metabolites act as anti‐inflammatory lipid mediators by binding to nuclear receptors and modulating DC functions. Although the role of cytokines in DC function has been studied extensively, the effects of prostanoids on DC biology have only recently become the focus of investigation. This review summarizes the current knowledge about the role of prostanoids and their receptors in modulating DC function and the subsequent immune responses.

Reciprocal crosstalk between dendritic cells and natural killer cells under the effects of PGE2 in immunity and immunopathology.

The reciprocal activating crosstalk between dendritic cells (DCs) and natural killer (NK) cells plays a pivotal role in regulating immune defense against viruses and tumors. The cytokine-producing capacity, Th-cell polarizing ability and chemokine expression, migration and stimulatory functions of DCs are regulated by activated NK cells. Conversely, the innate and effector functions of NK cells require close interactions with activated DCs. Cell membrane-associated molecules and soluble mediators, including cytokines and prostaglandins (PGs), contribute to the bidirectional crosstalk between DCs and NK cells. One of the most well-known and well-studied PGs is PGE2. Produced by many cell types, PGE2 has been shown to affect various aspects of the immune and inflammatory responses by acting on all components of the immune system. There is emerging evidence that PGE2 plays crucial roles in DC and NK cell biology. Several studies have shown that DCs are not only a source of PGE2, but also a target of its immunomodulatory action in normal immune response and during immune disorders. Although NK cells appear to be unable to produce PGE2, they are described as powerful PGE2-responding cells, as they express all PGE2 E-prostanoid (EP) receptors. Several NK cell functions (lysis, migration, proliferation, cytokine production) are influenced by PGE2. This review highlights the effects of PGE2 on DC–NK cell crosstalk and its subsequent impact on immune regulations in normal and immunopathological processes.

Leaf extracts from Phlomis crinita Cav. subs. mauritanica Munby affect immune cell functions in vitro

Aqueous and methanolic leaf extracts from Phlomis crinita subs. mauritanica Munby were investigated for their potential immunomodulatory activity on mouse lymphocytes and macrophages in vitro. The phagocytic activity of macrophages and the proliferation of lymphocytes in the absence and presence of mitogens (lipopolysaccharide, LPS or lectin) were assayed. Depending on the concentrations, the extracts affect macrophage functions by modulating their lysosomal enzyme activity and nitric oxide release. For lymphocyte proliferation assay, both extracts enhance significantly cell proliferation either with or without mitogen stimulation. These results suggest that leaf extracts from P. crinita subs. mauritanica Munby contain potent components such as flavonoids that potentially may be useful for modulating immune cell functions in physiological and pathological conditions.

Inhibition of proinflammatory macrophage responses and lymphocyte proliferation in vitro by ethyl acetate leaf extract from Daphne gnidium

Medicinal plants are considered immunomodulatory as they display various biological activities. There is no report addressing the anti-inflammatory effects of Daphne gnidium. In this study, we investigated the effects of D. gnidium ethyl acetate (EA) leaf extract on mice immune cell function in vitro. Production of pro-inflammatory cytokines (IL-1β and TNF-α), cyclooxygenase-2-derived prostaglandinE2 (PGE2) and iNOS-II-synthesised nitric oxide (NO) were examined. EA extract effect on mitogen-induced lymphocyte proliferation was also investigated. We reported for the first time that D. gnidium EA leaf extract dose-dependently inhibits macrophage proinflammatory function by reducing LPS-induced production of IL-1β, TNF-α, COX-2-derived PGE2 and iNOS-II-synthesised NO. Mitogen-induced lymphocyte proliferation was also dose-dependently inhibited by the extract. Lectin-induced response appears to be more sensitive to the suppressive effects of the extract than LPS-stimulated response. Collectively, these results demonstrate that D. gnidium EA leaf extract acts as an in vitro anti-inflammatory factor by inhibiting mice macrophage and lymphocyte activities.

CD40 engagement on dendritic cells induces cyclooxygenase-2 and EP2 receptor via p38 and ERK MAPKs

We have previously reported that cyclooxygenase (COX)‐2‐derived prostaglandin (PG)E2 critically regulates dendritic cell (DC) inflammatory phenotype and function through EP2/EP4 receptor subtypes. As genes activated by CD40 engagement are directly relevant to inflammation, we examined the effects of CD40 activation on inflammatory PGs in murine bone marrow‐derived DC (mBM‐DC). We showed for the first time that activation of mBM‐DC with agonist anti‐CD40 monoclonal antibody (anti‐CD40 mAb) dose dependently induces the synthesis of significant amounts of PGE2 via inducible expression of COX‐2 enzyme, as NS‐398, a COX‐2‐selective inhibitor reduces this upregulation. In contrast to lipopolysaccharide, which upregulates mBM‐DC surface levels of EP2 and EP4 receptors, CD40 crosslinking on mBM‐DC increases EP2, but not EP4, receptor expression. Flow cytometry analysis and radioligand‐binding assay showed that EP2 was the major EP receptor subtype, which binds to PGE2 at the surface of anti‐CD40‐activated mBM‐DC. Upregulation of COX‐2 and EP2 levels by CD40 engagement was accompanied by dose‐dependent phosphorylation of p38 and ERK1/2 mitogen‐activated protein kinase (MAPK) and was abrogated by inhibitors of both pathways. Collectively, we demonstrated that CD40 engagement on mBM‐DC upregulates COX‐2 and EP2 receptor expression through activation of p38 and ERK1/2 MAPK signaling. Triggering the PGE2/EP2 pathway by anti‐CD40 mAb resulted on the induction of Th2 immune response. Thus, CD40‐induced production of PGE2 by mBM‐DC could represent a negative feedback mechanism involving EP2 receptor and limiting the propagation of Th1 responses. Blocking CD40 pathway may represent a novel therapeutic pathway of inhibiting COX‐2‐derived prostanoids in chronically inflamed tissues (that is, arthritis).

Anti-inflammatory and antiulcerogenic activities of leaf extracts and sesquiterpene from Teucrium ramosissimum (Lamiaceae)

Teucrium ramosissimum (Lamiaceae) is a native and endemic medicinal plant from South of Tunisia traditionally used for the treatment of many diseases. The anti-inflammatory and antiulcerogenic activities of sesquiterpene (β-eudesmol), chloroform, and ethyl acetate leaf extracts from T. ramosissimum were assayed. Macrophage phagocytic activity and lymphocyte proliferation in the absence and presence of mitogens (lipopolysaccharide [LPS] or lectin) were investigated. Depending on the concentrations, the extracts affect macrophage functions by modulating their lysosomal enzyme activity and nitric oxide (NO) release. For lymphocyte proliferation assay, tested extracts enhance significantly cell proliferation either with or without mitogen stimulation. These results suggest that leaf extracts from T. ramosissimum contain potent components such as flavonoids that may be potentially useful for modulating immune cell functions in physiological and pathological conditions. Antiulcerogenic activity was examined on rat ethanol-induced ulcerogenic model. Compared with control (cimetidine), leaf extracts from T. ramosissimum exert different protective effects against ethanol-induced ulcerogenesis.