Siglec receptors and hiding plaques in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease. One hallmark of this disease is the continuous increase in the numbers and size of aggregating amyloid plaques. The accumulation of extracellular plaques is an immunologically interesting phenomenon since microglial cells, brain-specific macrophages, should be able to cleanse the aggregating material from the human brain. Immunotherapy targeting β-amyloid peptides in plaques with antibodies represents a promising therapy in AD. Recent progress in pattern recognition receptors of monocytes and macrophages has revealed that the sialic acid-binding, immunoglobulin-like lectin (Siglec) family of receptors is an important recognition receptor for sialylated glycoproteins and glycolipids. Interestingly, recent studies have revealed that microglial cells contain only one type of Siglec receptors, Siglec-11, which mediates immunosuppressive signals and thus inhibits the function of other microglial pattern recognition receptors, such as TLRs, NLRs, and RAGE receptors. We will review here the recent literature which clearly indicates that aggregating amyloid plaques are masked in AD by sialylated glycoproteins and gangliosides. Sialylation and glycosylation of plaques, mimicking the cell surface glycocalyx, can activate the immunosuppressive Siglec-11 receptors, as well as hiding the neuritic plaques, allowing them to evade the immune surveillance of microglial cells. This kind of immune evasion can prevent the microglial cleansing process of aggregating amyloid plaques in AD.     

The antigen recognized by MOMA-I is sialoadhesin

The monoclonal antibody MOMA-1 is a widely-used marker for marginal metallophilic macrophages in spleen and some other subsets of macrophages. The antigen recognized by MOMA-1 has yet to be characterized, but its expression pattern is similar to that of sialoadhesin (Sn, CD169, Siglec-1), a member of the sialic acid binding Ig-like lectin (Siglec) family. Using flow cytometry of Sn-transfected cells and staining of lymphoid tissue sections from Sn-deficient mice, we demonstrate here that the antigen recognized by MOMA-1 is Sn.     

FcγRI (CD64): An identity card for intestinal macrophages

Macrophages are becoming increasingly recognized as key cellular players in intestinal immune homeostasis. However, differentiating between macrophages and dendritic cells (DCs) is often difficult, and finding a specific phenotypic signature for intestinal macrophage identification has remained elusive. In this issue of the European Journal of Immunology, Tamoutounour et al. [Eur. J. Immunol. 2012. 42: 3150‐3166] identify CD64 as a specific macrophage marker that can be used to discriminate DCs from macrophages in the murine small and large intestine, under both steady‐state and inflammatory conditions. The authors also propose a sequential ‘monocyte‐waterfall’ model for intestinal macrophage differentiation, with implications for immune tolerance and inflammation at the gut mucosal interface. This Commentary will discuss the advantages and potential limitations of CD64 as a marker for intestinal macrophages.     

Siglec‐1 inhibits RSV‐induced interferon gamma production by adult T cells in contrast to newborn T cells

Interferon gamma (IFN‐γ) plays an important role in the antiviral immune response during respiratory syncytial virus (RSV) infections. Monocytes and T cells are recruited to the site of RSV infection, but it is unclear whether cell‐cell interactions between monocytes and T cells regulate IFN‐γ production. In this study, micro‐array data identified the upregulation of sialic acid‐binding immunoglobulin‐type lectin 1 (Siglec‐1) in human RSV‐infected infants. In vitro, RSV increased expression of Siglec‐1 on healthy newborn and adult monocytes. RSV‐induced Siglec‐1 on monocytes inhibited IFN‐γ production by adult CD4⁺ T cells. In contrast, IFN‐γ production by RSV in newborns was not affected by Siglec‐1. The ligand for Siglec‐1, CD43, is highly expressed on adult CD4⁺ T cells compared to newborns. Our data show that Siglec‐1 reduces IFN‐γ release by adult T cells possibly by binding to the highly expressed CD43. The Siglec‐1‐dependent inhibition of IFN‐γ in adults and the low expression of CD43 on newborn T cells provides a better understanding of the immune response against RSV in early life and adulthood.     

Siglec‐7 Defines a Highly Functional Natural Killer Cell Subset and Inhibits Cell‐Mediated Activities

Sialic acid‐binding immunoglobulin‐like lectin‐7 (Siglec‐7) is an inhibitory receptor expressed on natural killer (NK) cells. In this study, we investigated the relationship between Siglec‐7 expression and NK cell functions. Siglec‐7 was highly expressed on NK cells and was preferentially expressed by mature NK cells from peripheral blood of healthy adults. Siglec‐7⁺ NK cells displayed higher levels of activating receptors CD38, CD16, DNAM1, NKp30 and NKp46, but lower levels of inhibitory receptors such as NKG2A and CD158b, compared with Siglec‐7^– NK cells. Functional tests showed that Siglec‐7⁺ NK cells displayed more CD107a degranulation and IFN‐γ production than Siglec‐7^– NK cells. Siglec‐7 inhibited NK cell functions when interacting with specific antibodies. These data suggest that Siglec‐7 defines a highly functional NK cell subset and suppresses NK cell‐mediated functions when cross‐linked with specific antibodies.     

Siglec‐9 is upregulated in rheumatoid arthritis and suppresses collagen‐induced arthritis through reciprocal regulation of Th17‐/Treg‐cell differentiation

Sialic acid‐binding Ig‐like lectin‐9 (Siglec‐9) is a novel sialic acid‐binding member of the immunoglobulin superfamily and is broadly expressed on immune cells, which can inhibit both innate and adaptive immune responses through immunoreceptor tyrosine‐based inhibitory motifs. However, the exact role of Siglec‐9 in the pathogenesis of rheumatoid arthritis (RA) remains unknown. In this study, we determined soluble Siglec‐9 (sSiglec‐9) levels in the serum and synovial fluid of patients with RA and evaluated the relation between sSiglec‐9 levels and clinical factors. In addition, we investigated whether Siglec‐9 could alleviate collagen‐induced arthritis (CIA) development in mice and explored the molecular mechanisms involved. The results showed that, in the serum and synovial fluid of patients with RA, sSiglec‐9 levels were elevated. Thus, high sSiglec‐9 serum levels associated with disease severity in patients with RA. Furthermore, administration of recombinant human Siglec‐9 Fc chimera protein significantly attenuated collagen‐induced arthritis development in vivo and in vitro by reciprocal regulation of the differentiation of Th17 and Treg cells. Our findings collectively indicate that Siglec‐9 plays a potent immunosuppressive role in the pathogenesis of RA by reciprocal regulation of Th17‐/Treg‐cell differentiation. In conclusion, Siglec‐9 may serve as a potential diagnostic and therapeutic target for RA.     

The xanthine oxidase–NFAT5 pathway regulates macrophage activation and TLR‐induced inflammatory arthritis

NFAT5 (nuclear factor of activated T cells), a well‐known osmoprotective factor, can be activated by isotonic stimuli such as Toll‐like receptor (TLR) triggering. However, it is unclear how NFAT5 discriminates between isotonic and hypertonic stimuli to produce different functional and molecular outcomes. Here, we identified a novel XO–ROS–p38 MAPK–NFAT5 pathway (XO is xanthine oxidase, ROS is reactive oxygen species) that is activated in RAW 264.7 macrophages upon isotonic TLR stimulation. Unlike what is seen under hypertonic conditions, XO‐derived ROS were selectively required for the TLR‐induced NFAT5 activation and NFAT5 binding to the IL‐6 promoter in RAW 264.7 macrophages under isotonic conditions. In mouse peritoneal macrophages and human macrophages, TLR ligation also induced NFAT5 activation, which was dependent on XO and p38 kinase. The involvement of XO in NFAT5 activation by TLR was confirmed in RAW 264.7 macrophages implanted in BALB/c mice. Moreover, allopurinol, an XO inhibitor, suppressed arthritis severity and decreased the expression of NFAT5 and IL‐6 in splenic macrophages in C57BL/6 mice. Collectively, these data support a novel function of the XO–NFAT5 axis in macrophage activation and TLR‐induced arthritis, and suggest that XO inhibitor(s) could serve as a therapeutic agent for chronic inflammatory arthritis.     

IL‐23 protection against Plasmodium berghei infection in mice is partially dependent on IL‐17 from macrophages

Although IL‐12 is believed to contribute to protective immune responses, the role played by IL‐23 (a member of the IL‐12 family) in malaria is elusive. Here, we show that IL‐23 is produced during infection with Plasmodium berghei NK65. Mice deficient in IL‐23 (p19KO) had higher parasitemia and died earlier than wild‐type (WT) controls. Interestingly, p19KO mice had lower numbers of IL‐17‐producing splenic cells than their WT counterparts. Furthermore, mice deficient in IL‐17 (17KO) suffered higher parasitemia than the WT controls, indicating that IL‐23‐mediated protection is dependent on induction of IL‐17 during infection. We found that macrophages were responsible for IL‐17 production in response to IL‐23. We observed a striking reduction in splenic macrophages in the p19KO and 17KO mice, both of which became highly susceptible to infection. Thus, IL‐17 appears to be crucial for maintenance of splenic macrophages. Adoptive transfer of macrophages into macrophage‐depleted mice confirmed that macrophage‐derived IL‐17 is required for macrophage accumulation and parasite eradication in the recipient mice. We also found that IL‐17 induces CCL2/7, which recruit macrophages. Our findings reveal a novel protective mechanism whereby IL‐23, IL‐17, and macrophages reduce the severity of infection with blood‐stage malaria parasites.     

Obesity and dysregulated central and peripheral macrophage–neuron cross‐talk

The involvement of macrophages in the pathogenesis of obesity has been recognized since 2003. Early studies mostly focused on the role of macrophages in adipose tissue (AT) and in obesity‐associated chronic low‐grade inflammation. Lately, AT macrophages were shown to undergo intrinsic metabolic changes that affect their immune function (i.e., immunometabolism), corresponding to their unique properties along the range of pro‐ versus anti‐inflammatory activity. In parallel, recent studies in mice revealed critical neuronal–macrophage interactions, both in the CNS and in peripheral tissues, including in white and brown AT. These intercellular activities impinge on energy and metabolic homeostasis, partially by also engaging adipocytes in a neuronal–macrophage–adipocyte ménage à trois. Finally, neuropeptides (NP), such as NPY and appetite‐reducing NPFF, may prove as mediators in such intercellular network. In this concise review, we highlight some of these recent insights on adipose macrophage immunometabolism, as well as central and peripheral neuronal–macrophage interactions with emphasis on their impact on adipocyte biology and whole‐body metabolism. We also discuss the expanding view on the role of the NP, NPY and NPFF, in obesity.     

Characterization of the Specific Interaction between Sialoadhesin and Sialylated Campylobacter jejuni Lipooligosaccharides

In Campylobacter jejuni-induced Guillain-Barré syndrome (GBS), molecular mimicry between C. jejuni lipooligosaccharide (LOS) and host gangliosides leads to the production of cross-reactive antibodies directed against the peripheral nerves of the host. Currently, the presence of surface exposed sialylated LOS in C. jejuni is the single known bacterial pathogenesis factor associated with the development of GBS. Using a unique, well-characterized strain collection, we demonstrate that GBS-associated C. jejuni strains bind preferentially to sialoadhesin (Sn, Siglec-1, or CD169), a sialic acid receptor found on a subset of macrophages. In addition, using a whole-cell enzyme-linked immunosorbent assay (ELISA), C. jejuni strains with sialylated LOS bound exclusively to soluble Sn. Mass spectrometry revealed that binding was sialic acid-linkage specific with a preference for α(2,3)-linked sialic acid attached to the terminal galactose of the LOS chain as seen in the gangliosides GD1a, GM1b, and GM3. This molecular interaction was also related to functional consequences as a GBS-associated C. jejuni strain that bound Sn in a whole-cell ELISA adhered to surface-expressed Sn of Sn-transfected CHO cells but was unable to adhere to wild-type CHO cells. Moreover, a sialic acid-negative mutant of the same C. jejuni strain was unable to bind Sn-transfected CHO cells. This is the first report of the preferential binding of GBS-associated C. jejuni strains to the Sn immune receptor (P = 0.014). Moreover, because this binding is dependent on sialylated LOS, the main pathogenic factor in GBS progression, the present findings bring us closer to unraveling the mechanisms that lead to formation of cross-reactive antibodies in GBS disease.Campylobacter jejuni, a food-borne Gram-negative bacterium, is the major cause of bacterial gastroenteritis worldwide. In addition to enteritis, infection with C. jejuni may also lead to a neurological complication called the Guillain-Barré syndrome (GBS). GBS is an autoimmune disease affecting the peripheral nerves. Antibodies raised by the host during an infection with C. jejuni possess the capacity to cross-react with structures on human nerve tissue, resulting in neurological complications for the host (38). Further, high titers of anti-ganglioside antibodies are frequently found in the sera of GBS patients (24, 41). Gangliosides are glycosphingolipids with an extracellular sialylated oligosaccharide chain and a ceramide tail that is embedded in the outer leaflet of the plasma membrane. Although predominantly found in the nervous system, gangliosides are present on other cell surfaces as well.C. jejuni has lipooligosaccharide (LOS) structures on its outer membrane. Biochemical and structural analysis of LOS outer core oligosaccharides has identified sialylated moieties that are structurally similar to several gangliosides (6, 9, 30). During infection, the structural similarity between C. jejuni LOS and human gangliosides, also known as molecular mimicry, facilitates the induction of anti-ganglioside antibodies and the development of GBS (1, 29, 38, 40). The C. jejuni genes involved in ganglioside mimicry are located within the LOS biosynthesis locus, a gene cluster that is interchangeable between strains and is genetically highly diverse (18, 19). Therefore, several LOS classes (A through S) have been identified (31). LOS class, gene alterations, mutations, and mechanisms such as phase variation in the LOS locus contribute to structural variations in the ganglioside mimics produced (19). The presence of LOS biosynthesis locus-encoded genes responsible for synthesis, modification, and transfer of sialic acid, found in LOS classes A, B, and C, is crucial in the induction of anti-ganglioside antibodies and hence GBS (20, 36). Sialylated LOS is also involved in other aspects of C. jejuni pathogenesis. C. jejuni strains expressing sialylated LOS invade human epithelial intestinal cells significantly more frequently than strains expressing nonsialylated LOS (28). However, the receptor for C. jejuni attachment to human epithelial intestinal cells is unknown.Certain C. jejuni strains are known to bind to Siglec-7, a member of the sialic acid binding immunoglobulin-like lectin (Siglec) family (8). Siglecs are present on the cell surface of a range of immune-associated cells and are involved in cell to cell interactions and signaling. A subset of the Siglec family, the CD33-related Siglecs, can serve as regulators of the immune system through immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in their cytoplasmic tail (7, 11). In addition, several recently described human Siglecs, Siglecs-14, -15, and -16 can interact with the immunoreceptor tyrosine-based activation motif (ITAM) adaptor, DAP12 and therefore potentially mediate the activation of intracellular signaling (5, 10).Sialoadhesin (Sn, Siglec-1, or CD169) is a macrophage-restricted Siglec that has been associated with inflammatory and autoimmune diseases. For example, Sn levels are elevated on activated macrophages within the inflamed organs of several inflammatory disorders, including rheumatoid arthritis, experimental autoimmune encephalomyelitis (EAE), and experimental autoimmune uveoretinitis (EAU) (22, 32, 39). This elevated expression may have functional consequences since Sn-deficient mice show a reduced severity of EAE and EAU (25, 39). With a poorly conserved cytoplasmic tail and the absence of tyrosine-based signaling motifs, Sn seems to be more involved in cell-to-cell communication and ligand binding than intracellular immunoregulation. It has been shown that macrophages expressing Sn can bind and internalize sialylated Neisseria meningitidis in an Sn- and sialic acid-dependent manner (26). Further, HIV-1 can interact with Sn, probably via a sialic acid residue on gp120, with binding resulting in enhanced infectivity and facilitates transinfections in alpha interferon (IFN-α)-stimulated CD14⁺ monocytes. Furthermore, Sn gene expression is elevated in CD14⁺ monocytes from patients infected with HIV-1 (34).Each Siglec has a unique specificity for certain sialylated glycans, with Sn preferring sialic acid conjugates with an α(2,3)galactose (gal) linkage (12). This α(2,3)Gal linkage is often found on the LOS of GBS-associated C. jejuni strains.Because of the connection of C. jejuni infection with autoimmune disease such as GBS and its clinical variant Miller-Fisher syndrome (MFS), we investigated whether α(2,3)-linked sialic acid residues on the surface of C. jejuni strains could interact with Sn and whether this interaction was characteristic for GBS-associated strains.     

Siglec-8 on human eosinophils and mast cells, and Siglec-F on murine eosinophils, are functionally related inhibitory receptors

Siglecs (sialic acid‐binding, Ig‐like lectins) are a family of single‐pass transmembrane cell surface proteins found predominantly on leucocytes. Their unique structural characteristics include an N‐terminal carbohydrate‐binding (‘lectin’) domain that binds sialic acid, followed by a variable number of Ig‐like domains, hence these structures are a subset of the Ig gene superfamily. Another unique feature of Siglecs is that most, but not all, possess so‐called immunoreceptor tyrosine‐based inhibitory motifs in their cytoplasmic domains, suggesting that these molecules function in an inhibitory capacity. Siglec‐8, the eighth member identified at the time, was discovered as part of an effort initiated almost a decade ago to identify novel human eosinophil and mast cell proteins. Since that time, its selective expression on human eosinophils and mast cells has been confirmed. On eosinophils, Siglec‐8 engagement results in apoptosis, whereas on mast cells, inhibition of Fc?RI‐dependent mediator release, without apoptosis, is seen. It has subsequently been determined that the closest functional paralog in the mouse is Siglec‐F, selectively expressed by eosinophils but not expressed on mast cells. Despite only modest homology, both Siglec‐8 and Siglec‐F preferentially recognize a sulphated glycan ligand closely related to sialyl Lewis X, a common ligand for the selectin family of adhesion molecules. Murine experiments in normal, Siglec‐F‐deficient mice and hypereosinophilic mice have resulted in similar conclusions that Siglec‐F, like Siglec‐8, plays a distinctive and important role in regulating eosinophil accumulation and survival in vivo. Given the resurgent interest in eosinophil‐directed therapies for a variety of disorders, plus its unique additional ability to also target the mast cell, therapies focusing on Siglec‐8 could some day prove to be a useful adjunct to our current armamentarium for the treatment of asthma, allergies and related disorders where overproduction and overactivity of eosinophils and mast cells is occurring.     

Intravenous immunoglobulin (IVIg) or IVIg‐treated macrophages reduce DSS‐induced colitis by inducing macrophage IL‐10 production

Intravenous immunoglobulin (IVIg) is used to treat immune‐mediated diseases but its mechanism of action is poorly understood. We have reported that co‐treatment with IVIg and lipopolysaccharide activates macrophages to produce large amounts of anti‐inflammatory IL‐10 in vitro. Thus, we asked whether IVIg‐treated macrophages or IVIg could reduce intestinal inflammation in mice during dextran sulfate sodium (DSS)‐induced colitis by inducing macrophage IL‐10 production in vivo. Adoptive transfer of IVIg‐treated macrophages reduces intestinal inflammation in mice and collagen accumulation post‐DSS. IVIg treatment also reduces DSS‐induced intestinal inflammation and its activity is dependent on the Fc portion of the antibody. Ex vivo, IVIg induces IL‐10 production and reduces IL‐12/23p40 and IL‐1β production in colon explant cultures. Co‐staining tissues for mRNA, we demonstrate that macrophages are the source of IL‐10 in IVIg‐treated mice; and using IL‐10‐GFP reporter mice, we demonstrate that IVIg induces IL‐10 production by intestinal macrophages. Finally, IVIg‐mediated protection is lost in mice deficient in macrophage IL‐10 production (LysMcre^+/?IL‐10^fl/fl mice). Together, our data demonstrate a novel, in vivo mechanism of action for IVIg. IVIg‐treated macrophages or IVIg could be used to treat people with intestinal inflammation and may be particularly useful for people with inflammatory bowel disease, who are refractory to therapy.     

Innate IFN‐γ Production by Subsets of Natural Killer Cells,Natural Killer T Cells and γδ T Cells in Response to Dying Bacterial‐Infected Macrophages

Interferon‐γ (IFN‐γ) activation of macrophages is a crucial step in the early innate defence against bacterial infection. This innate IFN‐γ is thought to be produced mainly by natural killer (NK) cells through activation with interleukin (IL)‐12p70 secreted by macrophages and dendritic cells (DCs) that have sensed bacterial products. However, a number of reports have shown that bacterial stimuli are unable to induce macrophages and/or DCs to produce sufficient amounts of IL‐12p70 unless these cells are primed by IFN‐γ. It remains, therefore, unsettled how initial IFN‐γ is produced. In a previous study, we reported a novel IFN‐γ production pathway that was associated with cell death in macrophages caused by intracellular bacteria like Listeria monocytogenes (LM) and Shigella flexneri. In this study, we showed that cell death of bone‐marrow‐derived macrophage (BMM) cells following in vitro infection with Staphylococcus aureus (SA), an extracellular bacterium, can also stimulate this IFN‐γ production pathway. We also unequivocally demonstrated by using BMM cells from IL‐12‐deficient mice that the bacterial‐infected macrophage cell death‐mediated IFN‐γ production can occur without IL‐12 although the magnitude of the response is much smaller than that in the presence of IL‐12. The enhancing effect of IL‐12 on this response proved to be attributable to the negligible amounts (0.5～1.5 pg/ml) of IL‐12p70 but not to the large amounts of IL‐12p40 that were both secreted by SA‐ and LM‐infected macrophages. Taken all together, we propose that macrophage cell death caused by bacteria may trigger the initial IFN‐γ production at an early stage of bacterial infection.     

Enhanced,Sialoadhesin-Dependent Uptake of Guillain-Barré Syndrome-Associated Campylobacter jejuni Strains by Human Macrophages

Molecular mimicry between Campylobacter jejuni sialylated lipooligosaccharides (LOS) and human nerve gangliosides can trigger the production of cross-reactive antibodies which induce Guillain-Barré syndrome (GBS). To better understand the immune events leading to GBS, it is essential to know how sialylated LOS are recognized by the immune system. Here, we show that GBS-associated C. jejuni strains bind to human sialoadhesin (hSn), a conserved, mainly macrophage-restricted I-type lectin. Using hSn-transduced THP-1 cells, we observed that C. jejuni strains with α(2,3)-sialylated LOS, including strains expressing GM1a- and GD1a-like epitopes, bind to hSn. This observation is of importance, as these epitopes are frequently the targets of the cross-reactive antibodies detected in GBS patients. Interestingly, the Sn binding domains were not constitutively exposed on the surface of C. jejuni. Heat inactivation and the environmental conditions which food-borne C. jejuni encounters during its passage through the intestinal tract, such as low pH and contact with bile constituents, exposed LOS and facilitated Sn binding. Sn binding enhanced bacterial uptake and increased the production of interleukin-6 (IL-6) by primary human Sn-expressing monocyte-derived macrophages compared to control conditions, where Sn was blocked using neutralizing antibodies or when nonsialylated C. jejuni was used. Sn-mediated uptake has been reported to enhance humoral immune responses. As C. jejuni strains expressing ganglioside mimics GD1a and GM1a are closely associated with GBS, Sn binding may be a determining event in the production of cross-reactive antibodies and the development of GBS.     

IL‐33 delivery induces serous cavity macrophage proliferation independent of interleukin‐4 receptor alpha

IL‐33 plays an important role in the initiation of type‐2 immune responses, as well as the enhancement of type 2 effector functions. Engagement of the IL‐33 receptor on macrophages facilitates polarization to an alternative activation state by amplifying IL‐4 and IL‐13 signaling to IL‐4Rα. IL‐4 and IL‐13 also induce macrophage proliferation but IL‐33 involvement in this process has not been rigorously evaluated. As expected, in vivo delivery of IL‐33 induced IL‐4Rα‐dependent alternative macrophage activation in the serous cavities. IL‐33 delivery also induced macrophages to proliferate but, unexpectedly, this was independent of IL‐4Rα signaling. In a filarial nematode infection model in which IL‐4Rα‐dependent alternative activation and proliferation in the pleural cavity is well described, IL‐33R was essential for alternative activation but not macrophage proliferation. Similarly, during Alternaria alternata induced airway inflammation, which provokes strong IL‐33 responses, we observed that both IL‐4Rα and IL‐33R were required for alternative activation, while macrophage proliferation in the pleural cavity was still evident in the absence of either receptor alone. Our data show that IL‐33R and IL‐4Rα promote macrophage proliferation independently of each other, but both are essential for induction of alternative activation.     

Nodal promotes the generation of M2‐like macrophages and downregulates the expression of IL‐12

Nodal, a member of the TGF‐β superfamily, is an embryonic morphogen that is upregulated in different types of tumors. Nodal increases the tumorigenesis by inducing angiogenesis and promoting metastasis. Importantly, Nodal inhibition suppresses the growth and invasion of tumor. Since tumor‐associated macrophages (TAMs) are the major infiltrating leukocytes in most cancers, we investigated whether Nodal is involved in the differentiation of TAMs. Our results revealed that Nodal inhibition in tumor microenvironment upregulated the production of IL‐12 in macrophages and reversed TAMs to classically activated macrophage phenotype. In contrast, treatment with recombinant Nodal (rNodal) decreased the expression of IL‐12 in murine macrophages. Furthermore, rNodal promoted macrophage polarization to an alternatively activated macrophage‐like/TAM phenotype and modulated its function. These results suggest that Nodal may play an important role in macrophage polarization and downregulation of IL‐12. The rescued antitumor function of TAMs via the inhibition of Nodal expression could be a new therapeutic strategy for cancer treatment.