Daiji Sakata

Prostaglandin E2-EP4 signaling promotes immune inflammation through Th1 cell differentiation and Th17 cell expansion.

Two distinct helper T (TH) subsets, TH1 and TH17, mediate tissue damage and inflammation in animal models of various immune diseases such as multiple sclerosis, rheumatoid arthritis, inflammatory bowel diseases and allergic skin disorders. These experimental findings, and the implication of these TH subsets in human diseases, suggest the need for pharmacological measures to manipulate these TH subsets. Here we show that prostaglandin E2 (PGE2) acting on its receptor EP4 on T cells and dendritic cells not only facilitates TH1 cell differentiation but also amplifies interleukin-23–mediated TH17 cell expansion in vitro. Administration of an EP4-selective antagonist in vivo decreases accumulation of both TH1 and TH17 cells in regional lymph nodes and suppresses the disease progression in mice subjected to experimental autoimmune encephalomyelitis or contact hypersensitivity. Thus, PGE2-EP4 signaling promotes immune inflammation through TH1 differentiation and TH17 expansion, and EP4 antagonism may be therapeutically useful for various immune diseases.

Prostaglandin E2-EP4 signaling initiates skin immune responses by promoting migration and maturation of Langerhans cells.

Antigen-specific immune responses in the skin are initiated by antigen uptake into Langerhans cells and the subsequent migration of these cells to draining lymph nodes. Although prostaglandin E2 (PGE2) is produced substantially in skin exposed to antigen, its role remains unclear. Here we show that although Langerhans cells express all four PGE receptor subtypes, their migration to regional lymph nodes was decreased only in EP4-deficient (Ptger4−/−) mice and in wild-type mice treated with an EP4 antagonist. An EP4 agonist promoted the migration of Langerhans cells, increased their expression of costimulatory molecules and enhanced their ability to stimulate T cells in the mixed lymphocyte reaction in vitro. Contact hypersensitivity to antigen was impaired in Ptger4−/− mice and in wild-type mice treated with the EP4 antagonist during sensitization. PGE2-EP4 signaling thus facilitates initiation of skin immune responses by promoting the migration and maturation of Langerhans cells.

Thromboxane A2 modulates interaction of dendritic cells and T cells and regulates acquired immunity

Physical interaction of T cells and dendritic cells (DCs) is essential for T cell proliferation and differentiation, but it has been unclear how this interaction is regulated physiologically. Here we show that DCs produce thromboxane A2 (TXA2), whereas naive T cells express the thromboxane receptor (TP). In vitro, a TP agonist enhances random cell movement (chemokinesis) of naive but not memory T cells, impairs DC–T cell adhesion, and inhibits DC-dependent proliferation of T cells. In vivo, immune responses to foreign antigens are enhanced in TP-deficient mice, which also develop marked lymphadenopathy with age. Similar immune responses were seen in wild-type mice treated with a TP antagonist during the sensitization period. Thus, TXA2-TP signaling modulates acquired immunity by negatively regulating DC–T cell interactions.

Prostaglandin F 2α receptor signaling facilitates bleomycin-induced pulmonary fibrosis independently of transforming growth factor-β

Idiopathic pulmonary fibrosis (IPF) is a progressive disease characterized by fibroblast proliferation and excess deposition of collagen and other extracellular matrix (ECM) proteins, which lead to distorted lung architecture and function. Given that anti-inflammatory or immunosuppressive therapy currently used for IPF does not improve disease progression therapies targeted to blocking the mechanisms of fibrogenesis are needed. Although transforming growth factor-β (TGF-β) functions are crucial in fibrosis, antagonizing this pathway in bleomycin-induced pulmonary fibrosis, an animal model of IPF, does not prevent fibrosis completely, indicating an additional pathway also has a key role in fibrogenesis. Given that the loss of cytosolic phospholipase A2 (cPLA2) suppresses bleomycin-induced pulmonary fibrosis, we examined the roles of prostaglandins using mice lacking each prostoaglandin receptor. Here we show that loss of prostaglandin F (PGF) receptor (FP) selectively attenuates pulmonary fibrosis while maintaining similar levels of alveolar inflammation and TGF-β stimulation as compared to wild-type (WT) mice, and that FP deficiency and inhibition of TGF-β signaling additively decrease fibrosis. Furthermore, PGF2α is abundant in bronchoalveolar lavage fluid (BALF) of subjects with IPF and stimulates proliferation and collagen production of lung fibroblasts via FP, independently of TGF-β. These findings show that PGF2α-FP signaling facilitates pulmonary fibrosis independently of TGF-β and suggests this signaling pathway as a therapeutic target for IPF.

Prostaglandin E2–prostoglandin E receptor subtype 4 (EP4) signaling mediates UV irradiation-induced systemic immunosuppression

UV radiation induces systemic immunosuppression. Because nonsteroidal anti-inflammatory drugs suppress UV-induced immunosuppression, prostanoids have been suspected as a crucial mediator of this UV effect. However, the identity of the prostanoid involved and its mechanism of action remain unclear. Here, we addressed this issue by subjecting mice deficient in each prostanoid receptor individually or mice treated with a subtype-specific antagonist to UV irradiation. Mice treated with an antagonist for prostaglandin E receptor subtype 4 (EP4), but not those deficient in other prostanoid receptors, show impaired UV-induced immunosuppression, whereas administration of an EP4 agonist rescues the impairment of the UV-induced immunosuppression in indomethacin-treated mice. The EP4 antagonist treatment suppresses an increase in the number of CD4+/forkhead box P3-positive (Foxp3+) regulatory T cells (Treg cells) in the peripheral lymph nodes (LNs) and dendritic cells expressing DEC205 in the LNs and the skin after UV irradiation. Furthermore, the EP4 antagonist treatment down-regulates UV-induced expression of receptor activator of NF-κB ligand (RANKL) in skin keratinocytes. Finally, administration of anti-RANKL antibody abolishes the restoration of UV-induced immunosuppression by EP4 agonism in indomethacin-treated mice. Thus, prostaglandin E2 (PGE2)–EP4 signaling mediates UV-induced immunosuppression by elevating the number of Treg cells through regulation of RANKL expression in the epidermis.

Dual roles of PGE2-EP4 signaling in mouse experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is an animal model of multiple sclerosis (MS). Although prostaglandin (PG) concentrations are increased in cerebrospinal fluid of MS patients, the role of PGs in MS is unknown. We examined this issue by subjecting mice deficient in each PG receptor type or subtype to EAE induction and using agonists or antagonists selective for each of the four PGE receptor (EP) subtypes. Among PG receptor-deficient mice, only EP4−/− mice manifested significant suppression of EAE, which was mimicked in wild-type mice and to a greater extent, in EP2−/− mice by administration of the EP4 antagonist ONO-AE3-208 during the immunization phase. EP4 antagonism during immunization also suppressed the generation of antigen-specific T helper (Th) 1 and Th17 cells in wild-type mice and to a greater extent, in EP2−/− mice. ONO-AE3-208 administration at EAE onset had little effect on disease severity, and its administration throughout the experimental period did not cause significant reduction of the peak of disease, suggesting that, in addition to its facilitative action during the immunization phase, EP4 exerts a preventive action in the elicitation phase. Administration of the EP4 agonist ONO-AE1-329 at EAE onset delayed and suppressed disease progression as well as inhibited the associated increase in permeability of the blood–brain barrier. Thus, PGE2 exerts dual functions in EAE, facilitating Th1 and Th17 cell generation redundantly through EP4 and EP2 during immunization and attenuating invasion of these cells into the brain by protecting the blood–brain barrier through EP4.

Plasmodium falciparum Phospholipase C Hydrolyzing Sphingomyelin and Lysocholinephospholipids Is a Possible Target for Malaria Chemotherapy

Sphingomyelinase (SMase) is one of the principal enzymes in sphingomyelin (SM) metabolism. Here, we identified a Plasmodium falciparum gene (PfNSM) encoding a 46-kD protein, the amino acid sequence of which is ∼25% identical to that of bacteria SMases. Biochemical analyses of the recombinant protein GST-PfNSM, a fusion protein of the PfNSM product with glutathione-S-transferase, reveal that this enzyme retained similar characteristics in various aspects to SMase detected in P. falciparum–infected erythrocytes and isolated parasites. In addition, the recombinant protein retains hydrolyzing activity not only of SM but also of lysocholinephospholipids (LCPL) including lysophosphatidylcholine and lysoplatelet-activating factor, indicating that PfNSM encodes SM/LCPL-phospholipase C (PLC). Scyphostatin inhibited SM/LCPL-PLC activities of the PfNSM product as well as the intraerythrocytic proliferation of P. falciparum in a dose-dependent manner with ID50 values for SM/LCPL-PLC activities and the parasite growth at 3–5 μM and ∼7 μM, respectively. Morphological analysis demonstrated most severe impairment in the intraerythrocytic development with the addition of scyphostatin at trophozoite stage than at ring or schizont stages, suggesting its effect specifically on the stage progression from trophozoite to schizont, coinciding with the active transcription of PfNSM gene.

Facilitation of Th1-mediated immune response by prostaglandin E receptor EP1

Prostaglandin E2 (PGE2) exerts its actions via four subtypes of the PGE receptor, EP1–4. We show that mice deficient in EP1 exhibited significantly attenuated Th1 response in contact hypersensitivity induced by dinitrofluorobenzene (DNFB). This phenotype was recapitulated in wild-type mice by administration of an EP1-selective antagonist during the sensitization phase, and by adoptive transfer of T cells from sensitized EP1−/− mice. Conversely, an EP1-selective agonist facilitated Th1 differentiation of naive T cells in vitro. Finally, CD11c+ cells containing the inducible form of PGE synthase increased in number in the draining lymph nodes after DNFB application. These results suggest that PGE2 produced by dendritic cells in the lymph nodes acts on EP1 in naive T cells to promote Th1 differentiation.

SIRPα polymorphisms, but not the prion protein, control phagocytosis of apoptotic cells

The regulation of phagocytosis previously ascribed to prion protein PrPC is found to be controlled by the linked locus encoding SIRPα.

Prostaglandin I2–IP Signaling Promotes Th1 Differentiation in a Mouse Model of Contact Hypersensitivity

PGI2, which exerts its actions via its specific Gs-coupled I prostanoid receptor (IP), is known to be present in the lymph nodes, but its roles in acquired cutaneous immune responses remain unclear. To investigate the role of PGI2–IP signaling in cutaneous immune responses, we applied IP-deficient (Ptgir−/−) mice to contact hypersensitivity as a model of acquired immune response and found that Ptgir−/− mice exhibited a significantly decreased contact hypersensitivity response. Lymph node cells from sensitized Ptgir−/− mice exhibited decreased IFN-γ production and a smaller T-bet+ subset compared with control mice. PGI synthase and IP expression were detected in dendritic cells and T cells, respectively, by quantitative real-time PCR analysis, suggesting that PGI2 produced by dendritic cells acts on IP in T cells. In fact, in vitro Th1 differentiation was enhanced by an IP agonist, and this enhancement was nullified by protein kinase A inhibitor. These results suggest that PGI2–IP signaling promotes Th1 differentiation through a cAMP-protein kinase A pathway and thereby initiates acquired cutaneous immune responses.