Visualizing Macrophage Phenotypes and Polarization in Diseases: From Biomarkers to Molecular Probes

Macrophage is a kind of immune cell and performs multiple functions including pathogen phagocytosis, antigen presentation and tissue remodeling. To fulfill their functionally distinct roles, macrophages undergo polarization towards a spectrum of phenotypes, particularly the classically activated (M1) and alternatively activated (M2) subtypes. However, the binary M1/M2 phenotype fails to capture the complexity of macrophages subpopulations in vivo. Hence, it is crucial to employ spatiotemporal imaging techniques to visualize macrophage phenotypes and polarization, enabling the monitoring of disease progression and assessment of therapeutic responses to drug candidates. This review begins by discussing the origin, function and diversity of macrophage under physiological and pathological conditions. Subsequently, we summarize the identified macrophage phenotypes and their specific biomarkers. In addition, we present the imaging probes locating the lesions by visualizing macrophages with specific phenotype in vivo. Finally, we discuss the challenges and prospects associated with monitoring immune microenvironment and disease progression through imaging of macrophage phenotypes.

Communications between macrophages and cardiomyocytes

The heart is a muscular organ that pumps blood throughout the body and is one of the most vital organs in human body. While cardiomyocytes are essential for maintaining the normal function of the heart, a variety of cardiovascular diseases such as coronary artery occlusion, arrhythmia, and myocarditis can lead to cardiomyocyte death, resulting in deterioration of heart function. The adult mammalian heart is incapable of regenerating sufficient cardiomyocytes following cardiac injuries, eventually leading to heart failure and death. Cardiac macrophages are ubiquitously distributed in the healthy heart and accumulated at the site of injury. Macrophages play essential roles in regulating homeostasis and proliferation of cardiomyocyte, promoting electrical conduction, and removing dead cardiomyocytes and debris through direct and indirect cell-cell crosstalk. In this review, we summarize the latest insights into the role of macrophages in maintaining cardiac homeostasis and the macrophage-cardiomyocyte crosstalk in both healthy and injured scenarios. Video Abstract.

In vitro and in vivo evaluation of cerium oxide nanoparticles in respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the most common cause of viral bronchiolitis among children worldwide, yet there is no vaccine for RSV disease. This study investigates the potential of cube and sphere-shaped cerium oxide nanoparticles (CNP) to modulate reactive oxygen (ROS) and nitrogen (RNS) species and immune cell phenotypes in the presence of RSV infection in vitro and in vivo. Cube and sphere-shaped CNP were synthesized by hydrothermal and ultrasonication methods, respectively. Physico-chemical characterization confirmed the shape of sphere and cube CNP and effect of various parameters on their particle size distribution and zeta potential. In vitro results revealed that sphere and cube CNP differentially modulated ROS and RNS levels in J774 macrophages. Specifically, cube CNP significantly reduced RSV-induced ROS levels without affecting RNS levels while sphere CNP increased RSV-induced RNS levels with minimal effect on ROS levels. Cube CNP drove an M1 phenotype in RSV-infected macrophages in vitro by increasing macrophage surface expression of CD80 and CD86 with a concomitant increase in TNFα and IL-12p70, while simultaneously decreasing M2 CD206 expression. Intranasal administration of sphere and cube-CNP were well-tolerated with no observed toxicity in BALB/c mice. Notably, cube CNP preferentially accumulated in murine alveolar macrophages and induced their activation, avoiding enhanced uptake and activation of other inflammatory cells such as neutrophils, which are associated with RSV-mediated inflammation. In conclusion, we report that sphere and cube CNP modulate macrophage polarization and innate cellular responses during RSV infection.

PTEN inhibitor attenuates cardiac fibrosis by regulating the M2 macrophage phenotype via the PI3K/AKT/TGF-β/Smad 2/3 signaling pathway

Cardiac fibrosis is a key feature of hypertensive cardiac remodeling. In response to microenvironmental stimuli, phenotypic and functional changes in macrophages are considered important determinants of cardiac fibrosis attenuation. VO-OHpic, a phosphatase and tension homolog of chromosome 10 (PTEN) inhibitor, has been demonstrated to be cardioprotective in cardiac remodeling. However, whether VO-OHpic can improve cardiac fibrosis and macrophage polarization remains elusive. The interaction between VO-OHpic and the macrophage phenotype to attenuate cardiac fibrosis was studied in both spontaneously hypertensive rats in vivo and an Ang II-induced hypertension model in vitro. In vitro experiments showed that VO-OHpic promoted M2 macrophage polarization and markedly inhibited proinflammatory M1 macrophages, while VO-OHpic treatment of protein kinase B (AKT)-knockdown/LY294002 (a PI3K inhibitor) macrophages exerted a reduced effect. In a coculture system, culturing cardiac fibroblasts with VO-OHpic-treated macrophages led to significant suppression of proliferation, fibrotic marker expression, and transforming growth factor (TGF)-β and Smad 2/3 protein expression. Taken together, VO-OHpic mediated a fibro-protective effect and increased M2 macrophage polarization via the phosphatidylinositol 3-kinase (PI3K)/AKT/TGF-β/Smad2/3 pathway.

Re-establishment of macrophage homeostasis by titanium surface modification in type II diabetes promotes osseous healing

Titanium surface mediated immunomodulation may address compromised post-implantation bone healing in diabetes mellitus. To assess in vitro phenotypic changes, M1 and M2 polarised Type 2 diabetic rat (Goto Kakizaki, GK) macrophages were cultured on micro-rough (SLA) or hydrophilic nanostructured SLA (modSLA) titanium. The in vivo effects of the SLA and modSLA surfaces on macrophage phenotype, wound-associated protein expression and bone formation were investigated using a critical-sized calvarial defect model. Compared to healthy macrophages, GK M2 macrophage function was compromised, secreting significantly lower levels of the anti-inflammatory cytokine IL-10. The modSLA surface attenuated the pro-inflammatory cellular environment, reducing pro-inflammatory cytokine production and promoting M2 macrophage phenotype differentiation. ModSLA also suppressed gene expression associated with macrophage multinucleation and giant cell formation and stimulated pro-osteogenic genes in co-cultured osteoblasts. In vivo, modSLA enhanced osteogenesis compared to SLA in GK rats. During early healing, proteomic analysis of both surface adherent and wound exudate material showed that modSLA promoted an immunomodulatory pro-reparative environment. The modSLA surface therefore successfully compensated for the compromised M2 macrophage function in Type 2 diabetes by attenuating the pro-inflammatory response and promoting M2 macrophage activity, thus restoring macrophage homeostasis and resulting in a cellular environment favourable for enhanced osseous healing.

Isolation and Phenotypic Characterization of Inflammatory Cells from Clinical Samples: Purification of Macrophages from Trypanosoma cruzi-Infected Hearts

Trypanosoma cruzi, the causal agent of chronic Chagas cardiomyopathy, exhibits an important tropism for cardiac tissue. In consequence, T. cruzi experimental infection represents a unique model to study cardiac macrophage behavior and effector functions during either acute or chronic immune response. In this chapter we describe a protocol to isolate immune cells from T. cruzi-infected murine cardiac tissue and to determine the percentage, absolute number, phenotype, and functionality of monocytes and macrophages by using flow cytometry. Moreover, we describe the parameters to discriminate between resident and infiltrating mononuclear phagocytic cells within infected hearts. The investigations in this field will provide mechanistic insights about the roles of these innate immune cells in the context of a clinically relevant target tissue.

Harnessing Macrophages for Vascularization in Tissue Engineering

In this review, we explore the roles of macrophages both in vessel development and in vascularization of tissue engineered constructs. Upon the implantation of tissue engineered constructs into the body, macrophages respond, invade and orchestrate the host's immune response. By altering their phenotype, macrophages can adopt a variety of roles. They can promote inflammation at the site of the implanted construct; they can also promote tissue repair. Macrophages support tissue repair by promoting angiogenesis through the secretion of pro-angiogenic cytokines and by behaving as support cells for nascent vasculature. Thus, the ability to manipulate the macrophage phenotype may yield macrophages capable of supporting vessel development. Moreover, macrophages are an easily isolated autologous cell source. For the generation of vascularized constructs outside of the body, these isolated macrophages can also be skewed to adopt a pro-angiogenic phenotype and enhance blood vessel development in the presence of endothelial cells. To assess the influence of macrophages on vessel development, both in vivo and in vitro models have been developed. Additionally, several groups have demonstrated the pro-angiogenic roles of macrophages in vascularization of tissue engineered constructs through the manipulation of macrophage phenotypes. This review comments on the roles of macrophages in promoting vascularization within these contexts.

Promoting in vivo early angiogenesis with sub-micrometer strontium-contained bioactive microspheres through modulating macrophage phenotypes

Early vascularization capacity of biomaterials plays an essential role in efficient wound healing and tissue regeneration, especially in large tissue tension implanting position such as bone augmentation. Strontium-contained silica-based bioactive materials have shown the role of promoting angiogenesis by stimulating osteoblasts to secrete angiogenesis related cytokines. However, osteoblasts have little effect on early angiogenesis due to the inflammatory reaction of implantation site. Here, for the first time, we found that the monodispersed strontium-contained bioactive glasses microspheres (SrBGM) could significantly promote the early angiogenesis through regulating macrophage phenotypes. After being stimulated with SrBGM in vitro, RAW cells (macrophages) presented a trend towards to M2 phenotype and expressed high level of platelet-derived growth factor-BB (PDGF-BB). Moreover, the RAW conditioned medium of SrBGM significantly enhanced the angiogenic capacity of HUVECs. The in vivo early vascularization studies showed that significant new vessels were observed at the center of SrBGM-based scaffolds after implantation for 1 week in a bone defect model of rats, suggesting their enhanced early vascularization. Due to the efficient vascularization, the in vivo new bone formation was promoted significantly. Our study may provide a novel strategy to promote the early vascularization of biomaterials through modulating the microphage phenotypes, which has wide applications in various tissue regeneration and wound healing.

Cartilage inflammation and degeneration is enhanced by pro-inflammatory (M1) macrophages in vitro, but not inhibited directly by anti-inflammatory (M2) macrophages

OBJECTIVE

Macrophages play a crucial role in the progression of osteoarthritis (OA). Their phenotype may range from pro-inflammatory to anti-inflammatory. The aim of this study was to evaluate the direct effects of macrophage subtypes on cartilage by culturing macrophage conditioned medium (MCM) on human articular cartilage.

DESIGN

Human OA cartilage explants were cultured with MCM of pro-inflammatory M(IFNγ+TNFα), or anti-inflammatory M(IL-4) or M(IL-10) human monocyte-derived macrophages. To assess effects of anti-inflammatory macrophages, the cartilage was cultured with a combination of MCM phenotypes as well as pre-stimulated with IFNγ+TNFα cartilage before culture with MCM. The reactions of the explants were assessed by gene expression, nitric oxide (NO) production and release of glycosaminoglycans (GAGs).

RESULTS

M(IFNγ+TNFα) MCM affected OA cartilage by upregulation of IL1B (Interleukin 1β), IL6, MMP13 (Matrix Metalloproteinase-13) and ADAMTS5 (A Disintegrin And Metalloproteinase with Thrombospondin Motifs-5), while inhibiting ACAN (aggrecan) and COL2A1 (collagen type II). M(IL-10) upregulated IL1B and Suppressor of cytokine signaling 1 (SOCS1). NO production and GAG release by the cartilage was increased when cultured with M(IFNγ+TNFα) MCM. M(IL-4) and M(IL-10) did not inhibit the effects of M(IFNγ+TNFα) MCM of neither phenotype affected IFNγ+TNFα pre-stimulated cartilage, in which an inflammatory gene response was deliberately induced.

CONCLUSION

M(IFNγ+TNFα) macrophages have a prominent direct effect on OA cartilage, while M(IL-4) and M(IL-10) do not inhibit the effects of M(IFNγ+TNFα), or IFNγ+TNFα induced inflammation of the cartilage. Therapies aiming at inhibiting cartilage degeneration may take this into account by directing suppression of pro-inflammatory macrophages or stimulation of anti-inflammatory macrophages.

Guiding synovial inflammation by macrophage phenotype modulation: an in vitro study towards a therapy for osteoarthritis

OBJECTIVE

The aims of this study were to modulate inflammation in synovial explants with the compounds: dexamethasone, rapamycin, bone morphogenetic protein 7 (BMP-7) and pravastatin, and to investigate the modulatory capacity of the compounds on specific macrophage phenotypes.

DESIGN

Synovial explants from osteoarthritis (OA) patients were treated with 10(-6) M dexamethasone, 100 ng/mL rapamycin, 500 ng/mL BMP-7 or 50 μM pravastatin. Half of the explants were pre-stimulated with IFNγ + TNFα to simulate acute inflammation. Inflammatory state of the synovium was assessed with gene expression analysis. Primary human monocytes were isolated and stimulated towards macrophage phenotypes M(IFNγ + TNFα), M(IL-4) and M(IL-10) with the respective cytokines, followed by treatment with the compounds.

RESULTS

Dexamethasone had an anti-inflammatory effect on IFNγ + TNFα stimulated and osteoarthritic synovium, likely due to suppression of pro-inflammatory M(IFNγ + TNFα) macrophages while enhancing anti-inflammatory M(IL4) and M(IL10) macrophages. Rapamycin and BMP-7 further enhanced inflammation in stimulated synovium, but rapamycin did not have a clear effect on non-stimulated synovium. Rapamycin suppressed M(IL-4) and M(IL-10) macrophages without affecting M(IFNγ + TNFα). BMP-7 suppressed M(IFNγ + TNFα) and enhanced M(IL-10) in the macrophage cultures. Pravastatin did not affect synovium, but enhanced M(IL-10).

CONCLUSIONS

These data indicate that macrophage phenotype modulation can be used to guide joint inflammation and thereby contribute to the development of new therapies to delay the progression of OA. The varying effects of the compounds on synovium of different degrees of inflammation, indicate that the modulatory capacity of the compounds depends on OA stage and underlines the importance of identifying this stadium for adequate treatment.

Astaxanthin inhibits inflammation and fibrosis in the liver and adipose tissue of mouse models of diet-induced obesity and nonalcoholic steatohepatitis

The objective of this study was to determine if astaxanthin (ASTX), a xanthophyll carotenoid, can prevent obesity-associated metabolic abnormalities, inflammation and fibrosis in diet-induced obesity (DIO) and nonalcoholic steatohepatitis (NASH) mouse models. Male C57BL/6J mice were fed a low-fat (6% fat, w/w), a high-fat/high-sucrose control (HF/HS; 35% fat, 35% sucrose, w/w), or a HF/HS containing ASTX (AHF/HS; 0.03% ASTX, w/w) for 30 weeks. To induce NASH, another set of mice was fed a HF/HS diet containing 2% cholesterol (HF/HS/HC) a HF/HS/HC with 0.015% ASTX (AHF/HS/HC) for 18 weeks. Compared to LF, HF/HS significantly increased plasma total cholesterol, triglyceride and glucose, which were lowered by ASTX. ASTX decreased hepatic mRNA levels of markers of macrophages and fibrosis in both models. The effect of ASTX was more prominent in NASH than DIO mice. In epididymal fat, ASTX also decreased macrophage infiltration and M1 macrophage marker expression, and inhibited hypoxia-inducible factor 1-α and its downstream fibrogenic genes in both mouse models. ASTX significantly decreased tumor necrosis factor α mRNA in the splenocytes from DIO mice upon lipopolysaccharides stimulation compared with those from control mice fed an HF/HS diet. Additionally, ASTX significantly elevated the levels of genes that regulate fatty acid β-oxidation and mitochondrial biogenesis in the skeletal muscle compared with control obese mice, whereas no differences were noted in adipose lipogenic genes. Our results indicate that ASTX inhibits inflammation and fibrosis in the liver and adipose tissue and enhances the skeletal muscle's capacity for mitochondrial fatty acid oxidation in obese mice.

The Effects of Co-transplantation of Olfactory Ensheathing Cells and Schwann Cells on Local Inflammation Environment in the Contused Spinal Cord of Rats

Inflammatory response following spinal cord injury (SCI) is important in regulation of the repair process. Olfactory ensheathing cells (OECs) and Schwann cells (SCs) are important donor cells for repairing SCI in different animal models. However, synergistic or complementary effects of co-transplantation of both cells for this purpose have not been extensively investigated. In the present study, we investigated the effects of co-transplantation of OECs and SCs on expression of pro- or anti-inflammatory factor and polarization of macrophages in the injured spinal cord of rats. Mixed cell suspensions containing OECs and SCs were transplanted into the injured site at 7 days after contusion at the vertebral T10 level. Compared with the DMEM, SC, or OEC group, the co-transplantation group had a more extensive distribution of the grafted cells and significantly reduced number of astrocytes, microglia/macrophage infiltration, and expression of chemokines (CCL2 and CCL3) at the injured site. The co-transplantation group also significantly increased arginase⁺/CD206⁺ macrophages (IL-4) and decreased iNOS⁺/CD16/32⁺ macrophages (IFN-γ), which was followed by higher IL-10 and IL-13 and lower IL-6 and TNF-α in their expression levels, a smaller cystic cavity area, and improved motor functions. These results indicate that OEC and SC co-transplantation could promote the shift of the macrophage phenotype from M(IFN-γ) to M(IL-4), reduce inflammatory cell infiltration in the injured site, and regulate inflammatory factors and chemokine expression, which provide a better immune environment for SCI repair.

The neuron-derived orphan receptor 1 (NOR1) is induced upon human alternative macrophage polarization and stimulates the expression of markers of the M2 phenotype

BACKGROUND

Atherosclerosis is an inflammatory disease in which macrophages play a crucial role. Macrophages are present in different phenotypes, with at the extremes of the spectrum the classical M1 pro-inflammatory and the alternative M2 anti-inflammatory macrophages. The neuron-derived orphan receptor 1 (NOR1), together with Nur77 and Nurr1, are members of the NR4A orphan nuclear receptor family, expressed in human atherosclerotic lesion macrophages. However, the role of NOR1 in human macrophages has not been studied yet.

OBJECTIVES

To determine the expression and the functions of NOR1 in human alternative macrophages.

METHODS AND RESULTS

In vitro IL-4 polarization of primary monocytes into alternative M2 macrophages enhances NOR1 expression in human but not in mouse macrophages. Moreover, NOR1 expression is most abundant in CD68+MR+ alternative macrophage-enriched areas of human atherosclerotic plaques in vivo. Silencing NOR1 in human alternative macrophages decreases the expression of several M2 markers such as the Mannose Receptor (MR), Interleukin-1 Receptor antagonist (IL-1Ra), CD200 Receptor (CD200R), coagulation factor XIII A1 polypeptide (F13A1), Interleukin 10 (IL-10) and the Peroxisome Proliferator-Activated Receptor (PPAR)γ. Bioinformatical analysis identified F13A1, IL-1Ra, IL-10 and the Matrix Metalloproteinase-9 (MMP9) as potential target genes of NOR1 in human alternative macrophages. Moreover, expression and enzymatic activity of MMP9 are induced by silencing and repressed by NOR1 overexpression in M2 macrophages.

CONCLUSIONS

These data identify NOR1 as a transcription factor induced during alternative differentiation of human macrophages and demonstrate that NOR1 modifies the alternative macrophage phenotype.