Obstacles and opportunities for understanding macrophage polarization

Pro-angiogenic and anti-inflammatory regulation by functional peptides loaded in polymeric implants for soft tissue regeneration

Inflammation and angiogenesis are inevitable in vivo responses to biomaterial implants. Continuous progress has been made in biomaterial design to improve tissue interactions with an implant by either reducing inflammation or promoting angiogenesis. However, it has become increasingly clear that the physiological processes of inflammation and angiogenesis are interconnected through various molecular mechanisms. Hence, there is an unmet need for engineering functional tissues by simultaneous activation of pro-angiogenic and anti-inflammatory responses to biomaterial implants. In this work, the modulus and fibrinogen adsorption of porous scaffolds were tuned to meet the requirements (i.e., ~100 kPa and ~10 nm, respectively), for soft tissue regeneration by employing tyrosine-derived combinatorial polymers with polyethylene glycol crosslinkers. Two types of functional peptides (i.e., pro-angiogenic laminin-derived C16 and anti-inflammatory thymosin β4-derived Ac-SDKP) were loaded in porous scaffolds through collagen gel embedding so that peptides were released in a controlled fashion, mimicking degradation of the extracellular matrix. The results from (1) in vitro coculture of human umbilical vein endothelial cells and human blood-derived macrophages and (2) in vivo subcutaneous implantation revealed the directly proportional relationship between angiogenic activities (i.e., tubulogenesis and perfusion capacity) and inflammatory activities (i.e., phagocytosis and F4/80 expression) upon treatment with either type of peptide. Interestingly, cotreatment with both types of peptides upregulated the angiogenic responses, while downregulating the inflammatory responses. Also, anti-inflammatory Ac-SDKP peptides reduced production of pro-inflammatory cytokines (i.e., interleukin [IL]-1β, IL-6, IL-8, and tumor necrosis factor alpha) even when treated in combination with pro-angiogenic C16 peptides. In addition to independent regulation of angiogenesis and inflammation, this study suggests a promising approach to improve soft tissue regeneration (e.g., blood vessel and heart muscle) when inflammatory diseases (e.g., ischemic tissue fibrosis and atherosclerosis) limit the regeneration process.

Salvianolic Acid B reducing portal hypertension depends on macrophages in isolated portal perfused rat livers with chronic hepatitis

This study is aimed to investigate the effects of Sal B on portal hypertension (PH). PH with chronic hepatitis was induced by carbon tetrachloride (CCl₄) in rats. The model was confirmed with elevated portal pressures and increased serum CD163 levels. The inducible nitric oxide synthase (iNOS) or heme oxygenase-1 (HO-1) in portal triads was assessed. The isolated portal perfused rat liver (IPPRL) was performed at d₀, d₂₈, d₅₆ , and d₈₄ in the progression of chronic hepatitis. After constricting with phenylephrine, the portal veins were relaxed with Sal B. The EC₅₀ of Sal B for relaxing portal veins was −2.04 × 10⁻⁹, 7.28 × 10⁻¹¹, 1.52 × 10⁻¹¹, and 8.44 × 10⁻¹¹ mol/L at d₀, d₂₈, d₅₆, and d₈₄, respectively. More macrophages infiltrated in portal triads and expressed more iNOS or HO-1 as PH advanced. The areas under the curve (AUCs) of Sal B for reducing PH were positively correlated with the levels of iNOS or HO-1 in portal triads, and so did with serum CD163 levels. Sal B reduces PH in IPPRL with chronic hepatitis, via promoting portal relaxation due to macrophage-originated NO or CO in portal triads, partly at least.

Tumor-associated macrophages: function, phenotype, and link to prognosis in human lung cancer

Macrophages are the dominant leukocyte population found in the tumor microenvironment. Accumulating evidence suggests that these tumor-associated macrophages (TAMs) actively promote all aspects of tumor initiation, growth, and development. However, TAMs are not a single uniform population; instead, they are composed of multiple distinct pro- and anti-tumoral subpopulations with overlapping features depending on a variety of external factors. Defining and differentiating these subsets remains a challenging work-in-progress. These difficulties are apparent in prognostic studies in lung cancer that initially demonstrated conflicting evidence regarding the significance of TAMs but which have more recently clarified and confirmed the clinical importance of these subsets through improved phenotypic capabilities. Thus, these cells represent potential targets for cancer therapeutic initiatives through translational approaches. In this review, we summarize the current understanding of how the tumor microenvironment takes advantage of macrophage plasticity to mold an immunosuppressive population, the phenotypic heterogeneity of TAMs, and their link to prognosis in human lung cancer.

Immune-mediated necrotizing myopathy is characterized by a specific Th1-M1 polarized immune profile

Immune-mediated necrotizing myopathy (IMNM) is considered one of the idiopathic inflammatory myopathies, comprising dermatomyositis, polymyositis, and inclusion body myositis. The heterogeneous group of necrotizing myopathies shows a varying amount of necrotic muscle fibers, myophagocytosis, and a sparse inflammatory infiltrate. The underlying immune response in necrotizing myopathy has not yet been addressed in detail. Affected muscle tissue, obtained from 16 patients with IMNM, was analyzed compared with eight non-IMNM (nIMNM) tissues. Inflammatory cells were characterized by IHC, and immune mediators were assessed by quantitative real-time PCR. We demonstrate that immune- and non-immune-mediated disease can be distinguished by a specific immune profile with significantly more prominent major histocompatibility complex class I expression and complement deposition and a conspicuous inflammatory infiltrate. In addition, patients with IMNM exhibit a strong type 1 helper T cell (T1)/classically activated macrophage M1 response, with detection of elevated interferon-γ, tumor necrosis factor-α, IL-12, and STAT1 levels in the muscle tissue, which may serve as biomarkers and aid in diagnostic decisions. Furthermore, B cells and high expression of the chemoattractant CXCL13 were identified in a subgroup of patients with defined autoantibodies. Taken together, we propose a diagnostic armamentarium that allows for clear differentiation between IMNM and nIMNM. In addition, we have characterized a Th1-driven, M1-mediated immune response in most of the autoimmune necrotizing myopathies, which may guide therapeutic options in the future.

SOCS3 deficiency promotes M1 macrophage polarization and inflammation

Macrophages participate in both the amplification of inflammation at the time of injury and downregulation of the inflammatory response to avoid excess tissue damage. These divergent functions of macrophages are dictated by their microenvironment, especially cytokines, which promote a spectrum of macrophage phenotypes. The M1 proinflammatory phenotype is induced by LPS, IFN-γ, and GM-CSF, and IL-4, IL-13, and M-CSF induce anti-inflammatory M2 macrophages. Suppressors of cytokine signaling (SOCS) proteins function as feedback inhibitors of the JAK/STAT signaling pathway, and they can terminate innate and adaptive immune responses. In this study, we have evaluated the influence of SOCS3 on macrophage polarization and function. Macrophages obtained from LysMCre-SOCS3(fl/fl) mice, which lack SOCS3 in myeloid lineage cells, exhibit enhanced and prolonged activation of the JAK/STAT pathway compared with macrophages from SOCS3(fl/fl) mice. Furthermore, SOCS3-deficient macrophages have higher levels of the M1 genes IL-1β, IL-6, IL-12, IL-23, and inducible NO synthase owing to enhanced transcriptional activation and chromatin modifications. SOCS3-deficient M1 macrophages also have a stronger capacity to induce Th1 and Th17 cell differentiation than M1 macrophages from SOCS3(fl/fl) mice. Lastly, LPS-induced sepsis is exacerbated in LysMCre-SOCS3(fl/fl) mice and is associated with enhanced STAT1/3 activation and increased plasma levels of M1 cytokines/chemokines such as IL-1β, TNF-α, IL-6, CCL3, CCL4, and CXCL11. These findings collectively indicate that SOCS3 is involved in repressing the M1 proinflammatory phenotype, thereby deactivating inflammatory responses in macrophages.

Blood leukocytes and macrophages of various phenotypes have distinct abilities to form podosomes and to migrate in 3D environments

Leukocytes migrate through most tissues in the body, a process which takes place in 3D environments. We have previously shown that macrophages use the amoeboid migration mode in porous matrices such as fibrillar collagen I and the mesenchymal mode involving podosomes and matrix proteolysis in dense matrices such as Matrigel. Whether such a plasticity may apply to other leukocytes and to all subsets of macrophages is unknown. Here, we therefore provide a comparative analysis of the in vitro 3D migration modes adopted by primary human leukocytes. Blood-derived monocytes, neutrophils and T lymphocytes were found to use the amoeboid mode in a porous fibrillar collagen I matrix but were unable to infiltrate dense Matrigel and to form podosomes. M2-polarized macrophages and elicited peritoneal macrophages formed podosome rosettes, degraded the ECM and infiltrated both matrices. In contrast, M1 macrophages were motionless in 2D and 3D environments, whilst resident macrophages, devoid of podosomes, were only able to use the amoeboid mode. Thus, we conclude that whereas all leukocytes use the amoeboid mode to migrate through porous matrices, it is only certain macrophages that can adopt the mesenchymal mode that permits migration through dense matrices. Interestingly, the acquisition of mesenchymal migration capacity by macrophages correlates with the presence of podosomes and with their capacity to organize those as rosettes, which appears to be modulated by their differentiation and polarization states. As a perspective, specific control of the mesenchymal migration would be a potential target for therapeutic approaches aiming at decreasing macrophage tissue infiltration.

TAp73 is required for macrophage-mediated innate immunity and the resolution of inflammatory responses

The multiple isoforms of p73, a member of the p53 family, share the ability to modulate p53 activities but also have unique properties, leading to a complex and poorly understood functional network. In vivo, p73 isoforms have been implicated in tumor suppression (TAp73(-/-) mice), DNA damage (ΔNp73(-/-) mice) and development (p73(-/-) mice). In this study, we investigated whether TAp73 contributes to innate immunity and septic shock. In response to a lethal lipopolysaccharide (LPS) challenge, TAp73(-/-) mice showed higher blood levels of proinflammatory cytokines and greater mortality than their wild-type littermates. In vitro, TAp73(-/-) macrophages exhibited elevated production of tumor necrosis factor alpha , interleukin-6 and macrophage inflammatory protein-2 as well as prolonged survival, decreased phagocytosis and increased major histocompatibility complex class II expression. Mice depleted of endogenous macrophages and reconstituted with TAp73(-/-) macrophages showed increased sensitivity to LPS challenge. These results suggest that macrophage polarization is altered in the absence of TAp73 such that maintenance of the M1 effector phenotype is prolonged at the expense of the M2 phenotype, thus impairing resolution of the inflammatory response. Our data indicate that TAp73 has a role in macrophage polarization and innate immunity, enhancing the action field of this important regulatory molecule.

Phenotypic characterization of lung macrophages in asthmatic patients: overexpression of CCL17

BACKGROUND

Studies with monocyte-derived macrophages (MDMs) and animal models have suggested a role for alternatively activated (M2) macrophages in asthmatic inflammation, but in vivo evidence for this phenotype in human asthma is lacking.

OBJECTIVE

To characterize the phenotype of lung macrophages from asthmatic patients in relation to disease severity and treatment.

METHODS

M2 biomarkers were first identified by using MDMs exposed to T(H)2 cytokines and then used to phenotype sputum and bronchoalveolar lavage (BAL) macrophages from 12 healthy control subjects, 12 patients with mild asthma, and 14 patients with moderate asthma and to assess the effects of corticosteroids and phosphatidylinositol 3-kinase (PI3K) inhibitors.

RESULTS

Sputum macrophages from asthmatic patients expressed significantly more CCL17 mRNA but less CD163 than macrophages from healthy subjects. However, none of the other M2 biomarkers were differentially expressed in asthmatic patients, and ex vivo BAL cells spontaneously produced similar amounts of M2 cytokines/chemokines (IL-10, CCL17, and CCL22). CCL17 mRNA overexpression correlated weakly but significantly with sputum eosinophilia (P = .0252) and was also observed in macrophages from patients with moderate asthma treated with inhaled steroids, suggesting relative insensitivity to inhibition by corticosteroids. The PI3K inhibitor LY294002 inhibited basal CCL17 release from BAL cells and IL-4-stimulated release from MDMs.

CONCLUSIONS

This study does not support the existence in human asthma of the full M2 phenotype described to date but points to upregulation of CCL17 in both patients with mild and those with moderate asthma, providing a further source for this ligand of CCR4(+) cells that contributes to airways inflammation. CCL17 expression is corticosteroid resistant but suppressed by PI3K enzyme inhibitors.

Arginase-1 expression in granulomas of tuberculosis patients

Mycobacterium tuberculosis (Mtb) is an intracellular pathogen able to survive and multiply within macrophages. Several mechanisms allow this bacterium to escape macrophage microbicidal activity. Mtb may interfere with the ability of mouse macrophages to produce antibactericidal nitric oxide, by inducing the expression of arginase 1 (Arg1). It remains unclear whether this pathway has a role in humans infected with Mtb. In this study, we investigated the expression of Arg1 in granulomas of human lung tissues from patients with tuberculosis. We show that Arg1 is expressed not only in granuloma-associated macrophages, but also in type II pneumocytes.

Induction of IL-4Rα-dependent microRNAs identifies PI3K/Akt signaling as essential for IL-4-driven murine macrophage proliferation in vivo

Macrophage (MΦ) activation must be tightly controlled to preclude overzealous responses that cause self-damage. MicroRNAs promote classical MΦ activation by blocking antiinflammatory signals and transcription factors but also can prevent excessive TLR signaling. In contrast, the microRNA profile associated with alternatively activated MΦ and their role in regulating wound healing or antihelminthic responses has not been described. By using an in vivo model of alternative activation in which adult Brugia malayi nematodes are implanted surgically in the peritoneal cavity of mice, we identified differential expression of miR-125b-5p, miR-146a-5p, miR-199b-5p, and miR-378-3p in helminth-induced MΦ. In vitro experiments demonstrated that miR-378-3p was specifically induced by IL-4 and revealed the IL-4-receptor/PI3K/Akt-signaling pathway as a target. Chemical inhibition of this pathway showed that intact Akt signaling is an important enhancement factor for alternative activation in vitro and in vivo and is essential for IL-4-driven MΦ proliferation in vivo. Thus, identification of miR-378-3p as an IL-4Rα-induced microRNA led to the discovery that Akt regulates the newly discovered mechanism of IL-4-driven macrophage proliferation. Together, the data suggest that negative regulation of Akt signaling via microRNAs might play a central role in limiting MΦ expansion and alternative activation during type 2 inflammatory settings.

Transformation from a neuroprotective to a neurotoxic microglial phenotype in a mouse model of ALS

Neuroinflammation is a prominent pathological feature in the spinal cords of patients with amyotrophic lateral sclerosis (ALS), as well as in transgenic mouse models of inherited ALS, and is characterized by activated microglia. Earlier studies showed that activated microglia play important roles in both motoneuron protection and injury. More recent studies investigating the pathoprogression of disease in ALS mice have demonstrated that the in vivo activation states of microglia, including their anti- versus pro-inflammatory responses, are best characterized as a continuum between two extreme activation states which are represented as a neuroprotective M2 (alternatively-activated) phenotypic state or an injurious/toxic M1 (classically-activated) state; a more complete understanding and determination the temporal transformation of microglia activation states in the ALS disease pathoprogression is therefore warranted. In the current study, we demonstrated a phenotypic and functional transformation of adult ALS mice microglia that overexpress mutant superoxide dismutase (mSOD1). mSOD1 microglia isolated from ALS mice at disease onset expressed higher levels of Ym1, CD163 and BDNF (markers of M2) mRNA and lower levels of Nox2 (a marker of M1) mRNA compared with mSOD1 microglia isolated from ALS mice at end-stage disease. More importantly, when co-cultured with motoneurons, these mSOD1 M2 microglia were neuroprotective and enhanced motoneuron survival than similarly co-cultured mSOD1 M1 microglia; end-stage mSOD1 M1 microglia were toxic to motoneurons. Our study documents that adult microglia isolated from ALS mice at disease onset have an M2 phenotype and protect motoneurons whereas microglia isolated from end-stage disease ALS mice have adopted an M1 phenotype and are neurotoxic supporting the dual phenotypes of microglia and their transformation during disease pathoprogression in these mice. Thus, harnessing the neuroprotective potential of microglia may provide novel avenues for ALS therapies.

Principal component analysis of the cytokine and chemokine response to human traumatic brain injury

There is a growing realisation that neuro-inflammation plays a fundamental role in the pathology of Traumatic Brain Injury (TBI). This has led to the search for biomarkers that reflect these underlying inflammatory processes using techniques such as cerebral microdialysis. The interpretation of such biomarker data has been limited by the statistical methods used. When analysing data of this sort the multiple putative interactions between mediators need to be considered as well as the timing of production and high degree of statistical co-variance in levels of these mediators. Here we present a cytokine and chemokine dataset from human brain following human traumatic brain injury and use principal component analysis and partial least squares discriminant analysis to demonstrate the pattern of production following TBI, distinct phases of the humoral inflammatory response and the differing patterns of response in brain and in peripheral blood. This technique has the added advantage of making no assumptions about the Relative Recovery (RR) of microdialysis derived parameters. Taken together these techniques can be used in complex microdialysis datasets to summarise the data succinctly and generate hypotheses for future study.

Macrophages in tumor microenvironments and the progression of tumors

Macrophages are widely distributed innate immune cells that play indispensable roles in the innate and adaptive immune response to pathogens and in-tissue homeostasis. Macrophages can be activated by a variety of stimuli and polarized to functionally different phenotypes. Two distinct subsets of macrophages have been proposed, including classically activated (M1) and alternatively activated (M2) macrophages. M1 macrophages express a series of proinflammatory cytokines, chemokines, and effector molecules, such as IL-12, IL-23, TNF-α, iNOS and MHCI/II. In contrast, M2 macrophages express a wide array of anti-inflammatory molecules, such as IL-10, TGF-β, and arginase1. In most tumors, the infiltrated macrophages are considered to be of the M2 phenotype, which provides an immunosuppressive microenvironment for tumor growth. Furthermore, tumor-associated macrophages secrete many cytokines, chemokines, and proteases, which promote tumor angiogenesis, growth, metastasis, and immunosuppression. Recently, it was also found that tumor-associated macrophages interact with cancer stem cells. This interaction leads to tumorigenesis, metastasis, and drug resistance. So mediating macrophage to resist tumors is considered to be potential therapy.

The E3 ubiquitin ligase neuregulin receptor degradation protein 1 (Nrdp1) promotes M2 macrophage polarization by ubiquitinating and activating transcription factor CCAAT/enhancer-binding Protein β (C/EBPβ)

Macrophage activation, including classical (M1) activation and alternative (M2) activation, plays important roles in host immune response and pathogenesis of diseases. Ubiquitination has been shown to be involved in the differentiation of immune cells and in the regulation of immune responses. However, the role of ubiquitination during M1 versus M2 polarization is poorly explored. Here, we showed that arginase 1 (Arg1), a well recognized marker of M2 macrophages, is highly up-regulated in peritoneal macrophages derived from E3 ubiquitin ligase Nrdp1 transgenic (Nrdp1-TG) mice. Furthermore, other M2 feature markers such as MR, Ym1, and Fizz1, as well as Th2 cytokine IL-10, are also up-regulated in Nrdp1-TG macrophages after IL-4 stimulation. Knockdown of Nrdp1 expression effectively inhibits IL-4-induced expression of M2-related genes in macrophages. Moreover, Nrdp1 inhibits LPS-induced production of inducible NOS and pro-inflammatory cytokines TNF-α, IL-1β, and IL-6 in macrophages. Immunoprecipitation assays show that Nrdp1 interacts with and ubiquitinates transcriptional factor C/EBPβ via Lys-63-linked ubiquitination. Nrdp1 enhances C/EBPβ-triggered transcriptional activation of the Arg1 reporter gene in the presence of IL-4 stimulation. Thus, we demonstrate that Nrdp1-mediated ubiquitination and activation of C/EBPβ contributes to a ubiquitin-dependent nonproteolytic pathway that up-regulates Arg1 expression and promotes M2 macrophage polarization.

Distinctive effects of T cell subsets in neuronal injury induced by cocultured splenocytes in vitro and by in vivo stroke in mice

BACKGROUND AND PURPOSE

T cells and their subsets modulate ischemic brain injury. We studied the effects of the absence of T cell subsets on brain infarction after in vivo stroke and then used an in vitro coculture system of splenocytes and neurons to further identify the roles of T cell subsets in neuronal death.

METHODS

Stroke was induced by middle cerebral artery suture occlusion in mice and infarct sizes were measured 2 days poststroke. Splenocytes were cocultured with neurons, and neuronal survival was measured 3 days later.

RESULTS

A deficiency of both T and B cells (severe combined immunodeficiency) and the paucity of CD4 or CD8 T cells equally resulted in smaller infarct sizes as measured 2 days poststroke. Although a functional deficiency of regulatory T cells had no effect, impaired Th1 immunity reduced infarction and impaired Th2 immunity aggravated brain injury, which may be due to an inhibited and enhanced inflammatory response in mice deficient in Th1 and Th2 immunity, respectively. In the in vitro coculture system, wild-type splenocytes resulted in dose-dependent neuronal death. The neurotoxicity of splenocytes from these immunodeficient mice was consistent with their effects on stroke in vivo, except for the mice with the paucity of CD4 or CD8 T cells, which did not alter the ratio of neuronal death.

CONCLUSIONS

T cell subsets play critical roles in brain injury induced by stroke. The detrimental versus beneficial effects of Th1 cells and Th2 cells both in vivo and in vitro reveal differential therapeutic target strategies for stroke treatment.

Thioredoxin-1 Promotes Anti-Inflammatory Macrophages of the M2 Phenotype and Antagonizes Atherosclerosis

Objective—

Oxidative stress is believed to play a key role in cardiovascular disorders. Thioredoxin (Trx) is an oxidative stress-limiting protein with anti-inflammatory and antiapoptotic properties. Here, we analyzed whether Trx-1 might exert atheroprotective effects by promoting macrophage differentiation into the M2 anti-inflammatory phenotype.

Methods and Results—

Trx-1 at 1 μg/mL induced downregulation of p16 INK4a and significantly promoted the polarization of anti-inflammatory M2 macrophages in macrophages exposed to interleukin (IL)-4 at 15 ng/mL or IL-4/IL-13 (10 ng/mL each) in vitro, as evidenced by the expression of the CD206 and IL-10 markers. In addition, Trx-1 induced downregulation of nuclear translocation of activator protein-1 and Ref-1, and significantly reduced the lipopolysaccharide-induced differentiation of inflammatory M1 macrophages, as indicated by the decreased expression of the M1 cytokines, tumor necrosis factor-α and monocyte chemoattractant protein-1. Consistently, Trx-1 administered to hyperlipoproteinemic ApoE2.Ki mice at 30 μg/30 g body weight challenged either with lipopolysaccharide at 30 μg/30 g body weight or with IL-4 at 500 ng/30 g body weight significantly induced the M2 phenotype while inhibiting differentiation of macrophages into the M1 phenotype in liver and thymus. ApoE2.Ki mice challenged once weekly with lipopolysaccharide for 5 weeks developed severe atherosclerotic lesions enriched with macrophages expressing predominantly M1 over M2 markers. In contrast, however, daily injections of Trx-1 shifted the phenotype pattern of lesional macrophages in these animals to predominantly M2 over M1, and the aortic lesion area was significantly reduced (from 100%±18% to 62.8%±9.8%; n=8; P <0.01). Consistently, Trx-1 colocalized with M2 but not with M1 macrophage markers in human atherosclerotic vessel specimens.

Conclusion—

The ability of Trx-1 to promote differentiation of macrophages into an alternative, anti-inflammatory phenotype may explain its protective effects in cardiovascular diseases. These data provide novel insight into the link between oxidative stress and cardiovascular diseases.

Characteristics of suppressor macrophages induced by mycobacterial and protozoal infections in relation to alternatively activated M2 macrophages

In the advanced stages of mycobacterial infections, host immune systems tend to change from a Th1-type to Th2-type immune response, resulting in the abrogation of Th1 cell- and macrophage-mediated antimicrobial host protective immunity. Notably, this type of immune conversion is occasionally associated with the generation of certain types of suppressor macrophage populations. During the course of Mycobacterium tuberculosis (MTB) and Mycobacterium avium-intracellulare complex (MAC) infections, the generation of macrophages which possess strong suppressor activity against host T- and B-cell functions is frequently encountered. This paper describes the immunological properties of M1- and M2-type macrophages generated in tumor-bearing animals and those generated in hosts with certain microbial infections. In addition, this paper highlights the immunological and molecular biological characteristics of suppressor macrophages generated in hosts with mycobacterial infections, especially MAC infection.

Non-canonical alternatives: what a macrophage is 4

Macrophages play pleiotropic, niche-specific roles in all tissues and organs. As immune sentinels, tissue macrophages regulate immune activation and inflammation; in turn, their function is modulated by inflammatory mediators deriving from such activation. Recent papers have established unanticipated roles for interleukin 4 and the alternative activation of tissue macrophages in the organismal response to diverse environmental stressors.

Macrophage differentiation and function in atherosclerosis: opportunities for therapeutic intervention?

The macrophage is exquisitely sensitive to its microenvironment, as demonstrated primarily through in vitro study. Changes in macrophage phenotype and function within the atherosclerotic plaque have profound consequences for plaque biology, including rupture and arterial thrombosis leading to clinical events such as myocardial infarction. We review the evidence for dynamic changes in macrophage numbers and macrophage differentiation within the atherosclerotic plaque microenvironment and discuss potential approaches to target macrophage differentiation for therapeutic benefit in cardiovascular disease.

Regulation of macrophage function by adenosine

Following its release into the extracellular space in response to metabolic disturbances, the endogenous nucleoside adenosine exerts a range of immunomodulatory effects and cells of the mononuclear phagocyte system are among its major targets. Adenosine governs mononuclear phagocyte functions via 4 G-protein-coupled cell membrane receptors, which are denoted A(1), A(2A), A(2B), and A(3) receptors. Adenosine promotes osteoclast differentiation via A(1) receptors and alters monocyte to dendritic cell differentiation through A(2B) receptors. Adenosine downregulates classical macrophage activation mainly through A(2A) receptors. In contrast A(2B) receptor activation upregulates alternative macrophage activation. Adenosine promotes angiogenesis, which is mediated by inducing the production of vascular endothelial growth factor by mononuclear phagocytes through A(2A), A(2B), and A(3) receptors. By regulating mononuclear phagocyte function adenosine dictates the course of inflammatory and vascular diseases and cancer.

SOCS, Inflammation, and Autoimmunity

Cytokines play essential roles in innate and adaptive immunity. However, excess cytokines or dysregulation of cytokine signaling will cause a variety of diseases, including allergies, autoimmune diseases, inflammation, and cancer. Most cytokines utilize the so-called Janus kinase–signal transducers and activators of transcription pathway. This pathway is negatively regulated by various mechanisms including suppressors of cytokine signaling (SOCS) proteins. SOCS proteins bind to JAK or cytokine receptors, thereby suppressing further signaling events. Especially, suppressor of cytokine signaling-1 (SOCS1) and SOCS3 are strong inhibitors of JAKs, because these two contain kinase inhibitory region at the N-terminus. Studies using conditional knockout mice have shown that SOCS proteins are key physiological as well as pathological regulators of immune homeostasis. Recent studies have also demonstrated that SOCS1 and SOCS3 are important regulators of helper T cell differentiation and functions. This review focuses on the roles of SOCS1 and SOCS3 in T cell mediated inflammatory diseases.

Living with the enemy or uninvited guests: functional genomics approaches to investigating host resistance or tolerance traits to a protozoan parasite, Theileria annulata, in cattle

Many breeds of cattle with long histories of living in areas of endemic disease have evolved mechanisms that enable them to co-exist with specific pathogens. Understanding the genes that control tolerance and resistance could provide new strategies to improve the health and welfare of livestock. Around one sixth of the world cattle population is estimated to be at risk from one of the most debilitating tick-borne diseases of cattle, caused by the protozoan parasite, Theileria annulata. The parasite mainly infects cells of the myeloid lineage which are also the main producers of inflammatory cytokines. If an infectious or inflammatory insult is sufficiently great, inflammatory cytokines produced by macrophages enter the circulation and induce an acute phase proteins (APP) response. The Bos taurus Holstein breed produces higher and more prolonged levels of inflammatory cytokine induced APP than the Bos indicus Sahiwal breed in response to experimental infection with T. annulata. The Sahiwal exhibits significantly less pathology and survives infection, unlike the Holstein breed. Therefore, we hypothesised that the causal genes were likely to be expressed in macrophages and control the production of inflammatory cytokines. A functional genomics approach revealed that the transcriptome profile of the B. taurus macrophages was more associated with an inflammatory programme than the B. indicus macrophages. In particular the most differentially expressed gene was a member of the signal regulatory protein (SIRP) family. These are mainly expressed on myeloid cell surfaces and control inflammatory responses. Other differentially expressed genes included bovine major histocompatibility complex (MHC) (BoLA) class II genes, particularly BoLA DQ, and transforming growth factor (TGF)B2. We are now exploring whether sequence and functional differences in the bovine SIRP family may underlie the resistance or tolerance to T. annulata between the breeds. Potentially, our research may also have more general implications for the control of inflammatory processes against other pathogens. Genes controlling the balance between pathology and protection may determine how livestock can survive in the face of infectious onslaught. Next generation sequencing and RNAi methodologies for livestock species will bring new opportunities to link diversity at the genome level to functional differences in health traits in livestock species.

Immunology in the clinic review series; focus on cancer: tumour-associated macrophages: undisputed stars of the inflammatory tumour microenvironment

Mononuclear phagocytes are cells of the innate immunity that defend the host against harmful pathogens and heal tissues after injury. Contrary to expectations, in malignancies, tumour-associated macrophages (TAM) promote disease progression by supporting cancer cell survival, proliferation and invasion. TAM and related myeloid cells [Tie2(+) monocytes and myeloid-derived suppressor cells (MDSC)] also promote tumour angiogenesis and suppress adaptive immune responses. These divergent biological activities are mediated by macrophages/myeloid cells with distinct functional polarization, which are ultimately dictated by microenvironmental cues. Clinical and experimental evidence has shown that cancer tissues with high infiltration of TAM are associated with poor patient prognosis and resistance to therapies. Targeting of macrophages in tumours is considered a promising therapeutic strategy: depletion of TAM or their 're-education' as anti-tumour effectors is under clinical investigation and will hopefully contribute to the success of conventional anti-cancer treatments.

Macrophage plasticity and polarization: in vivo veritas

Diversity and plasticity are hallmarks of cells of the monocyte-macrophage lineage. In response to IFNs, Toll-like receptor engagement, or IL-4/IL-13 signaling, macrophages undergo M1 (classical) or M2 (alternative) activation, which represent extremes of a continuum in a universe of activation states. Progress has now been made in defining the signaling pathways, transcriptional networks, and epigenetic mechanisms underlying M1-M2 or M2-like polarized activation. Functional skewing of mononuclear phagocytes occurs in vivo under physiological conditions (e.g., ontogenesis and pregnancy) and in pathology (allergic and chronic inflammation, tissue repair, infection, and cancer). However, in selected preclinical and clinical conditions, coexistence of cells in different activation states and unique or mixed phenotypes have been observed, a reflection of dynamic changes and complex tissue-derived signals. The identification of mechanisms and molecules associated with macrophage plasticity and polarized activation provides a basis for macrophage-centered diagnostic and therapeutic strategies.

Adoptive transfer of IL-4Rα+ macrophages is sufficient to enhance eosinophilic inflammation in a mouse model of allergic lung inflammation

BACKGROUND

The IL-4 receptor α (IL-4Rα) chain has a broad expression pattern and participates in IL-4 and IL-13 signaling, allowing it to influence several pathological components of allergic lung inflammation. We previously reported that IL-4Rα expression on both bone marrow-derived and non-bone marrow-derived cells contributed to the severity of allergic lung inflammation. There was a correlation between the number of macrophages expressing the IL-4Rα, CD11b, and IA(d), and the degree of eosinophilia in ovalbumin challenged mice. The engagement of the IL-4Rα by IL-4 or IL-13 is able to stimulate the alternative activation of macrophages (AAM). The presence of AAM has been correlated with inflammatory responses to parasites and allergens. Therefore, we hypothesized that IL-4Rα⁺ AAM play an active role in allergic lung inflammation. To directly determine the role of AAM in allergic lung inflammation, M-CSF-dependent macrophages (BMM) were prepared from the bone-marrow of IL-4Rα positive and negative mice and transferred to IL-4RαxRAG2(-/-) mice. Wild type TH2 cells were provided exogenously.

RESULTS

Mice receiving IL-4Rα(+/+) BMM showed a marked increase in the recruitment of eosinophils to the lung after challenge with ovalbumin as compared to mice receiving IL-4Rα(-/-) BMM. As expected, the eosinophilic inflammation was dependent on the presence of TH2 cells. Furthermore, we observed an increase in cells expressing F4/80 and Mac3, and the AAM marker YM1/2 in the lungs of mice receiving IL-4Rα(+/+) BMM. The BAL fluid from these mice contained elevated levels of eotaxin-1, RANTES, and CCL2.

CONCLUSIONS

These results demonstrate that transfer of IL-4Rα + macrophages is sufficient to enhance TH2-driven, allergic inflammation. They further show that stimulation of macrophages through IL-4Rα leads to their alternative activation and positive contribution to the TH2-driven allergic inflammatory response in the lung. Since an increase in AAM and their products has been observed in patients with asthma exacerbations, these results suggest that AAM may be targeted to alleviate exacerbations.

The macrophage

Macrophages are a diverse phenotype of professional phagocytic cells derived from bone-marrow precursors and parent monocytes in the peripheral blood. They are essential for the maintenance and defence of host tissues, doing so by sensing and engulfing particulate matter and, when necessary, initiating a pro-inflammatory response. Playing such a vast number of roles in both health and disease, the activation phenotype of macrophages can vary greatly and is largely dependent on the surrounding microenvironment. These phenotypes can be mimicked in experimental macrophage models derived from monocytes and in conjunction with stimulatory factors, although given the complexity of in vivo tissue spaces these model cells are inherently imperfect. Furthermore, experimental observations generated in mice are not necessarily conserved in humans, which can hamper translational research. The following chapter aims to provide an overview of how macrophages and their parent cell-type, monocytes, are classified, their development through the myeloid lineage, and finally, the general function of macrophages.

Adenosine promotes alternative macrophage activation via A2A and A2B receptors

Adenosine has been implicated in suppressing the proinflammatory responses of classically activated macrophages induced by Th1 cytokines. Alternative macrophage activation is induced by the Th2 cytokines interleukin (IL)-4 and IL-13; however, the role of adenosine in governing alternative macrophage activation is unknown. We show here that adenosine treatment of IL-4- or IL-13-activated macrophages augments the expression of alternative macrophage markers arginase-1, tissue inhibitor of matrix metalloproteinase-1 (TIMP-1), and macrophage galactose-type C-type lectin-1. The stimulatory effect of adenosine required primarily A(2B) receptors because the nonselective adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased both arginase activity (EC(50)=261.8 nM) and TIMP-1 production (EC(50)=80.67 nM), and both pharmacologic and genetic blockade of A(2B) receptors prevented the effect of NECA. A(2A) receptors also contributed to the adenosine augmentation of IL-4-induced TIMP-1 release, as both adenosine and NECA were less efficacious in augmenting TIMP-1 release by A(2A) receptor-deficient than control macrophages. Of the transcription factors known to drive alternative macrophage activation, CCAAT-enhancer-binding protein β was required, while cAMP response element-binding protein and signal transducer and activator of transcription 6 were dispensable in mediating the effect of adenosine. We propose that adenosine receptor activation suppresses inflammation and promotes tissue restitution, in part, by promoting alternative macrophage activation.

Identification and manipulation of tumor associated macrophages in human cancers

Evading immune destruction and tumor promoting inflammation are important hallmarks in the development of cancer. Macrophages are present in most human tumors and are often associated with bad prognosis. Tumor associated macrophages come in many functional flavors ranging from what is known as classically activated macrophages (M1) associated with acute inflammation and T-cell immunity to immune suppressive macrophages (M2) associated with the promotion of tumor growth. The role of these functionally different myeloid cells is extensively studied in mice tumor models but dissimilarities in markers and receptors make the direct translation to human cancer difficult. This review focuses on recent reports discriminating the type of infiltrating macrophages in human tumors and the environmental cues present that steer their differentiation. Finally, immunotherapeutic approaches to interfere in this process are discussed.

Advanced age impairs macrophage polarization

Aging affects many aspects of the cellular function of macrophages. Macrophages play a critical role in innate immunity, acting as sentinels to fight pathogens, promoting wound healing, and orchestrating the development of the specific acquired immune response. However, little is known about how age influences the ability of macrophage to change phenotypes in response to environmental factors. This study examined the age-associated defects on macrophage polarization toward a pro-inflammatory (M1) or an anti-inflammatory (M2) phenotype. Adherent splenocytes enriched for macrophages were cultured with or without lipopolysaccharide (LPS), a combination of interferon (IFN)-γ and tumor necrosis factor (TNF)-α or interleukin (IL)-4. A panel of M1 markers, inducible nitric oxide synthase (iNOS), IL-6, IL-1β, and TNF-α, and M2 markers, including arginase-1 (Arg1), Ym1, and Found In Inflammatory Zone 1 (FIZZ1), were analyzed. IL-6 mRNA in cells from aged mice was decreased by 78% and 58% compared with young after stimulation with LPS or IFN-γ and TNF-α (P<0.05), respectively. Also, there was a marked reduction in the induced levels of iNOS, IL-1β, and TNF-α in cells from aged mice relative to young controls. Similarly, IL-4 exposure resulted in a reduction of M2 markers in adherent splenocytes from aged mice compared with younger animals. This was consistent with a 28% decrease in splenic F4/80(+)IL-4R(+) cells in aged mice relative to controls, although IL-4R expression on these cells did not vary between age groups. In contrast, levels of M1 and most M2 markers, save for FIZZ1, in bone marrow-derived macrophages were similar between the age groups, irrespective of stimuli. These data imply that impaired macrophage polarization in the elderly may dysregulate the development of the host response, making them more susceptible to infectious diseases and that the aging microenvironment may be a key modulator of these macrophage-elicited responses.

Chlamydia-induced ReA: immune imbalances and persistent pathogens

Reactive arthritis (ReA), an inflammatory arthritic condition that is commonly associated with Chlamydia infections, represents a significant health burden, yet is poorly understood. The enigma of this disease is reflected in its problematic name and in its ill-defined pathogenesis. The existence of persistent pathogens in the arthritic joint is acknowledged, but their relevance remains elusive. Progress is being made in understanding the underlying mechanisms of ReA, whereby an imbalance between type 1 and type 2 immune responses seems to be critical in determining susceptibility to disease. Such an imbalance occurs prior to the initiation of an adaptive immune response, suggesting that innate cellular and molecular mechanisms in ReA should be prioritized as fruitful areas for investigation.

Regulatory macrophages: setting the threshold for therapy

Macrophages exhibit remarkable plasticity and can change their phenotype in response to different environmental cues. They can become activated to kill intracellular microbes or they can assume regulatory properties to modulate immune responses. Regulatory macrophages are fundamentally different from classically activated, and we propose from non-classically activated macrophages; they arise in response to different stimuli and perform different physiological functions. They are likely to express unique biochemical markers that could be exploited to identify and potentially target these macrophage subsets in tissue. Furthermore, inducers of regulatory macrophages may have the potential to be used as anti-inflammatory therapeutics. Therefore, a better understanding of the various macrophage phenotypes may pave the way for new therapies that are directed at modulating macrophage functions or manipulating individual macrophage subsets.

Macrophage activation governs schistosomiasis-induced inflammation and fibrosis

Macrophages regulate the initiation, maintenance, and resolution of chronic inflammatory responses and their function depends on their activation status. Studies in mice infected with the helminth parasite Schistosoma mansoni have been particularly helpful in defining the in vivo function of classically and alternatively activated macrophages (AAMϕs). These studies have shown that AAMϕs protect mice from acute and chronic S. mansoni infection through distinct mechanisms, which are discussed in this Viewpoint.

Regulation of neutrophil survival/apoptosis by Mcl-1

Neutrophil granulocytes have the shortest lifespan among leukocytes in the circulation and die via apoptosis. At sites of infection or tissue injury, prolongation of neutrophil lifespan is critical for effective host defense. Apoptosis of inflammatory neutrophils and their clearance are critical control points for termination of the inflammatory response. Evasion of neutrophil apoptosis aggravates local injury and leads to persistent tissue damage. The short-lived prosurvival Bcl-2 family protein, Mcl-1 (myeloid cell leukemia-1), is instrumental in controlling apoptosis and consequently neutrophil lifespan in response to rapidly changing environmental cues during inflammation. This paper will focus on multiple levels of control of Mcl-1 expression and function and will discuss targeting Mcl-1 as a potential therapeutic strategy to enhance the resolution of inflammation through accelerating neutrophil apoptosis.

Human macrophages primed with angiogenic factors show dynamic plasticity, irrespective of extracellular matrix components

Macrophages are important in inflammation as well as in tissue repair processes. They can be activated by various stimuli and classified into two major groups: M1 (classically activated) or M2 (alternatively activated). Inflammation, angiogenesis and matrix remodeling play a major role in tissue repair. Here, we investigate the combined influence of a pro-angiogenic microenvironment and specific extracellular matrix (ECM) components or tissue culture polystyrene (TCPS) on the dynamics of human macrophage polarization. We established that human angiogenically primed macrophages cultured on different ECM components exhibit an M2-like polarization. These M2-like macrophages polarized to M1 and M2 macrophages with classical (LPS and IFNγ) stimuli and alternative (IL-4 and IL-13) stimuli respectively. Moreover, these M1 and M2 (primary) polarized macrophages rapidly underwent a secondary (re)polarization to M2 and M1 with conditioned media from M2 and M1 primary polarized macrophages respectively. In these initial priming and later (re)polarization processes the soluble factors had a dominant and orchestrating role, while the type of ECM (collagen I, fibronectin, versus tissue culture polystyrene) did not play a crucial role on the polarization of macrophages.

Phenotypic and functional plasticity of cells of innate immunity: macrophages, mast cells and neutrophils

Hematopoietic cells, including lymphoid and myeloid cells, can develop into phenotypically distinct 'subpopulations' with different functions. However, evidence indicates that some of these subpopulations can manifest substantial plasticity (that is, undergo changes in their phenotype and function). Here we focus on the occurrence of phenotypically distinct subpopulations in three lineages of myeloid cells with important roles in innate and acquired immunity: macrophages, mast cells and neutrophils. Cytokine signals, epigenetic modifications and other microenvironmental factors can substantially and, in some cases, rapidly and reversibly alter the phenotype of these cells and influence their function. This suggests that regulation of the phenotype and function of differentiated hematopoietic cells by microenvironmental factors, including those generated during immune responses, represents a common mechanism for modulating innate or adaptive immunity.

Protective and pathogenic functions of macrophage subsets

Macrophages are strategically located throughout the body tissues, where they ingest and process foreign materials, dead cells and debris and recruit additional macrophages in response to inflammatory signals. They are highly heterogeneous cells that can rapidly change their function in response to local microenvironmental signals. In this Review, we discuss the four stages of orderly inflammation mediated by macrophages: recruitment to tissues; differentiation and activation in situ; conversion to suppressive cells; and restoration of tissue homeostasis. We also discuss the protective and pathogenic functions of the various macrophage subsets in antimicrobial defence, antitumour immune responses, metabolism and obesity, allergy and asthma, tumorigenesis, autoimmunity, atherosclerosis, fibrosis and wound healing. Finally, we briefly discuss the characterization of macrophage heterogeneity in humans.

Protective and pathogenic functions of macrophage subsets

Macrophages are strategically located throughout the body tissues, where they ingest and process foreign materials, dead cells and debris and recruit additional macrophages in response to inflammatory signals. They are highly heterogeneous cells that can rapidly change their function in response to local microenvironmental signals. In this Review, we discuss the four stages of orderly inflammation mediated by macrophages: recruitment to tissues; differentiation and activation in situ; conversion to suppressive cells; and restoration of tissue homeostasis. We also discuss the protective and pathogenic functions of the various macrophage subsets in antimicrobial defence, antitumour immune responses, metabolism and obesity, allergy and asthma, tumorigenesis, autoimmunity, atherosclerosis, fibrosis and wound healing. Finally, we briefly discuss the characterization of macrophage heterogeneity in humans.

Macrophages in synovial inflammation

Synovial macrophages are one of the resident cell types in synovial tissue and while they remain relatively quiescent in the healthy joint, they become activated in the inflamed joint and, along with infiltrating monocytes/macrophages, regulate secretion of pro-inflammatory cytokines and enzymes involved in driving the inflammatory response and joint destruction. Synovial macrophages are positioned throughout the sub-lining layer and lining layer at the cartilage–pannus junction and mediate articular destruction. Sub-lining macrophages are now also considered as the most reliable biomarker for disease severity and response to therapy in rheumatoid arthritis (RA). There is a growing understanding of the molecular drivers of inflammation and an appreciation that the resolution of inflammation is an active process rather than a passive return to homeostasis, and this has implications for our understanding of the role of macrophages in inflammation. Macrophage phenotype determines the cytokine secretion profile and tissue destruction capabilities of these cells. Whereas inflammatory synovial macrophages have not yet been classified into one phenotype or another it is widely known that TNFα and IL-l, characteristically released by M1 macrophages, are abundant in RA while IL-10 activity, characteristic of M2 macrophages, is somewhat diminished. Here we will briefly review our current understanding of macrophages and macrophage polarization in RA as well as the elements implicated in controlling polarization, such as cytokines and transcription factors like NFκB, IRFs and NR4A, and pro-resolving factors, such as LXA4 and other lipid mediators which may promote a non-inflammatory, pro-resolving phenotype, and may represent a novel therapeutic paradigm.

Macrophages as a battleground for toxoplasma pathogenesis

In this issue of Cell Host & Microbe, Jensen et al. (2011) show that clonal lineages of Toxoplasma gondii have evolved distinct ways of subverting their favored host cell, the macrophage. The results suggest that T. gondii and the ROP kinases can be used to probe immune signaling pathways.

Macrophage polarization: convergence point targeted by mycobacterium tuberculosis and HIV

In the arms race of host–microbe co-evolution, macrophages (Mɸs) have been endowed with strategies to neutralize pathogenic challenge while preserving host integrity. During steady-states conditions, Mɸs perform multiple house-keeping functions governed by their differentiation state, tissue distribution, and signals from the microenvironment. In response to pathogenic challenge and host mediators, however, Mɸs undergo different programs of activation rendering them either pro-inflammatory and microbicidal (M1), or immunosuppressants and tissue repairers (M2). An excessive or prolonged polarization of either program may be detrimental to the host due to potential tissue injury or contribution to pathogenesis. Conversely, intracellular microbes that cause chronic diseases such as tuberculosis and acquired immunodeficiency syndrome exemplify strategies for survival in the host. Indeed, both Mycobacterium tuberculosis (Mtb) and human immunodeficiency virus (HIV-1) are successful intracellular microbes that thrive in Mɸs. Given these microbes not only co-circulate throughout the developing world but each has contributed to prevalence and mortality caused by the other, substantial insights into microbe physiology and host defenses then rest in the attempt to fully understand their influence on Mɸ polarization. This review addresses the role of Mɸ polarization in the immune response to, and pathogenesis of, Mtb and HIV.

Inflammatory signaling in macrophages: transitions from acute to tolerant and alternative activation states

Acute inflammatory activation of macrophages by Toll-like and related receptors is characterized by transient activation of MAPK-, NF-κB- and IRF-mediated signaling pathways and expression of pro-inflammatory genes. This activation state is inherently unstable and often transitions into a state of 'tolerance' characterized by diminished signaling, repressive chromatin modifications, and an alternative gene expression program. This Viewpoint describes signaling and epigenetic mechanisms associated with transition to tolerant states, which are proposed to correspond to alternative activation states programmed by the original inflammatory stimuli.

Scavenger receptor SR-BI in macrophage lipid metabolism

OBJECTIVE

To investigate the mechanisms by which macrophage scavenger receptor BI (SR-BI) regulates macrophage cholesterol homeostasis and protects against atherosclerosis.

METHODS AND RESULTS

The expression and function of SR-BI was investigated in cultured mouse bone marrow-derived macrophages (BMM). SR-BI, the other scavenger receptors SRA and CD36 and the ATP-binding cassette transporters ABCA1 and ABCG1 were each distinctly regulated during BMM differentiation. SR-BI levels increased transiently to significant levels during culture. SR-BI expression in BMM was reversibly down-regulated by lipid loading with modified LDL; SR-BI was shown to be present both on the cell surface as well as intracellularly. BMM exhibited selective HDL CE uptake, however, this was not dependent on SR-BI or another potential candidate glycosylphosphatidylinositol anchored high density lipoprotein binding protein 1 (GPIHBP1). SR-BI played a significant role in facilitating bidirectional cholesterol flux in non lipid-loaded cells. SR-BI expression enhanced both cell cholesterol efflux and cholesterol influx from HDL, but did not lead to altered cellular cholesterol mass. SR-BI-dependent efflux occurred to larger HDL particles but not to smaller HDL(3). Following cholesterol loading, ABCA1 and ABCG1 were up-regulated and served as the major contributors to cholesterol efflux, while SR-BI expression was down-regulated.

CONCLUSION

Our results suggest that SR-BI plays a significant role in macrophage cholesterol flux that may partly account for its effects on atherogenesis.