Macrophages are innate immune cells with well-established roles in the primary response to pathogens, but also in tissue homeostasis, coordination of the adaptive immune response, inflammation, resolution, and repair. These cells recognize danger signals through receptors capable of inducing specialized activation programs. The classically known macrophage activation is induced by IFN-gamma, which triggers a harsh proinflammatory response that is required to kill intracellular pathogens. Macrophages also undergo alternative activation by IL-4 and IL-13, which trigger a different phenotype that is important for the immune response to parasites. Here we review the cellular sources of these cytokines, receptor signaling pathways, and induced markers and gene signatures. We draw attention to discrepancies found between mouse and human models of alternative activation. The evidence for in vivo alternative activation of macrophages is also analyzed, with nematode infection as prototypic disease. Finally, we revisit the concept of macrophage activation in the context of the immune response.

Key Points• Human and mouse macrophages share partially conserved gene and protein expression programs in the resting or M2 activated state. • TGM2 is a novel M2 marker consistently induced in human and mouse M2 macrophages.The molecular repertoire of macrophages in health and disease can provide novel biomarkers for diagnosis, prognosis, and treatment. Th2-IL-4-activated macrophages (M2) have been associated with important diseases in mice, yet no specific markers are available for their detection in human tissues. Although mouse models are widely used for macrophage research, translation to the human can be problematic and the human macrophage system remains poorly described. In the present study, we analyzed and compared the transcriptome and proteome of human and murine macrophages under resting conditions (M0) and after IL-4 activation (M2). We provide a resource for tools enabling macrophage detection in human tissues by identifying a set of 87 macrophage-related genes. Furthermore, we extend current understanding of M2 activation in different species and identify Transglutaminase 2 as a conserved M2 marker that is highly expressed by human macrophages and monocytes in the prototypic Th2 pathology asthma. (Blood. 2013;121(9):e57-e69)

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Cell-cell fusion in sexually reproducing organisms is a mechanism to merge gamete genomes, and in multicellular organisms, it is a strategy to sculpt organs such as muscles, bones, and placenta. Moreover, this mechanism has been implicated in pathological conditions such as infection and cancer. Study of genetic model organisms has uncovered a unifying principle: cell fusion is a genetically programmed process. This process can be divided in three stages: (i) competence: cell induction and differentiation, (ii) commitment: cell determination, migration and adhesion, and (iii) cell fusion: membrane merging and cytoplasmic mixing. Recent work has led to the discovery of fusogens, cell fusion proteins that are necessary and sufficient to fuse cell membranes. Two unrelated families of fusogens have been discovered, one in mouse placenta and one in Caenorhabditis elegans (Syncytins and F proteins, respectively). Current research aims to identify new fusogens and determine the mechanisms by which fusogens merge membranes.

IntroductionThe steroid hormone 1␣,25-dihydroxyvitamin D 3 (1␣,25(OH) 2 D 3 ) is known for its important role in regulating calcium homeostasis and bone mineralization. 1 1␣,25(OH) 2 D 3 acts through a nuclear receptor, the vitamin D receptor (Vdr), which is a member of the steroid and thyroid hormone receptor superfamily. More recently, evidence has accumulated that the hormone can have important functions in the immune system. Expression of Vdr was found in different immune effector cells of the myeloid and lymphoid lineage under resting and activating conditions. 2,3 These findings contributed to the hypothesis that locally produced 1␣,25(OH) 2 D 3 may perform regulatory functions on those cells. Indeed, over the past few years it has been demonstrated that 1␣,25(OH) 2 D 3 can act as an important immunosuppressive modulator. 1␣,25(OH) 2 D 3 has been shown to suppress T-cell proliferation 4 and to decrease the production of the T helper type 1 (Th1) cytokines interleukin 2, interferon ␥ (IFN-␥), and tumor necrosis factor ␣ (TNF-␣), leading to the inhibition of Th1 cell development. 5 Besides its direct effects on T cells, 1␣,25(OH) 2 D 3 and its analogs are potent inhibitors of dendritic cell (DC) differentiation and maturation and can impair the capacity of DCs to induce alloreactive T-cell activation. 6,7 In line with this, Vdr-deficient mice have been shown to have an increased frequency of mature DCs in lymph nodes. 8 Additional support for the immunomodulatory role of 1␣,25(OH) 2 D 3 in vivo came from studies of autoimmune diseases in several different animal models. It has been demonstrated that 1␣,25(OH) 2 D 3 can prevent or suppress experimental autoimmune encephalomyelitis, 9 rheumatoid arthritis, 10 systemic lupus erythematosus, 11 type 1 diabetes, 12 and inflammatory bowel disease, 13,14 further supporting its potent suppressive effects on the immune system.In contrast to its well-characterized effects on adaptive immune responses, much less is known about the effects of 1␣,25(OH) 2 D 3 on effectors of innate immunity, especially on macrophages. It has been shown that 1␣,25(OH) 2 D 3 can induce the differentiation of myeloid progenitors into macrophages. 15,16 However, the effects of 1␣,25(OH) 2 D 3 on mature and activated macrophages that are involved in inflammatory reactions have not been characterized yet. Such possible effects might be of especial importance since it was demonstrated that macrophages can release biologically active 1␣,25(OH) 2 D 3 on activation with IFN-␥. 17,18 The production of 1␣,25(OH) 2 D 3 by activated macrophages is regulated by the IFN-␥-mediated induction of 1␣-hydroxylase expression, the enzyme controlling the last step of 1␣,25(OH) 2 D 3 synthesis. 17,18 In Supported by the National German Genome Network (NGFN; 01GR0439), EU FP5 project EUMORPHIIA (QLG2- CT-2002-00930), and Volkswagenstiftung.L.H. performed research and wrote the paper; J.B., J.E., and S.S. performed research; T.F. contributed reagents and analytical tools; R.G. and M.P.K analyzed data; R.B. initiate...

Rationale: Fetuin-A is a liver-derived plasma protein involved in the regulation of calcified matrix metabolism.Biochemical studies showed that fetuin-A is essential for the formation of protein-mineral complexes, called calciprotein particles (CPPs). CPPs must be cleared from circulation to prevent local deposition and pathological calcification.Objective: We studied CPP clearance in mice and in cell culture to identify the tissues, cells, and receptors involved in the clearance. Methods and Results:

SummaryMultinucleated giant cells (MGCs) form by fusion of macrophages and are presumed to contribute to the removal of debris from tissues. In a systematic in vitro analysis, we show that IL-4-induced MGCs phagocytosed large and complement-opsonized materials more effectively than their unfused M2 macrophage precursors. MGC expression of complement receptor 4 (CR4) was increased, but it functioned primarily as an adhesion integrin. In contrast, although expression of CR3 was not increased, it became functionally activated during fusion and was located on the extensive membrane ruffles created by excess plasma membrane arising from macrophage fusion. The combination of increased membrane area and activated CR3 specifically equips MGCs to engulf large complement-coated targets. Moreover, we demonstrate these features in vivo in the recently described complement-dependent therapeutic elimination of systemic amyloid deposits by MGCs. MGCs are evidently more than the sum of their macrophage parts.

Multinucleated giant cells, characteristic of granulomatous infections, originate from fusion of macrophages, however, little is known about the underlying mechanism. Alternative activation of macrophages by exposure to IL-4 and IL-13 induces macrophage homokaryon formation. We have established a new quantitative bifluorescent system to study IL-4-induced fusion of primary murine macrophages in vitro. Using this assay, we could show that macrophage fusion is not mediated by a single molecule, but involves multiple functional components. Although several murine macrophage populations were not competent to form giant cells, indicating that they fail to display the full fusion machinery, these non-fusogenic macrophages could fuse with fusion-competent macrophages in a heterophilic manner. Since IL-4 induced molecules were needed on both fusion partners, we conclude that at least two functionally distinct molecules mediate macrophage homokaryon formation with each present on one fusion partner. In addition, though IL-4 treatment led to induction of a fusogenic status, macrophages could only fuse efficiently when adherent to a permissive substratum. Based on our findings, we conclude that macrophage fusion is a multistage process involving multiple target molecules. The model we describe will allow analysis of the molecular basis of membrane fusion and possible insight into alternative activation of macrophages. IntroductionFusion of cells is a fundamental biological process, yet very little is known about its mechanistic basis [1]. Cells of the monocyte/macrophage lineage can fuse and form large multinucleated giant cells (MGC) that have long been recognised as a histopathological hallmark of tuberculosis, schistosomiasis and other granulomatous diseases [2,3].To date, the function of MGC and their potential contribution to disease progression remain ill defined. However, macrophage fusion is also characteristic of osteoclast differentiation and foreign body granuloma formation, where fused macrophages ensure optimal degradation of bone and foreign material, respectively [2,4]. While intracellular and viral membrane fusion has been studied in detail, much less is known about the machinery that mediates cell-to-cell fusion, in particular macrophage homokaryon formation [1,5]. By analogy to virus-cell fusion, fusion of macrophages might be mediated by cell surface proteins [1]. Several individual candidate macrophage fusion molecules including dendritic cell-specific transmembrane protein (DC-STAMP), TREM2/DAP-12 (triggering receptor expressed on myeloid cells 2), MFR (macrophage fusion receptor), CD47, the mannose/fucose receptor and CD44 have been proposed, based on utilization of different experimental models [6][7][8][9][10][11][12][13]. However, there is no knowledge of their potential interplay and redundancy in the process of MGC formation. Alternative activation of macrophages by exposure to the type 2 cytokines IL-4 or IL-13 induces the formation of MGC in vitro [14][15][16]. Alternative activation of macrop...