Metastasis is the primary cause of cancer death. Weichand et al. describe a new mechanism explaining how tumor-associated macrophages contribute to metastatic spread, which involves promoting tumor lymphangiogenesis via S1P receptor 1 and the NLRP3 inflammasome.

The lipid sphingosine-1-phosphate (S1P) is a chemokine for a variety of immune cells including lymphocytes and monocytes. Migration toward S1P is determined by the S1P receptor expression profile, with S1PR1/3 (where S1PR is S1P receptor) stimulating and S1PR2 attenuating migration. However, the impact and physiological significance of S1P-induced migration of macrophages is largely unclear. We observed that alternative activation of human macrophages, by IL-4 or apoptotic cells (ACs), enhanced S1PR1 expression. Moreover, ACs provoked macrophage migration toward S1P in an S1PR1-dependent manner as confirmed by pharmacological receptor inhibition and S1PR1-deficient murine macrophages. In a mouse model of resolving peritoneal inflammation, F4/80-driven deletion of S1PR1 reduced postinflammatory macrophage emigration from inflammatory sites. S1PR1 expression on macrophages might, therefore, be relevant for restoring tissue homeostasis during the resolution of inflammation. Keywords: Macrophage activation r Peritonitis r Resolution of inflammation r Sphingolipids r ZymosanAdditional supporting information may be found in the online version of this article at the publisher's web-site IntroductionMacrophages are innate immune cells that originate from primitive hematopoietic progenitors or blood-derived monocytes during inflammation [1]. Unlike resident macrophages, which largely regulate their numbers via local proliferation, mobilized monocytederived macrophages, like dendritic cells (DCs), rely on their high motility to extravasate tissues during inflammation. Resolution of inflammation requires clearance of monocyte-derived macrophages excess from the inflammation sites, which includes their migration to the lymphatic system [2]. The molecular mechanisms that guide macrophages toward the lymphatic system are unknown. One possible molecule involved in the process Correspondence: Dr. Bernhard Brüne e-mail: b.bruene@biochem.uni-frankfurt.de is sphingosine-1-phosphate (S1P). Detectable quantities of this sphingolipid are, at steady-state conditions, present only in blood and lymph of mammals [3]. Migration toward S1P is controlled by the relative expression of five specific S1P receptors (S1PR1-5), which are partially redundant in their cellular signaling capacity. S1P coupling to S1PR1 and S1PR3 favors migration, whereas S1PR2 counteracts this process [3].Mononuclear phagocytes express multiple S1PRs [4], which are species, cell type, and activation specific. Human monocytes express S1PR1, 2, and 4. Human macrophages express S1PR1-4 [5], while murine macrophages express mainly S1PR1 and 2 [6]. S1P plays a role in macrophage migration in physiology and pathophysiology [7]. In inflammatory settings, S1PR2 inhibited and * These authors contributed equally to this article.www.eji-journal.eu Eur. J. Immunol. 2013. 43: 3306-3313 Innate Immunity 3307 S1P3 promoted macrophage recruitment during thioglycollateinduced peritonitis as well as their motility in vitro [6,8]. During maturation, DCs change S1PR expression from S...

Tumor-associated macrophages (TAMs) are a major supportive component within neoplasms. Mechanisms of macrophage (MΦ) attraction and differentiation to a tumor-promoting phenotype, which is characterized by pronounced interleukin (IL)-10 production, are under investigation. We report that supernatants of dying cancer cells induced substantial IL-10 release from primary human MΦs, dependent on signaling through tyrosine kinase receptor A (TRKA or neurotrophic tyrosine kinase receptor type 1 (NTRK1)). Mechanistically, sphingosine-1-phosphate (S1P) release from apoptotic cancer cells triggered src-dependent shuttling of cytosolic TRKA to the plasma membrane via S1P receptor signaling. Plasma membrane-associated TRKA, which was activated by constitutively autocrine secreted nerve growth factor, used phosphatidylinositol 3-kinase (PI3K)/AKT and p38 mitogen-activated protein kinase (MAPK) signaling to induce IL-10. Interestingly, TRKA-dependent signaling was required for cytokine production by TAMs isolated from primary murine breast cancer tissue. Besides IL-10, this pathway initiated secretion of IL-6, tumor necrosis factor-α (TNF-α) and monocyte chemotactic protein-1 (MCP-1), indicating relevance in cancer-associated inflammation. Our findings highlight a fine-tuned regulatory system including S1P-dependent TRKA trafficking for executing TAM-like cell function in vitro as well as in vivo.

The sphingolipid sphingosine-1-phosphate (S1P) is an important regulator of immune cell functions in vivo. Besides recruiting lymphocytes to blood and lymph, it may promote immune cell survival and proliferation, but also interferes with their activation. Hereby, S1P may act as an intracellular second messenger or cofactor or, upon being secreted from cells, may bind to and activate a family of specific G-protein-coupled receptors (S1PR1-5). Extracellular versus intracellular S1P hereby might trigger synergistic/identical or fundamentally distinct responses. Furthermore, engagement of different S1PRs is connected to different functional outcome. This complexity is exemplified by the influence of S1P on the inflammatory potential of macrophages, shaping their role in inflammatory pathologies such as atherosclerosis and cancer. Here, we summarize the recent progress in understanding the impact of S1P signaling in macrophage biology, discuss its impact in solid as well as 'wet' tumors and elaborate potential options to interfere with S1P signaling in the context of cancer.

IntroductionMammalian hematopoiesis in the BM is regulated among others by the oxygen (O 2 ) availability. O 2 concentrations in the BM range from anoxia to 6% opposed to 4%-14% in well-oxygenated tissues, including the blood. 1,2 Recent data indicate that O 2 gradients within the BM participate in keeping hematopoietic stem cells (HSCs) in a low-replicating pluripotent state. HSCs are located in an extremely hypoxic niche as demonstrated by dye-perfusion and engraftment studies. 3,4 Hypoxia-inducible factors 1-3 (HIF-1-HIF-3) are stabilized by a low pO 2 to induce adaptive gene expression. They are heterodimers consisting of distinct O 2 -sensitive ␣-subunits and a stable common ␤-subunit, also known as aryl hydrocarbon receptor nuclear translocator (ARNT). 5 HIF-1 and HIF-2 were recently connected to HSC biology. In their hypoxic niche, HIF-1 maintains HSC quiescence, 6 whereas HIF-2 maintains their self-renewing capacity. 7 HIF-1 also affects embryonic hematopoiesis as demonstrated by defective myeloid and erythroid progenitor formation in HIF-1␣ Ϫ/Ϫ as well as ARNT Ϫ/Ϫ embryos. 8 In adult hematopoiesis, HIF-1 is essential for B-cell progenitor proliferation and mature B-cell subclass differentiation. 9 However, its involvement in mononuclear phagocyte development is unknown. Previous findings indicated defective development of human plasmacytoid dendritic cells (pDCs) under hypoxia in vitro. 10 Therefore, we asked whether HIF-1 regulates DC lineage differentiation in mice. Methods AnimalsHIF-1␣ fl/fl or HIF-2␣ fl/fl mice 11,12 were bred with LysM-Cre transgenic mice 13 in the C57BL/6 background. Age-matched C57BL/6 wild-type (WT) mice were controls. ID2 Ϫ/Ϫ mice and their respective WT control were in the NMRI background. 14 The guidelines of the Hessian animal care and use committee were followed. DC generation from BMFor DC generation in vitro, 2 ϫ 10 6 total BM cells/mL in RPMI 1640 with 10% FCS and 200 ng/mL recombinant murine fms-related tyrosine kinase 3-ligand (Flt3-L, PeproTech) were cultured in 6-well Ultra-Low attachment plates (Corning) for up to 9 days 15 at various O 2 levels as indicated, using a InVivo 2 400 hypoxia workstation (Ruskinn Technologies). Alternatively, cells were cultured with 100M dimethyloxallyl glycine (DMOG, from Biomol). Sorted monocyte/DC progenitors/common DC progenitors (MDPs/ CDPs; 10 4 cells/well) were cultured with 200 ng/mL Flt3-L in 24-well Ultra-Low attachment plates. Flow cytometry and cell sortingBM cells, spleen, or whole blood cells were stained with fluorochromeconjugated antibodies and analyzed on a LSRII/Fortessa flow cytometer (BD Biosciences). MDP/CDP were sorted from lineage Ϫ cell-enriched BM (lineage cell depletion kit and AutoMACS cell separator from Miltenyi Biotec) using a FACSAria III cell sorter (BD Biosciences). For details (antibodies, surface markers, intracellular transcription factor staining procedures), see supplemental Methods (available on the Blood Web site; see the Supplemental Materials link at the top of the online article). pDCs were i...

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Prostacyclin is an important mediator of peripheral pain sensation. Here, we investigated its potential participation in mediating neuropathic pain and found that prostacyclin receptor (IP) knockout mice exhibited markedly decreased pain behavior. Application of an IP antagonist to the injury site or selective IP deficiency in myeloid cells mimicked the antinociceptive effect observed in IP knockout mice. At the site of nerve injury, IP was expressed in interleukin (IL) 1β-containing resident macrophages, which were less common in IP knockout mice. Local administration of the IP agonist cicaprost inhibited macrophage migration in vitro and promoted accumulation of IP- and IL1β-expressing cells as well as an increase of IL1β concentrations at the application site in vivo. Fittingly, the IL1-receptor antagonist anakinra (IL-1ra) decreased neuropathic pain behavior in wild-type mice but not in IP knockout mice. Finally, continuous, but not single administration, of the cyclooxygenase inhibitor meloxicam early after nerve injury decreased pain behavior and the number of resident macrophages. Thus, early synthesis of prostacyclin at the site of injury causes accumulation of IL1β-expressing macrophages as a key step in neuropathic pain after traumatic injury.