Macrophages are present throughout the human body, constitute important immune effector cells, and have variable roles in a great number of pathological, but also physiological, settings. It is apparent that macrophages need to adjust their activation profile toward a steadily changing environment that requires altering their phenotype, a process known as macrophage polarization. Formation of reactive oxygen species (ROS), derived from NADPH-oxidases, mitochondria, or NO-producing enzymes, are not necessarily toxic, but rather compose a network signaling system, known as redox regulation. Formation of redox signals in classically versus alternatively activated macrophages, their action and interaction at the level of key targets, and the resulting physiology still are insufficiently understood. We review the identity, source, and biological activities of ROS produced during macrophage activation, and discuss how they shape the key transcriptional responses evoked by hypoxia-inducible transcription factors, nuclear-erythroid 2-p45-related factor 2 (Nrf2), and peroxisome proliferator-activated receptor-γ. We summarize the mechanisms how redox signals add to the process of macrophage polarization and reprogramming, how this is controlled by the interaction of macrophages with their environment, and addresses the outcome of the polarization process in health and disease. Future studies need to tackle the option whether we can use the knowledge of redox biology in macrophages to shape their mediator profile in pathophysiology, to accelerate healing in injured tissue, to fight the invading pathogens, or to eliminate settings of altered self in tumors.

The ATP-binding cassette (ABC) transporter associated with antigen processing (TAP) participates in immune surveillance by moving proteasomal products into the endoplasmic reticulum (ER) lumen for major histocompatibility complex class I loading and cell surface presentation to cytotoxic T cells. Here we delineate the mechanistic basis for antigen translocation. Notably, TAP works as a molecular diode, translocating peptide substrates against the gradient in a strict unidirectional way. We reveal the importance of the D-loop at the dimer interface of the two nucleotide-binding domains (NBDs) in coupling substrate translocation with ATP hydrolysis and defining transport vectoriality. Substitution of the conserved aspartate, which coordinates the ATP-binding site, decreases NBD dimerization affinity and turns the unidirectional primary active pump into a passive bidirectional nucleotide-gated facilitator. Thus, ATP hydrolysis is not required for translocation per se, but is essential for both active and unidirectional transport. Our data provide detailed mechanistic insight into how heterodimeric ABC exporters operate.

Tumor cell-derived factors skew macrophages toward a tumor-supporting phenotype associated with the secretion of protumorigenic mediators. Apoptosing tumor cells release sphingosine 1-phosphate (S1P), which stimulates the production of lipocalin 2 (LCN2) in tumor-associated macrophages and is associated with tumor metastasis. We explored the mechanism by which S1P induces LCN2 in macrophages and investigated how this contributed to tumor growth and metastasis. Knockdown of S1P receptor 1 (S1PR1) in primary human macrophages and experiments with bone marrow-derived macrophages from S1PR1-deficient mice showed that S1P signaled through S1PR1 to induce LCN2 expression. The LCN2 promoter contains a consensus sequence for signal transducer and activator of transcription 3 (STAT3), and deletion of the STAT3 recognition sequence reduced expression of an LCN2-controlled reporter gene. Conditioned medium from coculture experiments indicated that the release of LCN2 from macrophages induced tube formation and proliferation in cultures of primary human lymphatic endothelial cells in a manner dependent on the kinase PI3K and subsequent induction of the growth factor VEGFC, which functioned as an autocrine signal stimulating the receptor VEGFR3. Knockout of Lcn2 attenuated tumor-associated lymphangiogenesis and breast tumor metastasis both in the breast cancer model MMTV-PyMT mice and in mice bearing orthotopic wild-type tumors. Our findings indicate that macrophages respond to dying tumor cells by producing signals that promote lymphangiogenesis, which enables metastasis.

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...

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