The nuclear receptor LXRα controls the functional specialization of splenic macrophages.

Macrophages are professional phagocytic cells that orchestrate innate immune responses and have considerable phenotypic diversity at different anatomical locations. However, the mechanisms that control the heterogeneity of tissue macrophages are not well characterized. Here we found that the nuclear receptor LXRα was essential for the differentiation of macrophages in the marginal zone (MZ) of the spleen. LXR-deficient mice were defective in the generation of MZ and metallophilic macrophages, which resulted in abnormal responses to blood-borne antigens. Myeloid-specific expression of LXRα or adoptive transfer of wild-type monocytes restored the MZ microenvironment in LXRα-deficient mice. Our results demonstrate that signaling via LXRα in myeloid cells is crucial for the generation of splenic MZ macrophages and identify an unprecedented role for a nuclear receptor in the generation of specialized macrophage subsets.

Inhibition of LXR controls the polarization of human inflammatory macrophages through upregulation of MAFB.

Monocyte-derived macrophages contribute to pathogenesis in inflammatory diseases and their effector functions greatly depend on the prevailing extracellular milieu. Whereas M-CSF primes macrophages for acquisition of an anti-inflammatory profile, GM-CSF drives the generation of T cell-stimulatory and pro-inflammatory macrophages. Liver X Receptors (LXRα and LXRß) are nuclear receptors that control cholesterol metabolism and regulate differentiation of tissue-resident macrophages. Macrophages from rheumatoid arthritis and other inflammatory pathologies exhibit an enriched LXR pathway, and recent reports have shown that LXR activation raises pro-inflammatory effects and impairs the acquisition of the anti-Inflammatory profile of M-CSF-dependent monocyte-derived macrophages (M-MØ). We now report that LXR inhibition prompts the acquisition of an anti-inflammatory gene and functional profile of macrophages generated within a pathological environment (synovial fluid from Rheumatoid Arthritis patients) as well as during the GM-CSF-dependent differentiation of human monocyte-derived macrophages (GM-MØ). Mechanistically, inhibition of LXR results in macrophages with higher expression of the v-Maf Avian Musculoaponeurotic Fibrosarcoma Oncogene Homolog B (MAFB) transcription factor, which governs the macrophage anti-inflammatory profile, as well as over-expression of MAFB-regulated genes. Indeed, gene silencing experiments on human macrophages evidenced that MAFB is required for the LXR inhibitor to enhance the anti-inflammatory nature of human macrophages. As a whole, our results demonstrate that LXR inhibition prompts the acquisition of an anti-inflammatory transcriptional and functional profile of human macrophages in a MAFB-dependent manner, and propose the use of LXR antagonists as potential therapeutic alternatives in macrophage re-programming strategies during inflammatory responses.

Intravenous Immunoglobulins Promote an Expansion of Monocytic Myeloid-Derived Suppressor Cells (MDSC) in CVID Patients.

Common variable immunodeficiency disorders (CVID), the most common primary immune deficiency, includes heterogeneous syndromes characterized by hypogammaglobulinemia and impaired antibody responses. CVID patients frequently suffer from recurrent infections and inflammatory conditions. Currently, immunoglobulin replacement therapy (IgRT) is the first-line treatment to prevent infections and aminorate immune alterations in CVID patients. Intravenous Immunoglobulin (IVIg), a preparation of highly purified poly-specific IgG, is used for treatment of immunodeficiencies as well as for autoimmune and inflammatory disorders, as IVIg exerts immunoregulatory and anti-inflammatory actions on innate and adaptive immune cells. To determine the mechanism of action of IVIg in CVID in vivo, we determined the effect of IVIg infusion on the transcriptome of peripheral blood mononuclear cells from CVID patients, and found that peripheral blood monocytes are primary targets of IVIg in vivo, and that IVIg triggers the acquisition of an anti-inflammatory gene profile in human monocytes. Moreover, IVIg altered the relative proportions of peripheral blood monocyte subsets and enhanced the proportion of CD14+ cells with a transcriptional, phenotypic, and functional profile that resembles that of monocytic myeloid-derived suppressor cells (MDSC). Therefore, our results indicate that CD14 + MDSC-like cells might contribute to the immunoregulatory effects of IVIg in CVID and other inflammatory disorders.

CD28 is expressed by macrophages with anti-inflammatory potential and limits their T-cell activating capacity.

CD28 expression is generally considered to be T lymphocyte specific. We have previously shown CD28 mRNA expression in M-CSF-dependent anti-inflammatory monocyte-derived macrophages (M-MØ), and now demonstrate that CD28 cell surface expression is higher in M-MØ than in GM-CSF-dependent macrophages, and that macrophage CD28 expression is regulated by MAFB and activin A. In vivo, CD28 was found in tumor-associated macrophages and, to a lower extent, in pro-inflammatory synovial fluid macrophages from rheumatoid arthritis patients. Analysis of mouse macrophages confirmed Cd28 expression in bone-marrow derived M-MØ. Indeed, anti-CD28 antibodies triggered ERK1/2 phosphorylation in mouse M-MØ. At the functional level, Cd28KO M-MØ exhibited a significantly higher capacity to activate the OVA-specific proliferation of OT-II CD4+ T cells than WT M-MØ, as well as enhanced LPS-induced IL-6 production. Besides, the Cd28KO M-MØ transcriptome was significantly different from WT M-MØ regarding the expression IFN response, inflammatory response, and TGF-ß signaling related gene sets. Therefore, defective CD28 expression in mouse macrophages associates to changes in gene expression profile, what might contribute to the altered functionality displayed by Cd28KO M-MØ. Thus, CD28 expression appears as a hallmark of anti-inflammatory macrophages and might be a target for immunotherapy.

Serotonin (5-HT) Shapes the Macrophage Gene Profile through the 5-HT2B-Dependent Activation of the Aryl Hydrocarbon Receptor.

Macrophages can either promote or resolve inflammatory responses, and their polarization state is modulated by peripheral serotonin (5-hydroxytryptamine [5-HT]). In fact, pro- and anti-inflammatory macrophages differ in the expression of serotonin receptors, with 5-HT2B and 5-HT7 expression restricted to M-CSF-primed monocyte-derived macrophages (M-MØ). 5-HT7 drives the acquisition of profibrotic and anti-inflammatory functions in M-MØ, whereas 5-HT2B prevents the degeneration of spinal cord mononuclear phagocytes and modulates motility of murine microglial processes. Because 5-HT2B mediates clinically relevant 5-HT-related pathologies (valvular heart disease, pulmonary arterial hypertension) and is an off target of anesthetics, antiparkinsonian drugs, and selective serotonin reuptake inhibitors, we sought to determine the transcriptional consequences of 5-HT2B engagement in human macrophages, for which 5-HT2B signaling remains unknown. Assessment of the effects of specific agonists and antagonist revealed that 5-HT2B engagement modifies the cytokine and gene signature of anti-inflammatory M-MØ, upregulates the expression of aryl hydrocarbon receptor (AhR) target genes, and stimulates the transcriptional activation of AhR. Moreover, we found that 5-HT dose dependently upregulates the expression of AhR target genes in M-MØ and that the 5-HT-mediated activation of AhR is 5-HT2B dependent because it is abrogated by the 5-HT2B-specific antagonist SB204741. Altogether, our results demonstrate the existence of a functional 5-HT/5-HT2B/AhR axis in human macrophages and indicate that 5-HT potentiates the activity of a transcription factor (AhR) that regulates immune responses and the biological responses to xenobiotics.

Growth Hormone Reprograms Macrophages toward an Anti-Inflammatory and Reparative Profile in an MAFB-Dependent Manner.

Growth hormone (GH), a pleiotropic hormone secreted by the pituitary gland, regulates immune and inflammatory responses. In this study, we show that GH regulates the phenotypic and functional plasticity of macrophages both in vitro and in vivo. Specifically, GH treatment of GM-CSF-primed monocyte-derived macrophages promotes a significant enrichment of anti-inflammatory genes and dampens the proinflammatory cytokine profile through PI3K-mediated downregulation of activin A and upregulation of MAFB, a critical transcription factor for anti-inflammatory polarization of human macrophages. These in vitro data correlate with improved remission of inflammation and mucosal repair during recovery in the acute dextran sodium sulfate-induced colitis model in GH-overexpressing mice. In this model, in addition to the GH-mediated effects on other immune cells, we observed that macrophages from inflamed gut acquire an anti-inflammatory/reparative profile. Overall, these data indicate that GH reprograms inflammatory macrophages to an anti-inflammatory phenotype and improves resolution during pathologic inflammatory responses.

Transcriptomic Profiles of CD47 in Breast Tumors Predict Outcome and Are Associated with Immune Activation.

Targeting the innate immune system has attracted attention with the development of anti- CD47 antibodies. Anti-CD47 antibodies block the inhibition of the phagocytic activity of macrophages caused by the up-regulation of CD47 on tumor cells. In this study, public genomic data was used to identify genes highly expressed in breast tumors with elevated CD47 expression and analyzed the association between the presence of tumor immune infiltrates and the expression of the selected genes. We found that 142 genes positively correlated with CD47, of which 83 predicted favorable and 32 detrimental relapse-free survival (RFS). From those associated with favorable RFS, we selected the genes with immunologic biological functions and defined a CD47-immune signature composed of PTPRC, HLA-E, TGFBR2, PTGER4, ETS1, and OPTN. In the basal-like and HER2+ breast cancer subtypes, the expression of the CD47-immune signature predicted favorable outcome, correlated with the presence of tumor immune infiltrates, and with gene expression signatures of T cell activation. Moreover, CD47 up-regulated genes associated with favorable survival correlated with pro-tumoral macrophages. In summary, we described a CD47-immune gene signature composed of 6 genes associated with favorable prognosis, T cell activation, and pro-tumoral macrophages in breast cancer tumors expressing high levels of CD47.

IVIg Promote Cross-Tolerance against Inflammatory Stimuli In Vitro and In Vivo.

IVIg is an approved therapy for immunodeficiency and for several autoimmune and inflammatory diseases. However, the molecular basis for the IVIg anti-inflammatory activity remains to be fully explained and cannot be extrapolated from studies on animal models of disease. We now report that IVIg impairs the generation of human monocyte-derived anti-inflammatory macrophages by inducing JNK activation and activin A production and limits proinflammatory macrophage differentiation by inhibiting GM-CSF-driven STAT5 activation. In vivo, IVIg provokes a rapid increase in peripheral blood activin A, CCL2, and IL-6 levels, an effect that can be recapitulated in vitro on human monocytes. On differentiating monocytes, IVIg promotes the acquisition of altered transcriptional and cytokine profiles, reduces TLR expression and signaling, and upregulates negative regulators of TLR-initiated intracellular signaling. In line with these effects, in vivo IVIg infusion induces a state tolerant toward subsequent stimuli that results in reduced inflammatory cytokine production after LPS challenge in human peripheral blood and significant protection from LPS-induced death in mice. Therefore, IVIg conditions human macrophages toward the acquisition of a state of cross-tolerance against inflammatory stimuli, an effect that correlates with the net anti-inflammatory action of IVIg in vivo.

Role of TLR4 (Toll-Like Receptor 4) in N1/N2 Neutrophil Programming After Stroke.

Background and Purpose- After stroke, the population of infiltrated neutrophils in the brain is heterogeneous, including a population of alternative neutrophils (N2) that express M2 phenotype markers. We explored the role of TLR4 (toll-like receptor 4) on neutrophil infiltration and polarization in this setting. Methods- Focal cerebral ischemia was induced by occlusion of the middle cerebral artery occlusion in TLR4-KO and WT (wild type) mice. Infarct size was measured by Nissl staining and magnetic resonance imaging. Leukocyte infiltration was quantified 48 hours after middle cerebral artery occlusion by immunofluorescence and flow cytometry. To elucidate mechanisms underlying TLR4-mediated N2 phenotype, a cDNA microarray analysis was performed in neutrophils isolated from blood 48 hours after stroke in WT and TLR4-KO mice. Results- As demonstrated previously, TLR4-deficient mice presented lesser infarct volumes than WT mice. TLR4-deficient mice showed higher density of infiltrated neutrophils 48 hours after stroke compared with WT mice, concomitantly to neuroprotection. Furthermore, cytometric and stereological analyses revealed an increased number of N2 neutrophils (YM1+ cells) into the ischemic core in TLR4-deficient mice, suggesting a protective effect of this neutrophil subset that was corroborated by depleting peripheral neutrophils or using mice with TLR4 genetically ablated in the myeloid lineage. Finally, cDNA microarray analysis in neutrophils, confirmed by quantitative polymerase chain reaction, showed that TLR4 modulates several pathways associated with ischemia-induced inflammation, migration of neutrophils into the parenchyma, and their functional priming, which might explain the opposite effect on outcome of the different neutrophil subsets. Conclusions- TLR4 deficiency increased the levels of alternative neutrophils (N2)-an effect associated with neuroprotection after stroke-supporting that modulation of neutrophil polarization is a major target of TLR4 and highlighting the crucial role of TLR4 at the peripheral level after stroke. Visual Overview- An online visual overview is available for this article.

Palmitate Conditions Macrophages for Enhanced Responses toward Inflammatory Stimuli via JNK Activation.

Obesity is associated with low-grade inflammation and elevated levels of circulating saturated fatty acids, which trigger inflammatory responses by engaging pattern recognition receptors in macrophages. Because tissue homeostasis is maintained through an adequate balance of pro- and anti-inflammatory macrophages, we assessed the transcriptional and functional profile of M-CSF-dependent monocyte-derived human macrophages exposed to concentrations of saturated fatty acids found in obese individuals. We report that palmitate (C16:0, 200 µM) significantly modulates the macrophage gene signature, lowers the expression of transcription factors that positively regulate IL-10 expression (MAFB, AhR), and promotes a proinflammatory state whose acquisition requires JNK activation. Unlike LPS, palmitate exposure does not activate STAT1, and its transcriptional effects can be distinguished from those triggered by LPS, as both agents oppositely regulate the expression of CCL19 and TRIB3 Besides, palmitate conditions macrophages for exacerbated proinflammatory responses (lower IL-10 and CCL2, higher TNF-α, IL-6, and IL-1ß) toward pathogenic stimuli, a process also mediated by JNK activation. All of these effects of palmitate are fatty acid specific because oleate (C18:1, 200 µM) does not modify the macrophage transcriptional and functional profiles. Therefore, pathologic palmitate concentrations promote the acquisition of a specific polarization state in human macrophages and condition macrophages for enhanced responses toward inflammatory stimuli, with both effects being dependent on JNK activation. Our results provide further insight into the macrophage contribution to obesity-associated inflammation.

MAFB Determines Human Macrophage Anti-Inflammatory Polarization: Relevance for the Pathogenic Mechanisms Operating in Multicentric Carpotarsal Osteolysis.

Macrophage phenotypic and functional heterogeneity derives from tissue-specific transcriptional signatures shaped by the local microenvironment. Most studies addressing the molecular basis for macrophage heterogeneity have focused on murine cells, whereas the factors controlling the functional specialization of human macrophages are less known. M-CSF drives the generation of human monocyte-derived macrophages with a potent anti-inflammatory activity upon stimulation. We now report that knockdown of MAFB impairs the acquisition of the anti-inflammatory profile of human macrophages, identify the MAFB-dependent gene signature in human macrophages and illustrate the coexpression of MAFB and MAFB-target genes in CD163+ tissue-resident and tumor-associated macrophages. The contribution of MAFB to the homeostatic/anti-inflammatory macrophage profile is further supported by the skewed polarization of monocyte-derived macrophages from multicentric carpotarsal osteolysis (Online Mendelian Inheritance in Man #166300), a pathology caused by mutations in the MAFB gene. Our results demonstrate that MAFB critically determines the acquisition of the anti-inflammatory transcriptional and functional profiles of human macrophages.

Mice Lacking Endoglin in Macrophages Show an Impaired Immune Response.

Endoglin is an auxiliary receptor for members of the TGF-ß superfamily and plays an important role in the homeostasis of the vessel wall. Mutations in endoglin gene (ENG) or in the closely related TGF-ß receptor type I ACVRL1/ALK1 are responsible for a rare dominant vascular dysplasia, the Hereditary Hemorrhagic Telangiectasia (HHT), or Rendu-Osler-Weber syndrome. Endoglin is also expressed in human macrophages, but its role in macrophage function remains unknown. In this work, we show that endoglin expression is triggered during the monocyte-macrophage differentiation process, both in vitro and during the in vivo differentiation of blood monocytes recruited to foci of inflammation in wild-type C57BL/6 mice. To analyze the role of endoglin in macrophages in vivo, an endoglin myeloid lineage specific knock-out mouse line (Eng(fl/fl)LysMCre) was generated. These mice show a predisposition to develop spontaneous infections by opportunistic bacteria. Eng(fl/fl)LysMCre mice also display increased survival following LPS-induced peritonitis, suggesting a delayed immune response. Phagocytic activity is impaired in peritoneal macrophages, altering one of the main functions of macrophages which contributes to the initiation of the immune response. We also observed altered expression of TGF-ß1 target genes in endoglin deficient peritoneal macrophages. Overall, the altered immune activity of endoglin deficient macrophages could help to explain the higher rate of infectious diseases seen in HHT1 patients.

MMP-12, Secreted by Pro-Inflammatory Macrophages, Targets Endoglin in Human Macrophages and Endothelial Cells.

Upon inflammation, monocyte-derived macrophages (MΦ) infiltrate blood vessels to regulate several processes involved in vascular pathophysiology. However, little is known about the mediators involved. Macrophage polarization is crucial for a fast and efficient initial response (GM-MΦ) and a good resolution (M-MΦ) of the inflammatory process. The functional activity of polarized MΦ is exerted mainly through their secretome, which can target other cell types, including endothelial cells. Endoglin (CD105) is a cell surface receptor expressed by endothelial cells and MΦ that is markedly upregulated in inflammation and critically involved in angiogenesis. In addition, a soluble form of endoglin with anti-angiogenic activity has been described in inflammation-associated pathologies. The aim of this work was to identify components of the MΦ secretome involved in the shedding of soluble endoglin. We find that the GM-MΦ secretome contains metalloprotease 12 (MMP-12), a GM-MΦ specific marker that may account for the anti-angiogenic activity of the GM-MΦ secretome. Cell surface endoglin is present in both GM-MΦ and M-MΦ, but soluble endoglin is only detected in GM-MΦ culture supernatants. Moreover, MMP-12 is responsible for the shedding of soluble endoglin in vitro and in vivo by targeting membrane-bound endoglin in both MΦ and endothelial cells. These data demonstrate a direct correlation between GM-MΦ polarization, MMP-12, and soluble endoglin expression and function. By targeting endothelial cells, MMP-12 may represent a novel mediator involved in vascular homeostasis.

Evaluation of the potential therapeutic benefits of macrophage reprogramming in multiple myeloma.

Tumor-associated macrophages (TAM) are important components of the multiple myeloma (MM) microenvironment that support malignant plasma cell survival and resistance to therapy. It has been proposed that macrophages (MØ) retain the capacity to change in response to stimuli that can restore their antitumor functions. Here, we investigated several approaches to reprogram MØ as a novel therapeutic strategy in MM. First, we found tumor-limiting and tumor-supporting capabilities for monocyte-derived M1-like MØ and M2-like MØ, respectively, when mixed with MM cells, both in vitro and in vivo. Multicolor confocal microscopy revealed that MM-associated MØ displayed a predominant M2-like phenotype in the bone marrow of MM patient samples, and a high expression of the pro-M2 cytokine macrophage migration inhibitory factor (MIF). To reprogram the protumoral M2-like MØ present in MM toward antitumoral M1-like MØ, we tested the pro-M1 cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) plus blockade of the M2 cytokines macrophage colony-stimulating factor or MIF. The combination of GM-CSF plus the MIF inhibitor 4-iodo-6-phenyl-pyrimidine achieved the best reprogramming responses toward an M1 profile, at both gene and protein expression levels, as well as remarkable tumoricidal effects. Furthermore, this combined treatment elicited MØ-dependent therapeutic responses in MM xenograft mouse models, which were linked to upregulation of M1 and reciprocal downregulation of M2 MØ markers. Our results reveal the therapeutic potential of reprogramming MØ in the context of MM.

Surfactant Protein A Prevents IFN-γ/IFN-γ Receptor Interaction and Attenuates Classical Activation of Human Alveolar Macrophages.

Lung surfactant protein A (SP-A) plays an important function in modulating inflammation in the lung. However, the exact role of SP-A and the mechanism by which SP-A affects IFN-γ-induced activation of alveolar macrophages (aMϕs) remains unknown. To address these questions, we studied the effect of human SP-A on rat and human aMϕs stimulated with IFN-γ, LPS, and combinations thereof and measured the induction of proinflammatory mediators as well as SP-A's ability to bind to IFN-γ or IFN-γR1. We found that SP-A inhibited (IFN-γ + LPS)-induced TNF-α, iNOS, and CXCL10 production by rat aMϕs. When rat macrophages were stimulated with LPS and IFN-γ separately, SP-A inhibited both LPS-induced signaling and IFN-γ-elicited STAT1 phosphorylation. SP-A also decreased TNF-α and CXCL10 secretion by ex vivo-cultured human aMϕs and M-CSF-derived macrophages stimulated by either LPS or IFN-γ or both. Hence, SP-A inhibited upregulation of IFN-γ-inducible genes (CXCL10, RARRES3, and ETV7) as well as STAT1 phosphorylation in human M-CSF-derived macrophages. In addition, we found that SP-A bound to human IFN-γ (KD = 11 ± 0.5 nM) in a Ca(2+)-dependent manner and prevented IFN-γ interaction with IFN-γR1 on human aMϕs. We conclude that SP-A inhibition of (IFN-γ + LPS) stimulation is due to SP-A attenuation of both inflammatory agents and that the binding of SP-A to IFN-γ abrogates IFN-γ effects on human macrophages, suppressing their classical activation and subsequent inflammatory response.

Atypical Activin A and IL-10 Production Impairs Human CD16+ Monocyte Differentiation into Anti-Inflammatory Macrophages.

Human CD14(++)CD16(-) and CD14(+/lo)CD16(+) monocyte subsets comprise 85 and 15% of blood monocytes, respectively, and are thought to represent distinct stages in the monocyte differentiation pathway. However, the differentiation fates of both monocyte subsets along the macrophage (Mϕ) lineage have not yet been elucidated. We have now evaluated the potential of CD14(++) CD16(-) and CD16(+) monocytes to differentiate and to be primed toward pro- or anti-inflammatory Mϕs upon culture with GM-CSF or M-CSF, respectively (subsequently referred to as GM14, M14, GM16, or M16). Whereas GM16 and GM14 were phenotypic and functionally analogous, M16 displayed a more proinflammatory profile than did M14. Transcriptomic analyses evidenced that genes associated with M-CSF-driven Mϕ differentiation (including FOLR2, IL10, IGF1, and SERPINB2) are underrepresented in M16 with respect to M14. The preferential proinflammatory skewing of M16 relative to M14 was found to be mediated by the secretion of activin A and the low levels of IL-10 produced by M16. In fact, activin A receptor blockade during the M-CSF-driven differentiation of CD16(+) monocytes, or addition of IL-10-containing M14-conditioned medium, significantly enhanced their expression of anti-inflammatory-associated molecules while impairing their acquisition of proinflammatory-related markers. Thus, we propose that M-CSF drives CD14(++)CD16- monocyte differentiation into bona fide anti-inflammatory Mϕs in a self-autonomous manner, whereas M-CSF-treated CD16(+) monocytes generate Mϕs with a skewed proinflammatory profile by virtue of their high activin A expression unless additional anti-inflammatory stimuli such as IL-10 are provided.

Reshaping of Human Macrophage Polarization through Modulation of Glucose Catabolic Pathways.

Macrophages integrate information from the tissue microenvironment and adjust their effector functions according to the prevalent extracellular stimuli. Therefore, macrophages can acquire a variety of activation (polarization) states, and this functional plasticity allows the adequate initiation, regulation, and resolution of inflammatory responses. Modulation of the glucose metabolism contributes to the macrophage adaptation to the surrounding cytokine milieu, as exemplified by the distinct glucose catabolism of macrophages exposed to LPS/IFN-γ or IL-4. To dissect the acquisition of macrophage effector functions in the absence of activating cytokines, we assessed the bioenergetic profile of macrophages generated in the presence of GM-CSF (GM-MØ) or M-CSF (M-MØ), which do not release pro- or anti-inflammatory cytokines unless subjected to additional activating stimuli. Compared to M-MØ, GM-MØ displayed higher oxygen consumption rate and aerobic glycolysis (extracellular acidification rate [ECAR]), as well as higher expression of genes encoding glycolytic enzymes. However, M-MØ exhibited a significantly higher oxygen consumption rate/ECAR ratio. Surprisingly, whereas aerobic glycolysis positively regulated IL1B, TNF, and INHBA mRNA expression in both macrophage subtypes, mitochondrial respiration negatively affected IL6, IL1B, TNF, and CXCL10 mRNA expression in M-MØ. The physiological significance of these results became evident under low oxygen tensions, as hypoxia enhanced ECAR in M-MØ via HIF-1α and HIF-2α, increased expression of glycolytic enzymes and GM-MØ-specific genes, and diminished expression of M-MØ-associated genes. Therefore, our data indicate that GM-MØ and M-MØ display distinct bioenergetic profiles, and that hypoxia triggers a transcriptomic switch in macrophages by promoting a HIF-1α/HIF-2α-dependent increase in ECAR.

Expression of endoglin isoforms in the myeloid lineage and their role during aging and macrophage polarization.

Endoglin plays a crucial role in pathophysiological processes such as hereditary hemorrhagic telangiectasia (HHT), preeclampsia and cancer. Endoglin expression is upregulated during the monocyte-to-macrophage transition, but little is known about its regulation and function in these immune cells. Two different alternatively spliced isoforms of endoglin have been reported, L-endoglin and S-endoglin. Although L-endoglin is the predominant variant, here, we found that there was an increased expression of the S-endoglin isoform during senescence of the myeloid lineage in human and murine models. We performed a stable isotope labelling of amino acids in cell culture (SILAC) analysis of both L-endoglin and S-endoglin transfectants in the human promonocytic cell line U937. Analysis of differentially expressed protein clusters allowed the identification of cellular activities affected during aging. S-endoglin expression led to decreased cellular proliferation and a decreased survival response to granulocyte-macrophage colony-stimulating factor (GM-CSF)-induced apoptosis, as well as increased oxidative stress. Gene expression and functional studies suggested that there was a non-redundant role for each endoglin isoform in monocyte biology. In addition, we found that S-endoglin impairs the monocytic differentiation into the pro-inflammatory M1 phenotype and contributes to the compromised status of macrophage functions during aging.

Macrophages from the synovium of active rheumatoid arthritis exhibit an activin A-dependent pro-inflammatory profile.

Rheumatoid arthritis (RA) is a chronic inflammatory disease whose pathogenesis and severity correlates with the presence of macrophage-derived pro-inflammatory cytokines within the inflamed synovium. Macrophage-derived cytokines fuel the pathological processes in RA and are targets of clinically successful therapies. However, although macrophage polarization determines cytokine production, the polarization state of macrophages in RA joints remains poorly defined. To dissect the molecular basis for the tissue-damaging effects of macrophages in RA joints, we undertook the phenotypic and transcriptomic characterization of ex vivo isolated CD14(+) RA synovial fluid (RA-SF) macrophages. Flow cytometry and gene profiling indicated that RA-SF macrophages express pro-inflammatory polarization markers (MMP12, EGLN3, CCR2), lack expression of markers associated with homeostatic and anti-inflammatory polarization (IGF1, HTR2B) and exhibit a transcriptomic profile that resembles the activin A-dependent gene signature of pro-inflammatory in vitro-generated macrophages. In fact, high levels of Smad-activating activin A were found in RA-SF and, accordingly, the Smad signalling pathway was activated in ex vivo-isolated RA-SF macrophages. In vitro experiments on monocytes and macrophages indicated that RA-SF promoted the acquisition of pro-inflammatory markers (INHBA, MMP12, EGLN3, CCR2) but led to a significant reduction in the expression of genes associated with homeostasis and inflammation resolution (FOLR2, SERPINB2, IGF1, CD36), thus confirming the pro-inflammatory polarization ability of RA-SF. Importantly, the macrophage-polarizing ability of RA-SF was inhibited by an anti-activin A-neutralizing antibody, thus demonstrating that activin A mediates the pro-inflammatory macrophage-polarizing ability of RA-SF. Moreover, and in line with these findings, multicolour immunofluorescence evidenced that macrophages within RA synovial membranes (RA-SM) also express pro-inflammatory polarization markers whose expression is activin A-dependent. Altogether, our results demonstrate that macrophages from RA synovial fluids and membranes exhibit an MMP12(+) EGLN3(+) CCR2(+) pro-inflammatory polarization state whose acquisition is partly dependent on activin A from the synovial fluid.